Integration of Photonic Detectors in Standard SiGe HBT BiCMOS

For certain purposes SiGe-BiCMOS technology has become an accepted alternative to III/V based technologies. Potential applications can so far be found in the analog, RFand mixed signal market segments, which exploit the high transit frequencies and low-noise features offered by this technology. In the past years optical data communication and optical storage applications have triggered the monolithic integration of photodetectors (PD) in photodiode integrated circuits (PDICS). This has become necessary for technical reasons to obtain increased bandwidths due to the elimination of parasitic capacitances arising from the external wiring between PD and IC. And it was also a cost-driven process. The opportunity to place many parallel optical receivers on one chip might open the door to optical interconnects. Apart from classical optical communication applications using wavelengths in the near IR at 850nm and IR at 1.3-1.5μm, optical data storage applications like CD and DVD have proven to be very successful at 780nm and 650nm, respectively. The next technology of optical mass storage media has already been introduced by Toshiba (HDDVD) and will soon be followed by the blu-ray disc. Both technologies use blue light at 410nm and are extremely cost driven because they are intended for the mass market. In this paper we try to point out the advantages of a SiGe:C-BiCMOS process platform for the implementation of multi-wavelength sensitive photodetector devices. A schematic cross section through a PD implemented in austriamicrosystems 0.35 SiGe-process (1) is shown in Figure 1. The device consists of two vertically arranged pn-junctions formed by a SiGe:C-anode and a n-tub cathode, resp., an n+-buried layer(BL)-cathode versus the substrate anode. The conversion of short-wavelength light at 410nm is done by the SiGe:C-PD and turned out to be very efficient and extremely fast (2). This high performance with respect to bandwidth can be attributed to the abruptness of the junction obtained by epitaxial and in-situ doped growth of the SiGe-anode layer, the implementation of carbon hindering out diffusion of boron and allowing a higher Ge concentration. The graded Ge concentration results in a quasi-electric field in the conduction band accelerating photogenerated minority carriers (electrons) out of the highly doped anode making recombination more unlikely and therefore increasing both speed and responsivity. The second junction is used to harvest light of longer wavelengths (660nm and 785nm) and to make this device favorable for CD/DVD and HDDVD resp. blu-ray applications. The advantage of the chosen double photodiode(DPD) concept of two vertically arranged pn-junctions over other approaches like pin photodiodes (3) is the easy and straightforward process integration. No further process steps or mask levels are necessary inhibiting thus further process complexity and extensive requalification measures. The higher process complexity of a SiGe:C–BiCMOS process technology with respect to a bare CMOS technology turns out to be an advantage at a closer look. In fact it offers more flexibility to cope with problems that arise in conjunction with the integration of PDs. E.g. to overcome the poor optical quantum efficiency that results from mismatched refraction indices of silicon resp. SiGe and the “backend”-materials silicon dioxide and the nitride passivation. Here a silicon dioxide/silicon nitride stack that is originally used as etch stop for structuring the emitter poly within the SiGe:C-BiCMOS process turns out to be also a very effective antireflective coating. It significantly increases the optical quantum efficiency from 50-75% to 75%-90%. High responsivities in the blue spectral range might be interesting not only for blu-ray disc double-layer storage media with reduced reflected light intensity, but generally for applications with low light intensity. The specific setup of our SiGe:C photodiode offers an additional feature we attribute to avalanche multiplication. At sufficiently high reverse bias (> 5V) the PD’s n-tub cathode (Figure 1) turns out to be fully depleted resulting in a very high responsivity of up to 0.6 A/W. The high bias could be generated on chip (4) by the implementation of a voltage up converter. A further example how the alleged higher SiGe:C-BiCMOS process complexity can be beneficially used to enhance diode performance will be presented in conjunction with the reduction of dark currents. Originally we measured dark currents up to 0.8% of the photocurrent at 10V reverse bias. With the help of a p+-implant from the CMOS process module, we managed to drastically reduce leakage currents.