Focused microstructure analysis and fabrication for monolithic integration with high-speed MUTC photodiode

Abstract

Back-incidence high-speed photodiodes with small mesa diameter use back-integrated microstructures, which utilize the convergence and high transmittance function of the microstructures on the light beam to compensate for the incident light alignment deviation and improve the optical coupling efficiency. Two kinds of microstructures integrated with the modified uni-traveling-carrier photodiode (MUTC-PD), microlens and two-dimensional concentric annular convergence grating, are designed theoretically and compared by simulation. It is found that the microlens have better optical performance when integrated. InP microlenses meeting the requirements were fabricated using the photoresist hot melt method and inductively coupled plasma (ICP) etching. Testing revealed that the microlens could reduce the incident light spot size to one-third of its original size. By optimizing the thickness and doping concentration of the PD layer structure, and prepared a high-speed MUTC-PD with monolithic integrated InP microlens, with a mesa diameter of 18 μm. The measured 3 dB bandwidth of the integrated chip is 43 GHz. Experimental results using incident light fiber movement showed that the responsivity of the integrated microlens MUTC-PD increased by 66.76 % when horizontally offset by 10 μm compared to the non-integrated microlens case. When shifted vertically by 20 μm, the MUTC-PD responsivity of the integrated microlens decreases to 83.7 % of the maximum value, which is 18.2 % higher than that of the one without integrated microlens.