Sensors and Materials, Vol. 8 , No. 6 (1996) 409-4 21
MYUTokyo
S &M0254

Thermal Sensors Fabricated by CMOS andMicromachining
Henry Baltes and Oliver Paul
Physical Electronics Laboratory, ETH Zurich,
8093 Zurich, Switzerland(Received March 29, 1995; accepted December 29, 1995)
Key words: CMOS, micromachining, infrared sensor, power sensor, flow sensor, pressure sensor,
thermal material properties

Integrated thermal sensors can be made by industrial CMOS IC technology combinedwith subsequent bulk or surface micromachining for thermal insulation. We review recent
work on CMOS-based thermal radiation, AC power, mass flow, and pressure sensors withon-chip signal conditioning circuitry. We discuss diagnostic microstructures for themeasurement of process-dependent thermal properties of CMOS materials.

1.

Introduction

When the number of devices required is sufficiently large, it may be desirable to batch­
manufacture sensors at low cost with high reliability and link them to integrated electronic
circuitry. In this case the sensor designer will benefit from the knowledge of not only
physical or chemical sensor effects and related materials, but also silicon IC fabrication
technology and integrated circuitry. The task is to translate sensor effects into device
structures that can be manufactured largely by standard industrial IC technology, whichallows on-chip signal conditioning circuitry designed to enhance the sensor performance
decisively.< 1J
Indeed, silicon sensor chips for temperature, optical radiation, and magnetic<2l measurands
can be fabricated by industrial CMOS or bipolar IC technology without further siliconprocessing. Microsensors for a number of thermal, mechanical or chernical(3l measurands
can be made by combining established IC technologies with additional, compatible
processing steps specific to the sensor function. Preferably, such extra etching or deposi­tion steps are performed by post-processing, i.e., processing after completion of the regular

409

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Sensors and Materials, Vol. 8, No. 6 (1996)

1

4

12

13 7

Fig. 1. Illustration of CMOS-compatible sacrificial metal etching. Cross section of pressure sensor
after completion of CMOS IC process (a), photolithography (b), and metal etch (c). Silicon (1),silicon dioxide (2), first metal (3), intermetal dielectric (4), second metal (5), passivation (6), metalpad (7), entrance pad for metal etchant (8), sacrificial metal (9), photoresist (10), access port for gas(11), metal heater (12), heater interconnect (13).

dielectric membrane have been demonstrated.<6-9l Figure 2 shows their schematic structurewith an absorption area consisting of the silicon oxide/nitride passivation on an aluminum
mirror above the CVD and thermal oxide layers. The overall membrane thickness is about
4 µm. A temperature increase due to radiation focused on the absorption area is detectedby an n-polysilicon/p-polysilicon thermopile (300 µV /K Seebeck coefficient) with a totalinternal resistance of 2.5 MQ, Optimized devices reach 96 V /W responsivity, 4.5 x 107 cm
1Rz I W detectivity, and 31 ms rise time. The spectral response of the silicon oxide/
nitride passivation approximately matches the radiation spectrum of an intruder with about
10 µm peak wavelength. The angular response of the sensor follows the cosine law, About
1 K temperature difference between the intruder and the background which results in atemperature increase of the sensor membrane of 0.01 K, can be resolved.
Figure 3 shows a micrograph of a chip with two sensor elements (each on a separate
720 µm by 720 µm membrane) and an operational amplifier. Since intrusion alarm
detectors work in the frequency range of O.1 to 10 Hz, 1/f noise cancellation is crucial. Thiscan be achieved for example by the auto-zero technique. The low-noise amplifier consists

Sensors and Materials, Vol. 8, No. 6 (1996)

412

Circuitry

Thermopile

Absorber

membrane
Oxide/nitride mask
Fig. 2. Structure of CMOS-compatible thermoelectric infrared sensor on a closed m mbraneconsistmg of the CMOS dielectric layers with thermopile and metal mrrror sandwiched m etween.
•

•

•

•

•

•

•

•

•

I

Fig. 3. Optical micrograph of dual infrared sensor with on-chip operational amplifier. The two
sensor elements each consist of an absorbing area and an n-polysilicon/p-polysilicon thermopile on a
720 µm by 720 µm silicon oxide/nitride membrane.

