Application Specific Spectral Response With CMOS ... - IEEE Xplore

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 5, MAY 1999

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Application Specific Spectral Response with CMOS Compatible Photodiodes Michael L. Simpson, Senior Member, IEEE, M. Nance Ericson, Member, IEEE, Gerald E. Jellison, Jr., William B. Dress, Member, IEEE, Alan L. Wintenberg, Member, IEEE, and Miljko Bobrek, Member, IEEE Abstract—Several methods are presented for realizing photodiodes with independent spectral responses in a standard CMOS integrated circuit process. Only the masks, materials, and fabrication steps inherent to this standard process were used. The spectral responses of the photodiodes were controlled by 1) using the SiO2 and polycrystalline Si as thin-film optical filters, 2) using photodiodes with different junction depths, and 3) controlling the density of the interfacial trapping centers by choosing which oxide forms the Si/SiO2 interface. Also presented is an example method for constructing photo-spectrometers using these spectrally-independent photodiodes. This methods forms weighted sums of the photodiodes’ outputs to extract spectrographic information. Index Terms—Photodiodes, spectroscopy.

I. INTRODUCTION

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ECENTLY, there has been much interest in photodetectors and imagers realized in standard CMOS IC processes for applications such as motion detection [1], [2], orientation detectors [3], solid-state cameras [4]–[8], optical metrology [9], perception systems [10], and whole-cell biosensors [11]. CMOS technology is especially attractive for these applications since it allows a wide range of analog and digital signal processing circuits to reside on the same substrate with the photodetector. Furthermore, it has been noted that CMOS imagers overcome many of the disadvantages of CCD’s [12]. Most of the early work with CMOS photodiodes has been applied to imaging and has not been concerned with spectral response. The application of optical application specific integrated circuits (OASIC’s) to the field of integrated scientific instrumentation has not received a great deal of attention in the literature. Additionally, although a recent press release describes a color camera fabricated using a color MOS technology [8], the inherent spectral filtering capabilities of standard CMOS IC processes are rarely utilized. CMOS technology allows the realization of phototransistors, photodiodes, and photogates without any modification or additions to the standard processing steps. These devices, as normally used, have broad spectral responsivities that peak in the red/near infrared region. While Wolffenbuttel and De Graaf [13] and Seitz et al. [7] have shown that some spectral information can be obtained by using the Manuscript received May 19, 1998; revised November 23, 1998. The review of this paper was arranged by Editor P. K. Bhattacharya. M. L. Simpson, M. N. Ericson, G. E. Jellison, Jr., W. B. Dress, and A. L. Wintenberg are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6006 USA. M. Bobrek is with the University of Tennessee, Knoxville, TN 37996-2100 USA. Publisher Item Identifier S 0018-9383(99)03564-9.

two junction depths available in n- or p-well bulk CMOS processes, most efforts to achieve spectral separation with Si photodetectors use methods that are not compatible with or add steps to CMOS processes. These methods include the use of amorphous Si [14], multiple active region depths [15], or the addition of micro-machined elements [16], [17]. In this work, we present methods for realizing photodiodes with independent spectral responses using only the materials, masks, and fabrication steps inherent to a standard bulkCMOS process. In addition, an example method of realizing photo-spectrometers using these photodiodes is described. By remaining completely within the capabilities of the standard process, this method provides the desired spectral information, allows the realization of analog and digital processing circuits on the same substrate with the photodetectors, yet costs no more to produce than standard CMOS circuits. Integrated photo-spectrometers in combination with analog, digital, and wireless circuits on the same substrate can have numerous applications in chemical, biological, and physical scientific micro-instrumentation. II. CMOS PHOTODIODE EXTERNAL QUANTUM EFFICIENCY The external quantum efficiency of any photodetector has four components: 1) efficiency of light transmission to the detector (fraction of incident photons that reach the Si surface); 2) efficiency of light absorption by the detector (fraction of photons reaching the Si surface that produce electronhole pairs); 3) quantum yield (number of electron-hole pairs produced by each absorbed photon); 4) charge collection efficiency of the photodetector (fraction of optically-generated minority carriers that cross the pn junction before recombining). The first is assumed to be unity for all wavelengths considered in this work. However, the second, third, and fourth components can be manipulated in standard CMOS IC processes to create wavelength dependent structure in A. Transmission Efficiency The transmission efficiency is controlled by the material covering the photodetector. In a CMOS process there are three materials that can be placed above the detector: 1) aluminum (Al); 2) silicon dioxide (glass); 3) polycrystalline Si (poly-Si).

