AUTOMATION & ROBOTICS


AUTOMATED SINGLE WAFER ASHING OF COMPOUND SEMICONDUCTORS


Robotic handling protects wafers while superior temperature & process control provides complete, precision ashing A


shing, in which the light-sensitive coating known as photoresist is removed and


cleaned from an etched wafer, is one of the most important and frequently performed steps in chip fabrication. In this step, photoresist organics are “burned off” using a processing tool in which monatomic plasma is created by exposing oxygen or fluorine gas at low pressure to high-power radio waves. Previously, wafer ashing was largely done using batch processing techniques to achieve the required throughput. However, unlike silicon semiconductors,


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


manufacturers require automated, single wafer processing tools capable of fast ashing rates and high production levels.


“Today, semiconductor manufacturers are


increasingly looking for a single wafer ashing solution for both high-temperature photoresist removal and precision descum,” says Wolfgang Pleyer, Senior Application Engineer at Munich- based PVA TePla, a leading worldwide provider of 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 photoresists. RF plasma etches the surface through a physical process that essentially bombards the surface with plasma in a specific 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 photoresist is removed, the underlying layers of the wafer may be sensitive and could be damaged with RF.” Today microwave-based plasma tools produce


a very high concentration of chemically active species and low ion bombardment energy, ensuring both a fast ash rate and a damage-free plasma cleaning. “Microwave tends to be quicker and produce higher ash rates than RF,” says Pleyer.


TARGETED PHOTORESIST REMOVAL USING OXYGEN Advanced microwave-based plasma ashing systems from manufacturers 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 compatible with all types of wafer surfaces; some require a combination of gases. “There can be other materials on, or within,


the photoresist that cannot be stripped away completely with just oxygen alone,” says Pleyer. “To resolve this issue, we may add some fluorine chemistry, usually CF4, mixed with the oxygen.” Because of the trend of using different


materials in wafers, some metals are oxidized easily during the process, which is not desirable. Both hydrogen and oxygen gasses at low pressure can be used in such circumstances. “Adding hydrogen will prevent the metals


from oxidizing while the oxygen removes the photoresist,” said Pleyer. “This is one thing we control very tightly during wafer ashing, and it requires excellent temperature uniformity to accomplish this task.” Working with MEMS devices requires the


removal of SU-8 or similar epoxy-based negative photoresists. A challenge with negative photoresists is that parts exposed to UV become polymerized, while the remainder of the film remains soluble and can be washed away. Moreover, the chemical stability of SU-8 photoresist can also make it difficult to remove. Removing SU-8 must be performed at lower temperatures. According to Pleyer, “You need to be below 100 degrees, and


10 NOVEMBER 2022 | FACTORY&HANDLINGSOLUTIONS


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50