FR2596921A1 - METHOD OF FORMING A CONDUCTIVE DESIGN ON THE SURFACE OF A SEMICONDUCTOR AS IN PARTICULAR SILICON, GERMANIUM AND GALLIUM ARSENIUM - Google Patents

Abstract

A) PROCESS FOR FORMING A CONDUCTIVE DRAWING ON THE SURFACE OF A SEMICONDUCTOR. B) PROCESS CHARACTERIZED IN THAT IT CONSISTS OF EXPOSING PARTS OF THE SURFACE TO THE LIGHT OF A LASER OF PREDETERMINED POWER DENSITY, AND IN IMMERSING THIS SURFACE IN A PLATING SOLUTION OF A METAL 4 PLATE, WHICH SO ALLOWS TO PLATE A PLATING METAL ON PARTS OF THE SEMICONDUCTOR SURFACE THAT HAVE BEEN EXPOSED TO LASER LIGHT. C) THE INVENTION RELATES TO A PROCESS FOR FORMING A CONDUCTIVE DESIGN ON THE SURFACE OF A SEMICONDUCTOR.

Description

A method of forming a conductive pattern on the surface of a semi-

  conductor such as in particular silicon,

  germanium and gallium arsenium. "

  The invention relates to a method of forming a conductive pattern on a semiconductor surface. The solar cells are currently manufactured using a process using photolithography. In this process, coatings first of titanium, then of palladium and finally of silver are applied by an evaporation process on the surface of a doped silicon pellet. The pellet is then coated with a photoresist, then a sliver of glass is placed on the photoresistor and this photoresistor is exposed to ultraviolet light. Those portions of the photoresist that have been either exposed or unexposed to light are then removed, usually by dissolution in a solvent, and the exposed metal layers etched with acid. The remaining photoresistor is stripped and the thin layer is then silver plated

  to give it the required thickness.

  Although it makes it possible to obtain satisfactory conductive circuit patterns on silicon, this method is expensive and time consuming because it involves many steps. The cost of solar cells would be reduced by a lot and we would

  would increase their use by finding a method for forming conductive patterns on doped silicon without having to implement all the steps required by the photolithography process.

  To achieve this object, the invention relates to a method of forming a conductive pattern on the surface of a semiconductor, characterized in that it consists in exposing portions of the surface in the light of a laser of the predefined power density, and immersing this surface in a plating metal plating solution, thereby enabling a plating metal to be plated on those portions of the semiconductor surface which have been exposed to the

laser light.

  It has thus been discovered a process for forming conductive patterns on doped silicon for the production of solar cells, which does not require photolithography to be used, or even to necessarily deposit layers of titanium and palladium on the silicon. . That is, it was discovered, quite by accident, that a particular power density and wavelength laser light could activate a silicon surface in such a way that a pattern driver

  silver could be applied directly to the silicon surface. Previously, the direct application of silver on the silicon surface was not possible because silver does not adhere well to silicon.

  However, the exposure of the silicon surface to the laser light of particular power density and wavelength, in some way activates the exposed surface, so that the silver adheres

  to this one. It is therefore possible to suppress the application of photoresist on the surface, as well as the deposition of the titanium and palladium layers.

  However, it was also found that laser lights of different power densities activated the titanium and palladium layers in the same way, so that the silver only adhered to the titanium or palladium parts that had been exposed to laser light. Thus, it is also possible to form a conductive circuit on a titanium layer having been applied to the silicon, or to apply a conductive layer on a layer of palladium placed on the top of a titanium layer.

  formed on the silicon wafer.

  By eliminating the photolithography step as well as the application of the titanium and palladium layers, it is possible to form a solar cell by a much faster and beautiful method.

  cheaper than the previous photolithographic process.

  Even using the titanium and palladium layers, the process according to the invention is always less expensive

  and shorter than the photolithographic process, since the photoresist application and extraction steps are eliminated.

  The invention will be described in detiis with reference to the accompanying drawings in which: - Figures 1, 2 and 3 are perspective views, partially in section, illustrating three embodiments of solar cells; and - Figure 4 is a graph showing the relationship between current and voltage in a solar cell

  performed by the method according to the invention.

