Model of thermoelectric radiation sensors made by CMOS and micromachining

101

Sensors and Actuators A, 35 (1992) IOI- 106

Model of thermoelectric radiation sensors made by CMOS and micromachining T Elbel, R Lenggenhager

and H Baltes

Physrcal Electronrcs Laboratory, ETH Zurich, CH-8093 Zurrch (Swtzerland)

(ReceivedJune 2.0,1991,m revisedform January 7. 1992, accepted June 9, 1992)

Abstract The feaslblllty of thermoplle infrared detectors on CMOS slhcon oxide cantilever beams isolated by post-processmg amsotroplc etchmg 1s assessed by an analytlcal model The sensltlvlty and detectlvlty are calculated for a vanety of device geometries An optImaI design v&h a predxted sensltlvrty of 20 V/W for a recelvmg area of 0 5 mm2 IS proposed

1. Introduction Thermoelectnc radiation sensors are based on the Seebeck effect The apphcatlon of the Seebeck

effect for contact temperature measurements with wire thermocouples IS well known Figure 1 shows a typical thermocouple The Junction of two thermoelectrlc materials,, A and B, with different thermoelectnc forces, uA and &$,, is maintained at temperature T,, (hot pomt) and the Junctions between contact material C and A or B are kept at a lower temperature T, (cold point) The resulting electrical voltage is u =

@A

-

‘%z)(~II

-

c)

(1)

In non-contact temperature sensors the radiation to be measured 1s absorbed by the ‘hot point’, which 1s formed as the receiving area Generally, a thermoelectric radiation sensor consists of several thermocouples Interconnected m senes and formmg a thermoplle To achieve high sensltmty, the thermoplle must be designed as a thin-film system with mmlmlzed thermal losses between the hot and cold Junctions and between the hot Junctions and the environment In recent years improved thermoplle sensors have been developed using Bl/Sb as thermoelectric

materials combined with thin-film technology, photohthographlc patterning and mlcromachmmg [ 1 - 31 Although integrated sensors with these thermoelectrlc materials have been reahzed by hybn-d integration techniques [4, 51, the essential daad09244247/92/%5 00

vantage of sensors made of semimetals or compound semiconductors 1s their mcompatlblhty with slhcon integrated clrcult technology To satisfy the demand for inexpensive reliable sensors allowing the comtegratlon of circuitry, an alternative approach 1s sensor fabncatlon by an IC process followed by post-processmg such as film deposition or amsotroplc etching [6,7] For example, thermoelectric mlcrosensors realized m this way have been developed for measuring gas flow and thermal radiation using bipolar technology [S] and for gas flow using CMOS [9, lo] technology In this paper we examine whether the CMOS thermoplle on an oxide beam previously [9] used

Th

Rg 1 Typxal thermocouple conslstmg of two dG-%ent thermoelectnc materrals A and B and a conductor matenal C, wth Junctions at temperature Th (‘hot’) and T, (‘cold’) The thermoekctncal voltage (I IS proportional to the temperature d~flercnce T,, - T,

@ 1992 -

Elsevler Sequoia All rights reserved

for a gas-flow sensor can also be exploited for an infrared detector To this end, we model the output voltage and detectlvity of a variety of design geometries compatible with the CMOS process and subsequent mlcromachmmg

Polyslhcon

-

_

2. Sensor structure The CMOS thermoelectric mlcrosensor structure is shown m Fig 2 Alummmm and polyslhcon of an mdustnal CMOS IC process are used as thermoelectric materials The Al/poly thermocouples connected m series are placed on a thin cantdever beam The irradiated hot contacts of the thermoplle are posltloned at the tip of the beam, the cold contacts are arranged on the bulk s&on, which acts as a heat smk The cantilever beam 1s made of a sandwich of field oxide, CVD oxide/ nitride and CVD passlvatlon Its overall thickness 1s about 2 pm The pertinent design and manufacturing methodology are reviewed elsewhere [6,7] In contrast to the sensor developments described m refs l-5, mlcromachmmg 1s the last process step It IS realized by maskless amsotroplc slhcon etching from the front of the wafer, performed after completion of the thermoplle by the CMOS IC process [9] Usually the hot Junctions are coated with a black absorber film, whose dimensions define the receiving area of the sensor For particular apphcations, such as the detection of laser radiation, the sensor works without a special absorber film

