Study of the Acoustoelectric Effect for SAW Sensors Brian H. Fisher, Student Member, IEEE and Donald C. Malocha, Fellow, IEEE Abstract— Research has recently begun on the use of ultra thin films and nanoclusters as mechanisms for sensing of gases, liquids, etc., since the basic material parameters may change due to film morphology. As films of various materials are applied to the surface of surface acoustic wave (SAW) devices for sensors, the conductivity of the films may have a strong acoustoelectric effect, whether desired or not. The purpose of this paper is to reexamine the theory and predictions of the acoustoelectric effect for SAW interactions with thin conducting or semi-conducting films. The paper will summarize the theory, and predict the effects of thin film conductivity on SAW velocity and propagation loss versus frequency and substrate material. The theory predicts regions of conductivity which result in extremely high propagation loss, which also correspond to the mid-point between the open and short circuit velocities. As an example of the verification and possible usefulness of the acoustoelectric effect, recent experimental results of palladium (Pd) thin films on an YZ LiNbO 3 SAW delay line have shown large changes in propagation loss, depending on the Pd film thickness, and/or exposure to hydrogen gas. By proper design, a sensitive hydrogen leak detector SAW sensor can be designed.

I.

INTRODUCTION

In a piezoelectric media, the piezoelectric constant contributes to the elastic stiffening, and generates a synchronous electric field with a traveling elastic wave in the media. This phenomenon was first studied in the 1950’s and began to be extensively studied in the 1960’s. Most of the early studies were interested in elastic wave propagation interactions in semiconductor substrates or films. One of the earliest papers by Hutson and White in 1962 laid the mathematical foundation for the one dimensional propagating wave in a semiconducting solid [1]. In 1970 Ingebrigtsen considered attenuation of a surface acoustic wave (SAW) due to a semiconducting thin film [2]. The initial theoretical works with elastic wave and semiconductor interactions were motivated by the desire for providing useful amplification, but was abandoned since it was not practical. In 1972, Hemphill then assumed a similar set of equations could be applied to losses in thin metallic films [3]. In 1984, Hemphill reported further study of the effect of a SAW and thin Manuscript received May 20, 2009; accepted November 27, 2009. This work was supported through NASA-KSC STTR contract NNK07EA39C, and the McKnight Doctoral Fellowship Program. B. H. Fisher and D. C. Malocha are with the School of Electrical Engineering & Computer Science, University of Central Florida, Orlando, FL 32816-2450 (email: [email protected])

film vanadium dioxide phase transition metal interaction for possible use in a programmable SAW delay line [4]. Over the ensuing 30 years, there has been a wealth of publications on the SAW acoustoelectric effect (AE) interaction with palladium (Pd) films and its exposure to hydrogen gas [5-13]. To the author’s knowledge, all of the published works are for SAW-Pd hydrogen (H2) gas interactions using films of several hundred angstroms or more. In fact, some publications have indicated that Pd thick films are better for various parameters, such as sensitivity, compared to thin films [6]. The use of ultrathin film Pd films (less than 30 Å), on SAW substrates for H2 gas sensing capabilities, has only been recently reported [12, 13]. These ultra-thin films fall in the conductivity region where the AE effect is most dramatic. The purpose of this paper is to reexamine the theory and predictions of the AE effect for SAW interactions with thin conducting or semi-conducting films. The motivation of this work was to investigate ultra-thin film interactions with gases and the environment. Film layers at the nano scale have differing properties than the conventional thin films, and their resistivity may be large and variable. For a film with known and reproducible properties, it is possible to predict velocity and propagation loss versus film thickness, especially critical in the SAW high loss operating region. In addition, the device operating frequency determines the resistivity range to obtain the desired or optimum AE effect for thin film sensors. As an example of the usefulness of the AE effect, recent experimental results of ultra-thin palladium (Pd) films on a 123 MHz, YZ LiNbO3 SAW delay line show huge changes in propagation loss with small interaction lengths, depending on the Pd film thickness, and/or exposure to hydrogen gas. By proper device design and using ultra-thin Pd films, the goal is to produce a room temperature, quick response, reversible, passive, wireless, sensitive hydrogen leak detector SAW sensor. The paper is structured as follows: in section II the AE theory is summarized, and predicts the effects of thin film conductivity on SAW velocity and propagation loss versus frequency and substrate material. Section III discusses Pd-H thin film interactions and previous SAW H2 gas sensor work. In section IV recent experimental results on SAW-ultrathin Pd-H2 gas interactions are presented and discussed.

