Abstract: In the solar industry, the quality of wafer cutting directly affects the pass rate of silicon wafers and the yield of cell wafers. There are more cutting quality problems in the wafer cutting process. Rubber surface collapse is a common problem. With the development of the solar industry and technological advancement, the requirements for the quality of silicon wafers are also more stringent. This paper mainly analyzes the causes of rubber surface collapse to better solve this problem.


1 Introduction
After the degumming of the ingot, or when sorting the silicon wafer, it is often found that the surface of the silicon wafer is bright and the silicon layer is in the form of line-like falling off, and the distribution position is mostly near the chamfer of the rubber surface, sometimes Produced in the middle of the chamfer. Rubber surface collapse is a common silicon wafer defect, and its influencing factors are various. This paper mainly analyzes the three aspects of ingot bonding, silicon wafer cutting and pre-cleaning.


2. Ingot bonding effect
Before the silicon wafer is cut, the tray, the glass, the silicon block, the guide strip and the like need to be bonded into the ingot according to a certain process. The bonding process of the ingot and the use of the raw material will affect the collapse of the rubber surface, so the bonding process Operation is extremely critical.


The following is the ingot bonding process:


(1) Prepare the tray. Clean the tray to ensure that the surface of the tray is clean and free of dirt. There is no deformation of the tray loading position. Check whether the tray screws are loose, whether the steel plate bonding surface is flat, and whether there is a gap between the tray and the steel plate.


(2) Prepare frosted glass. Check whether the frost on the glass surface is uniform, the frosted surface is not used, whether the edge is neat, the size is qualified, and then according to the matching condition of the glass and the tray, the side end surface of the glass is scribed to facilitate accurate positioning when sticking the glass. Wipe the bonding surface of the tray and glass with alcohol and paper.


(3) Prepare glue for bonding glass. The commonly used glue is epoxy resin glue. After the glue is weighed, it is fully and quickly stirred, and the color is required to be uniform without obvious chromatic aberration. The total amount of each preparation is not allowed to be excessive, and the surface of the epoxy is prevented from being solidified by exposure to the air for a long time.


(4) Bonding glass. Apply the glue that has just been prepared to the clean and smooth steel plate. The glue should be as even as possible. Otherwise, the excess glue will flow onto the tray. Then turn the tray over and the adhesive surface will be fastened to the bonding surface of the glass. Tightly, the air bubbles between the glass and the aluminum plate are extruded and positioned according to the positioning line of the side end face of the glass. (Replace the movement of the glass and the tray position at any time).


(5) After curing for a period of time, the glass-coated tray is turned over for size positioning.


(6) Prepare the silicon block. Select the silicon block to be bonded, and the length of the silicon block at the positioning position should be the same as possible. Wipe the silicon block and glass bonding surface with alcohol and paper.


(7) Bonding silicon blocks. The prepared sticky glue is evenly coated on the glass, and is scraped flat with a spatula to evenly distribute the glue on the glass. The glass-bonded tray is placed close to the positioning fulcrum of the tooling, and the bonding surface of the silicon block is placed on the glass. , press firmly before and after, squeeze the air out, and then press the weight on it. Measure and adjust the overall length of the ingot so that the lengths of the same set of silicon blocks are the same.


(8) Squeegee. After bonding, a lot of glue will be extruded on the contact surface between the silicon block and the glass. The presence of the glue will affect the cutting, and it should be removed before the glue is cured. The squeegee time is such that after the heavy object is removed, if the interval is too long, the colloid is not easily removed after it is completely cured. When scraping the surface of the glue, the guide strips are moved in parallel at an angle of 10o-30o, and the guide strips face the glass at right angles. In addition, on the side of the silicon block, there may be glue on the tray, which needs to be wiped clean.


(9) Bonding guide strips. After bonding the silicon block for 30 minutes, the weight is removed, and two wood grain tapes are respectively adhered on both sides of the upper surface of the silicon block, so that the distance between the two tapes is equal to the width of the guide strip, and the prepared guide is applied in the gap of the tape. Apply glue and then stick the guide strip.


(10) Curing. The bonded silicon block needs to be cured for more than 4 hours before it can be cut into the wire sawing process.


In the process of bonding the ingot, the factors affecting the collapse of the rubber surface are:


(1) The surface of the frosted glass is unevenly frosted, which affects the flatness of the surface, thereby affecting the bonding effect of the silicon block and causing chipping.


(2) The bonding strength and hardness of the glue bonding the silicon block do not meet the requirements. When the steel wire passes through the bonding surface of the silicon block, the rubber surface collapses due to the stress. Generally, when the hardness of the glue is close to the hardness of the silicon block itself, it is advantageous for the bonding effect.


(3) The glue of the glue block is unevenly mixed, which affects the bonding strength; or it is equipped with more glue at one time, which causes the glue to harden before bonding the silicon block, and the fluidity of the glue is deteriorated after bonding the silicon block, and the glue distribution is uneven. The bonding effect is poor.


(4) The squeegee is uneven, the thickness of the glue is unevenly distributed, and delamination occurs. When the cutting to the knives position, the rubber surface will be collapsed. In severe cases, the wafer will break.


(5) After bonding the silicon block, the glue generates bubbles, and the bubbles are not accumulated, which will cause the rubber surface to collapse.


