IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 4, APRIL 2012 587

Preparation and Low-Temperature Sintering ofCu Nanoparticles for High-Power Devices

Shutesh Krishnan, Senior Member, IEEE, A. S. M. A. Haseeb, Member, IEEE, and Mohd Rafie Johan

Abstract— One of the fundamental requirements forhigh-temperature electronic packaging is reliable silicon attachwith low and stable electrical resistance. This paper presents astudy conducted on Cu nanoparticles as an alternative lead-freeinterconnect material for high-temperature applications. Cunanoparticles were prepared using pulsed wire evaporationtechnique in water medium. Pure Cu nanoparticles without anyorganic mixture were used in this paper. An economical approachto extract the nanoparticles from water was established. Insitu Cu nanoparticles oxide reduction was successfully doneusing forming gas (N2-5%H2). Cross-section analysis on bondedinterface shows onset of Cu nanoparticles sintering at 400 °C.We successfully demonstrated the possibilities of using Cunanoparticles as silicon die attach material for high-temperatureelectronic devices.

Index Terms— Cu nanoparticles, high-temperature device,lead-free, low-temperature sintering, pulsed wire evaporation.

I. INTRODUCTION

THE need for alternative lead-free die attach material withhigh electrical and thermal requirements is ever increas-

ing. Conventional lead (Pb) base solder alloys and highlyconductive epoxies are still widely used for these applications.However, these materials have limitations at high-temperatureoperations as their remelting/degradation temperatures arevery low. Most of the lead-free solders available currentlyare known to cause reliability problems like intermetalliccompound growth and solder joint fatigue [1]. Semiconductorpackaging researchers are putting effort on finding suitablealternative lead-free die attach material before the 2014 dead-line set by the European Union.

Nano-structured metals are being researched extensivelyas an alternative for many semiconductor interconnectionchallenges [2]. These researches are mainly focused on wafer-level bonding for future integrated circuit miniaturization withincreased interconnection density. Nanoscale interconnectionmaterials have huge potential to overcome fatigue failureproblems with bulk solder [3]. The increased surface areaof nanoparticles that provides higher surface energy andreactivity make it possible to be sintered at below the bulk

Manuscript received November 4, 2010; revised August 18, 2011; acceptedJanuary 13, 2012. Date of publication March 15, 2012; date of currentversion March 30, 2012. Recommended for publication by Associate EditorC. Gurumurthy upon evaluation of reviewers’ comments.

S. Krishnan is with ON Semiconductor and Department of MechanicalEngineering, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail:shutesh.k@onsemi.com).

A. S. M. A. Haseeb and M. R. Johan are with the Department of MechanicalEngineering, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail:haseeb@um.edu.my; mrafiej@um.edu.my).

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

Digital Object Identifier 10.1109/TCPMT.2012.2189208

melting temperature [4], [5]. Sintering temperature well belowthe melting point for nanoscale metals is a great advantagefor semiconductor packaging. Processing the devices at lowtemperature could avoid initial die level stresses and potentialcircuit damages.

Silver nanopaste is gaining popularity as an alternativesolution for high-power devices. Almost all the researchfor lead-free die attach materials to date were focused onsilver nanoparticles [6]–[8] for their electrical, thermal, andproven low-temperature sintering characteristics. These silvernanoparticles are typically mixed with an organic systemfor easy handling. However, studies had shown that use ofthese organic system-based nanoparticles may be limited tothermostable devices [9]. This is mainly due to the organicbinder and solvent in the silver nanopaste that would causeunstable volatilization at a higher heating rate. Moreover, animproper heating rate during the sintering process is likely toinduce cracks.

Synthesis of nanoparticles needs to be cost-effective for usein semiconductor assembly. The electrical wire evaporationhas become an attractive technique for low-cost nanoparti-cles fabrication. This technique produces nanoparticles withsmaller and narrow particle size distribution in comparisonwith that carried out in gas atmosphere [10], [11]. The com-plexity in chemical synthesis method with contamination fromby-product and particles agglomeration [12] is avoided in thistechnique. However, limitation in particles extraction from thewater medium remains a concern for volume production.

This paper presents a study conducted on Cu nanoparticlesas a potential alternative to lead-free die attach solders. Cuhas the highest thermal and electrical conductivity after Agand it is more cost-effective. The Cu nanoparticles used inthis experiment were produced using pulsed wire evaporation(PWE) technique in deionized (DI) water. These Cu nanopar-ticles were not mixed with any precursor or organic bindersystem. An economical nanoparticles extraction method formass volume production is also introduced in this paper. As Cunanoparticles start oxidizing almost immediately after synthe-sis, in situ oxide reduction with forming gas (N2H2) was usedto reduce the Cu oxide into pure metal powder just before thedie attach process. Findings from this paper will be used forfuture research in using Cu nanoparticles as viable alternativeto lead-free die attach solders for power discrete devices.

