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Which Photodiode to Use: a Comparison of CMOS-Compatible

Which Photodiode to Use: a Comparison of CMOS-Compatible

1 Which photodiode to use: a comparison of CMOS-compatible structures Kartikeya Murari, Student Member, IEEE, Ralph Etienne-Cummings, Senior Member, IEEE, Nitish Thakor, Fellow, IEEE, and Gert Cauwenberghs, Senior Member, IEEE

Abstract—While great advances have been made in optimizing and computational imagers that can perform stereo vision [8], fabrication process technologies for solid state image , the motion estimation [9] among others [10]. It was expected that need remains to be able to fabricate high quality photosensors CMOS imagers could share production lines with mainstream in standard CMOS processes. The quality metrics depend on both the pixel architecture and the photosensitive structure. This logic and memory fabrication, thereby delivering economies paper presents a comparison of three photodiode structures of scale. However, improving CMOS fabrication technology in terms of spectral sensitivity, and . The meant scaling down feature sizes. While this made CMOS + + three structures are n /p-sub, n-well/p-sub and p /n-well/p- imager fill factors comparable to CCDs, it introduced noise sub. All structures were fabricated in a 0.5 µm 3-metal, 2- and non-linear effects due to sub-micron features. Thus, to poly, n-well process and shared the same pixel and readout architectures. Two pixel structures were fabricated - the standard regain performance, CMOS fabrication technology had to be three active pixel , where the output depends optimized for imaging, offsetting the advantages of being able on the photodiode , and one incorporating an in- to fabricate in a standard CMOS process [11]. Today high per- pixel capacitive transimpedance amplifier where the output is formance is available in both CMOS and CCD technologies, dependent only on a designed feedback . The n-well/p- but with higher design complexity associated with CMOS than sub performed best in terms of sensitivity (an improvement of 3.5× and 1.6× over the n+/p-sub and p+/n-well/p-sub CCD technologies. respectively) and signal-to-noise ratio (1.5× and 1.2× Clearly, the need still exists for high performance CMOS improvement over the n+/p-sub and p+/n-well/p-sub diodes image sensors designed in standard CMOS processes. The key + respectively) while the p /n-well/p-sub diode had the minimum here is performance, which depends on the application. For (33% compared to other two structures) dark current for a given example, stroboscopic imaging requires very high sensitivity sensitivity. while biological fluorescence imaging requires the dark current Index Terms—CMOS, photodiodes, active pixel sensors. to be minimized. Since most of the parameters characterizing imagers are interdependent, application dependent tradeoffs I. INTRODUCTION need to be made for an optimal design. Certain applications are OLID state image sensors have come a long way since the also specific, requiring knowledge of the spectral S first CMOS [1] and CCD [2] sensors were described in sensitivity of the detector. the late 1960s. Since then, great advances have been made The at the heart of most CMOS image sen- in both modalities of sensors. CCD imagers took the lead sors is a photodiode. While the ultimate performance of the till the 1990s because CMOS fabrication technology was sensor also depends on the pixel and peripheral circuitry, the not sufficiently advanced to make use of the main advan- photodiode plays a limiting role. Material parameters control tage of CMOS imagers - the ability to integrate electronic photodiode performance and cannot be changed by a designer circuits on the focal plane, in the same die. Advancements unless the CMOS fabrication procedure itself is modified. in CCD technology have revolved around optimizing fabri- Prior work comparing different photodiode structures has cation techniques to improve the sensitivity, charge transfer not been very systematic. Bhadri et al. simulated but did not measure spectral sensitivity and dark current for n-well/p-sub, efficiency, dark current, and noise performance among other + specifications. By the mid 1990s, fabrication technology had n-well/p photodiodes and a pnp bipolar phototransistor in a improved to a point that there was a resurgence in CMOS 1.5 m CMOS process [12]. Odiot et al. compared different imaging systems [3], [4], [5] that offered compact, single- photodiode geometries for a n-well/p-sub photodiode [13]. Li et al. measured the broadband sensitivity and dark response chip, low-power devices. Innovative circuit design has led + to CMOS imagers capable of imaging at several thousand for n /p-sub and n-well/p-sub photodiodes using three and four transistor active pixel sensors in a 0.35 m CMOS frames per second [6], pixel pitches down to 1.4 m [7] process [14]. Fowler et al. measured a higher response for + Manuscript received December 16, 2008. This work was supported by the an n-well/p-sub photodiode than for an n /p-sub using a NIH grant R01AG029681. Chips were fabricated through the MOSIS foundry three transistor pixel in a 0.35 m CMOS process [15]. Tian service. + K. Murari and N. Thakor are with the Department of Biomedical Engineer- et al. compared n /p-sub and n-well/p-sub photodiodes in ing, Johns Hopkins University School of Medicine, Baltimore, MD 21205. 0.18 m CMOS process [16] where gate leakage currents on R. Etienne-Cummings is with the Department of Electrical and Computer the order of under normal lighting conditions Engineering, Johns Hopkins University. G. Cauwenberghs is with the Division of Biology, University of California, require optimizing design beyond the photodiode selection, San Diego. which is the focus of this work. We present a comparative 2

