Laser forward transfer using structured light Raymond C. Y. Auyeung,* Heungsoo Kim, Scott Mathews and Alberto Piqué Materials Science and Technology Division, Code 6364, Naval Research Laboratory, Washington, DC 20375, USA * [email protected]
Abstract: A digital micromirror device (DMD) is used to spatially structure a 532 nm laser beam to print features spatially congruent to the laser spot in a laser-induced forward transfer (LIFT) process known as laser decal transfer (LDT). The DMD is a binary (on/off) spatial light modulator and its resolution, half-toning and beam shaping properties are studied using LDT of silver nanopaste layers. Edge-enhanced “checkerboard” beam profiles led to a ~30% decrease in the laser transfer fluence threshold (compared to a reference “checkerboard” profile) for a 20-pixel bitmap pattern and its resulting 10-μm square feature. ©2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (230.6120) Spatial light modulators; (230.4110) Modulators; (140.3300) Laser beam shaping; (310.1860) Deposition and fabrication.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
K. K. B. Hon, L. Li, and I. M. Hutchings, “Direct writing technology - advances and developments,” CIRP Ann. Manuf. Technol. 57(2), 601–620 (2008). N. S. Kim and K. N. Han, “Future direction of direct writing,” J. Appl. Phys. 108(10), 102801 (2010). C. B. Arnold, P. Serra, and A. Piqué, “Laser direct-write techniques for printing of complex materials,” MRS Bull. 32(01), 23–32 (2007). B. Derby, “Inkjet printing of functional and structural materials: fluid property requirements, feature stability, and resolution,” Annu. Rev. Mater. Res. 40(1), 395–414 (2010). A. Mahajan, C. D. Frisbie, and L. F. Francis, “Optimization of aerosol jet printing for high-resolution, highaspect ratio silver lines,” ACS Appl. Mater. Interfaces 5(11), 4856–4864 (2013). N. A. Charipar, K. M. Charipar, H. Kim, M. A. Kirleis, R. C. Y. Auyeung, A. T. Smith, S. A. Mathews, and A. Piqué, “Laser processing of 2D and 3D metamaterial structures,” Proc. SPIE 8607, 86070T (2013). A. Piqué and H. Kim, “Laser-induced forward transfer of functional materials: advances and future directions,” J. Laser Micro Nanoeng. 9(3), 192–197 (2014). A. Khan, K. Rahman, D. S. Kim, and K. H. Choi, “Direct printing of copper conductive micro-tracks by multinozzle electrohydrodynamic inkjet printing process,” J. Mater. Process. Technol. 212(3), 700–706 (2012). R. Auyeung, H. Kim, N. Charipar, A. Birnbaum, S. Mathews, and A. Piqué, “Laser forward transfer based on a spatial light modulator,” Appl. Phys., A Mater. Sci. Process. 102(1), 21–26 (2011). A. Piqué, R. C. Y. Auyeung, H. Kim, K. M. Metkus, and S. A. Mathews, “Digital microfabrication by laser decal transfer,” J. Laser Micro Nanoeng. 3(3), 163–169 (2008). H. Kim, J. S. Melinger, A. Khachatrian, N. A. Charipar, R. C. Y. Auyeung, and A. Piqué, “Fabrication of terahertz metamaterials by laser printing,” Opt. Lett. 35(23), 4039–4041 (2010). Texas Instruments data sheet TI DN 2509699 Rev B (March 2009). R. Ulichney, Digital Halftoning (MIT, 1987). A. Piqué, R. Auyeung, K. Metkus, H. Kim, S. Mathews, T. Bailey, X. Chen, and L. Young, “Laser decal transfer of electronic materials with thin film characteristics,” Proc. SPIE 6879, 687911 (2008). S. A. Mathews, R. C. Y. Auyeung, H. Kim, N. A. Charipar, and A. Piqué, “High-speed video study of laserinduced forward transfer of silver nano-suspensions,” J. Appl. Phys. 114(6), 064910 (2013). L. Rapp, C. Constantinescu, Y. Larmande, A. Alloncle, and P. Delaporte, “Smart beam shaping for the deposition of solid polymeric material by laser forward transfer,” Appl. Phys., A Mater. Sci. Process. 117(1), 333–339 (2014).
