Kari Schjølberg-Henriksen

Strong, high-yield and low-temperature thermocompression silicon wafer-level bonding with gold

A systematic variation of process parameters for wafer-level thermocompression bonding with gold is presented for the first time. The process was optimized for high bond strength and high bond yield. In addition, the impact of the process temperature was investigated. A bond strength of 10.7 ± 4.5 MPa and a bond yield of 89% was achieved when bonding a wafer pair at 298 °C applying 4 MPa pressure for 45 min. A total of ten wafer pairs were bonded in a custom-built bonding tool and tested to establish the optimal process parameters. The bonded interface was found to be strong and dense enough for MEMS applications. The bonds were characterized using pull tests, transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS). The TEM inspections indicated that it is possible to form hermetic seals by using the presented bonding method.

Capacitive differential pressure sensor for harsh environments

Abstract A capacitive differential pressure sensor for the pressure range of 0–1 bar has been developed. The primary field of application is hydrodynamic flow measurements in hot petroleum wells. In these harsh environments the sensor has to survive high common mode pressure in the range of 1000 bar and temperatures up to 180°C. The pressure element is formed in a triple-stack of fusion-bonded silicon wafers. A bossed diaphragm etched in the upper wafer bends due to the differential pressure across it. The capacitance to the middle wafer is measured. A reference capacitor insensitive to the differential pressure enables compensation for capacitance shifts caused by ambient pressure and temperature changes. The lower silicon wafer is included to minimise the diaphragm stress from the package. An ASIC, certified for 230°C, which is developed at SINTEF, is used for signal read-out. The sensor is measured to have a low zero pressure signal drift, smaller than 2.5% of full-scale output, when the temperature is varied in the range 0–200°C and the ambient pressure in the range 0–1000 bar. The sensitivity of the sensor is sufficient for the application.

Sodium contamination of SiO2 caused by anodic bonding

In this paper we present an investigation of sodium contamination of SiO2 (oxide) during anodic bonding. Sodium contamination can be deleterious to the electrical properties of silicon structures. Silicon wafers with metal–oxide semiconductor (MOS) capacitors were bonded to Corning 7740 (Pyrex) glass wafers. The concentration of mobile ions was measured on capacitors outside and within glass cavities using the triangular voltage sweep method. Using secondary ion mass spectrometry analysis, it was confirmed that the ions were sodium. We found an increase in sodium concentration Nm between 1010 and 1013 cm−2, depending on the oxide location and the geometry of the glass cavity. The gate aluminium of the MOS capacitor was found to partly shield the oxide from contamination, causing a two to five times smaller increase in Nm. Reducing the bonding voltage from 800 to 500 V did not affect the increase in Nm significantly. In contrast, changing the ambient in the bonding chamber from vacuum to 1020 mbar air, reduced the contamination of capacitors situated outside the glass. A plasma-enhanced chemical vapour deposited Si3N4 film was found to be very beneficial in protecting the capacitors. The Si3N4 prevented sodium contamination of the capacitors situated within the glass cavities, and radically reduced the contamination of the capacitors situated outside the glass. The results suggest that the contaminating sodium originated from the bulk glass.

Protection of MOS capacitors during anodic bonding

We have investigated the electrical damage by anodic bonding on CMOS-quality gate oxide and methods to prevent this damage. n-type and p-type MOS capacitors were characterized by quasi-static and high-frequency CV-curves before and after anodic bonding. Capacitors that were bonded to a Pyrex wafer with 10 μm deep cavities enclosing the capacitors exhibited increased leakage current and interface trap density after bonding. Two different methods were successful in protecting the capacitors from such damage. Our first approach was to increase the cavity depth from 10 μm to 50 μm, thus reducing the electric field across the gate oxide during bonding from approximately 2 × 105 V cm−1 to 4 × 104 V cm−1. The second protection method was to coat the inside of a 10 μm deep Pyrex glass cavity with aluminium, forming a Faraday cage that removed the electric field across the cavity during anodic bonding. Both methods resulted in capacitors with decreased interface trap density and unchanged leakage current after bonding. No change in effective oxide charge or mobile ion contamination was observed on any of the capacitors in the study.

