Wafer Back Grinding

With the proliferation of smaller and thinner packages for portable and handheld products, the need for thinner semiconductor devices is increasing. What was once a process only in selected situations is now a required process for most applications, and thin wafer technology is becoming more and more critical. With the advent of 300mm thicker wafers, collision plates, stackable dies and ultra-thin packaging requirements, wafer backgrinding equipment and processes are becoming critical assembly considerations.

Wafer thinning options

There are several methods that are currently used to thin wafers on the wafer backside grinding, the most popular of which is the well-known mechanical backgrinding and polishing technique. This process is preferred in many cases because it is faster and less costly than newer chemical or plasma processes that have recently been developed. However, this has disadvantages applying mechanical stress and heat during the grinding process and causing scratches on the underside of the wafer. These scratches and the depth of the scratches on the wafer surface are directly proportional to the grain size and the pressure applied to the wafer during the grinding process. The scratch depth and surface roughness of the back of a semiconductor die are directly related to the strength of the die, so it is critical that the finished back surface of the wafer is as smooth (or polished) as possible.

Backgrinding process

In order to improve the productivity of the operation, a multi-step grinding operation is generally performed. In the first step, coarse grits are used to coarse the wafer and remove an excessive wafer thickness. In the second stage of polishing, the wafer is used to finer grain and to thoroughly grind the wafer to the required thickness. For wafers with a diameter of 200 mm, they typically start at a wafer thickness of about 720 µm and grind to a thickness of 150 µm or less. Rough grinding typically removes about 90 percent of the excess material. A typical two-step backgrinding operation uses twin spindles with grinding wheels mounted on each spindle.

Wafer scratches and strength

After back grinding, the wafer will have a scratch pattern on the back side. The depth of these scratches will depend on the grain size of the wheel and the amount of vertical pressure exerted during grinding. (The finer the grain, the smaller and shallower the scratches). As the strength of silicon is inversely proportional to the depth of the scratches, it is important to minimize the roughness of the tile surface.

We conducted an experiment to determine the difference in strength of the silicone matrix depending on the different grain sizes (or scratch depth). Samples with vertical scratches were taken from the waffle for a worst case scenario. For the 2000 grit grinding process, the stress required to break the die was 50 percent higher than the stress required to break the die in the (larger) grain 1200 grinding process. using various grits to grind silicon.

Sources of stress

The amount of stress applied to the die in actual applications depends on many factors, including the material of the substrate on which the die is mounted, the size of the die, the die attach material, and the temperature excursions that the die will be subjected to in an application. For many high-power applications, the die is mounted directly onto a high-conductivity material, such as copper or aluminum, and the thermal coefficient of expansion (TCE) mismatch between the silicon and the metal will result in significant mechanical stress on the die during temperature extremes. The actual stress on the die in such an application is similar to the stress induced by the test method during the experiment that was conducted.

For example, if the die is mounted on a copper heatsink or die attach pad, the large TCE (17.3 ppm/K) of a copper substrate vs. the small TCE (2.3 ppm/K) of a silicon die results in mechanical stress at the die edge.  A larger matrix causes more stress. In addition, if the metal heat sink or substrate is not thick enough to resist the expansion or contraction of the die, the material will bend and put even more stress on the die. Any scratch on the back face of the die can then propagate cracks under stress conditions that can trigger environmental tests or actual power applications.

Grinding equipment

To increase productivity, many equipment manufacturers produce equipment for thinning and processing multiple wafers. The design of these machines affects the quality of the thinned wafers as much as the selection of the grinding spindles. There are representations of some of the multi-plate handling machines that are currently available.

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