Patent Application
20130095603
The present invention relates to a process for treating a metal contact produced on a substrate, a dielectric layer possibly being provided between the substrate and the metal.
The process according to the invention may especially be applied during manufacture of a photovoltaic cell.
Specifically, in this sort of application, metal contacts are deposited on the front and back side of the substrate in order to allow electrons generated via the photoelectric effect in the substrate to be collected.
One production process commonly used in the photovoltaic industry comprises the following steps.
A substrate, for example made of p-type silicon, is first cut to the desired size.
In order to improve the quality of the surfaces cut out in this way, chemical etching, for example with alkalis, is then carried out.
Generally, a step of texturing the front side is carried out in order to form an optical structure with the aim of trapping the photons of incident light in the substrate so as to increase the efficiency of the cell. This may for example be pyramid-shaped optical structures produced by chemical etching with sodium hydroxide.
The surfaces of the substrate are then doped n-type, for example by diffusion of phosphorus. A prior step may consist in carrying out an etch with an acid so as to neutralize any residual alkalis and remove any impurities since the surfaces of the substrate must be clean before the doping is carried out.
Next, the n-type doping of the vertical edges is removed in order to isolate these edges. This is for example achieved by plasma etching.
A dielectric layer is then deposited over all of the front side of the substrate, so as to provide an antireflection function. This dielectric layer may be produced by vapor deposition of silicon nitride (SiN).
A metal contact is then produced on the front and back sides of the substrate.
In particular, on the back side, a paste is deposited comprising an aluminum powder mixed with a solvent. This deposition is generally obtained by screen printing. The paste is deposited in a chosen pattern, in the form of a mesh or a uniform layer.
Next, the paste is heated in order to remove the solvent and leave behind only the aluminum. The heating is generally carried out in an oven at a temperature between 100° C. and 200°°C., so as to remove solvents and organic compounds.
This technique for depositing the metal pattern is very advantageous in terms of cost and alignment of said pattern relative to the substrate.
Finally a high-temperature step of annealing the substrate thus equipped with the dielectric layer and the front and back side metal patterns is carried out.
An “anneal” is conventionally defined in metallurgy as being a heat treatment the temperature profile of which comprises at least one period at a temperature above the melting point of the material in question.
This step allows a durable metal contact to be formed between said patterns and the substrate while removing the last of the nonmetal residues.
This step is a delicate step because it is necessary to control the thermal profile of the heating operation depending on the nature and composition of the metal paste. In particular there is a risk, if the anneal is too long and/or carried out at too high a temperature, that the photovoltaic cell will be degraded, the contacts possibly passing through the active zone of the cell.
Although a high-quality metal contact can be achieved between the metal patterns and the substrate with the process described above, the electrical conductivity of the metal contacts nevertheless remains limited. This is especially a result of the process used to deposit the metal, which process is based on deposition of a metal paste formed by mixing a powder of the metal in question with a solvent.
Specifically, when the paste has dried, the metal pattern has a structure consisting of an agglomerate of particles, which does not make it easy to obtain a low electrical resistance in the metal contact. A metal pattern 10 made of aluminum deposited on a substrate 11 made of silicon is shown by way of example in
In addition, this particle agglomerate is particularly prone to oxidation because this structure consisting of an agglomerate of particles has a large surface area.
This is particularly disadvantageous in the context of application to manufacturing photovoltaic cells. Similar problems may however arise in other applications, provided that a metal contact is to be formed on a substrate using a step of depositing a paste formed by mixing a metal powder with a solvent.
Thus one object of the invention is to increase the electrical conductivity of a metal contact produced on a substrate, a dielectric layer possibly being provided between the substrate and the metal pattern.
Another object of the invention is to improve the ability of a metal contact to withstand oxidation effects, the metal part of which contact is obtained with a paste formed by mixing a metal powder with a solvent.
To achieve at least one of these objects, the invention provides a process for obtaining a metal contact on a substrate, comprising the following steps:
(a) depositing a metal pattern in the form of a paste formed from mixing a metal powder with a solvent;
(b) heating the assembly thus formed in step (a) so as to evaporate the solvent; and
(c) carrying out an anneal in order to form an metal contact between the metal pattern and the substrate, characterized in that it furthermore comprises a step (d) in which the metal contact is heated using a laser with an energy density between 0.5 J/cm2 and 15 J/cm2.
