Amorphous Silicon Waveguides and Interferometersfor Low-Cost Silicon Optoelectronics

Giuseppe Cocorullo 1,3 Francesco G. Della Corte , Rosario De Rosa2Ivo Rendina Aifredo Rubino 2 Ezio Terzini 2

1 National Research Council ofItalyInstitute of Research on Electromagnetism and Electronic Devices (IRECE-CNR)Via Diocleziano 328, 1-80124 Naples, Italy

2 Ente per le Nuove Tecnologie, 1'Energia e l'Ambiente — Centro di Portici (ENEA-CRP)Via Vecchio Macello, 1-80055 Portici, Naples, Italy

3 University of Calabria — Department of Electronics (DEIS), 1-87036 Rende, Cosenza, Italy

ABSTRACT

The present work reports on our recent achievements in the exploitation of a simple technology for the fabrication ofhydrogenated amorphous silicon (a-Si:H) based low-loss rib waveguides. In particular, waveguides with various widthshave been fabricated out of an a-SiC:HIa-Si:H stack deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD)at the relatively low temperature of 220 °C. The ribs were defmed by an anisotropic, CH4-based, Reactive Ion Etchingprocess. The devices have been subsequently characterized by cut-back technique. Even though a dependence ofattenuation parameter on the waveguide width was observed, propagation losses as low as 0.7 dB/cm could be measured atX=:l 3 m, in good agreement with the theoretical estimations based on the intrinsic absorption of the material. Startingfrom the same structure, a Fabry-Perot thermo optical modulator has been also fabricated and tested at the communicationwavelength of 1.3 .tm.

Keywords: silicon, silicon carbide, amorphous semiconductors, waveguides, integrated optics, silicon optoelectronics,Fabry-Perot interferometers

1. INTRODUCTION

Silicon is certainly an unrivalled material for the realization of electronic devices, and its technology has reached such adegree of refmement to allow today the integration of virtually any function, from sensing to complex data processing, onthe same chip. In the wide variety of new applications of this semiconductor, optoelectronic devices are among the mostrecent. In particular, there is a growing attention to the integration of optical functions on silicon chips, with the purpose ofadding optical communication capabilities to standard microelectronic circuits. These devices are expected to takeadvantage from the realization of on-chip or chip-to-chip optical data transfer, or clock signal distribution, resulting inhigher speed and more reliable operations. Moreover, the integration of optics and electronics on the same silicon substratewill certainly give a strong contribution in cutting the costs of office or home direct access to the worldwide optical fibernetwork, thus allowing the diffusion of wide band communication systems to the largest number of subscribers. Finally,other advantages will come from the possibility of a wider use of fibers for data transfer in noisy environments, like anaircraft or in modern microprocessor assisted car engines, brakes and suspensions.

F.G.D.C. (correspondence): Email: dellacor@irecel.irece.na.cnr.it; telephone: +39 81 5705999; fax: +39 81 5705734

