Pulsed Thermoelectric Cooling for Improved Suppression of a Germanium Hotspot

602 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 4, NO. 4, APRIL 2014

Pulsed Thermoelectric Cooling for ImprovedSuppression of a Germanium HotspotMichael Manno, Peng Wang, Member, IEEE, and Avram Bar-Cohen, Fellow, IEEE

Abstract— As hotspots become an increasingly important fac-tor in the design of electronic devices, it has become essentialto develop novel near-junction cooling methods. Steady-statethermoelectric cooling has previously been considered for theremoval of localized hotspots on various substrates. In thispaper, the transient behavior of a germanium thermoelectricself-cooler, in which the chip substrate is used as a leg ofthe thermoelectric circuit, is described. A 3-D thermoelectricnumerical model was created in the commercial FEA packageANSYS and is used to explore the effects of various initialconditions, current pulse durations, current pulse magnitudes,pulse shapes, and die thicknesses. The results suggest that pulsedtransient thermoelectric cooling has the potential to improvehotspot temperature reduction by approximately 30% relativeto what is achievable in steady state. In addition, it was foundthat larger currents generally cause more rapid thermoelectriccooling, but also result in large overshoot temperatures and thatthe applied current profile has a strong effect on the transientbehavior of the cooler.

Index Terms— Germanium, hotspot, self-cooling, thermoelec-tric, transient cooling.

I. INTRODUCTION

THE emergence of on-chip hotspots in recent generationsof nano-featured semiconductor devices has dramatically

altered the thermal management requirements for electroniccomponents and, as stated in the iNEMI Roadmap [1],necessitated new component design and thermal packagingtechniques. Previous studies have shown that site-specificthermoelectric coolers are an effective means of reducinghotspot temperatures and thus improving chip reliability andperformance [2]–[7]. Thermoelectric self-cooling of semicon-ductor chips and substrates, in which the chip itself is used asone branch of the thermoelectric circuit, is especially well-suited to hotspot remediation, reducing the cooling systemsize and eliminating the thermal contact resistance betweenthe microcooler and the hotspot [4]. In addition, since the‘microcooler’ section of the thermoelectric circuit is simply anarbitrary area of metallization, the self-cooling technique couldbe readily extended to an array of microcoolers for cooling asystem with multiple dynamic hotspots. A recent study found

Manuscript received April 3, 2013; revised September 12, 2013; acceptedSeptember 30, 2013. Date of publication November 1, 2013; date of currentversion March 28, 2014. Recommended for publication by Associate EditorA. Jain upon evaluation of reviewers’ comments.

The authors are with the TherPES Laboratory, Department of MechanicalEngineering University of Maryland, College Park, MA 20742 USA (e-mail:[email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TCPMT.2013.2286740

that heavily doped germanium offered a better power factor,and, therefore, better cooling performance, than silicon inthe self-cooling configuration [8]. Further work explored thepotential of growing germanium crystals on the back of siliconchips for improved thermal performance with easy integrationinto current electronic systems [9].

Although previous efforts have focused on the suppres-sion of steady-state hotspots with continuous thermoelectriccooling, this paper will explore the ability of dynamicallyswitched thermoelectric microcoolers to suppress hotspots in agermanium chip. It is anticipated that the temporal and spatialinterplay of Peltier cooling and Joule heating, respectively,can provide cooling beyond what is achievable under steady-state conditions. Peltier cooling is an interface effect andoccurs at the surface of the microcooler, while Joule heatingis a volumetric effect and occurs throughout the volumeof the thermoelectric element. Therefore, when a current isapplied, there is instantaneous temperature reduction at themicrocooler, while the Joule heating effect diffuses moreslowly toward the cooled (as well as heated) ends of thethermoelectric circuit. Owing to the spatial separation betweenthe origins of these effects, it is possible to create a shortperiod of super cooling (followed by super heating) at themicrocooler, during which the temperature is lower than whatis achievable in steady state.

