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Integrated High-Speed, High-Sensitivity Photodiodes and Optoelectronic Integrated Circuits

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Integrated High-Speed, High-Sensitivity Photodiodes and Optoelectronic Integrated Circuits Sensors and Materials, Vol. 13, No. 4 (2001) 189-206 MYUTokyo S &M0442 Integrated High-Speed, High-Sensitivity Photodiodes and Optoelectronic Integrated Circuits Horst Zimmermann Institut fiirElektrische Mess- und Schaltungstechnik, Technische Universitat Wien, Gusshausstrasse, A-1040 Wien, Austria (Received February 28, 2000; accepted February3, 2001) Key words: integrated circuits, integrated optoelectronics, optical receivers, optoelectronic de­ vices, PIN photodiode, double photodiode, silicon A review of the properties of photodiodes available through the use of standard silicon technologies is presented and some examples of how to improve monolithically integrated photodiodes are shown. The application of these photodiodes in optoelectronic integrated circuits (OEICs) is described. An innovative double photodiode requiring no process modificationsin complementary metal-oxide sem!conductor (CMOS) and bipolar CMOS (BiCMOS) technologies achieves a bandwidth in excess of 360 MHzand data rates exceeding 622 Mb/s. Furthermore, a new PIN photodiode requiring only one additional mask for the integration in a CMOS process is capable of handling a data rate of 1.1 Gb/s. Antireflection coating improves the quantum efficiencyof integrated photodiodes to values of more than 90%. Integrated optical receivers for data communication achieve a high bandwidth and a high sensitivity. Furthermore, an OEIC for application in optical storage systems is introduced. Author's e-mail address: [email protected] 189 l 90 Sensors and Materials, Vol. 13, No. 4 (2001) 1. Introduction Photons with an energy larger than the band gap generate electron-hole pairs in semiconductors. This photogeneration G obeys an exponential law: aP,0 G( x) = -- exp( -ax), Ahv (1) where xis the penetration depth coordinate, P0 is the nonreflectedportion of the incident optical power, A is the light-sensitive area of a photodiode, hv is the energy of the photon, and a is the wavelength-dependent absorption coefficient. Photogeneration, i.e., the optical absorption coefficient, and the transport mechanisms of photogenerated carriers determine the speed and the dynamic quantum efficiencyof photodiodes. There are two transport mechanisms for electrons and holes photogenerated in semi­ conductors: carrier drift and carrier diffusion. Drift is a fast mechanism; however, slow diffusionof photogenerated carriers usually limits the bandwidth and the dynamic quan­ tum efficiencyof pn photodiodes available in standard CMOS and Bi CMOS technology to several megahertz.<1.2J Measured results for the n+/p-substrate photodiode fabricated by a 0.8-µm BiCMOS technology confirm these findings. Other photodiodes in typical stan­ dard-buried-collector (SBC)-based BiCMOS technology only achieved a low quantum efficiency due to the thin epitaxial layer (n-collector) used as the intrinsic region of a PIN 3 4 photodiode.< . J For a high quantum efficiency,all carriers should ideally be photogenerated within the drift region. To increase the quantum efficiency and to avoid the slow diffusion of photogenerated carriers by extending the space-charge region, up to three additional masks (without masks for antireflection coating) for the integration of fast PIN photo­ 5 6 diodes with a thick intrinsic region were used in a SBC based bipolar process.C - l In this work, the innovative double photodiode approach is described, which does not require additional process complexity to achieve faster integrated photodiodes and to improve the quantum efficiency in the red region of the spectrum. In addition, a new PIN photodiode with a high quantum efficiencyrequiring only one additional mask in a CMQS process and extending the bandwidth further is shown. Results of PIN photodiodes integrated on p­ type and n-type substrates are compared and the innovative photodiodes are implemented in advanced OEICs. 2. Materials and Methods Direct band-gap IIlN semiconductor materials such as GaAs, InP and InGaAs possess a steep absorption edge for light.(?) The large absorption coefficients in the visible and near-infrared spectral range allow thin IIlNabsorption regions and a very fast photodetec­ tor response enabling data rates in excess of 20 Gb/s. csJ The optical absorption coefficient of theindirect semiconductor material Si is much lower forwavel engthslarger thanappro ximately 400 nm. <7J Thicker Si absorption regions are therefore necessary for a high quantum efficiency, resulting in a slower transient response. Sensors and Materials, Vol. 13, No. 4 (2001) 191 The crystal growth and process technologies of III/V materials, however, cannot compete with those of silicon. Defectdensities are much higher in III/V crystals, hindering the fabrication of highly integrated III/V circuits. The III/V chips, therefore, only contain approximately 105 transistors, whereas Si dynamic random access memory (DRAM) chips contain up to 109 transistors. The prices of III/V photodetectors and integrated circuits are much higher than