DE102012207764A1 - Method for boron doping silicon wafers in zone oven, involves diffusing boron from borosilicate glass layer in silicon wafer, cooling diffusion chamber at defined removal temperature, and removing silicon wafer out of chamber - Google Patents

Abstract

The method involves setting defined load temperature to load diffusion chamber with silicon wafers. The diffusion chamber is heated until defined target temperature is reached. The boron-containing reactive gas is introduced into diffusion chamber to form layer of borosilicate glass on surface of silicon wafer. The boron is diffused from borosilicate glass layer in silicon wafer. The diffusion chamber is cooled until defined removal temperature is reached. The silicon wafer is removed out of diffusion chamber.

Description

Solar cells based on silicon wafers play an increasingly important role in many areas of daily life. An essential process step in the production of such solar cells is the doping of the silicon wafer, in particular its n-type doping. Typical elements which are used as n- or p-doping atoms are phosphorus and boron, the latter in particular, for example, for the formation of the emitter ("collecting junction"), a "floating junction" or a "back surface field" an essential role plays.

Probably the most frequently used method for producing boron-doped silicon wafers in solar cell production is a diffusion method using a tube furnace ("OPEN TUBE SYSTEM"). In this case, liquid boron tribromide (BBr ₃ ) and oxygen (O ₂ ) together with nitrogen (N ₂ ) as a carrier gas in a reaction chamber of the tube furnace, in which the wafers are arranged, passed. In the tube furnace then the following reactions take place: 4 BBr ₃ (g) + 3 O ₂ (g) → 2 B ₂ O ₃ (l) + 6 Br ₂ (g) 2 B ₂ O ₃ (I) + 3 Si (s) → 4 B (s) + 3 SiO ₂ (s)

The letters added in parentheses in the above reaction equations to the respective substance data serve as an indication of the state of matter of the respective substance, wherein (g) is gaseous, (l) is liquid and (s) is solid.

The SiO ₂ formed in the second of the above-mentioned reaction steps is formed on the surface of the wafer and becomes partially in the liquid B ₂ O ₃ according to the reaction equation x SiO ₂ + y B ₂ O ₃ → x SiO ₂ · y B ₂ O ₃ dissolved so that a layer of borosilicate glass (BSG) is formed on the wafer surface.

According to the prior art, this borosilicate glass layer is used as a boron source for an at least partially downstream diffusion step in which boron is diffused into the wafers to provide the desired n-doped zone. For use in solar cells, a boron-diffused layer with a sheet resistance between 30 ohms / square and 200 ohms / square is required. In order to adjust this sheet resistance, it is typical to use diffusion temperatures between 890 ° C. and 1000 ° C., which are kept constant until the diffusion process is complete.

The methods known from the prior art comprise the steps of a) setting a defined loading temperature; b) loading the diffusion chamber with the silicon wafers when the defined loading temperature is set; c) heating the diffusion chamber until a defined target temperature is reached in each zone; d) introducing a boron-containing reaction gas into the diffusion chamber to form a layer of borosilicate glass on the surface of the silicon wafer; e) diffusing boron from the borosilicate glass layer into the silicon wafer, f) cooling the diffusion chamber until a defined removal temperature has been reached in each zone; and g) removing the silicon wafers from the diffusion chamber. There are two variants of the process control in the prior art:

On the one hand, it is for example from the US Pat. No. 6,548,378 B1 , the article "Charge carrier lifetime degradation in Cz silicon through the formation of a boron-rich layer during BBr3 diffusion processes", by MA Kessler et al., Semicond. Sci. Technol. 25 (2010), 055001 and the article "Water vapor as an oxidant in BBr3 open-tube silicon diffusion systems" by RF Lever et al., IBM J. Res. Develop. 1974, pp. 40-46 It is known to start the above-mentioned steps one after the other, but in step c) the target temperature is set to a lower temperature than that required for boron diffusion to a relevant extent and then in step d) the borosilicate glass layer is used as boron source for the later diffusion process is formed. Between the end of step d) and the beginning of step e), the zone furnace must therefore be heated to the required diffusion temperature in an additional step.

This leads to a procedure, as described below on the basis of 2 will be described in more detail. Since a well-defined source for the doping atoms is initially provided in this process control and the diffusion process only begins with the temperature increase, the parameters of the boron-doped layer of the silicon wafer produced are particularly easy to adjust.

