Eric G. Stevens

Low-Crosstalk and Low-Dark-Current CMOS Image-Sensor Technology Using a Hole-Based Detector

As the pixel size of CMOS image sensors (CIS) shrink, problems associated with crosstalk become more severe for devices built using mainstream CMOS processing. This high crosstalk increases the amount of noise added to the final image (via an increase of the off-diagonal terms in the color correction matrix (CCM)) and degrades the modulation transfer function (MTF). Reducing dark current has also been challenging for such CIS imagers. At present, the solution to these problems has been to switch to n-type substrates since they have been used for interline charge-coupled devices (CCDs) for decades.

An analytical, aperture, and two-layer carrier diffusion MTF and quantum efficiency model for solid-state image sensors

A two-dimensional analytical model is formulated for calculating the pixel response, modulation transfer function (MTF), and quantum efficiency of front-side illuminated, solid-state image sensors. Included in this unified model are the effects of lateral diffusion of charge carriers within a two-layer substrate and less than full pixel sampling apertures. The results of this model are compared to those of a numerical, three-dimensional Monte Carlo algorithm and to the analytical results reported by Blouke and Robinson. We find good agreement between the quantum efficiency and MTF calculated by the present model and by the three-dimensional Monte Carlo method. However, we find higher quantum efficiency and lower MTF than the previously reported analytical two-layer model. The unified aspect of the present model correctly combines the effects of sampling aperture and lateral diffusion. >

A 1-Megapixel, progressive-scan image sensor with antiblooming control and lag-free operation

A 1024-pixel*1024-pixel interline charge-coupled device (IL CCD) image sensor has been developed. It incorporates antiblooming and electronic exposure control while eliminating lag and obtaining a high responsivity. The novel features of this device include a noninterlaced, or progressive-scan, architecture and dual-horizontal registers that can be used to clock out the image area by one or two lines at a time. These features make it well suited for applications demanding high-resolution-image capture from a single, high-speed scan. The progressive-scan architecture of this device covers the same resolution in an electronic-camera application as that of a 2-million-element, interlaced device. >

A unified model of carrier diffusion and sampling aperture effects on MTF in solid-state image sensors

A unified model is used for calculating the modulation-transfer function (MTF) of a front-side-illuminated, solid-state image sensor including the effects of lateral diffusion of charge carriers within the substrate and less than full pixel sampling apertures. Moreover, it is found that, in general, these two effects cannot be treated independently and then combined, using the convolution theorem to arrive at the total sensor MTF as has often been done in the past. It is found that the actual MTF is higher than predicted by simply multiplying the diffusion and aperture MTFs together. This discrepancy is seen to increase as pixel-to-pixel crosstalk increases for a given aperture size due to an increase in diffusion length, a decrease in depletion depth, or a decrease in the substrates absorption (coefficient) and/or as the pixels aperture decreases with respect to the pixel size. >

A 1.4 million element, full frame CCD image sensor with vertical overflow drain for anti-blooming and low color crosstalk

Blooming and color crosstalk must be greatly suppressed in solid-state image sensors for nearly all imaging applications. A vertical overflow drain has been developed for a 1.4 megapixel image sensor for blooming suppression and low color crosstalk. The overflow drain is formed using a uniform flat p-well. This paper describes the modeling, fabrication, and experimental data associated with implementing vertical overflow in this device.

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 large area 1.3-megapixel full-frame CCD image sensor with a lateral-overflow drain and a transparent gate electrode

A large-area, 1.3 million pixel, full-frame CCD (charge coupled device) image sensor has been developed that incorporates both a lateral-overflow drain (LOD) for antiblooming control and a transparent indium-tin oxide (ITO) gate electrode for increased photosensitivity. The LOD offers high responsivity, extremely linear photoresponse, and ultrahigh optical overload protection. The replacement of one polysilicon phase with ITO increases the quantum efficiency at 400 nm to 15.8% from the 1.5% for the standard double polysilicon gate electrode process. The LOD design allows for antiblooming suppression in excess of 43000 times the saturation signal while maintaining better than 1% nonlinearity.<<ETX>>

Front-illuminated full-frame charge-coupled-device image sensor achieves 85% peak quantum efficiency

A high sensitivity front-illuminated charge-coupled device (CCD) technology has been developed by combining the transparent gate technology introduced by Kodak in 1999 with the microlens technology usually employed on interline CCDs. In this new architecture, the microlens is used to focus the incoming light onto the more transparent of the two electrodes. The new sensors offer significant increases in quantum efficiency while maintaining the performance advantages of front-illuminated full-frame CCDs including 3 pA/cm2 typical dark current at 25 degree(s)C, and 55 ke full well in a 6.8 micrometers pixel.

3.2-million-pixel full-frame true 2Φ CCD image sensor incorporating transparent gate technology

This paper describes the performance of an advanced high- resolution full-frame architecture CCD imaging device for use in scientific, medical and other high-performance monochromatic digital still imaging applications. Of particular interest is the replacement of the polysilicon 2nd gate electrode with that of a more spectrally transparent material thereby dramatically improving device sensitivity. This has been achieved without compromising performance in other areas such as dark current, noise, transfer efficiency and, most importantly, yield.

An LOD with improved breakdown voltage in full-frame CCD devices

In full-frame image sensors, lateral overflow drain (LOD) structures are typically formed along the vertical CCD shift registers to provide a means for preventing charge blooming in the imager pixels. In a conventional LOD structure, the n-type LOD implant is made through the thin gate dielectric stack in the device active area and adjacent to the thick field oxidation that isolates the vertical CCD columns of the imager. In this paper, a novel LOD structure is described in which the n-type LOD impurities are placed directly under the field oxidation and are, therefore, electrically isolated from the gate electrodes. By reducing the electrical fields that cause breakdown at the silicon surface, this new structure permits a larger amount of n-type impurities to be implanted for the purpose of increasing the LOD conductivity. As a consequence of the improved conductance, the LOD width can be significantly reduced, enabling the design of higher resolution imaging arrays without sacrificing charge capacity in the pixels. Numerical simulations with MEDICI of the LOD leakage current are presented that identify the breakdown mechanism, while three-dimensional solutions to Poissons equation are used to determine the charge capacity as a function of pixel dimension.