Mats Bexell

Assembling three-dimensional microstructures using gold-silicon eutectic bonding

Abstract An assembly method for three-dimensional microelements is presented. The assembly is done in situ with a micromanipulator in an SEM using Au-Si eutectic bonding. Microblocks bonded to larger silicon substrates are used for evaluation of the mechanical strength and a microarch is presented to demonstrate the possibilities of the technique. The microelements are fabricated by bulk micromachining, and sputter deposited with chromium and gold. Etched (111) faces have been successfully bonded. TEM investigation of samples from vacuum furnace experiments show large gold grains with smaller chromium silicide grains in the bonded region. The silicon in the eutectic liquid precipitates epitaxially on both silicon faces. Mechanical bending tests on the microblocks give sufficiently high fracture stresses for the intended applications in microrobotic systems. Average fracture stresses of 65 MPa are measured for one set of parameters. Problems encountered are misalignments of the microelements during processing and void formation in the bonds. It is believed this is connected to the experimental equipment and set-up. The microarch, which consists of three assembled microblocks, reaches a tensile stress of 16 MPa, encouraging further development. In conclusion, strong microbonds are achieved using a solidified gold-silicon eutectic melt as an adhesive, and it is demonstrated for the first time that three-dimensional microassembly by means of eutectic bonding of micromachined elements can be performed by manipulation and processing on a locally heated specimen table.

Fabrication and evaluation of a piezoelectric miniature motor

A piezoelectric miniature motor with a diameter of 4 mm has been fabricated and characterised. In comparison with a previous motor the design of the electrode pattern and the piezoceramic elements have been modified. This allowed a simplified assembly procedure and an enhanced performance. The demands on accuracy in the particular assembly procedure has been reduced to ±50 μm. The experimentally measured maximum torque was increased from 1.4 mNm to 3.75 mNm and the maximum rotational velocity was increased from 4.2 rpm to 65 rpm. The experimental results showed reasonable agreement with a previously derived analytic model. Investigations of limiting factors for motor performance show that the motor could possibly support a torque level in the order of 12 mNm and a rotational velocity above 455 rpm.

Characteristics of a piezoelectric miniature motor

The performance of a piezoelectric inchworm motor with a diameter of 4 mm has been evaluated on a miniature scale. The experiments have shown that some of the desired characteristics for a miniature motor such as a high output torque and linear control of rotational velocity with frequency can be achieved using a simple construction. The experimentally measured maximum output torque of 1.4 mN μm is one of the highest presented for a motor of this size. To prevent electrode failure in this first miniature prototype the maximum speed was limited to 4.2 rpm. The most critical parameter for motor performance is height deviations between piezoceramic elements which therefore should be minimised. Further, results show a reasonable agreement with a previously derived analytical model. An optimised motor design is expected to have maximum output torque and speed values as high as 10 mN μm and 120 rpm.

Characterization of an inchworm prototype motor

Abstract A new driving principle for an active joint intended for, e.g., a microrobot, has been evaluated. Piezoelectric bimorphs are used as actuator elements, and quasistatic positioning in combination with an inchworm type of repetition produces the rotation. The performance of a macroscopic prototype has been investigated and compared with an analytical model and with finite-element analysis. The agreement between the model and the prototype is good, and it is believed that the characteristics of a microsized active joint can be estimated from the analytical model. The difference in size between the prototype and the proposed micromotor is a factor of 100. A micromotor is expected to have a very high torque-to-volume ratio (3 kN m −2 ). Its power and speed limits are believed to be sufficient for an operational micromotor. Other characteristics that make it well suited for microrobotics are: no gliding contact causing wear, simple design and locked rotor when the voltage is turned off. A possible fabrication process, microassembly, is demonstrated. A bimorph microelement has been successfully bonded to a support on a substrate and the expected behaviour has been verified.

Design and fabrication of a gripping tool for micromanipulation

Abstract A tool for micromanipulation is presented in this paper. A titanium gripper is fabricated by a combination of electro-discharge machining and etching, the latter of which is to provide a material less prone to cracking by removing the heat-affected surface. Evaluation of the design has been carried out by finite-element analysis and the performance of the gripper has been qualitatively as well as quantitatively established. The manipulation system to which the tool belongs is also briefly described.

Microassembly of a Piezoelectric Miniature Motor

An assembly method for the fabrication of a piezoelectric miniature motor is presented. The fabrication is done by placement and soldering of piezoceramic elements onto a silicon substrate. The assembly of the miniature motor demands high precision (±5 μm), four-axes positioning which is fulfilled by a special micropositioning stage. A particular assembling sequence is used and critical process and performance parameters are evaluated. One of the important factors is the joint strength between the piezoceramic element and the silicon substrate. A joint strength as high as 100 MPa has been measured which would allow for torque values in the range of 14 mNm for a 4 mm diameter motor.

Microassembly of a piezoelectric miniature motor

Presented is an assembling method for the fabrication of a piezoelectric miniature motor. The fabrication is done by placement and soldering of actuator elements to a substrate. The assembly of the miniature motor demands high precision (plus or minus 1 micrometer), four-axes positioning which is fulfilled by a special micropositioning stage. A particular assembling sequence is used and critical process and performance parameters are evaluated. One of the important factors is the joint strength between actuator element and substrate. A joint strength as high as 100 MPa has been measured which would allow for torque values in the range of 15 mNm for a 4 mm diameter motor.