Wafer Cassettes Information
Wafer cassettes are used to transport and store wafers during semiconductor manufacturing operations. Also referred to as wafer carriers, they are designed to house or carry several wafers of the same size and are available in a range of materials to support specific applications, such as thermal processing.
A vacuum wand is typically used to pick up and deposit wafers in cassettes, which are then used to transfer the wafers between processing chambers and other storage containers. Custom cassettes may be designed to interface with a specific system or storage container.
Custom wafer carrier manufacturing. Video Credit: Pozzetta Products Inc.
Wafer cassettes can be characterized by a number of important dimensions such as overall length, width, and height, as well as specific dimensions such as:
- D1 dimension – the distance from the end of the cassette to the to the center line of the first pocket
- Pocket spacing – the distance between the center lines of each wafer pocket
- Pocket flat – the width of each wafer pocket at its narrowest distance
Wafer carriers are designed to fit a wide range of wafer sizes from 2.5 inches to 450mm.
As semiconductor fabrication technology has advanced to produce more sophisticated devices, wafer cassette design has kept pace. Semiconductor fabrication now involves higher automation levels, greater tool uptime and throughput, higher process temperatures, additional mask layers, and compound semiconductor materials. In response, wafer carriers have been designed to have better thermal capabilities, protection from electrostatic discharge, reduced outgassing at high temperatures, and improved dimensional stability and particulation.
Wafer cassettes are designed to suit a wide variety of applications and may possess the following qualities:
- Capability to hold from one to multiple wafers, commonly up to 25 for transport or storage.
- Open side wall configuration to allow easy access for wafer handling.
- Locating track notches at the center of the cassette, as well as pins and holes on the top rail for reliable operation with automated material handling systems and process tools.
- End wall configurations having a handle or flanges for material handling systems.
- Identification options such as colored ID tags, radio frequency tags, cardholders, bar code labels, and laser marking.
- Surface finishes such as anodized, alodine, decoral, electroless nickel plating, or hardcoat.
Wafer cassettes are manufactured from a variety of materials tailored to specific semiconductor applications.
Stainless steel wafer cassettes can withstand high semiconductor baking temperatures up to 450°C. They are fully conductive, non-deforming, non-contaminating, non-outgassing, allow uniform heat distribution, and have a Rockwell hardness rating much softer than silicon.
Plastics are often blended with other materials such as carbon for improved static dissipation to avoid wafer damage caused by a buildup and discharge of static electricity. Plastic materials include polyetheretherketone (PEEK) with carbon fiber or carbon nanotubes, and polypropylene and carbon powder.
Cassettes designed for wet chemical processing applications may use material such as perfluoroalkoxy alkane (PFA) that is resistant to degradation from aggressive chemicals.