(6) The scraping glue is not clean. Since the glue will overflow from both sides of the silicon block after bonding the silicon block, it remains on the chamfered portion exposed on both sides of the silicon block. If the residual glue is not cleaned, the heat generated by the cutting is caused when cutting to the knife. Poor softening, affecting the amount of sand carried by the steel wire, resulting in a large bow line, insufficient cutting ability, resulting in rubber surface collapse.


(7) Thickness of the glue layer. The instantaneous high temperature generated during the cutting process will soften the rubber layer, reduce the sand carrying capacity of the steel wire, increase the sawing edge of the knife edge and the edge of the cutting edge, and the friction of the edge sawing will cause the rubber surface to collapse, so the rubber layer The thinner the more, the better the reduction of the occurrence of rubber surface collapse.


(8) Adhesion of room temperature humidity. Since the temperature and humidity will affect the hardening and curing time of the glue, which will affect the bonding effect of the glue, measures must be taken to ensure the temperature and humidity of the bonding chamber.


3. Ingot cutting effect
The cutting process of the silicon wafer is completed by the steel wire and the mortar. The mortar is prepared by a certain ratio of silicon carbide and cutting liquid, and has a certain viscosity and density, and the silicon carbide particles actually play the cutting role. The steel wire is wound around the guide wheel of the cutting chamber to form a flat wire mesh. During the cutting process, the mortar rotates at a high speed, and the mortar flowing out from the slurry nozzle causes the silicon carbide particles to be wrapped on the surface of the steel wire due to the suspension of the cutting liquid, along with the steel wire. The rotation causes collision friction on the silicon block. Since the Mohs hardness of silicon carbide is 9.5, the Mohs hardness of silicon is 7, and the hardness of silicon carbide is greater than the hardness of silicon, thereby functioning as a cutting.


In order to reduce the production cost, the small groove distance thin steel wire cutting has become the trend of the current slicing industry, but due to the thinner diameter of the steel wire, the sand carrying capacity will be affected to some extent; on the other hand, the groove distance is reduced, the number of pieces is increased, and the cutting is increased. The increased area will also increase the wear and breakage of silicon carbide, thereby reducing the cutting ability of the mortar. In particular, the cutting to the cutting position, the edges of the silicon carbide are rounded, and the cutting ability is further reduced, so that there is inevitably a wire bow (generally, the wire bow is 5-7 mm). When the steel wire is cut to the bonding surface of the silicon block, it will pass through three types of media, namely silicon block, glue layer and glass. Due to the hardness of the three media, the steel wire is broken by stress during the passage of the three media, resulting in silicon falling. , resulting in rubber surface collapse. The other three media have different melting points. The high temperature and heat generated during the cutting process will soften the rubber layer and the glass, affecting the sand carrying capacity of the steel wire, and increasing the sawing marks, resulting in the collapse of the rubber surface.


The measures that can be taken to address this issue are:


(1) Adjust the cutting process to reduce the speed of the table in advance when the silicon block is cut to the cutting position, which is beneficial to increase the circulation of the mortar and make up for the problem of insufficient cutting ability of the mortar.


(2) Because the station speed is slow during the cutting process, if the mortar flow rate is too large, the impact force caused by the mortar flow will easily cause the steel wire to collapse and cause the rubber surface to collapse. Therefore, the mortar flow rate during the cutting process should be appropriately reduced.


(3) Improve the cutting ability of the mortar during the cutting process, which can be achieved by changing the way the mortar is updated. Usually, the mortar update is performed after the cutting of each saw wafer. In order to increase the cutting ability of the mortar after cutting, we can choose to update the mortar when each saw cuts to 90%, which can effectively reduce the exit bow and reduce Saw marks and the force on the steel wire.


4. Pre-cleaning effect
The pre-cleaning process is to remove the silicon wafer from the glass and remove the adhesive strip. The main steps are spray-spray-ultrasonic cleaning-degumming-water rinse.


The factors that affect the collapse of the rubber surface include:


(1) Spray water pressure. There are two rows of nozzles on both sides of the spray tank for spraying and cleaning the ingot. Since the silicon wafer is suspended upward on the pre-cleaning shelf, if the water pressure of the nozzle is too large, the impact on the ingot is too large. This will cause the chip surface to collapse, so ensure that the water pressure is within the required range.


(2) Water temperature influence. When the water temperature of the degumming tank is low, the rubber layer is not completely softened, and the silicon wafer cannot be automatically detached from the bonding surface. The employee will forcefully remove the silicon wafer by hand, which easily causes the silicon wafer to collapse. Therefore, to ensure the degumming temperature, the temperature range of lactic acid degumming is generally 70±10 °C. In addition, when the silicon wafer is moved from the degumming tank to the clean water rinsing tank, if the temperature of the water is too low and the silicon wafer enters the low temperature water from the higher temperature water, the surface of the silicon wafer is subjected to the stress caused by the water temperature, and the silicon sheet is brittle. Increase, easy to cause the rubber surface to collapse.


(3) Since the silicon wafer is usually cut by the split wire network, there are usually two silicon blocks on each crystal bar. Therefore, during the pre-cleaning process, the silicon wafer will be dumped on the side with less resistance, which will result in glue generation. The surface collapsed. In view of this, a silicon spacer which is suitable in size and size can be prepared, and the pre-cleaning is sandwiched in the middle of the ingot before the feeding, and the inclination of the silicon wafer is lowered to prevent the rubber surface from collapsing.


5. Summary
By analyzing the factors that cause the rubber surface collapse, the rubber surface collapse problem can be better solved, which is of great significance to improve the silicon wafer pass rate and reduce the production cost.


source
Light Energy Magazine
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