MTF Characterization of Small Pixel Pitch IR Cooled Photodiodes Using EBIC

Abstract

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 μm 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.