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Integration of a passive micro-mechanical
infrared sensor package with a commercial
smartphone camera system
Nathan Eigenfeld

Abstract— This report presents an integration
plan for a passive micro-mechanical infrared (IR)
sensor system with a standard smart phone camera
system. The advantage of utilizing a micromechanical system for IR detection is its ability to be
optically readout. If mechanically integrated with
the smartphone’s camera and light system, a very
simple, electronics-free device could be fabricated
and packaged to provide affordable IR imaging to
the consumer market.
I. PROJECT DEFINITION
THERMAL imaging has grown in popularity in
the last twenty years with the advancement of
semiconductor manufacturing for microelectromechanical systems (MEMS). Currently,
consumer IR imaging with 320x240 resolution
has a market price-point of $2000-$3000. With
obvious high-end military applications such as
thermal weapon sights, security, and weapons
detection, there is a need for even more
affordable devices to provide accessibility to the
general public. Consumer access to IR imaging,
will produce a wave of new applications not
limited to personal security systems or night
vision for everyone with a cell-phone. Thus far,
uncooled thermal detectors have emerged as the
dominant technology for marketable IR imaging.
They are affordable, low power devices because
no cooling is required to maintain
their
sensitivity. Specifically, the microbolometer has
been the pixel of choice for the last ten years for
both high-end military applications and
expensive consumer products. It has the ability to
be batch produced by standard semi-conductor
fabrication processes into focal plane arrays
(FPA) and coupled with CMOS circuitry.
Microsystems Project Final Report, May 2013
However, theoretical noise limitations are
impinging on its further improvement. One
emerging device to sub-route such limitations is
the micromechanical sensor (Fig. 1). The device
consists of a bimorph cantilever, which bends
when incident IR radiation is absorbed. These
devices may be readout capacitively, or optically,
the latter being of great advantage for a lownoise device and specifically for this project. For
an optical readout, one of the materials in the
bimorph must be reflective in the visible light
spectrum, upon which an illuminating light is
reflected. Devices in the research realm have
utilized high-performance CCD camera systems
coupled with Fourier Optic methods to measure
the pixel deflections in arrays of micromechanical devices based off interference
patterns [1]. As phone cameras become more
advanced, their capabilities for readout methods
will become viable for this readout method. They
may already be rudimentarily capable of
performing simple readout algorithms sufficient
for consumer applications.
Fig.1. Demonstration of the bimorph cantilever’s induced bending
upon absorbed power by a thermal expansion coefficient
mismatch in the materials [2].
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The inherent advantage of an optically
readout micro-mechanical device for this
project is the complete decoupling of the
sensor and electronics. The microsystem will
rely on the phone’s camera and light as well as
software to manage the readout task, making
the self-contained microsystem passive, and a
completely mechanical package. This
simplifies its integration substantially. It is
envisioned the system would resemble current
commercial lens attachments for smartphones.
This report will propose chip design and
mechanical package for the IR imager’s
integration with current smartphones, address
key integration issues regarding the packaged
system and also verifying the feasibility of
current camera phone performance to sense
minute optical interference changes in the
pixel arrays.
II. DESIGN CONCEPT
The housing and function of this
microsystem
is
completely
mechanical.
However, it must allow for dual-side
functionality, i.e. the chip must be able to view
IR light from one-side and optical light from the
other (Fig. 2.). The advantage of silicon based IR
sensor is silicon’s transparency in the IR. Thus,
the backside of the chip may face outwards
towards the viewing field. It must also be
integrated with an IR lens system, which is
generally an anti IR reflecting germanium or
sapphire window made into a simple optic. The
front side of the chip, facing the phone camera
and light, must have an optical lens to focus the
camera’s LED light onto the sensor. Once this
light is reflected, it is re-focused through the
LED optic and onto the camera sensor by the inhouse camera lens (Fig. 2). It is assumed, an
image analysis algorithm could be created to
determine changes in the phone’s camera pixel
intensities, which correspond to a mapping of
individual bimorph cantilever deflection
magnitudes. This readout scheme produces the
composite readout image displayed on the phone.
