Advances in Photodiodes Part 6 pdf

Methodology for Design, Measurements and Characterization of Optical Devices on Integrated Circuits 139 (a) (b) Fig. 24. Photodiode, structure N-Well/P-substrate: (a) cross section, (b) response on covered and not covered device as a photodiode is not too efficient due to crosstalk, degrading pixel’s characteristics as the dynamic range and increasing the fixed pattern noise. It is reported that crosstalk is a mechanism that is pronounced at longer wavelengths (Lee 2008) since light with these characteristics can go deep into silicon having a high probability to be reflected by the different layers present in the structure of the pixel. This can be confirmed with Fig. 24(b), where it is seen that the photocurrent of a covered photodiode increases as wavelength is increased. Furthermore, comparing Figs. 23 and 24(b), it can be concluded that the base-collector junction of the phototransistor operates as a barrier for carriers generated by light that penetrates beyond the surface region into the substrate. Since the photodiode has not this additional junction, it is collecting extra carriers, thus inconveniently reducing the difference between the response of covered and not covered devices. This should be considered also when designing pixel architectures, providing the pixel with surrounding materials with low dielectric constants and index of refraction, as long as the technology allows it. Otherwise, junction barriers as base-collector in phototransistors can play a similar role. Due to the importance of this mechanism over the performance of pixels, following an explanation is given regarding crosstalk. 6.1 Crosstalk mechanisms We have focused in the characterization and analysis of likely mechanisms that could contributes to the crosstalk on structures “P+/N-Well/P-substrate” (phototransistors) and “N-Well/P-substrate” (photodiodes). On these kinds of photo-devices, crosstalk has been defined and classified in two main mechanisms: (a) optical crosstalk and (b) electrical crosstalk (Brouk 2002, Kang 2002, Tabet 2002). Here, we introduce an additional classification of crosstalk, as is shown in the Fig. 25 and 26. The first classification is the lateral crosstalk mechanism which includes lateral optical crosstalk, and lateral electrical crosstalk. The second one is the vertical crosstalk mechanisms. Advances in Photodiodes 140 Crosstalk mechanisms Lateral optical crosstalk mechanisms Lateral crosstalk mechanisms Lateral electrical crosstalk mechanisms There aren’t vertical optical crosstalk mechanisms Vertical crosstalk mechanisms Vertical electrical crosstalk mechanisms Table 1. Crosstalk mechanisms classification 6.2 Lateral crosstalk mechanisms Lateral optical crosstalk is due to light traveling laterally among the layers up to the junction (near the surface of the device) acting as a waveguide, as is shown in the Fig. 25. Lateral electrical crosstalk is the phenomenon whereby photons generate carriers in the “ near to the surface region ”. That phenomenon has its origin in short wavelength light, mainly under of 650nm. Both, optical and lateral electric crosstalk, are present either in, phototransistors or photodiodes. However, in phototransistors this contribution is as a photocurrent collected by the base contact, which is tied to VDD, hence masking the effect. This is a first reason whereby the photo-current is larger in photodiodes than in phototransistors. Fig. 25(a) shows the way in which both, optical crosstalk and lateral electric crosstalk mechanisms, affect the response of phototransistors and Fig. 25(b) shows the corresponding for photodiodes. The effect is very strong since the current comes from all directions. (a) (b) Fig. 25. Lateral crosstalk mechanisms in phototransistor 6.3 Vertical crosstalk mechanisms Vertical crosstalk mechanism is shown in Fig. 17. It originates only due to electrical crosstalk, so it is called vertical electrical crosstalk. In the case of phototransistors, carriers generated along the substrate, as well as behind and outside the “N-Well”, are collected by the base contact since it is connected to a higher voltage than the emitter (see Fig. 17(a)). So, only majority carriers are collected by the base. In this case, diffusion of minority carriers is present as leakage current. Hence, little or no contribution to the spectral response of phototransistors is due to vertical electrical crosstalk. Methodology for Design, Measurements