1. Introduction Much of modern microelectronics technology is based on the physical or chemical deposition of thin films and their subsequent patterning by photolithography. This two-step approach requires extensive equipment investment and strict environmental controls (i.e. cleanroom) resulting in considerable cost, time and materials expense. Yet it continues to be used widely due to its parallel processing advantage, which once optimized, yields a high throughput #225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 422
capability. Direct write processes [1–3] such as inkjet [4], aerosol jet [5], microdispense [6] and laser-induced forward transfer (LIFT) [7] offer lower-cost non-lithographic alternatives that deposit and pattern material simultaneously under ambient conditions. They offer the additional advantage of easily reconfigurable patterns without the need to design and manufacture a new mask every time. These direct write processes are inherently serial in nature, as a pattern must be built up one ‘voxel’ (or volume element) at a time, although schemes have been devised to increase their throughput [8]. Recently, this limitation of serial processing was overcome by combining a modified LIFT process with a spatial light modulator to perform digital microfabrication in a highly parallel and dynamically reconfigurable manner, thus representing a paradigm shift in direct write technology [9]. A LIFT process using high-viscosity donor material, also known as laser decal transfer (LDT), results in laser-transferred voxels that are highly congruent in shape and size to the incident laser spot [10, 11]. By increasing the viscosity of the transferred voxels, wetting issues of the substrate and volume shrinkage of the voxels during curing can be minimized. The shape of the laser spot striking the donor layer is typically determined by the spatial light profile passing through an aperture or photomask, which is then projected with the desired demagnification onto the work surface. With a digital micromirror device (DMD), a loaded bitmap image onto the DMD takes the place of this aperture/mask and spatially modulates the beam profile reflected from the DMD. This laser spot is then imaged and demagnified onto the work surface as before. By using a DMD to shape the laser spot, this removes the need to fabricate a new aperture or mask each time a new pattern is desired. In addition, the DMD can “imprint” any bitmap pattern onto the laser beam, which is not entirely possible with an aperture. The high refresh rate (kHz) of the DMD also enables the printing of different patterns with each laser shot. All these features make the combination of LDT with a DMD a highly parallel, rapidly reconfigurable, digital printing technique for microfabrication. The previous work [9] reported by our group employed a pulsed 355 nm laser beam with a Gaussian beam profile, which necessitated a Gaussian to flat-top beam shaper. The current work employed a pulsed 532 nm beam with a flat-top beam profile, which ensured a more uniform illumination of the DMD array. To further understand the LDT-DMD technique, the effect of DMD pixelation on the laser beam profile (and laser transfer) was studied by examining the resolution and quality of laser transfers for different pixel sizes in the bitmap image. These results were then used to construct appropriate DMD bitmap images for beamshaping studies on laser transfers. The effects of laser beam shape on laser forward transfer will be discussed and compared to those results from a reference beam. 2. Experimental procedure A more compact optical setup for the study of the LDT-DMD process was used in this work. A schematic diagram of the setup used is shown in Fig. 1. A frequency-doubled Nd:YAG (Quantel ULTRA) pulsed multimode laser (λ = 532 nm, 10 ns FWHM) provided 8 mJ pulse energies at a maximum rep rate of 50 Hz. A pulse generator (Stanford Research Systems DG535) controlled the 532 nm pulse energies by varying the Q-switch delay to the laser. The output laser beam is expanded ~8X to illuminate the DMD array more uniformly. As in the previous work, the DMD array comprises a 1.8 cm (diagonal) spatial light modulator (Texas Instruments DLP® D4100 0.7” UV XGA Kit) optimized for UV wavelengths and consists of an aluminum micromirror array with 1024 x 768 mirror elements with a pitch of 13.68 µm. Although this DMD was optimized for the UV, the 532 nm throughput of 68% [12] was sufficient to perform laser transfer. Each micromirror can tilt at an angle of ± 12° about its diagonal and corresponds to one pixel on the bitmap image programmed on the DMD. The DMD has an active area of 10.506 x 14.008 mm and can generate up to 32,550 patterns per second. A desired bitmap image is loaded onto the DMD and a single laser pulse illuminates the entire array. The reflected beam from the DMD is
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 423
directed to the donor substrate (referred as the ‘ribbon’) and then imaged through a 20X microscope objective (Mitutoyo M Plan Apo NUV 20X) onto the donor or ribbon substrate located on an X-Y stage pair (Aerotech ATS150). DMD image loading, laser pulse-picking and stage movement can be controlled independently or with a computer. The spatial beam profile reflected from the DMD is re-imaged and monitored on a CCD beam profiler (Ophir Beamstar FX-50 or Spiricon SP620U camera) and typically matches the loaded bitmap pattern. Pulse energies were recorded after the objective on a pyroelectric energy sensor (Ophir PE9 or PE25).
Fig. 1. Schematic diagram of the laser transfer setup with a 532 nm laser pulse spatially modulated by a DMD.
The orientation of the DMD array relative to the incident and exiting laser beams was carefully aligned for optimum efficiency, minimal rotation and distortion in the imaged DMD pattern. The ratio of one pixel of the bitmap pattern on the DMD to its final size on the image plane of the objective is roughly equal to the ratio of the focal lengths of the imaging lens L and the microscope objective. This ratio, known as the demag factor, is ~24 for the 20X objective used in our optical setup. Alternatively, one pixel in the bitmap image of the DMD corresponds to ~0.56 µm in the image plane of the 20X objective. The ribbon was made from a 50 x 75 mm borosilicate glass (1.1 mm thick) onto which a layer of high-viscosity (80,000 - 100,000 cP) nanoparticle silver paste (Harima NPS, 3 - 7 nm particle size) was doctor-bladed. After blading, the nanopaste was dried in air to remove excess solvent from the paste for optimum decal transfer. The ribbon was placed with the ink layer side parallel to and facing the receiving substrate, separated by an adjustable gap of 1 to 50 µm. The ribbon/receiving substrate stack rested on the X-Y motion controlled stages, and its translation synchronized with the pulsing of the laser in order to print the pattern from the DMD image. Once transferred, the Ag patterns were oven cured at 100 °C for 30 minutes for scanning electron microscopy or SEM imaging. (Full cure conditions were 200 °C for 1 hour). Optical microscopy (Olympus BX51), confocal microscopy (Olympus LEXT OLS3000) and SEM (JEOL, JSM-7001F) were used to characterize the shape and morphology of the transferred patterns; and confocal microscopy was used to inspect the void left in the ribbon nanopaste layer after the transfer. 3. Results and discussion To fully exploit the digital control of the laser light and its influence on LDT, the effect of the individual micromirrors (and therefore pixels) of the DMD on the spatial properties of the
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 424
laser beam and quality of the laser transfer must be understood. The laser beam fluence, spatial uniformity and structure all play a part in the laser transfer threshold and any topographic features in the resulting transfer. The resolution of the laser printed features in our system will first be determined and then examined how it is affected by a modified beam profile. The optical configuration in this work images the DMD bitmap pattern directly onto the image plane of the microscope objective. It is also possible to use other types of spatial light modulators to manipulate the amplitude, phase, or polarization of the beam profile in space and time to generate more complex beam patterns, but those approaches tend to add extra complexity to the LIFT setup with perhaps a small benefit in return. Because the DMD is a binary device – the micromirrors (or pixels) are either on or off, a single pixel cannot be used to generate a grayscale. However, we can borrow from a technique, over a hundred years old and still being used today in publishing, displays and image processing, called half-toning [13]. This technique relies on the human eye to spatially average an assortment of sizes, shapes and spacings of ‘cells’ or dots into a continuous gradient of tones in an image. In this work, groups of pixels in the DMD modulate the spatial pattern of the incident beam, which is then imaged (with demag factor) by our optical system onto the ribbon nanopaste layer. We need to determine the smallest unit cell or pixel size of the DMD, which cannot be imaged in our optical system (and transferred pattern) and therefore can be used to generate grayscale
Fig. 2. Tracking of LDT process from initial DMD bitmap images of 4 different square sizes to their corresponding final transfer features. The top row shows (a) 125-px, (b) 51-px, (c) 23-px and (d) 8-px square bitmap images and the second row shows the corresponding measured beam profiles. (The inset in (d) is a magnified beam profile from the 8-px square pattern). The third row shows 3-D confocal microscopy images of the hole left in the ribbon after the transfer. Last row shows SEM images of the resulting transfer on silicon.
