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  Method of making thin silicon sheets for solar cellsUSPTO Application #: 20070093026Title: Method of making thin silicon sheets for solar cells Abstract: A thin layer of single-crystal silicon is produced by forming first trenches in a silicon substrate having (111) orientation; forming narrower second trenches at the base of the trenches; anisotropically etching lateral channels (4) from the second trenches, until adjacent etch fonts (16) substantially meet; and detaching said layer from the substrate. The trenches may be arranged so that the resultant layer has rows of slots, with the slots in adjacent rows being mutually offset. Solar cells may be formed on strips having two electrical contacts on the same face (6) of each strip (5). (end of abstract) Agent: Woodcock Washburn LLP - Philadelphia, PA, US Inventors: Andrew William Blakers, Klaus Johannes Weber USPTO Applicaton #: 20070093026 - Class: 438258000 (USPTO) Related Patent Categories: Semiconductor Device Manufacturing: Process, Making Field Effect Device Having Pair Of Active Regions Separated By Gate Structure By Formation Or Alteration Of Semiconductive Active Regions, Having Insulated Gate (e.g., Igfet, Misfet, Mosfet, Etc.), Having Additional Gate Electrode Surrounded By Dielectric (i.e., Floating Gate), Including Additional Field Effect Transistor (e.g., Sense Or Access Transistor, Etc.) The Patent Description & Claims data below is from USPTO Patent Application 20070093026. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a divisional application of U.S. application Ser. No. 10/497,369, filed Nov. 18, 2004, which is a 35 U.S.C. .sctn. 371(c) national stage application of PCT/AU02/01640, filed Dec. 4, 2002, which claims the benefit of Australian Patent Application Serial No. PR 9307 filed Dec. 4, 2001. The disclosures of each of these patent applications are hereby incorporated by reference in their entirety. FIELD OF THE INVENTION [0002] This invention relates to processes for producing thin crystalline layers of silicon, to the fabrication of solar cells from these layers, and to solar cells fabricated from these layers. BACKGROUND OF THE INVENTION [0003] In most areas of semiconductor processing, the cost of the starting substrate wafer is small compared to the value of the final, processed wafer. However, this is not always the case. For example, the photovoltaic solar cell industry is extremely cost sensitive, and the cost of a starting silicon wafer is typically nearly half of the value of the finished photovoltaic module. Thus, in this industry it is extremely important that the silicon substrates are used as efficiently as possible. These substrates are produced by sawing thin slices from a cylindrical boule of crystalline silicon. The thinnest slice that can be cut is determined by the mechanical properties of the silicon and the sawing technology, and is typically 300-400 .mu.m for the current generation of wafers, but is projected to be 200 .mu.m for the next wafer generation. However, the kerf loss in sawing the wafer is approximately 250 .mu.m, meaning that most of the boule ends up as powder. It is desired, therefore, to provide a more efficient way of producing thin single crystal silicon layers. SUMMARY OF THE INVENTION [0004] In accordance with the present invention, there is provided a process for producing a layer of single-crystal silicon, said process comprising the steps of: [0005] forming parallel, first trenches in a surface of a silicon substrate of substantially (111) orientation; [0006] applying an etch-resistant layer to the surface of said first trenches; [0007] forming a series of narrower second trenches inside said first trenches; [0008] contacting said second trenches with an etchant to which {111} planes of silicon are substantially resistant, and etching lateral channels from said second trenches until adjacent and opposing etch fronts at least substantially meet, thereby defining said layer of single-crystal silicon; and [0009] detaching said layer from said substrate. [0010] The layer of single-crystal silicon may comprise a plurality of strips of silicon, which optionally may be attached to the layer at one or both ends of the strips. The processes of the invention may further comprise the step of separating the strips from the layer of silicon. In a preferred form of the invention, the process further comprises the steps of forming a plurality of solar cells from the plurality of strips before or after the strips are separated. [0011] The invention further provides a layer of single-crystal silicon produced by a process of the invention. [0012] The invention still further provides a solar cell produced by a process of the invention. [0013] In a second embodiment, the invention provides a sheet of silicon having a thickness of up to about 100 microns, said sheet including a plurality of parallel rows of slots therethrough, each of said rows containing a plurality of said slots arranged endwise therein, wherein the slots in each row are offset with respect to the slots in an immediately is adjacent row. DETAILED DESCRIPTION OF THE INVENTION [0014] In the present invention, by "surface of a silicon substrate of substantially (111) orientation" is meant that the silicon substrate is either of precisely (111) orientation, or otherwise sufficiently close to (111) orientation that when the lateral channels have been etched, adjacent channels either meet or approach each other sufficiently closely that the layer may be detached from the substrate by the application of force substantially without