Carsten Christensen

Tantalum oxide thin films as protective coatings for sensors

Reactively sputtered tantalum oxide thin-films have been investigated as protective coating for aggressive media exposed sensors. Tantalum oxide is shown to be chemically very robust. The etch rate in aqueous potassium hydroxide with pH 11 at 140/spl deg/C is lower than 0.008 /spl Aring//h. Etching in liquids with pH values in the range from pH 2-11 have generally given etch rates below 0.04 /spl Aring//h. On the other hand patterning is possible in hydrofluoric acid. Further, the passivation behaviour of amorphous tantalum oxide and polycrystalline Ta/sub 2/O/sub 5/ is different in buffered hydrofluoric acid. By ex-situ annealing in O/sub 2/ the residual thin-film stress can be altered from compressive to tensile and annealing at 450/spl deg/C for 30 minutes gives a stress-free film. The step coverage of the sputter deposited amorphous tantalum oxide is reasonable, but metallisation lines are hard to cover. Sputtered tantalum oxide exhibits high dielectric strength and the pinhole density for 0.5 /spl mu/m thick films is below 3 cm/sup -2/.

Reliability of industrial packaging for microsystems

Abstract Packaging concepts for silicon-based micromachined sensors exposed to harsh environments are explored. By exposing the sensors directly to the media and applying protection at the wafer level the packaging and assembly will be simplified as compared to conventional methods of fabrication. Protective coatings of amorphous silicon carbide and tantalum oxide are suitable candidates with etch rates below 0.1 A/h in aqueous solutions with pH 11 at temperatures up to 140°C. Si-Ta-N films exhibit etch rates around 1 A/h. Parylene C coatings did not etch but peeled off after extended exposure times at elevated temperatures. The best diamond-like carbon films we tested did not etch, but delaminated due to local penetration of the etchants. Several glue types were investigated for chip mounting of the sensors. Hard epoxies, such as Epotek H77, on the one hand exhibit high bond strength and least degradation and leakage, but on the other hand introduce large sensor output drift with temperature changes. Softening of the Epo-tek H77 was observed at 70°C. An industrially attractive thin-film anodic silicon-to-silicon wafer bonding process was developed. Glass layers are deposited at 20 nm/s (1.2 μm/min) by electron-beam evaporation and bond strengths in excess of 25 N/mm 2 are obtained for bonding temperatures higher than 300°C. Through-hole electrical feedthroughs with a minimum line width of 20μm and a density of 250 wires per cm were obtained by applying electro-depositable photo-resist. Hermetically sealed feedthroughs were obtained using glass frits, which withstand pressures of 4000 bar.

Combined differential and static pressure sensor based on a double-bridged structure

A combined differential and static silicon microelectromechanical system pressure sensor based on a double piezoresistive Wheatstone bridge structure is presented. The developed sensor has a conventional (inner) bridge on a micromachined diaphragm and a secondary (outer) bridge on the chip substrate. A novel approach is demonstrated with a combined measurement of outputs from the two bridges, which results in a combined deduction of both differential and static media pressure. Also following this new approach, a significant improvement in differential pressure sensor accuracy is achieved. Output from the two bridges depends linearly on both differential and absolute (relative to atmospheric pressure) media pressure. Furthermore, the sensor stress distributions involved are studied by three-dimensional finite-element (FE) stress analysis. Furthermore, the FE analysis evaluates current and other potential piezoresistor positions on the outer Wheatstone bridge.

Room-temperature vacancy migration in crystalline Si from an ion-implanted surface layer

Migration of vacancies in crystalline, n-type silicon at room temperature from Ge+-implanted (150 keV, 5×109–1×1011 cm−2) surface layers was studied by tracing the presence of P–V pairs (E centers) in the underlying layer using deep level transient spectroscopy (DLTS). Under the conditions we have examined, the vacancies migrate to a maximum depth of about 1 μm and at least one vacancy per implanted Ge ion migrates into the silicon crystal. The annealing of the E centers is accompanied, in an almost one-to-one fashion, by the appearance of a new DLTS line corresponding to a level at EC−Et≈0.15 eV that has donor character. It is argued that the center associated with this line is most probably the P2–V complex; it anneals at about 550 K. A lower limit of the RT-diffusion coefficient of the doubly charged, negative vacancy is estimated to be 4×10−11 cm2/s.

Particle Precipitation in Connection with KOH Etching of Silicon

This paper considers the precipitation of iron oxide particles in connection with the KOH etching of cavities in silicon wafers. The findings presented in this paper suggest that the source to the particles is the KOH pellets used for making the etching solution. Experiments show that the precipitation is independent of KOH etching time, but that the amount of deposited material varies with dopant type and dopant concentration. The experiments also suggest that the precipitation occurs when the silicon wafers are removed from the KOH etching solution and not during the etching procedure. When not removed, the iron oxide particles cause etch pits on the Si surface when later processed and exposed to phosphoric acid. It has been found that the particles can be removed in an HCl solution, but not completely in an H2SO4-H2O2 solution. The paper discusses the involved precipitation mechanism in terms of the change in free energy of adsorption, the Pourbaix diagram, the electrochemical double-layer thickness and silicon dopant type, and concentration.

Cold-walled UHV/CVD batch reactor for the growth of Si1−xGex layers

Abstract A novel cold-walled, lamp-heated, ultrahigh vacuum chemical vapor deposition (UHV/CVD) batch system for the growth of SiGe layers is presented. This system combines the batch capability of the standard UHV/CVD furnace with the temperature processing available in rapid thermal processing (RTP) equipment. The first results are very encouraging: germanium contents up to 30% and boron doping levels up to 5×10 18 cm −3 have been obtained. The uniformity of the film thickness is around 10% both across a single wafer and through the boat load. Different surface passivation schemes were investigated using excess carrier lifetime measurements. These measurements show that diluted HF provides much better surface passivation than buffered HF.

Eutectic bonds on wafer scale by thin film multilayers

The use of gold based thin film multilayer systems for forming eutectic bonds on wafer scale is investigated and preliminary results will be presented. On polished 4 inch wafers different multilayer systems are developed using thin film techniques and bonded afterwards under reactive atmospheres and different bonding temperatures and forces. Pull tests are performed to extract the bonding strengths.