Cory Jung

Robust detection of defects in imaging arrays

As digital imagers continue to increase in size and pixel density, the detection of faults in the field becomes critical to delivering high quality output. Traditional schemes for defect detection utilize specialized hardware at the time of manufacture and are impractical for use in the field, while previously proposed software-based approaches tend to lead to quality-degrading false positive diagnoses. This paper presents an algorithm that utilizes statistical information extracted from a sequence of normally captured images to identify the location and type of defective pixels. Building on previous research, this algorithm utilizes data local to each pixel and Bayesian statistics to more accurately infer the likelihood of each defect, which successfully improves the detection time. Several defect types are considered, including pixels with one-half of the typical sensitivity and permanently stuck pixels. Monte Carlo simulations have shown that for defect densities of up to 0.5%, 50 ordinary images are sufficient to accurately identify all faults without falsely diagnosing good pixels as faulty. Testing also indicates that the algorithm can be extended to higher resolution imagers and to those with noisy stuck pixels, with only minimal cost to performance.

Noise analysis of fault tolerant active pixel sensors

As digital imagers grow in pixel count and area, the ability to correct for pixel defects becomes more important. A fault tolerant active pixel sensor (APS) has previously been designed and fabricated that can correct for stuck high and stuck low defects. Analyses of the pixel noise for a standard APS and a fault tolerant APS are presented that consider reset noise, photocurrent shot noise, dark current shot noise, transistor thermal noise, transistor flicker noise, operational amplifier noise, and feedback resistor thermal noise. Under worst case conditions (no illumination), the noise of the fault tolerant APS is 1.106 /spl times/ more than a standard APS. At a typical illumination level, the fault tolerant APS noise is nearly unchanged to that of a standard APS. Previous research has shown that the fault tolerant APS is more sensitive than a standard APS, thus the overall signal-to-noise ratio of the fault tolerant APS should be greater than the standard APS except under very low light conditions.

On-Line Mapping of In-Field Defects in Image Sensor Arrays

Continued increase in complexity of digital image sensors means that defects are more likely to develop in the field, but little concrete information is available on in-field defect growth. This paper presents an algorithm to help quantify the problem by identifying defects and potentially tracking defect growth. Building on previous research, this technique is extended to utilize a more realistic defect model suitable for analyzing real-world camera systems. Monte Carlo simulations show that abnormal sensitivity defects are successfully detected by analyzing only 40 typical photographs. Experimentation also indicates that this technique can be applied to imagers with up to 4% defect density, and that noisy images can be diagnosed successfully with only a small reduction in accuracy. Extension to colour imagers has been accomplished through independent analysis of image colour planes

Characteristics of fault-tolerant photodiode and photogate active pixel sensor (APS)

A fault-tolerant APS has been designed by splitting the APS pixel into two halves operating in parallel, where the photo sensing element has been divided in two and the readout transistors have been duplicated while maintaining a common row select transistor. This split design allows for a self correcting pixel scheme such that if one half of the pixel is faulty, the other half can be used to recover the entire output signal. The fault tolerant APS design has been implemented in a 0.18 /spl mu/m CMOS process for both a photodiode based and photogate based APS. Test results show that the fault tolerant pixels behave as expected where a non-faulty pixel behaves normally, and a half faulty pixel, where one half is either stuck low or high, produces roughly half the sensitivity. Preliminary results indicate that the sensitivity of a redundant pixel is approximately three times that of a traditional pixel for the photodiode APS and approximately twice that for the photogate APS.

