Total Dose Degradation of MEMS Optical Mirrors

T. F. Miyahira, *Member, IEEE*, H. N. Becker, S. S. McClure, L. D.
Edmonds,

A. H. Johnston, *Fellow, IEEE*, and Y. Hishinuma

*Abstract* -- **This paper discusses the effect of ionizing radiation on
two types of deformable MEMS mirrors. Little effect was observed in the
technology that was based on electrostatic deflection, consistent with
the structural design that does not contain insulators between the two
sections. Significant changes in the operating characteristics were
observed for the second type of mirror, which uses piezoelectric
material for actuation. The mirrors required higher total dose levels
before they were affected compared to MEMS accelerometers, which can be
explained by the larger interelement spacing used in the mirror
arrays.**

I. Introduction

There are many possible implementations of microelectromechanical (MEMS)
devices, including cantilevers, diaphragm assemblies, mirrors, springs,
and even motors. Many of these are of interest for space applications.
To date, however, radiation studies have been done on only a limited
number of MEMS structures \[1-5\]. The results of the earlier work have
shown that charge trapping in insulators within various MEMS devices can
cause a shift in the voltage required to activate mechanical motion, as
well as "stiction". Most MEMS devices are significantly degraded at
total dose levels between 30 and 100 krad, unless they are fabricated
without dielectric materials between the regions that are mechanically
actuated \[5\].

This paper reports the results of total dose degradation on deformable
optical mirror arrays that can be deflected by applying an external
voltage. Devices of this type are proposed for use in ultra-large
lightweight space telescopes \[6\]. The purpose of the present study is
to determine how mirror arrays of this type fabricated in different
processes are affected by space radiation, as well as on more general
insight into the susceptibility of optical MEMS devices to radiation.

\- - - -

Manuscript received July 21, 2003; revised September, 2003.

T. F. Miyahira, H. D. Becker, L. D Edmonds, A. H. Johnston and Y.
Hishnuma are with the Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA.

S. S. McClure is with Northrop Grumman, Los Angeles, CA.

This work was carried out at the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics
and Space Administration under the NASA Electronic Parts and Packaging
Program, Code Q.

II\. Description of Devices

A. Electrostatic Mirror Assemblies

The first type of device studied in this work was a commercial MEMS
deformable mirror array manufactured by Boston Micromachines Corporation
(BMC). The arrays are fabricated using silicon micromachining techniques
with structural silicon and sacrificial oxides. As shown in Fig. 1, they
consist of two polysilicon membranes, separated from each other by a
thin airgap. The top membrane provides the mirror surface. The lower
membrane is f![](media/image1.wmf){width="3.501388888888889in"
height="1.332638888888889in"}ixed at the edges to the substrate. The two
membranes are attached at the center with a silicon-dioxide spacer, 5 μm
thick. The lower membrane can be deflected by the electrostatic
attraction that results from applying a voltage to the lower electrode,
which induces an image charge in the membrane. Electrodes at the bottom
layer, formed on a silicon wafer isolated with a one-micron layer of
nitride, are individually addressable. Voltage is applied to the
individual lower electrodes to achieve the desired mirror contour. The
upper membranes are both grounded. Note that although this structure
contains silicon-dioxide, it is not present in the regions between the
top and bottom electrodes.

Fig. 1. Physical diagram of the membrane-based mirror segment of the
Boston Micromachine device. It is electrostatically activated.

The deflection of the mirror varies with distance from the edge, where
the "anchor" membrane is attached to the substrate. The mirrors were
designed for a typical center deflection of approximately 1.5 micron at
a deflection voltage of 140 V. The maximum voltage that can be applied
is 250 V. The peak deflection at the center of the mirror is a key
parameter for mirror applications. The deflection is nonlinear with
voltage, as shown in Fig. 2.

![](media/image2.wmf){width="3.45in" height="2.3513888888888888in"}

Fig. 2. Dependence of maximum deflection at the center of the mirror
membrane on applied voltage for a typical BMC micromirror.

These mirrors deflect in a highly nonlinear manner, and behave more like
a stretched membrane than a plane mirror. A typical deflection contour
for a mirror segment is shown in Fig. 3.

![](media/image3.png){width="3.2368055555555557in" height="2.21875in"}

Fig. 3. Typical deflection depth profile for a segment of the BMC mirror
with 140 V applied.

