# Patent

## Title
Method for forming patterned doping regions

## Abstract

A method for forming doping regions is disclosed, including providing a substrate, forming a first-type doping material on the substrate and forming a second-type doping material on the substrate, wherein the first-type doping material is separated from the second-type doping material by a gap; forming a covering layer to cover the substrate, the first-type doping material and the second-type doping material; and performing a thermal diffusion process to diffuse the first-type doping material and the second-type doping material into the substrate.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This present application is based on, and claims priority of Taiwan Patent Application No 101135229, filed on Sep. 26, 2012, the disclosure of which is hereby incorporated by reference herein in its entirety.
TECHNICAL FIELD
The technical field relates to a method for forming patterned doping regions, and more particularly to a method for forming patterned doping regions of a solar cell.
BACKGROUND
In recent years, due to rise of environmental protection and global warming, the green industry has been greatly developed. Solar cells are a major technology in the green industry. Methods for developing a high efficiency, low-cost solar cell have become an important focus of research. Solar cells can be disposed on buildings, such as houses, and movable apparatus, such as cars; indoors, or on portable electric devices, to convert light into electrical power.
The conventional art develops solar cells with selective emitters. Emitters with a low concentration between electrodes can reduce the recombination of carriers at the surface of the cell, while emitters with a higher doping concentration under electrodes can provide good contact. Therefore, compared to conventional solar cells with emitting structures that have a constant doping concentration, solar cells with selective emitters have higher open loop voltage (Voc) and short-circuit current (Isc) and thus higher photoelectric conversion efficiency.
In 2010, Sunpower Company developed a solar cell with a back electrode having a cross-finger shape and an efficiency of 24.2%. Although this solar cell has a good conversion efficiency, it has a very high manufacturing cost due to its requiring many high-temperature and photolithography processes.
Both the solar cells with selective emitters and the solar cells with a back electrode having a cross-finger shape employ patterned doping regions to increase conversion efficiency. However, the process for forming patterned doping regions is more complicated and requires a highly accurate process, such as lithography, thus increasing the cost of the solar cell.
SUMMARY
An embodiment of the present disclosure provides a method for forming doping regions, comprising providing a substrate, forming a first-type doping material on the substrate, forming a second-type doping material on the substrate, wherein the first-type doping material is separated from the second-type doping material by a gap, forming a covering layer to cover the substrate, the first-type doping material, and the second-type doping material, and performing a thermal diffusion process to diffuse the first-type doping material and the second-type doping material into the substrate.
Another embodiment of the disclosure provides a method for forming doping regions, comprising providing a substrate, forming a doping material on the substrate, forming at least one patterned covering layer on a portion of a surface of the doping material, wherein the portion of the doping material is covered by at least one patterned covering layer, and another portion of the doping material not covered by the patterned covering layer is exposed, and performing a thermal diffusion process to diffuse the doping material into the substrate, forming a first doping region in a portion of the substrate underlying the patterned covering layer and forming a second doping region in a portion of the substrate not covered by the patterned covering layer, wherein the doping concentration of the first doping region is greater than the doping concentration of the second doping region.



BRIEF DESCRIPTION OF DRAWINGS
The disclosure can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein,
FIG. 1A ̃FIG. 1G show cross sections of intermediate stages of a method for forming a structure comprising p-type regions and n-type regions.
FIG. 2A ̃FIG. 2D show cross sections of intermediate stages of a method for forming a structure comprising p-type or n-type patterned doping regions.
FIG. 3A shows curves with time as a function of temperature for various examples of the disclosure.
FIG. 3B shows resistance for examples under various heating conditions.



