Patent Application
20110079250
The invention relates to a new methods for post-texturing cleaning, surface conditioning, and rinsing silicon wafers or similar surfaces, with particular, although not exclusive, applicability in photovoltaic applications.
Texturing of the wafer surface is usually the first step of the single emitter photovoltaic (PV) manufacturing process for both mono- and multi-crystalline silicon wafers. The texturing process roughens the surface and reduces the reflection of the silicon surface by etching along crystal planes and grain boundaries to increase the surface area to provide more light trapping. In addition to texturing, the initial wet chemical process bath or baths also must remove saw-damage, undesirable contamination, and condition the silicon surface to be hydrophilic so as to allow uniform doping for the emitter formation.
Two of the most widely used processes, developed at the Energy Research Centre of the Netherlands (“ECN”) and the University of Konstanz (“UKN”) each begin with a concentrated HF/HNO3 texturing bath that also removes any saw damage. KOH with IPA or a surfactant or NaOH can also be used for texturing, either alone or after the HF/HNO3. The ECN and UKN processes are disclosed in A. W. Weeber, A. R. Burgers, M. J. A. A. Goris, M. Koppes, E. J. Kossen, N. C. Rieffe, W. J. Soppe, C. J. J. Tool, and J. H. Bultman, 19th European Photovoltaic Solar Energy Conference (2004), and Hauser, et al., U.S. Pat. No. 7,192,885, respectively.
The texturing process composition, used for multi-crystalline wafers, is comprised of 20% to 55% H2O, 10% to 40% concentrated HF (49 wt %) and 20% to 60% concentrated HNO3 (approximately 65 wt %), with additives such as acetic acid (H-AC) and a surfactant. The texturing process is carried out at ambient or lower temperature with a controllable etch rate. Other texturing composition ranges can also be used. Typically, the wafers are exposed to the chemical between 1 and 5 minutes.
For mono-crystalline silicon, the preferred texturing solution is alkaline-based. Dilute NaOH solutions or KOH solutions, with and without additives such as isopropyl alcohol (IPA) or ethylene glycol, are suitable for texturing, particularly for mono-crystalline silicon. Although acid texturing baths, including those described above for multi-crystalline wafers can be used, a KOH or NaOH bath is used subsequently to remove the porous silicon that remains on the surface. An HF/HCl bath is typically performed after the alkaline hydroxide step to facilitate removal of the mobile ions, metallic contamination, and to strip the chemical oxide. The final oxidation step creates a homogeneous hydrophilic surface allowing uniform phosphorus doping to create the emitter.
The use of dilute chemical baths in processing silicon wafers is known in the art as exemplified by Olesen, et al., U.S. Pat. No. 6,158,445, the techniques outlined by researchers at IMEC published in Proceedings of the 7th International Symposium on Cleaning, SCP Global Technologies, Boise, Id., May 1-3, 2000; Kashdoush, et al., U.S. Pat. No. 6,837,944; Tsao, et al., U.S. Pat. No. 6,165,279; and Puri, et al., U.S. Pat. No. 6,681,781. Moreover, it is well-known in the art to use a dilute HCl rinse after SC-1 baths in semiconductor manufacturing.
However, most such methods contemplate a separate water rinse or other neutralizing method performed after the dilute chemical bath. De-ionized water (DIW) rinsing or an equivalent is generally required because these methods are intended for use in semiconductor manufacturing where contamination levels on the wafer surface must be lower than those for solar cell manufacturing. There is no suggestion of how rinse water or the dilute chemical baths could be recycled and/or reused.
The objective of the current invention to provide a method for post-texturing and cleaning baths for silicon wafers and the like that reduces the amount of chemicals and water used in the process while retaining an acceptable level of cleansing and surface conditioning.