II. METHODOLOGY

A. Materials

1) Cu Nanoparticles: The experiments were conductedusing Cu nanoparticles synthesized in the lab using

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588 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 4, APRIL 2012

Fig. 1. Schematic diagram of the PWE system.

PWE system. This was done in DI water as explodingmedium. Particles produced using this method ranged from30 to 50 nm in size. The nanoparticles solution was thenmixed with acetone and sonicated for 5 h prior to theexperimental works, as the particles suspended in the solutionhave the tendency to agglomerate after synthesis due to theirvery high surface energy. During this sonication process, thesonic bath temperature was maintained at about 25 °C toavoid overheating and oxidation of the particles. Details ofthe particles synthesis is given in next section.

2) Test Vehicle: MOSFET devices are currently assembledusing solder or highly conductive epoxies. These devicesare coated with Ti/Ni/Ag back-metal layers. A similar high-temperature MOSFET device was used in this experiment.The device was singulated to 0.5 × 0.7 mm die size. Metalsubstrate used for the silicon die attach experiment was A42Cu alloy leadframe. Leadframes with Ag surface finishingwere used to study the adhesion strength. Dage-4000 die shearinstrument was used to measure the silicon die shear strength.

B. Experimental Setup

1) PWE Technique: The particles were produced using thelab scale NTI-miniP nanoparticle synthesizer from Nano TechInc. (South Korea). This equipment makes nano-sized powderby supplying electric power to a metallic wire for a veryshort time (microseconds, µs), sublimating a certain lengthof metallic wire and generating evaporation and condensation.

The PWE equipment was set up as shown in Fig. 1. Cuspool with 0.2 mm wire diameter was installed on the machineand the wire was supplied through the straightener and thefeeding roller. This wire was then fed into the explodingbottle consisting of two high-voltage extension bars and twoelectrodes. The exploding bottle was filled with 500 ml DIwater. The two electrodes were connected to a high-voltagepower source.

The wire was adjusted to be placed between the two polesand touching the electrodes in the exploding bottle (circledregion in Fig. 1). At this point the power was turned ON.

After about 5 s the wire exploded. The explosion makesa strong flash and noise. For our experiment a total of30 explosions were made. The Cu nanoparticles synthesizedusing this method have some traces of micrometer-sizedparticles that need to be removed.

During the PWE explosion, the nano-sized powder flies at2 km/s with a strong flash and the DI water in the explodingbottle interrupts the reaction [13]. The major factor determin-ing the particle size in the PWE process is superheating ofthe evaporated material. The particle size produced using thistechnique reduces considerably with increasing super heatingfactor of the metal, K

K = W

Ws

where W is the specific energy required for evaporation andWs is the sublimation energy of the wire.

Sublimation energy of Cu is 47.8 J/mm3. In general, thesize of the particles produced using PWE method depends onthe overheating factor K and not the energy applied to themetallic wire [14]. According to our experimental condition,the most proper value of K is 2. The length of the Cu wireevaporated in each explosion was 30 mm.

C. Cu Nanoparticles Extraction and Printing

The Cu nanoparticles remain suspended in 500 ml DIwater after the PWE process. After the explosion process,the solution was left in the exploding bottle for 10 minto let the micrometer-sized particles to settle down. Oncethe micrometer-sized particles settled down, the solution wastransferred from the exploding bottle to a beaker.

The Cu nanoparticles solution in the beaker was then mixedwith acetone at 50:50 by volume ratio. This mixture washeated at 40 °C for 2 h to reduce the solution volume to 25%of the initial volume. During this heating process, nanopar-ticles agglomeration was also observed. These agglomeratednanoparticles settled down at the bottom of the beaker leavingthe transparent acetone–water mixture at the top.

The agglomerated Cu nanoparticles were collected andrinsed using ethanol to remove any remaining traces of water.The Cu nanoparticles were then sonicated (Branson, USA)for 5 h to disperse the particles prior to dispensing onto theleadframe. This solution is now referred to as Cu nano-colloid.

