R.H. Poelma

A numerical experimental approach for characterizing the elastic properties of thin films: application of nanocantilevers

In this paper, a numerical experimental approach for measurement of the effective Youngs modulus and its size effect in homogeneous thin films and cantilevers is demonstrated. Cu thin films were used as a case study. The experiment consists in measuring the pull-in instability voltage of electro-static actuated Si nanocantilevers and bilayer Cu/Si nanocantilevers. An electro-mechanical coupled finite element model of the bilayer Cu/Si nanocantilevers was used to extract the effective Youngs modulus from the measured pull-in voltage. The fabricated samples consist of 340 nm thick Si cantilevers with 10 and 50 nm thick physical vapor deposited Cu films. White light interferometry was used to measure the cantilever curvature and Stoneys equation was used to calculate the thin film stress. It is shown that the pull-in instability experiment and the cantilever curvature measurement can be used for fast and easy determination of Youngs modulus and film stress of 10 and 50 nm thick Cu films, respectively.

Stretchable Binary Fresnel Lens for Focus Tuning

This paper presents a tuneable binary amplitude Fresnel lens produced by wafer-level microfabrication. The Fresnel lens is fabricated by encapsulating lithographically defined vertically aligned carbon nanotube (CNT) bundles inside a polydimethyl-siloxane (PDMS) layer. The composite lens material combines the excellent optical absorption properties of the CNT with the transparency and stretchability of the PDMS. By stretching the elastomeric composite in radial direction, the lens focal length is tuned. Good focusing response is demonstrated and a large focus change (≥24%) was achieved by stretching lenses up to 11.4%.

Multi-LED package design, fabrication and thermal analysis

An ultra-thin multi-LED package is designed, manufactured and its thermal performance is characterized. The objective of this study is to develop an efficient thermal modelling approach for this system which can be used for optimization of the thermal-performance of future ultra-thin designs. A high-resolution thermal imaging camera and thermocouples were used to measure the temperature distribution of the multi-LED package and the LED-die temperature for different operating powers. Finally, we compare the thermal measurements with the finite element simulation results. It is concluded that the modelling approach can assist in the thermal optimization of future multi-LED package designs.

Metallic Nanoparticle Based Interconnect for Heterogeneous 3D Integration

We explored the properties and performance of copper-based metallic nanoparticle paste (MNPs) for interconnects applications in 3D heterogeneous integration. A patterning method was developed to process micron sized sintered MNPs structures. This enables the fabrication of IC interconnect test structures to characterise specific resistivity sintered MNPs and the contact resistances of sintered MNP to bulk copper (bCu) which was respectively 78.4 mOhm.micrometer and 0.23 Ohm.micrometer2. In situ XRD analysis showed no oxidation of MNPs at processing temperatures below 100 °C. When Copper based MNPs are sintered under forming gas conditions, no oxidation of copper is measured. With in situ TEM at a temperature range of 220 - 260 oC local melting of copper nanoparticles was observed. This is in agreement with the electrical measurements, the resistivity and contact resistance are considerably reduced when MNPs is sintered in this temperature range. Copper-based MNPs is successfully applied as die attach and wafer to wafer (W2W) bonding. However, for W2W bonding, the specific contact resistance was 800 Ohm.micrometer2.

Tunable binary Fresnel lens based on stretchable PDMS/CNT composite

This paper presents a tunable micro Fresnel lens made by a polydimethylsiloxane/carbon nanotubes (PDMS/CNTs) configuration that can change its focal length by simply stretching the substrate. We believe this is the first time that this configuration has been realized and demonstrated. The Fresnel lens is formed by embedding vertically aligned CNTs bundles in a 2mm thick PDMS layer. It utilizes the transparency and flexibility of the PDMS and the excellent optical absorption properties of CNTs. The lens is fabricated using a straightforward process, which requires only one lithography step. Preliminary results show that this Fresnel lens has good optical properties, and focal length change can be realized by simply stretching the polymer substrate.

3D interconnect technology based on low temperature copper nanoparticle sintering

We explore a methodology for patterned copper nanoparticle paste for 3D interconnect applications in wafer to wafer (W2W) bonding. A novel fine pitch thermal compression bonding process (sintering) with coated copper nanoparticle paste was developed. Most of the particle size is between 10-30 nm. Lithographically defined stencil printing using photoresist and lift-off was used to apply and pattern the paste. Variations in sintering process parameters, such as: pressure, geometry and ambient atmosphere, were studied. Compared to Sn-Ag-Cu (SAC) microsolder bumps, we achieved better interconnect resistivity after sintering at 260 °C for 10 min, in a 700 mBar hydrogen forming gas (H2/N2) environment. The electrical resistivity was 7.84 ± 1.45 μΩ·cm, which is about 4.6 times that of bulk copper. In addition, metallic nanoparticle interconnect porosity can influence the electrical properties of the interconnect. Consequently, we investigated the porosity effect on conductivity using finite element simulation. A linear relationship between the equivalent conductivity and particle overlapping ratio was found.

