Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
A Closed-Loop Micromachined Accelerometer
Based on Thermal Convection
Alexandra Garraud, Philippe Combette, Benoît Charlot, Pierre Loisel* and Alain Giani,
Institut d’Electronique du Sud – UNIVERSITE MONTPELLIER 2 – CNRS UMR 5412
Place E. Bataillon, 34095 Montpellier, France.
*SAGEM D.S. 72-74 Rue de la Tour de Billy, BP 72, 95101-ARGENTEUIL
Abstract- In this work, we present the frequency
analysis of a micromachined thermal accelerometer
based on convection. Open-loop block diagram
representation is first introduced to explain the sensor
behavior. New sensor architecture is imagined to
enhance sensor characteristics: a closed-loop
configuration is designed by addition of two resistors
closed to detectors. Effects on thermal sensitivity and
bandwidth are investigated.
I. INTRODUCTION
In recent years, a new concept of accelerometer based on
thermal exchanges has been intensively studied. The
physical principle is based on a hot gas bubble acting as a
proof mass. Under acceleration, free-convection transfers
are modified and induce the bubble motion. Like other
transduction mechanisms, such as piezoelectricity,
piezoresistivity or capacitive sensing [1], it converts
acceleration into electrical signal. But the absence of
mechanical clamping between the gas and the chip induces
no stress concentration. This leads to higher shock
reliability. In addition, its simple structure allows low
fabrication costs and competitive performances [2]–[3].
Previous studies have focused on the influence of several
parameters on both sensitivity and frequency bandwidth of
the thermal accelerometer: nature and pressure of gas,
cavity volume, detectors’ dimensions [4–6]. The highest
– 3 dB bandwidth of a thermal accelerometer was 120 Hz
with a standard gas filled cavity configuration [7] and more
recently 320 Hz with a helium-filled cavity configuration
[6]. But these high bandwidths go with low sensitivities
due to a constant sensitivity-bandwidth product.
This present article will present a way to improve
bandwidth without reducing sensitivity by introducing a
closed-loop configuration in the sensor functioning. In a
first time, we will present the open-loop system and then
we will explain how to improve the system with feedback.
II. SENSOR PRINCIPLE
The thermal accelerometer, described in Fig. 1, is based
on natural free convection in a closed chamber containing a
gas. It contains a heating resistor suspended over a cavity
etched on silicon, providing thermal isolation. When the
resistor is supplied by an electrical current, it heats up the
surrounding gas creating a symmetrical temperature
distribution.
Fig. 1. Schematic diagram of the micromachined thermal accelerometer.
When no acceleration is applied, the system is balanced
so that two temperature detectors placed on either side give
the same value, as shown in Fig. 2 by the straight line.
When the sensor is subjected to an acceleration Γ, the
temperature profile shifts, as can be seen in Fig. 2, and the
balance in free-convection heat transfer is modified. The
two detectors don’t measure the same temperature anymore
and this temperature difference δT is correlated to the
acceleration by the sensitivity S equal to δT/Γ (°C/g).
Fig. 2. Schematic diagram of the micromachined thermal accelerometer.
Fig. 3 shows a SEM image of the device. We notice the
suspended wires standing over the micromachined cavity.
The device contains two pairs of suspended bridges on
each side of the heater resistor. The cavity is obtained by
KOH wet anisotropic etching of the silicon (100) oriented
and measures typically 1000 μm x 2000 μm for a depth of
800 μm. The heater (100 μm wide) and detectors (20 μm
wide) are made of a 300 nm thick platinum layer (including
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