The Cu nano-colloid was dispensed using a stainless steelstencil with 1.0 × 1.0 mm opening. The leadframe was thendried in a vacuum oven at 40 °C for 2 min. These printingand drying steps were repeated until a thin visible layer of Cunanofilm was formed on the surface of the leadframe (Fig. 2).

D. Oxide Reduction of Cu Nanoparticles

The synthesized Cu nanoparticles have a natural oxidecapping. This oxide layer needs to be removed prior to thebonding process. We used a modified conventional eutecticdie attach machine (with closed tunnel) for this purpose.A commercial grade high-purity forming gas N2-5%H2 waspurged into the tunnel at 6 l/min. The metal leadframe wasthen placed into this heated tunnel for 3 h. Temperaturein the tunnel was maintained at 300 °C during this period.

KRISHNAN et al.: PREPARATION AND LOW-TEMPERATURE SINTERING OF Cu NANOPARTICLES 589

Fig. 2. Cu nanoparticles after screen printing and vacuum dry on leadframesurface. SEM image of the screen printed Cu nanoparticles (inset).

TABLE I

EXPERIMENT MATRIX FOR SILICON DIE BOND AND ANNEALING

SampleBonding

temperature(°C)

Bondingpressure(MPa)

Bondingtime (min)

Annealing1 h@450 °C

A 350 10.0 20 Yes

B 350 5.0 20 Yes

C 400 10.0 15 Yes

D 400 5.0 15 Yes

E 400 10.0 15 No

The leadframe was then cooled down to room temperatureand transferred to a standard die attach machine for siliconattach experiment.

1) Silicon Attach Experiment: Singulated MOSFET die wasattached onto the Cu nanofilm coating (after oxide reduction)using a pick and place arm. The pick and place arm wasequipped with load cell. Once the die was placed onto theleadframe, pressure was applied. The temperature at the bondsite was then gradually increased at 10 °C/min. Samples werebonded at various bond pressures and temperatures.

Table I shows the variables used in the silicon die bondingexperiment. The die bonded samples were then subjected toannealing for 1 h at 450 °C in a N2-filled oven. Die sheartest was performed on all these samples. Samples were alsosubjected to cross-section microscopic investigation under ascanning electron microscope (SEM).

III. RESULTS AND DISCUSSION

The PWE synthesis in DI water gives a translucent green-blue solution. The solution slowly turns to opaque green-blue as the number of explosion increases. This green-bluecolor of the solution gradually disappeared when the solutionwas mixed with acetone (50:50) and heated to reduce thevolume and finally rinsed. SEM images of the particles areshown in Fig. 3. The particles are spherical. Average size ofthe Cu nanoparticles synthesized using this method is about

Fig. 3. TEM image of Cu nanoparticles as produced using PWE in DI water.

50 nm measured using SEM. The exploding medium (DIwater) temperature was 25 °C.

The green-blue color of the solution after explosion is thecharacteristic of Cu(II) ion in a water solution. Copper isreactive and when in contact with oxygen like the oxygendissolved in water, it oxidizes. Cu(OH)2 is formed duringthe explosion process in DI water. When this solution washeated, Cu(OH)2 turned into CuO. Studies have shown thatPWE in water medium produces smaller size nanoparticleswith narrow particle size distribution [15]. Using water asan exploding medium provides a high-density environmentaround the wire to suppress the plasma formation. The largeheat capacity of water also helps in fast cooling of theparticles. These characteristics make PWE in DI water anideal approach for metal nanoparticles fabrication. Besides,the amount of micrometer-sized particles is also minimal dueto the longer plasma formation time in water medium. Adetail study examining the DI water temperature and densityeffect on Cu nanoparticles size and structure is currently beingpursued and will be reported in a later date.

A. Cu Nanoparticles Extraction

The DI water with Cu nanoparticles was mixed with acetoneat 50:50 by volume ratio. This mixture is mainly to aid inreducing the boiling temperature of the solution. According tothe vapor–liquid equilibrium of acetone–water (50:50 ratio),the boiling point of the mixture is 59.87 °C [16]. The lowboiling point of the mixture hastened the evaporation processas the solution was heated at 40 °C. Usual centrifuging methodfor particle separation may not be cost-effective in massproduction.

The particles can now be easily collected for furtherprocessing. The PWE synthesis in DI water gives approxi-mately 0.0054 gm of Cu nanoparticles per explosion. Thisis equal to the yield of nanopowders produced in mass-scale synthesis using inert gas evaporation techniques reportedearlier [17], [18].

