sensors Article
In-Process Atomic-Force Microscopy (AFM) Based Inspection Samir Mekid Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia; [email protected]; Tel.: +966-13-860-7746 Academic Editor: Vittorio M. N. Passaro Received: 23 March 2017; Accepted: 19 May 2017; Published: 31 May 2017
Abstract: A new in-process atomic-force microscopy (AFM) based inspection is presented for nanolithography to compensate for any deviation such as instantaneous degradation of the lithography probe tip. Traditional method used the AFM probes for lithography work and retract to inspect the obtained feature but this practice degrades the probe tip shape and hence, affects the measurement quality. This paper suggests a second dedicated lithography probe that is positioned back-to-back to the AFM probe under two synchronized controllers to correct any deviation in the process compared to specifications. This method shows that the quality improvement of the nanomachining, in progress probe tip wear, and better understanding of nanomachining. The system is hosted in a recently developed nanomanipulator for educational and research purposes. Keywords: in-process inspection; AFM; nanomachining; probes; nanoscale inspection
1. Introduction Manufacture of small components with ever increasing accuracy is highly requested in the market of precision systems [1]. Some nanoscale features are machined with highly requested accuracy and are predicted to reach the subnanometer level in 2020 by Tanigushi [2]. The nanomachining and micromachining are however, associated with their own related phenomena. Effects of van der Waals force are more apparent at nanoscale, while adhesion between asperities, fracture, surface layer formation, and debris formation are important at microscale. It is known that phenomena of materials fracture are manifold since they are governed by the microscopic properties of a material that are varying from one material to another. Although ultra-precision machines are designed to achieve this ultra-precision [3], machining conserves its share of errors and needs compensation in in-process. Hence, fabricating at nanoscale and microscale may not be very deterministic if the functions of manufacturing e.g., material removing is carried out, the part is removed, and the obtained features are inspected and expected to be very close to specifications. At this scale, it becomes important to inspect while in-process and correct if there is a need to stay within the specifications for the reasons discussed previously. This approach of in-process inspection has been used in manufacturing for decades using different measurements techniques on machine tools for standard parts. A review has been carried out [4] for standard scale machines. Recent sensors have been proposed to measure specific features such as roundness [5,6], surface finish [7], and dimensional measurement [8]. The search of the public literature shows little contribution in the area of in-process inspection at nanoscale. The characterization of the nano-lithography is usually carried out by the same atomic-force microscope (AFM) probe used to remove material, showing quality measurement degradation [9] and using AFM for nano-patterning and imaging for mask repair based on the same principle [10]. Other work included multi pass scratching method with depth prediction using an AFM tip [11].
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The AFM probe can be used for specific lithography work and retract back to inspect the workSensors done,2017, but17,this 1194 practice will degrade the probe tip and hence, affects the measurement 2 ofquality. 10 To avoid this, we propose in this paper to add a second probe for lithography work that is designed The AFM probe can beto used specific lithography work and retract to inspect and assembled back-to-back an for AFM probe within one system. Theback AFM probe the willwork keep its done, but quality this practice will being degrade the probebytip and hence, affects the measurement quality. To measurement without disturbed the other degrading tasks. avoid this, we propose in this paper to add a second probe for lithography work that is designed and A new era has been triggered in engineering and science where material properties can be assembled back-to-back to an AFM probe within one system. The AFM probe will keep its designed and controlled at the level of individual atoms and molecules. The proposed in-process measurement quality without being disturbed by the other degrading tasks. inspectionAserves thehas following specificinobjectives: new era been triggered engineering and science where material properties can be (a)
designed and controlled at the level of individual atoms and molecules. The proposed in-process To provide nano-machining processes with in-process inspection using back-to-back probes one inspection serves the following specific objectives:
(b) (c)
(a) To provide nano-machining processes with in-process inspectioncharacteristics. using back-to-back probes one To build nano-devices with specific geometries and particular of which is AFMs. To inspect and characterize complex shapes at corresponding accuracies. (b) To build nano-devices with specific geometries and particular characteristics. (c) To inspect and characterize complex shapes at corresponding accuracies.
of which is AFMs.
2. Proposed Design System
2. Proposed Design System
2.1. Host System Description
2.1. Hoston System Descriptionreviews and objectives, we propose a recently developed in-process Based the previous inspection for onnanomachining based AFM probing system. host in-process system is a Based the previous reviews andon objectives, we propose a recently The developed inspection for nanomachining basedfor on sample AFM probing system. The system manipulation, is a nanomanipulator nanomanipulator having one side preparation andhost robotics and, the having one side for sample preparation and robotics manipulation, and, the other side for 1). other side for nanomachining such as nanolithography with in-process inspection (Figure nanomachining such as nanolithography with in-process inspection (Figure 1). This instrument This instrument encompasses nano-manufacturing with in-process inspection, assembly, and encompasses nano-manufacturing with in-process inspection, assembly, and inspection/characterization of material particles or parts at the nanoscale. The instrument comprises inspection/characterization of material particles or parts at the nanoscale. The instrument comprises all operations in one single rotating stage keeping the same datum for the samples that are under all operations in one single rotating stage keeping the same datum for the samples that are under investigation. This rotating stage has three zones. The first one is the sample preparation zone. investigation. This rotating stage has three zones. The first one is the sample preparation zone. The The second oneisisfor forsamples samples manipulation and equipped withrobotic three arms, robotic arms, each having second one manipulation and equipped with three each having three threedegrees degreesofoffreedom freedom(DOF) (DOF) with a nanometer resolution 10 mm range of motion. with a nanometer resolution overover a 10a mm range of motion. This This manipulation areaarea is monitored havingaaresolution resolution 1 µm. inspect manipulation is monitoredvia viaa aCCD CCDcamera camera having of of 1 µm. ThisThis can can inspect samples and probes. The third zone has the AFM inspection with in-process (i.e., nanomachining) samples and probes. The third zone has the AFM inspection with in-process (i.e., nanomachining) inspection. The nanomachining encompasses numerically controlled nanomachining e.g., inspection. The nanomachining encompasses numerically controlled nanomachining e.g., lithography with its probe mounted back-to-back an AFM for in-process inspection a new with lithography its probe mounted back-to-back to an AFMtoprobe forprobe in-process inspection as a as new feature. feature. A suction device is added to clean the on-going machining debris. A top view of the whole A suction device is added to clean the on-going machining debris. A top view of the whole instrument instrument is shown in Figure 2. Most parts are homemade except technology hardware e.g., AFM, is shown in Figure 2. Most parts are homemade except technology hardware e.g., AFM, robotic arms, robotic arms, rotating stage. rotating stage.
Figure 1. Schematic of the integrated manipulator and overall coordinate system. Figure 1. Schematic of the integrated manipulator and overall coordinate system.
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Figure 2. Nanomanipulator platform with nanomachining.
Figure 2. Nanomanipulator platform with nanomachining. Figure 2. Nanomanipulator platform with nanomachining. All components are controlled based on the control architecture described in Figure 3. The All components controlled based for on in-process the control architecture in Figure 3. The highlighted colorfulare controllers are mainly inspection. Some described vibrations can be noticed All components are controlled based on the control architecture described in Figure 3. The of the AFM and cutting probes to an for environment by Some isolating the nanomanipulator highlighted colorful controllers aredue mainly in-processminimized inspection. vibrations can be noticed of highlighted controllers are mainly for in-process inspection. Some vibrations can be noticed fromand thecolorful ground through controlled damping table. the AFM cuttingvibration probes due to anaenvironment minimized by isolating the nanomanipulator from
of the AFM and cutting probes due to an environment minimized by isolating the nanomanipulator the ground vibration through a controlled damping table. from the ground vibration through a controlled damping table.
Figure 3. Cross section of the nanomanipulator with in-process AFM inspection.