and Characterization of Optical Devices on Integrated Circuits 141 (a) (b) Fig. 26. Vertical crosstalk mechanisms in photodiode, (a) Phototransistors (b) photodiodes Vertical electrical crosstalk effect in photodiodes is illustrated in Fig. 26(b). Carriers generated by photons behind and outside the N-Well contribute to the spectral response of the photodiode with the leakage current coming from the substrate. Carriers generated deep in the substrate are due to longer wavelengths. This component of leakage current has a very strong effect over photodiodes but this is not the case for phototransistors, so the difference appreciated in Fig. 17 can be attributed to this. 7. Conclusions An architecture was proposed, from which characterization of photo-devices can be made, giving useful information for the performance evaluation of junction structures available in CMOS standard technologies. An adjustable gain amplifier, with a gain range of 10dB - 32dB, was configured allowing different biasing and operating points for photo-response measurement of different devices. Good agreement between simulated and experimental transfer function of the amplifier was obtained. The row-select transistor, M2, plays an important role in the operation of the amplifier. It was found that the aspect ratio of this transistor should be high in order to have a small channel resistance and to ensure an adjustable gain property to the amplifier. On the other hand, phototransistors (p+/N-well- /p-subs) and photodiodes (N-well/p-subs) were characterized for a 1.5µm technology, but the same methodology can be used with other silicon foundries. Structures have a maximum quantum efficiency of about 0.7 and a maximum sensitivity of almost 0.3A/W. Besides, photodiodes made with an N-well/P-subs junction, have shown a strong substrate leakage current contribution due to crosstalk that can affect parameters such as dynamic range and fixed pattern noise. So, depending on the features added to the architecture and the technology available, photodiodes may not be a good choice for image sensor arrays. 8. References Albert J.P. Theuwissen. (2008). CMOS image sensors: State-of-the-art. Solid-State Electronics, Vol. 52, (2008) page numbers (1401-1406), ISSN: 0038-1101 Chye Huat Aw and Bruce A. Wooley (1996). A 128x128-pixel Standard CMOS Image Sensor with Electronic Shutter. IEEE Journal of Solid-State Circuit, Vol. 31, No.12, December 1996 page numbers 1922-1930, ISSN: 0018-9200 Advances in Photodiodes 142 Graeme Storm, Robert Henderson, J. E. D. Hurwitz, David Renshaw, Keith Findlater, and Matthew Purcell. Extended Dynamic Range From a Combined Linear-Logarithmic CMOS Image Sensor. IEEE Journal of Solid-State Circuits, Vol. 41, No. 9, September. 2006, page numbers 2095-2106, ISSN: 0018-9200. Kareem A. Zaghloul, and Kwabena Boahen. (2004). Optic Nerve Signals in a Neuromorphic Chip I: Outer and Inner Retina Models. IEEE Transactions on biomedical engineering, Vol. 51, No. 4, (April 2004) page numbers (657-666), ISSN: 0018-9294. Ralf M. Philipp, David Orr, Viktor Gruev, Jan Van der Spiegel, and Ralph Etienne- Cummings. (2007). Linear Current-Mode Active Pixel Sensor. IEEE Journal of Solid- State Circuits, Vol. 42, No. 11, (November 2007) page numbers (2482-2491), ISSN: 0018-9200. R. J. Baker, Harry W. Li and David E. Boyce (2005). CMOS, Circuit Design, Layout, and Simulation. IEEE PRESS, of the Institute of Electrical and Electronics Engineers, Inc. ISBN: 0-7803-3416-7. 345 East 47 th Street, New York, NY 10017-2394. R. J. Perry and Krishna Arora (1996),. Using PSPICE to simulate the photoresponse of ideal CMOS Integrated Circuits Photodiodes. Proceedings of the IEEE Southeastcon Bringing Together Education, Science and Technology, (1996) page numbers (374-380), ISSN 0-7803-3088-9 Tae Gyoung Lee, Won Nam Kang, Young Ju Park, Eun Kyu Kim (2007). Fabry-Perot Interference Characteristics of the Photoluminescence in Nanoclustered SiNx:H Thick Films. Journal of the Korean Physical Society, Vol. 50, No. 3, (March 2007) page numbers (581-585). Wenjie Liang, Marc Bockrath, Dolores Bozovic, Jason H. Hafner, M., Tinkham and Hongkun Park (2001). “Fabry-Perot interference in a nanotube electron waveguide”. Journal of Nature, Vol. 411, No. 7, (2001) page numbers (665-669), ISSN Lee, Ji Soo and Mouli, Chandra (Boise, ID, US), Image Sensors with optical trench, United States Patent 7315014, January 2008. url: "http://www.freepatentsonline.com/7315014.html" I. Brouk, Y. Nemirovsky, S. Lachowicz, E. A. Gluszak, S. Hinckley, and K. Eshraghian (2002). Characterization of crosstalk between CMOS photodiodes. Solid-State Electronics, Vol. 46, (month and year of the edition) page numbers (first-last), ISSN In Man Kang (2002) “The simulation of the crosstalk between Photodiodes Fabricated Using the 0.18µm CMOS Process”, Semiconductor Materials and Devices Laboratory, School of Electrical Engineering and Computer Science, Seoul National University, Republic of Korea.SMDL Annual Report 2002. URL of the website is: http://smdl.snu.ac.kr/Research/annual/annual2002/2002pdf/ann_kim_2002.PDF. http://smdl.snu.ac.kr/Research/annual/annual2002/ M. Tabet (2002). Double Sampling Techniques for CMOS Image Sensors. Doctor of Philosophy thesis, University of Waterloo Electrical and Computer Engineering. Waterloo, Ontario, Canada, 2002. 7 Performance Improvement of CMOS APS Pixels using Photodiode Peripheral Utilization Method Suat U. Ay University of Idaho USA 1. Introduction Charge-coupled device (CCD) technology had been leading the field of solid-state imaging for over two decades, in terms of production yield and performance until a relatively new image sensor technology called active pixel sensor (APS) (Fossum, 1993), using existing CMOS facilities and processes, emerged as a potential replacement in the early 1990s. While CMOS APS technology was originally considered inferior, continuous improvements in cost, power consumption (Cho et al., 2000), dynamic range (Gonzo et al., 2002), blooming threshold, readout scheme and speed (Krymsky et al.,1999), low supply voltage operation (Cho et al., 2000), large array size (Meynants, 2005), radiation hardness (Eid et al., 2001), and smartness have achieved performance equal to or better than CCD technology (Agranov et al., 2005; Krymsky et al., 2003). Electro-optical performance of a photodiode (PD) type APS pixel is directly related to physical properties of photodiode diffusion layer. Doping concentration, junction depth, junction grading, biasing conditions, and physical shape of the photodiode diffusion layer determine the pixel full-well capacity, which is one of the main performance benchmarks of the PD-APS pixel. Pixel full-well capacity is related to sensitivity, charge capacity, charge saturation, dynamic range, noise performance, and the spectral response of the pixel (Theuwissen, 1995). Pixel dynamic range versus full well capacity for different pixel noise levels could be plotted as shown on Fig. 1. Thus, increasing full well capacity is desirable. In this chapter, so called photodiode peripheral utilization method (PPUM) is introduced addressing performance improvement of photodiode type CMOS APS pixels, (Ay, 2008). PPUM addresses the improvement of the metrics full well capacity and spectral response especially in blue spectrum (short wavelength). First, identification of junction and circuit parasitics and their use in improving the full-well capacity of a three-transistor (3T) PD-APS pixel through photodiode peripheral capacitance utilization is discussed. Next, spectral response improvement of PD-APS pixels by utilizing the lateral collection efficiency of the photodiode junction through PPUM is discussed. The PPUM method and its proposed benefits were proven on silicon by designing a multiple-test-pixel imager in a 0.5μm, 5V, 2P3M CMOS process. Measurement results and discussions are presented at the end of the chapter. Advances in Photodiodes 144 Fig. 1. Pixel dynamic range versus full well capacity and noise floor. 2. Photodiode Peripheral Utilization Method (PPUM) The theory behind the photodiode peripheral utilization method (PPUM) is that, if the pixel pitch is restricted to a certain size, then pixel full-well capacity could be increased by opening holes in the photodiode’s diffusion. These diffusion holes could be used to increase photodiode parasitic capacitance, by increasing the perimeter capacitance of the photodiode for certain process technologies shown on Fig 2. Diffusion holes also can increase spectral response of a photodiode by utilizing lateral collection of charges converted close to the semiconductor surface at the edges of photodiode, (Fossum, 1999; Lee and Hornsey, 2001). Fig. 2. Unit junction capacitance of CMOS processes, (Ay, 2004). Performance Improvement of CMOS APS Pixels using Photodiode Peripheral Utilization Method 145 A reverse-biased PN-junction diode is used in photodiode (PD) type CMOS APS pixels as a photon conversion and charge (electron) storage element. The total capacitance of the photodiode diffusion layer determines key pixel performance parameters. For example, wide-dynamic-range pixels require large pixel full-well capacity and low readout noise. Photodiode full-well capacity is comprised of two components: bottom plate (area) and side wall (peripheral) junction parasitic capacitance. Designer controls the size of the photodiode diffusion bottom plate, while peripheral junction depth and doping concentration are process and technology dependent. The photodiode’s unit area junction capacitance (C A ) and unit peripheral junction capacitance (C P ) are given in the following equations, (Theuwissen 1995), including technology and design parameters, for the first-order capacitance that contributes to total well capacity. PD A P CCACP = ⋅+ ⋅ (1) J0A J0SW PD MJ MJSW PD PD B BSW CA C P C V V 1 1 Φ Φ ⋅⋅ =+ ⎡⎤⎡ ⎤ − − ⎢⎥⎢ ⎥ ⎣⎦ ⎣⎦ (2) where C A , C P unit area junction capacitance and unit peripheral junction capacitances, respectively; C J0A , C J0SW unit zero-bias area and peripheral junction capacitances, respectively; A, P area and peripheral of the photodiode regions, respectively; Φ B , Φ BSW built-in potential of area and side-wall junctions, respectively; M J , M JSW junction grading coefficients of area and side-wall junctions, respectively; V PD photodiode junction voltage. Other parasitic capacitances due to the reset and readout transistors in pixel contributing to total photodiode junction capacitance are shown in Fig. 3. for a three-transistor (3T) PD-APS pixel. These parasitic capacitances contribute to total pixel capacitance differently in different modes of pixel operation, (Ay, 2004). Right after photodiode reset and during scene integration periods, overlap capacitances C O1 and C O2 and gate-to-body capacitance of the Fig. 3. Parasitic capacitances of photodiode type CMOS APS pixel. Advances in Photodiodes 146 readout transistor M2 (C B2 ) add to the total photodiode capacitance. During a readout period, miller capacitance C M2 and overlap capacitances C O1 and C O2 contribute to the total photodiode capacitance. Contribution of pixel circuit parasitic capacitances is described by the following equations during imaging (3) and readout (4): par,ima g in g M1 OL,M1 M2 M2 OL,M2 OX CWLWLLC ⎡⎤ ⎡⎤ =⋅ +⋅− ⋅ ⎣⎦ ⎣⎦ (3) par,read M2 M2 OL,M2 OX 2 CWL2L1GC 3 ⎡⎤ ⎡⎤ =⋅ ⋅ −⋅ ⋅− ⋅ ⎡⎤ ⎣⎦ ⎢⎥ ⎣⎦ ⎣⎦ (4) M1 OL,M1 M2 OL,M2 OX WL WL 2GC ⎡⎤ +⋅ +⋅ ⋅−⋅ ⎡⎤ ⎣⎦ ⎣⎦ where W 1 , W 2 channel width of the reset and source-follower transistors, respectively; L OL1 , L OL2 channel overlap length of the reset and source-follower transistors, respectively; C OX unit oxide capacitance, G pixel source follower gain factor. C A and C P of a few CMOS process technologies, with minimum feature sizes 2.0μm–0.18μm, is shown in Fig. 2., (Ay, 2004). Unit-area capacitance is larger for deep sub-micron devices with a minimum feature size <0.5μm, due to the increased channel-stop doping-level (for better device isolation, higher diffusion doping concentrations, and shallower junction depths) (Packan, 2000). Thus, peripheral junction capacitance could be better utilized in processes that have equal or more unit peripheral junction capacitances than in processes with <0.5μm feature sizes, by opening holes in the photodiode region. As will be shown in the next sections, this will not only improves the total full-well capacity of the pixel, but also improves the spectral response for detecting short wavelength photons. 3. Photodiode lateral collection improvement The photosensitive element in APS pixels, the photodiode (PD), works in charge integration- mode where pixels are accessed at the end of a time interval called the integration period. When it is accessed, photodiode is read and then cleared for next scene integration. Fig. 4. shows the cross-section of a PN-junction photodiode formed in a CMOS process; the photodiode is reverse-biased and formed by using the shallow N+ doped, drain-source diffusion of an NMOS device. A bias voltage applied to the N+ region forms a depletion region around the metallurgical PN-junction, which is free of any charge because of the electrical field. Any electron-hole pairs generated in this region see the electrical field as shown in the AA′ cross-section view of the photodiode in Fig.4. Electrons move in the opposite direction of the electric field (toward the N+ region), while holes move toward the P-region. As a result, electrons are collected in a charge pocket in the N+ region, while holes are recombined in the substrate. This type of photodiodes has been widely used in CMOS and early CCD-type image sensors as a photo conversion and collection element. There are two issues associated with using the N+ drain/source diffusion of an NMOS transistor as photosensitive element. First is the dark current induced by stress centres around the diffusion, (Theuwissen 1995). These stress centres are formed during the field Performance Improvement of CMOS APS Pixels using Photodiode Peripheral Utilization Method 147 Fig. 4. a)Cross-section and b)potential-well diagram of a PN-photodiode. oxide (FOX) formation in standard CMOS processes. The second issue is the surface-related dark current generated from the work function difference between the N+ diffusion surface and overlaying isolation oxide layer. This second one causes surface recombination centers and defects. Both of them absorb photo-generated electron-hole pairs close to the surface, resulting in quantum loss at shorter wavelengths. As a result, silicon photodiodes show less sensitivity in the blue spectrum (<400nm. Most blue photons are collected through lateral diffusion of the carriers generated on or in the vicinity of a photodiode peripheral—known as peripheral photoresponse or lateral photocurrent (Lee et al., 2003). Thus, increasing lateral collection centers or peripheral length of a photodiode potentially improves collection efficiency for short-wavelength photons (Fossum, 1999; Lee et al., 2001) as it is depicted in Fig. 5. This method was adopted for UV photodiode devices in P-well CMOS processes (Ghazi et al.,2000). Fig. 5. Improving lateral collection by increasing photodiode peripheral for blue photons. Advances in Photodiodes 148 4. CMOS pixel design using PPUM There are many ways to test CMOS imaging pixels using test vehicles. Some uses product grade imager platforms to test not only the performance of the imaging pixels, but also their performance in final product environment. Some uses very small array of dumb pixels to measure basic characteristics of the pixel under investigation. A commonly used architecture is called fully flexible open architecture (FFOA) that composes of sample and hold circuits, correlated double sampling (CDS) and differential delta sampling (DDS) circuits, and source follower amplifiers (Nixon et al, 1996; Mendis et al., 1997). Simple FFOA architecture gives very reliable and predictable signal path characteristics. It also allows multiple pixel types with different sizes to be integrated on the same chip. A test imager was designed containing reference and pixels utilizing PPUM as proof of concept. The reference or baseline three-transistor (3T) photodiode type (PD) APS reference pixel (REF) is shown in Fig. 6. It was designed to normalize measurement results of the test pixels with diffusion holes. A fairly large pixel size of 18μm × 18μm was chosen. It has circular-looking photodiode diffusion region for reducing overall dark current. Row select and reset signals were drawn on top of each other using horizontal metal-2 and metal-3 lines, and metal-1 was used on the vertical direction for routing pixel output and supply signals. The reference photodiode diffusion area and peripheral were 141.7μm2 and 44.6μm, respectively. Unit area and peripheral capacitance of the photodiode’s N+ diffusion layer in used process were 0.25fF/μm2 and 0.22fF/μm, respectively. Total pixel capacitance was calculated by including the Miller contribution of the source-follower transistor (M2) and other parasitic capacitances from equations (3) and (4). Miller contribution to the total photodiode capacitance at 0.75 source-follower gain was calculated to be 1.1fF; peripheral junction capacitance made up of 20 percent of the total photodiode capacitance, and the total calculated photodiode capacitance was about 47.5fF. Fig. 6. 3T CMOS APS reference pixel (REF) a) schematic, b) layout. Four test pixels with a number of circular diffusion openings were designed to model the peripheral utilization effect on pixel performance, with layouts shown in Fig. 7. Pixels have [...]