WO2010052541A2

WO2010052541A2 - Oxidation and cleaning process for silicon wafers

Info

Publication number: WO2010052541A2
Application number: PCT/IB2009/007310
Authority: WO
Inventors: Ainhoa Esturo-Breton; Steffen Keller
Original assignee: Centrotherm Photovoltaics Technology Gmbh
Priority date: 2008-11-07
Filing date: 2009-11-04
Publication date: 2010-05-14
Also published as: DE102008056455B3; WO2010052541A3; TW201027617A

Abstract

Oxidation and cleaning process for silicon wafers, in which the silicon wafers are provided with a silicon oxide layer on at least part of their surface (12; 24), before they are etched in an alkaline etching solution (14;, 26) and they are etched in a solution (16) which contains an acid which oxidises metallic impurities, whereby at least a portion of the silicon oxide layer is exposed unprotected to the etching solution and the acid, and in which the silicon wafers are rinsed in deionised water after the etching processes (18), whereby the at least one unprotected portion of the silicon oxide layer is at least partly left on the silicon wafers and the silicon wafers are dried (20) after rinsing (18; 32).

Description

Oxidation and cleaning process for silicon wafers

The invention concerns an oxidation and cleaning process for silicon wafers according to the preamble of claim 1 and also a solar cell produced using this process.

By far the majority of current solar cell production relates to crystalline silicon solar cells. The starting point for the manufacture of solar cells is silicon wafers. When these silicon wafers are produced by splitting silicon blocks, their crystal structure is badly damaged on the surface as a result of the sawing, which has a detrimental effect on the efficiency of the later solar cell. As a result, the saw damage described is usually removed, normally by overetching the silicon wafers. If, however, silicon wafers are used which have already been drawn from a silicon melt in the desired thickness, the sawing step is, at least in most cases, omitted. Nevertheless, these silicon film materials are often accompanied by surface impurities, which have a detrimental effect on the efficiency of the solar cell, so that an initial overetching is performed.

In addition or as an alternative to the overetching described, for the purpose of removing saw damage or surface impurities, the silicon wafers are often given a texture on their surface which reduces the reflection of the silicon wafer and so improves the light injection into the later solar cell. In industrial production, the surface texture is normally produced using wet chemical processes.

Regardless of whether or not there is already a surface texture, the manufacture of a solar cell always requires the formation of a pn-transition in the silicon wafer. Usually a doping agent is diffused into the silicon wafers for this purpose. This usually takes place at temperatures above 700⁰C. At such high temperatures, however, any impurities already present on the surface of the silicon wafer or in the diffusion device are likewise diffused into the silicon wafer. There they act as recombination centres for generated charged particles and in this way they substantially degrade the efficiency of the finished solar cell. For this reason, it is necessary to clean silicon wafers which are to be diffused before they are introduced into the diffusion device. This is normally done using wet chemical processes, where in principle various cleaning processes can be applied.

A cleaning process has become established in the industrial production of crystalline silicon solar cells in which the silicon wafers are firstly etched in an alkaline etching solution before they are placed in a metal-oxidising acid. Finally the silicon wafers are overetched in an aqueous hydrofluoric acid solution. Rinsing processes in water, most commonly in deionised water, can be provided between the individual steps. In each case, however, the silicon wafers are rinsed in deionised water after the overetching with hydrofluoric .acid. This serves to remove dissolved impurities from the silicon wafers and to dispose of any acid residues or at least to dilute them sufficiently. The most commonly used alkaline etching solutions are potassium hydroxide or sodium hydroxide solutions.

