FEATURE MEMS
Micro-isotropic etching for MEMS and optical MEMS devices
Yannick Pilloux at Plasma-Therm discusses how the latest process achieves MEMS trench sidewall smoothing through dry plasma High Density Radicals Flux (HDRF) technology
T
he semiconductor industry has been focused on dry etch and deposition
technologies for decades, while cleaning technologies have mainly focused on wet technology. Optical devices as well as MOEMS for optical Mems, require the sidewall profile of the device to be smooth in order to keep a good quality of optical characteristics. While the industry is currently using thermal oxidation, dry plasma etching is seen as an “innovative” technology to smooth the device sidewall. In response to market demand, Plasma-
Therm’s dry plasma technology called: HDRF (High Density Radicals Flux), provides an efficient cleaning solution without damaging sensitive layers like GaN and TiN, and keeps low temperature processing. In addition, HDRF technology is able to smooth scalloping after DRIE Bosch processing by keeping the characteristics of MEMS devices, as well as low temperature processing for Via last approach. Dry plasma cleaning/stripping was
introduced to the market many years ago with barrel Asher technology, for which wafers are located in the middle of the plasma chamber and are in contact with ions energy & photons. This effect can create damage on sensitive layers like GaN, TiN, as well as damaging sensitive devices by having an electrical charging effect. Initially, HDRF technology was
introduced to strip and clean any organic layers or residues. This unique technology, ICP (Inductive Couple Plasma) source is an ideal solution for low damage, low temperature photo-resist stripping without electrical charging effect. HDRF is a high density plasma source higher than 1E17 cm-3
density, while
conventional ICP source is in the range of 1E13 cm-3
Figure 1: . Such high plasma density allows
HDRF to clean any organic layer or residues in high aspect ratio structures up to 30:1. HDRF plasma characteristics are based on
O* radicals free of ions while conventional Asher has ions and photons. High plasma density of O* radicals allows the cleaning of organic layers and/or to remove the photo-resist for multiple applications in LED, MEMS, Photonics & wireless markets
16 AUTUMN 2017 | MICROMATTERS
Figure 1: Micro moulded parts
Micro moulded parts
still limitation in high aspect ratio structure, and/or when it is thick polymers. Due to HDRF high plasma density,
polymers are removed in any structure with O* radicals plasma, efficiency is measured by Energy Dispersive X-ray Spectroscopy (EDX) (“Fig. 1”) for a strip rate 10 times higher than conventional wet clean.
without damaging the device. Damage is identified as electrical charge effect, which can generate a collapse of the Mems membrane, for example. While shaping MEMS & MOEMS devices
Figure 2:
is commonly done by using Deep Reactive Ion Etching (DRIE) technology and the Bosch process, which is an alternative process of isotropic etching and sidewall passivation steps (called loop); Alternative Bosch processing generates scalloping, identified as peaks and valleys on the sidewall structure, having a polymer layer of 1 to 3µm thick on the etched sidewall. The Bosch process, by definition is a
Cleanroom facility
polymerised process, adding polymers on the sidewall which are C* and F* (Carbon and Fluor) to be removed mainly before packaging. While traditional wet technology can remove polymers, there is
Figure 1: Bosch polymers removal
Figure 2: Scallop smoothing
MOMEMS APPLICATIONS Then, for MOEMS applications as well as TSV (Though Silicon Via) for 3D interconnect, a step of smoothing the sidewall scallops is required to facilitate a conformal dielectric deposition on the Silicon sidewall and to avoid electrical leaks or arcing between Silicon and copper Via. Typical scalloping size after DRIE is from 80nm to 1.5µm. By using fluorine based chemistry, HDRF can smooth scallopings in high aspect ratio structure 30:1. It is called micro-isotropic etch steps, which is to etch mainly peaks without impacting structure dimension called CD loss (Control Dimension Loss). Scalloping from 50 to 800nm (peak to valley) can be reduced to few nanometre by HDRF plasma, within less than three minutes (Figure 2). The Semiconductor industry started to reduce die size, to add more dies per wafer, for which cleaning quality needs to be improved. While wet technology has limitations in high aspect ratio structures, high plasma density named HDRF can clean polymers, even in complex 30:1 aspect ratio structures. In addition, while low temperature processing is required for cleaning/ stripping, HDRF is a perfect fit to avoid device damage, due to low charging effect. Finally, Bosch polymer removal after DRIE can be cleaned by HDRF, followed by micro-isotropic etching to smooth scalloping. More information about the systems and technologies discussed here are available on the Plasma-Therm website.
Plasma-Therm
www.plasmatherm.com e:
sales@plasmatherm.com
/ MICROMATTERS
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