Process optimization with on-site mixing and controls is key to reducing costs in solar cell manufacturing.
The materials used in solar manufacturing are varied, depending upon the technology, application, and tool in practice. For example, cleaners may be procured as specialties (pre-formulated) or mixed on-site from low-cost commodities, depending upon the choice of the end-user and their tool capability for in situ mixing . In one application, the addition of metal inhibitors has resulted in virtual elimination of an entire step and a savings of several mts/day of IPA . The same options are encouraged with texturing, surface finishing, or thinning. Opportunities also exist with the use of polymeric coatings, such as temporary bonding in wafering or protective coating in laser scribing/dicing . These cases describe the use of simple and low-cost additives or commodities, mixed on-site, and integrated by the end-user for greater flexibility to lower cost, change technology, or upgrade equipment (Table 1).
Thinning and texturing
Chemical etching is the most common practice in the industry for reducing thickness by bulk silicon removal and surface texturing. Etch rates in HNA exceed those in alkaline mixtures, reaching much greater than an order of magnitude, depending upon composition and process conditions. For example, mixed acids are observed to etch at 15µm/min irrespective of Si crystal orientation , whereas KOH (20-40% = 3.5-7M, 80C) etches Si (100) at 1.1 – 1.4µm/min with the (110) and (111) being faster and slower, respectively . In some cases, HNA mixtures can produce Si etch rates well over 100µm/min, such as the popular Spinetch products from General Chemical .
As with most surface finishing operations, etch processes are diffusion limited. Species transport is hindered by increased viscosity, organic diluents (i.e. IPA, polyglycols, etc.), and increased concentration. This was shown in etch experiments with Si (100) monitoring etch rate and roughness (Fig. 1).
Etch rate and roughness in HNA chemistry is controlled with the use of viscous acids such as sulfuric and phosphoric. Adjusting the combined viscous acid concentration from 25% to 40% increases viscosity and subsequently drops the etch rate from ~170 to 130µm/min. The same effects have been proven with the addition of small amounts of a rheology modifier. With a proper modifier selection, 1-2% addition will increase viscosity similar to that observed from a 20% increase of viscous acid. Typical cost savings can be significant because additives of this sort will commonly exhibit a fraction of the costs as compared to phosphoric. Once prepared, controls are installed for refractive index (Fig. 2) and kinematic viscosity (Fig. 3), available in handheld or inline measurement equipment.
The choice in HNA chemistry over alkali is not only dependent upon etch rate and roughness. Depending upon composition and conditions, HNA typically etches isotropically, while KOH may be anisotropic and produce distinctive pyramidal features on Si (100), a property that is attractive to light-trapping . This approach may be done at a fraction of the cost of highly concentrated systems by diluting and adding rheological modifiers with controls.
Cleaning processes vary from simple water rinses to the more challenging removal of saw and laser debris, stencil and resists, and post-reactive ion etch (RIE) residue. A simple process may use a concentrate-form detergent, prepared on-site, and monitored/adjusted according to the process needs. Barring the drag-out of toxics, the waste is collected, neutralized, and with approvals, released to a local municipality. This approach has been qualified for panel cleans in removing post-RIE residue from aluminum pads.
While simple and low-cost cleaning practices may be attractive, their success in manufacturing must be measured by inspection processes that exhibit the same principles of reliability and value. One choice is with the use of the mercury probe, a tool that conducts a rapid and non-destructive electrical characterization . Its primary application has focused on measurements made in seconds of thin epitaxial layers , permittivity, doping, oxides, dielectric strength , and most recently in quantifying solar efficiency using illuminated I-V monitoring. The Hg-probe has also been proven to rapidly characterize clean surfaces and inhibitor films to below 5Å .
Although automated Hg-probe equipment exists, the basic R&D platform includes two glass vials containing mercury, whereby it makes contact with the sample facing down via a machined surface. In the center of the measurement platform is a small hole for the primary contact surrounded by an open annulus as a secondary contact. Both of the mercury contacts are surrounded by a vacuum ring.
For simple measurements, mercury-sample contacts should be ohmic (non-rectifying) allowing for the current-voltage instrumentation to be used to measure resistance, leakage currents, or current-voltage (I-V) characteristics. Resistance is measured on thin films composed of any material that does not react with mercury. Metals, semiconductors, oxides, and chemical coatings may be measured successfully. Results of Hg-probe tests following detergent cleans on metals have shown the presence of desirable thin protective films to prevent oxidation (Fig. 4).
