J.H. He

Fabrication and characterization of diamond-like carbon/Ni bimorph normally closed microcages

Normally closed microcages based on highly compressively stressed diamond-like carbon (DLC) and electroplated Ni bimorph structures have been simulated, fabricated and characterized. Finite-element and analytical models were used to simulate the device performance. It was found that the radius of curvature of the bimorph layer can be adjusted by varying the DLC film stress, the total layer thickness and the thickness ratio of the DLC to Ni layers. The angular deflection of the bimorph structures can also be adjusted by varying the finger length. The radius of curvature of the microcage was in the range of 18–50 µm, suitable for capturing and confining micro-objects with sizes of 20–100 µm. The operation of this type of device is very efficient due to the large difference in thermal expansion coefficients of the DLC and the Ni layers. Electrical tests have shown that these microcages can be opened by ~90° utilizing a power smaller than 20 mW. The operating temperatures of the devices under various pulsed currents were extracted through the change in electrical resistance of the devices. The results showed that an average temperature in the range of 400–450 °C is needed to open this type of microcage by ~90°, consistent with the results from analytical simulation and finite-element modelling.

Development of all metal electrothermal actuator and its applications

The in-plane motion of microelectrothermal actuator (heatuator) has been analysed for Si-based and metallic devices. It was found that the lateral deflection of a heatuator made of a Ni-metal is about -60% larger than that of a Si-based actuator under the same power consumption. Metals are much better for thermal actuators as they provide a relatively large deflection and large force, for a low operating temperature, and power consumption. Electroplated Ni films were used to fabricate heatuators. The electrical and mechanical properties of electroplated Ni thin films have been investigated as a function of temperature and plating current density, and the process conditions have been optimised to obtain stress-free films suitable for MEMS applications. Lateral thermal actuators have been successfully fabricated, and electrically tested. Microswitches and microtweezers utilising the heatuator have also been fabricated and tested.

Laser micromachining of thin films for optoelectronic devices and packages

Focused laser micromachining in an optical microscope system is used to prototype packages for optoelectronic devices and to investigate new materials with potential applications in packaging. Micromachined thin fims are proposed as mechanical components to locate fibers and other optical and electrical components on opto-assemblies. This paper reports prototype structures which are micromachined in silicon carbide to produce beams 5 μm thick by (1) laser cutting a track in a SiC coated Si wafer, (2) undercutting by anisotropic silicon etching using KOH in water, and (3) trimming if necessary with the laser system. This approach has the advantage of fast turn around and proof of concept. Mechanical test data are obtained from the prototype SiC beam package structures by testing with a stylus profilometer. The Youngs modulus obtained for chemical vapor deposited silicon carbide is 360 +/- 50 GPa indicating that it is a promising material for packaging applications.

MEMS/NEMS mechanical characterization on different thin film materials by scanning along micro-machined cantilevers

Mechanical characterization is vital for the design of MEMS/NEMS. Many methods have been developed to measure the mechanical properties of materials; however, most of them are either too complicated, or expensive for industrial application, or not accurate. This paper describes a new characterization method to extract the mechanical properties of the materials that is simple, inexpensive and applicable to a wide range of materials. The beams of the material under tests, are patterned by laser micromachining and released by KOH etch. Surface profilometer is used to scan along μ-machined cantilevers and produce a bending profile, from which the Young’s modulus can be extracted. The errors due to initial curling and anticlastic (width) effect have been carefully studied. A new ANSYS FEA model is developed to evaluate the effects and test structure designs. SiNx, Ni, SiC and nanocrystal diamond cantilevers have been fabricated and their mechanical properties, e.g. Young’s modulus have been evaluated as 154+/-12GPa, 202+/-16GPa, 360+/-50GPa and 504+/-50GPa, respectively. These results are consistent with those measured by nano-indentation. Residual stress gradient has also been extracted by surface profilometer, which is comparable with the results inferred from Zygo interferometer measurements. It is also possible to extract plate modulus and Poisson ratio with minimal error achieved. This method can be extended to AFM or nanometer-stylus profilometer for NEMS mechanical characterization.

Micro-mechanical characterisation for MEMS thin films by bending /spl mu/-machined cantilevers

A simple, cost-effective, μ-mechanical characterisation process is developed. It can be applied to a wide range of materials: including non-conductive material. This technique is based on μ-machined beam bending. The beams of the material under test, are patterned by laser micromachining and released by alkaline etching. A surface profilometer stylus is then used to scan along the cantilevers, deflecting them and yielding position-displacement traces, from which the thin film s elastic modulus can be extracted. A consistent data analysis method is developed to extract the modulus from such traces. LPCVD SiN cantilevers are fabricated and the Youngs modulus is extracted 155+/-20 GPa. Finite Element Modelling (FEM) is used to analyse the beam behaviour.

Nanomechanical characterisation of MEMS thin film materials

Mechanical characterisation of thin film is crucial for development of MEMS/NEMS devices. A simple mechanical characterisation method is developed using a surface profilometer. Surface profilometer is used to scan along the micromachined cantilevers and produce the bending profile, from which the Youngs modulus can be extracted. The following MEMS materials: SiN, Ni, Ni/SiN bimorph, Nanocrystal Diamond (ND), SiC have been characterised. The results are compared and in agreement with those obtained from nanoindentation and resonance frequency method. The advantages and disadvantages of these three methods are discussed.

A critical comparison and development of nano-mechanical characterization of MEMS/NEMS thin film materials

This paper compared the three different methods for determination of thin film modulus or MEMS applications: 1) scanning bending cantilever, 2) nanoindentation and 3) resonance frequency method. Surface profilometer was used to scan along the micro-machined cantilevers at different loads and produce the bending profile, from which the Youngss modulus can be extracted. Indentation profiled produced by Nano-indenter can deduce Youngs modulus and hardness of the thin film materials. AFM vibrometer is used to detect the resonance of the thin film cantilever, from which the stiffness, and therefore the Youngs modulus can be derived. The material properties of silicon nitride characterized by three methods are consistent and comparable with one another. The following MEMS materials: SiN, Ni, Ni/SiN bimorph, Nano-Diamond, SiC have been characterized and compared by using different method. Their advantages and disadvantages are also discussed.

Microgripper using a diamond like carbon/metal bilayer

A novel nonnally closed microcage has been fabricated and characterized. This device was made from a highly compressively stressed diamond like carbon (DLC) and electroplated Ni bimorph structure. The large stress in the DLC causes the bimorph layer to curve once it is released from the substrate. The radius of curvature is in the range of 18 ∼ 50μm, and can be controlled by varying the DLC and the Ni thicknesses. The devices can be operated in a pulsed mode current with low operation temperature, and can be opened by ∼60μm laterally with a power consumption of only ∼16mW.