286 SPIE Vol. 3278 • 0277-786X/98/$1O.OO

In the wake of these new hypothesized large scale application of silicon for optoelectronic, today there is a great interest inthe study of optical materials compatible with the technology of this semiconductor. Silica, for instance, is certainly animportant candidate for the realization of integrated waveguides and active optical components for the 1.3 and 1.55 iminfrared wavelength windows typical of the fiber optic communications. As a matter of facts, in recent times waveguidesexhibiting propagation losses lower than 0. 1 dB/cm have been successfully demonstrated [1] and subsequently utilized incommercial devices, while the thermo-optic effect has been exploited for the realization of low-speed switchingcomponents [2], and even amplification appears to be feasible. Finally, another clear advantage comes from the lowwaveguide-to-fiber coupling losses that can be achieved thanks to the refractive index matching and the possibility ofrealizing self aligned coupling thanks to proper V-grooves carved in the silicon substrate.However, though the fabrication process is cheap, it is not clear at the moment what is its degree of compatibility with thestandard microelectronic processes necessary to integrate electronic data processing functions on the same substrate. Inparticular, the high temperature required for the deposition of silica, usually in excess of 600 °C, and the very thick under-cladding, of the order of several tens of micrometers, necessary to obtain an effective optical confmement of the radiation,are unlikely to be effectively compatible with the fabrication ofVLSI circuits [3, 4].In order to overcome the drawbacks imposed by the silica-on-silicon technology, several other possibilities have beenconsidered. The Silicon-On-Insulator (SOl) or Silicon Implanted Oxide (SIMOX) approaches, for instance, are bothcertainly promising from the point of view of the thickness required to obtain low-loss waveguides [5]. Unfortunately theyalso require a rather expensive technology, which is even more unfair to the fabrication of low-cost CMOS devices.Moreover, the oxide layer behaves as a bottleneck to the heat flow in the active components based on the thermo-opticeffect, with the result ofslowing down the switching times [2].Optoelectronic devices based on the Silicon-On-Silicon technology have been also studied [6]. The good thermalconductivity of Silicon has allowed in this case to fabricate Fabry-Perot modulators exhibiting bandwidths wider than 1MHz. On the other side, however, the weak refractive index change between core and cladding, obtained by heavily dopingthe latter, produces a poor confmement of the radiation, which results in propagation losses usually in excess of severaldB/cm.As an alternative to these technologies, we recently proposed hydrogenated amorphous Silicon (a-Si:H) and other relatedalloys, like amorphous silicon carbide (a-SiC:H) for the realization ofgood quality waveguides, also suitable for fabricatingsimple interferometers [7]. Planar waveguides, in fact, were fabricated and tested, showing propagation losses lower than 2dB/cm, and simple Fabry-Perot light modulators, based on the thermo-optic effect, were also characterized.The present work reports on our most recent achievements in the exploitation of this simple technology for the fabricationof low loss rib waveguides.

2. EXPERIMENTAL

a-Si:H thin films can be obtained by several common techniques, involving either a physical deposition, like sputtering orevaporation, or chemical reactions of gas mixtures at the substrate surface. Among these latter techniques, the chemicalvapor deposition sustained by a plasma discharge, known as PECVD, has gained the widest diffusion for the goodelectronic quality of the semiconductor, its reliability and also because it is suitable for large area applications, like in thesolar cell industry. Another big advantage offered by this technique is that thin films of a-Si:H and its related alloys, like a-SiGe:H and a-SiC:H, can be deposited at relatively low temperatures, which makes it compatible with other technological

steps.The waveguide structure we examined is made of an undoped a-SiH core layer grown on an undoped a-SiC:H layer,forming the undercladding, while the overcladding is air.For the fabrication of the waveguide we started from a 4", <100> oriented, silicon wafer, which forms the substrate.Usually this substrate is heavily Sb-doped, in order to prevent the occurrence of any possible alternative path to the lightpropagating through the waveguide during the measurements. No effect on the device characteristics can be imagined forlow-doping substrates. After a short dip in an oxide-etch solution, the crystalline silicon wafer is loaded into our deposition

system, consisting of an Ultra High Vacuum (UHV), capacitively coupled, PECVD commercial system, equipped with

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three chambers communicating via a load-lock chamber. The substrate holder acts as the grounded electrode. The poweredelectrode faces the substrate from below, to avoid powder incorporation into the growing layer.The deposition of the a-SiC:H underciadding starts approximately one hour after charging the substrate, to allow thereaching of a base pressure of l0 Pa. and a uniform temperature of 180 °C. With SiH4 and CH4 flows of 20 and 46.7 sccm(standard cubic centimeters per minute) respectively, at a pressure of 93 Pa. and a RF power of 17 mW/cm2 at 13.56 MHz,the growth rate was 0.24 nm/s. The deposition of the a-Si:H core la' required to move the sample into another chamber.Here a SiH4 flow of 42 sccm, at a temperature of 220 °C and a RF lwer of 23 mW/cm, at the same pressure as before,allowed a growth rate of approximately 0.2 nm/s. It is worth noting that these growth parameters do not differ substantiallyfrom those used for the fabrication of thin-film solar cells.Waveguides with widths ranging between 6 and 15 jim have been defined by photolithography out of the described stack.Reactive Ion Etching (RIE) was used to obtain sharp walls of the ribs. The RIE system was a single chamber vacuumsystem. operated at room temperature. The process gas was a 8% 0,/CF4 mixture, with a flow of 30 sccm. at a pressure of39 Pa. The RF (13.56 MHz) power density was 98 W/cm. The resulting etch rate was I nm/s.A SEM view of one of the devices is reported in Fig. 1. The rib height is 1.2 jim. while the overall core thickness is3 jim.The underciadding is 400 nm thick.In order to obtain information on the deposited films, measurements were carried out on some samples grown in the sameconditions. The optical band gaps and refractive indexes were initially measured from transmittance and reflectancemeasurements. The a-Si:H showed an optical band gap E of 1.75 eV and a refractive index n of 3.4. while for a-SiC:H wemeasured E=2.0 and n=3.0. The photon absorption of a-Si:H was measured below the optical band gap by the ConstantPhotocurrent Method [8]. It showed a=0.08 cm* In spite of its wider band gap, the a-SiC:H layer was characterized by aslightly higher absorption. This is probably due to the fact that, for solar cell applications, this material is usually depositedin much thinner layers (10 nm), where the absorption plays a minor role, and therefore the technology is not optimized yet.

