Materials and Processing for the Realisation of Microhotplates Operated at Elevated Temperature
P. Fürjes, Cs. Dücso, M. Ádám,
I. Bársony
Res. Inst. for Technical Physics
and Materials Science – MFA
H-1525 Budapest, P.O. Box. 49,
Hungary
E-mail:ducso@mfa.kfki.hu
A. Morrissey
National Microelectronics
Research Centre – NMRC
University College, Lee Maltings,
Prospect Row, Cork
Ireland
E-mail: amorisey@nmrc.ucc.ie
Formation of integral micro-filaments and
micro-hotplates is the key element in the
development of integrated gas sensor array
chip used for olfactory gas detection. The
most perspective candidates for the monolith
integration of the sensing elements are the
micro-pellistors and the Taguchi type gas
sensors, however, both type has to be
operated at temperature of 200-900oC.
Critical steps in micro-hotplate processing
are the thermal isolation, the selection of
appropriate structural materials, formation
of stable contacts to the filaments and the
deposition of gas sensitive layers. In this
work the alternative processes and solutions
are encountered and
discussed with
emphasis on wiring and contact formation.
their stability and performance. One of the
most promising application is the integrated
micro-pellistor, which detects combustive
gases by measuring the generated heat
during their catalytic oxidation at typical
temperature of 300-600oC. The transduction
principle is the resistance change of a
catalysator coated filament caused by the
deliberated heat in the exothermal reaction.
Therefore the filament has to be formed
from chemically resistive material of high
thermal coefficient of the resistivity (TCR),
such as Pt in conventional discrete devices.
The need for minimisation of power
dissipation is twofold: enables the
construction of a battery operated device and
eliminates the risk of explosion even over
the low explosion limit (LEL) concentration
level.
1. Introduction
2. Results and Discussions
Abstract
Microelements operated at elevated
temperature of 200-900oC play essential role
in integrated gas and flow sensors, micro IR
sources and thermo-mechanical actuators.
Beside the high operational temperature,
these devices must withstand a large number
of heating-cooling cycles without loosing
2.1. Design, selection of materials
The low power dissipation requirement
determines the geometric parameters of the
hotplates. At steady state conditions one can
count with the next sources of heat loss [1,
2]:
•radiation
Pr= 2  a surf ( Tf – Tk )
2
4
4
•conduction via air
Pc, air = A l (Tf - Tk ) / x
•conduction via suspensions
Pc,h = n [(sivii)] (Tf - Tk)/lh
2.2.
Structure and processing
The single crystal silicon filament is
attached to a 100100m2 silicon-nitride
pad on top. The n-type silicon heater is
suspended and connected across a 60-80m
deep groove by two wires embedded in
silicon-nitride (Fig. 1).
•convection
Pconv = kconv a2 (Tf - Tk )
where
 - Stefan-Boltzmann constant, a – side of the
rectangular hotplate, surf- emissivity of the
hotplate material, x- cavity depth, - heat
conductivity, s,v,l
- dimensions of the
beams, i- index of materials, Nu- Nusselt
number, kkonv =  Nu / L, L- characteristic
length
The size of the hotplate is determined by
the targeted power consumption (<20mW at
500oC) and the thermal properties of the
construction material. Taking into account
it’s mechanical properties and process
compatibility, reduced stress SiN1.05 was
selected for this purpose. The hotplate size
has to be reduced to 100 x 100 m2
according to model calculations. For the
selection of the micro-filament material the
heating power (P= I2R) and the maximum
allowable current density have to be
considered,
which
is
limited
by
electronmigration induced degradation of
metals at ca. 1mA/m2. Therefore the
filament has to be formed from material of
high resistivity, which is obviously silicon in
this case. Bearing in mind the complicated
characteristics of TCR of polycrystalline
silicon [3] the filament material was formed
from single crystalline Si. For the selection
of contact material a few alternatives can be
considered, as discussed in section 2.3.
Fig. 1. SEM views of suspended heaters
(a) and a single crystalline silicon microfilament (b).
The selected SiN1.05 layer is deposited by
LPCVD
process
at
800oC
using
SiH2Cl2:NH3=4:1 mixture. The small
dissolution rate of the non-stoichiometric
silicon-nitride layer in 20%HF (6nm/min)
used in the porous silicon formation was
considered in geometric design.
The free suspension thermal isolation of
the heater is realised by removal of the
silicon under the filament by porous silicon
micromachining [4]. The sacrificial porous
Si technique is based on the selective porous
etching of p-type Si leaving the n-doped
regions intact providing their concentration
level is kept below 1018/cm3. In order to
maintain device integrity this is the very last
step in the process flow. The resistance of
the filament can be set in a wide range (102105 ) by doping and the three dimensional
shape.
2.3.