II.

REVIEW OF ACOUSTOELECTRIC EFFECT

A. Theoretical Background The following is a brief summary of the derivation of the acoustoelectric (AE) effect. The initial approach presents a simple 1-D summary analysis of wave propagation which can be applied to SAW phenomenon, as derived in [1]. The approach assumes volume wave propagation in an isotropic media and allows the volume to be conductive and to interact with the wave. This substrate can have a resistivity between zero and infinity. The substrate material is also assumed piezoelectric which allows a travelling potential in synchronous with the wave. Consider a wave propagating in the x direction. From classical physics, the force on a particle is the product of the mass times the acceleration. In the solid, a similar relationship can be derived using the stress-strain relations and result in the following 2 T     2 and S   (1) x x t where S is strain, T is stress, µ is displacement and ρ is the material mass density. Assume the substrate is characterized by a piezoelectric constant, e, and the strain, produces an electric field, E, in x. For the onedimensional problem, the constitutive relations are: T  c E S  eE (2)

D  eS  E where cE is the elastic constant at constant E, and ε is the dielectric permittivity at constant strain. From (2) T S E  cE e x x x D S E  e  x x x

(3)

D  E t

j(kx - ωt)

Assuming plane wave propagation, then D = D0e and E = E0ej(kx – ωt), and equation (4) can be solved for D and E as  (5) D  j E 

Substituting (5) into (2) and solving for E yields e S (6)  E  1 j   Define f r   as the dielectric relaxation frequency, 2 

f e S T  c 1  E  2  c  1   fr    f     

Assuming plane wave propagation as before, then

(7)

(8)

Since the effective stiffness constant is complex, and frequency is real, then the wave number, k, is complex. This implies a decaying propagating wave in the xdirection. The velocity must be real and is given as      E  2   c e 1  1   E  v  Re   2 k     c    f r   1         f   

(9)

 

The propagation loss coefficient, α, is given by     Im( k )    IM   c    eff

  fr     2 e   f  v 2 E 0 2c   f  1  r f    

     

(10)

Next, Ingebrigtsen similarly solved the 2-D equations for a SAW propagating in the x-direction, depicted in Fig. 1, interacting with a thin resistive film on the surface of a piezoelectric material [2]. The resulting equations are the same with substitution of the following:



4)

then substituting (6) into (2) and solving yields  fr    or T  ceff  S  2 1  j 

   0 e j kxt  and   2   ceff  k 2  

2   ( f   p )  and  p   11 22   12

For a conductive volume, the current density yields J

Figure 1. SAW propagation schematic for analysis.



1

2

(11)

where, εp and εf is the relative permittivity of the piezoelectric material and thin film respectively. B. AE Discussion and Results The AE theory predicts a relaxation frequency which is the frequency where maximum SAW attenuation will occur. The relaxation frequency is inversely proportional to the material’s dielectric constant and thin film resistivity, but independent of material coupling coefficient. Changing the substrate, film dielectric constant, or resistivity will change the relaxation frequency. A plot is shown in Fig. 2 of the relaxation frequency for three relative dielectric constants as a function of thin film resistivity. The plot suggests that given the frequency range of operation for typical SAW devices (10MHz to 10GHz), the maximum attenuation will occur at relatively high thin film resistivities (3 to 104Ω-cm), if the relative permittivity of these films is equal to one.

exposed to the desired analyte, which seldom occurs in most materials. Huge changes in loss can occur over one or two orders of magnitude change in resistivity.

