optimization of metallic microheaters for high-speed reconfigurable silicon photonics

Optimization of metallic microheatersfor high-speed reconfigurable silicon

photonics

A. H. Atabaki, E. Shah Hosseini, A. A. Eftekhar,S. Yegnanarayanan, and A. Adibi

School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA30332, USA

Abstract: The strong thermooptic effect in silicon enables low-power andlow-loss reconfiguration of large-scale silicon photonics. Thermal recon-figuration through the integration of metallic microheaters has been one ofthe more widely used reconfiguration techniques in silicon photonics. Inthis paper, structural and material optimizations are carried out through heattransport modeling to improve the reconfiguration speed of such devices,and the results are experimentally verified. Around 4 μs reconfigurationtime are shown for the optimized structures. Moreover, sub-microsecondreconfiguration time is experimentally demonstrated through the pulsedexcitation of the microheaters. The limitation of this pulsed excitationscheme is also discussed through an accurate system-level model developedfor the microheater response.

© 2010 Optical Society of America

OCIS codes: (130.0130) Integrated optics; (160.6840) Thermo-optical materials.

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#130933 - $15.00 USD Received 30 Jun 2010; revised 9 Aug 2010; accepted 9 Aug 2010; published 11 Aug 2010(C) 2010 OSA 16 August 2010 / Vol. 18, No. 17 / OPTICS EXPRESS 18312

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1. Introduction

The promise of silicon (Si) photonics for large-scale and low-cost integration of photonic de-vices for different applications such as, intra-chip optical interconnects, fiber optics compo-nents, and optical signal processing has attracted a lot of attention recently [1–3]. The possibil-ity of large-scale integration of photonic devices enables functionalities with an unprecedentedlevel of flexibility and scalability. A major requirement in such Si photonics devices is the pos-sibility of tuning of individual photonic devices not only to correct for fabrication inaccuraciesbut also for the reconfiguration of the characteristics of the system. Previous works for recon-figuration of Si photonic devices have been mainly focused on three major physical effects,namely, free-carrier-plasma dispersion [4, 5], electrooptic [6], and thermooptic [7–17] effects.Although the free-carrier-plasma dispersion effect enables a fast reconfiguration speed (typi-cally ≤1 ns), this fast reconfiguration comes at the cost of an inherent optical loss caused bythe injection of free carriers. For many applications, especially signal processing, the introducedloss can be detrimental to the performance of the device. Another method of reconfiguration isthrough hybrid Si-polymer devices based on the electrooptic effect, which has the advantageof being low-loss and low-power with relatively fast response time (typically, in the order ofnanoseconds) [6]. However, realization of these devices is technologically challenging as theyrequire large drive voltages and at the same time their fabrication is not CMOS-compatible. On


the other hand, the thermooptic effect, which is inherently lossless, can be utilized to efficientlytune photonic devices with negligible insertion loss. The only shortcoming of these devices istheir slow response time, which is usually limited to a few to tens of microseconds as a resultof slow heat diffusion process. The thermooptic effect in Si-based devices has been used ex-tensively before for the implementation of reconfigurable filters [7], reconfigurable add-dropmultiplexers (ROADMs) [8, 9], dispersion compensators [10, 11], and switches [12–17].

Heating of photonic devices for the utilization of the thermooptic effect has been mainlydemonstrated using two approaches: 1) direct Joule-heating of the Si device [18], and 2) inte-gration of metallic microheaters close to the Si device [7–17]. The former approach requiresdoping of the Si slab to reduce the resistivity of the intrinsic Si which leads to optical absorptionand higher insertion loss. Also, with moderate doping levels of the Si layer, heater electrical re-sistance is high and therefore, the required drive voltage is relatively high (typically, tens ofVolts) in these devices. The latter approach has been more widely used to this date as themetallic microheater can be placed far enough from the photonic device to reduce the opticalabsorption by the metal. However, this method usually has slower response time because of alarger heating volume and therefore, a larger heat capacity. This slow reconfiguration time canbe an obstacle in many signal processing application. Hence, a detailed study of heat transportin such structures is necessary for more optimum designs.

