T. Gessner

Optical properties and mechanical stress in SiO2/Nb2O5 multilayers

Abstract The aim of this work is the formation of stress-controlled reflection layers which are suitable for application on micromechanical elements without bending them out of shape. Thin films of Nb 2 O 5 and SiO 2 have been deposited by magnetron sputtering of an oxide target (SiO 2 ) and reactive sputtering of a niobium target in an oxygen containing atmosphere, respectively. Silicon and niobium oxide had been selected on the basis of preliminary experiments out of nine different thin film oxides since they yielded sufficiently low absorption coefficients of magnetron sputtered films. Both optical parameters and film stress have been investigated in dependence on deposition parameters. In particular, by varying the substrate bias voltage (for Nb 2 O 5 ) and the sputtering pressure (for SiO 2 ) the film stress could be influenced to a large extent without deterioration of the optical properties. In Nb 2 O 5 , the stress could be varied between compressive and zero stress, whereas in SiO 2 compressive stress was always obtained. Hence, a complete stress-compensation in the multilayers necessitates the application of an additional underlying metal layer having tensile stress. We found magnetron-sputtered chromium films most suitable for that purpose since they provided the highest line stress in comparison to their additional mass per area. To demonstrate our approach we present as an example a layer stack of six SiO 2 and six Nb 2 O 5 films which were designed for maximum reflection at the wavelengths 446, 532 and 629 nm. The film stack was formed after a chromium film (nominal thickness 143 nm) was deposited onto a thin (thickness 30 μm) single-crystal silicon mirror plate. Reflectivity at the wavelengths given above was between 96 and 98%. The multilayer was highly stress-compensated with a typical residual bow of 0.4 μm for a 3.5×3.5 mm 2 mirror.

Thermal conductivity of ultra low- k dielectrics

The so-called 3ω measurement technique (transient hot wire method) was established to determine the thermal conductivity of thin films. Measurements of standard substrates and films validate the found thermal conductivity values and agree with published, commonly accepted values. The method was successfully applied to determine the thermal conductivity of porous low-k dielectric materials using special test structure fabrication. The thermal conductivity of the porous low-k dielectrics thus measured is only between 7 and 13% of the thermal conductivity of thermally grown silicon dioxide.

A novel high aspect ratio technology for MEMS fabrication using standard silicon wafers

A novel CMOS-compatible technology for the fabrication of MEMS based on standard single crystal silicon wafers is available at the Center of Microtechnologies [1]. High Aspect Ratio Microstructures (HARMs) are manufactured using a three mask level technology and dry processing throughout. The released micromechanical components consist of monocrystalline silicon without additional thin films after processing. As a result of the novel process flow the structures are surrounded by air gaps and fixed by special anchors. These Air gap Insulated Microstructures (AIM) were fabricated and tested with respect to mechanical stability, temperature dependence and electrical behavior. The low production costs and high device performance of the fabricated sensors and actuators demonstrate the ability of the technology for high volume production.

Laser display technology

A new laser projection system is presented within this paper. The working principle and the main components are discussed. A cost effective and small sized laser projection system becomes possible by the use of solid-state lasers and the application of microsystem devices for laser beam deflection. The light deflecting devices fabricated by micromachining are an essential of this paper. Design, different technological approaches and measurement results of the vertical scanner and of the line scanning micromirror arrays are discussed.

Selective adhesive bonding with SU-8 for zero-level-packaging

An adhesive bonding technique for wafer-level encapsulation of high aspect ratio microstructures (HARMS) is presented. The adhesive material is spin coated on a cap wafer and structured prior to bonding. Thus sealed cavities of variable height are created in the bonding layer. SU-8 negative photoresist is used as the adhesive material in combination with miscellaneous surface materials: silicon, silicon dioxide and aluminum. The influences of the bonding process parameters - bonding pressure, bonding temperature and process time - as well as the SU-8 layer properties on the bond strength and the homogeneity of the bond have been investigated. To evaluate the process conditions the shear strength of the bond has been measured according to the ASTM standard D 1002 for adhesive bonds. Each bond interface was tested by 32 test specimens of 10 by 10 mm2 side length. With optimal process conditions shear strength of 19.2, 23.3 and 21.3 MPa have been obtained for silicon, silicon dioxide and aluminium respectively. The application of the selective adhesive bonding technique has been successfully demonstrated by encapsulating different types of single crystal silicon inertial sensors.