Figure 2. Relaxation frequency versus thin film resistivity for three dielectric constants, typical of common SAW substrates. The range of the relaxation frequency (y-axis) is the typical range for SAW device operation.

Figure 4. Propagation loss (dB/cm) versus resistivity (ohm-cm) for YZ LiNbO3 at 125, 250, and 500 MHz.

Figure 3. Predicted SAW attenuation (dB/λ) and velocity versus resistivity for YZ LiNbO3. Three SAW center frequencies, 125, 250 and 500 MHz are shown.

Fig. 3 shows the effect of film resistivity on propagation loss and velocity for a SAW on YZ LiNbO3. The propagation loss (in dB/λ) peak occurs at a specific resistivity value where the SAW operational frequency equals the relaxation frequency. At maximum attenuation the propagation velocity is also midway between the open and short circuit velocity. The AE velocity shift is greatest on high coupling materials; at 500 MHz being less than 0.1 ppm on quartz but greater than 200 ppm on YZ LiNbO3. The fractional velocity change being greatest at the peak attenuation is unfortunate for many applications. Figure 4 shows the YZ LiNbO3 propagation loss in dB/cm for 125, 250, and 500 MHz operation; providing a more physical interpretation. The peak attenuation rises rapidly with frequency and is strongly thin film resistivity dependent. In the low resistivity region (less than 1Ω-cm), thin film losses at low frequencies are negligible, but as frequency increases the loss can be appreciable, depending on device length and film resistivity. To traverse the high loss region, from the open to short circuit velocity, a film resistivity will need to change over several orders of magnitude, when

Early Experimental Work Given the theoretical background, early investigations were conducted by many researchers on many different waves, materials and embodiments. R. B. Hemphill, first in 1972 and then in 1984 [3, 4], studied the SAW AE effect on SAW materials still used today, quartz and lithium niobate substrates. His earliest work investigated thin metal film and SAW interactions and showed good correlation between his theory and experiments. In 1984, Hemphill investigated thin film transition metals effects on SAW propagation for the possible use in programmable devices. He recognized that some transition metals can have large changes in resistivity versus temperature and this could be used as an environmentally programmable element by noting the change in the SAW velocity or delay. He showed experimental results for vanadium dioxide films, and found good correlation to his predictions [4]. There has also been a wealth of research on the SAW AE effect on metals, oxides and overlay layers. Most notably: acoustic charge transport [14, 15], superconductivity [16], and quantum hall effects [17]. The work on acoustic charge transport may be useful in understanding the AE interaction with ultra-thin films on a sub-micron level. The previous discussion and presentation provides a foundation for the current efforts. III.

PALLADIUM FILM-HYDROGEN INTERACTIONS

Previous Work Thin films fall into three categories: ultra-thin, thin and thick films. These regimes are primarily distinguished by the film’s electrical conduction mechanisms. Some ultra-thin films are composed of a

