Resonant cavity thin film photodiode for compact displacement sensor

A new sub-wavelength thick photodiode for realizing a compact interferometer has been fabricated. The previous thin film photodiode has problems of low sensitivity and signal deformation caused by multiple reflections at interfaces. By setting the thin Si film to be λ/2 of the wavelength of incident light, the signal intensity increases due to resonant cavity enhancement. At the same time, the photocurrent signal shape is improved, because the resonant cavity works as an anti-reflection structure. The interference signal increases 32-fold, and its shape approaches a sinusoidal curve. The sensitivity obtained is about 10 mA/W. The noise of the 2nd order Fourier component in the interference signal is reduced from 41 to 25% at 632.8 nm and to 4.3% at 678.8 nm, showing good agreement with the theoretical estimation.