intensities. This concept will become important later in the discussion of beam-shaping with the DMD. In our previous work, the laser transferred patterns corresponded exactly to the shape and structure of the bitmap pattern on the DMD, whereas this work explores the use of a “structured” bitmap pattern (grayscale) in laser transfers for the first time.
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 425
The resolution of the LDT process with the DMD was first tested with progressively smaller squares that were laser printed from a ribbon with a 1.5 µm-thick silver nanopaste layer. Figure 2 follows the LDT process from the initial DMD pattern, through the measured beam profile and crater left in the ribbon, to the resultant transfer. The beam profile matched the loaded bitmap image down to ~20 pixels, but diffraction effects began to appear in bitmaps comprising of < 10 pixels. As the number of `on' pixels on the DMD was decreased, the size of the printed voxel became smaller until no transfer was observed for a square bitmap < 8 mirrors (or pixels) in the DMD, corresponding to a laser spot size of ~5 µm square. Note that this transfer resolution is the result of the combination of laser spot size with ribbon nanopaste thickness, spatial uniformity of the beam and the optical demag for the DMD. In fact, LDT has demonstrated laser-transferred resolutions as low as 2 x 2 µm in area but with a nanopaste thickness of 200 nm [14]. Moreover, as will be shown later, the imaged resolution in our optical system is observed to be higher than that for laser transfer. Conversely, the resolution limit of the negative image of the square pattern in Fig. 2 - a “square ring”, can be explored with decreasing hole size as seen in Fig. 3. As the images show, the transfers consisted of a frame whose borders became increasingly wider until the opening at the center of the frame disappeared. The resolution limit was reached for a hole comprising of 8x8 mirrors “off”, below which, a solid square pattern, devoid of a center was printed. This resolution limit of ~8 pixels in the current setup for laser transfer is consistent with that seen in Fig. 2. Therefore, bitmap patterns must be > 8 for successful laser transfer in the current optical system. It is important to note that these types of patterns are impossible to generate from masks made from metal foil cut-outs such as traditional stencils.
Fig. 3. Tracking of LDT process from initial DMD bitmap images of 4 different rings to their corresponding final transfer features. The top row shows 169-px square bitmap images with (a) 125-px, (b) 51-px, (c) 23-px and (d) 8-px ‘hole’ and the second row shows the corresponding measured beam profiles. The third row shows 3-D confocal microscopy images of the mesa left in the ribbon after the transfer. Last row shows SEM images of the resulting transfer on silicon.
The resolution limit of 8 pixels seems to indicate roughly the size of the unit cell of the DMD, but this must be examined further. The previous laser-transferred squares employed
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 426
bitmap square patterns that were all ‘on’ (i.e. light throughput from the DMD was maximized). In order to compare these transfers with those from a modified beam shape, the reference bitmap pattern must be made lower in intensity than the “all-on” pattern since DMD light throughput cannot exceed its inherent maximum. Therefore, in our beam-shaping studies, comparisons of transfers from a modified beam bitmap pattern should only be made with those from this reference bitmap pattern. In addition, in order to make valid comparisons, the overall quality of transfers from this reference pattern must be similar to those from a regular, unmodified, all “pixels-on” pattern. One method to decrease light throughput from a DMD is to turn off alternate pixels as in a “checkerboard” pattern. In Fig. 4, bitmap patterns of checkerboards with decreasing unit cell sizes are shown along with the resulting beam profiles, ribbon craters and transfers. Note that for a 10-pixel unit cell size, the checkerboard pattern in the transfer begins to average out. However, even for a 2-pixel unit cell, there are sub-micron surface features in the transfer, which are only completely removed for a 1-pixel unit cell. These fine features interestingly demonstrate a higher DMD resolution (≈micron) in texturing a surface than the ~5 µm resolution seen in laser transfer with our current setup. Note in Fig. 4, the surface quality of the transfer from a 1-pixel unit cell is equivalent to that from the 80-pixel (all pixels on) cell. Because the overall quality of these two transfers are the same, the 1x1 pixel checkerboard pattern can be chosen as the reference pattern and a fair comparison with it can be made for beam-shaping experiments.
Fig. 4. Tracking of LDT process from initial DMD bitmap images of “checkerboard” pattern to their corresponding final transfer features. The top row shows decreasing unit cell sizes of the 80-pixel square bitmap image loaded onto the DMD. The second row shows the measured beam profiles of the laser spot striking the ribbon. The third row shows 3-D confocal microscopy images of the hole left in the 1.5 µm-thick ribbon and the fourth row shows SEM images of the resulting transfer on silicon along with the laser transfer fluence. The last row shows higher resolution SEM images of the 2- and 1-pixel unit cell checkerboard pattern transfers.
