DE102010004497A1 - Method for increasing sheet resistance in partial area of doped semiconductor region e.g. emitter region, of solar cell e.g. silicon solar cell, involves carrying out oxidation of partial area in water-vapor containing environment - Google Patents

Abstract

The method involves carrying out oxidation (80) of a partial area (10) of a doped semiconductor region e.g. emitter region (3) in water-vapor containing environment, such that a silicon oxide layer (11) with thickness between 130 and 250 nanometers is formed. The oxide layer formed by the oxidation is removed by etching. The oxidation of the partial area is suppressed in a high-doping region (15) through formation of a masking e.g. silicon nitride (7), phosphor glass layer or borosilicate glass layer, on the high-doping region, where the masking is removed after the oxidation of the area. An independent claim is also included for a solar cell comprising an emitter.

Description

The invention relates to a method for increasing a sheet resistance in at least one subregion of a doped semiconductor region and to a solar cell according to the preamble of claim 13.

In the fabrication of semiconductor devices, it is sometimes necessary to increase the sheet resistance of a doped semiconductor region. For example, it is known from the field of solar cell production to etch back portions of a doped semiconductor layer acting as an emitter, there to increase the sheet resistance and thus to realize a two-stage doping, which is commonly referred to as a selective emitter.

It is an object of the present invention to provide an alternative to known methods for increasing the sheet resistance in at least one subregion of a doped semiconductor region.

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

The invention is based on the idea of oxidizing at least a portion of a doped semiconductor region in a water vapor-containing environment. Oxidation in a water vapor-containing environment allows the formation of comparatively thick oxide layers in a relatively short time. These oxide layers themselves are insulators. Parts of the doped semiconductor region remaining under the oxide layers have an increased sheet resistance compared to unoxidized parts of the doped semiconductor region.

It has been shown that doping with good homogeneity can be produced with the method according to the invention in regions located below the oxide layers. There are thus increased sheet resistances of good homogeneity.

As is known, semiconductor doping with high homogeneity in the production of solar cells is advantageous. For solar cells, increasing dispersion of the emitter layer resistance values as a result of distributed series resistances leads to reduced filling factors of the solar cells and thus ultimately to a lower efficiency. Particularly in the case of solar cells with a two-stage emitter doping, that is to say a so-called selective emitter, it is important that the lightly doped emitter regions have homogeneous sheet resistance values.

Since with the method according to the invention, as stated above, dopants with good homogeneity can be produced, the solar cell according to the invention according to claim 12 is thus based on the same general inventive concept as the method according to the invention. The solar cell according to the invention has an emitter. In high-doping-free portions of an emitter interface facing a front side of the solar cell, the ratio of the standard deviation of an emitter layer resistance to the mean value of the emitter layer resistance is smaller than 0.07. In an advantageous embodiment, this ratio is less than 0.05.

High-doping-free regions are to be understood as emitter regions in which a stronger doping is intentionally not provided than in surrounding emitter regions. For example, often in those areas in which the metallization is applied, a stronger doping is formed than in surrounding emitter areas. Accordingly, in the areas of metallization there is a high level of doping in the sense described. Further, the front side of the solar cell is that side which, when aligned with the incident light, provides the greatest current efficiency.

Further developments of the method according to the invention as well as the solar cell according to the invention are each the subject of dependent claims.

An embodiment variant of the method according to the invention provides that an oxide layer formed during the oxidation of the at least one subregion is removed. This is preferably done by means of etching and particularly preferably by means of a wet-chemical etching process. In this way, a fabricated semiconductor device is not affected by the oxide layer formed.

A further embodiment variant of the method according to the invention provides that an emitter region of a solar cell substrate is used as the doped semiconductor region. The term of the solar cell substrate is to be understood to mean that from this solar cell substrate in the course of a solar cell is made.

A development of the method according to the invention provides that the oxidation of the doped semiconductor region is suppressed in at least one high doping region. This is preferably done by forming a mask on this at least one high doping region. Under the suppression of oxidation is presently understood that the oxidation of the doped Semiconductor region is at least disabled, preferably completely prevented.