B. Piezoelectric Activated Mirror

Deformable mirrors that are activated by piezoelectric elements were
developed by JPL in conjunction with Pennsylvania State University, and
were included in the study even though they are not commercially
available. While the target applications for this device are the same as
for the BMC device, the JPL device has a completely different process
and structure. Rather than being electrostatically driven, mirror
segments in this device are controlled by a piezoelectric actuator.
Further the JPL device is fabricated using a wafer bonding silicon
membrane transfer technique. The resulting cross sectional structure,
shown in Fig. 4, consists of a silicon mirror membrane, connected by an
indium post to the controlling piezoelectric membrane. This membrane is
lead zirconium titanate (PZT) on silicon nitrite. The moving structure
is supported on silicon posts.

![](media/image4.png){width="3.2493055555555554in"
height="2.1798611111111112in"}

Fig. 4. Cross section of the JPL/Penn State piezoelectric deformable
mirror.

This device is controlled by applying a voltage across the piezoelectric
membrane. Flexure of this membrane moves the mirror surface. One
advantage of this structure is that the resulting device has a much
larger deflection sensitivity than the typical BMC device, operating at
much lower voltage.

The dependence of deflection on voltage for a typical piezoelectric
mirror segment is shown in Fig. 5. It is linear at low voltages, but
becomes sublinear at higher voltage.

![](media/image5.wmf){width="3.3756944444444446in"
height="2.526388888888889in"}

Fig. 5. Dependence of deflection on voltage for a typical PZT mirror.

III\. Experimental Procedure

Measurements of the peak deformation depth of the BMC micromirrors were
made using a Wyko model RST Plus Optical Profiler. This device scans the
surface of the device measuring the deflection by counting interference
fringes, a delicate experimental procedure. The maximum deflection
occurs at the center of the mirror. This deflection was measured at a
specific drive voltage by performing difference scans between the 0V and
140V applied condition. The control voltage was applied using a mirror
drive system provided by BMC. This is a time-consuming process,
requiring careful attention to alignment of the mirror assembly with the
laser measurement apparatus. The maximum voltage that can be applied to
the BMC device is 250 V. A voltage of 140 V, which produces a deflection
of approximately 1.5 µm, is typical of many applications, and
measurements at that voltage were used to determine how the mirror
actuation was affected by radiation.

A similar experimental approach was used for the PZT actuated
micromirrors, measuring them with the same optical profiling system. A
voltage of 20 V, 2/3 of the maximum operating voltage, was used as a
reference point for radiation characterization. The repeatability of the
measurements was about 0.02 µm. Two scans were made during each
measurement, averaging the results.

Figure 6 shows the results of a typical scan of one segment of the
mirror array. The deflection is measured at the center of the mirror
assembly. The left part of the figure shows the location of the scanned
mirror within the overall mirror array. The scan at the right is
produced by the Wyco measurement apparatus, which counts interference
fringes at each point in the scan.

![](media/image6.wmf){width="3.3680555555555554in" height="1.46875in"}

Fig. 6. Experimental results of a scan of the piezoelectric mirror with
the interferometer.

The irradiations were done with cobalt-60 gamma rays at the JPL
irradiation facility. The test samples were split into two groups. One
group was biased during irradiation, while the other was unbiased (all
pins at ground). Samples were removed after successive irradiations for
measurement. Control devices were tested before each set of
measurements.

IV\. Experimental Results

A. BMC Mirrors

For the BMC mirrors, testing was performed on two groups with five
mirror segments each, all located on a single device. One group of
segments was irradiated without bias (electrodes at ground), while the
other was irradiated with a deflection voltage of 140 volts. The device
was removed after each exposure run temporarily removing bias from the
segments that were biased, and measured with the optical profiler. This
required about one hour between successive irradiations.

Total dose testing of biased and unbiased mirror segments was done in
several steps to a maximum of 3 Mrad. Deflection data for both of the
test groups indicated no significant effects due to radiation. Data for
the biased devices, with measurement error bars is shown in Fig. 7.
Similar results were obtained for the unbiased segments. Although there
is a slight change at lower total dose levels, the change is within the
measurement accuracy (2%), as determined from measurements on the
unirradiated control samples. Thus, the radiation test results for the
BMC mirrors essentially produce a null result, even after irradiation to
very high total dose levels.