In this work, we experimentally and numerically study the effect of different geometricalparameters for the improvement of the reconfiguration speed of the thermally-tuned Si pho-tonic devices. It is shown that by using a cladding material with better thermal properties (e.g.,higher thermal conductivity as in low-pressure chemical vapor deposition (LPCVD) silicon ni-tride (SiN)) the speed of the device is improved. For more accurate device modeling, thermalproperties (i.e., thermal conductivity and specific heat capacity) of the deposited materials arefine-tuned by fitting the experimental and simulation data. In this work, a simple model is pre-sented, which can perfectly describe the transients of the thermal response of these devices. Itis also shown that through pulsed-excitation of these devices sub-microsecond reconfigurationtime is possible.

The organization of this paper is as follows. In Section 2, the architecture and numericalmodeling of microheaters are presented. Fabrication and experimental characterization detailsare presented in Section 3. Device optimization along with the comparison of modeling andexperimental results are discussed in Section 4. The system-level model for metallic micro-heaters along with their pulsed-excitation performance is studied in Section 5; and the paper isconcluded in Section 6.

2. Device architecture and numerical modeling

The performance of microheaters, which is mainly studied in terms of power consumption andtuning speed, is usually calculated either through analytical/semi-analytical methods [19, 20]or through numerical modeling of the heat-transport equation [12, 21]. Although the formerapproach provides physical insight into the heat transport, it is usually not accurate enoughfor the optimization purpose. Hence, in this work our modeling is performed by numericalsimulation of the heat-transport equation with material thermal properties, which are optimizedto minimize the error between simulation and experimental results.

Figure 1(a) shows the architecture of the waveguide-microheater configuration on an silicon-on-insulator (SOI) substrate, which is considered in this work. Here, the metallic microheateris placed on top of the Si waveguide and is separated from the Si device layer by a claddingmaterial with the thickness tclad to avoid optical loss. The details of structural parameters areshown in Fig. 1(b) and are tabulated in Table 1. Heat transport in this device is simulated bynumerically solving the heat conduction equation


(a) (b)

Fig. 1. (a) Architecture of the metallic microheater over a Si waveguide on an SOI wafer. (b)Distribution of temperature at the cross-section of an SOI waveguide as heat is generatedin the metallic microheater. White arrows shows the heat flux in this device.

∇.(−k∇T )+ρc∂T∂ t

= qs, (1)

using the finite-element method (FEM) [22] in the COMSOL software package by applyingappropriate boundary conditions. Here k, c, and ρ , are the thermal conductivity, specific heatcapacity, and density of the material, respectively; and qs and T are the density of the heat powergeneration and the temperature, respectively. In these simulations, the structure is assumed tohave translational symmetry and thus the cross-section of the device shown in Fig. 1(a) is mod-eled (see Fig. 1(b)). Heat convection boundary condition,−k ∂T/∂n = hair, is applied to thetop surface of the device, where n is the direction normal to the surface and hair is the convec-tion heat coefficient to the surrounding air. Also, the right, left and bottom boundaries of thesimulation window are placed 20 μm away from the microheater device so that the temperaturerise is negligible at these boundaries. This large simulation window allows approximating thetemperature at the boundaries with the ambient temperature (Tamb) with very good accuracy.

Table 1. Device Parameters

.

