Li Shiah Lim

Characterization of Silicon Nanowire Embedded in a MEMS Diaphragm Structure Within Large Compressive Strain Range

The characteristics of piezoresistive silicon nanowires (SiNWs) under compressive strain as large as 1.7% are reported. The SiNW is embedded in a multilayered diaphragm structure consisting of silicon nitride and silicon oxide. After leveraging the high fracture stress and intrinsic tensile stress of silicon nitride layer to produce a flat diaphragm, we can create large compressive strain to the SiNW without damaging the diaphragm. The relationship between SiNW resistance change and applied strain is measured and investigated with 2-μm and 5-μm SiNWs for both scientific and practical points of view. This approach demonstrates the validity to reveal the SiNW properties under large strain, and the exploration provides good reference for future SiNW-based MEMS sensor design.

A wirelessly powered and interrogated blood flow monitoring microsystem fully integrated with a prosthetic vascular graft for early failure detection

This paper presents an implantable blood flow monitoring microsystem embedded in a prosthetic graft for early graft failure detection. The microsystem consists of two MEMS pressure sensors, an inductively powered wireless sensor interface ASIC, two miniature coupling coils, and a flexible cable connecting them. The implantable microsystem is powered and configured by an external monitoring device using 13.56-MHz carrier frequency. The blood flow rate information is sensed in the form of an oscillation frequency and transmitted to the external monitoring device through backscattering. The ASIC fabricated in 0.18-μm CMOS process occupies a total area of 0.5 × 3.3 mm2 including pads and consumes a total power of 12.6 μW. With the high-efficiency design of coupling coils, rectifier and LDO, the wireless power link achieves an overall power efficiency of 2% through 5-cm-thick tissue. With the ultra low power consumption and high-efficiency power transfer, the ASIC can be powered by transmitting only 630-μW RF carrier from the external device. The measured performance of the blood flow monitoring microsystem demonstrates a 0.17-psi pressure resolution.

MEMS Tri-axial Tactile Sensor packaging using polymer fleible cable for sensorised guide wire application

This paper presents the integration process of tactile sensor system using polymer flexible cable for sensorised guide wire application and described the fabrication process flow of polyimide flexible cable, silicon holder and sensorised guide wire assembly processes. Polyimide flexible cable is used for the integration due its biocompatibility and suitable mechanical properties for flexible cable bending. Sensorised guide wire prototype was assembled using dummy sensor and ASIC chip.

Direct eutectic AuSn solder bumping on Al bond pad surface using laser solder ball jetting

Au-rich eutectic AuSn (Au80wt%-Sn20wt%) solder ball alloy is extensively used in MEMS and optoelectronics packaging, for providing flip-chip solder bump interconnections. In this paper, we will look into the possibility of using laser solder ball jetting process for direct eutectic AuSn solder bumping on Al bond pad surface, and compare with eutectic AuSn solder bumping on Al bond pad with Ti/Ni/Au UBM structure. The laser jetted eutectic AuSn solder bumps were observed to wet and form hemi-spherical bumps on the Al bond pad surface, with and without UBM structure. FIB-EDX analysis of the laser jetted eutectic AuSn solder bump on Al bond pad with UBM structure showed formation of dense islands of Au5Sn IMC layer from the top Au finishing layer of the UBM structure. On the other hand, only a few clusters of Au5Sn IMC were formed near to the solder joint of the laser jetted eutectic AuSn solder bump on Al bond pad surface. Ball shear test on the laser jetted eutectic AuSn solder bumps exhibited average solder shear strength of 4.52g/mil2 and 14.22g/mil2, on Al bond pad surface and Al bond pad with UBM structure respectively. Laser jetted eutectic AuSn solder bumps on Al bond pad surface displayed pad lift failure mode, as compared to failure at Al bond pad layer for Al bond pad with UBM structure. In conclusion, eutectic AuSn solder balls could be bumped onto Al bond pad surface via laser jetting.

Design, Fabrication and Characterization of Ultra Miniature Piezoresistive Pressure Sensors for Medical Implants

Pressure sensors using MEMS technology have been advanced due to their low cost, small size and high sensitivity, which is an advantage for biomedical applications. In this paper,silicon nanowire was proposed to be used as the piezoresistors due to the high sensitivity [1][2].The sensors were designed, and characterized for the use of medical devices for pressure monitoring. The pressure sensor size is 2mm x 2mm with embedded SiNWs of 90nm x150nm been fabricated. Additionally, the sensitivity of 0.0024 Pa-1 pressure sensor has been demonstrated.

Implantable blood flow sensor integrated on flexible circuit for vascular graft application

This paper reports an implantable blood flow sensor system integrated on flexible circuit that consists of pressure sensor and inductively powered wireless sensor interface Application Specific Integrated Circuit (ASIC) for early graft failure detection application. The proposed system was embedded within the vascular graft to have continuously monitoring the differential blood pressure change as an indication of stenosis build- up in the graft. The fabricated pressure sensor is showed the step of 0.5psi of pressure changes is clearly visible with the resistance change. The resolution of pressure change 8mmHg was achieved. [1] The whole system was successfully integrated on designed flexible circuit by optimizing method of flip-chip bonding with gold stud bump and underfill encapsulant at lower temperature. The assembled system was successfully demonstrated wirelessly.

Force transmitting element fabrication and characterization for MEMS tri-axial force sensor application

This paper presented the design, fabrication, and characterization of force transmitting element for MEMS tri-axial force sensor application. The transmitting element consists of two main parts: integrated silicon rod and mechanical stopper. The advantages of the design are the simplicity of fabrication process, the ability to detect force in all direction and to protect the sensing element from excessive force, thus increase the robustness of the devices.

Wireless sensor microsystems for emerging biomedical applications (Invited)

In this paper, two examples of wireless sensor microsystems for medical devices are presented: a wireless blood flow monitoring microsystem which is fully integrated with a prosthetic vascular graft for early failure detection, and a 100-channel wireless neural recording microsystem In the context of such biomedical applications, high-efficiency wireless transceiver circuit techniques for data communication and power transfer as well as low-power sensor interface circuit techniques are introduced and explained.

Characterization of metal pad condition after vapor HF release process for MEMS packaging application

This paper reports the characterization of metal pad condition after vapor hydrofluoric (VHF) release process for microelectromechanical systems (MEMS) packaging application. There was a few different stacks of dielectric materials with metal layer been proposed as the test vehicle for the characterization analysis. The surface micrograph, roughness and bond ability of metal pad been studied in order to characterize the of metal pad condition after VHF release. The bond ability of aluminium (AlSiCu) pad after VHF released been analyzed through different metrology measurement such as Scanning electron Microscopy (SEM), Atomic Force Microscopy (AFM), Auger Spectroscopy Electron (AES), bond shear and wire pull test after wire bonding process. The analysis results were summarized and reported in this paper.