The focus of this paper is on the in-process inspection zone. An AFM is mounted on XY precision stage (Figure 3) with locks for each axis, so it is possible to adjust the probe height by a few millimeters to accommodate sizes. piezoelectric scanner nanomachining is mounted close to Figure various 3. Cross Crosspart section of The the nanomanipulator nanomanipulator withfor in-process AFM inspection. inspection. Figure 3. section of the with in-process AFM the AFM on a separate platform that is firmly mounted of the supporting platform. A space under the probes is allowed to receive the rotating table and to place samples under the two probes when The focus of this paper is on on the the in-process in-process inspection inspection zone. An An AFM AFM is is mounted mounted on on XY XY precision precision needed.
stage (Figure 3) with locks for each axis, so it is is possible possible to to adjust adjust the the probe probe height height by by aa few few millimeters millimeters to accommodate various part sizes. The piezoelectric scanner for nanomachining is mounted close to the AFM on on aa separate separateplatform platformthat thatisisfirmly firmly mounted supporting platform. A space under mounted of of thethe supporting platform. A space under the the probes is allowed to receive the rotating tabletoand to samples place samples under two when probesneeded. when probes is allowed to receive the rotating table and place under the twothe probes needed.
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The two AFM and lithography probes are mounted back-to-back with physical distance Sensorstwo 2017, 17, 1194 and lithography probes are mounted back-to-back with physical4 of 10 The distance adjustment set AFM to 5 µm as a security distance to avoid any crash between the probes. The two heads adjustment set to 5 µm as a security distance to avoid any crash between the probes. The were littleThe modified withand covers removed and with little adjustment of the piezoscanner. Only two one two AFM lithography probes are mounted back-to-back with physical distance heads were little will modified with covers removed and little the piezoscanner. Only top view camera available for both systems aswith shown in adjustment Figure for of positioning. The detailed adjustment set to 5beµm as a security distance to avoid any crash between4 the probes. The two heads one top view camerainwill be available both systems asbetween shown inthe Figure 4 for The positions 4 and 5 for where distance is 5positioning. µm. Theone probes were are littleshown modified Figures with covers removed andthe with little adjustment of theprobes piezoscanner. Only detailed positions are shown in Figures 4 and 5 where the distance between the probes is 5 µm. top view camera will be available for both systems as shown Figure The 4 for sample positioning. Thetested detailed can safely move horizontally without crashing against eachin other. to be canThe be probes can safely move without crashing against each other. The is sample beprobes tested can positions are shown inprobes Figuresusing 4 and 5 where thethe distance between the probes 5 µm. to The adjusted underneath thehorizontally one axis of table. be adjusted underneath the probes usingcrashing one axisagainst of the each table.other. The sample to be tested can be can safely move horizontally without adjusted underneath the probes using one axis of the table.
Figure 4. Position AFMand andlithography lithography probes actual configuration. Figure 4. Position ofof AFM probeson onthe the actual configuration. Figure 4. Position of AFM and lithography probes on the actual configuration.
Figure 5. Bottom view of back-to-back probes probes (left: AFM cantilever, right: Lithography: Figure 5. Bottom view of back-to-back (left:probe AFMNCLR-10 probe NCLR-10 cantilever, right: Lithography: A-probe probe) cantilever). A-probe (Akiyama probe)(Akiyama cantilever).
Figure Bottom view of back-to-back probe NCLR-10 cantilever, right: The5.AFM inspection is carried out by aprobes Nanite (left: AFM AFM scan head with a maximum scan range of The AFM inspection is carried out by a Nanite AFM scan head with a maximum scan range of XY Lithography: A-probe (Akiyama probe) cantilever). XY 110 µm × 22 µm needing standard AFM cantilevers, while for the lithography, a scan head Nanite 110 µm × 22 µm needing standard AFM cantilevers, while for the lithography, a scan head Nanite SH SH A110 having similar range of measurement as the AFM head is used in front. The AFM is
A110The having similar range is ofcarried measurement the AFM headscan is used front.a The AFM isscan controlled AFM inspection out by as a Nanite AFM headinwith maximum range by of SPM200 controller while the lithography head is controlled by SPM100 with speeds of up to 60 ms/line. XY 110 µm × 22 µm needing standard AFM cantilevers, while for the lithography, a scan head Nanite
SH A110 having similar range of measurement as the AFM head is used in front. The AFM is
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controlled by SPM200 controller while the lithography head is controlled by SPM100 with speeds of controlled by SPM200 controller while the lithography head is controlled by SPM100 with speeds of up to 60 ms/line. Hence, both heads are independently operated but working under one script when up to 60 ms/line. Hence, both heads are independently operated but working under one script when Hence, both heads are independently operatedare butinworking one6script when operations both operations of lithography and in-process progressunder (Figures and 11). Theboth overall control both operations of lithography and in-process are in progress (Figures 6 and 11). The overall control of lithography and in-process are in progress (Figures 6 and 11). The overall control architecture is architecture is shown in Figure 6 where the both operation of lithography and inspection are architecture is shown in Figure 6 where the both operation of lithography and inspection are shown in Figure 6 where thecommands both operation lithography and inspection operating in a loop, operating in a loop, taking fromofhigh level control planned byare human interface. The operating in a loop, taking commands from high level control planned by human interface. The taking from high level control planned by human interface. samplecommands is controlled in position under the lithography and AFM probes. The sample is controlled in sample is controlled in position under the lithography and AFM probes. position under the lithography and AFM probes.
Figure 6. Overall control architecture for the manipulator. manipulator. Figure 6. Overall control architecture for the manipulator.
A probe stability test on the AFM probe has been carried out to measure vibrations of the probe A probe stability test on the AFM probe has been carried out to measure vibrations of the probe tip as shown in Figure 7. The average amplitude of vibration is less than 1 nm depending on the probe The average average amplitude amplitude of vibration is less than 1 nm nm depending depending on the probe tip as shown in Figure 7. The bending stiffness. bending stiffness.
Figure Figure 7. 7. AFM AFM tip tip vibration vibration measurement. measurement. Figure 7. AFM tip vibration measurement.
2.2. Nanomachining Modes and Inspecting Configurations Configurations 2.2. Nanomachining Modes and Inspecting Configurations Nanolithography probe cancan be carried out by mechanical and electrical means. Nanolithographyusing usingananAFM-like AFM-like probe be carried out by mechanical and electrical Nanolithography using an AFM-like probe can be carried out by mechanical and electrical The firstThe method needs aneeds high aflexural force, or a lateral force force from from the probe tip on the means. first method high flexural force, or a lateral the probe tipthe onsurface, the surface, means. The first method needs a high flexural force, or a lateral force from the probe tip on the surface, second method needs a high biased voltage is used to oxidize local surface. The scratching on the the second method needs a high biased voltage is used to oxidize local surface. The scratching on the second method needs a high biased voltage is used to oxidize local surface. The scratching on the polyimide film is processed by an an AFM AFM (Figure (Figure 9) 9) probe probe composed composed of of aa microscopic microscopic cantilever cantilever with polyimide film is processed by an AFM (Figure 9) probe composed of a microscopic cantilever with a specific flexural flexural stiffness stiffnessand andresonant resonantfrequency, frequency,and anda atip tipattached attached cantilever known toto thethe cantilever known by by its a specific flexural stiffness and resonant frequency, and a tip attached to the cantilever known by its its material with associated properties and dimensions e.g., apex radius, aspect ratio, hardness, and material with associated properties and dimensions e.g., apex radius, aspect ratio, material with associated properties and dimensions e.g., apex radius, aspect ratio, hardness, and stiffness. Two types of scratching scratching results can be be obtained obtained on on the the material. material. They can appear as either stiffness. Two types of scratching results can be obtained on the material. They can appear as either pile-up, or sink-in as shown in Figure 8a. The latter process results in neat sink-in shown in Figure 8a. neat grooves. grooves. To To distinguish distinguish pile-up, or sink-in as shown in Figure 8a. The latter process results in neat grooves. To distinguish between the two methods, two different loading behaviors are governing as it can be seen from between the two methods, two different loading behaviors are governing as it can be seen from
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Figure 8b. 8b. For For each each type, type, the the load load applied applied on on the the probe probe has has its its own own profile profile where where the the loading loading is is similar similar Figure Figure 8b. For each type, the load applied on the probe has its own profile where the loading is similar for both both but but the the unloading unloading is is different. different. for for both but the unloading is different.
