Measurement of Heat Dissipation and Thermal-Stability of Power Modules on DBC Substrates with Various Ceramics by Sic Micro-Heater Chip System and Ag Sinter Joining

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Article Measurement of Heat Dissipation and Thermal-Stability of Power Modules on DBC Substrates with Various Ceramics by SiC Micro-Heater Chip System and Ag Sinter Joining

Dongjin Kim 1,2, Yasuyuki Yamamoto 3, Shijo Nagao 2,* , Naoki Wakasugi 4, Chuantong Chen 2,* and Katsuaki Suganuma 2

1 Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan; [email protected] 2 The Institute of Scientiﬁc and Industrial Research, Osaka University, Osaka 567-0047, Japan; [email protected] 3 Specialty products development, Tokuyama Co., Yamaguchi 746-0006, Japan; [email protected] 4 Division of system development, Yamato Scientiﬁc Co., Ltd., Tokyo 135-0047, Japan; [email protected] * Correspondence: [email protected] (S.N.); [email protected] (C.C.); Tel.: +81-6-6879-8521 (S.N. & C.C.) Received: 4 October 2019; Accepted: 28 October 2019; Published: 31 October 2019

Abstract: This study introduced the SiC micro-heater chip as a novel thermal evaluation device for next-generation power modules and to evaluate the heat resistant performance of direct bonded copper (DBC) substrate with aluminum nitride (AlN-DBC), aluminum oxide (DBC-Al2O3) and silicon nitride (Si3N4-DBC) ceramics middle layer. The SiC micro-heater chips were structurally sound bonded on the two types of DBC substrates by Ag sinter paste and Au wire was used to interconnect the SiC and DBC substrate. The SiC micro-heater chip power modules were ﬁxed on a water-cooling plate by a thermal interface material (TIM), a steady-state thermal resistance measurement and a power cycling test were successfully conducted. As a result, the thermal resistance of the SiC micro-heater chip power modules on the DBC-Al2O3 substrate at power over 200 W was about twice higher than DBC-Si3N4 and also higher than DBC-AlN. In addition, during the power cycle test, DBC-Al2O3 was stopped after 1000 cycles due to Pt heater pattern line was partially broken induced by the excessive rise in thermal resistance, but DBC-Si3N4 and DBC-AlN specimens were subjected to more than 20,000 cycles and not noticeable physical failure was found in both of the SiC chip and DBC substrates by a x-ray observation. The results indicated that AlN-DBC can be as an optimization substrate for the best heat dissipation/durability in wide band-gap (WBG) power devices. Our results provide an important index for industries demanding higher power and temperature power electronics.

Keywords: power cycle test; SiC micro-heater chip; direct bonded copper (DBC) substrate; Ag sinter paste; wide band-gap (WBG); thermal resistance

1. Introduction Silicon carbide (SiC) and gallium nitride (GaN), are wide band-gap (WBG) semiconductors, are strongly recognized as the best materials for the power electronics applications demanding higher power and higher temperature [1,2]. Since these WBG semiconductors have superior properties, kind

Micromachines 2019, 10, 745; doi:10.3390/mi10110745 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 745 2 of 11 of a wide band gap (>3 eV), a high critical electric field (>3 MV/cm) and a high saturation velocity (>2 107 cm/s), SiC and GaN can enable to overcome the ultimate performances reached by silicon × (Si) based devices, in terms of power conversion efficiency [3]. In addition, WBG semiconductor devices can be operating much higher temperatures (>250 ◦C) than Si based devices (<150 ◦C) [4–8], this means massive, complex and heavy cooling systems can be eliminated from power conversion systems. Inverters and converters automotive components can be change to smaller by the simply heat dissipation design of in the high temperature environments [9]. In general, the structure of the power module has multi-layer structures of a semiconductor chip and a heat dissipation/insulation plate. All the layers have different material properties like a coefficient of thermal expansion (CTE), which causes thermos-mechanical stress during repetitive operating [10,11]. Besides, to withstand higher power and higher thermal density, how to insulate electricity and dissipate heat is one of the key issues of WBG power conversion systems. Therefore, there have been some issues, for instance how to interconnect these multi-layers to implement in high temperature and have a thermal-stable reliability. In addition, power electronic substrate plays an important role in dissipating heat to prevent power electronic module failure [12–18]. Heat dissipation/insulation substrate, which is a direct bonded copper (DBC) and a direct bonded aluminum (DBA), are existed between power semiconductor device and heat sink. Heat is transferred from surface of the power semiconductor device to the heat sink. The DBC and DBA were metallized both side of ceramic plate, to improve the thermal conductivity of ceramic substrates and form circuits [19], and were considered as the most promising substrates for power modules due to their excellent thermal conductivity, low thermal resistance, and high insulation voltage. However, the sandwich structure of the DBC and DBA substrate will cause thermo-mechanical stresses due to the CTE mismatch. It has been reported that heat resistance of the substrate depending on the type of ceramics and metals [20–22]. In this regard, our group carried out to evaluate thermo-mechanical stability of various ceramic applied active metal brazed (AMB) DBC substrates with the Ni plating layer during a harsh thermal shock cycling test at the temperature range of from 50 C to 250 C[23]. The DBC substrate with silicon nitride (Si N -DBC) ceramics middle layer − ◦ ◦ 3 4 indicate no serious damage within 1,000 cycles of thermal shock cycles, while those of an aluminum nitride (AlN) and alumina (Al2O3) caused by Cu layer severe delamination after the same cycles [23]. The results indicated that the failures may impress that the AlN and Al2O3 have critical disadvantage of higher CTE value and lower toughness than Si3N4. Since this previous study just investigated the DBC substrate itself, the failures must be induced by the mismatch between Cu and ceramic, meaning that the stress occurred from the DBC. Such thermal cycles give only homogeneous temperature change in the substrate specimens as shown in Figure1a. However, the realistic thermal distributions in a ceramic substrate must have a large temperature gradient to transfer the generated heat from surface of the device chips to the cooling system as shown in Figure1b. The stress induced by the CTE mismatch during the thermal conduction from chips to cooling system was more complexed than single DBC substrate. Therefore, the evaluation of such a practical temperature distribution is urgently needed for designing power module. In this context, power cycling test is the strongly useful evaluation methods to evaluate the reliability of a device packaging used in a condition similar to those in actual operations. In this process, the heat transfer performance of such a substrate mainly depends on the thermal properties of employed ceramic materials, e.g., Al2O3, AlN, and Si3N4. Power modules was required with a comprehensive combination of conductive Cu and insulation ceramics, involving the layer thicknesses and circuit patterns to achieve both thermal management and the least power loss at the same time. Usually, to measure the thermal resistance of devices after a power cycling test, it needs an equipment of power cycling system and also a T3ster system which means the Thermal Transient Tester system. It is a technology to monitor the thermal transport through the devices. However, both of the power cycling system and the T3ster system are very expensive and need to take up a lot of space. A simple, fast and miniaturized thermal measurement system is actually necessary. Micromachines 2018, 9, x 3 of 11 Micromachines 2019, 10, 745 3 of 11 Micromachines 2018, 9, x 3 of 10

