In–Bi low-temperature SLID bonding for piezoelectric materials

Abstract

Purpose This paper aims to demonstrate low-temperature bonding for piezoelectric materials at temperatures well below the relevant Curie temperatures so as to avoid depolarization of the piezoelectric material during bonding. Design/methodology/approach Au-coated test samples of lead zirconate titanate (PZT) are bonded to a WC-based resonant backing layer with In–Bi eutectic material in which the In–Bi metal system is a preform or thin, evaporated layers. The bonded samples are characterized using electrical impedance spectroscopy and cross-section microscopy. The first technique verifies the integrity of polarization and reveals the quality of the bondline in a non-destructive manner, particularly looking for voids and delaminations. The latter technique is destructive but gives more precise information and an overview of the structure. Findings Successful low-temperature (115°C) bonding with intact PZT polarization was demonstrated. The bondlines show a layered structure of Au/Au–In intermetallic compounds (with Bi inclusions)/Au, capable of withstanding temperatures as high as 271°C before remelting occurs. For bonded samples using In–Bi preform, repeatable bonds of high quality (very little voiding) were obtained, but the bonding time is long (1 h or more). For bonded samples using evaporated thin films of In–Bi, bonding can be performed in 30 min, but the process needs further optimization to be repeatable. Originality/value Low-temperature solid-liquid interdiffusion (SLID) bonding is a novel technique, merging the fields of low-temperature solder bonding with the SLID/transient liquid phase (TLP) approach, which is normally used for much higher temperatures.

Intermetallic Bonding for High-Temperature Microelectronics and Microsystems: Solid-Liquid Interdiffusion Bonding

“…Our preliminary experiments with thin-film-deposited In-Bi show that similar bond morphologies can be obtained, with decreased bonding times [39].…”

“…Au-In-Bi SLID bonding using thin-film (2.5-3 μm) thermally evaporated eutectic In-Bi on Au surfaces was also demonstrated [39]. This approach opens up for a much shorter process time, as well as being compatible with photolithographic patterning.…”

“…Bonded, acoustically active materials are tested with electrical impedance spectroscopy, confirming a very limited amount of voiding, compatible with ultrasonic applications [39]. Table 2 compares the different SLID systems that our group has worked with and what has been our group's research focus for each system.…”

Solid-Liquid InterDiffusion (SLID) Bonding, for Thermally Challenging Applications

“…Polarization will be lost if the material is exposed to temperatures above the Curie temperature, typically around 150 °C for PZT. We have demonstrated Au-In-Bi bonding of PZT to RBL [5]. Electrical impedance spectroscopy verifies that the polarization of PZT is intact after bonding, and that the voiding level is acceptable for the acoustic properties of the assembly.…”

“…SLID bonding can eliminate the need for a hierarchy of solder alloys with different melting temperatures for bonding of stacks or for bonding at different levels of integration, hence simplifying manufacturing processes. Using appropriate metal systems, low-temperature SLID bonding can be used for temperature-sensitive materials such as polymers or poled piezoelectrics, where the resulting bond can still be reliable at somewhat elevated temperatures [5].…”