Figure Pile-up andand sink-in; (b) load-displacement curves for elastoplastic material and viscoelastic Figure8.8. 8.(a)(a) (a) Pile-up and sink-in; (b) load-displacement load-displacement curves for elastoplastic elastoplastic material and Figure Pile-up sink-in; (b) curves for material and and plastic response on soft material. viscoelastic and plastic response on soft material. viscoelastic and plastic response on soft material.
Irrespective of of the the desired desired mode mode described described previously, previously, the the AFM AFM probe probe tip tip will will be be worn worn worn Irrespective progressively as as long long as as itit is is in in sliding contact with the opposite surface. Wear has been discussed in has been discussed in progressively as insliding slidingcontact contactwith withthe theopposite oppositesurface. surface.Wear Wear has been discussed [12] where the contact radius of the tip can degrade by increasing its radius in two phases; the first [12] where thethe contact radius in [12] where contact radiusofofthe thetip tipcan candegrade degradeby byincreasing increasingits itsradius radius in in two two phases; the first onewill willbe bevery veryfast fastwith withimmediate immediateincrease increaseofof ofthe theradius radiuswhen whenthe thetip tipbreaks, breaks, the second one takes one will be very fast with immediate increase the radius when the tip breaks, the second one takes one the second one takes a a longer time depending on the scanned distance. Figure 9a shows an example to probe tip wear with a longer time depending on the scanned distance. Figure 9a shows an example to probe tip wear with longer time depending on the scanned distance. Figure 9a shows an example to probe tip wear wearrates ratesexamples. examples.ItIt Itisis isrequired, required, depending on the mode of nanomanufacturing, nanomanufacturing, to compensate compensate wear rates examples. required, depending mode of to wear depending onon thethe mode of nanomanufacturing, to compensate for for this this wear andother for other other misadjustments. Figure 9b shows shows tip degradation shape degradation degradation after several several for and for misadjustments. Figure 9b tip shape after this wearwear and for misadjustments. Figure 9b shows tip shape after several passes passes fromto mm to 12 12the mm. the initial initial shape is shown shown at 00 mm. mm. The wear wear range canup goto up20to tonm 20 nm nm passes 11 mm to mm. the is at The range can go up 20 from 1 from mm 12 mm. initial shapeshape is shown at 0 mm. The wear range can go for for continuous continuous scan over 12 mm mm pass. The trajectory trajectory length used inpaper this paper paper wasthan less 1than than mm, for 12 length in this was less mm, continuous scanscan overover 12 mm pass.pass. The The trajectory length usedused in this was less mm,11 with withgood very good good probability probability tothe keep theintips tips ininitial their initial initial shapes. with very to keep the in their shapes. very probability to keep tips their shapes.
(a) (a)
(b) (b)
Figure 9. 9. (a) (a) Probe Probe tip tip wear; wear; (b) (b) Example Exampleof ofwear wearrate rateat atthe thetip. tip. Figure 9. Probe tip wear; (b) Example of wear rate at the tip. Figure (a)
2.3. In-Process In-Process Inspection Inspection 2.3. 2.3. In-Process Inspection The inspection inspection and and characterization characterization at at nanoscale nanoscale is is carried carried out out using using AFM/STM AFM/STM head head from from The The inspection and characterization at nanoscale is carried out using AFM/STM head from Nanosurf with a probe. Nanomachining is also facilitated by a piezoscanner with independent Nanosurf with with aa probe. probe. Nanomachining facilitated by by aa piezoscanner piezoscanner with with independent independent Nanosurf Nanomachining is is also also facilitated controlled probe probe assembled assembled back-to-back back-to-back to to the the inspection inspection AFM AFM probe. probe. One One is is an an inspection inspection probe, probe, controlled controlled probe assembled back-to-back to the inspection AFM probe. One is an inspection probe, the the second second one one is is aa processing, processing, i.e., i.e., scratching scratching probe. probe. The The two two probes probes are are located located back-to-back back-to-back within within the second one is a processing, i.e., scratching probe. The two probes are located back-to-back within a safe constant constant distance distance of of 55 µm µm and and having having aa scanning scanning range range of of aa 100 100 µm. µm. aa safe safe constant distance of 5 µm and having a scanning range of a 100 µm. The inspection inspection probe probe configuration configuration can can follow follow the the lithography lithography probe probe depending depending on on the the mode mode The The inspection probe configuration can follow the lithography probe depending on the mode selected for for the the lithography lithography by by being being back-to-back, back-to-back, or, or, close close lateral lateral (side) (side) ifif the the lithography lithography probe probe selected selected for the lithography by being back-to-back, or, close lateral (side) if the lithography probe uses uses its its lateral lateral deflection deflection that that is is higher higher than than the the bending. bending. Figure Figure 10 10 shows shows the the two two possible possible uses configurations. Only Only the the back-to-back back-to-back configuration configuration is is discussed discussed in in this this paper. paper. configurations.
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its lateral deflection that is higher than the bending. Figure 10 shows the two possible configurations. Sensorsthe 2017, 17, 1194 7 of 10 Only back-to-back configuration is discussed in this paper. Sensors 2017, 17, 1194
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(a) (a)
(b) (b)
Figure 10.Two Two configurations of in-process the in-process inspection. (a) Flexural mode (back-to-back); Figure 10. configurations of the inspection. (a) Flexural mode (back-to-back); (b) Lateral Figure 10. Two configurations of the in-process inspection. (a) Flexural mode (back-to-back); (b) Lateral mode (side-to-side). mode (side-to-side). (b) Lateral mode (side-to-side).
3. Nanomachining Inspection Experiments Experiments 3. Nanomachining and and In-Process In-Process Inspection 3. Nanomachining and In-Process Inspection Experiments Several Several tests tests have have been been carried carried out out to to trigger trigger proper proper nanomachining nanomachining and and to to observe observe the the benefits benefits Several tests have been carried out to triggertests proper nanomachining andwith to observe thetips—i.e., benefits of having back-to-back probes. The lithography included three probes different of having back-to-back probes. The lithography tests included three probes with different tips—i.e., of having back-to-back The lithography tests included three probes different tips—i.e., triangular, conical, and and probes. isosceles—that were various speeds andwith applied voltage on the the triangular, conical, isosceles—that were running running at at various speeds and applied voltage on triangular, conical, and isosceles—that were running at various speeds and applied voltage on the same material sample that is available commercially. same material sample that is available commercially. sameSince material results samplewere that is available commercially. not too different, different, two tests tests have have been been selected selected for for this this current current work work after after Since the the results were not too two Sincetrials; the results werepolyimide not too different, two tests have been selected for this current work after extensive controlled depth scratch and series of manufactured grooves. The purpose extensive trials; controlled polyimide depth scratch and series of manufactured grooves. The purpose extensive trials;in-process controlledinspection. polyimideThe depth scratchscript and series ofinmanufactured grooves. The purpose here is to show following shown an algorithm was used in Figure here is to show in-process inspection. The following script shown in an algorithm was used11. in here is to show in-process inspection. The following script shown in an algorithm was used in Figure 11. Figure 11.
Figure 11. In-process inspection script. Figure 11. In-process inspection script. Figure 11. In-process inspection script.
