GaAs pHEMT Single Pole Double Throw(SPDT) RF Switch Failure Analysis

SPDT radio frequency (RF) switch which fabricated in GaAs pHMET technology was field return. Failure analysis was performed on the typical sample. Some failed units can recover after baking in high temperature chamber, which seemed to be fake failure. During non destructive analysis such as X-ray & C-SAM, no abnormality was found. After decapsulation, die surface was clean and integrated. Through OBIRCH test, hot spot was detected on the surface of a Metal-insulator-Metal (MIM) capacitor. Then FIB cut was implemented on the spot, cross section SEM image shown insufficient passivation coverage and dielectric crack in the MIM cap. According to the failure phenomenon and pHEMT technology structure, the failure mechanism was discussed. In the end of the paper, design based corrective action and reliability test were recommended to improve product reliability.

I. Introduction

Mobile communication systems has developed rapidly in recent years. The 3^rd&4^th generation communication is now all around us and will out of stage soon, 5G has already come. Although the carried frequency, base band bandwidth are increasing, the structure of RF front end in Radio Remote Unit(RRU) will not change in short time. RF switch was one of the key components widely employed to build the transceiver chain. RF switch has many applications such as VSW detection, DPD alignment and Tx to Rx chain switching in TDD system. This paper studied a field SPDT switch failure case, the switch was fabricated in GaAs pHMET technology. Background investigation shown that the failed units had already worked for several months, and higher humidity areas get higher failure rate. After baking the failed samples in the chamber(125, 24hrs or longer), some of them can recover its function. Pressure cook test can re-occur the failure phenomenon. Non-destructive analysis found no obvious abnormality. After decapsulation, still no failure signs was detected. Then the samples were sent to IR-OBIRCH (Infra Red Optical Beam Induced Resistance Change) analysis, hot spot was found on the surface of MIM cap. After FIB cut on the hot spot SEM image shown insufficient passivation coat and dielectric crack in the MIM cap.