Jaakko Saarilahti

Capacitive microphone with low-stress polysilicon membrane and high-stress polysilicon backplate

Abstract A capacitive single-chip silicon microphone with very low-stress polysilicon membrane was fabricated. A mechanism for stress-releasing due to the high stress of the perforated membrane was introduced. With the achieved stress level of 2 MPa, a microphone with the membrane area of 1 mm 2 can be optimally designed, although the measured components did not show the optimal resolution due to excessive acoustic resistance. With a membrane area of 1 mm 2 , the acoustical sensitivity was 4 mV/Pa (at 1 kHz) and the noise equivalent sound level was 33.5 dB (A), which are adequate values for many applications. The packaged components were tested with a thermal cycle between −40°C and +60°C, and due to low packaging-related stresses, no buckling of the membranes was observed.

Implementing ALD layers in MEMS processing

Layers manufactured by the ALD technique have many interesting applications in microelectromechanical systems (MEMS), for example as protective layers for biocompatible coating, highdielectric-constant layers, or low-temperature conformal insulating layers. Before an ALD process can be successfully implemented in MEMS processing, several practical issues have to be solved, starting from patterning the layers and characterizing their behaviour in various chemical and thermal environments. Stress issues may not be forgotten. We have recently implemented two ALD processes, namely the trimethylaluminium/water process to deposit Al2O3 and the titanium tetrachloride/water process to deposit TiO2 in our MEMS processing line and carried out the necessary characterization, details of which are reported here. For us, ALD has been a truly enabling technology in the processing of a three-dimensional micromechanical compass based on the Lorentz force, where Al2O3 acted as a pinhole-free electrical insulation grown at low temperature.

Droplet Manipulation on a Superhydrophobic Surface for Microchemical Analysis

We report methods for importing, transporting, sorting, mixing, exporting and filtering liquid droplets on a superhydrophobic surface. The functions demonstrated serve as basic blocks for a microfluidic system capable of performing chemical analysis of droplets.

A novel methdod for processing capacitive micromechanical ultrasonic transducers (cMUT)

The cMUTs presented in this study are based on a new surface micromachining process, where part of the top electrode of the cMUT is fabricated of porous polysilicon. This method gives many advantages over the previously reported fabrication processes.

Low-temperature processes for MEMS device fabrication

The high temperatures typical in semiconductor and conventional MEMS fabrication limit the material choices in MEMS structures. This paper reviews some of the low-temperature processes and techniques available for MEMS fabrication and describes some characteristics of these techniques and practical process examples. The techniques described are plasma-enhanced chemical vapour deposition, atomic layer deposition, reactive sputtering, vapour phase hydrofluoric acid etching of low-temperature oxides, and low-temperature wafer bonding. As a practical example of the use of these techniques, the basic characteristics of a MEMS switch and other devices fabricated at VTT are presented.

Characterization and operation of different cMUT membranes in air

Capacitive micromachined ultrasonic transducers (cMUT) have emerged as an alternative technology to piezoelectric transducers. The paper presents characterization measurements of several membranes with different geometries and materials, aimed at an optimization of the membrane configuration for different applications. In parallel, suitable testing and readout electronics have been developed. We report electro-mechanical measurements and in-air uncoupled testing of devices fabricated in a dedicated process, appropriate for short-range ultrasound transmission.

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.

RBS channeling spectroscopy of Ge implanted epitaxial Si1-xGex layers

Si1-xGex layers were formed through high-dose germanium ion implantation into (100)Si substrates. Two alternative implantation techniques along with that of the single-energy Ge+ implantation were separately adopted: the double-energy Si+ and Ge+ method, and the double-energy Ge+ and Ge++ method. The purpose of the both double-energy methods was to form deeper amorphous layers by using relatively low-dose Si+ or Ge++ ion bombardment while the SiGe alloy layers were created by high dose Ge+ ion implantations. Furthermore, all the amorphized samples were epitaxialy regrown by conventional furnace annealing at temperature of 525 to 600°C. RBS channeling spectroscopy was used for optimizing these implantation processes. Measurements confirm that the double-energy Ge+ and Ge++ method is optimum because of generating fewer residual defects. Additionally, the preliminary result on the regrowth properties of the double-energy Ge+ and Ge++ implanted SiGe layer is also presented.