This disclosure relates in general to the field of photovoltaics and microelectronics, and more particularly to fabrication processes and manufacturing equipment for forming porous silicon layers on at least two surfaces of a plurality of silicon templates. This disclosure provides for such manufacturing at low costs and high throughputs.
Currently, crystalline silicon has the largest market share in the photovoltaics (PV) industry, accounting for over 80% of the overall PV market share. Although going to thinner crystalline silicon solar cells is long understood to be one of the most potent ways to reduce PV cost (because of the relatively high material cost of crystalline silicon wafers used in solar cells as a fraction of the total PV module cost), it is fraught with the problem of mechanical breakage due to the thin and large substrate sizes, and also to some extent that of light trapping in a thin structure (since silicon is an indirect bandgap semiconductor material). The requirement of high mechanical yield and reduced wafer breakage rate is further problematic with the realization that for cost-effectiveness, the yields in PV manufacturing factories must be very high. On a standalone crystalline silicon solar cell (without support), going even somewhat below the current thickness range of 140-250 microns starts to severely compromise mechanical yield during manufacturing. Thus, any solution to process very thin solar cell structures must either be fully supported by a host carrier throughout the cell process or use a novel self-supporting, standalone, substrate with an accompanying structural innovation.
Innovative manufacturing processes to produce solar cells at low costs using thin-film semiconductor substrates (TFSSs) as the active material have been disclosed in related applications. In some embodiments, the TFSSs comprise crystalline semiconductor (more specifically silicon in some embodiments). This technology platform produces solar cells from re-usable semiconductor templates. In certain embodiment, the surfaces of the templates have a periodic array of pre-structured 3-dimensional (3-D) structures. Examples of the 3-D TFSS based solar cells include but are not limited to prism honey-comb and inverted pyramidal cavities, which are described in earlier applications having common inventorship and/or ownership. The following are examples of related applications.
U.S. Patent Publications US2008/0157283 A1, US2008/0289684 A1, US2010/0148318 A1, US2010/0300518; U.S. patent application Ser. No. 13/057,104; PCT Application Serial Nos. PCT/US10/60591, PCT/US10/62614.
As disclosed by the above documents, the epitaxial substrate is grown on top of a reusable template and is subsequently dislodged. In one embodiment, the template and substrate each comprise monocrystalline silicon. Afterwards, the reusable template may be reused several times, with or without reconditioning, to grow more epitaxial substrates. The reusable template has a planar top surface or a top surface with pre-formed 3-D microstructures, such as hexagonal honey-comb cavities or inverted pyramidal cavities. The releasing of the 3-D TFSS is achieved with an interim sacrificial layer. The sacrificial layer should satisfy two important criteria. First, it needs to transfer the information on crystallinity from the reusable template to the epitaxial layer. Second, it should be able to be removed selectively compared to the substrate and the reusable template. One specific embodiment of the sacrificial layer is porous silicon, whose porosity can be modulated to achieve both the aforementioned critical functions.
As disclosed by the above documents, the planar or 3-D TFSSs are made and released from one-side, i.e., the top surface of a reusable template. In other words, one solar substrate is made from a reusable template in each of its reuse cycles. In those disclosures, the template and substrate making equipment, and the porous silicon forming and epitaxial silicon growing equipment is capable of processing on only one side of the silicon template. The equipment is capable of processing multiple wafers at a time in a batching processing mode, but only one side of each template is used.
Solar cell manufacturing requires much higher productivity with much lower costs compared with semiconductor processes. Therefore, there it may be advantageous to develop manufacturing processes and equipment capable of making thin semiconductor substrates from both sides of a reusable template simultaneously.
In order to reduce the thin-film crystalline semiconductor (such as silicon) substrate manufacturing costs and increase the production throughputs, this disclosure provides manufacturing methods and apparatus designs for forming porous silicon layers on at least two surfaces of a plurality of silicon templates, thus effectively increasing the substrate manufacturing throughput and reducing the substrate manufacturing cost. This approach also reduces the amortized starting template cost per manufactured substrate (TFSS) by about a factor of two for a given number of template reuse cycles.
