Optimization of Copper Thermocompression Diffusion

metals

Article Optimization of Copper Thermocompression Diﬀusion Bonding under Vacuum: Microstructural and Mechanical Characteristics

Michail Samouhos 1,*, Antonis Peppas 1 , Panagiotis Angelopoulos 1, Maria Taxiarchou 1 and Petros Tsakiridis 2 1 Laboratory of Pyrometallurgy, School of Mining and Metallurgical Engineering, National Technical University of Athens, 9 Heroon Polytechniou St, 15780 Athens, Greece; [email protected] (A.P.); [email protected] (P.A.); [email protected] (M.T.) 2 Laboratory of Physical Metallurgy, School of Mining and Metallurgical Engineering, National Technical University of Athens, 9 Heroon Polytechniou St, 15780 Athens, Greece; [email protected] * Correspondence: [email protected]; Tel.: +30-2107724482

Received: 11 August 2019; Accepted: 24 September 2019; Published: 26 September 2019

Abstract: The optimization of the autogenous diffusion copper bonding via thermocompression at vacuum environment was investigated. The influence of various bonding parameters on the interdiffusion efficiency was studied in detail at the micro (SEM-EBSD) and nano (TEM) scales. Bonding at 1000 ◦C for 90 min under pressure (10 MPa) presented optimum structural and mechanical results. Under these conditions, interdiffusion phenomena were observed at a significant extent through the swelling transformation of existing fine grains or the formation of equiaxed copper grains with an orientation parallel to the bond interface. Transmission electron microscopy revealed the importance of the grain size of the base material on the bond quality. In the regions with fine-sized copper grains, the formation of small equiaxed recrystallized twins was observed. Their length within the bonding zone was in the order of 200 and 400 nm. On the contrary, in the regions with coarse grains the interdiffusion was poorer. The processing temperature and duration presented a significant effect on the bonding strength (BS). BS exceeded 100 MPa in case of processing conditions of T 850 C and t 60 min, while the maximum BS value achieved ( 180 MPa) was comparable ≥ ◦ ≥ ≈ with the respective value of the base material. The microhardness of the optimum bond reached 55 HV—slightly higher in comparison to the hardness of the initial copper material. The results indicated that the proposed thermocompression process is appropriate for the production of Cu-Cu bonded structures that can be potentially used as electrical components under mechanical stress.

Keywords: copper; thermocompression; bonding; diﬀusion; microstructure characterization

1. Introduction Non-fusion bonding consists of a solid-state process in which the joining between two metal surfaces is achieved via interdiffusion and grain growth across the bonding interface [1,2]. In the case of Cu-Cu autogenous bonding, several non-fusion techniques have been developed, including low and high-temperature thermocompression, forge and roll, friction, explosion, electric resistance, and ultrasonic processing [1,3–6]. Vacuum thermocompression consists of the most widely followed process due to its sufficiently controlled conditions and the formation of Cu-Cu bonds with various physicochemical specifications according to respective applications. Low temperature thermocompression (performed between 150 and 450 ◦C) (LTT) and room temperature compression Cu-Cu bonding have been extensively studied due to their potential application in future three-dimensional integrated circuits. LTT processes impart high electric

Metals 2019, 9, 1044; doi:10.3390/met9101044 www.mdpi.com/journal/metals Metals 2019, 9, 1044 2 of 10 conductivity and low-medium mechanical strength properties, while they assure low thermal stress avoiding damages to the bonded microelectronics [7]. Bonding (tensile) strength (BS) values of up to 70 MPa have been reported in the case of thermocompression between 200 and 400 ◦C[8–10], while a BS value around 10 MPa is achieved at room temperature compression [11]. Exceptionally high BS values (>200 MPa) have been referred to in the case of annealed and pre-treated copper surfaces using H2/Ar plasma or formic acid vapor [12]. On the other hand, high-temperature Cu-Cu bonding has not been thoroughly investigated. A limited number of studies describe that the process is performed using a diﬀusion bonding furnace with hydraulic load equipment in the temperature range of 600–1000 ◦C[13,14]. Moreover, recently there has been an interest in Cu-Cu bonded structures with advanced mechanical properties that can be used for the construction of bar-wound stators in vehicles [15] or as components in particle accelerators [16,17]. High-temperature bonding technology is compatible with these applications as the existence of thermal deformation phenomena does not consist of a deterrent factor. The current investigation aims for the optimization of a simple, high-temperature (750–1000 ◦C) Cu-Cu bonding process using an induction vacuum furnace. The bond quality, in terms of bonding conditions, was studied via microscopy and mechanical tests, while the optimum accomplished bond was examined using scanning electron microscopy (SEM) in conjunction with electron backscatter diﬀraction (EBSD) and transmission electron microscopy (TEM).

