Quanfang Chen

Thickness-dependent mechanical properties of polydimethylsiloxane membranes

Polydimethylsiloxane (PDMS) has cross-linked network structures and its properties have been regarded as dimensionally independent. Here we demonstrate that both the mechanical strength and the Youngs modulus of the PDMS membranes are thickness dependent and the transition from bulk behavior to dimension dependent is predicted to occur at a membrane thickness of about 200 µm. The thickness-dependent phenomenon is attributed to shear stress during fabrication, which is proportional to the thickness-induced reorder of polymer chain coils to form stronger cross-linked networks.

Mesoscale actuator device: micro interlocking mechanism to transfer macro load

Abstract A novel proof-of-concept prototype Mesoscale Actuator Device (MAD) containing microscale components has been developed. The MAD is similar to piezoelectrically driven inchworm motors with the exception that mechanically interlocking microridges replace the traditional frictional clamping mechanisms. The interlocked microridges, fabricated from single crystal silicon, are designed to increase the load carrying capability of the device substantially. Tests conducted on the current design demonstrate that interlocked microridges fabricated with 30% KOH solution support a 9.6 MPa shear stress or that a pair of 5×5 mm locked chips supports a 500 N load. For high frequency operation, an open loop control signal is implemented to synchronize the locking and unlocking of the microridges with the elongating and contracting of the actuator. The system was successfully operated from 0.2 Hz to 500 Hz (or speeds from 2 μm/s to 5 mm/s). The upper limit (500 Hz) is imposed by software and hardware limitations and not related to physical limitations of the prototype device.

Mechanical properties of carbon nanotube–copper nanocomposites

The authors have characterized the mechanical properties of carbon nanotube (CNT) reinforced copper nanocomposites which were fabricated with an innovative electrochemical co-deposition process. The mechanical strength of Cu/CNT nanocomposites is found to be more than three times greater than that of pure copper. The increased strength is attributed to the good distribution of CNTs and good interfacial bonding between CNT and copper realized by the fabrication method.

Mesoscale actuator device with micro interlocking mechanism

A novel proof-of-concept prototype Mesoscale Actuator Device (MAD) has been developed. The MAD is similar to piezoelectric driven inchworm motors with the exception that mechanically interlocking microridges replace the traditional frictional clamping mechanisms. The interlocked microridges, fabricated from single crystal silicon, increase the load carrying capability of the device substantially. Tests conducted on the current design demonstrate that the interlocked microridges support 16 MPa in shear or that a 3/spl times/5 mm locked chip supports a 25 kgf load. To operate the MAD device at high frequencies an open loop control signal is implemented. Synchronizing the locking and unlocking of the microridges with the elongating and contracting actuator requires minor perturbations in the voltage signal supplied. The system was successfully operated from 0.2 Hz to 500 Hz (or speeds from 2 /spl mu/m/s to 5 mm/s). The upper limit (500 Hz) is imposed by software limitations and not related to physical limitations of the current device.

Low-stress ultra-thick SU-8 UV photolithography process for MEMS

Patterning thick SU-8 with conventional photolithography facilities is important for fabricating various MEMS structures. However, the fabrication of thick SU-8 MEMS has experienced severe problems such as cracks, distortions, or delaminations during the fabrication process and/or postservices, due to the large internal stress generated during the photolithography process. In this work, an in-depth finite element analysis (FEA) is performed to investigate the causes and effects of the internal stresses. Analytical results show that the post-exposure bake (PEB) temperature is the main factor in developing the resulted internal stress. Under the guidance of analytical results, an optimized UV photolithography process for the fabrication of ultra-thick low-stress SU-8 patterns is developed with conventional (simple) equipment. A low PEB temperature of 55°C reduces the internal stresses by more than 70% compared to those fabricated with the recommended procedure. Experimental results indicate that cracks, distortions, and delaminations are eliminated from the fabricated SU-8 structures using the newly developed procedure. In addition, the patterned SU-8 has a Youngs modulus of 2.5 GPa and an ultimate strength of 50 MPa, which is about 50% higher than previous reported values.

A silicon-on-insulator based micro check valve

In this paper we present a bulk micromachined check valve with very high frequency and extremely low leak rates. The valve is designed to have a hexagonal orifice, a hexagonal membrane flap and three flexible tethers. The three elbow-shaped flexible tethers are used to secure the membrane flap to the valve seat and to obtain large flap displacement in the forward flow direction. A silicon-on-insulator wafer and deep reactive ion etching technology are used to implement this microvalve. A very simple fabrication process has been developed, and only two photolithographic masks are required. Preliminary fluidics testing on a 1.44 mm size check valve was performed. A maximum flow rate (deionized water) of 35.6 ml min−1 was obtained at a forward pressure of 65.5 kPa, and only negligible leakage rate was observed at a reverse pressure of up to 600 kPa. The frequency response of the valve in air was also measured and its first resonance frequency was found at 17.7 kHz.

