Automated single wafer incineration of compound semiconductors

Incineration, 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 manufacturing.

During this step, the organic materials in the photoresist are “burned off” using a processing tool in which a monatomic plasma is created by exposing low pressure oxygen or fluorine gas to waves high power radio.

Previously, wafer incineration 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. whose size can vary between 100 and 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 need unique automated wafer processing tools capable of fast ashing rates and high production levels.

“Today, semiconductor manufacturers are increasingly looking for a unique wafer incineration solution for high-temperature photoresist removal and precision stripping,” said Wolfgang Pleyer, P.Eng. main application at Munich-based PVA TePla, one of the world’s leading suppliers of microwave and RF plasma systems. with US operations in California.

Microwave plasma incineration

For 50 years, most plasma tools have used RF (radio frequency) to strip 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 just increase the DC bias and take it all out,” Pleyer said. “But RF plasma is not as selective in attacking photoresist. Additionally, when the photoresist is removed, the underlying layers of the wafer may be sensitive and could be damaged by RF.

Today, microwave-based plasma tools produce a very high concentration of chemically active species and low ion bombardment energy, ensuring both fast ash and damage-free plasma cleaning.

“Microwaves tend to be faster and produce higher ash rates than RF,” Pleyer said.


Targeted photoresist removal using oxygen

Advanced microwave-based plasma incineration systems from manufacturers such as PVA TePla often use oxygen as the primary process gas. The oxygen burns the wafers very selectively and only attacks the photoresist, leaving the rest of the wafer intact.

Unfortunately, using a pure oxygen process is not always compatible with all types of wafer surfaces; some require a combination of gases.

“There may be other materials on or inside the photoresist that cannot be completely removed with just oxygen alone,” Pleyer said. “To solve this problem, we can add fluorine chemistry, usually CF4, mixed with oxygen.”

Due to the tendency to use different materials in wafers, some metals easily oxidize in the process, which is undesirable. Hydrogen and oxygen gas at low pressure can be used in such circumstances.

“Adding hydrogen will keep metals from oxidizing while oxygen strips photoresist,” Pleyer said. “This is something 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 the UV-exposed parts cure, while the rest of the film remains soluble and can be washed off.

Additionally, the chemical stability of SU-8 photoresist can also make it difficult to remove.

Removal of SU-8 should be done at lower temperatures. According to Pleyer, “You need to be below 100 degrees and in some cases below 50 degrees.

Greater flexibility in chemistry is also required, potentially including the use of fluorine and excellent temperature control. All of this is much easier to accomplish with single wafer processing.

According to Pleyer, customers may have photoresist on a metal surface deposited between two metal surfaces requiring the photoresist to be removed from the wafer side.

Due to its isotropic etching property, oxygen-based microwave plasma ash can remove photoresist between metal plates, unlike RF-based systems.

Ease of single slice automation

In manually loaded systems, the ashtray has a sliding door where the wafers rest on the heating or cooling plan mounted on the chamber entrance door. In automated systems, wafers are increasingly loaded into the chamber using robotic manipulation.

“Customers today want to reduce all human factors as chips continue to become more advanced,” Pleyer said. “It requires automatic handling and loading using robotics and full control by a host computer. In some cases, the operator only has to place the cassette on the loading port, which starts automatically.

PVA TePla, for example, designed its GIGAfab-A plasma system to be configurable for 200 or 300 mm wafers and a cluster tool with up to 3 process modules called GIGAfab Modular.

Both systems use open cassettes as well as standard front-opening or mechanical loading stations. Wafer processing is thermoelectrically controlled from RT to 250°C. A unique planar microwave plasma source provides high ash levels over a wide temperature range.

With thinner and thinner wafers, more reliable automated single wafer processing equipment handles fragile wafers.

“Trying to physically manipulate wafers without using robots can end badly,” said Ryan Blaik, director of semiconductor and medical device sales at PVA TePla in California,

Processing a single wafer also provides better temperature controls.

“With batch processing, microwave radiation must heat all the wafers in a quartz boat, and the temperature can fluctuate during processing,” Blaik said. “For a single-wafer processing system, wafers are only introduced into the chamber after preheating, which helps maintain a constant temperature during processing.”

In single slice processing, a descum process can be accomplished using the same tool. The main difference between the two processes is the temperature to which the wafer is exposed in the plasma chamber.

According to PVA TePla’s Pleyer, “For skimming we want low ash and good uniformity and process control. Because we are only aiming for residue disposal, a very high temperature incineration recipe will not work. It is easier to accomplish single wafer ashing using a microwave-based plasma system. »

As semiconductor device manufacturing continues to increase globally to meet an insatiable demand for chips, the need for control, efficiency and configurable solutions for wafer incineration will continue as as the chips themselves increase in complexity and decrease in size.

Automated single-wafer microwave plasma systems provide chipmakers with targeted, configurable incineration that meets the needs of a growing range of wafer types.

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