Hiroki Morimura

Novel surface structure and its fabrication process for MEMS fingerprint sensor

This paper describes a microelectromechanical systems (MEMS) fingerprint sensor that has novel protrusions on the sensor surface in order to detect clear fingerprint images for various finger surface conditions. A preliminary experiment clarified the importance of contact between soft finger ridges and the sensor surface in producing fingerprint images. Based on this, novel T-shaped protrusions on the sensor surface are proposed as an interface to mediate the contact. In order to fabricate the overhung form of the T-shaped structure, photosensitive polyimide and a copper sacrificial layer were used. Using the CMOS-compatible MEMS fabrication process, the arrayed T-shaped protrusions of polyimide are stacked on a CMOS large-scale integrated circuit. The sensor produced clear fingerprint images for various kinds of fingers. Measurement of the relationship between applied force and sensor output showed that the T-shaped protrusions achieved high sensitivity regardless of finger elasticity. Reliability tests confirmed that the sensor with the T-shaped protrusions has sufficient mechanical and electrical strength for wide applications.

MEMS fingerprint sensor immune to various finger surface conditions

This paper describes a new fingerprint sensor that detects the topography of finger ridges and valleys. We propose a microelectromechanical systems (MEMS) cavity structure for the pixels arrayed on the sensor surface and a fabrication process that stacks the cavity structures on a CMOS LSI. A thin film on top of the cavity structures is bent by finger ridges mechanically, which is detected by the sensing circuits below them electronically. Based on an analytical model, we designed a cavity structure suitable for fingerprint detection. We fabricated the cavity structures on the sensing circuits by a fabrication process that entails gold electroplating, sacrificial layer etching for cavities, and a sealing technique. The fabricated fingerprint sensor consists of about 57 334 pixels in the area of 11.2 mm /spl times/ 12.8 mm, which yields a high spatial resolution of 508 dots-per-inch (dpi). With these pixels working together, fingerprint images were obtained. The sensor produces clear fingerprint images regardless whether the finger was dry or wet, which confirms its potential for various practical applications.

RF CMOS-MEMS Switch with Low-Voltage Operation for Single-Chip RF LSIs

This paper describes a novel RF CMOS-MEMS switch that integrates RF MEMS switches and CMOS control circuits. A single-pole 8-through RF CMOS-MEMS switch was fabricated and its operation at 3.3 V supply voltage was achieved. The switch was encapsulated with a thin film at wafer level to prevent destruction during packaging. Experimental results confirm that the switch has mechanical reliability for more than 1 billion cycles

A new sensor structure and fabrication process for a single-chip fingerprint sensor/identifier LSI

We propose a new sensor structure and the fabrication process for a single-chip fingerprint sensor/identifier LSI in which the sensor is stacked on a 0.5 /spl mu/m CMOS LSI. We investigate the influence of electrostatic discharge (ESD), mechanical stress, and water penetration on the sensors reliability. The results reveal ESD tolerance is obtained at the value of 2.0 kV, the sensor is immune to mechanical stress under the condition of 1 MPa tapping tests, and it is protected against contamination by a passivation film. The tests confirm that the sensor has sufficient reliability for conventional identification usage.

Novel MEMS power generator with integrated thermoelectric and vibrational devices

This paper describes a novel concept of an integrated power generator chip that utilizes environmental temperature differences and vibrations as power sources. In the concept, thermoelectric devices produce voltage by the Seebeck effect and vibrational devices produce electrostatic energy from the external kinetic energy of vibration. A monolithic fabrication process for these microelectromechanical systems (MEMS) devices is proposed based on the seamless integration technology (SeaiT) consisting of thick-film forming processes with deep reactive ion etching (DRIE) of Si, electroplating of gold and ashing of a sacrificial layer. The fabricated thermoelectric devices generated its voltage outputs under temperature difference. The vibrational devices were also evaluated by applying an ac voltage which showed its resonant frequency characteristics. The integration of the different types of power generation paves the way for utilizing small amounts of energy from human and environmental sources.

Integrated MEMS LC Resonator with Sealed Air-Suspended Structure for Single-Chip RF LSIs

This paper describes an integrated MEMS LC resonator that consists of a MEMS inductor and varactor. The resonator features a sealed air-suspended structure: the inductor and varactor are suspended above a CMOS LSI to improve their performance, and they are sealed with a film to protect them during packaging. The quality factor of the inductor is four times higher than that of a CMOS top-layer-metal inductor, and the varactor has a tuning ratio of 100% at about 5-V applied voltage. The resonator is applied to a voltage- controlled oscillator to show its effectiveness for the development of single-chip RF LSIs.

MEMS fingerprint sensor with arrayed cavity structures

We propose a MEMS (Micro Electro Mechanical Systems) fingerprint sensor whose pixels have novel cavity structures. Each cavity structure has a sensitive plane of a thin film on top, and a sensing circuit below it. Ridges on a finger surface bend the thin film mechanically, which is detected by the sensing circuit electronically. We fabricated the sensor using a CMOS compatible process with our sealing technique. The sensor can obtain fingerprint images of even dry or wet fingers. It also shows sufficient mechanical strength against finger pressure. These results confirm that the MEMS fingerprint sensor has the potential for wide practical application.

Effect of Scratch Stress on the Surface Hardness and Inner Structures of a Capacitive Fingerprint Sensor LSI

The authors performed a scratch test in which a weighted needle applies scratch stress to the surface of a capacitive fingerprint sensor large scale integration (LSI), which has a grounded wall (GND wall) structure where each sensor plate is surrounded by a lattice-like wall. The scratch stress degrades not only the sensors surface but also the metal interlayer. Increasing the thickness of the surface passivation film and that of the interlayer reduce the degradation of the surface and inner structures. To confirm the influence of thick passivation film on the electrostatic discharge (ESD) hardness and the intensity of sensing of the fingerprint sensor LSI, the authors performed an air discharge test and fingerprint identification. The tests show that a thick passivation film and a thick interlayer are effective in preventing scratch stress with ESD hardness and the intensity of sensing of the fingerprint sensor LSI.

High sensitive structure and its fabrication process for MEMS fingerprint sensor to various fingers

This paper describes a novel structure of MEMS (microelectromechanical systems) and its fabrication process on the surface of the MEMS fingerprint sensor that works as an interface between a finger and the sensor. In order to detect fingerprints, whether a finger is soft or hard, novel T-shaped protrusions are proposed. The structure was fabricated by etching a sacrificial layer on the cavity structure that has a pair of electrodes. Structural calculations and experimental measurements showed that the proposed sensor bends the electrode most efficiently. This enables fingerprint detection regardless of finger elasticity, and enhances sensitivity for various finger surface conditions.

RF MEMS switches integrated with sealed suspended coplanar waveguides for reconfigurable RF circuits

This paper describes an RF-MEMS switch structure and its fabrication process for developing low-loss multiport RF switches that integrate multiple RF MEMS switches and CMOS control circuits. In our structure, RF MEMS switches and coplanar waveguides are seamlessly integrated, and they are suspended above a CMOS LSI to reduce the loss due to the lossy Si substrate. A gold multilayer stacking technique was used to fabricate the structures, and the STP technique was used to seal them for damage-free packaging. Switching operation of RF MEMS switches was achieved and low insertion loss of 0.07 dB/mm at 5 GHz was obtained for the suspended coplanar waveguides.