Vertical Overflow Drain Research Articles

Development of a ToF Pixel With VOD Shutter Mechanism, High IR QE, Four Storages, and CDS

In this paper we discuss the development of an indirect time-of-flight (ToF) pixel in the 0.11- $\mu \text{m}$ CMOS image sensor technology. The pixel design is based on a pinned-photodiode structure with a novel vertical overflow drain (VOD) shutter mechanism used for fast modulation. We present the second generation of the pixel, with a greatly improved VOD structure that enables a fast shutter efficiency better than 1:100 and a deeper photodiode collection depth for better quantum efficiency in the near-infrared wavelengths. We present a new 6.7- $\mu \text{m}$ pixel design with four pinned storage diodes (SDs) that feature in-pixel complete charge transfer and enable correlated-double-sampling readout as well as an almost simultaneous global shutter exposure of up to four interleaved frames to be used for the scene depth computation. The novel design features a low readout noise of 7.5e- and a full-well-capacity of 9500e- per SD (a total of 38 000e- per pixel).

A Fast-Gated CMOS Image Sensor With a Vertical Overflow Drain Shutter Mechanism

This paper presents a fast-gated CMOS image sensor (CIS) with a vertical overflow drain (VOD) shutter mechanism. The prototype imager includes two novel features: 1) the adaptation of the VOD shutter structure into a 0.18- $\mu \text{m}$ CIS process and 2) the application of the VOD shutter for the purposes of time-resolved imaging with down to 5-ns pulsewidth. A 360-pixel $\times $ 180-pixel array with several 5.4- $\mu \text{m} \times 5.4$ - $\mu \text{m}$ pixel types was implemented and tested, demonstrating 2-ns shutter rise/fall times and the 1:20 shutter contrast ratio for 850-nm pulsed illumination. These parameters, together with the uniformity of the shutter and the large full-well capacity of the pixel, are on par with the state-of-the-art of indirect time-of-flight and time-resolved imagers. The device structure and the special mode of operation that enables a fast gating are studied through the TCAD simulations and experimental results. Important design features that affect the pixel performance are illustrated in detail.

Effect and Limitation of Pinned Photodiode

The pinned photodiode (PPD) is the primary technology for image sensors and used in almost all charge-coupled device image sensors and CMOS image sensors. This paper discusses the effect and limitation of PPD, especially dark current and electronic shuttering. Even when the PPD is used and silicon surface is neutralized, the proposed model explains that generation–recombination (GR) centers at the silicon surface contribute the dark current. The temperature dependence is an activation type with activation energy, $E_{g}$ , not $E_{g}/2$ . The model sheds light on the importance of GR center reduction for dark current decrease even at PPD. It is also noted that the vertical overflow drain shutter combined with the PPD has potential of high-speed shuttering with small skew.

Dynamic Multispectral Imaging Using the Vertical Overflow Drain Structure

Multispectral imaging enables discrimination of spectra beyond the 3-D spectral space of human vision. Most multispectral systems are complex and bulky, and simpler monolithic designs usually require several pixels in order to generate one multispectral data point. Here, we use the vertical overflow drain (VOD) structure to enable monolithic multispectral imaging in a single pixel. We show that by controlling the substrate bias voltage it is possible to change the effective depth of the photodiode, thus enabling dynamic tuning of the pixel's spectral response. A small voltage change (<;5 V) is shown experimentally to reduce the red light quantum efficiency by ~ 40 %, while the blue light quantum efficiency is reduced by <;10%. Using this effect, we demonstrate an ability to discriminate between different monochromatic illumination sources with 20-nm spectral resolution in a single pixel. In addition, we present an RGB image taken using an off-the-shelf charge coupled devices with no color filters by relying solely on the VOD mechanism. Finally, we present process and device simulations suggesting that this mechanism can be implemented in any pinned photodiode pixel with a VOD barrier, including in CMOS image sensors fabricated on n-type starting material.

