MONOLITHIC MEMS VACUUM VALVES FOR MINIATURE CHEMICAL PRE-
C. Baker1, M-A. Schwab1, R. Moseley1 , R.R.A. Syms1,2 and E.M. Yeatman1,2
1Microsaic Systems Ltd., Woking, Surrey, UK
2Imperial College London, London, UK
A monolithic pneumatic valve is reported for use in a
miniature chemical pre-concentrator. The valve comprises
a perforated diaphragm above a substrate with offset
perforations. The diaphragm is closed electrostatically, can
be coated with adsorbent material for collecting the analyte
of interest, and heated ohmically to desorb the analyte into
the analytical system. The valve supports a high flow rate
when open, along with the ability to maintain closure
against over one bar of pressure, allowing its use with
vacuum based instruments such as mass spectrometers. The
fabrication process is described, and pneumatic and
thermal performance are reported.
Mass spectrometer, pneumatic valve, bonded silicon
Preconcentrators are well established devices for
enhancing the sensitivity of detection instruments. A
typical preconcentrator comprises a porous or perforated
structure which inherently, or by the addition of a suitable
coating material, absorbs the substance of interest from a
sampled gas flow. The material is then desorbed, usually
by heating, into a much smaller gas volume than that
sampled, and the resulting concentrated sample is injected
into an analysis instrument.
MEMS provides a suitable method for producing
miniaturized pre-concentrator structures, and a number
have been reported. Sandia National Laboratories have
developed coated diaphragm pre-concentrators with
integrated heating elements, which allow very rapid
desorption, and more recently have also reported flow-
through devices which increase the adsorption surface area
. These devices were designed for use with micro-
fabricated gas chromatography columns, as were the
devices of Zellers et al. . Martin et al. developed a
preconcentrator based on a perforated Si diaphragm, and
demonstrated its operation as an input to an ion mobility
spectrometer . The flow of the sampled and injected
gases must be controlled and timed, typically using valves.
If these are also micro-engineered, a very small dead
volume can be achieved, maximizing the concentration
ratio of the device .
By integrating the valve and pre-concentrator into a
single structure, the ultimate miniaturization can be
reached. Fig. 1 illustrates our device concept: a perforated
silicon diaphragm floats above a substrate with offset
perforations, such that pulling the diaphragm down
electrostatically closes the valve. The diaphragm can be
coated with a patterned absorbent material, and heated for
desorption by passing a current laterally across it.
Flow Adsorbing layer Membrane
Figure 1: Integrated pre-concentrator valve in (top to bottom)
sampling, closed and desorption states.
By assembling four of these devices in the
configuration shown in Fig. 2, with one valve pair
controlling the sampled inlet and exhaust, and the other the
flush gas (if required) and injection, a complete pre-
concentrator is achieved.
Figure 2: Four-valve pre-concentrator assembly.
In the sampling mode, inlet and pump valves are open,
the other valves are closed, and air is drawn through the
device at high rate. During this sampling phase it may be
necessary to heat the overall device to a modest level to
reduce adsorption on surfaces other than the inlet valve
coating. During the analysis phase, inlet and fan valves are
closed and the detector valve is open, the inlet valve
diaphragm is heated to release the analyte, which is then
drawn into the detector either by diffusion or by provision
of a carrier gas through the purge inlet.
The requirements for the valves depend strongly on the
analysis instrument. For ion mobility spectroscopy (IMS)
or gas chromatography-mass spectrometry (GC-MS), as
used with previously reported MEMS pre-concentrators,
the instrument inlet is at atmospheric pressure, so that the
valves will not usually need to withstand a large
differential pressure. Recently,
spectrometers (QMS) have been miniaturized using MEMS
approaches , and interfacing pre-concentrators directly
to these is highly attractive. Since QMS operates at
vacuum, this requires that the valves in the system of Fig. 2
can be held closed against one bar of differential pressure
with low leakage. Also, to allow a large air volume to be
sampled, the same valves must allow a high flow rate at an
acceptable driving pressure.
The MEMS valves are generated with a two-mask
process at wafer scale. The process is summarized in
Figure 3a to 3f. The starting point is a double side polished
bonded silicon-on-insulator (BSOI) wafer with a device
layer of thickness 30 µm and with low resistivity. An oxide
thickness of 4 µm separates the device layer and substrate
handle layer. Substrates of thickness 500 µm and again of
low resistivity were used. The BSOI wafer is initially
thermally oxidized to grow about 1 µm SiO2 on the device
and handle layers (Fig. 3a). Photoresist is then spin coated
and patterned consecutively on the two sides (Fig. 3b), and
the resist is used to pattern the oxide layer by dry etching.
These patterns are transferred into the corresponding
silicon layers using a Surface Technology Systems Single
Chamber Multiplex inductively coupled plasma (ICP)
etcher, operating a variant of the cyclic etch-passivate
process based on SF6 and C4F8 developed by Robert Bosch
GmbH  (Fig. 3c). The resist mask is then removed using
a commercial wet resist stripper followed by oxygen
plasma. Front to back alignment is carried out using a
Quintel IR-4000 mask aligner with through-wafer infrared
Once the valves have been patterned on both sides, the
die are singulated. The buried oxide is then removed by HF
vapor etching using a commercially available system .
