After the wafer has gone through the front-end process, the chip preparation is completed, and it needs to be cut to separate the chips on the wafer and finally packaged; in addition, there are also some processes and tests that require the pure silicon wafer substrate to be cut into small square pieces for use.
The difference between UV film and blue film
Before wafer dicing, a layer of film will be glued on the back of the wafer. The function of this layer of film is to stick the chip to the film, which can keep the grain intact during the cutting process and reduce the problems of breakage, displacement and falling during the cutting process.
In actual production, the film used to fix wafers and chips generally uses UV film or blue film.
UV film: mainly used in wafer thinning process;
Blue film: mainly used in wafer dicing process;
Figure 1. Characteristics of UV and blue films UV and blue films
Both UV film and blue film are sticky. The degree of stickiness is generally expressed by the sticky peeling degree, usually in units of N/20 mm or N/25 mm. 1 N/20 mm means that the test strip is 20 mm wide and the force required to peel it off the test board at a peeling angle of 180° is 1 N.
UV film is a special coating applied to the surface of a PET film substrate to block ultraviolet light and short-wavelength visible light. Figure 2 is a general UV film structure diagram.
Generally, UV film consists of three layers. The base material is polyvinyl chloride, the sticky layer is in the middle, and the release film is adjacent to the sticky layer. Some UV film models do not have this release film.
Figure 2. UV film structure
UV film is usually called ultraviolet irradiation tape. It is relatively expensive and has a short validity period when not used. It is divided into three types: high viscosity, medium viscosity and low viscosity.
The viscosity of high-viscosity UV film is 5000mN/20mm ~ 12000mN/20mm before ultraviolet irradiation. After ultraviolet irradiation, the peeling viscosity is below 1000mN/20mm;
The peeling viscosity of low-viscosity UV film is about 1000mN/20mm before ultraviolet irradiation. After ultraviolet irradiation, the peeling viscosity drops to about 100mN/20mm.
After ultraviolet irradiation, there will be no residual glue on the wafer surface of the low-viscosity UV film, and the grain is easy to remove.
The UV film has appropriate expansion, and water will not penetrate between the grain and the tape during the thinning and dicing process.
Blue film is usually called electronic grade tape. It is relatively cheap. It is a blue film with constant viscosity. Its viscosity peeling degree is generally 100~3000mN/20mm. It will produce residual adhesive due to the influence of temperature. In contrast, UV film is more stable and has less residual adhesive than blue film.
Compared with blue film, UV film has great advantages due to its variable viscosity peeling degree. Its main functions are: to fix the wafer during wafer thinning; to protect the chip during wafer dicing to prevent it from falling off or breaking the edge; to flip and transport the wafer to prevent the already diced chip from falling off.
Standardizing the use of various parameters of UV film and blue film, and selecting appropriate UV film or blue film according to the processing technology required by the chip can save costs and contribute to the industrial development of chips.
Blade dicing or blade sawing
Due to the large friction during cutting, DI water (deionized water) should be sprayed continuously from all directions. In addition, the impeller should be attached with diamond particles so that it can be sliced better.
At this time, the cut (blade thickness: groove width) must be uniform and must not exceed the width of the scribe groove. For a long time, sawing has been the most widely used traditional cutting method, and its biggest advantage is that it can cut a large number of wafers in a short time.
However, if the feeding speed of the slice is greatly increased, the possibility of the edge of the small chip peeling will increase. Therefore, the number of rotations of the impeller should be controlled at about 30,000 times per minute.
Blade Optimization
To take on new slicing challenges, collaboration between slicing systems and blades is necessary. This is especially true for high-end applications.
The blade plays a major role in process optimization. In addition to size, three key parameters determine blade characteristics: diamond (abrasive) size, diamond content and type of binder.
The binder is a matrix of various metals and/or diamond abrasives distributed in it. Other factors, such as feed rate and spindle speed, may also affect blade selection.
