Advances in SAW-based gas sensors

Smart Mater. Struct. 6 (1997) 689–699. Printed in the UK

PII: S0964-1726(97)87992-X

Advances in SAW-based gas sensors C Caliendo†, P Verardi†, E Verona†, A D’Amico‡, C Di Natale‡, G Saggio‡, M Serafini‡, R Paolesse§ and S E Huqk † CNR–Istituto di Acustica ‘O M Corbino’, via Cassia No 1216, 00189 Rome, Italy ‡ Dipartimento di Ingegneria Elettronica, Universita` di Roma ‘Tor Vergata’, via della Ricerca Scientifica, 00133 Rome, Italy § Dipartimento di Chimica, Universita` di Roma ‘Tor Vergata’, via della Ricerca Scientifica, 00133 Rome, Italy k Central Microstructure Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK Received 14 February 1997, accepted for publication 3 July 1997 Abstract. This paper deals with some advances in surface acoustic wave (SAW) sensor research relating to the use of both a new chemically interactive material (metal porphyrin) and new amplifiers for low-noise SAW oscillators, which are illustrated and discussed in some detail. The trend toward SAW matrixes, implemented on silicon, required for more sensitive chemical image systems, has led to the observation of coupling effects between adjacent SAW oscillators, even if the electronics were not implemented on Si. This effect must be considered at design level in order to achieve a higher resolution value in detecting either single chemical species or chemical images.

1. Introduction In recent years there has been growing attention to surface acoustic wave (SAW) devices for the detection of various physical and chemical parameters including temperature [1, 2], acceleration [3–8], force [9], pressure [10–13], DC and AC high voltages [14–16], electric fields [17], magnetic fields [18], weight percentage in solutions [19], ionic concentration [20], gas flow [21], dew point [22] and gas and vapour concentration [23–38]. Table 1 gives an idea of the research work done since 1981 on SAW chemical sensors. The use of SAWs in the field of sensors is favoured by a number of different mechanisms of interaction between the measurand and the acoustic propagation. These deal with many different linear and non-linear properties of the propagation medium which are related to mass density, elastic stiffness and, in piezoelectric material, also to electric and dielectric behaviour. Furthermore, as the acoustic energy is confined into a thin near-surface region of the substrate, the SAW propagation is very sensitive to surface perturbations and the whole propagation path is responsible for the response of the device. Suitable substrates are piezoelectric material such as quartz and LiNbO3 , even if silicon in combination with AlN or ZnO have been proved to represent an adequate solution for high-volume production as required by semiconductor industries, for which the full implementation of both SAW sensors and related microelectronics is important. This paper takes into consideration the recent work related to the use of SAW devices only in the chemical sensing area, even if the electronic circuit considerations can be applied as well as to physical sensors. In particular in this c 1997 IOP Publishing Ltd 0964-1726/97/060689+11$19.50

paper we only report recent new but preliminary results on SAW sensors obtained along two different research lines. The first is concerned with the study and synthesis of new chemical interactive materials (CIMs), named cobalto-porphyrin. The second research line deals with the optimization of a low-noise hybrid electronic circuit and the study of the interference phenomena which appear in a SAW sensor matrix fabricated on a silicon substrate. First results, dealing with new balanced amplification, are reported and commented, relative to SAW on silicon. The same experiment has been conducted also on quartz substrate for the sake of comparison, but not presented here. 2. Sensor design and fabrication The choice of studying the possibility of matrix integration in silicon follows the idea of electronic integration in the same chip. In this context we have at least two possibilities in constructing a multilayer system suitable for SAW propagation. Si/SiO2 /AlN represents one of them, but in this work the Si/SiO2 /ZnO configuration has been utilized as the one suitable for our purpose due to the full availability of the ZnO technology. The thin film of ZnO is utilized, as known, for the necessary electroacoustic transduction from the IDT to the substrate. From the acoustic propagation theory it turns out that these thin layers play an important role for the phase velocity of the SAW and for the efficiency of transduction, once the electrode configuration is designed. These electrodes can be implemented according to the configuration shown in figure 1, where we have illustrated four possible schemes, which differ from each other in the relative position of 689

C Caliendo et al Table 1. Some chemical species revealed with SAW sensors. Measurand