DETAILED DESCRIPTION
In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a through understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing.
An embodiment of the disclosure forms p-type or n-type patterned doping regions using a coating of phosphoric acid or boric acid. The embodiment uses screen printing to take off photolithography and further uses a coating layer to avoid the spread of coated phosphoric acid or boric acid. An embodiment of the disclosure can use a single thermal diffusing step to get a p-type pattern region or an n-type pattern region.
A method for forming a structure comprising p-type regions and n-type regions is illustrated FIG. 1A ̃FIG. 1G. In an embodiment of the disclosure, the p-type and n-type pattern doping regions are used to form the cross-finger back electrodes of a solar cell. First, referring to FIG. 1A, a substrate 102 is provided. The substrate 102 may comprise silicon, gallium arsenide, gallium nitride, strained silicon, silicon germanium, silicon carbide, diamond, an epitaxy layer, and/or other materials. Next, a first mask 104 comprising first openings 106 is provided overlying the substrate 102. In an embodiment of the disclosure, the first mask 104 can be a screen. Thereafter, a first-type doping material 108 is formed into the first openings 106 in the first mask 104. The method for forming the first-type doping material 108 can be spray coating, spin coating, or screen printing. In an embodiment in which the first-type doping material 108 is p-type doping material, the first-type doping material 108 can be a boride, aluminide or gallide. Furthermore, the first-type doping material 108 can be boron glass, and can be formed by the following steps. Boric acid is coated on the first mask 104, such as a screen, and is filled into the first opening 106. Next, a thermal process is performed to convert the liquid-state boric acid to a colloidal-state or a solid-state compound, such as a colloidal-state or a solid-state boride. The thermal process can have a temperature of 200° C. ̃600° C. and can further have a temperature of 250° C. ̃350° C. In an embodiment in which the first-type doping material 108 is n-type doping material, the first-type doping material 108 can be a phosphide, arsenide or telluride. Furthermore, the first-type doping material 108 can be phosphorous glass, and can be formed by the following steps. Phosphorous acid is coated on the first mask 104, such as a screen, and is filled into the first opening 106. Next, a thermal process is performed to convert the liquid-state phosphoric acid to a colloidal-state or a solid-state compound, such as a colloidal-state or a solid-state phosphide. The thermal process can have a temperature of 200° C. ̃600° C. and can further have a temperature of 250° C. ̃350° C.
Referring to FIG. 1C, a second mask layer 110 comprising second openings 111 is formed overlying the substrate 102, wherein the second mask 110 can be a screen. Thereafter, a second-type doping material 112 is formed into the second opening 111 in the second mask 110, wherein the method for forming the second-type doping material 112 can be spray coating, spin coating or screen printing. In an embodiment of the disclosure, the first-type doping material 108 can be p-type doping material, and the second-type doping material 112 can be n-type doping material. In another embodiment of the disclosure, the first-type doping material 108 can be n-type doping material, and the second-type doping material 112 can be p-type doping material.
In an embodiment in which the second-type doping material 112 is n-type doping material, the second-type doping material 112 can be a phosphide, arsenide or telluride. Furthermore, the second-type doping material 112 can be phosphorous glass, and can be formed by the following steps. Phosphorous acid is coated on the second mask 110, such as a screen, and is filled into the second opening 111. Next, a thermal process is performed to convert the liquid-state phosphoric acid to a colloidal-state or a solid-state compound, such as a colloidal-state or a solid-state phosphide. The thermal process can have a temperature of 200° C. ̃600° C. and can further have a temperature of 250° C. ̃350° C. In an embodiment in which the second-type doping material 112 is p-type doping material, the second-type doping material 112 can be a boride, aluminide or gallide. Furthermore, the second-type doping material 112 can be boron glass, and can be formed by the following steps. Boric acid is coated on the second mask 110, such as a screen, and is filled into the second opening 111. Next, a thermal process is performed to convert the liquid-state boric acid to a colloidal-state or a solid-state compound, such as a colloidal-state or a solid-state boride. The thermal process can have a temperature of 200° C. ̃600° C. and can further have a temperature of 250° C. ̃350° C.
Next, referring to FIG. 1D, the second mask 110 is removed. In an embodiment of the disclosure, the first-type doping material 108 and the second-type doping material 112 are separated by a distance d, and the distance d can be 5 μm ̃30 μm.
Referring to FIG. 1E, a covering layer 114 is formed on the substrate 102, the first-type doping material 108, and the second-type doping material 112. The method for forming the covering layer 114 can be spray coating, spin coating, screen printing, plasma enhanced chemical vapor deposition (PECVD), or atomic layer deposition (ALD). In an embodiment of the disclosure, the covering layer 114 can be SiNx or Al2O3. Al2O3 is a preferable material for the covering layer 114 since Al2O3 does not substantially react with boron doping material or phosphorous doping material, and Al2O3 is stable at high temperatures and is easily removed. Furthermore, Al2O3 does not substantially affect the diffusion of boron doping material and phosphorous doping material into the substrate. Compared to silicon oxide, using Al2O3 as the covering layer in the embodiment can avoid affecting the diffusion of boron or phosphorous.
