Method for simultaneous doping and oxidation of semiconductor substrates and the use thereof
The invention relates to a method for simultaneous doping and oxidation of semiconductor substrates and also to doped and oxidised semiconductor substrates produced in this manner. Furthermore, the invention relates to the use of this method for producing solar cells.
Modern solar cell concepts contain, on the one hand, doped regions close to the surface, for example for producing the p-ii junction or so-called front- or back-surface field. A phosphorus diffusion into p-doped silicon for example can be applied here for emitter production. Furthermore, excellent solar cells have dielectricalry passivated surface regions which suppress the recombination of the produced charge carrier pairs and also advantageously affect the optical properties of the semiconductor component. Layers of this type can be produced with PVD methods or by thermal processes. In the case of silicon dioxide on silicon, thermal oxidation is implemented in the presence of oxygen and, for a moist oxidation, with the additional presence of water vapour. Currently, these process steps are implemented seqxientially, as a result of which the production process for example of solar cells is made complicated since it contains at least one thermal diffusion process and
one oxidation process. If these steps are implemented sequentially, further additional steps must be accepted which ensure that, in the process steps, only the regions of the wafers provided for this purpose are processed, e.g. masking or etching steps.
The individual method steps which are of importance for the solar cell production are intended to be explained subsequently in brief.
Diffusion of doping atoms can be effected in different ways. What is common to all processes known from the state of the art is that a doping agent source is present, from which the doping agent is transferred into the silicon under suitable conditions, This doping source can be present in the gaseous atmosphere, e.g. POCls, or can be deposited by suitable methods, e.g. phosphoric acid can be sprayed on. Furthermore, CVD processes can be used in order to produce doped layers,
In the process of ion implantation, the doping atoms are implanted in the wafer. The wafer is subjected for this purpose to high-energy particle beams containing doping atoms. The atoms then penetrate into the wafer and the doping is activated in a subsequent annealing step at increased temperature and distributed around as desired. During the activation, the atoms forced into the crystal lattice move towards free lattice sites and then can serve as doping agent. During the distribution, by means of diffusion of the doping atoms, the concentration profile of the doping atoms is changed by diffusion within the semiconductor. In both cases, an external doping atom source is no longer present during the thermal treatment and the particle beam is switched off.
The thermal oxidation of silicon is widely used in semiconductor technology. Essentially, silicon located on the surface of the Si crystal is oxidised in an oxygen-containing atmosphere at increased
temperatures. This oxide forms an SiO2/Si interface with the silicon substrate located thereunder. During the oxide growth, silicon is converted into oxide and the interface is moved such that the SiO2 layer thickness increases. The growth rate thereby reduces since the oxidising atmosphere components must diffuse through constantly thickening oxide layers towards the SiO2/Si interface. The kinetics of this reaction depend upon the crystal orientation, doping and upon the oxidising atmosphere components. For example, by adding water vapour (moist oxidation), the oxidation can be accelerated. Also DCE (trans- 1.,2-dichloroethyiene) can influence the reaction speed (0. Schultz, High-Efficiency Multicrystalline Silicon Solar Cells, Dissertation at the University of Konstanz, Faculty of Physics (2005), p. 103). Furthermore, the kinetics are determined very greatly by the temperature which prevails during the oxidation.
The SiO2/Si interface can be configured with suitable process control such that it is passivated. This means that the recombination rate of the minority charge carriers is reduced relative to an unpassivated surface (O. Schultz, High-Efficiency Multicrystalline Silicon Solar Cells, Dissertation at the University of Konstanz, Faculty of Physics (2005), p. 104 ff.).
A process in which impurities can be transferred specifically from one region of the semiconductor into another is termed gettering (A. A. Istratov et al., Advanced Gettering Techniques in UL-SI Technology, MRS Bulletin (2000), pp. 33 - 38), This process can be performed by different methods. One is phosphorus gettering. During a phosphorus diffusion, silicon intermediate lattice atoms which increase the mobility of many types of impurities are produced. Due to the higher solubility of these components in highly-doped silicon regions, these collect during the high temperature step consequently in these areas and the volume of the semiconductor is cleaned,
Since no gettering is observed during pure oxidation, this process is particularly susceptible to impurities, which are located either on or in the substrate, in contaminated process and handling devices or in contaminated process gases or process aids.