The process according to the invention will possibly have other technical features according to the invention, whether alone or in combination, namely:
Other features, objects and advantages of the invention will be revealed by the detailed description, given below with reference to the following figures:
The invention relates to a process for treating a metal contact produced on a substrate, in which the contact has been obtained using the following steps (a), (b) and (c):
(a) depositing a metal pattern in the form of a paste formed from mixing a metal powder with a solvent;
(b) heating the assembly thus formed in step (a) so as to evaporate the solvent; and
(c) carrying out an anneal in order to form an metal contact between the metal pattern and the substrate.
Step (a) may be a step of screen printing.
The metal pattern deposited in step (a) may be at least 1 pm in thickness. The metal pattern obtained after steps (a) to (c) is an agglomerate of particles, as shown in
The metal pattern may take the form of a mesh or the form of a layer. The metal pattern may especially comprise silver, aluminum, or a silver-aluminum alloy.
The nature of the metal used in the paste is chosen depending on the type of metal contact desired. Thus, for a photovoltaic cell, a backside silver-aluminum alloy metal contact may be envisioned.
A dielectric layer may be provided between the metal pattern and the substrate. The process furthermore comprises a step (d) in which the metal contact is heated by a laser with an energy density between 0.5 J/cm2 and 15 J/cm2.
The electrical resistance of this contact is thus decreased, without damaging the metal contact or the substrate and without debonding the contact from the substrate.
As will be described in the rest of the description, a number of parameters may influence the value of the energy density obtained at the surface of the metal contact.
The laser 1, used in this device to heat the metal contact, may emit in the infrared range, for example at a wavelength of 1064 nm. This laser 1 may be a diode-pumped laser, such as a Nd:YAG laser emitting at 1064 nm and pumped at 808 nm by a laser diode.
The laser 1 described above is a laser that emits in the infrared range. Specifically, this wavelength range is the most critical for metal contacts produced on silicon substrates since silicon absorbs infrared radiation and risks being damaged by this radiation (strain induced by increasing volume).
As a variant, the laser used could be a laser that emits in the ultraviolet range or in the visible range (for example in the “green” at a wavelength of about 438 nm).
When the laser 1 is a laser-diode-pumped laser, the peak current drawn by the laser diode may be between 20 A and 30 A and preferably between 25 A and 28 A.
Above 30 A, the contact and the substrate run the risk of being damaged. Generally, in this case, partial ablation and then debonding of the contact is observed, with the contact being torn from the underlying substrate.
In this range of peak diode-current values, an energy density of between 0.5 and 15 J/cm2 may be obtained at the surface of the metal contact.
The electrical resistance of the metal contact is therefore substantially diminished, without damage. In addition, a metal contact that is durably fixed to the substrate is obtained, i.e. there is no risk of the contact and the substrate debonding (blistering effect).
The laser 1 may moreover be a pulsed laser.
In this case, the laser 1 may emit pulses at a repetition frequency of between 30 kHz and 60 kHz, and preferably between 40 kHz and 60 kHz.
This range of values for the repetition frequency promotes the decrease in the electrical resistance of the metal contact without adversely affecting the metal contact, the substrate or the connection between the two.
Moreover, the degree of coverage of the area of the metal contact between two pulses is at least 95% and preferably at least 97%. One of the following degrees of coverage may especially be envisioned: 97%, 98% or 99%.
The expression “degree of coverage” is understood to mean the percentage of the area of the contact that is subjected to two passes of the laser in succession in the scan direction. It will therefore be understood that these two passes are slightly shifted, perpendicular to the scan direction of the laser.
A high degree of coverage has the advantage of making it easier to obtain a minimum energy density and decreases the electrical resistance of the metal contact.
The scan rate of the laser may be lower than 10 m/s and preferably between 1 m/s and 10 m/s.
This rate range makes it possible to obtain a productivity that is acceptable from the industrial standpoint while preserving the metal contact and the substrate.
The pulse length of each pulse may moreover be between 1 ns and 1 μs.
The device shown in
Other possible designs may be conceived of with regard to
and to its thickness e by the relationship: Rc=
/e, expressed below in mΩ/carré.