Figure 1. Comparison of the test environments and its heat transfer path: (a) thermal shock or aging conditionFigure 1. Comparisonas non-driving of thecondition test environments and (b) actual and power its heat cycling transfer test path:as driving (a) thermal condition. shock or aging Figure 1. Comparison of the test environments and its heat transfer path: (a) thermal shock or aging condition as non-driving condition and (b) actual power cycling test as driving condition. conditionIn this study as non, SiC-driving micro condition-heater andchip (b was) actual designed power cycling and used test asto driving bond withcondition. the various type of ceramicIn this DBC study, substrates SiC micro-heater such as AlN chip and was Al designed2O3, and Si and3N used4. The to Ag bond sinter with joining the various was used type as of ceramica die to In this study, SiC micro-heater chip was designed and used to bond with the various type of attachDBC substrates material because such as which AlN and can Al withstand2O3, and the Si3N high4. The temperature Ag sinter a joiningpplications was usedand reported as a die toin attachmany ceramicpreviousmaterial DBC because studies. substrates which Steady cansuch-state withstand as thermal AlN and the resistance highAl2O temperature3, and from Si3N the4. applicationsThe SiC Ag micro sinter-heater and joining reported chip was to in used cooling many as previousa system die to attachwithstudies. different material Steady-state typebecause DBC thermal which were resistancecan measured. withstand from In theaddition, high SiC micro-heater temperature the power chip cyclingapplications to cooling test also and system wasreported performed with in di manyfferent to previousinvestigatedtype DBC studies. were the measured.high Steady tempe-staterature In thermal addition, reliability resistance the of power the from SiC cycling microthe SiC test-heater micro also chip was-heater die performed attachchip to structure cooling to investigated onsystem each withtypethe high differenDBC temperature substrate.t type DBC The reliability were failure measured. ofafter the power SiC In micro-heater addition, cycling wasthe chip power analyzed die cycling attach by a structuretest micro also-focus was on each performed3D computed type DBC to investigatedtomographysubstrate. The the (CT) failure high X- Raytemperature after system. power Treliability cyclinghis method was of the analyzedcan SiC significantly micro by a-heater micro-focus distinguish chip die 3D withattach computed traditio structure tomographynal onthermal each typecycling(CT) DBC X-Ray test substrate. system.because TheThis thermal failure method properties after can power significantly of materialcycling distinguish was can analyzed be considered with by traditional a micro during-focus thermal repetitive 3D cyclingcomputed thermal test tomographyenvironments.because thermal (CT) Our propertiesX- Raymethod system. ofof materialpower This method cycling can be cantest considered significantly with SiC during heater distinguish repetitive chip may with thermal provide traditional environments. in important thermal cyclingevaluationOur method test indexbecause of power of thermal futurecycling packagingpropertiestest with SiC of systems heater material chip with can may WBG be provide considered semiconductors in important during targetedevaluation repetitive for thermal index high of- environments.temperature/powerfuture packaging Our systems methoddensity with powerof power WBG modules. semiconductorscycling test with targeted SiC heater for high-temperaturechip may provide/power in important density evaluationpower modules. index of future packaging systems with WBG semiconductors targeted for high- temperature/power2. Materials and Methods density power modules. 2. Materials and Methods 2.2.1. Materials SiC Micro and-Heater Methods Chip and Direct Bonded Copper (DBC) Substrate 2.1. SiC Micro-Heater Chip and Direct Bonded Copper (DBC) Substrate Geometrically 3D designed SiC micro-heater chip was fabricated with n-doped 4H-SiC by using 2.1. SiCGeometrically Micro-Heater 3D Chip designed and Direct SiC Bonded micro-heater Copper chip(DBC was) Substrate fabricated with n-doped 4H-SiC by using a lithography technology (see Figure 2a), also it consists of a heater, a probe and an electrode, the a lithography technology (see Figure2a), also it consists of a heater, a probe and an electrode, the materialGeometrically of heater and 3D designedprobe are SiCplatinum micro -(Pt)heater. The chip DBC was substrate fabricated with with five n lands-doped of 4H the-SiC top byside using was material of heater and probe are platinum (Pt). The DBC substrate with five lands of the top side was aintroduced, lithography and technology the SiC (see heater Figure chip 2 a was), also die it consists attached of onto a heater, the center a probe land and and an electrode its geometric, the introduced, and the SiC heater chip was die attached onto the center land and its geometric dimension materialdimension of heateris as displayed and probe in are Fig platinumure 2b. At (Pt) the. Thebackside DBC substrateof SiC micro with-heater five lands chip ofand the the top top side side was of is as displayed in Figure2b. At the backside of SiC micro-heater chip and the top side of DBC substrates introduced,DBC substrates and were the metallized SiC heater in chip the order was dieof Ti attached and Ag l ontoayer by the a centersputtering land process. and its geometric dimensionwere metallized is as displayed in the order in Fig of Tiure and 2b Ag. At layer the backside by a sputtering of SiC micro process.-heater chip and the top side of DBC substrates were metallized in the order of Ti and Ag layer by a sputtering process.

