Mass sensitivity of Love-mode acoustic sensors incorporating silicon dioxide and silicon-oxy-fluoride guiding layers

Sensors and Actuators A 88 (2001) 20±28

Mass sensitivity of Love-mode acoustic sensors incorporating silicon dioxide and silicon-oxy-¯uoride guiding layers Geoffrey L. Harding* CSIRO Telecommunications and Industrial Physics, PO Box 218, Lind®eld, NSW 2070, Australia Received 18 May 1999; received in revised form 28 July 2000; accepted 7 August 2000

Abstract A detailed experimental study of the sensitivity to mass loading has been carried out for Love-mode acoustic sensors of frequency 110 MHz and wavelength 40 mm, consisting of ST-cut quartz overlaid with various thicknesses of sputtered silicon dioxide guiding layers. Measurements of the sensitivity at various positions on the surfaces of the devices showed that the relative sensitivity of the IDT electrodes and `sweet spot' (between the IDTs) depends strongly on guiding layer thickness. The manufacture and properties of devices incorporating a versatile new guiding layer material based on silicon-oxy-¯uorine has also been investigated. The characteristic velocity of shear acoustic waves in this material may be varied by controlling the ¯uorine content during deposition. A systematic study of the sensitivity and insertion loss has been carried out for devices incorporating guiding layers produced with various ¯uorine concentrations. Highly sensitive devices have been produced with guiding layer thickness 2 to 5.5 mm. Some preliminary results have also been obtained for devices incorporating multilayer or continuously graded silicon-oxy-¯uoride guiding layers. The properties of most of the devices are extremely stable in air at room temperature. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Acoustic sensors; Love-mode; Mass sensitivity; Guiding-layer material

1. Introduction Recently surface acoustic wave (SAW) devices based on plate modes [1±6] and on Love waves [7±24] have received increasing interest for potential sensing applications in gaseous and particularly in liquid media. In liquid sensing applications SH waves are advantageous as they do not radiate energy into the ¯uid. In plate mode devices an SH wave is one particular mode which can be excited in a thin piezoelectric plate. Love waves are similar to plate modes in that they propagate near the surface of a substrate material which supports shear horizontal (SH) waves when the surface is overlaid by a thin ®lm guiding layer in which the shear wave velocity is less than the shear velocity in the substrate. The sensitivity to mass loading of the Love-wave device is enhanced by a low density of the ®lm as well as a large difference between the shear velocities. Love-wave devices have been successfully produced incorporating guiding layers consisting of spun polymethylmethacrylate (PMMA), spun polystyrene and sputtered or CVD silicon dioxide overlaid on single crystal quartz. The polymeric *

Present address: 38 Lamette St. Chatswood, Sydney, NSW 2067, Australia. Tel.: ‡61-2-9413-7485; fax: ‡61-2-9413-7200. E-mail address: [email protected] (G.L. Harding).

materials have lower shear wave velocity and lower density than the silicon dioxide [7±11,17,19,23], providing the potential for higher sensitivity, but devices incorporating polymers suffer from the disadvantage of higher acoustic loss. Gizeli and co-workers [7±11] utilised PMMA layers spun onto Y-cut quartz with interdigital transducers (IDTs) of periodicity 45 mm at the quartz±PMMA interface. Kovacs and co-workers [12±15] and Du and co-workers [19±21] have utilised sputtered silicon dioxide layers on ST-cut quartz and Jakoby and Vellekoop [16] utilised plasma enhanced CVD silicon dioxide. Harding and Du [22,23] have recently investigated hybrid devices incorporating guiding layers consisting of sputtered silicon dioxide overlaid by spun PMMA or spun polystyrene. The hybrid devices exhibited higher sensitivity than devices based on a single layer of silica or PMMA. Both experimental measurements and modelling results [9,12,17±19,24] show that for a particular guiding layer material, the sensitivity to mass loading increases with layer thickness, peaks at an optimum layer thickness, then decreases with further increases in thickness. Du et al. [19] have carried out a systematic study of the sensitivity to mass loading in the `sweet spot', i.e. the area between the IDTs, as a function of the thickness of the silicon dioxide guiding layer for devices of wavelength

