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Monday, March 20, 2023

Opinion: Exploring Automated Single-Wafer Ashing of Compound Semiconductors

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Ashing, by which the sunshine–delicate coating generally known as photoresist is eliminated and cleaned from an etched wafer, is without doubt one of the most necessary and ceaselessly carried out steps in chip fabrication. On this step, photoresist organics are “burned off” utilizing a processing device by which monatomic plasma is created by exposing oxygen or fluorine fuel at low strain to excessive–energy radio waves. Beforehand, wafer ashing was largely accomplished utilizing batch–processing strategies to realize the required throughput.

Nonetheless, not like silicon semiconductors, by which wafers are mass–produced in a typical 300–mm measurement, compound semiconductors are made from silicon carbide, gallium nitride, gallium arsenide, and sapphire, which may range from 100 to 200 mm. When that is the case, considerably higher uniformity of photoresist removing is required, which implies higher temperature and course of controls. In consequence, most compound semiconductor wafer producers require automated, single-wafer–processing instruments able to quick ashing charges and excessive manufacturing ranges.

Right this moment, semiconductor producers are more and more on the lookout for a single-wafer–ashing resolution for each excessive–temperature photoresist removing and precision descum.

Microwave plasma ashing

For 50 years, most plasma instruments have used radio frequency (RF) for stripping photoresists. RF plasma etches the floor by a bodily course of that basically bombards the floor with plasma in a particular course.

Previously, you possibly can merely enhance the DC bias and take away the whole lot, however RF plasma will not be as selective in attacking photoresist. Additionally, when the photoresist is eliminated, the underlying layers of the wafer could also be delicate and may very well be broken with RF.

Right this moment, microwave–based mostly plasma instruments produce a really excessive focus of chemically energetic species and low ion bombardment power, guaranteeing each a quick ash price and a harm–free plasma cleansing.

Microwave tends to be faster and produces greater ash charges than RF.

Focused photoresist removing utilizing oxygen

Superior microwave–based mostly plasma ashing techniques from producers like PVA TePla typically make the most of oxygen as the first course of fuel. The oxygen ashes the wafers very selectively and assaults solely the photoresist, leaving the remainder of the wafer untouched.

Sadly, utilizing a pure oxygen course of will not be at all times appropriate with all forms of wafer surfaces; some require a mixture of gases.

There might be different supplies on or inside the photoresist that can not be stripped away fully with simply oxygen alone. To resolve this concern, we might add some fluorine chemistry, often CF4, combined with the oxygen.

Due to the pattern of utilizing completely different supplies in wafers, some metals are oxidized simply throughout the course of, which isn’t fascinating. Each hydrogen and oxygen gases at low strain can be utilized in such circumstances.

Including hydrogen will stop the metals from oxidizing whereas the oxygen removes the photoresist. That is one factor we management very tightly throughout wafer ashing, and it requires wonderful temperature uniformity to perform this job.

Working with MEMS gadgets requires the removing of SU–8 or related epoxy–based mostly unfavourable photoresists. A problem with unfavourable photoresists is that components uncovered to UV develop into polymerized, whereas the rest of the movie stays soluble and might be washed away. Furthermore, the chemical stability of SU–8 photoresist could make it tough to take away.

Eradicating SU–8 have to be carried out at decrease temperatures. It’s essential be beneath 100˚C, and in sure instances beneath 50˚C. Extra flexibility within the chemistry can be required, together with probably using fluorine and wonderful management of the temperatures. All of that is a lot simpler to perform with single–wafer processing.

Prospects might have a photoresist on a metallic floor deposited between two metallic surfaces, requiring the removing of the photoresist from the aspect of the wafer. As a result of its isotropic etch property, oxygen–based mostly microwave plasma ashers can take away the photoresist in between the metallic plates, not like RF–based mostly techniques.

Ease of single–wafer automation

In manually loaded techniques, the asher has a pull–out door, the place the wafers lie on the heating or cooling airplane mounted on the entry door of the chamber. In automated techniques, wafers are more and more loaded into the chamber using robotic dealing with.

Right this moment, prospects need to scale back all human elements as chips develop into extra superior. This requires automated dealing with and loading utilizing robotics and full management by a bunch laptop. In some instances, the operator solely wants to position the cassette onto the load port, which is able to begin robotically.

PVA TePla, for instance, has designed its GIGAfab–A plasma system to be configurable for 200– or 300–mm wafers and a cluster device with as much as three course of modules referred to as the GIGAfab Modular. Each techniques use open cassette, in addition to entrance opening or normal mechanical load stations. Wafer processing is thermoelectrically managed from RT to 250˚C. A singular planar microwave plasma supply gives excessive ash charges over a large temperature vary.

PVA TePla’s GIGAfab Modular platform (Supply: PVA TePla)

With wafers changing into thinner, extra dependable automated single-wafer–processing tools handles fragile wafers.

“Making an attempt to deal with the wafers bodily with out using robots can finish poorly,” mentioned Ryan Blaik of PVA TePla in California.

Single–wafer processing additionally gives higher temperature controls.

“With batch processing, microwave radiation should warmth all of the wafers in a quartz boat, and the temperature can fluctuate throughout processing,” Blaik mentioned. “For a single-wafer–processing system, wafers are introduced into the chamber solely after preheating, permitting a continuing temperature to be maintained throughout processing.”

In single–wafer processing, a descum course of might be achieved utilizing the identical device. The first distinction between the 2 processes is the temperature the wafer is uncovered to whereas within the plasma chamber.

For descum, we wish a low ash price and good uniformity and course of management. As a result of we’re solely concentrating on removing of residues, an ashing recipe at very excessive temperatures won’t work. It’s simpler to perform utilizing single–wafer ashing utilizing a microwave–based mostly plasma system.

As extra semiconductor machine fabrication continues to ramp up globally to fulfill an insatiable demand for chips, the necessity for management, effectivity, and configurable options for wafer ashing will proceed because the chips themselves enhance in complexity and reduce in measurement. Automated, single–wafer microwave plasma techniques present chip fabricators with focused and configurable ashing that meets the wants of an rising array of wafer sorts.

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