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Research Papers

# Enhanced Electrical and Thermal Interconnects by the Self-Assembly of Nanoparticle Necks Utilizing Capillary Bridging

[+] Author and Article Information
Thomas Brunschwiler

IBM Research–Zurich,
Rüschlikon 8803 ZH, Switzerland
e-mail: tbr@zurich.ibm.com

Gerd Schlottig

IBM Research–Zurich,
Rüschlikon 8803 ZH, Switzerland
e-mail: erd@zurich.ibm.com

Songbo Ni

IBM Research–Zurich,
Rüschlikon 8803 ZH, Switzerland
e-mail: nso@zurich.ibm.com

Yu Liu

IBM Research–Zurich,
Rüschlikon 8803 ZH, Switzerland
e-mail: liuyu@student.ethz.ch

Javier V. Goicochea

IBM Research–Zurich,
Rüschlikon 8803 ZH, Switzerland
e-mail: goicox@gmail.com

Jonas Zürcher

IBM Research–Zurich,
Rüschlikon 8803 ZH, Switzerland
e-mail: zur@zurich.ibm.com

Heiko Wolf

IBM Research–Zurich,
Rüschlikon 8803 ZH, Switzerland
e-mail: hwo@zurich.ibm.com

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received November 21, 2013; final manuscript received July 29, 2014; published online September 19, 2014. Assoc. Editor: Nils Hívik.

J. Electron. Packag 136(4), 041012 (Sep 19, 2014) (10 pages) Paper No: EP-13-1130; doi: 10.1115/1.4028332 History: Received November 21, 2013; Revised July 29, 2014

## Abstract

This work presents enhanced composite joints that support both electrical and thermal transport in electronic packages. The joints are sequentially formed by applying a nanoparticle suspension, evaporating a solvent, self-assembling of nanoparticles by capillary bridging, and the formation of so called “necks” between micrometer-sized features. This sequence is used to either form low temperature electrical joints under copper pillars or enhanced percolating thermal underfills (ePTU) with areal contacts between filler particles of the composite. The report discusses processing aspects of the capillary bridges evolution and of uniform neck formation, it discusses strategies to achieve mechanically stable necks, and it compares the performance of the achieved joints against state-of-the-art solutions. The capillary bridge evolution during liquid evaporation was investigated in copper pillar arrays and random particle beds. The vapor–liquid interface first penetrates locations of low pillar or particle density resulting in a dendritic fluid network. Once the network breaks up, individual necks form. For aqueous nanosuspensions, highly uniform necks with high yield were obtained by evaporation at 60 °C. Isothermal conditions were preferred to yield equal neck counts at the cavity's top and bottom surfaces. Mechanically stable silver necks required an annealing at only 150 °C, dielectric necks an annealing at 140 °C with a bimodal approach. Therein polystyrene (PS) nanoparticles occupy interstitial positions in densly packed alumina necks, then melt and adhere to the alumina. The electrical necks showed a shear strength of 16 MPa, equivalent to silver joints used in power electronic packages. The thermal necks yielded a thermal conductivity of up to 3.8 W/mK, five-fold higher than commercially available capillary thermal underfills.

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## Figures

Fig. 1

The NEI formed by electrically conductive necks between copper pillars and adjacent pads (left). The ePTU with dielectric necks formed between micrometer-sized filler particles for an improved thermal joint (right).

Fig. 2

Schematic of the ePTU neck formation process with capillary bridging that directs self-assembly. The evaporation front drives nanoparticles into the contact area of micrometer-sized filler particles, resulting in improved thermal contacts. Contacts between filler particles themselves turn into necks, contacts between filler particles and substrates turn into collars. Likewise, the NEI necks form between copper pillars and pads.

Fig. 3

SEM image of an electroformed copper pillar with a height of 40 μm (from base to the top of the dome) and diameter of 100 μm. The silicon surface is coated with a 2 μm thick polyimide layer.

Fig. 4

(a) Cross section and (b) top view of the silicon cavity test section for neck formation investigations. Centrifugal filling is used to result in a filler particle-bed in the cavity [11,14]. The solvent of the nanosuspension is injected through the port and can leave the cavity through the cavity inlet and outlet during the evaporation step. The diamond, square, and triangle shaped features indicate distinct observation locations.