Once complete, the device handle layers are physically
released from each other to form the basic structure of the
gas valve (Fig. 3d). Electrical isolation between the device
and handle layers is achieved by re-oxidizing the device at
1100°C for 48 hours (Fig. 3e). Finally, contact areas to
both layers are patterned into the oxide, and electrical
contacts are attached (Fig. 3f).
Figure 3: Fabrication process steps as described in the text.
Figure 4: Diaphragm surface with adsorbent polymer coating.
Fig. 4 shows a completed valve from the diaphragm
side. A polymer adsorbent layer has been patterned in the
central region between the perforations by inkjet
deposition. Fig. 5 shows a mounted device on a printed
Figure 5: Valve mounted on printed circuit board.
Figure 6 shows the high pneumatic performance
achieved. Devices were mounted in a pneumatic test rig
with flow and pressure measurement, and the flow rates
measured as a function of differential pressure for open and
closed states. In the closed state, the valve leakage is below
the limit of measurement of 0.01 sccm, for differential
pressures up to 1.1 bar. In the open state, flows up to 10
l/min are obtained for 500 mbar forward pressure, yielding
an open to closed flow ratio of at least 106. This can be
compared to a flow of 0.3 l/min at 500 mbar in . About
100 mbar forward pressure is required in the open state to
achieve substantial flow. In both open and closed states the
valves are oriented so that the differential pressure tends to
In a simple approximation, the maximum concentration
factor achievable in a pre-concentrator is given by the total
sampled volume divided by the “dead volume” of the
device. In practice this is limited principally by two
considerations: firstly, the collection efficiency for the
analyte of interest will be below 100%, and secondly, the
volume or surface area of sorbent material will place an
absolute (saturation) limit on the amount of analyte that
can be collected, so consequently the concentration factor
cannot be increased limitlessly simply by extending the
sampling time. For a device integrating four valves of the
type presented here, with sub-mm inter-valve spacings, an
enclosed volume of about 0.1 cc should be achievable,
which then sets the minimum dead volume. For one minute
of sampling at 10 l/min, this suggests a maximum volume
ratio as high as 105, which could in principle provide a
concentration factor above 104 if saturation is not reached.
Insensitivity to dust contamination has also been
demonstrated. To characterize the trapping of dust particles
by the device, a controlled flow of air from an office
environment was presented to a particle size analysis
instrument, with and without the valve in the flow. As can
be seen in Figure 7(a), a significant fraction of particles
above 1 µm are trapped in the device. The pneumatic
performance of otherwise equivalent devices was then
measured for nominally clean valves (indicated as CR),
and valves having passed 3 and 30 liters “dirty” air. As can
be seen in Figure 7(b), no change in open flow rate, or
closed leakage, was detected, despite the presence of
trapped dust particles.
Figure 6: Pneumatic performance of valve in open (o) and closed
0200 400 6008001000 1200
Clean room valve open
Clean room valve closed
After 1 minute in dusty atm open
After 1 minute in dusty atm closed
After 10 minutes in dusty atm open
After 10 minutes in dusty atm closed
Figure 7: (a) Dust particle size distribution for air before and
after passing through valve; (b) flow vs. pressure for valves
before and after passing dusty air as indicated.
Thermal performance was also evaluated. Figure 8
shows a thermal camera image of the diaphragm with the
heating Download full-text
uniformity is achieved, and the target desorption
temperature of 170ºC could be reached across most of the
surface. Uniformity is acceptable across most of the
surface, although lower temperatures are seen at the edges
in particular. Altering the geometry offers the possibility of
improving the temperature uniformity by altering both the
electrical current flow and the heat flow across the
current supplied. Reasonable temperature
Figure 8: Thermal image of valve with heating current applied.
The thermal response time was also measured, as
shown in Figure 9. Initially a high fixed heating current is
applied; the current is then stepped down in increments at 5
second intervals. In this case the target temperature of
180ºC is reached in about 5 s, and the difference between
the spatial maximum and average temperatures for the
diaphragm is about 10ºC. Closed-loop control of heating
should allow both more rapid stabilization, and reduction
of the overshoot amplitudes.
Figure 9: Thermal response of heated valve.
In summary, monolithic pneumatic MEMS valves have
been demonstrated, with integrated surface sorbent coating
and surface heating capability. Compatibility with vacuum
interfaces is shown by the low leakage under a forward
bias above one bar, and a high flow rate when open, for
forward pressures above 100 mbar, is achieved. Ohmic
heating provides diaphragm temperatures up to ≈ 180ºC
within several seconds, with acceptable spatial uniformity.
Further work is focused on demonstrating full pre-
concentrator functionality for various analytes and
analytical instrument types.
We are grateful to Jen Stepnowski and Andrew McGill
of the Naval Research Laboratory for assistance with
polymer coating. This work was supported by the UK
Ministry of Defence under the MEAD programme.
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