damaging the layer. Typically the surface of the silicon substrate is at an angle of from 0.degree. to about 10.degree. from the (111) orientation, more typically from 0.degree. to about 7.degree., more typically from 0.degree. to about 4.degree.. [0015] Preferably, adjacent and opposing etch fronts meet to provide for separation of the layer of silicon which will no longer be joined to the substrate when the etch fronts meet. Alternatively, the etching proceeds until adjacent and opposing etch fronts almost meet and the layer of silicon is then mechanically detached by breaking of the remaining connection(s) between the layers and the substrate. Thus, by "substantially meet" is meant that the adjacent and opposing etch fronts approach each other sufficiently closely that the layer of silicon present above the lateral channels remains attached to the substrate but is readily removable from it. [0016] In one form of the invention, the first and second trenches extend continuously from one edge of the silicon substrate to an opposite edge to produce a plurality of thin strips of silicon after the steps of etching the lateral channels and detachment of the layer. In another form of the invention, the first and second trenches do not extend as far as at least one edge, more typically both edges, of the silicon substrate so that a series of strips is formed by the etching step, but the strips remain attached either at one end or at both ends to part of the layer of silicon. The strips may be separated from the layer of silicon after they have been subjected to any further processing steps that may be desired, by cutting their end or ends from the remainder of the layer. In still another form of the invention, the trenches are arranged in parallel rows, each row of trenches containing a plurality of trenches arranged endwise in them and the trenches in each row being staggered or offset with respect to the trenches in an immediately adjacent row. None of the trenches extends the whole way across a surface of the silicon substrate, so that a single crystal silicon sheet is formed after etching and detachment. The single sheet is exhibits a pattern of slots identical to the pattern of trenches in the silicon substrate before the etching step. The rows of trenches are spaced from each other by a distance of less than 0.3 times the length of each trench. Each trench is typically et least about 0.5 mm long, more typically in the range of from 2 to 10 mm. [0017] The first trenches can, for example, be formed using a laser, a dicing saw or by an etching process such as plasma etching or wet etching in combination with a suitably patterned etch mask. The narrower trenches can for example be formed using a laser, a dicing saw or by an etching process such as plasma etching or wet etching in combination with a suitably patterned etch mask. [0018] The first trenches are typically from about 1 micron to about 100 microns deep, more typically from about 20 microns to about 70 microns deep, and typically range in width from about 50 microns to about 200 microns. Thus, the layer of single crystal silicon is typically from about 1 micron to about 100 microns in thickness. [0019] The first trenches are most usually about 0.1 mm wide. They may be spaced from about 0.1 mm to several millimetres apart. Typically, the spacing of the first trenches is from about 0.5 mm to about 5 mm, more typically from about 0.5 mm to about 2 mm. [0020] The narrower trenches range in depth from less than one micron to tens of microns, more typically from about 1 micron to about 5 microns. Usually, the narrower trenches occupy from about 25% to about 90% of the width of the first trenches. [0021] The etch-resistant layer formed on the first trenches is a layer of a material that is etched at a much slower rate by the etchant used to etch the lateral channels, compared to the rate of etching of the silicon. As a result, the etch-resistant layer is intact after the step of etching the lateral channels (although it may be thinned to some extent) so that the only places where the silicon is etched are in the second trenches, where there is no etch resistant layer on the silicon. [0022] Suitable anisotropic etchants for silicon to which {111} planes are etch-resistant are well known to persons of ordinary skill in the art and include aqueous potassium hydroxide solution, for example of from about 20-50 wt %, more usually about 30 wt %, concentration. Other etchants that may be used include tetramethylammonium hydroxide solutions and sodium hydroxide solutions. [0023] Suitable etch-resistant layer materials include silicon dioxide and silicon nitride. [0024] It will be apparent that the present invention allows ordinary silicon wafers to be processed to produce thin rectangular strips of silicon or a silicon sheet, the thickness of the strips or the sheet being much smaller than the wafer thickness. The individual strips or sheet can be processed into solar cells and then detached from the substrate. Following this process, a layer of substrate is removed which is much thinner than can be achieved by prior art cutting processes, with much less waste than using prior art processes. Because the substrate has been reduced in thickness by an amount much less than the original substrate thickness, the remainder can be re-used for the production of further thin silicon strips or sheet. Thus, from a single silicon wafer it is possible to obtain at least two, and typically several, thin layers for use (for instance) in solar cells. Continue reading... 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