An operated linear scratch was carried out on polyimide film with a normal force of 10 µN An 12). operated linearhad µN scratch was carried polyimide film with a normal forceThe of 10 (Figure The probe a stiffness of 18.5 out N/monwith resonant frequency of 220 kHz. probe (Figure 12).110 Theµm probe had aa with stiffness N/m resonant of 220 The depth probe stiffness of 18.5 18.5 N/m with kHz. length was equipped a Si of tip having apex radius betterfrequency than 10 nm. ThekHz. specified length waswas 110 150 µm nm equipped with a Si tip having apex nm.nm depth The specified of scratch but soon it was realized that theradius depthbetter was atthan only10135 average. This has of scratch was 150 nm but soon it was realized that the depth was at only 135 nm average. This has only 135 nm average. been corrected automatically towards the remaining 10 µm to 150 nm as it can be shown in Figure 12. been corrected corrected automatically automaticallytowards towardsthe theremaining remaining µm 150 it can shown in Figure 1010 µm toto 150 nmnm as as it can be be shown in Figure 12. 12.
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Figure 12. Scratch on polyimide film using 10 µN force (left to right). Figure 12. Scratch on polyimide film using 10 µN force (left to right). Figure 12. Scratch on polyimide film using 10 µN force (left to right).
The inspection of the scratch shown in Figure 11 was progressing as the lithography probe was The inspection of scratch in 11 was lithography probe Theby inspection of the theof scratch shown in Figure Figure 11when was progressing progressing as the lithography probe was was moving a gap distance 5 µm shown as this was the gap positioningas thethe probes back-to-back. The moving by a gap distance of 5 µm as this was the gap when positioning the probes back-to-back. moving by a gap distance of 5 µm as this was the gap when positioning the probes back-to-back. The correction introduced here was triggered when the AFM probe reports the incorrect depth (Figure The correction introduced here was triggered when the AFM probe reports the incorrect depth correction waslithography triggered when AFM probe reports thein-depth incorrectbydepth (Figure 13), hence introduced parametershere on the can the be updated to cut more the spotted (Figure 13),Ifparameters hence parameters the can be updated cut morein-depth in-depth by the theprobe spotted 13), hence theonlithography bebecause updated to to cut more by spotted difference. it is too lateon(always by lithography 5 µm at can least) of this gap, then the lithography is difference. If it is too late (always by 5 µm at least) because of this gap, then the lithography probe is difference. If it is too late (always by 5 µm at least) because of this gap, then the lithography probe is requested to return to where the defect has been detected and a correction will be then implemented requested to return to where the defect has been detected and a correction will be then implemented requested where the defect has been detected and a correction will be then implemented as it is seentoinreturn Figureto13a. as as it it is is seen seen in in Figure Figure 13a. 13a.
(a) (a)
(b) (b)
Figure 13. (a) Controlled lithography process on polyimide film (not to scale); (b) detailed process. Figure 13. (a) process on on polyimide polyimide film film (not (not to to scale); scale); (b) (b) detailed detailed process. process. Figure 13. (a) Controlled Controlled lithography lithography process
In the next test, a nanolithography process was executed on a series of PMMA material samples In themode next test, a nanolithography process wasofexecuted on a series of PMMA samples in sink-in as introduced previously. A series grooves were indented over 70material µm length with in sink-in previously. series of grooves were indented lengthmode with an averagemode depthasofintroduced 10 nm (Figure 14 top).AIn-process measurement by AFM over probe70inµm tapping an average depth of 10 nm (Figure 14 top). In-process measurement by AFM probe in tapping mode
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In the next test, a nanolithography process was executed on a series of PMMA material samples in sink-in mode as introduced previously. A series of grooves were indented over 70 µm length with Sensors 2017, 17, 1194 9 of 10 an average depth of 10 nm (Figure 14 top). In-process measurement by AFM probe in tapping mode compared executing lithography has shown little expected discrepancy (Figure (Figure 14 bottom) compared to tothe theprobe probe executing lithography has shown little expected discrepancy 14 but is still in good agreement with the initial request. bottom) but is still in good agreement with the initial request.
Figure 14. (Top) Indented material; (Bottom) measurement in-process using back-to-back probes. Figure 14. (Top) Indented material; (Bottom) measurement in-process using back-to-back probes.
4. Conclusions 4. Conclusions An in-process inspection system using back-to-back probes has been tested An in-processAFM AFMbased based inspection system using back-to-back probes has developed, been developed, and presented in this paper. This proposed configuration conserves theconserves AFM measurement tested and presented in this paper. This sensing proposed sensing configuration the AFM quality of the manufactured features; compensates for errors generated by various e.g., measurement quality of the manufactured features; compensates for errors generatedsources, by various continuous degradation of the probe tip; and obtains features that are nanomachined to specifications. sources, e.g., continuous degradation of the probe tip; and obtains features that are nanomachined to The nanomachining with in-process inspection has been proven to be very smooth and to have a specifications. unique of error compensation that can behas either while and monitoring Theadvantage nanomachining with in-process inspection beenmanual provenor toautomatic be very smooth to have the process. The distance between the two tips, although constant, will always delay the AFM tip to a unique advantage of error compensation that can be either manual or automatic while monitoring trigger any correction. Reducing this selecting rectangular example, the process. The distance between thegap twobytips, although constant,tips, willfor always delaywill theimprove AFM tipthe to quality of lithography to specifications. The in-process inspection will help also to better understand trigger any correction. Reducing this gap by selecting rectangular tips, for example, will improve the nano-manufacturing quality of lithographyprocesses. to specifications. The in-process inspection will help also to better understand The next focus of study would be to consider side-to-side probes position and to implement nano-manufacturing processes. an automated correction within process probes considering theand initial lithography The next focus of study wouldthe be manufacturing to consider side-to-side position to implement an specifications as a reference. automated correction within the manufacturing process considering the initial lithography specifications as a reference. Acknowledgments: The author would like to thank R. Jaafar from Nanosurf Switzerland for his technical support. The author also acknowledges the support provided by King Abdulaziz City for Science and Technology (KACST) Acknowledgments: The author would like thank (KFUPM) R. Jaafar for from Nanosurf Switzerland for his3021-04 technical through King Fahd University of Petroleum & to Minerals funding this project No. 12-NAN as part of theThe National Science, Technology, and Innovation support. author also acknowledges the support Plan. provided by King Abdulaziz City for Science and Technology through King Fahd University of Petroleum Conflicts of (KACST) Interest: The author declares no conflict of interest. & Minerals (KFUPM) for funding this project No. 12-NAN 3021-04 as part of the National Science, Technology, and Innovation Plan.
References Conflicts of Interest: The author declares no conflict of interest. 1.
Mekid, S.; Schlegel, T.; Aspragathos, N.; Teti, R. Foresight formulation in innovative production, automation and control systems. Foresight 2007, 9, 35–47. [CrossRef] Tanigushi, On the T.; Basic Concept of Nanotechnology. In Proceedings of theinInternational on Mekid, S.; N. Schlegel, Aspragathos, N.; Teti, R. Foresight formulation innovativeConference production, automation and control systems. Foresight 2007, 9,of35–47. Precision Engineering, Tokyo; Part II; Japan Society Precision Engineering: Tokyo, Japan, 1974. Tanigushi, N. On BasicS.Concept of Nanotechnology. In Proceedings the International on Vacharanukul, K.; the Mekid, In-process dimensional inspection sensors. ofMeasurement 2005,Conference 38, 204–218. Precision Engineering, Tokyo; Part II; Japan Society of Precision Engineering: Tokyo, Japan, 1974. [CrossRef] Vacharanukul, K.; Mekid, S. In-process dimensional inspection sensors. Measurement 2005, 38, 204–218. Vacharanukul, K.; Mekid, S. In-process out-of-roundness measurement probe for turned workpieces. Measurements 2011, 44, 762–766. Mekid, S.; Vacharanukul, K. Differential Laser Doppler based Non-Contact Sensor for Dimensional Inspection with Error Propagation Evaluation. Sensors 2006, 6, 546–556 Mekid, S. Design Strategy for Precision Engineering: Second Order Phenomena. J. Eng. Des. 2005, 1, 63–74.