On the other hand it is for example from the article Bening et al., Proceedings 24th European Solar Photovoltaic Solar Conference, 21-25 September 2009, Hamburg, Germany, p. 863-870. "High-efficiency N-type silicon solar cells with front side boron emitter" , from the article "N-type multicrystalline silicon solar cells: BBr3 diffusion and passivation of p + -diffused silicon surfaces" by J. Libal et al., Proceedings 20th European Photovoltaic Solar Energy Conference, 6-10 June 2005, Barcelona, Spain, p. 793 -796 and from the article "Effect of gettered iron on recombination in diffused regions of crystalline silicon wafers" by D. Macdonald et al., Appl. Phys. Lett. 88, 092105, 2006 It is known to carry out the above-mentioned steps with the exception of steps d) and e) one after the other. In particular, the desired diffusion temperature for step e) is already set as target temperature in step c) and, after reaching the target temperature, commenced with the introduction of the boron-containing reaction gas according to step d). As a result, the diffusion process according to step e) can already begin while the construction of the borosilicate glass layer is not yet completed. In other words, according to this method, the process steps d) and e) begin almost simultaneously and run side by side until a borosilicate glass layer of the desired thickness is formed.

This leads to a procedure, as described below on the basis of 3 will be described in more detail. Due to the fact that two of the required process steps run parallel to each other, time should be saved. Ultimately, however, the time gain achieved is low, which is not least due to the fact that the source concentration of boron is initially low because of borosilicate glass layer still under construction and thus little boron is diffused, although the required diffusion temperature has already been reached.

Due to the fierce competition in the field of the production of solar cells, there is a constant pursuit of improving the efficiency of the processes used and reducing the manufacturing costs. This can be achieved, in particular, by the use of time-saving processes.

The object of the invention is therefore to specify a method for boron doping of silicon wafers, which brings about a shortening of the duration of the manufacturing process and thereby enables a more efficient and less expensive production of boron-doped silicon wafers.

This object is achieved by a method for boron doping of silicon wafers with the features of claim 1. Advantageous developments of the method are the subject of the dependent claims.

• a) Einstellen einer definierten Beladungstemperatur;
• b) Beladen der Diffusionskammer mit den Silizium-Wafern, wenn die definierte Beladungstemperatur eingestellt ist;
• c) Aufheizen der Diffusionskammer, bis in jeder Zone eine definierte Zieltemperatur erreicht ist;
• d) Einbringen eines borhaltigen Reaktionsgases in die Diffusionskammer zur Bildung einer Schicht aus Borsilikatglas auf der Oberfläche der Silizium-Wafer;
• e) Eindiffundieren von Bor aus der Borsilikatglasschicht in den Silizium-Wafer
• f) Abkühlen der Diffusionskammer, bis in jeder Zone eine definierte Entnahmetemperatur erreicht ist; und
• g) Entnehmen der Silizium-Wafer aus der Diffusionskammer.

The method according to the invention for boron doping of silicon wafers in a zone furnace, which has a diffusion chamber with a plurality of zones which can be heated independently of one another, comprises the following steps:

• a) setting a defined loading temperature;
• b) loading the diffusion chamber with the silicon wafers when the defined loading temperature is set;
• c) heating the diffusion chamber until a defined target temperature is reached in each zone;
• d) introducing a boron-containing reaction gas into the diffusion chamber to form a layer of borosilicate glass on the surface of the silicon wafer;
• e) diffusing boron from the borosilicate glass layer into the silicon wafer
• f) cooling the diffusion chamber until a defined extraction temperature is reached in each zone; and
• g) removing the silicon wafer from the diffusion chamber.

It is essential to the invention that at least steps c) and d) are carried out at least partially simultaneously or that at least steps d) and f) are carried out at least partially simultaneously. In other words, the invention is characterized in that at least during a part of the time in which the diffusion chamber is heated and / or during a part of the time in which the diffusion chamber cools, boron-containing reaction gas is introduced into the diffusion chamber. Thus, even during these periods, during which there is no constant temperature in the diffusion chamber, the structure of the borosilicate glass layer.

It should be noted that the diffusion of dopant atoms, referred to as step e), is essentially temperature controlled. This means that this step starts as soon as a boron source is present and a sufficiently high temperature prevails. As a result, in the process according to the invention, if appropriate, steps d) and e) also run at least partially parallel to one another, wherein as a rule step e) is continued even after the end of step d).

