Reasons and solutions for the long polishing time of sapphire windows

The process flow of the sapphire window grinding and polishing stage is: rough grinding (uniformization)-fine grinding (planarization)-polishing (mirror surface). Among them, the rough grinding and fine grinding stages are carried out by the way of abrasive impact and crushing the sapphire surface. It is purely physical. The polishing generally uses alkaline silica sol to slightly corrode the sapphire surface while sweeping through the polishing pad. To achieve the mirror effect, it is CMP polishing, that is, chemical physical polishing. 

It can be seen from the above process that sapphire polishing is a corrosion-sweeping process, in which the thickness of the corrosion only reaches the angstrom level. After sapphire polishing, all parts of the lens should achieve mirror effect.

In daily processing, we often encounter the phenomenon of abnormally prolonged polishing of sapphire. The signs of too long time are as follows:  

1,   rectangle, especially the four corners of long strips, can not be polished;  

2,  , there are pits on the surface;  

3, Defects such as orange peel and water lines;  

4,  The surface is hazy and foggy. 

The above phenomena, except for the third point, are basically caused by previous grinding. 

As mentioned above, polishing is a gradual corrosion process, and the depth of corrosion is very subtle. The polishing time of sapphire is long, but the corrosion has not reached the overall situation. If you want to achieve the overall situation, you can only extend the polishing time. 

So what is the reason why it is difficult for corrosion to achieve the overall situation in a short time? There is no doubt that it is caused by the flatness of the polished lens.

Let's analyze the above phenomena step by step. 1,   corners are not transparent. Double-sided grinding is to fix the lens by the planetary wheel and move on the grinding disc of the grinder along with the operation of the planetary wheel. The grinding disc drives the grinding sand to impact and crush the surface to achieve the purpose of grinding. However, the rectangular lens, especially the elongated lens, is relatively fixed in the planetary wheel and cannot rotate. Even if the surface is uneven, it cannot be rotated by resistance like a disc to achieve a consistent result. Since the four corners, especially the two outer corners, are always running on the outer ring, the linear velocity is relatively fast, so the grinding removal amount is relatively large, resulting in corner collapse. During polishing, in order to polish the collapsed area, it can only be etched and swept for a long time, and it can be polished completely when the overall situation is flat. 

So how to deal with this phenomenon? Based on the processing experience of Taixing Hecheng Optoelectronics Technology Co., Ltd., I put forward several points for reference. First of all, if you carefully observe the placement of the planet wheels and the wafers, you will see that the outer wafers are relatively far apart, especially the wafers that are close to square, that is, the area of the outer part of the abrasive sand is small, and of course the amount of removal will be relatively large. quick. The solution is to add a piece of material of the same thickness (preferably a wafer) between the gaps. Secondly, the inconsistent removal rate of the inner and outer sides is caused by different linear speeds. Therefore, the inner and outer sides have to be adjusted multiple times during the grinding process to achieve the same removal amount by switching directions. During this period, the grinding operator must pay attention to control the rhythm, take multiple measurements, and not be afraid of trouble. 

2. Surface pits. There are two basic phenomena of surface pits, one is individual pits and there are almost every piece of the whole board, and the second is that there are dense pits on a single piece. The first phenomenon is caused by the pits caused by the extrusion of individual large particles of grinding sand. The method adopted is to relieve the pressure, that is, to reduce the pressure of the upper plate, and even use the back pressure (in order to increase the grinding speed, many companies adopt I can’t agree with the method of pressing or increasing the weight of the upper plate. It is necessary to know that the defects produced at this stage will be paid for by polishing, which is not worth the loss). At the same time, pay attention to analyze whether there are debris, iron filings or other large sands that are mixed in through the environment. If this phenomenon occurs, the machine must be cleaned and the sand liquid must be filtered. Generally speaking, a small amount of large-grain grinding sand will be mixed in, and it will be eliminated after a certain period of grinding time. See the next analysis for the second phenomenon and the dense pits on the entire surface. 

3. The surface is fogged and foggy. As mentioned above, grinding is divided into coarse grinding and fine grinding. The purpose of rough grinding is to make the thickness and flatness of the wafer entering the fine grinding uniform. In order to achieve this goal quickly, coarse sand with a sand number above W28 is generally used for grinding, and the purpose is only for consistency and leveling. However, if the processing method of the material is not considered, it will cause the problem of excessive roughness. For example, some materials are processed by diamond wire slicing, and others are ground by a surface grinder. Among them, the wafer polished by the grinder is observed under a microscope and densely covered with long and deep grooves. If only the product is considered for rough grinding Consistency does not consider the repair of roughness, then it takes a long time to eliminate it to the fine grinding stage. We generally believe that the rough grinding removal of the diamond wire slicing method is not less than 0.08-0.1mm, and the rough grinding removal of the material processed by the grinder is not less than 0.2-0.3mm. In the fine grinding stage, the removal volume generally does not exceed 0.05mm, and then it can be transferred to the polishing process. Therefore, the root cause of surface fogging and fogging is caused by the roughness after fine grinding. In order to eliminate fogging and fogging, the polishing time can only be prolonged. The solution is to understand the processing method of the lens material and determine the amount of rough grinding reasonably. 

The above experience is a bit of experience from Taixing Hechen Crystal Technology Co., Ltd. If there is anything wrong, you are welcome to correct me. Taixing Hechen Crystal Technology Co., Ltd. specializes in providing high-density sapphire windows, optical lenses, optical prisms and light guide blocks for beauty instruments. Welcome to consult! Tel (same number on WeChat) 159952167788. 

In recent years, sapphire products have become more and more widely used in optical systems, and higher requirements have been placed on the surface shape of sapphire windows. Taixing Hechen Crystal Technology Co., Ltd. is committed to the processing and research of sapphire high-surface windows, and has been able to process Sapphire window products of 1/5L-1/10L, follow-up we will sort out the processing technology and experience, and share with you.