**1. Introduction**

520 Acoustic Waves – From Microdevices to Helioseismology

Yu, L. S.; Harper, J. M. E.; Cuomo, J. J. & Smith, D. A. (1985). Alignment of Thin Films by

Zhang, Y.; Wang Z. & Cheeke, J. D. N. (2003). Resonant Spectrum Method to Characterize

*Contr.*, Vol. 58, No. 5, pp. 1062–1068.

*Contr.,* Vol. 50, No. 3, pp. 321–333.

9, pp. 932–933.

Wave Resonant Frequencies and Modes. *IEEE Trans. Ultrason., Ferroelect., Freq.* 

Glancing Angle Ion Bombardment During Deposition. *Appl. Phys. Lett.*, Vol. 47, No.

Piezoelectric Films in Composite Resonators. *IEEE Trans. Ultrason., Ferroelect., Freq.* 

Polymer coated gas-phase sensors using the classical Rayleigh-type surface acoustic wave (RSAW) mode have enjoyed considerable interest worldwide over the last two decades [1- 3]. This interest is motivated by their orders of magnitude higher sensitivity and larger dynamic range compared to bulk acoustic wave (BAW) sensors, fast response times, excellent overall stability, coming close to that of quartz crystal sensors, and low phase noise of the sensor system making high-resolution measurements possible [4]. Because of these features that are difficult to achieve with other technologies, RSAW based gas sensors have found successful application in a variety of industrial implementations such as electronic noses, systems for analysis of chemical and biological gases, medical diagnostics, environmental monitoring and protection, etc. [5-11]. On the other hand, surface transverse wave (STW) based gas sensors, even though sharing the same operation principle, have not been studied so extensively yet. The purpose of this article is to present and discuss systematic experimental data with both acoustic wave modes which will prove that STW based gas-phase sensors not only successfully compete with their RSAW counterparts but also complement them in applications where RSAW gas sensors reach their limits. Successful corrosion proof RSAW sensors using gold metallization for operation in highly reactive chemical environments will also be presented.

## **2. Operation principle of RSAW/STW based resonant gas phase sensors**

Both RSAW and STW based gas sensitive resonant sensors share the same operation principle illustrated in Fig. 1. The sensor device typically is a two-port RSAW or STW resonator on a temperature compensated rotated Y cut of quartz whose geometry has been optimized in such manner that the resonator retains a well behaved single-mode resonance and suffers minimum loss increase and Q-degradation after the gas sensitive layer, (typically a solid, semisolid or soft polymer film with good sorption properties), is deposited on its surface. On the other hand, the sensor has to have maximum active area in the centre of its geometry where the magnitude of the standing wave and deformation are maximized. Thus, strong interaction with the gas adsorbed in the polymer film occurs and maximum gas sensitivity is obtained. The sensor operation principle according to Fig. 1 is fairly simple. If a gas-phase analyte of a certain concentration is applied to its surface, gas molecules are absorbed by the sensing layer until thermodynamic equilibrium is achieved; i. e. the number

Polymer Coated Rayleigh SAW and STW Resonators for Gas Sensor Applications 523

frequency of the sensor (see marker positions in Fig. 2 a) and b)) then due to the high Q of the sensor device, low-noise oscillation with high short-term stability will be obtained. Any change in gas concentration will alter the resonance frequency and the output frequency *f0* of the sensor oscillator, accordingly. Thus *Δf* can be measured with a high precision using a high-resolution frequency counter, connected to the output of the sensor oscillator. At a given gas concentration *C*, measured in parts per million (ppm), the resolution *R* of the sensor system, also measured in ppm, will be limited only by the short-term stability of the

*)*, also called Allan's variation, for the measurement time

*) represents the flicker phase noise of the sensor oscillator in the time domain which is dominated by the actual flicker phase noise of the coated acoustic wave sensor.* The resolution *R* determines the minimum change in gas concentration that the system can detect and is, therefore, also

> <sup>0</sup> *RC f f* = Δ [ ( ) ]/ στ

To calculate *R* for a given gas concentration *C*, according to (1), it is sufficient to measure

counters operating in the typical 0,3 to 1,0 GHz RSAW/STW sensor range with 1 Hz

1 <sup>1</sup> ( ) ( ) 2( 1) *M y i i i y y <sup>M</sup>*

=

where *i* is an integer. In a well stabilized against thermal transients sensor oscillator