Fig 2 Cross sectIon of alurnlnlum-polyslllcon thermoplle on oxide beam over etch pit (CVD oxide between polys~hcon and metal 1s not shown)

etch

Rg 3 SchematIc top VPW of cantilever beam thermopde sensor with receivmg area from Y = 0 to x = L, and the thermopde from x = L, to x = L,

x < L, we obtain the relation d’(T(x) - T,) s,oA(T4(x) - T;(x)) dx2 IV

3. Sensitivity model

~4n4 Figure 3 shows a schematic top view of the thermoptle sensor In order to calculate the sensltlvlty, we have to know the temperature dlstnbutlon along the cantilever beam m the x-direction We start from the steady-state heat conduction equation dlv(A V(7’ - T,)) + N, = 0

bulk 4

pit

(2)

with T, denoting the environmental temperature, N, the heat-loss power per volume V and 2 the thermal conductlvlty Neglecting boundary dlsturbances m the y-direction and assuming a uniform thermal conductlvlty and a uniform temperature across the thm cantdever sandwich, for L, <

-

- Tel = () IV

(3)

Here E, denotes the emlsslvlty of the cantilever matenal, Q the Stefan-Boltzmann constant, y the heat-transfer coefficient, A the surface area and x the variable distance along the beam (see Fig 3) The thermal conductlvlty of a sandwich with film thicknesses t, and thermal conductlvltles 1, (s=1,2, ) 1s

(4) s

103

The heat-transfer coefficient for a quiet gas envlronment m a small package 1s [2,3] y=Iz,

($+f ) 2

where a, denotes the thermal conductlvlty of the gas atmosphere, d, the distance between the cantilever and the bottom of the etch pit and d2 the distance between the cantilever and package cap With (T - T,)/T, 4 1 and msertmg eqns (4) and (5), eqn (3) becomes

wth the n-radiance E and the emlsslvlty E, of the sensltlve area on the upper face of the beam (The emlsslvlty Q of the lower face of the beam may or may not be different from E, ) The heat flow from the recavmg area through the beam to the heat smk at the position x = L, IS determined by the absorbed u-radiance and the thermal losses of the sensitive area by radiation and dissipation due to the ambient gas From eqns (10) and (11) the resulting temperature Increase IS obtained as T,-T,=

d’(T(x) - r,) dx2 -

EsE 4(&,+ &,)crT,3+ y + ;

c US S

x (T(x) - T,) = 0

(6) With L, denoting the beam length, L, the length of the receiving area (see Fig 3), and T, the temperature of the recelvmg area, the followmg boundary condltlons apply (1) the heat-sink condltlon [T(x) - Tel,= L, = 0

(7)

and (11) the heat-source condltron [T(x) -

(8)

U=L, = T - T,

We assume that the receiving area at the tip of the beam has an isothermal collector layer and therefore shows a uniform temperature increase T, - T, From eqns (6) - (8) we get the temperaE,za

S=

c

)

c &ts coth[k(L, - L,)] I( s > (12)

The sensltlvlty 1s defined as the ratio of the signal voltage U to the mcldent radiation power N, namely S = U/N

(13)

According to eqn (l), the signal voltage for a thermoplle with z Junctions 1s U = zu(T, - T,)

(1B)

With W denoting the width of the cantilever and N=EWL,

(14)

the sensltlvlty S finally reads

(15)

[4(&s+ ~,)cT,3 + r] WL, + wk c Asts coth[QLo - LA \S

/

ture dlstnbutlon (T(x)-T,)=(T,-T,)s with

k=

The correspon&g temperature increase can be calculated using the heat-balance condltlon

= {EJ - [4@, + ~,)oT,3 It y](T, - T,)} WL, (11)

Generally, a given constant receiving area 1s the starting pomt of radiation thennoplle design, optlmlzatron of the sensitivity or detectlvlty 1s accomplished by varying the length of the legs [3] In the case of CMOS sensor fabncatlon, however, the achievable length of the cantilever beam 1s hmlted Therefore, when modelhng this sensor type, we must vary not only the length of the legs, L, - L,, but also the length of the receiving area L, at constant cantilever length L, The thermoelectnc power a of the alummmm/ polyslhcon thermocouples used m our sensor development was determmed as 58 ,uV/K [9] For the thermal conductlvlty of polynhcon, we used the value 1 = 50 W/(m K) [ 1l] The thermal con-

104

ductlvlty and the emrsslvlty of a s&on oxide/ nitride sandwch system were determined as ;1 = 2 W/(m K), E, = 0 2 and E, = 0 6, respectively

