Thermal Conduction Across Metal Dielectric Sidewall Interfaces
Research Articlewww.acsami.orgThermal Conduction across Metal Dielectric Sidewall InterfacesWoosung Park,† Takashi Kodama,†,‡ Joonsuk Park,§ Jungwan Cho,†, Aditya Sood,†,§Michael T. Barako,†, Mehdi Asheghi,† and Kenneth E. Goodson*,††Department of Mechanical Engineering and §Department of Materials Science and Engineering, Stanford University, Stanford,California 94305, United States‡Department of Mechanical Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Department of Mechanical Engineering, Kyung Hee University, 1732 Deogyoung-daero, Giheung-gu, Yongin-si, Gyeounggi-do446-701, South Korea NG Next, Northrop Grumman Aerospace Systems, Redondo Beach, California 90278, United StatesS Supporting Information*ABSTRACT: The heat flow at the interfaces of complexnanostructures is three-dimensional in part due to thenonplanarity of interfaces. One example common in nanosystems is the situation when a significant fraction of theinterfacial area is composed of sidewalls that are perpendicularto the principal plane, for example, in metallization structuresfor complementary metal-oxide semiconductor transistors. It isoften observed that such sidewall interfaces contain significantly higher levels of microstructural disorder, which impedesenergy carrier transport and leads to effective increases ininterfacial resistance. The impact of these sidewall interfacesneeds to be explored in greater depth for practical device engineering, and a related problem is that appropriate characterizationtechniques are not available. Here, we develop a novel electrothermal method and an intricate microfabricated structure toextract the thermal resistance of a sidewall interface between aluminum and silicon dioxide using suspended nanogratingstructures. The thermal resistance of the sidewall interface is measured to be 16 5 m2 K GW 1, which is twice as large as theequivalent horizontal planar interface comprising the same materials in the experimental sample. The rough sidewall interfacesare observed using transmission electron micrographs, which may be more extensive than at interfaces in the substrate plan in thesame nanostructure. A model based on a two-dimensional sinusoidal surface estimates the impact of the roughness on thermalresistance to be 2 m2 K GW 1. The large disparity between the model predictions and the experiments is attributed to theincomplete contact at the Al SiO2 sidewall interfaces, inferred by observation of underetching of the silicon substrate below thesidewall opening. This study suggests that sidewall interfaces must be considered separately from planar interfaces in thermalanalysis for nanostructured systems.KEYWORDS: nonplanar interface, thermal boundary resistance, roughness, incomplete contact, electron phonon coupling1. INTRODUCTIONThermal interface resistance can originate from both“intrinsic” sources derived from dissimilar properties of theenergy carriers between materials and “extrinsic” sources causedby the local microstructural quality of the interfaces. Thetheoretical lower bound of thermal interface resistance isdictated by the intrinsic sources, including the mismatch in theacoustic properties of the adjacent materials,9 and energytransfer between dissimilar energy carriers.9 12 The experimentally reported thermal interfacial resistance values are oftenlarger than theoretical predictions primarily because of“extrinsic” sources consisting of interfacial defects (roughness),13 18 intermixing of atomic species,19,20 crystallineInterfaces dictate thermal transport in many nanostructuredsystems, such as fin field-effect transistors (FinFETs)1,2 andinterconnects in microelectronics,3,4 and these interfaces areoften found to be nonplanar. A nonplanar interface comprisestwo types of orthogonal surfaces: those that are parallel to theprincipal plane and sidewalls that are perpendicular to it. Thequality of the sidewall interfaces can be lower than that ofplanar interfaces because of microstructural defects and hasbeen shown to impact the electrical5,6 and the opticaltransport.7,8 However, despite the increasing prevalence ofthree-dimensional (3D) nanostructures having a large arealfraction of sidewalls, the quantitative differences betweenthermal transport across planar and sidewall interfaces havenot been investigated. XXXX American Chemical SocietyReceived: May 11, 2017Accepted: August 8, 2017Published: August 8, 2017ADOI: 10.1021/acsami.7b06567ACS Appl. Mater. Interfaces XXXX, XXX, XXX XXX
