Constandinou, TG

Nikolic, K

Serb, A

IEEE Photonics Technology Letters

English

Konstantin Nikolic

Alexandru Serb

Timothy G. Constandinou

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An Optical Modulator in Unmodified, Commercially Available CMOS Technology

Spiral - Imperial College Digital Repository

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. XX, NO. YY, ZZ 2011 1An Optical Modulator in Unmodified,Commercially-Available CMOS TechnologyKonstantin Nikolic, Alexandru Serb, Student Member, IEEE, and Timothy G. Constandinou, Senior Member, IEEEAbstract—Here we present a method and structures formid-infrared, free-space optical communication using un-modified, commercially available complementary metal-oxide-semiconductor integrated circuits. The modulator is based onthe free carrier absorption in parasitic PN junction structuresunder reverse bias. Measured results demonstrate the proof-of-concept with speeds of 100bps (1.55µm wavelength), but atleast two orders of magnitude improvement can be achieved.This technology will enable non-galvanic chip-to-chip and chip-to-package communication as an alternative to wirebonding inapplications that benefit from a planar top chip surface, such aschemical sensing lab-on-chip systems as well as general sensorsand mid-infrared communication.Index Terms—light modulators, CMOS, electro-absorptioneffect, photonics, siliconI. INTRODUCTIONMONOLITHIC integration of optical components andintegrated circuits is a key driver in the field of siliconphotonics [1], with a strong focus on telecommunicationapplications. However there is a whole range of possibleapplications for optical communications which are not inthe realm of high-speed telecommunications, but which canbenefit from a simple, directly implementable technique whichuses light for communication. For example, a serious drawbackfor many lab-on-chip applications and other hybrid system isthe encapsulation of bond wires which drastically reduces theaccess to the top surface of the chip for any kind of sensorsand potential microfluidic system integration. The presenceof a simple circuitry on the existing semiconductor chips foroutputting information using light, even at moderate speeds,would be very useful. Hence the development of a methodfor a non-galvanic information output from a semiconductordevice or a chip within a low cost integrated technology istherefore of great interest to the industry.Poor light emitting properties of silicon limit the progress inthis area and the ideas for silicon light emitting diodes (LEDs)and lasers thus far [2] were all based on some modificationsof complementary metal-oxide-semiconductor (CMOS) fabri-cation processes, from minimal such as dislocation loops [3]to more significant such as silicon nanocrystals [4], additionof rare-earth materials, Raman effect [5], etc. Another idea ofhow to output data optically from a CMOS device is to usean external light source and modulate the light in the CMOSThe authors are with the Department of Electrical & Electronic Engi-neering and Centre for Bio-Inspired Technology, Institute of BiomedicalEngineering, Imperial College London, London SW7 2AZ, UK, email:{k.nikolic,alexandru.serb05,t.constandinou}@imperial.ac.ukThe authors greatfully acknowledge the support of the UK EPSRC, grantsEP/G070466/1 and EP/H024581/1.Manuscript received November 30, 2010; revised March 07, 2011.Fig. 1. Principle of operation of a mid-infrared free-space optical communica-tion system. The vertical incidence of the light beam allows two modes ofoperation: reflective, shown above (similar to the CD/DVD data read), or trans-mitted beam mode (without the metal reflector). The inset illustrates a cross-section of the communication parasitic PN junction, showing the incident andreflected paths of infra-red radiation through a silicon CMOS die.structure. The most common optical modulation techniquesin silicon are based on the plasma dispersion effect [6] andthus far were usually combined with interference effects, usingthe bias voltage [7]–[11] or photoexcitation by ultra-fast lowenergy pump pulses (in all-optical modulators [12]) to generatecarriers in Si. Although very successful in achieving very highcommunication speeds, this approach can be very sensitiveto fabrication tolerances and most of the solutions coupledwaveguides and separate structures. Treyz et.al [13] reportedan all-Si modulator based on free carrier absorption and with-out use of interference, with a modulation depth of -6.2 dB andresponse time 50 nsec. However, this technique is less efficient(carrier injection in [13] requires at least ten times more energyper bit than our solution) and needs dedicated rib waveguides.The vast majority of optoelectronic integrated circuits (ICs)require specialist semiconductor fabrication technologies oradditional process steps, which are not yet mainstream. Forsome applications there is a need to achieve the optical dataoutput using standard, commercially available CMOS ICs.Here we present a solution to free-space optical commu-nication based on electrically controlled modulation (electro-absorption, due to free carrier absorption) of mid-infraredlight sent through free space to a CMOS IC with parasiticPN