The transient behavior of thermoelectric coolers hasbeen documented in previous studies, notably [10]–[14].Ezzahri et al. [10] showed transient cooling of approximately2 K in a silicon microcooler similar in geometry to thegermanium cooler discussed in this paper. Yang et al. [11]demonstrated that transient cooling of up to 30 K couldbe achieved in a freestanding thermoelectric leg with noexternal heat fluxes applied. Similarly, Snyder et al. [12] exper-imentally demonstrated transient super cooling of approx-imately 10 K using a single p-n thermoelectric couple.Gupta et al. [14] numerically studied a thin-film thermoelectricmodule integrated into a chip and found that transient hotspotreductions of 6–7 K beyond the steady-state optimum werepossible. Finally, Bhattacharyya et al. [13] utilized transientthermoelectric heat transfer coupled with a shape memoryalloy to create an actuator. Although many of the exist-ing studies explore freestanding thermoelectric devises, thispaper presented here examines transient thermoelectric coolingof a chip hotspot with an integrated self-cooling substrate.There is only limited data available in the literature on theself-cooling configuration, especially pertaining to transientoperation.

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MANNO et al.: IMPROVED SUPPRESSION OF A GERMANIUM HOTSPOT 603

The objective of this research is to explore the possibilityof using transient pulsed thermoelectric cooling to temporar-ily suppress and/or remove hotspots, beyond what can beachieved in steady-state operation. Because electronic compo-nents typically operate in a dynamic mode, producing transienthotspots that may vary temporally in heat flux, temperature,and location, dynamic thermoelectric cooling could be used toefficiently cool a hotspot that has elevated heat dissipation for ashort period of time. Therefore, it is of engineering importanceto understand and apply transient thermoelectric self-coolingto such situations.

II. THERMOELECTRIC MODELING METHODS

A. Steady-State Behavior of a Germanium Self-Cooler

The transient behavior of a thermoelectric cooler is linkedto steady-state optimum conditions. Therefore, this presentpaper will reference the steady-state germanium self-coolingpreviously reported in [8]. In that study, a 12 mm × 12 mm diewas used, along with background and hotspot heat fluxes of70 and 700 W/cm2, respectively, applied to the bottom of thechip, and an effective heat transfer coefficient of 8700 W/m2K,representative of a finned heat sink, applied to the top. Fig. 1shows a top and sectional view of the germanium die withboundary conditions applied. The germanium substrate isassumed to be doped with arsenic, to a concentration ofapproximately 2.5 × 1018 cm−3. The doped germanium prop-erties utilized in this present paper were electrical resistivityof 5 × 10−5 �m, thermal conductivity of 60 W/m-K, Seebeckcoefficient of 467 × 10−6 W/K, density of 5323 kg/m3, andspecific heat of 320 J/kg-K [8]. An electrical contact resistanceof 1 × 10−6 �-cm2 was assumed at the metal–germaniuminterfaces [8].

As can be seen in Fig. 1, current is applied using aring electrode. Current flows through the germanium die andinto the center electrode. Peltier cooling occurs at the centerelectrode–germanium interface, creating a cold spot whichdraws heat from the surrounding substrate and reduces thehotspot temperature. Therefore, the center electrode is referredto as the microcooler. Prior work, notably [15] and [16], hasrevealed that for a given geometry there is an optimum currentthat provides the maximum reduction in steady-state hotspottemperature. To familiarize the reader with the previous self-cooling results. Fig. 2 shows the steady-state temperaturedistribution along the bottom of the 100-μm thick germaniumdie for various applied currents with a 600 μm by 600-μmmicrocooler. For this geometry, the optimal cooling currentis approximately 1.25 A and the peak hotspot temperature isreduced from 111.3 °C to 106.8 °C, a temperature reduction of4.5 K. It is clear that applying currents larger or smaller thanthe optimal current will result in less effective hotspot cooling.