Impact of SiO2 on Al–Al thermocompression wafer bonding

Al–Al thermocompression bonding suitable for wafer level sealing of MEMS devices has been investigated. This paper presents a comparison of thermocompression bonding of Al films deposited on Si with and without a thermal oxide (SiO2 film). Laminates of diameter 150 mm containing device sealing frames of width 200 µm were realized. The wafers were bonded by applying a bond force of 36 or 60 kN at bonding temperatures ranging from 300–550 °C for bonding times of 15, 30 or 60 min. The effects of these process variations on the quality of the bonded laminates have been studied. The bond quality was estimated by measurements of dicing yield, tensile strength, amount of cohesive fracture in Si and interfacial characterization. The mean bond strength of the tested structures ranged from 18–61 MPa. The laminates with an SiO2 film had higher dicing yield and bond strength than the laminates without SiO2 for a 400 °C bonding temperature. The bond strength increased with increasing bonding temperature and bond force. The laminates bonded for 30 and 60 min at 400 °C and 60 kN had similar bond strength and amount of cohesive fracture in the bulk silicon, while the laminates bonded for 15 min had significantly lower bond strength and amount of cohesive fracture in the bulk silicon.

Sensitive and Selective Photo Acoustic Gas Sensor Suitable for High Volume Manufacturing

Sensitive and selective gas measurements are crucial for a large variety of applications. This paper describes the manufacturing and characterisation of a photo acoustic gas sensor system. The system is based on a pressure sensor element with a sensitivity of 10 muV/V/Pa. 12 prototypes for measuring CO2 have been characterised. Detection limits ranging from 92 ppm to below 6 ppm CO2 were obtained, depending on the measurement time and photo acoustic cell design. No cross-sensitivity towards CO, CH4, or humidity could be observed in any of the sensors. The temperature drift of the uncompensated raw signal of two sensor designs was below 117 ppm CO2 in the range from 25degC to 50degC.

Miniaturised Sensor Node for Tire Pressure Monitoring (e-CUBES)

Tire pressure monitoring systems (TPMS) are beneficial for the environment and road and passenger safety. Miniaturizing the TPMS allows sensing of additional parameters. This paper presents a miniaturized TPMS with a volume less than 1 cm3, realised by 3D stacking and through-silicon via (TSV) technology. Suitable technologies with low electrical resistance and high bond strengths were evaluated for stacking the microcontroller, transceiver, pressure sensor and bulk acoustic resonator (BAR) in the TPMS. 60 μm deep W-filled TSVs with resistance 0.45 Ω and SnAg micro bumps with a bond strength of 53 MPa were used for stacking the transceiver to the microcontroller. TSVs through the whole wafer thickness with resistance 6 Ω were used for the pressure sensor. Au stud bumps were used for stacking the pressure sensor and BAR devices. The final TPMS stack was packaged in a moulded interconnect device (MID) package.

3D Integration of MEMS and IC: Design, Technology and Simulations

A 3D integrated silicon stack consisting of two MEMS devices and two IC devices is presented. The MEMS devices are a pressure sensor and a bulk acoustic resonator (BAR). The stack was constructed for a tire pressure monitoring system (TPMS) which was one out of three demonstrators for an EU funded project called e-CUBES. Thermal simulations were performed to check the level of thermo-mechanical stresses induced on the pressure sensor membrane during extreme environmental conditions. Additional simulations were made to calculate the exact temperature on the BAR device during operation as this was important for the operational frequency. This paper presents and discusses the technology choices made for the stacking of the pressure sensor and the BAR. Results are given from simulations, initial short-loop experiments and for the final stacking.

Sensitive and Selective Photoacoustic Gas Sensor Suitable for High-Volume Manufacturing

Sensitive and selective gas measurements are crucial for a large variety of applications. This paper describes the manufacturing and characterization of a photoacoustic gas sensor system. The system is based on a pressure sensor element with a sensitivity of 10 muV/V/Pa. To demonstrate and evaluate the concept, 12 prototypes for measuring CO2 have been manufactured and characterized. Detection limits ranging from 92 ppm to below 6 ppm CO2 were obtained with a path length of 10 cm, depending on the measurement time and photoacoustic cell design. Measurements showed no cross-sensitivity towards CO, CH4, or humidity in any of the sensors. The temperature drift of the uncompensated raw signal of two sensor designs was below 117 ppm CO2 in the range from 25degC to 50degC.

Resist evaluation for fabrication of diffractive optical elements (DOEs) with sub-micron resolution in a MEMS production line

Diffractive optical elements (DOEs) represent small, lightweight and potentially low-cost alternatives to conventional optical components. We have evaluated photoresists and processes for fabrication of silicon micro-machined DOEs with a sub-micron pattern using an MA150 (Suss) proximity aligner. The resists HiPR 6512 (Fuji film), AZ ECI 3007 (AZ Electronics Materials), IX335 H (JSR Micro) and UVIII (Rohm and Haas) were all able to resolve the desired 0.8 µm pattern, but the wall angle obtained with IX335H was a superior 86°. Double development of the resists proved possible in a KOH-based developer but unfeasible in a TMAH-based developer. The final DOE device was successfully realized based on the optimized photolithography process.