Method for patterning mo layer in a photovoltaic device comprising cigs material using an etch process

This application is a continuation of U.S. patent application Ser. No. 11/562,573, filed Jan. 22, 2006 which is incorporated herein.

Thin layers of material comprising Cu(In,Ga)Se, i.e. CIGS, are known to exhibit the highest photovoltaic conversion efficiency of any thin film material for a photovoltaic device (19.5%). See K. Ramanathan et al., “Properties of High-Efficiency CIGS Thin-Film Solar Cells,” 31^st IEEE Photovoltaics Specialists Conference and Exhibition, Lake Buena Vista, Fla., Jan. 3-7, 2005; and D. E. Tarrant et al., “CIS thin film development and product status at Shell Solar, May 2003,” Proc. of 3^rd WCPEC, Osaka, Japan, May 2003. Similar progress has been reported in the manufacturing area, where the efficiency of champion modules has exceeded 13% with yield above 80%. See M. Contreras et al., “High Efficiency Cu(In,Ga)Se₂-Based Solar Cells: Processing of Novel Absorber Structures,” Proc. of 1^st WCPEC, Hawaii, Dec. 5-9, 1994. Consequently, CIGS is considered by many in the art to be an attractive material for use in the manufacture of thin film photovoltaic panels.

In a typical solar cell module, as shown in FIG. 1A, a CIGS layer 106 is grown to a thickness of about 2 μm on an underlying metal layer 104 comprised of molybdenum (Mo). Mo provides good ohmic contact and has a similar thermal coefficient of expansion as CIGS, which can be important for the elevated temperature of CIGS processing. The Mo is usually deposited on a substrate material 102 which may be, for example, glass, stainless steel, or plastic. A range of concentrations of Cu, In, Ga and Se can be used in the CIGS layer, and sometimes the Ga is absent or S may be added. Accordingly, CIGS is used herein in its broadest possible meaning in the context of similar or related films. Moreover, other materials may be used in thin film photovoltaic modules, either individually or in combination, and either alternatively or additionally. These include CdTe, amorphous silicon (a:Si) and micro- or nano-crystal silicon (μc:Si). See generally, Y. Hamakawa (ed.), “Thin-Film Solar Cells,” Chapter 10 (2004), the contents of which are incorporated herein by reference.

While the above-mentioned reported efficiencies of thin-film photovoltaic modules including CIGS are promising, there is a large gap between those numbers and actually-obtained efficiencies of known commercial photovoltaic modules containing CIGS. One problem is that laser and mechanical scribes are commonly used to pattern and form interconnects in thin-film photovoltaic modules, and these prior art processes have a number of drawbacks that limit module efficiency. For example, they create wide scribes, defects, and shunt current paths. Furthermore, they provide limited means for wiring the module in series-parallel arrangements that might reduce sensitivity to series resistance, shading losses or non-uniformity.

For these and other reasons, some have considered using lithographic patterning processes to form thin-film photovoltaic module interconnects. However, these processes would require the ability to etch Mo, and, in some cases, to do so selectively so that, for example, the etch will not induce excessive undercut of an overlying CIGS layer. The prior art literature provides scant reference to etching Mo in a CIGS solar cell, and is otherwise insufficient to solve this problem.

Moreover, it was not even known to etch CIGS in a solar cell until the invention of U.S. patent application Ser. No. 11/395,080 (AMAT-10936), the contents of which are incorporated herein by reference. While this invention dramatically advanced the state of the art of thin-film photovoltaic modules, and also mentions etching Mo, additional problems have arisen that were not seriously addressed before that invention.

For example, as shown in FIG. 1B, certain CIGS growing processes can include selenium annealing at high temperature. In such processing, a MoSe₂ layer 108 is formed at the interface between the CIGS layer 106 and Mo layer 104.

When such a MoSe₂ layer is formed, both the Mo layer and this additional MoSe₂ layer need to be removed during processing, and ideally using an etch. Again, the prior art literature is insufficient for overcoming this newly-observed problem. For example, T. Ohmori et al., in their article entitled “pH Dependent Controlled patterning of p-MoSe₂ Surfaces by In-Situ Electrochemical Scanning Tunneling Microscopy,” Langmuir, 14 (21), 6287-6290 (1998) propose using a solution of 0.05M NH₃ and 0.025M KNO₃ with the assistance of a high electrical field induced between an Atomic Force Microscope (AFM) tip and a MoSe₂ surface. For a typical gap of 2 nm between the AFM tip and the substrate and with the reported etching threshold voltage of 0.3V, the electrical field is as high as 1.5×10⁸ V/m which is unsuitable for application to macro-scale processes such as photovoltaic module fabrication. Likewise, S. Chandra and S. N. Sahu, in their paper entitled “Electrodeposited semiconducting molybdenum selenide films: I. Preparatory technique and structural characterization,” J. Phys. D: App. Phys., Vol. 17 (1984), pp. 2115-2123, propose an electro-deposition method of MoSe₂ films. While the article implies a MoSe₂ etch in basic solutions, no etch recipe is given, and in any event it does not describe a useful process for photovoltaic module fabrication.

Therefore, there remains a need in the art to overcome many of the shortcomings of the conventional processes for etching an underlying metal Mo layer in a thin-film photovoltaic device having CIGS material. The present invention aims at doing this, among other things.

The present invention provides a method of patterning a MoSe₂ and/or Mo material, for example a layer of such material(s) in a thin-film structure. According to one aspect, the invention relates to etch solutions that can effectively etch through Mo and/or MoSe₂. According to another aspect, the invention relates to etching such materials when such materials are processed with other materials in a thin film photovoltaic device. According to other aspects, the invention includes a process of etching Mo and/or MoSe₂ with selectivity to a layer of CIGS material in an overall process flow. According to still further aspects, the invention relates to Mo and/or MoSe₂ etch solutions that are useful in an overall photolithographic process for forming a photovoltaic cell and/or interconnects and test structures in a photovoltaic device.

In furtherance of these and other objects, a method of processing a thin-film structure according to the invention includes etching a thin film layer in the thin-film structure, wherein the thin film layer comprises molybdenum. In certain embodiments, the etched thin film layer further comprises selenium. In other embodiments, the etched thin film layer comprises Mo and MoSe₂. In additional embodiments, the method further includes defining a masking layer so as to pattern the thin film layer. In other embodiments, the thin-film structure includes a photovoltaic film such as CIGS.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:

FIGS. 1A and 1B illustrate certain aspects of the processing of thin-film structures including CIGS and an underlying conductor;

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