Figure 2. SiC heater die-attached structure: (a) SiC micro-heater chip structure (b) SiC heater chip on Figurethe substrate. 2. SiC heater die-attached structure: (a) SiC micro-heater chip structure (b) SiC heater chip on the substrate. Figure 2. SiC heater die-attached structure: (a) SiC heater chip on the substrate (b) SiC micro-heater chip structure.

Micromachines 2018, 9, x 4 of 10 Micromachines 2019, 10, 745 4 of 11 2.2. Die Attach Material 2.2. DieTwo Attach types Material of Ag particles were mixed and used to make Ag paste. The first type comprised flake-Twoshaped types Ag ofparticles Ag particles with an were average mixed lateral and used diameter to make of 8 Ag μm paste. and a Thethickness first type of 260 comprised nm, as shownflake-shaped in Figure. Ag 3a particles (AgC-239, with Fukuda an average Metal lateral Foil and diameter Powder of Co., 8 µm Ltd., and Kyoto, a thickness Japan). of T 260he second nm, as typeshown comprised in Figure spherical3a (AgC-239,-shape Fukuda particles Metal withFoil an average and Powder diameter Co., o Ltd.,f 400Kyoto, nm, as Japan).shown in The Figure second 3b (FHD,type comprised MitsuiMining spherical-shape and Smelting particles Co., Ltd., with Tokyo, an average Japan).diameter The two kinds of 400 of nm, Ag as particles shown were in Figure mixed3b as(FHD, the Ag MitsuiMining filler with a and weight Smelting ratio Co.,of 1:1 Ltd., [24] Tokyo,. These Japan). hybridThe particles two kinds wereof then Ag stirred particles magnetically were mixed foras the10 Agmin filler and withvibrated a weight ultrasonically ratio of 1:1 for [24 ].30 These min with hybrid viscosity particles of were 100– then200 stirredPa·s solvent magnetically (alkylene for glycols10 min and polyols) vibrated using ultrasonically a hybrid formixer 30 min(HM with-500, viscosity Keyence ofCorporation 100–200 Pa, sOsaka, solvent Japan (alkylene) to fabricate glycols · Agand paste polyols) to achieve using aa hybriduniform mixer mixture. (HM-500, The amount Keyence of Corporation, the solvent Osaka,was maintained Japan) to at fabricate approximately Ag paste 10to wt achieve % in athe uniform paste to mixture. keep a Thesuitable amount viscosity of the solventof 150–250 was c maintainedps at room attemperature. approximately The 10Ag wt paste % in wasthe pastethen printed to keep onto a suitable the center viscosity land ofof 150–250the DBC centi substrate poises (cPs)which at were room p temperature.urchased from The Mitsubishi Ag paste Materialswas then Corporation printed onto. theFor centerthe bonding land of process, the DBC a substratesstep-by-step which profile were with purchased low pressure from Mitsubishi(0.4 MPa), includingMaterials 120 Corporation. °C and 180 For °C the for bonding 5 min as process, pre-heating, a step-by-step and then profile250 °C with for 60 low min, pressure was applied (0.4 MPa), to prevent pores formation caused by evaporation of organic solvent (see Figure 3b). Finally, the including 120 ◦C and 180 ◦C for 5 min as pre-heating, and then 250 ◦C for 60 min, was applied to sinteredprevent porescross section formation is formed caused byas shown evaporation in Figure of organic 3c. For solvent power (see supply, Figure 10EA3b). Finally,Au wires the of sintered 45 μm diametercross section (TANAKA is formed DENSHI as shown KOGYO, in Figure 3c.Yoshida, For power Japan) supply, on 10EA the Au four wires electrode of 45 µ werem diameter each intercon(TANAKAnected DENSHI by an KOGYO,ultrasonic Yoshida, wire bonding Japan) machine on the four (Kulicke electrode & Soffa were Industries each interconnected Inc, 4500 digital by an seriesultrasonic, Singapore wire bonding). machine (Kulicke & Soffa Industries Inc, 4500 digital series, Singapore).