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Fig. 1. Placement of poly-Si, glass, and Al layers in a double-poly, double-metal, CMOS IC process.

Fig. 2. Optical characterization test set-up.

The main optical application of Al is as a light shield or mask, while the glass layers can serve as spacers in stacks of thin-film optical filters. In addition, manipulation of the glass layers will later be shown to have an effect on the charge transport properties of the device. Finally, poly-Si is a wavelength sensitive reflector/absorber/transmitter of light Like the glass layers, polythat can be used to modify Si can also play a role in the manipulation of charge transport properties. The layers of polycrystalline silicon and SiO over the photodetectors behave as a stack of thin-film filters. The optical transmission of the assembly can be determined if one knows the polarization and angle of incidence of the light and one also knows the thickness and the index of refraction of each layer [18]. The circuit designer is free to specify the use and placement of the two layers of poly-Si and five layers of glass (the designer is not free to specify the removal of the PPOX layer) shown in Fig. 1 to form a stack of materials creating a thin-film filter. Although unconventional, this use of poly-Si and glass layers can be accomplished with no modifications to standard CMOS fabrication procedures. Several n-well/p-substrate diodes with various poly-Si/glass coverings were fabricated in a 1.2- m, n-well, bulk CMOS process. The spectral responsivities of these devices were measured in the wavelength range between 400 and 1100 nm in 10-nm steps using the test set-up shown in Fig. 2. The was calculated from these measurements by normalized dividing by the spectral responsivity of the NIST calibrated

Fig. 3. Response of CMOS photodiodes with coverings of 1) no poly-Si layers; 2) the first poly-Si layer; 3) the second poly-Si layer; and 4) both poly-Si layers. All poly-Si layers are over field oxide.

Si photodiode shown in Fig. 2. While absolute values were not obtained for these devices, other investigators have of CMOS photodiodes to be in the measured peak 50–80% range [9]. Because of the integrating sphere used in this test set-up, the angle between the incident light and the thin-film surface varied over the range from normal to nearly parallel. Since the angle of incidence has a strong effect on the thin-film filter response, it is important to calibrate the of the device in the same manner it is to be applied. of n-well/p-substrate Fig. 3 shows the normalized photodiodes with coverings of 1) all the glass layers, no poly-Si layers; 2) all glass layers and the first poly-Si layer; 3) all glass layers and the second poly-Si layer; 4) all glass layers and both poly-Si layers. In all four cases, the poly-Si layers were above field oxide. As this figure shows, the device without poly-Si coverings has a and exhibits only a slight interstrong short wavelength ference pattern (this pattern would be much more pronounced if the light entered with a normal angle of incidence). All devices with a poly-Si covering show a very attenuated short , with the double poly-Si device showing wavelength strong attenuation even at wavelengths greater than 550 nm. As will be shown in the next section, this is to be expected since poly-Si (like Si) more efficiently absorbs short wavelength

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Fig. 4. Response of a CMOS photodiode covered with both layers of poly-Si. The light enters the device with a normal angle of incidence.

Fig. 5. CMOS photodiodes covered with one or two layers of poly-Si. One set of detectors has poly-Si over field oxide, while the other set has poly-Si over gate oxide.

light. In addition to the attenuation at the shorter wavelengths, as expected the devices with poly-Si show a pronounced interference pattern. As Fig. 4 shows, this interference pattern has more structure for a normal angle of incidence. Fig. 5 shows the effect of using the glass layers as spacers in a poly-Si/glass thin-film filter. This figure shows the norof n-well/p-substrate photodiodes with the same malized three poly-Si coverings as Fig. 3. As in Fig. 3, three of the detectors have poly-Si over field oxide. However, the other three detectors have poly-Si over gate oxide. The use of gate oxide has the effect of moving the peaks and anti-peaks by 30 nm.

