GB2188774A - Method of forming a conductive pattern on a semiconductor surface - Google Patents

Abstract

Portions 5 of a semiconductor surface are exposed to light from a laser at a predetermined power density and a wavelength of e.g. 5000 angstroms and the exposed surface of the semiconductor is immersed in a plating solution of a plateable metal whereby a metal is plated 4 onto the exposed portions of the semiconductor. The method can be used where the semiconductor is first coated with a refractory metal, or a noble metal, or with a refractory metal followed by a noble metal. The metal may be electroplated or electroless plated. A solar cell may be formed. <IMAGE>

Description

SPECIFICATION Method of forming a conductive pattern on a semiconductor surface This invention relates to a method of forming a conductive pattern on a semiconductor surface.

Solar cells are presently being made by a method that involves photolithography. In that method, coatings, first of titanium and then of palladium, and finally of silver, are applied by an evaporation process to a surface of a doped silicon wafer. The wafer is then coated with a photo-resist, a glass mask is placed over the photo-resist, and the photo-resist is exposed to ultraviolet light. Those portions of the photo-resist that have either been exposed to the light or that have not been exposed to the light are then removed, usually by dissolution in a solvent, and the metal layers exposed are etched away. The remaining photo-resist is stripped off, and the sliver pattern is then plated with silver to build it up to the requisite thickness.

While this method produces satisfactory conducting circuit patterns on the silicon, it is expensive and time-consuming because so many steps are involved. It would greatly reduce the cost of solar cells and increase their usefulness if a method could be found to form conductive patterns on doped silicon that did not require all of the steps involved in the photolithography process.

Accordingly, a method of forming a method of forming a conductive pattern on the surface of a semiconductor comprising exposing portions of said surface to light from a laser at a predetermined power density; and immersing said surface in a plating solution of a plateable metal, whereby a plateable metal is plated onto said laser light-exposed portions of said semiconductor surface. F We have discovered a process for forming conductive patterns on doped silicon for use in making solar cells that does not involve photolithography, nor does it necessarily even involve the deposition of titanium and palladium layers onto the silicon.That is, we have discovered, quite by accident, that laser light of a particular power density and wavelength will activate a silicon surface in such a way that a conductive pattern of silver can be applied directly to the silicon surface. Previously, the direct application of silver to the silicon surface was not possible because silver did not adhere well to silicon. However, exposure of the silicon surface to laser light of particular power density and wavelength in some way activates the exposed surface so that the silver will adhere to it. We are therefore able to eliminate the application of a photoresist to the surface, as well as the deposition of the titanium and palladium layers.However, we have also found that laser light of differing power densities will activate the titanium and palladium layers as well, so that the silver will adhere only to those portions of the titanium or palladium that have been exposed to the laser light. Thus, it is also possible to form a conductive circuit on a layer of titanium that has been applied to the silicon or to apply a conductive layer on a layer of palladium on top of a layer of titanium on the silicon wafer.

By eliminating the photolithography step as well as the application of the titanium and palladium layers, we are able to form a solar cell by a method that is much less time consuming and less expensive than the prior photolithography method. Even if the titanium and palladium layers are used, the process of this invention is still less expensive and less time consuming than the photolithography method, because the application of the photoresist and its removal are eliminated.

In order that the invention can be more clearly understood, convenient embodiments thereof will now be described, by way of example, with reference to the accompanying drawings in which: Figures 1, 2, and 3 are isometric views, partially in section, illustrating three embodiments of solar cells, Figure 4 is a graph showing the relationship between current and voltage in a solar cell made according to a process of this invention.

In the embodiment of Figure 1, a silicon wafer 1 has a portion 2 doped negative (or positive) and another portion 3 oppositely doped. A plateable metal 4, such as silver, is applied directly upon those portions 5 of silicon wafer 1 that were exposed to laser light, forming circuit pattern 6 on the surface of the silicon wafer 1; anti-reflective coating 7 is applied over the remainder of the surface.

The embodiment of Figure 2 is identical to Figure 1 except that a very thin layer 8 of a refractory metal or a noble metal is applied to the surface of silicon wafer 1, and a layer 9 of a plateable metal is applied over those portions 10 of refractory or noble metal layer 8 that were exposed to laser light, thereby forming circuit pattern 11; antireflective coating 12 is applied in between circuit pattern 11.

The embodiment of Figure 3 is identical to Figure 2, except that a layer 13 of a noble metal is applied over refractory metal layer 8. Over those portions 14 of noble metal layer 13 that were exposed to laser light is applied plateable metal 15, forming circuit pattern 16; anti-reflective coating 17 fills spaces in the circuit pattern.

