CMOS Integrated Optical Sensor Using Phase Detection

Transcription

1 CMOS Integrated Optical Sensor Using Phase Detection Vamsy P. Chodavarapu Supriya P. Khanolkar Elizabeth C. Tehan Department of Chemistry ofnew York, Buffalo, USA Albert H. Titus Alexander N. Cartwright Frank V. Bright Department of Chemistry Abstract In this paper, we present an integrated fiber optic sensor based on a phose detection technique. The operation principle of the sensor as a xerogel bared oxygen sensor has been demonstrated. The integrated device includes a light emitting diode, an oxygen sensing probe and a CMOS Integrated Circuit (IC). The CMOS IC combines the LED driver circuity, optical detector and the phase detector circuitry on a single chip. The integrated sensor provides accurate, low cost and low power detection for quantifiing oxygen and for recording excited-state Iijetimes rangingfrom nanoseconds to millisecond. Keywords CMOS sensors, oxygen sensor, optical fiber sensor, phase detection. INTRODUCTION Recently, there has been considerable research interest in the development of optical sensors for medical, industrial and environmental applications. Most optical sensors are based on either measuring the changes in luminescence intensity or lifetime. Lifetime based sensing is superior in comparison to intensity based methods because they exhibit long term stability and because the results are independent of changes in the excitation source intensities, photobleaching and wash-out of the sensor probes, detector sensitivity and erroneous signals from surrounding environment [I, 21. It is well known that the luminescence lifetimes of a number of sensing elements is a function of concentration of certain target analytes. However, the direct measurement of these lifetimes often requires expensive and sophisticated instrumentation. Fortunately, frequency domain lifetime sensing based on an observed phase shift between the excitation and the emission luminescence is an inexpensive, versatile and robust strategy that can be used for detecting a wide range of chemical and biochemical agents [3]. The main goal of this work is to develop a miniaturized, portable and reliable integrated sensor for usage in a number of sensing applications involving fluorophores with lifetimes varying from nanoseconds to milliseconds. Toward this end, we have developed a LED driver circuitry, optical detector, and phase detector all on single silicon CMOS IC. The phase detector accurately determines the phase shift between the excitation signal and the signal emitted from the optical sensor probe [4, 5, 61. A block diagram of the integrated sensor system is presented in Figure 1. To demonstrate the device operation, we have constructed an optical fiber based oxygen sensing probe that can be integrated with the CMOS IC Figure I : Integrated Sensor System PHASE DETECTION TECHNIQUE Figure 2 shows the excitation and emission signals of the sensor when excited by a sinusoidal signal. The fluorescence lifetime of the sensing layer introduces a phase shift between the excitation and emission signals. More importantly, it is also possible to identify the fluorescent lifetime /03/$ IEEE 1266

2 of dyes (fluotophores) that is dependent on the concentration of an analyte that alters the lifetime. Under sinusoidal excitation, the relationship between the phase shift of the fluorescence signal (e),.the emission.lifetime of the fluorescence dye (T), and.the excitation modulation frequency v) is given by [3] : e = tan-' (~dr) 0) Clearly, io extract meaningful information Equation 1, the product of the modulation frequency and the fluorescence lifetime of the dye, Z$* r, should he less than n/2 radians. J Time (1) Exciiaaon signal Emission Signal Figure 2: Signals resulting from sinusoidal excitation of a fluorescent dye. tris(4,7-diphenyl-l,lo-pbenanthroline)ruthenium(li) chloride pentahydrate ([Ru(dpp),2'] (GFS Chemicals Inc.) that is sequestered within porous sol-gel-derived xerogel layer [7, 8, 91..A diagram of the fiber optic,sensor probe is shown in Figure 3. Sol-gel derived xerogels provide a thermally stable and optically transparent (with transmission over