Sensors and Materials, Vol. 8, No. 6 (1996)413

of an operational transconductance amplifier followed by a multirate switched capacitor
integrator.c13> Both are based on the same folded cascade p-channel input stage. They
differ in the biasing and the common mode feedback circuits. The transconductor has anadditional differential pair for the offset and low-frequency noise compensation. The
voltage gain is 60 dB. To overcome the high 1/f noise in MOSFET devices, one can also
design a lateral bipolar device fabricated by a standard CMOS process. It is possible toreach equivalent input noise densities of less than 0.1 µV / � for frequencies as low as
1 Hz.<6>
Prototypes were fabricated using a 1.2 µm CMOS IC process of AMS Austlia Mikro
Systeme on 100 mm wafers and a 2 µm CMOS IC process of EM Microelectronic Marinon 150 mm wafers. Subsequent to the latter process sequence, we performed the following
steps on whole wafers: deposition of stress-compensated passivation nitride, deposition ofetch protection layer on the wafer front, isotropic etch of the wafer back to achieve goodadhesive conditions, deposition of the back nitride mask, photolithography on the wafer
back, etch in a 6 M 95 °C KOH solution, and dicing of the wafer. The overall membraneyield after dicing is higher than 98%. As a result of the stress-controlled deposition of thesilicon nitride passivation,<9J low-stress membranes with less than 0.1 µrn buckling over thewhole 720 µm by 720 µm area were achieved.
The same n-polysilicon/p-polysilicon thermopiles can be used for CMOS thermal
convertersC 14> which measure the mean square value of an unknown AC signal by thermo­
electric conversion and contain signal conditioning circuitry on the same chip. Possible
applications are microwave instrumentation and thermal voltage standards. Compensationof the nonlinearity at high power levels and of the dependence on the ambient temperature
can be achieved by using two sensor elements in a first-order sigma-delta modulator.<14l

4.

Thermal Gas Flow Sensor

Improved open and closed CMOS rnicrostructures for the measurement of mass flow
have been reported recently.<1 5l As an example of an open structure, Fig. 4 shows a two­
dimensional thermal gas flow sensor based on two crossed microbridges. The operation of
the device is based on the flow-dependent heat transport from a central integrated heater
into the surrounding gas. A temperature difference is generated by the flow between
downstream and upstream locations on the device. Four integrated polysilicon resistors of
1.05 kQ each, symmetrically located around the center of the suspended bridges, provide
the heating power of a few mW required to operate the sensor. Temperature differences aremeasured with integrated polysilicon/alurninum thermopiles with cold contacts on thesilicon substrate and hot contacts on the suspended structure. Two pairs of parallel
thermopiles respond to the respective collinear velocity components of the gas flow. Eachthermopile produces a thermovoltage of 1.21 mV /K.
Figure 5 shows the differential output signals of both thermopile pairs as a function offlow rate. The flow was forced through a 0.8-mm-high, 8-mm-wide, and 30-mm-long flow
channel past the flow sensor mounted into the channel wall. One thermopile pair wasoriented parallel, the other perpendicular to the direction of flow. For rates below 250 seemSensors and Materials, Vol. 8, No. 6 (1996)

414

Fig. 4. SEM micrograph of thermal gas flow sensor for rate and direction measurements on a
microbridge consisting of the CMOS dielectric layers with four heating resistors (in the center) and
four thermopiles.

4
parallel
2 ,-

=

0

-2

•
••

--- ······

•
••

.

••
•
•
••

....••······. ::::::: ·········;···········f--••••••

-4
-400

••

•

•

perpendicular
I
I
200
-200
0
Flow rate (seem)

400

Fig. 5. Measured velocity response of the directional gas flow sensor shown in Fig. 4 in terms of thedifferential output voltages of the thermopile pairs oriented parallel and perpendicular to the gas flow
direction.

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Sensors and Materials, Vol. 8, No. 6 ( 1996)

the signal of the parallel thermopiles is linear with a sensitivity of0.19 V w-1 (m/s)-1• This
represents an appreciable improvement over previous micromachined flow sensors.cs.16J
For vector measurements, the dependence of the corresponding output voltages Ux and
Uy on the flow angle <.P at a flow rate of 50 seem shows the expected harmonic behavior(Fig. 6). From Ux and Uy the thermally measured flow direction <Pmeas is calculated using<Pmeas = arctan(Uy / Ux). An excellent correlation between <Pmeas and the geometricallydetermined <.P is found for this design; the mean deviation is only l.7 ° .c 15J

S.

