Before a wafer can become a true semiconductor chip, it must go through three stages of conversion: first, a block of ingots is cut into wafer sheets; second, the first step is to engrave transistors on the front side of the wafer sheet, and lastly, the wafer sheet is cut into a complete semiconductor chip for packaging. Encapsulation process is the latter, in which the wafer is cut into a number of hexahedral shaped individual chips. This process of obtaining individual chips is called "dicing", and the process of sawing the wafer into individual rectangles is called die-cutting. In recent years, with the improvement of semiconductor integration, the thickness of the wafer has become thinner and thinner, which, of course, to the "single cut" process has also brought a lot of difficulties.
First, the development of wafer scribing
The front-end and back-end processes need to be further studied and developed by analyzing the interaction of different teaching methods for various problems: the evolution of the back-end process can directly determine the structure and position of the small hexahedral chips separated from the die alone on the wafer, as well as the structure and position of the pads on the wafer (the path of the electrical connection to achieve this); on the contrary, the front-end technological process has changed the back-end process in the wafer in the process of evolution. In contrast, the evolution of front-end technology processes has changed the flow and methods of backside thinning and "Die Sawing" wafers in back-end processing. As a result, the growing sophistication of package designs will have a significant impact on back-end process companies. Furthermore, the number, procedure, and type of dicing will change depending on the appearance of the product's packaging material. See: The evolution of "thin and light" semiconductor packaging. Now, let's study the history of the evolution of "cut sheet" by controlling the chip, and look at the five cutting management methods.
In the early days, fracture by applying external force was the only cutting method that could divide a wafer into hexagonal cores.wafer testing However, this method had some drawbacks, such as chip edge fragmentation or breakage. Also, the cutting surface was rough because the burrs (burrs) on the surface of the metal layer were not completely removed.
To solve this problem, the "scribing" method cuts the wafer surface to about half its depth before "breaking" it. As the name implies, "slicing" refers to the use of an impeller in front of the wafer before cutting (half-cutting). In the early days, most wafers under 6 inches were cut by "slicing" them and then "breaking" them between slices.
Blade Cutting or Blade Sawing
The application of the "slicing" dicing method gradually began to evolve into the "blade dicing" dicing (or sawing) method, i.e., the method whereby two or three consecutive industrial research cuts are made through the use of a single blade. "Blade" cutting method design can effectively compensate for "scribing" after "breaking (Breaking)", the chip produces the phenomenon of peeling, can be in the The "Singulation" process can play a role in environmental protection and chip management. "Blade" cutting and the previous "slice" cutting ability to understand the difference, that is, after a "blade" cutting, instead of "Breaking ( Instead of "Breaking" after a "Blade" cut, we cut the material again with a blade. Therefore, it is also called "Step Dicing" method.
In order to protect the wafers from external damage during the dicing process, a film is applied to the wafers beforehand to ensure safer "slicing". In the "backside grinding" process, the film is applied to the front of the wafer. In contrast, the film is applied to the back of the wafer during "blade" cutting.wafer probe Backside milling determines the thickness of the wafer. During the bonding process, the film on the back side of the chip comes off automatically. Due to the high friction during the cutting process, deionized water is sprayed continuously in all directions. In addition, the impeller should have diamond particles attached so that it can slice better. At this point, the cut (blade thickness: width of the groove) must be uniform and must not exceed the width of the 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 feed rate of the chip is greatly increased, the possibility of chip edge peeling becomes greater. Therefore, the number of impeller revolutions should be controlled at about 30,000 per minute. As you can see, the technology of the semiconductor process tends to accumulate slowly through long periods of time and trial and error (in the next section on eutectic placement, we will discuss about cutting and DAF (placement film)).
Pre-mill Dicing: Cutting Sequence Change
When cutting on an 8-inch diameter wafer with a razor blade, there is no need to worry about chip edge peeling or cracking. However, as the wafer diameter increases to 21 inches, the thickness becomes extremely thin, and peeling and cracking begin to occur again. To greatly minimize the physical impact on the wafer during the dicing process, DBG's "cut and grind" method replaced the traditional dicing sequence. Unlike the traditional "blade" dicing method of continuous dicing, DBG performs the "blade" dicing first, and then gradually reduces the wafer thickness through successive backside thinning until the chip cracks. It can be said that DBG is the previous "blade" cutting method of the upgraded version, because it can reduce the impact of the second cut, so DBG method in the "wafer-level packaging" has been rapidly promoted.
Five, laser cutting (Laser Dicing)
Wafer-level chip chip level packaging (WLCSP) process mainly using laser cutting method. Laser dicing can reduce spalling and cracking to get better chips, but when the wafer thickness exceeds 100μm, the production efficiency will be greatly reduced. Therefore, they are mainly used for wafers with a thickness of less than 100 μm (relatively thin). Laser dicing is a process that utilizes a high-energy laser to cut grooves in a silicon wafer.probe holder However, conventional laser cutting requires a protective coating to be applied to the wafer surface. This physical contact creates grooves on the wafer surface by heating or irradiating the wafer with laser light, and the cut wafer adheres to the wafer surface. As can be seen, the traditional laser dicing method also cuts directly into the wafer surface, and in this respect it is similar to the "blade" dicing method.
Laser stealth dicing (SD) is a method of separating a chip by cutting the inside of the wafer with laser energy and then applying an external force to the tape on the backside to break it. When pressure is applied to the tape on the backside, the wafer will be lifted immediately due to the stretching of the tape, thereby separating the chip. Compared to conventional laser dicing methods, SD has the following advantages: first, there are no silicon chips; second, the slit (slit: the width of the cutting groove) is narrow, so that more chips can be obtained. In addition, using the SD method will greatly reduce peeling and cracking, which determine the overall quality of the cut. Therefore, the SD method is expected to be the most popular technique in the future.
Plasma dicing is a recently developed technique in which plasma etching is used for dicing during the fabrication process (Fab). The plasma dicing method uses a semi-gas material instead of a liquid, so it has relatively little impact on the environment. Moreover, the entire wafer is cut at once, so the "single cut" is faster. The plasma method to chemical reaction gas as raw material, etching process is very complex, so its process flow is relatively complex. However, compared with "blade" cutting and laser cutting, plasma cutting does not cause damage to the surface of the wafer, thus reducing the scrap rate and obtaining more chips.
Recently, the thickness of wafers has been reduced to 30 μm, and materials such as copper (Cu) or low dielectric constant (Low-k) are used in China. Therefore, in order to prevent burrs, plasma cutting methods are also favored. Of course, plasma cutting processing technology is also in constant development, I believe that their own near future, one day when etching can learn to no longer use the need to wear a special mask by, because this is the use of plasma cutting of a major economic development research direction.
As the wafer thickness increases from 100μm to 50μm, and then to 30μm, and thinner, the cutting method to obtain a separate chip is also changing and evolving from "fracture" and "blade" cutting to laser cutting and plasma cutting. Although the cutting process itself has increased production costs with increasingly sophisticated cutting methods, on the other hand, the production cost of a single chip has been trending downward by greatly reducing flaking and cracking in the cutting of semiconductor chips and increasing the number of chips obtained per wafer. Of course, the increase in gain per unit area of wafer is closely related to the reduction in the width of the dicing street slot. By plasma cutting, you can get 20% more chips than "blade" cutting, which is an important reason why people choose plasma cutting. With the development of wafer technology, slicing appearance and packaging methods, wafer processing technology and cutting technologies such as DBG have also emerged.