visible wavelength spectrum) sensing platform with tunable pore dimensions for encapsulating various sensing elements. The solgel processing solution that we used to form the current xerogels was prepared by mixing n-octyltrimethoxysilane, tetraethylorthosilane, ethanol, and 0.1M HCI (with a molar ratio of 1:3.4:0.006, respectively). This mixture was allowed to hydrolyze for one hour with stirring under ambient conditions. ([Ru(dpp)y] was dissolved in ethanol and was added to the sol-gel processing solution so that the final concentration of the Ru(dpp)y within the solution was 2mM. Standard 600pm core diameter optical fibers were stripped of the cladding chemically with a concentrated solution of HF. The exposed core of the fiber was then rinsed with deionized water and soaked in 1M NaOH for 24hr. The fibers were then rinsed clean with copious amounts of de-ionized water and allowed to dry in an oven for 1 hour. The optical fibers were then dip-coated with the Ru(dpp)y-doped solgel processing solution. The fibers were stored with the exposed core facing downward in the dark, under ambient conditions. Protectne In the system designed here, the CMOS IC provides a DC covering value at the output of the phase detector that corresponds to 4 the phase shin between the excitation signal and the emission signal from the optical sensor probe. The DC values obtained are independent of the absolute values of the input signal amplitudes. By using the phase shin information, the + lifetime of the dye at different analyte concentrations is cal- Core Clad Xerogel Layer culated (see Equation l). Figure 3; Fiber Optic Sensor Probe For the oxygen-sensing probe described in the present work, the concentration of oxygen can he calculated from the lifetime dependent oxygen quenching process described by the Stem-Volmer equation: 5 = 1 + K,,pO, r where 7. and 7 are the luminescence lifetimes in the absence of oxygen and at any particular oxygen concentration, respectively, hv is the Stem-Volmer constant, andpoz is the oxygen partial pressure. FIBER OPTIC OXYGEN SENSOR PROBE The fiber optic sensor probe is based on the evanescent wave excitation of the oxygen-sensitive luminophore, CMOS IC SYSTEM The CMOS IC for this sensor system is divided into two blocks: [i] the transmitter driver and [ii] the receiver. The transmitter driver block provides the necessary input excitation to modulate the light source (in this case a blue GaN LED). This circuit consists of a current buffer used as the LED Driver. The receiver is comprised of a photodetector and corresponding phase-detection circuitry. This photodetector detects the luminescence signal that derives from the sensor element and processes the signal to extract the luminescence phase shin between'the excitation and the emission. The IC. was fabricated through MOSIS ( using the AMI 0.5 pm technology. 1267

3 [i] Transmitter Driver Section: The high current drive buffer in the transmitter. driver section is used to couple the input modulated signal with the high output impedance to the off-chip load. The output buffer provides sufficient driving current for the LED and at the same time maintains the required DC bias voltage at the output. The circuit diagram for the buffer is presented in Figure 4. A quasi-complementary configuration [IO] is used for the output current buffer that consists of output common source FETs and operational amplifiers (labeled error amplifiers in Figure 4). Error amplifiers provide negative feedhack to reduce the output resistance of M1 and M2, allowing the buffer to source higher currents that are needed to drive the LED [13] Figure 4: Output Bufler Circuit Diugrum [ii] Receiver Section: The photodetector in the receiver section consists of a photodiode followed by a current-to-voltage (I-to-V) converter [14]. The semiconductor photodiode is designed with a single p-n junction formed between a lightly.doped substrate (p-) and a heavily doped diffusion region (n++). The preferred characteristics of the photodetector include high frequency response, small size, reliability and cost effectiveness. To maintain these characteristics we have made use of a 1x4 amy of