The overetching of the silicon wafers with hydrofluoric acid, or a solution .containing hydrofluoric acid, serves to remove any silicon oxide on the surface of the silicon wafers. This silicon oxide c'an either first be grown on with suitable process steps, for example wet chemical oxidation, or it is what is known as natural silicon oxide, which forms under ordinary environmental conditions under the effect of atmospheric oxygen. After removing the silicon oxide layer using hydrofluoric acid, a hydrophobic silicon surface remains. The result of this is that in the subsequent rinsing in deionised water, the water pearls off the surface of the silicon wafers, so that the silicon wafers can be placed in the diffusion device with little or no residue of the deionised water. Any carry-over of impurities into the diffusion device, for example the dissolved impurities described above which may be left in the deionised water, is thus largely avoided, thus largely preventing any impairment of the efficiency of the finished solar cells. The described application of hydrofluoric acid to create a hydrophobic silicon surface is therefore common practice nowadays in the industrial production of silicon solar cells. As a rule, not even a single overetching using hydrofluoric acid is left out, since it is assumed that the impurities hereby introduced into the diffusion tube will be at least partially enriched in the diffusion device, so that even in the case of silicon wafers inserted later into the diffusion device, which had previously been cleaned using hydrofluoric acid, there will be impurities introduced into the silicon wafers, which will ultimately have a negative effect on their efficiency.

This process, however, causes problems when silicon wafers are to be diffused with a silicon oxide layer, since this is removed during the cleaning by the hydrofluoric acid. It has, however, emerged that the presence of silicon oxide layers during a diffusion, which in the present case always means the diffusion of doping agent, can be advantageous. For example, in tested diffusion processes, such as those currently in common use, in order to produce emitters with sheet resistances in the range of 30 - 75 Ω/sq, sheet resistances in the range of between 70 and 200 Ω/sq are realised, when the diffusion of the doping agent occurs through a silicon oxide layer arranged on the surface of the silicon wafers. Such high sheet resistances have an advantageous effect on efficiency, provided, however, an electrical contact with the solar cell with low enough electrical resistance is guaranteed. A diffusion of doping agent through a silicon oxide layer could therefore be used advantageously in the manufacture of selective emitters.

Against this background, the invention is based on the problem of providing a cleaning process for silicon wafers which have a silicon oxide layer on at least part of their surface.

This problem is solved by a cleaning process with the features of claim 1.

The subject matter of the invention is also a solar cell with the features of claim 14.

Advantageous refinements are the subject matter of each of the dependent subclaims .

With respect to the cleaning process, until now the unanimous opinion has been that an overetching of the silicon wafers with hydrofluoric acid, or a solution containing hydrofluoric acid, is essential for the creation of a hydrophobic surface, in order to guarantee adequate cleaning of the silicon wafers. Surprisingly, however, it has emerged that an adequate cleaning effect can be achieved even without overetching the silicon wafers with hydrofluoric acid with the aim of creating a hydrophobic surface, so that solar cells with excellent efficiencies can be produced. The process according to the invention is therefore designed to provide the silicon wafers with a silicon oxide layer over at least part of their surface before they are etched. The etching then takes place in a solution which contains an acid which oxidises metallic impurities. At this stage, at least a portion of the silicon oxide layer of each solar cell is exposed unprotected to the acid. After the etching process, the silicon wafers are rinsed in deionised water. The at least one unprotected portion of the silicon oxide layer is at least partly left on the silicon wafers. Subsequently, the silicon wafers are dried after rinsing.

According to one refinement of the invention, before rinsing, the silicon wafers are etched in deionised water in an alkaline etching solution, when at least a portion of the silicon oxide layer is exposed unprotected to the etching solution. Should it be necessary, this can improve the cleaning effect.

The sequence of the various etching processes is not relevant. There may be intermediate rinsing steps between the individual etching processes.

Because the at least one unprotected portion of the silicon oxide layer is at least partially left on the silicon wafers, at least this portion is hydrophilic and thus not hydrophobic. A complete removal of the silicon oxide layer by means of a solution containing hydrofluoric acid, which was formerly done in the cleaning process to form hydrophobic surfaces, is therefore precluded. Furthermore, this also precludes the complete removal of the silicon oxide layer during the etching in the alkaline etching solution. This risk remains anyway only in principle, but with the alkaline etching solutions commonly used to clean silicon wafers and the etching periods which are normal in this connection, it is practically nonexistent. In the present invention, in any case, the etching rate of the alkaline etching solutions as well as the etching periods are adjusted in such a way that a complete removal of the silicon oxide layer does not occur.