B. Surface Oxide Reduction and Silicon Attach

The reduction of surface oxidation was observed throughchange of color of the particles. The particles turned from

590 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 4, APRIL 2012

(a)

(b)

Fig. 4. Leadframe surface at 100× magnification. (a) Cu nanoparticles (CuO)as dispensed (black) and (b) after oxide reduction (brown).

TABLE II

SILICON DIE ATTACH SHEAR STRENGTH FOR

VARIOUS BONDING CONDITIONS

Bondingtemperature

(°C)

Bondingpressure(MPa)

Bondingtime (min)

Annealing1 h@450 °C

Shearstrength(Kgf)

350 10.0 20 Yes 0.52

350 5.0 20 Yes 0.35

400 10.0 15 Yes 1.25

400 5.0 15 Yes 0.90

400 10.0 15 No 1.05

black (CuO) during printing to metallic brown (Cu) after 3 hof forming gas treatment. Fig. 4 shows the high magnificationimage of the leadframe surface before and after Cu nanopar-ticles oxide reduction. Since the rate of Cu oxide reduction informing gas is much slower than that of pure hydrogen gas,longer treatment duration was needed in our experiment (3 h).Most of the studies on nanoparticles oxide reduction in the pastwere conducted in pure hydrogen environment at a very high-temperature [19]–[21]. In these cases, the concentration of H2required for oxide reduction is greater than the flammabilitycriteria of H2 in air.

In a manufacturing environment, the maximum allow-able hydrogen percentage is about 7%. Successful Cu oxide

Si Chip

Ag plated Cu substrate

Ti/Ni/Ag backmetal

(a)

.u

.v

.w

Ti/Ni/Ag backmetal

Ag plated Cu substrate

Void

(b)

Fig. 5. Cross-section image of sample bonded at 400 °C with postbondannealing. (a) Si/Cu substrate interface and (b) high magnification of sinteredCu nanolayer.

reduction was also reported using formic acid at 250 °C for80 min [22]. Nonetheless, forming gas will be the preferredsolution in semiconductor assembly for safety reasons andthe equipment platforms are readily fitted with forming gashandling capability.

Silicon attach experiment was conducted using conventionaleutectic bonding machine in a N2-5%H2-filled tunnel. Silicondie shear strength was measured and tabulated in Table IIbelow. The silicon die shear strength shows a drastic increaseas the bonding temperature increases. Bonding pressure of10.0 MPa at 400 °C with postbond annealing gives the highestshear strength. A repeated run at 400 °C without postbondannealing shows drop in the shear strength compared to asimilar run with postbond annealing. Inspection on the shearedsamples showed gross cohesive failure. The sample that wasbonded at 400 °C with postbond annealing and showing thehighest shear value was subjected to cross-sectional micro-scopic analysis. Fig. 5 displays the cross-sectional SEM imageof the sample.

Cross-sectional SEM image in Fig. 5(a) denotes theconnection of the sintering neck. However, there are voids

KRISHNAN et al.: PREPARATION AND LOW-TEMPERATURE SINTERING OF Cu NANOPARTICLES 591

seen between the sintered regions. The voids are mainly dueto the poor densification of the nanoparticles. The printingand drying process only deposited the Cu nanoparticles on theleadframe surface and there was no force applied for particlescompaction. Hence, it is presumed that there are regions withinthe Cu nanoparticles printed area that have low particle density.At locations where Cu particles were not densely deposited,voids formed. Though we used multiple layer screen printingand drying approach, due to lack of compaction force duringparticles deposition, these voids remained within the layers.During particles sintering, a high external pressure is neededto alleviate these voids since grain growth also occurs simul-taneously. Therefore, particles densification before sinteringprocess is crucial to minimize pores and voids formation.

In Fig. 5(b), intermetallic fusion between sintered Cu andAg layers at wafer backmetal interface (marked “u”) seemsto have taken place. Similar fusion is also observed at thesubstrate interface (marked “v”). Most of these joints wereseen along the cross-section region where the particle densityis higher. There were no pores observed in the sintered regions(marked “w”). The smallest grain measured in the sinteredregion was about 600 nm, which has grown to a size of oneorder of magnitude larger than the initial particle size. Thelarger grains could result from the consumption of severalsmall grains.