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 427
Modifying the beam spatial profile can improve the quality, process conditions and window of successful laser transfers. In a recent work [15], it was found that the threshold fluence required to release a voxel from the nanopaste layer varied inversely as the voxel size since more of the laser energy was expended at the voxel’s perimeter for smaller voxels. This result was confirmed by the increase in laser fluence in Fig. 4 as the unit cell decreased from 80- to 10-pixels. This decrease in unit cell size was accompanied by an increase in the total perimeter of the voxels and a decrease in the size of the individual voxels, which required an increase in the laser transfer fluence. (If the laser transfer process solely depended on the total area transferred, then the fluences should have scaled accordingly). For unit cell sizes less than 10 pixels, the transfers were not congruent to the incident beam profile and any firm conclusions regarding their transfer fluences could not be drawn. In order to redistribute more of the laser energy towards the perimeter of a voxel, our target beam profiles were designed for higher laser intensity at the edges than in the interior. Figure 5 shows bitmap 1x1 checkerboard patterns of 80-, 40- and 20-pixel squares with a 510% border of “all-on” pixels. This border resulted in a two- to three-fold increase of the edge intensity over that of the interior as seen in the beam profiles in Fig. 5 and as evidenced by the dark thin border inside the crater in the ribbon. The resulting transfers were congruent in shape and size as the laser spot, and their morphology were not adversely affected by the edge-enhanced beam profile.
Fig. 5. Tracking of LDT process using edge-enhanced beams created with a) 80-pixel, b) 40pixel and c) 20-pixel square ‘checkerboard’ bitmap images with a 5-10% border width as shown in the top row. The second row shows the measured beam profiles of the laser spot striking the ribbon. Confocal microscopy images of the hole left in the ribbon and the resulting fully-cured transfer on silicon are shown in the third and fourth row respectively. Laser transfer fluence thresholds for these edge-enhanced checkerboard patterns are a) 39, b) 57 and c) 96 mJ/cm2 respectively.
This 1x1 checkerboard shaped-beam profile can be used with a reference (unmodified) 1x1 checkerboard beam profile to study its effect on the laser transfer threshold fluence. For each of the three bitmap patterns in Fig. 5, a 3x3 array of silver nanopaste squares were transferred onto silicon as a function of incident laser energy. The LDT threshold was determined to be the fluence at which ~90% of the transfers were intact and congruent in shape and size to the laser spot. Figure 6 shows the transfer threshold fluence for three spot
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 428
sizes using the reference and modified checkerboard beam profiles. A significant threshold decrease of ~30% occurred for a laser spot size approaching 10 µm (20-pixel square bitmap), whereas transfers using larger spot sizes were not affected by the modified beam profile, as was expected. This decrease is less than that observed for laser transfers of polymer membranes using an edge-enhanced beam profile [16] but may be due to differences in materials properties and their interaction with laser light. Furthermore, our transfer process employs ribbons with a single ink layer, which is a simpler configuration than ones that use a dynamic release layer (DRL). This DRL also has a minimum threshold fluence for successful transfer and therefore the use of a DMD-structured beam with a single ink layer gives more control over the resulting transfers. No transfers were successful for the 5 µm (10-pixel square pattern) in the current setup, but may be due to ribbon ink thickness and beam nonuniformity (see Fig. 2). It is important to note that the absolute fluence values in Fig. 6 are not important and should not be compared with those found in ‘typical’ laser transfers that employ the full unmodulated beam profile. The checkerboard concept used in this work allows modified and reference beam profiles to be compared fairly and equally in an imaged DMD system. The relative change in transfer threshold for a shaped vs. non-shaped beam is what is most important and demonstrates the feasibility of using halftoning to simulate grayscale in a DMD pattern to shape a laser beam for laser transfers.
Fig. 6. Laser transfer fluence threshold with laser spot size for checkerboard reference and checkerboard edge-enhanced beam profiles. The unit cell for the checkerboard bitmap pattern is 1-pixel square. Threshold fluences are measured for square laser transfers of silver nanopaste. The solid and dashed lines are visual guides only.
4. Conclusion The laser beam profile of a 532 nm laser was spatially modulated by a DMD and its effect on LDT of silver nanopaste layers has been studied. Bitmap square patterns and their negative counterpart were “imprinted” onto the laser beam profile, imaged through a 20X microscope objective, and transferred intact silver structures from ~70 to 10 μm square in our optical setup. Finer features have been demonstrated elsewhere with LDT, but were not the goal of
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 429
this study, and should be possible with improved beam uniformity and optimized ribbon ink thickness and uniformity. The “half-toning” concept was used on the DMD to simulate “grayscaling” of the laser beam profile. A resolution limit near ~10 pixels in the DMD bitmap (imaged size ~5 µm) was found for laser forward transfer of silver nanopaste in our setup. However, surfaces of the transferred pattern showed even finer structure arising from a number of single pixels in the DMD bitmap. Ultimately, a unit cell of 1x1 pixel formed the basis of the half-tone (or ‘checkerboard’) pattern. By using this pattern with an all-white pixel border, the initial interior profile of the beam intensity was decreased while the edge intensity was increased by a factor of ~2-3 over that of the interior with edge widths ~5 to 10% of the full beam width. For a laser spot and transfer size of 12 μm, a decrease of ~30% in laser transfer fluence threshold was observed for an edge-enhanced beam profile compared to a reference checkerboard beam profile. This result shows that a DMD can effectively shape the spatial profile of a laser beam using the grayscale (or “half-toning”) concept and that in general, spatially structuring the laser beam can benefit the laser transfer process. Although not studied in this work, beam-shaping may allow laser transfers with thicker ribbon ink layers, smaller feature sizes, more fragile (organic or brittle) materials and more complex shapes. The use of a spatial light modulator such as a DMD in laser transfer provides a highly parallel, rapidly reconfigurable technique for laser digital microfabrication. It is a natural extension to use a DMD with a spatially congruent laser transfer process such as LDT to directly map pixels from the bitmap pattern into transferred voxels. In addition, by modifying the beam profile using half-toning techniques in the DMD bitmap, LDT now has the potential to operate with a wider process window, more complex shapes and increased reliability. Acknowledgment This work was funded by the Office of Naval Research through the Naval Research Laboratory Basic Research Program.
