Performance of micromachined quartz gravimetric sensors upon electrochemical adsorption of monolayers Ping Kao1, Ashish Patwardhan1, David Allara2,3, Srinivas Tadigadapa1,3*
Jörg Strutwolf, Damien Arrigan Tyndall National Institute University College Cork Lee Maltings, Prospect Row, Cork, Ireland
1
Department of Electrical Engineering, 2Department of Chemistry, 3Materials Research Institute The Pennsylvania State University University Park, PA 16802, USA. *e-mail:
[email protected]
Abstract— This paper presents the results of electrochemical calibration experiments performed on micromachined quartz gravimetric sensors. The absolute mass sensitivity of bulk acoustic quartz crystal microbalance (QCM) can be improved into the sub-10-12 g range upon miniaturization of the resonator thickness and area. Using plasma etching, we have fabricated miniaturized QCMs with thicknesses of ~29 μm and diameters of 500 μm with f0=58 MHz. Resonators with 60 nm thick Ti/Pt top electrodes were used to study the electrochemically induced oxide layer formation on the metal surface, the adsorption of hydrogen, and underpotential deposition (UPD) of Cu on Pt electrodes. The performance of microQCM is compared with the performance of a frequency matched overtone mode of a commercial 5 MHz resonator. Micromachined QCMs showed expected sensitivity improvement to UPD of Cu, however an unexpected hundred fold enhancement to oxygen and ~28 times enhancement in the sensitivity to hydrogen adsorption was observed which may be due to the roughness/porosity of the electrodes.
I.
INTRODUCTION
Gravimetric sensors offer the possibility of high-sensitivity, label free detection for chemical and biochemical sensing. Gravimetric sensors are mass measuring systems and recent advances in micro and nanoscale fabrication techniques have shown that in properly designed systems it is possible to measure mass into the 10-21 grams (zeptograms), range allowing the possibility of single atom/molecule detection [1]. Most of the high sensitivity gravimetric sensors are based upon micromachined cantilever designs which detect the attachment of mass through either a change in the resonance frequency or through change in static deflection resulting from the attachment of the molecules which are measured using impedance or optical techniques [2, 3]. Recent advances in micromachining techniques have also led to the miniaturization of quartz crystal microbalances (QCM) which now allow for the measurement of mass in the picogram to femtograms range [4-7]. Unlike the cantilever array gravimetric devices, QCM arrays have the advantage that they can readily operate when one of the faces is immersed in liquids. This allows for a more
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robust and versatile platform to realize chemical and biochemical sensors. A typical commercial QCM consists of a ~333-μm-thick and ~25-mm-diameter disk made from an ATcut quartz crystal with gold electrodes on each face, has a shear resonance frequency in the 5-MHz range, and exhibits a sensitivity of ~17 ng cm−2Hz−1. Under the conditions that the adsorbed film material is: (i) rigid, (ii) sufficiently thin compared to the quartz crystal, and (iii) attached to the sensor surface under a no-slip condition; the dependence of the frequency change (Δf) of a resonating quartz crystal to the mass loading (Δm) is given quite accurately by the Sauerbrey equation: ⎛2 f 2 Δf = −⎜ 0 ⎜ μq ρq ⎝
⎞ Δm ⎟ ⎟ A ⎠
(1)
where f0 is the fundamental resonant frequency with no attached mass, µq is the shear modulus of the quartz (2.947 × 1010 N m−2 for AT-cut quartz), ρq is the density of quartz (2.648 × 103 kg m−3), and A is the area of the electrode on the quartz crystal. The negative sign indicates a reduction in the resonance frequency upon mass loading. A very important aspect of eq (1) is that the change in frequency (Δf) for the same surface mass density (Δm/A) loading increases as the square of the decrease in the thickness of the crystal. This provides a possible way of making large enhancements in the performance of quartz crystal resonators if methods of fabrication can be developed to miniaturize them. Using microfabrication techniques we have fabricated quartz resonator arrays with fundamental resonance frequency of up to 95 MHz and while their basic functionality has been explored in air and vacuum ambient, in this paper we present the results of electrochemical calibration experiments performed in liquids. Electrochemical quartz crystal microbalance (EQCM) technique has been used for the detection of mass adsorbed on a QCM electrode for quite some time now [8, 9], however the relatively low mass sensitivity of commercial crystals make interpretation of data for adsorption of hydrogen particularly challenging [10, 11]. In this paper we
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IEEE SENSORS 2008 Conference
will present the initial data on the performance of 58 MHz micromachined QCM and their possible use for interpretation of interface phenomenon and surface adsorption in electrochemical measurements. II.
the DIP package using silver epoxy and wire bonded to one of the pads while the back-side electrode of the resonator was directly wire bonded to any other available pad in the package.
