Laser Sources For Next-Generation Solar Cells By Finlay Colville, Coherent, Inc. As solar cell and panel manufacturers emerge from the credit crisis, new process tools are set to enter production lines for increased module efficiency. This market-driven pull demands improved laser-based process tools and a broader range of laser types than used historically by the solar industry. Final laser source selection depends ultimately on a range of different factors being satisfied. Introduction Laser sources and laser-based process tools have been employed consistently during the past decade for a variety of production stages within the manufacturing of both crystalline silicon (c- Si) and thin-film solar cells and panels. Consequently, a diverse range of applications can be found within many of the review articles published recently covering lasers within the solar industry. 1,2 However, some of the applications appear to have different sources recommended for the same basic process. In fact, as more laser source and tool manufacturers are attracted by the potential revenues associated with the high-growth status of the solar industry, the match between qualified laser process and tangible market-pull can often appear rather confusing. This article explains the fundamental drivers that form the basis of laser source adoption within the solar industry. Where is the “pull”? What are the timescales involved? Why have certain laser sources been prioritized within production lines to date? And what are the trends that are most likely to change the equipment-type landscape during the next 10 years?3 Market Dynamics Drive Technology Preferences Adoption rates for laser sources within the solar industry are dictated by a range of different factors, including: • Supply-demand market dynamics; 4 • Maturity and qualification of new process steps disseminated from within the research community;2 • Return on investment (ROI) of laser-based processing versus alternative (non-laser) technologies available to cell manufacturers; As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 2 • The extent to which production equipment utilized is supplied by complete (turnkey) full production line manufacturers, compared to discrete (customized) modular inline tooling; • Availability of 24/7 production-qualified laser sources to commercialize research proven laser-based process stages; • Availability of turnkey laser-based tools with wide process windows, optimum process quality, and acceptable cost entry points.5 By far, the most important adoption criterion is that of the supply-demand market dynamics prevalent at any given time. This is confirmed quickly when reviewing when and why laser- based processes have entered solar manufacturing, both for c-Si and thin-film cells. A review article explaining this in more detail was recently published.3 While end-user demand for solar modules outstripped the collective production output of the supply chain (up to the end of 2008), almost every solar cell manufactured was guaranteed an automatic sale — a fact that enabled low-efficiency cells to be made in high volume. Consequently, c-Si cells (which up until end-2008 were accounting for ∼90% of sold product) could be manufactured using legacy production lines; tooling that largely bypassed any laser- based process stage. Laser-based process tools were found in only two areas of c-Si cell production: (i) for a routine loss-preventative step during junction isolation6; and (ii) within the trailblazing “Saturn” lines from BP-Solar during the industry’s first full-blown effort to produce high-efficiency selective emitter c-Si cell types.7 Within thin-film production, everything produced during this time period did utilize lasers for the thin-film patterning process (the so- called P1, P2, and P3 steps) — lasers being the de facto tooling immediately after the early R&D conducted within the United States and Japan during the 1980s and early 1990s. Figure 1 illustrates these three legacy laser applications. Figure 1: The three main applications historically for lasers used in solar cell manufacturing have been (left to right) laser edge isolation, laser scribing for buried contacts, and thin-film selective removal. As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 3 During 2009, changes in regional legislation (mainly from Spain), coupled with the onset of the credit crisis, would result in a 180-degree shift in the supply-demand curve.8 The capacity- expansion glory days of 2005–2008 were quickly replaced by a significant oversupply to the market; shifting low-efficiency (or high-cost) product was no longer a done deal. Efficiency enhancement became the buzzword at the trade shows during 2009, and with this, the requirement to implement new production steps moving forward. Laser-based processing for “advanced” c-Si cells now had a direct market pull. The remainder of this article discusses what kind of laser sources are used by the solar industry, both those deployed until the end of 2008 and those currently being qualified within R&D and pilot production lines for next-generation cell types. Early Laser Adoption First, let’s summarize those applications used extensively by the solar industry, in which laser- based process tools featured heavily until the end of 2008: 1. Junction isolation (also known as laser edge isolation) has become a well-proven laser- based process in c-Si lines as the final step at the back-end of cell manufacturing. Here, lasers scribe an electrical isolation groove around the perimeter of the front surface, between the finger grid and the edge of the cell.5 2. Finger groove formation and mask creation (for both heavy diffusion and metallization) was applied extensively by BP-Solar within the well-known laser grooved buried contact (LGBC) c-Si cells. Lasers again performed scribing5 — this time along finger and busbar lines on the front surface. 