Mats Bergh

Formation of Silicon Structures by Plasma‐Activated Wafer Bonding

A room temperature wafer bonding process based on oxygen plasma treatment or argon plasma treatment has been studied for surfaces of silicon, silicon dioxide, and crystalline quartz. The surface energy of the bonded samples was observed at different storage times. Atomic force microscope measurements, multiple internal reflection infrared spectroscopy, electrical characterization, secondary ion mass spectroscopy, and post‐processing steps were performed to evaluate the plasma‐treated surfaces and the formed bonded interfaces. Electrical measurements were used to investigate the usefulness of plasma‐bonded interfaces for electronic devices. The bonded interfaces exhibit high surface energies, comparable to what can be achieved with annealing steps in the range of 600–800°C using normal wet chemical activation before bonding. The high mechanical stability obtained after bonding at room temperature is explained by an increased dynamic in water removal from the bonded interface allowing covalent bonds to be formed.

Applications of aluminium nitride films deposited by reactive sputtering to silicon-on-insulator materials

Self-heating effects in silicon-on-insulator (SOI) devices limit the applicability of SOI materials in electronics in cases where high power dissipation is expected. Aluminium nitride as a potential candidate for buried insulator material in future SOI-structures is investigated. Reactive sputtering was used to manufacture the aluminium nitride films. The deposited films exhibit low stress and fairly low surface roughness. Further, resistivities above 1014 Ωcm as well as low thermal resistances were obtained. Interfacial problems at the interface between silicon and aluminium nitride were handled by adding a thin (a few nm) film of thermally grown silicon dioxide to that interface. The deposited films could be bonded both directly and through an electrostatic technique to silicon wafers. The presented results show that it is possible to make SOI structures with aluminium nitride as buried insulator by means of wafer bonding and subsequent etch-back.

The Effects of HF Cleaning Prior to Silicon Wafer Bonding

The effects of preparation of silicon surfaces in hydrofluoric acid (HF) solutions, prior to direct wafer bonding, is investigated. Surface analysis with atomic force microscopy, electron spectroscopy for chemical analysis, and estimation of the surface particle density is made. This is related to results from room temperature bonding experiments. A diluted (1-10%) HF solution is most favorable for hydrophobic silicon wafer bonding. The subsequent water rinse should be omitted, or performed in a careful way, to avoid particle contamination. HF:NH 4 F solutions generally are not favorable for bonding. The initial room temperature bonding is attributed to the relatively weak van der Waals forces, which makes the bonding sensitive to the surface roughness and particle density. The surface chemistry appears to have a second order influence in hydrophobic bonding

Silicon-on-diamond MOS-transistors with thermally grown gate oxide

Summary form only given. Self-heating in Silicon-On-Insulator (SOI) devices has during the past years attracted lots of attention and is a problem that remains to be solved. It has, furthermore, been shown that in smart power devices, thick buried oxides of 3 /spl mu/m or more are desired to prevent the substrate potential to lower breakdown voltages. However, these thicker buried oxides will only aggravate the thermal limitations imposed by the buried oxide. Due to the outstanding thermal properties of diamond compared to silicon dioxide, it would consequently be advantageous if silicon dioxide could be replaced with diamond in future SOI materials. Even though it has been shown that diamond is compatible with conventional silicon processing, no MOS-transistors with thermally grown gate oxide has been manufactured up to date, due to the difficulty in protecting diamond during furnace oxidations. In this paper Silicon-On-Diamond (S-O-D) MOS-transistors with thermally grown gate oxide are presented for the first time.

Silicon on aluminum nitride structures formed by wafer bonding

This paper deals with the use of reactively sputtered aluminum nitride (AlN) films as insulators for Bond and Etch-back Silicon-On-Insulator (BESOI) materials. In SOI-applications where high power is dissipated in the silicon SOI-film the low thermal conductivity of the buried silicon dioxide layer may cause a temperature rise in the silicon film detrimentally affecting the device performance. An attractive alternative would be to replace the silicon dioxide of the SOI structure with another material, like diamond, silicon carbide or aluminum nitride. The thermal conductivity of AlN is considerably larger than that of Si0/sub 2/. This paper presents results on how sputter deposition of AlN may be combined with wafer bonding for the creation of highly thermally conductive SOI structures.

High frequency losses in transmission lines made on SIMOX, bulk silicon and depleted silicon/silicon structures formed by wafer bonding

Wafer bonding and etch-back has been used to manufacture a silicon material intended as substrate for high frequency applications. The space charge region surrounding the bonded silicon/silicon interface depletes the silicon, thereby causing semi-insulating behaviour at high frequencies. The formed material was characterized using measurements on metal transmission lines and the results were compared to similar measurements on SIMOX and bulk silicon wafers.

Integration of Silicon and Diamond, Aluminum Nitride or Aluminum Oxide for Electronic Materials

Material integration for the formation of advanced silicon-on-insulator materials by wafer bonding and etch-back will be discussed. Wafer bonding allows combining materials that may not be possible to grow on top of each other by any other technique. In our experiments, polycrystalline diamond, aluminum nitride or aluminum oxide films with thickness of 0.1-5 µm were deposited on silicon wafers. Bonding experiments were made with these films to bare silicon wafers with the goal of forming silicon-on-insulator structures with buried films of polycrystalline diamond, aluminum nitride or aluminum oxide. These silicon-on-insulator structures were aimed to address self-heating effects in conventional silicon-on-insulator materials with buried layers of silicon dioxide. The surfaces of the deposited diamond films were, by order of magnitude, too rough to allow direct bonding to a silicon wafer. In contrast the deposited aluminum nitride and aluminum oxide films did allow direct bonding to silicon. Bonding of the diamond surface to silicon was instead made through a deposited and polished layer of polycrystalline silicon on top of the diamond. In the case of the aluminum nitride electrostatic bonding was also demonstrated. Further, the compatibility of these insulators to silicon process technology was investigated.