0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 4 9 1 - X

G.L. Harding / Sensors and Actuators A 88 (2001) 20±28

40 mm. This study showed that the sensitivity peaks sharply at 5.5 mm thickness of the guiding layer with a rapid falloff in sensitivity for larger thicknesses, whereas modelling results predicted a more gradual decrease in sensitivity for thicknesses greater than the optimum value. In this manuscript a more detailed experimental investigation of the sensitivity to mass loading of the silicon dioxide-based devices is presented. The average sensitivity of the entire Love-wave device (incorporating IDTs and `sweet spot') has been determined as a function of guiding layer thickness, and in addition the details of the sensitivity of devices as a function of position on the device has been investigated for selected layer thicknesses. A problem of considerable interest is the development of new Love-wave guiding layer materials with appropriate properties. Both spun PMMA and sputtered silicon dioxide layers have shear velocities which are somewhat sensitive to deposition conditions, but in our experience, the properties are not easily controllable. The properties of sputtered silicon dioxide, for example, are quite sensitive to the subtleties of the sputter deposition conditions which require careful tuning to achieve both low acoustic loss and low characteristic acoustic shear wave velocity as required for optimum devices. An ideal guiding layer material would be capable of reproducible deposition with stable properties, including low acoustic loss, relatively low density, and with a shear velocity which could be varied continuously over a large range. Such a material could facilitate fabrication of devices, and provide the possibility for production of highly sensitive Love-wave devices incorporating a single (homogenous) layer, multiple layers with stepped values of acoustic velocity, or graded guiding layers with continuously varying acoustic velocity. In this manuscript we also present the results of measurements of the sensitivity and insertion loss for Love-wave devices incorporating a newly developed guiding layer material based on sputtered silicon-oxy-¯uorine which has some of the desired properties. 2. Experimental techniques The experimental techniques relating to the fabrication of devices based on sputtered silicon dioxide guiding layers, and measurements of sensitivity to mass loading using the novel use of ultra-thin gold, are described in detail elsewhere [19], but a few important points will be mentioned here. The Love-wave delay line devices were fabricated on ST-cut quartz substrates with wave propagation perpendicular to the crystallographic X-axis. The transmit and receive IDT electrodes consisted of sputtered chromium of thickness 0.24 mm with periodicity 40 mm. The IDT dimensions were 3 mm  3 mm and the IDT centre-to-centre spacing was 6 mm. Thus, the dimensions of an entire Love-wave delay line are 9 mm  3 mm and the `sweet spot' (between the IDTs) is 3 mm  3 mm. The basic SAW device (with no guiding layer) supports an SH mode of frequency

21

124.3 MHz and exhibits an insertion loss of 24 dB. A broadband ampli®er was used to feed energy back from the receive IDT to the transmit IDT to compensate for the loss of the delay line and to maintain oscillation. Frequency of oscillation was measured using a Philips PM6669 frequency counter and insertion loss was measured using a Tektronix 2710 spectrum analyser. Two identical delay lines were fabricated (centres 9 mm apart) on the same quartz substrate allowing the measurement of the difference frequency in experiments designed to determine device sensitivity. The difference frequency has the advantage of being relatively insensitive to environmental in¯uences such as temperature, and being of the order of a few tens of kHz, is more easily measured than the full oscillation frequency. Guiding layers consisting of silicon dioxide were produced using an rf magnetron reactive sputtering technique with an argon±oxygen gas mixture. In our experience careful control of the deposition conditions is necessary in order to produce high quality silicon dioxide with low acoustic loss and low shear velocity. Devices incorporating silicon dioxide guiding layers of thickness 2.2±7.2 mm were available from previous work [19]. These devices were used in the present experiments for the detailed study of mass sensitivity. Vacuum-based deposition techniques have been used elsewhere for experimental sensitivity measurements on plate mode devices, using evaporated silver [1] and CVD of polyamic acid [5]. In the present experiments sensitivity to mass loading was determined by depositing an ultra-thin (0.5 nm nominal thickness) sputtered gold ®lm through an aperture in a polyimide polymer mask, onto a selected area of one of the delay lines. Such ultra-thin gold ®lms are well known to condense with a discontinuous island structure on a substrate such as silica. Our measurements of sheet resistance have shown that the gold ®lms become continuous at a thickness of 3 nm, considerably larger than the nominal thicknesses used in our sensitivity measurements. Consequently the metal ®lm should not interact with the wave via acoustoelectric coupling. Several gold ®lms of this thickness could be deposited sequentially producing a linear frequency response. Fig. 1 shows schematically the three types of aperture used: A, for coating the entire area of a delay line; B, for coating a 2.5 mm wide area in the sweet spot and C, a narrow aperture 1.2 mm wide, which could be positioned anywhere along the device for the investigation of local sensitivity as a function of position along the delay line. Calibration of the gold deposition rate, and measurement of the difference frequency change after deposition of an ultra-thin gold ®lm allowed determination of the mass sensitivity (Hz/ng) for the various areas studied. Our approach in plotting sensitivity in the units Hz/ng effectively allows comparison of the frequency response when a 1ng mass is deposited onto various well de®ned areas of the delay line, for example, within a narrow strip at some location along the length of the delay line, within the entire area of the `sweet spot', or onto the

22

G.L. Harding / Sensors and Actuators A 88 (2001) 20±28

Fig. 1. Schematic of a dual delay line Love-wave device showing the location, relative to the IDTs, of the apertures in the masks used to deposit thin gold for determination of mass sensitivity: (A) aperture 10 mm  5 mm for the entire delay line sensitivity; (B) aperture 2:5 mm  4 mm for sensitivity within the `sweet spot'; (C) aperture 1:2 mm  3:5 mm which can be located at various positions along the delay line to determine sensitivity as a function of position.