BOX thickness tBOX 1 μmcladding thickness tclad 1 μm

microheater thickness th 100 nmmicroheater width Wh 1 μm

waveguide thickness twg 220 nmwaveguide width Wwg 480 nm

Figure 1(b) shows the distribution of temperature at the cross-section of waveguide-microheater configuration. White arrows show the heat flux in this structure. The thicknessesof the buried oxide (BOX) layer and the cladding layer are both 1 μm and the waveguide cross-section is 220×480 (nm)2, in this example. The cladding material is SiO2 and the microheatermaterial is nickel (Ni). To maximize the overlap of microheater temperature profile with the op-tical mode of the waveguide, a simple single-strip microheater, which is laterally co-centeredwith the waveguide, is considered in this work as shown in Fig. 1(a). The list of physical param-eters used in our simulations can be found in Table 2. Thermal conductivity and specific heat


1 2 3 0

5

10

15

BOX thickness ( μm)

rise

/ fa

ll tim

e (μ

sec)

rise time

fall time

(a) (b)

Fig. 2. (a) Simulation results of the effect of BOX thickness on the rise-time and fall-timeof temperature at the center of waveguide (b) Simulation result for the temperature rise atthe center of the Si device for mW/μm power dissipation density over the waveguide. Thewidth of the microheater is 0.5 μm in these simulation.

Table 2. Modeling Parameters

.

Material ρ( kgm3 ) c( J

KgK ) k( WmK )

Si 2330 703 163Thermal SiO2 2203 733 1.38PECVD SiO2 2203 650 1LPCVD SiN 2500 170 14

Ni 1300 800 70

capacity of the cladding material depend on the deposition condition [23] and are optimized byfitting the modeling to the experimental results presented in Section 3.

Two of the most important geometrical parameters in the optimization of the response ofmetallic microheater devices are the thicknesses of the BOX and the cladding layers. Figures2(a) and 2(b) show the rise/fall time and the steady-state temperature rise at the center of theSi device for different thicknesses of the BOX layer, respectively; while the thickness of thecladding layer is fixed at tclad = 1 μm. The rise-time (fall-time) is defined as the time by whichthe temperature rises (falls) from 10% to 90% (90% to 10%) of the steady-state value when astep signal is applied to the microheater. It should be noted the heat transport equation is lin-ear and time-invariant and therefore, the rise-time and fall-time of the device are equal. Thisis observed in Fig. 2(a), in which the rise/fall-time curves lie on top of each other. It is seenin Figs. 2(a) and 2(b) that the response of the microheater becomes faster at the price of lesstemperature rise or higher power consumption. Since, in this work we are aiming to increasethe reconfiguration speed of microheaters, thin BOX layers are chosen. It should also be notedthat the BOX should be kept thick enough to avoid leakage of the optical field into the under-neath Si substrate. The BOX thickness required to keep the propagation loss below 0.08 dB/cm(corresponding to a resonator intrinsic quality factor (Q) of 107) is around 700 nm [28]. Wehave chosen a BOX thickness of tBOX =1 μm, because of the availability of SOI wafers withthis BOX layer thickness, with a little compromise over the device performance. Moreover,the thickness of the cladding layer, tclad , has to be above 800 nm to satisfy the same level ofpropagation loss (i.e., 0.08 dB/cm) caused by the absorption by the metallic microheaters [28].Throughout this work, we have chosen a slightly larger cladding thickness , tclad=1 μm, sothat the possible variations in the cladding thickness in the fabrication process do not introduce


10 m10 m10 m

bus waveguide

microring

(a) (b)

Fig. 3. (a) Optical micrograph of a 20 μm diameter microring with a 0.5 μm wide micro-heater on top. Resonator is side-coupled to a bus waveguide with width of 480 nm. (b) SEMof the microheater of the same device shown in (a).

metal absorption loss in the device.