discontinuous network of atomic clusters, in which the primary method of electrical conduction is thermally activated quantum tunneling across the discontinuities. Thin and thick films however, depend on electron scattering as the primary means of electrical conduction. The two regimes are classified by the ratio of film thickness to the mean free electron path; for thin films this ratio is less than 10%; conversely for thick film this ratio is greater than 10% [18]. Pd films 500Å and greater have been reported to behave like bulk materials [19]. In terms of the palladium-hydrogen (Pd-H) interaction thin and thick films behave equivalently, ultra-thin films however behave differently. 1) Pd Thin/Thick Film Background Review A wealth of studies has been performed on the Pd-H interaction as a function of temperature, H2 gas concentration and pressure [19, 20]. This strong temperature and pressure dependence prompted scientist to analyze the interaction as function of the interaction enthalpies which is a weak function of temperature [21]. For this discussion the analysis of the Pd-H interaction is most intuitively understood in terms of H concentration and pressure. It is necessary to bound the discussion to room temperature and low hydrogen concentrations (below the lower explosive limit of approximately 4%) since after all, this is the intended area of operation. The absorption of hydrogen by Pd was first observed in 1868 by Thomas Grahab [22]. Since then, Pd has been widely used for hydrogen purification and sensing since it has the highest hydrogen solubility of any element at atmospheric pressure. H2 dissociates on Pd surfaces; additional H atoms diffuse into Pd to form palladium hydride (PdHx) [21]. This causes an increase in the mass and electrical resistivity of the Pd. The increase in mass and resistivity continues until chemical equilibrium between the gas phase, adsorbed and absorbed hydrogen is reached. The magnitude of the change varies with H concentration. The process may or may not be reversible at room temperature depending on the type of chemical bond that is formed [23]. For Pd films (200Å to 50μm), a 10% increase in Pd resistivity is reported for exposure to 4% hydrogen gas concentrations[24, 25]. The reaction times of these sensors are dependent on H2 gas concentration and flow rate and film thickness [6]. Typical reported times at range from five minutes to hours in some cases. 2) Pd Ultra-Thin Film Background Review The focus of this Pd thin film research was to characterize films in the ultra-thin film range, from approximately 1-100Å in thickness. Almost all previous publications on Pd hydrogen sensors have had films in the 200-10,000Å thickness range [5-11, 22, 25-29]. Films in the ultra-thin regime behave much differently than thicker films due to film morphology, different

physical conduction mechanisms, and different gas interactions. Under appropriate growth conditions ultra-thin films may have a nano-clustered morphology [30, 31]. Electrical conduction is achieved via thermally activated quantum tunneling between the nano-clusters [32]. When exposed to hydrogen gas the Pd clusters “swell” due to hydrogen induced lattice expansion (HILE), consequently creating a greater number of conductive pathways and decreasing the tunneling gap, thus causing a dramatic decrease in electrical resistivity [33-35]. This behavior typically occurs in the high resistivity range where the AE effect is most dominant. Humphrey et al. [36] reported a frequency dependent resistivity for nano-clustered films. Humphrey’s experiments showed that the resistivity decreased by as much as three orders of magnitude as frequency is increased from 10 kHz to 200MHz. This behavior, he postulated, is due to capacitance effects between the nano-clusters. Ultra-thin metal films have also been reported to have an “anomalously high” effective permittivity (105 to 108) [37-40]. Theoretically, this is due to oppositely polarized nano-clusters which create dipole pairs. The effects of a low RF resistivity and high effective permittivity on the AE response are complementary, since a decrease in resistivity shifts the dielectric relaxation frequency upward, while a high effective permittivity shifts the relaxation frequency downward. The SAW-film interaction occurs at the micron and submicron level due to the SAW wavelength and film morphology, thus it is not clear if the RF resistivity and effective permittivity observed using the EM waves [36-39] will be the same when observed with a SAW. These parameters change when an ultra-thin Pd film is exposed to hydrogen gas, and may dramatically alter the AE response of the SAW. Furthermore, these films have a very small volume, which allows hydrogen gas to diffuse in and out very rapidly. Depending on the Pd-gas interaction, the resistivity will change rapidly and may be reversible [34, 35]. The purpose of this work is to study the ultra-thin Pd films, using the AE effect to increase the sensitivity of the gas, film and SAW interaction. 3) SAW Pd Thick Film H2 Sensors The use of SAW devices as sensors was introduced in the 1970’s and continues to be explored to date. The use of SAW with Pd thin films for as H2 sensors has also had a wealth of publications to date and only a few examples are referenced here [5-13]. It has been recognized that Pd absorbs hydrogen and that this absorption may change the mechanical or AE parameters of the SAW-film interaction. The first SAW based hydrogen sensor was demonstrated by D’Amico et al. in 1982 [5-7]. In his report, D’Amico utilized SAW single and dual delay line oscillators in order to observe the frequency shift, due to mass loading caused by a thick Pd film (1900-7600Å) in the delay path.