Next, referring to FIG. 1F, a thermal diffusion process is performed to diffuse the first-type doping material 108 into the substrate 102 to form a first doping region 116, and diffuse the second-type doping material 112 into the substrate 102 to form a second doping region 118. In an embodiment of the disclosure, the temperature of the thermal diffusion process can be 800° C. to 1000° C., and further can be 700° C. to 1200° C. It is noted that since the first-type doping material 108, the second-type doping material 112, and the substrate 102 are covered by the covering layer 114 in the embodiment, the lateral diffusion of the first-type doping material 108 and the second-type doping material 112 is limited. Therefore, the first-type doping material 108 and the second-type doping material 112 substantially diffuse downward to increase the accuracy of the first doping region 116 and the second doping region 118. Thereafter, referring to FIG. 1G, the covering layer 114, the first-type doping material 108 and the second-type doping material 112 are removed. In an embodiment of the disclosure, removing the covering layer 114, the first-type doping material 108 and the second-type doping material 112 can be accomplished by immersion in a fluorine-containing solution.
A method for forming a structure comprising p-type or n-type patterned doping regions is illustrated FIG. 2A ̃FIG. 2D. In an embodiment of the disclosure, the p-type regions or n-type patterned doping regions are used to form selective emitters of a solar cell. First, referring to FIG. 2A, a substrate 202 is provided. The substrate 202 may comprise silicon, gallium arsenide, gallium nitride, strained silicon, silicon germanium, silicon carbide, diamond, an epitaxy layer, and/or other materials. Next, a doping material 204 is formed on the substrate 202. In an embodiment in which the doping material 204 is p-type doping material, the doping material can be a boride, aluminide or gallide. Furthermore, the doping material 204 can be boron glass, and can be formed by the following steps. Boric acid is coated on the substrate. Next, a thermal process is performed to convert the liquid-state boric acid to a solid-state compound, such as a solid-state boride. In an embodiment in which the doping material 304 is n-type doping material, the doping material can be a phosphide, arsenide or telluride. Furthermore, the doping material can be phosphorous glass, and can be formed by the following steps: Phosphorous acid is coated on the substrate, and next a thermal process is performed to convert the liquid-state phosphoric acid to a solid-state compound, such as a solid state phosphide. Thereafter, referring to FIG. 2B, a patterned covering layer 206 is formed on the doping material 204, wherein the patterned covering layer 206 can be formed by screen printing, and can be SiNx or Al2O3. Al2O3 is a preferable material for the patterned covering layer 206 since Al2O3 does not react substantially with the doping material 204, and does not substantially affect the diffusion of the doping material 204.
Thereafter, referring to FIG. 2C, a thermal process is performed to diffuse the doping material 204 into the substrate 202, forming a first doping region 208 in a portion of the substrate 202 under the patterned covering layer 206 and forming a second doping region 210 in a portion of the substrate 202 uncovered by the patterned covering layer 206. It is noted that since the doping material 204 under the patterned covering layer 206 is covered, the doping material 204 does not evaporate to diffuse upward, thus the major portion of the doping material 204 covered by the patterned covering layer 206 diffuse downward, but the doping material 204 not covered by the patterned covering layer 206 can evaporate to diffuse upward. Therefore, the first doping region 208 under the patterned covering layer 206 has a higher doping concentration and lower resistance than the second doping region 210 uncovered by the patterned covering layer 206. Therefore, the embodiment can form the first doping region 208 and the second doping region 210 with different doping concentrations and resistance using a single thermal diffusion process. Next, referring to FIG. 2D, the doping material 204 is removed. In an embodiment of the disclosure, the doping material 204 can be removed by doping a solution containing fluorine.
Accordingly, with the formation of the patterned covering layer 206 on the doping material 204, the embodiment can form the first doping region 208 and the second doping region 210 with a different doping concentration using a single thermal diffusion process. The relations between condition of thermal diffusion and resistance of the first doping region 208 and the second doping region 210 are illustrated in accordance with FIG. 3A and FIG. 3B. FIG. 3A shows curves with time as a function of temperature for various examples of the disclosure. FIG. 3B shows resistance of the examples under various heating conditions. In FIG. 3A and FIG. 3B, A0 corresponds to resistance of the first doping region covered with a patterned covering layer. A1 corresponds to resistance of the second doping region uncovered with a patterned covering layer and with a heating condition to raise the temperature to 875° C. in one minute. A3.5 corresponds to resistance of the second doping region uncovered with a patterned covering layer and with a heating condition to raise the temperature to 875° C. in 3.5 minutes. A5 corresponds to resistance of the second doping region uncovered with a patterned covering layer and with a heating condition to raise the temperature to 875° C. in 5 minutes. A7 corresponds to resistance of the second doping region uncovered with a patterned covering layer and with a heating condition to raise the temperature to 875° C. in 7 minutes. According to FIGS. 3A and 3B, the second doping region covered with a patterned covering layer has the lowest resistance, and the condition has a longer rising time to 875° C., presenting a higher resistance. According to the results from FIG. 3A and FIG. 3B, the embodiment can fine tune the duration to the rise in temperature to a degree, such as 875° C., for the first doping region and the second doping region to achieve a predetermined resistance. In addition, the disclosure can further adjust the thickness and concentration of the doping material to tune the concentration of the first doping region and the second doping region.
While the disclosure has been described by way of example and in terms of the embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments. It is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