The correct choice of the sensing layer suitable for the chosen acoustic mode is the key to proper sensor operation and good sensitivity and dynamic range [13, 14]. A sensing layer is considered as "good" if it has an excellent adhesion to the surface of the acoustic device for proper interaction with the acoustic wave, can easily adsorb and restlessly desorb large amounts of probing gases without chemically reacting with them, has good temperature stability and low ageing and does not change its sensitivity and sorption characteristics over thousands of measurement cycles. It is also desirable that the layer provides some selectivity to a certain chemical compound, i. e. it absorbs larger amounts of that compound than other compounds. Finally, the layer should not significantly degrade the Q, loss and the shape of

Because of their complicated net structure, many polymers feature excellent physical sorption, as required for reproducible sensor performance and this makes them appropriate for gas sensing applications [15-19]. If some of them have also appropriate viscoelastic properties for good interaction with the RSAW or STW mode, then they will provide the required performance of the acoustic wave sensor, accordingly. Layers with appropriate viscoelastic properties are those that follow the deformation of the surface as a result of the wave propagation without causing significant propagation loss and conversion of the

−

σ*y(*τ

typically 20 to 50 consecutive measurements of *f0* are enough to calculate

**4. Chemosensitive layers for RSAW/STW based gas sensors** 

 τ

τ

1/2 <sup>1</sup> <sup>2</sup> 1

+

τ

σ*y(*τ*)* with

*<sup>y</sup>* (1)

*)* can be calculated from a finite number *M* of

= − <sup>−</sup> , (2)

which is normally 1s for most frequency

*. The value of* 

sensor oscillator

σ*y(*τ

σ*y(*τ σ*y(*τ

resolution. Then, according to [12],

called detection limit. It is calculated as follows:

*)* of the sensor oscillator for the time interval

σ τ

consecutive frequency measurements *yi* of *f0* as:

sufficient accuracy for practical sensor applications.

the resonance after deposition onto the acoustic device.

of adsorbed molecules becomes equal to the number of desorbed ones. Due to adsorption, the layer becomes heavier and this increases the mass loading on the sensor surface. As a result of that, the acoustic wave propagation velocity *v* decreases and causes a concentration proportional frequency down shift *Δf* of the sensor's resonance, called sensor signal. The resonance frequency shift of RSAW gas sensors coated with a polyisobutilene (PIB) polymer film is shown in Fig. 2 a) and b) for two different concentrations of tetrachloroethilene vapors. If the vapor concentration is small (0,1% in Fig. 2 a)) then the resonance shifts down by 83 ppm without degradation in loss or Q. At large concentrations of the gas vapors (0,7% in Fig. 2 b)), the 550 ppm of observed frequency down shift is accompanied by a 2 dB loss increase due to the heavy mass loading. However, the sensor device retains a high loaded Q, (above 2000 in Fig. 2 a) versus >4000 in Fig. 2 b)) and a steep phase slope in a well behaved single-mode resonance without distortion or excitation of undesired longitudinal modes.

Fig. 1. Operation principle of RSAW/STW based resonant gas phase sensors

Fig. 2. Frequency (upper curves and phase (lower curves) responses of PIB coated RSAW sensors prior to (right) and after (left) tetrachloroethilene vapor probing at a) 0,1% and b) 0,7% concentration

### **3. Measurement resolution of RSAW/STW gas phase sensor systems**

If a sensor device as the ones from Fig. 2 a) and b) is used as a frequency stabilizing element in the feedback loop of an oscillator circuit and its frequency *f0* is adjusted at the resonance

of adsorbed molecules becomes equal to the number of desorbed ones. Due to adsorption, the layer becomes heavier and this increases the mass loading on the sensor surface. As a result of that, the acoustic wave propagation velocity *v* decreases and causes a concentration proportional frequency down shift *Δf* of the sensor's resonance, called sensor signal. The resonance frequency shift of RSAW gas sensors coated with a polyisobutilene (PIB) polymer film is shown in Fig. 2 a) and b) for two different concentrations of tetrachloroethilene vapors. If the vapor concentration is small (0,1% in Fig. 2 a)) then the resonance shifts down by 83 ppm without degradation in loss or Q. At large concentrations of the gas vapors (0,7% in Fig. 2 b)), the 550 ppm of observed frequency down shift is accompanied by a 2 dB loss increase due to the heavy mass loading. However, the sensor device retains a high loaded Q, (above 2000 in Fig. 2 a) versus >4000 in Fig. 2 b)) and a steep phase slope in a well behaved single-mode resonance without distortion or excitation of undesired longitudinal

