Anu Kärkkäinen

MEMS-based AC voltage reference

An ac root-mean-square (RMS) voltage reference based on a microelectromechanical system (MEMS) component is presented. The device stability is investigated in various experiments. A time stability at a level of a few /spl mu/V/V in 24 h was measured using an accelerometer MEMS component at an operating frequency of 100 kHz.

Optimized design and process for making a DC voltage reference based on MEMS

A micromechanical moving plate capacitor has been designed and fabricated for use as a dc voltage reference. The reference is based on the characteristic pull-in property of a capacitive microelectromechanical system (MEMS) component. The design is optimized for stability. A new silicon-on-insulator (SOI) process has been developed to manufacture the component. We also report on improved feedback electronics and the latest measurement results.

Stability of Electrostatic Actuation of MEMS

The increased electrostatic stability of MEMS sensors enables new application areas for the sensors, and decreases the manufacturing costs of existing products. Especially in the applications where the MEMS component is operated under bias voltage close to the pull-in point, the undesired instability phenomenon becomes a major source of inaccuracy. We demonstrate that biasing the sensor to the pull-in point using AC voltage is significantly more stable than the conventionally used DC voltage biasing.

Wideband Microwave Power Sensor Based on MEMS Technology

An optimized transmission-type MEMS RF power sensor is presented. The design is based on microwave filter theory. It is shown that the thermal resolution of the sensor can be reduced below -50 dBm for RF bandwidths up to les 40 GHz

AC Voltage Reference Based on a Capacitive Micromechanical Component

The design and characterization of a high stability capacitive MEMS device intended for an AC voltage reference at 100 kHz or higher frequencies is presented. Preliminary results from a first prototype device show that the pull-in voltage of the device is stable to within 10 ppm over 60 hours. We discuss an optimised device design which is expected to show greater stability

Demonstration of 50-Hz electrical active power measurement using a micromechanical magnetometer

This paper describes a demonstration where a micromechanical Lorentz force magnetometer is used for measurement of 50 Hz active power. The magnetometer reacts to the external current and voltage, where current coupling is magnetic and voltage coupling is galvanic. The output is proportional to their product. Even though the component used for this demonstration is not optimal for the purpose, a clear response to active power was obtained.

Integration of a capacitive pressure sensing system into the outer catheter wall for coronary artery FFR measurements

The deadliest disease in the world is coronary artery disease (CAD), which is related to a narrowing (stenosis) of blood vessels due to fatty deposits, plaque, on the arterial walls. The level of stenosis in the coronary arteries can be assessed by Fractional Flow Reserve (FFR) measurements. This involves determining the ratio between the maximum achievable blood flow in a diseased coronary artery and the theoretical maximum flow in a normal coronary artery. The blood flow is represented by a pressure drop, thus a pressure wire or pressure sensor integrated in a catheter can be used to calculate the ratio between the coronary pressure distal to the stenosis and the normal coronary pressure. A 2 Fr (0.67mm) outer diameter catheter was used, which required a high level of microelectronics miniaturisation to fit a pressure sensing system into the outer wall. The catheter has an eccentric guidewire lumen with a diameter of 0.43mm, which implies that the thickest catheter wall section provides less than 210 microns height for flex assembly integration consisting of two dies, a capacitive MEMS pressure sensor and an ASIC. In order to achieve this a very thin circuit flex was used, and the two chips were thinned down to 75 microns and flip chip mounted face down on the flex. Many challenges were involved in obtaining a flex layout that could wrap into a small tube without getting the dies damaged, while still maintaining enough flexibility for the catheter to navigate the arterial system.

A novel MEMS gas sensor based on ultrasonic resonance cavity

We present a novel low-cost and low-power MEMS gas sensor concept based on an ultrasonic resonance cavity. The sensor consists of a capacitive micromachined ultrasonic transducer (CMUT) embedded to an acoustic resonance cavity. The sensor operation was demonstrated with carbon dioxide CO2 and methane CH4, the lowest resolvable concentrations are about 10 - 20 ppm - a competitive result with the existing commercially available CO2 sensors. In addition, the sensor is able to measure gas concentration and humidity independently, and thus can be used as a combo sensor for gas concentrations and humidity.

3D flip chip packaging of MEMS sensor

Advanced 3D packaging of a Micro Electro Mechanical Systems (MEMS) chip and a CMOS/ASIC Chip was studied. We successfully introduced redistribution process applying two spin coated polybenzoxazole (PBO) polymer layers and two metal layers on 200 mm ASIC wafer. Both MEMS and ASIC bump pad openings were set to 60 μm in diameter. Sputtering and electrochemical plating (ECP) techniques were utilized for metallization. On the Al pads of the sensor Au stud bumps were created. The redistributed ASIC pads were coated with sputtered Au on top of the ECP nickel metal layer and thus Au-Au flip chip bonding was accomplished. The MEMS sensor element in this study was capacitive pressure sensing diaphragm. The diaphragm was made of poly-Si. The pressure range tested was typical barometric range from 35 kPa to 115 kPa. The device operating temperature range from - 40 °C to + 85 °C was tested. Along with the packaging process, solder ball transfer jig was fabricated using bulk silicon wafer. It enabled transfer of eight solder balls to the Chip Scale Packaging (CSP) at one time. The solder ball landing pad was sputtered Au as well. The solder ball pad openings were 300 μm in diameter. Two different size of solder balls were used, 310 μm and 410 μm to ensure enough clearance between CSP and Printed Circuit Board (PCB). Solder balls were consisted of polymer core ball with SnAgCu (SAC) solder metal layers. Several thermo compression bondings were carried out and fine-tune solder ball connections. Functionality was verified by electrical device measurements. To improve productivity, replacement of the Au stud bumps was demonstrated using wafer level ECP to make SnAg μbumps. The plating quality attained within 1 μm height uniformity inside a bonding chip area. SEM observation showed that connection of SnAg micro bump to Au-pad metal was realized.

A DC Voltage Reference Based on MEMS

A micromechanical moving plate capacitor has been designed and fabricated for use as the key component in a DC voltage reference. The reference is based on the characteristic pull-in property of a capacitive MEMS device. New device design and characterization results are presented