Figure 3. (a) Ag particles, (b) sintering condition and (c) schematic diagram of the assembled image. Figure 3. (a) Ag particles, (b) sintering condition and (c) schematic diagram of the assembled image. 2.3. Test Machine and Method 2.3. Test Machine and Method Figure4 shows the machine of power cycling test and thermal resistance measurement and its component.Figure 4The shows machine the machine consists of of power four parts, cycling power test supplier,and thermal controller, resistance monitor measurement and water-cooling and its component.system. Over The 200 machine W of power consist iss supplied of four fromparts, the power power supplier, source tocontroller, the electrode, monitor and and when water heat- cooling system. Over 200 W of power is supplied from the power source to the electrode, and when is generated from the top of the heater chip, heat is transferred to the heat sink (25 ◦C) through the heatdie attachis generated and the from DBC the substrate. top of the At heater this time,chip, SiCheat heater is transferred/DBC die-attached to the heat specimensink (25 °C) is mountedthrough theon die the attach water coolingand the DBC plate substrate. by diamond At this thermal time, grease SiC heater/DBC (thermal interface die-attached material, specimen TIM). is To mounted prevent ondelamination the water cooling between plate specimen by diam andond coolingthermal plategrease during (thermal power interface cycling, material, the DBC TIM). substrate To prevent was delaminationﬁxed to the water-cooling between specimen plate and with cooling a force plate of 20 during N. Then, power∆T cycling,is calculated the DBC by substrate the temperature was fixed of to the water-cooling plate with a force of 20 N. Then, ∆T is calculated by the temperature of the top surface of the SiC heater chip (Tchip) and the temperature of the thermocouple attached to the water- cooling plate (Tthermocouple), the equation is as follows:

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the top surface of the SiC heater chip (Tchip) and the temperature of the thermocouple attached to the Micromachines 2018, 9, x 5 of 10 water-cooling plate (Tthermocouple), the equation is as follows:

∆T =∆TT = Tchip T– Tthermocouple (1)(1) chip − thermocouple Here, the thermal resistance (Rth) can be calculated by dividing this temperature difference (∆T) Here, the thermal resistance (Rth) can be calculated by dividing this temperature diﬀerence (∆T) by the input power (Q) as follows: by the input power (Q) as follows: RthR=th =∆ T∆/TQ/Q (2)(2) where,where,Q Q(W) (W is) Jouleis Joule heating heating by the by electrical the electrical power of power SiC micro-heater of SiC micro chip-heater and T chipis the and temperature T is the (K).temperature (K).

FigureFigure 4.4. Power cycling test system with SiC micro-heatermicro-heater chip.chip. 3. Results and Discussion 3. Results and Discussion 3.1. Steady-State Thermal Resistance 3.1. Steady-State Thermal Resistance For thermal resistances measurement, a specimen was packaged by the method introduced in the previousFor sectionthermal (see resistance Figure3s). measurement After the die, bonding,a specimen the was specimen packaged interconnected by the method without introduced noticeable in defectsthe previous such as section oxidation, (see andFigure its actual3). After manufactured the die bonding and, interconnected the specimen SiCinterconnected heater die attached without specimennoticeable are defects shown such in Figureas oxidation,5a. Furthermore, and its actual the manufactured electrode and temperatureand interconnected sensor SiC of SiC heater heater die chipattached was specimen Au wire bondedare shown with in Figure the DBC 5a. substrate Furthermore, after the die-attach electrode bonding. and temperature The Au wiresensor bonded of SiC withheater the chip DBC was substrate Au wire was bonded given at wi upperth the left DBC corner substrate in Figure after5a. dieIn addition,-attach bonding an enlarged. The image Au wire of thebonded packaged with the SiC DBC micro-heater substrate chipwas circuitgiven at is upper shown left in Figurecorner5 inb. Figure When 5a. the In power addition, is supplied, an enlarged the heatimage transfer of the pathpackag is showned SiC in micro Figure-heater6a, where chip each circuit layer is shown has a thermal in Figure resistance. 5b. When Here, the sincepower all is materialssupplied, exceptthe heat ceramic transfer are path the same, is shown input in power Figure dependent 6a, where thermal each layer resistance has a performancethermal resistance. can be directlyHere, since compared all materials according except to ceramic ceramic type. are The the results same, ofinput thermal power resistance dependent performance thermal are resistance shown performance can be directly compared according to ceramic type. The results of thermal resistance in Figure6b. The thermal resistance of the SiC micro-heater chip power modules on the DBC-Al 2O3 performance are shown in Figure 6b. The thermal resistance of the SiC micro-heater chip power substrate at power over 200 W was about twice higher than DBC-Si3N4 and also higher than DBC-AlN. modules on the DBC-Al2O3 substrate at power over 200 W was about twice higher than DBC-Si3N4 The thermal resistance of DBC-AlN was lower than that of DBC-Si3N4. In addition, thermal resistance and also higher than DBC-AlN. The thermal resistance of DBC-AlN was lower than that of DBC- of the SiC micro-heater chip power modules on the DBC-Al2O3 substrate tended to increase thermal resistanceSi3N4. In addition, signiﬁcantly thermal with resistance increasing of power the SiC but micro there- isheater almost chip not power so much modules change on for the the DBC DBC-AlN-Al2O3 substrate tended to increase thermal resistance significantly with increasing power but there is almost and DBC-Si3N4 with the increased power. The reason of the lowest thermal resistance of AlN is that not so much change for the DBC-AlN and1 DBC1 -Si3N4 with the increased power. The reason of the the thermal conductivity of AlN (285 W m− K− , for single crystals) is signiﬁcantly higher than Al2O3 lowest thermal1 1 resistance of AlN is that·1 the·1 thermal conductivity of AlN (285 W·m−1·K −1, for single (25 W m− K− ) and Si3N4 (60–90 W m− K− )[21,25]. Therefore, AlN-DBC can be considered to have crystals)· is· significantly higher than· Al· 2O3 (25 W·m−1·K −1) and Si3N4 (60–90 W·m−1·K −1) [21,25]. Therefore, AlN-DBC can be considered to have the best heat dissipation performance of an excellent power module when applied properly in the area where heat dissipation is intensively required.