The invention is based on the surprising finding that the properties of boron-doped silicon wafers, which are produced by using the method, in contrast to the previous assumption, not only well-defined and well reproducible, if the formation of the borosilicate glass layer and the Eindiffusionsvorgang at constant temperature can be carried out, but also if these processes take place at changing temperature. This makes it possible to use at least parts of the necessary heating and cooling phases already for the production of the doped silicon wafer, which leads to a significant time savings, since typical temperature change rates for commercially available zone furnaces at about 4 ° C per minute for heating and 10 ° C per minute when cooling.

However, it has been found that it makes sense to allow a few minutes to elapse between the end of the loading of the diffusion chamber and the beginning of the introduction of the boron-containing reaction gas, so that the temperature distribution in the interior of the diffusion chamber can homogenize.

It has proved to be advantageous if at least step d) is passed at least once during step c) and during step f). This makes it possible to finer adjust the doping properties and contributes to a homogenization of the properties of the obtained doped silicon layer.

With regard to a simple process control, it is particularly advantageous if step f) is performed immediately after step c). This means that there are no more phases during the process in which the temperature of the diffusion chamber must be kept constant with high control effort. It has proved to be expedient to select the target temperature according to step c), which is also the highest temperature which is achieved in carrying out the method, higher than that which is necessary for producing a boron-doped layer with comparable properties was used during the diffusion step according to the hitherto conventional methods in which the borosilicate glass formation and the diffusion of boron at constant temperature took place.

Conveniently, the target temperature according to step c) is higher than the loading temperature selected in step a).

Local temperature inhomogeneities within the zone furnace can be at least partially compensated for by setting the loading temperature, the target temperature and the removal temperature for different zones of the zone furnace individually.

Particularly good results are obtained if the loading temperature and the removal temperature are selected from the temperature range between 700 ° C and 900 ° C and that the target temperature is selected from the temperature range between 850 ° C and 1200 ° C.

Furthermore, during the heating in step c) a temperature increase rate between 0.5 ° C per minute and 15 ° C per minute should be used and when cooling in step f) a temperature cooling rate between 0.1 ° C per minute and 10 ° C per minute is used become. Again, to compensate for inhomogeneities between the zones of the zone furnace when heating in step c) the temperature increase rate for different zones of the zone furnace are each set individually and / or the temperature cooling rate for different zones of the zone furnace are set individually during cooling in step f) ,

As is apparent from the above-mentioned possible variations, the method of the present invention can be used to adjust and control the properties of the resulting boron-doped layers of the silicon wafers to a considerable extent.

Advantageous means for supplying the boron-containing reaction gas in the diffusion chamber is a BBr ₃ -Bubbler or a BCl ₃ -Bubbler.

Particularly good results are achieved when the boron-containing reaction gas O ₂ and either BBr ₃ or BCl ₃ , which is supplied with N ₂ or Ar as a carrier gas.

The invention will be explained in more detail with reference to figures. Show it:

1 A zone furnace known in the art;

2 a first known from the prior art method for boron doping of silicon wafer, shown in a temperature-time diagram;

3 a second method known from the prior art for boron doping of silicon wafers, shown in a temperature-time diagram; and

4 : An embodiment of the method according to the invention, shown in a temperature-time diagram.

1 shows a known, commercially available zone furnace 100 as it can be used in carrying out the method according to the invention. The zone oven 100 has a diffusion chamber 101 on, passing through a tubular section inside a quartz mantle 108 is formed. At one end of the zone furnace 100 are a gas inflow 104 into the diffusion chamber 101 and a gas drain 105 from the diffusion chamber 101 available; at the opposite end is the oven door 106 arranged. In this example, the oven has five heatable zones 107a . 107b . 107c . 107d and 107e that are independently controllable.

For arranging the silicon wafers 103 in the zone oven 100 These will be on a ship 102 made of quartz arranged with open oven door 106 in the zone furnace 100 introduced what after closing the oven door 106 to the in 1 illustrated situation leads. However, in the interest of clarity and clarity of presentation according to 1 the number of silicon wafers drawn 103 smaller than she is in reality.

2 shows the process in a first known from the prior art method for boron doping of silicon wafers using a zone furnace 100 in a temperature-time diagram. Here are, as well as in the 3 and 4 , the points at which process steps are initiated and / or terminated marked with dots.