[121 With these expenmental values and using eqn ( 15), we calculate the sensltlvlty S, the signal voltage U with respect to an lrradlance of 1 W/m* and the detectlvlty as functions of the length of the legs or the length of the recelvmg area, respectively We determme the detectlmty D* = NEP - ‘(A Af) “2, where NEP IS the noise equlvalent power, A the detector area and Af the bandwidth, under the assumption that thermal noise IS the mam source of noise and neglectmg all other noise sources Figures 4 and 5 show the resulting slgnal voltage U, sensitivity S and detectlvlty D* for the small cantilever thermoplle used m ref 9 with an overall length L, = 0 23 mm and a width W = 0 11 mm with 20 thermocouples Figures 6 and 7 show the correspondmg results for a cantilever with the dimensions Lo = 1 mm, W = 1 mm and 200 thermocouples As demonstrated m Figs 4-7, it IS possible to optlmlze the signal voltage U and the detectlvlty D* by varlatlon of the length of the legs or of the length of the receiving area, respectively The posltlon of the maximum slgnal voltage or detectlvlty depends on the overall length of the cantilever Figure 8 shows the optimum length of the legs for maxnnum signal voltage and for maximum detectlvlty as a function of the overall beam length Thus Fig 8 enables the designer to optimize the position of the ‘hot’ contacts for a given total beam length

Fig 5 Detectwlty D* vs length (cantdever length IS 0 23 mm)

of thermoplle

legs L,-

60

L,

20

.

SIgnalVoltage u

;, 105 3

00

02

04

06

Lo-L,

08

10

12

0

[nunI

Fig 6 Senswlty S and signal voltage U vs length of thermopde legs L, - L, (cantilever length IS 1 mm)

02 l

Sqnal Voltage U

20 s 2 v,

012 5 10

DO ai

02

Lo-L,

03

00

02

04

[n-ml

Fig 4 Sensltwrty S and slgnal voltage (I vs length of thermopde legs L, - L, (cantdever length IS 0 23 mm)

06 4,-L,

Rg 7 Detectwlty D* vs (cantlever length IS 1 mm)

08

10

12

Inun1

length

of

thermoptle

legs

L, - L,

105

Fig 6 as a function of the thermal conducttvlty of the gas atmosphere for CMOS sensors fabncated by etching from the front (thermoplle on oxide beam [9]) or from the rear of the wafer (thermoplle on oxide membrane) For the latter sensor geometry the influence of the gas 1s appreciably lower The influence of radlatlon losses on the sensitivity 1s small (about 1%)

“I

04-

203.

y

02-

s

. Ol-

4. Conclusions ‘-00

02

04

06 Lo

08

10

12

[mm1

Fig 8 Optimum length of legs L, -L, as a function of the length of the cantilever beam L, The case L, = L,/2 IS shown for comparison (dashed hnc)

Van Herwaarden and Sarro [8] found a maxlmum detectlvlty at L, = L,/2 for then 10 pm thick &con cantilever beam thermoplles fabricated by bipolar IC technology In the case of cantilever beam sensors fabncated m CMOS technology, this result holds only for very short beams, say L,dO3mm This difference m behavlour 1s due to the fact that heat conduction losses m the thick slhcon beam prevail m the bipolar case In the CMOS case, however, the influence of the envn-onmental gas cannot be neglected m view of the small dlstance between the cantilever beam and the bottom of the etch pit Our calculations were carried out for a nitrogen atmosphere Figure 9 shows the sensitivity for the geometry optimized according to

40 etching

fromthe

rear

- 30

3 2 m

20

10 t

003

002

004

006 kg

008

010

012

[W/mKl

Fig 9 Sensltwty S for the optrmlzed geometry as a function of the thennal conductwty ,I, of the gas atmosphere for a sensor structure etched from the front side and a structure etched from the rear side

We have calculated the performance figures of thermoplle radiation sensors on &con oxide beams or membranes based on CMOS technology combined with mlcromachmmg Our results show the feaslblhty of such sensors and provide the designer with optimal geometry parameters Prehmmary measurements based on lrradlatmg the tip of the previous [9] CMOS flow sensor with a laser beam are m support of our calculations Work on optlmlzed CMOS radiation sensor designs IS in progress