  In the embodiment of FIG. 1, a silicon wafer 1 comprises a negatively (or positively) doped part 2 and another doped part 3 in the opposite polarity. A metal 4 which is flat to be plated such as silver, is applied directly to the portions of the silicon wafer I which have been exposed to the laser light, to form the circuit pattern 6 on the surface of the silicon wafer 1; an anti-reflective coating 7 is

applied on the remaining surface.

  The embodiment of FIG. 2 is identical to that of FIG. 1 except that a very thin layer 8 of a refractory metal or of a noble metal is applied to the surface of the silicon wafer 1, and that a layer 9 of a clearable metal is applied to the portions of the refractory metal or noble metal layer 8 which have been exposed to the laser light to thereby form the circuit pattern 11; an anti-reflective coating 12 is applied

between the circuit drawings 11.

  The embodiment of FIG. 3 is identical to that of FIG. 2 except that a layer 13 of noble metal is applied to the refractory metal layer 8. On the parts 14 of the noble metal layer 13 which have been exposed to the laser light, a plating metal 15 is applied to form the circuit pattern 16; an antireflective coating

  17 fills the spaces of the circuit design.

  The process according to the invention can be applied to any semiconductor material such as, for example, silicon, germanium, and gallium arsenide. Silicon is the preferred semiconductor material, since it has been found that the method according to the invention applies perfectly well to silicon. The silicon must be monocrystalline silicon, but can be formed by a wide variety of processes including the Czochralski process, the process

  floating zone, or the dendritic layer process.

  Silicon can be doped with different dopants of

  p and n types including boron, phosphorus, nitrogen, etc. The semiconductor material may have substantially

  any surface configuration including flat or curved configurations, as well as any size or shape, to the extent that the areas on which the conductive pattern is to be formed may be exposed to laser light.

  In the preferred method according to the invention, the conductive pattern of the metal which can be piaquered is formed directly on the silicon. However, under certain circumstances, it may be desirable to form a layer of a refractory metal, a nail of a noble metal, or a layer of a refractory metal followed by a layer of a noble metal, on the semiconductor material before forming the conductive pattern by the plating metal, which increases the adhesion of the plating metal to the semi-conductive material. These refractory metal or refractory metal and noble metal layers are preferably absent because they increase the manufacturing cost of the solar cell and it does not appear on time.

  current, significant advantages in using them.

  The refractory metal layer, however, is intended to serve as a diffusion barrier and may be desirable when the solar cell is to be exposed to temperatures that can cause diffusion of the plating metal into the semiconductor material. Although any refractory metal including titanium, tantalum and tungsten can be used to form the diffusion barrier, titanium is preferable because it has a high affinity for oxygen and thus forms a very good bond with a silicon surface even though this surface of

  silicon is covered with a layer of silicon dioxide. The refractory metal layer is preferably formed by evaporation of the refractory and condensing metal.

  the semiconductor material, but this layer could also be formed by projection or by any other method. A thickness of about 300 to 1500 Angstroms is preferable because thinner layers may lead to non-uniform coverage, and thicker layers are not necessary. Although the plating metal can be applied directly to the refractory metal after portions thereof have been exposed to the laser, it may be desirable in some cases to form a galvanic screen between the diffusion barrier and the metal. plating metal to prevent corrosion between the metal layers due to their differences in potentials in the scale of electromotive forces. The galvanic screen may consist of a noble metal, such as gold, platinum, palladium, ruthenium or rhodium, but it is preferably constituted by palladium because the plating metals, such as that ar20 gent, adhere very well to palladium. The noble metal layer that forms the galvanic screen is preferably formed by evaporation, but this layer can also be formed by other techniques such as projection. The thickness of the noble metal layer is preferably between 300 x 1-10m and 1500 x 1-10m because thinner layers may not cover

  uniformly the refractory metal layer, and thicker layers are not needed, do not provide additional benefits, and increase the cost of the product.