As for the sensor pixels, a very simple 320x240
array (current low-price consumer resolution) is
acceptable. Since this device is consumer
oriented, high-performance is not of initial
concern.
The bimorph cantilevers should be
optimized for sensitivity, not speed of response
(two main performance metrics for IR imaging).
This is accomplished by substantially thermally
isolating the cantilevers from the substrate to
achieve the maximum temperature rise in the
structure causing the largest deflection. This will
help ensure the phone camera can read the
optical interference created by deflecting pixels.
The FPA may require specific “angling” as well
as the LED lens to ensure the light is focused
correctly onto the FPA and reflected into the
phone’s camera. The FPA package must be
transparent on the LED side to allow visible light
transmission back and forth from the LED to
FPA and FPA to camera.
Fig.2. A simple schematic of components in the microsystem
mechanical package including a micro-mechanical pixel.
III. KEY INTEGRATION ISSUES
Several integration issues arise for this
design concept for a practical device. One
challenge of this project would be retro-fitting of
the mechanical housing to a variety of
commercial cameras. Different phones have a
variety of lens and light arrangement, from
vertical to horizontal camera/light orientation,
and one, two or multiple lights in different
configurations. It is envisioned that the variances
in camera and light orientation across different
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smartphones could be compensated for in the
readout algorithm software rather than creating
different microsystem packages for individual
phones. The mechanical package would be
designed to be a universal “snap” on fit for all
phones (Fig. 3.).
Fig.3. A simple layout including the active components in
the microsystem. The universal mechanical mount (orange)
contains the chip sensor with frontside and backside windows. It
mounts directly over the camera and light portion of a given
phone.
The optic for the visible light interference
poses another integration issue. It must be
advanced enough to be integrated with the
camera’s LED to focus light onto the FPA. Thus,
the f-number and area of the lens must
correspond to realistic dimensions for the
mechanical package. The lens must minimize
chromatic and spherical aberrations to optimize
the readout by the camera and software. An
achromatic doublet lens is feasible by its
commercial availability and practical use by
optical designers.
Next, it must be verified that the LED can
provide enough power through the lens, to the
array so that the reflected power back to the CCD
camera is above the photometric exposure for
standard CCD pixels. More specifically, the
pixels must be able to sense minute differences
in the baseline reflected illuminance from the
LED by cantilever deflection in the IR FPA (Sec.
IV Analysis).
Another important consideration for
micromechanical device performance is ambient
temperature effect. The bimorph cantilevers
deflect even in ambient temperature changes,
convoluting the actual signal or IR intensity
absorbed in the pixel. The temperature sensor of
the phone must be utilized to compensate for this
in the software package to obtain correct IR
intensities from the pixels (Fig. 3.).
Further, these devices are sensitive to
mechanical vibrations which induce additional
noise into the sensor. The accelerometer in the
phone could be utilized to sense jerk or shock
events and compensate for them in the readout
software for more accurate imaging (Fig. 3.).
IV. ANALYSIS
To ensure this is a feasible IR detection
readout method, simple calculations will be done
to demonstrate the variances in reflected power
by the deformed cantilevers are detectable by a
standard CCD camera in a phone. Below is a
table with standard dimensions and light
intensities. This analysis will avoid complicated
noise considerations as it is outside the scope of
the project.
LED Radius
LED Power
LED Lens F-number
FPA Resolution
Pixel Pitch
Reflection Coefficient
CCD Lens Radius
CCD Lens F-number
CCD Pixel Pitch
CCD
Photometric
Threshold
Frame Rate
15 mm
25 mW
1
320x240
50 μm
0.9
30 mm
1
5 μm
0.1 lux*sec
30 Hz
Table 1: Device parameters for assessment of a basic
example.