photodiodes that measure 50 ~ 5 pni 0 The output currents of these photodiodes are summed to provide the total signal. As mentioned earlier, the phase detector is used to translate the luminescence decay time to a proportional electrical quantity. This phase detector is based on the multiply and integrate principle and is realized by using an analog multiplier and a lowpass filter. The analog multiplier and the lowpass filter use a balanced single-input Gilbert cell mixer [11, 121 and OTA-C structure, [IS, 161 respectively. Phase detection is carried out synchronously with the inputexcitation signal (VceF) from the transmitter section. The output from the Gilbert cell mixer is a DC component whose magnitude depends on the phase difference, and an AC signal that is at twice the frequency of the excitation signal. The DC component is extracted,using the OTA-C lowpass filter with a cut off frequency that is much lower than the excitation signal frequency (om). The phase detection process is described as follows: Assume the input excitation signal is represented by v,~= v, COS (w,t + e,) (3) where, V, is the excitation signal amplitude, o, is the angular signal frequency and 8, is the initial phase angle of the signal. The resulting fluorescence signal is then represented as: v,i, = v, cos (0,t + e,) (4) where, V, is the sensor signal amplih;de, and Els is the phase of the fluorescence signal. In this case, the output of the phase detector is given by the product signal as [ 141. I VDC = (v,v,) [COS (e, - e,)] + E,, (5) where, E,, is the offset of the phase detector. In operation, -i:i! a DC value is obtained that corresponds to the phase difference between the excitation and emission as shown in Figure 5. g d 1.09 P 5 $ , In@ Phase Oicterence(0ewer) Figure 5: Plot of output DC versus inputphase diference The obtained DC values are specific for a particular set of input signal amplitudes. Hence, the system needs recalibration for each new set of measurements, thereby making the measurements tedious and time consuming. To overcome this problem our system has been modified to provide output DC values that are independent of input signal amplitudes. The block diagram of the modified phase detector is shown in Figure 6. This system consists of two matched parallel paths each comprising a Gilbert cell mixer and.an OTA-C filter, respectively. The output from the I-to-V converter is applied simultaneously to,both the paths. However, 1268

4 the reference excitation signal, applied to only one path, is intentionally 90" out of phase with respect to the other. Under these conditions, the pair of DC values obtained at the output is given by: VDC,=%(V,VJ[COS (e, -CIS)]+ V,, (6) VDC, = % (V,V,) [sin (e, - Os)] + V,, (7) Dividing the above equations we obtain: The phase difference can be directly obtained from these DC values as follows: VDCy - Vm (e, - e,) = tan-' (9) EXPERIMENTAL RESULTS FROM THE FIBER OPTIC OXYGEN SENSOR Initial luminescence lifetime studies on the fiber optic probe were performed using the 400nm frequency doubled pulsed output from of a femtosecond pulse regenerative amplifier. The luminescence lifetimes were monitored as a function of varying gaseous 02/N2 concentrations by using a streak camera. The measured calibration curve for the lifetimes of ([R~(dpp)~]~') is shown in Figure 8.. For the phase measurements, the luminescence signal from the fiber optic sensing probe was detected by a silicon photodetector. The phase difference between the input signal and the output of the photodetector was,measured using a lock-in amplifier. Figure 9 shows a direct comparison of the measured emission phase angle by using the sensor at a modulation frequency of 20 khz and the estimated phase exlracted from the lifetime readings ' cosq xmht 8- Mixer, Lwpass Fiter, sm,j - SimJ Figure 6: Phase Detector Block Diagram.% 0, Figure 8: Calibration curve for the luminescence l@times of [Ru(dpp))$+ in a xerogel matrix. t R1seMea~.emmlfr"msor Figure 7: Micrograph of the CMOS IC A micrograph of the CMOS IC is shown in Figure 7. This prototype is packaged in a 40-pin DIP. Currently, we are in the process of hlly characterizing the CMOS IC. Figure 9; Comparison between the measured phase (solid circles) from the sensor and the estimatedphose (solid squares) from the lifetime measurements. 1269

