Tin Oxide Microsensors - ACS Publications - American Chemical Society

3 Tin Oxide Microsensors Shih-Chia Chang and David B. Hicks Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch003

Electronics Department, General Motors Research Laboratories, Warren, MI 48090-9056

Tin oxide based semiconductor gas sensors were fabricated on silicon wafers using conventional micro -electronic processing technology. Thin film tin oxide was delineated by a reactive ion etching technique using SiCl as the etch gas. The overall device size is 450 μm x 350μmwith a sensing element feature size of 50μmx 50 μm. The sensor resistance changes significantly when the sensor is exposed to alcohol vapor or NO in ambient air. At sensor temperatures ranging from 200 to 300°C, the values of R(air)/ R(200 ppm of alcohol vapor in air) are 50 and 140 for tin oxide (SnO) and palladium-gold/SnO (PdAu/SnO) sensors, respectively; while the values of R(50 ppm of NO in air)/R(air) are 60 and 5 for SnO and PdAu/SnO sensors, respectively. Possible sensing mechanisms based on XPS, Kelvin probe, Hall and electrical conductivity measurements are also discussed. 4

X

x

X

x

x

x

x

The resistivity of certain semiconductors such as tin oxide (Sn0) and zinc oxide (ZnO) can be strongly modulated by the presence of certain gaseous species in the ambient. Several gas sensors have been developed based on such material characteristics (1-5). The principal advantages of semiconductor gas sensors are: (a) relative simplicity of fabrication; (b) relative simplicity of operation; (c) low cost (fabrication and maintenance). However, the major drawback of these sensors is their low sensing selectivity among various gases. Sensor selectivity can be enhanced by devising a multi-sensor scheme in conjunction with logic/control circuitry to form an inte grated sensing system (6£7). In such a system the selectivity would be achieved either by operating the individual sensing elements at different temperatures, by altering or modifying the materials used x

0097-6156/ 86/ 0309-0058S06.00/ 0 © 1986 American Chemical Society

In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

CHANG AND

3.

Tin Oxide Microsensors

HICKS

59

i n the s e n s i n g elements, o r by u s i n g a c o m b i n a t i o n o f both approaches. For example, i n our r e c e n t t e s t s on p a l l a d i u m - g o l d / t i n o x i d e s e n s o r s , we found t h a t the s e n s o r s responded q u i t e d i f f e r e n t l y to the presence o f N0 (50 ppm) and a l c o h o l (C2H5OH, 200 ppm) i n a i r when they were o p e r a t e d a t d i f f e r e n t temperatures as i n d i c a t e d i n Table I . X

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch003

Table I .

R e s i s t a n c e o f PdAu/SnO Sensor i n Three D i f f e r e n t Ambients a t Two D i f f e r e n t Sensor Temperatures x

Gas T h i n F i l m PdAu/SnO

Sensor

x

Sensor r e s i s t a n c e a t 220°C Sensor r e s i s t a n c e a t 280°C

N0

Species in A i r M0 C H OH 2

X

increase + increase +

5

X

+ C H OH 2

5

increase t decrease +

decrease decrease

By s i m u l t a n e o u s l y m o n i t o r i n g the r e s i s t a n c e change o f the PdAu/SnO f i l m s a t two d i f f e r e n t sensor t e m p e r a t u r e s , p o t e n t i a l l y we can d i f f e r e n t i a t e t h r e e d i f f e r e n t ambient c o n d i t i o n s : (a) N0 p r e s e n t ; (b) C HcjOH vapor p r e s e n t ; ( c ) both N0 and C H^0H vapor p r e s e n t . An i n t e g r a t e d m u l t i - s e n s o r system r e q u i r e s p r e c i s e c o n t r o l o f sensor c h a r a c t e r i s t i c s , and may r e q u i r e 10 o r more s e n s o r s i n c l o s e p r o x i m i t y . To a c h i e v e t h i s , we have t o r e l y on m i c r o e l e c t r o n i c p r o c e s s e s i n o r d e r t o f a b r i c a t e s e n s o r s w i t h s m a l l and p r e c i s e l y cont r o l l e d f e a t u r e s i z e s on s i l i c o n . In t h i s paper, the a p p l i c a t i o n o f m i c r o e l e c t r o n i c p r o c e s s i n g t e c h n o l o g y t o the f a b r i c a t i o n o f SnO and PdAu/SnO m i c r o s e n s o r s on s i l i c o n wafers i s d e s c r i b e d , s e n s o r r e s p o n s e s t o v a r i o u s gases i n a i r a r e p r e s e n t e d , and the p o s s i b l e s e n s i n g mechanisms a r e b r i e f l y discussed. x