study of three photodiode structures fabricated in a 0.5 m level using a reset signal. The capacitance Cj of the junction 3-metal, 2-poly, n-well CMOS process. As elaborated in the is given by: following sections, the response of a pixel depends on the A Cj = ǫ (4) photodiode and the photodiode sense node capacitance. If W not decoupled, one would measure the characteristics of the where W is defined in Eq. 2. Clearly, the output voltage pixel rather than the photodiode. In this work, two pixel signal of the photodiode will depend on the and circuits were implemented, one of which allowed decoupling on the junction capacitance. From Eqs. 1-4, these depend on the photodiode capacitance from the pixel, allowing a true the doping concentrations of the p and n type regions of the photodiode comparison. Section II discusses issues that control junction. the sensitivity of a detector and describes the three structures In addition to the junction characteristics, the location of the tested. Sections III and IV detail the pixel circuits and chip junction also contributes to spectral sensitivity. As mentioned architecture. Section V summarizes the results and Sec. VI in Sec. II, charge carriers can diffuse a diffusion length before concludes the paper. they recombine and are lost. In the case of a shallow junction, a fraction of the EHP generated below the junction will start diffusing deeper into the material, away from the junction, II. PHOTOTRANSDUCTIONANDPHOTODIODESTRUCTURES and will not be collected. However, the EHP generated above Phototransduction starts with incidence on a detec- the junction cannot diffuse away into the bulk of the material tor. If the photon energy is greater than the bandgap of the ma- and are much more likely to get collected. Thus, a deeper terial, hν >Eg, -hole pairs (EHP) are generated. The junction improves the collection efficiency leading to a higher quantum efficiency (QE) of a detector is defined as the percent- sensitivity. This effect is enhanced for long wavelength pho- age of hitting the photoreactive surface that produce tons which penetrate more into the material and generate EHP an EHP. QE is only part of the measure of sensitivity of the deeper in the bulk [19]. The processing steps needed to create detector. Also important is the collection efficiency which is a highly doped region are different from those required to the fraction of generated EHP that contribute to a current flow create a lightly doped one. Thus, the depth of a junction is external to the detector. Considering phototransduction in a pn indirectly related to the doping concentration of the material junction, EHP are generated all over the p and n regions that used to create it. Typically, low doping is achieved by diffusion form the junction. In the bulk of the junction, these electron which results in deeper junctions while regions of high dopant and holes have a high probability of recombining and are concentration are created using ion implantation, leading to thereby lost. In the and a diffusion width shallower junctions [20]. on either side of it, however, due to the electric field existing Apart from the sensitivity, other photodiode parameters like across the depletion region, the and holes are swept dark current, thermal and shot noise also depend on the diode away, leading to a useful photocurrent [17]. If W , Lp, Ln material. Dark current is a result of random thermal EHP denote the depletion region width, hole diffusion length, and generation in the absence of . EHP are generated all over electron diffusion length respectively, the photocurrent IOp can the material but the most significant contribution comes from be written as [18]: surface states at the face of the material [21]. If the thermal rate of EHP generation is gTh, similar to Eq. 1, IOp = qgOpA (W + Ln + Lp) (1) the dark current can be written as [18]: where gOp is the light induced rate of EHP generation, IDk = qgThA (W + Ln + Lp) (5) q is the electronic charge and A is the total area of the g depends on the absolute temperature and also on certain junction including the bottom and sidewalls. The width of Th material properties like the defect density in the crystal struc- the depletion region depends on N and N , the respective a d ture, which, in turn, depends on the photodiode material and doping concentrations of the p and n type materials used for structure. Thermal noise on the photodiode capacitance due to the junction and on the voltage V applied across it [18]: the thermal agitation of electrons is given by: 2ǫ (V − V ) 1 1 W = bi + (2) kT q N N vn = (6) s  a d  s Cj where ǫ is the permitivitty of . Vbi is the built-in diode Shot noise is due to the quantized nature of the phototrans- potential and is given by: duction and is related to the photocurrent:

kT NaNd i = 2qI ∆f (7) V = ln (3) n Op bi q n2 i where the noise is measuredp over a bandwidth of ∆f. Since where ni is the intrinsic carrier concentration of silicon. k, IOp depends on the photodiode, so does the shot noise. q and T denote the Boltzmann constant, electronic charge From the above discussion, it is obvious that several param- and absolute temperature respectively. In voltage mode active eters of photosensing using pn junction diodes depend on the pixel sensors, the photocurrent then discharges the photodiode structure and type of the junction. In standard n-well CMOS capacitance which had been precharged to some reference processes, there are three ways to create a pn junction. We 3

(a) (b) (c)

