AVIA DPSS Lasers: Advanced Design for Increased Process Throughput

White Paper AVIA DPSS Lasers: Advanced Design for Increased Process Throughput The Q-switched, diode-pumped, solid-state (DPSS) laser has become a widely employed tool in a broad range of industrial micromachining and micro materials processing g tasks. This is because these lasers offer a unique combination of desirable operating and optical characteristics, including infrared, visible or ultraviolet output, high pulse repetition rates, excellent stability and superb reliability. However, in many of the applications for DPSS lasers, including solar cell production, touch screen patterning and microelectronics manufacturing, there is a relentless drive to increase process throughput and efficiency, and thus lower production costs. Coherent has designed its AVIA series of Q-switched, DPSS lasers specifically to support this trend, in particular, by offering higher average power and increased repetition rates, without any sacrifice in lifetime or reliability. This whitepaper briefly reviews some of the most prominent applications for DPSS lasers and their requirements, and then examines the design factors that enable AVIA to meet their developing needs. AVIA Laser Characteristics Coherent AVIA series Q-switched, DPSS lasers are available at wavelengths of 532 nm, 355 nm and 266 nm. These lasers, which range in power from 3W to 45W, are all characterized by extremely high quality beam characteristics. Specifically, this means TEM 00 output with M 2 of less than 1.3, pulse durations in the 20 ns to 40 ns range, pulse energies from 20 µj to 300 µj. These optical attributes make it possible to focus the laser down to diffraction limited spot sizes in the micron size range, which together with their short pulse duration and high energy to achieve high peak fluence, enables efficient, high precision material removal of a wide range of dielectric, semiconductor and conductive materials. Additionally, AVIA series lasers offer advanced features that work with on-board sensors to drive closed loop monitoring and control of critical operation processes to deliver exceptional stability and reliability. These features include ThermEQ, which ensures uniform pulse energy across a burst of pulses, PulseTrack which provides precise, on-the-fly control of pulse energy, and PosiLock which provides simultaneous beam pointing and power stabilization. Together, these features allow the user to achieve exceptional process consistency and repeatability. Finally, AVIA lasers are also constructed for long lifetime, extended maintenance intervals, and ease of repair. All these factors, along with their low power consumption and minimal consumables costs, have thus made these AVIA Q-switched DPSS lasers a popular choice for demanding, high precision microprocessing tasks in which throughput and operating costs are also a significant consideration. Micromachining Applications One important developing application area for AVIA lasers is solar cell processing. In fact, there are actually several different techniques in which AVIA lasers are applied in solar fabrication, in both crystalline silicon (csi) and thin film technologies. For example, these lasers offer an alternative to conventional chemical and thermal processing methods for the phosphosilica glass (PSG) doping process sometimes used in the manufacture of csi devices. Here, the laser is used to heat the solar cell in a highly localized way in order to change the concentration of dopants over a specific, limited area. This creates a series of so called selective emitters which ultimately improve cell efficiency. Another technique in csi device fabrication, currently in widespread production use, is edge isolation. In this case, the laser scribes a groove, typically 10 µm to 20 µm deep, in order to electrically isolate the front and rear surfaces of the solar cell by cutting through electrically conductive shunt pathways at the device edges. Touchscreen patterning is also a major application for Q-switched DPSS lasers, which offer greater speed, lower cost and a more environmentally friendly option than lithographic techniques. Touchscreen production involves deposition of a layer of transparent conductive oxide (TCO) on a substrate. This conductive film must then be scribed through to create a pattern of electrically isolated areas. Touchscreens with more sophisticated functionality may require numerous scribe www.coherent.com I tech.sales@coherent.com I (800) 527-3786 I (408) 764-4983 1

