JPH0812849B2 - Solar cell manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element having a PN junction in a semiconductor, particularly to a method for manufacturing a solar cell using a Si semiconductor.

[0002]

2. Description of the Related Art When a solid diffusion source is used for diffusion, conventionally, a single type solid diffusion source is used. In this method, the diffusion process needs to be performed twice when two different types of impurity diffusion are performed. In addition, “Unimproved Boron Nitride Glass Process” Solid State Technology (1980)
(An Improved Boron Nitrid
e Glass Process "" Solid Sta
te Technology (1980) ”), when a solid diffusion source in which boron nitride (BN) is fired in a wafer shape with a binder of boron oxide (B ₂ O ₃ ) is used, a large amount of impurities and minority carriers after diffusion are used. It was difficult to keep the lifetime high.

On the other hand, "Characterization of Pyrolytic Boron as a Diffusion Source for Silicon", Solid State Electronics, Vol. 21, 987-988 (1
978) ("CHARACTERIZATION OF
PYROLYTIC BORON ASA DIFF
USION SOURCE FOR SILICO
N "" Solid State Electronic
s, Vol. 21 ,. pp. 987-988 (197)
8) ") and the use of a pyrolytic boron nitride (PBN) wafer formed by chemical vapor deposition (CVD) as a diffusion source of boron as described in JP-A-62-101026. Is known to be possible.

[0004]

As a method for forming a boron diffusion layer on the surface of a semiconductor, there are thermal diffusion from a liquid phase, a solid phase, a gas phase, diffusion by an ion implantation method, and the like. When a boron solid diffusion source is used as a diffusion method from a gas phase using the solid diffusion source, a diffusion process needs to be performed twice when two different types of impurity diffusion are performed as described above. This complicates the process and makes it difficult to shorten the construction period and reduce the cost.

Further, when diffusion is performed using a BN wafer, a diffusion source containing a large amount of impurities such as heavy metals has been conventionally used. This makes it difficult to prevent crystal defects due to impurities in the boron layer formed by diffusion and a decrease in minority carrier lifetime in the diffusion layer due to trap centers.

Therefore, PBN formed by the CVD method
Boron diffusion using a solid diffusion source is considered. However, when the surface of BN is oxidized and then boron diffusion is simply performed on the surface of the substrate in an inert gas atmosphere, 1000 ° C.
In the following diffusion, variations in the sheet resistance of the boron diffusion layer occur, and in the diffusion at a high temperature of 1100 ° C. or higher, a decrease in lifetime becomes a problem. Therefore, the photoelectric conversion efficiency was about 18% in the single crystal silicon solar cell in which the junction was formed by the conventional method.

Such a decrease in lifetime is remarkable in the diffusion using the conventional BN solid-state diffusion source,
In particular, when forming a high-low junction structure for creating a surface electric field, the surface impurity concentration of the boron diffusion layer is set to 5
X10 ¹⁹ cm ^-3 or more, and in this case P
Even if BN is used, it is necessary to carry out diffusion at a high temperature of about 900 ° C. for a long time of 30 minutes or more, so that it is difficult to maintain the lifetime. Further, when it is necessary to remove the boron-silicon layer after diffusion, if the removal is performed by low temperature oxidation (LTO), the surface concentration of the boron diffusion layer decreases, so that the surface concentration during diffusion is 5 × 10 ²⁰ cm ⁻³ or more. Must be In order to carry out such diffusion, it becomes difficult to maintain the lifetime because diffusion at higher temperature is required.