The polysilicon structures reported here are similar to the resonant-cavity photodiode reported by Bean et al. [19]. However, there are several key differences. 1) The Bean structure places the absorbing material between the two reflectors, while the devices reported in this work place the photodiode below the resonant cavity. 2) The Bean photodiode has narrow response peaks, while the photodiodes reported here have much broader peaks. 3) The Bean structure was designed for high quantum efficiency and fast response time, while the devices

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Fig. 6. The absorption coefficient for Si.

reported here are intended for low-frequency microinstrumentation requiring spectral selectivity. 4) The devices reported here were fabricated in a commercially-available CMOS IC process with no modifications to the process, while the Bean photodiode is not realizable in these processes without significant modification to the fabrication steps. Bean et al., report a 40% peak external quantum efficiency that is somewhat lower than the 50–80% reported for CMOS photodiodes [9]. However, considering absorption by the polysilof the Bean icon structures used in this work, the peak photodiode and the devices reported here are comparable. Fig. 7. Double-junction photodiode in an n-well CMOS process.

B. Absorption Light is absorbed in the photodetector by electrons in the valence band that become mobile carriers in the conduction band. This process also leaves behind a mobile hole in the valence band. The number of absorbed photons is given by (1) where number of absorbed photons; number of photons incident on the Si surface; fraction of photons reflected from the surface; absorption coefficient; wavelength of the incident light; depth into the photodetector. As Fig. 6 shows, between the direct and indirect band edges is a strong function of wavelength. Short wavelength light is absorbed near the Si surface, while longer wavelength light is absorbed deeper in the material. For Si is essentially zero for wavelength longer than 1100 nm and these photons are for poly-Si is very similar to that of not absorbed. The crystalline Si [20].

Wolffenbuttel and De Graaf [13] and Seitz et al. [7] have shown how this depth dependent absorption of light can be used to determine spectral content with photodiodes fabricated in a standard IC process. Fig. 7 shows a double-junction photodiode realized in a n-well bulk CMOS process. The shallow junction device preferentially collects charge created by shorter wavelength light, while the deeper junction collects the charge created by longer wavelength light. Some standard IC processes may have a third useful junction depth. Unfortunately, the number of useful junction depths is limited to just a few in any standard process. of p-diff/n-well and Fig. 8 shows the normalized n-well/p-substrate/p-diff photodiodes fabricated in a standard 1.2- m n-well bulk CMOS process. As expected, the shorter wavelengths predominantly produce current in the p-diff/nwell diode, while the longer wavelengths predominately produce current in the n-well/p-substrate diode. The p-diff/n-well peaks at 530 nm, while the n-well/p-substrate/p-diff response peaks at 710 nm. The ratio of the two photodiode currents can be used to measure the wavelength in monochromatic or narrow bandwidth applications as Fig. 9 demonstrates.

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Fig. 8. Response of the p-diff/n-well and n-well/p-substrate photodiodes realized in an n-well CMOS process.

Fig. 9. Ip-di /In-well as a function of wavelength. This ratio depends only on wavelength.

C. Charge Transport Once this photo-generated mobile charge is created, it either recombines or crosses a pn junction. Since the charge that crosses the pn junction creates a useful signal and the charge that recombines is lost, the charge transport properties have of the photodetector. Other a significant effect on the authors (see [21] for example) have presented calculations of the internal quantum efficiency (fraction of optically-generated

minority carriers which cross the pn junction before recombining). A few key equations from the derivation in [21] for the simple device shown in Fig. 10 will be repeated below. The rate of photo-generation of holes, , in the n-well as a function of wavelength and depth is given by (2) where the variable definitions are the same as those given for (1). This generation of holes creates an excess hole density in

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Fig. 10.

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 5, MAY 1999

Structure used for one-dimensional photodiode calculation.

the n-well,

, that is subject to the boundary conditions (3)

and (4) s the diffusion constant for holes in the n-well and where is the surface recombination velocity and is proportional to the density of interfacial traps. Equation (3) indicates that the excess hole concentration goes to zero at the edge of the depletion region, while (4) is a balance between carriers flowing to the surface and carries recombining at the surface. With the above boundary conditions, the internal quantum efficiency (defined as spectral response SR in [21]) is (5) and , where is the diffusion if length of holes in the n-well. Equation (3) and (5) illustrate key points: the amount of charge that recombines via the interfacial trapping centers (and therefore not seen as signal) is proportional to the excess minority carrier concentration at the Si/SiO interface, and this surface minority carrier concentration is higher at shorter wavelengths (since is larger at shorter wavelengths). Therefore, as is well known, interfacial traps affect short wavelength SR more strongly than long wavelength SR. Also, as (5) shows, because of boundary condition (3), a shallow junction (small ) improves the short wavelength SR. Thus, shallow p- or n-diffusion devices have better short wavelength SR than an n-well device. As a two-dimensional analysis shows [22], making the device narrower has an effect similar to making the device shallow. A shallow device allows more photo-generated surface charge to reach the bottom junction, while a narrow device allows more photo-generated surface charge to reach the sidewall junctions. The density of surface states is controlled by the “quality” of the oxide layer that terminates the Si lattice. In CMOS processes this initial oxide layer is supposed to be either a carefully grown thin oxide (gate oxide), or a relatively thick grown oxide (field oxide). However, by manipulating the process in a manner not usually allowed by CMOS design rules but possible with no changes to the standard fabrication process, the terminating layer can be a deposited or spun-on oxide. Each of these four oxides provides a different density of surface states and strongly influences the response of the photodetector in the near ultraviolet, blue, and green regions.

