CN102206866A - Hydrogen plasma passivation method by preventing discharge with medium - Google Patents

Abstract

The invention relates to a dielectric barrier discharge hydrogen plasma passivation method. The passivation method is as follows: put the sample on the sample stage, and evacuate the cavity to a certain vacuum through the vacuum system; pass hydrogen gas into the cavity through the hydrogen source and adjust it to a certain pressure; conduct hydrogen plasma passivation through dielectric barrier discharge processing. Compared with the existing technology: the present invention can directly process the prepared battery sheet, the equipment is simple, and the processing time is short. Compared with plasma immersion ion implantation (PIII) technology, under the same ion state and treatment effect, no external ICP or ECR excitation source is needed, and the acceleration electric field can also be realized by using a sinusoidal power supply that is easier to implement.

Description

Dielectric Barrier Discharge Hydrogen Plasma Passivation Method

technical field

The invention relates to the technical field of hydrogen ion implantation of solar cells and silicon wafers, in particular to a dielectric barrier discharge hydrogen plasma passivation method.

Background technique

Crystalline silicon is currently the most widely used solar cell material, but there are a large number of deep-level recombination center (SHR) recombination centers in monocrystalline silicon, especially polycrystalline silicon, which restricts the further improvement of device conversion efficiency. Hydrogen passivation can improve the minority carrier lifetime of silicon chips and the conversion efficiency of cells. The current methods to achieve hydrogen passivation are:

1. Hydrogen plasma implantation of PECVD source: The ion energy of this method is small, on the order of several eV. The ion penetration ability is weak, and the application prospect is limited.

2. Si ₃ N ₄ film prepared by PECVD: The preparation of Si ₃ N ₄ amorphous silicon film on the surface of solar cells has two functions: first, the film of about 80nm can play the role of anti-reflection of visible light; second, Si ₃ N ₄ non-crystalline silicon The crystalline silicon thin film is rich in hydrogen, and hydrogen can diffuse into the matrix under certain heat treatment (350°C) conditions. The main process disadvantage: the process is before the two-step high-temperature treatment, and the subsequent high-temperature treatment (over 400°C) will cause the precipitation of hydrogen. Moreover, the injection depth of this method is limited, and it has no effect on the cells that have been coated.

3. Ion gun injection: Ion sources (such as Kaufmann, Hall plasma sources) and ion confinement accelerating electric fields are required. The implant energy, implant area, implant direction and flux can be controlled. The disadvantages include low production capacity, high vacuum requirements, and relatively complicated equipment. It is often used for doping in the semiconductor industry.

4. High-pressure high-temperature annealing: annealing is carried out in a hydrogen atmosphere with GPa-level pressure. This method can process a large number of silicon wafers at one time. The disadvantage is that the equipment requirements are high, and the working gas is high-pressure hydrogen, which is flammable and explosive. The processing time is too long (in hours).

5. Plasma immersion ion implantation (PIII): widely used in surface modification of metal materials. The injection part consists of an ion source (ECR or ICP) and a kV-level pulse acceleration voltage. It is an ideal hydrogen ion implantation method at present. This method is relatively close to the method in this paper in terms of performance and vacuum system conditions, but the principle of plasma generation and power supply are quite different.

Contents of the invention

The technical problem to be solved by the present invention is to provide a dielectric barrier discharge hydrogen plasma passivation method with simple equipment, which can directly process the prepared cells and has a short processing time.

The technical solution adopted by the present invention to solve the technical problem is: a dielectric barrier discharge hydrogen plasma passivation method, which has the following steps:

1) Place the SiN _X side of the finished crystalline silicon solar cell on the lower metal electrode as the sample stage, and evacuate the cavity to a certain vacuum through the vacuum system;

2) Pass hydrogen gas into the chamber (4) through the hydrogen source and adjust the pressure to 0.1-10mTorr;

3) Turn on the excitation power supply, and slowly increase the voltage to cause the hydrogen as the working medium to undergo glow discharge;

4) Adjust the voltage and frequency of the excitation power supply so that the hydrogen ions can obtain enough energy without damaging the SiN _X film, and carry out hydrogen plasma passivation treatment.