B. Transient Behavior of a Germanium Self-Cooler

The commercial software ANSYS was used to build a finite-element model for the previously identified optimal coolergeometry, i.e., 12 mm × 12 mm × 100-μm thick die, and a600 μm × 600 μm microcooler. Building on previous studiesof hotspot cooling, background and hotspot heat fluxes of

Fig. 1. Germanium self-cooler schematic (a) top view and (b) cross sectionview (heat sink added for clarity).

70 and 700 W/cm2, respectively, were applied to the bottomof the chip and an effective heat transfer coefficient of8700 W/m2K, was applied to the top as indicated in Fig. 1[8]. Solid69 thermal-electric elements were used in thesimulation and the full model consisted of approximately400 000 elements. The mesh was constructed such that itis densest around the microcooler and hotspot, where thelargest temperature gradients occur. Owing to symmetry inthe geometry, it was possible to analyze just a quarter ofthe structure and the number of elements was reduced toapproximately 120 000 elements.

A constant current condition was assumed at the micro-cooler and ground electrode. The Peltier heating and coolingwere calculated based on the applied current and operatingtemperature and input into the model as heat flux boundaryconditions while ANSYS was used to calculate the nonuniformjoule heating resulting from the applied current. A meshrefinement showed that increasing the quarter model element


Fig. 2. Steady-state temperature distribution along the bottom of a self-cooledgermanium chip for various currents.

count from 120 000 to 440 000 elements resulted in just a0.09 K discrepancy in the calculated hotspot temperature.Thus, a quarter model of approximately 120 000 elementswas utilized in this paper and the results were validatedqualitatively with published ANSYS Inc. results and withpublished experimental data [11], [12], [17].

As can be seen in Fig. 3(a), Snyder et al. [12] performedan experimental study on a single bulk thermoelectric n-pcouple made of an n-BiTeSe element and a p-BiSbTe elementattached by a copper strap. The n-type and p-type elementswere each 1 mm × 1 mm × 5.8 mm tall. The temperatureof the cold junction of the couple was monitored with athermocouple and a plot of the temperature response to ashort current pulse was obtained [12]. Similarly, Fig. 3(b)shows the transient temperature response of a thermoelectricdevice to current pulses of various pulse magnitudes obtainedby Yang et al. [11] Again, a single n-p couple, consisting oftwo 1 mm × 1 mm × 6 mm tall elements connected with acopper foil, was analyzed [11].

In Fig. 3(a) and (b), the thermal mass of the system ismuch larger than that in this present paper; thus, the ther-mal responses are much slower. However, the experimentaltemperature responses do show qualitative agreement withthe numerical results, consisting of an initial decrease intemperature due to the additional Peltier cooling resulting fromthe increase in current, followed by a temperature increasedue to the increased Joule heating. In the Snyder study, I0represents normalized current; thus, Fig. 3(b) indicates thatlarger currents initially result in a more rapid decrease intemperature, but also lead to larger temperature overshoots.A similar result will be shown in Section III-3 of this presentpaper for a step change or square pulse of current.

To effectively capture the competing aspects of the transientresponse of a hotspot to a dynamically controlled germaniumself-cooler, beyond limiting the maximum temperature andtemporarily reducing the hotspot temperature, several novel

Fig. 3. Cold junction temperature response to an applied current pulseexperimentally determined by (a) Snyder et al. [12] and (b) Yang et al. [11].

metrics were used and are described in Table I, as wellas displayed in Fig. 4. The transient advantage (TA) metricintegrates the super cooling and holding time into one met-ric, providing the cumulative temperature reduction benefitachieved by the cooler, relative to the steady-state temperature.Similarly, the transient penalty (TP) incorporates overshoottemperature and settling time into one metric, providing thecumulative temperature increase over the steady-state valueand representative of the price paid for the dynamic operationof the microcooler at larger currents.