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Vacharanukul, K.; Mekid, S. In-process out-of-roundness measurement probe for turned workpieces. Measurements 2011, 44, 762–766. Mekid, S.; Vacharanukul, K. Differential Laser Doppler based Non-Contact Sensor for Dimensional Inspection with Error Propagation Evaluation. Sensors 2006, 6, 546–556. [CrossRef] Mekid, S. Design Strategy for Precision Engineering: Second Order Phenomena. J. Eng. Des. 2005, 1, 63–74. [CrossRef] García Plaza, E.; Núñez López, P.J. Surface roughness monitoring by singular spectrum analysis of vibration signals. Mech. Syst. Signal Proc. 2017, 84, 516–530. [CrossRef] Koleva, S.; Enchev, M.; Szecsi, T. Automatic dimension measurement on CNC lathes using the cutting tool. Procedia CIRP 2015, 33, 568–575. [CrossRef] Fang, T.-H.; Weng, C.-I.; Chang, J.-G. Machining characterization of the nano-lithography process using atomic force microscopy. Nanotechnology 2000, 11, 181–187. [CrossRef] Keyvania, A.; Tamera, M.; van Es, M.H.; Sadeghian, H. Simultaneous AFM Nano-Patterning and Imaging for Photomask Repair. In Proceedings of the Metrology Inspection, and Process Control for Microlithography XXX, San Jose, CA, USA, 21 February 2016. [CrossRef] Geng, Y.; Yan, Y.; Yu, B.; Li, J.; Zhang, Q.; Hu, Z.; Zhao, X. Depth prediction model of nano-grooves fabricated by AFM-based multi-passes scratching method. Appl. Surf. Sci. 2014, 313, 615–623. [CrossRef] Killgore, J.P.; Geiss, R.H.; Hurley, D.C. Continuous measurement of atomic force microscope tip wear by contact resonance force microscopy. Small 2011, 7, 1018–1022. [CrossRef] [PubMed] © 2017 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
In-Process Atomic-Force Microscopy (AFM) Based Inspection Samir Mekid Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia; [email protected]; Tel.: +966-13-860-7746 Academic Editor: Vittorio M. N. Passaro Received: 23 March 2017; Accepted: 19 May 2017; Published: 31 May 2017
Abstract: A new in-process atomic-force microscopy (AFM) based inspection is presented for nanolithography to compensate for any deviation such as instantaneous degradation of the lithography probe tip. Traditional method used the AFM probes for lithography work and retract to inspect the obtained feature but this practice degrades the probe tip shape and hence, affects the measurement quality. This paper suggests a second dedicated lithography probe that is positioned back-to-back to the AFM probe under two synchronized controllers to correct any deviation in the process compared to specifications. This method shows that the quality improvement of the nanomachining, in progress probe tip wear, and better understanding of nanomachining. The system is hosted in a recently developed nanomanipulator for educational and research purposes. Keywords: in-process inspection; AFM; nanomachining; probes; nanoscale inspection
1. Introduction Manufacture of small components with ever increasing accuracy is highly requested in the market of precision systems [1]. Some nanoscale features are machined with highly requested accuracy and are predicted to reach the subnanometer level in 2020 by Tanigushi [2]. The nanomachining and micromachining are however, associated with their own related phenomena. Effects of van der Waals force are more apparent at nanoscale, while adhesion between asperities, fracture, surface layer formation, and debris formation are important at microscale. It is known that phenomena of materials fracture are manifold since they are governed by the microscopic properties of a material that are varying from one material to another. Although ultra-precision machines are designed to achieve this ultra-precision [3], machining conserves its share of errors and needs compensation in in-process. Hence, fabricating at nanoscale and microscale may not be very deterministic if the functions of manufacturing e.g., material removing is carried out, the part is removed, and the obtained features are inspected and expected to be very close to specifications. At this scale, it becomes important to inspect while in-process and correct if there is a need to stay within the specifications for the reasons discussed previously. This approach of in-process inspection has been used in manufacturing for decades using different measurements techniques on machine tools for standard parts. A review has been carried out [4] for standard scale machines. Recent sensors have been proposed to measure specific features such as roundness [5,6], surface finish [7], and dimensional measurement [8]. The search of the public literature shows little contribution in the area of in-process inspection at nanoscale. The characterization of the nano-lithography is usually carried out by the same atomic-force microscope (AFM) probe used to remove material, showing quality measurement degradation [9] and using AFM for nano-patterning and imaging for mask repair based on the same principle [10]. Other work included multi pass scratching method with depth prediction using an AFM tip [11].
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The AFM probe can be used for specific lithography work and retract back to inspect the workSensors done,2017, but17,this 1194 practice will degrade the probe tip and hence, affects the measurement 2 ofquality. 10 To avoid this, we propose in this paper to add a second probe for lithography work that is designed The AFM probe can beto used specific lithography work and retract to inspect and assembled back-to-back an for AFM probe within one system. Theback AFM probe the willwork keep its done, but quality this practice will being degrade the probebytip and hence, affects the measurement quality. To measurement without disturbed the other degrading tasks. avoid this, we propose in this paper to add a second probe for lithography work that is designed and A new era has been triggered in engineering and science where material properties can be assembled back-to-back to an AFM probe within one system. The AFM probe will keep its designed and controlled at the level of individual atoms and molecules. The proposed in-process measurement quality without being disturbed by the other degrading tasks. inspectionAserves thehas following specificinobjectives: new era been triggered engineering and science where material properties can be (a)
designed and controlled at the level of individual atoms and molecules. The proposed in-process To provide nano-machining processes with in-process inspection using back-to-back probes one inspection serves the following specific objectives:
(b) (c)
(a) To provide nano-machining processes with in-process inspectioncharacteristics. using back-to-back probes one To build nano-devices with specific geometries and particular of which is AFMs. To inspect and characterize complex shapes at corresponding accuracies. (b) To build nano-devices with specific geometries and particular characteristics. (c) To inspect and characterize complex shapes at corresponding accuracies.
of which is AFMs.
2. Proposed Design System
2. Proposed Design System
2.1. Host System Description
2.1. Hoston System Descriptionreviews and objectives, we propose a recently developed in-process Based the previous inspection for onnanomachining based AFM probing system. host in-process system is a Based the previous reviews andon objectives, we propose a recently The developed inspection for nanomachining basedfor on sample AFM probing system. The system manipulation, is a nanomanipulator nanomanipulator having one side preparation andhost robotics and, the having one side for sample preparation and robotics manipulation, and, the other side for 1). other side for nanomachining such as nanolithography with in-process inspection (Figure nanomachining such as nanolithography with in-process inspection (Figure 1). This instrument This instrument encompasses nano-manufacturing with in-process inspection, assembly, and encompasses nano-manufacturing with in-process inspection, assembly, and inspection/characterization of material particles or parts at the nanoscale. The instrument comprises inspection/characterization of material particles or parts at the nanoscale. The instrument comprises all operations in one single rotating stage keeping the same datum for the samples that are under all operations in one single rotating stage keeping the same datum for the samples that are under investigation. This rotating stage has three zones. The first one is the sample preparation zone. investigation. This rotating stage has three zones. The first one is the sample preparation zone. The The second oneisisfor forsamples samples manipulation and equipped withrobotic three arms, robotic arms, each having second one manipulation and equipped with three each having three threedegrees degreesofoffreedom freedom(DOF) (DOF) with a nanometer resolution 10 mm range of motion. with a nanometer resolution overover a 10a mm range of motion. This This manipulation areaarea is monitored havingaaresolution resolution 1 µm. inspect manipulation is monitoredvia viaa aCCD CCDcamera camera having of of 1 µm. ThisThis can can inspect samples and probes. The third zone has the AFM inspection with in-process (i.e., nanomachining) samples and probes. The third zone has the AFM inspection with in-process (i.e., nanomachining) inspection. The nanomachining encompasses numerically controlled nanomachining e.g., inspection. The nanomachining encompasses numerically controlled nanomachining e.g., lithography with its probe mounted back-to-back an AFM for in-process inspection a new with lithography its probe mounted back-to-back to an AFMtoprobe forprobe in-process inspection as a as new feature. feature. A suction device is added to clean the on-going machining debris. A top view of the whole A suction device is added to clean the on-going machining debris. A top view of the whole instrument instrument is shown in Figure 2. Most parts are homemade except technology hardware e.g., AFM, is shown in Figure 2. Most parts are homemade except technology hardware e.g., AFM, robotic arms, robotic arms, rotating stage. rotating stage.