![](media/image7.wmf){width="3.45in" height="2.3513888888888888in"}

Fig. 7. Change in mirror deflection due to radiation for biased segments
of the BMC mirror array.

B. Piezoelectric Mirrors

A bias voltage of 20 V was applied to samples of the piezoelectric
mirrors that were irradiated in a biased condition. Additional samples
were irradiated without bias (all pins at ground). Typical results for
samples from both groups are shown in Fig. 8. In contrast to the results
for the BMC mirrors, there are significant shifts in deflection
sensitivity. The biased devices are affected at much lower levels
compared to the unbiased samples, but both groups show some change in
deflection sensitivity after irradiation.

![](media/image8.wmf){width="3.1333333333333333in" height="2.2125in"}

Fig. 8. Change in mirror deflection due to radiation for a typical
piezoelectric mirror.

IV\. Discussion

A. Previous Models of Charge Trapping in MEMS Structures

It has been previously shown for other electrostatically actuated
devices that the presence of charge trapping dielectric materials can
produce permanent shifts in the voltage conditions required to actuate
MEMS devices \[1-5\]. For MEMS accelerometers significant changes
occurred in actuation voltage at about 20 krad(Si), and there was
evidence of "stiction" effects at about 30 krad(Si). Edmonds, *et al*.
developed a quantitative model for this device based on a model for
charge trapping in the dielectric material below the movable sensor. The
model incorporated parameters that describe the balance between charge
deposition and secondary emission, which competes with charge deposition
for this structure.

B. BMC Micromirrors

Ideally, there is no dielectric between the lower and upper electrodes
of the BMC mirror actuator. However, during fabrication the entire
region between the electrodes is filled with silicon dioxide. This
material is removed by chemical etching through a manifold of very small
holes in the achor membrane. If some of this material remains, it could
trap charge between the electrodes during irradiation. The null result
obtained for several different mirror segments suggests that this did
not occur. The electric field required to operate these devices is 3
MV/cm. To generate this field strength would require the presence of a
substantial dielectric layer.

C. Piezoelectric Micromirrors

In contrast to the BMC device the JPL/Penn State micromirror has an
insulating material, PZT, between the top and bottom electrodes. The
thickness of the PZT film in the piezoelectric mirrors is 1.5 µm. The
lateral dimension of the PZT between different mirror segments is 500
µm. PZT has a dielectric constant of approximately 1700. Note that in
these structures the PZT film is continuous. Nevertheless,
electrostriction causes highly localized deformation of the PZT
material, deflecting the mirror downward when either a positive or
negative voltage is applied between the two electrodes. Note that only
the top electrode has a defined pattern.

Prior to irradiation the leakage current through the PZT layer was on
the order of a few microamps. After irradiation the leakage current
increased, as shown in Fig. 9.

![](media/image9.wmf){width="3.375in" height="2.435416666666667in"}

Fig. 9. Increase in leakage current through PZT layer after irradiation.

Tests of the PZT micromirrors showed that the change in deflection
sensitivity was the same regardless of whether a positive or negative
actuation voltage was used to deflect the mirror. This contrasts with
results in \[5\] for GaAs MEMS switches where the change in deflection
sensitivity was different for the two polarities.

For cobalt-60 gamma rays the mass absorption coefficient is essentially
the same for materials with differing densities, nominally 0.03 cm^2^/g.
This allows us to determine absorption in PZT from the density of the
material, which is 7.5.

## V. Conclusions

This paper has shown the first results for optical MEMS devices.
Although the first type of micromirror was not affected by radiation,
that was consistent with the structure of the device which does not
include any oxides or similar insulating material in the path between
the top and bottom electrodes of the device, provided that the
sacrificial silicon dioxide used during the fabrication process is
completely removed by etching.

The PZT element used on the second type of MEMS micromirror is a
dielectric, and charge trapping within this dielectric can potentially
affect device operation.

Micromirrors are an important new class of devices that are of interest
for space applications. Thorough radiation characterization studies are
required to determine the possible radiation effects. The tests reported
herein are part of a continuing effort to evaluate these effects.

## References

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\[3\] L. P. Shanwald, et al., "Radiation Effects on Surface
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\[4\] L. D. Edmonds, G. M. Swift and C. I. Lee, "Radaiton Response of a
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\[5\] S. S. McClure, et al., "Radiation Effects in
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\[6\] E. H. Yang, et al, "Concept, Modeling and Fabrication Techniques
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