The silicon wafers therefore exist in at least partly hydrophilic condition, hence they are dried after rinsing in deionised water. The actual drying process can be realised in any way known in the art.

It has emerged that an overetching with hydrofluoric acid can in principle be completely dispensed with, without impairing the efficiencies of the final finished solar cells. One preferred variant embodiment of the process according to the invention therefore allows for the etching of the silicon wafers with a medium containing hydrofluoric acid, in particular with a solution containing hydrofluoric acid, to be omitted.

One advantageous variant embodiment of the invention provides the silicon wafers with a silicon oxide layer by means of deposition from the vapour phase. In this case, low-pressure (LPCVD) , atmospheric pressure (APCVD) or plasma-enhanced (PECVD) deposition processes can be applied. In one especially advantageous variant embodiment, the silicon oxide layer is formed by means of a wet thermal oxidation.

Provided that a silicon oxide layer applied to the silicon wafers is sufficiently etched off during the cleaning process, the silicon oxide layer can in principle be applied in any thickness. It has, however, emerged that the longer the etching processes of the cleaning method last, in particular the action of the alkaline etching solution, the more severe the damage to the surface of the silicon wafers. One refinement of the invention is therefore designed to apply the silicon oxide layer in a thickness of between 2 nm and 70 nm, preferably in a thickness of between 10 nm and 70 nm.

In another variant embodiment of the invention, provision is made accordingly so that the thickness of the silicon oxide layer is reduced by less than 25% of its initial thickness during the etching processes.

For preference, an alkali hydroxide solution, in particular an aqueous alkali hydroxide solution, is used as alkaline etching solution, and especially preferably a potassium or sodium hydroxide solution.

^' According to one refinement of the invention, the alkaline etching solution has a silicon oxide etching rate of less than 25 nm per minute. These etching rates have been proven in practice.

One refinement of the process according to the invention is designed so that any saw damage is removed from the silicon wafers using wet chemical methods, before they are provided with the silicon oxide layer. Any of the saw-damage etching processes known in the art can be used for this purpose. In particular, the saw damage can be removed using an alkali hydroxide solution. In addition, acidic etching solutions can also be used.

A further advantageous refinement of the invention is designed so that on at least part of the surface of the silicon wafers, a texture is formed using wet chemical processes, before the silicon wafers are provided with the silicon oxide layer. To form the texture using wet chemical processes, all texture etching solutions known in the art can be used, in particular alkaline or acidic texture etching solutions. The wet chemical formation of a texture can be provided in addition or as an alternative to the wet chemical removal of the saw damage.

An alternative variant embodiment to the complete omission of etching with hydrofluoric acid is designed so that the silicon wafers are additionally etched in a heavily diluted or buffered hydrofluoric acid solution, which has a silicon oxide etching rate of less than 25 nm per minute. This is advantageous for example when alkaline etching media are to be avoided or if very thin silicon oxides in isolated areas, for example natural silicon oxides, are to be removed. The etching period should then obviously be adapted to the etching rate. The etching described, in a heavily diluted or buffered hydrofluoric acid solution can be provided in conjunction with etching in the acid which oxidises metallic impurities alone or in conjunction with the etching in the alkaline etching solution.

Advantageously, the silicon wafers are etched in a hydrofluoric acid solution for less than one minute, preferably less than 30 seconds and especially preferably for less than 15 seconds.

The process according to the invention can advantageously be used in the production of solar cells.

The invention will next be explained in more detail with the aid of figures. Where relevant, the elements with the same action are accompanied by the same reference numbers. The' figures show:' Figure 1 A first embodiment of the process according to the invention in diagrammatic view

Figure 2 Diagrammatic view of a further embodiment of the process according to the invention

Figure 3 A further embodiment of the process according to the invention in a schematic view

Figure 4 A further embodiment of the process according to the invention.

Figure 1 shows a diagrammatic view of a first embodiment of the process according to the invention. In this case, any saw damage is first removed by etching 10. Consequently, this process is especially suitable for silicon wafers sawn from blocks. It can, however, also easily be used for other silicon materials, for example for silicon film materials. Following the removal 10 of the saw damage, a silicon oxide layer is applied to the silicon wafers using APCVD 12, although other silicon oxide deposition processes can also be used, in particular LPCVD or PECVD processes or a thermal oxidation.