  In the next step of the process according to the invention, different parts of the surface of the semiconductor material (or refractory metal, if this refractory metal constitutes the top layer, or noble metal, if this noble metal constitutes the layer superior) are exposed to laser light. The plating metal preferably adheres only to those portions of the metal surface that have been exposed to laser light. Because a laser is used, no mask is needed, and the pattern of the circuit can be formed either by moving the laser light on the surface or by moving the surface under the laser light. It is better to move the laser light,

  because it is faster and can be controlled electrC10 only more easily and with greater precision.

  If no refractory metal or noble metal layer is present, the laser light must have a power density in the range of 3.9 × 10 5 and 6.4 × 10 5 Joules / cm 2 and a wavelength of approximately 5000 x 10 m. It has been found experimentally that the use of lower power densities does not sufficiently activate the surface of the semiconductor material for the veneer to adhere properly thereto. By using powers of densities higher than 6.4 x 105 Watts / cm2, the resolution becomes Low and the quality of the solar cell can be degraded by

  laser-induced damage in the semiconductor material.

  A refractory metal is applied to the semiconductor material, or it is applied to this semiconductor material both a refractory metal and a noble metal plated on the top of the refractory metal, or even if the noble metal directly on the semiconductor material, the laser light must have a wavelength of about 5'00 x 1-10 m and a power density between

  4.3 x 105 and 7.6 x 10 Watts / cm2 Using wavelengths outside of this range or den-

  Because of the higher power requirements, the plating metal can adhere to both the unexposed and exposed portions of the surface and the resolution becomes low. By using lower power densities, the plating metal may not adhere to the exposed portions. In the next step of the process according to the invention, the upper layer formed on the solar cell, which may be constituted by the semiconductor material itself, the refractory metal or the noble metal, is plated with a metal of veneer such as, for example, silver, copper or gold. The preferred plating metal is silver because it has excellent conductivity and excellent adhesion. The plating can be carried out conventionally using electroless plating or electroplating. Electroplating is preferred because it has proved very practical to implement. The plating should be continued until the thickness of the plating metal layer is between two and ten microns. If the plating metal layer is thinner, this layer may not be able to conduct the current properly, leading to a large voltage drop across the solar cell; on the other hand, thicknesses greater than 10 microns are not

usually not necessary.

  In the next step of the process according to the invention, the layers of refractory metal and / or noble metal included in the intervals of the plating metal circuit pattern are removed. This result can be obtained in a well known manner by attack

  with acid, for example using aqua regia.

  If several solar cells have been formed on the same wafer, it is necessary to carry out a "mesa" attack which consists in separating the solar cells from the wafer by attacking a part of the layer of semiconductor material so that the properties of each cell can be measured separately. The cells are then tested and if desired, an anti-reflective coating consisting of, for example, zinc selenide or magnesium fluoride, is sprayed onto the surface to increase the efficiency of the solar cell; this is a process

well known from the prior art.

  The cells are preferably agglomerated to increase the adhesion of the metals to the semiconductor material placed below. Agglomeration is typically performed at 300 to 450 ° C. lower temperatures appear to be ineffective and higher temperatures may diffuse

  metals in the semiconductor material.

  In addition to the production of solar cells, the process according to the invention can also

  be used to form integrated small scale circu baked connecting parts, as well as other products.

  The invention will now be illustrated with reference to the example below.

EXAMPLE

  Monocrystalline silicon wafers of 50.8 mm diameter and 0.3 mm thickness made by the floating zone method, were divided into 12 zones to form solar cells

  one centimeter by one centimeter in each zone.

  In these experiments, an argon laser with a peak output wavelength of 5145 x 1-10 m and a maximum power of 18 watts was used to

  expose the pads to the test drawings.

