Investigations in the guiding efficiency in a modified ion beam sputtering process Sina Malobabic,1,2,* Marco Jupé,1,2 and Detlev Ristau1,2 1 2
Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany
Quest: Centre of Quantum Engineering and Space-Time Research, Leibniz Universität Hannover, Germany *Corresponding author: [email protected] Received 4 September 2013; revised 30 October 2013; accepted 30 October 2013; posted 31 October 2013 (Doc. ID 197045); published 22 November 2013
Ion beam sputtering (IBS) is an established deposition process used in the production of optical coatings. In this study, a modification of the IBS process, based on additional electromagnetic fields, is examined in an effort to improve the technology. The reported experiments reveal the underlying effects of electromagnetic fields on the distribution of the coating material sputtered from the target. An increase in local deposition rate is observed and discussed in the context of the interaction between the introduced fields and the species in the target area. First approaches toward an optimization of the observed bunching effect on the deposition material are illustrated. © 2013 Optical Society of America OCIS codes: (170.0110) Imaging systems; (170.3010) Image reconstruction techniques; (170.3660) Light propagation in tissues. http://dx.doi.org/10.1364/AO.52.008212
1. Introduction
The common ion beam sputtering (IBS) process is a deposition technique well qualified for the production of optical coatings with lowest optical losses. However, to keep pace with the rapid development of optical technology, it is desirable to achieve further progress toward an even higher level of quality presently achievable with IBS. Unfortunately, recent, mainly empirical optimization concepts of the classical IBS process seem to accomplish only incremental improvements. In this context, significant modifications of the IBS process involving additional electromagnetic fields can be considered a novel approach to guide the process plasma, exert influence on the optical properties, as well as to adjust the flux geometry of the sputtered species. The implementation of electromagnetic fields into a deposition process is already known from the vacuum arc deposition (VAD) process, where such fields were effectively 1559-128X/13/348212-08$15.00/0 © 2013 Optical Society of America 8212
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applied to clean vacuum arc-generated plasma from macroparticles of the cathode material, as well as from neutral atoms [1]. A comparable macroparticle generation in the state-of-the-art IBS process can be avoided using empirically optimized coating conditions. Nevertheless, technology should allow for tailored controlling of the coating process. However, the effect of a guiding device in highly developed IBS processes may be relatively problematic, because the ionization rate of the material flux sputtered from the target is two orders of magnitude lower than in a VAD process. Recently, it was proven that the ions within the particle flux of an IBS process can be controlled by electromagnetic fields to regulate the trajectory of the elementary particles [2]. The magnetic fields can be applied to influence the charged content of the material emitted from the target, as well as from the process environment. Furthermore, it can be guided along the axial magnetic field lines. In the classic IBS process, the material is distributed as a cosine-like polynomial. This geometry of the sputtered species flux can be
changed by collimating the deposition material emitted from the target. The quantity of the charged plasma content determines the efficiency of those effects. The present paper is particularly concerned with the potential to optimize the deposition rate distribution in an IBS process. In the presented experiments, two guiding devices, with different setups, are applied to investigate the guiding effect. The paper comprises a detailed investigation of the change in deposition rate in dependence of the electromagnetic field strength, under the variation of essential process parameters like oxygen supply and sputter ion source settings. Supplementing the results of [2], the present paper delivers an insight into the interaction of deposition distribution and electromagnetic fields. 2. Guiding Effect
Generally, electromagnetic fields can only influence charged particles. Therefore, it can be expected that no significant change in trajectory of uncharged material is induced. Consequently, the ion density in the sputtered coating material is an important parameter for the plasma guiding effect. Unfortunately, the sputtering process at the target surface is only reaching an ionization rate of about 1% for the emitted species [3,4]. For economic applications, the emitted coating material has to be ionized to a higher degree to gain an appropriate higher guiding efficiency. The high energy electrons ejected from the neutralizer, as well as the hot electrons of the plasma beam, are able to ionize the sputtered coating material in front of the target. In addition, the electrons can be trapped along the axial magnetic field lines. This confinement of electrons prevents a recombination of electrons and ions and, accordingly, a reneutralization. During the time of flight from the target to the substrate, electrons can ionize the sputtered coating species via collision ionization. Finally, coherent exchanging charge transfer effects can also change the proportion of electrically charged particles and uncharged material [5]. Subsequently, the combination of a high current of hot electrons, ejected by the neutralizer and the magnetic field in the solenoid, leads to the creation of additional ions in the coating process, amplifying the guiding effect. Thus, an axial magnetic field induces a sufficient degree of ionization within the plasma making it possible to guide more coating material toward the substrate. The magnetic fields providing the guiding effect can be generated by the implementation of a solenoid into the conventional IBS process. The provided magnetic field is described by the formula B
μNI ; 2πrsolenoid
(1)
where I is the coil current, N is the number of turns, and rsolenoid is the inner radius of the coil,
respectively. A value of the magnetic field in the range of 30–60 mT is in agreement with the requirement (Khizhnyak, 1965; Vojtsenya et al. 1967) H>
mv⊥ c →B≥ qer
q 2 mhEkin ic2 μ0 ∕e2 r2solenoid ;
(2)
[1], where the symbol m denotes the mass of the electron (or of the ion or molecule). v⊥ relates to the velocity component perpendicular to the magnetic field, Ekin is the mean kinetic energy of the ions and q represents the charge factor in this equation. Typically, e stands for the elementary charge, c stands for the velocity of light in vacuum, and μ0 is the absolute permeability of the vacuum. If the condition is fulfilled, then a guiding of the plasma inside the coil takes place [1]. In first order, the electromagnetic fields are not sufficiently strong to change the trajectory of the ions significantly. Field strength values in the range of some Telsa (T) would be necessary to confine the ions [6]. In contrast to this, electrons, with their mass three orders of magnitude smaller than ions, can be influenced efficiently by moderate electromagnetic fields. The cross field motion of the electrons is confined by the electromagnetic force of an axial magnetic field. The velocity component of the electron movement perpendicular to the magnetic field line leads to a circular path, caused by the Lorentz force. As a consequence of this effect, the electrons tend to move in a circular trajectory, and the dimension of the free movement space is characterized by the Larmor radius [6,7,8]. relec
mv⊥ : qB
(3)
The electrons within the plasma are confined as a result of the magnetic fields, however, the ions are not confined. Therefore, application of the magnetic field allows the creation of a self-consistent electrostatic potential within the system, because the electrons are shifted from the ions [9], which forces the ions to follow the magnetic fields lines. Consequently, this charge separation guides the ions through the solenoid because the ions follow the electrons to stay in a state of quasi-neutrality [10,11]. If the Larmor radius is smaller than the radius of the coil (relec < rsolenoid ), then the electrons are trapped, because they move circularly along the magnetic field lines. The Larmor radius of the ions, however, is much larger (rsolenoid < rion ), therefore the ions are not trapped [10]. If the relation relec < rsolenoid < rion ;
(4)
is fulfilled [1,11], meaning the Larmor radius of the electrons relec has to be smaller than the radius of the solenoid rsolenoid [which, in addition, has to be smaller 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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than the Larmor radius of the ions (rion )], then guiding is possible. Furthermore, an additional electrostatic field can be applied to enhance plasma guiding. This is caused by static fields parallel to the target provide a positive potential, reflecting the ions and preventing the plasma from diffusing [12]. The electrostatic field can be provided by a bias plate that is energized with several tens of volts. A.