2. Materials and Methods

2.1. Cu-Cu Thermocompression Bonding Commercially pure (99.9% purity) Cu foils (200 µm thickness, 2 cm width) were used for the bonding tests. The foils were initially placed for 10 s in a hydrochloric acid bath (5% v/v) and subsequently were cleaned with acetone aiming to the removal of a potential thin oxide film, which could impede the interdiffusion mechanism during the bonding process. A stainless steel clamp with an appropriately designed recess was used for the tightening of 28 foils (Figures1 and2a). During its screwing, the clamp with the sample was compressed at 10 Mpa. Literature data and preliminary tests showed that the compression between 7 and 10 MPa is efficient for Cu-Cu bonding performance. The superficial interdiffusion between the foils and the clamp was avoided by the coating of the clamp’s recess with a thin, fine powdered graphite layer. Bonding tests were performed using a customized Termolab induction graphite vacuum furnace in the temperature range between 750 and 1000 ◦C under 5 10 2 mbar pressure. The furnace’s chamber consists of a vertical cylindrical graphite tube (internal × − dimensions: d = 24 cm, h = 21 cm) capped by bottom and top graphite elements ensuring heating uniformityMetals 2019 (Figure, 9, x FOR2b). PEER REVIEW 3 of 10

Figure 1. Technical drawing of the clamp used for the autogenous copper bonding. Figure 1. Technical drawing of the clamp used for the autogenous copper bonding.

Figure 2. Processing steps of autogenous Cu bonding and mechanical characterization; (a) Cu foils in the pressing clamp, (b) placement of samples in the graphite furnace, (c) Cu foils after bonding and (d) bonding strength test.

2.2. Diffusion Bonding Characterization The microstructure of the copper bonds was initially examined by optical metallurgical microscopy (BX41M model, Olympus, Shinjuku, Japan), while a more detailed investigation was performed using a JEOL 6380 LV Scanning Electron Microscope (JEOL, Tokyo, Japan) at 20 kV, coupled with Energy Dispersive System (SEM-EDS). Electron backscatter diffraction (EBSD) analysis was carried out, focusing on the grains generation and crystals orientation in the diffusion zone, using a Nordlys EBSD detector (Oxford Instruments, Abingdon, England). The sample under investigation was tilted 70° to the electron beam and placed at 15 mm of working distance, whereas the EBSD orientation maps were recorded from a selected region of the diffusion zone at 20 kV, with a scan step size of 300 nm. Prior to the SEM examination and the EBSD characterization, all samples were

Metals 2019, 9, x FOR PEER REVIEW 3 of 10

Metals 2019, 9, 1044 3 of 10 Figure 1. Technical drawing of the clamp used for the autogenous copper bonding.

Figure 2. Processing steps of autoge autogenousnous Cu bonding and mechan mechanicalical characterization; (a) Cu foils in the pressing clamp, (b) placement of samples in the graphite furnace, (c) Cu foils after bondingbonding and (d) bonding strengthstrength test.test.

