The Science of Wafer Grinding

Grinding is one of the most important flattening processes in semiconductor fabrication. It largely affects line-width capability, process latitude and yield, as well as throughput.

Silicon wafers are brittle and susceptible to subsurface cracks (SSCs). SSCs can damage the surface integrity of a wafer during grinding, leading to the chipping of large portions of its surface.

The Spindle

The spindle is one of the most important components of a wafer grinding machine. It is responsible for rotating and moving the grind wheel to perform the grinding process, and it also serves as the source of a tool's power.

Many grinding machines include a dual spindle configuration that includes both coarse and fine grind wheels. This allows a grinding process to be performed simultaneously, which reduces cycle time and increases productivity.

The ability to use a grind spindle with an air bearing helps solve the problem of spindle vibration, which can negatively affect the machining quality and the surface integrity of a work piece. This type of spindle is especially effective for grinding hard-to-cut materials such as sapphire, because the air bearing can reduce the force placed on the wafer by the spinning grinding wheel.

Various techniques have been developed to address this problem, including thusing a ring groove grinding wheel with air bearings or a porous restrictor that helps stribute the load. These methods provide excellent accuracy, but can be costly.

The Chuck

The chuck is the central component of the wafer grinding system. It carries the wafer from the grinding table to the first and second grind units and provides the support to the wheel. This is a critical step in the wafer grinding process as it prevents breakage and debris accumulation.

In the conventional wafer grinding process, a wafer is placed on a chuck made of porous ceramic material. This is done to protect the front surface of the wafer from contamination by silicon dust during the grinding process. In addition, the chuck is reground periodically to ensure that the flatness of the chuck is consistent at a fixed level.

To carry the wafer from the grind table to the first and second grind units, a vacuum pump is connected to a duct through an intake and exhaust hole formed in the body of the chuck. The duct is supplied with deionized water and air to provide cooling and wash away material particles generated during the grinding process.

The first and second grind units 150a and 150b each include: a grinder 151 with diamond resigns 153 serially mounted on the bottom thereof along its periphery for grinding the rear face of the wafer; and a wheel 155 for rotating the grinder 151. The grinder 151 is coupled with the bottom of the wheel 155 and is driven by a motor.

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The Wafer

Wafers are a core raw material used to make semiconductor ICs. They look like round discs and have several parts, called die, etched on their surface that are integrated into chips after wafer grinding.

Before a wafer can be used in manufacturing, it needs to undergo many processes to ensure that its surface is smooth and perfect for circuit layouts. These processes involve machines and abrasive chemicals that clean the wafer’s rough surface to create a flawless, mirror-like finish.

First, the wafer’s crystal structure is determined. It is typically aligned in one of several relative directions known as crystal orientations, with (100) and (111) facing faces being the most common. The orientation is important for determining the depth at which ions can be implanted.

Next, the polycrystalline material is purified to remove 99.99% of impurities, and it is heated to a high temperature so that it will become a highly crystalline substance. This is done using a process called metallurgical purification.

The Wheel

The wheel is responsible for grinding the wafer and removing chips from it. This is a critical step for generating the final product, and one that can be extremely costly if the grind system fails.

Grain size is particularly important when grinding thin wafers, as it can have an impact on how long the grind tool can be used and how many wafers can be processed. This can affect the amount of money that is spent on grinding equipment and the quality of the wafers produced.

The grain size of the grinding wheel can also have an effect on the abrasive particle distribution, which is vital for ensuring that the grind surface is uniform. When abrasive grains are spread evenly throughout the grinding wheel, this improves the quality of the finished wafer, and reduces the likelihood that defects will occur.

While many abrasives are available for grinding wheels, only certain grains have been shown to be capable of providing the quality and performance required for wafer grinding. These include aluminum oxide, silicon carbide and diamond. Each of these grains has its own characteristics that help ensure optimal performance and longevity.