Fig. 1. Schematic drawings of the three photodiode structures - (a) n+/p-sub, (b) n-well/p-sub and (c) p+/n-well/p-sub. Note the larger depletion region and the deeper junction in (b,c) and the pinned detector surface in (c). Fabrication design rules require larger minimum sizes and separation for n-wells. Thus, n+/p-sub photodiodes can be more compact than the other two structures. now briefly describe the structures and the motivation behind ‘pinned’ structure was first developed for CCD imagers [22] choosing these three. and subsequently reported for CMOS imagers as well [23]. The diode is drawn in Fig. 1(c). This implant serves two A. n+/p-sub purposes. Firstly, in the same area, there are now two pn + Fig. 1(a) shows the most straightforward structure used to junctions - the p /n-well and the n-well/p-sub. This creates create a photodiode. In terms of design rules, this structure is an effective depletion region even larger than the n-well/p- the most compact. It is formed by creating a highly doped n sub diode and should lead to the highest collection efficiency region in the p substrate. Due to the high doping concentration among the three structures. However, depletion of the n+ implant (compared to an n-well diffusion), from from the two junctions add in parallel, lowering the charge-to- Eq. 2, the depletion region width W is small. This leads to a voltage conversion. Secondly, as mentioned before, the main reduction in the collection efficiency. Since W is small, from source of dark current is from the interface states at the surface of the junction. If the free charge carrier concentration at the Eq. 4, the junction capacitance Cj is large. This results in a low interface is high, the interface states will be occupied and not charge-to-voltage conversion. Since the n region is created by + ion implantation, the junction is relatively close to the surface. contribute to EHP generation. Since the p layer has a high This causes a further reduction in the collection efficiency, hole concentration, we expect this photodiode to have lower specially for longer . We considered the n+/p dark current than the n-well/p-sub structure where the surface photodiode as the reference design and expect improvements layer does not have a high free carrier concentration. over it in the other two structures. III. PIXEL CIRCUITS B. n-well/p-sub As alluded to in Sec. II, the current output of a pixel depends This photodiode uses the lightly doped n-well diffusion to on the QE of the photodiode that fixes the number of EHP per create a pn junction in the p substrate, as shown in Fig. 1(b). photon and the collection efficiency that decides the fraction The lower doping concentration of the n-well (compared to of the generated EHP that contribute to the photocurrent. an n+ implant) increases the depletion width W and this The subsequent voltage output of the pixel depends on the decreases the junction capacitance. The larger depletion region photocurrent and the photodiode capacitance. In order to should lead to a better collection efficiency and the smaller compare photodiodes sensitivities per se, the effect of the capacitance should improve the charge-to-voltage conversion. capacitance must be removed. One of the two implemented Since the n-well diffusion is deeper than an n+ implant, circuits accomplishes this by unlinking the photodiode capac- the junction is deeper and more efficient at capturing long itance from the pixel operation. We now present the two pixel wavelength photons compared to a n+/p-sub junction. The architectures that were used in this work. One is the standard increased depth also creates significant depletion regions along three transistor active pixel sensor and the other is a pixel that the sidewalls of the junction, further improving the collection includes a capacitive transimpedance amplifier within itself. efficiency. However, this increased junction area, caused by higher sidewalls due to the deeper junction, will also increase A. 3 transistor (3T) pixel the junction capacitance Cj , offsetting some of the improve- ment in the charge-to-voltage conversion resulting from the The circuit for the three transistor active pixel sensor [1] is smaller W . Design rules require larger minimum spacing and shown in Fig. 2(a). The pixel is reset using transistor M1 which + minimum widths for n-wells compared to n regions. Thus, charges the photodiode capacitance CPD almost to VDD. After given a constant size, a pixel with an n-well photodiode will the reset is released, the photocurrent IPD discharges the have a lower fill factor than one with an n+/p-sub photodiode. capacitance. The output of the pixel can be accessed through the source follower transistor M2 and the access transistor M3. + C. p /n-well/p-sub The output vout can be written as: This photodiode is similar to the n-well/p-sub diode above, 1 + v = G I dt (8) but adds a p implant covering the n-well diffusion. This out SF C PD PD Z 4

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1111111111111111111111111111111111116666665555555522266 222222222222222222222222222211111111111111111111111111111111111555555556666 11111111111666666666 111111111111111111111111111111111111666666 66 222222222222222222222222222211111111116666 (e) (f) (g) (h) Fig. 2. Pixel circuits and layouts. (a) is the 3T APS and (b,c,d) are layouts of the 3T APS with an n+/p-sub, n-well/p-sub and p+/n-well/p-sub photodiode respectively. (e) is a simplified schematic of the CTIA APS and (f,g,h) are layouts of the CTIA APS with an n+/p-sub, n-well/p-sub and p+/n-well/p-sub photodiode respectively. All pixels were sized 30 µm × 30 µm. Note the different photodiode geometries due to different design rules. The differences are summarized in Table I.