X

2

X

2

x

x

Experimental Device F a b r i c a t i o n . A four-mask p r o c e s s was used f o r t h e f a b r i c a t i o n o f t i n o x i d e m i c r o s e n s o r s on s i l i c o n s u b s t r a t e s . The major p r o c e s s i n g s t e p s a r e shown i n F i g u r e 1: 1.

A 1 ym t h i c k S i 0 l a y e r was t h e r m a l l y grown on a s i l i c o n wafer. A 1 ym t h i c k p o l y c r y s t a l l i n e s i l i c o n ( p o l y s i l i c o n ) l a y e r was then d e p o s i t e d by c h e m i c a l vapor d e p o s i t i o n (CVD). Phosphorus d o p i n g o f p o l y s i l i c o n was done by i o n i m p l a n t a t i o n w i t h a dosage o f 1 0 cm" and a v o l t a g e o f 200 keV. The p o l y s i l i c o n s h e e t r e s i s t a n c e o f 50 fl/ • was o b t a i n e d a f t e r p o s t - i m p l a n t a c t i v a t i o n ( F i g u r e 1a). 2

l b

2


60 2.

F U N D A M E N T A L S A N D APPLICATIONS O F C H E M I C A L SENSORS

P o l y s i l i c o n (used as a sensor h e a t e r ) was d e l i n e a t e d by e t c h i n g i n a SF^ plasma. A 1 ym l a y e r o f CVD S i 0 was d e p o s i t e d , and a 100 nm t i n o x i d e f i l m was s u b s e q u e n t l y s p u t t e r - d e p o s i t e d ( F i g u r e 1b). The t i n o x i d e t h i n f i l m was p a t t e r n e d by r e a c t i v e i o n e t c h i n g (RIE) u s i n g e i t h e r S i C l j j o r 7% H i n N as t h e e t c h gas. The p o l y s i l i c o n c o n t a c t h o l e s were opened by w e t - c h e m i c a l e t c h i n g i n b u f f e r e d h y d r o f l u o r i c a c i d (BHF). A d o u b l e - l a y e r m e t a l l i z a t i o n (Cr -50 nm p l u s A l ~1 ym) was done by e l e c t r o n beam e v a p o r a t i o n t o form t h e e l e c t r i c a l i n t e r c o n n e c t i o n ( F i g u r e 1 c ) . The m e t a l i n t e r c o n n e c t f e a t u r e was d e f i n e d by wet chemical e t c h i n g . A l a y e r o f PdAu, -2.5 nm t h i c k w i t h a c o m p o s i t i o n o f Pd/Au = 4/6, was f l a s h d e p o s i t e d on some o f t h e samples t o enhance t h e sensor s e n s i t i v i t y t o C H^0H ( F i g u r e 1d). F i g u r e 2 i s t h e SEM p i c t u r e o f a completed t i n o x i d e m i c r o s e n s o r w i t h f o u r m e t a l bonding pads. The complete s e n s i n g d e v i c e has a d i m e n s i o n o f 450 ym x 350 ym w i t h an a c t i v e s e n s i n g element ( e i t h e r S n 0 o r PdAu/Sn0 ) f e a t u r e s i z e o f 50 ym x 50 ym. 2

3.


2

4.

2

2

x

x

The s p u t t e r - d e p o s i t i o n o f t h e t i n o x i d e t h i n f i l m (-100 nm t h i c k ) was done under t h e f o l l o w i n g c o n d i t i o n s : a. b. c. d. e. f.

t a r g e t m a t e r i a l : s i n t e r e d t i n oxide RF power: 150 W s u b s t r a t e - t o - t a r g e t d i s t a n c e : -7.5 cm argon p r e s s u r e : -1 Pa; oxygen p r e s s u r e : -0.2 Pa s p u t t e r t i m e : 10 min s u b s t r a t e t e m p e r a t u r e :

CO

2 (6.7x10 ) -

1 (2.5x10 ) Time F i g u r e 3b. Sensor responses to 200 ppm of a l c o h o l vapor, 200 ppm of a l c o h o l vapor p l u s 200 ppm of p r o p y l e n e , 200 ppm of p r o p y l e n e , and s a t u r a t e d water vapor i n a i r f o r PdAu/SnO sensor. Sensor temperature ~250 °C. x




C H A N G A N D HICKS

F i g u r e 4.