... the minority carriers cannot 166 Advances in Photodiodes form an induced photocurrent Hence, Fig 9 also depicts that spectral responses begin to decay dramatically in the infrared region Figure 10 displays the simulations associated with the excess minority carrier densities in p-substrate and p-epitaxial/p+substrate From this figure, the excess minority carrier density in the p-substrate region is... responses of four CMOS photodiodes in Fig 4(a) under zero biased voltage  164 Advances in Photodiodes Fig 6 Simulated spectral responses of n-/p-sub under different surface recombination velocities where Sp indicates a surface recombination velocity Fig 7 Simulated result of the relationship between the incident light wavelength and absorption length ⎛ 84.732 α =⎜ ⎝ λ 2 ⎞ − 76. 417 ⎟ ⎠ (1) The reciprocal... process, which increase the manufacture cost 2 Most of incident light is absorbed and reflected during passing through the color filter that decreases the photo-response of a photodiode Accordingly, one extra process of micro lens is included for the commonly-used color filter to focus incident light with the purpose of increasing the induced current 3 In order to meet the requirement of sensing different... similar except for those in the short wavelength region The reason for this phenomenon is that the excess minority carriers in the device surface, which are excited by incident light with short wavelengths, recombine rapidly owing to heavy doping in the p+ layer The surface recombination velocity is significantly influenced by process factors such as surface roughness, surface contamination and oxidation... not easily controlled [35], [ 36] Omitting reflection coefficients, Fig 6 depicts the simulated spectral responses of the n-/p-sub under different surface recombination velocities Since excess minority carriers in the device surface are excited by incident light with short wavelengths, and the recombination probability of these excited carriers increases with surface recombination velocity, the degraded... depicted in Fig 1 Each photodiode is connected to an amplifier that transfers the captured image signal into an electrical signal Additionally, an overall photodiode array is established by using a pattern with red, green and blue photodiodes as shown in Fig 1 according to the human perceptual principle [1], [2] As for the signal processing part, it comprises a decoder, timing-control unit, compensating... length of the minority carriers in the p-epitaxial region can also be as high as several hundred micrometers, most minority carriers in this region are transferred from p-epitaxial to p+substrate via drifting or diffusion However, as the diffusion length of minority carriers in p+substrate is only several micrometers, most minority carriers in p+substrate are rapidly recombined so that the minority carriers... Nixon, R H., Kemeny, S E., Pain, B., Staller, C O., Fossum, E R., (19 96) 256x2 56 CMOS active pixel sensor camera-on-a-chip, IEEE Journal of Solid-State Circuits, vol 31, no 12, December 19 96, pp 20 46- 2050, ISSN:00189200 Packan, P.A., (2000) Scaling Transistors into the Deep-Submicron Regime, MRS Bulletin, Volume 25, No 6, p 18, June 2000 Theuwissen, A.J.P (1995) Solid-State Imaging with Charge-Coupled Devices,... response in the space-charge region increases with the width of the region, while that in n- and p-substrate falls as the effective charge collection regions decrease in these two layers In Fig 13(a), the increase in the spectral response of the space- Color-Selective CMOS Photodiodes Based on Junction Structures and Process Recipes 167 (a) (b) Fig 10 Simulated excess minority carrier densities in (a)... applications, photodiodes in CMOS image sensors have been designed using different process recipes and junction structures Particularly, a CMOS photodiode with an n-/p-sub or p+/n- junction is often applied to CMOS image sensors [15], [23]-[ 26] Since the spectral response of silicon covers visible light and infrared regions, n-/p-sub and p+/n-/p-sub photodiodes can be used in commercial CMOS image sensors Owing . Optic Nerve Signals in a Neuromorphic Chip I: Outer and Inner Retina Models. IEEE Transactions on biomedical engineering, Vol. 51, No. 4, (April 2004) page numbers (65 7 -66 6), ISSN: 0018-9294 structures available in CMOS standard technologies. An adjustable gain amplifier, with a gain range of 10dB - 32dB, was configured allowing different biasing and operating points for photo-response. Jason H. Hafner, M., Tinkham and Hongkun Park (2001). “Fabry-Perot interference in a nanotube electron waveguide”. Journal of Nature, Vol. 411, No. 7, (2001) page numbers (66 5 -66 9), ISSN Lee, Ji