The method of this invention can be applied to any semiconducting material such as, for example, silicon, germanium, and gallium-arsenide. Silicon is the preferred semiconducting material because the method of this invention has been found to work very well with silicon. The silicon should be single crystal silicon, but it can be formed by a variety of methods, including the Czochralski method, the float-zone method, or the dendritic web method. The silicon can be doped with various p and n type dopants including boron, phosphorus, nitrogen, etc. The semiconducting material can have almost any surface configuration including flat or curved as well as any size or shape, so long as the areas on which the conducting pattern is to be formed can be exposed to the laser light.

In the preferred process of this invention, the conducting pattern of the plateable metal is formed directly on the silicon. However, under certain circumstances, it may be desirable to form a layer of a refractory metal, a layer of a noble metal, or a layer of a refractory metal followed by a layer of a noble metal, on the semiconducting material prior to forming the conducting pattern with the plateable metal, which increases the adherence of the plateable metal to the semiconducting material. These layers of refractory metal or of refractory metal and noble metal are preferably not present as they increase the cost of forming the solar cell and, at the present time, there does not appear to be any significant advantage to using them.

The layer of refractory metal does serve the purpose of acting as a diffusion barrier, however, and it may be desirable, where the solar cell will be exposed to temperatures that may cause the diffusion of the plateable metal into the semiconducting material. While any refractory metal, including titanium, tantalum, and tungsten, could be used to form the diffusion barrier, titanium is preferred because it has a strong affinity for oxygen and therefore will bond well to a silicon surface even if the silicon surface has a silicon dioxide layer on it. The layer of refractory metal is preferably formed by evaporation of the refractory metal and its subsequent condensation on the semiconducting material, but it could also be formed by sputtering or other methods.A thickness of about 300 to about 1500 angstroms is preferred because thinner layers may result in non-uniform coverage and thicker layers are unnecessary.

While the plateable metal can be applied directly over the refractory metal after portions of it have been exposed in the laser, in some instances it may be desirable to form a galvanic buffer between the diffusion barrier and the plateable metal in order to prevent corrosion between the metal layers due to differences in their potentials in the electromotive series. The galvanic buffer can be formed from a noble metal such as gold, platinum, palladium, ruthenium, or rhodium, but it is preferably formed of palladium because plateable metals, such as silver, adhere well to palladium. The layer of noble metal that forms the galvanic buffer is preferably formed by evaporation, but it can also be formed by other techniques such as sputtering.The thickness of the noble metal layer is preferably from 300 to 1500 angstroms because thinner layers may not uniformly cover the refractory metal layer and thicker layers are unnecessary, offer no additional benefit, and add to the cost of the product.

In the next step of the process of this invention, portions of a surface of the semiconducting material (or of the refractory metal if the refractory metal is the top layer, or of the noble metal if the noble metal is the top layer) are exposed to laser light. The plateable metal will adhere preferentially only to those portions of the metal surface that have been exposed to the laser light. Because a laser is used, no mask is required, and the circuit pattern can be formed either by moving the laser light over the surface or by moving the surface under the laser light. It is preferable to move the laser light as this is more rapid and can be electronically controlled more easily and precisely. If no refractory metal or noble metal layers are present, the laser light should have a power density of from 3.9 x 105 to 6.4 x 105 joules/cm2 and a wavelength of about 5000 angstroms.We have found experimentally that the use of lesser power densities will not sufficiently activate the surface of the semiconducting material so that the plateable metal will adequately adhere to it. If power densities greater than 6.4 x 105 watts/cm2 are used, the resolution will be poor and the quality of the solar cell may be degraded by laserinduced damage to the semiconducting material.

If a refractory metal is applied to the semiconducting material, or both a refractory metal and a noble metal over top of a refractory metal are applied to the semiconducting material, or the noble metal is applied directly to the semiconducting material, the laser light should have a wavelength of about 5000 angstroms and a power density of from 4.3 x 105 to 7.6 x 105 watts/cm2. If wavelengths outside of this range are used or greater power densities are used, the plateable metal may adhere to unexposed as well as to exposed portions of the surface and the resolution will be poor. If lesser power densities are used, the plateable metal may not adhere to the exposed portions.

In the next step of the process according to this invention, the top layer on the solar cell, which may be the semiconducting material by itself, the refractory metal, or the noble metal, is plated with a plateable metal such as, for example, silver, copper, or gold. The preferred plateable metal is silver because it has the excellent conductivity and adherence. Plating can be accomplished in a conventional manner using electroless plating or electroplating. Electroplating is preferred as it has been found to work very well. Plating should be continued until the layer of plateable metal is from 2 to 10 microns thick. If the plateable metal layer is any thinner, it may not be able to conduct current well, which will result in a large voltage drop across the solar cell; thicknesses greater than 10 microns are usually unnecessary.

