Gerald Meinhardt

Blue-Enhanced PIN Finger Photodiodes in a 0.35-

Finger photodiodes in PIN technology are introduced to enhance the responsivity for blue and ultraviolet light. A thick low doped epitaxial layer results in high responsivity and high bandwidth also for red and near-infrared light. Results of PIN finger photodiodes are compared to that of PIN photodiodes for 10- and 15-mum epitaxial intrinsic layer thickness. The cathode finger structure results in a high responsivity of 0.20 A/W (quantum efficiency 61%) for 410-nm light and a bandwidth of 1.25 GHz for 10- mum epi thickness at a reverse bias voltage of 3 V. The rise and fall times with an epitaxial layer thickness of 15 mum are below 1 ns for the wavelength range from 410 to 785 nm.

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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.

SiGe BiCMOS Technology

We present the design and optimization of 80-channel, 50-GHz Si3N4 based AWG. The AWG was designed for TM-polarized light with a central wavelength of 850 nm. The simulations showed that, while the standard channel count AWGs (up to 40) feature gut optical properties and are relatively easy to design, increasing the channel counts (> 40 channels) leads to a rapid increase in the AWG size and this, in turn causes the deterioration of optical performance like higher insertion loss and, in particular, higher channel crosstalk. Optimizing the design we are able to design 80-channel, 50-GHz AWG with satisfying optical properties.

Integration of Photonic Detectors in Standard SiGe HBT BiCMOS

We present the design of 20-channel, 50-GHz Si3N4 based AWG applying our proprietary AWG-Parameters tool. For the simulations of the AWG layout we used PHASAR photonics tool from Optiwave. The simulated transmission characteristics were then evaluated applying our AWG-Analyzer tool. We studied the influence of one of the design parameters – the separation between input/output waveguides, dx on the channel crosstalk. The results show that there is some minimum waveguide separation necessary to keep the crosstalk between transmitting channels low. The AWGs were designed for TM-polarized light with a central wavelength of 850 nm. They will later be used in a photonic integrated circuit dedicated to medical diagnostic imaging applications.

Design and Optimization of High-Channel Si3N4 Based AWGs for Medical Applications

The impressive progress of silicon photonic integrated device technology during the past fifteen years has been primarily driven by the requirements of optical data- and telecommunication. Research and development in silicon photonics has therefore been focused on the telecom wavelengths in the 1.55 μm and 1.31 μm regions and on silicon-on-insulator (SOI) material as waveguide integration platform. The rising cost burden of the traditional healthcare system as well as the increasing health consciousness among people is stimulating the decentralization of healthcare and is creating a strong demand for novel medical diagnostic devices suitable for point-of-care testing. This opens up new possibilities for integrated nanophotonic sensing devices operating in the visible and <; 1.1 μm near infrared region. In this talk, we will present our ongoing research activities on the development of a CMOS-compatible photonic integrated circuit technology platform. This platform relies on silicon nitride waveguides fabricated by low-temperature plasma enhanced chemical vapor deposition (PECVD), which allows their monolithic co-integration with silicon photodiodes and CMOS based electronic read-out circuitry. We have achieved propagation losses of less than 1 dB/cm at a wavelength of 850nm in silicon nitride waveguides processed directly on an optoelectronic CMOS chip employing chemical-mechanical planarization (CMP). We will present the design and experimental validation of various nanophotonic building blocks required for the implementation of medical diagnostic sensing devices. We will show results of optical biosensing experiments based on integrated Mach-Zehnder interferometers and demonstrate how inkjet material printing technology can be effectively used to locally functionalize the optical waveguide transducer components. Moreover, we will discuss the potential of this silicon nitride waveguide based nanophotonic integration platform for the miniaturization of optical coherence tomography systems.

Design and simulation of 20-channel 50-GHz Si3N4-based arrayed waveguide grating applying AWG-parameters tool

Electronics traditionally takes place in horizontal layers on the surface of a wafer. This is also valid for integrated photonics, where the wave guides are built from layers that are parallel to the wafer surface. The transfer of signals in-between such entities is critical in respect to power loss. This is especially valid for the coupling of light, e.g. the coupling from fibers into or out of the integrated wave guide layers, coupling of light in-between boards and the two sides of a wafer. But also for HF electrical signals the transfer from one side of the wafer to the other side of the wafer by TSVs can be challenging because of high parasitics like the capacitance between the TSV metallization and the wafer substrate. This paper presents a new approach for the parallel processing of TSVs enabling optical TSVs and electrical TSVs capable of data rates up to 40 Gbit/s.

Silicon nitride waveguide integration platform for medical diagnostic applications

We present an improvement of monolithically integrated photodiodes in a p-type substrate of a commercial high-speed 0.35μm SiGe heterojunction bipolar transistor (HBT) BiCMOS technology. These photodetectors (PDs) combine low capacitance with high bandwidth and responsivity. Slight process modifications of the standard HBT process have been introduced in order to decrease leakage currents and enhance reach-through stability of the PDs. These modifications have been chosen carefully in order not to alter any other transistor parameters as shown in [1]. To enable low capacitances of the PDs very lightly p-doped epitaxially grown layers of different thicknesses over highly p-doped substrates have been investigated. The improvement becomes manifest, e.g. in a bandwidth of 557MHz and a responsivity of 0.19A/W of a finger photodiode at blue light and a reverse bias voltage of 4V in a 10μm cathode digit-spacing configuration. The capacitance of this finger photodiode is 150fF, overtopping the regular PIN photodiode published in [2] for the same light-sensitive area with a capacitance of 225fF. Results of detectors with interdigitated cathode distances of 5μm, 10μm, 15μm and 30μm are presented over the wide spectrum of technologically significant optical wavelengths from near-infrared to blue and ultraviolet. These detectors fulfil the requirements demanded by photodiode integrated circuits for universal backward compatible optical storage systems.