where GSF is the sub-unity gain of the source follower. The Parasitic capacitances due to M1 do not effect Cfb because output of the pixel depends on the photodiode capacitance they can be lumped into either the photodiode or the load CPD. Thus, this pixel, while extremely simple in design and capacitance of the circuit. Any process dependent mismatch operation, cannot be used to compare photodiodes without in Cfb will manifest as gain error across pixels. the knowledge of their capacitance. Junction capacitances are generally not known accurately. Since this is the oldest and IV. CHIP ARCHITECTURE most common pixel circuit, we included the 3T APS in the In the last two sections, we described three photodiode comparison and present this data as representative of the pixels structures and two pixel circuits. Two versions of the CTIA and not of the photodiodes. pixel were designed, with the feedback capacitance being 5 and 10 fF. The chip was designed in a 3 metal, 2 poly B. Capacitive trans-impedance amplifier (CTIA) pixel 0.5 m n-well CMOS technology. Each of the three circuits were laid out with the three kinds of photodiodes resulting A simplified schematic for the CTIA pixel [24], [25] is in nine pixel arrays. Fig. 2(b-d) shows the layouts of the shown in Fig. 2(e). The amplifier A was realized as a single- three transistor APS using an n+/p-sub, n-well/p-sub and stage cascoded common source amplifier. At a bias current of p+/n-well/p-sub photodiode respectively. Fig. 2(f-h) shows 200 nA, 1 pF capacitive load and 3.3 V supply, simulations the layout of the CTIA APS using an n+/p-sub, n-well/p- indicate a gain of 85 dB and a gain-bandwidth product of sub and p+/n-well/p-sub photodiode respectively. This CTIA 675 kHz. Transistor M1 acts as the reset , charging the pixel had Cfb=5 fF. The transistor sizing within each pixel photodiode capacitance to the inversion point of the amplifier. circuit was kept the same. The pixel pitch was kept constant at This sets the reverse bias across the photodiode. The capacitor 30 m across all the pixels. Due to differences in design rules Cfb acts in negative feedback. With a sufficiently high gain and given the same pixel size, the photodiode size across all amplifier and Cfb << CPD, the circuit effectively pins the structures was not constant. Table I summarizes the photodiode photodiode output node and forces the photocurrent to charge geometries in all the pixels. Cfb. The output vout can be written as: In order to obtain statistically significant comparisons, 7×7 arrays of each pixel were designed. For each array type, a 1 v = − I dt (9) two-pixel wide border was not considered in the analysis to out C PD fb Z minimize effects from the peripheral pixels that see a different The circuit cancels out the effect of the photodiode capacitance surrounding environment from the central pixels. Using an and the output signal depends only on the photocurrent and analog consisting of several transmission gates, a known capacitance which is a circuit design parameter. The the output of each individual pixel in the center 3×3 elements negative sign comes from the inverting nature of the amplifier. of each array could be connected to an output buffer. In all, 5

(a) (b)

(c) (d)

Fig. 3. (a) Micrograph of the 1.5 mm × 1.5 mm die showing arrays of the test pixels and other test structures. (b,c,d) are measured pixel outputs from the 3T APS, 10 fF CTIA APS and the 5 fF CTIA APS respectively. All APS pixels are implemented with n-well/p-sub photodiodes. The CTIA APS was covered by an OD 1.8 neutral density filter attenuating incident light by a factor of 62.5.

TABLE I PHOTODIODEGEOMETRIESACROSSPIXELS light was controlled using a Fluorolog-3 spectrofluorometer (Jobin Yvon, NJ). Light intensity was measured using a model Photodiode type APS type Area Perimeter Fill factor 1930 optical power meter (Newport, NY). Fig. 3(b-d) show 2 (µm ) (µm) (%) the measured pixel response of the 3T APS, 10 fF CTIA 3T 671.85 120.8 74.7 n+/p-sub CTIA 10 fF 512.86 102.5 57.0 APS and the 5 fF CTIA APS respectively, under broadband CTIA 5 fF 523.89 101.6 58.2 illumination. All pixels had the n-well/p-sub photodiode. The 3T 531.81 99.8 59.1 exposure time was kept constant at 6 ms and a neutral density n-well/p-sub CTIA 10 fF 513.54 96.4 57.1 filter was used to attenuate the incident light by a factor of CTIA 5 fF 519.52 95.9 57.7 62.5 (OD 1.8) for the CTIA APS. Note the negative slope for 3T 541.08 105.2 60.1 p+/n-well/p-sub CTIA 10 fF 485.10 97.5 53.9 the CTIA APS as predicted by Eq. 9. CTIA 5 fF 494.01 97.5 54.9 The analog output of the chip was digitized to 16 bits using a NI6031 data acquisition card (National Instruments, TX) and read into a computer for analysis. The slope of the voltage 81 pixels (9 arrays of 3×3 pixels each) were individually output of the pixel was averaged for a thousand exposures as interrogated. a measure of the sensitivity. The standard deviation in the slope measurements was taken as the noise of the detector. It should V. RESULTS be noted that this measure takes into account the electrical Fig. 3(a) shows the micrograph of the 1.5 mm × 1.5 mm noise added by the readout circuitry. However, a single readout die with all the test structures. The chip was powered by path was shared by all the 3T APS and one by all the CTIA two independently regulated 3.3 V supplies - one for all APS. The data were collected at illumination levels leading the pixel arrays and one for all the digital circuits. For to noise beyond the read noise floor. Thus, we expect the characterization, a computer controlled the digital inputs to the measurements to be representative of the inherent photodiode chip, connecting a specified pixel to the output buffer. Incident noise. All data were averaged across the nine pixels of each 6

35 1.2 +

−12 n /p 30 n−well/p 10 0.8 × + ) p /n−well/p

−2 25 0.4 +

/ Wcm 20 n /p

−2 n−well/p 0

cm 15 p+/n−well/p −1 −0.4 10

5 −0.8 Normalised incremental sensitivity Sensitivity (Vs 0 −1.2 400 500 600 700 800 900 400 500 600 700 800 900 λ (nm) λ (nm) (a) 3.5 Fig. 5. Incremental sensitivity of the photodiodes calculated using the data n+/p from the CTIA APS. All data were normalized to the maximum incremental 3 n−well/p sensitivity across all photodiodes. ) + −2 p /n−well/p 2.5 0.64. By dividing the measured responses of the pixels by the / Wcm