These devices were exposed to 0.001%, 0.1% and 1% concetrations of H2 gas (balance N2). The fractional change in frequency was found to be proportional to film thickness; a 1900Å film caused 15ppm shift while the 7600Å film caused a 100ppm frequency shift. The reaction rates ranged from 0.8 to 21Hz/s depending on gas concentration and flow rate. The total rise time ranged from 5 to 70 minutes depending on gas concentration and flow rate. In general, the rise time increased with flow rate but decreased with gas concentration. Though the measurements were performed at room temperature and pressure, O2 gas was used to dissociate the H2 molecules from the Pd, thus it is not clear if the process is completely reversible without O2. Jakubik et al. [8, 10, 11] also implemented a SAW dual delay line oscillator for H2 gas sensing, with the distiction of using a bilayer structure in the delay path. A 1200Å dielectric film—copper phthalocycanine, (CuPc), nickel phthalocycanine, (NiPc), or metal-free phthalocycanine, (H2Pc)—was placed between the SAW substrate and a 200Å Pd film. The dielectric prevented the Pd film from shorting out the AE response of the SAW. The mass loading effect of hydrogenated CuPc, NiPc, and H2Pc and 200Å Pd films are small when compared to the electrical response, thus, the AE response is the dominant sensing mechanism. These devices were exposed to H2 gas concentrations from 1.5-4%, from which a 40ppm shift in center frequency resulted. The reaction rates varied linearly with H2 gas concentration where a 1.5% H2 gas concentration caused a 2.1Hz/s reaction rate and 6.7 min total rise time and a 4% H2 gas produced 21Hz/s with a total rise time of 1 minute. The fall times were all typically 5minutes and these devices were reversible at room temperature and pressure. Yamanaka et al. [9] introduced the ball SAW device hydrogen sensor. This consisted of a piezoelectric quartz crystal sphere with a single interdigitated transducer at its equator and a 200Å Pd film in the SAW propagation path. When exposed to 3% H2 gas, a reversible 10ppm change in velocity with a response time of around 60 seconds was observed. There was also a small decrease in amplitude due to H2 gas exposure, the magnitude of which was appeared to be frequency dependent. The authors did not know the precise cause of the attenuation but theorized that it will be used as a supplementary H2 sensing mechanism. Sensor Application The devices designed by D’Amico and Jakubik are active and wired and comprise the majority of the SAWbased hydrogen sensing designs found in literature. Yamanaka’s ball SAW sensor designed by may be implented passively and wirelessly, but unfortunately, it is relatively difficult and expensive to fabricate. Thus, a

different approach was need in order to implent a passive, wireless, SAW based hydrogen gas sensor.

Figure 5. Example SAW OFC RFID hydrogen sensor embodiment. The device is being tested on an RF probe station and the reflector diffraction pattern is apparent from the OFC reflectors at differing frequencies.

The ultimate goal of the research is to produce a wireless, passive gas hydrogen sensor with the use of an ultrathin Pd film. One basic embodiment is shown in Fig. 5, where a Pd thin film is placed in the propagation path between a transducer and an orthogonal frequency coded (OFC) reflector that can affect the amplitude or delay of the reflected signal [41]. A second OFC reflector has no Pd film in the path and acts as a reference. In order to build an operational device, first the Pd film needs investigation and then the SAW Pdfilm interaction needs to be characterized. IV.

PALLADIUM THIN FILM EXPERIMENTAL STUDY

Pd Resistivity vs. Thickness Characterization An investigation into the deposition of thin and ultrathin Pd films was conducted. The purpose was to establish reproducible and controlled Pd film depositions. Thin films of Pd were deposited using an electron beam evaporation system. A base pressure of approximately 10-6 Torr was achieved before evaporation. The substrate temperature was approximately 400C. A quartz crystal thickness monitor was used to record the inferred metal thickness and was located in close proximity to the samples. The thickness monitor was calibrated as carefully as possible to attempt reproducibility and accuracy. Hundreds of deposition runs were used to remove sources of error, and many more deposition runs were made to verify reproducibility. Fig. 6 shows a typical data run of the measurement of Pd film resistivity versus thickness. The curve is obtained under vacuum by measuring the resistivity between two thin film metal plates of gold. The in-situ measurement is recorded using a programmable multimeter and deposition monitor. The deposition rate is maintained at approximately 0.1 Å/sec. The plot shows an exponential change in resistivity versus film thickness when films are less than

approximately 30 Å. The ultra-thin film resistivity is within the range of interest, as previous discussed and shown in Figs. 2 to 4. This is fortuitous for possible use as H2 gas sensors if the gas affects the film’s properties.

