Failure Analysis of RF VGA in Base Station System and GaAs HBT Process Improvement

Alarm messages about radio frequency (RF) gain occur when the mobile communication base station works in the field. According to the log information, it is judged that the variable gain amplifier (VGA) of the receiving link is abnormal. The returned samples’ gain on board evaluation was much lower than the good unit, which confirms that the variable gain amplifier failed. The failed samples showed no abnormality through appearance inspection, X-ray, CSAM, and other non-destructive analyses. The emission microscope probe test found abnormal hot spots on the HBT, and focus ion beam processing was performed on the hot spot transistor; scanning electron microscopy (SEM) observed that the distance between the base metal 1(M1) and the emitter metal 2(M2) is about 0.33um, which is much smaller than the baseline(1.0um). Root cause analysis found that the M2 metal fabrication process requires two times lithography; the second lithography process mask alignment shift and without CD monitoring, resulting in a protective layer of polyimide over etched. After confirming the root cause, the foundry updated the process of M2 deposition by two times lithography to one time, and the corresponding photoresist thickness was optimized to 5.2um. After optimizing the process, the consistency of the M2 metal layer of the device is enhanced, and the yield of known good die (KGD) is increased by 0.41%.