By: Norbert Lorenz
In recent years the interest in micro-devices, including Micro-Electro-Mechanical-Systems (MEMS), from research institutions, industry and the press has risen considerably. However one of the persisting challenges in the fabrication of such devices is the packaging process. A number of different bonding techniques have been developed but in general they require the entire device to be heated to high temperatures. In particular for direct bonding techniques like Si-fusion and anodic bonding, temperatures in excess of 1000°C and strong electric fields (1000-2000 V) are essential for successful packaging. As a consequence the use of temperature-sensitive materials within the package is restricted and problems are generated in multi-stage packaging processes where several heating cycles are carried out in sequence; parts joined in an earlier heating step can disassemble in a later one. Furthermore it is clearly important that the package should not affect the performance of the device or cause any damage. Often hermetic and/or vacuum packaging is required which makes the process application specific and expensive. Therefore it can easily account for up to 50% of the overall device cost and can even reach as much as 90%.
Our solution is to combine the benefits of intermediate layer bonding – considerably lower bonding temperatures and absence of electric fields – with the highly localised heating possible with a laser. In addition to localised heating, appropriate heatsinking is required due to the long time-constants of the process (tens of seconds). The device is hence placed in good thermal contact with a heat sink to draw the excess heat away through the base of the device (figure 1).
Two particularly promising intermediate layers we investigate are (i) a thermosetting polymer called benzocyclobutene (BCB) and (ii) glass frit. BCB benefits from low out-gassing of the polymer during the bonding process, low bonding temperatures, and bio-compatibility, required for bio-medical applications. It provides near-hermetic seals, which is sufficient for most MEMS applications. We show laser-based packaging of silicon to glass – materials commonly used in MEMS fabrication – on a near/close to wafer-level process, where nine devices are packaged on a single silicon wafer (figure 2).
For applications where full hermetic seals are desired (e.g. vacuum packaging) we instead use glass frit which can provide full hermetic seals with long-term stability in a simple and robust bonding process which is tolerant to moderately rough bonding surfaces with roughness of up to a few hundred microns. We have investigated the feasibility of our laser-based glass frit joining process with 4 different miniature packages (figure 3):
(i) LCC (leadless chip carrier) package. This consists of an alumina-based
substrate containing a recess, to which a kovar lid is bonded
(ii) Planar LTCC (low temperature co-fired ceramic), an alumina-based ceramic to which a kovar “top-hat” lid is bonded
(iii) Planar AlN (Aluminium Nitride) to which a kovar “top-hat” lid is bonded
(iv) Planar Si substrate to which either a kovar or Nickel cap is bonded
Subsequent to bonding all packages were Helium-leak tested and all devices passed with full hermetic seals according to MIL-STD-883G. In case of the LCC package online temperature monitoring during the joining process has shown that the temperature in the centre of the device can be kept below 140°C throughout the entire process despite a temperature of 375°C in the joining area (figure 4).
In summary, we have demonstrated the feasibility of laser-based intermediate layer bonding of a range of miniature packages, important for MEMS and related applications. The laser provides localised heat energy, preventing damage to temperature-sensitive materials or dis-assembly of other components of the overall system.