Thin-Wafer Handling: Spin Chuck Designs for Thinned Substrates
Thin-Wafer Processing Trends
Thin and fragile substrates are now common throughout the semiconductor industry. Thicknesses of the substrates often range from 50 to 100 µm (0.001 to 0.004 inch) and are cut into various shapes (round, square, and rectangular). Safely handling these fragile materials requires specially designed spin chucks and thin-wafer handling techniques.
Thinned substrates can be made of a wide variety of materials such as flexible polymer films (fluorinated ethylene propylene [FEP] and polyester [PET]) and metal foils (titanium, aluminum, and steel). Other substrate materials include silicon, gallium arsenide (GaAs), gallium nitride (GaN), indium phosphide (InP), and silicon carbide (SiC). These CS and III-V materials are extremely brittle and far more sensitive to both mechanical and thermal shock.
Issues with Standard Vacuum Spin Chucks
Process engineers often encounter a major hurdle with standard vacuum chucks that use a series of concentric circles and/or small-diameter perforated holes to supply vacuum through the spin chuck surface. These designs distribute the vacuum unevenly across the surface and cause dimples, deflection, and/or, in worst-case scenarios, cracking. Furthermore, all of these detrimental anomalies will lead to less-than-optimal film characteristics across the substrate. A film frame support structure will assist in handling these delicate films; however, any irregular chuck topography will detrimentally affect total thickness variation (TTV). Therefore, specialized spin chucks are recommended to safely contact thinned substrates without risk to delicate structures and subsequent film uniformity.
Alternative Spin Chuck Designs
For thinned substrates (< 250 µm thick), we have developed a porous ceramic insert design that has a distinct advantage of completely supporting the backside of any given substrate dimension. The chuck distributes the vacuum equally through a porous surface and mitigates any potential deflection, eliminating detrimental effects to your substrate or coat quality. These chucks are design specific and available for a wide array of shapes and sizes.
Film frames are commonly used to support silicon (Si) and CS materials. A device wafer can be mounted onto a film frame after backside processing but while it is still supported by its carrier, as recommended in the mechanical debonding process, or following separation (debonding) from the carrier, as in the thermal slide debonding process, for subsequent transport, cleaning, and packaging.
The porous ceramic design can also be adapted for thinned substrates mounted to film frames. Mechanical clamps and a porous ceramic insert combine for spin processing thinned substrates (< 250 µm thick) that have been taped to frames. The ceramic insert ensures complete and uniform backside support, while it distributes the vacuum source across the taped surface. This design also utilizes vacuum O-rings and mechanical clamps for securing the outer film frame to the chuck assembly and maintaining positive lock.