Nov/Dec, 2022


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some fluorine chemistry, usually CF4, mixed with the oxygen.” Because of the trend of using different


materials in wafers, some metals are oxi- dized easily during the process, which is not desirable. Both hydrogen and oxygen gases at low pressure can be used in such circum- stances.


“Adding hydrogen will prevent the met-


als 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 chal- lenge with negative photoresists is that parts exposed to UV become polymerized, while the remainder of the film remains soluble and can be washed away. The chemical stability of SU-8 photoresist can also make it diffi- cult to remove. Removing SU-8 must be per-


formed at lower temperatures. According to Pleyer, “You need to be below 100 degrees, and in cer- tain cases below 50 degrees. More flexibility in the chemistry is also required, including poten- tially the use of fluorine and excellent control of the tempera- tures. All of this is much easier to accomplish with single wafer processing.” According to Pleyer, cus-


tomers may have a photoresist on a metal surface deposited be - tween two metal surfaces requir- ing the removal of the photoresist from the side of the wafer. Due to its isotropic etch property, oxy- gen-based microwave plasma ash- ers can remove the photoresist in between the metal plates, unlike RF-based systems.


Single Wafer Automation In manually loaded systems,


the asher has a pull-out door where the wafers lie on the heat- ing or cooling plane mounted on the entry door of the chamber. In automated systems, wafers are increasingly loaded into the chamber utilizing robotic han- dling.


“Today, customers want to


reduce all human factors as chips continue to become more ad - vanced,” said Pleyer. “This requires automatic handling and loading using robotics and full control by a host computer. In some cases, the operator only needs to place the cassette onto the load port, which will start automatically.” PVA TePla, for example, has


designed its GIGAfab-A plasma system to be configurable for 200 or 300 mm wafers and a cluster tool with up to three process modules called the GIGAfab Modular. Both systems use open cassette as well as front opening or standard mechanical load sta- tions. Wafer processing is ther-


moelectrically controlled from RT to 482°F (250°C). A unique planar microwave plasma source provides high ash rates over a wide temperature range. With wafers becoming thinner, more


reliable automated single wafer processing equipment handles fragile wafers. “Trying to handle the wafers physically


without the use of robots can end poorly,” says Ryan Blaik, sales manager of semicon- ductor and medical devices at PVA TePla in California. Single wafer processing also pro- vides better temperature controls. “With batch processing, microwave radi-


ation must heat all the wafers in a quartz boat, and the temperature can fluctuate dur- ing processing,” says Blaik. “For a single-


Page 55


Automated Single Wafer Ashing of Compound Semiconductors Continued from previous page


wafer processing system, wafers are brought into the chamber only after pre-heating, allowing a constant temperature to be main- tained during processing.” In single wafer processing, a descum


process can be accomplished using the same tool. The primary difference between the two processes is the temperature the wafer is exposed to while in the plasma chamber. According to Pleyer, “For descum, we want


a low ash rate and good uniformity and process control. Because we are only targeting removal of residues, an ashing recipe at very high tem- peratures will not work. It is easier to accom- plish using single wafer ashing using a microwave-based plasma system.”


Continued on page 58


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