DE3643898A1 - METHOD FOR FORMING A CONDUCTIVE PATTERN ON A SEMICONDUCTOR SURFACE - Google Patents

Description

The invention relates to a method for forming a conductive pattern on a semiconductor surface.

Solar cells are currently produced by a process that includes photolithography. With this procedure become layers, first of titanium and then of palladium and finally from silver through an evaporation process on a surface of a doped silicon wafer upset. The plate is then covered with a photoresist coated, a glass mask is placed over the photoresist layer applied and the photoresist layer of ultraviolet light exposed. The parts of the photoresist that either have been exposed to the light or have not been exposed to the light are then removed, usually by up solution in a solvent, and the exposed metal layers etched away. The remaining photoresist material is removed and the silver pattern is then covered with silver plat animals to build this up to the desired thickness.

Satisfactory conductive circuit during this procedure circular pattern on the silicon, it is expensive and  time consuming, because of the many necessary procedural steps te. It would significantly reduce the cost of solar cells and increase their utility when a process could be found to dope conductive patterns on a to form silicon that does not require all the steps which are necessary in the photolithographic process.

The object of the invention is to create such Procedure.

The task is solved according to the characteristic feature of the main claim, i.e. through an educational process of a conductive pattern on the surface of a half conductor consisting of exposing parts of the surface the light of a laser with a predetermined power density; and immersing the surface in a plating solution of a plateable metal, which makes a plateable Metal on the parts of the half exposed to the laser light conductor surface to be plated.

A method has been found to make conductive patterns to form doped silicon for use in manufacturing Position of solar cells, which process photolithography does not include nor necessarily the deposit itself of titanium and palladium layers on the silicon. That means that the inventors determined it quite randomly was the laser light of a very specific power density and wavelength a silicon surface in one Way activated that a conductive pattern of silver can be applied directly to this silicon surface. So far, the direct application of silver to the Silicon surface not possible because silver on the Silicon does not adhere well. The effect of laser light very specific power density and wavelength on the Silicon surface activates the in some way exposed surface so that the silver adheres to it.  

As a result, the inventors were able to use Eliminate photoresist on the surface, like that Deposition of the titanium and palladium layers. It was however also found that laser light from various Power densities the titanium and palladium layers too activated so that the silver only on these parts of the titanium or palladium that is exposed to laser light have been. Thus it is also possible to work on one layer of titanium to form a conductive circuit which Layer on which silicon was applied, or a conductive layer on a layer of palladium over the Apply layer of titanium on the silicon wafer.

By eliminating the photolitho process step graphics as well as the application of titanium and palladium it becomes possible to layer a solar cell through a ver drive to form that much less time consuming and less is more complex than the previous photolithographic Ver drive. Even if titanium and palladium layers are used, the process according to the invention is still less expensive and less time consuming than that photolithographic process because the application of the Photoresist material and its removal can be removed.

The invention is explained below with reference to exemplary embodiments play explained in more detail, which are shown in the drawings are. Show it:

Figures 1, 2 and 3 isometric views, partly in section, for explaining three embodiments of solar cells.

Fig. 4 is a graphical representation of the relationship between current and voltage in a solar cell, which was produced according to the inventive method.

In the embodiment of FIG. 1, a silicon wafer 1 has a part 2 which is doped negatively (or positively), and another part 3 of opposite Doation. A plateable metal 4 , such as silver, is applied directly to the parts 5 of the silicon wafer 1 which have been exposed to laser light, whereby a circuit pattern 6 is formed on the surface of the silicon wafer 1 ; an anti-reflective coating 7 is applied over the rest of the surface.

The embodiment of FIG. 2 is identical to FIG. 1, except that a very thin layer 8 of a heat-resistant metal or a noble metal is applied to the surface of the silicon wafer 1 , and a layer 9 of a plateable metal over these parts 10 of the layer 8 of heat-resistant metal or precious metal is applied, which were exposed to the laser light, whereby a circuit pattern 11 is formed; after which an anti-reflective coating 12 Be applied between the circuit pattern 11 .

The embodiment of FIG. 3 is identical to that of FIG. 2, with the exception that a layer 13 of noble metal is applied over the heat-resistant metal layer 8 . On the parts 14 of the noble metal layer 13 , which were exposed to laser light, a plateable metal 15 is brought on, whereby the circuit pattern 16 is formed. An anti-reflective coating 17 fills the spaces in the circuit pattern.