Stiction-Induced Sealing of Surface Micromachined Channels

We established a technique to achieve low temperature hermetic sealing of surface micromachined channels by exploiting stiction. Using a wing-shaped structural layer, microchannels automatically seal during the drying step that follows the sacrificial etch. This avoids the need of plugging layers to close the apertures of the sacrificial etch. To demonstrate the technique, we designed a surface micromachined channel with a structural layer made of polycrystalline silicon carbide (poly-SiC). This layer integrates an array of anchored pillars to achieve long and wide microchannels (5.4 mm × 0.43 mm × 0.001 mm). To dimension the sealing-wing, we estimate the minimum adhesion energy between the poly-SiC and silicon. This is done by analytical modeling and experimental characterization of test structures in the shape of centrally-supported circular plates. The bending and adhesion of the sealing-wing is followed in situ by optical microscopy. The closed microchannels are annealed at 100 °C. Some structures are exposed to thermal stress at 760 °C. The results are inspected by white light interferometry, scanning electron microscopy, and helium leak testing. Microchannels result hermetically sealed showing leak rates below the detection limit (4×10-9 Pa·m3/s). The seal is effective to at least 600 kPa.

Buckling analysis of carbon nanotubes and the influence of defect position

In this paper, the buckling behavior of fixed-fixed, both single- and multi- wall carbon nanotubes (CNTs) under axial compressive loads, are studied using analytical continuum theory and molecular dynamics (MD). An approach based on the tethering of atoms, is used to apply the boundary conditions and extract the reaction forces during the MD simulation. Both the analytical and the MD results agree well for slender CNTs (length=diameter = L/D ≥ 9), that show global buckling. The critical buckling load of non slender CNTs (L/D < 9) is overestimated by the analytical model due to the local buckling. Moreover, the effects of the vacancy defect position on the critical buckling load are studied at room temperature and at low temperature (1 K). It is concluded that the defects at the ends of the CNT and close to the middle of the CNT significantly reduce the critical buckling load and strain of CNTs at 1 K. However, the influence of vacancy defects on the critical buckling load and strain appears to be small at room temperature. The MD results can be used for developing more computationally efficient and accurate continuum descriptions of the CNT mechanics in future work.

Multi-scale numerical-experimental method to determine the size dependent elastic properties of bilayer silicon copper nanocantilevers using an electrostatic pull in experiment

Thin metal films are widely used in modern electro mechanical systems. The need for more integrated functionality and minimization of material and energy consumption leads to miniaturization of these systems. As a consequence, materials are processed on the micro- and nanometer scale. On this scale, material properties become a function of size. To predict performance and reliability, knowledge on the size dependence of material properties is imperative. In this work the unknown size dependence of the copper Youngs modulus is determined by electrostatic pull-in experiments performed on bilayer copper-silicon nanocantilevers. The size effect is also predicted with a multi-scale (MS) method. In this method atomistic simulations predict the bulk elastic and surface properties of mono-crystalline silicon (Si) and poly-crystalline copper (Cu). These results are combined to represent the bilayer nanocantilevers of the experiment in a continuum model. The model is verified by comparison with a well documented size effect of the effective Si Youngs modulus. It is shown that the experimental method can be used for determining the Youngs modulus of thin Cu films in the 10 to 50 nm range. Both the experimental results and the MS simulation results show that there is a strong size effect present in Si and Cu.

Effects of Conformal Nanoscale Coatings on Thermal Performance of Vertically Aligned Carbon Nanotubes

The high aspect ratio and the porous nature of spatially oriented forest-like carbon nanotube (CNT) structures represent a unique opportunity to engineer a novel class of nanoscale assemblies. By combining CNTs and conformal coatings, a 3D lightweight scaffold with tailored behavior can be achieved. The effect of nanoscale coatings, aluminum oxide (Al2 O3 ) and nonstoichiometric amorphous silicon carbide (a-SiC), on the thermal transport efficiency of high aspect ratio vertically aligned CNTs, is reported herein. The thermal performance of the CNT-based nanostructure strongly depends on the achieved porosity, the coating material and its infiltration within the nanotube network. An unprecedented enhancement in terms of effective thermal conductivity in a-SiC coated CNTs has been obtained: 181% compared to the as-grown CNTs and Al2 O3 coated CNTs. Furthermore, the integration of coated high aspect ratio CNTs in an epoxy molding compound demonstrates that, next to the required thermal conductivity, the mechanical compliance for thermal interface applications can also be achieved through coating infiltration into foam-like CNT forests.