The present invention discloses an improved rising process for the post-texturing and cleaning baths for silicon wafers and similar surfaces that is especially well-suited to silicon wafers intended to be used for photovoltaic applications. The inventors recognized that for photovoltaic and similar applications a dilute bath used for one step in the process can be reused in other steps because the purity and surface conditioning requirements are not as stringent as with semiconductor wafers. Thus, after the texturing step, rather than using concentrated chemical baths for cleaning and water rinses with new rinse water after these post-texturing baths, the present invention replaces the concentrated chemical baths and subsequent water rinses with combined rinsing and cleaning steps using dilute chemical baths. This replacement also allows for the reuse of chemicals and water recycled from other steps, with or without the addition, or “spiking”, of relatively small amounts of additional chemicals. Furthermore, by using appropriately selected chemicals in a particular order, the current invention allows the number of chemical baths to be reduced. The invention can generally be applied to wet cleaning processes that involve a plurality of chemicals, in individual tanks, with a rinsing step after each chemical step.
In one embodiment, post-texturing chemical baths of the current invention are composed of dilute HF/HCl, followed by dilute SC-1 or O3/DIW (deionized water) or dilute H2O2 or similar dilute formulations. Thus, the concentrated HF/HCl bath and DIW rinse of conventional cleaning processes may be replaced with a dilute HF/HCl rinse, and the final alkaline bath and DIW rinse may be replaced with a dilute SC-1, H2O2, or O3 deionized water (O3/DIW) rinse or other combination including HF/O3/DIW.
The use of these dilute rinses allow for the rinsing water to be reused in a controlled manner by cycling the water “upstream” from each rinsing bath. The apparatus for the baths are designed to use the rinsing chemical baths starting with the oxidizing bath (O3, SC-1 or H2O2) going upstream and reusing the water by spiking with HF and/or HCl, and then upstream for rinsing after the KOH, NaOH bath, and finally using this rinsing water with chemicals after the HF/HNO3 bath, or any combination of KOH and NaOH baths. For certain steps, temperature or time in the bath can be increased to counteract the decrease in reactivity when using the dilute chemicals.
A pre-cleaning and/or post-cleaning process can also be included to remove contamination, as well as metallic, mobile ions and particles, from the surface and to create an oxidized or a oxide-passivated} state. The pre-cleaning step can include a dilute SC-1 rinse followed by a dilute HCl rinse, with or without HF added. The post cleaning step may follow the alkaline or O3/DIW step with a dilute HF bath with or without HCl added, and then another dilute SC-1 or other alkaline or O3/DIW bath.
The apparatus include an input conveyer 110, 210, or similar which bring the silicon 120, 220 or other materials to a linear transfer robot 125, 225 for conveying the silicon 120, 220 to the various baths and rinses. An output conveyer 130, 230 can be used to remove the silicon 120, 220 from the apparatus 100, 200 when the processing is complete. It will be understood by those of ordinary skill in the art that all the apparatus discussed would include exhaust means 170 at appropriate locations as well as means (not shown) conventionally used to introduce and remove the chemical baths and DIW rinses all of which could use cooled, ambient, or heated water, depending on the needs of the process.
Further, it will be understood that rinsing can be accomplished by any of a variety of well-known methods, including cascade overflow, quick dump rinsing, and spray rinsing or any combination of these methods. Multiple cycles of these rinsing methods may be used, including full or partial dump rinsing cycles, all of which are well-known in the art. Processing can be dual sided; front and back surface or single sided; front or back surface only. The equipment used for this process can be in-line rollers, immersion, or any applicable equipment that allows dilute processing or can be modified for dilute processing or similar means.
Additions to the process are possible. For example, the rinse water may be subject to gasification or degasification; the rinse may include megasonic cleaning, possibly with the addition of CO2 to acidify the water and/or N2 or O2; additives that gives water unique properties at low concentrations or chelators may be used. Further, filtering of the rinsing water can be performed to remove particles and other undesirable contamination
In
In
It will be understood that while there are a variety of chemicals and concentrations that can be used to obtain the desired surface condition effects, to properly implement this invention the chemical baths that can be reused or recycled must be chosen to meet the combined goal. For example, at the time of transfer of the O3/DIW bath to the HF/HCl bath, there are residual amounts of O3 in the bath, but because the concentration of the HF/HCl is more than the O3 and because O3 decomposes to molecular oxygen (O2) with no byproduct of the reaction, it is a preferable bath for both functionality, in that it provides a hydrophilic surface, and reactivity, in that it does not interfere with the functionality of the HF/HCl bath.