These inputs will provide general dimensions for
the optic lens radius/placement and FPA
placement. Firstly, the average LED power
output is 25 mW, which is focused by the LED
lens at f=1 onto the FPA. The f-number is the
ratio of lens diameter to the focal point. For the
lens to capture all the power of the LED, this
means the diameter must be ~30mm and the FPA
must be ~30 mm away. This results in ~20 nW of
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reflected power off each pixel in the FPA.
Disregarding the angled reflection needed to
reach the camera, the corresponding optical path
length is 30mm back to the camera lens and ~10
mm focusing length from the CCD lens to the
CCD FPA assuming f=1.
Fig. 4: Demonstration of basic optical power shifts by IR
pixel deflection and detected interference by CCD pixel
Now, the relative shift of the baseline
reflection upon cantilever deflection must be
addressed for the extreme case (Fig. 4). For
micromechanical pixels in research the
approximate mechanical responsivity of the bimorph cantilever is 0.25 μm/K [3]. For a device
capable of sensing 5mK differences in incident
radiation (not including noise), the vertical
deflection of the cantilever is ~1 nm. This 1 nm
deflection is produces a 30μm shift in the
reflected ray on the CCD camera lens. After
focusing, this corresponds to ~2 μm shift of the
ray on the CCD pixel. Given the 20 nW of
reflected power, this corresponds to a relative
change in power of ~40%. So for the minimum
resolvable temperature difference (excluding
noise) of 5mK, the CCD pixel will see a 40%
change in incident power levels of ~2 nW. For
imaging at 30Hz, this is ~100 lux*sec, which is
illuminance*seconds. For commercial 5 μm pixel
cameras, the minimum lux*sec is on the order or
10 [4]. Thus, the CCD is very capable of
detecting the slight deflections of the
microcantilevers. This is without noise included,
which could decrease performance by 1-2 orders
of magnitude. Even so, the minimum detectable
temperature difference would still be ~500mK
which is acceptable for a low-cost commercial
device.
Note this simple analysis does not address
the actual software algorithm to readout the
relative power shifts per pixel. It was merely a
reality check for utilizing the in house camera
and light to detect optical power shifts from the
deflecting cantilevers.
V. CONCLUSION
A device design for a purely mechanical IR
camera cell-phone attachment was presented and
analyzed. This cell-phone attachment could
revolutionize IR imaging by its accessibility to
the general public. Key integration issues revolve
around the universality of the mechanical mount
to address various phone designs and the optical
LED lens integration. This design is feasible with
current camera phone/light technology and the
resulting work is opto-mechanical packaging.
Based off the analysis of optical path lengths, the
active components could be < 50 x 50 mm2,
leaving room for a mechanical package that
could be < 2 x 2 cm2. This simple phone
attachment could bring IR imaging to millions of
consumers at a < $500 price point. The main
price reduction surrounds the lack of electronic
component integration. The applications created
by exposure to such an immense user-base are
envisioned to be endless.
References:
[1] J. Zhao, “High sensitivity photomechanical
MW-LWIR imaging using an uncooled MEMS
microcantilever array and optical readout
(Invited Paper),” Proceedings of SPIE, vol. 5783,
pp. 506-513, 2005.
[2] J. Lai, T. Perazzo, Z. Shi, and a Majumdar,
“Optimization and performance of highresolution
micro-optomechanical
thermal
sensors,” Sensors and Actuators A: Physical, vol.
58, no. 2, pp. 113-119, Feb. 1997.
[3] Scott R. Hunter ; Gregory S. Maurer ; Gregory
Simelgor ; Shankar Radhakrishnan ; John Gray;
High-sensitivity 25μm and 50μm pitch
microcantilever IR imaging arrays. Proc. SPIE
6542, Infrared Technology and Applications
XXXIII, 65421F (May 14, 2007).
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[4] Joyce Farrell ; Feng Xiao ; Sam Kavusi;
Resolution and light sensitivity tradeoff with
pixel size. Proc. SPIE 6069, Digital Photography
II, 60690N (February 10, 2006).