entire area of the delay line. Alternative sensitivity units, for example Hz cm2/ng, are less appropriate for this comparison. Repeated measurements on one device, and estimates of the possible uncertainty in the mass deposition rate indicate that sensitivities could usually be determined within 5%. For mask C there was also a possible uncertainty of 0.25 mm in the positioning of the mask. A few experiments have also been carried out to allow comparison of the mass sensitivity results obtained using the gold deposition technique with results obtained using a protein deposit relevant to many biosensing experiments. Various areas of a Love-wave delay line were coated with the protein Sheep IgG which was immobilised on a robust silanation layer on the silica surface of the sensor [21]. The IgG was applied in concentrated (5 mg/ml) droplets of volume 10±20 ml on a particular area of the delay line. The hydrophobic nature of the surface allowed the areas exposed to be de®ned probably within 20%. After an exposure time of 1 h, the IgG droplet was gently washed away using DI water and the surface blown dry. The protein layer so produced is generally thicker than a monolayer due to the tendency for the protein molecules to form clumps on the surface to some extent, rendering the estimate of mass deposited per unit area uncertain, however, a value of 440 ng/cm2 has been estimated in our previous work [21]. The values of frequency change associated with the protein deposited on the `sweet spot', the `sweet spot' plus the half of each IDT closest to the `sweet spot', and the full Love-wave delay line, were compared with the sensitivity results obtained using the gold ®lms. The silicon-oxy-¯uorine ®lms were produced by a rf magnetron reactive sputtering technique similar to that used for silicon dioxide, but using argon±oxygen±carbon tetra¯uoride gas mixtures. Our previous development work on metal-oxy-¯uorine ®lms for optical applications [25,26] showed that these ®lms generally contain negligible carbon due to its removal from the sputtering chamber as CO and CO2. Films with higher/lower characteristic shear acoustic velocities were produced by decreasing/increasing the par-

tial pressure of carbon tetra¯uoride in the sputtering chamber during deposition. In this manuscript, the partial pressure of CF4 is expressed as a percentage of the total gas pressure (0.3 Pa of argon plus oxygen plus carbon tetra¯uoride) in the sputtering chamber. The systematic evaluation of mass sensitivity of Love-wave devices for a range of thicknesses of the guiding layer, and a particular ¯uorine content of the guiding layer material, was determined by increasing the thickness of the guiding layer on a device in sequential deposition steps. Measurement of mass sensitivity (both in the `sweet spot' and for the whole delay line area) and insertion loss was made after deposition of each step. This method allows ef®cient accumulation of data as only a limited number of devices are required, but introduces some uncertainty because the interfaces between adjacent layers may become contaminated during the mass sensitivity measurements (despite attempts at complete removal of the gold deposits by chemical etching and gentle abrasion). Furthermore, adjacent layers may have slightly different compositions due to uncontrollable variations in deposition conditions, with possible deleterious effects on insertion loss and sensitivity. Measurements of the compositions of the silicon-oxy¯uoride ®lms were made by energy dispersive X-ray analysis using a JSM 5400 LV scanning electron microscope with an Oxford ISIS EDX analyser operating with 20 kV accelerating voltage, and using virtual standards. 3. Results and discussion 3.1. Sensitivity of devices incorporating silica films Fig. 2 shows the mass sensitivity (Hz/ng) measured in the `sweet spot', using mask B and for the entire delay line, using mask A, as a function of thickness of the silicon dioxide guiding layer (all the devices have wavelength 40 mm). Both sensitivity curves peak near a thickness of 5.5 mm, corresponding to thickness/wavelength 0.14, however, the sensitivity in the `sweet spot' decreases sharply for larger and smaller thicknesses while the sensitivity of the entire delay line decreases much less rapidly on either side of the maximum. These results indicate that the relative sensitivity of `sweet spot' and IDT areas must change signi®cantly with varying guiding layer thickness. In particular, the sensitivity of the IDT areas must increase for thicknesses less than or more than 5.5 mm in order to compensate for the rapidly decreasing sensitivity in the `sweet spot' area. Fig. 3 shows the sensitivity (Hz/ng) plotted as a function of position for three Love-wave devices incorporating silica thicknesses 2.2, 5.5 and 7.2 mm. This data was obtained using the mask (C) with 1.2 mm aperture. The dashed lines at 3 and 6 mm indicate the edges of the IDTs; (0±3 mm constitutes the transmit IDT and 6±9 mm constitutes the receive IDTs). The sensitivities at the extremities of the delay line (i.e. at 0 and 9 mm) have been

G.L. Harding / Sensors and Actuators A 88 (2001) 20±28

23

Fig. 3. Mass sensitivity vs. position on the Love-wave delay line for three thicknesses of silica guiding layer. The dashed lines show the positions of the edges of the IDTs. The open triangles refer to the device incorporating 7.2 mm of silica with transmit and receive IDTs reversed. Fig. 2. Mass sensitivity of the `sweet spot' and for the entire delay line, as a function of guiding layer (overlay) thickness for devices with wavelength 40 mm incorporating silicon dioxide.