Micromachines 2019, 10, 745 6 of 11 the best heat dissipation performance of an excellent power module when applied properly in the area where heat dissipation is intensively required. Micromachines 2018, 9, x 6 of 11

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Figure 5. Test specimen description: (a) interconnected SiC heater chip on direct bonded copper (DBC) substrateFigure 5. andTest ( specimenb) its SEM description: image of detail (a) interconnected components. SiC heater chip on direct bonded copper (DBC) Figure 5. Test specimen description: (a) interconnected SiC heater chip on direct bonded copper (DBC) substrate and (b) its SEM image of detail components. substrate and (b) its SEM image of detail components.

Figure 6. (a) Schematic diagram of heat path and its thermal resistance measurement during power Figurecycling 6. (b ()a results) Schematic of various diagram ceramics of heat of path thermal and resistance.its thermal resistance measurement during power cycling (b) results of various ceramics of thermal resistance. 3.2. PowerFigure Cycling 6. (a) Schematic Test diagram of heat path and its thermal resistance measurement during power cycling (b) results of various ceramics of thermal resistance. 3.2. PowerFor the Cycling power Test cycling test, the temperature of the cooling plate was constantly controlled to be 503.2.◦C. ForPower The the waterCycling power ﬂow Testcycling rate wastest, 5.4the Ltemperature/min. The constant of the cooling current plate for heating was constantly were tested, controlled and constant to be 50current °C. The is necessarywater flow to rate suppress was 5.4 the L/min. surge The spike constant when heatingcurrent for was heating started. were The tested, typical and temperature constant currentproﬁleFor ofis thethenecessary powerpower tocyclingcycling suppress istest, plotted the the surge temperature in Figure spike7 .when Theof the input heating cooling electric was plate currentstarted. was constantly inThe the typical test wascontrolled temperature 2.1 A. Theto be profilecondition50 °C. of The the of water powerpower flow cyclingcycling rate testwasis plotted was5.4 L/min. applied in Fig Theure 2 s 7constant ON. The and input 5current s OFFelectric atfor the currentheating input inwere power the tested,test of was 200 and W.2.1 constantA. The current is necessary to suppress the surge spike when heating was started. The typical temperature conditionThe resultsof pow indicateder cycling thattest Alwas2O applied3 was stopped 2 s ON duringand 5 s theOFF power at the cycleinput testpower due of to 200 excessive W. rise profile of the power cycling is plotted in Figure 7. The input electric current in the test was 2.1 A. The in thermalThe results resistance indicated before that 1000 Al2 cycles,O3 was Sistopped3N4 and during AlN specimens the power werecycle each test due subjected to excessive to more rise than in condition of power cycling test was applied 2 s ON and 5 s OFF at the input power of 200 W. thermal20,000 cycles resistance of power before cycle 1000 tests. cycles, In the Si3 caseN4 and of AlAlN2O 3specimensstopping, were die-attach each andsubjected ceramic to componentmore than The results indicated that Al2O3 was stopped during the power cycle test due to excessive rise in 20,000were not cycles any degradationof power cycle but tests. the Pt In heater the case pattern of Al line2O3 wasstopping, partially die broken-attach asand shown ceramic in Figure component8a due thermal resistance before 1000 cycles, Si3N4 and AlN specimens were each subjected to more than wereto the not poor any heat degradation dissipation but performance the Pt heater of Al pattern2O3. The line Pt was heater partially pattern broken line accumulated as shown in a lotFigure of heat, 8a 20,000 cycles of power cycle tests. In the case of Al2O3 stopping, die-attach and ceramic component due to the poor heat dissipation performance of Al2O3. The Pt heater pattern line accumulated a lot ofwere heat, not leading any degradation to the temperature but the increasePt heater significantly, pattern line leadingwas partially to the brokenfilm of aspattern shown line in partiallyFigure 8a melteddue to as the shown poor inheat Figure dissipation 8b. performance of Al2O3. The Pt heater pattern line accumulated a lot of heat, leading to the temperature increase significantly, leading to the film of pattern line partially Figure 9a shows temperature swing behavior of the Si3N4-DBC substrate, the heater temperature atmelted the beginning as shown was in Figure about 8b.240 °C, and when it reached 20,000 cycles, the temperature was about 260 °C. In the case of AlN-DBC substrate, the starting temperature was about 220 °C, and the temperature after 20,000 cycles was about 280 °C (see Figure 9b). The temperature ranges of Si3N4 and AlN are not significantly different, but the temperature change tendency was remarkable. Unlike