Experimental Setup
To investigate the influence of an electromagnetic field guiding the deposition material, several setups were implemented into the common IBS process. The first setup was designed to achieve a complete overview of possible impacting physical parameters; whereas, the second one allows detailed investigations on the influence of the magnetic DC field inside the separator. Setup 1, as illustrated in Fig. 1(a), was applied for the first experimental series. As mentioned above, the purpose of the experiments was to study the influence of different fields on the guiding process. Setup 2 is an upgrade of setup 1, mainly to produce a higher magnetic field. Basically, the guiding device of setup 1 generates a combination of an electrical and a magnetic field, provided by a solenoid and a bias plate, respectively. The solenoid is a spirally shaped, water-cooled copper tube with a diameter of 25 cm. It consists of a precollimating and a guiding coil. The entrance of the precollimating coil is positioned in front of the ion source. It collimates the plasma beam onto the
Fig. 1. Overview of applied setups: (a) solenoid combined of precollimation coil and guiding coil and (b) double-wound coil. 8214
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target. The major aim of the precollimation is to generate magnetic field lines parallel to the target and force the sputter material in the direction of the guiding coil. The guiding coil leads the sputtered species emitted from the target to the substrates and allows a line-of-sight trajectory from the target to the exit of the solenoid. However, both coils are bent in a V shape, with an angle of approximately 140°. It has to be noted that the angle between the normal of the target and the center line of the guiding coil is approximately 60°. To allow isolated investigations of the precollimating effect, the setup can be driven with or without the respective coil. The solenoid generates a magnetic field strength of maximal 30 mT at 340 A coil current inside the solenoid. The experiments show that most of the plasma is confined and is guided along the magnetic field lines to the substrate. Furthermore, a bias plate is implemented that covers a quarter of the internal area of the solenoid at the outlet of the solenoid [12]. A voltage of approximately 80 V was applied to the bias plate. The experiments using setup 1 reveal various aspects of plasma guiding. Furthermore, it has to be mentioned that small variations in geometry lead to significant changes in guiding. For instance, a change of the angle between the coils can lead to a shielding of neutralizer electrons. However, a disadvantage of setup 1 is that only 30 mT are achievable at the maximum. Therefore, an investigation of the dependence of magnetic field strength and deposition rate was only possible to a limited degree. Consequently, the effect of the magnetic field was investigated in a specially constructed setup. The new developed solenoid was mounted inside the chamber, which guarantees high accuracy and result reproducibility. The double windings of the coil covering a cylindrical glass tube provide sufficient stability. Moreover, the tube can be replaced and cleaned to remove internal coatings. The structure of the upgraded setup can be seen in Fig. 1(b), presented as setup 2. In detail, the core component of the guidance in setup 2 is a coil providing axial magnetic fields. In contrast with setup 1, the axis of the coil is oriented about 20° with respect to the normal at the target’s surface. The distance between target and coil is 10 cm. In total, the path length from the target surface to the exit of the coil adds up to approximately 40 cm, including a length of 30 cm of the guiding coil. Since the coil is now positioned directly in the sputter yield, the major part of the sputter yield is directed toward the coil. The precollimation coil and the bias plate are not integrated in setup 2. The B-field was varied proportionally to the coil current in the range of 0–340 A. The general operating current of the coil was 300 A. Contrary to setup 1, the solenoid in setup 2 allocates a higher maximal magnetic field strength, accounting for 60 mT, to investigate its influence. However, both setups were used in a specially constructed cluster of vacuum chambers; whereas, the process chamber is currently used for the
3. Results and Influences of the Guiding Effect
In the discussion of the setup, the electromagnetic device is presented as an addition to the classic IBS process. As a baseline for discussion of the guiding effects, the specifications of the sputter process without electromagnetic guiding tools are characterized. These measurements allow for an interpretation of the guiding experiment results. Figure 2 shows the dependency of the deposition rate, which was measured with the oscillating crystal in the common IBS process, without electromagnetic fields, as a function of the voltage and the current of the ion source. The oscillating crystal was mounted at the solenoid entrance in setup 2. The variation of the ion source parameters were recorded for aluminum as the target material in a reactive process. The deposition rate varies strongly in relation to the ion source and to the beam voltage. The current dependency, with a constant beam voltage of 800 V, can be approximated linearly in the range of interest, between 300 and 700 mA. In contrast, the rate dependency on the beam voltage for a constant beam current of 300 mA is growing almost linearly up to
150 200 250 300 350 400 450 500 550 44 Deposition rate in dependence of
Deposition rate [a.U.]
experiments. The process chamber is equipped with a high-power ion source (Veeco, 16 cm RF Ion Source). In this configuration, the ion source possesses an optimized elliptical grid focusing the plasma beam on the target with a diameter of approximately 6 cm. As is common in the IBS process, the source is combined with a neutralizer (electron emitter). Pure metal targets were used in the sputtering process. Consequently, the process was performed in a reactive mode, applying additional oxygen. The reactive gas can be supplied to the process via the ion source as well as via an additional pipe. Generally, argon was used as the sputter gas and for the RF neutralizer. All data presented in this paper were collected by the following methods of measurements. The layers provided by the particular setups were deposited on Borofloat 33 substrates from Schott, mounted at the end of the solenoid for the purpose of monitoring distribution of the coating material. The optical and physical thicknesses of the coatings, at well-defined positions on the substrate, were determined ex situ by evaluating transmission spectra. Consequently, spectral photometry was used for the measurements, and the thicknesses were determined by a particular thin film calculation software. The evaluation of the lateral coating distribution required a large number of single measurements and data reduction procedures. The transmission measurement was performed by a fiber-spectrometer (Avantis, AvaSpec2048 × 14) in combination with an optimized beam line providing the white light. With respect to the inhomogeneity of the layers, the spot size at the measurement position was approximately 1 mm in diameter. In an automatic step, the data were converted and transferred to the thin film software “Spectrum” [13], which performed the evaluation.
40
the Variation of Ion Source Voltage taken at constant current (300mA)
36
the Variation of Ion Source Current
Current of Ion Source [mA]
32 taken at a constant beam voltage (800V) 28 24 20 16
Oscillating crystal measurement Target material: Aluminum
12 8 400
600
800
1000
1200
1400
Voltage of Ion Source [V]
Fig. 2. Dependency of the coating rate with respect to beam voltage and beam current when aluminum is used as the target material.
V beam 800 V, and the curve flattens out above V beam 1000 V. In the deposition experiments, the guiding effect has been investigated under a well-defined set of parameters. For these experiments, titania and alumina single layers were deposited on Borofloat glass substrates in a reactive process using metal targets. Alumina can be used as low refractive index and titania as high-index material in a layer stack. Silica is the most common low refractive index material and was investigated in a previous study [2]. First, the influence of high-energy electrons from the neutralizer on the guiding and, second, the dependence on the oxygen flux, are discussed. Those two investigations were performed in the first setup. Finally, the influence of the magnetic field strength is analyzed, essentially employing the second setup. Comparing the lateral deposition distribution on the substrates in setup 1, represented by the photocopies in Figs. 3–5, the samples in Fig. 3 have a lateral distribution that is different to the samples shown in Figs. 4 and 5. This is traced back using the precollimation coil that was applied for the coating measurements illustrated in Figs. 4 and 5, but not for those coatings displayed in Fig. 3. The magnetic field of the precollimation coil was switched off
Fig. 3. Influence of highly energetic electrons emitted from the neutralizer on the lateral coating distribution, using titanium as the target material. 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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Fig. 4. Influence of oxygen content on the lateral coating distribution, using titanium as the target material.