junction structures, Fig. 1. Both the light emitter and thephotodetector are of conventional readily available type andlocated outside the electronic chip. The proposed technique isnot based on interference effects and therefore it is very robustand can be achieved using LEDs and commercial CMOS ICs.II. THEORETICAL EXPECTATIONSThe signal that we measure is the output light intensity (I),either reflected as in Fig. 1 or transmitted as in experimentsshown here, which will be attenuated due to free carrier2 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. XX, NO. YY, ZZ 20110 1 2 3 4 5 6 7 8 9 10T=(1+V)pV=sin(2pf t) 0Frequency (kHz)p =2/3p =1/22nd harmonic|YT(f)|[A.U.]ND [cm-3] 1020 1020 1019 1018 1020 NA [cm-3] 1020 1019 1019 1018 5·1017 ΔI/Imin  [ppm]  2600 600 250 25 ~10  Step Junction model ExponentiallyGraded Fig. 2. Table shows theoretical estimates for the maximum modulationdue to ∆V = 5V for the step junction model and for the exponentiallygraded doping profile assuming n+ diffusion (with surface donor concentrationND) into a p-well (NA). Graph shows amplitude spectrums for the depletionregion width modulation, i.e. the output signal, for: step junction, p = 1/2(approximates the well-to-well junction), and an exponentially graded dopingprofile, p = 2/3 (approximation for diffusion-to-well structures).absorption. The coefficient of absorption (α) depends on thefree carrier concentration, as described by Soref [14], [15] andLipson [10]. The free carrier concentration can be changedin the region of the PN junction by changing the appliedreverse bias voltage V and thus altering the depletion regionwidth w. A detailed analysis of how w and consequentlythe relative change of light intensity δI = ∆I/Imin and themodulation depth Imax/Imin depend on V are given in ourtheoretical exposition [16]. Here we summarise the theoreticalresults in Fig. 2. We assume two types of PN junctions: well-well and diffusion-into-well, the former was approximatedby a step junction and the latter by an exponential dopingprofile. For a step junction there is a text-book solution [17]:w ∼√Vbi − V where Vbi is the built-in voltage. If I0 is theinput light intensity, the output at distance L is approximately:I = I0 exp(−αL), what gives δI ∼ (Vbi − V )1/2 (assumingL = L0 − w and αw << 1). The exponential doping profilerequires more calculations and the numerical results [16] showδI ∼ (Vbi − V )2/3. In the experiments we applied a reversebias to the PN junctions V = VDC + V0 sin (2πf0t), hence:δI ∼ [V1 − V0 sin(2πf0t)]p , where V1 = Vbi − VDC. Thespectrum of approximate output δI is given in Fig. 2. Althoughthe real structures deviate from these perfect cases, the theoryindicates the limits and suggests that a very sensitive techniquemay be needed in order to observe this effect.III. METHODOLOGY, INSTRUMENTATION AND RESULTSA. CMOS-based optical modulator design. Instrumentation.A microphotograph of the test chip together with details ofspecific optical modulator structures is shown in Fig.3.The experimental setup describing the individual parts isgiven in Fig. 4. To demonstrate the modulation we em-ployed the phase-lock technique to detect the signal (after atransimpedance amplifier) using the 2nd harmonic. The 2ndharmonic was specifically targeted to exclude any feedthrougha)b) Structure #4d) p-substrate (bulkCMOS technology)Depletionregionp+ p+ p+ContactsDiffusionDie-thickness 300μmp-wellp-well on p-substrate (bulk CMOS technology)n+ n+Inter-metal dielectric (SiO  )   2Passivation layer (Si N  )             4         3Standard metalisation, eg. AlLight illumination (1.3-8μm)Side view Plan viewp-substrate (bulkCMOS technology)DepletionregionInter-metal dielectric (SiO  )   2Passivation layer (Si N  )             4         3DiffusionDie-thickness 300μmLight illumination (1.3-8μm)n+ n+ n+p+ p+ p+ p+contacts n-welln-welln-welln-welln-welln-welln-welln-welln-wellp-wellp-well on p-substrate (bulk CMOS technology)c) Structure #52010191018101710161015101.0 2.00Depth (μm)N (cm-3)n++/p-well/p-subp-well/p-subFig. 3. CMOS Integrated Circuit. (a) Microphotograph of the test chip withnine different PN-junction structures, fabricated in a commercially available0.35µm CMOS technology. The size of each structure is 500x500µm. (b)Test structure #4: side and plan views of the device consisting of multiplen-diffusion stripes within a single p-well, i.e. PN-junctions are n+/p-well. Thisstructure is made up of 12 stripes of n-diffusion of 37x480µm. (c) Structure#5: an array of n-well regions within a p-well/p-substrate, made up of 624n-wells of size 15x15µm each. (d) Possible doping profile for n+/p-well. Thelight wavelength (λ) is in the range 1.3-8µm, where bulk silicon is largelytransparent and free-carrier absorption is dominant (α ∼ λ2, e.g. see [14]).and it offered the best signal-to-noise ratio in our setup. As themodulation is non-linearly proportional to the applied (reverse)bias, δI(V ), any signal recovered on the 2nd harmonic is aconsequence of the modulation of the free-carrier absorption.The input voltage modulation is in the form of a sine wave offrequency f and a square wave amplitude modulation of fenv.B. Measured ResultsThe results demonstrating the achievement of light modu-lation