Next, the silicon wafers are etched in an aqueous sodium hydroxide (NaOH) solution 14, followed by etching 16 in hydrochloric acid (HCl) . During the etching processes 14, 16, as also during the etching processes described in the following embodiments, the silicon oxide layer is exposed unprotected to the etching media. No masks, for example with the aid of lacquers or dielectrics, are applied to protect the silicon oxide layer from the etching media. So the silicon oxide layer is completely unprotected. Insofar as special ^• manufacturing processes require and the process flow permits, in particular where the purity requirements are met, there is, however, in principle the possibility of at least partially protecting the silicon oxide layer against the etching media, for example by using appropriate dielectrics.

Instead of etching in NaOH solution 14 followed by etching in HCl 16, as mentioned above, there may also be a first etching in HCl. After the etching processes, the silicon wafers are rinsed in deionised water 18. Contrary to the former opinion, no etching with hydrofluoric acid takes place, so that the silicon oxide layer is at least partially left on the silicon wafers. Although in principle there is the possibility of removing the silicon layer during etching 14 in an alkaline etching solution, in this as in all following embodiments, the etching rate of the alkaline etching solutions as well as the etching periods are adjusted so that this does not happen. For preference, in all embodiments an alkaline etching solution, in particular an aqueous NaOH or potassium hydroxide (KOH) solution, is used, which has a silicon oxide etching rate of less than 25 nm per minute. After rinsing 18 the silicon wafers are dried 20. Since, in the process according to the invention, these are at least partially hydrophilic, the drying process 20 may be more expensive than in the cleaning processes formerly in common use. In principle, all drying processes known in the art can be applied. For example a dried gas such as nitrogen, preferably under additional heat exposure, can be used.

Figure 2 illustrates in diagrammatic form a further embodiment of the process according to the invention. This differs from the embodiment in Figure 1 firstly in that after removing the saw damage 10, a texture is formed on at least part of the surface of the silicon wafers using wet chemical means 22. This process is usually referred to as texture etching. A further difference lies in the fact that the silicon oxide layer here is applied by means of a thermal oxidation 24. Moreover, instead of a NaOH solution as alkaline etching medium, an aqueous potassium hydroxide (KOH) solution is provided, in which the silicon wafers are etched 26. To assist drying 20, in the embodiment in Figure 2, centrifuging 28 takes place before the final drying process 20. This involves the silicon wafers being spun while arranged, for example, in a holder. The centrifuging 28 assists the subsequent drying 20 and can accelerate it.

The embodiment in Figure 3 omits any initial removal 10 of the saw damage and in this way differs from the embodiment in Figure 2. A further difference lies in the fact that the rinsing step 18 following the etching processes 26, 16 is followed by etching 30 in buffered hydrofluoric acid (HF) solution. This additional etching step 30 can provide certainty in the case of very badly contaminated silicon wafers or can be used when initial saw damage etching is omitted. The buffering guarantees that any silicon oxide 24 which has been applied will at best be partly removed during the etching 30-. In this case, the buffering is selected such that a silicon oxide etching rate of less than 25 nm per minute is guaranteed. This can also be assured with a sufficiently diluted HF solution, so that this can always be used as an alternative to a buffered HF solution. Obviously the etching period must be suitably adapted, in order to prevent the silicon oxide being completely removed. The etching 30 in buffered HF is followed by ah additional rinsing step 32, in order to remove HF residues.

Instead of the centrifuging 28 from the embodiment in Figure 2, the variant embodiment in Figure 3 makes provision for blowing-off 34. This involves a gas stream blowing water mechanically down off the silicon wafers. This is, again, followed by drying 20 of the silicon wafers.

