Pulse control in high-power UV laser enables new micromachining options

Transcription

1 INDUSTRIAL LASERS Pulse control in high-power UV laser enables new micromachining options RAJESH PATEL, JAMES BOVATSEK, and ASHWINI TAMHANKAR Manufacturing mobile consumer electronics requires increasingly precise laser-based processing capabilities that can be met by a hybrid ultraviolet (UV) fiber laser with software-adjustable pulsewidth control, pulse-splitting, and pulse-shaping capabilities. Mobile consumer electronic devices such as smart phones and tablets represent one of the largest and fastest evolving segments in the global manufacturing industry. 1 Continual technology development efforts are required to build up advanced precision laser manufacturing processes for key components such as semiconductor chips, microelectronic packages, touch-screen displays, and printed circuit boards. The current generation of diode-pumped solid-state (DPSS) lasers has, in many cases, played an enabling role with new techniques that make it possible to produce sophisticated products at mass-market costs. However, just as the mobile devices grow more capable and complex, so must manufacturing processes evolve. Given the variety of materials and processes that lasers address in this manufacturing space, it can be diffi cult to choose laser characteristics enabling broad advances in high-quality, highspeed micromachining. Such processes are strongly infl u- enced by a number of laser parameters, including wavelength, pulsewidth, average power, beam quality (M 2 ), pulse repetition frequency (PRF), and pulse-topulse energy stability. And, with the advent of pulse shaping and pulse splitting, the list of ingredients in an optimized process recipe grows longer. Lasers having shorter wavelength, shorter pulsewidth, and low M 2 benefit micromachining processes by creating a tightly focused spot and minimizing heat affected zone (HAZ). In general, shorter wavelengths are strongly absorbed by many materials, making them suitable for machining a wide variety of organics, semiconductors, ceramics, and metals. High energy absorption, as well as high on-target irradiance levels created by short pulses, vaporizes material quickly, reducing HAZ and charring. The small, focused beam spot enables machining smaller features with higher precision. Higher power, higher PRF, FIGURE 1. Scribe depth vs. speed for silicon, illustrating the process optimization benefi t possible using TimeShift technology. Depth (µm) % increase in speed 0 94% increase in speed pulse-shaping, and pulse-splitting capabilities all can contribute to improved micromachining throughput. And consistent higher pulse-to-pulse stability ensures process repeatability and helps achieve higher process yield. Although UV Q-switched (Q-SW) DPSS lasers have met manufacturing demands, they have limitations in achieving higher speeds. A common approach to increase the processing speed is boosting the PRF of the laser while holding other process parameters fi xed. However, for a typical Q-SW DPSS laser, this is really not achievable. For these lasers, the average power and pulse energy decrease quite rapidly as the PRF increases. Also, the laser pulsewidth and pulseto-pulse energy variability tend to increase significantly. Since the variation in these laser parameters affects the micromachining speed, feature size, and accuracy achieved, simply increasing PRF is TimeShift technology Speed (mm/s) With TimeShift technology 1 25 ns (PRF optimized) 1 25 ns Laser Focus World April

2 INDUSTRIAL LASERS continued often not enough to maintain process results as we attempt to scale up throughput. The real and sought-after solution to overcoming these limitations is a laser that not only maintains high average power at the higher PRFs but also continues to deliver shorter pulsewidths with low pulse-to-pulse energy variation, pulse-shaping, and pulse-splitting capabilities, to achieve both higher throughput and cleaner ablation results. To meet the need for such laser technology, we developed a UV hybrid fiber laser, the Quasar , which combines higher power at higher PRF. It also features a software-adjustable technology called TimeShift for setting a wide range of pulse energies and pulsewidths. This technology also gives the process engineer the ability to create pulse intensity waveforms in the time domain through techniques such as pulse shaping, pulse splitting, and burst-mode operation. The laser has an output power of >40 W of 355 nm wavelength at 2 khz PRF and eliminates the dependence of pulsewidth on PRF, making it possible to hold all characteristics of the laser output steady as throughput is scaled up by increasing the repetition rate. The combination of higher power at high PRF, independently adjustable pulsewidth, and pulse-manipulation capabilities are proving benefi cial for micromachining of various microelectronics materials. Here, we present initial results achieved Depth (µm) ns pulse 52% 5 5 ns split pulse TimeShift technology Fluence (J/cm 2 ) 77% FIGURE 2. Scribe depth vs. fl uence for silicon, illustrating TimeShift technology benefi t. in silicon, glass, and printed circuit board (PCB) materials. Silicon dicing in semiconductor fabrication In microelectronics manufacturing, hundreds or thousands of integrated circuits or other micro-devices are fabricated on a single semiconductor (silicon) wafer and are fi nally separated or diced into individual chips. Laser dicing of silicon wafers has been in use for several years as an alternative to conventional dicing with a precision rotary saw. As the wafers become thinner and as lasers become more reliable and powerful, the advantages over saw-based dicing increase dramatically. Moreover, with the advanced features now available, it becomes easy to manage thermal loading in the material while maintaining high micromachining throughput. This is especially important in silicon dicing, where thermal effects can either lead to micro-cracks along the diced chip edges (which decrease mechanical die strength) or possibly damage the electronic devices themselves. An alternative to through-dicing is to create a laser-ablated groove or scribe line in the wafer streets between chips. This is done either as a fi rst step in a two-step scribe-and-cleave process or as a step before saw dicing when delicate or brittle materials are present. At our applications lab, we demonstrated scribing of ~100-µm-thick polished single-crystal (<100>) silicon wafers using the Quasar laser. Very high scribing speeds were achieved, while avoiding any thermal damage to the material. In a fi rst experiment, we compared the scribe depth obtained with the laser operating at three different conditions: a single pulse per focal spot location, having April 13 Laser Focus World