assumed to be 0. The sensitivity of the device incorporating 5.5 mm silica is extremely high within the `sweet spot' and falls away sharply near the edges of the IDT areas. Conversely, the results for the devices incorporating silica of thickness 2.2 and 7.2 mm exhibit considerably lower sensitivities in the `sweet spots', but substantial sensitivity in the IDT areas. The sensitivities plotted in Fig. 3, when integrated either over the `sweet spot' or over the full delay line, agree extremely well (usually within a few %) with the sensitivities shown in Fig. 2 where the entire delay line or most of the `sweet spot' is coated (see Table 1). Fig. 3 shows that the sensitivity curves for the 2.2 and 5.5 mm devices are reasonably symmetric about the geometric centre of the devices while the curve for the 7.2 mm device is slightly asymmetric. This result was found to be consistent in repeated measurements but as a further check the 7.2 mm device was reversed so that the transmit IDT became the receive IDT and vice versa, and the sensitivity was remeasured at the centre of each IDT. The sensitivity results were substantially the same as before (see open

triangles in Fig. 3) con®rming that the asymmetry is a genuine property of this particular device possibly due to some variation in guiding layer thickness over the device or a subtle variation in uniformity of the IDTs. Table 2 shows the results of coating particular areas of the 5.5 mm device with IgG. Frequency changes for the three areas coated mirror the sensitivity results in Fig. 3, i.e. a large frequency response for the coated `sweet spot', and only marginally increased frequency when half of each IDT, or when the entire delay line is coated. However, the variable amount of protein deposited in such experiments can result in uncertainties of at least 20% in the frequency response, so this technique is at best semi-quantitative. Using the surface mass density 440 ng/cm2 for the IgG, approximate sensitivity values 300, 170 and 115 Hz/ng are calculated for the `sweet spot', `sweet spot' plus half of each IDT, and entire delay line, respectively, in reasonable agreement with the results in Fig. 2 and Table 1. Fig. 4 shows the measured sensitivity for the entire Lovewave delay line (sensitivity expressed in cm2/g, see [19]), and the sensitivity predicted by Ogilvy in some recent modelling work [24], as a function of guiding layer thickness for devices of wavelength 40 mm. There is reasonable

Table 1 Comparison of average sensitivities (calculated from the local sensitivities in Fig. 3) and measured sensitivities (Fig. 2) for the `sweet spot' and entire delay line, for devices with three silica thicknesses Silica thickness (mm)

Average sensitivity calculated for `sweet spot' (Hz/ng)

Measured sensitivity for `sweet spot' (Hz/ng)

Average sensitivity calculated for delay line (Hz/ng)

Measured sensitivity for delay line (Hz/ng)

2.2 5.5 7.2

84 270 100

86 240 95

81 118 95

80 118 100

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G.L. Harding / Sensors and Actuators A 88 (2001) 20±28

Table 2 Frequency response of Love-wave device with 5.5 mm silica when particular areas are coated with the protein Sheep IgGa Area of 5.5 mm device coated with IgG

Frequency change (kHz)

Sensitivity (Hz/ng)

`Sweet spot' (3 mm  3 mm) `Sweet spot' plus half of each IDT (6 mm  3 mm) Entire delay line

12.3 13.4 13.7

300 170b 115

a b

Sensitivities of the areas are estimated using 440 ng/cm2 for the IgG. See Table 1 for comparison with sensitivities obtained via gold deposition. Sensitivity determined from Fig. 3 for this area is 172 Hz/ng.

agreement between measured and predicted sensitivities. However, the present models do not account for the signi®cant variations of sensitivity with position on the devices. The results shown in Fig. 3 appear to indicate a genuine spatial variation, over the length of the delay line, of the mass loading sensitivity of these devices. The following pieces of evidence indicate that the results are probably not due to an artefact of the measurement technique.  As noted above, the sensitivities measured by loading the delay line with a narrow gold strip (using mask C) can be added to give, to a very good approximation, both the sensitivity of the `sweet spot' and that for the entire delay line, when the summation is carried out over the appropriate regions. This additivity of the measured results indicates that the form of the sensitivity variation shown in Fig. 3 is not due to effects of scattering at the edges of the gold loading layer.  Ogilvy [24] has discussed other possible effects of the gold loading layer, such as electrical effects and induced stresses, and concluded that any such effects should be negligible. As noted in Section 2, the gold film is sufficiently thin and inhomogeneous that it is non-conducting.  Coating with sheep IgG gave results that were consistent with those for the gold films.

