Status of HgCdTe Bicolor and Dual-Band Infrared Focal Arrays at LETI

Abstract

In this paper we show the latest achievements of HgCdTe-based infrared bispectral focal plane arrays (FPAs) at LETI infrared laboratory. We present and compare the two different pixel architectures that are studied now in our laboratory, named “NPN” and “pseudo-planar”. With these two technologies, a wide range of system applications in dual-band detection can be covered. Advantages of both architectures will be pointed out. We also review performances obtained with these different architectures. The first one has been studied for several years in our laboratory, and we review results obtained on FPAs of size 256 × 256 pixels on a 25 μm pitch, in the MWIR/MWIR (3 μm/5 μm) range. Very high noise equivalent temperature difference (NETD) operability is obtained, at 99.8% for the λc = 3 μm band and 98.7% for the λc = 5 μm band. The second one has been developed more recently, to address other applications that need temporal coherence as well as spatial coherence. We show detailed performances measured on pseudo-planar type FPAs of size 256 × 256 pixels on a 30 μm pitch, in the MWIR/LWIR (5 μm/9 μm) range. The results are also very promising for these prototypes, with NETD as low as 15 mK for an integration time as short as 1 ms, and good operability. The main manufacturing issues are also presented and discussed for both pixel architectures. Challenging process steps are, firstly, molecular beam epitaxy (MBE) HgCdTe heterostructure growth, on large substrates (cadmium zinc telluride) and heterosubstrates (germanium), and, secondly, detector array fabrication on a nonplanar surface. In particular, trenches or hole etching steps, photolithography and hybridization are crucial to improve uniformity, number of defects and performances. Some results of surface, structural and electrical characterizations are shown to illustrate these issues. On the basis of these results, the short-term and long-term objectives and trends for our research and development are presented, in terms of pixel pitch reduction, wavelengths, and dual-band FPA size.