SOI-CMOS based single crystal silicon micro-heaters for gas sensors

Abstract

Here we report on novel high temperature gas sensors that have been fabricated using an SOI (silicon-on-insulator) -CMOS process and deep RIE back-etching. These sensors offer ultra-low power consumption, low unit cost, and excellent thermal stability. The highly-doped single crystal silicon (SCS) layer of a standard SOI-CMOS process, which is traditionally used to form the source and drain regions of a MOSFET, is used, for the first time, to form a resistive heater of a micro-hotplate in a high-temperature gas sensor. Our sensors have a power consumption of only 12-30 mW at a temperature of 500degC. We have observed that the drift in resistance of a SCS heater held at 500degC for 500 hours, without burn-in, was less than 1%. SCS micro-hotplates are not only suitable for chemoresistive sensors, as described here, but also calorimetric gas sensors that require these high operating temperatures. Tungsten oxide nanorods have been deposited onto our micro-hotplates by atmospheric chemical vapour deposition and have shown reasonable sensitivity to ethanol vapour in air.

Full text

IEEE
SENSORS
2006,
EXCO,
Daegu,
Korea
/
October
22-25,
2006
SQI-CMOS
based
single
crystal
silicon
micro-
heaters
for
gas
sensors
T.
Iwaki,
J.A.
Covington
and
J.W.
Gardner
School
of
Engineering
University
of
Warwick
Coventry
CV4
7AL,
UK
T.Iwakigwarwick.ac.uk
F.
Udrea
Department
of Engineering
University
of
Cambridge
Cambridge
CB2
lPZ,
UK
C.S.
Blackman
and
I.P.
Parkin
Department
of
Chemistry
University
College
London
London
WC1H
OAJ,
UK
Abstract-
Here
we
report
on
novel
high
temperature
gas
sensors
that
have been
fabricated
using
an
SOI
(Silicon-on-
insulator)
-CMOS
process
and
deep
RIE
back-etching.
These
sensors
offer
ultra-low
power
consumption,
low
unit
cost,
and
excellent
thermal
stability.
The
highly-doped
single
crystal
silicon
(SCS)
layer
of
a
standard
SOI-CMOS
process,
which
is
traditionally
used
to
form
the
source
and
drain
regions
of
a
MOSFET,
is
used,
for
the
first
time,
to
form
a
resistive
heater
of
a
micro-hotplate
in
a
high-temperature
gas
sensor.
Our
sensors
have
a
power
consumption
of
only
12-30
mW
at
a
temperature
of
500
°C.
We
have
observed
that
the
drift
in
resistance
of
a
SCS
heater
held
at
500
°C
for
500
hours,
without
burn-in,
was
less
than
1
%.
SCS
micro-hotplates
are
not
only
suitable
for
chemoresistive
sensors,
as
described
here,
but
also
calorimetric
gas
sensors
that
require
these
high
operating
temperatures.
Tungsten
oxide
nanorods
have
been
deposited
onto
our
micro-hotplates
by
atmospheric
chemical
vapour
deposition
and
have
shown
reasoanble
sensitivity
to
ethanol
vapour
in
air.
I.
INTRODUCTION
Although
there
is
an
increasing
demand
from
the
environmental,
automotive
and
medical
industries
for
portable,
handheld
gas
monitors,
presently
the
gas
sensor
market
is
still
relatively
small.
The
small
uptake
for
these
low-cost
battery
powered
applications,
can
be
accounted
for
by
two
major
problems.
Firstly,
the
relatively
high
power
consumption
and
secondly
the
high
price
of
commercial
gas
sensors.
For
example,
commercially
available
pellistors
have
a
typical
power
consumption
of
350
to
850
mW
for
an
operating
temperature
of
500
°C
[1].
Taguchi
type
resistive
gas
sensors,
(the
most
commonly
used
gas
sensor)
require
an
operating
temperature
of
between
300
and
400
°C
with
a
power
budget
of
800
mW
[2].
