Introduction to the wafer Back Grinding process
1. The purpose of Back Grinding
In the process of making semiconductors from wafers, the appearance of wafers constantly changes. First, in the wafer manufacturing process, the Edge and surface of the wafer are polished, a process that usually grinds both sides of the wafer. After the end of the front-end process, you can start the backside grinding process that only grinds the back of the wafer, which can remove the chemical contamination in the front-end process and reduce the thickness of the chip, which is very suitable for the production of thin chips mounted on IC cards or mobile devices. In addition, this process has the advantages of reducing resistance, reducing power consumption, increasing thermal conductivity and rapidly dissipating heat to the back of the wafer. But at the same time, because the wafer is thin, it is easy to be broken or warped by external forces, making the processing step more difficult.
2. Back Grinding (Back Grinding) detailed process
Back grinding can be divided into the following three steps: first, paste protective Tape Lamination on the wafer; Second, grind the back of the wafer; Third, before separating the chip from the Wafer, the wafer needs to be placed on the Wafer Mounting that protects the tape. The wafer patch process is the preparation stage for separating the chip (cutting the chip) and therefore can also be included in the cutting process. In recent years, as chips have become thinner, the process sequence may also change, and the process steps have become more refined.
3. Tape Lamination process for wafer protection
The first step in the back grinding is the coating. This is a coating process that sticks tape to the front of the wafer. When grinding on the back, the silicon compounds will spread around, and the wafer may also crack or warp due to external forces during this process, and the larger the wafer area, the more susceptible to this phenomenon. Therefore, before grinding the back, a thin Ultra Violet (UV) blue film is attached to protect the wafer.
When applying the film, in order to make no gap or air bubbles between the wafer and the tape, it is necessary to increase the adhesive force. However, after grinding on the back, the tape on the wafer should be irradiated by ultraviolet light to reduce the adhesive force. After stripping, tape residue must not remain on the wafer surface. Sometimes, the process will use a weak adhesion and prone to bubble non-ultraviolet reducing membrane treatment, although many disadvantages, but inexpensive. In addition, Bump films, which are twice as thick as UV reduction membranes, are also used, and are expected to be used with increasing frequency in the future.
4. The wafer thickness is inversely proportional to the chip package
Wafer thickness after backside grinding is generally reduced from 800-700 µm to 80-70 µm. Wafers thinned down to a tenth can stack four to six layers. Recently, wafers can even be thinned to about 20 millimeters by a two-grind process, thereby stacking them to 16 to 32 layers, a multi-layer semiconductor structure known as a multi-chip package (MCP). In this case, despite the use of multiple layers, the total height of the finished package must not exceed a certain thickness, which is why thinner grinding wafers are always pursued. The thinner the wafer, the more defects there are, and the more difficult the next process is. Therefore, advanced technology is needed to improve this problem.
5. Change of back grinding method
By cutting wafers as thin as possible to overcome the limitations of processing techniques, backside grinding technology continues to evolve. For common wafers with a thickness of 50 or greater, backside Grinding involves three steps: a Rough Grinding and then a Fine Grinding, where the wafer is cut and polished after two grinding sessions. At this point, similar to Chemical Mechanical Polishing (CMP), Slurry and Deionized Water are usually applied between the polishing pad and the wafer. This polishing work can reduce the friction between the wafer and the polishing pad, and make the surface bright. When the wafer is thicker, Super Fine Grinding can be used, but the thinner the wafer, the more polishing is required.
If the wafer becomes thinner, it is prone to external defects during the cutting process. Therefore, if the thickness of the wafer is 50 µm or less, the process sequence can be changed. At this time, the DBG (Dicing Before Grinding) method is used, that is, the wafer is cut in half before the first grinding. The chip is safely separated from the wafer in the order of Dicing, grinding, and slicing. In addition, there are special grinding methods that use a strong glass plate to prevent the wafer from breaking.
With the increasing demand for integration in the miniaturization of electrical appliances, backside grinding technology should not only overcome its limitations, but also continue to develop. At the same time, it is not only necessary to solve the defect problem of the wafer, but also to prepare for new problems that may arise in the future process. In order to solve these problems, it may be necessary to switch the process sequence, or introduce chemical etching technology applied to the semiconductor front-end process, and fully develop new processing methods. In order to solve the inherent defects of large-area wafers, a variety of grinding methods are being explored. In addition, research is being conducted on how to recycle the silicon slag produced after grinding the wafers.
Post time: Jul-14-2023