Wafer Wheel

Overview of Wafer Wheel

Grinding is an essential process for manufacturing of silicon wafer, and the type of grinding wheel used to carry out the grinding process is called the wafer wheel, or wafer grinding wheel. Wafer grinding wheels are commonly used in the in-feed grinding process of semiconductor wafer such as SiC, sapphire, TSV package, Si, reclaimed wafers, etc. 

The in-feed grinding process is made up of both the fine grinding processes and rough grinding processes. Wafer wheels are made of diamond abrasives and customized vitrified bonds in a special porous microstructure. The diamond size for rough grinding processes are about #325 to #1000, while finish grinding processes are about #2000 to #8000.

When it comes to grinding wheels used in the silicon wafer industry, they are typically demanded to be high quality and low price. Some of the major demands of these wafer grinding wheels include self-dressing ability, low damage on ground surfaces, consistent performance, long product life, and low prices. In this article, we’ll specifically touch base on the grinding wheels for silicon wafers.
 

What Exactly Is a Wafer

A wafer is a term used in electronics which represents a thin slice of semiconductor, such as a crystalline silicon, used for fabricating integrated circuits, and manufacturing solar cells in photovoltaics. The wafer acts as the substrate for microelectronic devices built upon or in the wafer. The wafer goes through multiple microfabrication processes, namely ion implantation, doping, etching, thin-film deposition of a variety of materials, and photolithographic patterning. Finally, the individual microcircuits are divided by wafer dicing and packaged as integrated circuits. 
 

Abrasives for Wafer Wheels

There are generally four types of abrasives used for grinding wheels, which include aluminum oxide, silicon carbide, cubic boron nitride, and diamond. For the purpose of this article, we’ll only address the diamond abrasives because they are virtually the exclusive abrasives option for wafer grinding wheels.

Diamonds present several advantages, such as high heat conductivity, superb hardness, high corrosion resistance, and low friction, making them one of the most prevalent choices for silicon wafer grinding. There are generally two types of diamonds – natural diamonds and synthetic diamonds, both of which are used as abrasives for wafer wheels. One key disadvantage of diamond abrasive is that they often transform into graphite during sintering when surrounded by high working temperature. This also tends to occur when the grinding temperature is too high as well.
 

The Backgrinding Process

The backgrinding process using a grinding wheel is one of the most popular grinding techniques for thinning wafers. The first step is to use a large grit to thin down the wafer coarsely and reduce the thickness of the wafer. The second step involves the use of a fine grit to polish the wafer and grind down the wafer to the designated thickness accurately. To provide an example, a wafer with the thickness of roughly 720 µm should have diameters of 200 mm, and is then grinded to a thickness of 150 µm or less. The coarse grinding process is typically capable of removing about 90 percent of the excessive material in a large bulk. If a two-spindle grinding wheel is used, the typical two-step backgrinding is adopted to complement the dual spindles. Some of the major parameters that must be monitored during the backgrinding process of wafer include:

● Thickness variation among the wafers
● Average roughness of the wafer
● The variation of total thickness within the wafer
● Average roughness
● Die strength
● Warpage and bowing
● Final thickness of the wafer
● Wafer breakage
 

Other Wafer Thinning Processes

If you do not have a wafer grinding wheel available for the wafer thinning, that is fine because there are alternatives, but may be costlier. Chemical or plasma etching are another way to thin wafers, and despite the cost, they do have some advantages over wafer wheels. Some of these advantages of plasma thinning include:

1. Scratches are less likely to be produced during the process.

2. Less mechanical stress and heat are exerted on the wafer during the thinning process.

3. The capability of producing even thinner wafers down to as thin as 50 µm because of the absence of mechanical stress applied to the wafer during the thinning process. You can also employ more dies in stacks in a thin package if the end product is required to be ultra-thin. 

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