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 430
Abstract: A digital micromirror device (DMD) is used to spatially structure a 532 nm laser beam to print features spatially congruent to the laser spot in a laser-induced forward transfer (LIFT) process known as laser decal transfer (LDT). The DMD is a binary (on/off) spatial light modulator and its resolution, half-toning and beam shaping properties are studied using LDT of silver nanopaste layers. Edge-enhanced “checkerboard” beam profiles led to a ~30% decrease in the laser transfer fluence threshold (compared to a reference “checkerboard” profile) for a 20-pixel bitmap pattern and its resulting 10-μm square feature. ©2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (230.6120) Spatial light modulators; (230.4110) Modulators; (140.3300) Laser beam shaping; (310.1860) Deposition and fabrication.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
K. K. B. Hon, L. Li, and I. M. Hutchings, “Direct writing technology - advances and developments,” CIRP Ann. Manuf. Technol. 57(2), 601–620 (2008). N. S. Kim and K. N. Han, “Future direction of direct writing,” J. Appl. Phys. 108(10), 102801 (2010). C. B. Arnold, P. Serra, and A. Piqué, “Laser direct-write techniques for printing of complex materials,” MRS Bull. 32(01), 23–32 (2007). B. Derby, “Inkjet printing of functional and structural materials: fluid property requirements, feature stability, and resolution,” Annu. Rev. Mater. Res. 40(1), 395–414 (2010). A. Mahajan, C. D. Frisbie, and L. F. Francis, “Optimization of aerosol jet printing for high-resolution, highaspect ratio silver lines,” ACS Appl. Mater. Interfaces 5(11), 4856–4864 (2013). N. A. Charipar, K. M. Charipar, H. Kim, M. A. Kirleis, R. C. Y. Auyeung, A. T. Smith, S. A. Mathews, and A. Piqué, “Laser processing of 2D and 3D metamaterial structures,” Proc. SPIE 8607, 86070T (2013). A. Piqué and H. Kim, “Laser-induced forward transfer of functional materials: advances and future directions,” J. Laser Micro Nanoeng. 9(3), 192–197 (2014). A. Khan, K. Rahman, D. S. Kim, and K. H. Choi, “Direct printing of copper conductive micro-tracks by multinozzle electrohydrodynamic inkjet printing process,” J. Mater. Process. Technol. 212(3), 700–706 (2012). R. Auyeung, H. Kim, N. Charipar, A. Birnbaum, S. Mathews, and A. Piqué, “Laser forward transfer based on a spatial light modulator,” Appl. Phys., A Mater. Sci. Process. 102(1), 21–26 (2011). A. Piqué, R. C. Y. Auyeung, H. Kim, K. M. Metkus, and S. A. Mathews, “Digital microfabrication by laser decal transfer,” J. Laser Micro Nanoeng. 3(3), 163–169 (2008). H. Kim, J. S. Melinger, A. Khachatrian, N. A. Charipar, R. C. Y. Auyeung, and A. Piqué, “Fabrication of terahertz metamaterials by laser printing,” Opt. Lett. 35(23), 4039–4041 (2010). Texas Instruments data sheet TI DN 2509699 Rev B (March 2009). R. Ulichney, Digital Halftoning (MIT, 1987). A. Piqué, R. Auyeung, K. Metkus, H. Kim, S. Mathews, T. Bailey, X. Chen, and L. Young, “Laser decal transfer of electronic materials with thin film characteristics,” Proc. SPIE 6879, 687911 (2008). S. A. Mathews, R. C. Y. Auyeung, H. Kim, N. A. Charipar, and A. Piqué, “High-speed video study of laserinduced forward transfer of silver nano-suspensions,” J. Appl. Phys. 114(6), 064910 (2013). L. Rapp, C. Constantinescu, Y. Larmande, A. Alloncle, and P. Delaporte, “Smart beam shaping for the deposition of solid polymeric material by laser forward transfer,” Appl. Phys., A Mater. Sci. Process. 117(1), 333–339 (2014).
1. Introduction Much of modern microelectronics technology is based on the physical or chemical deposition of thin films and their subsequent patterning by photolithography. This two-step approach requires extensive equipment investment and strict environmental controls (i.e. cleanroom) resulting in considerable cost, time and materials expense. Yet it continues to be used widely due to its parallel processing advantage, which once optimized, yields a high throughput #225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 422
capability. Direct write processes [1–3] such as inkjet [4], aerosol jet [5], microdispense [6] and laser-induced forward transfer (LIFT) [7] offer lower-cost non-lithographic alternatives that deposit and pattern material simultaneously under ambient conditions. They offer the additional advantage of easily reconfigurable patterns without the need to design and manufacture a new mask every time. These direct write processes are inherently serial in nature, as a pattern must be built up one ‘voxel’ (or volume element) at a time, although schemes have been devised to increase their throughput [8]. Recently, this limitation of serial processing was overcome by combining a modified LIFT process with a spatial light modulator to perform digital microfabrication in a highly parallel and dynamically reconfigurable manner, thus representing a paradigm shift in direct write technology [9]. A LIFT process using high-viscosity donor material, also known as laser decal transfer (LDT), results in laser-transferred voxels that are highly congruent in shape and size to the incident laser spot [10, 11]. By increasing the viscosity of the transferred voxels, wetting issues of the substrate and volume shrinkage of the voxels during curing can be minimized. The shape of the laser spot striking the donor layer is typically determined by the spatial light profile passing through an aperture or photomask, which is then projected with the desired demagnification onto the work surface. With a digital micromirror device (DMD), a loaded bitmap image onto the DMD takes the place of this aperture/mask and spatially modulates the beam profile reflected from the DMD. This laser spot is then imaged and demagnified onto the work surface as before. By using a DMD to shape the laser spot, this removes the need to fabricate a new aperture or mask each time a new pattern is desired. In addition, the DMD can “imprint” any bitmap pattern onto the laser beam, which is not entirely possible with an aperture. The high refresh rate (kHz) of the DMD also enables the printing of different patterns with each laser shot. All these features make the combination of LDT with a DMD a highly parallel, rapidly reconfigurable, digital printing technique for microfabrication. The previous work [9] reported by our group employed a pulsed 355 nm laser beam with a Gaussian beam profile, which necessitated a Gaussian to flat-top beam shaper. The current work employed a pulsed 532 nm beam with a flat-top beam profile, which ensured a more uniform illumination of the DMD array. To further understand the LDT-DMD technique, the effect of DMD pixelation on the laser beam profile (and laser transfer) was studied by examining the resolution and quality of laser transfers for different pixel sizes in the bitmap image. These results were then used