At this stage the speci®c mechanism leading to the results shown in Figs. 2 and 3 is not clear. However, the results show clearly that effects of the IDTs on the wave propagation are signi®cant, and must be taken into account in calculations of the mass loading sensitivity. Previous theoretical models (e.g. [24]) have assumed an in®nite, homogeneous waveguide. Such models can only predict a spatially uniform sensitivity, which is clearly inconsistent with the measurements. The most signi®cant result is not that the IDTs affect the wave propagation, and hence the mass loading sensitivity, in their vicinity Ð as the generators and receivers of the waves this must be the case Ð but that their effect varies very strongly with the thickness of the guiding layer. Two possible mechanisms by which the IDTs could in¯uence the wave propagation, and which may contribute to the measured results are the following. 1. The finite apertures of the transducers, and their nature as arrays of line sources. Finite aperture effects are probably small, because all sides of the rectangular aperture are many wavelengths (75) in length. However, modelling of the possible effects of the finite apertures, and of side lobes and grating lobes, is required to confirm this. 2. Scattering of the waves by the IDT fingers. In the devices used for these measurements the IDT fingers consisted of sputtered chromium 0.24 mm thick, with centre-to-centre spacing of l/2 (20 mm). The acoustic impedance of the fingers (29 MRayl for waves of SH polarisation, using constants for bulk chromium) is substantially greater than that of the SiO2 guiding layer in which they are embedded (8.1 MRayl). Thus, the scattering cross-section of each finger may be substantial. Furthermore, the finger spacing satisfies the Bragg scattering condition which ensures that both the back- and forward-scattered waves from the individual fingers will interfere constructively. This effect also requires further evaluation. 3.2. Sensitivity of devices incorporating silicon-oxyfluorine guiding layers

Fig. 4. Measured sensitivity for the entire delay line and predicted sensitivity [24] for Love-wave devices of wavelength 40 mm incorporating various thicknesses of silica guiding layer.

Fig. 5 shows the Love-wave oscillation frequency as a function of guiding layer thickness for (40 mm wavelength) devices manufactured using 0% (i.e. pure silica), 2.5, 5, 8 and 36% CF4 in the sputtering gas. As the partial pressure of

G.L. Harding / Sensors and Actuators A 88 (2001) 20±28

25

Fig. 5. Love-wave frequency vs. guiding layer (overlay) thickness for devices of wavelength 40 mm incorporating silicon-oxy-fluorine guiding layers produced using various partial pressures of carbon tetrafluoride in the sputter gas mixture.

CF4 increases, the ¯uorine content of the guiding layer ®lms should increase, and the Love-wave frequency decreases more rapidly with thickness, indicating that the shear velocity decreases as the ¯uorine content of the ®lms increases. Apparently the presence of ¯uorine in the amorphous silicon dioxide reduces the magnitudes of the elastic moduli, and so reduces the acoustic shear velocity. Some devices were also manufactured with 20 and 15% CF4, but the relationships between frequency and guiding layer thickness were not signi®cantly different from the result for 36% CF4. This result suggests that the ¯uorine content may saturate and consequently the acoustic properties become relatively constant for 15% CF4. Fig. 6 shows the mass sensitivity versus guiding layer thickness for silicon-oxy-¯uorine guiding layers produced using various CF4 contents in the sputter gas. The upper graph shows the sensitivity in the `sweet spot' and the lower graph shows the average sensitivity of the entire delay line. In both graphs the peak sensitivity shifts to smaller guiding layer thickness as the ¯uorine content increases, and the magnitude of the sensitivity maximum for the entire delay line generally increases with increasing ¯uorine content. The peak sensitivity for the 36% CF4 device is 20% higher than for the device incorporating pure silica. Some similar characteristics are evident for each Si±O±F composition. In particular the sensitivity in the `sweet spot' decreases sharply with increasing thickness after reaching a maximum, while the sensitivity of the entire delay line decreases more slowly. Obviously the relative sensitivity of `sweet spot' and IDTs changes in similar fashion to the results described for pure silica. The peak sensitivity of the `sweet spot' appears to decrease as the ¯uorine content increases, however, the sensitivities vary very rapidly with guiding layer thickness, particularly for 5, 8 and 36% CF4, resulting in considerable dif®culty in experimental resolution of the maximum sensitivity. A characteristic of some interest for all the Si±O±F devices is that the sensitivity maximum for the entire delay

Fig. 6. Mass sensitivity of the `sweet spot' (upper) and the entire delay line (lower) vs. guiding layer (overlay) thickness for Love-wave devices of wavelength 40 mm incorporating silicon-oxy-fluorine layers produced using various partial pressures of carbon tetrafluoride in the sputter gas mixture.