Research ArticleACS Applied Materials & InterfacesFigure 1. TEM images for (a) nonetched planar, (b) etched planar, and (c) sidewall interfaces between Al and SiO2. The wavelength of roughnessand the peak peak roughness is defined as L and δ as shown in (d). The wavelength of the roughness is estimated as the distance between theadjacent peaks. We assume that L 15 nm for both (b) and (c) and δ 1 and 6 nm for (b) and (c), respectively.defects,21 23 and interfacial bonding strength.24 28 For sidewallinterfaces in the present study, there is a significantcontribution to the thermal interfacial resistance from extrinsicsources owing to the anisotropic fabrication processes, such asreactive ion etching (RIE) and directional material depositionthat tend to degrade the quality of the microstructure at thesidewall.7,29While thermal transport across planar interfaces has beenextensively studied,9,30 39 limited attention is given to theimpact of sidewall interface. In this article, we develop a noveltechnique to characterize the thermal interface resistance ofsidewall interfaces between metals and dielectrics, a commonmaterial combination in electronics such as interconnects. Thisprovides a fundamental understanding on sources of interfacialresistance and quantification of the disparity in the thermalresistance of sidewall and planar interfaces.2. THERMAL CHARACTERIZATION OF INTERFACESWe explore thermal transport across three types of interfacesbetween Al and SiO2: planar, planar and etched, and sidewallinterfaces as shown in Figure 1a c, respectively. The planarinterface provides a baseline value for interfacial resistancesbetween Al and SiO2, where Al is deposited on thermally grownSiO2 using evaporation. The planar-etched interface is treatedwith RIE process followed by the same metal deposition.Sidewall interfaces are fabricated using the identical etching andmetallization processes as those for the planar-etched interfaces.The etching process degrades the quality of the interface,specifically for sidewall interfaces, and the roughened interfacesare observed using transmission electron microscopy (TEM) asshown in Figure 1 (see the Supporting Information for samplefabrication). We characterize the planar and etched planarinterfaces using the time-domain thermoreflectance (TDTR)technique.21,40,41 Conducting measurements on sidewallinterfaces is significantly more challenging and requireselaborate microfabricated structures.Here, we develop an electrothermal method and an intricatemicrofabricated structure containing a high density of sidewallinterfaces along the conduction path to extract the thermalresistance of a sidewall interface between Al and SiO2. Wedesign and fabricate the suspended nanograting structures ofalternating Al and SiO2 for the electrothermal characterizationof sidewall interfaces as shown in Figure 2. Four of the Al linesare selectively patterned into the four-probe measurementconfigurations to serve as either a Joule heater or localthermometers, and these lines are connected to contact pads toprovide electrical access. The length of the grating structuresperpendicular to the metal lines is 10 times more longer than inFigure 2. Sample geometry for the thermal resistance of a sidewallinterface measurement structure. (a) Scanning electron microscopy(SEM) image depicting the entire suspended region of structure. Thedark region indicates the suspended region, and the measurementregion is between dashed lines. (b) Higher magnification SEM imageshows the location of the heater (false color red line) in the center ofthe grating structure, and the grating structure is symmetric withrespect to the heater line. Thermometers (false color blue lines redline) are located at 0, 4, 10, and 18 unit cells and referred to as T0, T4,T10, and T18, respectively. (c) SEM images for various combinations ofthe material width, and the images are taken prior to devicesuspension.the parallel direction. The high aspect ratio of the gratingstructures and suspended film confines heat to travel laterallythrough suspended alternating lines of Al and SiO2 andtherefore through each of the sidewall interfaces.42 Thesymmetry of the grating structures with respect to the centralheater line ensures that half of the heat flows outward in eachdirection. The width of the SiO2 layers varied between 150 and300 nm enables us to subtract out the volumetric contributionof the SiO2 layers, which is similar to the transmission linemeasurement for electrical contact resistance.43 The range ofSiO2 layer widths is designed to set the volumetric thermalresistance of each SiO2 layer to be comparable to the thermalresistance of a sidewall interface.BDOI: 10.1021/acsami.7b06567ACS Appl. Mater. Interfaces XXXX, XXX, XXX XXX
Research ArticleACS Applied Materials & InterfacesFor thermal characterization, a dc electrical current is appliedto generate Joule heating on the central heater line, and thetemperatures at four different locations are measured using theheater and the thermometers. These thermometers are locatedat n 0, 4, 10, and 18, where n is the integer number of unitcells (i.e., an Al and SiO2 bilayer) separating the thermometer nand the heater. The locations are shown in Figure 2b. Withincreasing power, the temperature differences between theheater and thermometers are measured. The temperaturedifference between the heater and the thermometer n is givenby ΔT0 n T(0) T(n), and for the constant thermalresistance of the nanograting structure, ΔT0 n linearly increaseswith applied heating power. The temperature differencesbetween the