  In preliminary experiments, 1500 x 1 10m of titanium followed by 500 x 1 -10m of palladium was evaporated on some of the silicon wafers. Twelve comb-shaped solar cell metallization patterns were laser-drawn on the wafer using an argon continuous laser focused at about 50 microns, and X-Y scanning mirrors to perform the raster scan of the beam. Each comb design consisted of five horizontal teeth 9 mm long, separated by 2 mm and connected by a vertical line 9 mm long, with a contact tab 2 mm by 1 mm centered on the vertical line. Each line was drawn using a single scan with a 7.7W laser power and a scanning speed of 20 cm / sec. The contact pad was drawn at the same power with a scan rate of 0.2 cm / sec and a scan coverage of%. No mark corresponding to the laser scan was 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 by applying a 10 mA plating current, the hitherto invisible contact tabs were instantaneously plated. The drawn lines

  at higher speeds took longer to be tackled.

  A study of plating thickness versus laser power was performed on the same wafer. The lines were drawn at laser powers ranging from about 8.5 W to 1-2.5 W, and then plated for two hours using a 10 mA plating current. The plating thicknesses were between 7 and 9 microns without depending more on the laser power. The use of speeds

  The lower scanning rates led to higher veneer speeds.

  It has also been tried copper plating on silicon coated with titaniumpalladium drawn by laser. The contact tabs were drawn using laser powers ranging between 7.7 W and 12.5 W. At higher powers, visible damage was observed. The laser-patterned wafer was placed in copper sulphate plating solution. The use of a 1 mA plating current made it possible to obtain the selective plating of the copper on the laser-drawn areas. Visible deterioration areas were plated faster. The selectivity of copper plating on silicon coated with titanium-pa! Ladiunr

  drawn by laser, was therefore also demonstrated.

  Laser-drawn drawings on titanium-coated silicon and bare silicon were also selectively plated in a silver cyanide plating bath. In the case of rau silicon, the plating silver did not adhere conveniently. On the contrary, the silver plated on the surface covered with titanium adhered perfectly well. This result is very

  It is promising for application to solar cells because it can lead to the removal of the evaporated palladium layer, which corresponds to a significant reduction in production costs.

  To demonstrate the feasibility of metallization devices utilizing this selective plating technique, solar cell comb designs were laser-drawn on platelets coated with 1500 x 1-10m titanium and 500 x 10 palladium. A laser power of 7.7 W and a scanning speed of 0.2 cm / s were used for both the lines and the contact tabs to achieve uniform plating speeds. After a silver cage run for three hours using a 10 mA plating stream, the palladium and titanium on the remainder of the wafer were etched. The surface of the silver pattern has been oxidized in the regal water used to attack the paila5 dium. This oxide was removed by immersion in the silver cyanide plating solution, followed by a 30-minute plating to restore the thickness. The thickness

  final plating was measured at the value of 25 microns.

  A second wafer was only plated for 15 minu-10 tes at 10 mA, and showed a plating thickness of 4.6 microns. "Mesas" were then defined photolithographically around the drawings to isolate

cells from each other.

  Current-voltage measurements were made in the presence of light and in the dark to characterize the cells. The IV (current) data in the presence of light are shown in Table I, and the IV (current-voltage) data in the dark are shown in Table II, before and after hydrogen capping at 450 ° C. minutes. It was found that the yields of cells not covered with an antireflective coating amounted to 11.15%, which compares favorably with the best metallised base cells by conventional evaporation and photolithography. Agglomeration improves the series resistance and, consequently, cell efficiency. The highest yield obtained after agglomeration is 11.63%, which is 0.5% higher than any of the base cell yields. The performance of this cell was

  increased to 16.5% by evaporating a double-layer anti-reflective coating.

  Figure 4 is a graph showing the current versus voltage for the illuminated cell. Figure 4 shows that the cell

  works as well or better than cells made by photolithographic processes, with a yield of 16.5% after application of an anti-reflective coating.

w CD rv A r o o

TABLE I

  I-V data in the presence of light for selectively plated solar cells.

