Patent Application
20090288708
The invention relates to a method for passivating at least a part of a surface of a semiconductor substrate.
Such a method is known from practice. In the known method, a substrate surface of a semiconductor substrate is passivated by realizing a SiOx layer, for instance a layer of silicon oxide, on that surface. Here, for instance, use can be made of an oxidation method in an oven. Another known method comprises sputtering the SiOx layer. Further, it is known from practice to deposit silicon oxide on a substrate by means of chemical vapor deposition.
The extent of surface passivation is usually expressed by the surface recombination velocity (SRV). A good surface passivation of the semiconductor substrate usually means a relatively low surface recombination velocity.
From the article “Plasma-enhanced chemical-vapor-deposited oxide for low surface recombination velocity and high effective lifetime in silicon”, Chen et al, Journal of Applied Physics 74(4), Aug. 15, 1993, pp. 2856-2859, a method is known in which a low SRV (<2 cm/s) can be obtained with virtually intrinsic silicon substrates, which have relatively high resistivities (>500 Ωcm). In this known method, use is made of a direct plasma enhanced chemical-vapor deposition (PECVD) and a subsequent thermal anneal in a forming gas at preferably 350° C.
Up to now, it has still been found a problem to properly passivate a substrate with a relatively low resistivity, at least such that a relatively low surface recombination velocity can be reached, in particular using deposition of a SiOx layer. Such a passivated semiconductor substrate is, for instance, desired for the manufacture of solar cells.
The present invention contemplates obviating above-mentioned drawbacks of the known method. In particular, the invention contemplates a method for passivating a semiconductor substrate, in which a SiOx layer obtained with the method has a relatively low surface recombination velocity, while the substrate particularly has a relatively low resistivity.
To this end, the method according to the invention is characterized in that at least one layer comprising at least one SiOx layer is realized on above-mentioned part of the substrate surface by:
while at least the at least one layer realized on the substrate is subjected to a temperature treatment in a gas environment, while the temperature treatment particularly comprises a forming gas anneal treatment.
It is found that, in this manner, a good surface passivation of the substrate, at least of above-mentioned part of the substrate surface, can be obtained, in particular when the semiconductor substrate inherently has a relatively low resistivity. Preferably, during above-mentioned temperature treatment, at least above-mentioned SiOx layer realized on the substrate is maintained at a treatment temperature which is higher than 350° C. It is found that, with such a temperature treatment, particularly good results can be obtained. The treatment temperature may, for instance, be in the range of approximately 250° C.-1000° C., in particular in the range of approximately 500° C.-700° C., more in particular in the range of approximately 550° C.-650° C. The temperature treatment may, for instance, take less than approximately 20 min.
After that temperature treatment, the substrate may, for instance, be cooled, optionally in a forced manner. In addition, preferably, a gas flow is supplied to above-mentioned substrate or at least the SiOx layer realized and the substrate during above-mentioned temperature treatment, to provide above-mentioned gas environment. Thus, the temperature treatment may, for instance, comprise a forming gas anneal treatment. The gas environment may, for instance, be provided by supplying a mixture of nitrogen and hydrogen to the substrate and/or the at least one layer realized on the substrate. In that case, the gas environment may, for instance, substantially comprise a hydrogen-nitrogen environment. On the other hand, the gas environment may, for instance, substantially comprise hydrogen gas, for instance by supplying a hydrogen gas flow to the substrate and/or the at least one layer realized on the substrate. The gas or gas mixture then preferably has substantially the same above-mentioned substrate treatment temperature.
According to one aspect of the invention, a method for passivating at least a part of a surface of a semiconductor substrate is characterized in that at least one layer comprising at least one SiOx layer is realized on above-mentioned part of the substrate surface by:
placing the substrate in a process chamber;
maintaining the pressure in the process chamber at a relatively low value;
maintaining the substrate at a specific treatment temperature;
generating a plasma by means of at least one source mounted on the process chamber at a specific distance from the substrate surface;
contacting at least a part of the plasma generated by each source with the above-mentioned part of the substrate surface; and
supplying at least one precursor suitable for SiOx realization to the above-mentioned part of the plasma;
while then H2 or a mixture of H2 with an inert gas, for instance N2 or Ar, is supplied to above-mentioned plasma, in particular for annealing the at least one layer and/or for increasing the diffusion of H2 in the at least one layer.
The invention further provides a solar cell, which is provided with at least a part of a substrate at least obtained with a method according to the invention. Such a solar cell can use improved properties in an advantageous manner, for instance a relatively low surface recombination velocity of the substrate surface, which is favorable to the performance of the solar cell.