5 The systematic bias between the two readings can be attrib- dye, Sensors and Actuators 9, vol. 29, pp , uted to a combination of the changes in ambient temperature that affects the luminescence lifetimes [I71 and the level of modulated pump light that reaches the detector. [7] E. J. Cho, F. V. Bright, Optical Sensor Array and CONCLUSION We have presented the design of a miniaturized, portable, and low cost integrated sensor system that can be used with various dyes having different lifetimes. This design relies on a single CMOS IC that drives the light source and processes the received signal. The phase detector portion produces an output signal that is independent of the signal amplitude, which simplifies the signal analysis. The system is low power, with most of thepower uskd to drive ihe LED. The fabricated IC is currently under test and we will combine this IC with the fiber optic sensor. In addition, we will extend this concept to simultaneous multi-analyte detection. ACKNOWLEDGEMENTS We would like to acknowledge the financial support of A. N. Cartwright s National Science Foundation (NSF) CAREER Award #ECS and Office of Naval Research Grant #NOOO and A. H. Titus NSF CAREER Award #ECS In addition, we would lie to thank MOSIS ( for fabricating the CMOS IC s. REFERENCES [l] G. Boisdk A. Harmer, Chemical and Biochemical Sensing With Optical Fibers and Waveguides, Artect House Publishers, Massachusetts, [2] J. R. Lakowicz, Principles of fluorescence spectroscopy, Kluwer AcademiclF lenum, New York, [3] M.E. Lippitsch, S. Draxler, Luminescence decaytime-based optical sensors: principles and problems, Sensors and Actuators B. vol. 29, pp , [4] W. Trettnak, C. Kolle, F. Reininger, C. Dolezal, P. O Leary, Miniaturized luminescence lifetime-based oxygen sensor instrumentation utilizing a phase modulation technique, Sensors and Actuators: B, vol , pp , [5] C. McDonagh, C. Kolle, A. K: McEvoy, D. L. Dowling, A. A. Cafolla, S. J. Cullen, B. D..MacCraith, Phase flurometric dissolved oxygen sensor, Sensors and Actuators 9, vol. 74, pp ,2001. [6] G. O Keeffe, B. D. MacCraith, A. K. McKvoy, C. M. McDonagh, J.. F. McGlip, Development of a LED-based phase fluorimetric oxygen sensor using evanescent wave, exciation of a sol-gel iinmobilized Integrated Light Source, Analytical Chemistry, Vol. 73. PO [8] M. Lechna, 1. Holowacz, A:Ulatowska, H. Podbielska, Optical Properties of sol-gel coatings for fiheroptic sensors, Surface and Coatings Technology, Vol I,. 42~ P 299-wn 2002~ [91 C. J. Brinker, G. W. Scherer, Sol-Gel Science: the physics and chemistry of sol-gel processing, Academic Press, New York, [IO] R. J. Baker, H. W. Li, D..E. Boyce, CMOS: Circuit Design, Layout and Simulation, IEEE Press, New York, [Ill B. Razavi, Design of Analog CMOS Integrated Circuits, McGraw-Hill International Editions. [I21 R. F. Salem, S. H. Galal, M. S. Tawfik, H. F. Ragaie, A new highly linear CMOS mixer suitable for deep submicron technologies Electronics, IEEE Circuits and Systems, Vol. I, Pp ,2002. [I31 Y. HA, M. F. Li, A. Q. Liu, A New CMOS Buffer Amplifier Design Used in Low Voltage MEMS Interface Circuits, Analog Integrated Circuits and Signal Processing, 27,7-17,2001. [ 141 M. Fortsch, K. Schneider, H. Zimmermann, Simple Current-To-Voltage Converter without Stability Prohlems,: Semiconductor Conference, CAS 2002 Proceedings. Intemational, Vol. I, pp vol.1, IEEE , * [IS] E. Sanchez-Sinencio, J. Silva-Martinez; CMOS, trans, conductance amplifiers, architectures aid active filters: a tutorial, Circuits, Devices and. System&:IEE -Pro, ieedings, Vol. 147Issue: l, pp. 3-12: Feb. 2000:.... [I61 R. L. Geiger, E. Sanchez-Sinencio, Active Filter De-,. sign Using Operational Transconductance Amplifiers:. A Tutorial, IEEE Circuits. and Devices Magazine, Vol. 1: pp.20-32, March [17] J: Hradi1;C; Davis, K. Mongey, C. McDonagh, B. D. MacCraith, Temperature-corrected pressure-sensitive : paint measurement using a single camera and a duallifetime approach, Measurement Science Technology, Vol. 13,pp ,