  Cells Short circuit current ID 3 sc (MA) Metal laser # 3-4 (after agglomeration) 27.42 Metal laser # 3-5 (after agglomeration) 27.40 Laser # 5-14 (after agglomeration) 25.17 Laser # 5-15 (after agglomeration) 25.76

  ___________________________________________-

  --__-___-______-___- Open circuit voltage Voc (V)

_________ _______

Fill Factor Filling

__--- - - - ___ - _ --- ___ ______

0.528 0.538 0.572

0.577 0.583 0.768

8.35 8.58 11.05 11.63

______________.

0.576 0.784

  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _-_ _ _ _ _ _ _

  r->. tn LI J do 0%% o r w rKj% in r'J kr O

TABLE II

  I-V data in the dark for selectively plated solar cells

to + to

  ID cell Series Resistance Parallel Resistance 301 (A / cm2) normalized (Y-cm2) normalized (KA-cm2) 02 Metal (after Metal (after Laser (after Laser (after laser 3-4 agglomeration) laser * 3-5 agglomeration) 14 Agglomerations Agglomerations 2,34,138.9 1.70 3, 3 1.4 × 10-11 1, 7 × 0.01, 1.5 × 10 -6 1, 2 x 10 5 3.3 x 10-6 6.7 x 10- 1 0, 3 13.7 3.6 12 3.6x i0 0.48

  * j represents the current e f'd.te, DnS the mass + J represertte the leakage current dtans a zone separation.

0% \ 0 N)

R E V ENDICATIONS

  1) Method for forming a conductive pattern on the surface of a semiconductor, characterized in that it consists in exposing portions of the surface to the light of a laser of predetermined power density, and immersing this surface in a plating solution of a plating metal (4), thereby making it possible to press a plating metal onto the

  parts of the semiconductor surface that have been exposed to laser light.

  2) Method according to claim 1, characterized in that the laser light has a power density of 4.3 x 105 to 6.6 x 105watts / cm. 30) Method according to any one

  Claims 1 and 2, characterized in that the interface is exposed to laser light when

found in the plating solution.

  4) Process according to claim 1, characterized in that before exposure to light of a power density laser 3.9 x 105 to 6.4 x 105 watts / cm2, the surface of the semiconductor is covered by a layer of refractory metal (8) or noble metal; in that the coated surface of the coating is placed in a bath of a plating metal so that the plating metal is plated on those parts of the coating which have been exposed to the laser light; and in that the parts of the coating which have not been covered with the plating metal are etched with acid. 5) A method according to claim 4, characterized in that the thickness of the refractory metal is between 300 x 1-10 m and 1500 x 1-10 m, and in that the thickness of the plating metal is between 12 and

microns.

  6) Process according to any one

  Claims 1 to 5, characterized in that, as a last step, an anti-reflective coating 17 is applied to the surface, and agglomeration is performed

between 300 and 4500C.

  7) Method according to claim 1, characterized in that before exposure to light of a power density laser 3.9 x 105 to 6.4 x 105 watts / cm2, a diffusion barrier is formed on the surface covering the latter with a refractory metal, and a galvanic screen is formed by covering the surface of this refractory metal with a noble metal; that the surface is placed in a plating metal bath so that the plating metal is plated on the portions of the noble metal surface that have been exposed to the laser light; and in that the parts of the noble metal and the refractory metal which are not covered by the plating metal,

are attacked with acid.

  8) The method of claim 7, characterized in that the thickness of the refractory metal coating is 300 x -1'mo to 1500 x 1-10m ee that the thickness of the noble metal coating is 300x 1 -

  at 1500 x 1-10 m, and in that the thickness of the plating metal plating is 2 to 10 microns.

  9) Process according to any one

  claims 7 and 8, characterized in that the

  refractory metal is titanium and in that the metal

noble is palladium.

   Process according to any one of claims 4 to 9, characterized in that the

Refractory metal is titanium.

  11) Method according to any one of claims 1 to 10, characterized in that the

  Veneer metal is electroplated on the surface.

  12) Process according to any one

  claims I to 11, characterized in that the

  semiconductor is mono-crystalline silicon.

   Method according to one of Claims 1 to 12, characterized in that the

Veneer metal is money.

  14) Method according to any one of claims 1 to 13, characterized in that the

  laser light has a wavelength of about

5000 x 1-10 m.