Fig. 11. Response of CMOS photodiodes with specified layers: 1) metal 1/poly-Si via but no poly-Si; and 2) metal 1/poly-Si via and one poly-Si layer.

Fig. 1 shows the placement of the poly-Si, glass, and metal layers used in a double-poly-Si, double-metal, standard CMOS process. The poly-Si/metal 1 oxide layer usually separates the poly-Si layers from metal. However, this glass layer is etched away if a poly-Si/metal 1 via is specified. If this via is specified with no poly-Si layer present, the metal 1 layer is placed directly over field or gate oxide. If this metal 1 layer is then etched away, the field or gate oxide layer may also be removed. In this case, the deposited oxide that usually separates metal 1 from metal 2 forms the Si/SiO interface. This deposited oxide layer is not as well controlled as the grown gate or field oxides, and more surface trapping states would be expected. of n-well/p-substrate Fig. 11 shows the normalized photodiodes with the following layers specified: 1) metal 1/poly-Si via but no poly-Si; and 2) metal-1/poly-Si via and one poly-Si layer. As predicted by the above discussion, the of the first photodiode is greatly short wavelength attenuated by the surface states. However, the short waveof the second photodiode is relatively large. This length indicates that the etch step that removes the metal 1 also removes the poly-Si layer. However, the presence of the polySi layer protects the oxide layer (or possibly protects the Si surface from the Al), and fewer surface states are formed.

III. PHOTO-SPECTROSCOPY While the above discussion describes how to fabricate photodetectors with independent spectral responses in a standard CMOS IC process, the responses of the devices taken individually are not very useful. Therefore, some additional signal processing must be performed to obtain spectral information from the incident light. The discussion below shows one method for combining photodiode outputs to extract spectrographic information. In general, a CMOS spectrometer can be represented by a block diagram given in Fig. 12. In this spectrometer, photodetectors with different spectral responses are used to different estimate spectral density of the incident light in , sub-bands. For a known input with the spectral density

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Fig. 12.

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CMOS photo-spectrometer.

the detector outputs are given by (6) is the spectral response of the th detector. The where above equation can be approximated by (7) is sufficiently large (i.e., if with an acceptable error if is sufficiently small). In (7), represents the spectral response , while is the input of the th detector at A spectrometer like the signal power in a small band one in Fig. 12 must perform a transform that is inverse to (7) sub(i.e., it must estimate values of the input power, , in detectors). This bands given the output readings, , from transform is given by the following matrix equation: (8) where the spectral response matrix, , is determined during the detector calibration process. In most applications (8) cannot be used in a straightforward manner because of different limitations that may occur. First of all, one may wish to estimate the input spectrum in a number of points that is larger than the number of detectors. Then (8) becomes an underdetermined problem and has an infinite number of solutions. Use of singular value decompoleads to a minimum norm solution that sition to find rarely corresponds to the desired input spectrum. To solve the problem, some additional constraints need to be imposed such as nonnegativity of the solution, restricted wavelength bands, or certain characteristics of the input spectral shape. The following example shows how additional information about the input spectrum can be used to solve an underdetermined problem. Let the additional information about the input spectrum be its slowly varying shape over a wide range of wavelengths. To simulate such an input, a Gaussian function defined over the 400–1030 nm band centered at 700 nm is used. To estimate the input spectrum at every 10 nm, (i.e., in 64 spectral points), a spectrometer with eight detectors is used. The detectors were selected out of more than 60 detectors developed and

Fig. 13. Normalized responses of CMOS photodiodes used in photospectrometer example.

calibrated during the research. They were calibrated with the same resolution over the 400–1030 nm band, and their responses are given in Fig. 13. normalized Because of the slowly varying shape of the input spectrum, it can be estimated in 64 points using only eight samples and the sinc interpolation according to

(9) Replacing

in (7) by (9) leads to

(10) problem. After which represents a critically determined the eight input spectrum points are determined using (10), the rest of 64 values can be found using (9). As Fig. 14 shows, this procedure gives an acceptable approximation of the input spectrum.