3. Fabrication and characterization

The performance of the microheater-waveguide configuration studied in the previous sectioncan be characterized by fabricating a device based on this configuration and by monitoring itstransmission properties as heat is dissipated in the microheater. Here, we fabricated 20 μm di-ameter microring resonators with same radial cross-section as shown in Fig. 1(b). As the radiusof the bend is much larger than the variations of both optical field and temperature distribution,previous simulations for devices with translational symmetry can be used for the microring de-vice with cylindrical symmetry (numerical modeling results showed very small error (≈ 0.5%)).The device in fabricated on Soitec SOI wafer with Si slab thickness of 220 nm, and a BOX layerof 1 μm thickness. The widths of the bus waveguide and the microring are 480 nm to assuresingle-mode operation. First, the pattern of the device is written on ZEP electron-beam resist us-ing electron-beam lithography (JEOL 9300) and etched into Si by inductively-coupled-plasmaetching (STS ICP) using a combination of Cl2 and HBr gases. After this step, 1 μm SiO2 is de-posited using plasma-enhanced chemical-vapor-deposition (PECVD) and microheater patternsare defined by a lift-off process using poly(methyl methacrylate)(PMMA) electron-beam resistand electron-beam evaporation. Microheaters are formed by the deposition of 75 nm thick Nilayer, and contact pads are covered with 150 nm gold (Au) for better electrical contact. To in-crease the yield in fabrication, we perform one step lift-off of both Ni and Au at the locationsof microheaters and contact pads. Later, in another lithography step, areas over photonic de-vice are opened using ZEP resist, where Au is removed using a Ni-safe Au etchant, GE-8148(Transcene Inc.). This Au is removed for higher electric resistance and higher power dissipationover the photonic device. Figure 3(a) shows the optical micrograph of the fabricated microringwith integrated microheaters. Dashed lines depict the edges of the photonic device. Figure 3(b)shows the scanning-electron micrograph of a 500 nm wide microheater over the microring.

The performance of the microheater is characterized by measuring the transmission of thefabricated microring using a standard optical characterization test setup similar to the one de-scribed in [29], while different drive signals are applied to the microheater. The optical trans-mission is measured by coupling the TE-polarized light from a swept-wavelength tunable laserinto the input waveguide, while the output of the device is coupled into a photodetector and thedata is transferred to a PC using a data-acquisition card. The drive signal of the microheater isapplied using an RF probe (Microtech Inc.). Figure 4(a) shows the transmission spectra of themicroring for different power dissipations in the 500 nm wide microheater. The intrinsic Q of


1.5512 1.5516 1.552 1.5524 1.5527

-16

-12

-8

-4

0

Nor

mal

ized

tran

smis

sion

(dB)

0 mW 0.38 mW

0.77 mW 1.45 mW1.1 mW

Wavelength (μm)

(a)

-10 0 10 20 30 40 50 60 70 80

0

0.2

0.4

0.6

0.8

1.0

1.2

Time (μs)

Nor

mal

ized

tem

pera

ture

experiment simulation

(b)

Fig. 4. (a) Normalized transmission of the microring shown in Fig. 3(a) for different powerdissipations in the microheater. (b) Experimental and simulation results of the normalizedstep response of the same microheater as in (a).

(a)

0.5 1 1.5 2

4

4.5

5

5.5

Heater width ( μm)

rise time(simulation) fall time(simulation) rise time(experiment ) fall time(experiment )

Res

pons

e tim

e(μ

s)

(b)

Fig. 5. (a) Experimental and simulation results for the temperature rise in the core of a20 μm diameter microring versus microheater width. Vertical axis on the right shows theredshift in the resonance frequency (b) Experimental and simulation results of temperaturerise-time and fall-time of microheaters versus microheater width. The rest of the deviceparameters are the same as those in Table 2.

the fabricated microring resonator is around 60k. It is observed that resonance wavelength ofmicroring is red-shifted as power dissipation is increased in the microheater. The resistance ofthis microheater is 590 Ω at small power dissipations and increases almost linearly with powerconsumption by 34 Ω/mW. This can be translated to 0.36 K/Ω change in the temperature of themicroheater. Hence, the microheater can also be used as a thermistor in this device [24].

To measure the step-response of the microheater, laser wavelength is fixed at the linear regionof the microring resonance line-shape. Then a small-signal step voltage is applied to the mi-croheater and the output of the microring is monitored on an oscilloscope. The applied signalshould be small enough so that the laser wavelength remains in the linear region of the res-onance. Figure 4(b) depicts the measured normalized step-response of the microheater alongwith the simulation results. Good agreement is obtained between measurement and simulationresults. It is observed that the rise-time and fall-time of the microheater is around 4 μs.


