Cause Analysis and Improvement of Cashmere White Spot on Diamond Wire Cutting Single Crystal Wafer

1 Introduction
Diamond wire cutting technology is also known as fixed abrasive cutting technology. It uses the method of electroplating or resin bonding to consolidate the diamond abrasive on the surface of the steel wire, and directly applies the diamond wire to the surface of the silicon rod or the silicon ingot to produce grinding, thereby achieving the cutting effect. Diamond wire cutting has the characteristics of fast cutting speed, high cutting precision and low material loss.
At present, the silicon wafer cutting silicon wafer has been completely accepted in the single crystal market, but it has also been encountered in the process of advancement, in which the whitening of the velvet is the most common problem. In view of this, this paper focuses on how to prevent the problem of velvet whitening of diamond-cut single crystal silicon wafers.
The cleaning process of the diamond wire cutting single crystal silicon wafer is to remove the silicon wafer cut by the wire saw machine from the resin plate, and after removing the rubber strip, the silicon wafer is cleaned. The cleaning equipment is mainly a pre-cleaning machine (deglasing machine) and a washing machine. The main cleaning process of the pre-cleaning machine is: feeding-spraying-spraying-ultrasonic cleaning-degumming-water rinsing-cutting. The main cleaning process of the washing machine is: feeding-pure water rinsing-pure water rinsing-alkali washing-alkali washing-pure water rinsing-pure water rinsing-pre-dehydration (slow pulling)-drying-cutting.

2. Principle of single crystal fleece
Monocrystalline silicon wafers are made by using an alkali solution to anisotropically etch a single crystal silicon wafer to prepare a suede. The reaction principle is as follows: chemical reaction equation Si+2NaOH+H2O=Na2SiO3+2H2↑
In essence, the formation process of the suede is: the corrosion rate of the NaOH solution is different for different crystal faces, and the corrosion rate of the (100) plane is more than ten times larger than the (111) plane, so the (100) crystal orientation of the single crystal silicon wafer After anisotropic etching, a lot of four-sided square pyramids with a surface of (111) are formed on the surface, that is, a "pyramid" structure (Fig. 1). After this structure is formed, when light is incident on a pyramidal slope of a certain angle, the light will be reflected to the slope of another angle to form two or more absorptions, thereby reducing the reflectivity of the surface of the silicon wafer, that is, the light trapping effect (as shown in the figure). 2). The better the size and uniformity of the "pyramid" structure, the more obvious the light trapping effect, and the lower the surface emissivity of the silicon wafer. 3. Analysis of the reasons for whitening of single crystal
Scanning electron microscopy was performed on the whitish silicon wafer. It was found that the pyramidal microstructure after the velvet was not formed, and the surface seemed to have a "waxy" residue, while the same silicon The pyramid structure of the suede in which the whitish area does not appear is formed better (Fig. 3). If there is residue in the local area of ​​the surface of the silicon wafer, there will be residual area "pyramid" structure on the surface of the silicon wafer. The size and uniformity of the structure and the effect are insufficient compared with the normal area, resulting in a residual area of ​​the textured surface. A region with a high reflectance is visually whitish compared to a normal region. It can be seen from the distribution of the whitish area that it does not appear in a regular or regular shape over a large area, but only in a local area, the local contaminants on the surface of the silicon wafer should not be cleaned, or the surface of the silicon wafer. The situation was caused by secondary pollution. The diamond-cut silicon wafer has a smoother surface and less damage (Fig. 4). Compared with the mortar wafer, the reaction speed of the lye and diamond-cut silicon wafer surface is higher than that of the mortar-cut single crystal wafer. Slow, so the effect of surface residue on the texturing effect is more obvious. 4. The main residual source of diamond wire cutting silicon surface
(1) Coolant: The main components of the diamond wire cutting coolant are surfactants, dispersants, defoamers, and water. The cutting fluid with excellent performance has good suspension, dispersibility and easy cleaning. Surfactants are generally more hydrophilic and easier to clean during wafer cleaning. These additives are continuously stirred and circulated in the water to generate a large amount of foam, which causes a decrease in the flow rate of the coolant, an influence on the cooling performance, a serious bubble or even a problem of foam overflow, which seriously affects the use. Therefore, the coolant is usually used in conjunction with the defoamer. Conventional silicone and polyether defoamers are used to ensure defoaming performance. Defoamers are generally poor in hydrophilicity and are easily emulsified in a solvent in water. They are easily adsorbed and remain on the surface of the wafer during subsequent cleaning. Produce a problem of whitening white spots.
It can not be compatible with the main component of the coolant, so it must be made into two components. When diluted, the main component and antifoaming agent should be added to the water to be diluted. In the process of use, it should be supplemented according to the foam condition, and it cannot be quantitatively controlled. The use and addition amount of the defoaming agent can easily make the defoaming agent excessively used, resulting in an increase in the surface residue of the silicon wafer, and the handling is also inconvenient, but since the main component raw material and the defoaming agent raw material are relatively inexpensive, Most of the domestic coolants use this formula system; the other coolant uses a new type of defoamer, which can be well compatible with the main components, does not need to be added, can effectively quantitatively control its dosage, and can effectively prevent excessive It is also very convenient to use, and with the appropriate cleaning process, the residue can be controlled to a very low level. A few manufacturers in Japan and China use this formula system, but because of its high raw material cost, its price advantage is not obvious. .
(2) Glue and resin version: At the end of the diamond cutting process, the silicon wafer near the incoming end has been cut through in advance, and the silicon wafer at the outlet end has not been cut through. The diamond thread cut in advance has begun to cut into the adhesive layer and resin plate. Since the silicon rod glue and the resin board are all epoxy resin products, the softening point is basically between 55-95 ° C. If the softening point of the rubber layer or the resin board is low, it is easy to heat up during the cutting process. Soft melt, attached to the surface of the steel wire and the silicon wafer, causing the cutting ability of the diamond wire to decrease, or the silicon wafer is contaminated by the resin, and once it is attached, it is difficult to clean it. Such contamination occurs mostly in the exit surface of the silicon wafer. Near the edge.
(3) Silicon powder: A large amount of silicon powder will be produced during the cutting process of the diamond wire. As the cutting progresses, the content of the fine powder in the mortar cooling liquid will become higher and higher. When the amount of the fine powder is large enough, it will adhere to the silicon. The surface of the sheet, and the grain size and particle size of the silicon powder produced by the diamond wire cutting, make it easier to adsorb on the surface of the silicon wafer, making it difficult to clean. Therefore, when the silicon block is cut, the amount and quality of the coolant are ensured, and the content of the fine powder in the coolant is lowered.
(4) Cleaning agent: At present, most of the manufacturers who use diamond wire cutting are cutting with mortar at the same time. Most of them are still pre-rinsing, cleaning process and cleaning agent used in mortar cutting. The single-gold wire cutting technology is from the cutting mechanism and supporting line. There is a big difference between the coolant and the mortar cutting, so the corresponding cleaning process, the amount of the cleaning agent, the formulation, etc. should be adjusted accordingly for the diamond cutting. The cleaning agent is an important aspect. The surfactant and alkalinity in the original cleaning agent formulation are not necessarily suitable for cleaning the silicon wafer cut by the diamond wire. The surface condition of the diamond wire and the composition in the coolant should be In the case of surface residues, a targeted cleaning agent is developed and a cleaning process is provided in conjunction therewith. The defoamer component as described above is an unwanted component in the mortar cutting.
(5) Water: Diamond wire cutting, pre-rinsing and cleaning overflow water may be adsorbed on the surface of the silicon wafer if it contains impurities.

