WO2013087071A1

WO2013087071A1 - Method for smoothing a silicon substrate by etching on one side

Info

Publication number: WO2013087071A1
Application number: PCT/DE2012/100380
Authority: WO
Inventors: Katrin Weise; Cornelia Klein
Original assignee: Rena Gmbh
Priority date: 2011-12-15
Filing date: 2012-12-14
Publication date: 2013-06-20
Also published as: TW201334054A; DE102011056495A1

Abstract

Method for smoothing a silicon substrate (50) by etching (16) on one side, wherein a protective layer (54; 56) is formed (12; 24) at least on a first side of the silicon substrate, then a second side of the silicon substrate (50) is smoothed by etching (16) by means of an alkaline etching solution having a temperature in the range of between 50°C and 90°C, and, during this process of smoothing by etching (16), those regions of the silicon substrate (50) on which the protective layer (54; 56) has been formed are protected against action of the etching solution by means of the protective layer (54; 56).

Description

Method for one-sided smooth etching of a silicon substrate

The invention relates to a method for one-sided smooth etching of a silicon substrate. In particular, the production of modern solar cell types requires the smoothest possible backside of a silicon substrate used as a solar cell substrate. Under the rear side of the solar cell that large or silicon substrate is to be understood, which usually averted incident light during operation of the fin- ished solar cell is ranked ^¬. Possible smooth backs allow a re ^¬ production of recombination of generated charge carriers at the back of the silicon substrate or solar cells. Compared to solar cell types with a non-smoothed back side, in conjunction with a passivation of surface states on the back surface, efficiency increases of up to 1% can be achieved. In particular, the smoothing textured backs of solar cell substrates can cause significant increases in efficiency.

So far, the backside of silicon substrates is smooth etched by means of an acidic polish etching solution. In various solar cell production processes, however, it has been found that the use of an acidic polish etching solution entails undesirable side effects or is not possible. This is in particular ^¬ sondere due to the fact that in etching the back other areas of the solar cell substrate can be attacked by means of an acidic Politurätzlösung. In particular, it has been found that areas provided with emitter doping can be severely damaged. As a result, an acidic polish etch solution can be used in many cases only prior to formation of emitter doping. This is but not compatible with all solar cell manufacturing processes.

Against this background, the present invention has for its object, a versatile method for a smooth etching free ^¬ side of a silicon substrate to provide.

This object is achieved by a method having the features of claim 1.

Advantageous developments are the subject of dependent claims. The method according to the invention for the single-sided smooth etching of a silicon substrate provides for forming a protective layer at least on a first side of the silicon substrate. Subsequently, a second side of the silicon substrate is smooth etched with ^{¬ means of} an alkaline etching solution, the temperature in the range between 50 ° C and 90 ° C has. During this Glattätzens those areas of the Siliziumsub ^¬ strats, which has been formed on the protective layer are protected by the protective layer against an action of the etching solution.

Under a smooth etching free in the sense of the present invention is meant an at least partial flattening of a surface of Silizi ^¬ umsubstrats. In other words, the smooth etching means at least partially leveling the surface of the silicon substrate.

As a result of the comparatively high temperature of the alkaline etching solution, an isotropic etching behavior of the alkaline etching solution can be realized. This allows the flat etching of the second side of the silicon substrate. The first side of the silicon ^¬ ziumsubstrats can be low-cost protected against exposure to the alkaline etching solution, for example by means of a silicate glass layer or a silicon nitride layer. Furthermore, it has been shown that by means of the OF INVENTION ^¬ to the invention process, the second side can be glattgeätzt cost-effective than with an acidic Politurätzlösung. This expense advantage results from lower chemical costs and a longer service life of the alkaline etching solution compared to acidic polish etching solutions.

As an alkaline etching solution, an aqueous NaOH or an aqueous KOH solution have been proven. Preferably, the first side of the second side is ^opposite . Particularly preferably, it is at the first side to a front side and the second side to a back print ^¬ te a solar cell substrate. In the sense described above, the rear side is understood to mean that side of the solar cell substrate which, when operating the finished solar cell, is arranged facing away from incident light. However, the front side is the side of the solar cell substrate which incident light is aligned supplied ^¬ Wandt during operation of the finished solar cell.