Fig. 1. Operation principle of RSAW/STW based resonant gas phase sensors

 Meas1:Mkr2 432.513 MHz -10.498dB

 Meas2:Mkr1 432.551 MHz -32.576

a) b) Fig. 2. Frequency (upper curves and phase (lower curves) responses of PIB coated RSAW sensors prior to (right) and after (left) tetrachloroethilene vapor probing at a) 0,1% and b)

1:

2:

Center 433.508 MHz Span 1.000 MHz

2

2

1M1

2M2

2:Transmission &MPhase 90.0 / Ref 0.00

1:Transmission &MLog Mag 1.0 dB/ Ref -9.75 dB

 Meas1:Mkr1 433.270 MHz -11.808dB

 Meas2:Mkr1 433.270 MHz -29.038

1

1

1M1

2

If a sensor device as the ones from Fig. 2 a) and b) is used as a frequency stabilizing element in the feedback loop of an oscillator circuit and its frequency *f0* is adjusted at the resonance

**3. Measurement resolution of RSAW/STW gas phase sensor systems** 

modes.

1:

2:

0,7% concentration

Center 432.551 MHz Span 1.000 MHz

1 2

2:Transmission Phase 90.0 / Ref 0.00

2

1:Transmission &MLog Mag 1.0 dB/ Ref -10.43 dB

1

frequency of the sensor (see marker positions in Fig. 2 a) and b)) then due to the high Q of the sensor device, low-noise oscillation with high short-term stability will be obtained. Any change in gas concentration will alter the resonance frequency and the output frequency *f0* of the sensor oscillator, accordingly. Thus *Δf* can be measured with a high precision using a high-resolution frequency counter, connected to the output of the sensor oscillator. At a given gas concentration *C*, measured in parts per million (ppm), the resolution *R* of the sensor system, also measured in ppm, will be limited only by the short-term stability of the sensor oscillator σ*y(*τ*)*, also called Allan's variation, for the measurement time τ*. The value of*  σ*y(*τ*) represents the flicker phase noise of the sensor oscillator in the time domain which is dominated by the actual flicker phase noise of the coated acoustic wave sensor.* The resolution *R* determines the minimum change in gas concentration that the system can detect and is, therefore, also called detection limit. It is calculated as follows:

$$R = \left[ \mathbb{C} \sigma\_y(\mathfrak{r}) f\_0 \mathfrak{r} \right] / \Delta f \tag{1}$$

To calculate *R* for a given gas concentration *C*, according to (1), it is sufficient to measure σ*y(*τ*)* of the sensor oscillator for the time interval τ which is normally 1s for most frequency counters operating in the typical 0,3 to 1,0 GHz RSAW/STW sensor range with 1 Hz resolution. Then, according to [12], σ*y(*τ*)* can be calculated from a finite number *M* of consecutive frequency measurements *yi* of *f0* as:

$$\sigma\_y(\mathbf{r}) = \left[ \frac{1}{2(M-1)} \sum\_{i=1}^{M-1} (y\_{i+1} - y\_i)^2 \right]^{1/2} \tag{2}$$

where *i* is an integer. In a well stabilized against thermal transients sensor oscillator typically 20 to 50 consecutive measurements of *f0* are enough to calculate σ*y(*τ*)* with sufficient accuracy for practical sensor applications.

### **4. Chemosensitive layers for RSAW/STW based gas sensors**

The correct choice of the sensing layer suitable for the chosen acoustic mode is the key to proper sensor operation and good sensitivity and dynamic range [13, 14]. A sensing layer is considered as "good" if it has an excellent adhesion to the surface of the acoustic device for proper interaction with the acoustic wave, can easily adsorb and restlessly desorb large amounts of probing gases without chemically reacting with them, has good temperature stability and low ageing and does not change its sensitivity and sorption characteristics over thousands of measurement cycles. It is also desirable that the layer provides some selectivity to a certain chemical compound, i. e. it absorbs larger amounts of that compound than other compounds. Finally, the layer should not significantly degrade the Q, loss and the shape of the resonance after deposition onto the acoustic device.