MICROMACHINED QCM ARRAY
We use an inverted mesa design for the quartz resonator array. Figure 1 shows the top-view and cross-sectional profiles of a single QCM. In this design, the thin resonator regions are etched out of a thicker quartz plate. The thicker regions allow for easy handling of the device, and these regions are essentially glued onto a package to allow for the liquid testing. To allow for easy contact to the electrode on the etched side, the etched regions of the pixels were extended to the edge of the chip as shown. The unetched surface was designed to be the reaction surface and has the “front-side” electrode. This configuration allows the top (reaction)-side electrode to be at low potential while the resonator was excited via the “backside” electrode. Top View Top Electrode
Figure 2. Optical photgraph of the experimental set-up for electrochemistry measurements showing the reference (white), counter (red) and working (green) electrodes inside a Faraday cage. Insets shows the packaged device (left) and bare sensor pixel (right). Cross-sectional View
AT Cut Quartz Etched Region Bottom Electrode
Electrode consists of 100nm Gold on 20nm Ti Or 30 nm Pt on 30 nm Ti
Figure 1. Schematic drawing showing the top and cross-sectional view (along the dotted line) of the inverted mesa micromachined QCM.
A. Fabrication of the QCM Array An inductively coupled plasma (ICP) etch process was used for quartz etching. The process uses a mixture of SF6 and Ar as the etching gases. Mirror finish with average surface roughness less than 2 nm was achieved even after an etch depth of 75 μm by etching at a very low base pressure of 1 mTorr. Electroplated nickel on a Au/Cr seed layer was used as the etching mask. The patterned nickel hard mask layer was deposited by electroplating. The quartz crystal was thereafter etched in a high-density ICP RIE etcher using SF6 + Ar gases. High selectivity (~10) for the Ni mask and high etch rates (~0.5 μm/min) using an ICP etcher have been already reported by our and other groups [12, 13]. The hard nickel mask was then stripped and lithographic patterning and etching performed on the bottom side Cr/Au electrodes. Finally the top-side electrodes were aligned and patterned using liftoff to complete the QCM device as shown in Fig. 2(inset). A 5 × 5 mm2 square hole was machined in a 24-pin, dual in-line ceramic package (DIP). The plane (unetched) side of the resonator having the front-side electrode where the mass sensing experiments are to be performed was placed facing the machined hole in the package and attached using roomtemperature vulcanized silicone elastomer adhesive. The frontside electrode was electrically connected to the gold layer on
B. Experimental Set-up HP 4195A impedance analyzer was used for all the reported characterization of the resonance frequency and Qfactor. 600 Series potentiostat model from CH Instruments was used for performing cyclic voltammetry and chronoamperometry measurements reported in this work. 1 μF capacitor was used to isolate the dc potential from the potentiostat from the transmission path of the 4195A. Fast frequency sweeps with 800ms sweep time were performed and recorded using Labview software in real time during the voltammetry and chronoamperometry cycles. The resonance frequency and Q-factor were determined by fitting a Lorentzian curve to the measured resonance data. Typical voltammetry sweeps were performed at a potential scan rate rate of 10 mV/s while chronoamperometry cycles were performed with a potential step period of 120s to allow for adequate data acquisition time from the QCM. Ag/AgCl, 3M KCl reference electrode and a Pt wire counter electrode were used as in this work. For the commercial QCM a stainless steel mesh electrode served as the counter electrode. Deionized water (18 MΩ) from Barnstead Nanopore was used in the preparation of the aqueous electrolytes in this work. 0.5 M H2SO4 solution was used for hydrogen and oxygen adsorption studies and 1 mM CuSO4 in 0.5 M H2SO4 solution was used for the copper deposition study. III.
EXPERIMENTAL RESULTS AND DISCUSSION
Measurements were made on the micromachined QCM as well as on a commercial 5 MHz Ti/Pt QCM (Maxtek Inc.) which was operated in the 11th mode (55MHz) to match the
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resonance frequency of the micromachined device to allow for an accurate comparison between the two devices. The first measurements were made for copper under potential deposition (UPD) which was expected to provide a good baseline for the calibration of the QCM. Figure 3 shows the results from chronoamperometric steps spanning the potential range for copper UPD. Assuming the deposition of the copper monolayer constitutes a rigid, non-slip layer, the sensitivity of the micromachined QCM is expected to be ~10.5 times higher than the commercial resonator and matches well with the observed sensitivity enhancement of 13.6 times. Furthermore, assuming a surface Pt site density of 1.5x1015 cm-2, and the deposition of a monolayer of copper, the frequency change for the commercial sensor is expected to be 105 Hz and matches well with the observed value of 100 Hz. For the micromachined QCM under the same assumptions, the expected frequency change is calculated to be 1285 Hz which compares well with the observed value of 1360 Hz.