line occurs at somewhat greater thickness than the maximum in `sweet spot' sensitivity. Thus, for the devices produced with 5, 8 and 36% CF4 the entire delay line sensitivities peak when the `sweet spot' sensitivities have fallen to relatively low values, indicating that the IDT areas must exhibit relatively high sensitivity for these devices when the sensitivity of the entire delay line is at a maximum. Fig. 6 also includes results for a multilayer or graded device for which the CF4 partial pressure was increased sequentially for each layer deposited, starting with a 2.2 mm thick layer of pure silica (0% CF4) and ®nishing with a layer produced using 8% CF4. This device is of some interest, exhibiting high sensitivity in both the `sweet spot' and for the entire delay line for a thickness of about 5 mm. A second device (not plotted as the ®lm was deposited in one coating operation) has been fabricated in which the guiding layer of thickness 4.4 mm was continuously graded by varying the CF4 partial pressure from 0 to 20%. This device has a relatively high sensitivity for the entire delay line of 139 Hz/ng, but the `sweet spot' sensitivity is relatively low at 68 Hz/ng. Insertion losses for the various devices are shown in Table 3. The trends for the Si±O±F devices are similar to those observed previously for the silica devices [19]. The magnitude of the insertion loss decreases initially (from the value 24 dB for the basic SH SAW device) as guiding layer thickness increases, then increases to 20 dB for larger thicknesses. The relatively low insertion losses for devices incorporating quite thick Si±O±F guiding layers indicate

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G.L. Harding / Sensors and Actuators A 88 (2001) 20±28

Table 3 Insertion losses of Love-wave devices of wavelength 40 mm incorporating various thicknesses of guiding layer produced using various CF4 partial pressuresa CF4 pressure (%)

Thickness (mm)

Insertion loss (dB)

CF4 pressure (%)

Thickness (mm)

Insertion loss (dB)

0

2.2 3.2 4.1 5.5 6.5 7.2

11 14 17.5 19.5 20 18

2.5

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.3

15.5 16.4 16.8 20 22 23 23 23

5

2.2 2.6 2.95 3.3 3.8

13.5 13 12 13 14

8

1.5 1.8 2.1 2.35 2.6 3.0

18 18 15 16 19.5 23

36

1.08 1.29 1.5 1.71 1.9

11.3 13.5 10 14 19.8

Graded

2.2 2.6 3.0 3.4 3.9 4.5 5.1 5.7

11 14 15.5 18.5 19 20.5 21.5 22.5

a Partial pressure expressed as a percentage of the total sputter gas pressure. The ``graded'' film was produced by increasing the CF4 pressure in sequential steps from 0 to 8%.

that the acoustic losses in the Si±O±F ®lms are probably comparable to that in silicon dioxide. The abrasion resistance of the devices incorporating Si±O±F ®lms seems comparable to devices incorporating silicon dioxide. Some of the latter devices have been cleaned in our laboratory using various chemicals and gentle abrasion, and reused several hundred times. Most of the devices described in this work have exhibited excellent stability in dry air at room temperature subsequent to their manufacture. The oscillation frequencies and insertion losses of all devices were remeasured 12±15 months after manufacture. The properties were unchanged (i.e. oscillation frequencies identical within a few kHz and insertion losses identical within 1 dB) except for the device fabricated using 36% CF4, for which the oscillation frequency had increased by 2 MHz (from 116 to 118 MHz) and the insertion loss changed by 1.5 dB (from 20 to 18.5 dB). The latter device, produced using a high CF4 partial pressure in the sputter chamber, probably slowly desorbs some ¯uorine subsequent to manufacture with a consequent change in properties. The graded (multilayer) device shown in Fig. 6 and Table 1 was also unchanged after 12 months, however, the continuously graded device referred to above exhibited a small increase in frequency (107.4±107.8 MHz), probably because a relatively high CF4 partial pressure (20%) was used in the ®nal stages of the fabrication. The mass sensitivity of the devices was not remeasured but the unchanged oscillation frequency for most of the devices strongly implies that their sensitivity remains unchanged.

3.3. Composition of the Si±O±F films Table 4 summarises the results of the analyses of a silicon dioxide ®lm (produced without CF4), and four Si±O±F ®lms deposited on the Love-wave devices using the CF4 partial pressures described above. The analyses were performed 12±15 months after manufacture of the ®lms, however, the measurements of oscillation frequency described above suggest that only the ®lm produced using 36% CF4 may have changed its composition subsequent to manufacture. Both absolute and relative atomic compositions have signi®cant uncertainty due to the low atomic numbers of the elements involved (particularly carbon and oxygen), the different thicknesses of the ®lms, and the relatively small thickness of the ®lm produced using 36% CF4. The possible uncertainties are 5 at.% for C and O, 3 at.% for F and 1 at.% for Si. The analyses indicate the existence of a signi®cant ¯uorine content and that some carbon is present in all the ®lms, but in relatively small quantities. The ®lm Table 4 Compositions of films produced using various CF4 partial pressures CF4 pressure (%)

Film thickness (mm)

Si (at.%)

O (at.%)

F (at.%)

C (at.%)