by bridging the current of the solenoid. The precollimation coil between the target and the ion source generates additional magnetic fields lines. The magnetic fields of those two coils superimpose and change the confinement of the plasma. Obviously, the magnetic field lines and, in turn, the trajectories of the electrons, significantly affect the lateral distribution of the coating. As described above it is assumed that the guiding effect depends inter alia on the current of highly energized electrons emitted by the neutralizer, referred to as the neutralizer current. Regarding this assumption, it seems evident that a higher neutralizer current leads to a higher efficiency of the guiding effect. Thus, the neutralizer current was varied to investigate in the influence on the plasma guiding effect. In these experiments, the neutralizer current was set to a fraction of the ion source current. The ion source parameters constituted I Source 300 mA and U Beam 800 V, respectively. The gas fluxes were kept stable on the base parameters that accounted for 6–23–20–0 [sccm]. This nomenclature implies that the neutralizer operates with 6 sccm argon gas; the ion source is operated with 23 sccm argon gas in combination with 20 sccm oxygen, and the lateral inlet supplies 0 sccm oxygen. The results of the coating experiments for titania with 20% neutralization (I neutralizer 60 mA) and with 200% neutralization (I neutralizer 600 mA), are displayed in Fig. 3, where all surface plots are equally scaled. The gain factor in deposition rate can be determined by evaluating the lateral thickness distribution of coating material on the substrates
Fig. 5. Influence of reactive gas inlet positioning on the lateral coating distribution, using titanium as the target material. 8216
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(230 mm × 230 mm × 0.5 mm). A comparison of the maximum deposition rates indicates a significant increase with rising electron flux, due to the neutralizer current. The maximum rate of r200% 0.035 nm∕s was achieved with a setting of I neutralizer ∕I source 200%. Additionally, Fig. 3(a) displays the current ratio as I neutralizer ∕I source 20%, which yields a rate value of approximately 0.0244 nm∕s and results in a gain factor in the local deposition rate of 1.5. As mentioned above, in these experiments, the precollimation coil was not in operation, but for the investigation of the influence of oxygen content and the oxygen activation, this coil was in operation. Figure 4 shows exemplarily, coatings from the measurement series using setup 1, which was designed to study the influence of oxygen content on the deposition rate distribution. For most of the investigations, a flux of 20 sccm of activated oxygen, injected through the ion source, was used. Applying partial oxygen pressure, a stoichiometric titania layer was deposited in the reactive process. In the experiment illustrated by Fig. 4, the oxygen flux has been increased from 20 to 50 sccm. An increase in deposition rate was observed that accounted for a gain factor of approximately 1.65. The reason for the increase in deposition rate might be explained by the theory that higher oxygen content increases the oxidation of the target and decreases the sputter efficiency. Therefore, the presence of oxygen decreases the amount of ions that can be influenced by the guiding device. Furthermore, Fig. 5 shows the lateral coating distribution of a process using setup 1 and compares activated oxygen with nonactivated oxygen. The nonactivated oxygen is provided via the lateral inlet, close to the target [Fig. 5(a)], and the activated oxygen is provided directly through the ion source [Fig. 5(b)], respectively. Both samples were coated in a deposition run with 50 sccm oxygen. It should be mentioned that the type of gas inlet used significantly influenced the deposition rates. Feeding the reactive oxygen through the lateral inlet leads to a gain factor in deposition rate of 1.2. Additionally, an increase in ion confinement can be obtained by the presence of a positive bias plate operating at 80 V. This positive potential rejects the ions moving toward the bias plate [2,12]. All coatings manufactured in setup 1 were produced under the operation of such a bias plate. Detailed investigations regarding the influence of the bias plate and guiding efficiency have been presented in a previous study [2]. The bias plate reveals the highest gain factor of up to 1.9 in deposition rate [2]. Nonetheless, the main parameter for the guiding effect is the magnetic field strength. To conclusively determine the dependency of the guiding efficiency on the strength of the magnetic field, the influence of higher magnetic fields has to be investigated. Because of the limitation of the available magnetic field strength of setup 1, setup 2 was applied for this
Fig. 6. Lateral distribution of deposition rates for different coil currents for alumina.
0.13
Deposition rate [nm/s]
investigation, as illustrated in Fig. 1(b). The guiding coil consists of a double-wound and closely spaced coil and provides a magnetic field strength of up to 60 mT. In fact, the magnetic field strength is proportional to the coil current. In the course of this paper, the coil current is used as an optimization parameter, instead of the magnetic field strength. This is more reproducible since the magnetic field might not be uniform. In the previous deposition experiments, titanium was used as the target material; however, the target material was switched to aluminum for the following experiments. The evaluation of the lateral thickness distribution of the samples is displayed in Fig. 6 for a series of measurements with an increasing coil current. The first column of samples in Fig. 6 depicts the original photocopies of the samples. The refractive index of alumina does not differ significantly from the refractive index of the Borofloat substrate. Therefore, the interference rings are not distinctive and are shown reworked with a graphics program in the second column. The series in Fig. 6 demonstrates the influence of the magnetic field on the thickness distribution. The deposition rate is expressed in units of gain factor related to a reference layer, deposited without field influence. The increase of the B-field provokes a shift of the coating material concentration toward the center of the sample. Additionally, the higher magnetic field leads to local increases in coating material.
Al2O 3 TiO2
0.12
Saturation
0.11 0.10 0.09 0.08
-x
~y= a-b
0.07 0.06
Limited growth in deposition rate
0.05 0.04 0.03 -50
0
50
100 150 200 250 300 350
Coil Current [A] Fig. 7. Local maxima of deposition rates as a function of coil current using oxides of aluminum (blue square plot) and titanium (red triangle plot) as target materials.
The maximum deposition rate without the application of an operating coil was measured to be approximately 0.074 nm∕s, as a reference. The lateral distribution of this reference sample is governed by a shadowing effect of coating material, given by the shape of the solenoid and the angle with respect to the target normal. The B-field generated by a 50 A coil current is sufficient to guide the plasma through the coil. Figure 7 displays the dependence of the local maxima of deposition rates for alumina and titania; whereas, the data were extracted by evaluating the maximum alumina deposition rates from Fig. 6 against the coil current. Its application results in a deposition rate approximately two times higher than the reference coating when using titanium as the target material, and it results in a deposition rate 1.32 times higher using aluminum as the target material. Consequently, the current through the coil does not necessarily have to be high as long as it generates a magnetic field of at least a couple of tens milliTesla (mT). Regarding alumina, the application of a stronger magnetic field, twice as strong as in setup 1, causes a better guiding effect along the center axis of the coil, with an observed deposition rate of approximately 0.098 nm∕s. Optimization of the deposition rate by increasing the coil current and, thereby, the magnetic field
Fig. 8. Lateral distribution of deposition rate in conjunction with the corresponding parameters set on the ion source. 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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Table 1
Summary of Parameters with Clear Influence on the Guiding Efficiency and Achieved Gain Factors
I Coil 300 A ∼30 mT
0 A–300 A ∼max: 60 mT
Influence
Gain Factor in Deposition Rate
Set-up Config./Target Material
Neutralizer Oxygen content Reactive/Nonreactive Bias plate (show in [2]) Magnetic field strength
1.50 1.65 1.20 1.90 1.75
Set-up 1/Titania