Fig. 5

Cavities (60 μm high) filled with monodispersed silica spheres of 53 μm diameter (a) and faceted diamond powder with a size distribution of 30–40 μm (b) views through the top glass

Fig. 6

Top and side view micrographs of the meniscus evolution (a–d) during evaporation within a cavity, which is populated with copper pillars (pitch: 210 μm, diameter: 130 μm). Water is initially dispensed into the cavity formed by the NEI test chip and a glass slide. The meniscus penetrates areas with large pillar spacing first ((a) and (b)), resulting in liquid bridges between the dense pillar row. They finally break-up and form individual capillary-bridges between copper pillar and glass ((c) and (d)). The periphery of the copper pillar can be seen at the top half of the micrograph (d) and should not be confused with its reflection in the bottom half of the image. Meniscus radii of 142 μm and 139 μm were identified for R1 and R2, respectively.

Fig. 7

Microscopic views through the cover glass plate of the specimen (Fig. 4) showing the evolution of capillary bridges during the evaporation process. The time given is the time elapsed since the start of the evaporation. Water was initially injected into the bed of 53 μm diameter particles. A sequence of two different evaporation patterns was identified: (1) dendritic network growth (left) and (2) collapse of capillary bridges (right). Two experiments at an evaporation temperature of 60 °C ((a)–(d)) and 100 °C ((e) and (f)) are reported. The evaporation front is indicated by a dashed line in the local close-ups (c, d).

Fig. 8

SEM image of sintered necks formed between copper pillars and pads. Ag ink with 2.4 vol. % silver nanoparticles in TGME was squeezed between the chip and carrier. The solvent evaporation was performed at 100 °C. The necks were annealed at 150 °C.

Fig. 9

SEM photograph close-ups and overviews (inset) of necks formed by capillary bridging within particle beds in cavities with spherical (a) (dneck/dp ∼ 0.2) and irregular shapes (b) (dneck/dp < 0.07). Nanosuspension: Ag ink 4.8 vol. % in TGME. Ag sintered at 150 °C.

Fig. 10

Bimodal necks to achieve mechanically stable dielectric areal interconnects. Two possible topologies, the interstitial neck ((a) and (c)) and the core-shell neck ((b) and (d)) are shown. ((a) and (b)) Schematics of the simultaneous and the sequential process. Large and small particles indicate the thermally conductive ceramic and the thermoplast material, respectively. The resulting necks are shown in the SEM images below (c) and (d). The core-shell neck is formed by two subsequent capillary bridging processes of two different nanosuspensions. Here, an inner alumina core neck and the outer PS shell neck (d).

Fig. 11

Core-shell necks and collars of 100 nm PS and 300 nm alumina nanoparticles after the annealing step of 140 °C for 15 min. (a) The PS shell forms a continuous and smooth layer around the core structure. (b) A percolation path between the top and bottom surface of the cavity with enhanced thermal interconnects is formed by two collars and one neck.

Fig. 12

SEM top views towards the bottom surface of a mechanically opened cavity showing the assembled PS collars. The silica filler particles with 53 μm diameter were removed prior to this inspection. The visible collars result from the capillary bridging of an aqueous nanosuspension (100 nm PS 1.25 vol. %) at 60 °C ((a) and (b)) and 100 °C (c).

Fig. 13

Collar yield on the bottom silicon surface after evaporation of the aqueous nanosuspension (100 nm PS 1.25 vol. %). The evaporation process was performed on a hot plate at three different temperatures. The collar yield at three different positions in the cavity is indicated and is a measure of the collar formation uniformity.

Fig. 14

Size and yield of the assembled collars at the bottom silicon plate versus nanoparticle concentration in the nanosuspension. The 60 μm-high cavity was filled with 53 μm silica spheres. The self-assembly was performed with an aqueous suspension (100 nm PS spheres) on a hot plate at an evaporation temperature of 60 °C and with silicon as the bottom plate.

Fig. 15

(a) Schematic of the die shear test showing the chisel displacement and shear direction as well as the degrees of freedom in the clamping chosen. (b) SEM image of the chip after breaking of the interconnects (copper pillar footprint and broken neck area is indicated).

Fig. 16

Benchmarking of measured effective thermal conductivities of capillary UF, percolating thermal underfills with filler particles only (centrifugal filling) [11], and percolating thermal underfills with enhanced thermal contacts (centrifugal + necking).

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