top of the reflector. The Pd-path device provides propagation loss data while the Pd-reflector response

Figure 6. In-situ vacuum, measured Pd film resistivity versus thickness, as inferred from crystal monitor.

Figure 8. Plots of the S21 time domain reflector response of a simple delay line on YZ LiNbO 3. Control device has lowest loss as reference. Curves show effect of multiple hydrogen exposures.

Figure 7. Plots of the S21 time domain reflector response of a simple delay line on YZ LiNbO3 for three cases: 1) control device without Pd film, 2) device with Pd film on the Al reflector grating and 3) device with Pd film in the propagation path.

SAW Pd Thin Film Experimental Results In order to examine the affects of the SAW-Pd thin film interaction, with and without hydrogen exposure, a series of test devices were designed and fabricated. The devices presented were fabricated on YZ LiNbO3 substrates at approximately 123MHz. The structures were made of aluminum films with the transducer sampled at 4f0 and the reflector was composed of 25, ¼ wavelength opened electrodes. Three devices were tested, a control wafer without a Pd film, and two designs having Pd either in the propagation path or on

would yield a change in the shorting of the Al structure. The devices S21 frequency responses were obtained via a RF probe station and network analyzer and then transformed into the time domain (Figs. 7 & 8). The control device without Pd showed the lowest loss of approximately 23dB, the device with Pd in the propagation path had approximately 46dB loss and the device with Pd on the reflector had approximately 31dB loss. The Pd film length was 1.27mm in the propagation direction, yielding a loss of approximately 88 dB/cm. The Pd thin film thickness is estimated to be 12-15Å, with a resistivity of approximately 0.125Ω-cm based on the in-situ thin film resistivity measurements. Next, the devices with the Pd films were exposed multiple times to a 2% hydrogen and 98% nitrogen gas mixture for approximately 10 seconds per exposure, at room temperature. The gas exposure was accomplished on the RF probe station via a small poly tube to deliver the gas to the substrate area. The films reacted immediately to gas exposure, as recorded by sight. As seen, multiple gas exposures resulted in decreasing measured loss; approaching that of the control device (Fig. 8). The change in insertion loss for the Pd-delay path device is approximately 20dB after final H2 gas exposure and the propagation loss was 9.4dB/cm. Since the loss curve in the AE model is symmetrical about the

peak loss (Figs. 3-4, & 9), it is necessary to determine on which side of the peak the measured loss occurs in

that the film may be forming PdHx, which is known to increase the film resistivity. Moreover, this apparent Table 1. Summary of measured propagation loss, velocity and the fractional change in group delay beneath Pd film on 123-MHz delay line.

increase in resistivity opposes change predicted by HILE for nanoclustered Pd films, consequently, suggesting that the AE effect is more sensitive to another characteristic of the ultra-thin Pd film. Figure 9. Plot of AE model prediction of propagation loss of SAW at 123MHz as a function of film resistivity. The change in loss indicates a resistivity increase of approximately an order of magnitude.