Further elaborations of the invention are described in the subclaims. The invention will now be elucidated on the basis of a non-limitative exemplary embodiment and with reference to the drawing, in which:
In this patent application, same or corresponding measures are designated by same or corresponding reference symbols. In the present application, a value provided with a term like “approximately”, “substantially”, “about” or a similar term can be understood as being in a range between that value minus 5% of that value on the one hand and that value plus 5% of that value on the other hand.
As shown in
The apparatus is particularly provided with supply means 6, 7 for supplying flows of suitable treatment fluids to the plasma P in, for instance, the anode plate 12 of the source 3 and/or in the process chamber 5. Such supply means may in themselves be designed in different manners, which will be clear to a skilled person. In the exemplary embodiment, the supply means comprise, for instance, an injector 6 designed for introducing one or more treatment fluids into the plasma P near the plasma source 3. The supply means further comprise, for instance, a shower head 7 for supplying one or more treatment fluids to the plasma P downstream of the above-mentioned plasma outflow opening 4 near the substrate 1. Alternatively, such a shower head 7 is, for instance, arranged near or below the plasma source 3 in the process chamber 5. The apparatus is provided with sources (not shown) which are connected to the above-mentioned supply means 6, 7 via flow control means, for supplying specific desired treatment fluids thereto. In the exemplary embodiment, during use, preferably no reactive gases, such as silane, hydrogen and/or oxygen, are supplied to the plasma in the plasma source 3, so that the source 3 cannot be affected by such gases.
For the purpose of passivating the substrate 1, during use, the cascade source 3 generates a plasma P in the described manner, such that the plasma P contacts the substrate surface of the substrate 1. Flows of treatment fluids suitable for SiOx deposition are supplied to the plasma P in a suitable ratio via the supply means 6, 7. The process parameters of the plasma treatment process, at least the above-mentioned process chamber pressure, the substrate temperature, the distance. L between the plasma source 3 and the substrate 1, and the flow rates of the treatment fluids are preferably such that the apparatus deposits the SiOx layer on the substrate 1 at an advantageous velocity which is, for instance, in the range of approximately 1-15 nm/s, which, for instance, depends on the velocity of the apparatus.
The above-mentioned substrate temperature, at least during the deposition of SiOx, may, for instance, be in the range of 250-550° C., more in particular in the range of 380-420° C.
Above-mentioned treatment fluids may comprise various precursors suitable for SiOx deposition. Thus, for instance, D4 and O2 can be supplied to above-mentioned part of the plasma P, for depositing a SiOx layer on the substrate surface, which is found to yield good results. The D4 (also known as octamethyltetrasiloxane) may, for instance, be liquid, and, for instance, be supplied to the plasma via above-mentioned shower head 7. The O2 may, for instance, be gaseous and be supplied to the plasma via above-mentioned injector 6.
Further, the at least one precursor may, for instance, be selected from the group consisting of: SiH4, O2, NO2, CH3SiH3 (1MS), 2(CH3)SiH2 (2MS), 3(CH3)SiH (3MS), siloxanes, hexamethylsiloxane, octamethyltrisiloxane (OMTS), bis(trimethylsiloxy)methylsilane (BTMS), octamethyltetrasiloxane (OMCTS, D4) and TEOS. The precursor may further comprise one or more other substances suitable for SiOx.
Since the plasma cascade source operates with DC voltage for generating the plasma, the SiOx layer can simply be grown at a constant growth rate, substantially without adjustment during deposition. This is advantageous compared to use of a plasma source driven with AC. Further, with a DC plasma cascade source, a relatively high growth rate can be obtained. It is found that, with this apparatus, a particularly good surface-passivated substrate can be obtained, in particular with a SiOx layer, with a relatively low substrate resistivity.
Optionally, on above-mentioned SiOx layer, for instance, a SiNx may be provided, which may, for instance, form an anti-reflection coating. Further, such a SiNx layer may, for instance, supply hydrogen to the deposited SiOx layer for the purpose of passivation of the substrate. Preferably, the SiOx layer and SiNx layer are successively deposited on the substrate 1 by the same deposition apparatus. To this end, for instance, precursors suitable for SiNx deposition (for instance NH3, SiH4, N2, and/or other precursors) can be supplied to the plasma P via above-mentioned supply means 6, 7 after deposition of the SiOx layer. The layer stack comprising at least one SiOx layer and at least one SiNx layer is also called a SiOx/SiNx stack. The thickness of above-mentioned SiNx layer is preferably in the range of approximately 25 to 100 nm, and may, for instance, be approximately 80 nm.