Vacuum Sensors

CMOS-based vacuum sensors have been demonstrated using bulkc1, l and surface<10--12>
micromachining. By bulk micromachining from the wafer front, beam-type thermoelectricand resonant microsensors can be combined on the same chip (see Fig. 7). In the firstdevice, a polysilicon thermopile separated from a heating resistor by an air gap measures
the temperature increase resulting from the heat conduction of the air. In the second, the air
damping of a thermally excited vibrating beam is detected through piezoresistors inte­
grated into its hingesY 8l The two sensor principles are complementary and together cover
17

0.6
Q)

:::,.
Q)

0

-0.6

0

90

180

270

Flow direction [ 0]360

Fig. 6. Measured angular response (dots and squares) of the directional gas flow sensor shown in
Fig. 4 in terms of the differential thermopile voltages as functions of gas flow direction, comparedwith harmonic fits (solid lines).

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Sensors and Materials, Vol. 8, No. 6 (1996)

Fig. 7. SEM micrograph of thermally excited beam resonator (bottom) and thermoelectric pressure
sensor (top) combined on the same CMOS chip. The thermoelectric sensor includes thermopiles onboth beams and a heating resistor at the tip of the right beam.

the so-called Pirani range between 0.1 and 105 Pa.
A recent front-etched CMOS device spanning the pressure range from 102 to 106 Pa wasfabricated based on two parallel polysilicon heater filaments.ci 9J A different type of CMOS
thermal pressure sensor for the same range00-12l has been fabricated by the sacrificial-metal
surface-rnicromachining procedure outlined in section 2. The corresponding design crosssection is given schematically in Fig. 1, while Fig. 8 shows a finished device with 200 µm
diameter. The sensor consists of a circular membrane composed of the intermetal isolationoxide and the passivation layer fabricated by the CMOS IC process. Sandwiched between
the isolation oxide and the passivation is a 3-µm-wide meandering metal line, made of thesecond CMOS metal, which serves as a heater with a resistance of 50 Q. A 0.65-µm-wide
air gap separates the membrane from the lower CMOS dielectrics and the substrate.
During pressure measurements the air pressure in the gap is stabilized at the ambient value
through eight lateral access ports. The operation of the sensor is based on the pressure­dependent heat conduction across the air gap. The power required to keep the membrane ata certain temperature above that of the substrate depends on the gas pressure. The
measured thermal response agrees with the predictions of kinetic gas theory.<20J
The above sensor was integrated with a pressure-insensitive reference structure and asimple read-out circuit consisting of an opamp and two n-channel transistorsY 2l Standard
cells from the circuit library provided by the CMOS manufacturer were used. The circuit
forces the sensor to track the resistance of the reference structure, which results in a

Sensors and Materials, Vol. 8, No. 6 (1996)

417

Fig. 8. SEM micrograph of surface-micromachined CMOS thermal pressure sensor with a circularmembrane supporting an integrated meandering heater and eight access ports to the micromachined
gap underneath the membrane.

constant temperature difference !Y..T of the order of 6 K between sensor and reference. For
the temperature stabilization of the pressure sensor the temperature coefficient of resis­
tance (TC of R) of0.34% /K of the heater was exploited. Figure 9 shows the corresponding
output voltage as a function of air pressure. The rnicrosystem shows maximum sensitivity
between 102 and 106 Pa. In view of the unrestricted CMOS compatibility of the surface
rnicromachining procedure used here, signal processing of the nonlinear output withdedicated CMOS circuitry can be incorporated easily.

6.

Thermal Properties of CMOS Materials

The various responses of the rnicrosensors discussed in the previous sections are
determined by the thermal and electronic properties of their constitutive materials. In viewof the "unconventional" uses of CMOS layers in microsensors, many sensor-relevant thinfilm properties are not routinely monitored in commercial IC processes. Knowledge of
their values, however, is a prerequisite for thermal sensor design and, in particular, theoptimization of such devices by numerical simulation. Table 1 summarizes the specificways in which CMOS layers are used in the described thermal devices. Among therequired material parameters are the thermal conductivities Kand heat capacities c of theindivictual layers. Of equal importance are electronic transport properties of the thermopile

418

Sensors and Materials, Vol. 8, No. 6 (1996)

600

.....