References 1 Tb Elbel, J E Muller and F Volklem, Mmlatunslerte thernusche Strahhmgs-scnsoren Die neue Thermosiule TS-50 1, Ferngeratetechnrk, 34 (1985) 113-115 2 T Elbel, Mmlatunzed thermoelectnc radlatlon sensors covenng a wide range wth respect to sensltwty or tmx constant, Sensors and Actuators A, 25-27 (1991) 653-656 3 T Elbel, Mmlaturlzed thermoelectnc radlatlon sensors, Sensors Mater, 3 (1991) 97-109 4 J Muller, T Elbel, S Poser and F Volklem, Thermoclektnsche Strahhmgssensoren mlt neuartlpr Sensor-Archltektur, Fezngerurerechmk, 40 (1991) 9-10 5 T Elbel, H _I Just, S Poser and N Wecke, Hyhndmtegnerte thermoelektrwhe Strahlungssensoren, Inr Conf Temperatur ‘92, Dusseldorf, Germany, Ott 8-9, 1992, VDI-Benchre No 982 m press 6 H Baltes, Design and slmulatlon of sensors, m H Rexhl (ed ), Mwo System Technologzes 90, Sprmger, Berhn, 1991, pp I17124 7 H Baltes, Mlcrotransducers by mdustnal IC technology and rmcromachmmg, Tech Dzgesr, 10th Sensor Symp, Tokyo, Japm, May 30-31, 1991, IEE Japan, pp 17-23 8 A W van Herwaarden and P M Sarro, Thermal sensors based on the Seebeck effect, Sensors and Acrmors, IO (1986) 321-346 9 D Moser, R Lenggenhager

usmg mdustnal

and H Baltes, SIllcon gas flow sensor CMOS and bipolar IC technology, Sensors and

Actuators A, 25-27

(1991) 577-581

10 G Wachutka, R Lenggenhager, D Moser and H Bakes, Analytlcal ZD-model of CMOS mlcromachmed gas flow sensors, Proc 6th Int Conf Soled-State Sensors and Actuators (Transducers ‘9I), Son Francmo, CA, USA, June 24-27, 1991, pp 22-25

106

11 G R LahqI and K D Wm., A batch-fabncated s~hcon thermoplIeInfrared detector, IEEE Tram Electron Devrces, ED-29 (1982) 14-22 12 F Volklem, Thermal conductwlty and dlffuswty of a thm film SIO, -4,N,-sandwich-system. Thm Sohd Fdms, 188 (1990) 2733

From 1983 to 1985 he worked at the fabrlcatlon and telecommumcatlon department of Landis & Gyr Corporation In 1990 he obtamed his M SC degree m physics from the Swiss Federal Institute of Technology Zurich and 1s now a Ph D student at the Physical Electromcs Laboratory

Biographies

Henry Baltes received the D SC degree from ETH Zurich m 1971 In the followmg years he was on the faculty of the Frele Umversltat Berlin, the University of Dusseldorf, the Umverslty of Waterloo, Ontano and Ecole Polytechmque Federale Lausanne In 1974, he Jomed Landis & Gyr Corporation, where he was m charge of the solid-state device laboratory From 1983 to 1988, he held the Henry Marshall Tory Chair at the Umverslty of Alberta, Edmonton, Canada From 1986 to 1988, he was a Dlrector of LSI Logic Corporation of Canada In 1988, he was appointed professor of physlcal electronics at the Swiss Federal Institute of Technology Zurich, where he dmxts the Phyncal Electromcs Laboratory Professor Baltes 1s a member of the IEEE, the Optical Society of America, the Institute of Physics (London), the Electrochemical Society, the SWISS Information Technology Society, the Swiss Electrotechmcal Assoclatlon, the SWISSSociety for Sensor Technology and the Swiss Physical Society

Thomas Elbel obtained both hu B SC and D SC degrees m electrical engmeermg from the Institute of Technology of Ilmenau (then GDR) m 1969 and 1985, respectively, and became a research associate of the former Academy of Sciences of the GDR, where he speclallzed m thermoelectric mlcrosensors In sprmg 1991 he worked as an invited vlsltmg scientist at the Swiss Federal Institute of Technology Zurich He Jomed Phyakahsch-Techmsche Bundesanstalt Braunschwelg, Germany, m May 1991 In autumn 1991 he was appomted professor of electrical engmeermg and measuring technique at the Fachhochschule Hannover, Germany In summer 1992 he was visiting professor at the Swiss Federal Institute of Technology Zurich, where he taught a course on thermal sensors Rent! Lenggenhager received a B SC degree m electrical engmeermg from NTB Buchs m 1983