For the purpose of smooth etching the back surface of the silicon substrate, the silicon substrate may be partially or completely immersed in the etching solution. In particular, the Glattät ^¬ zen can advertising realized by means of a one-sided etching method to, for example, by only the second side is wetted by the etching solution, the silicon substrate ^¬ net such angeord, the first side of the silicon substrate, however, the upper ^¬ half a liquid level of the etching solution located. Several silicon substrates can be stacked or continuously smoothed. be etched. The method is therefore compatible with Batch ar ^¬ beitenden systems as well as with continuous manufacturing lines. Preferably, a silicate glass layer or a silicon nitride layer is formed as a protective layer. In this way, the protective layer can be formed with technologies known from solar cell manufacturing. In particular, phosphorosilicate glass layers and borosilicate glass layers have proven to be useful as a protective layer.

In one process variant, a solar cell substrate is a ^¬ glattgeätzt side and forms a protective layer in a Emit ^¬ terdiffusion a dopant-silicate glass layer excluded. In this way, the protective layer can be formed inexpensively in the context of an emitter diffusion step which is required anyway for the solar cell production. When do ^¬ animal-containing silicate glass layer, in particular a phosphorus silicate glass layer or a borosilicate glass have proven layer.

The emitter diffusion can be realized in any fundamental in itself be ^¬ knew Art. It can unilateral or multilateral ^¬ side emitter diffusion processes are used. In particular, tube diffusion has proven itself. Will be ^¬ forms within the emitter diffusion, for example if it is carried out as tubes ^¬ diffusion on the second side of the Silizi ^¬ umsubstrats the dopant-silicate glass layer, it will be advantageously removed prior to smooth etching free. This can be achieved, for example, by means of a one-sided etching process known per se, in particular using weakly concentrated hydrofluoric acid solutions. Preferably, the dopant-containing silicate glass layer from the second side of the silicon substrate in the context of a Kan- tenisolationsschrittes removed. In this way, no additional process step is required over solar cell manufacturing processes which provide an edge isolation step. As edge isolation step, a process step is commonly referred to, wherein the surface is electrically insulated on the second side of the silicon substrate by the Emit ^¬ terdotierung. For this purpose, the emitter doping can be etched away from the second side of the silicon substrate beispielswei ^¬ se, so that in the course of this etching, the do animal-containing silicate glass layer can be cost-effectively removed from the second side of the silicon substrate.

Advantageously, the dopant-containing silicate glass layer formed as part of the emitter diffusion is removed from the second side of the silicon substrate by wet-chemical etching. Particularly preferably, this is done by means of a silicon katglasät zlösung first type containing water, 10 grams per liter (g / 1) to 50 g / 1 hydrofluoric acid, 200 g / 1 to 1000 g / 1 swiveling ^¬ ric acid and / or 250 g / 1 to 500 g / 1 nitric acid. In this way, it is optionally possible to simultaneously remove an emitter doping from the second side of the silicon substrate by means of the silicate glass etching solution of the first type. A combination of removing the dopant-containing silicate glass layer from the second side of the silicon substrate with an edge insulation is thereby made possible.

Advantageously, after the flat etching of the second side of the silicon substrate, the dopant-containing silicate glass layer formed on the first side is removed from the first side of the silicon substrate by means of a second-type silicate glassate, thereby over-etching the second side of the silicon substrate by means of the second-type silicate glassyzate. In this way any structures remaining on the second side of the silicon substrate after the smooth-etching process can be further extended. be leveled. Remaining edges can be rounded off. As Silikatglasät zlösung second type, an aqueous solution has been proven, which has hydrofluoric acid and nitric acid. In another variant of the method in which a solar cell substrate is glattgeätzt one side, a silicon nitride layer is formed on the first side of the silicon substrate as a protective layer ^¬. This silicon nitride layer is subsequently left as an antireflection layer on the silicon substrate. This variant of the method thus represents an alternative possibility of a cost-effective design of the protective layer, since the formation of a silicon nitride layer as an antireflection layer is provided anyway in a large number of solar cell production processes.

The production of a solar cell requires the formation ei ^¬ nes emitter. Advantageously, an emitter diffusion is therefore carried out prior to the formation of the silicon nitride layer, and in this case a dopant-containing silicate glass layer is formed on the silicon substrate. This dotierstoffhalt strength Sili ^¬ katglasschicht is completely removed prior to forming the silicon nitride layer ^¬. In this way, the silicon nitride layer provided as a protective layer can be left as an antireflection layer on the silicon substrate. The do- animal-containing silicate glass layer is preferably removed nassche ^¬ mixed, for example with the Silikatglasät zlösung second type. A possible edge isolation as zlösung above in the context of the Silikatglasät first kind described, is preferably carried out also before the formation of the Si liziumnitridschicht.