The excitation power (1) is a sinusoidal alternating current of 500 to 10 kV, the frequency is 500 Hz-20 kHz, and the hydrogen plasma passivation treatment time is 1-10 min.

In step 4, in order to eliminate the crystal lattice damage caused by implantation, heating at 300-350° C. may be applied to the treated battery sheet.

In order to increase the yield of the high-energy part of the plasma, the sine wave power supply can be replaced by a nanosecond pulse power supply.

The beneficial effects of the present invention are: compare with existing technology:

1) The technology of preparing Si ₃ N ₄ film by PECVD is not suitable for the direct processing of cells, and the post-processing will cause hydrogen desorption. Correspondingly, the present invention can directly process the prepared battery sheet.

2) The ion gun implantation technology equipment is complex in design and has a small production capacity, which is not suitable for solar cell production. The invention is simple in design, and the equipment complexity and production capacity are even better than the existing PECVD devices.

3) The processing time of hydrogen plasma injection of PECVD source, preparation of Si ₃ N ₄ film by PECVD, and high-pressure high-temperature annealing technology is too long, measured in hours, and the application is also restricted. The processing time of the present invention only needs 1-10 minutes.

4) Plasma immersion ion implantation (PIII) technology has short processing time, and the ion density, implantation depth and flux are all controllable, which is ideal, especially the ion energy can reach 4000eV. In the case of the same ion state and treatment effect, the present invention does not need an external excitation source (ICP or ECR), and an easy-to-implement sinusoidal power supply can also be used to realize the accelerating electric field. Compared with the plasma immersion ion implantation (PIII) technique, the equipment requirement of the present invention is relatively simple.

Description of drawings

Below in conjunction with accompanying drawing and embodiment the present invention is further described;

Fig. 1 is the structural representation of hydrogen plasma passivation equipment of the present invention;

In the figure, 1. excitation power supply, 2. upper metal electrode, 3. barrier medium, 4. cavity, 5. sample, 6. lower metal electrode.

Detailed ways

The present invention adopts the method of dielectric barrier discharge as the method of plasma generation and injection, and the schematic diagram of the equipment is shown in FIG. 1 . The dielectric barrier discharge hydrogen plasma passivation equipment of the present invention is composed of: including a chamber 4, a hydrogen source, a vacuum system, an excitation power supply 1 for generating a dielectric barrier discharge, and two upper and lower metal electrodes 2 and lower metal electrodes6.

The cavity 4 is a vacuumable metal cavity whose inner wall is coated with an insulating medium. The cavity needs to be equipped with various air sources and its auxiliary valves and flow control devices.

The hydrogen source and the vacuum system are in communication with the interior of the chamber 4 .

A pair of parallel upper metal electrodes 2 and lower metal electrodes 6 are arranged in the cavity 4 and connected to the excitation power source 1. The surface of the upper metal electrode 2 is covered with a barrier medium 3, and the surface of the lower metal electrode 6 is also covered with a barrier medium 3. The upper and lower The width of the air gap between the two blocking media 3 covered by the opposite surfaces of the metal electrodes 2 and 6 is controlled at 5-10 mm. The barrier medium 3 can be made of insulating materials such as glass, alumina, and quartz, and the greater the dielectric constant, the lower the ignition voltage. The barrier medium 3 on the surface of the lower metal electrode 6 is not necessary, and the lower metal electrode 6 can directly serve as a sample platform.

The excitation power supply 1 is a pulse voltage source or a sinusoidal voltage source. According to the needs, the excitation voltage can be selected from 1-20kV, and the power frequency can be selected from 1kHz to 100kHz.

The samples to be processed refer to solar cells and silicon wafers.