those of Si photodetectors and integrated circuits. Silicon, therefore, is the first choice in microelectronics. A low price for optical sensor chips is important for high volume applications in automation and consumer electronics. Silicon allows the integration of photodetectors and signal-processing circuits monolithically on one chip, resulting in a low price and a high reliability. The monolithic integration improves immunity against electromagnetic interference(EMI) compared to two-chip solutions with a photodetector chip and an amplifying or signal-processing chip. This improvement is due to a very short connection between the photodetector and the amplifier in an OEIC. A one-chip solution, i.e., an OEIC, avoids bond pads and their parasitic capacitance, thereby increasing the bandwidth. For some applications, the die area of the integrated photodiodes is smaller than the die area which would be needed for bond pads, resulting in a cost advantage. The OEICs for optical storage systems such as CD-ROMs and digital­ versatile-disks (DVDs), for instance, are such examples, where the die area for photo­ diodes is smaller than for the corresponding number of bond pads. Receivers for short­ range optical communication and data transmission in the wavelength range of 630-850 nm via plastic optical fibers (POFs) and optical silica-based fibers in local area networks are another application of silicon OEICs. This article presents the results of integrated photodiodes and OEICs for these two applications. Although silicon cannot compete with the bandwidths of III/V photodetectors, it will be shown that silicon photodetectors·and OEICs can achieve data rates of more than 1 Gb/s and bandwiths of more than 1 GHz. The development of OEICs is supported by process and device simulation programs such as SUPREM, TSUPREM, MEDICI and ATLAS with implemented photogeneration. Process technologies, therefore, can be investigated by simulation with respect to the properties of photodiodes available in these technologies. Circuit simulation programs are extensively used for chip development in universities, research institutes and microelec­ tronic companies. Examples of frameworksincluding circuit simulation, layout, extrac­ tion and verificationtools are CADENCE and MENTOR. For the development of ASICs, circuits and layouts are available to customers of foundries in libraries. It can be concluded that there is much know-how and that there are many tools that can be exploited to develop OEICs. No special simulation tools have to be created forthe investigation of integrated photodiodes and for the development of OEICs. Some very interesting results for integrated photodiodes and OEICs can now be described in detail. 3. Results 3.1 n+/p-substrate photodiode Most modem CMOS and Bi CMOS processes implement self-adjusting well formation and require only one lithography step for the formation of both n- and p-type wells. Therefore, throughout a wafer or a chip, either a p-type well or an n-type well is present. 192 Sensors and Materials, Vol. 13, No. 4 (2001) The p-type doping concentration in an n+/p-substrate photodiode (Fig. 1) consequently is 16 3 higher than 10 cm- , and the space-charge region, i.e., the drift region for photo generated carriers, is very thin. In tum, a large portion of the carriers is photogenerated outside the space-charge region and a large contribution of slow carrier diffusion to the photocurrent results (Fig. 2). For measurements, an emitter coupled logic (ECL) generator was used to modulate a diode laser with a wavelength of 638 nm. The laser light was coupled into the photodiode on a waferprober by an optical single-mode fiber. The transient response of the photodiode was measured with a digital sampling oscilloscope HP54750/51 and a picoprobe (pp) with an input capacitance of 100 fF and a bandwidth of 3 GHz. The frequency response of the photodiode was measured using the picoprobe and a network analyzer HP8751A, which also modulated the laser in this case. A 500-Q polysilicon resistor was integrated with the photodiode to allow its transient characterization. The rise and fall times determined from Fig. 2 are 26 ns and 28 ns, respectively. The measured-3 dB frequencyis only 6.7 MHz.C9l Anode Anode Cathode Cathode + n+ + pwell p-p+ substrate Fig. 1. Cross section of an n+/p-substrate photodiode obtained through the use of a technology based on self-adjusting wells. 80 70 60 .s> 50 :g, 40 � 30 20 10 0 0 50 100 150 200 Time (ns) Fig. 2. Transient response of an n+/p-substrate photodiode fabricated by an industrial 0.8-µm BiCMOS technology based on self-adjusting wells. Sensors and Materials, Vol. 13, No. 4 (2001) 193 3.2 Double photodiode To avoid the slow diffusion of photogenerated carriers for wavelengths shorter than approximately 650 nm without additional process steps, an innovative double photodiode (DPD) is suggested (Fig. 3). The limitation of slow carrier diffusion is avoided using two vertically arranged p/njunctions. One p/njunction is formed by a p+-source/drain region and an n-well in CMOS or BiCMOS technology. The other p/njunction is composed of the n well and the p substrate. Both anodes of the double photodiode are connected to the ground.
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