Starting herefrom, it was the object of the present invention to optimise the known methods for producing solar cells with respect to the individual method steps and to make possible simpler production.
This object is achieved by the method having the features of claim 1 and the accordingly produced doped and oxidised semiconductor substrate having the features of claim 29, Furthermore, the use having the features of claim 30 is provided. The further dependent claims reveal advantageous developments.
According to the invention a method for simultaneous doping and oxidation of semiconductor substrates is provided, in which at least one surface of the semiconductor substrate is coated at least in regions with at least one layer comprising a doping agent. Also a plurality of doping agents can be comprised in the at least one doping agent layer. Subsequently, a thermal treatment is then effected in an atmosphere comprising an oxidant for the semiconductor material, as a result of which diffusion of the doping agent into the volume of the semiconductor substrate is made possible. During the thermal treatment, a partial oxidation of the surface regions of the semiconductor substrate which are not coated with the doping agent layer is likewise effected, Thus two process steps can be combined in a simple manner, which leads to simplification of the overall process.
Preferably, the layer containing the doping agent consists of a material or comprises a material which is selected from the group consisting of amorphous silicon, silicon dioxide, silicon carbide, silicon nitride, aluminium oxide, titanium dioxide, tantalum oxide, dielectric materials,
ceramic materials comprising organic compounds which can be altered chemically in the diffusion process, non-stoichiometric modifications of these materials and mixtures of these materials. There should be understood by non-stoichiometric modifications, for example with respect to silicon nitride, compounds which deviate from the stoichiometric ratio SiaN4.
It is likewise possible, as is known from semiconductor technology, to use substances which are present for example initially in liquid or pasty form. These are then deposited on the semiconductor, for example by centrifugation, spraying, dip coating, printing or CVD. Subsequently, a drying step can then follow in which a part of the organic components escape. In a further step, the substance can then be converted into a glass-like consistency which then serves, in the subsequent high-temperature process, as diffusion source or also as barrier. Substances of this type can be produced and processed according to the known sol-gel method.
The doping agent is preferably selected from the group consisting of phosphorus, boron, arsenic, aluminium and gallium.
Preferably, the layer comprising the doping agent has a concentration gradient with respect to the doping agent, a higher doping agent concentration prevailing in the region orientated towards the semiconductor substrate.
Various alternatives exist with respect to the coating of the semiconductor substrate. Thus a first preferred variant provides that the semiconductor substrate is coated continuously on one surface with a layer comprising a doping agent and subsequently, by thermal treatment with an atmosphere containing an oxidant, a partial oxidation of the non-coated surfaces, e.g. the rear-side of the semiconductor substrate, is effected, Another variant provides that one
or more surfaces of the semiconductor substrate are coated merely in regions with a layer comprising a doping agent, as a result of which also uncoated regions remain. In the subsequent oxidation step, a partial oxidation of the non-coated surfaces of the semiconductor substrate is then effected.
Basically, it is the case that the method according to the invention can be combined at any time with any process steps which are known from processing semiconductor substrates and in particular in the production of solar cells,. Hence it is for example possible that for the semiconductor substrate to have been treated at least in regions before the coating with the layer comprising the doping agent. However it is likewise possible also that a treatment is implemented after the coating with the layer comprising the doping agent and before the thermal treatment. The treatment steps are hereby selected preferably from the group consisting of wet-chemical or dry-chemical processing, thermal processing, coating, mechanical processing, laser technology processing, metallisation, silicon processing, cleaning, wet- or dry-chemical texturing, removal of texturing and combinations of the mentioned treatment steps, There are here a large number of combinations between the mentioned treatment steps. For example, the semiconductor substrates can be processed after the coating with the doping agent -with the aim of preparing the uncoated regions for the thermal treatment. This can include for example that existing textures are levelled entirely or partially or that different cleaning processes are implemented. The cleaning can thereby be both of a wet-chemical and dry-chemical nature. Another example concerns the removal at least in regions of existing coatings with the aim of achieving a structuring of the coating or else in order to remove parasitic coatings on for example the rear-side,
A further preferred variant provides that the coated semiconductor substrate is treated wet- or dry-chemically before the thermal

treatment. Likewise the possibility exists of etching the uncoated parts of the semiconductor whilst the coating masks the remaining semiconductor, In this way, suitable starting conditions for the thermal oxidation can be created, in particular a very high passivation quality can be achieved.
