The post correction uniformity performance of infrared 2D starring arrays is a key factor to ensure the best IR image quality at the camera level. SOFRADIR has conducted several studies to improve both the post correction uniformity performance and the correction tables stability over the time periods, the readout integrated circuit configuration and the environmental conditions. Indeed, works have been performed on the homogeneity technology deposits, on the improvement of the readout circuit linearity and on the optimization of the dewar design to reduce the parasitical fluxes. Thanks to these improvements, Sofradir offers to its customers high level post correction uniformity performances as well as excellent correction tables stability for the mid wave and long wave infrared band. Thus, the calibrations constraints are reduced at the camera level and the image quality is optimized over a large camera utilization conditions.
HgCdTe (Mercury Cadmium Telluride / MCT) staring arrays for infrared detection do show constant improvements regarding their compactness and performances. New detectors are now proposed offering system solutions in the different IR wavebands and profiting of the latest technology improvements as well as MCT performance advantages and cost reduction. As a matter of fact, the size of MCT wafer has grown to 4", the pixel pitch was lowered to 15μm while maintaining outstanding results on the Focal Plane Arrays (FPA) uniformity. New functions as the Analog to Digital Conversion (ADC) are added to read-out circuits.
Results are presented concerning Non Uniformity Corrections (NUC) stabilities for two Sofradir products. Then results from developments of integrated ADC are addressed and finally, the Jupiter 1280x1024 mid-wave (MWIR) MCT detector performance results are presented.
The possibility to grow HgCdTe by Molecular Beam Epitaxy (MBE) on large alternative substrates opens
the way of increasing the size and reducing the cost of infrared FPAs operating in the Medium Wave
InfraRed (MWIR) bands. Germanium was chosen several years ago at Leti because its 'in situ' and 'ex
situ' surface preparations are much easier to control compared to the more conventionally used silicon
alternative substrate. Moreover extremely high quality germanium "epiready" substrates are commercially
available at a reasonable cost for wafer sizes up to 8 inches. MWIR HgCdTe wafers grown today by Defir
(LETI/Sofradir joint laboratory) on germanium (up to 4 inches diameter) using MBE, exhibit electrical and
physical properties that enables the fabrication of FPAs with various sizes (320×256, 640×512, 1280×1024)
and pitches (from 30μm to 15μm) with electro-optical performances similar to the standard process based
on the more conventional epilayers of HgCdTe grown on CdZnTe by Liquid Phase Epitaxy (LPE). Due to
the low microscopic and macroscopic defect density that can be obtained on such wafers, operabilities
above 99.9% are reached today.
A status of this MBE growth technology is presented as well as the FPAs performances, including
conventional industrial products manufactured such as 320×256 (pitch 30μm), 640×512 (pitch 15μm) and
the largest 1280×1024 (pitch of 15μm) more recently available.
As far as high performance IR detector are concerned, MWIR cooled 2D arrays are more and more used for high quality systems which need the uniformity and quality of the response in order to offer a good non uniformity corrections (NUC) and a stable IR detector calibrations. The aim of this paper is to demonstrate the state of the art of Sofradir's detectors regarding the NUC.
We will start by presenting all the present performances (Residual Fixed Pattern Noise, two or three points corrections, linearity, stability of the correction versus the time and versus ageing, stability with the focal plane temperature, fast cool down applications, etc.).
We will then make a comparison with other 2d arrays' performances, using the same methods of calculation as presented in the datasheet.
Finally, we will present a perspective about the advanced studies carried on at Sofradir to increase the performances in this critical area in terms of technological progress or calculation method.