Mass sensitivity of temperature-stabilized surface acoustic wave delay lines on GaAs

Mass sensitivity of temperature-stabilized surface acoustic wave delay lines on GaAs

ELSEVIER Sensors and Actuators B 24-25 (1995) 65-68 CHEMICAL Mass sensitivity of temperature-stabilized surface acoustic wave delay lines on GaAs ...

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ELSEVIER

Sensors and Actuators

B 24-25 (1995) 65-68

CHEMICAL

Mass sensitivity of temperature-stabilized surface acoustic wave delay lines on GaAs J. Enderlein a, S. Makarov b, E. Chilla a, H.-J. FrGhlich a b Saint

’ Paul-Dmde-Imtihctfilr Fe&&peelektrcmik, Hawogteipkm 5-7, LX10117 Berlin, &many Petersbowg State University,Faculty of Mathematics and Mechanics, Bibliotechmya pL 2, 198904 Saint Petenbourg - Peterhof Russia

Abstract Sensors using monolithically integrated acoustic and electronic components assembled on the same substrate require special arrangements for the temperature stabilization of the oscillator frequencies. We discuss the temperature stabilization and mass sensitivity of Rayleigh waves on (100) GaAs crystals cut in the (110) propagation direction. The temperature compensation is investigated for a GaAs/SiOa/Au layered structure. Additionally, a mass-sensitive organic layer is considered. We show that a temperature-compensated mass-sensitive device can be realized. The temperature coefficient of delay (TCD) and the temperature coefficient of frequency (TCF) are calculated. Better temperature compensation than for ST-X quartz can be achieved. Keywmk Delay lines; Gallium

arsenide; Sulfate

acoustic waves; Temperature

1. Introduction The piezoelectric semiconductor GaAs is a very interesting material for acoustic sensor applications. It offers the possibility of monolithic integration of fast active electronic components as well as highly sensitive surface acoustic wave (SAW) modules on the same substrate. Because of the favourable epitaxial growth, the (100) cut is preferred for integration of electronic devices. Fortunately in the (110) direction of this cut Rayleigh waves can effectively be generated. The piezoelectric coupling coefficient is about 70% of that of ST-quartz [l], which is at present the most favourable material in SAW sensor techniques. But in contrast to ST-X quartz, which is temperature stable, the temperature coefficient of delay (TCD) of GaAs has a value of +52 ppm “C-’ [Z]. Heat sources like electronic components could lead to local temperature differences and thus have an essential influence on the propagation characteristics of the SAW. Therefore, whenever acoustic and electronic devices must be integrated on the same substrate, the TCD must be taken into consideration. To compensate for temperature effects in sensor techniques, a parallel dual-delay-line oscillator containing a sensing channel and a reference channel is [email protected]~5/$09.50 Q 1995 Elsevier Science S.A. All rights reserved SSDI 0925-4005(94)01317-B

stabilization

often used. The difference frequency of these two oscillators is assumed to be temperature independent [3]. However, this arrangement cannot be expected to work well for highly sensitive devices, especially if heat sources are located in the immediate vicinity of the channels. In order to avoid the effect of temperature dependence, the TCD should vanish. Furthermore, both channels should be arranged as close together as possible. The TCD compensation has been widely discussed in the context of applications of SAW devices in communication techniques [4-71. It was found that temperature compensation can be realized for materials with a positive TCD by a silicon dioxide (SiO,) film overlayer [4]. For GaAs TCD =O was realized by a combined SiO,/Au layered structure [8]. Recently, it has been shown that in a multilayer system a maximum mass sensitivity exists for an optimal layer-thickness configuration [9,10], which is larger than the sensitivity of an acoustic plate-mode device [ll]. However, this configuration was not optimized for temperature compensation. Here, we analyse the mass sensitivity and temperature behaviour of an SiOJAu layered system on (100) GaAs. We show that temperature compensation as well as mass-sensitivity optimization can

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et nl. I Sensors and Actuators B 24-25 (195’5) 65-66

simultaneously be obtained for the (110) propagation direction.

