(CMUT) as a chemical sensor for DMMP detection - Khuri-Yakub ...

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K.K. Park et al. / Sensors and Actuators B 160 (2011) 1120–1127 1123of a 50 nm polymer film on the quality factor and the resonant frequencyof the device is negligible based on the input impedancemeasurement. The parallel resonant frequency slightly is increasedby 0.64% while the quality factor is decreased by 7 after the polymerdeposition.Compared to the spin coating method, the inkjet-dispensingmethod tends to have less uniformity across the active area. However,the inkjet-dispensing method can selectively pattern areasof interest, e.g., top side of CMUT plates of a single device, andcoat different polymers on multiple devices in a single die for arrayimplementation [11].2.5. Detection systemFig. 4. Photograph of inkjet polymer coating. Cylinder on the top of the picture ismicro-capillary used as droplet ejection. In this photograph, the solvent containinga polymer on the center element is not dried yet.that of parallel resonant frequency (140) due to the resonator-toresonatornon-uniformity. To avoid low Q at the series resonantfrequency, the oscillator circuit is designed to oscillate at the parallelresonant frequency.2.4. Chemical functionalizationThe resonators must be functionalized with a sorbent film toabsorb analytes in air. This sorbent film should have high sensitivityand good selectivity to the analyte of interest. Among several commerciallyavailable polymers such as polyallylamine hydrochloride(PAAM), polyethylene glycol (PEG), and polyvinyl alcohol (PVA), weselected polyisobutylene (PIB) as the sorbent film to detect DMMPbecause of its availability and higher sensitivity to DMMP [20].Diluted droplets of PIB (in toluene, 1 mg/ml), each with avolume of 0.2 nl, were ejected on the 18 MHz device using aninkjet-dispensing system (model MD-P-801, Microdrop, Norderstedt,Germany) [21]. The ejected droplets conformally coated thesurface of the CMUT (Fig. 4). Then the solvent evaporated in a fewseconds leaving a thin layer of polymer. It is challenging to measurethe thickness of this polymer film using optical methods becausethe coated film is thin and optically transparent; only a particlepattern (Fig. 5) indicates the edge of the polymer coating. Thus,using atomic force microscope (AFM), operated in tapping mode,we measured the thickness of the polymer to be 50 nm. The effectFig. 5. Optical picture of a CMUT after functionalization. Small dotted line representsthe edge of the polymer coating.An accurate measurement of the resonant frequency is an essentialpart of a highly sensitive sensor system; lower frequencynoise will result in lower limit of detection level of the system.The resonant frequency can be determined through the electricalinput impedance measurement. However, this method suffersfrom limited frequency-sweep resolution and slow response time.Therefore, we designed a low noise oscillator circuit that tracks theresonant frequency of the device. This method allows for accuratereal-time measurements using a digital frequency counter connectedto the output of the oscillator circuit.2.5.1. Oscillator circuit designThe oscillator circuit is composed of the CMUT as well as otherdiscrete circuit components. For an optimal design, the circuit couldbe simulated and optimized in design steps. During these steps,the measured input impedance (Fig. 3a) could be modeled in anappropriate format and incorporated in the circuit simulation. Inthis work, we modeled the input impedance of the device as a6-elements circuit model as shown in Fig. 3b. The model consistsof the conventional 4-element RLC model (Rx, Lx, Cx, and Co) andadditional two elements, Cc, and Rc. The 4-element RLC model isknown as Butterworth Van Dyke model and commonly used as anequivalent circuit model for MEMS resonators. The additional twoelements (i.e. Cc and Rc) represent the contact capacitance and thecontact resistance to the substrate. The values of six elements wereextracted from the measured input impedance based on a leastsquares fitting method (Fig. 3a).Compared to other MEMS resonators and quartz-based resonators,CMUT resonators have two characteristics that govern theoscillator design. First, the CMUT is a one-port resonator, i.e., it hastwo electrical pads, a signal pad and a ground pad. This configurationgives more limitation of the topology of the oscillation circuit,compared to other two-port MEMS resonators [22,23]. Second, allthe resonator units (elements) share the substrate as a commonground. Therefore the oscillator circuit, which is required to becompatible with the multi-channel oscillator, can access only oneelectrode of the CMUT, and leave the other electrode connectedto ground. Last, as previously mentioned, the quality factor of theparallel resonant frequency of the CMUT is higher than that of theseries resonant frequency. The circuit is designed to oscillate at theparallel resonant frequency to take advantage of higher Q.Based on the above criteria, an oscillator circuit was designedand implemented (Fig. 6). The CMUT resonator was interfaced tothe circuit through a voltage divider. The value of the capacitor inthis stage was chosen to optimize the tradeoff between in-circuitQ and the gain of the voltage divider. The gain stage and band-passfilter were implemented using low noise op-amps (AD8045 andOPA657) to satisfy the Barkhausen criteria. The implemented circuitrequires only two op-amps inside the oscillation loop, with atotal power consumption of 80 mW. The loop gain is sufficientlylarger than unity around the resonant frequency to start the oscillationfrom in-circuit noise components, and thus the system does