2.2. Diffusion Diffusion Bonding Characterization The microstructuremicrostructure of of the the copper copper bonds bonds was initially was initially examined examined by optical by metallurgical optical metallurgical microscopy (BX41Mmicroscopy model, (BX41M Olympus, model, Shinjuku, Olympus, Japan), Shinjuku, while aJapan), more detailed while a investigation more detailed was investigation performed using was aperformed JEOL 6380 using LV Scanning a JEOL Electron6380 LV MicroscopeScanning Electr (JEOL,on Tokyo,Microscope Japan) (JEOL, at 20 kV,Tokyo, coupled Japan) with at Energy20 kV, Dispersivecoupled with System Energy (SEM-EDS). Dispersive Sy Electronstem (SEM-EDS). backscatter Electron diffraction backscat (EBSD)ter diffraction analysis was (EBSD) carried analysis out, focusingwas carried on out, the grainsfocusing generation on the grains and crystals generation orientation and crystals in the orientation diffusion zone, in the using diffusion a Nordlys zone, EBSDusing detectora Nordlys (Oxford EBSD detector Instruments, (Oxford Abingdon, Instruments, England). Abingdon, The sampleEngland). under The investigationsample under was investigation tilted 70◦ towas the tilted electron 70° to beam the andelectron placed beam at 15 and mm placed of working at 15 distance,mm of working whereas distance, the EBSD whereas orientation the EBSD maps wereorientation recorded maps from were a selected recorded region from ofa theselected diffusion region zone of the at 20 diffusion kV, with zone a scan at step20 kV, size with of 300 a scan nm. Priorstep size to the of SEM300 nm. examination Prior to the and SEM the EBSDexamination characterization, and the EBSD all samples characterization, were prepared all samples by grinding were and mechanical polishing. SEM samples were polished with a 6 µm and 1 µm diamond paste, and then successfully etched with a NH4OH and H2O2 solution. The EBSD sample (optimum bond) was subjected to a final polishing stage using a 0.1 µm diameter colloidal silica suspension. Nanoscale investigation of the optimum bonded specimen was performed with a high resolution JEOL JEM-2100 LaB6 transmission electron microscope (HRTEM) (JEOL, Tokyo, Japan), operating at 200 kV. Ion-beam milling at 5 kV in a Gatan Precision Ion Polishing system (PIPS, Gatan Inc., Pleasanton, CA, USA) was used for the sample’s thinning. Bonding strength measurements were carried out at room temperature and the stress-strain curves were obtained using a universal testing machine (Model 4482, max load: 100 kN) at a crosshead speed of 1 mm/min (Figure2c,d). Vickers microhardness tests were conducted with a Shimadzu type M microhardness tester (Shimadzu, Kyoto, Japan), working at a magnification of 500 . × 3. Results and Discussion The bonding consists of a multi-factorial process mainly affected by the applied pressure on the sample, the heating temperature/duration, and the vacuum quality. According to the literature, these parameters dramatically affect the plastic deformation of the surface asperities, resulting in the increase of contact between the surfaces and control of the diffusion mechanism of the atoms across Metals 2019, 9, x FOR PEER REVIEW 4 of 10 prepared by grinding and mechanical polishing. SEM samples were polished with a 6 μm and 1 μm diamond paste, and then successfully etched with a NH4OH and H2O2 solution. The EBSD sample (optimum bond) was subjected to a final polishing stage using a 0.1 μm diameter colloidal silica suspension. Nanoscale investigation of the optimum bonded specimen was performed with a high resolution JEOL JEM-2100 LaB6 transmission electron microscope (HRTEM) (JEOL, Tokyo, Japan), operating at 200 kV. Ion-beam milling at 5 kV in a Gatan Precision Ion Polishing system (PIPS, Gatan Inc., Pleasanton, CA, USA) was used for the sample’s thinning. Bonding strength measurements were carried out at room temperature and the stress-strain curves were obtained using a universal testing machine (Model 4482, max load: 100 kN) at a crosshead speed of 1 mm/min (Figure 2c,d). Vickers microhardness tests were conducted with a Shimadzu type M microhardness tester (Shimadzu, Kyoto, Japan), working at a magnification of 500×.

3. Results and Discussion The bonding consists of a multi-factorial process mainly affected by the applied pressure on the sample, the heating temperature/duration, and the vacuum quality. According to the literature, these Metals 2019, 9, 1044 4 of 10 parameters dramatically affect the plastic deformation of the surface asperities, resulting in the increase of contact between the surfaces and control of the diffusion mechanism of the atoms across the interface.interface. The The quality quality of of thermocompressed Cu-Cu Cu-Cu bonds, in respect to temperature and timetime processing, is examined in the current study. The parametric tests were performed under a standard loading pressurepressure (10(10 MPa)MPa) andand vacuumvacuum degreedegree valuevalue (5(5 × 10−22 mbar).mbar). × − 3.1. Microstructural Investigation In order to investigate the eeffectﬀect of temperature and duration on bonding, several samples were subjected to morphological examination by means of optical,optical, scanning,scanning, andand transmissiontransmission electronelectron microscopy. TheThe sectionsection micrographs micrographs of of Figure Figure3(a1,b1) 3(a1,b1) presents, presents, at horizontal at horizontal direction, direction, the bondthe bond line formationsline formations between between 5 foils 5 foils bonded bond ated 935 at ◦935C for °C 45 for and 45 and 90 min, 90 min, respectively. respectively. As can As becan seen, be seen, the jointhe qualityjoin quality is improved is improved in the in case the ofcase a more of a prolongedmore prolonged processing processing time. Figuretime. 3Figure(a2,b2) 3(a2,b2) shows, atshows, higher at magniﬁcation,higher magnification, the bond the line bond number line number 1 at the 1 at same the same conditions. conditions. Undesirable Undesirable intermediate intermediate voids voids are clearlyare clearly visible visible in the in case the ofcase the of sample the sample submitted subm toitted bonding to bonding for 45 min for (Figure45 min3 (Figure(a2)). On 3(a2)). the contrary, On the voidscontrary, are decreased,voids are decreased, a recrystallization a recrystallization process is taking process place, is taking and elongated place, and grains elongated are grown grains in theare interfacegrown in ofthe the interface sample of processed the sample for processed 90 min (Figure for 903(b2)). min (Figure 3(b2)).

