Optical detector in standard CMOS SOI

Standard Complementary Metal Oxide Semiconductor (CMOS) Silicon on Insulator (SOI) technologies are not very well suited to absorb electromagnetic radiation because silicon has an absorption length of several hundred micrometers at the interesting wavelengths and the absorbing layers are in the range of several ten nanometers. Therefore, special techniques are required to enhance the absorption in the active silicon layer. While some groups utilize Germanium, which is also available in some standard SOI technologies, we use geometric resonant structures to maximize the optical path length and utilize resonances.

Simulations of the a two-dimensional resonant grating show an absorption of close to seventy percent compared to six percent without a resonant structure in the same layer configuration. This means an increase of a factor eleven by using resonant gratings in theory.

Evaluations have shown that the metal fill which is used in the process to secure planar surfaces and prevent out-of-focus conditions during mask processing shields the active region of the detector from incident radiation. Moreover, we assume that a Silicide layer covers the active area and prevents radiation to reach the absorbing layer. Results show that the peak of absorption exists but is slightly shifted.

A minimum rise time of 8.5 picoseconds has been measured for two-dimensional gratings, about three times higher than one-dimensional gratings. At maximum responsitivity the rise time is 42 picoseconds.

[1] N. Moll, T. Morf, M. Fertig, T. Stoeferle, B. Trauter, R. Mahrt, J. Weiss, T. Pflueger, "Polarization-Independent Photo-Detectors with Enhanced Responsivity in a Standard SOI CMOS Process", IBM Research Report on Electrical Engineering, May 18th 2009.
[2] N. Moll, T. Morf, M. Fertig, T. Stoferle, B. Trauter, R.F. Mahrt, J. Weiss, T. Pfluger, K.-H. Brenner, "Polarization-Independent Photodetectors With Enhanced Responsivity in a Standard Silicon-on-Insulator Complementary Metal–Oxide–Semiconductor Process", IEEE Journal of Lightwave Technology Vol. 27, Issue 21, pp. 4892-4896 (2009).