Sensor r e s i s t a n c e vs c o n c e n t r a t i o n ( i n ppm) o f a l c o h o l vapor ( O ) , C^H^ (•) and CO ( A ) : (a) f o r SnO s e n s o r , (b) f o r PdAu/SnO s e n s o r . Sensor temperature ~250°C. x

x


66

F U N D A M E N T A L S A N D APPLICATIONS O F C H E M I C A L SENSORS

4x10' CO


E

O C CO

3x10' -

CO CD

oc

2x10

o CO

c CD

CO 1x10' -

Time F i g u r e 5.

Sensor responses t o 50 ppm o f N 0 i n a i r f o r SnO and PdAu/SnO s e n s o r s . Sensor temperature ~250°C. X

x

x

2

0

2

-2 -2 o- 0- 0" 0

2

2

0" 0-

2

Depletion Layer Polycrystalline Thin Film Tin Oxide

(a)

Grain Boundary

F i g u r e 6.

( a ) The f o r m a t i o n o f d e p l e t i o n l a y e r s i n t h e s u r f a c e and g r a i n boundary r e g i o n s due t o oxygen c h e m i s o r p t i o n reduces the c a r r i e r c o n c e n t r a t i o n ( n ) ; ( b ) t h e f o r m a t i o n o f p o t e n t i a l b a r r i e r s a t t h e g r a i n b o u n d a r i e s reduces t h e c a r r i e r m o b i l i t y ( y ^ ) . When a l c o h o l vapor was i n t r o d u c e d , b o t h n and y i n c r e a s e d . H


3.

Step ( 2 ) : C

H

67


C H A N G A N D HICKS

C-H-OH (gas) - CH CH0 + 2H*(ads) o

0 H

i s

2 5 producing Step ( 3 ) :

d

i

s

s

o

c

i

a

t

i

v

e

l

y chemisorbed on t h e SnO a c t i v e hydrogen H*.

2H* (ads) + 0 "

2

x

surface,

(ads) - H 0 + 2e~ 2

2

2H* r e a c t s w i t h 0~ , and t h e r e l e a s e d e l e c t r o n s a r e i n j e c t e d back t o t h e SnO c o n d u c t i o n band, c a u s i n g t h e d e p l e t i o n l a y e r w i d t h t o decrease. Hence, both t h e c a r r i e r c o n c e n t r a t i o n and c a r r i e r mobility increase.