5. Suggestions for reducing the problem of velvet whitening of diamond-cut single crystal silicon wafers
(1) Using a dispersing coolant, it is required to use a low-residue defoamer for the coolant to reduce the residual of the coolant component on the surface of the silicon wafer;
(2) Use suitable glue and resin plates to reduce contamination of the silicon wafer;
(3) The coolant is diluted with pure water to ensure that there is no residual impurities in the water used;
(4) For the surface condition of the silicon wafer cut by the diamond wire, use a cleaning agent which is more suitable for the activity and the cleaning effect;
(5) Use the diamond wire coolant online recovery system to reduce the silicon powder content during the cutting process, thereby effectively controlling the silicon powder residue on the surface of the silicon wafer, and also increasing the water temperature, flow rate and time in the pre-flushing to ensure silicon. Flush the powder in time;
(6) Once the wafer is placed on the cleaning station, it must be processed immediately to keep the wafer wet during the entire cleaning process.
(7) The silicon wafer keeps the surface moist during the degumming process and cannot be naturally dried.
(8) The silicon wafer should be exposed to the air as much as possible during the cleaning process to prevent the wafer from being generated on the surface of the wafer.
(9) The cleaning staff should not directly touch the surface of the silicon wafer during the whole cleaning process, and must wear rubber gloves to avoid fingerprint printing.
(10) The battery end uses hydrogen peroxide H2O2+ alkali NaOH according to the volume ratio of 1:26 (3% NaOH solution) cleaning process, which can effectively reduce the occurrence of the problem of whitening. The principle is similar to the SC1 cleaning solution for semiconductor silicon wafers (commonly known as No. 1 liquid). Its main mechanism of action: the surface of the silicon wafer is formed by the oxidation of H2O2, which is corroded by NaOH. Oxidation occurs immediately after corrosion, oxidation and corrosion are repeated, so particles attached to the surface of the silicon wafer (such as silicon powder, The resin, metal, etc. also fall into the cleaning liquid along with the etching layer; due to the oxidation of H2O2, the organic matter on the surface of the wafer is decomposed into CO2 and H2O and removed. At present, there are already silicon-line-cut single-crystal silicon wafers that silicon wafer manufacturers use to process and clean the silicon wafer manufacturers. The silicon wafers have been used in batches of domestic and Taiwan battery manufacturers for complaints about the problem of whitening and whitening. There are also battery manufacturers that use similar pre-cleaning processes for texturing, and also effectively control the appearance of whitening. It can be seen that this cleaning process is added in the silicon wafer cleaning step to remove the silicon wafer residue, thereby effectively solving the problem of whitening and whittling of the battery sheet end.

6. Summary
At present, diamond wire cutting has become the main processing technology in the field of single crystal cutting, but the problem of whitening and whitish in the process of propulsion has been plagued by silicon wafers and battery manufacturers, causing battery manufacturers to have some conflicts with silicon wafers cut by diamond wires. . Through the comparison analysis of the whitish area, it is concluded that the velvet whit is mainly caused by the residue of the silicon wafer. In order to better prevent the problem of the whitening of the silicon wafer at the end of the battery sheet, this paper analyzes the possible sources of surface contamination of the silicon wafer, as well as suggestions and measures for improvement in production. The cause analysis and improvement can be carried out according to the number, region, shape and the like of the occurrence of white spots. It is especially recommended to use the cleaning and cleaning process of hydrogen peroxide + alkali. The successful experience has proved that it can effectively prevent the problem of velvet whitening of diamond-cut silicon wafers for the reference of the industry and manufacturers.

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