Preben Storås

Strong, high-yield and low-temperature thermocompression silicon wafer-level bonding with gold

A systematic variation of process parameters for wafer-level thermocompression bonding with gold is presented for the first time. The process was optimized for high bond strength and high bond yield. In addition, the impact of the process temperature was investigated. A bond strength of 10.7 ± 4.5 MPa and a bond yield of 89% was achieved when bonding a wafer pair at 298 °C applying 4 MPa pressure for 45 min. A total of ten wafer pairs were bonded in a custom-built bonding tool and tested to establish the optimal process parameters. The bonded interface was found to be strong and dense enough for MEMS applications. The bonds were characterized using pull tests, transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS). The TEM inspections indicated that it is possible to form hermetic seals by using the presented bonding method.

First fabrication of full 3D-detectors at SINTEF

3D-detectors, with electrodes penetrating through the entire substrates have drawn great interests for high energy physics and medical imaging applications. Since its introduction by C. Kenney et al in 1995, many laboratories have begun research on different 3D-detector structures to simplify and industrialise the fabrication process. SINTEF MiNaLab joined the 3D collaboration in 2006 and started the first 3D fabrication run in 2007. This is the first step in an effort to fabricate affordable 3D-detectors in small to medium size production volumes. The first run was fully completed in February 2008 and preliminary results are promising. Good p-n junction characteristics have been shown on selected devices at the chip level with a leakage current of less than 0.5 nA per pixel. Thus SINTEF is the second laboratory in the world after the Stanford Nanofabrication Facility that has succeeded in demonstrating full 3D-detectors with active edge. A full 3D-stacked detector system were formed by bump-bonding the detectors to the ATLAS readout electronics, and successful particle hit maps using an Am-241 source were recorded. Most modules, however, showed largely increased leakage currents after assembly, which is due to the active edge and p-spray acting as part of the total chip pn-junction and not as a depletion stop. This paper describes the first fabrication and the encountered processing issues. The preliminary measurements on both the individual detector chips and the integrated 3D-stacked modules are discussed. A new lot has now been started on p-type wafers, which offers a more robust configuration with the active edge acting as depletion stop instead of part of the pn-junction.

Development of cost-effective high-density through-wafer interconnects for 3D microsystems

High-density through-wafer interconnects are of great interest for fabricating real 3D microsystems. A complete solution for realizing through-wafer interconnects is presented. The proposed solution is believed to be cost effective and easy to integrate in a device process flow. A deep reactive ion etch process was developed to etch 20 x 20 μm 2 via holes through 300 μm thick silicon wafers. Thermal oxide is used to insulate the vias from the bulk silicon and heavily doped polysilicon is used as the conductor. Aluminum metallization is provided on both sides of the wafer. The electrical resistance of a single through-wafer via is close to 30 Ω.

Sodium distribution in thin-film anodic bonding

Depth profiles of the sodium distribution at bonded interfaces of silicon wafers, bonded with an intermediate borosilicate glass film, are recorded by secondary ion mass spectrometry (SIMS). A layer of dielectric material is present under the thin layer of glass, either a layer of silicon oxide or a combined layer of silicon oxide and silicon nitride. The glass layers are sputtered onto the wafers. Sodium is found to pile-up at interfaces. In the structures where a layer of silicon nitride is included, the amount of pile-up is reduced. Sputtered glass films are analysed with energy dispersive X-ray spectroscopy (EDX), and with a microprobe, in order to estimate the content of sodium in the films. Glass films deposited by e-beam evaporation are included in the EDX measurements for comparison purposes.

High aspect ratio deep RIE for novel 3D radiation sensors in high energy physics applications

3D detectors with electrodes penetrating through the entire silicon substrate have many advantages over conventional planar silicon technology, for example, high radiation tolerance. High aspect ratio through-wafer holes are essential in such fabrication, and deep reactive ion etching (DRIE) is used. A series of DRIE processes were tested and optimised to achieve the required aspect ratio, and in 5-¿m wide trenches, aspect ratios of 58:1 were achieved.