3. Thin-film patterning uses lasers to selectively remove layers of materials during the various deposition stages employed to fabricate thin-film solar panels. These scribe lines provide cell isolation and interconnection across the panels. Laser types used for these applications have been dominated by nanosecond diode-pumped solid-state (DPSS) sources operating at 1064 nm (IR), 532 nm (green) and 355 nm (UV). This is confirmed when looking at the revenue split for lasers sold into the solar industry during 2008 (Figure 2).9 Indeed, each of the applications listed above came to prominence during the late 1990s (in the case of thin-film patterning) and the mid-2000s (laser edge isolation) — a period when DPSS lasers were gaining traction throughout analogous market segments (microelectronics and flat panel displays). The quality of scribing is optimized by choosing laser wavelengths to overlap with the absorption bands of the materials being ablated. In those niche cases where process windows for nanosecond material removal prove too challenging for reliable and repeatable industry implementation, some researchers are currently exploring the use of sub-nanosecond (in particular picosecond) laser types. As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 4 Figure 2: Breakdown of relative revenues (in U.S. $) for lasers sold into solar cell manufacturing during 2008 showing a strong bias towards nanosecond DPSS lasers. The advent of “turnkey” a:Si production-lines with nameplate fab capacities in excess of 50MW certainly prioritized lasers as key tools within the solar industry. The phrase “laser scriber” then emerged as a somewhat crude generalization for any piece of equipment containing a laser.4 Announcements of turnkey a:Si fab sales were a frequent occurrence during 2006–2008, and with laser source revenues running at around $0.5–1M per 50MW-fab, it was no great surprise that many within the laser industry viewed the thin-film sector (especially a:Si) as the one to target within the solar industry. But, first looks can be deceptive: for much of the capacity announced during this period within the thin-film community has not been directly contributing to finished products, and most are still at the build or qualification phase. As a result, most of the lasers sold to thin-film capacity expansions during 2006–2008 are yet to be run in anger — the exceptions to this being First Solar (the undisputed champions of the thin-film space by some margin), Kaneka, and Unisolar. Lasers used in c-Si fabs during this same time period received far less attention. Skepticism had been raised over laser technology in general when LGBC cell types failed to reach mass production levels (more on account of market dynamics then at play, coupled with the absence of any equipment supply-chain in place6). And lasers were up against both dry (plasma) and wet (chemical) etching solutions for junction isolation5 — technologies preferred by much of the equipment supply chain at that time. Interestingly, laser scribing (or groove formation) on the front surface of c-Si cells to depths of 10-20 µm represents one of the easiest laser-based processes considered today for advanced c-Si cell concepts! IR, green, and UV DPSS lasers can be found in laser-based tools supplied to the industry for these applications. The conclusion reached early was that improved scribe quality was provided by moving to shorter wavelengths — a direct consequence of the well-known absorption properties of silicon with improved “coupling” of UV light into highly localized regions within the bulk (see Figure 3). “Quality” in solar cell manufacturing is in part at the surface (less debris, less melting, reduced “dead-area”) but more critically subsurface (minimized heat affected zones, less damage sites, minimized recombination losses). Each of these can very quickly reduce the overall cell efficiency if laser output parameters are less than optimal. Perhaps a case here of the solar industry learning what many laser-based tool suppliers to the semiconductor industry had established as a “given” some ten years earlier: move to short-wavelengths when applying lasers for silicon processing. As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 5 Figure 3: Surface (and subsurface) quality for all front surface c-Si laser-based applications improves with the use of shorter wavelengths from the lasers applied. Here, a comparison is shown on the surface of a c- Si cell between IR (left) and UV (right) nanosecond DPSS lasers, after the edge isolation process. New Laser Sources For Efficiency Enhancement Today, the outlook for lasers within solar cell manufacturing is very different to that encountered during 2006–2008 — thanks to changing market dynamics at the end of 2008. For many equipment suppliers who had enjoyed unprecedented revenue growth until