lines. A narrow scribe that cuts completely through the TCO layer is necessary for this application, and the ability of AVIA lasers to achieve small focused spot sizes makes them ideal for this purpose. Depending on the application, TCO scribing on touchscreens is performed with UV DPSS wavelengths. (Despite TCO s transparency at visible wavelengths, UV wavelength lasers can successfully process these materials with tight focus and high laser fluence.) AVIA Model (wavelengthpower) 266-3 355-7 355-10 355-14 355-20 355-23 355-23-250 355-28 532-23 532-30 532-38 532-45 µvia Hole Drilling Flex Circuit Cutting Si Wafer Dicing/Scribing Touch-Screen Patterning LED wafer scribing csi Solar Cell PSG Doping csi Solar Cell Edge Isolation Thin-Film Solar Cell Patterning Figure 1. Summary of the major applications for AVIA lasers, and the output wavelength/power combinations typically employed for each. The production of small holes, called vias, in printed circuit boards, is another significant commercial application for AVIA lasers (and, in fact, the one for which they were originally developed). These holes are typically less than 70 µm in diameter (CO 2 lasers are used to drill holes larger than this size), and must have a high aspect ratio (steep sides). It is not practical to produce holes of this description using mechanical drills. Again, the small focused spot size of the AVIA laser makes it ideal for this use, plus the 355 nm wavelength is efficiently absorbed by the most common PC board materials, including resin coated copper (RCC), polytetrafluoroethylene (PTFE) and copper. The ever shrinking dimensions of integrated circuits, together with a shift in materials, are making laser scribing (cutting) an increasingly attractive and economically viable alternative to mechanical sawing. For example, in LED fabrication the laser is focused down to a spot size of a few microns or less, resulting in a laser scribe that is much narrower than achievable with a saw cut, and with significantly less edge damage (cracking and chipping). This means that LED devices can be packed closer together with narrower gaps, increasing process utilization. Plus, the high quality laser cut edge eliminates the need for post processing. These features translate into higher yields, and therefore lower unit cost. Similarly, laser cutting is gaining popularity for use with integrated circuits that incorporate so called low-κ materials. These porous materials are notoriously difficult to cut by mechanical means. Most commonly, a hybrid approach is employed in which the laser is used to remove the lowκ layer in the streets between devices that would be severely delaminated by a mechanical saw. After removal of the low-κ and other layers by the laser, a mechanical saw is used to complete the through cut of the bulk silicon. Increasing Throughput All of the AVIA applications just described are cost sensitive processes, where manufacturers are seeking to maximize throughput, yield and process utilization. Furthermore, the dynamic nature of the technologies involved, together with tremendous downward price pressure from the market, make efficiency goals something of a moving target. That is, manufacturers require continual improvements in laser-based process efficiency in order for those methods to remain viable. To meet this need, supporting increased throughput is a major goal for AVIA product development efforts at Coherent. AVIA lasers operate in a pulsed mode, typically with repetition rates in the 20 khz to 300 khz range. For most applications in which the beam is scanned across the workpiece (scribing as opposed to drilling), the laser spot from each pulse is typically overlapped with the previous pulse in the 30% to 70% range so as to create a continuous cut (rather than a discontinuous perforation). Many applications, especially semiconductor scribing, use multiple scans of the same www.coherent.com I tech.sales@coherent.com I (800) 527-3786 I (408) 764-4983 2

area. Percussive drilling is always accomplished with multiple pulses. Because of these output characteristics, there are two basic ways in which process speeds can be increased, once the necessary pulse energy for a given technique is established. First, the pulse energy from the laser can be increased sufficiently so that the output can be split, thus allowing multiple processes to run in parallel. Second, if individual pulse energy stays the same, repetition rate can be increased so that scanning speeds can be raised without reducing the pulse-topulse overlap. This translates into higher average output power from the laser. Coherent has aggressively developed the AVIA product portfolio to address both methods for increasing throughput, and to keep pace with the ever increasing production demands in microelectronics and solar manufacturing. Three important aspects of achieving this are the use of end pumping, employment of a master oscillator/power amplifier (MOPA) configuration for power scaling, and the utilization of external cavity frequency multiplication for improved lifetime and efficiency. Each of these features merits some discussion. End Pumping Q-switched TEM 00 lasers are based on crystals such as Nd:YVO4 (Vanadate) or Nd:YAG that are pumped by high-power diode lasers. The main challenge in increasing output power in these solid-state lasers is maintaining TEM 00 mode operation while scaling up pump diode power. However, as pump power increases, the tendency to excite higher-order transverse modes increases. Lasers with higher-order modes or higher M² values cannot be focused to spot sizes as small as true TEM 00 mode lasers having low M² values. And, the ability to be finely focused is a fundamental laser requirement for micromachining the very small features required in microelectronics and solar applications. The best way to minimize the tendency to excite higher order modes in a DPSS laser is to use end pumping. End pumping is a longitudinal pumping geometry in which dichroic optics are employed to introduce the shorter wavelength pumping beam so that it is collinear with the longer wavelength laser beam, as shown in the accompanying figure. The advantage of this method is that the laser gain volume excited by the pump beam can be well matched to the intended TEM 00 mode volume, therefore reducing the possibility of supporting multimode operation. Also, because there is little pump power wasted in pumping unneeded parts of the laser rod, system efficiency is maximized and the amount of waste heat generated in the laser crystal is minimized. Master Oscillator Adjustable End Mirror Pump Laser Diode Pump Light Q-Switch Imaging Optics Fiber Delivery Dichroic Mirror Laser Crystal Output Coupler Power Amplifier Output to Harmonic Generator Figure 2. A simplified schematic of a MOPA DPSS laser based on a dual end pumped geometry. Pumping the laser rod from both sides (dual pumping) increases the amount of power that can be extracted from the gain medium. Further increasing output power can be accomplished by increasing the total pump light power. However, increasing the pump power delivered into a small volume in the laser crystal exacerbates thermal effects. These include thermal lensing, thermal stress (that could lead to crystal fracture), thermal depolarization (for isotropic materials like Nd:YAG), and thermally-induced non-spherical optical distortion. Without suitable techniques to alleviate and compensate such thermal effects, the laser will become unstable, resulting in multimode operation. One technique used in the AVIA to compensate for thermal lensing is an adjustable end mirror. Specifically, the rear, flat, high reflector in the cavity moves longitudinally towards or away from the source of thermal lensing so as to maintain constant output beam characteristics. This even enables compensation for thermal lensing under variations in pump power, or with changes in repetition rate, duty cycle or pulsing sequence (bursts, etc.). MOPA Architecture Even with clever methods to reduce the impact of thermal loading on the laser crystal, there is still a limit to the amount of power that can be extracted from a single, end-pumped laser oscillator. The next step in increasing power is to add another gain stage. This additional stage can be placed within the first laser resonator, so that both gain stages are between the high reflecting end mirror and the output coupling mirror. Alternately, the second laser gain stage can be put after the output coupler and used to amplify the output of the laser oscillator in a single pass. www.coherent.com I tech.sales@coherent.com I (800) 527-3786 I (408) 764-4983 3

