Alignment tolerance enlargement of a high-speed photodiode by a self-positioned microball lens

By integrating a commercially available microball lens on the InGaAs p-i-n photodiode chip, we have achieved at least 5- and 16-fold improvements of the optical alignment tolerance in the transverse and longitudinal axes, respectively, without sacrificing the diode efficiency. The optical alignment during lens/chip integration is achieved by a self-positioning process, which provides an on-chip ball-lens socket in concentric circles to the photodiode aperture to sustain the drop-in microball lens. The photodiode after lens integration exhibits a 3-dB bandwidth larger than 9 GHz and a responsivity larger than 0.9 A/W, both at 1310-nm wavelength. In addition, the InGaAs photodiode exhibits a dark current density of less than 3 μA/cm
²
, both before and after lens positioning, indicating a proper integration.