# Micro-Optoelectromechanical Tilt Sensor

Authors: Timothy G. Constandinou; Julius Georgiou
Journal: Journal of Sensors
 (2008)

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## Abstract

This paper presents a novel hybrid CMOS/MEMS tilt sensor with a5∘ resolution over a 300∘ range. The device uses a MEMS-based semicircular mass suspended from a rigid body, projecting a shadow onto the CMOS-based optical sensor surface. A one-dimensional photodiode array arranged as a uniformly segmented ring is then used to determine the tilt angle by detecting the position of the semicircular mass. The complete sensor occupies an area of under 2.5 mm × 2.5 mm.


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## Body

## 1. Introduction

An
increasing number of medical devices including implantable prosthetics and body
worn instrumentation are incorporating sense systems within and around the
body. Physical constraints demand such devices to be compact and lightweight,
and the need for autonomy imposes stringent power budgets on such systems. One
such sensor is theinclinometer (or
tilt sensor), which senses its orientation with respect to gravity.Sensing
tilt, in personalised medicine is becoming increasingly important, especially
when combined with other sensor modalities. For example, a tilt sensor
incorporated within a heart monitor can provide a metric on the patients’
activity, providing additional insights for interpreting the electrocardiogram
data. In other biomedical applications, real-time, accurate tilt data can be
used in therapy, for example, in correcting vestibular balance disorders.The vast
majority of tilt sensors developed to date are capacitive (or conductive) in
nature, often with electrolytic fluid-filled cavities. Such systems have been
developed by Bretterklieber et al. [1] and more recently using MEMS
technology [2, 3]. Zhao and Yeatman [4] have developed a digital MEMS-based tilt sensor offering
the possibility of simplifying the interface electronics. It has also been
reported possible to implement inclinometers using modified accelerometers [5, 6] but such systems are typically limited in range and/or sensitivity. Other
approaches include a MEMS convection-based [7] and optical
microfilter-based [8] tilt sensors.In this paper,
we present a novel ultra-low-power tilt sensor generating a digital output.
This has been implemented as part of a hybrid two-chip solution (MEMS
microstructure/CMOS vision chip) and has been fabricated.


## 2. System Architecture and Circuit Implementation

This tilt sensor
uses a micromechanical semicircular mass to express the gravitational field
optically and then custom CMOS vision chip to detect and resolve the vector.
This concept is illustrated in Figure1.Figure 1
The micro-optoelectromechanical
tilt sensor concept, based on a two-tier stack comprising of MEMS structure on
top of CMOS die. The optical detector has 66 photodetector elements arranged in
5° segments to cover a 330° field-of-view (FOV).The
design splits the 330° field of view into two 165° half-planes,
which we have chosen to refer to as left- and right-hand-side (LHS, RHS)
horizons. This is illustrated in the top-level system architecture, shown in Figure 2. As the micromechanical semicircular mass moves, it forms a shadow that is projected
onto the surface of the CMOS chip, the edge of which crosses both the LHS and
RHS horizons. Therefore, each horizon
detector is required to sense the position (or tilt angle) of these edges.
By taking the difference, the output can encode the tilt angle using a signed,
digital representation.Figure 2
The top-level
system architecture comprising of two half-plane horizon detectors, clock phase
generator, and 2’s compliment subtraction circuit.


## 3. Horizon Detector Circuit

The circuit implementation of the
half-plane horizon detectors is shown in Figure3. This consists of 33
photodiode elements positioned equidistantly around an arc to detect the
position of the semicircular mass. Between each two adjacent photodiodes, we
use a discrete edge detector element (that is in total, 32 edge detectors per
horizon) to compare photocurrent magnitudes and if sufficiently different flag
an edge-detect signal. The output is captured using a sequence of operations.
Firstly, the address bus is initialised using pull-up devices to precharge the
parasitic capacitance. Next, a DRIVE signal is propagated through the one-dimensional array, that is, 1×32 elements,
until an edge signal is encountered. On detecting an edge condition, the
current position is asserted onto the address bus via the address encoder,
constituting (for each element) the relevant pull-down switches. Subsequently,
the DRIVE signal is terminated and
the address bus is sampled onto a 5-bit register.Figure 3
Circuit schematic
for the half-plane horizon detector. Each side of the tilt detector uses this
horizon detector—comprising of 33 photodetector elements, and 32 edge detector
cells—sequentially interrogated until the horizon (i.e., edge) is detected.
The address is then asserted onto a precharged bus to encode a digital position
output. The shaded region is repeated so as to form a 32-element array.