Because of their complicated net structure, many polymers feature excellent physical sorption, as required for reproducible sensor performance and this makes them appropriate for gas sensing applications [15-19]. If some of them have also appropriate viscoelastic properties for good interaction with the RSAW or STW mode, then they will provide the required performance of the acoustic wave sensor, accordingly. Layers with appropriate viscoelastic properties are those that follow the deformation of the surface as a result of the wave propagation without causing significant propagation loss and conversion of the

Polymer Coated Rayleigh SAW and STW Resonators for Gas Sensor Applications 525

performance of both modes under identical real-life conditions. Such a performance comparison would be correct only if it is carried out with sensor devices of both modes operating on the same acoustic wave length for the following reason: If both types of devices are fabricated on the same piezoelectric material and cut orientation (AT-cut quartz in this case), use the same device geometry, are coated with the same sensing layer of the same thickness and are probed with identical gases and concentrations, then the only factors responsible for the differences in electrical and sensor performance would be the type of motion for each mode, (elliptical for the RSAW and shear horizontal for the STW) and the way the acoustic wave interacts with the sensing layer. The results presented in the next sections were performed with RSAW and STW sensors whose electrical characteristics in the

**5.1 Electrical performance of STW/RSAW sensor resonators coated with solid and** 

Acoustic wave mode STW RSAW Acoustic wave length 7,22 μm 7,22 μm Sensor resonator frequency 433 MHz 701 MHz Device insertion loss 5-7 dB 6-7 dB Loaded Q-factor 3000-4000 5000-6000 Side lobe suppression > 8 dB >12 dB Metallisation Al Al

Table 1. Electrical characteristics of the uncoated STW/RSAW sensor resonators used in the

In addition, the RSAW device retains a well behaved single-mode resonance with excellent side lobe suppression as required for stable operation of the sensor oscillator. The STW device shows a different behavior. Its frequency shifts down by 4 MHz (6100 ppm) which accounts for about 2 times higher relative mass loading sensitivity than its RSAW counterpart. The insertion loss increases by just about 3 dB versus 5,7 dB for the RSAW mode which implies that the STW mode tolerates solid films better in terms of loss increase. On the other hand, excitation of a second higher-order Love wave mode [20] about 7 MHz higher than the main STW mode at 697 MHz is observed. Since a 180 deg. phase reversal at this Love mode occurs, (see the lower data plot in Fig. 3 b)), it is not very likely to degrade the performance of the sensor oscillator. A more serious problem, however, is the distortion at the main STW mode that indeed can cause the sensor oscillator to jump onto an adjacent peak during the measurement. That is why, coating STW sensor resonators with excessively thick solid films as the 190 nm HMDSO from Fig. 3 should be stopped before distortion and multiple peak behavior on the main STW mode occurs. As far as the higher-order Love mode at 704 MHz is concerned, we have noticed that its gas sensitivity is orders of magnitude lower than the STW mode on the right side. This lack of sensitivity is explained by the fact that the Love mode scatters its energy into the bulk of the sensing layer [20].

The frequency and phase responses of the STW and RSAW sensor resonators from Table 1 prior to and after coating with the solid HMDSO are compared in Fig. 3. After film deposition, the frequency of the RSAW device shifts down by about 1,5 MHz (3500 ppm), its insertion loss increases by 5,7 dB and the loaded Q decreases from 6000 to about 2000.

uncoated state are summarized in Table 1.

**semisolid sensing layers** 

comparative studies

acoustic energy into undesired modes that decay into the bulk of the substrate and may cause degradation of sensor performance.

An important parameter of the sensing film, except for its viscoelastic properties is its solidness. On one hand, the parameter "solidness" determines the sorption properties of the film and the amount of gas that the layer can accommodate before saturation is reached. On the other hand, it determines the way in which the polymer film interacts with the acoustic wave. Therefore, the film solidness will determine the sensitivity, dynamic range and detection limit of the sensor. Based on their solidness, there are three types of polymer films that are appropriate for RSAW/STW sensors:


### **5. Comparative characteristics of polymer coated RSAW and STW gas sensors operating at the same acoustic wave length**

To identify the advantages and disadvantages of the STW mode versus its RSAW counterpart on quartz for gas sensor applications it is necessary to compare the sensor

acoustic energy into undesired modes that decay into the bulk of the substrate and may