400

200

4
10° 10 1 102 103 10 105 106

Pressure p [Pa]Fig. 9. Pressure response of a rnicrosystem consisting of the thermal vacuum sensor shown in Fig. 8,
a pressure-insensitive reference structure, and CMOS read-out circuity.

materials, such as the Seebeck coefficient a, resistivity p, charge carrier densities n or p,
and their respective temperature coefficients. In view of their process dependence, theproperties have to be measured systematically for each IC process sequence used.
For this purpose we developed special CMOS-compatible micromachined diagnostic
structures.<21-24J Each structure is optimized for one physical parameter while cross­sensitivities to other effects are minimized. Figure 10 shows a thermal microbeam usedto
measure the Seebeck coefficient of CMOS polysilicon against aluminum. The beam
consists of the entire sandwich of CMOS dielectrics and contains an integrated heater, atemperature monitor, and a contacted sample of the characterized polysilicon layer. Other
thermal structures have been designed to measure the thermal conductivities of all CMOS
layers and heat capacities of some of them.<2 1• • lTable 2 lists measured thermal and electronic transport coefficients at 300 K. Results
for the double-poly double-metal CMOS processes of Austria Mikro Systeme (AMS) andMicroelectronic Marin (EM) with respective gate lengths of 1.2 and 2 µm are compiled.
We routinely measure such properties in the temperature range from 100 to 420 K, which
also allows derivation of the required temperature coefficients.
22 24

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Sensors and Materials, Vol. 8, No. 6 (1996)
Table 1
Sensor-specific uses of CMOS IC layers for the thermal microsensors discussed in this paper.
Material

Use

Silicon dioxide

Thermal isolation
Thermal mass
Mechanical building block

Polysilicon

Thermopile component
Heating resistor
Temperature monitor
Thermal conductor
The1mal mass
Mechanical building block

Metal layers

Thermopile component
Thermal conductor
Thermal mass
Mechanical building block
IR reflector

Passivation

The1mal isolation
Thermal mass
Mechanical building block
IR absorberFig. 10. SEM micrograph of a thermal diagnostic microstructure for measuring the Seebeckcoefficient of CMOS polysilicon.

Sensors and Materials, Vol. 8, No. 6 (1996)

420

Table 2
Measured physical properties of materials in a 1.2 µm process of AMS Austria Mikro Systeme and a
2 µm process of EM Microelectronic Marin. Compiled properties are thermal conductivity IC, heat
capacity c, Seebeck coefficient a, sheet resistance p, temperature coefficient of resistance (TCofR),and majority carrier densities n and p.
Material

AMS

EM[W/mK][J/cm3K]

1.25±0.1
1.05 ±0.1

n.n.
n.n.

[W/mK][µV/K][Q/Sq.][10-3K-1][1020cm-3]

28±2, 19±2
-120±5, 190±525,2150.860, - 0.140
3.4, 1.6

17 ±2, 20±2
-111±5,330±10
32,4250.825,-0.586
1.8, 0.33

p

TCofR

[W/mK][mQ/Sq.][l0-3K-1]

194±8
65
2.96

250±1044
4.38

Kn+ ' KP+
Pn•, Pp•
TCofR[W/mK][Q/Sq.][ l0 3K-1]
-

173±8
36
3.01

240± 1029
4.28[W/mK]

1.54 ± 0.05
2.7 ± 0.4

n.n.

Property,c
SiO2(thermal, CVD) c (th.)
Poly-Si

+

'ICn

'

"l(P-t-Cln+ ' ap+
P.•, pp•
TCofR
+

n ' p

Metal 1

Metal 2

Passivation

,c

1(

C

7.

+

(th.)

[J/cm3K]

1.9 ±0.1

Outlook

While the recent progress in thermal sensors made by CMOS IC technology combinedwith post-processing bulk or surface micromachining is impressive, it should be remem­
bered that the translation of working prototypes into full-scale industrial production is nottrivial. Even more difficult is the incorporation of the successful sensor chip into the final
application system, in particular with respect to encapsulation.c25l
While many CMOS manufacturers fabricate IC wafers and some are versed in CMOS­
compatible micromachining, very few oversee the engineering of the whole system
including reliability and electromagnetic compatibility, which is decisive for a successful
final sensor product. On the research side this means that early collaboration between thesensor designer, CMOS manufacturer, and final system manufacturer is imperative.

Sensors and Materials, Vol. 8, No. 6 (1996)

421

Acknowledgments
The authors would like to thank Mr. D. Jaeggi, ETH Zurich, for contributing tosection 3. The financial support of the Swiss Confederation through the Priority Program
LESIT and the ESPRIT III Program of the European Union (BBW grant 93.0161) isgratefully acknowledged.

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