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The waffle grinder is needed to grind the wafer or, also referred to as "back" or "thinning the wafer". This is a process where the back side of the wafer is ground to create a thinner wafer that allows more layers and higher density of ICs to be placed in a smaller package. Additional reasons for creating ultra-thin plates can be flexibility or heat dissipation. Most silicon wafers are manufactured to a thickness of around 750 µm, but can be reduced to as little as 50 µm or less by back grinding.
The wafers usually undergo a thorough cleaning and surface lamination process prior to being sanded again. At the stage of lamination, a protective tape is applied to the surface of the wafer, which protects it against mechanical damage and contamination with grinding fluid and debris.
To support wafers during "ultra-fine" grinding of wafers and other post-grinding operations, the 3M wafer support system is often used. In this process, a UV-curable adhesive is applied to the wafer surfaces and used as a binder between the glass backing and the tiles. Grinding stress on the tile is minimized as the adhesive flows into and supports the topography of the circuit patterns on the front of the tiles. After back-grinding and any post-grinding operations, the glass surfaces are removed by laser bond removal.
The laminated wafers are then loaded into the wafer cassettes, which in turn are loaded into an automated backgrinding machine. This machine uses a robotic arm to pick up the wafers and place them backside up under high-precision, computer-controlled grinding wheels.
To achieve ultra-thin thicknesses or specific surface roughness requirements, a variety of grinding wheels can be used, starting with coarse grit and progressing to grinding wheels with finer and finer grits. For the final surface finishing process, extremely fine diamond grit is often used.
Continuous washing with deionized water is used to remove contamination from the wafers during the regrinding process. In addition, to prevent contamination from external sources, back grinding is usually done in clean environments, generally grade 1000 or better.
Other wafer thinning methods are also available, including chemical and plasma processes. While these procedures allow the production of even thinner wafers than backgrinding, they are generally more expensive. Back grinding remains the most popular and widely used method of thinning wafers.
To produce even thinner wafers, thinner than 50 μm, two or more processes are often combined. For this purpose, the 3M Wafer Support SystemTM can be used in conjunction with SEZ etching and / or CMP (Chemical-Mechanical Planarization) polishing. SEZ pickling and CMP polishing are wet chemical processes to gently remove silica material without adding mechanical stress to the tiles.
There are several methods that are currently used to thin wafers, 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, it has the disadvantages of applying mechanical stress and heat during the grinding process and causing scratches on the back side 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.
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 most of the excess thickness of the wafer. In the second step, a finer grain is used to polish the wafer and finely grind it 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.
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