−2 2 respective gains, the photodiode outputs were calculated for subsequent analysis. 1.5 A. Spectral sensitivity 1 The spectrofluorometer was programmed to step the wave- Sensitivity (Acm 0.5 length of light from 400 to 860 nm in steps of 5 nm. Incident light was measured for each of the wavelengths. The 0 slope of the pixel output (for the 3T APS) and the photocurrent 400 500 600 700 800 900 λ (nm) (for the CTIA APS) was normalized by the incident irradiance −2 (b) (Wm ) and the photodiode area. At this point, we would like to explain the rationale behind normalizing by the irradiance Fig. 4. Comparison of the spectral sensitivities of the three photodiodes. (a) and not by the radiant flux (W) on the pixel which can be ob- compares the photodiode capacitance dependent sensitivity measured from the 3T APS and (b) compares circuit-independent sensitivities of the three tained by multiplying the irradiance and the pixel area. Using photodiodes measured from the CTIA APS. the radiant flux assumes that none of the collected EHP were generated beyond the perimeter of the pixel. Since electron and hole diffusion lengths are larger than our pixel dimensions, kind. each photodiode collects from an unknown area larger than Prior to photodiode comparison, the readout paths of the itself. Thus, one can not calculate the optical power (W) which pixels were characterized. While the simplified CTIA pixel is responsible for the current through one photodiode. While shown in Fig. 2(e) is self-sufficient, the 3T APS shown in one can measure the response of a photodiode by optically Fig. 2(a) requires a current sink to be attached to the output confining the illumination to only one pixel, that is not the node for the source follower to work. Also, while the 3T APS normal mode of operation of imager arrays. Therefore, we output decreases as the photogenerated EHP are collected, used the irradiance as a measure of the incident power. the CTIA output increases due to the inverting nature of the Fig. 4(a) shows the spectral sensitivities for all three pho- amplifier. Due to these differences in the APS circuits, all todiodes using the three transistor APS. This data is repre- CTIA pixels shared one readout circuit and all 3T pixels shared sentative of the pixels and not of the photodiodes due to the a different one. photodiode capacitance figuring in the output (Eq. 8). The In order to measure the dc gains of the two different higher sensitivity of the n-well/p-sub diode over the n+/p readout paths, the output nodes of the pixels were driven by a diode is due to a combination of better quantum and collection triangular wave generated using a function generator [16]. The efficiencies and smaller capacitance. While the response of the outputs of the pixels were acquired using the data acquisition p+/n-well/p-sub diode was expected to be better than that of card. For the CTIA pixels, the readout path consisted only the n+/p-sub diode, this is not the case, probably due to the of a large PMOS transistor configured as a common source increased photodiode capacitance. At short wavelengths, the amplifier, buffering the output. The measured gain was 0.82. p+/n-well/p-sub sensitivity is actually lower, possibly due to The 3T APS had an NMOS transistor of the same size as a the p+ implant shielding some of the blue photons. voltage buffer, but also had an in-pixel source follower that Fig. 4(b) shows the same data for the 10 fF CTIA APS. contributed additional gain (Eq. 8). The measured gain was For this pixel, since the integrating capacitance is known, it 7

4 3.5 50 n+/p n+/p 3T APS 3.5 n−well/p 3 n−well/p 5 fF CTIA APS 10 fF CTIA APS p+/n−well/p p+/n−well/p 40 3 2.5

2.5 30 2 2 1.5 1.5 20 Normalized units Normalized units Normalized units 1 1 10 0.5 0.5

0 0 0 Sensitivity Noise SNR Sensitivity Noise SNR Sensitivity Noise SNR (a) (b) (c)