Furthermore,
the
semi-manual
method
by
which
they
are
fabricated
makes
them
expensive
even
though
production
volumes
are
significant.
Over
the
last
15
years,
considerable
effort
has
been
directed
towards
reducing
both
the
power
consumption
and
the
manufacturing
costs
by
replacing
conventional
sensors
with
micro-hotplate
type
sensors
based
on
silicon
processes
[3,
4,
6,
7].
However,
all
of
these
designs
have
some
disadvantages
and
have
not
yet
enjoyed
great
commercial
success.
The
most
widely
investigated
resistive
heater
material
for
micro-hotplates
is
platinum
[3].
It
is
not
difficult
to
develop
a
micro-hotplate
using
this
material,
as
it
is
an
inactive
noble
metal
and
thus
has
excellent
thermal
stability.
However,
platinum
is
not
a
CMOS
compatible
material
and
thus
such
designs
cannot
be
fully
integrated
with
drive/detection
circuitry
or
take
advantage
of
the
low
production
costs
associated
with
CMOS
processes.
CMOS
compatible
polysilicon
heaters
have
also
been
widely
investigated
[4].
However,
their
long
term
stability
has
been
reported
to
be
poor
due
to
the
highly
reactive
grain
boundary
[5].
Furthermore,
several
studies
have
been
made
on
highly
boron
doped
single
crystal
silicon
based
micro-hotplates
formed
by
anisotropic
selective
wet
silicon
etching
[6].
These
are
CMOS
compatible
and
the
heater
material
itself
is
believed
to
be
thermally
stable
without
the
grain
boundary
issue,
which
causes
the
resistance
drift
in
polysilicon.
However,
the
wet
etching
process
makes
it
impossible
to
passivate
the
bottom
of
the
heaters
within
CMOS
process.
Therefore,
the
heaters
are
easily
contaminated
and
the
commercial
exploitation
is
difficult.
Micro-hotplates
using
FETs
have
recently
been
proposed
[7].
The
FET
heater
can
be
controlled
to
have
a
higher
resistivity
than
equivalent
resistive
metals
(for
the
same
area),
which
makes
it
possible
to
reduce
the
heater
size
further.
Thus
lower
power
consumption
can
be
achieved.
However
its
operating
temperature
is
limited
to
less
than
400
°C,
because of
the
bipolar
turn-on
of
the
MOSFET
and
the
use
of
aluminium
interconnects.
Thus,
there
is
a
need
for
a
CMOS
compatible,
1-4244-0376-6/06/$20.00
}2006
IEEE
460

IEEE
SENSORS
2006,
EXCO,
Daegu,
Korea
/
October
22-25,
2006
high
temperature
and
reliable
heater
structure
for
gas
sensors.
Recently,
we
have
proposed,
simulated
and
designed
SOI
(Silicon-on-insulator)
-CMOS
based
highly
doped
single
crystal
silicon
(SCS)
micro-hotplates
to
achieve
these
goals
[8,
9].
Using
SOI
technology,
it
is
possible
to
fully
passivate
the
SCS
heaters
unlike
those
reported
in
previous
work
[6].
We
are
also
aiming
at
lower
power
consumption
with
extremely
small
heater
radii.
This
paper
reports
on
the
first
experimental
results
of
our
SOI
CMOS
SCS
micro-
hotplates.
II.
SOI-CMOS
GAS
SENSOR
STRUCTURES
Figure
1
shows
the
design
of
an
SOI
resistive
gas
sensor
employing
an
SCS
micro-hotplate.
The
micro-hotplate
is
comprised
of
a
silicon
nitride/silicon
dioxide
membrane
in
which
a
highly
doped
SCS
resistive
heater
is
sandwiched.
The
shapes
of
both
membrane
and
heater
were
designed
to
be
circular
to
reduce
the
possibility
of
membrane
failure
due
to
mechanical
stress
(which
is
more
pronounced
at
high
operating
temperatures).