4. Microheater Optimization

A single-strip microheater co-centered with the underlying photonic device is studied in thiswork as the microheater of choice. The effect of the width of this microheater is studied inthis section. Also, as shown in Section 2, to assure faster reconfiguration time, the minimumpossible thickness of the BOX and cladding layers (≈ 1 μm) should be used. Although thethermal properties of the BOX layer are always fixed, the choice of the cladding material willaffect the thermal response of the device, and its effect is studied in this section.

4.1. Effect of Microheater Width

Figure 5(a) shows the simulation and experimental results of temperature rise in the centerof the Si device for 1 mW power dissipation in the microheaters with different widths (Wh)over a 20 μm diameter microring. It is observed that temperature rise is higher for the nar-rower microheaters and hence they are more power efficient. To compare the experimental andsimulation results, power dissipation in Au pads and thin-film connectors are extracted and nu-merical results are adjusted to take this power loss into account. The thermooptic coefficient ofSi, dnSi/dT = 1.8× 10−4, is used to relate the resonance wavelength shift to the temperaturechange of the microring. We see that relatively good agreement between simulation and experi-mental results is obtained. The vertical axis on the right side of Fig. 5(a) also shows the amountof frequency shift of the microring for 1 mW power dissipation.

Rise-time and fall-time of microheaters are also measured and shown in Fig. 5(b) for dif-ferent microheater widths along with the simulation results. It is observed that as the width ofthe microheater is reduced, its reconfiguration time is decreased as a result of smaller heatingvolume. The frequency response of these microheaters are also measured. For this measure-ment, a small-signal sinusoidal voltage with frequency f is applied to the microheater and theoptical output of the microring which has a 2 f frequency content (optical response is linearwith respect to power dissipation) is locked to the double frequency of the drive signal in alock-in amplifier. From these measurements, the 3dB bandwidth of microheaters with widthsof 2 μm, 1 μm, and 0.5 μm are measured to be 109 kHz, 132 kHz, and 139 kHz, respectively.These results support our previous observation that narrower microheaters reconfigure fasteralong with more heating efficiency.

It is observed from the temperature response of the microheater device (Figs. 5(a) and 5(b))that the performance measures of such a device do not change considerably with the micro-heater width. This is mainly caused by the lateral diffusion of the heat in the cladding and BOXlayers (approximately 1-2 μm on each side of the microheater), which is comparable to the sizeof the microheater [19]. Hence, to further reduce the heating volume, trenches can be etchedon the sides of the microheater. The effect of side trenches is significant in the performance ofthe thermooptic device and has to be studied for different applications. For instance, a 500 nmwide microheater with two 1 μm wide trenches located 500 nm to the sides of the microheaterand etched down to the underlying Si substrate shows an increase in the rise/fall time to 6.2 μswhile the power consumption is reduced by half compared to the case without trenches. Othersimulations on the effect of trenches showed an increase in the response time of the device andsince, the focus of this work is to increase the reconfiguration speed, we avoid trenches on thesides of the microheater.

4.2. Effect of cladding material

One other important factor in the performance of microheaters that is usually ignored is theeffect of cladding material. Usually, PECVD SiO2 is used for cladding because of the ease offabrication and CMOS-compatibility. An alternative material that can be used as the claddingis SiN. It has been shown before that SiN films deposited using LPCVD have high thermal


(a)

Nor

mal

ized

tem

pera

ture

0 5 10

0

0.5

1

T ime ( μs)50 55 60

T ime ( μs)

Rise time Fall time

SiN cladding

Si O 2 cladding

(b)

Fig. 6. (a) Frequency response of microheaters shown in Fig. 3(a) with the width of Wh =1 μm with PECVD SiO2 and LPCVD SiN cladding. (b) The normalized step-response ofthe same microheaters as in (a) at the rise and fall edge of the drive signal.