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Micromachines 2018, 9, x 7 of 11 leading to the temperature increase significantly, leading to the film of pattern line partially melted as shownFigure in Figure 9a shows8b. temperature swing behavior of the Si3N4-DBC substrate, the heater temperature at the beginning was about 240 °C, and when it reached 20,000 cycles, the temperature was about Figure9a shows temperature swing behavior of the Si 3N4-DBC substrate, the heater temperature 260 °C. In the case of AlN-DBC substrate, the starting temperature was about 220 °C, and the at the beginning was about 240 ◦C, and when it reached 20,000 cycles, the temperature was about 260 ◦C. temperature after 20,000 cycles was about 280 °C (see Figure 9b). The temperature ranges of Si3N4 InMicromachines the case of 2018 AlN-DBC, 9, x substrate, the starting temperature was about 220 ◦C, and the temperature7 of 10 and AlN are not significantly different, but the temperature change tendency was remarkable. Unlike after 20,000 cycles was about 280 ◦C (see Figure9b). The temperature ranges of Si 3N4 and AlN are not Si3N4 substrate, which has a small change in temperature, AlN was largely divided into two stages. significantlySi3N4 substrate, different, which but has the a temperaturesmall change change in temperature, tendency wasAlN remarkable.was largely Unlikedivided Si into3N4 twosubstrate, stages. The stage 1 is from the starting point to 4000 cycles, it reached up to saturation temperature. The whichThe stage has a small1 is from change the instarting temperature, point to AlN 4000 was cycles, largely it dividedreached into up twoto saturation stages. The temperature. stage 1 is from The stage 2 is from 4000 cycles to 20,000 cycles, at a relatively stable temperature, the temperature slowly thestage starting 2 is from point 4000 to 4000cycles cycles, to 20,000 it reached cycles, upat a to relatively saturation stable temperature. temperature, The the stage temperature 2 is from slowly 4000 reached its peak point. cyclesreached to 20,000 its peak cycles, point. at a relatively stable temperature, the temperature slowly reached its peak point.

Figure 7. Temperature swing during power cycling tests. FigureFigure 7. 7.Temperature Temperature swing swing during during power power cycling cycling tests. tests.

a b

Figure 8. Pt heater pattern line was partly broken ( a) and its magniﬁedmagnified view (b). Figure 8. Pt heater pattern line was partly broken (a) and its magnified view (b).

Figure 9. Temperature swing behavior during power cycling tests: (a) Si3N4 and (b) AlN.

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Si3N4 substrate, which has a small change in temperature, AlN was largely divided into two stages. The stage 1 is from the starting point to 4000 cycles, it reached up to saturation temperature. The stage 2 is from 4000 cycles to 20,000 cycles, at a relatively stable temperature, the temperature slowly reached its peak point.

Figure 7. Temperature swing during power cycling tests.

a b

Micromachines 2019Figure, 10, 745 8. Pt heater pattern line was partly broken (a) and its magnified view (b). 8 of 11

Figure 9. Temperature swing behavior during power cycling tests: (a) Si3N4 and (b) AlN. Figure 9. Temperature swing behavior during power cycling tests: (a) Si3N4 and (b) AlN. To investigate a factor of different temperature behavior, a nondestructive test was performed by a micro-focus 3D computed tomography (CT) X-Ray system (XVA-160N, Uni-Hite System Corporation, Yamato City, Japan) on the specimens after power cycling tests. Figure 10 displays nondestructive test results (X-ray) after power cycling tests after power cycling tests (20,000 cycles). Figure 10a,b show the SiC micro-heater chip power modules on the DBC-Si3N4 substrate and its high magnification image, respectively. Figure 10b,c the SiC micro-heater chip power modules on the DBC-Si3N4 substrate of AlN and its high magnification image Figure 10d, respectively. There was no relationship between the different temperature behavior of the two substrates and the substantial damage to the die attach structure. Therefore, no delamination of the ceramic or degradation of the Ag sinter joint occurred during power cycling tests, and it was clearly found that the temperature rise did not cause physical failure. Despite the excellent thermal conductivity of AlN, the cause of temperature rise shown in Figure9b can be regarded as an issue of the TIM. The mechanism of air entrainment into the TIM are shown in Figure 11. In the initial state, the DBC substrate and the water cooling plate were closely adhered by TIM (see Figure 11a), after experiencing expansion and shrinkage, there should be some gaps generation between the TIM and substrate, the adhesion strength changed to lower with a little gap generation as displayed in Figure 11b,c. Air entrainment into the TIM also can lead to the temperature swing increase. Micromachines 2018, 9, x 8 of 10

To investigate a factor of different temperature behavior, a nondestructive test was performed by a micro-focus 3D computed tomography (CT) X-Ray system (XVA-160N, Uni-Hite System Corporation, Yamato City, Japan) on the specimens after power cycling tests. Figure 10 displays nondestructive test results (X-ray) after power cycling tests after power cycling tests (20,000 cycles). Figure 10a,b show the SiC micro-heater chip power modules on the DBC-Si3N4 substrate and its high magnification image, respectively. Figure 10b,c the SiC micro-heater chip power modules on the DBC-Si3N4 substrate of AlN and its high magnification image Figure 10d, respectively. There was no relationship between the different temperature behavior of the two substrates and the substantial damage to the die attach structure. Therefore, no delamination of the ceramic or degradation of the Ag sinter joint occurred during power cycling tests, and it was clearly found that the temperature rise did not cause physical failure. Despite the excellent thermal conductivity of AlN, the cause of temperature rise shown in Figure 9b can be regarded as an issue of the TIM. The mechanism of air entrainment into the TIM are shown in Figure 11. In the initial state, the DBC substrate and the water cooling plate were closely adhered by TIM (see Figure 11a), after experiencing expansion and Micromachines 2018, 9, x 9 of 11 shrinkage, there should be some gaps generation between the TIM and substrate, the adhesion strength changed to lower with a little gap generation as displayed in Figure 11b,c. Air entrainment Micromachines 2019, 10, 745 9 of 11 into the TIM also can lead to the temperature swing increase.