## 4. Edge Detector Circuit

Figure4 shows the circuit schematic of
the edge detector element. This has been adapted from Constandinou et al. [9, 10] to the expected contrast ratio and photocurrent levels. A global tuning
mechanism is maintained by driving the Ibias and Itune references
off-chip and internally duplicating and distributing to the individual edge
detector elements. The DRIVEIN and DRIVEOUT are used to test an individual
edge detectors state before propagating to test the next element. Additionally,
an artificial propagation delay is implemented within the output buffer to
guarantee robustness.Figure 4
Circuit schematic
for the edge detector element. The shaded regions (photodiodes D1-D2 and load
devices Q1-Q4) are external to the edge detector block.


## 5. Photodiode Design

The photodiodes are implemented using an
n-well/p-substrate junction as illustrated in Figure5. The different photodiode
elements (66 devices) have been individually custom designed (to comply with
design rule geometry) to fit within a 550 μm to 600 μm radius arc in 5° segments. The active sensor surface for each photodiode element comes to: 4245 μm2.
The photodiode is reverse-biased by stacking two diode-connected PMOS devices
to Vdd. The photocurrent range for the given device under the expected light
levels is from 1 pA (dark current) to 100 nA. The photodiode achieves a spectral
responsivity of 0.15 A/W, 0.25 A/W, and 0.3 A/W for wavengths λ=475 nm (blue),
λ=510 nm (green), and λ=630 nm (red), respectively, when being uniformly
illuminated at 5 μW/mm2.Structure of
n-well/p-substrate photodiode showing (a) side, and (b) plan view (not shown to
scale or profile).
(a)(b)


## 6. 4-Phase Clock Generation Circuit

The operation of the tilt sensor is
centrally regulated via a 4-phase clock sequence, generated using the circuit
shown in Figure6. This implements a looping 4-element state machine to
generate the required control signals. Phase actions are: φ1 to precharge
the bus, φ2 to interrogate
the LHS and RHS horizons and drive the bus, φ3 to sample the
bus result on horizon register, and φ4 to sample the
difference (i.e., subtraction) on the output register.Figure 6
Circuit schematic
for the 4-phase clock generation used to sequence actions required to sense the
tilt angle.


## 7. CMOS/MEMS Integrated Circuits

Figures7 and 8 show the CMOS and MEMS microphotographs,
respectively.Figure 7
Microphotograph of
the tilt sensor electronics implemented in AMS 0.35μm 2P4M CMOS technology. The
dimensions are: 1250 μm × 1250 μm (for core), and 2.5 mm × 2.5 mm die dimensions, that
is, including padring and bondpads (not shown).Figure 8
SEM photograph of
the micromechanical section of the tilt sensor implemented in the Tronics
SOI/HARM technology. The dimensions are: 2.0 mm × 2.0 mm.The MEMS component has been designed such that the pivot/axel used to suspend the mass is smooth and has little friction. In addition, we patterned the release holes in the semicircular mass with a specific pattern recommended by the Tronics foundry, which forms bumps which ensure that there is enough space between the main surface area and the sidewall to avoid stiction [11]. Furthermore, the mass includes
antisticking holes to ensure release, based on technical data provided from
Tronics foundry. During fabrication, the design incorporates a number of struts
to secure the mass during processing and release. After manufacture, these
struts have been intentionally designed to be breakable, thus releasing the
semicircular mass into operation.