A preferred variant provides that a further coating is applied on the semiconductor substrate. Thus for example the layer comprising the doping agent on the side orientated away from the semiconductor substrate can be provided with a cover layer as diffusion barrier for the doping agent in order to prevent escape of the doping agent. This cover layer preferably comprises a material which is selected from the group consisting of amorphous silicon, silicon dioxide, silicon carbide, silicon nitride, aluminium oxide, titanium dioxide, tantalum oxide, dielectric materials, ceramic materials, materials comprising organic compounds which can be altered chemically in the diffusion process, non-stoichiometric modifications of these materials and mixtures of these materials. In a further preferred variant, the cover layer can also have a multilayer construction, the different layers comprising different materials,
In a preferred variant the at least one coating can be effected such that the coating material is deposited in liquid or pasty form on the semiconductor substrate or on the coatings already applied on the semiconductor substrate. This can be effected preferably by centrifugation, spraying, dip coating, printing or CVD methods. Subsequently, a drying step can be effected, in which a part of the organic components is removed. In a further step, the coating material can then be converted into a glass-like consistency and serves, during the subsequent high-temperature process, as diffusion source or also as barrier. Coating materials of this type can also be produced and processed according to the sol-gel method. However, also coating methods and doping methods, as are known from the state of the art,
can likewise be applied, In this respect, reference is made to S.K. Ghandi, VLSI Fabrication Principles: Silicon and Gallium Arsenide, 2nd edition (1994) chapter 8, pp, 510 - 586 .
A further variant according to the invention provides that, between semiconductor substrate and the at least one doping agent layer, at least one further layer is applied, through which the diffusion of the doping agent into the volume of the semiconductor substrate is not
completely suppressed or obstructed. For example, normally a native
\ silicon dioxide layer is formed on silicon, said silicon dioxide layer being
so thin that doping of the silicon cannot be masked thereby. It is also possible that other layers are still present from preceding processes or process steps by means of which the diffusion is however not suppressed.
The thermal treatment in the method according to the invention is effected preferably in a tubular furnace or a continuous furnace. However, it is also basically conceivable that the thermal treatment is implemented directly in a PBCVD reactor. The thermal treatment is thereby effected preferably at temperatures in the range of 600 to 1150°C.
Various method variants exist with respect to the oxidation step. Thus a dry oxidation can be implemented using oxygen as oxidant. A further preferred variant provides that a moist oxidation is implemented, i.e. oxygen is used as oxidant in the presence of water vapour. The atmosphere used for the oxidation can contain in addition further compounds for controlling the oxidation process. Likewise, compounds can be added to the atmosphere for maintaining the cleanliness of the same. There is included for this purpose in particular trans-1,2-dichloroethane.
The semiconductor substrate preferably consists of silicon, germanium or gallium arsenide. Likewise, already doped semiconductor substrates, which are doped e.g. with phosphorus, boron, arsenic, aluminium and/or gallium, can also be used. However it is preferred in particular that the semiconductor substrate in the regions close to the surface has, in addition to already present dopings, at most a slight doping which stems from the previously deposited doping agent source and has been formed by an additional thermal treatment before the simultaneous diffusion and oxidation. In the final thermal treatment, the diffusion of these doping agents is then reinforced.
It is likewise possible that the semiconductor substrate, even before implementation of the method according to the invention has structures at least in regions, e.g. in the form of masking, which suppress or obstruct thermal oxidation of the semiconductor substrate in these regions.