In accordance with the present disclosure, an apparatus for forming porous silicon layers on at least two surfaces of a plurality of silicon templates in a batch electrochemical anodic etch process is provided. The apparatus comprises a plurality of edge-sealing template mounts operable to prevent formation of porous silicon at the edges of a plurality of templates. An electrolyte is disposed among the plurality of templates. The apparatus further comprises a power supply operable to switch polarity, change current intensity, and control etching time to produce the porous silicon layers.
In yet another embodiment, the apparatus for forming porous silicon layers on at least two surfaces of a plurality of silicon templates in a batch electrochemical anodic etch process further comprises an electrolyte temperature controller.
In yet another embodiment, the apparatus for forming porous silicon layers on at least two surfaces of a plurality of silicon templates in a batch electrochemical anodic etch process further comprises an external liquid circulation and gas bubble extractor.
In yet another embodiment, an apparatus for forming porous silicon layers on at least two surfaces of a plurality of silicon templates in a batch electrochemical anodic etch process is provided. The apparatus comprises a plurality of edge- sealing template mounts operable to prevent formation of porous silicon at the edges of a plurality of templates. An electrolyte is disposed among the plurality of templates. The apparatus further comprises an electrolyte temperature controller. The apparatus further comprises an external liquid circulation and gas bubble extractor. The apparatus further comprises a power supply operable to switch polarity, change current intensity, and control etching time to produce the porous silicon layers.
In yet another embodiment, the template may be reconditioned and cleaned as needed followed by a reuse cycle. Each template in this disclosure may be used multiple cycles until it becomes too thin to handle or until it loses its structural properties. In those cases, the used templates may be recycled.
The features, nature, and advantages of the disclosed subject matter will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference numerals indicate like features and wherein:
Although the present disclosure is described with reference to specific embodiments, one skilled in the art could apply the principles discussed herein to other areas and/or embodiments without undue experimentation.
The epitaxial layer on the wafer edge may be removed (as shown in
Alternatively, the epitaxial growth is prevented from growing at template edges by mechanical shadowing. In another alternative method, the TFSS edges are defined by laser trench cutting with the cutting depth is no larger than approximately the epitaxial layer thickness. In this method, the remaining epitaxial silicon layers at template edges may be removed after TFSS release by template edge lapping, grinding or polishing. As illustrated in
Alternatively, the epitaxial layer is prevented from growing at template edges by mechanical shadowing. In another alternative method, the TFSS edges are defined by laser trench cutting with the cutting depth no larger than approximately the epitaxial layer thickness. In this method, the remaining epitaxial silicon layers at template edges may be removed after the TFSS releasing by template edge lapping, grinding or polishing.
As illustrated in
At step 52, a porous silicon layer is formed by electrochemical anodic HF etching of silicon on both the front and back sides of the template surfaces. During the porous silicon forming in an HF/IPA (or HF/acetic acid) solution, the electrical current polarity is periodically switched between positive and negative currents so that each template side is successively etched in order to form the bilayer or multi-layer porous silicon structure on both template sides. In addition to the periodical current polarity switching, the current intensity is also changed in a controlled manner to form a porous silicon bilayer or multilayer that consists of the desired different (low and high) porosities. The first thin porous silicon layer is on the top and is first formed from the bulk silicon wafer. The first thin layer has a lower porosity of approximately 15% to 30%. The second thin porous silicon layer is directly grown from the bulk silicon and is underneath the first thin layer of porous silicon. The second thin porous silicon layer may have a higher porosity in the range of approximately 40%-80%. The top lower porosity layer is used as a crystalline seed layer for high quality epitaxial silicon growth, and the underneath higher porosity porous silicon layer is used for facilitating TFSS releasing due to its low-density physical connections (between the epitaxial and bulk silicon interfaces) and its weak mechanical strength.