An advantageous embodiment variant of the invention provides that the masking is initially also formed on the at least one subregion and is locally removed there before oxidation. This has the advantage that the masking can be made comparatively inexpensive.

In one embodiment variant of the method according to the invention, for the purpose of locally removing the masking in the at least one partial region, an etching medium is applied to a region of the masking covering the at least one partial region. For example, an etching paste can be applied by means of a printing process known per se, in particular by means of a screen printing process. Furthermore, it is conceivable to apply an etching liquid locally to the masking region covering the at least one subregion. This could for example be done conveniently by means of a known ink jet printing technology. In an alternative embodiment variant of the method according to the invention, the masking is removed locally by means of laser beam evaporation.

As already explained above, by means of the method according to the invention an increased sheet resistance can be produced in the at least one subregion of a doped semiconductor region, which has a good homogeneity. As a result, there is a homogeneous doping. The method according to the invention can therefore advantageously be used for homogenizing a doping in at least one subregion of a doped semiconductor region.

For this purpose, the method according to the invention is used in at least one subregion of a doped semiconductor region of insufficient homogeneity. If the homogeneity of the doping in the entire doped semiconductor region is to be improved, then the method according to the invention can be applied to the entire doped semiconductor region.

In a preferred embodiment of the solar cell according to the invention, the solar cell is designed as a silicon solar cell.

In an advantageous development of the solar cell according to the invention, the emitter is formed by a two-stage doping. There is consequently a so-called selective emitter. The more heavily doped regions of this two-stage doping represent high doping regions in the sense of the present invention.

In the following the invention will be explained in more detail with reference to figures. Where appropriate, elements having equivalent effect are provided with like reference numerals. Show it:

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

2 A second embodiment of a method according to the invention in a schematic representation

3 Schematic representation of the integration of the method according to the invention in a solar cell production process

4 Schematic representation of the integration of the method according to the invention in the production process of solar cells with a boron-doped back

5 Integration of the method according to the invention in the production of i-PERC solar cells in a schematic representation

6 Schematic representation of an embodiment of a solar cell according to the invention

1 shows a schematic representation of a first embodiment of the method according to the invention. Starting point of the representations in 1 is an emitter diffusion 70 in which on a silicon substrate 1 an emitter area 3 and a phosphosilicate glass 5 on the emitter area 3 is trained. The emitter area 3 represents in the embodiment of 1 a doped semiconductor region. In the following, the phosphosilicate glass 5 etched 72 , and removed.

Subsequently, silicon nitride 7 on the emitter area 3 upset 74 , This silicon nitride 7 serves as a mask to suppress the oxidation of the emitter region 3 in high-doping areas 15 , As the representation of 1 can be removed, the masking 7 initially in both high-end areas 15 as well as on subareas 10 of the emitter area 3 educated 74 , The silicon nitride 7 is preferably applied by means of a plasma-driven chemical vapor deposition (PECVD).

Subsequently, an etching paste 9 on the subareas 10 covering regions of the silicon nitride layer 7 upset 72 , This can be done by means of printing methods known per se and is preferably realized by means of a screen printing technology. With the help of the etching paste 9 will continue, and so on before oxidizing 80 , the silicon nitride 7 in the subareas 10 locally removed 78 ,

This is followed by oxidation 80 of the subareas 10 in a steam environment. As in all the described embodiments, a silicon substrate 1 is used, the oxidation 80 carried out at a temperature between 750 ° C and 950 ° C, preferably at a temperature between 790 ° C and 850 ° C. Due to the masking of the high-doping regions 15 with the remaining silicon nitride 7 are here only in the subareas 10 silicon oxide 11 educated. Below these silicon oxide layers 11 the emitter region has a higher sheet resistance than in the high doping regions 15 which during the oxidation 80 through the remnants of the silicon nitride 7 were protected.

In the following, the remaining radicals of the silicon nitride forming the masking become 7 together with the silicon oxide layers 11 completely etched 82 and removed. This leaves an emitter with high doping regions 15 and subregions with increased sheet resistance, ie with respect to the high doping regions 15 weaker doping. The subareas 13 with the increased sheet resistance and the high doping ranges 15 thus form a two-stage emitter, which is also referred to as a selective emitter. This two-stage emitter is in principle already before removal 82 the silicon oxide layers 11 given.

