Understanding the Wafer Manufacturing Process:

Wafer manufacturing process can be tailored by: a) wire saw and wire b) abrasive, c) carrier, d) wafer thickness, and slurry viscosity.

a) WIRE SAW and WIRE:

The wiresaw consists of one wire moving, either uni-directionally or bi-directionally, on the surface of silicon ingot. Major wire saw manufacturers are: NTC, HTC, and Mayer-Berger. The single wire is wound carefully on wire guides with grooves of constant pitch to form a "wire-web". The wire guides are rotated by a pair of master-slave drives, causing the entire wire web to move at high speed (typically 9-14m/s) while carrying the abrasive. The ingot is fed in direction perpendicular to the wire web. The wire is maintained at a consistent tension (typically 10-20N for 120 micron wire) during the cutting process. A spool of steel wire is used to continuously supply the wire necessary for cutting. Today, industry uses wires of 100-160micron.

Industry trend is towards thinner wire to maximize number of wafers per cut. Thinner wire size at constant wafer thickness gives smaller kerf slot width and less Si waste. However, thinner wire is more costly, more likely to break, and requires lower cutting speeds.

b) ABRASIVE:

The abrasive "grit" is made of SiC. Commonly used abrasive types are: JIS 1200, JIS 1500, F500, F600, F800. Wafer manufacturers use particle size to control wafer quality and wafer thickness. Typical abrasive particle size is 6-15microns. The wire is coated with an abrasive grit suspension, which supplies the abrasion to the ingot with the moving wire within the wire-saw. The abrasive particles adhere to the cutting wire, which can become "wound-up" within the wire saw.

Industry trend is toward smaller abrasive particle size to minimize kerf slot width and ensure less Si waste.

c) CARRIER:

To maintain a uniform cutting edge throughout the process, the abrasive grit is suspended in a high viscosity medium, called "carrier". Examples of carriers used are mineral oil, polyethylene glycol 200, polyethylene glycol 300, polyethers, and others. Various additives may be added to carrier in order to stabilize the suspension and increase its uniformity.

Industry trend is carrier with additives or water based carriers to improve yield and wafer surface parameters.

d) WAFER:

Today, wafers are produced ranging in thickness from 160-240 micron with a small kerf loss and high yield. Thinner wafers result in more cut wafers per ingot. At the same time, thinner wafers pose challenges as they cause increased breakage & requirements for specialty handling equipment.

Industry trend is towards thinner wafer to maximize number of wafers per ingot.

e) SLURRY VISCOSITY:

Abrasive in carrier is called slurry. Every wafer manufacturing process has its optimum slurry viscosity. Too low slurry viscosity causes thin wire coating, hence lower abrasive level per unit wire length, which can stall cutting speed (known as wire starvation). Too high slurry viscosity may increase wire drag and vibration within kerf slot and reduces uniform volume and smooth flow of slurry into small kerf slots. In addition, too high slurry viscosity may cause non-uniform slurry curtain, and harder pump circulation. Slurry viscosity can be decreased by adding more carrier, decreasing abrasive loading, and/or increasing slurry temperature during cutting. Slurry viscosity can be increased by increasing abrasive loading, using additives, and/or lowering slurry temperature during cutting.

Industry trend is towards lower viscosity slurries.

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