Fig. I SEM view of one of the a-SiC:HIa-Si:H realized waveguides. The rib height is 1.2 jim.

EHT=30.ee kU (JD= 25 }thg= 2.50 K Xløpa Detector= SE1

3. MEASUREMENTS

Waveguides of various lengths were obtained by cleavage of the crystalline substrate. Tanks to the good adhesion of thedeposited layers to the substrate, the waveguide end faces were always flat and sharp, although the amorphous materials donot have crystalline planes. The radiation of a 1 mW, 1.3 rim, DFB laser diode, pigtailed to a 5-tim-core monomode fiber,was butt coupled to each waveguide for testing. The transmitted light was detected at the output by means of an InGaAs

photodiode.The propagation losses were estimated with the cut-back technique. The longest waveguides measured 2 cm, and they wereshortened down to 2.5 mm by successive cleavages, in a sequence of four steps at least. At any step, the optimal couplingbetween the fiber and the device under test was reached before the measure commenced. We estimated that the errors risingfrom the signal fluctuations at the photodiode, due to the above coupling procedure, were limited to 20%. This value can beused to estimate the error margin of the measurements.The propagation losses for a set of waveguides with different widths are summarized in Fig 2. These data show a strongdependence of the light attenuation from the changing geometric factor, as a sharp increase of the losses at narrower ribwidths is observed. We believe that this behavior must interpreted as due to the irregular profiles of the narrower structures.As a matter of fact, we did not succeeded yet in defming waveguides less than 6 m in width, as they always appeared to

have bottleneck-like shapes, responsible of very high losses.The best performances were shown by the 15-pm-wide device, which performed an c ofO.7 dB/cm. It is worth noting thatthis value is in good agreement with the theoretical attenuation calculated by means of a numerical waveguide designsoftware, based on the effective index method. On the other hand, losses as low as 0.4 dB/cm have been calculated for the8-jim-wide structure, in evident contrast with the measured value of 5dB/cm.This theoretical analysis has also evidenced that all ofthe realized waveguides are strongly multimode. For the l5-tm-wideone, for instance, the TE01 and the TE02 modes propagates respectively with attenuations of 0.98 and 1.02 dB/cm. This classof waveguides can fmd application in the fabrication of photonic devices, especially when the transport of a large amountof energy is required or when wide-spectrum, non-coherent light sources are available.

? 1O

I 68101214rib width [urn]

Fig. 2 Measured waveguide propagation losses as a function of the rib width.

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4. THERMO-OPTICAL MODULATION

Light switching systems are required for the construction of optical communication links in local area networks and also inphotonic inter-module connections. In these fields, of course, silicon is a promising material due to the fact that itstechnology is mature, cheap, and it could allow the monolithic integration between high speed Si electronic circuits and Si-based guided wave devices. Moreover, in the those applications where high bit rates are not required, such as in fiber-to-the-home networks and automotive products, the use of robust and low-cost optical components, compatible with the

present microelectronic technology, is greatly preferred to that of the high-performance high-cost 111-V optoelectronicdevices. For this reason, in the last few years, an increasing interest has been devoted to the fabrication of all-silicon lightswitches or modulators.