heater and the individual thermometers linearlyincrease with increasing heat input, as shown in Figure 3a. Weobtain the thermal resistance for the different number of unitcells from the power temperature slope and further extract thevalue for a single unit cell. The thermal resistance between theheater and the thermometer n, Rth,0 n″, is″ n R th,0dR e,0 dT0dR e, n dTn· ·dq″ dR e,0dq″ dR e, n(1)where the subscripts e and th denote electrical and thermalresistances, respectively, and dT0/dRe,0 and dTn/dRth,n areobtained from separate temperature-dependent electricalresistance measurements between 295 and 320 K. We thenextract the thermal resistance per unit cell Rth,unit″by measuringthe slope of the linear function Rth,0 n″versus the number of unitcells n.The thermal resistance of a single unit cell comprises threecomponents, which include the volumetric thermal resistances″ 2 , the Al layer Rth,Alof the SiO2 layer R th,SiO″ , and the thermalresistances at both sidewall interfaces R″Int,SW″″ R th,SiO″ 2 2·RInt,SW″R th,unit,total R th,Al(2)The contribution of the Al and SiO2 layers needs to beseparated out to extract the thermal boundary resistance (TBR)of sidewall interfaces. We estimate the thermal conductivity ofthe Al layer to be 120 W m 1 K 1 from the measuredelectrical resistivity using the Wiedemann Franz law with theuniversal Lorentz number of 2.44 10 8 W Ω K 2. Thecontribution from the Al layer (note that Rth,Al″ is less than 1%of R″th,unit) is then subtracted from the total thermal resistance.This simplifies eq 2 to contain only the volumetric thermalresistance of the SiO2 layer and the two thermal resistancesfrom the sidewall interfaces 1 ·wSiO 2·RInt,SW″″R th,unit 2 k SiO2 (3)We obtain Rth,unit″with four different width combinationsbetween Al and SiO2 as shown in Figure 2c. The thermalresistance of the sidewall interface is extracted by extrapolatingthe resistance of a unit cell with the width of SiO2, wSiO2, asshown in Figure 3b. The y-intercept of the linear fit equalstwice the sidewall TBR. The sidewall thermal interfaceresistance between the Al and thermal SiO2 layers is found tobe 16 5 m2 K 1 GW 1. The slope of eq 2 yields the inverseof the thermal conductivity of SiO2, and we extract a value ofkSiO2 1.4 0.1 W m 1 K 1, which agrees with the literaturevalue for thermally grown SiO2 to within 1%.44 46 Thisconfirms the accuracy of the electrothermal measurements ofthis work. The thermal conductivity of the SiO2 layer isindependent of the layer width because the spatial dimensionsin this measurement are much greater than the SiO2 phononmean free paths (MFPs).44,47 The uncertainty in Rth,SW″ispredominantly due to the uncertainty in wSiO2, which comesfrom the interfacial roughness and TEM resolution limit. Theuncertainty in wSiO2 is calculated to be 13 nm, and thisuncertainty in wSiO2 propagates to 4.4 m2 K GW 1 ofuncertainty in Rth,SW″ . Other sources of uncertainty, such asparasitic heat loss along the grating structure and radiation,contribute less than 6% error to the measurement (see theSupporting Information).Two planar interface samples are prepared using the sameoxide growth and Al deposition conditions as the nanogratingstructure for thermal resistance measurements of sidewallinterfaces. One planar interface is treated by etching SiO2before Al deposition with the same etching conditions as thenanograting structure for sidewall thermal interface resistanceFigure 3. (a) Experimentally measured temperature differencebetween the heater and the thermometers, denoted as ΔT0 n, wheren is the number of unit cells separating the thermometer from theheater. (b) Measured thermal resistances of the unit cells with variouswidths of SiO2, where the contribution of Al has been removed. Thedashed line is a linear fit to the data points. The slope of the trend lineis the inverse thermal conductivity of SiO2, kSiO2, and the y-interceptindicates the total TBR contribution from both sidewalls between Aland SiO2.CDOI: 10.1021/acsami.7b06567ACS Appl. Mater. Interfaces XXXX, XXX, XXX XXX
Research ArticleACS Applied Materials & Interfacesmeasurements, whereas the other has no treatment on theinterface. This is followed by depositing 91 nm thick Al usingelectron-beam evaporation on top of the thermally grown SiO2(without etching process). The planar interface between Al andSiO2 is investigated using TDTR, an optical pump probetechnique for measuring the thermal interface resistance inmultilayer thin-film structures.21,40 We find the thermalinterface resistance across the planar Al SiO2 interface to be 6.6 2.2 m2 K GW 1 for the unetched region and 6.7 1.9m2 K GW 1 for the etched region. Uncertainties in theextracted values are propagated from the uncertainties in the Altransducer layer thickness and the thermal properties of the Aland SiO2 layers (see the Supporting Information).3. RESULTS AND DISCUSSIONThe measured planar thermal interface resistance values areonly 40% of the thermal resistance of the sidewall interface.We attribute this discrepancy primarily to extrinsic factors,increased