A further variant according to the invention provides that, during the process, a gettering process is implemented by enriching impurities in doped regions in the semiconductor substrate. This is possible in particular during the doping with phosphorus in the thermal process. Gettering takes place during the phosphorus diffusion as a side effect. The impurities diffuse into the regions of high phosphorus concentrations since there they dissolve better than in the remaining volume. They have less influence on the semiconductor component there. In the case of a pure oxidation process, as is known from the state of the art, no gettering process results so that very high purity requirements must be maintained here. Hence the method according to the invention, relative to the state of the art, also has the advantage that, with respect to the purity conditions, high requirements of this type do not require to be maintained, which can be attributed to the gettering process taking place in parallel.

According to the invention, a doped and oxidised semiconductor substrate which can be produced according to the above-described method is likewise provided.
The above-described method is used in particular in the production of solar cells.
The invention is intended to be represented subsequent^ by the concrete example of a boron-doped silicon substrate as semiconductor substrate and a phosphorus-containing silicon dioxide as doping agent source.
The silicon wafer 1 is coated on one side for example in a so-called PECVD coating plant with a phosphorus-containing silicon oxide 2 (Fig. 1).
The silicon oxide 2 serves as phosphorus source and layer 3 as barrier against escaping phosphorus. The other side of the disc remains uncoated. The thus-uncoated disc can now be cleaned again in order to pretreat the uncoated side for the subsequent thermal process. This cleaning can be implemented by wet- or dry technology. If steps which attack the layer 3 are included in this cleaning, these steps must then be chosen to be brief such that the properly of the layer 3 to serve as diffusion barrier is not lost. Correspondingly, the layer can also be formed to be suitably thick.
A high-temperature step now follows subsequently. This step is essentially characterised in that, on the side coated with layer 2, the phosphorus from layer 2 penetrates into the silicon and a suitable doping concentration 4 is achieved in the wafer. Simultaneously a thermally grown silicon dioxide 5 is formed on the non-coated regions of the wafer (Fig. 2). This silicon dioxide is produced if the atmosphere in the furnace in which the high-temperature process is implemented
contains oxygen. In addition to the oxygen, also water vapour or other suitable substances can be contained in the atmosphere, which enable the oxidation process or have an advantageous effect, for example acceleration. The above-mentioned layers 2 and 3 can also be combined to form one layer which has a suitable course of the concentration of the doping agent so that the latter is prevented from escaping from the layer into the process atmosphere to an undesired extent. Essentially, the layer must ensure in this way merely that the side to be oxidised is not disadvantageous^ effected by escaping doping agent.
As already described above, coating in regions is also possible. This can be effected by using corresponding masks or even by targeted back-etching, In Fig. 3, a silicon wafer 1 is represented before the thermal treatment for simultaneous diffusion and oxidation. A first surface here has regions with a phosphorus-containing silicon oxide layer 2. The silicon oxide 2 thereby serves as phosphorus source. At the same time, cover layers made of silicon dioxide 3 are deposited on these regions. Due to the thermal treatment for diffusion and oxidation, a structure is then obtained as is represented in Fig. 4. This high-temperature step has the effect that the phosphorus from layer 2 penetrates into the silicon wafer 1 on the side coated with layer 2 and a suitable doping concentration 4 in the wafer is achieved. At the same time, a thermally grown silicon dioxide 5 is formed on the non-coated regions of the wafer.
The above-described invention can be used in various ways, for example for the production of solar cells. Two possible process variants are represented subsequently:
Process variant A
Firstly a rear-side suitable cover layer is applied and thereafter an etching step is implemented, in which the layers 2 and 3 are removed. The cover layer thereby protects the layer 5 situated thereunder. The material choice for this layer is very wide. The layer can comprise for example a dielectric, a metal, a ceramic material or a layer system. Subsequently, an antireflection coating 7 is deposited on the front-side of the wafer {Fig, 5),
Thereafter, the rear-side layer system is opened locally with a suitable method, e.g. with a laser (Fig, 6).