According to the in 2 The method described is the loading of the zone furnace 100 carried out at a loading temperature of 800 ° C and then the zone furnace 100 heated to a temperature of 850 ° C. After reaching this temperature, the supply of boron-containing reaction gas begins until about 40 minutes after loading the formation of the desired borosilicate glass layer is completed. During the formation of this layer, the temperature is kept at 850 ° C. Subsequently, the heating takes place to the target temperature of about 920 ° C, which is then held for about 60 minutes to perform the indiffusion step. Finally, the zone oven 100 cooled to the discharge temperature and the silicon wafer 103 are from the zone furnace 100 taken.

3 shows the course of the process in a second known from the prior art method for boron doping of silicon wafers using a zone furnace 100 in a temperature-time diagram. After loading the zone furnace 100 at a loading temperature of 800 ° C, the zone furnace 100 heated to a temperature of 940 ° C, which also corresponds to the target temperature for the step of Eindiffundens. After reaching this temperature, the supply of boron-containing reaction gas begins until about 40 minutes after loading or 25 minutes after the beginning of the supply of the boron-containing reaction gas, the formation of the desired borosilicate glass layer is completed. During the formation of this layer, the temperature is also maintained at 940 ° C as during the subsequent indiffusion step, which lasts another 55 minutes. Finally, the zone oven 100 cooled to the discharge temperature and the silicon wafer 103 are from the zone furnace 100 taken.

4 shows the real temperature-time diagram of a zone furnace 100 during the implementation of the method according to the invention, wherein the predetermined temperature values for the individual zones 107a to 107e the zone furnace 100 Table 1 below shows: Zone Loading temperature (° C) Target temperature (° C) Withdrawal temperature (° C) 107a 844 969 850 107b 841 968 850 107c 843 968 850 107d 845 974 850 107e 865 980 850 Table 1

The diffusion chamber 101 becomes at the given loading temperature with the wafers 103 loaded and then heated to the specified target temperature. After all zones 107a to 107e have reached the respective predetermined target temperature, the zone furnace 100 cooled to the predetermined discharge temperature.

As in 4 can be seen, the supply of boron-containing reaction gas is started 7 minutes after the start of heating and ends during the cooling process 10 minutes before reaching the discharge temperature. Thus, in this embodiment, both the heating step and the introduction of the reaction gas, so steps c) and d) and the introduction of the reaction gas and the cooling step, ie steps d) and f), at least partially carried out simultaneously for the inventive method. As a result, the total duration of the process is only 54 minutes versus about 130 minutes in the prior art processes incorporated in the 2 and 3 are shown.

In the course of the procedure, based on the 4 has been described, the boron-containing reaction gas was fed through a BBr ₃ -Bubbler, while the admixture of oxygen in the diffusion chamber 101 took place. For a diffusion chamber volume of 200 liters, particularly suitable parameters are gas flows of 20 slm (standard liters per minute) for the nitrogen carrier gas; 0.1 slm for BBr ₃ and 0.1 slm for O ₂ .

In order to make the obtained silicon wafers suitable for use in solar cells, the etching of the remaining borosilicate glass layer from the silicon wafers is necessary according to the method according to the invention, as well as according to the known methods. The average sheet resistance of the resulting boron-doped layer was 59 ohms / square with a maximum variation on a wafer of +/- 5.9 ohms / square and maximum variation across the wafers on the ship 102 from + / 1 to 2.1 ohms / square.

In order to ensure that the method according to the invention is not only time-saving but results in boron-doped silicon wafers with competitive properties, the following experiment was carried out:

On identical monocrystalline, n-doped silicon wafers, two groups of identically constructed solar cells were produced, namely solar cells, in which the boron-doped layer forms a homogeneous p ⁺ emitter on the front side of the solar cell and the back with one by means of phosphorus doping generated back surface field is provided. The only difference between the two groups was the method used for boron doping. While a group with the basis of the 4 explained, the inventive method was prepared, was in the second group based on the 3 described method-wise, which, as mentioned in detail in the introduction of the description, is shown in printed prior art as the method of choice.

Subsequently, the performance of the solar cells belonging to the respective groups was investigated, with regard to the short-circuit current density Jsc, the open clamp voltage (Voc), fill factor (FF) and efficiency (η) of the solar cells. The results are shown in Table 2 below: Jsc (mA / cm ² ) Voc (mV) FF (%) η (%) Process duration (min) Layer resistance (ohms / sq) State of the art 38.9 639 74.3 18.4 130 64 +/- 3.6 invention 39.0 642 76.1 19.0 54 59 +/- 3.4 Table 2

From Table 2, it is clear that the performance of solar cells produced according to the invention, the performance of solar cells, according to the in 3 have been prepared, not only achieved but even marginally exceeded in all relevant operating parameters, while less than half the time was required for their production. The invention thus allows the production of slightly better solar cells at a much lower cost.