An important parameter of the sensing film, except for its viscoelastic properties is its solidness. On one hand, the parameter "solidness" determines the sorption properties of the film and the amount of gas that the layer can accommodate before saturation is reached. On the other hand, it determines the way in which the polymer film interacts with the acoustic wave. Therefore, the film solidness will determine the sensitivity, dynamic range and detection limit of the sensor. Based on their solidness, there are three types of polymer films

a. *Solid polymer films*. In fact, these films are solid as glass. That is why, they are often called "glassy polymer films" and have a stiffness value close to that of the sensor's quartz substrate that they are deposited on. If used with the STW mode, due to the lower propagation velocity, these solid films trap the wave energy to the substrate surface and the acoustic wave propagates with low loss. That is why, solid films work much better with the STW mode than with the RSAW one. When their thickness becomes too high, a second slightly faster mode, called "Love mode" gets excited and multimoding occurs. Solid polymer films feature surface sorption and become easily saturated by the adsorbed gas but on the other hand, they feature very fast response times and are very sensitive if the sensor is operated far below saturation. That is why they are appropriate for high resolution measurements at low gas concentrations, (typically below 0,1%). A typical representative of the solid polymer family is the hexamethyldissiloxane (HMDSO),

b. *Soft polymer films*. These films are soft and elastic just like rubber. That is why they are referred to as "rubbery" or "jelly-like" films. Typically, they are deposited using spin coating or more advanced techniques such as airbrush or electro spray methods that provide good control over film thickness and uniformity. Since these soft polymers provide profound bulk sorption, they are capable of adsorbing large amounts of gas and are appropriate for measurements at high gas concentrations, (typically above 0,1%). They are well tolerated by the RSAW mode but do not work so well with STW. The reason is that they cause energy leakage of the STW into the bulk of the soft layer which results in increased loss and Q-degradation of the sensor resonator. Polymers like polyisobutilene (PIB), poly-(2-hydroxyethylmethacrylate) (PHEMA) and poly-(n-

c. *Semisolid polymer films*. These light and highly elastic films are also typically obtained in a plasma polymerization process [17, 18] for good reproducibility of the film parameters and have a structure very similar to polystyrene, the material used in plastic bags. They are highly resistant to almost all aggressive chemicals such as acids, bases and organic solvents and this makes them appropriate for environmental sensing applications. They are well tolerated by both, the RSAW and STW mode and often feature sensitivities comparable to those of the soft polymer films. The two semisolid films used in this study are styrene (ST) and allylalkohol (AA) synthesized in a plasma polymerization reactor.

obtained in a glow-discharge plasma polymerization process [19].

butylmethacrylate) (PBMA) are often used in RSAW based gas sensors.

**5. Comparative characteristics of polymer coated RSAW and STW gas** 

To identify the advantages and disadvantages of the STW mode versus its RSAW counterpart on quartz for gas sensor applications it is necessary to compare the sensor

**sensors operating at the same acoustic wave length** 

cause degradation of sensor performance.

that are appropriate for RSAW/STW sensors:

performance of both modes under identical real-life conditions. Such a performance comparison would be correct only if it is carried out with sensor devices of both modes operating on the same acoustic wave length for the following reason: If both types of devices are fabricated on the same piezoelectric material and cut orientation (AT-cut quartz in this case), use the same device geometry, are coated with the same sensing layer of the same thickness and are probed with identical gases and concentrations, then the only factors responsible for the differences in electrical and sensor performance would be the type of motion for each mode, (elliptical for the RSAW and shear horizontal for the STW) and the way the acoustic wave interacts with the sensing layer. The results presented in the next sections were performed with RSAW and STW sensors whose electrical characteristics in the uncoated state are summarized in Table 1.

### **5.1 Electrical performance of STW/RSAW sensor resonators coated with solid and semisolid sensing layers**

The frequency and phase responses of the STW and RSAW sensor resonators from Table 1 prior to and after coating with the solid HMDSO are compared in Fig. 3. After film deposition, the frequency of the RSAW device shifts down by about 1,5 MHz (3500 ppm), its insertion loss increases by 5,7 dB and the loaded Q decreases from 6000 to about 2000.