[0008]

Means for solving the above problem will be described with reference to FIGS. When high-purity pyrolytic boron nitride (PBN) is used and PBN1 is arranged adjacent to the semiconductor 2 in the reaction tube 5 as shown in FIG. 1 to diffuse boron, other than boron or nitride contained in PBN1. The content of impurities, especially heavy metals and alkali metals, is 1
The lifetime after diffusion can be lengthened by suppressing the content to below ppm. However, if the diffusion is simply performed in an inert atmosphere 3 such as nitrogen, argon, or helium, the boron-containing gas 4 that diffuses from PBN to the semiconductor 2 in the vapor phase 4
Is small. For this reason, the thickness of the boron glass 9 formed by the boron-containing gas 4 reaching the semiconductor 2 varies as shown in FIG. 2A, and the sheet resistance of the diffusion layer 8 after diffusion is large, and particularly At a temperature of 1000 ° C. or less, it becomes ± 20% or more. For example, when it is used for forming a channel of a MOS diode, an emitter and a base of a bipolar diode, or diffusion of a light receiving portion such as an optical sensor or a photoelectric conversion element, minority carrier lifetime, crystal defect, interface state of this diffusion layer. A relatively low concentration of diffusion is performed to reduce the density. In this case, the above-mentioned variation in sheet resistance becomes particularly large, so it is necessary to reduce this. As a solution to this problem
It is possible to form a thicker boron glass 9 uniform on the surface of the substrate 7 as shown in FIG increasing concentrations of boron-containing gas 4 2 (b) by mixing a small amount of hydrogen and oxygen.
The diffusion layer 8 thus formed has a diffusion temperature of 10
The resistance variation was about 5% or less even when the high resistance layer having a sheet resistance of 50 Ω / □ or more was formed at 00 ° C. or less. In addition, by mixing oxygen into the diffusion atmosphere 3, it is possible to prevent contamination of heavy metals and the like during diffusion. This is because boron is diffused to the semiconductor surface only at the initial stage of diffusion, and thereafter the oxide film formed on the surface prevents diffusion of impurities such as heavy metals. At the same time, the boron high-concentration diffusion layer is oxidized and the formation of the boron-silicon layer after diffusion can be prevented.

[0009]

As described above, by making the diffusion atmosphere 3 an oxidizing atmosphere, the surface of the boron nitride diffusion source 1 is oxidized, and the vapor pressure of this oxide increases, so that a large amount of boron glass adheres to the substrate surface. To do. A mixed gas of hydrogen and oxygen can be used as the oxidizing atmosphere. In this case, if the hydrogen concentration is 0.1% or more of the diffusion atmosphere and the oxygen concentration is 0.05% or more, which is half the oxygen concentration, the amount of boron gas evaporated can be sufficiently increased. Further, water vapor may be mixed with the diffusion gas 3 without using the mixed gas of hydrogen and oxygen. In this case, it is necessary that the water vapor is evenly mixed in the diffusion gas. The purpose of these hydrogen, oxygen, and water vapor can be sufficiently achieved only by using the first few seconds to 1 minute of diffusion. The thickness of the boron glass formed on the surface of the substrate must be equal to or less than the thickness that allows boron to pass through during diffusion.

When oxygen is mixed in the diffusion atmosphere 3, an oxide film is formed during the diffusion as described above. Therefore, as shown in FIG. 3, the boron glass formed before the oxide film 10 is formed ( The diffusion layer 8 is formed only by diffusion from 300 Å or less). After this, the oxide film 10 (500
~ 1000Å) prevents diffusion of impurities such as heavy metals, so that the boron diffusion layer 8 with less contamination is formed. For this purpose, it is desirable that the oxygen concentration in the diffusion atmosphere 3 be about 1/10 or more of the inert gas in the diffusion atmosphere.

A high quality boron diffusion layer can be obtained by using these diffusion methods. Further, even in the diffusion at a high temperature, the diffusion can be performed without generating crystal defects. Such high quality diffusion layer not only forms a PN junction, is also effective for the formation of high-low junction such as P + P the impurity concentration to form a Higher diffusion layer in the same conductivity type as the bulk of the device is there. When used for these diffusions, the impurity surface concentration of the diffusion layer needs to be 5 × 10 ¹⁹ cm ³ or more. For this purpose, diffusion at high temperature and high concentration is required, but it was difficult to generate defects by the conventional diffusion method. However, by performing the above-mentioned diffusion using PBN, such diffusion is performed. It became possible to form layers. Furthermore, when the boron-silicon layer is oxidized by low-temperature oxidation after these diffusions and removed by etching, the surface concentration is reduced by about one digit. In this case, the surface concentration immediately after diffusion needs to be 5 × 10 ²⁰ cm to ³ .