to construct appropriate DMD bitmap images for beamshaping studies on laser transfers. The effects of laser beam shape on laser forward transfer will be discussed and compared to those results from a reference beam. 2. Experimental procedure A more compact optical setup for the study of the LDT-DMD process was used in this work. A schematic diagram of the setup used is shown in Fig. 1. A frequency-doubled Nd:YAG (Quantel ULTRA) pulsed multimode laser (λ = 532 nm, 10 ns FWHM) provided 8 mJ pulse energies at a maximum rep rate of 50 Hz. A pulse generator (Stanford Research Systems DG535) controlled the 532 nm pulse energies by varying the Q-switch delay to the laser. The output laser beam is expanded ~8X to illuminate the DMD array more uniformly. As in the previous work, the DMD array comprises a 1.8 cm (diagonal) spatial light modulator (Texas Instruments DLP® D4100 0.7” UV XGA Kit) optimized for UV wavelengths and consists of an aluminum micromirror array with 1024 x 768 mirror elements with a pitch of 13.68 µm. Although this DMD was optimized for the UV, the 532 nm throughput of 68% [12] was sufficient to perform laser transfer. Each micromirror can tilt at an angle of ± 12° about its diagonal and corresponds to one pixel on the bitmap image programmed on the DMD. The DMD has an active area of 10.506 x 14.008 mm and can generate up to 32,550 patterns per second. A desired bitmap image is loaded onto the DMD and a single laser pulse illuminates the entire array. The reflected beam from the DMD is
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 423
directed to the donor substrate (referred as the ‘ribbon’) and then imaged through a 20X microscope objective (Mitutoyo M Plan Apo NUV 20X) onto the donor or ribbon substrate located on an X-Y stage pair (Aerotech ATS150). DMD image loading, laser pulse-picking and stage movement can be controlled independently or with a computer. The spatial beam profile reflected from the DMD is re-imaged and monitored on a CCD beam profiler (Ophir Beamstar FX-50 or Spiricon SP620U camera) and typically matches the loaded bitmap pattern. Pulse energies were recorded after the objective on a pyroelectric energy sensor (Ophir PE9 or PE25).
Fig. 1. Schematic diagram of the laser transfer setup with a 532 nm laser pulse spatially modulated by a DMD.
The orientation of the DMD array relative to the incident and exiting laser beams was carefully aligned for optimum efficiency, minimal rotation and distortion in the imaged DMD pattern. The ratio of one pixel of the bitmap pattern on the DMD to its final size on the image plane of the objective is roughly equal to the ratio of the focal lengths of the imaging lens L and the microscope objective. This ratio, known as the demag factor, is ~24 for the 20X objective used in our optical setup. Alternatively, one pixel in the bitmap image of the DMD corresponds to ~0.56 µm in the image plane of the 20X objective. The ribbon was made from a 50 x 75 mm borosilicate glass (1.1 mm thick) onto which a layer of high-viscosity (80,000 - 100,000 cP) nanoparticle silver paste (Harima NPS, 3 - 7 nm particle size) was doctor-bladed. After blading, the nanopaste was dried in air to remove excess solvent from the paste for optimum decal transfer. The ribbon was placed with the ink layer side parallel to and facing the receiving substrate, separated by an adjustable gap of 1 to 50 µm. The ribbon/receiving substrate stack rested on the X-Y motion controlled stages, and its translation synchronized with the pulsing of the laser in order to print the pattern from the DMD image. Once transferred, the Ag patterns were oven cured at 100 °C for 30 minutes for scanning electron microscopy or SEM imaging. (Full cure conditions were 200 °C for 1 hour). Optical microscopy (Olympus BX51), confocal microscopy (Olympus LEXT OLS3000) and SEM (JEOL, JSM-7001F) were used to characterize the shape and morphology of the transferred patterns; and confocal microscopy was used to inspect the void left in the ribbon nanopaste layer after the transfer. 3. Results and discussion To fully exploit the digital control of the laser light and its influence on LDT, the effect of the individual micromirrors (and therefore pixels) of the DMD on the spatial properties of the
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 424
laser beam and quality of the laser transfer must be understood. The laser beam fluence, spatial uniformity and structure all play a part in the laser transfer threshold and any topographic features in the resulting transfer. The resolution of the laser printed features in our system will first be determined and then examined how it is affected by a modified beam profile. The optical configuration in this work images the DMD bitmap pattern directly onto the image plane of the microscope objective. It is also possible to use other types of spatial light modulators to manipulate the amplitude, phase, or polarization of the beam profile in space and time to generate more complex beam patterns, but those approaches tend to add extra complexity to the LIFT setup with perhaps a small benefit in return. Because the DMD is a binary device – the micromirrors (or pixels) are either on or off, a single pixel cannot be used to generate a grayscale. However, we can borrow from a technique, over a hundred years old and still being used today in publishing, displays and image processing, called half-toning [13]. This technique relies on the human eye to spatially average an assortment of sizes, shapes and spacings of ‘cells’ or dots into a continuous gradient of tones in an image. In this work, groups of pixels in the DMD modulate the spatial pattern of the incident beam, which is then imaged (with demag factor) by our optical system onto the ribbon nanopaste layer. We need to determine the smallest unit cell or pixel size of the DMD, which cannot be imaged in our optical system (and transferred pattern) and therefore can be used to generate grayscale
Fig. 2. Tracking of LDT process from initial DMD bitmap images of 4 different square sizes to their corresponding final transfer features. The top row shows (a) 125-px, (b) 51-px, (c) 23-px and (d) 8-px square bitmap images and the second row shows the corresponding measured beam profiles. (The inset in (d) is a magnified beam profile from the 8-px square pattern). The third row shows 3-D confocal microscopy images of the hole left in the ribbon after the transfer. Last row shows SEM images of the resulting transfer on silicon.
intensities. This concept will become important later in the discussion of beam-shaping with the DMD. In our previous work, the laser transferred patterns corresponded exactly to the shape and structure of the bitmap pattern on the DMD, whereas this work explores the use of a “structured” bitmap pattern (grayscale) in laser transfers for the first time.