Subsequently, a suitable contact paste is disposed, e.g. by means of screen printing, with a suitable method on the front-side and on the rear-side in a freely selectable sequence. Pastes which allow a simple subsequent wiring of the solar cells in modules can also be combined on the rear-side (Fig. 7).
In the subsequent step, the contacts are formed in that the silicon disc is subjected to a suitable thermal process. This so-called contact sintering can be implemented for example in a sintering furnace, as is known already at the present time in solar cell production technology (Fig. 8).
The production process of the solar cell is now essentially concluded, Further process steps with which the component is finished can also be introduced or added here. For example, wet chemical surface treatments can take place initially in order to reduce the reflection of the silicon disc by means of a so-called texturing. In addition, thermal healing steps or laser processes for edge insulation can be applied.
Process variant B
After deposition of the antirefiection coating according to Pig. 3 in variant A, the contact paste is disposed here on the front-side. The disc is subsequently treated in a suitable thermal process, the front-side contact being formed (Fig. 9).
Subsequently, a suitable metal layer is disposed on the rear-side of the solar cell. This step can also be combined with the preceding. However, it is essential here that the metal layer does not penetrate the layer sequence situated thereunder as far as the silicon (Fig. 10).
Finally, the rear-side metal layer is processed with a laser in such a manner that it penetrates the layer sequence situated thereunder on regions provided for this purpose and produces an electrical contact to the silicon, If the metal layer is for example aluminium-containing, then it can also form a local p++ doping at the points of the laser processing.
The production process of the solar cell is now essentially concluded. Further process steps with which the component is finished can also be introduced or added here. For example wet chemical surface treatments can take place initially in order to reduce the reflection of the silicon disc by means of a so-called texturing. Furthermore, thermal healing steps or laser processes for edge insulation can be applied.

Patent claims
1. Method for simultaneous doping and oxidation of semiconductor
substrates, in which at least one surface of the semiconductor
substrate is coated at least in regions with at least one layer
comprising at least one doping agent and subsequently, by means
of a thermal treatment in an atmosphere comprising an oxidant
for the semiconductor material, a diffusion of the doping agent
into the volume of the semiconductor substrate is made possible
and oxidation of the surface regions of the semiconductor
substrate which are not coated with the doping agent layer is
effected,
2. Method according to claim 1,
characterised in that the layer comprising the doping agent consists of a material or comprises a material which is selected from the group consisting of amorphous silicon, silicon dioxide, silicon carbide, silicon nitride, aluminium oxide, titanium dioxide, tantalum oxide, dielectric materials, ceramic materials, materials comprising organic compounds which can be altered chemically in the diffusion process, non-stoichiometric modifications of these materials and mixtures of these materials.
3. Method according to one of the preceding claims,
characterised in that the doping agent is selected from the group
consisting of phosphorus, boron, arsenic, aluminium and gallium,
4. Method according to one of the preceding claims,
characterised in that the layer comprising the doping agent has a
concentration gradient with respeci to the doping agent, a higher

doping agent concentration prevailing in the region orientated towards the semiconductor substrate.
5. Method according to one of the preceding claims,
characterised in that the semiconductor substrate is coated on a
surface of the semiconductor substrate with at least one layer
comprising at least one doping agent and subsequently, by means
of a thermal treatment in an atmosphere comprising an oxidant
for the semiconductor material, a diffusion of the doping agent
into the volume of the semiconductor substrate is made possible
and oxidation of the surfaces of the semiconductor substrate
which are not coated with the doping agent layer is effected.
6. Method according to one of the preceding claims,
characterised in that at least one surface of the semiconductor
substrate is coated in regions with at least one layer containing at
least one doping agent and subsequently, by means of a thermal
treatment in an atmosphere comprising an oxidant for the
semiconductor material, a diffusion of the doping agent into the
volume of the semiconductor substrate is made possible and
oxidation of the non-coated surface regions of the semiconductor
substrate is effected.