Here,
two
micro-heater
designs
have
been
produced
with
different
geometries
as
shown
in
Figure
2.
The
radii
of
the
membrane
and
the
heater
are
282
gm
and
75
tm
for
the
large
micro-heaters
and
150
gm
and
12
tm
for
the
small
micro-heaters,
respectively.
To
supply
the
power
to
the
micro-heaters,
metal
tracks
were
used
to
reduce
both
the Joule
heat
generated
and
the
conduction
heat
loss.
The
second
CMOS
metal
layer
is
used
to
form
heat
spreading
plates
to
improve
the
temperature
uniformity
of
the
sensing
material,
and
placed
above
the
heaters.
The
electrodes
to
measure
the
resistances
of
the
sensing
materials
are
made
of
the
third
CMOS
metal
layer.
These
electrodes
have
interdigitated
structures
with
aspect
ratios
of
typically
16
and
1.7
for
large
and
small
micro-
hotplates,
respectively.
The
fabrication
process
of
the
above
structures
strictly
follows
the
standard
SOI-CMOS
process.
An
SOI
wafer
was
used
as
an
initial
substrate.
The
SCS
heaters
were
formed
by
trench
etching
and
then
doped
simultaneously
with
source
or
drain
by
ion
implantation
of
boron
(p+)
or
arsenic
(n+),
thus
no
additional
processing
was
needed.
The
concentration
of
p+
and
n+
are
both
ca.
7
x
10
'9
cm-3.
After
the
SOI-CMOS
process,
the
wafer
was
back
etched
by
deep
reactive
ion
etching
(DRIE).
Tungsten
was
used
for
metallization
rather
than
aluminium
as
to
avoid
electro-migration
at
high
temperatures.
The
thickness
of
the
membrane
is
ca.
5
ptm.
Polysilicon
heaters,
which
have
the
same
shape
as
SCS
heaters,
were
fabricated
on
the
same
wafer
for
comparison.
Atmospheric
pressure
chemical
vapour
deposition
(APCVD)
based
WO3-X
films
with
nanorod
structures,
developed
by
University
College
London
(UCL)
[10],
were
directly
deposited
onto
the
micro-heaters.
The
film
was
deposited
using
WCl6
with
co-reactant
of
ethanol
at
a
temperature
of
625
°C.
The
nanorod
structure
is
confirmed
by
SEM
as
shown
in
Figure
3.
Similar
types
of
metal
oxide
nanorod
films
have been
previously
studied
due
to
their
large
surface
to
volume
ratio
[
1]
hence
high
sensitivity.
However,
they
were
synthesized
by
high
temperature
(e.g.
900
°C)
annealing
of
metal
or
metal
oxide
powder
and
thus
the
direct
deposition
onto
CMOS
based
micro-hotplates
was
difficult.
The
relatively
low
temperature
of
our
CVD
method
and
tungsten
metallization
of
our
micro-hotplates
made
this
possible.
To
the
best
of
our
knowledge,
this
is
the
first
time
that
the
metal
oxide
nanorods
have
been
deposited
onto
CMOS
based
micro-hotplates
by
CVD.
The
film
was
oxidised
at
450
°C
after
deposition.
Gas
sensing
area
Electronic
IC
area
Sensitive
II
-
/material
D
C
.-4e
U,
NTTAn
',!
4n!
Figure
1.
Schematic
cross-section
of
a
resistive
gas
sensor
employing
SCS
micro-hotplate
with
metal
tracks
with
integrated
interface
circuitry.
Figure
2.
Photographs
of
(a)
large
and
(b)
small
micro-hotplate.
Figure
3.
SEM
photograph
of
as
deposited
W03-,
flm.
1-4244-0376-6/06/$20.00
}2006
IEEE
461

IEEE
SENSORS
2006,
EXCO,
Daegu,
Korea
/
October
22-25,
2006
III.