The method according to this invention can be applied to each half conductor material can be applied, such as on Silicon, germanium, and gallium arsenide. Silicon is that preferably semiconducting material because it was found that this method according to the invention is very good for silicon is working. The silicon should be a single crystal silicon however, it can be formed by a variety of methods  , including the Czochralski method, the Flo tation zone method, or the dendritic tissue Method. The silicon can be with different p and n-type dopants, including Boron, phosphorus, nitrogen, etc. The semiconducting material can have any surface configuration, a finally flat or curved, as well as any size or shape as long as only the surfaces on which the conductive Pattern is to be formed, exposed to laser light can be.

In a preferred process according to the invention, the conductive pattern is formed from the plateable metal directly on the silicon. However, in some circumstances it may be desirable to form a layer of refractory or refractory metal, a layer of precious metal, or a layer of refractory metal followed by a layer of precious metal on the semiconductor material before the conductive pattern with the plateable metal is formed, which increases the adhesion of the plateable metal to the semiconductor material. These layers of heat-resistant metal or heat-resistant metal and precious metal are preferably not present, since they increase the cost of education of the solar cell, and it seems their use - to be of no significant advantage - at least at the present time.

However, the layer of the heat-resistant metal serves the purpose to act as a diffusion barrier and it may well be worthwhile where the solar cell temperatures exposed to diffusion of the clad Metal in the semiconductor material could cause this To provide a barrier. Although any heat-resistant metal including titanium, tantalum and tungsten, usable to the To form a diffusion barrier, but titanium is preferred, because it has a strong affinity for oxygen and therefore binds well to the silicon surface, even if the  Silicon surface on top of a silicon dioxide layer owns. The layer of heat-resistant metal is preferred by evaporation of the heat-resistant metal and its subsequent condensation on the semiconductor material ge forms, but it could also by spraying or by means other procedures. A thickness of about 300 to about 1500 angstroms is preferred because of thinner layers non-uniform coverage and thicker Layers are not necessary.

The plateable metal can be directly over the heat solid layer can be applied after parts of it Lasers have been exposed, but in some cases it may be desirable to add a galvanic buffer between the To provide a diffusion barrier and the clad metal, to prevent corrosion between the metal layers due to Differences in their potential in the electromotive Prevent series. The galvanic buffer can be formed are made from a precious metal, such as gold, Platinum, palladium, ruthenium or rhodium, but it will preferred to form this from palladium because it can be plated Metals, such as silver, adhere well to palladium be liable. The layer of the precious metal that the galvanic Forms buffer, is preferably formed by evaporation, however, it can also be done by other techniques such as spraying be educated. The thickness of the precious metal layer is before preferably 300 to 1500 angstroms because the layers are thinner Do not uniformly cover the layer of heat-resistant metal could, and because thicker layers are not necessary, offer no additional benefit and only the cost of Increase the product.

In the next process step of the process according to the invention, parts of the surface of the semiconducting material (or that of the heat-resistant metal if heat-resistant metal is the top layer, or of the noble metal if the noble metal is the top layer) are exposed to the laser light. The plateable metal will preferably only adhere to those parts of the metal surface which 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 because the laser beam is faster to move and can be controlled more easily and accurately electronically. If there is no layer of heat-resistant metal or noble metal, the laser light should have a power density of 3.9 × 10 ⁵ to 6.4 × 10 ⁵ joules / cm ² and a wavelength of about 5000 angstroms. It has been found experimentally that the use of lower power densities does not activate the surface of the semiconductor material sufficiently, so that the plated metal then no longer adheres sufficiently. If power densities of more than 6.4 × 10 ⁵ watts / cm ² are used, the resolution is poor and the quality of the solar cell can deteriorate due to laser-induced destruction of the semiconductor material.

If a refractory metal is applied to the semiconductor material, or both a refractory metal and a noble metal are applied over the refractory metal to the semiconductor material, or if the noble metal is applied directly to the semiconductor material, the laser light should have a wavelength of about 5000 angstroms and have a power density of about 4.3 × 10 ⁵ to 7.6 × 10 ⁵ watts / cm ² . If wavelengths outside this range are used or if higher power densities are used, the plateable metal could also adhere to unexposed as well as exposed parts of the surface and the resolution would be poor. If lower power densities are used, the plateable metal will not adhere to the exposed parts.