Placing both gain stages within the laser oscillator may be attractive if intra-cavity harmonic generation is desired. However, this approach is extremely difficult to manufacture reliably due to the need to compensate for the thermally induced aberration differences between the two gain stages. Therefore, Coherent has chosen to use a single pass power amplification stage external to the laser oscillator in a so called master oscillator/power amplifier (MOPA) configuration. Specifically, the output beam from the laser resonator (master oscillator) is fed into an end-pumped, laser amplified module (power amplifier) that is similar in construction to that used in the oscillator. A single pass through the power amplifier can as much as double the average output power. Careful selection of laser cavity parameters and the use of various thermal management techniques have enabled Coherent to maximize the optical-to-optical conversion efficiency from each stage in the MOPA architecture, while still delivering superior reliability. As a result, Coherent currently produces the highest power, Q-switched, TEM 00 green and UV solid-state lasers. Wavelength Conversion The final, and perhaps most difficult, technological challenge to generating high power, TEM 00 green (532 nm) and UV (355 nm and 266 nm) output from a Q- switched laser which operates at a 1064 nm fundamental is the design and production of the harmonic crystals stages. The first design choice that must be made is whether to use an intracavity or extracavity harmonics stage. Whereas intracavity generation has the advantage of a simple design not requiring crystal translation, extracavity harmonic generation provides a straightforward route to power scaling. For these latter reasons, Coherent utilizes extracavity frequency multiplication in the AVIA product line. Another important practical advantage of extracavity harmonic generation is that this configuration makes it substantially more practical to physically move the nonlinear crystal to expose an unused area when degradation starts to occur at a given spot. This is extremely difficult to accomplish with an intracavity nonlinear crystal because this optic also acts as the output coupler; thus, moving it tends to alter laser output characteristics. Moving the crystal to expose fresh areas greatly extends the lifetime of the UV stage and maintains more constant power over time, as shown in figure 3. Figure 3. Improvements in long term output power and stability when the THG crystal is shifted to expose a new area after natural degradation occurs. Coherent has also developed the most rigorous nonlinear crystal growth protocols, crystal testing methods, coating designs, and handling methods to minimize crystal degradation mechanisms and thus further extending spot lifetimes. Finally, Coherent maintains the highest standards for manufacturing in a cleanroom environment and utilizes low outgassing construction materials inside the sealed laser to reduce deposition of airborne molecules onto the non-linear crystal surfaces. These are key factors enabling AVIA s industry leading lifetime of over 22,000 hours for intracavity optics and non-linear crystals. Results The net result of all these design and construction techniques is evidenced by the product specifications for AVIA models. Currently, AVIA lasers offer the highest average power available for a TEM 00 laser at 355 nm by delivering 28W, and also set the benchmark at 532 nm with 45W of power; in neither case is reliability sacrificed, and both these high power lasers are backed by the same 24 month warranty as all AVIA products. Both of these lasers use the MOPA configuration to scale up the output of a lower power AVIA model and thus support increased throughput. For example, the AVIA 355-28 nearly doubles the pulse energy of the AVIA 355-14 at 100 khz, allowing the laser beam to be split for parallel processing and doubling throughput. In conclusion, precision laser micromachining processes that are utilized throughout high technology manufacturing place stringent demands on laser www.coherent.com I tech.sales@coherent.com I (800) 527-3786 I (408) 764-4983 4

performance. Furthermore, users continually demand lasers that support increased process throughput, yet are still sensitive to cost, reliability and downtime issues. The MOPA platform, employed in the Coherent AVIA high power product line, has proven to be a robust, practical way to meet these demands today, and is well poised for success moving into the future. Figure 4. The Coherent AVIA 355-28, delivers 28W of output at 355 nm, at a repetition rate of 110 khz, making it particularly well suited for high throughput, precision microprocessing tasks. www.coherent.com I tech.sales@coherent.com I (800) 527-3786 I (408) 764-4983 5