In the next step of the process of this invention, the refractory and/or noble metal layers that are in between the plateable metal circuit pattern are removed. This can be accomplished by etching, as is well known in the art, using, for example, aqua regia.

If several solar cells have been formed on a single wafer, it is necessary to do a "mesa", etch, which consists of separating the solar cells on the wafer by etching away a portion of the layer of the semiconducting material so that the properties of each cell can be separately measured. The cells are then tested and, if desirable, an anti-reflective coating of, for example, zinc selenide or magnesium fluoride, is spun on to increase the efficiency of the solar cell; this is a process well known in the art.

The cells are preferably sintered to increase the adherence of the metals to the underlying semiconductive material. Sintering is typically performed at from 300 to 4500C as lower temperatures seem to be ineffective and higher temperatures may diffuse the metals into the semiconducting material.

In addition to producing solar cells, the process of this invention can also be used to make interconnecting parts for small scale integrated circuits, as well as other products.

The invention will now be illustrated with reference to the following Example.

EXAMPLE Two inch diameter single crystal silicon wafers 0.3 millimeters thick, made by the float-zone method, were divided into 12 areas, for making one centimeter by one centimeter solar cells in each area. In these experiments, an argon-ion laser having a peak wavelength output of 5145 angstroms and a maximum power of 18 watts was used to expose the wafers to test patterns.

In preliminary experiments, 1500 angstroms of titanium, followed by 500 angstroms of palladium were evaporated onto some of the silicon wafers. Twelve solar cell comb-shaped metallization patterns were laser-written on the wafer using a continuous wave argon-ion laser focused to approximately 50 Am and X-Y scanning mirrors to raster the beam. Each comb pattern consisted of five 9 mm long horizontal teeth, 2 mm apart, connected by a 9 mm long vertical line, with a 2 mm by 1 mm contact pad centered on the vertical line. Each line was written using a single scan, a laser power of 7.7 W, and a scan speed of 20 cm/s. The contact pad was written at the same power with a scan speed of 0.2 cm/s and a scan overlap of 60%.

No markings corresponding to the laser scans were visible on the palladium-coated surface even when examined under a high-power Nomarski microscope. However, when the wafer was immersed in a silver cyanide plating bath with a 10 mA plating current applied, the hitherto invisible contact pads instantly plated up. The lines, written at higher speeds, took much longer to plate.

A study of plated thickness as a function of laser power was carried out on the same wafer.

Lines were written at laser powers ranging from 8.5 W to 12.5 W, and then plated for two hours using a plating current of 10 mA. The plated thicknesses ranged from 7-9 Am, with not much dependence on laser power. Using lower scan speeds resulted in increased plating rates.

Copper plating on laser-written titaniumpalladium-coated silicon was also tried. Contact pads were written using laser powers ranging from 7.7 W to 12.5 W. At the higher powers visible damage was observed. The laser-written wafer was placed in a copper sulphate plating solution.

Using a 1 mA plating current ensured the selective plating of copper on the laser-written regions. The regions of visible damage plated up most rapidly. Selectivity of copper plating on laser-written titaniumpalladium-coated silicon was therefore also demonstrated.

Laser-written patterns on titanium-coated silicon and bare silicon also plated selectively in a silver cyanide plating bath. In the case of bare silicon, the plated silver did not adhere well. The silver that plated onto the titanium-coated surface was very adherent, however. This result is very promising for solar cell applications, because it could lead to the elimination of the evaporated palladium layer, which would mean a significant reduction in processing costs.

To demonstrate the feasibility of metallizing devices using this selective plating technique, solar cell comb patterns were laser written on wafers coated with 1500 angstroms titanium and 500 angstroms palladium. A laser power of 7.7 W and a scan speed of 0.2 cm/s were used for both the lines and contact pads to ensure uniform plating rates. After silver plating for three hours using a plating current of 10 mA, the palladium and titanium over the rest of the wafer was etched off. The surface of the silver pattern was oxidized in the aqua regia used to etch palladium. This oxide was removed by immersion in the silver cyanide plating solution, followed by 30 minutes of plating to build back the thickness. The final plated thickness was measured to be 25 ,um. A second wafer was plated for only 15 minutes at 10 mA, and found to have a plated thickness of 4.6 ,um.Mesas were then photolithographically defined around the patterns to isolate the cells from each other.

Lighted and dark current-voltage measurements were made to characterize the cells. The lighted I-V data is shown in Table I, and she dark I-V (current-voltage) data is shown in Table II, before and after sintering in hydrogen at 4500C for 30 minutes. The non-anti-reflective-coated cell efficiencies are seen to be as high as 11.15%, which compares favorably with the best of the baseline cells metallized by conventional evaporation and photolithography. Sintering improves the series resistance, and, therefore, the cell efficiency. The highest efficiency obtained after sintering is 11.63%, which is half a percent higher than any of the baseline cell efficiencies.