Figure 1. Schematic of the integrated manipulator and overall coordinate system. Figure 1. Schematic of the integrated manipulator and overall coordinate system.
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Figure 2. Nanomanipulator platform with nanomachining.
Figure 2. Nanomanipulator platform with nanomachining. Figure 2. Nanomanipulator platform with nanomachining. All components are controlled based on the control architecture described in Figure 3. The All components controlled based for on in-process the control architecture in Figure 3. The highlighted colorfulare controllers are mainly inspection. Some described vibrations can be noticed All components are controlled based on the control architecture described in Figure 3. The of the AFM and cutting probes to an for environment by Some isolating the nanomanipulator highlighted colorful controllers aredue mainly in-processminimized inspection. vibrations can be noticed of highlighted controllers are mainly for in-process inspection. Some vibrations can be noticed fromand thecolorful ground through controlled damping table. the AFM cuttingvibration probes due to anaenvironment minimized by isolating the nanomanipulator from
of the AFM and cutting probes due to an environment minimized by isolating the nanomanipulator the ground vibration through a controlled damping table. from the ground vibration through a controlled damping table.
Figure 3. Cross section of the nanomanipulator with in-process AFM inspection.
The focus of this paper is on the in-process inspection zone. An AFM is mounted on XY precision stage (Figure 3) with locks for each axis, so it is possible to adjust the probe height by a few millimeters to accommodate sizes. piezoelectric scanner nanomachining is mounted close to Figure various 3. Cross Crosspart section of The the nanomanipulator nanomanipulator withfor in-process AFM inspection. inspection. Figure 3. section of the with in-process AFM the AFM on a separate platform that is firmly mounted of the supporting platform. A space under the probes is allowed to receive the rotating table and to place samples under the two probes when The focus of this paper is on on the the in-process in-process inspection inspection zone. An An AFM AFM is is mounted mounted on on XY XY precision precision needed.
stage (Figure 3) with locks for each axis, so it is is possible possible to to adjust adjust the the probe probe height height by by aa few few millimeters millimeters to accommodate various part sizes. The piezoelectric scanner for nanomachining is mounted close to the AFM on on aa separate separateplatform platformthat thatisisfirmly firmly mounted supporting platform. A space under mounted of of thethe supporting platform. A space under the the probes is allowed to receive the rotating tabletoand to samples place samples under two when probesneeded. when probes is allowed to receive the rotating table and place under the twothe probes needed.
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The two AFM and lithography probes are mounted back-to-back with physical distance Sensorstwo 2017, 17, 1194 and lithography probes are mounted back-to-back with physical4 of 10 The distance adjustment set AFM to 5 µm as a security distance to avoid any crash between the probes. The two heads adjustment set to 5 µm as a security distance to avoid any crash between the probes. The were littleThe modified withand covers removed and with little adjustment of the piezoscanner. Only two one two AFM lithography probes are mounted back-to-back with physical distance heads were little will modified with covers removed and little the piezoscanner. Only top view camera available for both systems aswith shown in adjustment Figure for of positioning. The detailed adjustment set to 5beµm as a security distance to avoid any crash between4 the probes. The two heads one top view camerainwill be available both systems asbetween shown inthe Figure 4 for The positions 4 and 5 for where distance is 5positioning. µm. Theone probes were are littleshown modified Figures with covers removed andthe with little adjustment of theprobes piezoscanner. Only detailed positions are shown in Figures 4 and 5 where the distance between the probes is 5 µm. top view camera will be available for both systems as shown Figure The 4 for sample positioning. Thetested detailed can safely move horizontally without crashing against eachin other. to be canThe be probes can safely move without crashing against each other. The is sample beprobes tested can positions are shown inprobes Figuresusing 4 and 5 where thethe distance between the probes 5 µm. to The adjusted underneath thehorizontally one axis of table. be adjusted underneath the probes usingcrashing one axisagainst of the each table.other. The sample to be tested can be can safely move horizontally without adjusted underneath the probes using one axis of the table.
Figure 4. Position AFMand andlithography lithography probes actual configuration. Figure 4. Position ofof AFM probeson onthe the actual configuration. Figure 4. Position of AFM and lithography probes on the actual configuration.
Figure 5. Bottom view of back-to-back probes probes (left: AFM cantilever, right: Lithography: Figure 5. Bottom view of back-to-back (left:probe AFMNCLR-10 probe NCLR-10 cantilever, right: Lithography: A-probe probe) cantilever). A-probe (Akiyama probe)(Akiyama cantilever).
Figure Bottom view of back-to-back probe NCLR-10 cantilever, right: The5.AFM inspection is carried out by aprobes Nanite (left: AFM AFM scan head with a maximum scan range of The AFM inspection is carried out by a Nanite AFM scan head with a maximum scan range of XY Lithography: A-probe (Akiyama probe) cantilever). XY 110 µm × 22 µm needing standard AFM cantilevers, while for the lithography, a scan head Nanite 110 µm × 22 µm needing standard AFM cantilevers, while for the lithography, a scan head Nanite SH SH A110 having similar range of measurement as the AFM head is used in front. The AFM is
A110The having similar range is ofcarried measurement the AFM headscan is used front.a The AFM isscan controlled AFM inspection out by as a Nanite AFM headinwith maximum range by of SPM200 controller while the lithography head is controlled by SPM100 with speeds of up to 60 ms/line. XY 110 µm × 22 µm needing standard AFM cantilevers, while for the lithography, a scan head Nanite
SH A110 having similar range of measurement as the AFM head is used in front. The AFM is
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controlled by SPM200 controller while the lithography head is controlled by SPM100 with speeds of controlled by SPM200 controller while the lithography head is controlled by SPM100 with speeds of up to 60 ms/line. Hence, both heads are independently operated but working under one script when up to 60 ms/line. Hence, both heads are independently operated but working under one script when Hence, both heads are independently operatedare butinworking one6script when operations both operations of lithography and in-process progressunder (Figures and 11). Theboth overall control both operations of lithography and in-process are in progress (Figures 6 and 11). The overall control of lithography and in-process are in progress (Figures 6 and 11). The overall control architecture is architecture is shown in Figure 6 where the both operation of lithography and inspection are architecture is shown in Figure 6 where the both operation of lithography and inspection are shown in Figure 6 where thecommands both operation lithography and inspection operating in a loop, operating in a loop, taking fromofhigh level control planned byare human interface. The operating in a loop, taking commands from high level control planned by human interface. The taking from high level control planned by human interface. samplecommands is controlled in position under the lithography and AFM probes. The sample is controlled in sample is controlled in position under the lithography and AFM probes. position under the lithography and AFM probes.
Figure 6. Overall control architecture for the manipulator. manipulator. Figure 6. Overall control architecture for the manipulator.
A probe stability test on the AFM probe has been carried out to measure vibrations of the probe A probe stability test on the AFM probe has been carried out to measure vibrations of the probe tip as shown in Figure 7. The average amplitude of vibration is less than 1 nm depending on the probe The average average amplitude amplitude of vibration is less than 1 nm nm depending depending on the probe tip as shown in Figure 7. The bending stiffness. bending stiffness.
Figure Figure 7. 7. AFM AFM tip tip vibration vibration measurement. measurement. Figure 7. AFM tip vibration measurement.