0 2.5 5.0 8.0 36.0

5.5 6.3 3.8 3.0 1.9

25 18 23 22 21

68 57 53 52 60

± 18 21 23 14

7 7 3 3 5

G.L. Harding / Sensors and Actuators A 88 (2001) 20±28

produced without CF4 contains some carbon suggesting that carbon can be derived from oil vapour in the vacuum chamber as well as from CF4. The ¯uorine content increases and oxygen content decreases as the CF4 partial pressure is increased from 2.5 to 8%, although the variation is relatively small. The results for the ®lm produced using 36% CF4 are almost certainly unreliable due to the small thickness of the ®lm. An analysis technique such as RBS, which can be used on relatively thin ®lms, may be more suitable for these materials [26]. 4. Concluding remarks The mass sensitivity of some particular Love-wave devices of wavelength 40 mm incorporating silicon dioxide guiding layers has been determined experimentally in some detail using a technique based on mass loading by ultra-thin gold ®lms. Signi®cant variations have been observed in the relative sensitivity of IDT and `sweet spot' areas for devices of different guiding layer thickness. Values of local sensitivity when integrated along the devices agree well with the sensitivities measured on larger areas such as the `sweet spot' and entire delay line. Further measurements of sensitivity to mass loading by layers of protein also agree reasonably well with the results obtained using gold ®lms. However, these experimental measurements have been carried out on a particular set of devices manufactured with certain dimensions and materials (in particular Cr IDTs which exhibit higher acoustic re¯ectance compared to Al) and consequently sensitivity results may be qualitatively rather than quantitatively similar for devices manufactured elsewhere. Nevertheless, this work may have important implications for the design of some sensors where often only part of the sensor channel, e.g. the `sweet spot', is used for sensing purposes. We suggest that the origins of the positional dependence of the sensitivity may lie with the acoustic re¯ectance associated with the IDTs, however, the complexity of the results illustrates the need for more detailed modelling and understanding of the Love-wave (and other) SAW transducers to elucidate the subtleties of device sensitivity. Measurements made on devices incorporating the newly developed silicon-oxy-¯uorine guiding layers show that the deposition conditions strongly in¯uence the shear velocity, allowing high sensitivity to be achieved for relatively small layer thicknesses. This new guiding layer material should facilitate fabrication of highly sensitive devices by alleviating some of the dif®culties which can be experienced in depositing sputtered silicon dioxide with low shear wave velocity, and allowing highly sensitive devices to be fabricated with relatively thin guiding layers. The insertion losses measured for Si±O±F devices are comparable to those for silica-based devices, suggesting that operation of particular devices should be possible in water and buffer solutions. The preliminary work on multilayer or graded Si±O±F guiding

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layers has shown some promise for further improvements in device sensitivity. Further detailed study of such devices produced using CF4, and other ¯uorine containing gases, as well as a full characterisation of the composition and properties of the Si±O±F materials would be of considerable interest. Similar properties of the guiding layer should be achievable for silicon-oxy-¯uoride produced by alternative deposition techniques such as plasma enhanced CVD. Some preliminary work has also been carried out on Love-wave devices incorporating silicon-oxy-chlorine guiding layers sputter deposited using argon±oxygen±carbon tetrachloride gas mixtures. Our initial results showed that shear wave velocity could be similarly varied and the insertion losses were low, but the Si±O±Cl ®lms were considerably less stable than Si±O±F. Nevertheless, a more detailed evaluation of silicon-oxy-chlorine may be worthwhile. In addition, the incorporation of elements other than halogens in the basic silicon dioxide guiding layer, for example, sulphur, may be worthy of investigation. Some dielectric materials such as aluminium nitride have high acoustic shear wave velocity which renders them unsuitable for the guiding layers of Love-wave devices. The incorporation of elements such as ¯uorine, chlorine and sulphur may reduce the acoustic wave velocity and thus open up the possibility of new Love-wave guiding layer materials. Acknowledgements The author would like to thank Drs. D.C. Price and A.J. Richards for useful suggestions and discussion of the manuscript, Dr. J. Du and Mr. V. Mohan for assistance in producing SH SAW devices, and Dr. C. Horrigan for performing the analyses of the ®lms. Mr. P. Dencher is acknowledged for design of the electronics and advice on impedance measurements. References [1] S.J. Martin, A.J. Ricco, T.M. Niemczyk, G.C. Frye, Characterization of SH acoustic plate mode liquid sensors, Sens. Actuators A 20 (1989) 253±258. [2] S.J. Martin, A.J. Ricco, Monitoring photo-polymerization of thin films using SH acoustic plate mode sensors, Sens. Actuators A 21±23 (1990) 712±718. [3] J.C. Andle, J.F. Vetelino, M.W. Lade, D.J. McAllister, An acoustic plate mode biosensor, Sens. Actuators B 8 (1992) 191±198. [4] J.C. Andle, J.F. Vetelino, Acoustic wave biosensors, Sens. Actuators A 44 (1994) 167±176. [5] F. Josse, R. Dahint, J. Schumacher, M. Grunze, J.C. Andle, J.F. Vetelino, On the mass sensitivity of acoustic-plate-mode sensors, Sens. Actuators A 53 (1996) 243±248. [6] M.G. Schweyer, J.C. Andle, D.J. McAllister, J.F. Vetelino, An acoustic plate mode sensor for aqueous mercury, Sens. Actuators B 35/36 (1996) 170±175. [7] E. Gizeli, N.J. Goddard, C.R. Lowe, A.C. Stevenson, A Love-plate biosensor utilizing a polymer layer, Sens. Actuators B 6 (1992) 131± 137.