conductivities [25] (10 to 20 times higher than PECVD SiO2). Our numerical results show thatif the thermal conductivity of cladding is increased while its specific heat capacity is kept un-changed, response of the microheater becomes faster at the price of higher power consumption.Since the physical parameters of LPCVD SiN films that were reported in the literature werenot consistent as a result of different deposition conditions, we fabricated the same devices asexplained in Section 3 with LPCVD SiN cladding and then we measured the thermal propertiesof the deposited SiN. Figure 6(a) shows the frequency response of the 1 μm wide microheaterwith PECVD SiO2 and LPCVD SiN claddings. It is seen that the 3dB bandwidth is increasedby 23% in the LPCVD SiN cladding device to 162 kHz. Figure 6(b) compares the normal-ized step-response of these microheaters at the rise and fall of the drive signal. It is observedthat the response of these devices are almost the same at large time-scales. However, deviceswith LPCVD SiN cladding showed faster response at small time-scales which can be advanta-geous for pulsed excitation of microheaters as discussed in Section 5. The steady-state heatingproperty of these devices are also measured, and it is found that devices with SiO2 claddinghave 35% less power consumption compared to devices with SiN cladding as a result of lowerthermal conductivity of the SiO2 layer.

5. System-level model and pulsed excitation of microheaters

One approach to increase the reconfiguration speed of microheaters is through applying high-energy-pulsed drive signals [18,26]. To predict the performance of microheaters using this typeof excitation, an accurate model capable of predicting the microheater response at short time-scales (hundreds of nanosecond) is needed. Despite the good agreement between the simulationand experimental results at t ≥ 1 μs (as shown in Section 3 and Fig. 4(b)), the accuracy ofmodeling reduces for t ≤ 1 μs as a result of second order heat propagation effects. In thissection, a system-level model is introduced which is capable of perfectly predicting the responseof microheaters in the whole time-span.

Figure 7(a) shows the proposed model, which is composed of a block with a delay-likeresponse cascaded by a first-order linear-time-invariant (LTI) system. The delay is caused bythe heat propagation from the microheater to the Si waveguide whose response is chosen so thatthe model best fits the experimental data. The effective delay of this block is τdelay and is shownin the delay response. Also, the first-order LTI system with a time-constant of τdi f f models theheat diffusion in a simple layered structure. The parameters of this model are extracted for a1 μm wide microheater by fitting the experimentally measured response to that of the model.


delay

1

jω – 1/ diff

heat propagation delay

heat diffusion

heater excitation

waveguide temperature

h(t)

(a)

0 2 4 6 8 10

0

0.5

1

Time (μs )

experiment

model

725

nsec

Nor

mal

ized

impu

lse

resp

onse

(b)

Fig. 7. (a) System-level model for heat transport in the microheater. (b) Experimental resultof the normalized impulse response of the microheater with a width of 1 μm and that ofthe fitted model shown in (a).

As a result τdelay and τdi f f are found to be 400 ns and 1.5 μs, respectively. Figure 7(b) showsthe experimental result for the normalized impulse-response of the described microheater alongwith the impulse-response of the proposed model. Experimental impulse response is evaluatedby taking the derivative of the step-response of the system. It is observed that there is a goodagreement between the experimental and modeling results. We also observe a 725 ns delay inthe impulse-response of this structure. This reveals that even by applying a high-energy pulseat the beginning of the excitation, reconfiguration time less than 725 ns is impractical. Thislimitation which is shown for the first time in this work is the ultimate reconfiguration speedlimit in these type of microheaters for the given device parameters.

0 2 4 6 8 10 0

0.5

1

Nor

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ized

tem

pera

ture

No pulse

0.5 μsec pulse

0 5 10 0

2

4

Pow

er (a

. u.)

Time (μs )

Fig. 8. Experimental results of the response of 1 μm wide microheater to a step signal with(blue curve) and without (red curve) pulsed-excitation. Inset shows the power dissipationsignals for the two cases.