3 25 ns pulsewidth at 0 khz PRF; using TimeShift to maintain that single 25 ns pulse at higher PRF of 300 khz; and c) a burst of pulses using the TimeShift pulse-splitting feature (10 subpulses of 5 ns, separated by 10 ns), with bursts repeated at 255 khz frequency. In Fig. 1, the curve for single 25 ns pulses at 0 khz establishes the basic trend that as scribe speed increases the scribe depth decreases. Using the high UV output power from the laser, these parameters deliver impressive depth vs. speed results. To increase scribe speed while maintaining a given depth, the TimeShift feature was used to generate the single 25 ns pulse at even higher PRF of 300 khz. We observed a 22% increase in the scribe speed for a -µmdeep scribe. Finally, we further optimized the process by using pulse splitting to achieve a 94% increase in the speed over a single 25 ns pulse-scribing condition for a -µm-deep scribe. We believe additional improvement is possible by optimizing features such as adjustable pulsewidth and pulse shape. In a second experiment, we investigated possibilities for increasing the scribe depth obtained at various work-surface fluences using the pulse-splitting capability. Varying the pulse energy, we generated laser scribes at various fluence levels, while holding scanning speed at 2 mm/s and PRF at 0 khz. One set of data was collected with a pulsewidth of 25 ns, while in a second set we split the pulse into five 5-ns sub-pulses separated by 10 ns. Figure 2 shows the clear advantage of using sub-pulse burst micromachining over single-pulse machining. By splitting the energy available in a single 25 ns pulse into five sub-pulses, an increase in ablation depth between 52 and 77% was obtained, depending on fluence level. We note that this is consistent with other studies fi nding enhanced material-removal rates for burst-mode ablation. 2-5 We also compared the visual appearance of debris and HAZ for the two cases. Figure 3 shows representative topview images of the scribes. Burst-mode scribes demonstrate high ablation quality with less loose debris on the top surface, despite the fact that the depth achieved was 25% greater than that achieved using single pulses. We believe that the burst-mode process creates gentler ablation with each lower-energy subpulse, transferring less heat into the material compared to the single-pulse case. Glass cutting in flatpanel displays In the display manufacturing process, touch-screen and LCD modules mostly require straight cuts for singulating small pieces of glass from large production panels. As glass substrates used in consumer electronics displays continue to become thinner and harder, laser glass-machining tools are showing great potential for providing high-quality cuts and high throughput while mitigating yield losses associated with the 25 µm 25 µm FIGURE 3. Silicon scribing quality with Quasar laser: top view of the scribe using a single pulse and top view of the scribe using fi ve sub-pulses. DC-DC Converters ULTRA Miniature From 0.5" x 0.5" x 0.3" High Reliability Greater than 1,000,000 hrs Mil Hbk 217F 2V to 10,000 VDC Outputs Watt Modules Isolated/Regulated/Proportional Programmable/Single/Dual Outputs Military Upgrades Expanded operating temperature -55 to 85c, no derating required Environmental Screening Selected Screening, Mil Std 883 Custom Models US Manufactured AS 9100 Approved Surface Mount/Thru-Hole See PICO s full line catalog YOUR PROVEN SOURCE Over 35 years providing reliable products Over 2,0 Standard Modules PICO ELECTRONICS, Inc. 143 Sparks Ave., Pelham, New York Call Toll Free FAX E Mail: info@picoelectronics.com Laser Focus World April 13 51