Fig. 6. Comparison of sensitivity, noise and SNR averaged over all wavelengths for all photodiodes using data from (a) photodiode capacitance dependent 3T APS and (b) circuit-independent CTIA APS. (c) compares the same metrics across different APS topologies using an n-well/p-sub photodiode. is possible to calculate the photocurrent from the output. The photodiodes, the average sensitivity and noise of each structure sensitivity is calculated as the photocurrent per unit area of the were calculated over the entire wavelength range. Fig. 6(a) photodiode for a given irradiance. A similar calculation was shows this data for the three transistor pixel. Sensitivity, made for the 5 fF CTIA pixel. The data were within 5% of noise and SNR were separately normalized by the respec- the 10 fF CTIA pixel for all wavelengths and are not shown. tive measurements for the n+/p photodiode. Again, these The CTIA pixels allows a true comparison of photodiode measurements are representative of the pixel and not of the sensitivity, irrespective of the APS design. The improvement photodiode. As expected from the spectral sensitivity data, in sensitivity of the n-well/p-sub diode over the n+/p diode the n-well/p-sub photodiode performs the best due to higher is now totally due to the increase in quantum and collection quantum and collection efficiencies and a small capacitance. efficiencies. Since the effect of the photodiode capacitance is While, it also has the maximum noise, possibly due to a eliminated, the p+/n-well/p-sub diode can be seen to be more larger photocurrent and smaller capacitance, the SNR for the sensitive than the n+/p-sub diode. n-well/p-sub diode is still the highest. The p+/n-well/p-sub Another point to note from the data is the different rates and n+/p perform almost similar because while the former of changes of the sensitivities as a function of the wavelength has better collection efficiency, it also has a larger capacitance of the incident light for different photodiodes. Fig. 5 shows offsetting the advantage. this trend by plotting the normalized ‘incremental’ sensitivity for the three photodiodes using data from the CTIA APS. To perform circuit-independent photodiode comparison, Incremental sensitivity (Si) was defined as the derivative of data were used from the 10 fF CTIA pixel to compute the the sensitivity (S) with respect to the wavelength (λ). The data same metrics. This is shown in Fig. 6(b). While the n-well/p- + were normalized as Si = [∂S/∂λ] / [max(abs(∂S/∂λ))] sub diode still outperforms the other two, the p /n-well/p- N + where a single maximum was calculated for the normalization sub diode is clearly more sensitive than the n /p. The same across all three photodiodes. metrics were also calculated from the data for the 5 fF CTIA It can be seen from the data that the dependence of the pixel. The results were within 5% of the above and are not sensitivity on the wavelength is not the same across all the shown. photodiodes. If that were the case, one would expect to see i i i Fig. 6(c) shows the normalized sensitivity, noise and SNR similar trends in S and S i.e. |Sn−well/p| > |Sp+/n−well/p| > i statistics for the three different pixel structures - the 3T APS |S + | where |.| denotes the absolute value. Although this n /p and the CTIA APS with C set to 5 and 10 fF. Since was generally true, as the light wavelength was changed fb i i photodiode capacitance was not known for the 3T APS, sensi- from 400 nm to 580 nm, |S + | > |S |. This p /n−well/p n−well/p tivities for all APS were calculated as the slope of the voltage indicates that the sensitivity of the + -well -sub diode is p /n /p output. All three pixels used n-well/p-sub photodiodes. As increasing faster than that of the n-well/p-sub diode. This can be seen, the effect of the photodiode capacitance plays a supports our conjecture that the + implant shields some of the p very important role in the pixel output. For an n-well/p-sub short wavelength light incident on the pixel. As the wavelength photodiode of the relevant geometry, C >> 10 fF. Thus, and thereby the penetration depth increases, the structure PD the response of the 3T APS which depends on CPD is much recovers from the aforementioned disadvantage, leading to a lower than that of the CTIA APS. Between the two CTIA higher rate of increase. The incremental sensitivity data from pixels, the ratio of the sensitivities is related to the ratio of the the 3T APS is not as instructive, due to the confounding factor feedback capacitances and is approximately equal to 2. One of the photodiode capacitance and is not shown. would expect the noise of the CTIA APS to be higher since the sensitivity is higher. However, the CTIA pixels employ an B. Sensitivity, Noise and SNR active reset that attenuates the reset noise which is a major To obtain a relative comparison of the sensitivity, noise component of the total noise. An analysis of the noise [26], and signal-to-noise ratio (SNR) of the different pixels and [27] is beyond the scope of this paper. 8