1124 K.K. Park et al. / Sensors and Actuators B 160 (2011) 1120–1127Fig. 6. (a) Schematic of the oscillator circuit including a CMUT. (b) Optical picture of a populated oscillator circuit on PCB.not require an additional start-up circuit. The CMUT is directlymounted on a printed circuit board (PCB) without a chip-carrierand wire-bonded on the PCB to reduce parasitic capacitance.2.5.2. Performance of the oscillatorThe free running oscillation frequency of the circuit was measuredusing an external digital frequency counter (Model SRS620,Stanford Research System, Sunnyvale, CA) with a typical gate timeof 10 ms, i.e., 100 frequency samples per a second. Frequency informationis transferred to an external PC for signal processing. Thetime delay between the counter and the PC is less than 100 ms,which is short enough for a real-time measurement of the frequencyshift due chemical absorption.The amount of frequency shift is proportional to the amount ofchemical absorption. As a result, the limit of detection of the outputsignal, i.e., frequency noise level, decides the limit of detectionof analyte concentration in air. In order to quantify the frequencynoise level, we used the Allan deviation method [24], which is acommon method to measure the short-term frequency stabilityof timing references. The Allan deviation number, y (n 0 ,M), fora given gate time, n 0 , is√√ y (n 0 , M) =√ 12n 2 (M − 2n + 1)∑M−2n j+n−1( ¯f i+n − ¯f i ) 2 , (4)j=0∑where 0 is the gate time of the frequency measurement, M is totalnumber of samples and ¯f i is the averaged frequency of n-samples.This method averages neighboring frequency samples to computedeviation at a given averaging time, n 0 , (i.e., effective gate time). Ina short averaging time, the Allan deviation tends to decrease withmore averaging time. However, the Allan deviation increases whenthe averaging time is larger than a certain value, because the driftof the resonant frequency (i.e., mid-term frequency noise) startsto dominate the frequency variation. We measured a large numberof finely spaced frequency samples and computed the Allandeviation for various averaging times to find the optimal averagingtime (Fig. 7). The oscillator has the lowest Allan deviation( y /f) of 2.74 × 10 −8 at 50 ms averaging time, which correspondsto a frequency noise level of 0.5 Hz. Based on Eq. (3), the sensor’sminimum detection limit of loaded mass per unit area ( min /A) is1.92 × 10 −11 kg/m 2 or 0.192 ag/m 2 with 3- confidence levels.i=jand bubblers [11,25]. Purified air from a zero air generator (Model76-803, Balston Parker, Haverhill, MA) was used to provide flowthrough two MFCs. These two MFCs separately control the flow rateof the carrier air and flow rate of the air injected into a bubbler. Inthe bubbler, the air flows through a liquid-phase analyte, resultingin evaporation of the analyte into the air. The air containinganalyte molecules is then mixed with the carrier air. This mixturegas is transferred into a 3 cm 3 glass chamber, which encloses thepolymer-coated CMUT resonator.The typical flow rates of the bubbler air is 1–10 ml/min and theflow rate of diluted air is adjusted in 499–490 ml/min to regulatethe total flow rate to 500 ml/min. The volume concentration of ananalyte is calculated based on three parameters: the flow rate of theair through the bubbler, flow rate of the carrier air and the vaporpressure of each analyte, assuming the output of the bubbler is fullysaturated with the analyte.3. Results3.1. Sensitivity and limit of detectionWe chose DMMP as our main analyte and ethanol and wateras reference analytes. Fig. 8 shows transient responses of the frequencyto different concentrations of three different analytes. Foreach measurement, the analyte molecules were injected into thechamber at 60 s by flowing air through the bubbler containingDMMP. The PIB coating absorbs the analyte molecules until itreaches an equilibrium condition for the given analyte concentration.The mass of the absorbed molecules shifts the resonant2.6. Experimental setupThe volume sensitivity is measured using an in-house chemicalset-up that can deliver low vapor concentrations of DMMP. Theexperimental setup consists of several mass flow controllers (MFC)Fig. 7. Allan deviation y of the oscillator circuit in Fig. 6(b).
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