Edvard Kälvesten

Low-temperature full wafer adhesive bonding

We have systematically investigated the influence of different bonding parameters on void formation in a low-temperature adhesive bonding process. As a result of these studies we present guidelines ...

Three dimensional silicon triple-hot-wire anemometer based on polyimide joints

The first three dimensional (3D) flow sensor probe based on polyimide joints has been fabricated and successfully tested. The new 3D sensor, which is specially designed for turbulent gas flow measurements, is based on the anemometer principle, i.e. gas cooling of electrically heated hot-wires. The sensor probe consists of three perpendicular 500 /spl mu/m/spl times/5 /spl mu/m/spl times/2 /spl mu/m polysilicon hot-wires giving a measuring volume sufficiently small to resolve the small eddies in a turbulent flow. A bulk micromachining process in combination with sacrificial etching is used to form the hot-wire probes. The hot-wires are connected to 30 /spl mu/m thick bulk silicon beams which are defined by double sided KOH etching. Two wires (x and y) are located in the wafer plane and the third z-wire is rotated out of the plane using a new robust micro-joint. The self-assembly micro-joint with small bending radius is based on thermal shrinkage of polyimide in V-grooves. High flow sensitivity for the anemometric hot-wires has been measured. Time constants of 120 and 330 /spl mu/s were measured for the cooling and heating of the hot-wires, respectively.

An integrated pressure—flow sensor for correlation measurements in turbulent gas flows

A new integrated pressure—flow sensor has been specially designed for measurements in turbulent gas flows. The pressure sensor is based on polysilicon diaphragm technology and the flow sensor on the gas cooling of a polyimide-insulated heated mass. With a pressure-sensor diaphragm area of 100 μm × 100 μm, a flow-sensor hot-chip area of 300 μm × 60 μm and an edge-to-edge distance of 100 μm between the different sensor areas, the smallest eddies in technically interesting turbulent flows can be resolved and measured. The pressure-sensor design shows a flat frequency response curve within ±2 dB between 10 Hz and 10 kHz with an acoustic sensitivity of 0.9 μV Pa−1 for a supply voltage of 10 V. The flow sensor has a thermal response with a time constant of 7 ms and a response time of 25 μs when the sensor is operated at constant temperature using feedback electronics. The measured steady-state flow-sensor power dissipation in a turbulent wall boundary layer at an overtemperature of 100 °C was P = 34 + 0.4τ00.47 mW where τ0 is the time-average flow-dependent wall shear stress. The integrated sensor has been used for simultaneous measurement of fluctuating pressure and wall shear stress in a turbulent boundary layer yielding pressure—wall shear stress correlation coefficients never previously presented.

Low-temperature wafer-level transfer bonding

In this paper, we present a new wafer-level transfer bonding technology. The technology can be used to transfer devices or films from one substrate wafer (sacrificial device wafer) to another substrate wafer (target wafer). The transfer bonding technology includes only low-temperature processes; thus, it is compatible with integrated circuits. The process flow consists of low-temperature adhesive bonding followed by sacrificially thinning of the device wafer. The transferred devices/films can be electrically interconnected to the target wafer (e.g., a CMOS wafer) if required. We present three example devices for which we have used the transfer bonding technology. The examples include two polycrystalline silicon structures and a test device for temperature coefficient of resistance measurements of thin-film materials. One of the main advantages of the new transfer bonding technology is that transducers and integrated circuits can be independently processed and optimized on different wafers before integrating the transducers on the integrated circuit wafer. Thus, the transducers can be made of, e.g., monocrystalline silicon or other high-temperature annealed, high-performance materials. Wafer-level transfer bonding can be a competitive alternative to flip-chip bonding, especially for thin-film devices with small feature sizes and when small electrical interconnections (<3/spl times/3 /spl mu/m/sup 2/) between the devices and the target wafer are required.

New small radius joints based on thermal shrinkage of polyimide in V-grooves for robust self-assembly 3D microstructures

A novel and simple technology for making robust three-dimensional (3D) silicon structures with small radii of bending has been developed and investigated. The proposed self-assembly method of bending 3D structures out of plane such that they stay bent without any interlocking braces is based on thermal shrinkage of polyimide in V-grooves. A wide range of bending angles for the permanent out-of-plane rotated structure can be chosen by varying the curing temperature of the polyimide. The relatively large thermal expansion of polyimide makes it possible to use the structure in a dynamic mode useful for compensation of undesired process variations. The proposed technique is compatible with both IC based surface micromachining and batch fabrication. Therefore, structures based on the new polyimide joint have general applications in micromachining and can be used in many new 3D sensors or actuators having detailed features in all three dimensions. For the tested polyimide V-groove joints, static bending angles between and have been achieved with a maximum bending angle of per V-groove. Bending radii smaller than 60 m for a 30 m thick out-of-plane rotated silicon plate have been measured. Electrical connections to the assembled structures have been investigated. It has been shown that the aluminium conductors crossing the V-grooves in the polyimide joint are not affected by the out-of-plane rotation.