This present paper began with an initial baseline simulationof hotspot temperature for various cooling currents, with aninitial condition of no thermoelectric cooling. The effect ofpulse duration, pulse shape, pulse magnitude, and substratethickness with the initial condition of optimal steady-statethermoelectric cooling was then examined.

III. RESULTS AND DISCUSSION

A. Microcooler Hotspot Suppression Baseline

A hotspot suppression baseline for dynamic thermoelec-tric self-cooling in a germanium chip was established as a


TABLE I

METRICS FOR CHARACTERIZING THE TRANSIENT BEHAVIOR OF A GERMANIUM SELF-COOLER

standard of comparison for this paper of pulsed thermoelectriccooling. The steady-state temperature distribution, producedby a hotspot with no thermoelectric cooling, served as theinitial condition for the transient model. Fig. 5 shows thetransient behavior of the hotspot center temperature for variousapplied microcooler currents, using a step change in current.An immediate steep drop in temperature is observed for allfour currents, followed by a continued asymptotic temperaturedecline for the two lower currents and an increase in tempera-ture for the two higher currents. Thus, for 0.5 and 1.25 A, theminimum temperature of 108.5 °C and 106.4 °C, representing2.7 and 4.8 K of cooling, are achieved at the new steady-state conditions, while for 2 and 2.75 A the initial minimumtemperature of 106.4 °C and 106.7 °C, respectively, is rapidlyexceeded as the temperature rises toward the new steady-statevalues of 108.1 °C and 111.8 °C, respectively.

As previously mentioned, the optimal steady-state coolingcurrent for this configuration is 1.25 A. It can be seenthat applying a current, i.e., less than the steady-state opti-mal current causes the hotspot temperature to asymptoticallyapproach a temperature, i.e., warmer than produced by theoptimal current, while applying a current, i.e., larger than theoptimum steady-state value, results in the hotspot temperaturedecreasing more quickly than in the steady-state optimumcase. However, this larger current produces additional Jouleheating which, within approximately 5 ms, causes the hotspottemperature to rise. Moreover, the hotspot temperature forthe larger current approaches a steady-state temperature, i.e.,significantly greater than the steady-state optimal hotspottemperature.

In addition to current magnitude, the die thickness andcooler size were also varied using this initial condition.


Fig. 4. Metrics used for quantifying the transient behavior of a germaniumself-cooler.

Fig. 5. Hotspot temperature for various applied currents with no current/nocooling as initial condition.

It was found that decreasing the cooler size or die thicknesscauses the hotspot temperature to decrease more quickly,but results in a warmer hotspot temperature than achievedwith the respective steady-state optimum current for eachgeometry. Conversely, when the cooler size or die thicknesswas increased, the temperature response was slower, but againthe hotspot temperature did not decrease to values below thatachieved by that geometry’s steady-state optimum current.The faster thermal response with thinner dies is due to thesmaller thermal mass of the thinner dies as well as the factthat the cooler is a shorter distance away from the hotspot. It isthus apparent that for a delayed activation of the microcooler,after the hotspot has been established, the optimal steady-state current provides the largest hotspot temperature reductionachievable for a specific geometry.

Fig. 6. Temperature as a function of time at the hotspot and microcoolerduring a current pulse.

B. Pulsed Microcooler Hotspot Suppression

The objective of this portion of the study was to examinethe impact that pulsed operation of the self-cooler has on thehotspot temperature. A germanium die, with a thickness of100 μm, and a 600 μm by 600 μm microcooler was used, withthe steady-state temperature created by the optimum steady-state current of 1.25 A serving as the initial condition for thisanalysis. A current square pulse of magnitude greater than1.25 A was applied to the model for a short period of time,and then the current was lowered back to the optimal steady-state current of 1.25 A.