EXPERIMENTS
The
power
consumption
of
the
large
and
small
micro-
hotplates
has
been
measured
and
the
results
shown
in
Figure
4.
For
this
purpose,
the
SCS
heater
was
used
as
both
the
heater
and
the
temperature
sensor.
The
temperature
is
related
to
the
SCS
resistance
by:
R(T)
=
R27
{l+
a(T
-
27)
+/3(T
27)2
},
40%
z
30%
;
20%
6%
.0
CD
O°J
(1)
where
a
=1.46X
l0-
[1/K]
and
X
=
9.
x
10-7
[1/K2]
(p+)
are
temperature
coefficients
of
resistance
obtained
by
the
measurement
of
resistance
in
a
temperature
controlled
furnace
(Carbolite/AAF1100).
Hence
we
found
that
the
large
and
small
heaters
require
only
30
mW
and
12
mW,
respectively,
to
operate
at
500
°C.
Reliability
tests
were
also
performed.
The
micro-
hotplates
with
SCS
(n+,
p+)
and
polysilicon
(for
comparison)
heaters
were
continuously
operated
with
a
constant
voltage.
The
operating
temperatures
were
350
°C
(for
chemoresistive
type)
and
500
°C
(for
calorimetric
type)
for
500
hours.
The
results
are
shown
in
Figures
5
and
6.
It
was
found
that
the
p+
doped
SCS
heater
was
the
most
stable
at
both
350
and
500
°C.
The
drift
of
the
p+
doped
SCS
heater
was
less
than
1
%0
after
being
operated
at
500
°C
for
500
hours.
E
0
E
.,
.
U)
C
0
0~
35
30
25
20
15
10
5
0
-16%
0
°J
0
100
200 300 400
500
Time
[hour]
Figure
6.
Resistance
drift
when
operated
at
500
'C.
Figure
7
shows
the
response
of
a
resistive
gas
sensor
coated
with
WO3_,nanorods
to
ethanol
vapour
in
air.
The
gas
sensor
chip
was
tested
in
a
chamber
at
30
°C
with
a
background
humidity
of
3000
ppm
(r.h.
7.1
°0
at
30
°C),
and
the
micro-hotplate
was
operated
at
a
temperature
of
ca.
350
°C.
The
gas
response
follows
the
power
law
where
the
exponents
are
0.63,
which
is
typical
for
metal
oxide
gas
sensors
[12],
indicating
that
the
sensor
is
working
properly.
100%
CD
U1)
@
10%
c
Q0
1%
100
0
100
200
300
400
500 600
Temperature
[00]
Figure
4.
Observed
power
consumption
of
SOI-CMOS
micro-hotplates.
8%
oCs
(P+)
.SCS
(P+)
6
G%
-
A
SCS
(n+)
SCS
(n+)
=
4%-o
polysilicon
0
polysilicon
-2%
-
-2%
0
100
200
300
400
500
Time
[hour]
Figure
5.
Resistance
drift
when
operated
at
350
'C.
1000
Concentration
(C)
[ppm]
10000
Figure
7.
Typical
response
of
APCVD
based
W03-,
nanorods
at
350
'C.
IV.
DISCUSSION
The
12
mW
power
consumption
of
these
SCS
heaters
at
500
°C
and
8
mW
at
350
°C
are,
as
far
as
we
know,
the
smallest
that
have
ever
been
reported.
Previous
work
by
Sheng,
reports
a
power
consumption
of
12
mW
to
operate
at
300
°C
[4].
The
extremely
large
ratio
of
membrane
to
heater
radii
makes
this
possible.
The
main
power
loss
of
the
proposed
micro-hotplate
is
by
conduction
through
the
membrane
[8],
which
is
given
by
the
following
(for
a
circular
membrane):
(2)
Qconduction
oc
1/ln(rt
lrh
),
where
rm
and
rh
denote
the
radii
of
the
heated
and
membrane
area
[13].