Sintering of Cu nanoparticles typically goes through threestages: appearance of the neck, growth of the neck, anddensification [23]. After the oxide reduction, most particlesare activated. During bonding, as we gradually increase theleadframe temperature to 10 °C/min, the local temperature inthe particles also raises. The high local temperature enhancesthe diffusivity of the copper atoms. At this stage, sinteringnecks form at the surface of the particles. As the leadframetemperature reaches the peak (400 °C), the sintering neck willgrow and connect with the adjacent group of particles. Bondpressure applied at this point enhances particles densification.Improvement in the adhesion strength with postbond annealinghas been studied before. The bonded Cu grain structurereaches steady state after postbond annealing. Higher postbondannealing temperature gives Cu atoms more energy to diffuseand allows grains to grow. It was reported that for wafer-levelbonding using blanket Cu wafer, higher bonding temperature(350–400 °C) and postannealing time (30–60 min) improvethe interfacial strength [24].

IV. CONCLUSION

We demonstrated the potential use of Cu nanoparticles asan alternative lead-free die attach material for high-powersemiconductor devices. MOSFET device with Ti/Ni/Ag backmetal was successfully bonded on Ag-plated Cu substrateusing Cu nanoparticles at 400 °C and 10 MPa bond pressure.An economical method for synthesis of Cu nanoparticles in thesize range of 30–50 nm using PWE in DI water and extractionof the particles for further use was established. We have shownin situ Cu oxide reduction using industry standard forminggas N2-5%H2 with existing assembly equipment platform.Sintering of Cu nanoparticles was done in N2 environment at

400 °C. The Cu joint adhesion strength was further enhancedby 20% with postbond annealing for 1 h at 450 °C. The Cuparticle printing and densification prior to silicon attach needto be improved to eliminate voids formation after sintering.Currently, we are working on gang press approach to mitigatethe void issue at sintering stage and to make it compatible withexisting manufacturing process. The flow of this gang pressapproach is as follows.

1) Apply Cu nanoparticles on leadframe at room tempera-ture (screen print or dispensing).

2) Die bonding on all units of the leadframe using stan-dard die attach machine at usual speed (for temporaryadhesion).

3) Transfer the die bonded leadframe onto the “Gang Press”system.

4) Preheating with applied pressure of 0.1 MPa providesthe densification needed, before sintering takes placewith the supplied N2H2. This minimizes the pores/voids.This also eliminates the long cycle time needed for oxidereduction since it is done in a closed cavity and followedby die bonding.

5) Increase the chase temperature to 300–400 °C withhigher pressure applied for particles sintering. Postbondannealing may not be needed with this.

However, we believe a certain level of pores should bemaintained within the sintered structure to absorb the internalstresses during reliability cycling. We are evaluating thisapproach and would present the results with electrical andreliability performance of Cu nanoparticles as an alternativeto lead-free die attach solder in another paper. Cu joints wouldnot have intermetallic associated reliability issues as with thesolder joints currently being used in electronic packages. AsCu has a very high melting point of 1083 °C, the resultingCu joints are expected to achieve higher operating tempera-tures. The oxide-free Cu interface is also expected to exhibitelectrical and thermal conductivity that is superior to existingsolders.

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Shutesh Krishnan (M’03–SM’09) received the Bachelors degree from theScience University of Malaysia, Alor Setar, Malaysia, and the Masters degreefrom the University of Malaya, Kuala Lumpur, Malaysia, in 1999 and 2008,respectively. He is currently pursuing the Ph.D. degree with the University ofMalaya.

He was a Package Development Engineer with ST Microelectronics,Chennai, India. He is currently with ON Semiconductor Package Innovationand Development Center, Kuala Lumpur. His primary functions are newproduct design and materials development. His current research interestsinclude nano-structured materials for microelectronic packaging.

A. S. M. A. Haseeb (M’09) received the Bachelors and Masters degrees inmetallurgical engineering from the Bangladesh University of Engineering andTechnology (BUET), Dhaka, Bangladesh, and the Ph.D. degree in metallurgyand materials engineering from the Catholic University of Leuven, Leuven,Belgium, in 1984, 1986, and 1992, respectively.

He has been a Professor of materials engineering, University of Malaya(UM), Kuala Lumpur, Malaysia, since 2006. Prior to joining UM, he wasa Professor at BUET. His current research interests include electronic mate-rials, nano-structured alloys for LIGA microelectromechanical systems, andmaterials degradation.

Mohd Rafie Johan received the Bachelors, Masters, and Ph.D. degrees fromthe University of Malaya (UM), Kuala Lumpur, Malaysia, in 1993, 1997, and2005, respectively.

He is currently a Lecturer with the Faculty of Mechanical Engineering,UM. He specializes in advanced materials and theoretical physics. His currentresearch interests include meta-materials, nanocomposite polymeric materialsand computational materials, and quantum field theory.