Figure 3. Metallographic micrographs of Cu-Cu bond cross sections presenting the interfaces between

the copper layers. Bonding conditions: sample (a) 935 ◦C/45 min, sample (b) 935 ◦C/90 min (the numbering corresponds to the bonding lines).

The quality of bonding zones in terms of processing temperature is vividly revealed by scanning electron microscopy. After the sample’s processing at 750 ◦C, the existence of a well-visible continuous interface between the copper layers indicates the formation of a low strength bond (Figure4(a1,a2)). By increasing processing temperature to 935 ◦C, the interdiffusion process among the grains becomes more intense, and the unbonded zone is limited in specific regions (Figure4(b1,b2)). Further temperature increments at 1000 ◦C lead to the creation of a uniform diffusion zone along the bond interface with minor visible unbonded regions. At this temperature, the interfacial discontinuity has been eliminated and a recrystallization has been achieved. Scanning electron microscopy indicates the absence of annealing-related recrystallization phenomena by temperature rising from 750 to 1000 ◦C. In order to understand the grains evolution and their final orientation in the bonding zone, the examination of the cross-sectional thermocompressed specimens, produced under optimum bonding conditions (i.e., heating for 90 min at 1000 ◦C), was carried out by electron backscatter diffraction (EBSD) analysis (Figure5). Metals 2019, 9, x FOR PEER REVIEW 5 of 10

Figure 3. Metallographic micrographs of Cu-Cu bond cross sections presenting the interfaces between the copper layers. Bonding conditions: sample (a) 935 °C/45 min, sample (b) 935 °C/90 min (the numbering corresponds to the bonding lines).

Metals 2019, 9, x FOR PEER REVIEW 5 of 10 The quality of bonding zones in terms of processing temperature is vividly revealed by scanning electronFigure microscopy. 3. Metallographic After micrographs the sample’s of Cu-Cu processing bond cro atss sections750 °C, presenting the existence the interfaces of a betweenwell-visible continuousthe copper interface layers. between Bonding the conditions: copper layers sample indica (a) 935tes the°C/45 formation min, sample of a low(b) 935strength °C/90 bondmin (the (Figure 4(a1,a2)).numbering By increasing corresponds processing to the bonding temperature lines). to 935 °C, the interdiffusion process among the grains becomes more intense, and the unbonded zone is limited in specific regions (Figure 4(b1,b2)). Further temperatureThe quality increments of bonding at 1000 zones °C in lead terms to ofthe processing creation of temperature a uniform diffusionis vividly zonerevealed along by thescanning bond interfaceelectron withmicroscopy. minor visible After unbonded the sample’s regions. processing At this temperature,at 750 °C, the interfacialexistence discontinuityof a well-visible has beencontinuous eliminated interface and betweena recrystallization the copper has layers been indica achieved.tes the Scanningformation electron of a low microscopystrength bond indicates (Figure the4(a1,a2)). absence By of increasing annealing-related processing recrystallization temperature to phenomena 935 °C, the interdiffusion by temperature process rising amongfrom 750 the to grains 1000 °C.becomes more intense, and the unbonded zone is limited in specific regions (Figure 4(b1,b2)). Further temperature increments at 1000 °C lead to the creation of a uniform diffusion zone along the bond interface with minor visible unbonded regions. At this temperature, the interfacial discontinuity has been eliminated and a recrystallization has been achieved. Scanning electron microscopy indicates

Metalsthe absence2019, 9, 1044of annealing-related recrystallization phenomena by temperature rising from 750 to5 1000 of 10 °C.

Figure 4. SEM micrographs of Cu-Cu bond cross sections presenting the interfaces between the copper layers. Bonding conditions: sample (a) 750 °C/90 min, sample (b) 935 °C/90 min and sample (c) 1000 °C/90 min.

In order to understand the grains evolution and their final orientation in the bonding zone, the examinationFigureFigure 4.4. of SEM the cross-sectionalmicrographs of thermocompre Cu-Cu bond crossssed sectionssectio specimens,ns presenting produced the interfacesinterfaces under optimum betweenbetween bonding thethe conditions (i.e. heating for 90 min at 1000 °C), was carried out by electron backscatter diffraction coppercopper layers.layers. Bonding conditions: sample ( a) 750 ◦°C/90C/90 min, sample (b) 935935 ◦°C/90C/90 minmin andand samplesample (EBSD) analysis (Figure 5). ((cc)) 10001000 ◦°C/90C/90 min.min.