2 shows a further embodiment of a method according to the invention. This differs from the method according to 1 essentially in that instead of a separately applied silicon nitride 7 this as a result of the emitter diffusion 70 formed phosphosilicate glass is used as a masking.

Accordingly, the emitter diffusion is followed by application 86 the etching paste 9 on the subareas 10 covering regions of phosphosilicate glass 5 at. This in turn can be done by means of a screen printing technology. Furthermore, the phosphosilicate glass 5 in the subareas 10 locally removed 84 , The remaining remains of the phosphosilicate glass 5 serve during the subsequent oxidation 80 of the subareas 10 in a water vapor-containing environment in the present embodiment as a masking of the high-doping regions 15 , After oxidation 80 and the concomitant formation of the silicon oxide layers 11 thus results in analogy to the embodiment of 1 a selective emitter with high doping regions 15 and subregions with increased sheet resistance 13 , After the in 2 like in 1 provided optional etching of the silicon oxide layers 11 and the masking, ie in the present case the remains of the phosphosilicate glass 5 , The subregions are located with increased sheet resistance as well as the high doping areas 15 open for any further treatment.

Instead of an etching paste can in the embodiments of the 1 and 2 also find other etching media use, in particular etching liquids, which are applied for example by means of suitable inkjet printing technologies. Instead of etching for the purpose of locally opening the mask is in the embodiments of the 1 and 2 also a laser beam evaporation of the respective masking in the subregions 10 conceivable.

The outline of the 3 illustrates how the method according to the invention can be integrated into the production process of solar cells with a flat back contact. Illustrated in this way 3 at the same time two further embodiments of the method according to the invention. The production process for solar cells according to the 3 is designed for the use of silicon blocks sawn silicon solar cell substrates, as initially a Sägeschadenätzen 30 is provided. This can be done at the same time or before a texture etching 30 be performed. In the following, the method steps of 1 or 2 go through what's in the 3 is symbolized by an ellipse with the reference symbol F1 / 2. If the method steps according to 1 go through, so is silicon nitride 7 used as masking, when using the method steps 2 however, the phosphosilicate glass 5 ,

In addition, in a conventional manner silicon nitride 7 deposited on the front of the silicon solar cell substrate used 32 , a front side metallization screen printed 34 and a flat aluminum backside metallization by screen printing 36 applied to the back of the solar cell substrate. By a fire 38 Finally, the front and back contacts are formed in a known manner. As 3 shows, the inventive method can be easily integrated into a manufacturing process for industrial standard solar cells with planar back-side metallization.

The fact that the method according to the invention can also be used advantageously in more mature solar cell production processes is illustrated 4 , In the illustrated embodiment, the inventive method F1 / 2 are from the figures 1 or 2 integrated into a manufacturing process for silicon solar cells with a boron backside field. After that already off 3 known Sägeschaden- and texture etching 30 will be at the Solar cell production according to 4 Subsequently, the texture on the back of the solar cell substrate removed 40 , This is followed by a known on-board boron diffusion in conjunction with an etching 42 a formed borosilicate glass. This is followed by oxidation 44 a surface of the silicon substrate adjoins a silicon nitride deposition 46 on the back of the silicon substrate before removing the front silicon oxide 48 , Will in the previous oxidation step 44 only the front surface of the silicon substrate is oxidized, so the removal step 48 of the silicon oxide on the front obviously omitted.

At the now reached point of the manufacturing process, the process steps according to the 1 or 2 be inserted advantageously, which by the reference F1 / 2 and the associated ellipse in 4 is symbolized. The etching 82 the masking of silicon nitride or phosphorus silicate glass and the silicon oxide layers has in the embodiment of 4 be made such that here deposited on the back 46 Silicon nitride is at least not completely removed. Preferably, the etching is 82 Therefore, the masking and the silicon oxide layers to confine to the front side of the solar cell substrate. In addition, they are already following 3 known steps of silicon nitride deposition 32 on the front, screen printing 34 a front side metallization, screen printing 36 backside metallization and firing 38 , However, in the present embodiment, the screen printing differs 36 the backside metallization insofar of the corresponding process step the 3 in that usually no planar rear-side metallization is applied, since as a result of the boron diffusion, a planar rear-side field is already present.

