For crystalline silicon solar cell production PECVD Technical Progress
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Date: Thu, 21 Jan 2010 |
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In order to reduce the efficiency of crystalline silicon solar cells, solar cells are usually the need to reduce the surface reflection, but also the need for crystalline silicon surface passivation to reduce surface defects for the minority carrier compound effect.
The refractive index of silicon is 3.8, if the smooth silicon surface directly placed in the refractive index of 1.0 in the air, its light reflection rate can reach about 30%. People to use the surface texture of lower part of the reflection, but it is still difficult to drop very low reflectivity, especially for polysilicon, the use of isotropic acid etching solution, if the corrosion too deep, it will affect the PN junction leakage current, so the right to reduce the effect of surface reflection is not obvious. Thus, considering the silicon surface and the air layer inserted between the transmittance of dielectric thin film refractive index of medium to lower surface reflection, in the industrial applications, SiNx film was chosen as the silicon surface of the antireflection film, SiNx IBM THINKPAD T20 Laptop Battery refractive index of As the x value will be different, varying from 1.9 to 2.3 or so, this is more appropriate in the 3.8 silicon and 1.0 in the air to reduce reflection of visible light design is a more excellent antireflection coating.
On the other hand, silicon surface, there are many dangling bonds, for N-type emitter of the non-equilibrium carriers have a strong appeal, making minority-carrier complex role of the place, thus reducing the current. Therefore, atoms or molecules need to use some of these surface dangling bond saturation. It was found that hydrogen for the silicon surface of the SiNx film has a strong passivation effect, reducing the surface unsaturated dangling bonds, reducing the surface level.
Taken together, SiNx film was prepared on the surface of silicon has played two of the most used, one is to reduce the surface reflection of visible light; Second, the surface passivation effect.
2 Classification of PECVD technology
Preparation of SiNx film is used There are many ways, including: chemical vapor deposition (CVD method), plasma enhanced chemical vapor deposition (PECVD method), low pressure chemical vapor deposition (LPCVD method). Commonly used in the current industry is the PECVD method.
PECVD method according to the plasma deposition chamber and the relationship between source and sample can be divided into two types:
Direct method: direct contact with the plasma samples, sample or samples of the support body is part of the electrode.
Indirect method: or called away from the domain method. The samples to be deposited outside the plasma region, plasma does not directly hit the sample surface, sample or its supporting body is not part of the electrode.
The direct method is divided into two kinds:
(1) pipe PECVD system: that is, using the same tube as the proliferation of quartz tube as the deposition chamber, using resistance furnace used for heating body can be placed over a silicon chip inserted into the quartz tube in the graphite boat deposition. The device's major manufacturers of Germany's Centrotherm Corporation, China's 48th Research Institute, Seven Star Chinese start-up.
(2) plate PECVD system: the forthcoming multi-chip silicon wafer placed in a graphite or carbon fiber stent placed in a metal deposition chamber, the chamber has plate-shaped electrode, and the sample supports the formation of a discharge circuit, in the cavity room in the process gas exchange between the two plates under the effect of electric field formed in space plasma, decomposition of SiH4 in the Si and H, and NH3 formation of SiNx deposition of N species into the silicon surface. This deposition system is currently mainly produced during Shimadzu Corporation.
Indirect method is divided into two kinds:
(1) Microwave method: use of microwave plasma as the excitation spectrum. Microwave source placed in the sample area, the first ammonia ionization, and then bombarded silane gas, resulting in SiNx molecules deposited on the sample surface. The device is a major manufacturer for the German company Roth & Rau.
(2) DC law: the use of DC-excited plasma source, and further ionization of ammonia and silane gas. No contact with the plasma samples. This equipment from the Dutch company OTB.
At present, China Microwave PECVD system occupy the mainstream of the market, while the tube PECVD systems also occupy a lot of share, while the plate system, Shimadzu only 5 ~ 6 production lines in use. DC-PECVD system, France has not yet entered the Chinese market.
In addition to these several modes of PECVD system, the United States, Applied Material has also developed a magnetron sputtering PECVD system, the system uses the magnetron sputtering source bombardment of high-purity silicon target, in an atmosphere of ammonia reactive sputtering, the formation of SiNx molecular deposition to the sample surface. The advantage of this technology does not use explosive of silane gas, security has greatly improved, while a high deposition rate.
If the PECVD system used in accordance with the frequency range, can also be divided into the following categories:
0 Hz: DC indirect method - OTB Corporation
40 KHz: Centrotherm's tube direct method PECVD and Applied Material's magnetron sputtering system
250 kHz: Shimadzu's direct method the system plate
440 kHz: Semco's plate direct method
460 kHz: Centrotherm's tube direct method
13.6 MHz: Semco and MVSystem the company's direct method the system plate
2450 MHz: Roth & Rau's plate indirect method system.
Comparison of three advantages and disadvantages of various methods
Various methods have their ACER LCBTP03003 Laptop Battery: From the big perspective, the direct PECVD method, the sample surface damage, will increase the surface of the composite declining birthrate, but also precisely because of its role in the bombardment of the surface, you can remove some of the natural surface oxide layer, making the surface impurity atoms to be inhibited, while the direct method can make hydrogen atom or hydrogen ions more deeply into the grain boundaries in polycrystalline silicon, making the grain boundary passivation more complete.
PECVD system using different frequencies, but also have certain advantages and disadvantages:
(1) The higher the frequency, even the smaller size, the more difficult to achieve large area uniformity.
(2) frequency, the lower surface of the wafer the more serious damage.
(3) ions into the silicon chip, the lower the frequency the deeper the better for polysilicon grain boundary passivation.
The uniformity of deposition chamber with the electrodes and the design is very relevant. Tube-type PECVD system, the graphite boat because of its hollow center, so the use of silicon as a part of the electrode, so the characteristics of glow discharge characteristics with silicon surfaces have a certain relationship, such as silicon-based surface texture generated by gold tower on the edge of the state of an impact on the plasma discharge, while the current silicon also affects the conductivity of the different plasma-field uniformity. Another tube PECVD airflow is from one end of the introduction of quartz tube, this can also cause uneven distribution of process gas.
Plate PECVD system uses a substrate plate as the electrode, and the adoption of uniform gas Shower system, but due to the heating of the substrate plate in the long run there will be a slight warpage, resulting in inconsistent spacing parallel plate electrodes, can also cause uneven between films. In addition, the plasma deposition of a large area in the direct method would result in high-frequency wavelengths arising due to the additional non-uniformity.
Various methods the quality of the films is slightly different, in principle, due to the direct method in the plasma directly on the silicon surface, so uniformity is better, while the indirect method of plasma ionized plasma is formed after the SiNx-proliferation to the silicon surface, the film quality is more crisp, while the magnetron sputtering because of its methods of work because the film is most crisp. For the dense films, their passivation characteristics and anti-reflective properties to be far superior.
Of course, this comparison also in certain deposition conditions, a general comparison of changes in deposition conditions can change the film characteristics.
4 Conclusion
At present, the industrialization of SiNx coating technology continues to develop, each technology has its own characteristic points, also have its shortcomings. Solar cell structures and processes toward the new direction of special development, silicon nitride film made a number of new requirements.
For example, there is a new type of solar cells required double-sided coating, positive-plated silicon nitride, silicon dioxide and silicon nitride coating on the back, in which case, a flexible microwave indirect method have a larger advantage. Other technical requirements in the preparation of a silicon nitride film also carried out after the wet photochemical metal coating process, this silicon nitride film density and quality requirements a lot higher, therefore, the direct method in particular, the direct method there will be a tube-type a great HP PAVILION DV4000 Laptop Battery.
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