Polymers make-up the majority of adhesives and coatings designed for a specific process. Simple materials exist with glass transition (Tg) values >400?C, 100% solids, good transparency, and may even be used in temporary processes where they are subsequently cleaned-up (dissolved) in aqueous detergents. Wafering uses adhesives to hold firm the polysilicon ingot on a sacrificial substrate during sawing. This bonding material, typically an epoxy, must resist vibration, tensile stress, and heat from the saw, and must also break down and dissolve in simple and safe chemistries. As pressure mounts to reduce cost by producing more wafers per ingot (i.e., thinner wafers), and facilitate the use of new technologies [12, 13], there begins a drive to investigate alternative adhesives. Many options exist that cure rapidly, perform to the task, and are removed in seconds.
As wafers are thinned to 100µm or less, it is understood that the material of choice must not only operate as an ingot adhesive, but also as a support for delicate substrates. The same approaches and processing are being explored in semiconductor wafer manufacturing. One option includes the use of encapsulation techniques. A version of the encapsulation process uses substrates (“tiles”) that are populated on a temporary support and held firmly by imbedding the tiles in-place. Encapsulation and cure may be completed within seconds, depending upon the process configuration (Fig. 5).
Once in place, the tiles appear in a “mosaic” pattern and are subsequently processed by typical machining (grind, polish), coating, etching, cleans, or any of a variety of applications. Once complete, the tiles may be released from the substrate directly to their final support (Fig. 6). The total practice involves simple polymer systems that may be prepared on site with controls. Variations on this theme have been demonstrated in thinning to <50μm with backside processing .
The author would like to thank Materials Development Corporation for its support. The author is also grateful to the staff at Daetec, especially to Marissa Lechman, Jared Petit, Agnes Tan, and Chuong Vu for their support in making this work possible.
Spinetch is a registered trademark of General Chemical.
- J. Moore, “Meeting Selectivity Needs with Unique Corrosion Inhibitors in Cleaning and Surface Finishing Practices,” Ultrapure Fluid Handling and Substrate Cleaning Conf., February (2008).
- J. Moore, “Formulating for Extreme PR Stripping, Achieving Performance, Selectivity, Stability,” Proc. ECS-International Semi. Tech. Conf., #C14, January (2008).
- J. Moore, et al., “A Novel Water-Washable Coating for Avoiding Contamination During Dry Laser Dicing Operations,” Proc. for GaAs ManTech Conf., pp. 317-320, May (2007).
- A. Stoller, et al., RCA Rev., 31, 265 (1970).
- H. Seidel, et al., J. Electrochem. Soc., vol. 137, p. 3626-32 (1990).
- Spinetch products were originally developed by Merck KgaA (Ger), are licensed by General Chemical, LLC, www.genchemcorp.com.
- E. Ryabova, “A Review of Solar Wafer Cleaning and Texturing Methods,” Photovoltaics World, pp. 12-15, May/June (2009).
- Model 802B-150 Hg-Probe is produced by Materials Development Corporation, www.mdc4cv.com.
- F.D. Hughes, “The Characterization of Epitaxial GaAs using Schottky Barriers,” Acta Electronica, 15, 43 (1972).
- R.S. Nakhamanson, et al., “Investigations of MIS Structure Inhomogeneities Using a Scanning Mercury Probe,” Phys. Stat. Sol., 19, 225 (1973).
- J. Moore, et al., “A Review of Semiconductor Manufacturing Applications Using Triazole-Based Inhibitors for Copper and Related Metals,” 24th Proc. Semi. Pure Water and Chem. Conf., pp. 155-164, (2005).
- J. Lillian, “A Sea Change in Wafering Technology,” Solar Industry, vol. 2, no. 8, pp. 16-19, (2009).
- A. Skumanich, “The Art of Wafer Cutting in the PV Industry,” Photovoltaics World, pp. 28-32, May/June (2009).
- J. Moore, “Temporary Support Systems to Enable Thin Wafer Handling During Grinding, Backside Processing, and Laser Dicing,” Int. Wafer Level Pkg. Conf., October, (2009).
John Moore is the founder of Diversified Applications Engineering Technologies (Daetec), LLC, 1227 Flynn Rod., Unit 310, Camarillo, CA 93012 USA; ph.: 805-484-5546; email [email protected]; www.daetec.com