An Improved CCD Register Driven from Overflow Drain through Barrier.

Conventional CCD registers for full frame or frame transfer scheme image sensors have disadvantages such as low light sensitivity due to light absorption in electrodes, high dark current generated at Si-Si02 interfaces, and small charge handling capability that results from the surface pinning mode of operation. To solve these problems, we proposed CCD register driven through a barrier in February 2000. The register cell was an inverted version of the photo-diode with a vertical overflow drain that is used as the driving electrode. In simulating the register's characteristics in back illumination mode, we found that all the above-disadvantages disappeared, but the characteristics were rather sensitive to the height, width, and location of SiO2 film required to isolate electrodes. Therefore, some difficulty remains in fabrication processes. This paper proposes a new version of DTB-CCD where a specific-feature SiO2 layer covers a part of the barrier surface. This layer not only helps to minimize the structure sensitiveness of performance but also makes it possible to employ the conventional overlapping poly-Silicon electrode process. A thin SiO2 layer between electrodes and optimized width and thickness of the Si02 layer on the barrier minimizes potential barrier height in channels and thus improve transfer efficiency. The simulated charge handling capability of the improved DTB-CCD was 3-4 times greater than that of a two-phase CCD register driven by surface pinning mode.

Technologies to improve photo-sensitivity and reduce VOD shutter voltage for CCD image sensors

New technologies to increase the photo-sensitivity and reduce the shutter voltage of the vertical over-flow-drain (VOD) have been developed for CCD image sensors. The photo-sensitivity was increased 40% by forming an anti-reflection film over the photodiode and reducing the thickness of the p/sup +/-layer formed at the photodiode surface. The VOD shutter voltage was reduced from 31 to 18 V by using an epitaxially grown substrate with double impurity concentration layers.

A Nonlinearity Consideration on Photoelectric Conversion Characteristics in CCD Image Sensor

The photodiodes with a vertical overflow drain are generally used in CCD image sensors. During the charge storage time, the minimum potential point in the barrier region moves toward the photo-diode surface. Almost incident light of short wavelength is absorbed within the surface side shallower than the minimum potential point. Therefore, nonlinearity between the incident light intensity and the output signal does not occur. However, for long wavelength light, since considerable light absorption takes place around the minimum potential point, the nonlinearity arises. The measured nonlinearity for the red incident light was about 5%. The nonlinearity obtained by the calculation coincided qualitatively with that of the measurements. Also, the calculation made clear that the wider barrier width, the low barrier impurity concentration, the low substrate impurity concentration, narrow storage region width, and low substrate bias enhance the nonlinearity.

固体撮像とその関連技術 ＣＣＤイメージセンサにおける基板不純物密度揺らぎの測定

It is well known that the saturation signal level of a CCD image sensor including photo diode with vertical overflow drain (VOD) varies strongly with substrate impurity concentration. However, analytical results between the saturation signal level and substrate impurity concentration have not been obtained yet. In this paper, the relation between the saturation signal level and substrate impurity concentration was calculated using the drift-diffusion model as a function of photo diode structures and operation conditions.By comparing analytical and experimental results obtained from CCD image sensor chips, we found that the impurity fluctuations at the peripherals of CZ, MCZ, and epitaxial wafers were +11%, ±16%, and ±6%, respectively. Those at the center were ±7%, ±9%, and ±6%, respectively. The distinctive features of the impurity fluctuation pattern for these wafers were therefore clarified. The minimum impurity fluctuation level that can be detected is almost ±0.1%.