## Claims

1. A method for forming doping regions, comprising:
providing a substrate;
forming a first-type doping material on the substrate, wherein the step of forming the first-type doping material on the substrate comprises providing a first mask comprising a first opening on the substrate, coating the first-type doping material on the mask and filling the first opening, performing a thermal process to convert the first-type doping material to a colloidal-state or solid-state compound, and removing the first mask;
forming a second-type doping material on the substrate, wherein the first-type doping material is separated from the second-type doping material by a gap, wherein the step of forming the second-type doping material on the substrate comprises providing a second mask comprising a second opening on the substrate, coating the second-type doping material on the mask and filling the second opening, performing a thermal process to convert the second-type doping material to a colloidal-state or solid-state compound, and removing the second mask;
forming a covering layer to cover the substrate, the first-type doping material, and the second-type doping material; and
performing a thermal diffusion process to diffuse the first-type doping material and the second-type doping material into the substrate.

2. The method for forming doping regions as claimed in claim 1, wherein the first-type doping material is n-type doping material or p-type doping material, the second-type doping material is p-type doping material or n-type doping material, and the first-type doping material and the second-type doping material have reverse conductivity type.

3. The method for forming doping regions as claimed in claim 2, wherein the p-type doping material is a boride, aluminide or gallide, and the n-type doping material is a phosphide, arsenide or telluride.

4. The method for forming doping regions as claimed in claim 1, wherein the covering layer is SiNx or Al2O3.

5. The method for forming doping regions as claimed in claim 1, wherein the step of forming the covering layer comprises spray coating, spin coating, screen printing, plasma enhanced chemical vapor deposition (PECVD), or atomic layer deposition (ALD).