The realization steps of the present invention are as follows:

1) Open the metal cavity, and place the sample 5 to be processed on the sample stage, which can be the lower metal electrode 6 or the barrier medium 3 on the lower metal electrode 6 according to the design.

2) Close the metal cavity, open the vacuum system, and pump the vacuum in the metal cavity to 0.01mTorr;

3) Turn on the hydrogen source, continue ventilation for 1min, adjust the valve, and keep the system pressure at 0.1-10mTorr;

4) Turn on the excitation power supply 1, adjust the power supply voltage above 5000V until it starts to glow (depending on the width of the air gap and the barrier dielectric material, the value needs to be adjusted accordingly);

5) Adjust the appropriate voltage and pressure to react continuously for 1-10 minutes.

Under the pressure of 0.1-10mTorr, the order of free path of molecules and ions is slightly less than 5mm, so it is considered that more than half of the ions can move to the wall before a collision occurs. Assuming that the peak value of the external power supply voltage is U=4kV, the electrons And monovalent ions can reach a maximum energy of 4keV under the acceleration of this electric field. In fact, when the dielectric barrier discharge occurs, the actual acceleration energy of some ions may be higher than 4keV. This method can realize the conditions achieved by the plasma immersion method (PIII) from the perspective of ion energy, and can not use ECR or ICP plasma source and current relatively complicated pulse acceleration power supply.

Example 1:

In this embodiment, the vacuum degree in the cavity of the system is evacuated to 0.01mTorr first, then hydrogen gas is introduced and the pressure is increased to 2mTorr, and then a sinusoidal AC power supply with a voltage of 4kV and a frequency of 10kHz is applied to the system through the excitation power supply. In the embodiment, there is no need to apply an additional heat source to the sample stage, and the temperature of the sample 5 itself can reach above 80° C. due to ion bombardment. Plasma treatment time 10min. Sample 5 processed in this embodiment is a P-type polysilicon cell. The resistivity of the original P-type polycrystalline silicon wafer is 0.5-3Ωcm. The preparation process of the cell is basically as follows: first, the silicon wafer is textured to form a pyramid-shaped texture on the surface of the silicon wafer; the textured surface is diffused and sintered with POCl ₃ at 900°C; after cleaning, the diffusion surface is treated with PECVD An amorphous SiN:H anti-reflection film of about 80nm is plated; finally, screen printing is performed and sintered at 850°C.

Example 2:

This embodiment is aimed at a P-type substrate single crystal cell. The cell preparation process is basically the same as that of the polycrystalline cell in Example 1, and the plasma treatment process and main conditions are also the same.

Claims (4)

1. dielectric barrier discharge hydrogen plasma passivating method is characterized in that: have following steps:

1) with the SiN of the crystal silicon solar cell sheet that completes _XFacing up places on the following metal electrode (6) as sample table, and by pumped vacuum systems cavity volume (4) is evacuated to certain vacuum;

2) feed hydrogen and regulate pressure to cavity volume (4) by sources of hydrogen to 0.1-10mTorr;

3) open excitation power supply (1), slowly boosted voltage makes the hydrogen generation glow discharge as working medium;

4) voltage of adjusting excitation power supply (1) and frequency make hydrogen ion obtain enough energy and don't can damage SiN _XFilm carries out hydrogen plasma Passivation Treatment.

2. dielectric barrier discharge hydrogen plasma passivating method according to claim 1 is characterized in that: described excitation power supply (1) is 500 to 10kV sinusoidal ac, frequency 500Hz-20kHz, and the hydrogen plasma Passivation Treatment time is 1-10min.

3. according to claim 2 or 3 described dielectric barrier discharge hydrogen plasma passivating methods, it is characterized in that: in step 4, processing battery sheet is applied 300-350 ℃ heating.

4. dielectric barrier discharge hydrogen plasma passivating method according to claim 1 is characterized in that: described excitation power supply (1) is a nanosecond pulse power supply.

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