Raytheon Vision Systems (RVS) has developed scanning, high-speed (<3klps), all digital, with on-chip Analog-to-Digital Conversion (ADC), mid-wave infrared (MWIR 3-5mm) focal plane arrays (FPA) with excellent modulation transfer function (MTF) performance. Using secondary ion mass spectrometry (SIMS) data and detailed models of the mesa geometry, RVS modeled the predicted detector MTF performance of detectors. These detectors have a mesa structure and geometry for improved MTF performance compared to planar HgCdTe and InSb detector structures and other similar detector structures such as nBn. The modeled data is compared to measured MTF data obtained from edge spread measurements and shows good agreement, Figure 1. The measured data was obtained using a custom advanced test set with 1µm precision alignment and automatic data acquisition for report generation in less than five minutes per FPA. The measured MTF values of 83 unique parts, Figure 2, had a standard deviation of 0.0094 and a mean absolute deviation of 0.0066 at half Nyquist frequency, showing excellent process repeatability and a design that supports high MTF with good repeatability.
In ultra-low light conditions the presence of dark current becomes a major source of noise for a CMOS sensor. Standard
dark current compensation techniques, such as using a dark reference frame, bring significant improvements to dark
noise in typical applications. However, applications requiring long integration times mean that such techniques cannot
always be used. This paper presents a differential dark current compensating pixel. The pixel is made up of a differential
amplifier and two photodiodes: one light shielded photodiode connected to the non-inverting input of the opamp and a
light detecting photodiode connected to the inverting input of the opamp. An integrating capacitor is used in the feedback
loop to convert photocurrent to voltage, and a switched capacitor network is present in parallel with the light shielded
pixel, which is used to satisfy the output equation to compensate the dark current. The pixel uses 150 μm x 150 μm
photodiodes and is fabricated in a standard 0.18 μm, 6M1P, CMOS process. The results show that the pixel is light
sensitive and has a linear output as expected. However, the dark current is not predictably controlled. Further work will
be carried out on the pixel design, and particularly the switched capacitor circuit, to determine the cause of the non-predictability
of the pixel output.