In order to understand the grains evolution and their final orientation in the bonding zone, the examination of the cross-sectional thermocompressed specimens, produced under optimum bonding conditions (i.e. heating for 90 min at 1000 °C), was carried out by electron backscatter diffraction (EBSD) analysis (Figure 5).

Figure 5. EBSD Band Contrast (left) and Inverse Pole Figure (IPF) (right) maps in the bonding zone. Figure 5. EBSD Band Contrast (left) and Inverse Pole Figure (IPF) (right) maps in the bonding zone. Bonding conditions of specimen: 1000 C/90 min. Bonding conditions of specimen: 1000 °C/90◦ min. It is evident that the grains size distribution of copper base alloy has not been affected by the It is evident that the grains size distribution of copper base alloy has not been affected by the thermocompression process. The observed interface is strongly related to the size of the primary thermocompression process. The observed interface is strongly related to the size of the primary copper grains. The smaller Cu primary grains support either the evolution of new finer equiaxed grainsFigure (2–5 µ5.m) EBSD of similar Band Contrast orientation (left) withand Inverse the initial Pole matrix, Figure (IPF) or subserve (right) maps the swellingin the bonding transformation zone. of theBonding existing conditions ones and of their specimen: interdi ff1000usion °C/90 between min. the neighbor plate matrix (3–10 µm); consequently, the cohesion of the bonded plates is improved. Under the optimum thermocompression bonding conditions,It is evident frequent that atomic the grains and size interface distribution migration of copper between base contiguous alloy has not copper been grains affected has by been the observed,thermocompression mainly due process. to the temperatureThe observed increase interface under is strongly pressure, related leading to the to thesize improvement of the primary of grain boundaries and the microstructure refining. On the other hand, in the case of large Cu grains, the diffusion zone in the interface is relatively limited ( 1 µm) and the consistency seems to be ≈ restrained. To clarify further, the copper bonding zone morphology and to investigate the interfacial microstructure formed by the diffusion bonding process, the interface of the optimum bonded specimen was additionally studied via TEM in cross-section. Interface regions with coarse (100–200 µm) and fine (<50 µm) grains were isolated for observation (Figure6). In the case of coarser copper grains, the thermocompression process does not significantly affect the copper diffusion mechanism, as the detected interface zone is very limited and presents a size in the range of 200 nm and 500 nm. A closer inspection revealed the existence of short vacancies, exhibiting a width of about 5–10 nm. On the contrary, in the case of the smaller neighbor copper grains, the bonding interface seems to be much higher (3–5 µm). The microstructure was composed of small equiaxed grains, with recrystallization twins in places. Some of the developed grains length within Metals 2019, 9, x FOR PEER REVIEW 6 of 10

copper grains. The smaller Cu primary grains support either the evolution of new finer equiaxed grains (2–5 μm) of similar orientation with the initial matrix, or subserve the swelling transformation of the existing ones and their interdiffusion between the neighbor plate matrix (3–10 μm); consequently, the cohesion of the bonded plates is improved. Under the optimum thermocompression bonding conditions, frequent atomic and interface migration between contiguous copper grains has been observed, mainly due to the temperature increase under pressure, leading to the improvement of grain boundaries and the microstructure refining. On the other hand, Metalsin2019 the, 9 case, 1044 of large Cu grains, the diffusion zone in the interface is relatively limited (≈1 μm) and the6 of 10 consistency seems to be restrained. To clarify further, the copper bonding zone morphology and to investigate the interfacial microstructure formed by the diffusion bonding process, the interface of the bondingthe optimum zone bonded was in specimen the order was of additionally 200 and 400 stud nm,ied while via TEM dislocations in cross-section. were Interface observed regions randomly dispersedwith coarse in the (100–200 copper matrix.μm) and fine (<50 μm) grains were isolated for observation (Figure 6).