## 8. Simulation Results

The circuit was simulated using the Cadence Spectre
(5.1.41isr1) simulator with foundry supplied BSIM3v3 models. Parametric
operating point simulations have been performed to determine the onset of edge
detection for various bias conditions. These results are illustrated in Figure9. The family of curves (for different values of Ibias) illustrates the minimum contrast ratio (Iphoto/δIphoto)
required to flag an edge detection condition. The tunability (for different
values of Itune) is
represented in the y-axis variation. These results show main trends of operation:
(i) for Ibias<Iphoto the tunable range is not
significantly effected by bias current level, and (ii) for Ibias>Iphoto,
increasing the bias current level significantly increases the tunable range (in
particular extending the contrast ratio). It therefore envisaged that if the
nonuniformity of incident light in addition to any component mismatch due to
technology variation is maintained to be within the tunable limits, then
relatively low bias current levels can be used (i.e., Ibias=5 to
10 nA).Figure 9
Operational range
(for the onset of edge detection) for the edge detector core for various bias
conditions (assumingIphoto,on=25 nA).A
top-level transient simulation illustrates the system-level operation of the
tilt sensor, presented in Figure10. This simulation assumes an average
photocurrent Iphoto of 25 nA, an ambient off-current of 1 nA, and uses a system clock of 500 kHz. The total
power consumption is 33 μW.Figure 10
Top-level
transient simulation for a tilt angle of −15°. Results shown are
(a) LHS horizon bus (5-bit), (b) LHS horizon output (5-bit), (c) RHS horizon
bus (5-bit), (d) RHS horizon output (5-bit), (e) subtraction result (6-bit),
(f) final output (6-bit), (g) clock, (h)–(k) φ1–φ4, (l) current
supply, and (m) integral of current supply.The functioning of the system can easily be traced by following the transient
simulation results shown in Figure10 as follows: (i) on the rising edge of φ1, the two buses
are precharged (result=31). (ii) On the rising edge of φ2, the horizon
detector results are asserted onto the buses (results of 9 and 14 for LHS and
RHS, resp.). (iii) On the rising edge of φ3, the values on
the buses are sampled to the horizon output registers and finally, (iv) on the
rising edge of φ4, the subtracted
result (from asynchronous adder) is sampled on the output register.


## 9. Discussion

One
aspect of the sensor that has been omitted from the scope of this p is the
light source. To ensure robustness in dark and fluctuating light conditions,
the sensor is envisaged to incorporate a dedicated light source. This can be a
small area, focused (i.e., narrow viewing area) light-emitting diode (LED) to
stimulate the photodetectors within the vision chip. The composite
emitter/sensor can then be packaged (e.g., as in optoisolators) to seal the
components from environmental exposure.Assuming
a typically red (GaAsP) LED has a forward bias of 2 V and the current is limited
to 1 mA, the emitter power consumption is 2 mW. For a 2 mm2 area
emitter with 1 mm emitter/sensor separation, a 15° beam divergence
and 5% luminous efficiency, this equates to 28 μW/mm2 incident light
power on the CMOS surface. Therefore, for a photodiode responsivity of
approximately 0.3 A/W (at λ=630 nm), and photodiode area of 4245 μm2,
the generated photocurrent in each illuminated segment will be 36 nA, well within the operating range
of the edge detector [9].To minimise
power consumption, the light source can be biased at reduced intensity as the
photodetectors cover a large sense area and is therefore extremely efficient. Alternatively
the sensor can be chosen to operate to a duty cycle, that is, pulsing the
emitter in synchronisation with the clock, and choosing to power the emitter
only at the required refresh rate. For example, if only a 10 Hz tilt response is
required, the sensor can be configured to operate for a single acquisition, that
is, for four clock cycles (400 microseconds using a 10 kHz clock), every 100 milliseconds,
thus reducing power consumption by 99.6%. Fundamentally, the minimum power
consumption is only limited by: (i) the dark current of the photodiodes, (ii)
the minimum detectable contrast ratio (for reliable edge detection), and (iii)
the required sensory response time.


## 10. Conclusion

In this
paper, we have presented a hybrid CMOS/MEMS inclinometer that uses a
photodetector array to detect the position of a suspended microsystem. With a
uniform array of radially positioned photodiodes, the tilt sensor achieves a 5° resolution over a 330° range. The system power consumption depends
highly on the incident light intensity but expected to be within the range 10–35 μW. The target system
specifications are given in Table 1.Table 1
Target design specifications.
CMOS technologyAMS 0.35μm 2P4M CMOSMEMS technologyTronics SOI/HARMSupply voltage3.3 VCircuit size2500μm × 2500 μmDevice count4312Input light intensity (range)10 nW/mm2 to 100 μW/mm2Input system clock (range)100 Hz to 1 MHzInput current bias (range)1 nA to 50 nATilt angle range330° (±165°)Tilt resolution5°Effective dynamic range36 dB (6 bits)System power consumption†10–35 μW†Excluding light emitter consumption.



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*Source: 782764-2008-06-09.xml*