strength, is limited. Increasing the magnetic field strength to arbitrary levels does not lead to higher deposition rates. Figure 7 shows a saturation initiating around 200 A coil current. The same behavior can also be seen for titania, which generally tends toward smaller deposition rate values. However, the observed saturation at approximately 200 A coil current might be explained by the fact that all available deposition material sputtered from the target is guided by a certain magnetic field level already; therefore, the quantity of charged particles is limited. Nonetheless, a further increase in deposition rate can be achieved by adjusting the parameters of the ion source to create more charged material. Taking into account the dependencies indicated in Fig. 2, a significant increase in deposition rate is yielded by increasing the current as well as the voltage of the ion source, without an application of magnetic fields. In Fig. 8, the standard parameters of the ion source (I Beam 300 mA and V Beam 800 V) have been increased to I Beam 500 mA and V Beam 1300 V, respectively. In conjunction with an additional magnetic field strength, induced by a coil current of 300 A, an increase in local deposition rates from 0.11 to 0.323 nm∕s, according to a gain factor of approximately 3, can be achieved. However, the increase in deposition rate by augmented ion source parameters is actually not a result of the guiding effect, but shows that all charged material that is allocated by the IBS process is guided through the coil. In summary, in Table 1, the parameters, with clear influence on the guiding efficiency, are displayed for the respective setups and target materials. The application of the bias plate yields, with 1.9 the highest gain factor, followed by the application of a low oxygen content, generates a factor of approximately 1.65 in deposition rate. The weakest influence is observed for the application of a nonreactive, instead of a reactive process, which still raises the deposition rate by a factor of 1.2. 4. Conclusion and Discussion
The application of electromagnetic fields in the common IBS process allows for a manipulation of the deposition material, since it can be guided from the target to the substrate. The guiding effect can be explained by the assumption that the electrons, trapped by the magnetic fields, create an electrostatic potential, and that the ions, which are not directly influenced by the electromagnetic field, follow the electrons. 8218
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Set-up 2/Alumina
This guiding effect increases the local deposition rate of the modified IBS process at the exit of the solenoid. The magnetic fields can be applied to collimate the sputtered species within the plasma. Consequently, the original cosine-like distribution shape changes its geometry. Finally, a significant content of the deposition material is restricted along the axial magnetic field. An increase of local deposition rate has been shown for titania (high refractive index coating material) and alumina (low refractive index material). Since both materials experienced a similar gain in deposition rate, it can be assumed that the increase in deposition rate can be achieved independently of the deposition material. In accordance to the present state of the art, the deposition rate can be raised by increasing the current as well as the beam voltage of the sputter source; in addition, it was shown that a higher power ion source leads to an increased deposition rate. Tuning the ion source parameters causes an increased sputter rate from the target. This increased quantity of deposition material cannot be traced back to the guiding effect directly, since tuning the ion source parameters does not influence the collimation or sputter distribution. However, increasing the source parameters leads to a higher quantity of the available charged material. These investigations show that, if more material is generated by the sputtering process, then more charged material is available that can be directly transferred to an increase in the deposition rate at the exit of the solenoid. An increase in both the ion source current and the ion source beam voltage increases the sputter flux of sputtered material, as well as the output rate of the guiding device, by allocating more sputter material. Thereby, a gain factor of three could be recorded for the alumina deposition in this configuration. In the present study, the magnetic field strength has the most significant effect on the guiding effect. Generally, the efficiency of the guidance was observed to increase with magnetic field strength, but the growth in deposition rate is limited for high magnetic field strengths due to the limited quantity of available sputtered material. For alumina, a gain factor of 1.75 and, for titania, a gain factor of 2.7, are observed by varying the magnetic field strength. A further optimization of the guiding effect can be achieved by a change of several parameters in the modified process. This includes an increase of the neutralizer current, the application of additional magnetic fields provided by a precollimation coil, and a decrease of the oxygen content, each leading
to a significant gain in deposition rate. First, a higher neutralizer current, as well as the magnetic fields of the precollimation coil, raises the degree of plasma ionization. Therefore, the amount of ions of the sputter material within the plasma representing the manipulatable content of the plasma is raised. Consequently, the quantity of deposition material affected by the application of the electromagnetic fields is increased, and more material falls under the scope of the guiding effect. Second, a decrease of the oxygen content within the process leads to an increase in deposition rate. Finally, a small increase in deposition rate is also observed by changing the reactive oxygen to nonreactive oxygen. Applying lower reactive oxygen content, or nonreactive oxygen, might degrade the level of oxidation that increases sputter efficiency at the target, making a higher quantity of deposition material available for guiding. In summary, these results indicate that an improvement of plasma guiding is possible and, hence, that electromagnetic fields are able to manipulate the coating process, depending on the adjustment of the plasma conditions. 5. Outlook
The current guiding procedure generates a lateral distribution in the substrate plane, which is similar to a Gaussian distribution. However, the lateral distribution might be optimized by application of additional magnetic or electrical fields to provide coatings of higher homogeneity. Improvements in local deposition rates have been shown for both high refractive index materials and low refractive index materials. This demonstrates the potential of the present approach for an improvement also for layer systems. In a subsequent research step, the optical properties of the coating itself have to be considered and related to the parameters of the electromagnetic fields. Besides this specific analysis of the optical properties, the next level of research has to reveal whether the optical properties of the coating itself can also be adjusted by the application of electromagnetic fields. In this paper, it is proven that the plasma flux can be directed by electromagnetic fields. Therefore, particular species, such as unwanted deposition material, might be separated to improve the optical properties. Specific components of deposition
material can be separated spatially by directing the charged deposition material in a curvilinear manner; whereas, the trajectories of clusters, or neutral material, will not be changed. Nonetheless, this investigation of the guidance of coating material is the first step toward an enhanced, new generation of IBS processes. These first attempts toward implementing a guiding device require further research and investigations to achieve fully applicable, standardized guiding procedures that can be employed in commercial applications. This work was financially supported by the Deutsche Forschungsgesellschaft (DFG) within the cluster of excellence 201 Quest. References 1. I. I. Aksenov, A. N. Belokhvostikov, V. G. Padalka, N. S. Repalov, and V. M. Khoroshikh, “Plasma flux motion in a toroidal plasma guide,” Plasma Phys. Controlled Fusion 28, 761–770 (1986). 2. S. Malobabic, M. Jupé, and D. Ristau, “Towards an electromagnetic field separation of deposited material implemented in an ion beam sputter process,” Appl. Phys. Lett. 102, 221604 (2013). 3. W. F. Van der Weg and P. K. Rol, “On the excited state of sputtered particles,” Nucl. Instrum. Methods 38, 274–276 (1965). 4. G. Betz and K. Wien, “Energy and angular distributions of sputtered particles,” Int. J. Mass Spectrom. 140, 1–110 (1994). 5. B. Schroeder, R. Peter, J. Harhausen, and A. Ohl, “Modelling and simulation of the advanced plasma source,” J. Appl. Phys. 110, 043305 (2011). 6. R. L. Boxman and S. Goldsmith, “Macroparticle contamination in cathodic arc coatings: generation, transport and control,” Surf. Coat. Technol. 52, 39–50 (1992). 7. M. A. Lieberman and A. J. Lichtenberg, Principles of Plasma Discharges and Materials Processing (Wiley, 1994), p. 132. 8. D. K. Kwok, P. K. Chu, M. M. M. Bilek, I. G. Brown, and A. Vizir, “Ion mean charge state in a biased vacuum arc plasma duct,” IEEE Trans. Plasma Sci. 28, 2194–2201 (2000). 9. C. A. Davis and I. J. Donnelly, “Simulation of ion transport through curved‐solenoid macroparticle filters,” J. Appl. Phys. 72, 1740–1747 (1992). 10. P. J. Martin and A. Bendavid, “Review of the filtered vacuum arc process and materials deposition,” Thin Solid Films 394, 1–14 (2001). 11. D. B. Boercker, S. Falabella, and D. M. Sanders, “Plasma transport in a new cathodic arc ion source: theory and experiment,” Surf. Coat. Technol. 53, 239–242 (1992). 12. D. T. K. Kwok, T. Zhang, P. K. Chu, M. M. M. Bilek, A. Vizir, and I. G. Brown, “Experimental investigation of electron oscillation inside the filter of a vacuum arc plasma source,” Appl. Phys. Lett. 78, 422–424 (2001). 13. M. Dieckmann, “Spectrum, Thin film software,” Laser Zentrum Hannover.
