Opinion: Exploring Automated Single-Wafer Ashing of Compound Semiconductors


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Ashing, wherein the sunshine–delicate coating referred to as photoresist is eliminated and cleaned from an etched wafer, is among the most vital and continuously carried out steps in chip fabrication. On this step, photoresist organics are “burned off” utilizing a processing software wherein monatomic plasma is created by exposing oxygen or fluorine gasoline at low strain to excessive–energy radio waves. Beforehand, wafer ashing was largely carried out utilizing batch–processing strategies to attain the required throughput.

Nevertheless, in contrast to silicon semiconductors, wherein wafers are mass–produced in an ordinary 300–mm dimension, compound semiconductors are made from silicon carbide, gallium nitride, gallium arsenide, and sapphire, which might range from 100 to 200 mm. When that is the case, considerably higher uniformity of photoresist removing is required, which suggests 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.

At this time, 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 via a bodily course of that primarily bombards the floor with plasma in a particular course.

Prior to now, you could possibly merely improve the DC bias and take away every little thing, however RF plasma is just not 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.

At this time, microwave–primarily based plasma instruments produce a really excessive focus of chemically lively species and low ion bombardment power, making certain each a quick ash fee and a harm–free plasma cleansing.

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

Focused photoresist removing utilizing oxygen

Superior microwave–primarily based plasma ashing methods from producers like PVA TePla usually make the most of oxygen as the first course of gasoline. 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 is just not at all times suitable with all forms of wafer surfaces; some require a mix of gases.

There might be different supplies on or throughout the photoresist that can’t be stripped away fully with simply oxygen alone. To resolve this problem, we could add some fluorine chemistry, often CF4, combined with the oxygen.

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

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

Working with MEMS gadgets requires the removing of SU–8 or related epoxy–primarily based damaging photoresists. A problem with damaging photoresists is that elements 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 should be carried out at decrease temperatures. You might want to be under 100˚C, and in sure instances under 50˚C. Extra flexibility within the chemistry can also be required, together with doubtlessly the usage of fluorine and glorious management of the temperatures. All of that is a lot simpler to perform with single–wafer processing.

Prospects could have a photoresist on a metallic floor deposited between two metallic surfaces, requiring the removing of the photoresist from the aspect of the wafer. Because of its isotropic etch property, oxygen–primarily based microwave plasma ashers can take away the photoresist in between the metallic plates, in contrast to RF–primarily based methods.

Ease of single–wafer automation

In manually loaded methods, 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 methods, wafers are more and more loaded into the chamber using robotic dealing with.

At this time, prospects wish to cut back all human components as chips develop into extra superior. This requires computerized dealing with and loading utilizing robotics and full management by a number laptop. In some instances, the operator solely wants to put the cassette onto the load port, which can begin mechanically.

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

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

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

“Attempting to deal with the wafers bodily with out the usage of robots can finish poorly,” stated Ryan Blaik of PVA TePla in California.

Single–wafer processing additionally supplies 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 stated. “For a single-wafer–processing system, wafers are introduced into the chamber solely after preheating, permitting a relentless temperature to be maintained throughout processing.”

In single–wafer processing, a descum course of might be completed utilizing the identical software. 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 fee 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 is not going to work. It’s simpler to perform utilizing single–wafer ashing utilizing a microwave–primarily based 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 improve in complexity and reduce in dimension. Automated, single–wafer microwave plasma methods present chip fabricators with focused and configurable ashing that meets the wants of an rising array of wafer sorts.


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