2.2. Nanomachining Modes and Inspecting Configurations Configurations 2.2. Nanomachining Modes and Inspecting Configurations Nanolithography probe cancan be carried out by mechanical and electrical means. Nanolithographyusing usingananAFM-like AFM-like probe be carried out by mechanical and electrical Nanolithography using an AFM-like probe can be carried out by mechanical and electrical The firstThe method needs aneeds high aflexural force, or a lateral force force from from the probe tip on the means. first method high flexural force, or a lateral the probe tipthe onsurface, the surface, means. The first method needs a high flexural force, or a lateral force from the probe tip on the surface, second method needs a high biased voltage is used to oxidize local surface. The scratching on the the second method needs a high biased voltage is used to oxidize local surface. The scratching on the second method needs a high biased voltage is used to oxidize local surface. The scratching on the polyimide film is processed by an an AFM AFM (Figure (Figure 9) 9) probe probe composed composed of of aa microscopic microscopic cantilever cantilever with polyimide film is processed by an AFM (Figure 9) probe composed of a microscopic cantilever with a specific flexural flexural stiffness stiffnessand andresonant resonantfrequency, frequency,and anda atip tipattached attached cantilever known toto thethe cantilever known by by its a specific flexural stiffness and resonant frequency, and a tip attached to the cantilever known by its its material with associated properties and dimensions e.g., apex radius, aspect ratio, hardness, and material with associated properties and dimensions e.g., apex radius, aspect ratio, material with associated properties and dimensions e.g., apex radius, aspect ratio, hardness, and stiffness. Two types of scratching scratching results can be be obtained obtained on on the the material. material. They can appear as either stiffness. Two types of scratching results can be obtained on the material. They can appear as either pile-up, or sink-in as shown in Figure 8a. The latter process results in neat sink-in shown in Figure 8a. neat grooves. grooves. To To distinguish distinguish pile-up, or sink-in as shown in Figure 8a. The latter process results in neat grooves. To distinguish between the two methods, two different loading behaviors are governing as it can be seen from between the two methods, two different loading behaviors are governing as it can be seen from
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Figure 8b. 8b. For For each each type, type, the the load load applied applied on on the the probe probe has has its its own own profile profile where where the the loading loading is is similar similar Figure Figure 8b. For each type, the load applied on the probe has its own profile where the loading is similar for both both but but the the unloading unloading is is different. different. for for both but the unloading is different.
Figure Pile-up andand sink-in; (b) load-displacement curves for elastoplastic material and viscoelastic Figure8.8. 8.(a)(a) (a) Pile-up and sink-in; (b) load-displacement load-displacement curves for elastoplastic elastoplastic material and Figure Pile-up sink-in; (b) curves for material and and plastic response on soft material. viscoelastic and plastic response on soft material. viscoelastic and plastic response on soft material.
Irrespective of of the the desired desired mode mode described described previously, previously, the the AFM AFM probe probe tip tip will will be be worn worn worn Irrespective progressively as as long long as as itit is is in in sliding contact with the opposite surface. Wear has been discussed in has been discussed in progressively as insliding slidingcontact contactwith withthe theopposite oppositesurface. surface.Wear Wear has been discussed [12] where the contact radius of the tip can degrade by increasing its radius in two phases; the first [12] where thethe contact radius in [12] where contact radiusofofthe thetip tipcan candegrade degradeby byincreasing increasingits itsradius radius in in two two phases; the first onewill willbe bevery veryfast fastwith withimmediate immediateincrease increaseofof ofthe theradius radiuswhen whenthe thetip tipbreaks, breaks, the second one takes one will be very fast with immediate increase the radius when the tip breaks, the second one takes one the second one takes a a longer time depending on the scanned distance. Figure 9a shows an example to probe tip wear with a longer time depending on the scanned distance. Figure 9a shows an example to probe tip wear with longer time depending on the scanned distance. Figure 9a shows an example to probe tip wear wearrates ratesexamples. examples.ItIt Itisis isrequired, required, depending on the mode of nanomanufacturing, nanomanufacturing, to compensate compensate wear rates examples. required, depending mode of to wear depending onon thethe mode of nanomanufacturing, to compensate for for this this wear andother for other other misadjustments. Figure 9b shows shows tip degradation shape degradation degradation after several several for and for misadjustments. Figure 9b tip shape after this wearwear and for misadjustments. Figure 9b shows tip shape after several passes passes fromto mm to 12 12the mm. the initial initial shape is shown shown at 00 mm. mm. The wear wear range canup goto up20to tonm 20 nm nm passes 11 mm to mm. the is at The range can go up 20 from 1 from mm 12 mm. initial shapeshape is shown at 0 mm. The wear range can go for for continuous continuous scan over 12 mm mm pass. The trajectory trajectory length used inpaper this paper paper wasthan less 1than than mm, for 12 length in this was less mm, continuous scanscan overover 12 mm pass.pass. The The trajectory length usedused in this was less mm,11 with withgood very good good probability probability tothe keep theintips tips ininitial their initial initial shapes. with very to keep the in their shapes. very probability to keep tips their shapes.
(a) (a)
(b) (b)
Figure 9. 9. (a) (a) Probe Probe tip tip wear; wear; (b) (b) Example Exampleof ofwear wearrate rateat atthe thetip. tip. Figure 9. Probe tip wear; (b) Example of wear rate at the tip. Figure (a)
2.3. In-Process In-Process Inspection Inspection 2.3. 2.3. In-Process Inspection The inspection inspection and and characterization characterization at at nanoscale nanoscale is is carried carried out out using using AFM/STM AFM/STM head head from from The The inspection and characterization at nanoscale is carried out using AFM/STM head from Nanosurf with a probe. Nanomachining is also facilitated by a piezoscanner with independent Nanosurf with with aa probe. probe. Nanomachining facilitated by by aa piezoscanner piezoscanner with with independent independent Nanosurf Nanomachining is is also also facilitated controlled probe probe assembled assembled back-to-back back-to-back to to the the inspection inspection AFM AFM probe. probe. One One is is an an inspection inspection probe, probe, controlled controlled probe assembled back-to-back to the inspection AFM probe. One is an inspection probe, the the second second one one is is aa processing, processing, i.e., i.e., scratching scratching probe. probe. The The two two probes probes are are located located back-to-back back-to-back within within the second one is a processing, i.e., scratching probe. The two probes are located back-to-back within a safe constant constant distance distance of of 55 µm µm and and having having aa scanning scanning range range of of aa 100 100 µm. µm. aa safe safe constant distance of 5 µm and having a scanning range of a 100 µm. The inspection inspection probe probe configuration configuration can can follow follow the the lithography lithography probe probe depending depending on on the the mode mode The The inspection probe configuration can follow the lithography probe depending on the mode selected for for the the lithography lithography by by being being back-to-back, back-to-back, or, or, close close lateral lateral (side) (side) ifif the the lithography lithography probe probe selected selected for the lithography by being back-to-back, or, close lateral (side) if the lithography probe uses uses its its lateral lateral deflection deflection that that is is higher higher than than the the bending. bending. Figure Figure 10 10 shows shows the the two two possible possible uses configurations. Only Only the the back-to-back back-to-back configuration configuration is is discussed discussed in in this this paper. paper. configurations.
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its lateral deflection that is higher than the bending. Figure 10 shows the two possible configurations. Sensorsthe 2017, 17, 1194 7 of 10 Only back-to-back configuration is discussed in this paper. Sensors 2017, 17, 1194
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(a) (a)
(b) (b)
Figure 10.Two Two configurations of in-process the in-process inspection. (a) Flexural mode (back-to-back); Figure 10. configurations of the inspection. (a) Flexural mode (back-to-back); (b) Lateral Figure 10. Two configurations of the in-process inspection. (a) Flexural mode (back-to-back); (b) Lateral mode (side-to-side). mode (side-to-side). (b) Lateral mode (side-to-side).