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Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany
Quest: Centre of Quantum Engineering and Space-Time Research, Leibniz Universität Hannover, Germany *Corresponding author: [email protected] Received 4 September 2013; revised 30 October 2013; accepted 30 October 2013; posted 31 October 2013 (Doc. ID 197045); published 22 November 2013
Ion beam sputtering (IBS) is an established deposition process used in the production of optical coatings. In this study, a modification of the IBS process, based on additional electromagnetic fields, is examined in an effort to improve the technology. The reported experiments reveal the underlying effects of electromagnetic fields on the distribution of the coating material sputtered from the target. An increase in local deposition rate is observed and discussed in the context of the interaction between the introduced fields and the species in the target area. First approaches toward an optimization of the observed bunching effect on the deposition material are illustrated. © 2013 Optical Society of America OCIS codes: (170.0110) Imaging systems; (170.3010) Image reconstruction techniques; (170.3660) Light propagation in tissues. http://dx.doi.org/10.1364/AO.52.008212
1. Introduction
The common ion beam sputtering (IBS) process is a deposition technique well qualified for the production of optical coatings with lowest optical losses. However, to keep pace with the rapid development of optical technology, it is desirable to achieve further progress toward an even higher level of quality presently achievable with IBS. Unfortunately, recent, mainly empirical optimization concepts of the classical IBS process seem to accomplish only incremental improvements. In this context, significant modifications of the IBS process involving additional electromagnetic fields can be considered a novel approach to guide the process plasma, exert influence on the optical properties, as well as to adjust the flux geometry of the sputtered species. The implementation of electromagnetic fields into a deposition process is already known from the vacuum arc deposition (VAD) process, where such fields were effectively 1559-128X/13/348212-08$15.00/0 © 2013 Optical Society of America 8212
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applied to clean vacuum arc-generated plasma from macroparticles of the cathode material, as well as from neutral atoms [1]. A comparable macroparticle generation in the state-of-the-art IBS process can be avoided using empirically optimized coating conditions. Nevertheless, technology should allow for tailored controlling of the coating process. However, the effect of a guiding device in highly developed IBS processes may be relatively problematic, because the ionization rate of the material flux sputtered from the target is two orders of magnitude lower than in a VAD process. Recently, it was proven that the ions within the particle flux of an IBS process can be controlled by electromagnetic fields to regulate the trajectory of the elementary particles [2]. The magnetic fields can be applied to influence the charged content of the material emitted from the target, as well as from the process environment. Furthermore, it can be guided along the axial magnetic field lines. In the classic IBS process, the material is distributed as a cosine-like polynomial. This geometry of the sputtered species flux can be
changed by collimating the deposition material emitted from the target. The quantity of the charged plasma content determines the efficiency of those effects. The present paper is particularly concerned with the potential to optimize the deposition rate distribution in an IBS process. In the presented experiments, two guiding devices, with different setups, are applied to investigate the guiding effect. The paper comprises a detailed investigation of the change in deposition rate in dependence of the electromagnetic field strength, under the variation of essential process parameters like oxygen supply and sputter ion source settings. Supplementing the results of [2], the present paper delivers an insight into the interaction of deposition distribution and electromagnetic fields. 2. Guiding Effect
Generally, electromagnetic fields can only influence charged particles. Therefore, it can be expected that no significant change in trajectory of uncharged material is induced. Consequently, the ion density in the sputtered coating material is an important parameter for the plasma guiding effect. Unfortunately, the sputtering process at the target surface is only reaching an ionization rate of about 1% for the emitted species [3,4]. For economic applications, the emitted coating material has to be ionized to a higher degree to gain an appropriate higher guiding efficiency. The high energy electrons ejected from the neutralizer, as well as the hot electrons of the plasma beam, are able to ionize the sputtered coating material in front of the target. In addition, the electrons can be trapped along the axial magnetic field lines. This confinement of electrons prevents a recombination of electrons and ions and, accordingly, a reneutralization. During the time of flight from the target to the substrate, electrons can ionize the sputtered coating species via collision ionization. Finally, coherent exchanging charge transfer effects can also change the proportion of electrically charged particles and uncharged material [5]. Subsequently, the combination of a high current of hot electrons, ejected by the neutralizer and the magnetic field in the solenoid, leads to the creation of additional ions in the coating process, amplifying the guiding effect. Thus, an axial magnetic field induces a sufficient degree of ionization within the plasma making it possible to guide more coating material toward the substrate. The magnetic fields providing the guiding effect can be generated by the implementation of a solenoid into the conventional IBS process. The provided magnetic field is described by the formula B
μNI ; 2πrsolenoid
(1)
where I is the coil current, N is the number of turns, and rsolenoid is the inner radius of the coil,
respectively. A value of the magnetic field in the range of 30–60 mT is in agreement with the requirement (Khizhnyak, 1965; Vojtsenya et al. 1967) H>
mv⊥ c →B≥ qer
q 2 mhEkin ic2 μ0 ∕e2 r2solenoid ;
(2)
[1], where the symbol m denotes the mass of the electron (or of the ion or molecule). v⊥ relates to the velocity component perpendicular to the magnetic field, Ekin is the mean kinetic energy of the ions and q represents the charge factor in this equation. Typically, e stands for the elementary charge, c stands for the velocity of light in vacuum, and μ0 is the absolute permeability of the vacuum. If the condition is fulfilled, then a guiding of the plasma inside the coil takes place [1]. In first order, the electromagnetic fields are not sufficiently strong to change the trajectory of the ions significantly. Field strength values in the range of some Telsa (T) would be necessary to confine the ions [6]. In contrast to this, electrons, with their mass three orders of magnitude smaller than ions, can be influenced efficiently by moderate electromagnetic fields. The cross field motion of the electrons is confined by the electromagnetic force of an axial magnetic field. The velocity component of the electron movement perpendicular to the magnetic field line leads to a circular path, caused by the Lorentz force. As a consequence of this effect, the electrons tend to move in a circular trajectory, and the dimension of the free movement space is characterized by the Larmor radius [6,7,8]. relec
mv⊥ : qB
(3)
The electrons within the plasma are confined as a result of the magnetic fields, however, the ions are not confined. Therefore, application of the magnetic field allows the creation of a self-consistent electrostatic potential within the system, because the electrons are shifted from the ions [9], which forces the ions to follow the magnetic fields lines. Consequently, this charge separation guides the ions through the solenoid because the ions follow the electrons to stay in a state of quasi-neutrality [10,11]. If the Larmor radius is smaller than the radius of the coil (relec < rsolenoid ), then the electrons are trapped, because they move circularly along the magnetic field lines. The Larmor radius of the ions, however, is much larger (rsolenoid < rion ), therefore the ions are not trapped [10]. If the relation relec < rsolenoid < rion ;
(4)
is fulfilled [1,11], meaning the Larmor radius of the electrons relec has to be smaller than the radius of the solenoid rsolenoid [which, in addition, has to be smaller 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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than the Larmor radius of the ions (rion )], then guiding is possible. Furthermore, an additional electrostatic field can be applied to enhance plasma guiding. This is caused by static fields parallel to the target provide a positive potential, reflecting the ions and preventing the plasma from diffusing [12]. The electrostatic field can be provided by a bias plate that is energized with several tens of volts. A.