The embodiment in Figure 4 largely corresponds to that in Figure 2. Only the etching 26 in a KOH solution is replaced by an etching 36 in a buffered HF solution. As already discussed in connection with Figure 3, the buffering and hence the etching rate as well as the etching period are selected such that a previously applied 24 silicon oxide layer is only partially removed during the etching 36 in the buffered HF solution. A silicon oxide etching rate of less than 25 nm per minute is appropriate for this. This can also be assured with a sufficiently diluted HF solution, in particular an agueous HF solution, so that this can always be used as an alternative to a buffered HF solution. An etching 36 in a buffered HF solution, or a suitably diluted HF solution, can for example be advantageous when etching in an alkaline etching solution is to be avoided. The other method steps correspond to those of the embodiment from Figure 2, so that reference may be made to the corresponding discussions above.

List of reference numbers

10 Removal of saw damage

12 Application of silicon oxide layer by means of APCVD

14 Etching in NaOH solution 16 Etching in hydrochloric acid

18 Rinsing in deionised water

20 Drying

22 Wet chemical formation of a texture 24 Application of silicon oxide layer by means of thermal oxidation

26 Etching in KOH solution

28 Centrifuging

30 Etching in buffered HF solution

32 Rinsing in deionised water

34 Blowing off

36 Etching in buffered HF solution

Claims

1. Oxidation and cleaning process for silicon wafers, in which the silicon wafers - are provided with a silicon oxide layer on at least part of their surface (12; 24), before

- they are etched in a solution (16) which contains an acid which oxidises metallic impurities,

- while at least a portion of the silicon oxide layer is exposed unprotected to the acid, and

- in which the silicon wafers are rinsed in deionised water (18) after the etching process (16) c h a r a c t e r i s e d i n t h a t the at least one unprotected portion of the silicon oxide layer is at least partially left on the silicon wafers and after rinsing (18; 32) the silicon wafers are dried (20) .

2. Process according to claim 1, c h a r a c t e r i s e d i n t h a t before rinsing (18) in deionised water, the silicon wafers are etched in an alkaline etching solution (14; 26), with at least one portion of the silicon oxide layer being exposed unprotected to the etching solution.

3. Process according to one of the preceding claims, c h a r a c t e r i s e d i n . . t h a t the silicon wafers in at least partly hydrophilic condition are rinsed in the deionised water (18; 32) .

4. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t the silicon wafers are provided with a silicon oxide layer by means of a process (12; 24) which is part of a group which includes the application of a silicon oxide layer by means of thermal oxidation (24), by means of wet thermal oxidation, by means of deposition from the vapour phase (12), by means of UV light exposure in an ozone atmosphere and that of the silicon oxide layer.

5. Process according to one of the preceding claims, c h a r a c t e r i s e d _.i n t h a t the silicon oxide layer is applied in a thickness of between 2 nm and 70 nm, preferably in a thickness of between 10 nm and 70 nm (12; 24) .

6. Process according to one of claims 2 to 5, c h a r a c t e r i s e d i n t h a t an alkali hydroxide solution is used as alkaline etching solution (14; 26), preferably a potassium or sodium hydroxide solution.

7. Process according to one of claims 2 to 6, c h a r a c t e r i s e d i n t h a t an alkaline etching solution with a silicon oxide etching rate of less than 25 nm per minute is used (14; 26) .

8. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t hydrochloric acid is used as the acid which oxidises metallic impurities (16) .

9. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t the thickness of the silicon oxide layer is reduced by less than 25% of its final thickness during the etching processes (14, 16; 26) .

10. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t etching of the silicon wafers with a medium containing hydrofluoric acid is omitted.

11. Process according to one of claims 1 to 9, c h a r a c t e r i s e d i n t h a t the silicon wafers are additionally etched in a heavily diluted or buffered hydrofluoric acid solution (30; 36), which has a silicon oxide etching rate of less than 25 nia per minute.

12. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t the silicon wafers are blown off (34) or centrifuged (28) before drying (20) .

13. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t saw damage is removed from the silicon wafers using wet chemical processes (10), before they are provided with the silicon oxide layer (12; 24) .

14. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t a texture is formed at least on part of the surface of the silicon wafers using wet ■ chemical processes (22), before the silicon wafers are provided with the silicon oxide layer (12; 24) .

15. Solar cell produced using a process according to one of claims 1 to 14.