The outline of the 5 shows that the inventive method can also be easily integrated into modern and complex industrial manufacturing processes for silicon solar cells with very high efficiencies. Thus, with the method according to the 5 industrially produced solar cells with a passivated emitter and a passivated back, so-called i-PERC solar cells. The sake of clarity are already out 1 known process steps in the width compared to the other process steps shown depreciated.

As in the case of 3 and 4 The manufacturing process begins with a saw damage and texture etching 30 , It is obvious that in the embodiments of the 3 to 5 The texture etching can also be omitted, but this leads to solar cells with lower efficiencies. Furthermore, a saw damage etching is known to be omitted if other solar cell substrates sawn from a silicon block are used.

Furthermore, if present, the texture on the back of the solar cell substrate is removed again 50 , In addition, they are already following 1 known method steps of emitter diffusion 70 , the etching of the phosphosilicate glass 72 , of applying 74 of silicon nitride on the front, applying 76 the etching paste as well as the local removal 78 of silicon nitride in some areas and oxidizing 80 of the sub-areas in steam-containing environment.

In addition, silicon nitride is applied to the back side of the solar cell substrate 90 , This silicon nitride is again locally removed on the backside 92 , This is what this excludes 1 known etching 82 the silicon nitride masking and the silicon oxide layers on the front side of the solar cell substrate. The deposited on the back of the solar cell substrate silicon nitride is not removed.

Subsequently, silicon nitride is deposited again on the front side 32 Screening the front side metallization 34 and the backside metallization applied by screen printing 36 , In this application 36 In the backside metallization, the metallization is introduced at least into the openings formed locally in the backside silicon nitride. In addition, a screen printing paste is used without Glasfritteanteile, so that the paste is not fired through the backside silicon nitride, but only reaches the silicon substrate through the openings formed on the back. Finally, the firing takes place again 38 ,

In the illustrated embodiment of the 5 is, in analogy to the embodiment of 1 , Silicon nitride applied as a mask 70 , In principle, however, it is also conceivable, in analogy to 2 to use the formed during emitter diffusion phosphosilicate glass as masking. In this case, the step of phosphosilicate glass etching would be omitted 72 , Furthermore, locally in the subregions, the phosphosilicate glass would be removed instead of the silicon nitride. In addition, instead of silicon nitride masking, the phosphor silicate glass would have to be etched on the front side and removed during this process.

In the embodiments of the 3 to 5 Both front and back metallizations are applied by screen printing. Basically, this can also be done using other technologies.

With the in connection with the 3 to 5 described production processes can be produced in each case low-cost solar cells with a selective emitter.

6 shows an embodiment of a solar cell according to the invention 20 , This is a silicon solar cell on a silicon substrate 1 based. The solar cell 20 has an emitter which passes through high doping regions 15 and subareas 13 is formed with increased sheet resistance, ie weaker doping. There is thus a selective emitter 13 . 15 in front. In high-emission-free sections of a front page 22 the solar cell 20 facing interface 23 of the emitter, the ratio of the standard deviation of an emitter layer resistance to the mean value of the emitter layer resistance is less than 0.05. In the embodiment of 6 are the high-emission-free sections of the front 22 the solar cell 20 facing interface 23 those sections of the interface 23 which are in the subareas 13 lie with increased sheet resistance.

On the high doping areas 15 of the emitter 13 . 15 are contact fingers 17 arranged the front side metallization. These contact fingers 17 are by a silicon nitride layer applied as antireflection coating 21 by fired. On the back of the solar cell 20 is a flat backside metallization 19 intended.

The in the 6 illustrated solar cell 20 For example, with the related 3 be manufactured described manufacturing process.