2. Model of the mass-sensitive and temperaturecompensated sensor structure The structure of the mass-sensitive sensor can be described by a four-layer system consisting of the GaAs substrate, an SiO, coating (Sl), an Au layer (S2), and a twofold sensitive layer having an inner passive part (S3) and an outer active part (S4) (Fig. 1). The chosen orientation of GaAs for the considered SAW sensor is the (100) cut, where Rayleigh waves can propagate in the (110) direction. The amorphous SiO, coating (Sl) has a negative TCD of about -78 ppm ‘C--l, which balances the positive TCD of GaAs for a ftxed layer thickness [5]. A mass loading by the Au layer (S2) is required to prevent the wave energy from leaking into the substrate [8]. In contrast to GaAs, where the TCD is calculated using the temperature coefficients up to the third order and the linear thermal expansion coefficients, the TCD of the layers is taken into consideration by using only the first-order temperature coefficients of elastic stiffness. In our model the sensitivelayer material is the widely used organic copper phthal-

ocyanine (CuPC), which is sensitive with respect to NO, adsorption as well as absorption. The contribution of the organic layer to the temperature behaviour is neglected. The mass-loading effect is simulated by increasing the density p of the upper part of the sensitive layer (S4) by Ap [lo]. It is assumed that only the active part (S4) of this layer is influenced by the gas. The thicknesses of the layers (Sl, S2) can be varied in order to achieve a zero TCD. Under that condition the dependence of the mass sensitivity on the layer thicknesses is calculated to find the maximum. The relative velocity shift caused by temperature variation, (Av/II)~, and the shift effected by mass loading, (Av/w),, are calculated as a function of the normalized layer thicknesses using a matrix algorithm introduced by Adler

P21. 3. Temperature

coefficient of delay

The first-order temperature coefficient of delay is defined as the fractional change in delay per degree. A propagating wave is delayed by a change of the propagation length and of the velocity Av 1 P

CuPC,* c I, = 1.2 10 LaN/n.2

s4

cd4

= 5.10

9 N/n’,

@ = 1630

Cu-Phthalocymne

53

Au,

$2 c4*

cII=

= 299 ,

cl2

KUPC) 2,202

101lN/n

10’“N/n,[email protected]=

SiO2, c ,,:

Sl

kg/n3

2

19300 kg/n:

7,797 lOlo N/n2

C,(I-,=C,,[l+C;;)(T-25)+Cj;‘(T-25)*

:

+ C1;3)(T- 25)3]

xE :

s4 0.J

e

where a= 6.86X 10e6 is the thermal expansion coefficient of GaAs and AT is the temperature variation. The second term is mainly influenced by the temperature dependence of the elastic constants C,. We have taken into account the temperature dependence of the elastic stiffness constants of GaAs up to the third order:

= 1.577 1010N/n,2e = 2202 kq/n (100)cut GaAs

s

Z

LZ

;

S3

S2 Sl s

GUAS

propagationdirection of Rayleighmes

Euler

angle: 45,0,0

(2)

where Cl;“) is the nth-order temperature coefficient of C, evaluated at 25 “C [13]. Since GaAs is a weakly coupling material, the temperature dependence of the piezoelectric constants has been neglected. Fig. 2 shows the contour plot of TCD as a function of the thicknesses h(Si0,) and h(Au) multiplied by the wave number k= 2rr/h, (ho = 10 pm). Temperature-compensated devices can be realized in the whole considered temperature range 25-35 “C by layer-thickness combinations represented by the bold line TCD = 0.

4. Mass sensitivity of the temperature-compensated delay line

45,0,90 Fig. 1. Model of the four-layer sensor structure consisting of the {lOO} &As substrate with the (110) propagation direction of the SAW and the layer components.

The mass sensitivity S, of an acoustic delay line is defined as the relative change of phase velocity v due to mass loading m (ng) of the surface A (cm”)

J. End&in

et al. I Sensors and Actuators B 24-25 (1995) 65-68

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5. Temperature coefficient of frequency For the purpose of using SAW delay-line structures in oscillators, we are interested in the variation of the temperature coefficient of frequency (TCF) in the vicinity of TCD = 0: TCF(T)

=

f(T)-fcTd = @tp(13 - @fTd f(To)

I

Oo

I

I

0.04

0.08

I

kh(Au1

0.12

I 0.16

0.2

Fig. 2. Contour plot of the temperature coeficient of delay (TCD) of the GaAs/Si02/Au/CuPC/CuPC* structure as a function of the normalized layer thicknesses of the SiOz and Au. The thicknesses of both CuPC layers are wl(CuPC), Wr(CuPC*)=O.OZ (A,,= 10 pm).