It is important to note that the 725 ns is the delay in heat transfer from the microheater to the


Si layer, and it is not necessarily the response time of the microheater. It is known that an LTIsystem can reach its steady state step response fast, if it is excited by a short pulse (ideally animpulse) combined with a step function. We have used our theoretical model to optimize thepulsed excitation signal. Using the results of our model we have developed an experiment to testthe effect of such excitations. Figure 8 shows the experimental results for the structure shown inFig. 3(a) with a microheater width of 1 μm. The other geometrical parameters of the structureare the same as those in Table 2. The two curves in Fig. 8 show the normalized temperatureat the center of the Si microring for the excitation with a step function with and without anadded pulse. For the pulsed-excitation, we have considered a 500 ns high-energy pulse at thebeginning of the excitation signal to enhance the rise-time of the system (inset of Fig. 8). It isobserved that the rise-time of the device is reduced from 4.2 μs to below 1 μs through pulsed-excitation. By shortening the duration of the pulse and increasing its peak-power, rise-times aslow as 725 ns can be achieved with practical peak-power levels.

Fig. 9. Modeling result for the heat propagation delay for different cladding material ther-mal diffusivity constants. The width of the microheater is 500 nm and the rest of the deviceparameters are the same as in Table 2. The arrows show the location of different materialson the thermal diffusivity axis.

The heat propagation delay from the microheater to the device layer plays a crucial rolein the thermal reconfiguration time and device optimization for reducing the heat propagationdelay is of great significance. We studied the impact of the following device parameters onthe heat propagation delay (i) microheater width and (ii) thermal conductivity of the claddingmaterial. As seen in Section 4.1, the width of the microheater does not play a major role inthe settling time of the device. On the other hand, the thermal diffusivity (α = k/ρc) of thecladding material plays a significant factor in reducing the heat propagation delay. Figure 9shows the modeling results for heat propagation delay of a 500 nm wide microheater versusthe cladding material thermal diffusivity constant. All other device parameters are the sameas in Table 2. It is observed that the heat propagation delay is inversely proportional to thethermal diffusivity of the material, which is in agreement with the analytical results for a one-dimensional system [22]. The locations of PECVD SiO2, LPCVD SiN and crystalline Si (c-Si)on the thermal diffusivity axis are marked. As observed, heat propagation delay of a claddingmaterial with a heat diffusivity close to c-Si can result into heat propagation delay of lessthan 10 ns. We have experimentally demonstrated in Ref. [27] that we can utilize this to ouradvantage through a novel thermally reconfigured device, in which the high thermal diffusivityof c-Si is used to reduce the heat propagation delay and also the reconfiguration time to below75 ns.

It should be noted that the pulsed excitation of microheaters can be used to reduce the rise-


time of the device. However, this approach cannot be used to reduce the fall-time and coolingusually proceeds at the natural thermal time constant of the device. However, as shown in[27], through differentially addressable device architecture, pulsed-excitation can be used toreconfigure the device in opposite directions at high speed.

6. Conclusion

In this work, a numerical study of heat transport in metallic microheater structures is performedfor the purpose of improving reconfigurable speed of Si photonic devices. It is experimentallyshown that through geometrical optimizations, ≈ 4 μs rise/fall times are achieved in such struc-tures. Also, the effect of the cladding material is studied and it is shown that high-thermally-conductive LPCVD SiN cladding can improve the reconfiguration speed compared to the con-ventional SiO2 cladding by approximately 20%. Moreover, sub-microsecond reconfigurationtime is experimentally demonstrated through pulsed-excitation of microheaters. A modifiedmodel is also proposed that can accurately predict the transients of the microheater response.Using this model, it is shown that the fastest reconfiguration time for the demonstrated micro-heater structure is around 725 ns using pulsed-excitation.

Acknowledgments

This work was supported by the Air Force Office of Scientific Research under Grant No.FA9550-09-1-0572 (G. Pomrenke) and by Defense Advanced Research Projects Agency un-der Grant No. HR0011-09-1-0014 (M. Haney).


optimization of metallic microheaters for high-speed reconfigurable silicon photonics

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