In the next stage of the process according to the invention  the top layer of the solar cell, which is the semiconductor mat rial itself, or the heat-resistant metal, or that Precious metal, with a plateable metal such as silver, copper or gold plated. That is preferable plated metal is silver because it is excellent conductive shows ability and attachment. The plating can be done in one conventional way are done using electro lysis-free plating or by electroplating. Electroplating is preferred because it has been found that she works very well. The plating should continue be until the layer of plated metal has a thickness from 2 to 10 microns. If the clad Metal layer is thinner in some way, she doesn't like it to be able to conduct electricity well, resulting in a strong voltage drop across the solar cell. Thick more than 10 microns are usually not necessary.

In the next stage of the process according to this invention the layers of heat-resistant and / or precious metal, between the circuit pattern of dopable metal are present removed. This can be achieved through Etching as is well known in the art, for example Aqua Regia (gold separation water or aqua regia) is used becomes.

If there are numerous solar cells on a single plate were formed, it is necessary to carry out a "mesa" etch feed, which consists of the solar cells on the plate Chen by etching away part of the layer of the semiconductor to separate materials in such a way that the properties of a each cell can be measured separately. The cells are then tested and, if desired, an anti-reflective tive coating of, for example, zinc selenide or Magnesium fluoride spun to the efficiency of the Increase solar cell. This is well-known to the person skilled in the art known process.  

The cells are preferably sintered to adhere of the metal on the underlying semiconductor material enlarge. Sintering is typically done at Tempe temperatures of 300 to 450 ° C because of lower temperatures Doors appear to be ineffective and at higher temperatures The metals diffuse into the semiconductor material can dieren.

In addition to the production of solar cells, the inventor Process according to the invention can also be used to interconnect parts for integrated circuits of small dimensions manufacture, as well as other products.

The invention will now be explained in more detail using the following example explained.

example

A crystal silicon wafer with a diameter of 5 cm and a thickness of 0.3 mm made by the flota zone processes, were divided into 12 areas in order Solar cells with a dimension of 1 cm × 1 cm in each Area. An argon was used in these experiments ion laser with a maximum output at one wavelength of 5145 angstroms and a maximum output of 18 watts used to irradiate the wafers with test patterns.

In previous experiments, 1500 angstroms of titanium, followed by 500 angstroms of palladium on some of the sili evaporated zium platelets. Twelve comb-shaped solar cells Metallization patterns were applied to the platelets using the Lasers written down, taking a continuous wave of argon ions laser was used, which was fossilized to approximately 50 µm, as well as X-Y deflecting mirrors to grid the beam move. Each comb pattern consisted of five 9 mm long horizontal teeth, 2 mm apart by a 9 mm long vertical line, with a 2 mm × 1 mm  large contact pads centered on the vertical line. Each line was described using a single one Scanning with a laser power of 7.7 watts, and with a scanning speed of 20 cm / sec. The contact pad was written with the same performance with a Scanning speed of 0.2 cm / sec and a scanning speed lapping of 60%. There were no markings to match the laser scans on the one coated with palladium Surface visible even if you put it under a high performance Nomarski microscope. However, if that Immersed in a silver cyanide plating bath with an applied plating current of 10 mA the previously invisible contact plate opens immediately. The Lines that have been written at higher speeds took much longer to plate.

An examination of the plated thickness as a function laser power was measured on the same plate guided. The lines were written with laser powers, which ranged from 8.5 watts to 12.5 watts, and then for two Hours of plating, with a plating current of 10 mA was used. The plated thickness ranged from 7 to 9 µm, with no strong dependence on laser power. At the use of lower scanning speeds there were increased plating rates.

There was also copper plating on with laser light described titanium-palladium-coated silicon ver looking for. Contact pads have been described using of laser powers ranging from 7.7 watts to 12.5 watts At the higher performances there was visible damage to observe. The laser-written plate was placed in a Copper sulfate plating solution immersed. The usage a plating current of 1 mA was the selective one Plating copper on the laser-inscribed areas for sure. The areas with visible damage plated the fastest. The selectivity of the copper  coating on the laser-written titanium-palladium coated silicon was therefore also shown.

Laser-written patterns of titanium-coated silicon and bare silicon was also selectively plated in one Silver cyanide plating bath. In the case of bare silicon the plated silver did not adhere well. The silver that on however, the titanium coated surface was plated strongly adherent. This result is very promising for solar cell applications as it eliminates the evaporated palladium layer leads which would mean that the process costs are significantly reduced.