FigureFigure 6. Transmission 6. Transmission electron electron microcopy microcopy imagesimages in bo bondingnding zone zone applied applied in grains in grains with with different different size:size: (a,b )(a grain,b) grain size size 100–200 100–200µm, μm, (c (,dc,)d) grain grain sizesize ≤5050 μµm.m. ≤ 3.2. MechanicalIn the case Properties of coarser copper grains, the thermocompression process does not significantly affect the copper diffusion mechanism, as the detected interface zone is very limited and presents a size in Bonding strength consists of a critical parameter for the utilization of autogenous Cu bonded the range of 200 nm and 500 nm. A closer inspection revealed the existence of short vacancies, componentsexhibiting in a industrialwidth of about applications. 5–10 nm. On BS the values contra ofry, specimens in the case processed of the smaller under neighbor different copper bonding temperaturegrains, the and bonding time conditionsinterface seems were to measured.be much higher The (3–5 increase μm). The of the microstructure processing was temperature composed from 750 toof 1000small◦ equiaxedC leads tograins, the increasewith recrystallization of BS value twin froms in 50 places. to 180 Some MPa of (Figure the developed7). The grains notable length Cu-Cu diffusionwithin improvement the bonding zone by the was temperature in the order rising of 200 between and 400 935nm, andwhile 1000 dislocations◦C can bewere attributed observed to the increaserandomly of the dispersed kinetic energy in the copper of copper matrix. atoms as the temperature is relatively close to their melting point. Computational investigation at an atomic level is necessary in order for the copper interdiffusion 3.2. Mechanical Properties to progress in relation to temperature increase to be interpreted on a theoretical basis. Relative studies have shownBonding that strength displacement consists of of thea critical copper parameter atoms onfor the a substrate utilization can of autogenous take place Cu even bonded at gentle experimentalcomponents conditions in industrial [18 ].applications. Correspondingly, BS values an of increasespecimens of processed the processing under different duration bonding from 20 to temperature and time conditions were measured. The increase of the processing temperature from 90 min at 935 ◦C raises the BS value from 27 to 120 MPa (Figure8). The results are in accordance 750 to 1000 °C leads to the increase of BS value from 50 to 180 MPa (Figure 7). The notable Cu-Cu with the microstructural examination as it was previously described, indicating a strong correlation diffusion improvement by the temperature rising between 935 and 1000 °C can be attributed to the between the temperature/duration of the bonding and the interdiffusion efficiency. The highest BS increase of the kinetic energy of copper atoms as the temperature is relatively close to their melting valuepoint. was achievedComputational in the caseinvestigation of processing at an at atomic 1000 ◦ Clevel for 90is min.necessary Under in these order conditions, for the copper the tensile failureinterdiffusion occurred at to 180 progress MPa, a in value relation which to temperature is comparable increase with to the be respectiveinterpreted valueon a theoretical of the base basis. material (BM) [19]. The morphology of the bonding strength curves indicates a good ductile behavior in case of processing temperatures up to 1000 C for 60 min. More aggressive diffusion bonding conditions ◦ ≤ resulted in the increasing of a final BS value. Figure9 presents the fractographs of Cu-Cu bonds fabricated at 750 and 1000 ◦C. According to Figure9a,b, it can be observed that the solid-state di ffusion reaction successfully proceeded in the case of bonding at 1000 ◦C. The grains of the Cu plates have been interspersed themselves and a “cup and cone” pattern caused by microvoid coalescence, which is characteristic of a ductile fracture mechanism. That is why the bond under investigation exhibited similar strength value with the corresponding of the base metal. In all cases the area presented extensive plastic deformation with large dimples, accompanied by void formation between the Cu plates. It should be noted that in a soft matrix (here pure Cu), the void nucleation predominately occurs by grains decohesion. Furthermore, due to heating of the whole parts, no distinct heat-affected zone has been detected. On the other hand, Metals 2019, 9, x FOR PEER REVIEW 7 of 10 Metals 2019, 9, x FOR PEER REVIEW 7 of 10 Relative studies have shown that displacement of the copper atoms on a substrate can take place even Relative studies have shown that displacement of the copper atoms on a substrate can take place even at gentle experimental conditions [18]. Correspondingly, an increase of the processing duration from at gentle experimental conditions [18]. Correspondingly, an increase of the processing duration from 20 to 90 min at 935 °C raises the BS value from 27 to 120 MPa (Figure 8). The results are in accordance 20 to 90 min at 935 °C raises the BS value from 27 to 120 MPa (Figure 8). The results are in accordance with the microstructural examination as it was previously described, indicating a strong correlation with the microstructural examination as it was previously described, indicating a strong correlation between the temperature/duration of the bonding and the interdiffusion efficiency. The highest BS between the temperature/duration of the bonding and the interdiffusion efficiency. The highest BS value was achieved in the case of processing at 1000 °C for 90 min. Under these conditions, the tensile value was achieved in the case of processing at 1000 °C for 90 min. Under these conditions, the tensile Metalsfailure2019 occurred, 9, 1044 at 180 MPa, a value which is comparable with the respective value of the 7base of 10 failure occurred at 180 MPa, a value which is comparable with the respective value of the base material (BM) [19]. The morphology of the bonding strength curves indicates a good ductile behavior material (BM) [19]. The morphology of the bonding strength curves indicates a good ductile behavior in case of processing temperatures up to 1000 °C for ≤60 min. More aggressive diffusion bonding in thecase case of processing of bonding temperatures at 750 C (Figure up9 toc,d), 1000 rounded, °C for dendritic≤60 min. structuresMore aggressive were observed, diffusion which bonding are conditions resulted in the increasing◦ of a final BS value. conditionsindicative ofresulted an incipient in the diincreasingffusion mainly of a final due BS to deficientvalue. heat treatment.