Alternatively, a single porosity release layer with a progressively increased porosity from top to bottom can also be used. In this case, the top portion of the porous silicon layer has a low porosity of approximately 15% to 30%, and the lower portion of the porous silicon layer has a high porosity of approximately 40% to 80%, with a region of graded porosity in between.
At step 54, and before the epitaxial silicon growth, the wafer is baked in a high temperature (at approximately 950° C. to 1200° C., and more specifically in some embodiments in the range of 1050° C. to 1150° C.) hydrogen environment within the epitaxial silicon deposition reactor in order to form coalesced structures (with relatively large voids) within the higher-porosity buried porous silicon layer while forming a continuous surface seed layer of crystalline silicon on the top of the lower-porosity porous silicon layer.
Next, at step 56, a mono-crystalline epitaxial silicon layer is concurrently deposited on both sides of the template, preferably in a high-throughput large-batch epitaxial furnace. The epitaxial layer may be in-situ doped. For example for solar cell applications, the bulk base of the epitaxial layer may be n (phosphorous) doped, the inner layer may be n+ (phosphorous) doped to form the FSF, and the outer layer may be p (boron) doped to form the emitter region of the solar cell. The thickness of the epitaxial layer is in the range of 1 to 100 microns.
In optional step 58, a thin oxide layer is thermally grown on the epitaxial surface. The thin oxide layer may be used for solar cell surface passivation or a masking layer for subsequent selective surface openings.
In optional step 60, temporary or permanent reinforcement plates are mounted on the front and back silicon surfaces. The reinforcement plates also serve as carriers for enabling handling and processing thin TFSSs using commercially available solar cell manufacturing equipment, as disclosed in PCT application serial nos. PCT/US10/60591 and PCT/US10/62614.
Next, at step 62, the epitaxial layer on the wafer edge is removed by template edge lapping, grinding or polishing, to expose the buried porous silicon layer so that the epitaxial layers may be released from the template. Alternatively, the epitaxial growth is prevented from growing at template edges by mechanical shadowing. In another alternative method, as disclosed in related applications, the TFSS edges are defined by laser trench cutting the cutting depth no larger than approximately the epitaxial layer thickness.
In step 64, the optionally reinforced epitaxial layer of silicon is released/separated from the template, and the released epitaxial silicon layer is therefore referred as a thin film silicon substrate. Related applications disclose detailed methods of releasing the epitaxial layer to form a TFSS. In one of the disclosed methods, the TFSS is released in an ultrasonic DI-water bath. In another disclosed method, the TFSS is released by direct pulling with the wafer backside and the top epitaxial layer vacuum chucked.
In step 66, the released TFSS backside surface is cleaned by short silicon etching using KOH or TMAH solutions to remove the silicon debris and fully or partially remove the quasi-monocrystalline silicon (QMS) layer. In the meantime and after removal of the edge epitaxial silicon layer from the template, the template is cleaned by using diluted HF and diluted wet silicon etch solution, such as TMAH and/or KOH to remove the remaining porous silicon layers and silicon particles. Then the template is further cleaned by conventional silicon wafer cleaning methods, such as SC1 and SC2 wet cleaning (or the so-called RCA cleaning process) to removal possible organic and metallic contaminations. Finally, after proper rinsing with DI water and N2 drying, the template is ready for another re-use cycle.
The template fabrication process starts at step 90 with a mono-crystalline (100) silicon wafer. In step 92, photoresist layers 93 are coated and patterned on both the front and back side surfaces of the template. Alternatively, resist layers could be screen printed.
Next, at step 94, the front and back side silicon trenches are etched in a deep-reactive-ion-etch (DRIE) or a reactive-ion-etch (RIE) process using silicon etching gases, such as SF6. In the DRIE process, periodic surface passivation using C4F8 gas is performed to control the trench sidewall surfaces. The trench etching is time-controlled. After the trench etching, the remaining resist layer is removed at step 96, and all the exposed silicon surfaces are fully cleaned at step 98. Optionally, the trench sidewall rough surfaces are slightly etched to make them smoother. At step 100, the reusable template is complete.