The effect of pulse magnitude, pulse duration, and diethickness was studied and the transient behavior of both thehotspot and the microcooler will be discussed. Fig. 6 showsthat the center temperature of the hotspot and microcoolerrespond similarly when a current pulse is applied. Therefore,it is reasonable to use the metrics described in Fig. 4 andTable I to characterize the behavior at both the hotspot andmicrocooler. It will be shown that the transient thermoelectriceffect is more pronounced at the microcooler than at thehotspot. This is due to the nature of the transient thermoelectriceffect as described in the first section of this paper. When thecurrent pulse is applied, cooling occurs instantaneously at themicrocooler and the transient overcooling results from the timeit takes for the Joule heating, produced throughout the volume,to diffuse to the microcooler. The hotspot is a small distanceaway from the cooler. Therefore, the cooling at the hotspotis somewhat delayed, relative to the instant when current isapplied, resulting in slightly less super cooling at the hotspotthan at the microcooler.

1) Evolution of Temperature Profile With Time: Fig. 7shows the temperature distribution along the bottom of agermanium chip, with a thickness of 100 μm and a coolersize of 600 μm × 600 μm, subjected to a temporarily elevatedcurrent. For additional clarity, Fig. 8 displays contour plots oftemperature for the area surrounding the hotspot at time stepsof interest. A current pulse of 2.5 A for a duration of 5 ms,followed by a reduction in the optimal 1.25 A, was applied inthis part of this paper. Fig. 7 shows the hotspot temperature


Fig. 7. Temperature distribution along the bottom of the germanium chipfor several time steps when a current pulse of magnitude 2.5 A and durationof 5 ms is applied.

Fig. 8. Contour plots of temperature along the bottom of the germanium chip.To show the most relevant area, only a portion of the quarter model is shown.Thus, the small square in the lower right corner is the hotspot, and the plotshows the temperature distribution up to a radial distance of approximately2500 μm.

distribution along the bottom of the chip (at t = 0) for nothermoelectric cooling, along with the steady-state profile forthe optimal 1.25 A current, and the temperature distribution

Fig. 9. Transient hotspot temperature resulting from a 5-ms current pulse.

associated with the minimum and maximum hotspot temper-atures resulting from the 5 ms, 2.5 A current pulse. It can beexpected that after reaching the maximum value of 108.0 °C,the hotspot temperature will decrease and approach that ofthe optimum steady-state condition. Figs. 7 and 8 reveal thatthe thermoelectric device can provide approximately 5 °C ofcooling in steady-state operation, completely removing thehotspot, although at the expense of slightly raised temperatureselsewhere. If a higher current is applied, the hotspot can besuper cooled by up to an additional 1.5 °C, suggesting that ashort duration, transient hotspot of higher heat flux or largersize could be suppressed with pulsed cooling.

2) Effect of Current Pulse Magnitude: The magnitude ofthe current pulse has a significant effect on transient coolingand the hotspot temperature over time for pulse magnitudesof 1.5, 2.5, and 3.5 A is shown in Fig. 9. It can be seen thatincreasing the current magnitude of the pulse initially produceslarger and more rapid hotspot temperature reductions, butalso leads to hotter overshoot temperatures. For each pulseconsidered, once the pulse ends, and the current returns tothe optimal level, there is a sudden sharp rise in temperature.This behavior is due to the diminished Peltier cooling, butthe continuing diffusion of additional Joule heating (producedby the higher current) toward the hotspot. However, sincethe current has been reduced, the cooling and heating beginto reequilibrate and after a short rise time, the hotspot tem-perature reaches its maximum overshoot and then begins todecrease toward the original steady-state temperature. A pulseduration of 5 ms was used for all of the cases, unless statedotherwise.

Fig. 10(a) shows the dependence of super cooling on thepulse magnitude. Increasing the pulse magnitude decreasesthe minimum hotspot and cooler temperature. However, forthis pulse shape, there are diminishing returns, especially atthe hotspot. Owing to the amount of volumetric Joule heatingproduced, using a pulse magnitude greater than 4.5 A wouldresult in very little additional super cooling and would incur asignificantly higher overshoot temperature. For this geometry,i.e., a 600 μm by 600 μm cooler and 100-μm thick die, theoptimum steady-state temperature reduction is 4.8 K at thehotspot and 4.9 K at the microcooler.