Frequency-selective silicon vibration sensor with direct electrostatic stiffness modulation

Vibration monitoring has become an important mean for wear state recognition at cutting tools, bearings, gears, engines and other highly stressed machine components [1], [2]. The majority of mechanical vibration used to identify the wear state is found in the frequency range from several Hertzto 10kHz [2]. At present, vibration measurement systems are usually based on wideband piezoelectric sensors completed with sophisticated analyzing electronics to observe the spectrum. Because of high costs permanent monitoring is only practicable in safety related applications or at extremely expensive machinery. Future developments in the field of vibration measurement equipment are expected to lead to smart sensors with fully digital interface, self-test functionality and on-board storage [3]. In this paper we present a frequency selective capacitive sensor for vibration detection with electrically tunable band selectivity fabricated using near-surface silicon micromechanics (SCREAM). The selectivity is based on the mechanical resonance of the structure, the center frequency of which is variable by direct electrostatic stiffness modulation. This represents a capability of resonance frequency tuning by a control voltage to adjust the measurement range to the desired value. Linearity of the sensor characteristics has been achieved by an optimization of the detection and tuning comb capacitors including FEM-analysis and MATLAB optimization algorithms. By grouping sensor structures with stepped base frequencies into an array the frequency range can be largely extended. To be used as a measurement system the sensor array requires a control unit such as a microcontroller. It handles tasks like cell selection, AD-conversion of the conditioned measurement signal and the generation of the tuning voltage. This measurement system will match the idea of a smart sensor since it includes calibration, self test and a digital interface to the outer world. Furthermore it can store data and even draw decissions based on the measured data and in- system algorithms for fault detection.

In situ high temperature synchrotron-radiation diffraction studies of silicidation processes in nanoscale Ni layers

The formation of nickel silicides has been studied by X-ray diffraction experiments using synchrotron radiation (SR). A high temperature chamber was used to investigate the phase formation and transition processes under quasi-static conditions at temperatures from 200 to 650°C. Samples with different dopants, several metal layer thicknesses as well as samples with and without a 150 A TiN capping layer on single-crystal (001) and polycrystalline silicon substrates were examined. While n-type dopants like P and As had no significant impact on the silicidation processes, boron decreased the range of thermal stability of the low-resistivity phase NiSi. A TiN capping layer shifts both these formation and transition temperatures to higher values.

Spin-on Si-based low-k materials

Publisher Summary The chapter provides an overview of silsesquioxane (SSQ) materials, and introduces deposition, properties, and selected integration results of porous SiO 2 like films. All materials described are based on silicon and deposited by using the spin-on process. The review on SSQ materials includes polymerization and integration issues of various types of SSQ. Different suppliers of precursors for depositing SSQ based low-kdielectrics are also summarized. The SiO 2 -like porous dielectrics are introduced via a short excursion to fundamentals related to introduction of porosity to dielectric materials. In this context, the chapter examines the routes to deposit this group of low-κ dielectric films using spin-on processing before focusing on the deposition and hydrophobisation post-treatment of SiO 2 aerogel thin films. The chapter also explains the main film properties required for the integration by highlighting film morphology, chemical composition and structure, electrical, mechanical, and thermal properties. Dependence of properties on different porosities is also illustrated and the basic material and process compatibility issues are discussed for the porous SiO 2 aerogel films. This comprises lithography compatibility, cap layer and hard mask deposition, etching, diffusion barrier deposition and chemical mechanical polishing of Copper at the porous films.

A Frequency Selective Silicon Vibration Sensor with Direct Electrostatic Stiffness Modulation

In the field of industrial machinery an increasing demand for low cost vibration measurement equipment exists. The described frequency selective structures represent a low-cost alternative to the commonly used wide band accelerometers. To continuously cover a wide frequency range with a small number of resonators the sensor structures possess a resonance frequency tuning capability. Electrostatic forces applied to the seismic mass lead to a softening of the system and thus a lowering of the resonance frequency. Experimental results gained from fabricated test structures demonstrate the suitability of the tuning principle for wide range resonance tuning and the obtained linearity of the sensor behaviour. By grouping eight cells with stepped base frequencies and overlapping tuning ranges into an array one decade from 1 to 10 kHz is continuously covered with tuning voltages below 40 V.

Tunable resonators with electrostatic self test functionality for frequency selective vibration measurements

In this paper we present an array of tunable resonators for frequency selective vibration measurements. The resonance frequency tuning is carried out by electrostatic forces directly applied to the seismic mass affecting the total stiffness. Frequency selective sensor systems require the exact knowledge of the resonance frequency. Therefore a method of in-system calibration using electrostatic excitation and capacitive vibration detection was investigated. Coupling problems of the excitation signal into the detection path were solved by using two excitation signals 180/spl deg/ out-of-phase and by contacting the substrate with a backside metallization.