Recently, an increased interest in metal-semiconductor-metal photodetector (MSM) is observed due to high performances regarding bandwidth and responsivity that can be achieved by the application in fabrication of advanced technologies such as electron beam lithography (EBL) which can offer the possibility to realize nanoscale devices and a new concept such as nanoscale grating.  

The paper presents the fabrication of an MSM type photodetector with an active area of 100x100 µm² on silicon on insulating wafers (SOI) with an absorbing layer thickness of  4 µm and oxide box of 2-4 µm. The interdigitated electrodes with 100µm length and 80 nm finger widths and three different fingers spacing of 80 nm, 100 nm, and 200 nm have been experimented in order to analyze the enhancement of the response due to subwavelength grating. The oxide box limits the thickness of optically active region of the structure for a high speed response and nanoscale electrodes leads to an efficient coupling of the light in the absorption layer.

The fabrication steps consist in:

- configuration of contacting pads over an insulating layer of  SiO₂ layer by conventional photolithography;

- direct electron beam writing in polymethylmethacrylate (PMMA) of nanoscale fingers;

- Ti/Au evaporation and lift-off process;

Intrinsec transit time and R-C time constant calculated taking into account the experimented finger spacing evidenced that MSM on SOI substrate can be achieved values less than 1ps for transit time and less than 2 pF for structure capacitance. A good Schottky contact was obtained for Au/n-Si with Ti as interlayer for a good adherence. The responsivity of the structure is improved compared with conventional MSM structure.

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