Fig. 10. (a) Transient temperature reduction and (b) pulsed coolingenhancement as a function of pulse magnitude.

Fig. 10(b) shows the pulsed cooling enhancement as afunction of pulse magnitude and it can be seen that increasingthe pulse magnitude increases the pulsed cooling enhancement.Fig. 11 shows the TA and TP as a function of pulse magnitudefor several different pulse durations. If a pulse duration greaterthan 1 ms is used, the optimum transient cooling advantageoccurs at a pulse magnitude of 2.5 A, while the TP increasesexponentially with pulse magnitude for all pulse durations.It can be seen that the ratio of TA to TP, representative ofthe overall transient effect, is in the order of 0.1–0.2 for allthese cases, indicating that TP is typically nearly an order ofmagnitude greater than the TA for a square pulse.

3) Effect of Current Pulse Duration: The effect of currentpulse duration was also explored, and Fig. 12 shows thetemporal variation of the hotspot temperature for 1, 5, and

Fig. 11. (a) Transient advantage and (b) penalty as a function of pulsemagnitude for several different pulse durations.

10-ms pulses of 2.5 A, followed by a decrease to the optimalcurrent of 1.25 A. The hotspot temperature history for aconstant current of 2.5 A is also displayed and it can beseen that the temperature–time histories for the different pulsedurations follow the same curve for the early part of thetransient, decreasing for the first 2.5 ms, and then increasingwith time past the minimum temperature. Since the differentpulse durations follow the same curve, the length of the pulsedoes not have a significant effect on the minimum temperatureachieved. However, the longer pulses produce more Jouleheating; thus, causing higher peak temperatures and longersettling times.

Fig. 13 shows the effect of pulse duration on TA andTP for 1.5, 2.5, and 3.5 A pulses. It can be seen that forshort times, increasing the pulse duration causes an increase


Fig. 12. Hotspot temperature as a function of time for a 2.5 A current pulse.The cooler size is 600 μm by 600 μm and the die is 100-μm thick.

in TA, due to the prolonged holding time at reduced tem-perature, but for longer times—when Joule heating beginsto dominate—no additional TA is achieved with increasedduration. TP increases nearly linearly with pulse duration dueto the increased overshoot temperature and extended settlingtime. This behavior is a result of the increased Joule heatingassociated with longer duration current pulses. These resultssuggest that pulse magnitude plays a larger role in increasingTA than pulse duration. The optimal pulse duration for thisgeometry and set of material properties is approximately 5 ms,after which the holding time shows no additional benefitfrom a longer pulse. Progressing past this optimum will haveno benefit, in terms of minimum transient temperature andholding time, and will have large adverse effects in termsof overshoot temperature and settling time. This can be seenin Fig. 13, where there is no increase in TA after 5 ms,while TP increases linearly with increasing pulse duration. Asmentioned in the previous section, TP is approximately oneorder of magnitude larger than the TA, resulting in an overalltransient effect (ratio of TA to TP) of substantially less thanunity.

4) Effect of Current Pulse Shape: As previously notedfor the square pulse shape considered, thus far, TA is oftensubstantially outweighed by TP resulting in a ratio of TAto TP of substantially less than unity. However, the time–temperature history described above implies that a squarecurrent pulse may, in fact, be the least efficient profile fortransient thermoelectric remediation of on-chip hotspots. Inaddition to the step current profile previously discussed, a sim-ple ramp up, ramp down, and isosceles profile were explored,as shown in Fig. 14. The ramp down current profile initiallyresults in a hotspot temperature similar to that of the stepprofile. The current continuously decreases during the rampdown profile, reducing the Peltier cooling; thus, the minimumhotspot temperature reached is slightly warmer than that ofthe step current. However, the continuous decrease in currentproduces less joule heating throughout the substrate, thus TPresulting from the ramp down profile is significantly lowerthan that of the step current. Similarly, the isosceles profilealso provides a comparable amount of TA with less TP, ascompared to the step current.

