ETTC'2003 - SEE

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F. Mailly et Al. ETTC 2003
MICROMACHINED THERMAL ACCELEROMETER WITHOUT SEISMIC MASS
F. Mailly*, A. Martinez, A. Giani, P. Combette et A. Boyer.
Centre d’Electronique et de Micro-optoélectronique de Montpellier, Unité mixte de Recherche du CNRS n°
5507, Université Montpellier II, Place E. Bataillon, 34095 Montpellier - France.
* Tel : + 33 04 67 14 37 85, fax : + 33 04 67 54 71 34,
E-mail: mailly@cem2.univ-montp2.fr
Abstract : The techniques of micromachining silicon are used for the manufacturing of a thermal accelerometer. This sensor
requires no solid proof mass and has a low cost production. A heating resistor creates a symmetrical temperature profile in an
hermetic cavity and two temperature detectors are placed on both sides. When an acceleration is applied, the temperature
profile becomes asymmetric and the two detectors measure the differential temperature. Platinum resistors deposited by
electron beam evaporation on a SiN x membrane are used as heater and temperature sensors. This paper presents sensitivity
measurements according to the distance heater-detector, the power supplied, the room temperature, the cavity volume and the
gas properties.
1. Introduction
Since a few years, the silicon micromachining
technology has been widely developed for the market of
inertial sensors because of its common base with standard
CMOS technology, which permits to achieve low cost,
sensor miniaturization, mass manufacturing and
monolithic electronic integration. The conventional
accelerometers generally measure the displacement or the
deformation of a proof mass (or seismic mass), whose
mobility is the principal disadvantage and is responsible
for their low shock survival rating. Piezoresistive or
capacitive detection are the most common used principles
to convert the acceleration into output voltage but they
have some disadvantages : high temperature dependence
and great influence of mounting stress for the
piezoresistive accelerometers, electromagnetic
interference and parasitic capacitance for the capacitive
ones.
The first thermal accelerometers have been studied
by Dauderstädt and coworkers [1-3] but they have still
involved a seismic mass : this one is positioned about a
heat source and its displacement under acceleration
influences the heat flow between the two elements and
therefore the temperature of the source. More recently,
thermal accelerometers without proof mass were studied
[4-14] and could provide a shock survival up to 50000 g.
The principle of these sensors was first described by Dao
and coworkers [4]: a suspended heating resistor creates a
symmetrical temperature profile and two temperature
detectors, placed symmetrically on both sides of the
heater, measure a differential temperature ∆Tdet (figure 1).
When an acceleration is applied on the sensitive axis x of
the sensor, the convection heat transfer and the
temperature profile become asymmetric and the
differential temperature ∆T det was shown to be
proportional to the acceleration.
2. Theory of operation
For our sensor, the temperature detectors are
platinum resistors which present a linear dependence to
their temperatures given by:
R ( T°
C ) = R0°
C ( 1+
α T°
C )
(1)
with T °C , detector average temperature (°C), R(T °C),
electrical resistance, R 0°C , electrical resistance at 0°C and
α, temperature coefficient of the resistance.
A "push-pull" Wheatstone bridge supplied with a
constant current I permits to convert the resistance
variations into a voltage output and the sensor sensitivity
S is given by:
I
S R0
C Tdet
4
∆ = α (2)
°
Since the detectors’ differential temperature ∆Tdet is
due to free convection, Leung et al. [7, 8] have developed
a simple model suggesting that the response of thermal
accelerometers is linearly proportional to the Grashof
number :
2
3
3
ρ g β ∆Tl
g β ∆T
l (3)
∆Tdet
∝ Gr =
=
2
2 2
µ Pr
a
with g , acceleration (or earth gravity), ρ , gas density, β ,
gas coefficient of expansion, ∆T , heater temperature rise,
l , linear dimension, µ , gas viscosity, Pr, Prandtl number
and a , gas thermal diffusivity.
Temperature
Without acceleration
With acceleration
Γ
Detector Heater Detector
Figure 1 : Principle of the sensor.
∆ T det
x
1