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Ashing, through which the sunshine–delicate coating referred to as photoresist is eliminated and cleaned from an etched wafer, is without doubt one of the most essential and continuously carried out steps in chip fabrication. On this step, photoresist organics are “burned off” utilizing a processing instrument through which monatomic plasma is created by exposing oxygen or fluorine gasoline at low stress to excessive–energy radio waves. Beforehand, wafer ashing was largely carried out utilizing batch–processing strategies to realize the required throughput.
Nonetheless, in contrast to silicon semiconductors, through which 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 fluctuate 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. Because of this, most compound semiconductor wafer producers require automated, single-wafer–processing instruments able to quick ashing charges and excessive manufacturing ranges.
At the moment, semiconductor producers are more and more on the lookout for a single-wafer–ashing answer 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 improve the DC bias and take away all the things, however RF plasma shouldn’t be as selective in attacking photoresist. Additionally, when the photoresist is eliminated, the underlying layers of the wafer could also be delicate and might be broken with RF.
At the moment, microwave–based mostly plasma instruments produce a really excessive focus of chemically lively species and low ion bombardment power, guaranteeing each a quick ash charge 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–based mostly plasma ashing techniques 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 shouldn’t be at all times suitable with all forms of wafer surfaces; some require a mixture 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 subject, we could add some fluorine chemistry, normally CF4, blended with the oxygen.
Due to the development of utilizing completely different supplies in wafers, some metals are oxidized simply in the course of the course of, which isn’t fascinating. Each hydrogen and oxygen gases at low stress 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 wonderful temperature uniformity to perform this job.
Working with MEMS units requires the removing of SU–8 or comparable epoxy–based mostly adverse photoresists. A problem with adverse photoresists is that components uncovered to UV turn out to be 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. You could be beneath 100˚C, and in sure circumstances beneath 50˚C. Extra flexibility within the chemistry can also 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.
Clients could have a photoresist on a steel floor deposited between two steel surfaces, requiring the removing of the photoresist from the facet of the wafer. Because of its isotropic etch property, oxygen–based mostly microwave plasma ashers can take away the photoresist in between the steel plates, in contrast to 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 aircraft mounted on the entry door of the chamber. In automated techniques, wafers are more and more loaded into the chamber using robotic dealing with.
At the moment, prospects wish to cut back all human elements as chips turn out to be extra superior. This requires computerized dealing with and loading utilizing robotics and full management by a bunch pc. In some circumstances, the operator solely wants to put the cassette onto the load port, which can 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 instrument with as much as three course of modules known as the GIGAfab Modular. Each techniques 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.
With wafers turning into thinner, extra dependable automated single-wafer–processing gear handles fragile wafers.
“Making an attempt to deal with the wafers bodily with out using 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 continuing temperature to be maintained throughout processing.”
In single–wafer processing, a descum course of might be achieved utilizing the identical instrument. The first distinction between the 2 processes is the temperature the wafer is uncovered to whereas within the plasma chamber.
For descum, we would like a low ash charge and good uniformity and course of management. As a result of we’re solely focusing 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–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 improve in complexity and reduce in dimension. Automated, single–wafer microwave plasma techniques present chip fabricators with focused and configurable ashing that meets the wants of an rising array of wafer varieties.