By using this diffusion source, it is possible to form a diffusion layer having a conductivity type different from that of the boron diffusion layer simultaneously with the boron diffusion. This will be described with reference to FIG. When forming the diffusion layer, the boron diffusion source 1 is arranged adjacent to the semiconductor 2 and the boron diffusion layer 1 is not arranged on the opposite side of the semiconductor 2. The semiconductor layer 2 Tsunoshirube conductivity <br/> type in one thermal diffusion by using a gas containing diffusion source of a different conductivity type boron diffusion layer in the diffusion atmosphere 3 in this state both sides of the semiconductor 2 Can be formed.

Further, by using the solid diffusion source 11 shown in FIG. 4 as a diffusion source having a conductivity type different from that of the boron diffusion layer, the first diffusion atmosphere 4 and the second diffusion atmosphere 6 are formed on both sides of the semiconductor 2. Can be reached with almost no mixing.

By producing a solar cell using these diffusion methods, characteristics such as lifetime and resistance variation of the boron diffusion layer are improved, and the production process is simplified, resulting in high conversion efficiency and low cost. The solar cell can be manufactured.

Until now, PBN has been used as a boron diffusion source.
Although only the diffusion source has been described, it goes without saying that these can also be applied to other boron diffusion sources that contain impurities equal to or less than the PBN diffusion source.

Needless to say, the diffusion method of the present invention is effective not only in the production of solar cells, but also in the production of photosensors, photoelectric conversion elements, transistors, thyristors, GTOs, diodes and the like.

The semiconductor for diffusion may be a single crystal semiconductor such as Si or Ge or a polycrystalline semiconductor. Further, as the conduction type diffusion source different from the boron diffusion layer, a gas containing phosphorus, histo, antimony, or the like, a solid diffusion source containing these, or the like can be used.

[0018]

EXAMPLE One example of the present invention will be described with reference to FIG.
As the semiconductor substrate 2, single crystal Si was used. PBN was used for the boron diffusion source 1. The substrate and the boron diffusion source substrate were arranged as shown in the figure. Argon gas was caused to flow into this, and diffusion was performed at 900 ° C. for 40 minutes. At the beginning of diffusion, 2% hydrogen and 5% oxygen were flushed for 2 minutes. As a result, a sheet resistance of 200Ω / □ was obtained. Also, 1000 ℃
When it was diffused at 20 Ω / □ was obtained. The surface impurity concentration at this time was about 1 × 10 ²⁰ cm ⁻³ . The variation in sheet resistance was 5% or less. The minority carrier lifetime after diffusion was about 50 μs. This result was the same when nitrogen gas or helium gas was passed instead of argon.

When nitrogen: oxygen was used as the diffusion gas 3 at a ratio of 3: 1, the sheet resistance after diffusion was increased by about an order of magnitude, but the minority carrier lifetime was also increased by about 3 times. This is because the diffusion of boron from the solid source progresses in the initial stage of diffusion, but thereafter the oxidation of the Si surface by oxygen in the diffusion atmosphere proceeds, and the diffusion of boron is slower than this. This is because only re-diffusion occurs and impurities other than boron are prevented from diffusing into the Si semiconductor by the thick oxide film 10.

When nitrogen and nitrogen carrier gas passing through a bubbler of POCl ₃ were used as the diffusion gas 3, the semiconductor substrate 2 had a p-type semiconductor on the boron diffusion source side and an n-type semiconductor on the opposite side. In this case, the formation of the p-type layer was ensured by using only nitrogen gas in the initial stage of diffusion.

Here, PO is used as the p-type diffusion source.
Although a nitrogen carrier gas passed through a Cl ₃ bubbler is taken as an example, it goes without saying that a bubbler using another compound or a gas such as phosphine may be used.

[0022] An example of using a solid diffusion source 6 as the second conductivity type impurity diffusion source in FIG. A boron solid diffusion source 4 and a phosphorus solid diffusion source 6 are provided on both sides of the semiconductor 2.
In the case of using, the p-type layer and the n-type layer could be formed on both surfaces of the semiconductor by one heat treatment.