The efficiency of this cell was increased to 16.5% by evaporating a double-layer antireflective coating.

Figure 4 is a graph that plots current versus voltage for the lighted cell. Figure 4 shows that the cell functions as well or better than cells made by photolithographic methods, with an efficiency of 16.5% after applying an antireflective coating.

TABLE I Lighted I-V Data For Selectively Plated Solar Cells Short-Circuit Open-Circuit Cell l.D. Current Jsc (MA) Voltage Voc (V) Fill Factor Efficiency (%) Laser Metal &num;3-4 27.42 0.528 0.577 8.35 (After Sintering) Laser Metal &num;3-5 27.40 0.538 0.583 8.58 (After Sintering) Laser &num;5-14 25.17 0.572 0.768 11.05 (After Sintering) Laser #5-15 25.76 0.576 0.784 11.63 (After Sintering) TABLE II Dark I-V Data For Selectively Plated Solar Cells Cell l.D. Normalized Series Normalized Shunt Jol (A/cm2) tJo2 (A/cm2) Resistance (cm2) Resistance (KQ-cm2) Laser Metal &num;3-4 2.34 138.9 1.4x10 11 1.5x10-6 (After Sintering) Laser Metal &num;3-5 1.70 3.3 1.7x10 11 1.2x10--5 (After Sintering) Laser Metal #5-14 0.63 13.7 4.1x10--'2 3.3x106 (After Sintering) Laser Metal &num;5-15 0.48 > 103 3.6Y10-i2 6.7x10-7 (After Sintering) J is representative of leakage current in the bulk region +J is representative of leakage current in the disection region

Claims (17)

1. A method of forming a conductive pattern on the surface of a semiconductor comprising exposing portions of said surface to light from a laser at a predetermined power density; and immersing said surface in a plating solution of a plateable metal, whereby a plateable metal is plated onto said laser light-exposed portions of said semiconductor surface.

2. A method according to claim 1, wherein the light from the laser has a power density of 4.3 x 105 to 6.6 to 105 watts/cm2.

3. A method according to claim 1, wherein the surface is exposed to the laser light when it is in the plating solution.

4. A method according to claim 1, wherein prior to exposure to light from a laser at a power density of 3.9 x 105 to 6.4 x 105 watts/cm2, the semiconductor surface is coated with a layer of a retractory or noble metal; the coated surface is placed in a bath of a plateable metal, whereby a plateable metal is plated onto said laser light-exposed portions of said coating; and portions of said coating not coated with said plateable metal are etched away.

5. A method according to claim 4, wherein the thickness of the refractory metal is from 300 to 1500 angstroms, and the thickness of the plateable metal is from 12 to 10 microns.

6. A method according to any of claims 1 to 5, wherein as a last step an anti-reflective coating is applied over the surface, and sintering at 300 to 450 C.

7. A method according to claim 1, wherein prior to exposure to light from a laser at a power density of 3.9 x 105 to 6.4 x 105 wtts/cm2, a diffusion barrier is formed on the surface by coating the latter with a refractory metal and a galvanic buffer is formed by coating the surface of said refractory metal with a noble metal; the surface is placed in a bath of a plateable metal, whereby a plateable metal is plated onto laser light-exposed portions of said noble metal surface; and portions of said noble metal and said refractory metal not coated with said plateable metal are etched away.

8. A method according to claim 7, wherein the thickness of the coating of refractory metal is 300 to 1500 angstroms, the thickness of the coating of noble metal is 300 to 1500 angstroms, and the thickness of the plating of plateable metal is 2 to 10 microns.

9. A method according to claim 7 or 8, wherein said refractory metal is titanium, said noble metal is palladium.

10. A method according to any of claims 4 to 9, wherein the refractory metal is titanium.

11. A method according to any of claims 1 to 10, wherein plateable metal is electroplated onto the surface.

12. A method according to any of claims 1 to 11, wherein the semiconductor is single crystal silicon.

13. A method according to any of claims 1 to 12, wherein the plateable metal is silver.

14. A method according to any of claims 1 to 13, wherein the laser light has a wavelength of about 5000 Angstroms.

15. A method of forming a conductive pattern on the surface of a semiconductor, said method being substantially as described herein with particular reference to the foregoing Example.

16. Solar cells comprising a product when made by a method according to any of claims 1 to 15.

17. Solar cells substantially as described herein with particular reference to Figure 1, 2 or 3 of the accompanying drawings.