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May, 2019 Plasma Wafer Processing Tools By Jeff Elliott D
uring manufacturing of integrated circuits (ICs), it is necessary to perform many depo- sition and etching steps to build the internal
structures. Among the most common are ashing, or stripping, to remove photoresist from etched wafers, as well as the “descum” process to remove any residue. Although solvents can be used to remove pho-
toresist in a “wet” process, alternatives that allow for dry processing are often sought out, due to re- ducing chemical handling and acquisition costs. Among the alternatives for dry processing are plasma tools that apply ionized energy for wafer ashing and descum. Advanced and automated plasma
etchers have been available since the ear- liest days of the semiconductor industry. However, plasma processing has been ex- panded to include the removal of polymers, SU-8 removal and sacrificial layers for wafer cleaning, chip carrier cleaning, en- capsulation and flip chip underfill, and for wafer stress relief, chip side healing (CSH) and passivation for ultra-thin wafer tech- nologies. Given the maturity of the market and
the refinement of processing techniques, plasma etchers — at least on the front-end — are relatively standardized, with fixed format, off-the-shelf options. However, with the addition of more back-end applications along with the explosion of MEMs, mi- crofluidic devices, power devices, HB-LED, PLED, and photovoltaics that utilize similar manufactur- ing techniques and tools, more semi-customized so- lutions are now required to adapt to the varying sizes, substrates and other manufacturing chal- lenges.
Plasma Ashing and Descum Plasma ashing is the process of removing the
photoresist by “burning off” the organics. Using a processing tool, monatomic plasma is created by exposing oxygen or fluorine gas at a low pressure to high-power radio waves, which ionize it. This process is performed under vacuum to create a plasma that turns the photoresist to ash. Two forms of plasma ashing are typically per-
formed on wafers. High-temperature ashing, or stripping, is performed to remove as much photore-
however, the choice of tool and semi-customization required is illustrated in selecting units for even these similar processes. PVA TePla America specializes in advanced
plasma systems and has an extensive history in the U.S., Europe and Asia dating back more than 50 years. The company originally started out as In- ternational Plasma Corporation (ICP) and then later through acquisitions and mergers was known as Dionex, Gasonix, Metro Line, and TePla. As mentioned, high-powered radio waves ion-
ize the oxygen or fluorine gas. For photoresist re- moval, this leads to a critical decision be- tween radio frequency (RF) or microwave- based units. When photoresist is removed, the underlying layers may be sensitive and could be damaged if the right choice is not made. “Some devices are sensitive to plasma
damage,” says Nafis. “If the device is not sen- sitive, you can use an RF generator, but if it is then you can use microwaves.” In general, RF-based units are superior for stripping photoresist. RF plasma etches the surface through a physical process achieved by es- sentially bombarding the surface with plas- ma in a specific direction. Microwave-based units, on the other
Plasma systems are used for wafer etching, stripping, cleaning, and surface preparation.
sist as possible, while the “descum” process re- moves residual photoresist. The main difference between the two processes is the temperature the wafer is exposed to while in an ashing chamber. According to Suraiya Nafis, head of sales for semiconductor equipment for PVA TePla America,
hand, are gentler because the plasma appli- cation does not have the same physical as-
pect to it. Instead, microwaves mainly deliver an isotropic chemical approach, which means they permeate everywhere. This also makes it more ideal for remov-
ing material from the underside of items, such as flip chips, that cannot be reached through direct physical bombardment. Microwaves are also specifically useful for removing SU-8, a commonly
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UNIC GV Special features:
• Easy handling • Fast cycle time • No tool change neccessary • Prepared for max. 23,000 ferrules • 5 different cross sections available - change by touchscreen
• Interface for integration in fully auto- matic production lines
AM 02 CK Special features:
• Easy tool change • Short tooling time • Crimp force analyzer • Quality control of crimping ferrules • Crimp quality conforms to DIN 46228
Just in ONE step
Zoller + Fröhlich GmbH Simoniusstrasse 22 88239 Wangen im Allgäu Germany
Tel. +49 7522 9308-0
www.zofre.de
Z+F USA, inc.
700 Old Pond Road Suite 606 Bridgeville, PA 15017 USA
Tel.: +1 412 257 8575
www.zf-usa.com
See at EWPTE, Booth 1138
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