x

Thus, t h e r e s i s t a n c e o f t h e SnO i s h i g h e r i n an ambient w i t h h i g h e r oxygen c o n c e n t r a t i o n ( o r i n an ambient c o n t a i n i n g s t r o n g o x i d a n t s , such as N 0 ) , ( S t e p 1). The r e s i s t a n c e o f SnO i s lowered by t h e presence o f C H^0H vapor, due t o t h e r e a c t i o n between H* and 0" , which r e l e a s e s t h e l o c a l i z e d e l e c t r o n back t o t h e SnO c o n d u c t i o n band (Step 3 ) . The s e n s i n g mechanisms p r e s e n t e d above a r e s u b s t a n t i a t e d by t h e e x p e r i m e n t a l r e s u l t s o b t a i n e d from t h e H a l l measurements (Table I I ) . T a b l e I I g i v e s t h e measured c a r r i e r c o n c e n t r a t i o n ( n ) , and e l e c t r o n H a l l m o b i l i t y ( y ) o f a SnO sensor under d i f f e r e n t ambient c o n d i t i o n s . As p r e d i c t e d , b o t h e l e c t r o n c o n c e n t r a t i o n and e l e c t r o n m o b i l i t y i n c r e a s e ( o r d e c r e a s e ) when a l c o h o l vapor ( o r N 0 ) i s p r e s e n t i n a i r . [ I n t h e H a l l e x p e r i m e n t , c e r t a i n hydrocarbon gaseous s p e c i e s were r e l e a s e d from t h e f i b e r g l a s s / r e s i n sample h o l d e r due t o t h e sensor h e a t i n g , c a u s i n g t h e sensor c o n d u c t i v i t y i n " a i r " t o be u n u s u a l l y h i g h as l i s t e d i n t h e T a b l e I I ] . The most l i k e l y e f f e c t o f PdAu d e p o s i t e d on t h e PdAu/SnO sensor s u r f a c e i s t h e promotion o f t h e d i s s o c i a t i v e a d s o r p t i o n o f C H^0H (Step 2) due t o t h e s t r o n g c a t a l y t i c s t r e n g t h o f Pd on hydrocarbon adsorbates. Hence, more a c t i v e hydrogen s p e c i e s (H*) a r e c r e a t e d by Pd, and more l o c a l i z e d e l e c t r o n s [ 0 ~ ( a d s ) ] a r e r e l e a s e d and i n j e c t e d back t o t h e SnO c o n d u c t i o n band (12,13). Another p o s s i b l e e f f e c t o f PdAu d e p o s i t s on PdAu/SnO s e n s o r s i s through t h e f o r m a t i o n o f a S c h o t t k y b a r r i e r between PdAu and SnO , as i n t h e case o f t h e Pd/CdS hydrogen s e n s o r . I f such a b a r r i e r i s formed, then a d e p l e t i o n l a y e r i s c r e a t e d i n s i d e t h e s e m i c o n d u c t o r t i n o x i d e . S i n c e t h e Pd work f u n c t i o n can be reduced by hydrogen a b s o r p t i o n t h r o u g h d i p o l e o r h y d r i d e f o r m a t i o n (14,15), t h e w i d t h o f the d e p l e t i o n l a y e r i n t i n o x i d e may be reduced. The r e d u c t i o n o f the d e p l e t i o n l a y e r w i d t h causes t h e sample r e s i s t a n c e t o d e c r e a s e . Such a p o s s i b i l i t y was checked and was r u l e d out, because a good ohmic c o n t a c t was o b t a i n e d between Pd (-50 nm t h i c k ) and SnO . I t i s a l s o commonly known t h a t g o l d forms an ohmic c o n t a c t w i t h t i n o x i d e . R e c e n t l y , Yamazoe e t a l . observed an extremely h i g h hydrogen s e n s i t i v i t y f o r Ag-Sn0 s e n s o r s (_^6). They a t t r i b u t e d t h i s t o t h e Fermi l e v e l o f SnO b e i n g pinned a t t h e redox p o t e n t i a l o f Ag /Ag° when t h e sensor was i n a i r , and a t t h e work f u n c t i o n o f Ag° ( m e t a l l i c ) when t h e sensor was i n a i r c o n t a i n i n g hydrogen. I n o u r x

X

x

2

2

x

H

x

X

x

2

2

x

x

x

x

2

+

x



3.14

UH (cm /V.s)

2

1 8

5

5.5

2

8.15 • 1 0

7.2

7

5.43

UH (cm /V.s)

2

2.23 • 1 0

2 • 10™

1 .10™

n (cm" )

3

5

1 8

1 9

1000 ppm C H OH 2000 ppm C H OH In Air in Air

4.9

7.2 • 1 0

5

Air

1 8

2

1000 ppm C H OH 2000 ppm C 2 H 5 OH in Air in Air

Ambient

Sensor at 230°C

2

3.6 • 1 0

n (cm )

-3

Air

Ambient

Sensor at 167°C

3

2.4 • 1 0

x

1 8

x

1 8

50 ppm NO in Air

2

3.54 • 1 0

50 ppm NO in Air

1 8

2.8

1.25 • 1 0

x

x

1 8

100 ppm NO in Air

1.54

2.5 • 1 0

100 ppm NO in Air

Table I I . E x p e r i m e n t a l R e s u l t s Obtained from the H a l l Measurements.


3.

CHANG AND HICKS


69

earlier study of PdAu/SnOx sensors (]2), the observed chemical state of Pd (XPS data) was Pd° state (metallic) after the sensor was heat treated in air at 350°C. Hence, the electronic effect suggested by Yamazoe et al. for the Ag-Sn02 sensor may not be applied to the PdAu/SnOx sensor discussed here.