Fault Tolerant Active Pixel Sensors in 0.18 and 0.35 Micron Technologies

A fault tolerant active pixel sensor (FTAPS) has been designed and fabricated to correct for point defects that occur in CMOS image sensors both at manufacturing and over the lifetime of the sensor. For some time it has been known that fabrication of CMOS image sensors in processes less than 0.35mum would generate significant performance changes, yet imagers are being fabricated in 0.18mum technology or smaller. Therefore the characteristics of the FTAPS are presented for pixels fabricated in both a standard 0.18mum and 0.35mum CMOS process and compared for consistency

On-line identification of faults in fault-tolerant imagers

Detection of defective pixels that develop on-line is a vital part of fault tolerant schemes for repairing imagers during operation. This paper presents a new algorithm for the identification of stuck low, stuck high and partially stuck pixels in both regular and fault tolerant APS systems. The algorithm does not require specialized illuminations but instead operates on a sequence of regular images and uses statistical information extracted from each image to decide the state of each pixel. Unlike previous techniques, simulations of this technique show that it can find all faulty pixels without misidentifying good pixels as faulty. Under typical conditions, the algorithm successfully converges on the correct result within 238 images for a fault tolerant APS, and 16 images for a regular APS. More extensive simulations have shown that these results can be extended to high-resolution sensors and complex defect models that include hot pixels, without a significant decline in performance.

Fault tolerant photodiode and photogate active pixel sensors

As the pixel counts of digital imagers increase, the challenge of maintaining high yields and ensuring reliability over an imager’s lifetime increases. A fault tolerant active pixel sensor (APS) has been designed to meet this need by splitting an APS in half and operating both halves in parallel. The fault tolerant APS will perform normally in the no defect case and will produce approximately half the output for single defects. Thus, the entire signal can be recovered by multiplying the output by two. Since pixels containing multiple defects are rare, this design can correct for most defects allowing for higher production yields. Fault tolerant photodiode and photogate APS’ were fabricated in 0.18-micron technology. Testing showed that the photodiode APS could correct for optically induced and electrically induced faults, within experimental error. The photogate APS was only tested for optically induced defects and also corrects for defects within experimental error. Further testing showed that the sensitivity of fault tolerant pixels was approximately 2-3 times more sensitive than the normal pixels. HSpice simulations of the fault tolerant APS circuit did not show increased sensitivity, however an equivalent normal APS circuit with twice width readout and row transistors was 1.90 times more sensitive than a normal pixel.

A Fault-Tolerant Active Pixel Sensor for Mitigating Hot Pixel Defects

Hot pixel defects are unavoidable in many solid-state image sensors. Affected pixels accumulate dark signal over the course of an exposure, grossly diminishing dynamic range and often rendering measurements unusable. Experiments suggest the mechanisms causing hot pixels are highly localized and the defect will be confined to a single pixel. A redundant, fault-tolerant active pixel sensor architecture that has previously been applied to other defect types is investigated for the suppression of hot pixels. A recovery scheme using minimal computational power is also described.

Noise analysis of fault tolerant active pixel sensors with and without defects

As the sizes of imaging arrays become larger both in pixel count and area, the possibility of pixel defects increases during manufacturing and packaging, and over the lifetime of the sensor. To correct for these possible pixel defects, a Fault Tolerant Active Pixel Sensor (FTAPS) with redundancy at the pixel level has been designed and fabricated with only a small cost in area. The noise of the standard Active Pixel Sensor (APS) and FTAPS, under normal operating conditions as well as under the presence of optically stuck high and low faults, is analyzed and compared. The analysis shows that under typical illumination conditions the total noise of both the standard APS and FTAPS is dominated by the photocurrent shot noise. In the worst case (no illumination) the total mean squared noise of the FTAPS is only 15.5% larger than for the standard APS, while under typical illumination conditions the FTAPS noise increases by less than 0.1%. In the presence of half stuck faults the noise of the FTAPS compared to the standard APS stays the same as for the FTAPS without defects. However, simulation and experimental results have shown that the FTAPS sensitivity is greater than two times that of the standard APS leading to an increased SNR by more than twice for the FTAPS with no defects. Moreover, the SNR of a faulty standard APS is zero whereas the SNR of the FTAPS is reduced by less than half.