Experimental Setup
To investigate the influence of an electromagnetic field guiding the deposition material, several setups were implemented into the common IBS process. The first setup was designed to achieve a complete overview of possible impacting physical parameters; whereas, the second one allows detailed investigations on the influence of the magnetic DC field inside the separator. Setup 1, as illustrated in Fig. 1(a), was applied for the first experimental series. As mentioned above, the purpose of the experiments was to study the influence of different fields on the guiding process. Setup 2 is an upgrade of setup 1, mainly to produce a higher magnetic field. Basically, the guiding device of setup 1 generates a combination of an electrical and a magnetic field, provided by a solenoid and a bias plate, respectively. The solenoid is a spirally shaped, water-cooled copper tube with a diameter of 25 cm. It consists of a precollimating and a guiding coil. The entrance of the precollimating coil is positioned in front of the ion source. It collimates the plasma beam onto the
Fig. 1. Overview of applied setups: (a) solenoid combined of precollimation coil and guiding coil and (b) double-wound coil. 8214
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target. The major aim of the precollimation is to generate magnetic field lines parallel to the target and force the sputter material in the direction of the guiding coil. The guiding coil leads the sputtered species emitted from the target to the substrates and allows a line-of-sight trajectory from the target to the exit of the solenoid. However, both coils are bent in a V shape, with an angle of approximately 140°. It has to be noted that the angle between the normal of the target and the center line of the guiding coil is approximately 60°. To allow isolated investigations of the precollimating effect, the setup can be driven with or without the respective coil. The solenoid generates a magnetic field strength of maximal 30 mT at 340 A coil current inside the solenoid. The experiments show that most of the plasma is confined and is guided along the magnetic field lines to the substrate. Furthermore, a bias plate is implemented that covers a quarter of the internal area of the solenoid at the outlet of the solenoid [12]. A voltage of approximately 80 V was applied to the bias plate. The experiments using setup 1 reveal various aspects of plasma guiding. Furthermore, it has to be mentioned that small variations in geometry lead to significant changes in guiding. For instance, a change of the angle between the coils can lead to a shielding of neutralizer electrons. However, a disadvantage of setup 1 is that only 30 mT are achievable at the maximum. Therefore, an investigation of the dependence of magnetic field strength and deposition rate was only possible to a limited degree. Consequently, the effect of the magnetic field was investigated in a specially constructed setup. The new developed solenoid was mounted inside the chamber, which guarantees high accuracy and result reproducibility. The double windings of the coil covering a cylindrical glass tube provide sufficient stability. Moreover, the tube can be replaced and cleaned to remove internal coatings. The structure of the upgraded setup can be seen in Fig. 1(b), presented as setup 2. In detail, the core component of the guidance in setup 2 is a coil providing axial magnetic fields. In contrast with setup 1, the axis of the coil is oriented about 20° with respect to the normal at the target’s surface. The distance between target and coil is 10 cm. In total, the path length from the target surface to the exit of the coil adds up to approximately 40 cm, including a length of 30 cm of the guiding coil. Since the coil is now positioned directly in the sputter yield, the major part of the sputter yield is directed toward the coil. The precollimation coil and the bias plate are not integrated in setup 2. The B-field was varied proportionally to the coil current in the range of 0–340 A. The general operating current of the coil was 300 A. Contrary to setup 1, the solenoid in setup 2 allocates a higher maximal magnetic field strength, accounting for 60 mT, to investigate its influence. However, both setups were used in a specially constructed cluster of vacuum chambers; whereas, the process chamber is currently used for the
3. Results and Influences of the Guiding Effect
In the discussion of the setup, the electromagnetic device is presented as an addition to the classic IBS process. As a baseline for discussion of the guiding effects, the specifications of the sputter process without electromagnetic guiding tools are characterized. These measurements allow for an interpretation of the guiding experiment results. Figure 2 shows the dependency of the deposition rate, which was measured with the oscillating crystal in the common IBS process, without electromagnetic fields, as a function of the voltage and the current of the ion source. The oscillating crystal was mounted at the solenoid entrance in setup 2. The variation of the ion source parameters were recorded for aluminum as the target material in a reactive process. The deposition rate varies strongly in relation to the ion source and to the beam voltage. The current dependency, with a constant beam voltage of 800 V, can be approximated linearly in the range of interest, between 300 and 700 mA. In contrast, the rate dependency on the beam voltage for a constant beam current of 300 mA is growing almost linearly up to
150 200 250 300 350 400 450 500 550 44 Deposition rate in dependence of
Deposition rate [a.U.]
experiments. The process chamber is equipped with a high-power ion source (Veeco, 16 cm RF Ion Source). In this configuration, the ion source possesses an optimized elliptical grid focusing the plasma beam on the target with a diameter of approximately 6 cm. As is common in the IBS process, the source is combined with a neutralizer (electron emitter). Pure metal targets were used in the sputtering process. Consequently, the process was performed in a reactive mode, applying additional oxygen. The reactive gas can be supplied to the process via the ion source as well as via an additional pipe. Generally, argon was used as the sputter gas and for the RF neutralizer. All data presented in this paper were collected by the following methods of measurements. The layers provided by the particular setups were deposited on Borofloat 33 substrates from Schott, mounted at the end of the solenoid for the purpose of monitoring distribution of the coating material. The optical and physical thicknesses of the coatings, at well-defined positions on the substrate, were determined ex situ by evaluating transmission spectra. Consequently, spectral photometry was used for the measurements, and the thicknesses were determined by a particular thin film calculation software. The evaluation of the lateral coating distribution required a large number of single measurements and data reduction procedures. The transmission measurement was performed by a fiber-spectrometer (Avantis, AvaSpec2048 × 14) in combination with an optimized beam line providing the white light. With respect to the inhomogeneity of the layers, the spot size at the measurement position was approximately 1 mm in diameter. In an automatic step, the data were converted and transferred to the thin film software “Spectrum” [13], which performed the evaluation.
40
the Variation of Ion Source Voltage taken at constant current (300mA)
36
the Variation of Ion Source Current
Current of Ion Source [mA]
32 taken at a constant beam voltage (800V) 28 24 20 16
Oscillating crystal measurement Target material: Aluminum
12 8 400
600
800
1000
1200
1400
Voltage of Ion Source [V]
Fig. 2. Dependency of the coating rate with respect to beam voltage and beam current when aluminum is used as the target material.
V beam 800 V, and the curve flattens out above V beam 1000 V. In the deposition experiments, the guiding effect has been investigated under a well-defined set of parameters. For these experiments, titania and alumina single layers were deposited on Borofloat glass substrates in a reactive process using metal targets. Alumina can be used as low refractive index and titania as high-index material in a layer stack. Silica is the most common low refractive index material and was investigated in a previous study [2]. First, the influence of high-energy electrons from the neutralizer on the guiding and, second, the dependence on the oxygen flux, are discussed. Those two investigations were performed in the first setup. Finally, the influence of the magnetic field strength is analyzed, essentially employing the second setup. Comparing the lateral deposition distribution on the substrates in setup 1, represented by the photocopies in Figs. 3–5, the samples in Fig. 3 have a lateral distribution that is different to the samples shown in Figs. 4 and 5. This is traced back using the precollimation coil that was applied for the coating measurements illustrated in Figs. 4 and 5, but not for those coatings displayed in Fig. 3. The magnetic field of the precollimation coil was switched off
Fig. 3. Influence of highly energetic electrons emitted from the neutralizer on the lateral coating distribution, using titanium as the target material. 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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Fig. 4. Influence of oxygen content on the lateral coating distribution, using titanium as the target material.
