MTF Characterization of Small Pixel Pitch IR Cooled Photodiodes Using EBIC
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https://doi.org/10.1007/s11664-020-08253-0 Ó 2020 The Minerals, Metals & Materials Society
TOPICAL COLLECTION: U.S. WORKSHOP ON PHYSICS AND CHEMISTRY OF II-VI MATERIALS 2019
MTF Characterization of Small Pixel Pitch IR Cooled Photodiodes Using EBIC ` CHE,1 O. GRAVRAND ,1,2 A. FERRON,1 F. BOULARD,1 A. YE S. BISOTTO,1 F. ROCHETTE,1 and J. ABERGEL1 1.—CEA LETI, 17 rue des martyrs, 38054 Grenoble, France. 2.—e-mail: [email protected]
Experimental assessment of small pitch cooled infrared focal plane arrays (FPA) modulation transfer function (MTF) is becoming an important issue. Indeed, the pitch approaches the typical carrier diffusion length of minority carriers in the absorber material. Therefore, the MTF is an important figure of merit of those arrays as it may be degraded by lateral diffusion. Moreover, the pitch also approaches the sensed wavelength so direct MTF measurement using optical projections becomes difficult. In this paper, we propose the use of electron beam induced current to experimentally characterize the MTF of small pitch cooled FPAs. Practically, the device is mounted inside a scanning electron microscope onto a cooled sample stage. The diode area is then scanned by the electron beam instead of the optical beam in classical MTF measurement. Because of the very narrow electron beam, the MTF can then be estimated with an excellent precision. First scans of mid-wave 7.5 lm pitch HgCdTe diodes are shown, demonstrating a 55% MTF at the Nyquist frequency, consistent with 3D electro-optical modeling. The relevance of this estimation is also discussed, in comparison with classical projection methods. Key words: IR FPA, MTF measurement, EBIC, small pitch, MCT
INTRODUCTION Infrared (IR) detector developments are strongly pushing toward small pixel pitches. Today’s standard is 15 lm, moving to 10 lm1,2 at Lynred (formerly Sofradir) and other detector manufacturers. CEA-LETI has recently demonstrated 7.5 lm pitch arrays in mid-wave (MW) and test arrays at 5 lm pitches.3 In a way, infrared image resolution requirement is driven by the incredibly high resolution of visible imagers and displays available for mass market applications such as smartphone cameras. Megapixel formats are, therefore, seen as a standard minimum format. To produce such large formats at a reasonable cost, high pixel density (i.e. small pixel pitches) are mandatory, together with
(Received February 3, 2020; accepted June 1, 2020)
large wafer formats. However, this increase in resolution need is also driven by more rational considerations. Indeed, the increase in range for identification and recognition might be substantial for tactical applications.4 This latter reference demonstrates an ultimate range for very small pixel pitches (regarding the wavelength), down to 3 lm for MW. However, this optimum range is subject to various conditions. First, the shrink to lower pixel pitch must be done maintaining diodes first order figures of merit (FOM): quantum efficiency (QE) must remain high, while diode output noise must remain backg
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