I-V and noise performance in MWIR to VLWIR large area Hg 1-x Cd x Te photodiodes

Abstract

The National Polar-orbiting Operational Environmental Satellite System (NPOESS), is overseen by the Integrated Program Office (IPO), a joint effort of the Department of Defense, Department of Commerce and NASA. One of the instruments on the NPOESS satellite is the Cross-track Infrared Sounder (CrIS) instrument. CrIS is a Fourier Transform interferometric infrared (FTIR) sensor used to measure earth radiance at high spectral resolution to derive pressure, temperature, and moisture profiles of the atmosphere from the ground on up. Each CrIS instrument contains three different cutoff wavelength (λ_c)focal plane modules (FPMs): an SWIR FPM [λ_c(98 K) ~ 5 mm], MWIR FPM [λ_c(98 K) ~ 9 mm] and a LWIR FPM [λ_c(81 K) ~ 15.5 mm]. There are nine large (850 mm diameter) photodiodes per FPM, the nine detectors being arranged in a 3 x 3 array. The nine detectors are placed under tight tolerances in the X, Y, and Z dimensions. The steps involved in the transfer of photodiodes as part of a newly fabricated wafer to the mounting of the photodiodes on the FPM involves many processing steps including a significant amount of dicing, cleaning, wire bonding and baking at elevated temperatures. Quantum efficiency and 1/f noise in Hg_1-xCd_xTe photodiodes are critical parameters that limit the sensitivity of infrared sounders. The ratio α, defined as the noise current in unit bandwidth i_n(f = 1 Hz, V_d, Δf = 1 Hz) to the dark current I_d(V_d), that is, α = i_n/I_d is one of the parameters used to select photodiodes for placement in FPMs. α is equivalent to √α_H/N that appears in the well-known Hooge expression. For the sixty-one, λ_c ~ 9 μm photodiodes measured at 60 mV reverse bias and at 98 K, the average value of α_dark = 1.3 x 10^-4 in the dark and α_PHOTO = i_n/I_PHOTO is ~ 2 x 10^-6 under illuminated conditions. These values of α are a factor of two lower than that reported previously. The λ_c ~ 15.5 μm photodiodes have average α_dark = 1.3 x 10^-5 with the highest performance, diffusion current limited photodiodes having values of α_dark in the mid 10^-6 range. All of the 850 μm diameter, λ_c ~ 15.5 μm photodiodes measured have excess low frequency noise, with the best performers having i_n(f = 100 Hz, V_d =-60 mV , Δf = 1 Hz) ~ 2 x 10^-11 A/Hz^1/2 and the best photodiode α_dark = 3.92 x 10^-6. I-V measurements, noise, and visual inspections are performed at several steps in the photodiodes manufacturing process. It was observed, following FPM fabrication, photodiode dark current and noise had increased from the initial pre-mounting leadless chip carrier (LCC) measurements for some of the nine photodiodes. The performance degradation observed led to an investigation into the cause (baking at elevated temperatures, mechanical handling, electrical stress etc.) of photodiode degradation that occurred between LCC and FPM testing. Correlations between I-V, noise and surface visual defects have been performed on some λ_c ~ 15.5 mm photodiodes. This paper outlines the results of the study, correlating the electrical performance observed to visual defects on the surface and to defects seen following cross sectioning of degraded photodiodes. In addition, other lessons learned and the corrective actions implemented that led to the successful manufacture of SWIR, MWIR and LWIR large photodiodes from the material growth to insertion into and successful demonstration of flight FPMs for the CrIS program are described.