are shown in Fig. 5. When the light source (LED) isOFF (Fig. 5 top, left of diagram), then the modulation of thedepletion width, caused by bias voltage shown in trace 3 (red),has no effect on the output. When light source is ON, after ashort transitional period, the bias voltage on the PN junction inthe shape of a square wave (modulated sine wave) is faithfullyrecovered at the output. The result for a well-well structureshows much stronger signal (Fig. 5-bottom), indicating betterutilisation of the PN junction area than the junction createdby diffusion. The demonstrated speeds are consequence of theNIKOLIC et al.: CMOS PHOTONICS - AN OPTICAL MODULATOR IN UNMODIFIED, COMMERCIALLY-AVAILABLE TECHNOLOGY 3IR LEDPN junction(on the chip)PhotodetectoroutputSignal generator(input)TransimpedanceAmplifier GfLock-inAmplifier 2fReferencefrequencyLow Pass FilterxDetectorLEDChipa)b)c)Fig. 4. The experimental setup: (a) Simplified schematic, (b) Cross-section ofthe printed circuit board (PCB) stack. Nine different PCBs #8 were fabricated,each with a small hole allowing only one structure to be illuminated on thechip, (c) a photo of the complete set-up.Fig. 5. Measured results. (top) Structure #4: trace no.1 is the output, no.3is the input modulation on the PN junctions. DC offset VDC=-3.15V, Vpp =4.4V, f=1 kHz, fenv =1Hz. Lock-in to 2nd harmonic, integration 3ms. (bot.)Struc. #5: tr-1 output, tr-2 input, VDC =−4.5V, Vpp = 7V, fenv = 10Hz,int.1ms. LED: λ=1.55µm, I = 90 mA, V = 1.28 V, optical output ≈4 mW.instrumentation and the speed of modulation is fundamentallylimited by the junction capacitance.IV. CONCLUSIONHere we have demonstrated a solution for the unresolvedneed for optical communication compatible with unmodi-fied, mainstream silicon-based VLSI technology. Implement-ing this technology together with some standard solutions forpower/data-in will enable CMOS chips to achieve both powerand data transfer without any bond wires. Apart from thesilicon photonics, this concept can also address packaging andencapsulation challenges faced in many lab-on-chip applica-tions, for example a multichannel System on Chip for rapidpoint-of-care gene detection [18]. Demonstrated technique canreadily achieve a relatively low bandwidth single communica-tion channel (single bitstream) sufficient for the lab-on-chip orsimilar applications. Speeds can be significantly improved byusing narrow band-pass filters (instead of Lock-in amplifier)and by deploying several channels in parallel. Multi-channelapplications have to address the question of cross-talk. In thatcase it would be possible to utilise the inherent advantageof this technology to be operational for different wavelengths(1.3-8µm) as well as various modulation techniques for theinput voltage, with appropriate optical or electrical filtering atthe output. The energy per bit is in the region of 1-10 nJ/biton the CMOS chip and the LED can consume up to 100mW,but this component is off chip. A very important aspect of ourdesign is that it is fully implementable in any commercially-available CMOS technology. Many techniques in the literaturethat are presented as CMOS-compatible refer to the fact thata CMOS process flow can be easily modified to accommodatethe extra processing steps required. Our intention however, isto have no customisation whatsoever but to use unmodifieddigital IC technology. Given these aims and constraints (nocontrol to doping levels, fabrication sequence, process steps,etc), we have successfully demonstrated a device structure andplatform concept that achieves optical modulation.REFERENCES[1] R. Soref, “Mid-infrared photonics in silicon and germanium,” NaturePhotonics, vol. 4, no. 8, pp. 495–497, 2010.[2] J. Di Liang, “Recent progress in lasers on silicon,” Nature Photonics,vol. 4, no. 8, pp. 511–517, 2010.[3] W. Ng, M. Lourenco, R. Gwilliam, S. Ledain, G. Shao, and K. Home-wood, “An efficient room-temperature silicon-based light-emittingdiode,” Nature, vol. 410, no. 6825, pp. 192–194, 2001.[4] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, and F. 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A CMOS-based light modulator for contactless data transfer: theory,”

A high-speed silicon optical modulator based on a metal–oxide–semiconductor capacitor,”

A multichannel DNA SoC for rapid point-of-care gene detection,”

An efﬁcient room-temperature silicon-based light-emitting diode,”

Compact electro-optic modulators on a silicon chip,” Sel. Top.Quantum Elec.,

Electrooptical effects in silicon,”

Gbit/s silicon optical modulator for highspeed applications,”

Highly efﬁcient optical phase modulator in SOI waveguides,”

Liang, “Recent progress in lasers on silicon,”

Micrometre-scale silicon electro-optic modulator,”

Mid-infrared photonics in silicon and germanium,”

Optical gain in silicon nanocrystals,”

Physics of semiconductor devices.

Silicon optical modulators at 1.3m based on free-carrier absorption,”

Silicon optical modulators,”

Silicon-based optoelectronics,”

Ultrafast alloptical modulation on a silicon chip,”

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An Optical Modulator in Unmodified, Commercially Available CMOS Technology

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