Fig. 13. Transient (a) advantage and (b) penalty as a function of pulseduration.

Fig. 15 displays TA and TP (left axis) and the overalltransient effect, which is equal to the ratio of TA to TP (rightaxis). As previously discussed, the step current has the poorestoverall transient effect, in the order of 0.2. However, the rampdown and isosceles profiles have overall transient effects of1.19 and 1.46, respectively, indicating that TA is larger thanTP. The current profiles used were not meant to exhaust all thepossible variations, but rather to illustrate the ability of pulsedthermoelectric operation to generate a net positive super cool-ing effect. It is to be expected that optimization of the pulseshape for a given hotspot dissipation profile or sensor drivenoperation of the thermoelectric cooler to respond to a variabledissipation profile could result in further improvements in thenet benefit of pulsed thermoelectric cooling.

5) Effect of Die Thickness: The effect of die thickness onTA and TP was also investigated for a square current profile.


Fig. 14. Alternative applied current profiles considered and the resultinghotspot temperature over time.

Fig. 15. Transient advantage and penalty and overall transient effect forvarious current shapes.

Changing the geometry of the die alters the optimum steady-state current and optimum steady-state temperature reduction.It is, therefore, necessary to find the optimum steady-statecurrent, which minimizes the hotspot temperature, for eachdie thickness. The parameters associated with the optimumsteady-state cooling were then used as the initial condition forthe transient analysis. Fig. 16 shows the steady state, tran-sient super cooling, and total transient temperature reductionat the hotspot and cooler as the die thickness varies from25 to 200 μm, while the cooler size remains constant at600 μm by 600 μm. At very small die thicknesses, thehotspot and cooler approach the same temperature. As the diethicknesses increases, the cooler and hotspot are physicallyfurther apart, and thus their respective temperatures divergefrom each other.

Thicker dies result in a larger temperature reduction at thecooler because the thickness of the die insulates the coolerfrom the hotspot. However, this weakens the effectiveness ofthe cooler in reducing the hotspot temperature. Thus, as diethickness increases, the steady state and transient temperaturereduction at the hotspot decreases. For die thicknesses of

Fig. 16. Optimal steady-state cooling, super cooling, and total transienttemperature reduction (steady-state cooling plus super cooling) of the hotspotand microcooler as a function of die thickness.

Fig. 17. Transient advantage and penalty as a function of die thickness.

25–100 μm the pulsed cooling enhancement at the hotspotstays nearly constant, meaning that increasing the die thicknessaffects the steady state and transient cooling equally. However,at die thickness greater than 100 μm, the pulsed coolingenhancement begins to decrease, meaning that the increaseddie thickness is degrading the super cooling relatively morethan the steady-state temperature reduction. As previouslystated, super cooling occurs because the cooling from thecooler reaches the hotspot more quickly than the bulk jouleheating. Consequently, moving the cooler further away fromthe hotspot, by increasing the die thickness, results in reducedsuper cooling and, therefore, reduced pulsed cooling enhance-ment. Fig. 17 shows TA and TP as a function of die thickness.The maximum TA occurs at a die thickness of 100 μm.Unfortunately, the TP also reaches a maximum at a diethickness close to this value.