by bridging the current of the solenoid. The precollimation coil between the target and the ion source generates additional magnetic fields lines. The magnetic fields of those two coils superimpose and change the confinement of the plasma. Obviously, the magnetic field lines and, in turn, the trajectories of the electrons, significantly affect the lateral distribution of the coating. As described above it is assumed that the guiding effect depends inter alia on the current of highly energized electrons emitted by the neutralizer, referred to as the neutralizer current. Regarding this assumption, it seems evident that a higher neutralizer current leads to a higher efficiency of the guiding effect. Thus, the neutralizer current was varied to investigate in the influence on the plasma guiding effect. In these experiments, the neutralizer current was set to a fraction of the ion source current. The ion source parameters constituted I Source 300 mA and U Beam 800 V, respectively. The gas fluxes were kept stable on the base parameters that accounted for 6–23–20–0 [sccm]. This nomenclature implies that the neutralizer operates with 6 sccm argon gas; the ion source is operated with 23 sccm argon gas in combination with 20 sccm oxygen, and the lateral inlet supplies 0 sccm oxygen. The results of the coating experiments for titania with 20% neutralization (I neutralizer 60 mA) and with 200% neutralization (I neutralizer 600 mA), are displayed in Fig. 3, where all surface plots are equally scaled. The gain factor in deposition rate can be determined by evaluating the lateral thickness distribution of coating material on the substrates
Fig. 5. Influence of reactive gas inlet positioning on the lateral coating distribution, using titanium as the target material. 8216
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(230 mm × 230 mm × 0.5 mm). A comparison of the maximum deposition rates indicates a significant increase with rising electron flux, due to the neutralizer current. The maximum rate of r200% 0.035 nm∕s was achieved with a setting of I neutralizer ∕I source 200%. Additionally, Fig. 3(a) displays the current ratio as I neutralizer ∕I source 20%, which yields a rate value of approximately 0.0244 nm∕s and results in a gain factor in the local deposition rate of 1.5. As mentioned above, in these experiments, the precollimation coil was not in operation, but for the investigation of the influence of oxygen content and the oxygen activation, this coil was in operation. Figure 4 shows exemplarily, coatings from the measurement series using setup 1, which was designed to study the influence of oxygen content on the deposition rate distribution. For most of the investigations, a flux of 20 sccm of activated oxygen, injected through the ion source, was used. Applying partial oxygen pressure, a stoichiometric titania layer was deposited in the reactive process. In the experiment illustrated by Fig. 4, the oxygen flux has been increased from 20 to 50 sccm. An increase in deposition rate was observed that accounted for a gain factor of approximately 1.65. The reason for the increase in deposition rate might be explained by the theory that higher oxygen content increases the oxidation of the target and decreases the sputter efficiency. Therefore, the presence of oxygen decreases the amount of ions that can be influenced by the guiding device. Furthermore, Fig. 5 shows the lateral coating distribution of a process using setup 1 and compares activated oxygen with nonactivated oxygen. The nonactivated oxygen is provided via the lateral inlet, close to the target [Fig. 5(a)], and the activated oxygen is provided directly through the ion source [Fig. 5(b)], respectively. Both samples were coated in a deposition run with 50 sccm oxygen. It should be mentioned that the type of gas inlet used significantly influenced the deposition rates. Feeding the reactive oxygen through the lateral inlet leads to a gain factor in deposition rate of 1.2. Additionally, an increase in ion confinement can be obtained by the presence of a positive bias plate operating at 80 V. This positive potential rejects the ions moving toward the bias plate [2,12]. All coatings manufactured in setup 1 were produced under the operation of such a bias plate. Detailed investigations regarding the influence of the bias plate and guiding efficiency have been presented in a previous study [2]. The bias plate reveals the highest gain factor of up to 1.9 in deposition rate [2]. Nonetheless, the main parameter for the guiding effect is the magnetic field strength. To conclusively determine the dependency of the guiding efficiency on the strength of the magnetic field, the influence of higher magnetic fields has to be investigated. Because of the limitation of the available magnetic field strength of setup 1, setup 2 was applied for this
Fig. 6. Lateral distribution of deposition rates for different coil currents for alumina.
0.13
Deposition rate [nm/s]
investigation, as illustrated in Fig. 1(b). The guiding coil consists of a double-wound and closely spaced coil and provides a magnetic field strength of up to 60 mT. In fact, the magnetic field strength is proportional to the coil current. In the course of this paper, the coil current is used as an optimization parameter, instead of the magnetic field strength. This is more reproducible since the magnetic field might not be uniform. In the previous deposition experiments, titanium was used as the target material; however, the target material was switched to aluminum for the following experiments. The evaluation of the lateral thickness distribution of the samples is displayed in Fig. 6 for a series of measurements with an increasing coil current. The first column of samples in Fig. 6 depicts the original photocopies of the samples. The refractive index of alumina does not differ significantly from the refractive index of the Borofloat substrate. Therefore, the interference rings are not distinctive and are shown reworked with a graphics program in the second column. The series in Fig. 6 demonstrates the influence of the magnetic field on the thickness distribution. The deposition rate is expressed in units of gain factor related to a reference layer, deposited without field influence. The increase of the B-field provokes a shift of the coating material concentration toward the center of the sample. Additionally, the higher magnetic field leads to local increases in coating material.
Al2O 3 TiO2
0.12
Saturation
0.11 0.10 0.09 0.08
-x
~y= a-b
0.07 0.06
Limited growth in deposition rate
0.05 0.04 0.03 -50
0
50
100 150 200 250 300 350
Coil Current [A] Fig. 7. Local maxima of deposition rates as a function of coil current using oxides of aluminum (blue square plot) and titanium (red triangle plot) as target materials.
The maximum deposition rate without the application of an operating coil was measured to be approximately 0.074 nm∕s, as a reference. The lateral distribution of this reference sample is governed by a shadowing effect of coating material, given by the shape of the solenoid and the angle with respect to the target normal. The B-field generated by a 50 A coil current is sufficient to guide the plasma through the coil. Figure 7 displays the dependence of the local maxima of deposition rates for alumina and titania; whereas, the data were extracted by evaluating the maximum alumina deposition rates from Fig. 6 against the coil current. Its application results in a deposition rate approximately two times higher than the reference coating when using titanium as the target material, and it results in a deposition rate 1.32 times higher using aluminum as the target material. Consequently, the current through the coil does not necessarily have to be high as long as it generates a magnetic field of at least a couple of tens milliTesla (mT). Regarding alumina, the application of a stronger magnetic field, twice as strong as in setup 1, causes a better guiding effect along the center axis of the coil, with an observed deposition rate of approximately 0.098 nm∕s. Optimization of the deposition rate by increasing the coil current and, thereby, the magnetic field
Fig. 8. Lateral distribution of deposition rate in conjunction with the corresponding parameters set on the ion source. 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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Table 1
Summary of Parameters with Clear Influence on the Guiding Efficiency and Achieved Gain Factors
I Coil 300 A ∼30 mT
0 A–300 A ∼max: 60 mT
Influence
Gain Factor in Deposition Rate
Set-up Config./Target Material
Neutralizer Oxygen content Reactive/Nonreactive Bias plate (show in [2]) Magnetic field strength
1.50 1.65 1.20 1.90 1.75
Set-up 1/Titania