ＶＯＤ付きホトダイオードにＮ｀＋´側壁を設けることによる蓄積電荷量増加法の提案

We propose a new photodiode structure to increase the charge handling capability of solid-state image sensors. This photodiode includes an N+ sidewall that surrounds the charge storage region of a conventional diode with a vertical overflow drain structure. The operation and performance of the diode were analyzed using three-dimensional numerical analyses. We found that an abrupt potential increase, produced around the boundary of the channel stop and the N+ sidewall, create a flat potential profile that spreads all over the storage region, at the beginning of the charge storage. This profile suppresses electron overflow into the substrate that occurs due to small number of signal electrons concentrated around the center of the storage region. Therefore, the N+ sidewall increases the storable charge. For example, the charge handling capability of a conventional 3.0-μm diameter photo diode was improved by more than a factor four by adding the N+ sidewall. The validity of the analyses was confirmed from the results of one-dimensional analyses on the relationship between incident light intensity and stored charge and on the potential barrier height at signal charge saturation.

Photo response analysis in CCD image sensors with a VOD structure

Photo response in CCD image sensors with Vertical-Overflow-Drain (VOD) was analyzed in an attempt to discover a way to lessen the photo response rise that accompanies increasing incident light intensity in the saturation region. A photo response analysis based on transistor I-V characteristics revealed that the extent of rise in the saturation region is uniquely determined by the non-ideality factor and temperature. Calculation of the non-ideality factor and its dependence on P-well impurity concentration and layer thickness further revealed that fabrication of P-wells with lower impurity concentrations and thicker layers would be effective in suppressing photo response rise.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of photoelectric conversion characteristics under knee control operation for CCD imagers

A high-light signal suppression operation, knee control operation, was evaluated in order to apply to a STACK-CCD of 2/3-in 2 M-pixel high definition television (HDTV) imager overlaid with an amorphous silicon photoconversion layer. A smooth bend was observed in the measured photoconversion characteristics near a knee point. It was analyzed theoretically by applying a subthreshold current of the MOSFET in weak inversion for skimming excess charges and reading out signal charges through a field shift gate. The calculated curve showed good agreement with the measured curve. Effectiveness of the knee control operation using the field shift gate was shown experimentally and theoretically over that using a conventional vertical overflow drain for IT-CCD imagers. >

The tacking CCD: a new CCD concept

The tacking CCD is a new type of charge transport mechanism that is suitable for junction- and MOS-type CCDs. A specific form, the trenched tacking CCD (TTCCD), promises high pixel density and high charge handling capability per unit of surface area. The charge handling capability is improved by using a trench to increase the charge storage area. With the new design concept it becomes possible to put the gates entirely into trenches, while simultaneously using the trenches as channel stops. The TTCCD structure is suitable for making new types of solid-state image sensors with increased light sensitivity, and it may be possible to incorporate a vertical overflow drain. First samples of the TTCCD have been realized, and its functionality has been confirmed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Photoresponse nonlinearity of solid-state image sensors with antiblooming protection

The physical mechanism for photoresponse nonlinearity in solid-state image sensors with antiblooming protection is described and analyzed. This mechanism is the premature turn-on of the antiblooming structure, and can be characterized by its nonideality factor. It is shown that a lower nonideality factor results in a more linear response. Electrostatic modeling results and measurements show that devices with low-overflow drain (LOD) antiblooming structures can achieve nonideality factors very close to unity and, therefore, offer superior photoresponse linearity. This is especially true for devices with large pixels having low charge capacity, since the maximum voltage swing across the detector is small. Conversely, devices with vertical-overflow drain (VOD) structures are seen to have nonideality factors at best around two and in some other cases even higher, resulting in severe response nonlinearity. >

