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.acceptedVersio

Environmental Stress Testing of Wafer-Level Al-Al Thermocompression Bonds: Strength and Hermeticity

Hermeticity, reliability and strength of Al-Al thermocompression bonds realized by applying different bonding parameters have been investigated. Laminates of diameter 150 mm were realized by bonding wafers containing membrane structures to wafers with patterned bonding frames. The laminates were bonded applying a bond force of 36 or 60 kN at temperatures ranging from 300 to 400°C for 15, 30 or 60 minutes. The hermetic properties were estimated by membrane deflection measurements with white-light interferometry after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. Bond strength was measured by shear test and pull tests. Laminates bonded applying a bond force of 60 kN at temperatures of 350 or 400°C resulted in hermetic bonds. No significant change in membrane deflection was observed after the steady-state life test or the thermal shock test. However, a gross leakage was observed in 1–11% of the dies after exposure to the moisture resistance test. The maximum leakage rate (MLR) estimated from membrane deflection measurements was below 10−11 mbar·l·s−1 for all laminates. The measured average bond strength of dies from selected laminates ranged from 28 to 190 MPa.acceptedVersio

Al-Al thermocompression bonding for wafer-level MEMS sealing

Al–Al thermocompression bonding has been studied using test structures relevant for wafer level sealing of MEMS devices. Si wafers with protruding frame structures were bonded to planar Si wafers, both covered with a sputtered Al film of 1 μm thickness. The varied bonding process variables were the bonding temperature (400, 450 and 550 °C) and the bonding force (18, 36 and 60 kN). Frame widths 100 μm, 200 μm, with rounded or sharp frame corners were used. After bonding, laminates were diced into single chips and pull tested. The effect of process and design parameters was studied systematically with respect to dicing yield, bond strength and resulting fractured surfaces. The test structures showed an average strength of 20–50 MPa for bonding at or above 450 °C for all three bonding forces or bonding at 400 °C with 60 kN bond force. The current study indicates that strong AlAl thermocompression bonds can be achieved either at or above 450 °C bonding temperature for low (18 kN) and medium (36 kN) bond force or by high bond force (60 kN) at 400 °C. The results show that an increased bond force is required to compensate for a reduced bonding temperature for AlAl thermocompression bonding in the studied temperature regimeacceptedVersio

3D Integration of MEMS and IC: Design, technology and simulations

* 3D integration: Opportunities and trends* e-CUBES: Tire pressure monitoring system (TPMS)* Package design including thermo-mechanical modeling* Technology development* Sensor packaging concept* Gold stud bump bonding* Device characterization and testing* Summary and outlook3D Integration of MEMS and IC: Design, technology and simulation

Non-destructive wafer-level bond defect identification by scanning acoustic microscopy

Metal-based thermocompression bonding enables the creation of hermetic seals formed at relatively low processing temperatures and occupying a small portion of the device area. In the current study we have investigated the application of scanning acoustic microscopy (SAM) for assessing the quality of metal thermocompression bonds, both by evaluating its capabilities of localizing areas of poor bonding, and by finding defects in the integrity of the bond seal. Wafer laminates containing a test vehicle of sealing frames with pre-defined defects in the bond metal layer were sealed by Au–Au and Al–Al thermocompression bonding. Employing SAM, an area of five chips of poor bonding was identified non-destructively on the Al–Al laminate. Line defects of width 3.6 µm and point defects of diameter 22.4 µm have also been identified by SAM. The dicing yield for sealing frames was above 96 % for all frames of widths 100–400 µm and for both bond metal systems. The average bond strength was 31.5 ± 11.9 MPa for Al–Al thermocompression bonds and 37.3 ± 9.7 MPa for Au–Au thermocompression bonds. Scanning acoustic microscopy operates non-destructively and proved to be an extremely useful tool complementing current state-of-the-art methods for bond quality assessment.acceptedVersio

Metal Films for MEMS Pressure Sensors: Comparison of Al, Ti, Al-Ti Alloy and Al/Ti Film Stacks

Thermo-mechanical stability of metal structures is one of the key factors affecting accuracy of micro-electromechanical (MEMS) piezoresistive pressure sensors. In this work, we present the measurement results of stress and hysteresis for the following metals deposited in the same sputtering equipment -Al, Ti, Al-Ti alloy and stacks of Al/Ti films-enabling, for the first time, a direct comparison between their thermo-mechanical properties supported with analysis of surface morphology (grain size, hillocks and voids).acceptedVersio

Low-Temperature Aluminum-Aluminum Wafer Bonding

Aluminum-aluminum thermo-compression wafer bonding is becoming increasingly important in the production of microelectromechanical systems (MEMS) devices. As the chemically highly stable aluminum oxide layer acts as a diffusion barrier between the two aluminum metallization layers, up to now the process has required bonding temperatures of 300°C or more. By using the EVG®580 ComBond® system, in which a surface treatment and subsequent wafer bonding are both performed in a high vacuum cluster, for the first time successful Al-Al wafer bonding was possible at a temperature of 100°C. The bonded interfaces of blank Al wafers and Al wafers with patterned frames were characterized using C-mode scanning acoustic microscopy (C-SAM) and transmission electron microscopy (TEM) as well as dicing yield and pull tests representative for the bonding strength. The investigations revealed areas of oxide-free, atomic contact at the Al-Al bonded interface.acceptedVersio

Environmental Stress Testing of Wafer-Level Au-Au Thermocompression Bonds Realized at Low Temperature: Strength and Hermeticity

Hermeticity, reliability and strength of four laminates bonded at different temperatures by Au-Au thermocompression bonding have been investigated. Laminates with a diameter of 150 mm were realized by bonding a wafer containing membrane structures to a Si wafer with patterned bond frames. A bond tool pressure of 2266 mbar was applied for 15 minutes at temperatures ranging from 150–300°C. The hermetic properties were estimated by membrane deflection measurements applying white-light interferometry after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. Bond strength was estimated by pull test measurements. A dicing yield above 90% was obtained for all laminates. Laminates bonded at 200°C and above had significantly higher hermetic yield than the laminate bonded at 150°C. No degradation in hermeticity was observed after the reliability tests. The maximum leakage rate (MLR) was estimated from two measurements of membrane deflection executed at two different times and was below 10−11 mbar ⋅ l ⋅ s−1. The average bond strength ranged from 44 to 175 MPa

Al-Al Wafer-Level Thermocompression Bonding applied for MEMS

Wafer-level thermocompression bonding (TCB) using aluminum (Al) is presented as a hermetic sealing method for MEMS. The process is a CMOS compatible alternative to TCB using metals like gold (Au) and copper (Cu), which are problematic with respect to cross contamination in labs. Au and Cu are commonly used for TCB and the oxidation of these metals is limited (Au) or easily controlled (Cu). However, despite Al oxidation, our experimental results and theoretical considerations show that TCB using Al is feasible even at temperatures down to 300−350 °C using a commercial bonder without in-situ surface treatment capability.acceptedVersio