Figure 10.10 . NondestructiveNondestructive test results (X (X-ray)-ray) after power cycling tests (20,000 cycles): ( a) specimen specimen of Si N and (b) its high magniﬁcation image, (c) specimen of AlN and (d) its high magniﬁcation image. Si3N44 and (b) its high magnification image,image, (c) specimen of AlN and (d) its high magnification image.

Thus, to avoid this phenomenon, it can be prevented by using a soft TIM or warpage suppression Thus, to avoid this phenomenon, it can be prevented by using a soft TIM or warpage suppression mechanism that does not harden under repeated substrate warpage behavior. The results of the mechanism that does not harden under repeated substrate substrate warpage behavior. The results of the power cycling test using the SiC micro-heater chip show the heat resistance performance of the DBC power cycling test using the SiC micro-heaheaterter chip show the heat resistanceresistance performance of the DBC substrate, are signiﬁcantly diﬀerent from the traditional thermal shock test. Consequently, the AlN-DBC substrate,, are significantly different from the traditional thermal shock test. Consequently, the AlN- substrates may have the best heat dissipation and durability, perhaps by optimizing TIM performance DBC substrates may have the best heat dissipation and durability, perhaps by optimizing TIM for WBG power conversion systems. performance for WBG power conversion systems.

Figure 11. Schematic diagram of AlN temperature rise mechanism by mechanical behavior during powerFigure cycling11. Schematic test: (a )diagram initial state of AlN (b) expansion temperature (c) shrinkage.rise mechanism by mechanical behavior during power cycling test: (a) initial state (b) expansion (c) shrinkage.

Micromachines 2019, 10, 745 10 of 11

4. Conclusions In this study, a power cycling test system based on a SiC micro-heater chip was developed to simulate the active SiC devices where the temperature of the heater chip can be recorded through the power cycling test in real-time. The new approach is useful for the evaluation of high-power devices with a similar condition to the real packaging system. The thermal resistance of of the SiC micro-heater chip power modules on DBC-AlN was lower than that of DBC-Al2O3 and DBC-Si3N4. In addition, thermal resistance of the SiC micro-heater chip power modules on the DBC-Al2O3 substrate tended to increase thermal resistance signiﬁcantly with increasing power but there is almost not so much change for the DBC-AlN and DBC-Si3N4 with the same increased power. Furthermore, no delamination of the ceramic or degradation of the Ag sinter joint occurred during power cycling tests for both DBC-AlN and DBC-Si3N4, and it was clearly found that the temperature rise did not cause physical failure. The results exhibit that both DBC-AlN and DBC-Si3N4 are excellent candidates for high power modules with WBG semiconductor devices, suitable for industries demanding high energy eﬃciency, when the packaging like the die-attach and the cooling system is well optimized.

Author Contributions: Conceptualization, S.N. and K.S.; methodology, Y.Y.; software, S.N.; validation, C.C.; formal analysis, N.W., D.K.; investigation, D.K.; writing—original draft preparation, D.K.; writing—review and editing, C.C. and S.N.; visualization, D.K., C.C. and S.N.; supervision, K.S.; project administration, S.N. and K.S.; funding acquisition, S.N. and K.S. Funding: The present study is supported by the METI project: Standardization for Energy Conservation as International Standardization of Thermal Property Measurements for Next Generation Power Electronics. The authors are grateful to all the members of the standardization project. The present study is supported by Yamato Scientific. Co. Ltd. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Gao, Y.; Zhang, H.; Li, W.; Jiu, J.; Nagao, S.; Sugahara, T.; Suganuma, K. Die bonding performance using bimodal Cu particle paste under diﬀerent sintering atmospheres. J. Electron. Mater. 2017, 46, 4575–4581. [CrossRef] 2. Wong, K.Y.; Chen, W.; Liu, X.; Zhou, C.; Chen, K.J. GaN smart power IC technology. Phys. Status Solidi Basic Res. 2010, 247, 1732–1734. [CrossRef] 3. Roccaforte, F.; Giannazzo, F.; Iucolano, F.; Eriksson, J.; Weng, M.H.; Raineri, V. Surface and interface issues in wide band gap semiconductor electronics. Appl. Surf. Sci. 2010, 256, 5727–5735. [CrossRef] 4. Kim, D.; Chen, C.; Pei, C.; Zhang, Z.; Nagao, S.; Suetake, A.; Sugahara, T.; Suganuma, K. Thermal shock reliability of a GaN die-attach module on DBA substrate with Ti/Ag metallization by using micron/submicron Ag sinter paste. Jpn. J. Appl. Phys. 2019, 58, SBBD15. [CrossRef] 5. Noh, S.; Choe, C.; Chen, C.; Suganuma, K. Heat-resistant die-attach with cold-rolled Ag sheet. Appl. Phys. Express 2018, 11, 016501. [CrossRef] 6. Kunimune, T.; Kuramoto, M.; Ogawa, S.; Sugahara, T.; Nagao, S.; Suganuma, K. Ultra thermal stability of LED die-attach achieved by pressureless Ag stress-migration bonding at low temperature. Acta Mater. 2015, 89, 133–140. [CrossRef] 7. Millan, J.; Godignon, P.; Perpina, X.; Perez-Tomas, A.; Rebollo, J. A survey of wide bandgap power semiconductor devices. IEEE Trans. Power Electron. 2014, 29, 2155–2163. [CrossRef] 8. Noh, S.; Choe, C.; Chen, C.; Zhang, H.; Suganuma, K. Printed wire interconnection using Ag sinter paste for wide band gap power semiconductors. J. Mater. Sci. Mater. Electron. 2018, 29, 15223–15232. [CrossRef] 9. Pérez-Tomás, A.; Fontserè, A.; Placidi, M.; Jennings, M.R.; Gammon, P.M.Modelling the metal–semiconductor band structure in implanted ohmic contacts to GaN and SiC. Model. Simul. Mater. Sci. Eng. 2013, 21, 035004. [CrossRef] 10. Hung, T.Y.; Chiang, S.Y.; Huang, C.J.; Lee, C.C.; Chiang, K.N. Thermal-mechanical behavior of the bonding wire for a power module subjected to the power cycling test. Microelectron. Reliab. 2011, 51, 1819–1823. [CrossRef] Micromachines 2019, 10, 745 11 of 11