the commercial sensor for oxidation of Pt can be calculated to be 26 Hz as compared to the observed frequency change of 180Hz. This implies a surface site density enhancement of ~7 times and is within the range observed by other investigators [9]. The observed enhancement has been attributed to surface roughness factor of the electrode metal. However it is interesting to note that the surface roughness factor is observed for oxidation of Pt but not for the deposition of copper. For the micromachined QCM, the expected frequency change is 325 Hz and is a factor of 100 smaller than the observed frequency change of ~32 kHz. We believe that our e-beam deposited Pt film quality may be of poor quality with high pin-hole density and porosity allowing for the oxidation of the underlying Ti film. This could easily result in the observed enhancement.
Figure 4. Cyclic voltammograms for (a) commercial Pt electrode QCM operated at 55 MHz(11th mode) and (b) micromachined QCM with a resonance frequency of 58 MHz. Figure 3. Chronoamperometry curves spanning the copper UPD potential range for (a) commercial QCM operated at 65 MHz and (b) Micromachined QCM with a resonance frequency of 58 MHz. The micromachined device shows ~14 times larger signal as compared to the commercial QCM.
Figure 4 shows the cyclic voltammetry scan in 0.5 M H2SO4 solution. The commercial Pt electrode QCM was operated at 55 MHz (11th mode) for this experiment. Under the same surface site density assumption, the frequency change for
Furthermore, the cyclic voltammograms could not be used for any quantitative analysis since they seem to exhibit unusual behavior. We believe that this unexpected behavior in the CV curves results due to the large area, electrically contacted screen printed nickel/gold layer on the DIP package which is directly exposed to the electrolyte. Due to these reasons, no quantitative analysis based on electrochemical charge transfer has been attempted.
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Enhanced sensitivity was also observed for the adsorption of hydrogen on Pt electrodes where a saw-tooth like frequency change behavior was observed upon performing chronoamperometry as shown in Fig. 5. For a commercial
Furthermore, it can be concluded that the Pt electrode on the QCM is highly porous implying large surface site (area) enhancement which in turn results in larger than expected signals for both oxygen and hydrogen whereas, Cu atoms seem unable to penetrate the porosity of the Pt electrode and deposit as a monolayer therefore resulting in excellent frequency change agreement. ACKNOWLEDGMENT The authors acknowledge partial financial support from the NSF funded PSU Center for Nanoscale Science (MRSEC DMR-0080019) and the use of facilities at the PSU Site of the NSF NNIN under Agreement 0335765. S.T. acknowledges Walton Fellowship from the Science Foundation of Ireland. REFERENCES
Figure 5. Shows the response of the microQCM in a chronoamperometry experiment in 0.5 M H2SO4 solution between VHigh=0.3V and VLow=-0.05V. A period of 32s was used for each step of the cycle. The red dashed line has been superposed to aid viewing of the trends in the graph.
QCM operating at 55 MHz, the expected frequency change is 1.7 Hz for adsorption of a monolayer of hydrogen, however, such a small frequency change could not be detected using our set-up. On the other hand, for the micromachined QCM the expected frequency change is 20 Hz as compared to the observed maximum frequency change of ~560 Hz resulting in an enhancement factor of ~28. Unlike in the oxygen case, we believe that the adsorption of hydrogen on Ti is negligible and the observed enhancement is mainly due to the roughness and porosity of the Pt layer of the electrode. The use of chronoamperometry makes it easy to see the frequency change which can be easily smeared out in a CV curve. Table 1 summarizes the observed results. TABLE I.
PERFORMANCE SUMMARY OF THE 58MHZ PT ELECTRODE MICROMACHINED QCM IN ELECTROCHEMISTRY EXPERIMENTS.
Expected Δf* Measured Δf* Expected Δf
+
Measured Δf
+
Cu UPD
Oxygen
Hydrogen
105 Hz
26 Hz
1.7 Hz
100 Hz
180 Hz
Noise!
1285 Hz
325 Hz
20 Hz
1360 Hz
32 kHz
560 Hz
*For Ti/Pt electrode, f0=5 MHz commercial QCM operated at 11th overtone (55 MHz). +
IV.
For Ti/Pt electrode, f0=58 MHz microQCM
CONCLUSIONS
In summary the performance of a 58 MHz fundamental resonance frequency micromachined QCM for electrochemical applications has been studied. Overall, the sensitivity of the micromachined QCM was found to be higher than that of a commercial QCM operated at similar frequency. However, due to packaging related interference, the CV curves could not be used for any quantitative evaluation of the surface sites.
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