IV. CONCLUSION

In this paper, the transient behavior of a germanium ther-moelectric self-cooler was presented. The effects of variousinitial conditions, current magnitudes, current pulse durations,current profiles, and die thicknesses were explored. A previ-ously optimized germanium cooler served as the steady-statebaseline design, and it was determined that pulsed germaniumself-cooling can provide a maximum of 7.5 °C temperaturereduction at the cooler and a 6.5 °C temperature reduction atthe 70 μm × 70 μm 700 W/cm2 hotspot on 100-μm-thick ger-manium chip. The super cooling results in an increase in tem-perature reduction of 50% at the cooler and 30% at the hotspot,relative to steady-state operation. A new TA metric was intro-duced to more clearly illustrate the transient behavior of ger-manium self-coolers, and it was shown that the applied currentprofile is a critical factor in maximizing the overall transienteffect of a pulsed Peltier cooler. Current profiles such as a rampdown or isosceles produced more TA than TP, resulting in anet positive transient effect. From all of the results presentedit is clear that transient cooling can be used advantageously toprovide bursts of additional Peltier cooling to augment a self-cooler already operating at steady-state cooling conditions,allowing for the suppression of higher flux hotspots and/orreducing the energy consumed by the bulk cooling system.

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Michael Manno received the B.S. degree inmechanical engineering from Tufts University, Med-ford, MA, USA, in 2010. His research focusedon anodic bonding for MEMS applications andrack level cooling systems for data centers. Since2010, he has been pursuing the Ph.D. degree inmechanical engineering with the University of Mary-land, College Park, MD, under the supervision ofProf. A. Bar-Cohen.

His current research interests include the novelmethods of cooling high heat flux hotspots, including

thermoelectric devices.Mr. Manno is a member of ASME.

Peng Wang (M’07) received the Ph.D. degreein mechanical engineering from the University ofMaryland at College Park, College Park, MD, USA,in 2007.

He has been focused on advanced cooling tech-nologies for chip-level, package-level, and system-level thermal management of electronics since 2000.His experience includes air cooling, solid statecooling, liquid cooling, two-phase evaporative coldplate, thermosyphon, and heat pipe for high powerelectronics. Currently, he is a Thermal Engineer

with Qualcomm, San Diego, CA, USA. He has published one book, onebook chapter, and more than 40 journal and conference papers on thermalmanagement of electronics.

Dr. Wang is a member of ASME.

Avram Bar-Cohen (M’85–SM’87–F’93) isan Internationally Recognized Leader in thedevelopment and application of thermal science andengineering to microelectronic and optoelectronicsystems. In his role at the Defense AdvancedProjects Agency (DARPA) and through his profes-sional service in IEEE and ASME, he has helpedto define and guide the field of thermal packagingand facilitated the emergence of high reliabilityconsumer electronics, computing platforms, andmicrowave communication and radar systems. He

has co-authored Dielectric Liquid Cooling of Immersed Components (WSPC,2012). He was the Founding Chair of the IEEE Intersociety Conference onThermal Management in Electronic Equipment (ITHERM) in 1988 and wasrecognized with the IEEE CPMT Society’s Outstanding Sustained TechnicalContributions Award in 2002, the ASME/IEEE ITHERM Achievement Awardin 1998 and the THERMI Award from the IEEE/Semi-Therm Conference in1997. Design and Analysis of Heat Sinks (Wiley, 1995), and Thermal Analysisand Control of Electronic Equipment (McGraw-Hill, 1983). He has co-edited16 books. He has authored or co-authored over 400 journal papers, refereedproceedings papers, and chapters in books; has delivered 65 keynote, plenaryand invited lectures at major technical conferences and institutions; and heholds eight U.S. and three Japanese patents. He has advised to completion65 Ph.D. and master’s students at the University of Maryland, College Park,MD, USA, the University of Minnesota, Minneapolis, MN, USA, and BenGurion University, Beer Sheva, Israel. He is a Distinguished UniversityProfessor with the Department of Mechanical Engineering, Universityof Maryland. From 2001 to 2010, he served as the Chair of mechanicalengineering at the University of Maryland and is currently with DARPA.

Prof. Cohen is an Honorary Member of ASME. He is the Editor-in-Chiefof the Encyclopedia of Thermal Packaging (WSPC, 2012).

Documents

Pulsed Thermoelectric Cooling for Improved Suppression of a Germanium Hotspot