The 3-D TFSS manufacturing process of using the double-sided honeycomb template is substantially the same as the planar TFSS making process, except the reinforcement plates may not be needed prior to the TFSS releasing. For simplicity, the 3-D TFSS making process is not shown in figures.
Next, at step 114, photolithographic defined or screen-printed photoresist pattern is applied on the front and back wafer surface. The photolithography process includes photoresist coating, baking, exposure, developing and post baking. The photoresist pattern consists of staggered large square openings and small square openings. It is important the square-opening patterns are precisely aligned to the wafer <100> direction on the front surface.
The patterned resist pattern is then transferred at step 116 to the hard masking layer, i.e. the front oxide layer. The pattern transferring from the photoresist layer to the oxide layer is achieved by controlled oxide etching in a buffered HF solution. During the HF wet etching, the front and back oxide opening are etched in the same time and the edge oxide layer is fully protected and kept at its original thickness. The oxide pattern on the wafer surfaces also consists of staggered large and small square openings that are aligned to the <100> crystallographic directions.
After the pattern transfer, at step 118, the remaining photoresist layer is removed by wet or dry photoresist removal methods. Next, at step 120, silicon anisotropic etching is conducted by a time-controlled KOH, NaOH, or TMAH etching that results in large pyramidal silicon cavities. The etching temperature may be in the range of approximately 50° C. to 80° C. The exposed silicon surfaces on the front and back sides are etched in the same time and the wafer edge surfaces are fully protected by the un-patterned oxide layer. The KOH etch may be time-controlled so that a certain pyramidal cavity depth may be reached. Alternatively, the KOH etching may be self-terminated when the four pyramid (111) sidewalls meet at the cavity bottom.
After the KOH etching, the remaining oxide layer is thinner than before the etching since the oxide is also etched to some extent in the KOH or TMAH solution, albeit with a much slower etch rate than silicon etch. The remaining oxide layer is then removed at step 122 in a diluted HF solution followed by standard wafer cleaning in SC2 and SC2, DI water rinsing and N2 drying. At step 124, reusable pyramidal double-sided template is finished.
The 3-D TFSS manufacturing process of using the double-sided pyramidal template is same as the planar TFSS making process, except the reinforcement plates may not be needed prior to the TFSS releasing. For simplicity, the 3-D TFSS making process is not shown in figures.
This edge-holding method prevents broken porous silicon debris from accumulating during the template loading and unloading process. The loaded templates are spaced uniformly in the reaction chamber. With the distributed gas injection layout, reactive gases are evenly delivered from gas injector 144 to all the exposed silicon surfaces and exhausted via exhaust 146, enabling uniform epitaxial growth within a single wafer and among wafers. The reaction chamber is heated to a high temperature in the range of 950° C. to 1200° C. during the process. The heating and its control function are not shown in
Those with ordinary skill in the art will recognize that the disclosed embodiments have relevance to a wide variety of areas in addition to those specific examples described above.
The foregoing description of the exemplary embodiments is provided to enable any person skilled in the art to make or use the claimed subject matter. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the innovative faculty. Thus, the claimed subject matter is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
It is intended that all such additional systems, methods, features, and advantages that are included within this description be within the scope of the claims.
This application is a continuation of U.S. Non-Provisional Ser. No. 13/026,239, filed Feb. 12, 2011 which claims priority to U.S. Provisional Patent Application Ser. No. 61/304,340, filed Feb. 12, 2010, both of which are hereby incorporated by reference in its entirety. Other related applications having common inventorship and/or ownership are mentioned throughout this disclosure, and they are also incorporated by reference in their entirety.
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Number | Date | Country | |
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20130171808 A1 | Jul 2013 | US |
Number | Date | Country | |
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61304340 | Feb 2010 | US |
Number | Date | Country | |
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Parent | 13026239 | Feb 2011 | US |
Child | 13554103 | US |