Substrate

Acetone Methanol H2

YZ-LiNbO3 STX-SiO2 Si/SiO2 /ZnO

CH3 CHOH H2 O H2 S

STX-SiO2 YZ-LiNbO3 YZ-LiNbO3 YZ-LiNbO3 RCY-quartz Si/SiO2 /ZnO STX-SiO2

CO CO2 CH4 SO2

Antigen/antibody reactions SO2 Toluene

STX-SiO2 YZ-LiNbO3 STX-SiO2 quartz YZ-LiNbO3 STX-SiO2 YX-LiNbO3 quartz Si/SiO2 /ZnO LiNbO3 STX-SiO2 RCY-quartz z cut LiNbO3 yz LiNbO3 ZnO/Al/Six Ny

CH2 Cl2 Humidity Organic vapours Vapours

STX-SiO2 ZnO/SiO2 /Si ZnO/Si

NO2

NH3

Chemically interactive material (CIM)

Reference

Hydroxybutyl methyl cellulose Hydroxybutyl methyl cellulose Palladium Palladium Palladium Hydroxybutyl methyl cellulose Phthalocyanine (PC) WO3 TEA WO3 WO3 WO3 Phthalocyanine (PC) Phthalocyanine (PC) Phthalocyanine (PC) Triethanolamine (TEA) Phthalocyanine (PC) Heteropolysiloxane Lead phthalocyanine

[39] [39] [40] [41] [42] [39] [43] [44] [44] [45] [46] [47] [48] [48] [48] [49] [48] [50] [51] [52] [53] [48] [48] [54] [55] [46] [56] [49] [57] [39] [39] [58] [59] [60]

Lead phthalocyanine Copper phthalocyanine Copper phthalocyanine none Platinum WO3 biologic film Triethanolamine (TEA) Poly(dimethylsiloxane) Ethylene/vinylacetate Polycarbonate resin Polyethynyfluoreno Polymers Polymers

Figure 1. Possible implementation schemes for the IDT.

fingers, ZnO thin film and floating electrodes, whose presence are utilized to give an advantage in confining the electric field lines to the piezoelectric region. In practical application, in order to reduce unwanted common mode signals such as those from temperature, pressure and humidity variation, a differential scheme is preferred such as that shown in figure 2, where only one 690

path is covered by chemical interactive material (CIM). Electric screens, connected to ground, are present to reduce direct electric coupling effects. The non-sensitive path should be covered by a material having electromechanical properties as close as possible to those of the CIM. This is not frequently considered, even if a positive contribution could be achieved for the

Advances in SAW-based gas sensors

Figure 2. Differential scheme with only one acoustic path covered by the CIM.

Figure 3. Transduction coefficient/relative velocity variation versus thickness normalized to λ.

improvement of the overall performances of the differential SAW sensor. The behaviour of the transduction coefficient K 2 (electroacoustic coupling factor) versus the thickness of the piezo-material, normalized to the wavelength λ, shows relative maxima for low h/λ values (0.05), as shown in figure 3. This allows us, once λ is chosen on the basis of the transduction periodicity, to grow thin piezo-films whose thickness is small compared to the λ value. Dimensions of the IDT, as obtained by design considerations (including Rayleigh wave velocity in the range of 4000–4500 m s−1 , centre band amplification frequency of about 70 MHz), are:

spatial periodicity λ = 60 µm, number of finger pairs N = 15, finger overlapping w = 3000 µm. The elements of the equivalent circuit of the IDT must be known as they play a role for a suitable oscillator design. In our case with reference to figure 4, the static capacitance value is approximately given by C0 =

N wi di ε0 εr 1X 2 i=1 h

(1)

where wi and di = λi /4 are the length and the width of the ith finger pair, respectively; ε0 εr the dielectric constant 691

C Caliendo et al

Figure 4. IDT/floating electrode configuration implemented.

Figure 6. Molecular skeleton of porphyrins.

Figure 5. Series equivalent circuit of the IDT.

Table 2. ZnO deposition parameters. Target–substrate spacing Rf power Substrate—type Substrate—temperature Gas—residual pressure Gas—working pressure Gas—composition Auto-polarization voltage Growth rate

55 mm 500 W Si(100) 250 ◦ C