Among the various techniques explored to realize active devices, those based on interference principles have been shown tobe more effective [9]. In particular, the strong dependence of the refractive index of Silicon on temperature, i.e. the thermooptic effect (TOE), has been exploited to fabricate Si-based light modulators. The first prototypic was developed by Treyzin 1991 [2]. The device, a Mach-Zehnder guided-wave modulator realized in 501 technology, exhibited a bandwidth of afew tens of kilohertz when heated by means of an electrical power dissipating in a resistive layer covering one of the twoarm ofthe interferometric structure. An analogous modulator, but exploiting a guided-wave film structure in GeSi1 on Si,was proposed in 1992 and showed bandwidths up to about 90 kIlz [10]. An optimization, mainly of the waveguidingcharacteristics, ofthe Treyz's device was then proposed by Fisher et al. [1 1]. They in particular reported switching times ofabout 5 ts in a large cross section 501 rib guided-wave single-mode structure.Recently, however, we reported the encouraging results of a micromachined all-silicon Fabry-Perot thermo-opticmodulators which extend the capability of thermally controlled silicon switches at bandwidths beyond 1 MHz [12].Unfortunately these devices, based on a Silicon-on-Silicon waveguiding structure, showed high insertion losses, due to thepoor confmement of the radiation obtained by doping the cladding layers. In order to overcome this problem, the superioroptical characteristics of an a-SiC:HIa-Si:H structure can be considered.

A simple Fabry-Perot thermo optic modulator was therefore realized by gluing a 2.52-mm-long waveguide, of the typedescribed in the previous section, onto a Peltier heat pump. The device was then heated and the substrate temperaturemonitored by means of a calibrated thermistor held in contact with the heat pump. Using a setup similar to the onedescribed for the cut-back measurements, the light transmitted through the waveguide was measured as a function of thetemperature. A typical modulation pattern is shown in Fig. 3 . The periodic amplitude modulation is clearly due to thesetting up of interference inside the waveguide. Because of the strong thermo-optic effect, the refractive index of Siliconrapidly changes with temperature, determining the tuning, or the detuning, of the interference filter with respect to theX=1.3 tm probing light, and therefore a transmitted intensity described by the well known Airy's formula:

II = IO1+ 2J(1)

where I is the light intensity impinging on the front mirror, FR is the reflecting finesse of the cavity, given by:71• JR/(1—R) , with R the reflectance of the two mirrors, and the phase factor , fornormal radiation is: q$ = (2irnSIl)/2with 1 and 2 respectively the cavity length and the radiation wavelength. In this equation both ns and 1 vary with

temperature, inducing a periodicity in I,.Defming the modulation depth M as the ratio (imImm)/Im, with Im 2lfld 1mm the maximum and the minimum photocurrentrespectively at the photodiode detector, we fmd for M a value of 30 5 %, which is approximately half the valuepredictable by theory if the ideal reflectance R=(nsj-n)2/(nsj+nafr)2 0.3 is assumed at the Si/air interface. This discrepancy

0000G)(I)

E0z

Temperature (°K)

Fig. 3 Experimental thermo-optic modulation pattern of a Fabry-Perot mterferometer.

is rather normal, and clearly dependent on the low finesse of our cavity, strongly affected by the poor degree of flatness andparallelism of the two a-Si mirrors, which were simply obtained by cleaving the crystal silicon substrate, and did notreceive any smoothing or polishing treatment.

I

0.9

0.8

0.7

0.6

297 299 301 303 305

5. CONCLUSIONS

The mature technology of low-temperature Plasma Enhanced Chemical Vapor Deposition has been used to fabricate low-loss optical rib waveguides on a crystal silicon substrate. Starting from these structures, a thermo-optical modulator of lightintensity has been realized, based on a Fabry-Perot interferometer. As the technological steps necessary for the fabricationof the device are compatible with those typical of the CMOS industry, this may help in the integration of optical andelectronic functions on the same silicon chip.

6. AKNOWLEDGEMENTS

This work was supported by Regione Campania —Assessorato alla Ricerca Scientifica (L. 4 1/94)

7. REFERENCES

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