~To)

(4)

where f(r) is the oscillation frequency at the temperature T and T,-,=2.5 “C is the reference temperature [14]. The phase delay dependence on temperature is given by the relation

where Lo is the temperature-independent length of the delay line. In Fig. 4 we display a contour plot of the TCF in thevicinity of the temperature-compensated layer-thickness combination wI(Au) = 0.1 and wl(Si0,) = 1.436. In Fig. 4(a) the layer thickness of SiO, is varied for a

Fig. 3. Mass sensitivity of Rayleigh waves on {lOO}GaAs propagating in the (110) direction as a function of normalized layer thickness of the Si02 and Au. A mass loading of 1 ng cm-’ has been simulated by a density change Ap=pXlO-” multiplied by the layer thickness h(CuPC*)=0.03 pm of the outer active layer.

The sensitivity is a negative value, because mass loading always decreases the velocity. We have calculated the mass sensitivity for different layer combinations assuming a mass loading of 1 ng cm-‘. In Fig. 3 a contour plot of S, as a function of kh(Au) and kh(Si0,) is displayed. Additionally, the TCD=O line from Fig. 2 has been drawn in order to display the condition for temperature compensation. The mass sensitivity at this line achieves its maximum of S, = - 0.55 ppm for the layer combination kh(Au) =O.l and wZ(SiO,)= 1.436. S, depends only weakly on the layer thicknesses. For an uncovered GaAs surface S, is about - 0.34 ppm. That is, temperature compensation as well as mass-sensitivity optimization can be realized simultaneously, even leading to an increase in sensitivity.

@)

khiAul

Fig. 4. (a) Contour plot of the temperature coefficient of frequency (TCF) as a function of the thickness !&(Si02) and the temperature. The thickness of the Au layer is ti(Au)=O.l. (b) Cuntour plot of the temperature coefficient of frequency (TCF) as a function of the thickness kh(Au) and the temperature. The thickness of the SiOl layer is M(Si02)= 1.436.

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30

35

40 ,0CT5 T

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Fig. 5. Temperature coefficient of frequency (TCF) as a function of temperature for TCD=O (solid line) and for error-limited layerthickness combinations (dashed lines) in comparison with ST-Xquartz (dash-dot line).

given Au layer (kh(Au)=O.l). It is clearly seen that the temperature stability depends strongly on the precision of the thickness. For a thickness inaccuracy of about 0.5% with respect to the temperature-compensated thickness, the upper limit of temperature compensation is between 28 “C and more than 40 “C, if a frequency error of 1 ppm is admitted. Therefore, the calculated value S,= -0.55 ppm can only be achieved by high-precision layer deposition. In Fig. 4(b) the Au thickness is varied. The TCF is much less temperature dependent for Au than for SiO, layer-thickness variation. It should be noted that kh can also be changed by adjusting the oscillator frequency within the same limits of 0.5% and in this way a known deviation of the thicknesses can bc compensated. In Fig. 5 the TCF of the temperature-compensated layer combination (solid line) discussed in Fig. 4 is displayed as a function of temperature. Dashed lines represent the error limits of the layer-thickness combinations. In comparison with the widely applied STX quartz marked by the dash-dotted line, the temperature behaviour for GaAs should be better even for temperatures higher than 35 “C.

and Au layers. The temperature coefficient of delay (TCD) and the temperature coefficient of frequency (TCF) have been calculated. The maximum mass sensitivity for the temperature-compensated system is S, = -0.55 ppm. The frequency shift caused by temperature variation can be limited to the same magnitude if the accuracy of layer thicknesses is better than 0.5%. For known deviations of the layer thicknesses from the temperature-compensated layer-thickness combination, the temperature stability can be increased by adjusting the SAW frequency. References

PI PI 141

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[71

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[91

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6. Conclusions PI

SAW sensors on GaAs offer the possibility of monolithic integration of electronic and acoustic components. However, temperature differences at the substrate might affect the behaviour of the device. We have found that temperature compensation and mass-sensitivity optimization can simultaneously be realized by SiO,

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