Medical Design Briefs - December 2022 - 32

Auster_W03_19T_43min_ER:500nm/min
blocks of 5° and 25 °C. A full
factorial DOE is a simple system350
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Field
Clear
Undercut
Pump out of
remaining
SiF4
atic design style that allows for
estimation of main effects and
interactions. The etch rate and
uniformity were established by
a 10-point measurement of the
oxide undercut over the device
wafer to calculate the etch rate
for a fixed etch time. The uniformity
was calculated as half
the range of the etch rate divided
by the average etch rate,
expressed as a percentage. The
DOE was repeated three times
to improve the statistical significance
of the results.
Time (sec)
Fig. 2 - NDIR process monitor trace showing the characteristic etching stages.
With this configuration, multiple connected silicon dioxide
(SiO2) or glass layers can be etched in a single process step, and
a final hydrophobic SAM coating can be applied, without a vacuum
break, to prevent stiction or device reliability issues. This
means that the device will guarantee reliable long-term performance
in a high-humidity environment. The other key benefits
of a single-wafer processing tool like the ORBIS 3000 are that
the within-wafer uniformity can be repeatably controlled, and
the process is not subject to the wafer-to-wafer uniformity issues
observed with batch processing tools (which are further compounded
by varying batch sizes).
To develop the new MEMS sensor devices, Hahn-Schickard
first designed a now-patented MEMS device with a moveable
shuttle, gearwheel, and driving and escapement rods. These
devices were then fabricated with a surface micromachining
process using a 2-µm oxide silicon-on-insulator (SOI) wafer
substrate with moveable silicon component parts. The device
was perforated with a resist-patterned silicon deep reactive ion
etching (Si DRIE) process, allowing access for a single-step
v-HF release process to realize the functioning device. (Note: A
hydrophobic SAM coating using perfluorodecyltrichlorosilane
(FDTS) can be optionally applied to ensure that the device is
repeatably fully stiction free.)
The device operates with a shape memory alloy (SMA) wire
bending activated by a phase change over the temperature
range of the sterilization cycle, which moves the push rod to
the next tooth of the gearwheel, thus counting one sterilization
cycle. Different design variants of the gearwheel were also designed
and tested with an on-wafer simulated sterilization temperature
cycle using a hot plate.
n Experimental Procedure
The v-HF process used by Hahn-Schickard was developed
by completing an initial screening experiment, followed by
a full three-level factorial design of experiments (DOE) for
HF flow and process pressure over two chuck temperature
32
The process had an etch rate
range between <100 and >900
nm/min and uniformities varying
between 70 and 95 percent.
The uniformity shows more temperature
dependence at 25 °C. A
chosen optimal process at 25 °C used a high HF flow and high
process pressure and yielded an etch rate of 900 nm/min and
95 percent uniformity. Higher pressure improves both the
etch rate and uniformity. Aluminium (Al) is present on the
devices for labeling, and this does not appear to be damaged
by the v-HF processing. The fully released device gearwheel
with a 20-µm undercut beyond the Si contours is shown in
Figure 1.
The ORBIS 3000 tool used by Hahn-Schickard allows effective
process monitoring of any v-HF etch process on the
memsstar ORBIS or ALPHA™ process chambers. The tool was
equipped with a nondispersive infrared (NDIR) process etch
monitor, which can be used not only to monitor the progress
of the etching process but also, in some cases, to control an
end-pointed process. In this case, a typical undercut process
etch monitor trace was observed where there was an initial
field oxide etch step with a strong silicon tetrafluoride (SiF4)
signal, which took about 10 minutes to complete. Next came
the longer undercut etch to release the device, which took
around 30 minutes to complete. The result achieved was the
desired 20-µm oxide undercut (see Figure 2).
n Conclusion
A medical instrument equipped with a device such as the
one Hahn-Schickard developed utilizing the memsstar ORBIS
system can record the number of sterilization cycles individually,
and the MEMS sterilization cycle counter operates
without the need of batteries. The function of the device has
been verified within an autoclave demonstrating the feasibility
of the concept. Current work focuses on manufacturing
aspects in order to obtain robust counter devices with reproducible
characteristics.
This article was developed from a paper presented at
MikroSystemTechnik Kongress 2021, November 8-10, 2021,
Stuttgart-Ludwigsburg. For more information, visit https://
www.memstarusa.com.
www.medicaldesignbriefs.com
Medical Design Briefs, December 2022
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Medical Design Briefs - December 2022

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