In addition, the data presented in
In
A reference is shown by the dashed line in
This reference was measured after a metal contact had been produced using the prior-art process, the metal of said contact being aluminum in contact with a silicon substrate, a dielectric layer being provided between the two.
The reference metal contact was therefore not subjected to a laser treatment.
In other words, the reference metal contact was especially subjected to steps (a) to (c) but not to step (d), in contrast to the other tests carried out.
In this particular instance the reference sheet resistance was measured to be 10.5 mΩ/carré.
For all the tests carried out, the electrical resistance of the contact decreased relative to the reference for the entire range of pulse frequencies tested, namely from 30 kHz to 60 kHz, and moreover whether the value of the degree of coverage was 97%, 98% or 99%.
More precisely, the values of the electrical resistance obtained were between 5.1 and 8.7 mΩ/carré, i.e. a decrease of between −51.4% and −17.1% relative to the reference value. In particular, the lowest resistance was obtained at a frequency of 30 kHz and for a degree of coverage of 99%.
In
The reference (metal contact not subjected to laser treatment) is still shown by a dashed line in
For all of the tests carried out, the electrical resistance of the contact decreased relative to the reference for the entire range of tested pulse frequencies, namely 30 kHz to 60 kHz, and moreover whether the value of the degree of coverage was 97%, 98% or 99%.
More precisely, the values obtained for the electrical resistance were between 8.1 and 10.3 mΩcarré, i.e. a decrease of between about −22.9% and −2% relative to the reference value.
Generally, the resistance obtained for the metal contact with the tests shown in
This is related to the fact that the increase in the scan rate decreases the energy density impacting the metal contact.
For this scan rate of 5 m/s, a degree of coverage of 99% will preferably be chosen, which allowed the lowest resistances to be obtained in the entire range of frequencies tested.
In
The reference (metal contact not subjected to laser treatment) is still shown by a dashed line in
With a higher diode current value, relative to the tests shown in
By way of summary, if the laser is a pulsed laser-diode-pumped laser emitting in the infrared range, a person skilled in the art will be able to use the following conditions to obtain an energy density of between 0.5 J/cm2 and 15 J/cm2:
For the data shown in
After the tests shown in
Thus, the laser treatment had the effect of changing the particle-agglomerate structure into a structure that was observed to be more continuous than the particle-agglomerate structure.
This may be seen in appended
a) shows a cross-sectional view of a metal pattern 10 made of aluminum, obtained after implementing steps (a) to (c) of the process. This metal pattern was formed by a porous agglomerate of particles. In other words, there were free spaces between the metal particles, these spaces promoting oxidation of the surface of the metal particles.
b) shows the same cross-sectional view of this same pattern 10 after the laser treatment step (d) according to the invention had been carried out. In this particular instance, step (d) was carried out with a degree of coverage of 95% and a diode current of 26 A.
The metal pattern 10 thus obtained had a densified surface layer 101, qualified as being continuous since there were no longer any spaces allowing gas to penetrate into the core of the metal pattern. This surface layer was therefore not formed by an agglomerate of particles. Moreover, it will be observed that the size of the particles just below this continuous surface layer is generally larger than that of those in
This is to be related to the results shown in
Specifically, it will be understood that there is a link between the increase in the electrical conductivity of the metal contact and the existence of this densified surface layer, which may be qualified as continuous, on the metal pattern. In addition, this continuous surface layer forms a barrier to external air, thereby limiting oxidation effects over time and, therefore, allowing a good electrical conductivity to be preserved in use.
These figures all show cross-sectional views of a metal pattern 10 obtained by implementing steps (a) to (d) of the process under the same conditions, only the diode current differing. The degree of coverage was in particular kept at 95%.
Specifically, for
It will be observed that the thickness of the surface layer, which may be qualified as continuous, increases with the increase in the diode current. It will thus be understood that the more the diode current is increased, the higher the energy density of the laser beam becomes, and the more the thickness of the densified zone increases.
The treatment of the metal contact described above will advantageously be applied to the manufacture of photovoltaic cells.
The laser described above is a pulsed laser emitting in the infrared range. As a variant, it could however be envisioned to use a laser emitting light continuously, in the infrared, visible or ultraviolet range irrespectively.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1001007 | Mar 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB11/51042 | 3/11/2011 | WO | 00 | 12/3/2012 |