A variant of the method provides that the silicon substrate is at least partially provided with egg ^¬ ner surface texture prior to forming the protective layer. In the case of a solar cell lens substrate can be increased in this way, for example, the Lichtein ^¬ coupling in the solar cell substrate and thus the efficiency of the finished solar cell. Preferably, the surface texture is formed on the first and second sides of the silicon substrate. On the second side of the Sili ^¬ ziumsubstrats formed constituents of the surface texture are then during the subsequent Glattätzens eingeeb ^¬ net. The surface texture is preferably formed by wet-chemical texture etching.

Advantageously, the second side of the silicon substrate is smooth-etched by means of an aqueous KOH solution. Particularly be ^¬ lasts an aqueous KOH solution having a KOH concentration of 10 weight percent to 30 weight percent has.

The second side of the silicon substrate is preferred glattgeätzt using an alkaline etching solution having a tempera ture ^¬ in the range between 70 ° C and 85 ° C. This Tempe ^¬ raturbereich has proven itself in practice especially.

Furthermore, it has proven useful, s, etch the second side while ei ^¬ ner period of 10 s to 300 s, preferably for a period of 30 s to 300 smooth. It has been found, that the inventive method can be before ^¬ geous, used in the manufacture of a solar cell having a passivated ^¬ fourth back or in the manufacture of a solar cell with a passivated emitter and passivated back, a so-called PERC cell. Under a passivated emitter are electrically passivated surface states in the emitter region, under a passivated back passivated surface states on the back of So ^¬ larzelle to understand. Furthermore, the invention will be explained in more detail with reference to figures. Where appropriate, are provided herein Ele ^¬ elements the same effect by the same reference numerals. The invention is not limited to the Ausführungsbeispie ^¬ le shown in the figures - not even in terms of functional features. The previous description as well as the following figures ^¬ description contain numerous features that are shown in the dependent subclaims partially summarized in several. However, those features, as well as all the other features disclosed above and in the following description of the figures, will also be considered individually by one skilled in the art and put together to meaningful further combinations. In particular, these features are combined in each case individually and in any geeigne ^¬ ter combination with the method of the independent claim. Show it:

Figure 1 Schematic representation of a first embodiment of the method according to the invention

Figure 2 is a schematic diagram of a second embodiment of the method according to the invention

Figure 3 Schematic representation of a method according to

1 continuous solar cell substrate during the various process steps

Figure 4 Schematic representation of the method according to

2 continuous solar cell substrate during the various process steps

Figures 1 and 3 illustrate a first Ausführungsbei ^¬ game of the method according to the invention. In this, a silicon solar cell substrate is to ^¬ next 50 texturgeätzt 10. Since ^¬ at a WEI in the illustrations of Figure 3 upwards In addition, the remaining surface areas of the silicon solar cell substrate are also provided with the surface texture 51. The front side of the silicon solar cell substrate 50 is provided with a surface texture 51 as well as a back side of the silicon solar cell substrate 50 facing down in the illustrations of FIG. In the representations of FIGS. 3 and 4, for the sake of clarity, however, a representation of the surface texture 51 is dispensed with at the edges of the silicon solar cell substrate 50. The Texturät- zen 10 of the silicon solar cell substrate 50 is carried vorzugswei ^¬ se by means of a wet chemical Texturätzlösung.

Furthermore, a tube emitter diffusion 12 is performed. This is to be understood as a tube diffusion in which an emitter doping 52 is formed. This is in the

Representations of Figure 3 by a dashed line sche ^¬ matically represented. In the context of tube emitter diffusion, a dopant-containing silicate glass layer 54 is ausgebil ^¬ det. This is preferably a phosphosilicate glass layer or a borosilicate glass layer.