TABLE II PHOTODIODE DARK CURRENT COMPARISON One question not addressed in this work is the effect of scaling down the pixels. In this comparison, the pixel size was Dark current DTOP a relative large 30 m in 0.5 m technology. This allowed Photodiode type 2 2 (nA/cm ) (nW/cm ) comparable fill factors for all three kinds of diodes, even n+/p-sub 96.2 0.14 n-well/p-sub 363.4 0.15 though the design rules vary considerably. This might not p+/n-well/p-sub 90.3 0.05 be the case in a high density array where a small pixel size neccesitates a small photodiode. A comparison across several technologies would also be very useful. C. Dark current Nevertheless, to the best of our knowledge, this is the first report of a thorough comparison of photodiodes in a Table II compares the dark currents for the photodiodes. standard n-well CMOS process. As outlined earlier, different Since all the parameters in Eq. 9 are known, the dark current measures of photodiode performance are interlinked. Often, can be calculated as current per unit area using the CTIA a tradeoff needs to be made between these parameters. As pixels. Dark current erodes the dynamic range and the low- an example, although the n-well/p-sub and p+/n-well/p-sub light sensitivity of a detector. However, a simple analysis photodiodes seem to be superior to the n+/p-sub photodiode, reveals that the ratio of the dark current to the sensitivity is design rules dictate them to occupy more area on silicon. Thus, a truer metric of photodiode performance. The ratio gives the if the application calls for very high resolution imaging and minimum optical power above which the photocurrent is larger illumination is not at a premium, the n+/p-sub photodiode than the dark current. Ideally, this quantity should be as small is the most suitable. On the other hand, fluorescence imaging as possible to allow low intensity and high dynamic range is a photon-starved process. Increasing incident light intensity imaging. We term this ratio the dark threshold optical power is not advisable since it causes rapid photobleaching of the (DTOP). Using photodiode sensitivity data of the CTIA pixels dyes. Thus, the photodiode of choice would be one with from Sec. V-B, Table II also reports the DTOP values for all high sensitivity and low dark current - the p+/n-well/p-sub the structures. Note that the argument regarding incident power photodiode. The aim of this work was to compare photodiodes measurement presented in Section V-A is applicable here as in a standard CMOS process, thereby quantifying the tradeoffs well, leading to DTOP values being measures of irradiance in the different structures. In closing, we hope this work to be rather than radiant flux. useful to CMOS designers, looking to choose the The deeper junction in the n-well/p-sub diode probably most appropriate photodiode based on the target application. accounts for increase of dark current over the n+/p diode because virtually all dark EHP generated above the junction are collected. As discussed in the motivation behind the p+/n- ACKNOWLEDGMENTS well/p-sub diode, the p+ protective layer leads to a reduction The authors wish to thank Philippe Pouliquen and Mohsen in the dark current over the n-well/p-sub by reducing the Mollazadeh for discussions on phototransduction and analog number of free interface states. This reduction is due to the circuit design. high free carrier concentration of the p+ layer that causes the interface states to be occupied and thereby unavailable for REFERENCES EHP generation. In terms of the DTOP, the improved quantum + [1] P. Noble, “Self-scanned silicon image detector arrays,” Electron Devices, and collection efficiencies of the p /n-well/p-sub diode over IEEE Transactions on, vol. 15, pp. 202–209, Apr 1968. + the n /p diode and its lower dark current compared to the [2] G. Weckler, “Operation of p-n junction in a photon flux n-well/p-sub diode give it the edge over the other two. integrating mode,” Solid-State Circuits, IEEE Journal of, vol. 2, pp. 65– 73, Sep 1967. [3] O. Yadid-Pecht, R. Ginosar, and Y. Shacham-Diamand, “A random VI. CONCLUSIONS access photodiode array for intelligent image capture,” Electron Devices, IEEE Transactions on, vol. 38, no. 8, pp. 1772–1780, 1991. We have presented a comparison of n+/p-sub, n-well/p- [4] N. Ricquier and B. Dierickx, “Pixel structure with logarithmic response sub and p+/n-well/p-sub photodiodes fabricated in a 0.5 m for intelligent and flexible imager architectures,” Microelectron. Eng, vol. 19, no. 1-4, pp. 631–634, 1992. 3-metal, 2-poly, n-well CMOS process. The photodiodes were [5] E. Fossum, “Active pixel sensors: are CCDs dinosaurs?,” in Proceedings characterized using two APS designs - the three transistor of SPIE, vol. 2, SPIE, 1993. APS where the measurements depend on unknown photodiode [6] S. Kleinfelder, S. Lim, X. Liu, and A. El Gamal, “A 10000 frames/s CMOS digital pixel sensor,” Solid-State Circuits, IEEE Journal of, capacitances and a capacitive trans-impedance amplifier APS vol. 36, pp. 2049–2059, Dec 2001. where a designed capacitance allows the photocurrents to [7] G. Agranov, J. Ladd, T. Gilton, R. Mauritzson, U. Boettiger, X. Fan, be inferred. The measured trends were in agreement with and X. Li, “Small pixel development for novel CMOS image sensors,” in Proceedings of SPIE, vol. 7001, p. 700108, SPIE, 2008. theoretical predictions based on the physical characteristics of [8] R. Philipp and R. Etienne-Cummings, “A 128 x 128 33 mW 30 frames/s the photodiodes. In terms of sensitivity, the n-well/p-sub diode single-chip stereo imager,” Solid-State Circuits Conference, 2006. ISSCC performs the best, with an improvement of 3.5 and 1.6 times 2006. Digest of Technical Papers. IEEE International, pp. 2050–2059, + + 6-9, 2006. over the n /p-sub and p /n-well/p-sub diodes respectively. [9] Y. Chi, U. Mallik, M. Clapp, E. Choi, G. Cauwenberghs, and R. Etienne- The signal-to-noise ratio was also higher by a factor of 1.5 Cummings, “CMOS With In-Pixel Temporal Change Detection and 1.2 respectively. The p+/n-well/p-sub photodiode had and ADC,” Solid-State Circuits, IEEE Journal of, vol. 42, pp. 2187– 2196, Oct. 2007. 67% lower dark current per unit sensitivity compared to the [10] M. Bigas, E. Cabruja, J. Forest, and J. Salvi, “Review of CMOS image other two diodes. sensors,” Microelectronics Journal, vol. 37, no. 5, pp. 433–451, 2006. 9