roughness, and incomplete contacts at sidewallinterfaces. A comparison between the planar (Figure 1a) andthe planar-etched (Figure 1b) interfaces shows that the etchingprocess increases the interface roughness, and the roughnessbecome more pronounced on the sidewall interfaces, as shownin Figure 1c. While the planar interfaces of both the etched andnonetched samples have perfect interfaces and 1 nm peak peak roughness, respectively, the sidewall interfaces exhibit 6nm peak peak roughness as measured using TEM imagesshown in Figure 1.This increased roughness impacts the transport properties ofenergy carriers and defects in the interfacial region. In additionto the roughness, the quality of the contact between Al andSiO2 at the sidewall interfaces is imperfect as evidenced by theundercut silicon below the sidewall interfaces, indicating thatpathways exist at the sidewall interfaces that allow etchants topass through (see the Supporting Information). The incomplete contact at the sidewall interfaces constitutes an additionalsource of sidewall thermal interface resistance.To understand the impact of roughness and the incompletecontact, we deconstruct the thermal interface resistancedepending on its source as followsFigure 4. Thermal interface resistance values for both sidewall andplanar interfaces of Al/SiO2, where δ is peak peak roughness. Thesolid lines are obtained from model predictions, and the dashed arrowindicates the contribution of incomplete contact to the thermalresistance of sidewall interfaces.and also use the Debye approximation (see the SupportingInformation).To estimate the contribution of electron phonon couplingand near interfacial defects to thermal interface resistance, wefirst model the rough surface as having a two-dimensional″ RInt,p″ p RInt,e″ p(δ) RExt,defects″″R total(δ) RExt,contact(4)where the argument δ is the peak peak roughness, thesubscript p p indicates phonon phonon energy transfer acrossan interface, e p indicates electron phonon coupling resistance in Al, defects indicates the interfacial structural defects, andcontact indicates incomplete contact. The components R″Int,p pand RInt,e p″are intrinsic sources of thermal interface resistancecorresponding to imperfect energy transfer between dissimilarmaterials. The remaining terms correspond to extrinsicresistance factors because of the interfacial microstructure.We individually estimate the components in thermalresistances from both intrinsic and extrinsic sources as shownin Figure 4, which provides a quantitative comparison betweenthe planar and the sidewall thermal interface resistances. Weuse the diffuse mismatch model to assess the phonon phononresistance R″Int,p p, giving a value of 1.6 m2 K GW 1 for theAl SiO2 interface, which is 25 and 10% of the thermalresistance for the planar and sidewall interfaces, respectively. Inthis model, we assume that SiO2 has the same acousticcharacteristics as quartz due to their similar atomic compositionFigure 5. (a) A roughness model in 3D, showing 2 2 unit cells,where L is the wavelength of a unit cell, and δ is peak peak roughness.(b) Schematic showing increased phonon boundary scattering in thevalley between rough structures. The red filled circle representsphonons at a distance of a phonon MFP Λp from the surface. L/2 isthe mean lateral characteristic length scale induced by rough geometry.(c) Schematic of incomplete contact of sidewall interfaces, whichbecomes a pathway for XeF2 gas to reach the underlying Si.sinusoidal profile as shown in Figure 5a. The local height f withrespect to the interfacial plane is given byf (x , y ) 2π 2π δsin x sin y 2 L L (5)where L is the wavelength in both x and y directions and δ isthe peak peak amplitude of roughness. We use TEM toestimate L and δ as shown in Figure 1. L is 15 nm for bothDDOI: 10.1021/acsami.7b06567ACS Appl. Mater. Interfaces XXXX, XXX, XXX XXX
Research ArticleACS Applied Materials & Interfacesthe etched planar and sidewall interfaces, and δ is 1 and 6nm for the etched planar and the sidewall interfaces,respectively.We evaluate the thermal resistance of electron phononcoupling in Al R″Int,e p with rough interfaces because thedominant thermal energy carriers are dissimilar between Al andSiO2. We begin by estimating the lattice thermal conductivity ofAl, which is a key parameter in R″Int,e p. The lattice thermalconductivity of pure Al (i.e., only phonon phonon andelectron phonon scattering exist) kp,Al is reported to be 6 Wm 1 K 1 based on ab initio calculations.48 However, the latticethermal conductivity in this work is smaller than that of theideal material as phonons experience additional scattering withgrains and rough structure of the interface. The average grainsize
Thermal Conduction across Metal Dielectric Sidewall Interfaces . thermal transport across planar and sidewall interfaces have not been investigated. . contribution to the thermal interfacial resistance from extrinsic sources owing to the anisotropic fabrication processes, such as
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