As shown in
The invention can also be implemented with a pre-cleaning step before the texturing bath. Precleaning is often desirable since the sawing of the wafers slices exposes the pristine silicon to the saw material and also to lubrication, although in certain circumstances and processes, incoming wafers may have sufficiently low contamination levels so that a pre-clean is not needed. Without precleaning, copper and organic lubricating oils build-up in the texturing bath over time and can be distributed throughout the system, even with optimized rinsing. By cleaning the wafer before the texturing process with a dilute bath or series of dilute baths, the life of the entire process and each individual bath can be increased.
The pre-cleaning process is typically composed of a SC-1 bath, either concentrated or dilute, followed by a SC-2 or dilute HCl bath or rinse. But, since a pre-clean with concentrated chemicals may affect the final texturing by exposing grain boundaries or etching the silicon that is detrimental to the final roughness desired, dilute chemical rinses are preferable because they do not affect the texture of the surface. Each of the concentrated SC-1 bath and SC-2 bath is followed by a DIW rinse. However, the dilute HCl rinsed wafers can proceed without additional rinsing into the texturing bath. Hydrofluoric acid may also be spiked into the water or used alone in place of HCl. The “pre-cleaning” baths may also be included after the texturing bath and subsequent rinses; however, the texturing bath would then still be subject to increased contamination. Also, additional precleaning and surface conditioning steps can be added if needed; for example, the sequence SC-1 bath to HF/HCl bath to SC-1 bath to render the surface hydrophilic to allow easier wetting of the surface prior to the texturing bath.
The apparatus structures 900, 1000 shown in
It should be noted that he invention does not preclude additions to the processing, such as additional cleaning baths to remove particles, an additional final bath to render the surface hydrophilic instead of hydrophobic; or the inclusion of sonic agitation or other physical effect.
As demonstrated by the following Table 1, the current invention is anticipated to result in significant reductions in chemicals and water used in the processing of silicon wafers for solar cell applications from the conventional concentrated chemical method.
The numbers for the concentrated process are actual bath volumes and rinsing cycles currently performed with concentrated chemicals using three dump rinse cycles on MTS equipment. The simplified process numbers include the anticipated volume reduction resulting from the elimination of one of the rinsing tanks. Additional reductions are achieved through the reuse of rinsing water.
As might be expected, the reuse of the water, cycling from one rinse tank to another, reduces the amount of water needed. Further, concentrated chemicals require large amounts of water to rinse the chemicals off the wafers to reach the desired low concentration levels, as the carry-over of chemical into the rinse water can be significant. Due to carry-over, the first stage of rinsing required that chemical to be neutralized and diluted to prevent further process, and then more water is used to render the wafer surface free of chemical. Less water is needed to rinse the concentrated chemicals; dilute baths are used that act as the rinsing process.
Furthermore, apparatus associated with the claimed method use less energy, exhaust, and materials, and have simpler facilitation requirements. The use of fewer baths and processing steps equates to a smaller system footprint and lower processing costs. Thus, an environmentally greener process is obtained when compared with the concentrated chemical process.
The invention may have applicability in other processes where multiple baths are needed, ultrahigh purity of water is not an issue and there are not high tolerances for surface cleanliness requirements of particles, metallic, or organic contamination. Such applications might include (i) silicon ingot, raw silicon, and poly silicon manufacturing; (ii) medical device cleaning; (iii) manufactured or milled parts cleaning; and (iv) panel cleaning, for LCD screens and solar panels or other similar parts. Therefore, while the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form, connection, and detail may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.