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in other applications were fabrication cost, power consumption, and extreme portability are important. ACKNOWLEDGMENT The authors gratefully acknowledge useful discussions with our colleagues Dr. M. J. Paulus, and Dr. D. H. Lowndes of the Oak Ridge National Laboratory, and Dr. M. J. Roberts of the University of Tennessee. REFERENCES

Fig. 14.

CMOS photo-spectrometer results.

Special attention needs to be paid to the condition of the matrix from (12) to avoid possibilities of having an ill-posed problem. Since the spectral responses of the CMOS detectors are far from being orthogonal, the condition number of the is larger than one. The relative error in the input matrix spectrum estimate due to a small change of in the matrix or the noise in the detector readings can be significant if the condition number of the matrix is large. In [23], this error is given by

(11) and are the absolute where is the condition number, errors during the calibration process and the detector readings, respectively. Therefore, a separate selection procedure was photodetectors out of more than 60 available used to select detectors that form the matrix detectors. In that procedure, with the smallest condition number were chosen. IV. CONCLUSIONS Standard CMOS IC processes provide the means to realize several photodiodes with independent spectral responsivities. The responsivities of these devices are controlled by using the poly-Si and glass coatings as thin-film optical filters, by taking advantage of both the diffusion and well junctions, and by manipulating the density of interfacial trapping centers by controlling which oxide terminates the Si lattice. In addition, variations in the widths of n-well photodiodes can be used to vary the short wavelength response of these devices. For appropriately constrained problems, these photodiodes can be used to form photo-spectrometers. These devices could be combined with on-chip analog, digital, and wireless circuits to form complete instruments-on-a-chip in a standard, lowcost, CMOS IC process. It is important to note that no changes to the standard CMOS fabrication process are required to realize these photodiodes. These micro-instruments would have applications in environmental measurements, drug discovery instrumentation, in situ, in vivo, or in vitro measurements, and