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 425
The resolution of the LDT process with the DMD was first tested with progressively smaller squares that were laser printed from a ribbon with a 1.5 µm-thick silver nanopaste layer. Figure 2 follows the LDT process from the initial DMD pattern, through the measured beam profile and crater left in the ribbon, to the resultant transfer. The beam profile matched the loaded bitmap image down to ~20 pixels, but diffraction effects began to appear in bitmaps comprising of < 10 pixels. As the number of `on' pixels on the DMD was decreased, the size of the printed voxel became smaller until no transfer was observed for a square bitmap < 8 mirrors (or pixels) in the DMD, corresponding to a laser spot size of ~5 µm square. Note that this transfer resolution is the result of the combination of laser spot size with ribbon nanopaste thickness, spatial uniformity of the beam and the optical demag for the DMD. In fact, LDT has demonstrated laser-transferred resolutions as low as 2 x 2 µm in area but with a nanopaste thickness of 200 nm [14]. Moreover, as will be shown later, the imaged resolution in our optical system is observed to be higher than that for laser transfer. Conversely, the resolution limit of the negative image of the square pattern in Fig. 2 - a “square ring”, can be explored with decreasing hole size as seen in Fig. 3. As the images show, the transfers consisted of a frame whose borders became increasingly wider until the opening at the center of the frame disappeared. The resolution limit was reached for a hole comprising of 8x8 mirrors “off”, below which, a solid square pattern, devoid of a center was printed. This resolution limit of ~8 pixels in the current setup for laser transfer is consistent with that seen in Fig. 2. Therefore, bitmap patterns must be > 8 for successful laser transfer in the current optical system. It is important to note that these types of patterns are impossible to generate from masks made from metal foil cut-outs such as traditional stencils.
Fig. 3. Tracking of LDT process from initial DMD bitmap images of 4 different rings to their corresponding final transfer features. The top row shows 169-px square bitmap images with (a) 125-px, (b) 51-px, (c) 23-px and (d) 8-px ‘hole’ and the second row shows the corresponding measured beam profiles. The third row shows 3-D confocal microscopy images of the mesa left in the ribbon after the transfer. Last row shows SEM images of the resulting transfer on silicon.
The resolution limit of 8 pixels seems to indicate roughly the size of the unit cell of the DMD, but this must be examined further. The previous laser-transferred squares employed
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 426
bitmap square patterns that were all ‘on’ (i.e. light throughput from the DMD was maximized). In order to compare these transfers with those from a modified beam shape, the reference bitmap pattern must be made lower in intensity than the “all-on” pattern since DMD light throughput cannot exceed its inherent maximum. Therefore, in our beam-shaping studies, comparisons of transfers from a modified beam bitmap pattern should only be made with those from this reference bitmap pattern. In addition, in order to make valid comparisons, the overall quality of transfers from this reference pattern must be similar to those from a regular, unmodified, all “pixels-on” pattern. One method to decrease light throughput from a DMD is to turn off alternate pixels as in a “checkerboard” pattern. In Fig. 4, bitmap patterns of checkerboards with decreasing unit cell sizes are shown along with the resulting beam profiles, ribbon craters and transfers. Note that for a 10-pixel unit cell size, the checkerboard pattern in the transfer begins to average out. However, even for a 2-pixel unit cell, there are sub-micron surface features in the transfer, which are only completely removed for a 1-pixel unit cell. These fine features interestingly demonstrate a higher DMD resolution (≈micron) in texturing a surface than the ~5 µm resolution seen in laser transfer with our current setup. Note in Fig. 4, the surface quality of the transfer from a 1-pixel unit cell is equivalent to that from the 80-pixel (all pixels on) cell. Because the overall quality of these two transfers are the same, the 1x1 pixel checkerboard pattern can be chosen as the reference pattern and a fair comparison with it can be made for beam-shaping experiments.
Fig. 4. Tracking of LDT process from initial DMD bitmap images of “checkerboard” pattern to their corresponding final transfer features. The top row shows decreasing unit cell sizes of the 80-pixel square bitmap image loaded onto the DMD. The second row shows the measured beam profiles of the laser spot striking the ribbon. The third row shows 3-D confocal microscopy images of the hole left in the 1.5 µm-thick ribbon and the fourth row shows SEM images of the resulting transfer on silicon along with the laser transfer fluence. The last row shows higher resolution SEM images of the 2- and 1-pixel unit cell checkerboard pattern transfers.
