Erik Poppe

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.

Interfacial characterization of Al-Al thermocompression bonds

Interfaces formed by Al-Al thermocompression bonding were studied by the transmission electron microscopy. Si wafer pairs having patterned bonding frames were bonded using Al films deposited on Si or SiO2 as intermediate bonding media. A bond force of 36 or 60 kN at bonding temperatures ranging from 400–550 °C was applied for a duration of 60 min. Differences in the bonded interfaces of 200 μm wide sealing frames were investigated. It was observed that the interface had voids for bonding with 36 kN at 400 °C for Al deposited both on Si and on SiO2. However, the dicing yield was 33% for Al on Si and 98% for Al on SiO2, attesting for the higher quality of the latter bonds. Both a bond force of 60 kN applied at 400 °C and a bond force of 36 kN applied at 550 °C resulted in completely bonded frames with dicing yields of, respectively, 100% and 96%. A high density of long dislocations in the Al grains was observed for the 60 kN case, while the higher temperature resulted in grain boundary rotation away from th...

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.

Bond strength of conductive Si-Si fusion bonded seals

High temperature silicon direct (fusion) wafer bonding is a process with many application areas. Depending on the application, perfect insulation or zero resistance across the bonded interface is desired, but high bond strength is needed in both cases. Recently, we have presented a hydrophilic bonding process which resulted in ohmic behaviour and negligible electrical resistance of the bonding interface. This paper is an investigation of the bond strength of conductive hydrophilic high-temperature silicon direct wafer bonds. Dicing yield and pull test measurements have been performed. Bonding frames of widths of 100, 200, and 400 μm were fabricated. The measured resistance of chips from boron implanted wafers was 0.35-0.38 Ω, and the resistance of chips from three non-implanted wafers was below 0.68 Ω. The dicing yield was above 89 % for frame widths of 200 μm or wider. Bond strengths of 10.5-13.5 MPa were measured on frames of 400 μm width. There was no significant difference in bond strength between implanted wafers, non-implanted wafers, and wafers with an intentional 60 nm thick SiO2 at the bond interface. The results show that directly bonded silicon bond frames of 200 and 400 μm widths can be conductive and show ohmic behavior while they also have sufficient yield and bond strength for application as device seals in industrial products.

High-frequency CMUT arrays with phase-steering for in vivo ultrasound imaging

Capacitive micromachined ultrasonic transducers (CMUTs) have prospective clinical applications in ultrasound diagnostics and therapy. We have modeled, fabricated, and characterized CMUT arrays suitable for intravascular and gastrointestinal use. The elements of the CMUT arrays are connected from the back side of the array by through-silicon-vias (TSVs) with a pitch of 25 μm. In the reported work, the presence of TSVs does not reduce the fill factor of the array. Two different membrane designs were fabricated, resulting in a resonance frequency in air of 35.6 and 41.0 MHz, respectively. The measurements were in good agreement with the simulated performance. The realized devices are promising for the fabrication of a 3D-ready and bio-compatible device, suitable for in vivo ultrasound applications.

Anisotropic conductive film interconnects for fine-pitch MEMS

The flip-chip interconnection technology based on anisotropic conductive films (ACFs) has recently become an attractive solution for the assembly of micro-electromechanical systems (MEMS) and application specific integrated circuits (ASIC) in MEMS packages. In the present work, we have studied the fine pitch capability of ACF interconnects for MEMS applications such as fingerprint sensors and capacitive micromachined ultrasonic transducers, in which interconnects spread around MEMS and ASIC surface. The silicon test chips and substrates with different interconnect pitch were assembled using a single layer ACF. The electrical performance of ACF interconnects with varying pitch from 110 to 200 μm was compared. Furthermore, the distribution of conductive particles and the electrical resistance of ACF interconnects at both peripheral and central parts of the chips were evaluated. Effect of thermal shock cycling test (−40 to +125 °C) on samples was investigated. The results showed insignificant difference in the electrical performance between ACF interconnects with pitch varying from 110 to 200 μm. The particle distribution and the electrical resistance of ACF interconnects at different chip regions were similar. No significant effect of the thermal shock cycling test was observed. No failures (open/short circuit) occurred, both before and after the thermal shock cycling test.

Die shear strength as a function of bond frame geometry — Au-Au thermo-compression bonding

The die shear strength has been studied as a function of bond frame geometry for wafer-level Au-Au thermo-compression bonds. The shear strength of samples with a 100 and 200 μm wide bond frame (bond area 1–2 mm2) was higher and more uniform than that of samples with a 400 μm wide bond frame (bond area 4 mm2). Bond frames with rounded corners had higher bond strength than samples with right angled corners (strength increased by 10–20 %). Three different fracture modes were observed. These trends were supported by finite element analysis (FEA) simulations. Conservative bonding parameters (temperature ≥ 400 °C, time ≥ 15 min, pressure 21 MPa) were applied to assure a uniform bond quality for the inspected samples. Shear testing as a method to quantify bond strength was discussed in general and in particular with respect to observed effects of bond frame geometries.

Passivation of silicon wafer patterned by aluminum for micromachining

Low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD) have been used for depositing silicon nitride (SiN) as passivation layer in microfabrications. SiN deposited by LPCVD and optimized PECVD can perfectly mask Si from etching attack in TMAH-water solution. After Al metallization, pinholes are always formed on SiN because of the Al crystal hillocks. Due to poor step coverage of PECVD SiN on Al, the edge etching of the Al patterns is the main reason for Si etching underneath of the Al patterns although the Al etching via the pinholes on SiN contributes. By structure designing and process tuning, we have achieved passivation techniques for micromachining after Al metallization.