Summary We have demonstrated that tin oxide-based microsensors can be fabricated on silicon wafers using microelectronic processing technologies. Alcohol vapor with a concentration of 50 ppm in air (which is about 1/10 of the alcohol concentration in the breath of a driver legally regarded as intoxicated) can be easily detected by the fabricated SnOx and PdAu/SnOx sensors. The PdAu deposit on tin oxide thin films enhances alcohol vapor as well as propylene sensitivity appreciably. Such enhancement is mainly attributed to the strong catalytic effect of Pd on alcohol vapor and propylene. Although the SnOx sensor showed reasonably high selectivity to alcohol vapor in the presence of water vapor, C^H^, CO, or CH^, further improvement in alcohol selectivity is needed in order to make it a practicable and reliable alcohol detector. This is especially true with regard to the interference gases of hydrocarbons and CO which are the major constituents in cigarette smoke. One of the approaches to attain high selectivity and reliability is by devising a multi-sensor scheme in conjunction with logic/control circuitry to form an integrated sensing system. To achieve this, there are two prerequisites: A) a detailed (atomic scale) understanding of sensing mechanisms, B) an effective thermal-isolation scheme to minimize heat transfer to the surrounding environment. The benefit of such a scheme is two-fold: It reduces sensor power consumption and, at the same time, protects adjacent logic/control circuitry from extreme temperatures generated by the sensor heater. Both sensors also showed high sensitivity to N0X (NO + N02) content in air. N0X with a concentration of 10 ppm in air is readily detected by both SnOx and PdAu/SnOx sensors. While a PdAu deposit on tin oxide enhances alcohol vapor and propylene sensitivity, it depresses the sensitivity to N0X. Acknowledgments We would like to thank Cheryl Puzio for the preparation of the mask set used in this work, John Biafora for the ion implantation doping of polysilicon, and W. Lange of the Analytical Chemistry Department for the SEM work. Literature Cited 1. Seiyama, T.; Kato, A.; Fujiishi, K.; Nagatani, M. Anal. Chem. 1962, 34, 1502.


70 2. 3. 4. 5. 6.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS

Nitta, M.; Kanefusa, S.; Haradome, M. J. Electrochem. Soc. 1978, 125, 1676. Chang, S. C. IEEE Trans. Electron Devices 1979, ED-26, 1875. Yamamoto, N.; Tonomura, S.; Matsuoka, T.; Tsubomura, H. Jap. J. Appl. Phys. 1981, 20, 721. Heiland, G. Sensors and Actuators 1982, 2, 343. Ko, W. H.; Fung, C. D.; Yu, D.; Yu, Y. H. Proc. of the Intl. Mtg. on Chemical Sensors, Seiyama, T.; Fueki, K.; Shiokawa, J.; Suzuki, S., Eds.; 1983, p. 496. Clifford, P. K. Proc. of the Intl. Mtg. on Chemical Sensors, Seiyama, T.; Fueki, K.; Shiokawa, J.; Suzuki, S., Eds.; 1983, p. 153. Chang, S. C. GM Research Publication GMR-4727, 1984. Seiyama, T.; Yamazoe, N.; Futada, H. Denki Kagaku 1971, 21, 53. Windischmann, H.; Mark, P. J. Electrochem. Soc. 1979, 126, 627. Chang, S. C. Proc. of the Intl. Mtg. on Chemical Sensors, Seiyama, T.; Fueki, K.; Shiokawa, J.; Suzuki, S., Eds.; 1983, p. 78. Chang, S. C. J. Vac. Sci. Technol. 1983, Al, 296. Seiyama, T.; Futada, H.; Eva, F.; Yamazoe, N. Denki Kagaku 1972, 40, 244. Steele, M. C.; Hile, J. W.; MacIver, B. A. J. Appl. Phys. 1976, 49, 2537. Lundstrom, I. Sensors and Actuators 1981, 1, 403. Yamazoe, N.; Kurokawa, Y.; Seiyama, T. Proc. of the Intl. Mtg. on Chemical Sensors, Seiyama, T.; Fueki, K.; Shiokawa, J.; Suzuki, S., Eds.; 1983, p. 35.

RECEIVED December 12, 1985


Tin Oxide Microsensors - ACS Publications - American Chemical Society

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