Further, for instance, an apparatus may be provided for subjecting the substrate to a temperature treatment after deposition of one or more of above-mentioned layers, for instance to a forming gas anneal treatment in which suitable gases are supplied to the substrate. In this manner, the temperature treatment can be carried out in a gas environment, at least such that at least one surface of the at least one layer realized on the substrate is contacted with those gases. A gas mixture suitable for the temperature treatment is, for instance, a hydrogen-nitrogen mixture. Such a temperature treatment may, for instance, be carried out in a separate thermal treatment apparatus, a separate forming gas anneal apparatus, or the like.
Further, during use of the apparatus, for instance H2 or a mixture of H2 and an inert gas such as N2 or Ar may be supplied to a plasma P, after the at least one layer has been realized on the substrate, for instance for increasing the diffusion of H2 in the SiOx layer or the SiOx/SiNx stack. Such a plasma treatment may, for instance, be carried out instead of the above-mentioned temperature treatment or in combination with such a temperature treatment.
A silicon oxide (SiO) layer was deposited on a substrate surface of a monocrystalline silicon with a method according to the invention, using an above-described apparatus shown in the Figures. The layer thickness was approximately 100 nm. The substrate inherently had a relatively low resistivity, for instance a resistivity of less than approximately 10 Ωcm. In particular, use was made of an n-doped silicon wafer with a resistivity of 1.4 Ωcm.
In order to deposit the above-mentioned silicon oxide layer on the substrate surface, in this example, use was made of flows of the precursors D4 and O2. Here, for instance, a D4 flow rate of approximately 5-10 grams per hour was used and a O2 flow rate of approximately 200 sccm (standard cm−3 per minute). The deposition treatment temperature of the substrate was, during the SiOx deposition, approximately 400° C.
The deposited silicon oxide layer was optionally provided with a SiNx layer, using N2H3 and SiH4, for forming a SiOx/SiNx stack on the substrate. The SiNx layer may, for instance, supply hydrogen to the deposited SiOx layer, and also serve as an anti-reflection layer.
After the deposition of above-mentioned layer/layers, the substrate was subjected to a temperature treatment, for instance using a suitable forming gas anneal apparatus. During this temperature treatment, the deposited SiOx layer and/or SiNx layer was maintained at a treatment temperature of approximately 600° C. for a treatment period of approximately 15 min, and in particular at an atmospheric pressure. In addition, during the temperature treatment, a 90% N2-10% H2 gas mixture was supplied to a surface of the SiOx layer and/or the SiOx/SiNx stack for obtaining a forming gas anneal. After above-mentioned approximately 15 min, the substrate and the at least one layer provided thereon were cooled.
The SiOx layer or SiOx/SiN2 stack thus obtained was found to have a stable, particularly low surface recombination velocity of approximately 50 cm/s. Therefore, the substrate treated in this manner, which has a very low resistivity, is particularly well suitable for use as, for instance, a ‘building block’ of solar cells.
It goes without saying that various modifications are possible within the framework of the invention as it is set forth in the following claims.
Thus, substrates of various semiconductor materials can be used to be passivated by the method according to the invention.
In addition, the method may, for instance, be carried out using more than one plasma source mounted on a process chamber.
Further, the substrate may, for instance, be loaded into the process chamber 5 from a vacuum environment, such as a load lock brought to a vacuum and mounted to the process chamber. In that case, the pressure in the process chamber 5 can maintain a desired low value during loading. In addition, the substrate may, for instance, be introduced into the process chamber 5 when that chamber 5 is at an atmospheric pressure, while the chamber 5 is then closed and pumped to the desired pressure by the pumping means.
In addition, the plasma source may, for instance, generate a plasma which exclusively contains argon.
Further, for instance, one or more layers can be applied to the substrate, for instance one or more SiOx layers and one or more optional other layers such as for instance SiNx layers.
Further, preferably, a whole surface of a substrate is passivated by means of a method according to the invention. Alternatively, for instance, only a part of the surface may be passivated by means of the method.
Further, the thickness of the SiOx layer realized on the substrate by means of the plasma treatment process may, for instance, be in the range of 10-1000 nm.
Further, a plasma with O2 as a precursor may, for instance, modify a substrate surface of the substrate, or part of the surface, into SiOx, so that above-mentioned SiOx layer is realized. In that case, the SiOx layer is, in particular, not realized by means of deposition, but by means of modification. The SiOx layer may also be realized in a different manner.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1029647 | Jul 2005 | NL | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NL2006/000393 | 7/28/2006 | WO | 00 | 6/26/2009 |