11. Chen, C.; Suganuma, K.; Iwashige, T.; Sugiura, K.; Tsuruta, K. High-temperature reliability of sintered microporous Ag on electroplated Ag, Au, and sputtered Ag metallization substrates. J. Mater. Sci. Mater. Electron. 2018, 29, 1785–1797. [CrossRef] 12. Park, S.; Nagao, S.; Kato, Y.; Ishino, H.; Sugiura, K.; Tsuruta, K.; Suganuma, K. High-temperature die attachment using Sn-plated Zn solder for power electronics. IEEE Trans. Compon. Packag. Manuf. Technol. 2015, 5, 902–909. [CrossRef] 13. Yu, F.; Cui, J.; Zhou, Z.; Fang, K.; Johnson, R.W.; Hamilton, M.C. Reliability of Ag sintering for power semiconductor die attach in high-temperature applications. IEEE Trans. Power Electron. 2017, 32, 7083–7095. [CrossRef] 14. Chen, C.; Suganuma, K. Microstructure and mechanical properties of sintered Ag particles with ﬂake and spherical shape from nano to micro size. Mater. Des. 2019, 162, 311–321. [CrossRef] 15. Zhang, Z.; Chen, C.; Yang, Y.; Zhang, H.; Kim, D.; Sugahara, T.; Nagao, S.; Suganuma, K. Low-temperature and pressureless sinter joining of Cu with micron/submicron Ag particle paste in air. J. Alloys Compd. 2019, 780, 435–442. [CrossRef] 16. Park, S.; Nagao, S.; Sugahara, T.; Suganuma, K. Mechanical stabilities of ultrasonic Al ribbon bonding on electroless nickel immersion gold ﬁnished Cu substrates. Jpn. J. Appl. Phys. 2014, 53, 2–9. [CrossRef] 17. Park, S.; Nagao, S.; Sugahara, T.; Suganuma, K. Heel crack propagation mechanism of cold-rolled Cu/Al clad ribbon bonding in harsh environment. J. Mater. Sci. Mater. Electron. 2015, 26, 7277–7289. [CrossRef] 18. Jiu, J.; Zhang, H.; Nagao, S.; Sugahara, T.; Kagami, N.; Suzuki, Y.; Akai, Y.; Suganuma, K. Die-attaching silver paste based on a novel solvent for high-power semiconductor devices. J. Mater. Sci. 2016, 51, 3422–3430. [CrossRef] 19. Lindemann, A.; Strauch, G. Properties of direct aluminium bonded substrates for power semiconductor components. PESC Rec. IEEE Annu. Power Electron. Spec. Conf. 2004, 6, 4171–4177. 20. Xu, L.; Wang, M.; Zhou, Y.; Qian, Z.; Liu, S. An optimal structural design to improve the reliability of

Al2O3-DBC substrates under thermal cycling. Microelectron. Reliab. 2016, 56, 101–108. [CrossRef] 21. Fei, M.; Fu, R.; Agathopoulos, S.; Fang, J.; Wang, C.; Zhu, H. A preparation method for Al/AlN ceramics substrates by using a CuO interlayer. Mater. Des. 2017, 130, 373–380. [CrossRef] 22. Lin, C.Y.; Tuan, W.H. Direct bonding of aluminum to alumina for thermal dissipation purposes. Int. J. Appl. Ceram. Technol. 2016, 13, 170–176. [CrossRef] 23. Choe, C.; Chen, C.; Noh, S. Thermal shock performance of DBA/AMB substrates plated by Ni and Ni—P layers for high-temperature applications of power device modules. Materials 2018, 11, 2394. [CrossRef] [PubMed] 24. Chen, C.; Nagao, S.; Suganuma, K.; Jiu, J.; Sugahara, T.; Zhang, H.; Iwashige, T.; Sugiura, K.; Tsurutab, K. Macroscale and microscale fracture toughness of microporous sintered Ag for applications in power electronic devices. Acta Mater. 2017, 129, 41–51. [CrossRef] 25. Ning, X.S.; Lin, Y.; Xu, W.; Peng, R.; Zhou, H.; Chen, K. Development of a directly bonded aluminum/alumina power electronic substrate. Mater. Sci. Eng. B Solid State Mater. Adv. Technol. 2003, 99, 479–482. [CrossRef]

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