A 1920(H)*1035(V) pixel high-definition CCD image sensor

A 1920(H)*1035(V)-pixel high-definition CCD (charge-coupled-device) image sensor compatible with an 1125-scanning-lines and 16:9 aspect-ratio television system is described. The device basically uses an interline scheme with a vertical overflow drain. To maintain 74.25-MHz ultrahigh-data-rate operation, the device adopts a dual-channel configuration for the horizontal CCD (H-CCD) register. To accomplish both vertical signal charge transfer in the vertical CCD (V-CCD) register and signal charge distribution from the V-CCD registers into the dual-channel horizontal CCD registers simultaneously within a 3.77- mu s short horizontal blanking period, a one-horizontal-period signal storage memory electrode and optical black memory are introduced. Bipolar buffer transistor chips are hybridized in the same package as the device, so as to reduce parasitic capacitance at CCD output terminals and maintain a wide-bandwidth operation. The device operates successfully and 1000-TV-line limiting resolution was obtained for both vertical and horizontal directions. Total random noise was evaluated to be 41 electrons. Dynamic range reached 66 dB. Signal-to-noise ratio for a black/white (B/W) camera was 51.5 dB under F4.0 and 2000-lux illumination conditions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

1/2-in 768(H)*492(V) pixel CCD image sensor

A sensor with unit cell dimensions as small as 8.4(H)*9.8(V) mu has been developed. In order to realize such a high-density device, the following technologies have been developed: (1) dual-channel horizontal CCD registers, (2) single p-well vertical overflow drain, (3) high-capacitance p-well structure, and (4) inverted-LOCOS channel isolation process. A horizontal resolution of more than 500 TV lines has been obtained. Maximum signal charge for the vertical CCD register is as much as 1.5*10/sup 5/ electrons, corresponding to 270-nA output current, in spite of a narrow-channel vertical CCD register with a mask width of only 1.8 mu m. The total random noise is less than 20 electrons after correlated double sampling. The dynamic range reaches 77 dB, and the photosensitivity is 0.092 mu A/ mu W. Experimental results also show that the technologies used here are effective for realizing a high-definition CCD image sensor for future use.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A 1/2-in. CCD image sensor overlaid with a hydrogenated amorphous silicon

A half-inch size CCD image sensor overlaid with a hydrogen-erated amorphous silicon (α-Si:H) as a photodetector has been developed. The array consists of 506V × 404H picture elements. The glow-discharged α-Si:H film has high quantum efficiency of 0.75-0.8 in the visible wavelength range and low dark current of 0.2 nA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and is formed on the CCD scanner with vertical overflow drain. This CCD image sensor has a sensitivity of 0.014 µA/lx (3200 K)and a S/N ratio of 73 and 68 dB for fixed-pattern noise and random noise, respectively. Smearing signal is suppressed to below 5 percent at incident light intensity of 1000 times saturation exposure. The blooming and highlight lag are completely suppressed by the vertical overflow drain structure.

A CCD image sensor with 768 × 490 pixels

A ⅔-in 768(H) × 490(V) element interline CCD image sensor has been successfully developed. The device adopts a vertical overflow drain principle, a buried,channel amplifier, three-level polysilicon technology, and 1.5-µm-rule fine-pattern process. The device operates with an NTSC format. The 560 TV lines limiting resolution is obtained in the horizontal direction. No significant loss in transfer efficiency is observed in the horizontal register, even at the 14.32 MHz clock rate. Optimal photosensitivity spectrum response is obtained and the peak response appears at 550 nm. The noise equivalent signal is reduced to 48 electrons, using correlated double sampling. Then, the dynamic range reaches 68 dB. The correlated double sampling, combined with buried-channel amplifier technology is found to be also effective for great reduction in horizontal line noise.

Interline CCD image sensor with an antiblooming structure

A ⅔-in 384 (H) × 490 (V) element interline CCD image sensor with a new antiblooming structure was developed. Blooming was suppressed without sacrificing photosensitivity and dynamic range by means of a vertical overflow drain positioned under (rather than beside) a photodiode. For 10-percent vertical height illumination the smear signal was reduced to 0.05 percent of the illumination signal. Well-balanced performance, namely, large dynamic range (72 dB), low random noise (65 rms noise electrons per charge packet), high-contrast transfer functions for horizontal and vertical directions, and a spectral response similar to the luminous efficiency curve were obtained under moderate operating conditions.