Characterization study of bonded and unbonded polydimethylsiloxane aimed for bio-micro-electromechanical systems-related applications

Authors have demonstrated that by controlling the mixing ratio of polydimethylsiloxanes (PDMSs) two components-base polymer (part A) and a curing agent (part B)-different mechanical properties of PDMS can be achieved. Test results show that the Youngs modulus decreases as the increasing of mixing ratios (A:B). However, there is a transitional mixing ratio (part A:part B=10) after which the Youngs modulus is almost independent of the mixing ratio. The PDMSs thickness plays an important role in determining the mechanical properties. The results show that the thinner the PDMS, the stiffer it behaves. The bonding strength between two cured PDMS parts with different mixing ratios shows that it depends on the mixing ratio. A maximum bonding strength of 130 kPa occurs on a bonded couple with mixing ratios of 30A:1B and 3A:1B, respectively. The fracture on bonded specimens does not occur at the bonding interfaces. Instead it occurs at the side with a larger portion of part A. The intermediate material property formed at the interface is attributed to the diffusion layer formed.

Design and fabrication of in situ UV-LIGA assembled robust nickel micro check valves for compact hydraulic actuators

Compact robust hydraulic actuators (pumps) are very important for space-related applications because of their capabilities in producing much larger forces per unit volume/mass than existing technologies. PZT stacks (pusher) have been widely identified for developing compact actuators (pump), but valves to match the inherent properties of PZT, in terms of high frequency (up to 100 kHz), load support (up to 100 MPa) and desired flow rate, are missing. The authors have developed novel robust solid nickel microvalve arrays using an in situ UV-LIGA assembling process. A high frequency and large pressure support capability has been realized analytically by elaborating the scaling law, in combination with a stiff material such as nickel. Large flow rate was realized by utilizing a microvalve array, instead of using a single valve. By doing this, all major requirements, in terms of frequency match, load support, as well as small amount of valve mass (payload), have been met. The reliability of the valve is assured by using a curved valve flap so that low stress can be maintained, in combination with a mechanical robust nickel valve stopper.

Investigating the influence of fabrication process and crystal orientation on shear strength of silicon microcomponents

A summary of the influence of microfabrication processes (wet and dry etching) and crystal orientation on the effective shear strength of microridges is addressed in this paper. Test results indicate that both crystal orientation and geometry plays an important role in determining the strength. The largest shear strengths obtained were for triangular and rectangular ridges fabricated with wet etching and deep RIE respectively. Both of these structures had similar crystal orientations. These strength values were approximately 3.5 times larger than the lowest strengths measured for wet etching structures. Using Chlorine RIE, we were able to demonstrate the influence of crystal orientation on strength, with microridges of {110} sidewall made on a (100) wafer the largest. For wet etching, we found that the strength was concentration dependent. For example, a 45% KOH fabricated structure produced strength values 65% higher than 30% KOH fabricated ones (note crystal orientation the same). This was attributed to a geometric effect, that is the 45% KOH solution had a “V” shaped bottom while the 30% KOH had a flat bottom. EDP and TMAH values had similar strengths to the 30% KOH solution (note similar crystal orientation). Therefore, microcomponent strength is strongly dependent upon fabrication process as well as crystal orientation.

Development of Mesoscale Actuator Device with Microinterlocking Mechanism

A novel proof-of-concept prototype Mesoscale Actuator Device (MAD) is described in this paper. The MAD is similar to piezoelectric driven inchworm motors with the exception that mechanically interlocking microridges replace the traditional frictional clamping mechanisms. The interlocked microridges, microfabricated from single crystal silicon, are shown to support macroscopic loads with exact values dependent upon manufacturing processes. Tests conducted on the current design demonstrate that the interlocked microridges support 16 MPa in shear or that two sets of 3 x 5 mm locked chips support a 50 kgf. Operation of a prototype MAD device containing microridges is accomplished at relatively large frequencies using an open loop control signal. Synchronizing the locking and unlocking of the microridges with the elongating and contracting actuator requires a dedicated waveform in the voltage signal supplied and permitted large operational frequencies. The system was successfully operated from 0.2 Hz to 500 Hz corresponding to speeds from 2,um/s to 5 mm/s. The upper limit (500 Hz) was imposed by software limitations and not related to physical limitations of the current device.