Table 1. Electrical characteristics of the uncoated STW/RSAW sensor resonators used in the comparative studies

In addition, the RSAW device retains a well behaved single-mode resonance with excellent side lobe suppression as required for stable operation of the sensor oscillator. The STW device shows a different behavior. Its frequency shifts down by 4 MHz (6100 ppm) which accounts for about 2 times higher relative mass loading sensitivity than its RSAW counterpart. The insertion loss increases by just about 3 dB versus 5,7 dB for the RSAW mode which implies that the STW mode tolerates solid films better in terms of loss increase. On the other hand, excitation of a second higher-order Love wave mode [20] about 7 MHz higher than the main STW mode at 697 MHz is observed. Since a 180 deg. phase reversal at this Love mode occurs, (see the lower data plot in Fig. 3 b)), it is not very likely to degrade the performance of the sensor oscillator. A more serious problem, however, is the distortion at the main STW mode that indeed can cause the sensor oscillator to jump onto an adjacent peak during the measurement. That is why, coating STW sensor resonators with excessively thick solid films as the 190 nm HMDSO from Fig. 3 should be stopped before distortion and multiple peak behavior on the main STW mode occurs. As far as the higher-order Love mode at 704 MHz is concerned, we have noticed that its gas sensitivity is orders of magnitude lower than the STW mode on the right side. This lack of sensitivity is explained by the fact that the Love mode scatters its energy into the bulk of the sensing layer [20].

Polymer Coated Rayleigh SAW and STW Resonators for Gas Sensor Applications 527

that maximum sensor sensitivity and dynamic range are obtained at minimum degradation of the electrical resonator performance (insertion loss, loaded Q, side lobe suppression and distortion) as required for stable low-noise operation of the sensor oscillator. A very efficient method for film thickness optimization using a controlled plasma deposition of the semisolid polymer Parylene C is described in [21]. This material has viscoelastic properties very similar to practical solid and semisolid layers but has the unique feature that it can polymerize directly on the surface of the acoustic devices at room temperature, thus avoiding undesired thermal frequency drifts. Also the actual deposition is performed in a chamber separate from the plasma reactor where the devices are protected from the high electric fields of the main plasma generator. This allows direct measurement of their frequency and phase responses with a network analyzer in the process of film deposition while the film thickness is measured with a quartz crystal microbalance (QCM). The results from a Parylene C deposition on a 433 MHz RSAW resonator and real time measurements of its electrical characteristics are shown in Fig. 4 for a polymer thickness ranging from 0 to 700 nm. From this measurement it is possible to identify the thickness range in which optimum sensor performance is expected and to extract information on the behavior of important

sensor parameters in the process of polymer coating as follows: • the down shift of the resonant frequency versus film thickness;

range is between 100 and 300 nm with an average of 200 nm.

• the behavior of the adjacent longitudinal modes in the process of deposition;

• the thickness range over which the sensor demonstrates maximum mass sensitivity of frequency while retaining good electrical performance. In Fig. 4 this optimum thickness

Fig. 4. Parylene C coating behavior of a 433 MHz RSAW sensor resonator in the 0 to 700 nm

As evident from Fig. 4, in the optimum thickness range (100 to 300 nm) a maximum linear frequency shift, (maximum mass sensitivity), is accompanied by just about 7 dB loss increase. The Q remains high as shown by the sharp resonance while the first longitudinal mode on the left side of the resonance remains suppressed by at least 12 dB. If the sensor is

• the loss increase with film thickness; • the loaded Q decrease with film thickness;

polymer thickness range

Fig. 3. Frequency (upper curves) and phase responses (lower curves) of the a) RSAW and b) STW sensor resonators from Table 1 prior to (upper plots) and after (lower plots) 190 nm HMDSO solid film deposition

### **5.2 Electrical performance of STW/RSAW sensor resonators coated with soft polymer films**

A similar comparison between both acoustic wave modes was performed by coating the devices from Table 1 with the soft polymer film PIB using the micro drop deposition method. The data obtained shows quite the opposite tendency compared to the solid film behavior from Section 5.1. The STW devices suffered a 5 dB increase in insertion loss and rather distorted frequency responses even at fairly thin soft layers. Only a moderate frequency down shift of 1330 ppm was obtain as a result of film coating. As evident from the frequency responses in Fig. 2 the RSAW devices were found to provide a much better performance at the same film thickness. They retain a high loaded Q and low insertion loss, as well as an undistorted single-mode resonance. These data imply that RSAW sensors will work better with soft polymer films while the STW mode will provide better performance with solid films as long as they are not excessively thick to cause distortion.