Wiring and contacts
The main issue in the selection of contact
material is the high temperature of contacts
during subsequent processing and functional
operation. There are several alternatives,
however, each suffers from disadvantageous
properties, such as high thermal and low
electrical conductivity, unfavourable TCR
dependence vs. temperature, high residual
stress, non-stability of contacts at high
temperature and non-compatibility with
silicon processing. All the materials
considered is soluble in HF (in case of Pt the
adhesive Ti layer), therefore the wires have
to be encapsulated by another SiN1.05 layer.
(Table 1). Unfortunately, the high deposition
temperature makes the application of Al
obsolete. NiFe is not compatible with silicon
processing.
Fig. 2. Ti/Pt/silicon contact as deposited
(a) and degraded after annealing at
800oC for 2 hrs. (b). Microscopic views.
 Polycrystalline silicon
R [kOhm]
Although
the
contacts
between
polycrystalline silicon and the filament are
inherently perfect, the high resistance in
combination with the high positive TCR of
Table 1. Guide for selection of
the poly-Si limits the operation temperature
materials for wiring
Al Ti/Pt p-Si TiSi2 NiFe of the structure. At low temperature the
resistance of the filament is typically ten
heat transfer
a
a
a
a
times higher, while increasing the input
resistivity/TCR ++ ++ +
++
power (and temperature) the resistance of
contact
++ ++ +
the polysilicon wires starts to dominate. This
stress
+
+
+
+
a
effect results in a malfunction of the device.
e- migration
a
a
+
+
a
The filament is heated up first, but at higher
chemistry
+
++ a
input power the major part of the heat is
++ excellent, + good, a acceptable, - not acceptable
generated in the polysilicon wiring:
 Ti/Pt
therefore one starts to heat the suspensions
The excellent chemical compatibility and instead of the hotplate. (Fig. 3.)
low resistivity of Pt offer the most attractive
30
solution for wiring. The generated heat is
25
negligible in the Pt wires while the heat
20
transfer can be kept at low level by forming
15
of minimised cross section in the suspension
beams. Nevertheless, the high operational
10
Heater
temperature of the contacts between the Si
5
Si
heater and Pt results in a continuous
poly-Si
0
deterioration by the formation and lateral
0
20
40
60
80
100
120
creeping of silicide phase (Fig.2.).
P [mW]
Application of a conductive diffusion
Fig. 3. Resistance of the polysilicon
barrier, such as TiN may eliminate the
contacted hotplate vs. input power. The
problem, providing the barrier layer can polysilicon wiring is heated over 60mW.
stand the high temperature and cycling
without loosing its mechanical integrity.
Nevertheless, this combination can be used
up to ca. 300oC. Most of the Taguchi-type
sensors operates in this temperature range.
 TiSi2
TiSi2 and other refractory silicides are the
most promising materials because of their
low resisitivity, low contact resistance,
stability at high temperature and chemical
compatibility. Due to the superior
electronmigration resistance of TiSi2 thin
layers can be formed with reduced stress
and thermal conductivity.
2.4. Thermal characterisation
The temperature vs. input power
characteristics is the most important feature
of the structure. The inflexion of TCR of
silicon and the contact problems mentioned
above make the temperature calibration
indispensable. The most obvious way is the
integration of an independent thermometer,
e.g. a Pt resistor in the hotplate. However,
the wiring of the thermometer results in
extra heat loss and more complex multilayer
structure. Unfortunately this method doesn’t
provide any information about the
temperature distribution of the structure,
therefore the elaboration of a non-contact
temperature mapping is essential. This is the
analysis of the emission spectrum. Taking
into consideration the dimensions of the
Fig. 4. SEM and IR camera views of a
micro- hotplate. The temperature of the
core is ca. 300oC
hotplate and the characteristic wavelengths
of the temperature range investigated one
can make rough mapping, however the small
size of the structure and the lack of
emissivity data makes the calibration
difficult. In Fig. 4. the limitation of an IR
camera is demonstrated. The temperature
scale still has to be calibrated.
3. Conclusions
The crucial step in processing of
minimised power consumption silicon
micro-filament is the formation of stable
contacts operated at elevated temperature.
There are two alternatives: application of
conductive diffusion barrier between metal
and silicon, or selection of appropriate
silicide for metallisation.
4. Acknowledgements
The assistance of A. L. Tóth in SEM
analysis is gratefully acknowledged. This
work was supported by the European
Community in the frame of the ”SAFEGAS”
project (Project G1RD-CT-1999-00167 of
the FW5 programme) and the Hungarian
OTKA grants (T 031727, T 033094) .
5. References
[1] U. Dibbern, “A Substrate for Thinfilm Gas Sensors in Microelectronic
Technology”, S & A B, 2 (1990) pp. 63-69.
[2]
J.G.E
Gardeniers,
“Silicon
Micromachined Microheater”, “Progress
Report of Copernicus PORSIS project”,
January, 1995
[3] J.W.Seto, “The electrical properties of
polycrystalline silicon films”, J. Appl.
Phys.,46 (1975) pp.5247-5254
[4]Cs.Dücső, et al.:”Porous Si Bulk
Micro-machining for Thermally Isolated
Membrane Formation”, S & A A 60, (1997)
pp.235-239.