[11] D. Litwiller, “CMOS vs. CCD: Maturing Technologies, Maturing Mar- Ralph Etienne-Cummings (S’02) received the kets,” Spectra, vol. 39, no. 8, pp. 54–58, 2005. B.Tech. degree in electrical engineering from the In- [12] P. Bhadri, P. Mal, S. Konanki, and F. Beyette, “Implementation of CMOS dian Institue of Technology, Madras, India, in 2002 photodetectors in optoelectronic circuits,” in Annual Meeting of the IEEE and the M.S. degree in biomedical engineering from Lasers and Electro-Optics Society, vol. 2, pp. 683–684, Nov. 2002. Johns Hopkins University, Baltimore, Maryland, in [13] F. Odiot, J. Bonnouvrier, C. Augier, J. Raynor, R. Central, S. Mi- 2004. He is currently working towards the Ph.D. croelectron, and F. Crolles, “Test structures for quantum efficiency degree in biomedical engineering at Johns Hopkins characterization for silicon image sensors,” in Microelectronic Test School of Medicine. His research interests lie in Structures, 2003. International Conference on, pp. 29–33, 2003. mixed-signal VLSI design for biomedical applica- [14] D.-Y. Li, V. Gaudet, and A. Basu, “Test results of various CMOS image tions in sensing, imaging and power management sensor pixels,” in Canadian Conference on Electrical and Computer and in neuromorphic systems. Engineering, pp. 2017–2020, May 2005. [15] B. Fowler, D. Yang, H. Min, and A. El Gamal, “Single Pixel Test Structures for Characterization and Comparative Analysis of CMOS Image Sensors,” in Proceedings of IEEE Workshop on Charge Coupled Devices, 1997. [16] H. Tian, X. Liu, S. Lim, S. Kleinfelder, A. El Gamal, et al., “Active Pixel Sensors Fabricated in a Standard 0.18 µm CMOS Technology,” in Proc. SPIE, vol. 4306, pp. 441–449, 2001. [17] O. Yadid-Pecht and R. Etienne-Cummings, CMOS imagers: from photo- transduction to image processi. Kluwer Academic Publishers Norwell, MA, USA, 2004. [18] B. G. Streetman and S. Banerjee, Solid State Electronic Devices. New Jersey: Prentice Hall International, Inc, 1999. [19] S. Sze, “Physics of semiconductor devices,” New York, Wiley- Interscience, 1981. 878 p., 1981. Nitish Thakor (S’78-M’81-SM’89-F’97) is a Pro- [20] R. Baker, CMOS Circuit Design, Layout, and Simulation. Wiley-IEEE fessor of Biomedical Engineering and the Director Press, 2004. of the Laboratory for Medical Instrumentation and [21] N. S. Saks, “A technique for suppressing dark current generated by Neuroengineering at Johns Hopkins University. He interface states in buried channel CCD imagers,” Electron Device is also the Director of the Neuroengineering Training Letters, IEEE, vol. 1, no. 7, pp. 131–133, 1980. Program at Johns Hopkins. His technical expertise [22] B. C. Burkey, W. C. Chang, J. Littlehale, T. H. Lee, T. J. Tredwell, is in the areas of neural diagnostic instrumenta- J. P. Lavine, and E. A. Trabka, “The pinned photodiode for an interline- tion, neural signal processing, optical imaging of transfer CCD image sensor,” in 1984 International Electron Devices the nervous system, and neural prosthesis. He has Meeting, vol. 30, 1984. published more than 180 refereed journal papers and [23] P. Lee, R. Gee, M. Guidash, T. Lee, and E. R. Fossum, “An active generated 6 patents. He is the Editor in Chief of pixel sensor fabricated using CMOS/CCD process technology,” in 1995 IEEE Transactions on Neural and Rehabilitation Engineering. Dr. Thakor is IEEE Workshop on CCDs and Advanced Image Sensors, Dana Point, a recipient of a Research Career Award from NIH and a Presidential Young CA, 1995. Investigator Award from the NSF, and is a Fellow of the AIMBE, IEEE and the [24] L. Kozlowski, D. Standley, J. Luo, A. Tomasini, A. Gallagher, R. Mann, Biomedical Engineering Society, and is a recipient of the Centennial Medal B. Hsieh, T. Liu, and W. Kleinhans, “Theoretical basis and experimental from the University of Wisconsin School of Engineering. confirmation: why a CMOS imager is superior to a CCD,” in Proceed- ings of SPIE, vol. 3698, p. 388, SPIE, 1999. [25] B. Fowler, J. Balicki, D. How, M. Godfrey, and P. Inc, “Low-FPN high- gain capacitive transimpedance amplifier for low-noise CMOS image sensors,” in Proceedings of SPIE, vol. 4306, p. 68, SPIE, 2001. [26] H. Tian, B. Fowler, and A. El Gamal, “Analysis of temporal noise in CMOS APS,” in Proceedings of SPIE, vol. 3649, p. 177, SPIE, 1999. [27] B. Fowler, M. Godfrey, and S. Mims, “Reset Noise Reduction in Capacitive Sensors,” Circuits and Systems I, IEEE Transactions on, vol. 53, no. 8, p. 1658, 2006.

Gert Cauwenberghs (S’02) received the B.Tech. degree in electrical engineering from the Indian In- stitue of Technology, Madras, India, in 2002 and the M.S. degree in biomedical engineering from Johns Hopkins University, Baltimore, Maryland, in 2004. Kartikeya Murari (S’02) received the B.Tech. de- He is currently working towards the Ph.D. degree gree in electrical engineering from the Indian In- in biomedical engineering at Johns Hopkins School stitue of Technology, Madras, India, in 2002 and the of Medicine. His research interests lie in mixed- M.S. degree in biomedical engineering from Johns signal VLSI design for biomedical applications in Hopkins University, Baltimore, Maryland, in 2004. sensing, imaging and power management and in He is currently working towards the Ph.D. degree neuromorphic systems. in biomedical engineering at Johns Hopkins School of Medicine. His research interests lie in mixed- signal VLSI design for biomedical applications in sensing, imaging and power management and in neuromorphic systems.

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