A method of manufacturing a solar cell using the diffusion method of the present invention will be described with reference to FIG. The boron diffusion layer 8 and the phosphorus diffusion layer 13 were formed by using the method of the above embodiment. Diffusion was carried out at 950 ° C. with N2 gas, backside boron diffusion layer 8 is surface concentration 1 × 10 ²⁰ cm ^- was ³ diffusion depth 0.5 [mu] m. The surface phosphorus diffusion layer 10 has a surface concentration of 1 × 10 ^21.
It was cm ⁻³ and the diffusion depth was 0.8 μm. An oxide film 10 is formed on both surfaces of the diffused substrate by an ordinary thermal oxidation method.
Then, the back surface electrode 12 connected to the back surface diffusion layer 8 and the front surface electrode 14 connected to the surface diffusion layer 13 were formed in a part of the film with a thickness of 0 nm. By using this method, the long-wavelength sensitivity was significantly improved as compared with the solar cell manufactured by the conventional boron diffusion method, and a photoelectric conversion efficiency of 19.5% could be achieved.

[0024]

According to the method of the present invention, it is possible to form a boron diffusion layer having a small variation in resistance and lifetime and a long minority carrier lifetime after diffusion. Also,
It becomes possible to form high-quality p-type and n-type diffusion layers on both surfaces of the semiconductor by one-time diffusion, and the long wavelength sensitivity of the solar cell is improved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a diffusion method of the present invention.

FIG. 2 is a schematic diagram showing a diffusion method of the present invention.

FIG. 3 is a schematic diagram showing a diffusion method of the present invention.

FIG. 4 is a schematic diagram showing a diffusion method of the present invention.

FIG. 5 is a schematic view showing an example of the diffusion method of the present invention.

FIG. 6 is a schematic view showing an example of the diffusion method of the present invention.

FIG. 7 is a schematic view showing an example of a solar cell manufactured by the diffusion method of the present invention.

[Explanation of symbols]

1 ... Boron solid diffusion source, 2 ... Semiconductor, 3 ... Diffusion atmosphere, 4 ... Diffusion gas, 5 ... Reaction tube, 6 ... Second
Diffusion gas, 7 ... Substrate, 8 ... Diffusion layer, 9 ... Boron glass, 10 ... Oxide film, 11 ... Second solid diffusion source, 12 ... Back electrode, 13 ... Second diffusion Layer, 14 ・
・ Surface electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuo Tanaka 1-280, Higashi Koikekubo, Kokubunji, Tokyo (56) References, Hitachi, Ltd. Central Research Laboratory (56) Reference JP-A-2-77118 (JP, A) JP-A-54 -98189 (JP, A) JP-A-51-25512 (JP, A)

Claims (3)

[Claims] 1. A semiconductor substrate in a reaction tube and the semiconductor substrate
Adjacent to only one side of the
A body diffusion source is placed, and an inert gas in the reaction tube, and
Conducting a diffusion layer formed when diffusing into the semiconductor substrate
Boron diffusion layer whose electrotype is formed by diffusion of the above boron
Of a diffusion source containing an impurity element having a conductivity type opposite to that of
Is introduced into one surface of the semiconductor substrate to introduce p-type diffusion.
Layer, a method for manufacturing a solar cell in which an n-type diffusion layer is formed on the other surface
Method, which is 0.1% or more and 2% or less of hydrogen in the diffusion atmosphere
And 0.05% or more and 5% or less oxygen for 2 minutes in the initial stage of diffusion
Introduced for the following time, the gas of the diffusion source and the hydrogen and
The introduction of oxygen started a little later than the start of the introduction of oxygen, and one heat expansion
To form the p-type diffusion layer and the n-type diffusion layer.
And a method for manufacturing a solar cell. 2. A single crystal Si substrate is used as the semiconductor substrate.
The method of manufacturing a solar cell according to claim 1. 3. The boron concentration on the surface of the boron diffusion layer is 5 ×.
The solar power according to claim 1 or 2, which is 10 ¹⁹ cm to ³ or more.
Pond manufacturing method.