then, this was nothing but doom and gloom. For technologies looking for an inflection point in terms of how solar cells would be made in the future, it was great news!10 Almost all high-efficiency c-Si solar cells, which had been championed by solar research institutes for over 20 years, use laser processes for essential, efficiency-enhancing steps.11 New equipment for production lines began to emerge with a redefined market-driven pull for more laser sources in c-Si cell manufacturing. By contrast, changes to laser types required by the thin-film industry have been more subtle — a consequence of increased diversification in the technologies used by thin-film panel manufacturers today, and uncertainty as to when most a:Si fabs will reach full utilization, at which point capacity expansions may be considered. For simplification, all laser-based processes forming part of efficiency enhancement stages within c-Si cell production can be allocated to one of four categories, defined as: 1. Assisting in mask writing for subsequent metallization12, secondary diffusion in selective emitters13, or surface etching to texture14; 2. Localized secondary diffusion (laser doping via phosphorous- or boron-contained precursor layers10); 3. Contact preparation (finger and busbar grooves6, through-silicon-vias15, interdigitated structuring16); As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 6 4. Contact forming (sintering or firing17). In the context of c-Si cell production as a whole, junction isolation now encompasses laser edge isolation and rear contact isolation (RCI) on back-contact cell types. Remembering that junction isolation is essentially loss prevention, it is prudent to decouple it from the efficiency enhancement steps above, which are specific to next-generation cell concepts. So, what do these new process steps mean in terms of laser source selection? Do they call for different laser types — or indeed, for new laser sources altogether? These are certainly the most important questions today for those engaged in laser process qualification within the c-Si segment of the solar industry. Can the laser sources used in c-Si cell production for laser-scribing historically (10–20W power nanosecond DPSS lasers) be applied here? Laser sources are now being asked to perform more precise (or higher-finesse/quality) micro materials processing within these new advanced c-Si cell concepts. Examples include (i) highly- selective thin dielectric layer removal or modification without changing any physical or chemical parameters associated with subsurface material9, and (ii) highly localized and controlled changes to doping concentration levels through melt-induced diffusion.10 In one of the new process steps, for example, lasers are required to selectively remove just 70–80 nm of SiNx without causing excessive damage subsurface (see Figure 4), or without adverse changes to the phosphorous concentration levels within the 200-nm-thick emitter layer directly underneath. These types of processes certainly don’t come without their challenges — but they are well within the capabilities of tool suppliers experienced in dealing with (often) more-challenging semiconductor fab requirements. Figure 4. Thin-film ablation of sub-micron thick layers (on the front or rear surfaces of c-Si cells) is currently an area of active research within the solar industry. Short-wavelength and/or short-pulsewidth lasers are showing the greatest promise today for this application. As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 7 Selection of laser sources for these new high-efficiency c-Si cell production lines is therefore comprised of a new subset of acceptance criteria. The following statements highlight the guidelines currently being applied throughout the solar equipment supply-chain for lasers here: • Don’t predetermine the laser source type based solely upon legacy laser scribing for junction isolation. More demanding materials processing generally requires a wider range of laser source types, other than ns DPSS lasers. • The ideal laser source is almost certainly not the lowest cost! (Many observers of the solar industry conclude erroneously that driving down the $/W of solar energy means that production equipment has to have the lowest capital cost.) • The ideal laser is that source which, when integrated within the optimum laser-based process tool, provides the maximum ROI to the cell manufacturer, defined by amortized per-wafer cost and associated efficiency gain compared to the standard cell type. Depending on a range of different factors, this may be a DPSS laser, it may be an excimer laser, it may involve a process tool with multiple lasers integrated, etc. Any glance at semiconductor and display laser-based tooling will clarify this issue. The c-Si solar equipment supply-chain has yet to grasp this issue to its full benefit. • Damage can only be minimized to levels where it does not adversely affect overall cell performance — it is never completely eliminated. This above-all promotes short- wavelength lasers, and almost completely rules out laser operating at 1 µm. • Short-term production tooling used in cell manufacturing lines is likely to be satisfied by laser sources that have been qualified already within analogous 24/7 high-tech operating environments. The success of laser-based tooling in c-Si cell manufacturing now depends