In the further course of the process, the dopant-Si ^¬ likatglasschicht 54 is removed 14 from the back side of the silicon solar cells ^¬ substrate 50. This can for example take place by means egg ner Silikatglasät zlösung second kind. In other areas, the dopant-containing silicate glass layer 54 remains on the silicon solar cell substrate 50 and serves as a protective ^¬ layer against a subsequent smooth etching 16. This smooth etching 16 is carried out in an aqueous KOH solution, which at a temperature in the range of 70 ° C to 85 ° C. located. For the purpose of the Glattätzens 16 Siliziumso ^¬ larzellensubstrat 50 can be completely immersed in the aqueous KOH solution. During the smooth etching 16, the upper Surface texture 51 on the back of the silicon solar cell Sub ^¬ strats 50 leveled by the isotropic etching of the KOH solution located at said temperature. On the other hand, away from the rear side of the silicon solar cell substrate 50, the silicon solar cell substrate 50 is protected by the dopant-containing silicate glass layer 54 from an action of the KOH solution, so that the surface texture 51 is retained. Subsequently, remaining parts of the dopant-containing silicate glass layer 54 are removed 18. This can be done for example by means of a Silikatätzlösung second type. In addition, the backside of the silicon solar cell substrate 50 is etched over ^¬, which in the case of not fully eingeebne- th surface on the back of Siliziumsolarzellensub ^¬ strats 50 it can be leveled going on.

As a result, a silicon solar cell substrate 50 provided with an emitter doping 52 remains, which has a surface texture 51 on its front side and whose rear side is leveled. This silicon solar cell substrate 50 can now be further processed in known manner to a solar cell wei ^¬. In particular, solar cells with a passivated rear side, for example PERC solar cells, can be produced.

Figures 2 and 4 illustrate a further Ausführungsbei ^¬ game of the method according to the invention. Again, the silicon solar cell substrate 50 is first texture-etched 10. This is followed again by a tube-emitter diffusion 12 with formation of the dopant-containing silicate glass layer 54. Deviating from the embodiment of Figures 1 and 3 it ^¬ follows hereinafter a wet chemical edge isolation 20, wherein in cost-effective manner, both the dotierstoffhal- As well as the emitter doping 52 is removed from the back of the silicon solar cell substrate 50. This can be achieved, for example, by means of the silicate glass solution of the first type. In the exemplary embodiment of FIGS. 1 and 3, the removal 14 of the dopant-containing silicate glass layer from the rear side can be easily replaced by the step of wet-chemical edge insulation 20 from the exemplary embodiment of FIGS. 2 and 4. In this way, a further embodiment of the inventive method results.

In the embodiment of Figures 2 and 4, a complete Entfer ^¬ nen 22 of the dopant-silicate glass layer 54 connects to the wet-chemical edge isolation 20th This can be achieved for example by means of a Silikatglasät zlösung second type.

Subsequently, a silicon nitride layer 56 is deposited on the front side of the silicon ^solar cell substrate 50. This can be done by means of chemical vapor deposition methods known per se (CVD method). Since the dopant-containing silicate glass layer 54 has been completely removed 22 before the deposition 24 of the silicon nitride layer 56, the silicon nitride layer 56 may furthermore remain on the silicon solar cell substrate 50 as an antireflection layer. Except where the respective solar cell manufacturing process, removal of the dopant-silicate glass layer 54 is not required, the silicon nitride layer can also remain open ^¬ as an antireflective layer visibly 56 when the dopant-silicate glass layer was 54 not previously removed completeness, ^¬ dig.

Furthermore, the rear side of the silicon solar cell sub ^¬ strate 50 in which is at a temperature in the range between For example, for this purpose, the silicon solar cell substrate 50 may be completely immersed in the KOH solution. In this smooth etching free 16 the back of the Siliziumsolarzellensub- is leveled strats 50 while the silicon nitride layer 56 acts in the other areas as the protective layer and the Silizi ^¬ umsolarzellensubstrat 50 protects against an action of the KOH solution. The result is thus a silicon solar cell substrate 50, which is provided on its front side with a surface texture 51 and an emitter doping 52 and a silicon nitride layer 56 as an antireflection layer and also has a flattened back. This Siliziumsolarzel- lensubstrat 50 can be further processed in a conventional manner to a ferti ^¬ gen solar cell. Metal contacts for contacting the emitter doping on the front side of the silicon solar cell substrate 50 can be fired therethrough ^¬ play for this purpose in through the silicon nitride layer in a known per se. The flattened back of the silicon solar cell substrate 50 provides ideal Vorausset ^¬ tions for the production of a solar cell with a passivated back. As can be realized by the silicon nitride layer 56 and a passivation of the emitter, and a solar cell having passivated emitter and passivated ^¬ fourth back can be manufactured. Reference sign list

