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Stripping Costs From Photoresist Processing
By Ed Sullivan
ring stripping of photoresist. This is the light-sen- sitive material (liquid or film) that is deposited during various steps of wafer production. Reexam- ining the wet process of stripping of thick photore- sist, which occurs at the back-end of wafer pro- cessing, can significantly reduce the amount of chemicals required, as well as related disposal costs.
S Photoresist materials are designed to
mask, or “resist,” UV light to accomplish back- end-of-line tasks, such as the etching and elec- troplating of circuits and copper pillars used as bonding pads for wafer packaging. In recent years, wafer foundries, as well
as semiconductor and compound semiconduc- tor manufacturers, have begun to incorporate copper pillars into their fabrication processes. The advantages of copper over solder have be- come increasingly clear, enabling higher pin counts and interconnect densities. Copper also offers higher reliability and improved electri- cal and thermal performance. It is worth noting that back-end processes
require the use of solvents while front-end-of- line processes typically employ acids, including sulfuric acid and peroxide. These would attack sur- faces, such as copper pillars, and damage them. Al- so, back-end processes use much thicker photore- sist materials, and because the solvents used for back-end stripping are less aggressive, chunks of undissolved resist residue often accumulate in the bath. These chunks can block bath circulation and filtration, shortening bath life and increasing sol- vent chemistry consumption substantially.
JST Manufacturing’s semiconductor processing applications lab.
stemmed from the chemical bath tool that was in- tegral to the stripping process. “Our customer, an amplifier manufacturer,
was dealing with a 50 to 100 micron thick resist film on its wafers, about 15 times thicker than re- sist used on front-end processes,” explains Ryan Zrno, chief technical officer of JST Manufacturing,
ilicon and compound semiconductor wafers undergo many critical procedures during the microfabrication process, including the recur-
Evaluating Wet Processing Challenges After deciding to adopt copper pillars for in-
house wafer production, one wafer manufacturer ran into some obstacles. Two situations caused the manufacturer to rethink the process by which it stripped the “thick” resist from its wafers during the copper pillar attachment process. Both issues
a specialist in wet processing equipment for the MEMS, nano, photovoltaic, wafer, and related in- dustries. “The traditional solvent chemistry was leaving large amounts of chunky resist residue in the bath, which was interfering with both circula- tion and filtration. This was causing increased bath changes, resulting in production delays and excessive use of expensive chemical solvents.” Zrno adds that the customer also wanted
to find an alternative to the solvent that had been used in the past, one based on TMAH (tetramethylammonium hydroxide). Although not used in toxic amounts, this solvent had a strong odor and was not the most effective chemical for resist stripping of wafers with cop- per pillars. To evaluate the problems and propose a
solution, Zrno invited the customer to visit JST’s applications lab and run some tests with the engineering staff. The company also coordi- nated a series of tests with appropriate sol- vents made to order from Diamalloy, the cus- tomer’s chemical supplier.
Tailoring a Solution “We ran a set of three different kinds of
tests, each in our standard down-flow bath tool,”
Zrno says. “Each time we would learn something valuable about possible solutions. It was a three- way development team composed of the customer’s staff, the chemical company and our engineers.” After a few weeks of testing, the proposed so-
lution was a new wet processing tool that did not leave large deposits of solubilized resist in the bath. Instead, a new chemistry was recommended along with a series of screens that were incorporat-
Continued on next page
February, 2018
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