Wafer-level membrane transfer bonding of polycrystalline silicon bolometers for use in infrared focal plane arrays

In this paper we present a new, innovative technology for fabrication and integration of free-hanging transducers. The transducer structures are processed on the original substrate wafer (sacrificial device wafer) and then transferred to a new substrate wafer (target wafer). The technology consists only of low-temperature processes, thus it is compatible with integrated circuits. We have applied the new membrane transfer bonding technology to the fabrication of infrared bolometers for use in uncooled infrared focal plane arrays (IRFPAs). In the future this may allow bolometers to be integrated with high-temperature-annealed, high-performance thermistor materials on CMOS-based uncooled IRFPAs. Membrane transfer bonding is based on low-temperature adhesive bonding of the sacrificial device wafer to the target wafer. The device wafer is sacrificially removed by etching or by a combination of grinding and etching, while the transducer structures remain on the target wafer. The transducer structures are mechanically and electrically contacted to the target wafer and the adhesive bonding material is sacrificially removed. The free-hanging transducers remain on the target wafer. One of the unique advantages of this technology is the ability to fabricate and integrate free-hanging transducers with very small feature sizes. In principle, membrane transfer bonding can be applied to any type of free-hanging transducer including ferroelectric infrared detectors, movable micro-mirrors and RF MEMS devices.

Dynamic actuation of polyimide V-groove joints by electrical heating

Abstract New robust three-dimensional silicon structures with integrated heaters for dynamic actuation have been fabricated and tested. The structures can be used in both static and dynamic modes for 3-D sensor and actuator applications. The proposed technique for bending 3-D structures out of the wafer plane is based on thermal shrinkage of polyimide in V-grooves, producing static bending. This static angle can be chosen in a wide range by varying the curing temperature. The relatively large thermal expansion of polyimide makes it possible to use the structure in dynamic mode. A local power dissipation in the joint is achieved by using a current through an integrated heater. This makes it possible to control the bending angle of each individual structure. For the tested polyimide V-groove joints, static bending angles of 35 ° and dynamic angles of 3 ° per V-groove have been achieved.

A new silicon gas-flow sensor based on lift force

This paper presents the first silicon-flow sensor based on lift force. The sensor is a bulk-micromachined airfoil structure that uses the lift force as a sensing principle. The lift force acts normal to the flow in contrast to drag-force sensor types, where the force acts in the flow direction. The sensor utilizes the special distribution of the lift force along the length of the sensor structure. Since the sensor, like an airfoil, is mounted at a small angle to the flow, it induces very little flow disturbance. The sensor consists of two plates connected to a center beam. Each plate is 5/spl times/5-mm square with a thickness of 30 /spl mu/m. The flow-induced forces deflect the two plates in the same direction, but with different magnitude. The deflections are detected by polysilicon strain gauges. The differential mode bridge makes the sensor insensitive to common mode deflection, e.g., acceleration forces. The lift-force principle is characterized using fundamental airfoil theory. The sensor has been experimentally verified, and a flow sensitivity of 7.4 /spl mu/V/V/(m/s)/sup 2/ has been measured in both flow directions.

Void-free full wafer adhesive bonding

In this paper we present guidelines for void free adhesive bonding of 10 cm diameter wafers. We have systematically investigated the influence of different bonding parameters on void formation in the bond. The layer thicknesses of the tested polymer coatings are between 1 /spl mu/m and 12 /spl mu/m. The polymer material, the bonding pressure and the pre-curing time and temperature for the polymer has shown significant influence on void formation in the bond. High bonding pressure and pre-curing times/temperatures that are specific to the polymer material counteract void formation. Process parameters for achieving void-free bonds using benzocyclobutene (BCB) and photoresist coatings as adhesive materials are given. Excellent bonding results have been achieved with BCB as bonding material.

An Integrated Silicon based Wall Pressure-Shear Stress Sensor for Measurements in Turbulent Flows

By the introduction of silicon micro machining into fluid mechanics the experimentalists have been given a new incentive to carry out measurements of fundamental parameters in turbulent flows. One such unknown parameter is the pressure-velocity correlation (PVC) which turns out to be a key term in the kinetic energy budget as well as in the transport equations for the Reynolds stresses.