LIST OF REFERENCE NUMBERS

1

silicon substrate

3

emitter region

5

PSG

7

silicon nitride

9

etching paste

10

subregions

11

silicon oxide

13

Subregions with increased sheet resistance

15

High doping regions

17

Contact fingers of the front metallization

19

backside metallization

20

solar cell

21

silicon nitride

22

front

23

Front-facing emitter interface

30

Sägeschaden- / Texturätzen

32

Silicon nitride deposition on front side

34

Screen printing front side metallization

36

Screen printing backside metallization

38

To fire

40

Remove texture on back

42

Back boron diffusion and borosilicate glass etch

44

Oxidize surface of silicon substrate

46

Silicon nitride deposition on reverse side

48

Remove silica on front

50

Remove texture on back

70

emitter diffusion

72

Etch phosphosilicate glass

74

Apply silicon nitride

76

Apply etching paste to subregions covering regions of the silicon nitride layer

78

Local removal of silicon nitride in partial areas

80

Oxidizing the subregions in a water vapor-containing environment

82

Etching masking and silicon oxide layers

84

Local removal phosphosilicate glass

86

Application of etching paste to partial regions covering the phosphor silicate glass

90

Apply silicon nitride on reverse

92

Locally remove silicon nitride on reverse side

F1 / 2

Process steps according to 1 or 2

Claims (15)

Method for increasing a sheet resistance in at least one subregion ( 10 ) of a doped semiconductor region ( 3 ), characterized by oxidation ( 80 ) of the at least one subregion ( 10 ) in a steam environment. Method according to claim 1, characterized in that a doped silicon region ( 3 ) as a doped semiconductor region ( 3 ) is used. Method according to one of the preceding claims, characterized in that by means of oxidizing ( 80 ) an oxide layer ( 11 ) is formed with a thickness between 100 nm and 400 nm, preferably with a thickness between 100 nm and 300 nm, and particularly preferably with a thickness between 130 nm and 250 nm. Method according to one of the preceding claims, characterized in that one during the oxidation ( 80 ) of the at least one subregion ( 10 ) formed oxide layer ( 11 ) Will get removed ( 82 ), preferably by means of etching. Method according to one of the preceding claims, characterized in that an emitter region ( 3 ) of a solar cell substrate as a doped semiconductor region ( 3 ) is used. Method according to one of the preceding claims, characterized in that the oxidation ( 80 ) of the doped semiconductor region ( 3 ) in at least one high-doping range ( 15 ) is suppressed, preferably by forming ( 74 ; 70 ) of a mask ( 7 ; 5 ) on this at least one high-doping range ( 15 ). Method according to claim 6, characterized in that as masking ( 7 ; 5 ) a dielectric ( 7 ; 5 ), preferably silicon nitride ( 7 ). Method according to one of claims 6 to 7, characterized in that as masking ( 5 ) a silicate glass layer ( 5 ) is formed, preferably a phosphorus ( 5 ) or borosilicate glass layer. Method according to one of claims 6 to 8, characterized in that the masking ( 7 ; 5 ) initially also on the at least one subregion ( 10 ) is formed and there before oxidation ( 80 ) is removed locally ( 84 ). Method according to claim 9, characterized in that for the purpose of local removal ( 84 ) of masking ( 7 ; 5 ) in the at least one subregion ( 10 ) an etching medium ( 9 ) to at least one subregion ( 10 ) covering region of masking ( 7 ; 5 ) is applied ( 76 ). Method according to one of claims 6 to 10, characterized in that the masking ( 7 ; 5 ) after oxidation ( 80 ), preferably together with the oxide layer ( 11 ). Method according to claim 5 and one of claims 6 to 11, characterized in that by increasing the sheet resistance in the at least one subregion ( 10 ) an emitter ( 13 . 15 ) with a two-stage doping ( 13 . 15 ) is formed. Solar cell ( 20 ) with an emitter ( 13 . 15 ), characterized in that in high-doping-free sections of a front ( 22 ) of the solar cell ( 20 ) facing interface ( 23 ) of the emitter ( 13 . 15 ) the ratio of the standard deviation of an emitter layer resistance to the mean value of the emitter layer resistance is less than 0.07. Solar cell ( 20 ) according to claim 13, characterized in that the emitter ( 13 . 15 ) by a two-stage doping ( 13 . 15 ) is formed. Use of the method according to one of claims 1 to 12 for homogenizing a doping in at least one partial area ( 10 ) of a doped semiconductor region ( 3 ).

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