critically on immediate ROI; risk is generally minimized by eliminating any teething problems adopting brand new laser technologies. For sure, the landscape for laser sources integrated within laser-based tooling used for advanced c-Si cell production lines will differ from that deployed before year-ending 2008.8 Expect to see more high-power, short-wavelength lasers operating in the UV spectral region (Figure 5), more advanced optical arrangements for adapting laser output profiles at the wafer surface, and different material removal mechanisms compared to single-pulse ablation with overlapped, focused Gaussian beam profile spots. An example of a current state-of-the-art laser- based tool for next-generation c-Si cells is shown in Figure 6. As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 8 Figure 5: Next-generation laser sources for advanced, high-efficiency c-Si cell concepts are likely to use a wider range of short-wavelength laser sources, more aligned with analogous laser-type adoption seen TM throughout the semiconductor and flat panel display industries: (a) Avia (266 and 355 nm, nanosecond), TM TM (b) Paladin (355 nm, quasi-CW), (c) Talisker (355 nm, picosecond), and (d) LPX excimer (157, 193, 248, 308, and 351 nm, nanosecond). Figure 6: The Coherent-Equinox-Fab-TT laser-based tool utilizes multiple laser sources integrated within a single platform with the ability to process four wafers simultaneously. As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 9 Anticipating how laser sources will evolve for thin-film is less obvious. Part of this is due to the current low utilization rates of thin-film fabs18 (in particular for a:Si panels), but also because widely different panel sizes and laser scribing arrangements have been rolled out in the past few years. However, one new laser-based process receiving widespread acceptance in thin-film fabs is a process called border (or edge) deletion, in which lasers ablate (comparatively) wide lines around the perimeter of near-finished thin-film panels. New high-power (several 100s of Watts) Q-Switched ns lasers are currently at evaluation phase for this application. Conclusions Having been used primarily during routine production line stages within both c-Si and thin-film fabs in the past, laser sources for tomorrow’s solar fabs are poised to play a more pivotal role as part of cell efficiency enhancement. Understanding the basic technical and ROI constraints governing the acceptance of new process stages is now more important than before. This will ultimately call for a wider range of laser sources and laser-based process tools, perhaps exploiting lessons learned from legacy semiconductor and flat-panel display production line stages. Finally, then, the solar industry equipment supply chain will mature considerably in its acceptance of laser-based process stages as a standard tool type within fabs. About The Author Finlay Colville (Finlay.Colville@coherent.com; +44-7802-238- 775) is Director of Marketing for Solar at Coherent Inc. in Santa- Clara, CA. Additional information can be found via the Solar web pages at www.Coherent.com/Solar. References: 1 F. Colville et al., “Existing and Emerging Laser Applications within PV Manufacturing,” Photovoltaics International, 1, 2008. 2 F. Colville, “Redefined Value: High Efficiency Cell Roadmap Demands Clear Dialogue,” InterPV, September 2009. 3 th S. O’Rourke, “Solar PV Competitiveness and Market Dynamics,” Keynote session at 34 IEEE PVSC, Philadelphia, 2009 4 F. Colville, “Supply-demand Dynamics Call Time on Legacy Cell Production Technology,” PVTimes, July 2009. 5 F. Colville, “Laser Scribing Exposed: the Role of Laser Based Tools in the Solar Industry,” Photovoltaics International, 3, 2009. 6 F. Colville, “Lasers Scribing Tools Edge in Front,” Global Solar Technology, Vol. 2, No. 2, March/April 2009. 7 F. Colville, “Into the Groove: How Buried Contacts Brought Lasers to Life in Solar,” InterPV, June 2009. 8 P. Mints, “The Global Economy Fell Down and Went Boom – Will Solar Follow?,” PVTimes, January 2009. 9 F. Colville, “A Sunny Outlook for Lasers in Solar”, Laser Focus World, Vol. 45, Issue 5, May 2009. As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter. 10 10 F. Colville, “EU PVSEC Highlights New Equipment Landscape for Solar Cell Production,” PVTimes, September 2009. 11 N. Mason, “High-Efficiency Crystalline Silicon PV Cell Manufacture: Status and Prospects,” PVSAT-5, Glyndŵr, 2009. 12 F. Colville, “Selective Criteria: Lasers Go Short for Dielectric Ablation of Silicon Solar Cells,” Solar: A PV Management Magazine, Issue 2, 2009. 13 F. Colville, “Laser Assisted Selective Emitters and the Role of Laser Doping,” Photovoltaics International, 5, 2009. 14 D. Niinobe et al., “Honeycomb-structured Multi-crystalline Silicon Solar Cells with 18.6% Efficiency Via rd Industrially Applicable Laser Processes”, 23 EUPVSEC, Valencia, 2008. 15 F. Colville, “The Hole Story: Lasers Take the Wrap,” InterPV, April 2009. 16 th A. Schoonerdbeek et al., “Laser Technology for Cost Reduction in Silicon Solar Cell Production,” 25 ICALEO, LIA, Scottsdale, 2006. 17 th E. Schneiderlöchner et al., “Laser-fired Contacts.” 17 EUPVSEC, Munich, 2001. 18 P. Mints, “As Demand for Solar Tech Deepens, Where Do Thin Films Stand,” PVTimes, June 2009. As seen in the 11/25/09 edition of the Photonics Online (www.photonicsonline.com) newsletter.