[1] A. Simoni et al., “A single-chip optical sensor with analog memory for motion detection,” IEEE J. Solid-State Circuits, vol. 30, pp. 800–805, July 1995. [2] X. Arreguit et al., “A CMOS motion detector system for pointing devices,” IEEE J. Solid-State Circuits, vol. 31, pp. 1916–1921, Dec. 1996. [3] P. Vernier et al., “An integrated cortical layer for orientation enhancement,” IEEE J. Solid-State Circuits, vol. 32, pp. 177–186, Feb. 1997. [4] C. H. Aw and B. A. Wooley. “A 128 128-pixel standard CMOS image sensor with electronic shutter,” IEEE J. Solid-State Circuits, vol. 31, pp. 1922–1930, Dec. 1996. [5] S. K. Mendis et al., “CMOS active pixel image sensors for highly integrated imaging systems,” IEEE J. Solid-State Circuits, vol. 32, pp. 177–186, Feb. 1997. [6] N. Ricquier and B. Dierickx. “Random addressable CMOS image sensor for industrial applications,” Sens. Actuators A, vol. 44, pp. 29–35, 1994. [7] P. Seitz et al., “Smart optical and image sensors fabricated with industrial CMOS/CCD semiconductor processes,” SPIE, vol. 1900, pp. 30–39, 1993. [8] First NTSC color camera on a single chip [Online] (Mar. 11, 1997). www: http://www.vvl.co.uk/release/405.htm, VISION press release. [9] J. Kramer, P. Seitz, and H. Baltes. “Industrial CMOS technology for the integration of optical metrology systems (photo-ASIC’s),” Sens. Actuators A, vol. 34, pp. 21–30, 1992. [10] T. Delbruck and C. A. Mead. “Analog VLSI phototransduction by continuous-time, adaptive, logarithmic photoreceptor circuits,” California Institute of Technology, Computation and Neural Systems Program, CNS Memo no. 30, July 14, 1994. [11] M. L. Simpson, G. S. Sayler, S. Ripp, D. E. Nivens, B. M. Applegate, M. J. Paulus, and G. E. Jellison Jr. “Bioluminescent-bioreporter integrated circuits form novel whole-cell biosensors,” Trends Biotechnology, vol. 16, pp. 332–338, Aug. 1998. [12] E. R. Fossum, “Active pixel sensors: Are CCD’s dinosaurs?” SPIE, vol. 1900, pp. 2–14, July 1993. [13] R. F. Wolffenbuttel and G. de Graaf, “Performance of an integrated silicon color sensor with a digital output in terms of response to colors in the color triangle,” Sens. Actuators, vol. A21–A23, pp. 574–580, 1990. [14] M. Weling and V. Malhotra. “Color detection using amorphous silicon Schottky photodiode,” Sens. Actuators A, vol. 29, pp. 195–200, 1991. [15] Spire Corporation, SBIR 95-1, proposal no. 08.03-6000B. [16] J. H. Jerman, D. J. Clift, and S. R. Mallisnson, “A miniature FabryPerot interferometer with a corrugated diaphragm support,” in Tech. Dig. IEEE Solid State Sensor and Actuator Workshop, Hilton Head Island, SC, 1990, pp. 140–144. [17] G. M. Yee, P. A. Hing, N. I. Maluf, and G. T. A. Kovacs. “Miniature spectrometers for biochemical analysis,” in Tech. Dig.Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, 1996, pp. 64–67. [18] J. A. Dobrowolski, “Optical properties of films and coatings,” in The Handbook of Optics. New York: McGraw-Hill, 1995, vol. 1. [19] J. C. Bean, J. Qi, C. L. Schow, R. Li, H. Nie, J. Schaub, and J. C. Campbell. “High-speed polysilicon resonant-cavity photodiode with SiO2 –Si Bragg reflectors,” IEEE Photon. Technol. Lett., vol. 9, pp. 806–808, June 1997. [20] G. E. Jellison, M. F. Chisholm, and S. M. Borbatkin. “Optical functions of chemical vapor deposition thin-film silicon determined by spectroscopic ellipsometry,” Appl. Phys. Lett., vol. 62, no. 25, pp. 3348–3350, June 21, 1993. [21] S. M. Sze, Physics of Semiconductor Devices, 2nd Edition. New York, Wiley, 1981, pp. 799–805. [22] A. G. Strollo and G. Vitale. “A closed-form two-dimensional model of a laser grooved solar cell,” Solid-State Electron., vol. 35, no. 8, pp. 1109–1118, 1992. [23] G. Strang, Introduction to Linear Algebra. Wellesley, MA: WellesleyCambridge Press, 1993.

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Michael L. Simpson (M’89–SM’95) received the Ph.D. degree in electrical engineering from the University of Tennessee (UT), Knoxville, in 1991. In 1992, he joined the Monolithic Systems Research Group at Oak Ridge National Laboratory (ORNL), Oak Ridge, TN. His present research interests include integrated sensors, whole-cell biosensors, and molecular electronics. He is an Adjunct Associate Professor of Electrical Engineering at UT, and the ORNL Coordinator of the UT/ORNL Joint Program in Mixed-Signal VLSI and Monolithic Sensors. He has published more than 50 technical papers, holds four patents, and has three patents pending. Dr. Simpson is a member of the Eta Kappa Nu Electrical Engineering Honor Society, and is listed in the fourth edition of Who’s Who in Science and Engineering.

M. Nance Ericson (S’86–M’94) received the B.S. degree in electrical engineering from Christian Brothers University, Memphis, TN, in 1987, and the M.S. degree from the University of Tennessee, Knoxville, in 1993. He is currently purusing the Ph.D. degree at the University of Tennessee with a focus on RF integrated circuits. In 1987, he joined Oak Ridge National Laboratory, Oak Ridge, TA, as a Research and Development Engineer in the Monolithic Systems Development Group. In this capacity, he has been involved in a variety of development projects with focus on the design of mixed-signal CMOS integrated circuits and integrated sensors. Presently, he is involved in the design of implantable instrumentation incorporating wireless integrated circuits, and high-density monolithic front-end electronics and data readout systems for the PHENIX detector. His research interests include physiological/biological sensing, wireless, silicon-based optical detectors, MOS device characterization, and high-temperature CMOS. He has authored or coauthored over 35 technical publications. He holds one patent in the area of wide-temperature logiarthmic current measurement and has three patents pending. Mr. Ericson is a member of Tau Beta Pi. He was the recipient of the IEEE Nuclear and Plasma Sciences Society Early Achievement Award in 1997.