### **6. A practical method for film thickness optimization of RSAW/STW gas sensors coated with solid and semisolid sensing layers**

The most important step in designing practical RSAW/STW resonant sensors is the selection of an optimum thickness of the sensing layer. It should be selected in such manner that maximum sensor sensitivity and dynamic range are obtained at minimum degradation of the electrical resonator performance (insertion loss, loaded Q, side lobe suppression and distortion) as required for stable low-noise operation of the sensor oscillator. A very efficient method for film thickness optimization using a controlled plasma deposition of the semisolid polymer Parylene C is described in [21]. This material has viscoelastic properties very similar to practical solid and semisolid layers but has the unique feature that it can polymerize directly on the surface of the acoustic devices at room temperature, thus avoiding undesired thermal frequency drifts. Also the actual deposition is performed in a chamber separate from the plasma reactor where the devices are protected from the high electric fields of the main plasma generator. This allows direct measurement of their frequency and phase responses with a network analyzer in the process of film deposition while the film thickness is measured with a quartz crystal microbalance (QCM). The results from a Parylene C deposition on a 433 MHz RSAW resonator and real time measurements of its electrical characteristics are shown in Fig. 4 for a polymer thickness ranging from 0 to 700 nm. From this measurement it is possible to identify the thickness range in which optimum sensor performance is expected and to extract information on the behavior of important sensor parameters in the process of polymer coating as follows:


526 Acoustic Waves – From Microdevices to Helioseismology

a) b) Fig. 3. Frequency (upper curves) and phase responses (lower curves) of the a) RSAW and b) STW sensor resonators from Table 1 prior to (upper plots) and after (lower plots) 190 nm

**5.2 Electrical performance of STW/RSAW sensor resonators coated with soft polymer** 

A similar comparison between both acoustic wave modes was performed by coating the devices from Table 1 with the soft polymer film PIB using the micro drop deposition method. The data obtained shows quite the opposite tendency compared to the solid film behavior from Section 5.1. The STW devices suffered a 5 dB increase in insertion loss and rather distorted frequency responses even at fairly thin soft layers. Only a moderate frequency down shift of 1330 ppm was obtain as a result of film coating. As evident from the frequency responses in Fig. 2 the RSAW devices were found to provide a much better performance at the same film thickness. They retain a high loaded Q and low insertion loss, as well as an undistorted single-mode resonance. These data imply that RSAW sensors will work better with soft polymer films while the STW mode will provide better performance

with solid films as long as they are not excessively thick to cause distortion.

**sensors coated with solid and semisolid sensing layers** 

**6. A practical method for film thickness optimization of RSAW/STW gas** 

The most important step in designing practical RSAW/STW resonant sensors is the selection of an optimum thickness of the sensing layer. It should be selected in such manner

MKR( 250): 701.33MHz

MKR( 219): 697.08MHz

MAGTD ( ) -5.40dB 5dB/ -30.90dB PHASE ( ) 60.6deg 100deg/ 330.0deg

MAGTD ( ) -8.38dB 5dB/ -32.80dB PHASE ( ) 5.8deg 100deg/ 313.9deg

CF: 701.33MHz SPAN: 20MHz

CF: 698.32MHz SPAN: 20MHz

p MKR( 250): 433.86MHz

MKR( 250): 431.34MHz

MAGTD ( ) -7.49dB 5dB/ -31.75dB PHASE ( ) 159.4deg 100deg/ 320.0deg

CF: 433.86MHz SPAN: 10MHz

p

CF: 431.34MHz SPAN: 10MHz

HMDSO solid film deposition

**films** 

MAGTD ( ) -13.18dB 5dB/ -31.75dB PHASE ( ) 174.5deg 100deg/ 320.0deg


Fig. 4. Parylene C coating behavior of a 433 MHz RSAW sensor resonator in the 0 to 700 nm polymer thickness range

As evident from Fig. 4, in the optimum thickness range (100 to 300 nm) a maximum linear frequency shift, (maximum mass sensitivity), is accompanied by just about 7 dB loss increase. The Q remains high as shown by the sharp resonance while the first longitudinal mode on the left side of the resonance remains suppressed by at least 12 dB. If the sensor is

Polymer Coated Rayleigh SAW and STW Resonators for Gas Sensor Applications 529

The purpose of the gas probing tests is to identify which acoustic wave mode provides

a) b)