About the usefulness of metallizing objects using this selective plating method show, solar cell comb patterns were placed on platelets wrote that with 1500 angstroms titanium and 500 angstroms Palladium were coated. A laser power of 7.7 watts and a scan speed of 0.2 cm / sec was used to bring in both the lines and the contact pads to ensure uniform plating rates. After silver plating under for three hours Using a plating current of 10 mA Palladium and the titanium over the rest of the plate away etched. The surface of the silver pattern was in Aqua Regia oxidized, which was used to etch the palladium. This Oxide was plated by immersion in the silver cyanide solution removed, followed by 30 min plating to the Rebuild thickness. The final clad thickness was measured at 25 µm. A second tile was created for plated for only 15 min at 10 mA and it was found that it had a plated thickness of 4.6 µm. Then Mesas became diffused photolithographically around the pattern to the cells isolate from each other. Illuminated and dark electricity Voltage measurements were made to cha the cells characterize. The I-V data under light are in Table 1 reproduced, and the I-V (voltage-current) data below  Darkness are shown in Table II, before and after sintering in hydrogen at 450 ° C for a period of time from 30 min. The efficiencies of not being anti-reflective coated cells, as can be seen, are up to 11.15%, which is cheap compared to the best base is line cells by conventional evaporation and photolithographic processes were metallized. A Sintering improves the series resistance, and therefore the Cell efficiency. The highest efficiency, according to the Sintering was achieved was 11.63%, which was a 1/2% higher is as any of the efficiencies of baseline cells. The efficiency of this cell was increased to 16.5% by a double layer anti-reflective coating was evaporated.

Figure 4 is a graph showing the current versus voltage for the exposed cell. Fig. 4 shows that the cell works as well or better as cells, which were produced by means of photolithographic processes, with an efficiency of 16.5% after application of an antireflective coating.

Table I

IV data under light for selectively plated solar cells

Table II

IV data under darkness for selectively plated solar cells

Claims (14)

1. A method of forming a conductive pattern on the surface of a semiconductor, characterized by exposing parts of the surface to the light of a laser with a predetermined power density; and immersing the surface in a plating solution of a plating metal, wherein a plating metal is plated on the parts of the semiconductor surface exposed to the laser light. 2. The method according to claim 1, characterized in that the light from the laser has a power density of 4.3 × 10 ⁵ to 6.6 × 10 ⁵ watts / cm ² . 3. The method according to claim 1, characterized in that the surface is exposed to laser light when it is is in the plating solution. 4. The method according to claim 1, characterized in that before exposure to light from a laser with a power density of 3.9 × 10 ⁵ to 6.4 × 10 ⁵ watt / cm ^2, the semiconductor surface with a layer of a heat-resistant metal or a precious metal is coated; the coated surface is placed in a bath of a plateable metal, a plateable metal being plated onto the parts of the coating exposed to the laser; and parts of the coating that are not covered with the plateable metal are etched away. 5. The method according to claim 4, characterized in that the thickness of the heat-resistant metal between 300 and 1500 Angstrom lies, and that the thickness of the clad Metal is between 12 and 10 microns. 6. The method according to any one of claims 1 to 5, characterized characterized in that a anti-reflective coating applied over the surface is brought, and a sintering at 300 to 450 ° C. he follows. 7. The method according to claim 1, characterized in that before exposure to light from a laser with a power density of 3.9 × 10 ⁵ to 6.4 × 10 ⁵ watt / cm ^2, a diffusion barrier is applied to the surface of the coating , the latter with a heat-resistant metal, and a galvanic buffer is formed by coating the surface of the heat-resistant metal with a noble metal; wherein the surface is immersed in a bath of a plateable metal, whereby a plateable metal is plated on the parts of the precious metal surface exposed to the laser; and the parts of the noble metal and the heat-resistant metal that are not coated with the plateable metal are etched away. 8. The method according to claim 7, characterized in that the thickness of the heat-resistant metal coating 300  The thickness of the coating is up to 1500 angstroms made of precious metal is 300 to 1500 angstroms and the Plating metal cladding thickness 2 to 10 Is micron. 9. The method according to claim 7 or 8, characterized net that the heat-resistant metal is titanium, and that the Precious metal is palladium. 10. The method according to any one of claims 4 to 9, characterized characterized in that the heat-resistant metal is titanium. 11. A method according to any one of claims 1 to 10, characterized in that the plateable metal the surface is electroplated. 12. The method according to any one of claims 1 to 11, characterized characterized in that the semiconductor single crystal silicon is. 13. The method according to any one of claims 1 to 12, characterized characterized in that the plateable metal is silver. 14. The method according to any one of claims 1 to 13, characterized characterized in that the laser light has a wavelength of has about 5000 angstroms.