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[8] E. Gizeli, A.C. Stevenson, N.J. Goddard, C.R. Lowe, A novel Loveplate acoustic sensor utilizing polymer overlayers, IEEE Trans. UFFC 39 (1992) 657±659. [9] A.C. Stevenson, E. Gizeli, N.J. Goddard, C.R. Lowe, Acoustic Loveplate sensors: a theoretical model for the optimization of the surface mass sensitivity, Sens. Actuators B 13/14 (1993) 635±637. [10] E. Gizeli, A.C. Stevenson, N.J. Goddard, C.R. Lowe, Acoustic Loveplate sensors: comparison with other acoustic devices utilizing surface SH waves, Sens. Actuators B 13/14 (1993) 638±639. [11] E. Gizeli, M. Liley, C.R. Lowe, Detection of supported lipid layers by utilizing the acoustic Love-waveguide device: application to biosensors, in: Proceedings of the 8th International Conference on Solid State Sensors and Actuators and Eurosensors IX, Technical Digest, Vol. 2, Stockholm, 1996, pp. 521±523. [12] G. Kovacs, G.W. Lubking, M.J. Vellekoop, A. Venema, Love waves for (bio)chemical sensing in liquids, in: Proceedings of the IEEE Ultrasonics Symposium, Tucson, AZ, USA, 20±23 October 1992, pp. 281±285. [13] G. Kovacs, M.J. Vellekoop, R. Haueis, G.W. Lubking, A. Venema, A Love-wave sensor for (bio)chemical sensing in liquids, Sens. Actuators A 43 (1994) 38±43. [14] R. Haueis, M.J. Vellekoop, G. Kovacs, G.W. Lubking, A. Venema, A Love-wave based oscillator for sensing in liquids, in: Proceedings of the 5th International Meeting on Chemical Sensors, Technical Digest, Vol. 1, Rome, 11±14 July 1994, pp. 126±129. [15] G. Kovacs, A. Venema, Theoretical comparison of sensitivities of acoustic shear wave modes for (bio)chemical sensing in liquids, Appl. Phys. Lett. 61 (1992) 639±641. [16] B. Jakoby, M.J. Vellekoop, Viscosity sensing using a Love-wave device, Sens. Actuators A 68 (1998) 275±281. [17] Z. Wang, J.D.N. Cheeke, C.K. Jen, Sensitivity analysis for Lovemode acoustic gravimetric sensors, Appl. Phys. Lett. 64 (1994) 2940±2942. [18] J. Enderlein, E. Chilla, H.-J. FroÈhlich, Comparison of the mass sensitivity of Love and Rayleigh waves in a three-layer system, Sens.

Actuators A 41/42 (1994) 472±475. [19] J. Du, G.L. Harding, J.A. Ogilvy, P.R. Dencher, M. Lake, A study of Love-wave acoustic sensors, Sens. Actuators A 56 (1996) 211±219. [20] J. Du, G.L. Harding, A.F. Collings, P.R. Dencher, An experimental study of Love-wave acoustic sensors operating in liquids, Sens. Actuators A 60 (1997) 54±61. [21] G.L. Harding, J. Du, P.R. Dencher, D. Barnett, E. Howe, Love-wave acoustic immunosensor operating in liquid, Sens. Actuators A 61 (1997) 279±286. [22] G.L. Harding, J. Du, Design and properties of quartz-based Lovewave acoustic sensors incorporating silicon dioxide and PMMA guiding layers, Smart Mater. Struct. 6 (1997) 716±720. [23] J. Du, G.L. Harding, A multi-layer structure for Love-mode acoustic sensors, Sens. Actuators A 65 (1998) 152±159. [24] J.A. Ogilvy, The mass-loading sensitivity of acoustic Love-wave biosensors in air, J. Phys. D: Appl. Phys. 30 (1997) 2497±2501. [25] G.L. Harding, High rate dc reactively sputtered metal oxy-fluorine dielectric materials, Thin Solid Films 138 (1986) 279±287. [26] G.L. Harding, Production and properties of high rate sputtered low index transparent dielectric materials based on aluminium-oxyfluorine, Solar Energy Mater. 12 (1985) 169±186.

Biography Geoffrey L. Harding is a graduate of Monash University, Australia (B.Sc. (Hons.), 1969) and the University of Cambridge, UK (Ph.D., 1973). Between 1974 and 1985 he worked as a lecturer at the University of Sydney, Australia, developing an advanced evacuated glass tubular solar energy collector which is now in mass production. He subsequently joined the CSIRO Division of Telecommunications and Industrial Physics where until recently he was a Principal Research Scientist, working on a number of projects involving applications of thin films, including development and applications of Love-wave acoustic sensors.