Gerald E. Jellison, Jr. received the B.S. degree from Bowdoin College, Brunswick, ME, in 1968, and the Ph.D. degree from Brown University, Providence, RI, in 1976. From 1976 to 1978, he held a National Research Council fellowship at the Naval Research Laboratory. In 1978, he joined Oak Ridge National Laboratory, Oak Ridge, TN, where he is currently a Senior Scientist in the Solid State Division. His work has included investigations of pulsed laser annealing using time-resolved optical techniques, spectroscopic ellipsometry measurements of the optical functions of thin films and bulk materials, and photovoltaics. His present interest is in the use of spectroscopic ellipsometry as a diagnostic tool for thin film growth, (both ex situ and in situ) for process monitoring and process control. Recently, he has developed a model for the optical functions of amorphous materials and used it to interpret spectroscopic ellipsometry data from many forms of thin-film amorphous materials, including amorphous silicon, silicon nitride, and carbon; this has led to optimization of silicon nitride for antireflection coatings on solar cells. He is the author of over 120 publications.

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William B. Dress (M’85–SM’97) received the Ph.D. degree in physics from Harvard University, Cambridge, MA, in 1968. He has over 30 years experience in experimental physics, sensor development, and computer modeling and simulation. Currently, he is a Senior Development Staff Member of the Instrumentation and Controls Division at Oak Ridge National Laboratory, Oak Ridge, TN. He is the author of over 60 publications ranging from experimental physics to artificial neural networks. His current interests are in the field of advanced signal-processing methods, wavelets, and Bayesian signal estimation.

Alan L. Wintenberg (S’86–M’88) received the B.S. degree in engineering physics and the M.S. and Ph.D. degrees in electrical engineering, all from the University of Tennessee, Knoxville. His dissertation concerned the development of wideband, low-noise current preamplifiers for measurement of low-level prebreakdown current in dielectrics and was done under the direction of Dr. T. V. Blalock. While working on the M.S. degree, he worked part-time at the Fusion Energy Division, Oak Ridge National Laboratory (ORNL), Oak Ridge, TN, where he developed several NIM and CAMAC-based instruments. As an undergraduate co-op student, he was assigned to the ORNL Physics Division Van de Graaf accelerator lab and was involved in operation and maintenance of the 6-MV tandem. Currently, he is a Member of the Development Staff of the Instrumentation and Controls Division of Oak Ridge National Laboratory and an Adjunct Professor of the Electrical Engineering Department at the University of Tennessee. Since joining ORNL, his responsibilities have included the development of a number of custom integrated circuits for physics research, biological and optical sensing, and robotics applications as well as the development of more conventional analog electronics for a BGO calorimeter for heavy ion physics. He served as Local Engineer for several projects involving robotic sensing electronics and several projects developing heavy ion physics instrumentation. Current projects include development of optical sensing IC’s for biochips, wireless transmitter IC’s for remote sensing and specialized IC’s for heavy ion physics experiments. His professional interests include analog electronics, instrumentation, remote sensing, computer simulation of circuits and wireless design. He is the author of a number of papers concerning instrumentation for dielectric current measurements, heavy-ion physics, and robotic proximity sensing.

Milijko Bobrek (S’95–M’96) received the B.S. and M.S. degrees in electrical engineering from the University of Banja Luka, Bosnia, in 1976 and 1982, respectively, and the Ph.D. degree from the Department of Electrical Engineering, University of Tennessee, Knoxville, in August 1996. From 1976 to 1989, he was with “Rudi Cajavec,” Consumer Electronics Company, Banja Luka. In 1989, he joined the University of Banja Luka as a Research and Teaching Assistant. After spending a year at the University of Tennessee as a Research Assistant Professor, he joined the Monolithic Systems Research Group at the Oak Ridge National Laboratory (ORNL), Oak Ridge, TN. Since 1994, he has been teaching undergraduate and graduate courses in electrical engineering at the University of Tennessee. His research interests include development and implementation of advanced methods in signal processing and digital communications. He has authored and coauthored articles in several signal processing and VLSI design journals. Dr. Bobrek is a member of the Phi Kappa Phi.