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 427
Modifying the beam spatial profile can improve the quality, process conditions and window of successful laser transfers. In a recent work [15], it was found that the threshold fluence required to release a voxel from the nanopaste layer varied inversely as the voxel size since more of the laser energy was expended at the voxel’s perimeter for smaller voxels. This result was confirmed by the increase in laser fluence in Fig. 4 as the unit cell decreased from 80- to 10-pixels. This decrease in unit cell size was accompanied by an increase in the total perimeter of the voxels and a decrease in the size of the individual voxels, which required an increase in the laser transfer fluence. (If the laser transfer process solely depended on the total area transferred, then the fluences should have scaled accordingly). For unit cell sizes less than 10 pixels, the transfers were not congruent to the incident beam profile and any firm conclusions regarding their transfer fluences could not be drawn. In order to redistribute more of the laser energy towards the perimeter of a voxel, our target beam profiles were designed for higher laser intensity at the edges than in the interior. Figure 5 shows bitmap 1x1 checkerboard patterns of 80-, 40- and 20-pixel squares with a 510% border of “all-on” pixels. This border resulted in a two- to three-fold increase of the edge intensity over that of the interior as seen in the beam profiles in Fig. 5 and as evidenced by the dark thin border inside the crater in the ribbon. The resulting transfers were congruent in shape and size as the laser spot, and their morphology were not adversely affected by the edge-enhanced beam profile.
Fig. 5. Tracking of LDT process using edge-enhanced beams created with a) 80-pixel, b) 40pixel and c) 20-pixel square ‘checkerboard’ bitmap images with a 5-10% border width as shown in the top row. The second row shows the measured beam profiles of the laser spot striking the ribbon. Confocal microscopy images of the hole left in the ribbon and the resulting fully-cured transfer on silicon are shown in the third and fourth row respectively. Laser transfer fluence thresholds for these edge-enhanced checkerboard patterns are a) 39, b) 57 and c) 96 mJ/cm2 respectively.
This 1x1 checkerboard shaped-beam profile can be used with a reference (unmodified) 1x1 checkerboard beam profile to study its effect on the laser transfer threshold fluence. For each of the three bitmap patterns in Fig. 5, a 3x3 array of silver nanopaste squares were transferred onto silicon as a function of incident laser energy. The LDT threshold was determined to be the fluence at which ~90% of the transfers were intact and congruent in shape and size to the laser spot. Figure 6 shows the transfer threshold fluence for three spot
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 428
sizes using the reference and modified checkerboard beam profiles. A significant threshold decrease of ~30% occurred for a laser spot size approaching 10 µm (20-pixel square bitmap), whereas transfers using larger spot sizes were not affected by the modified beam profile, as was expected. This decrease is less than that observed for laser transfers of polymer membranes using an edge-enhanced beam profile [16] but may be due to differences in materials properties and their interaction with laser light. Furthermore, our transfer process employs ribbons with a single ink layer, which is a simpler configuration than ones that use a dynamic release layer (DRL). This DRL also has a minimum threshold fluence for successful transfer and therefore the use of a DMD-structured beam with a single ink layer gives more control over the resulting transfers. No transfers were successful for the 5 µm (10-pixel square pattern) in the current setup, but may be due to ribbon ink thickness and beam nonuniformity (see Fig. 2). It is important to note that the absolute fluence values in Fig. 6 are not important and should not be compared with those found in ‘typical’ laser transfers that employ the full unmodulated beam profile. The checkerboard concept used in this work allows modified and reference beam profiles to be compared fairly and equally in an imaged DMD system. The relative change in transfer threshold for a shaped vs. non-shaped beam is what is most important and demonstrates the feasibility of using halftoning to simulate grayscale in a DMD pattern to shape a laser beam for laser transfers.
Fig. 6. Laser transfer fluence threshold with laser spot size for checkerboard reference and checkerboard edge-enhanced beam profiles. The unit cell for the checkerboard bitmap pattern is 1-pixel square. Threshold fluences are measured for square laser transfers of silver nanopaste. The solid and dashed lines are visual guides only.
4. Conclusion The laser beam profile of a 532 nm laser was spatially modulated by a DMD and its effect on LDT of silver nanopaste layers has been studied. Bitmap square patterns and their negative counterpart were “imprinted” onto the laser beam profile, imaged through a 20X microscope objective, and transferred intact silver structures from ~70 to 10 μm square in our optical setup. Finer features have been demonstrated elsewhere with LDT, but were not the goal of
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 429
this study, and should be possible with improved beam uniformity and optimized ribbon ink thickness and uniformity. The “half-toning” concept was used on the DMD to simulate “grayscaling” of the laser beam profile. A resolution limit near ~10 pixels in the DMD bitmap (imaged size ~5 µm) was found for laser forward transfer of silver nanopaste in our setup. However, surfaces of the transferred pattern showed even finer structure arising from a number of single pixels in the DMD bitmap. Ultimately, a unit cell of 1x1 pixel formed the basis of the half-tone (or ‘checkerboard’) pattern. By using this pattern with an all-white pixel border, the initial interior profile of the beam intensity was decreased while the edge intensity was increased by a factor of ~2-3 over that of the interior with edge widths ~5 to 10% of the full beam width. For a laser spot and transfer size of 12 μm, a decrease of ~30% in laser transfer fluence threshold was observed for an edge-enhanced beam profile compared to a reference checkerboard beam profile. This result shows that a DMD can effectively shape the spatial profile of a laser beam using the grayscale (or “half-toning”) concept and that in general, spatially structuring the laser beam can benefit the laser transfer process. Although not studied in this work, beam-shaping may allow laser transfers with thicker ribbon ink layers, smaller feature sizes, more fragile (organic or brittle) materials and more complex shapes. The use of a spatial light modulator such as a DMD in laser transfer provides a highly parallel, rapidly reconfigurable technique for laser digital microfabrication. It is a natural extension to use a DMD with a spatially congruent laser transfer process such as LDT to directly map pixels from the bitmap pattern into transferred voxels. In addition, by modifying the beam profile using half-toning techniques in the DMD bitmap, LDT now has the potential to operate with a wider process window, more complex shapes and increased reliability. Acknowledgment This work was funded by the Office of Naval Research through the Naval Research Laboratory Basic Research Program.
#225490 - $15.00 USD (C) 2015 OSA
Received 22 Oct 2014; revised 5 Dec 2014; accepted 8 Dec 2014; published 8 Jan 2015 12 Jan 2015 | Vol. 23, No. 1 | DOI:10.1364/OE.23.000422 | OPTICS EXPRESS 430
