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Nov/Dec, 2022


Automated Single Wafer Ashing of Compound Semiconductors


By Del Williams


organics are “burned off” using a processing tool in which monatomic plasma is created by exposing oxygen or fluorine gas at low pres- sure to high-power radio waves. However, unlike silicon semiconductors,


A


where wafers are mass-produced in a stan- dard 300 mm size, compound semiconductors are made of silicon carbide, galli- um arsenide, gallium nitride, and sapphire which can vary in size between 100 to 200 millimeters. When this is the case, significant- ly better uniformity of photoresist removal is required, which means better temperature and process controls. As a result, most compound


semiconductor wafer manufactur- ers require automated, single wafer processing tools capable of fast ashing rates and high produc- tion levels. “Today, semiconductor manu-


facturers are increasingly looking for a single wafer ashing solution for both high-temperature pho- toresist removal and precision des- cum,” says Wolfgang Pleyer, sen- ior application engineer at Munich-based PVA TePla, a lead- ing worldwide provider of


shing is one of the most important and frequently performed steps in chip fab- rication. In this step, photoresist


microwave and RF plasma systems with U.S. operations in California.


Microwave Plasma Ashing For 50 years, most plasma tools have


used RF (radio frequency) for stripping pho- toresists. RF plasma etches the surface through a physical process that essentially bombards the surface with plasma in a spe- cific direction.


“In the past, you could simply increase


the DC bias and remove everything,” says Pleyer. “But RF plasma is not as selective in attacking photoresist. Also, when the pho- toresist is removed, the underlying layers of the wafer may be sensitive and could be dam- aged with RF.” Today microwave-based plasma tools


produce a very high concentration of chemi- cally active species and low ion bombardment energy, ensuring both a fast ash rate and a damage-free plasma cleaning.


Targeted Photoresist Removal Advanced microwave-based


plasma ashing systems from man- ufacturers such as PVA TePla often utilize oxygen as the primary process gas. The oxygen ashes the wafers very selectively and attacks only the photoresist, leaving the rest of the wafer untouched. Unfortunately, using a pure


oxygen process is not always com- patible with all types of wafer sur- faces; some require a combination of gases. “There can be other materials


GIGAfab Modular plasma ashing system.


on, or within, the photoresist that cannot be stripped away completely with just oxygen alone,” says Pleyer. “To resolve this issue, we may add


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