strength, is limited. Increasing the magnetic field strength to arbitrary levels does not lead to higher deposition rates. Figure 7 shows a saturation initiating around 200 A coil current. The same behavior can also be seen for titania, which generally tends toward smaller deposition rate values. However, the observed saturation at approximately 200 A coil current might be explained by the fact that all available deposition material sputtered from the target is guided by a certain magnetic field level already; therefore, the quantity of charged particles is limited. Nonetheless, a further increase in deposition rate can be achieved by adjusting the parameters of the ion source to create more charged material. Taking into account the dependencies indicated in Fig. 2, a significant increase in deposition rate is yielded by increasing the current as well as the voltage of the ion source, without an application of magnetic fields. In Fig. 8, the standard parameters of the ion source (I Beam 300 mA and V Beam 800 V) have been increased to I Beam 500 mA and V Beam 1300 V, respectively. In conjunction with an additional magnetic field strength, induced by a coil current of 300 A, an increase in local deposition rates from 0.11 to 0.323 nm∕s, according to a gain factor of approximately 3, can be achieved. However, the increase in deposition rate by augmented ion source parameters is actually not a result of the guiding effect, but shows that all charged material that is allocated by the IBS process is guided through the coil. In summary, in Table 1, the parameters, with clear influence on the guiding efficiency, are displayed for the respective setups and target materials. The application of the bias plate yields, with 1.9 the highest gain factor, followed by the application of a low oxygen content, generates a factor of approximately 1.65 in deposition rate. The weakest influence is observed for the application of a nonreactive, instead of a reactive process, which still raises the deposition rate by a factor of 1.2. 4. Conclusion and Discussion
The application of electromagnetic fields in the common IBS process allows for a manipulation of the deposition material, since it can be guided from the target to the substrate. The guiding effect can be explained by the assumption that the electrons, trapped by the magnetic fields, create an electrostatic potential, and that the ions, which are not directly influenced by the electromagnetic field, follow the electrons. 8218
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Set-up 2/Alumina
This guiding effect increases the local deposition rate of the modified IBS process at the exit of the solenoid. The magnetic fields can be applied to collimate the sputtered species within the plasma. Consequently, the original cosine-like distribution shape changes its geometry. Finally, a significant content of the deposition material is restricted along the axial magnetic field. An increase of local deposition rate has been shown for titania (high refractive index coating material) and alumina (low refractive index material). Since both materials experienced a similar gain in deposition rate, it can be assumed that the increase in deposition rate can be achieved independently of the deposition material. In accordance to the present state of the art, the deposition rate can be raised by increasing the current as well as the beam voltage of the sputter source; in addition, it was shown that a higher power ion source leads to an increased deposition rate. Tuning the ion source parameters causes an increased sputter rate from the target. This increased quantity of deposition material cannot be traced back to the guiding effect directly, since tuning the ion source parameters does not influence the collimation or sputter distribution. However, increasing the source parameters leads to a higher quantity of the available charged material. These investigations show that, if more material is generated by the sputtering process, then more charged material is available that can be directly transferred to an increase in the deposition rate at the exit of the solenoid. An increase in both the ion source current and the ion source beam voltage increases the sputter flux of sputtered material, as well as the output rate of the guiding device, by allocating more sputter material. Thereby, a gain factor of three could be recorded for the alumina deposition in this configuration. In the present study, the magnetic field strength has the most significant effect on the guiding effect. Generally, the efficiency of the guidance was observed to increase with magnetic field strength, but the growth in deposition rate is limited for high magnetic field strengths due to the limited quantity of available sputtered material. For alumina, a gain factor of 1.75 and, for titania, a gain factor of 2.7, are observed by varying the magnetic field strength. A further optimization of the guiding effect can be achieved by a change of several parameters in the modified process. This includes an increase of the neutralizer current, the application of additional magnetic fields provided by a precollimation coil, and a decrease of the oxygen content, each leading
to a significant gain in deposition rate. First, a higher neutralizer current, as well as the magnetic fields of the precollimation coil, raises the degree of plasma ionization. Therefore, the amount of ions of the sputter material within the plasma representing the manipulatable content of the plasma is raised. Consequently, the quantity of deposition material affected by the application of the electromagnetic fields is increased, and more material falls under the scope of the guiding effect. Second, a decrease of the oxygen content within the process leads to an increase in deposition rate. Finally, a small increase in deposition rate is also observed by changing the reactive oxygen to nonreactive oxygen. Applying lower reactive oxygen content, or nonreactive oxygen, might degrade the level of oxidation that increases sputter efficiency at the target, making a higher quantity of deposition material available for guiding. In summary, these results indicate that an improvement of plasma guiding is possible and, hence, that electromagnetic fields are able to manipulate the coating process, depending on the adjustment of the plasma conditions. 5. Outlook
The current guiding procedure generates a lateral distribution in the substrate plane, which is similar to a Gaussian distribution. However, the lateral distribution might be optimized by application of additional magnetic or electrical fields to provide coatings of higher homogeneity. Improvements in local deposition rates have been shown for both high refractive index materials and low refractive index materials. This demonstrates the potential of the present approach for an improvement also for layer systems. In a subsequent research step, the optical properties of the coating itself have to be considered and related to the parameters of the electromagnetic fields. Besides this specific analysis of the optical properties, the next level of research has to reveal whether the optical properties of the coating itself can also be adjusted by the application of electromagnetic fields. In this paper, it is proven that the plasma flux can be directed by electromagnetic fields. Therefore, particular species, such as unwanted deposition material, might be separated to improve the optical properties. Specific components of deposition
material can be separated spatially by directing the charged deposition material in a curvilinear manner; whereas, the trajectories of clusters, or neutral material, will not be changed. Nonetheless, this investigation of the guidance of coating material is the first step toward an enhanced, new generation of IBS processes. These first attempts toward implementing a guiding device require further research and investigations to achieve fully applicable, standardized guiding procedures that can be employed in commercial applications. This work was financially supported by the Deutsche Forschungsgesellschaft (DFG) within the cluster of excellence 201 Quest. References 1. I. I. Aksenov, A. N. Belokhvostikov, V. G. Padalka, N. S. Repalov, and V. M. Khoroshikh, “Plasma flux motion in a toroidal plasma guide,” Plasma Phys. Controlled Fusion 28, 761–770 (1986). 2. S. Malobabic, M. Jupé, and D. Ristau, “Towards an electromagnetic field separation of deposited material implemented in an ion beam sputter process,” Appl. Phys. Lett. 102, 221604 (2013). 3. W. F. Van der Weg and P. K. Rol, “On the excited state of sputtered particles,” Nucl. Instrum. Methods 38, 274–276 (1965). 4. G. Betz and K. Wien, “Energy and angular distributions of sputtered particles,” Int. J. Mass Spectrom. 140, 1–110 (1994). 5. B. Schroeder, R. Peter, J. Harhausen, and A. Ohl, “Modelling and simulation of the advanced plasma source,” J. Appl. Phys. 110, 043305 (2011). 6. R. L. Boxman and S. Goldsmith, “Macroparticle contamination in cathodic arc coatings: generation, transport and control,” Surf. Coat. Technol. 52, 39–50 (1992). 7. M. A. Lieberman and A. J. Lichtenberg, Principles of Plasma Discharges and Materials Processing (Wiley, 1994), p. 132. 8. D. K. Kwok, P. K. Chu, M. M. M. Bilek, I. G. Brown, and A. Vizir, “Ion mean charge state in a biased vacuum arc plasma duct,” IEEE Trans. Plasma Sci. 28, 2194–2201 (2000). 9. C. A. Davis and I. J. Donnelly, “Simulation of ion transport through curved‐solenoid macroparticle filters,” J. Appl. Phys. 72, 1740–1747 (1992). 10. P. J. Martin and A. Bendavid, “Review of the filtered vacuum arc process and materials deposition,” Thin Solid Films 394, 1–14 (2001). 11. D. B. Boercker, S. Falabella, and D. M. Sanders, “Plasma transport in a new cathodic arc ion source: theory and experiment,” Surf. Coat. Technol. 53, 239–242 (1992). 12. D. T. K. Kwok, T. Zhang, P. K. Chu, M. M. M. Bilek, A. Vizir, and I. G. Brown, “Experimental investigation of electron oscillation inside the filter of a vacuum arc plasma source,” Appl. Phys. Lett. 78, 422–424 (2001). 13. M. Dieckmann, “Spectrum, Thin film software,” Laser Zentrum Hannover.
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