10 texture etching silicon solar cell substrate

12 Tubular emitter diffusion with formation of dopant-containing silicate glass layer

14 Removal of dopant-containing silicate glass layer from the rear

16 smooth etching backside in KOH solution with temperature of

70 ° C to 85 ° C

18 Removal of remaining parts of the dopant-containing silicate glass layer and overetching rear side

20 Wet chemical edge insulation

22 Complete removal of the dopant-containing silicate glass layer

24 depositing silicon nitride layer on front side

50 silicon solar cell substrate

51 surface texture

52 emitter doping

54 dopant-containing silicate glass layer

56 silicon nitride layer

Claims

claims

A method of single-sided smooth etching (16) of a silicon substrate (50), in which

- at least on a first side of the silicon substrate, a protective layer (54; 56) is formed (12; 24),

- Subsequently, a second side of the silicon substrate (50) is smoothly etched by means of an alkaline etching solution (16), which has a temperature in the range between 50 ° C and 90 ° C,

- during this Glattätzens (16) those areas of the silicon substrate (50) on which the protective layer (54; 56) has been formed, the protective layer by means ^¬ (54; 56) are protected against an action of the etching solution,

characterized ,

that as a protective layer (54; 56) a silicate glass layer

(54) or a silicon nitride layer (56) is formed

(12; 24).

2. The method according to claim 1,

characterized ,

in that a phosphosilicate glass layer or a borosilicate glass layer is formed as the protective layer (54) (12).

3. Method according to one of the preceding claims,

characterized ,

a solar cell substrate (50) is smoothly etched on one side (16).

Method according to claim 3,

characterized ,

that as a protective layer (54) in the context of an emitter diffusion one (12), a dopant-silicate glass layer (54), preferably a phosphosilicate glass film or a boron ^¬ silicate glass layer formed (12).

Method according to claim 4,

characterized ,

that in the framework of the emitter diffusion (12) formed is removed dopant-silicate glass layer (54) before the Glattät ^¬ zen (16) from the second side of the silicon substrate (50) (14).

Method according to claim 5,

characterized ,

that in the framework of the emitter diffusion (12) (54) is removed by nasschemi ^¬ chemical etching (14) from the second side of the Siliziumsub ^¬ strats formed dopant-silicate glass layer (14), preferably by means of a silicon katglasät zlösung first type which comprises water, 10 g / 1 to 50 g / 1 hydrofluoric acid, 200 g / 1 to 1000 g / 1 sulfuric acid and / or 250 g / 1 to 500 g / 1 nitric acid.

Method according to one of claims 4 to 6,

characterized ,

is that after the smooth etching free (16) of the second side of the silicon substrate (50) formed on the first side do ^¬ animal-containing silicate glass layer (54) from the first side of the silicon substrate (50) etching solution by means of a silicate glass of the second type is removed (18) and In this case, the second side of the silicon substrate is over-etched by means of the second-type silicate glass etching solution (18).

Method according to claim 3,

characterized ,

in that a silicon nitride layer (56) is used as protective layer (56). formed on the first side of the silicon substrate (50) and will be blank as the antireflective layer on the silicon substrate ^¬ (24).

9. The method according to claim 8,

characterized ,

that

- Before the formation (24) of the silicon nitride layer (56) an emitter diffusion (12) takes place and in this case a do ^¬ animalstoffhaltige silicate glass layer (54) on the Silizi ^¬ umsubstrat (50) is formed (12) and

- This dopant-containing silicate glass layer (54) before forming (24) of the silicon nitride layer (56) is completely removed ^¬ dig (22).

10. The method according to any one of the preceding claims,

characterized ,

in that prior to forming (12; 24) the protective layer (54; 56) the silicon substrate (50) is at least partially provided with a surface texture (51) (10), wherein the surface texture (51) is preferably on the first and second sides of the Silicon substrate (50) is formed (10).

11. The method according to any one of the preceding claims,

characterized ,

that the second side of the silicon substrate (50) is glattgeätzt means of an aqueous KOH solution (16), preferably ^¬ by means of an aqueous KOH solution having a KOH concentration of 10 weight percent to 30 Gewichtspro ^¬ center.

12. Method according to one of the preceding claims, characterized in that

in that the second side of the silicon substrate (50) is smoothly etched (16) by means of the alkaline etching solution and the etching solution has a temperature in the range between 70 ° C and 85 ° C (16).

13. The method according to any one of the preceding claims,

characterized ,

the second side is smooth etched for a period of 10 seconds to 300 seconds (16), preferably for a period of 30 seconds to 300 seconds.

14. The method according to any one of claims 3 to 13,

characterized ,

in that a solar cell with passivated rear side is produced from the solar cell substrate (50), preferably a solar cell with passivated emitter and passivated rear side.