In this paper, a novel wafer-level hermetic packaging technology for heterogeneous device integration is presented. Hermetic sealing is achieved by low-temperature thermo-compression bonding using electroplated Au micro-sealing frame planarized by single-point diamond fly-cutting. The proposed technology has significant advantages compared to other established processes in terms of integration of micro-structured wafer, vacuum encapsulation and electrical interconnection, which can be achieved at the same time. Furthermore, the technology is also achievable for a bonding frame width as narrow as 30 μm, giving it an advantage from a geometry perspective, and bonding temperatures as low as 300 °C, making it advantageous for temperature-sensitive devices. Outgassing in vacuum sealed cavities is studied and a cavity pressure below 500 Pa is achieved by introducing annealing steps prior to bonding. The pressure of the sealed cavity is measured by zero-balance method utilizing diaphragm-structured bonding test devices. The leak rate into the packages is determined by long-term sealed cavity pressure measurement for 1500 h to be less than Pa m3s−1. In addition, the bonding shear strength is also evaluated to be higher than 100 MPa.

https://iopscience.iop.org/article/10.1088/1361-6439/27/1/015029/pdf

Wafer-level hermetic thermo-compression bonding using electroplated gold sealing frame planarized by fly-cutting

Surface activated bonding of Ti/Au and Ti/Pt/Au films after vacuum annealing for MEMS packaging

Wafers with smooth Au thin films (rms surface roughness: < 0.5 nm, thickness: < 50 nm) were successfully bonded in ambient air at room temperature after an Ar radio frequency plasma activation process. The room temperature bonded glass wafers without any heat treatment showed a sufficiently high die-shear strength of 47–70 MPa. Transmission electron microscopy observations showed that direct bonding on the atomic scale was achieved. This surface-activated bonding method is expected to be a useful technique for future heterogeneous photonic integration. key words: heterogeneous integration, room-temperature bonding, surface-activated bonding, Au-Au bonding

Room-Temperature Bonding of Wafers with Smooth Au Thin Films in Ambient Air Using a Surface-Activated Bonding Method

Using the recently developed method to directly measure thermal boundary conductance (TBC) across bonded interfaces, we report the measurements of TBC at interfaces bonded by surface activated bonding at room temperature. The TBC of as-bonded silicon-silicon interface is limited to 1.3 × 102 MW m−2 K−1, which is equivalent to thermal conductance of micrometer-thick bulk silicon. We further show that the TBC can be greatly improved by recrystallizing the amorphous interlayer, which here is realized by thermal annealing. The dependence of the TBC on the annealing temperature is highly nonlinear, which can be explained in terms of thermal activation of crystal growth.

Thermal conductance of silicon interfaces directly bonded by room-temperature surface activation

We newly introduce a compliant rim to realize hermetic sealing of electronic components at low temperature. The compliant rim easily deforms under pressing load owing to its cone-shaped cross section and, therefore, intermetallic bonding can be performed at low temperature. We demonstrate the room-temperature vacuum sealing using the compliant rim made of Au with the aid of ultrasonic vibration of submicron amplitude. A test vehicle fabricated using silicon and glass showed that the air leak rate of the room-temperature sealing was well below 1 ? 10?12 Pa?m3/s, which is sufficiently low for use in vacuum packaging.

http://iopscience.iop.org/article/10.7567/JJAP.53.06JM05/pdf

Room-temperature hermetic sealing by ultrasonic bonding with Au compliant rim

Low-temperature hermetic bonding based on surface activation is useful for optical microsystem packaging because high bonding temperatures may degrade microsystem performance and sensitivity. However, surface-activated bonding (SAB) is usually performed under ultra-high-vacuum conditions, and the bonding environment cannot be chosen freely. In this study, thin Au sealing rings (300–500 nm thick, and 100 μm wide) were used as bonding layers for SAB at atmospheric pressure. A sufficiently high die-shear strength was achieved via surface activation using an argon radio-frequency plasma treatment. On examination of the fracture surfaces of the broken seal after the die-shear test, we observed that the fractures typically occurred at the deposited interface or partially inside the bulk substrates. Hermeticity was evaluated by measuring the resonance characteristics of photothermally excited microcantilevers inside the cavities. The samples bonded at the low temperature of 150 °C under the application of a bonding pressure of 313 MPa for 30 s showed leakage rates of less than 5.0 × 10−9 Pa·m3 s−1, which is the rejection limit defined by the MIL-STD-883G specification.

https://iopscience.iop.org/article/10.1088/0960-1317/22/5/055026/pdf

Shin Ichi Yamamoto

Papers

Low-temperature hermetic packaging for microsystems using Au–Au surface-activated bonding at atmospheric pressure