order to decipher whether the resistivity of the Pd film increases or decreases with exposure to H2 gas. This was accomplished by analyzing the velocity shift beneath the Pd film on the delay devices and the loss trend in the Pd-reflector devices. When the reflector devices were exposed to H2 gas there was as increase in reflectivity of approximately 7dB. Since the grating reflectivity, which is overlaid with a Pd film, increased with H2 exposure, it suggests that the Pd film resistivity may be increasing, which would tend to open circuit the region between reflector electrodes. Initially, the reflector response is damped as compared to the case without the Pd film and then increases with each H2 gas cycle. Assuming that damping is due to an electrical shorting effect, the Pd film resistivity would increase with each successive H2 gas cycle, suggesting that the measured loss falls to the right of the peak loss predictions of the AE model. The group delay of the Pd-delay path device was extracted by gating the reflector S21 time response (Fig. 8), removing the transducer response, and performing a Fourier transform. Using this frequency domain response, the group delay was calculated by finding the derivative of the phase with respect to frequency (dθ/dω), from which, the velocity beneath the Pd film was extracted; the results are summarized in table 1. The table shows that the mean velocity beneath the Pd film is less than of free surface, and increases when the Pd film is exposed to H2 gas. A positive velocity change suggests that the film resistivity increases when exposed to H2 gas, once again indicating that the measured loss falls to the right of the peak loss. This suggests that the hydrogen gas is increasing, rather than reducing, the film resistivity. This seems to indicate

Using the measured propagation losses from the Pddelay line device (i.e. Fig. 8, 88 dB/cm, etc.), the AE model predicts an increase in resistivity from 1.4x103Ωcm (before H2 gas exposure) to 1.4x104 Ω-cm (after final H2 gas exposure) with a fractional velocity increase of 823ppm after the final H2 gas cycle (Fig 9). This a promising result as the SAW velocity as determined by the AE model is close to values extracted from the measured data (Table 1), signifying good agreement between theory and experiment. A resistivity of 1.4x103Ω-cm suggests a film which is approximately 1 to 2 Å thick, based on the in-situ resistivity vs. thickness curve of Fig.6. Moreover, 1.4x103Ω-cm is 4 orders of magnitude larger than the in-situ measured DC resistivity of 0.125Ω-cm. It is unlikely that the film is 1 to 2Å thick since its thickness was approximated using carefully calibrated, water cooled, dual quartz microbalances, which infers film thickness from the mass loading on the crystal. The film monitor and deposition system are believed to be provide very reproducible results after making over 100 depositions over 2 years. The Pd films for these devices are believed to be 12 to 15 Å. The inferred increase in resistivity due to 2% H2 gas exposure may be due to the formation of PdHx, however the magnitude of the increase (10x) is anomalously high and unreported in literature. Sakamoto et al. [25] measured a maximum of 2x increase in the resistivity of thick Pd films when exposed to large concentrations of H2 gas. These results are in agreement with at least 9 other authors cited by Sakamoto. This suggests that there are secondary mechanisms in the SAW-Pd-H2 interaction that need further investigation. The anomalous resistivity inferred from the data may be due to a high effective permittivity of the ultra-thin Pd film. Based on Humphrey’s report [36], assuming the RF resistivity of the film is an order of magnitude lower than the DC value (of 0.125Ω-cm), is a conservative

estimate. Using the RF resistivity value (of 0.0125Ωcm) and the measured propagation loss data, the AE model implies an effective permittivity of 7x106 before hydrogen gas exposure and an increase to 7x107 after final hydrogen gas exposure. Interestingly, these values are within the ranges reported by Boltaev and Krupka [38, 40] for discontinuous metal films. Further device characterization is required, including Pd ultra-thin film morphology, RF resistivity, effective permittivity, aging effects in atmosphere, and actual SAW/film interaction at the microscopic level. At room temperature, the devices showed no reversibility to their pre-hydrogen exposure operational levels. However, both device structures show strong sensitivity to hydrogen gas exposure and could be used as an on-off gas sensor when integrated into a wireless SAW sensor embodiment. V.