The block diagram of the computer controlled system for measuring the gas sensing characteristics of the RSAW/STW polymer coated sensors is shown in Fig. 6. For correct comparison of the gas probing performance of both acoustic wave modes four pairs of devices (one RSAW and one STW sensor in each pair, coated with the same polymer to the same film thickness and in the same deposition process) are mounted in open TO 92 packages and placed in the sensor head which can accommodate a total of eight sensors. Each device is connected to one of the 8 sensor oscillator circuits in the head. During gas probing each of the 8 oscillators is turned on for a short period of time to take the measurement. The oscillators are operated one at a time and multiplexed consecutively to avoid possible injection locking. Their output frequency is down converted to an intermediate frequency in the 4-9 MHz range by means of a stable heterodyne reference oscillator to allow fast high-resolution measurements with a reciprocal frequency counter. The chemical compounds 1 through 4 used for gas probing are vapors from the 4 liquidphase analytes in the 4 containers. A permeation cell is placed on top of each container to allow a defined vapor pressure which is controlled by the rotation speed of the pump. By a switch block of valves the vapors of each analyte are then consecutively fed to the sensor head where they interact with the sensors. After the measurements at each analyte are completed the sensors are flushed with dry air passing through a silica gel integrator which provides also a homogenous air and gas flow. The entire system is controlled by a computer which performs measurements in probe-flush cycles over time and provides real-time sensor data on the computer screen. The data in Fig. 7 is the gas probing performance of a 700 MHz STW styrene coated sensor probed with dichloroethane vapors at 6500 ppm concentration in 100 s probe-flush cycles. It should be noted that prior to this measurement the sensor was probed with a different compound (xylene) for 62 hours and 40 minutes. Note the excellent reproducibility of the noise free sensor signal with a magnitude

Fig. 5. Critical thickness in a) STW and b) RSAW devices in the process of Parylene C

**7.1 Computer controlled automatic system for gas probing measurements** 

Critically coated SAW devices

better performance in real-world gas sensing conditions.

coating

intended for high-resolution measurements at low gas concentrations, then a thickness close to 100 nm should be chosen due to the highest Q and lowest loss. If measurements at higher gas concentrations are expected then a 300 nm thickness may be more appropriate since the thicker layer may adsorb larger amounts of gas without film saturation.

### **6.1 Critical thickness in RSAW/STW based sensor resonators coated with solid and semisolid sensing layers**

As shown in the previous sections, the sensing layer does not only shift the resonant frequency down, increase the loss and decrease the loaded Q as a result of mass loading but it also influences the longitudinal modes supported by the resonator geometry that appear on the left side of the main resonance. In the uncoated resonator these modes are well enough suppressed (typically by 5 to 15 dB) and do not cause any problems when the resonator is operated in an oscillator circuit. As soon as a sensing layer is deposited on the surface, it will change the phase conditions along the device topology and this will cause the adjacent longitudinal modes to arise in magnitude at the expense of the main resonance. This situation gets worse at thick solid films for both, the STW and the RSAW mode. At a certain thickness which we call "critical thickness" the magnitude of the first adjacent lowfrequency longitudinal mode on the left becomes equal to the magnitude of the higherfrequency main resonance. This creates a potential for instability in the sensor oscillator stabilized with this sensor since it can easily jump from the main resonance onto the left longitudinal mode during gas probing which will ruin the measurement. The critical thickness situation is illustrated in Fig. 5 a) and b) for a STW and a RSAW device from Table 1, accordingly, in the process of Parylene C deposition as described in Section 6. As evident from Fig. 5 a), at thickness values above 185 nm the first longitudinal mode on the left starts rapidly growing until its magnitude becomes equal to the main resonance. The critical thickness at which this happens is about 350 nm. At this thickness also a strong Love mode excitation on the right is observed. The RSAW device in Fig. 5 b) reaches its critical thickness at about 650 nm. From these data we can draw the conclusion that the devices from Table 1 can be usable as Parylene C coated sensors as long the film thickness is lower than 300 nm and 600 nm for the STW and RSAW devices, respectively. Comparing the coating behavior of both modes in Fig. 5 we see that the STW mode retains a much better behaved resonance than its RSAW counterpart until the critical thickness is reached. At that thickness the STW device has a loss of 14 dB (Fig. 5 a)), versus 35 dB for the RSAW device (Fig. 5 b)). Therefore, the STW mode tolerates solid and semisolid sensing films much better than the RSAW one and is more appropriate for operation with such films in practical gas sensors.