7. Method according to one of the preceding claims,
characterised in that the semiconductor substrate has been
treated at least in regions before the coating with the layer
comprising the doping agent.
8. Method according to one of the preceding claims,
characterised in that, after the coating with the layer comprising
the doping agent and before the thermal treatment, at least one
further treatment step of the semiconductor substrate is effected.

9. Method according to one of the two preceding claims,
characterised in that the treatment steps are selected from the
group consisting of wet-chemical or dry-chemical processing,
thermal processing, coating, mechanical processing, laser
technology processing, metallisation, silicon processing, cleaning,
wet- or dry-chemical. texturing, removal of texturing and also
combinations of the mentioned treatment steps,
10. Method according to one of the preceding claims,
characterised in that at least one further coating is applied on the
semiconductor substrate.
11. Method according to one of the preceding claims,
characterised in that the layer comprising the doping agent on the
side orientated away from the semiconductor substrate is
provided with a cover layer as diffusion barrier for the doping
agent.
12. Method according to the preceding claim,
characterised in that the cover layer consists of a material or comprises a material which is selected from the group consisting of amorphous silicon, silicon dioxide, silicon carbide, silicon nitride, aluminium oxide, titanium dioxide, tantalum oxide, dielectric materials, ceramic materials, materials comprising organic compounds which can be altered chemically in the diffusion process, non-stoichiometric modifications of these materials and mixtures of these materials.
13. Method according to one of the two preceding claims,
characterised in that the cover layer has a multilayer
construction.
14. Method according to one of the preceding claims,
characterised in that the at least one coating is effected such that
the coating material is deposited in liquid or pasty form on the
semiconductor substrate or on already present coatings.
15. Method according to the preceding claim,
characterised in that the deposited layer is dried and also converted subsequently into a glass-like consistency.
16. Method according to the preceding claim,
characterised in that the deposition of the coating material is effected by centrifugation, spraying, dip coating, printing and/or CVD.
17. Method according to one of the two preceding claims,
characterised in that at least one coating material consists of a
sol-gel,
18. Method according to one of the preceding claims,
characterised in that, between semiconductor substrate and the
at least one doping agent layer, at least one further layer is
applied, through which the diffusion of the doping agent into the
volume of the semiconductor substrate is not completely
suppressed.
19. Method according to one of the preceding claims,
characterised in that the thermal treatment is effected in a
tubular furnace or in a continuous furnace.
20. Method according to one of the preceding claims,
characterised in that the thermal treatment is implemented at a
temperature in the range of 600 to 1150°C.

2L Method according to one of the preceding claims,
characterised in that a dry oxidation is implemented using oxygen as oxidant.
22. Method according to one of the preceding claims,
characterised in that a moist oxidation is implemented using
oxygen as oxidant in the presence of water vapour,
23. Method according to one of the preceding claims,
characterised in that the atmosphere used for the oxidation
comprises further compounds for controlling the oxidation or for
maintaining the cleanliness of the atmosphere, in particular
trans-1,2-dichloroethane.
24. Method according to one of the preceding claims,
characterised in that the semiconductor substrate consists of
silicon, germanium or gallium arsenide.
25. Method according to one of the preceding claims,
characterised in that the semiconductor substrate is doped with
phosphorus, boron, arsenic, aluminium and/or gallium,
26. Method according to one of the preceding claims,
characterised in that the semiconductor substrate in the regions
close to the surface has, in addition to already present dopings, at
most a slight doping which stems from the previously deposited
doping agent source and has been formed by an additional
thermal treatment before the simultaneous diffusion and
oxidation.
27. Method according to one of the preceding claims,
characterised in that the semiconductor substrate has structures
at least in regions which suppress or obstruct thermal oxidation
of the semiconductor substrate in these regions.
28. Method according to one of the preceding claims,
characterised in that, during the process, a gettering process is
effected by enriching impurities in the doped regions in the
semiconductor substrate.
29. Doped and oxidised semiconductor substrate producible
according to one of the preceding claims.
30. Use of the method according to one of the claims 1 to 28 for the
production of solar cells.