4 INDUSTRIAL LASERS continued µm 5 µm FIGURE 4. Glass cut quality: top view of the cut using TimeShift technology and cross-section of the cut showing roughness. 52 conventional mechanical scribe-andcleave process. In our applications lab studies, the Quasar was used to make clean, quality linear cuts at high throughput on 0.55-mm-thick display glass. Control of pulse temporal characteristics facilitated tailoring of the individual laser pulses to reduce thermal loading in the material, which resulted in good quality cuts at faster scanning speeds, improving overall processing throughput. We achieved linear cutting speed of 1 m/s, highly desired in the manufacturing process. Figure 4a shows a top view of the cut edge, seen from the front side of the glass. Clean edge quality is evident, with very minimal chipping (<10 µm). Figure 4b shows a side view of the cut edge, where we observed roughness within the acceptable range of typical manufacturing requirements. FermionicssOpto-Technology w w w.fermionics.com 4555RunwaySt.SimiValley,CA93063Tel(805) Fax(805) PCB processing In manufacturing consumer electronic devices, lasers routinely perform a variety of machining processes on PCB materials. These processes include blind- or through-via drilling, depaneling, profi l- ing, laser direct patterning, repair, trimming, marking, and skiving. PCBs may comprise a variety of materials, layers, and thicknesses, and each material type and process is likely to have specific requirements for optimal laser parameters; consequently, fl exibility in tailoring laser pulses is important. For example, a typical blind-via drilling application involves clean removal of thin resin material to expose an underlying copper layer. Figure 5 shows laser-drilled 60-µm-diameter blind vias in PCB material comprising a -µmthick resin layered over copper. In this particular material, shorter pulsewidths (< ns) are better suited to achieve cleaner resin removal, fully exposing undamaged copper at the bottom of vias. Also, high power at higher PRF is desirable to achieve higher throughput. With conventional Q-SW UV DPSS lasers, throughput scaling is limited by insufficient pulse energy at high PRF. If highest power is restricted to lower PRFs, maximum throughput is achieved only with the use of a split-beam/parallel processing approach something that is often undesirable from a system cost and complexity standpoint. High power and shorter pulsewidths are available with the Quasar laser at higher PRFs, allowing for throughput maximization by operating at the higher PRF and lowering the exposure time. The result is clean blind vias with minimal damage or oxidation to the exposed copper surface and minimal sidewall taper. Both of these process results are important for assuring good adhesion of the subsequent copper plating layer. Depending on the via diameter and resin thickness, throughputs approaching 4000 holes/s are possible. In summary, results for our lab tests demonstrated that with proper parameter optimization, high quality and April 13 Laser Focus World

5 < ns pulse Clean Cu Via surface Exposed copper REFERENCES 1. ReportLinker web site, Global Consumer Electronics Market Outlook 15 ; ly/yhkfke. 2. P.R. Herman, R. Marjoribanks, and A. Oettl, Burst-Ultrafast Laser Machining Method, US Patent No. 6,522,301 (03). 3. B.R. Campbell, T.M. Lehecka, J.G. Thomas, and V.V. Semak, A Study of material removal rates using the double pulse format with nanosecond pulse laser on metals, Proc. ICALEO 08, W. Hu, Y.C. Shin, and G. King, Modeling of multi-burst mode pico-second laser ablation for improved material removal rate, Appl. Phys. A, 98, 2, (February 10). 5. R. Knappe, H. Haloui, A. Seifert, A. Weis, and A. Nebel, Scaling ablation rates for picosecond lasers using burst micromachining, Proc. SPIE, Laser-based Micro- and Nanopackaging and Assembly IV, 7585 (10). > ns pulse Dirty Cu 65 µm 65 µm FIGURE 5. Drilling of 60-µm-diameter blind vias in non-reinforced resin layer on PCB material: surface view and bottom view, focused on copper substrate. Longer pulses result in residual resin remaining in the hole. cromachining processes to meet the manufacturing challenges of consumer electronics products.!"# high throughput can be achieved with a UV nanosecond-pulsed laser source, extending the capabilities of laser mi- Rajesh Patel is director, segment marketing, James Bovatsek is applications lab manager, and Ashwini Tamhankar is senior applications engineer at Spectra Physics, 3635 Peterson Way, Santa Clara, CA 954; s: raj.patel@newport.com, jim.bovatsek@newport. com, and ashwini.tamhankar@newport.com; 4++*5&#$5.6(7.&8$+,9:8,. Biophotonics!"#$%&'()*&#$+, (-./$,+(0*1#2&3. OEM Partner for Biochips Typical applications Environmental monitoring Crop and food analyzing Life science Medicine Target Molecules Bioreceptors Linker Chemistry Planar Waveguide with Grating Substrate Optics Balzers AG Balzers/Liechtenstein Optics Balzers Jena GmbH Jena/Germany Laser World of Photonics Munich, Hall B2, Booth No. 305 Laser Focus World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pril 13 53