DISCUSSION AND CONCLUSIONS

This paper presented background on the pioneering work on the acoustoelectric effect for SAW devices. Early investigators provided the theoretical basis and the earliest experimental verification, as well as some of the first sensors. This paper has presented experimental measurements of Pd ultra-thin film resistivity and SAWPd propagation loss. The measurements of propagation loss confirmed a large propagation loss effect at critical film thickness, as predicted by the AE theory. The present results of the SAW device exposure to hydrogen gas imply an approximately equivalent 10x change in the Pd thin film resistivity or effective permittivity. This is an unexpectedly large change, which could suggest a significant change in film morphology and electrical characteristics. The films have not yet been inspected or characterized for initial morphology or for post gas exposure morphology. The large change in SAW propagation loss with only 2% H2 gas exposure demonstrates a very large sensitivity, ideal as a sensor. A SAW device embodiment operating in a differential mode with Pd in one path could operate as a warning “switch” sensor. The 2% hydrogen gas concentration is below the ignition threshold, which is often important for such a sensor. The ultimate sensitivity to both low and high hydrogen gas concentrations needs further investigation. These are initial device results and are extremely encouraging, but leave many questions unanswered. The measured devices infer a much larger Pd film resistivity than is measured in vacuum after deposition. This will require further investigation to resolve the mechanisms for the predicted and measured resistivity differences. Device fabrication processes are critical in achieving Pd film stability and hydrogen sensitivity. The devices currently operate irreversibly at room temperature, but perhaps reversibility is achievable with a different film morphology. The film and device

physics needs further work to understand all the mechanisms causing the SAW film interactions. It is noteworthy to realize that the SAW device is a contactless method to evaluate thin film properties and itself, is a sensor that may be very useful. Both the Pd film conductivity and dielectric properties require further study to determine the SAW, ultra-thin Pd film and H2 interactions. The SAW/Pd interaction occurs at the micron and sub-micron level due to the SAW wavelength and the film morphology. The frequency dependent effects need further study with the goal of a sensitive, reversible room temperature SAW wireless, passive hydrogen gas sensor. ACKNOWLEDGMENT The authors are most grateful to Dr. Robert Youngquist, NASA-KSC for his continuing support discussions and suggestions. The authors acknowledge support through NASA-KSC STTR contract NNK07EA39C, and the McKnight Doctoral Fellowship Program. REFERENCES [1]

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Brian H. Fisher (S’05) was born in St. Mary, Jamaica in November 1982. He received the B.S. degree in electrical engineering from Florida Atlantic University, Boca Raton, FL in 2005, and a M.S. degree from the University of Central Florida, Orlando, FL in 2007. He is currently pursuing his Ph.D. at the University of Central Florida under the advisement of Dr. Donald Malocha. His research interests include SAW cryogenic liquid and hydrogen gas sensors. Mr. Fisher has been a member of IEEE and UFFC since 2005 and 2006 respectively. He is a McKnight Doctoral Fellowship recipient. Donald C. Malocha received his B.S. in Electrical Engineering & Computer Science and, M.S. and Ph.D. degrees in Electrical Engineering from the University of Illinois, Urbana, in 1972, 1974 and 1977, respectively. He is currently a Professor in the School of Electrical Engineering and Computer Science at the University of Central Florida (UCF). He has held research positions at Texas Instruments, Sawtek, and Motorola, and has been a visiting scholar at ETH, Switzerland, and the University of Linz, Austria. Don is an Associate Editor of the IEEE Ultrasonics, Ferroelectrics and Frequency Control Transactions, UFFC Standards chair, and is past president of the IEEE UFFC society. He serves on the technical program committee (TPC) of the IEEE International Ultrasonics Symposium, International Frequency Control Symposium, and has served on the TPC of the IEEE MTT Symposium and European Frequency & Time Forum. He has numerous publications in acoustoelectric and RF technology, and has multiple patents issued and pending. He has received the IEEE UFFC 2008 Distinguished Service Award, the 2005 J. Staudte Memorial Award, the 2000 IEEE Third Millennium Medal, the 1998 Electronic Industries Association’s (EIA) David P. Larsen Award, and is the 2004 UCF Distinguished Researcher of the Year. His UCF research group works in many aspects of surface and bulk acoustic wave (SAW and BAW) technology; current emphasis is on wireless, passive SAW sensor and RFID tag technology. The group has expertise in materials, modeling, simulation, fabrication, and device design and system implementation.