LIST OF REFERENCE NUMBERS

100

zone furnace

101

diffusion chamber

102

ship

103

Silicon wafer

104

gas flow

105

gas drainage

106

oven door

107a, b, c, d, e

zones

108

Quartz-Coat

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 6548378 B1 [0007]

Cited non-patent literature

• "Charge carrier lifetime degradation in Cz silicon through the formation of a boron-rich layer during BBr3 diffusion processes", by MA Kessler et al., Semicond. Sci. Technol. 25 (2010), 055001 and the article "Water vapor as an oxidant in BBr3 open-tube silicon diffusion systems" by RF Lever et al., IBM J. Res. Develop. 1974, pp. 40-46 [0007]
• Benign et al., Proceedings 24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany, pp. 863-870 [0009] "High-efficiency N-type silicon solar cells with front side boron emitters" by J. Benick et al.
• "N-type multicrystalline silicon solar cells: BBr3 diffusion and passivation of p + -diffused silicon surfaces" by J. Libal et al., Proceedings 20th European Photovoltaic Solar Energy Conference, 6-10 June 2005, Barcelona, Spain, p. 793 -796 [0009]
• "Effect of gettered iron on recombination in diffused regions of crystalline silicon wafers" by D. Macdonald et al., Appl. Phys. Lett. 88, 092105, 2006 [0009]

Claims (11)

Method for boron doping of silicon wafers ( 103 ) in a zone furnace ( 100 ), which has a diffusion chamber ( 101 ) with several independently heated zones ( 107a . 107b . 107c . 107d . 107e ), comprising the steps of: a) setting a defined loading temperature; b) Loading the diffusion chamber ( 101 ) with the silicon wafers ( 103 ) when the defined loading temperature is set; c) heating the diffusion chamber ( 101 ), in every zone ( 107a . 107b . 107c . 107d . 107e ) a defined target temperature is reached; d) introduction of a boron-containing reaction gas into the diffusion chamber ( 101 ) for forming a layer of borosilicate glass on the surface of the silicon wafers ( 103 ); e) diffusing boron from the borosilicate glass layer into the silicon wafer ( 103 f) cooling the diffusion chamber ( 101 ), in every zone ( 107a . 107b . 107c . 107d . 107e ) has reached a defined extraction temperature; and g) removing the silicon wafers ( 103 ) from the diffusion chamber ( 101 ), characterized in that at least steps c) and d) are carried out at least partially simultaneously or that at least steps d) and f) are carried out at least partially simultaneously. A method according to claim 1, characterized in that during step c) and during step f) at least step d) is in each case at least once through. A method according to claim 1 or 2, characterized in that step f) is performed immediately after step c). Method according to one of the preceding claims, characterized in that the target temperature according to step c) is higher than the loading temperature according to step a). Method according to one of the preceding claims, characterized in that the loading temperature, the target temperature and the discharge temperature for different zones ( 107a . 107b . 107c . 107d . 107e ) of the zone furnace ( 100 ) are set individually. Method according to one of the preceding claims, characterized in that the loading temperature and the removal temperature are selected from the temperature range between 700 ° C and 900 ° C and that the target temperature is selected from the temperature range between 850 ° C and 1200 ° C. Method according to one of the preceding claims, characterized in that during heating in step c) a temperature increase rate between 0.5 ° C per minute and 15 ° C per minute is used and that during cooling in step f) a temperature cooling rate between 0.1 ° C. per minute and 10 ° C per minute is used. Method according to one of the preceding claims, characterized in that during heating in step c) the temperature increase rate for different zones ( 107a . 107b . 107c . 107d . 107e ) of the zone furnace ( 100 ) is set individually. Method according to any preceding claim, characterized in that, during cooling in step f), the temperature cooling rate for different zones ( 107a . 107b . 107c . 107d . 107e ) of the zone furnace ( 100 ) is set individually. Method according to one of the preceding claims, characterized in that the boron-containing reaction gas is introduced into the diffusion chamber through a BBr ₃ bubbler or a BCl ₃ bubbler ( 101 ) is introduced. A method according to any preceding claim, characterized in that the boron-containing reaction gas comprises O ₂ and either BBr ₃ or BCl ₃ supplied with N ₂ or Ar as the carrier gas.

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