200 200

150 150

100 100 BS (MPa)BS BS (MPa)BS BM 750 °C 50 BM 750 °C 50 850 °C 935 °C 850 °C 935 °C 1000 °C 1000 °C 0 0 0 10203040 0 10203040 Elongation(%) Elongation(%) Figure 7. Bonding strength of the Cu-Cu bond in relation to the bonding temperature (processing for Figure 7. Bonding strength of the Cu-Cu bond in relati relationon to the bonding temperature (processing for 90 min); BM: base material. 90 min); BM: base material.

200 200

150 150

100 100 BS (MPa) BS (MPa) BM 20min 50 BM 20min 50 30 min 45 min 30 min 45 min 60 min 90 min 60 min 90 min 0 0 0 10203040 0 10203040 Elongation(%) Elongation(%) FigureFigure 8. 8.BondingBonding strength strength of the of theCu-Cu Cu-Cu bond bond in relation in relation to the to bonding the bonding duration duration (processing (processing at 935 at°C). Figure 8. Bonding strength of the Cu-Cu bond in relation to the bonding duration (processing at 935 °C). Metals 2019, 9, x FOR PEER REVIEW 8 of 10 935 ◦C). Figure 9 presents the fractographs of Cu-Cu bonds fabricated at 750 and 1000 °C. According to Figure 9 presents the fractographs of Cu-Cu bonds fabricated at 750 and 1000 °C. According to Figure 9a,b, it can be observed that the solid-state diffusion reaction successfully proceeded in the Figure 9a,b, it can be observed that the solid-state diffusion reaction successfully proceeded in the case of bonding at 1000 °C. The grains of the Cu plates have been interspersed themselves and a “cup case of bonding at 1000 °C. The grains of the Cu plates have been interspersed themselves and a “cup and cone” pattern caused by microvoid coalescence, which is characteristic of a ductile fracture and cone” pattern caused by microvoid coalescence, which is characteristic of a ductile fracture mechanism. That is why the bond under investigation exhibited similar strength value with the mechanism. That is why the bond under investigation exhibited similar strength value with the corresponding of the base metal. In all cases the area presented extensive plastic deformation with corresponding of the base metal. In all cases the area presented extensive plastic deformation with large dimples, accompanied by void formation between the Cu plates. It should be noted that in a large dimples, accompanied by void formation between the Cu plates. It should be noted that in a soft matrix (here pure Cu), the void nucleation predominately occurs by grains decohesion. soft matrix (here pure Cu), the void nucleation predominately occurs by grains decohesion. Furthermore, due to heating of the whole parts, no distinct heat-affected zone has been detected. On Furthermore, due to heating of the whole parts, no distinct heat-affected zone has been detected. On the other hand, in the case of bonding at 750 °C (Figure 9c,d), rounded, dendritic structures were the other hand, in the case of bonding at 750 °C (Figure 9c,d), rounded, dendritic structures were observed, which are indicative of an incipient diffusion mainly due to deficient heat treatment. observed, which are indicative of an incipient diffusion mainly due to deficient heat treatment.

FigureFigure 9. SEM 9. SEM fractographs fractographs of of coppercopper bonds; bonds; (a (,ba):,b Interface): Interface of bond of bond fabricated fabricated at 1000 at°C/90 1000 min◦C (the/90 min (the arrowsarrows showshow the interface orientation orientation between between the the partially partially detached detached Cu Cuplates), plates), (c,d): (c fractured,d): fractured surfaces of Cu plates bonded at 750 °C/90 min. surfaces of Cu plates bonded at 750 ◦C/90 min. The microhardness distribution across the bonding region of the samples processed for 90 min at 935 and 1000 °C (samples b and c, respectively, according to Figure 4) is presented in Figure 10. The measurements, in the case of the optimum sample (90 min at 1000 °C), reveal an increase of the hardness in the bonding interlayer, whereas the corresponding values for the base material (BM) are lower. The hardening of the interlayer can be attributed to the microstructure refinement, after the thermocompression process, and to the development of significant amount of equiaxed finer size grains, due to copper diffusion between the primary metallic plates. The results are in accordance with the EBSD micrograph, in which the presence of fine equiaxed grains distribution in the primary metallic copper plates was observed.

