INSTRUMENTATION • ELECTRONICS
Silicon and compound semiconductor wafers undergo many critical procedures during the microfabrication process
Zrno adds that the customer also
solvent that had been used in the past. Although not used in toxic amounts, this stripping around wafer with copper pillars.
tool,” Zrno says. “Each time we would learn something valuable about possible solutions. It was a three-way development chemical company and our engineers.” After a few weeks of testing, the
A redesigned wet processing tool
Stripping the costs out of photoresist processing S
ilicon and compound semiconductor wafers undergo many critical procedures during the microfabrication process, including the
recurring stripping of photoresist, the is deposited during various steps of wafer production. Re-examining the wet process of stripping of thick photoresist, which occurs at the back-end of wafer processing, can considerably reduce the amount of chemicals required, as well as related disposal costs. Photoresist materials are designed to
mask, or ‘resist’ the UV light to accomplish back-end-of-line tasks such as the etching and electroplating of circuits and copper pillars used as bonding pads for wafer packaging. In recent years, wafer foundries as
well as semiconductor and compound semiconductor manufacturers have begun to incorporate copper pillars into their fabrication processes. Back-end processes require the use of solvents while front- end-of-line processes typically employ acids such as sulphuric acid and peroxide. These would attack surfaces such as copper
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pillars in a destructive manner. Also, back- end processes use much thicker photoresist materials and because the solvents used for back-end stripping are less aggressive, chunks of un-dissolved resist residue often accumulate in the bath.
EVALUATING WET
PROCESSING CHALLENGES After recently deciding to adopt copper pillars for the wafers it produces in-house, a wafer manufacturer unexpectedly ran into some production obstacles. Both hurdles were connected directly to the chemical bath tool that was integral to the stripping process.
manufacturer, was dealing with a 50-100 about 15 times thicker than resist used on front-end processes,” explains Ryan Zrno, CTO of JST Manufacturing. “Using the traditional solvent chemistry was leaving large amounts of chunky resist residue in the bath, which was interfering with causing increased bath changes resulting in production delays and excessive use of expensive chemical solvents.”
proposed solution was a new wet processing tool that did not leave large deposits of solubilised resist in the bath. Instead, a new chemistry was recommended along with a series of screens that were incorporated into quick-dump exchanges. This meant that the bath solvent
chemistry was circulated in such a manner any large clusters of resist before they could become totally solubilised and bath solvent. Removal of resist clusters also meant that they were no longer a threat to The next step was to build a test
module, which included a bath with a single series of screens, a reservoir, basic control system, and a pump. “Once completed, the customer came back out and we did testing again,” Zrno says. “After successful testing of the module, JST designed and built a fully automated production tool featuring a 6-gallon bath and 20-gallon reservoir. We also added other proprietary components that enabled the tool to meet the customer’s production requirements.” The customer ordered two of the new
production tools to run parallel processes and meet throughput requirements. Chemical usage dropped by two-thirds
at the customer’s resists stripping stations, mainly due to the increased bath life. Also requirements for changing of baths, which normally took 30 to 60 minutes. Those were also reduced by two-thirds, as was the associated downtime to drain and recharge bath solutions.
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