3. Nanomachining Inspection Experiments Experiments 3. Nanomachining and and In-Process In-Process Inspection 3. Nanomachining and In-Process Inspection Experiments Several Several tests tests have have been been carried carried out out to to trigger trigger proper proper nanomachining nanomachining and and to to observe observe the the benefits benefits Several tests have been carried out to triggertests proper nanomachining andwith to observe thetips—i.e., benefits of having back-to-back probes. The lithography included three probes different of having back-to-back probes. The lithography tests included three probes with different tips—i.e., of having back-to-back The lithography tests included three probes different tips—i.e., triangular, conical, and and probes. isosceles—that were various speeds andwith applied voltage on the the triangular, conical, isosceles—that were running running at at various speeds and applied voltage on triangular, conical, and isosceles—that were running at various speeds and applied voltage on the same material sample that is available commercially. same material sample that is available commercially. sameSince material results samplewere that is available commercially. not too different, different, two tests tests have have been been selected selected for for this this current current work work after after Since the the results were not too two Sincetrials; the results werepolyimide not too different, two tests have been selected for this current work after extensive controlled depth scratch and series of manufactured grooves. The purpose extensive trials; controlled polyimide depth scratch and series of manufactured grooves. The purpose extensive trials;in-process controlledinspection. polyimideThe depth scratchscript and series ofinmanufactured grooves. The purpose here is to show following shown an algorithm was used in Figure here is to show in-process inspection. The following script shown in an algorithm was used11. in here is to show in-process inspection. The following script shown in an algorithm was used in Figure 11. Figure 11.
Figure 11. In-process inspection script. Figure 11. In-process inspection script. Figure 11. In-process inspection script.
An operated linear scratch was carried out on polyimide film with a normal force of 10 µN An 12). operated linearhad µN scratch was carried polyimide film with a normal forceThe of 10 (Figure The probe a stiffness of 18.5 out N/monwith resonant frequency of 220 kHz. probe (Figure 12).110 Theµm probe had aa with stiffness N/m resonant of 220 The depth probe stiffness of 18.5 18.5 N/m with kHz. length was equipped a Si of tip having apex radius betterfrequency than 10 nm. ThekHz. specified length waswas 110 150 µm nm equipped with a Si tip having apex nm.nm depth The specified of scratch but soon it was realized that theradius depthbetter was atthan only10135 average. This has of scratch was 150 nm but soon it was realized that the depth was at only 135 nm average. This has only 135 nm average. been corrected automatically towards the remaining 10 µm to 150 nm as it can be shown in Figure 12. been corrected corrected automatically automaticallytowards towardsthe theremaining remaining µm 150 it can shown in Figure 1010 µm toto 150 nmnm as as it can be be shown in Figure 12. 12.
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Figure 12. Scratch on polyimide film using 10 µN force (left to right). Figure 12. Scratch on polyimide film using 10 µN force (left to right). Figure 12. Scratch on polyimide film using 10 µN force (left to right).
The inspection of the scratch shown in Figure 11 was progressing as the lithography probe was The inspection of scratch in 11 was lithography probe Theby inspection of the theof scratch shown in Figure Figure 11when was progressing progressing as the lithography probe was was moving a gap distance 5 µm shown as this was the gap positioningas thethe probes back-to-back. The moving by a gap distance of 5 µm as this was the gap when positioning the probes back-to-back. moving by a gap distance of 5 µm as this was the gap when positioning the probes back-to-back. The correction introduced here was triggered when the AFM probe reports the incorrect depth (Figure The correction introduced here was triggered when the AFM probe reports the incorrect depth correction waslithography triggered when AFM probe reports thein-depth incorrectbydepth (Figure 13), hence introduced parametershere on the can the be updated to cut more the spotted (Figure 13),Ifparameters hence parameters the can be updated cut morein-depth in-depth by the theprobe spotted 13), hence theonlithography bebecause updated to to cut more by spotted difference. it is too lateon(always by lithography 5 µm at can least) of this gap, then the lithography is difference. If it is too late (always by 5 µm at least) because of this gap, then the lithography probe is difference. If it is too late (always by 5 µm at least) because of this gap, then the lithography probe is requested to return to where the defect has been detected and a correction will be then implemented requested to return to where the defect has been detected and a correction will be then implemented requested where the defect has been detected and a correction will be then implemented as it is seentoinreturn Figureto13a. as as it it is is seen seen in in Figure Figure 13a. 13a.
(a) (a)
(b) (b)
Figure 13. (a) Controlled lithography process on polyimide film (not to scale); (b) detailed process. Figure 13. (a) process on on polyimide polyimide film film (not (not to to scale); scale); (b) (b) detailed detailed process. process. Figure 13. (a) Controlled Controlled lithography lithography process
In the next test, a nanolithography process was executed on a series of PMMA material samples In themode next test, a nanolithography process wasofexecuted on a series of PMMA samples in sink-in as introduced previously. A series grooves were indented over 70material µm length with in sink-in previously. series of grooves were indented lengthmode with an averagemode depthasofintroduced 10 nm (Figure 14 top).AIn-process measurement by AFM over probe70inµm tapping an average depth of 10 nm (Figure 14 top). In-process measurement by AFM probe in tapping mode
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In the next test, a nanolithography process was executed on a series of PMMA material samples in sink-in mode as introduced previously. A series of grooves were indented over 70 µm length with Sensors 2017, 17, 1194 9 of 10 an average depth of 10 nm (Figure 14 top). In-process measurement by AFM probe in tapping mode compared executing lithography has shown little expected discrepancy (Figure (Figure 14 bottom) compared to tothe theprobe probe executing lithography has shown little expected discrepancy 14 but is still in good agreement with the initial request. bottom) but is still in good agreement with the initial request.
Figure 14. (Top) Indented material; (Bottom) measurement in-process using back-to-back probes. Figure 14. (Top) Indented material; (Bottom) measurement in-process using back-to-back probes.
4. Conclusions 4. Conclusions An in-process inspection system using back-to-back probes has been tested An in-processAFM AFMbased based inspection system using back-to-back probes has developed, been developed, and presented in this paper. This proposed configuration conserves theconserves AFM measurement tested and presented in this paper. This sensing proposed sensing configuration the AFM quality of the manufactured features; compensates for errors generated by various e.g., measurement quality of the manufactured features; compensates for errors generatedsources, by various continuous degradation of the probe tip; and obtains features that are nanomachined to specifications. sources, e.g., continuous degradation of the probe tip; and obtains features that are nanomachined to The nanomachining with in-process inspection has been proven to be very smooth and to have a specifications. unique of error compensation that can behas either while and monitoring Theadvantage nanomachining with in-process inspection beenmanual provenor toautomatic be very smooth to have the process. The distance between the two tips, although constant, will always delay the AFM tip to a unique advantage of error compensation that can be either manual or automatic while monitoring trigger any correction. Reducing this selecting rectangular example, the process. The distance between thegap twobytips, although constant,tips, willfor always delaywill theimprove AFM tipthe to quality of lithography to specifications. The in-process inspection will help also to better understand trigger any correction. Reducing this gap by selecting rectangular tips, for example, will improve the nano-manufacturing quality of lithographyprocesses. to specifications. The in-process inspection will help also to better understand The next focus of study would be to consider side-to-side probes position and to implement nano-manufacturing processes. an automated correction within process probes considering theand initial lithography The next focus of study wouldthe be manufacturing to consider side-to-side position to implement an specifications as a reference. automated correction within the manufacturing process considering the initial lithography specifications as a reference. Acknowledgments: The author would like to thank R. Jaafar from Nanosurf Switzerland for his technical support. The author also acknowledges the support provided by King Abdulaziz City for Science and Technology (KACST) Acknowledgments: The author would like thank (KFUPM) R. Jaafar for from Nanosurf Switzerland for his3021-04 technical through King Fahd University of Petroleum & to Minerals funding this project No. 12-NAN as part of theThe National Science, Technology, and Innovation support. author also acknowledges the support Plan. provided by King Abdulaziz City for Science and Technology through King Fahd University of Petroleum Conflicts of (KACST) Interest: The author declares no conflict of interest. & Minerals (KFUPM) for funding this project No. 12-NAN 3021-04 as part of the National Science, Technology, and Innovation Plan.
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