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The microhardness distribution across the bonding region of the samples processed for 90 min at 935 and 1000 ◦C (samples b and c, respectively, according to Figure4) is presented in Figure 10. The measurements, in the case of the optimum sample (90 min at 1000 ◦C), reveal an increase of the hardness in the bonding interlayer, whereas the corresponding values for the base material (BM) are lower. The hardening of the interlayer can be attributed to the microstructure refinement, after the thermocompression process, and to the development of significant amount of equiaxed finer size grains, due to copper diffusion between the primary metallic plates. The results are in accordance with the EBSD micrograph, in which the presence of fine equiaxed grains distribution in the primary metallicMetals 2019 copper, 9, x FOR plates PEER wasREVIEW observed. 9 of 10

FigureFigure 10.10. Transvesre HardnessHardness distributiondistribution across the bondingbonding region; (a)) processingprocessing forfor 9090 minmin atat 935935 ◦°C,C, ((bb)) processingprocessing forfor 9090 minmin atat 10001000◦ °CC (inter:(inter: interlayerinterlayer betweenbetween thethe foils,foils, central:central: thethe centralcentral pointpoint ofof thethe foilfoil alongalong itsits thickness).thickness). 4. Conclusions 4. Conclusions The aim of the current study was the optimization of the autogenous high-temperature copper The aim of the current study was the optimization of the autogenous high-temperature copper diffusion bonding process. The micro and nano structure of the bonds, as well as their mechanical diffusion bonding process. The micro and nano structure of the bonds, as well as their mechanical properties were investigated. The main outcomes derived by the experimental procedure are as follows: properties were investigated. The main outcomes derived by the experimental procedure are as 1.follows:Cu-Cu bonds with advanced mechanical properties were successfully formatted through a thermocompression process under vacuum. Commercial copper foils with 200 µm thickness were 1. Cu-Cu bonds with advanced mechanical properties were successfully formatted through a used for the bonding tests, while a pressure of 10 MPa was applied. thermocompression process under vacuum. Commercial copper foils with 200 μm thickness 2. The increase of the bonding temperature, from 750 to 1000 C, and the duration, from 20 to were used for the bonding tests, while a pressure of 10 MPa was◦ applied. 90 min, significantly improved the bonding strength of the copper bonds. The highest ultimate BS 2. The increase of the bonding temperature, from 750 to 1000 °C, and the duration, from 20 to 90 (180 MPa) approached the respective value of the base material, and was achieved at 1000 C for min, significantly improved the bonding strength of the copper bonds. The highest ultimate◦ BS 90 min. The transverse microhardness of the bond reached 55 HV, slightly higher in comparison (180 MPa) approached the respective value of the base material, and was achieved at 1000 °C for to the respective value of the base material. 90 min. The transverse microhardness of the bond reached 55 HV, slightly higher in comparison 3. toElectron the respective backscatter value diff ofraction the base (EBSD) material. investigation, of the optimum bonded specimen, reveals a 3. Electronsatisfactory backscatter interdiffusion diffraction growth (EBSD) in regions investig withation, finer of grains. the optimum Transmission bonded electron specimen, microscopy reveals a satisfactory interdiffusion growth in regions with finer grains. Transmission electron microscopy revealed the formation of small equiaxed recrystallized twin crystals in the bonding zone with a size ranging from 200 to 400 nm. 4. The above mechanical and microstructural data indicated that the developed copper-bonded structure could be successfully used in devices that are submitted to intense mechanical stress (i.e., bar-wound stators in vehicles).

Author Contributions: M.S., A.P., P.A. and P.T. designed and performed the experiments. M.S. additionally wrote the manuscript. M.T. contributed to the discussion of the experimental data.

Funding: This research received no external funding.

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revealed the formation of small equiaxed recrystallized twin crystals in the bonding zone with a size ranging from 200 to 400 nm. 4. The above mechanical and microstructural data indicated that the developed copper-bonded structure could be successfully used in devices that are submitted to intense mechanical stress (i.e., bar-wound stators in vehicles).

Author Contributions: M.S., A.P., P.A. and P.T. designed and performed the experiments. M.S. additionally wrote the manuscript. M.T. contributed to the discussion of the experimental data. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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