Harry Hedler

Thermo-mechanical Design of Resilient Contact Systems for Wafer Level Packaging

Wafer level packaging (WLP) technologies are cost effective packaging solutions which are used increasingly. Second level reliability, i.e. mainly the thermo-mechanical reliability during thermal cycling, is a major concern of WLP. To avoid excessive solder straining, solder balls have been replaced by resilient interconnects, which can adopt the main part of the thermal mismatch deformation. One solution combining an increased reliability on module level with advantages in processing and the capability of full wafer level test and burn-in is ELASTecreg (ELASTec hArr Elastic-bump on Silicon Technology), particularly developed for memory products. The new failure risks are mainly related to fatigue of the metallic redistribution layer (RDL). Parametric studies using finite element analyses (FEA) were performed to avoid excessive straining of the metal lines. A balance of metal straining and solder straining had to be achieved. Comparisons were made for different soft bump layouts and RDL patterns. Optimal solutions figured out by FEA were also investigated experimentally by thermal cycle tests. However, the thermo-mechanical characteristics like stress-strain behaviour and fatigue resistance of the metallic films are the most important parameters for reliability predictions. In particular, the elastic-plastic properties of thin metallic Cu and Ni films are shown to depend on features like film thickness, grain size and orientation, resulting in a thin film strength exceeding the bulk strength of the same metal by several hundred percent

Strong transmittance above the light line in mid-infrared two-dimensional photonic crystals

The mid-infrared region of the electromagnetic spectrum between 3 and 8 μm hosts absorption lines of gases relevant for chemical and biological sensing. 2D photonic crystal structures capable of guiding light in this region of the spectrum have been widely studied, and their implementation into miniaturized sensors has been proposed. However, light guiding in conventional 2D photonic crystals is usually restricted to a frequency range below the light line, which is the dispersion relation of light in the media surrounding the structures. These structures rely on total internal reflection for confinement of the light in z-direction normal to the lattice plane. In this work, 2D mid-infrared photonic crystals consisting of microtube arrays that mitigate these limitations have been developed. Due to their high aspect ratios of ∼1:30, they are perceived as semi-infinite in the z-direction. Light transmission experiments in the 5–8 μm range reveal attenuations as low as 0.27 dB/100 μm, surpassing the limitations for light guiding above the light line in conventional 2D photonic crystals. Fair agreement is obtained between these experiments, 2D band structure and transmission simulations.

High performance 3D interconnects based on electrochemical etch and liquid metal fill

Besides the usually known TSV technologies to etch and fill Si interconnects there is a powerful photo-assisted electrochemical etch technology for fine pitch TSV, which goes hand in hand with a liquid fill metallization. Both together allow the generation of high density wiring on and through thick self-carrying interposers, where other technologies fail. Together with any etching technology the liquid fill technology allows an extremely simple process flow to realize conductor lines inside buried channels. Taking the right material combinations there is very low stress inside the substrate, which allows a high design freedom for larger via or systematic aligned via. Using the fill technology for stacked dies an extremely simple and compact 3D wiring is possible. It is less complex than conventional ones since it does not need any seed layer or additional balls between chips. The fill technology has the potential of complete wiring of multichip systems.

Combining Photonic Crystal and Optical Monte Carlo Simulations: Implementation, Validation and Application in a Positron Emission Tomography Detector

This paper presents a novel approach towards incorporating photonic crystals (PhCs) into optical Monte Carlo (MC) simulations. This approach affords modeling the full diffractive nature of PhCs including their reflection and transmission behavior as well as the manipulation of the photon trajectories through light scattering. The main purpose of this tool is to study the impact of PhCs on the light yield and timing performance of scintillator-based detectors for positron emission tomography (PET). To this end, the PhCs are translated into look-up tables and implemented into the optical MC algorithm. Our simulations are validated in optical experiments using PhC samples fabricated with electron beam lithography. The experimental results indicate that the simulations match the measurements within the accuracy of the experiments. The application of the combined simulation technique to a PET detector module predicts an increase of the total light yield by up to 23% for PhC coatings versus the reference without PhCs. Timing calculations reveal an improvement of the coincident resolving time by up to 6%. The results underline the potential of PhCs to improve light yield and timing of PET detector modules.

Implementation of photonic crystal simulations into a Monte Carlo code to investigate light extraction from scintillators

Introduction: Nanostructures are used in various forms to increase light extraction from materials with a high refractive index [1,2]. Recently, photonic crystals (PhC) have been proposed to improve the light yield of scintillators used in high energy physics and medical imaging applications [3]. PhCs consisting of a thin slab with a periodically modulated refractive index serve to reduce total internal reflection at the scintillator extraction face. PhC coatings influence the reflection and transmission coefficients of photons impinging on the interface and the photon trajectory. A careful evaluation of the impact of PhCs on the total light yield and the propagation time distribution of extracted photons requires combined simulations in two regimes: i) interaction of electromagnetic waves with PhCs with dimensions similar to the wavelength which requires a full vectorial Maxwell solver and ii) propagation of photons inside the scintillator which is commonly derived from a Monte Carlo (MC) code using laws of geometric optics. This work examines the characteristics of PhCs using Rigorous Coupled Wave Analysis (GD-Calc, KJ Innovation) and ray-tracing (Radiant Zemax) for optical MC simulations.