The
membrane
to
heater
ratio
of
our
small
micro-
hotplate
is
12.5,
whereas
that
of
previous
work
was
ca.
2
[4].
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Large
micro-hotplates
lh'
(radius:
282
pm)
N\\"
,"*
,f,4
411--l.
V1.
1#1
-------
lw.
Small
micro-hotpl;
(radius-
150
pn
462

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/
October
22-25,
2006
Although
it
has
been
observed
that
the
thermal
stability
of
the
p+
SCS
is
much
higher
than
the
n+
SCS
or
polysilicon,
the
precise
cause
is
not
known.
A
possible
reason,
for
this
resistance
drift
of
the
SCS
heater,
is
the
stress
induced
by
the
deformation
of
the
membrane
at
high
temperatures.
Optical
interferometer
(Wyko/NT2000)
was
used
to
measure
this
deformation
and
it
was
found
that
the
membrane
deforms
significantly
at
high
temperatures
(maximum
deflection
is
ca.
10
gtm
at
500
°C)
the
results
are
shown
in
Figure
8.
This
is
due
to
the
difference
in
temperature
expansivity
of
the
layers
within
the
SOI
membrane.
This
deformation
induces
a
stress
in
the
SCS
heater.
Interestingly,
Demenet
et
al.
reported
that
the
yield
stress
of
n-type
heavily
doped
SCS
is
much
lower
than
that
of
intrinsic
silicon,
whereas
p-type
doping
has
only
a
small
effect
on
yield
stress
[14].
The
reported
values
of
yield
stress
at
500
°C
are
60,
180
and 210
MPa
for
n-type
(6x018
cm-3),
p-type
(x1
08
cm3),
and
intrinsic
silicon.
This
lower
n-type
value
is
natural
because
the
dislocation
velocity
of
heavily
doped
n-type
silicon
is
much
faster
(ca.
10
x)
than
that
of
p-type
silicon
at
500
°C
[
15].
Therefore,
it
is
possible
that
the
membrane
deformation
is
causing
an
increased
plastic
deformation
of
n+
SCS
over
p+
SCS
during
operation.
Furthermore,
it
is
also
known
that
the
dislocation
motion
during
plastic
deformation
causes
multiplication
of
dislocations
[16],
and
it
is
these
dislocations
that
cause
the
increase
in
the
resistivity,
hence
resistance
[17].
This
would
result
in
an
increased
drift
in
n-
type
SCS
heaters
over
p-type,
as
observed
here.
5
E
0
a)
4)
a)
a)
-Q
E
0
-5
10
-15
-400 -300
-20C
-100
0
100
Displacement
[pm]
200
300
400
Figure
8.
Membrane
deformation
at
RT,
350
and
500
'C.
V.
CONCLUSIONS
Here,
novel
gas
sensors
based
on
highly
doped
SCS
micro-hotplate
have
been
designed
and
characterised.
These
gas
sensors
were
fabricated
using
a
standard
SOI-CMOS
process
that
could
offer
low
production
costs,
with
a
power
consumption
of
only
12-30
mW
to
operate
at
500
°C.
The
drift
of
resistance
for
p-type
SCS
was
found
to
be
significantly
less
than
that
of
polysilicon
or
n-type,
(less
than
1
0o
when
operated
at
500
°C
for
500
hours).
APCVD
based
novel
tungsten
oxide
nanorods
were
deposited
directly
onto
the
micro-hotplate
to
form
a
resistive
gas
sensor,
and
its
operation
has
been
demonstrated.
ACKNOWLEDGEMENT
Professor
I.
P.
Parkin
thanks
the
Royal
Society-
Wolfson
trust
for
a
merrit
award.
Dr
Udrea
acknowledges
the
award
of
the
Philip
Leverhulme
Prize.
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1-4244-0376-6/06/$20.00
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IEEE
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