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Ashing, by which the sunshine–delicate coating generally known as photoresist is eliminated and cleaned from an etched wafer, is among the most necessary and often 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 gasoline at low strain to excessive–energy radio waves. Beforehand, wafer ashing was largely carried out utilizing batch–processing strategies to realize the required throughput.
Nonetheless, not like silicon semiconductors, by 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 elimination 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.
Right this moment, semiconductor producers are more and more in search of a single-wafer–ashing resolution for each excessive–temperature photoresist elimination 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 way of a bodily course of that primarily bombards the floor with plasma in a particular course.
Prior to now, you would merely improve the DC bias and take away all the things, 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 could possibly be broken with RF.
Right this moment, microwave–primarily based plasma instruments produce a really excessive focus of chemically energetic species and low ion bombardment power, guaranteeing each a quick ash fee and a harm–free plasma cleansing.
Microwave tends to be faster and produces larger ash charges than RF.
Focused photoresist elimination 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 will not be all the time suitable with all kinds of wafer surfaces; some require a mixture of gases.
There will be different supplies on or throughout the photoresist that can not be stripped away fully with simply oxygen alone. To resolve this challenge, we might add some fluorine chemistry, often CF4, blended with the oxygen.
Due to the pattern 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 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 process.
Working with MEMS gadgets requires the elimination of SU–8 or related epoxy–primarily based detrimental photoresists. A problem with detrimental photoresists is that elements uncovered to UV develop into polymerized, whereas the rest of the movie stays soluble and will 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 circumstances beneath 50˚C. Extra flexibility within the chemistry can be required, together with doubtlessly 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 elimination of the photoresist from the facet 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, not like 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.
Right this moment, 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 bunch laptop. In some circumstances, the operator solely wants to position 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 device with as much as three course of modules known 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 offers excessive ash charges over a large temperature vary.
With wafers changing into thinner, extra dependable automated single-wafer–processing tools handles fragile wafers.
“Attempting 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 offers 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 will 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 fee and good uniformity and course of management. As a result of we’re solely focusing on elimination 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 system fabrication continues to ramp up globally to satisfy 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 varieties.
