Beam steering
In a project funded by the Swedish Foundation for Strategic Research, our role was to provide a suitable packaging technique for a small optical system consisting of a fabricated lens component and a VCSEL (vertical-cavity surface-emitting laser). The LTCC (low-temperature co-fired ceramic) material that composes the substrate uses a multi-layer glass-composite technique, which makes it possible to fabricate 3D structures, such as well-defined cavities in combination with electrical connections. In recent years, LTCC has emerged as one of the most promising packaging techniques for commercial MEMS devices.
LTCC substrates were manufactured by VTT (Finland) using DuPont 951 tape material following the design rules provided by VTT. Four layers of tape were used for the carrier with the cavity spanning half way through the substrate, i.e., two layers down. The cavity depth was designed to exactly match the height of the VCSEL chip (250 µm). In order to ensure surface flatness, the electrical connectors were placed in the inner layers instead of on the top layer. The VCSEL was assembled and connected to ground with a silver-filled epoxy adhesive, and the remaining electrical connections to the substrate were created by wire bonding. Gold bond pads were used, and all the other connectors were made of AgPd. The lens component was assembled on top of the LTCC carrier and centered above the VCSEL, leaving the bond pads and wires intact in the etched cavity below the lens.
Figure 1. A schematic illustration of the VCSEL mounted in the LTCC
cavity with the lens component aligned and assembled
Adhesives
Sensor packaging greatly influences sensor performance. In some applications, such as gyroscopes, it is important to induce as little stress as possible in the sensor but the sensor must still stay in its calibrated position. In a study we performed, different adhesives were tested by measuring the capacitance changes in a gyroscope that are translated into displacement of sensor structures. Soft room-temperature curing systems are promising.
Stress can result from the fabrication process, CTE mismatch in the die attachment material, lid sealing or shrinkage during adhesive curing. For example, interfacial stress can develop within the MEMS package. This can be controlled to an extent by using a die-attachment material with a relatively low modulus of elasticity. The low Young’s modulus of silicones compensates for the CTE mismatch between the die and the circuit board and so prevents the stress from being transferred to the die. Silicone-based materials have unique flexibility and stress-relieving characteristics that make them attractive for absorbing CTE mismatches during thermal cycling.
Silicon-to-silicon bonding with intermediate glass layer A new concept for joining two silicon wafers that we have developed. The glass paste developed by DuPont based on Corning 7070 glass was screen printed on one of the wafers, polished, and then joined to the other wafer using anodic bonding. This technique results in a hermetic seal that is suitable for packaging different kinds of sensors.
LTCC
We are at present investigating lower cost methods for interconnections, packaging, and assembly, for example, by using anodic bonding in combination with LTCC (low-temperature co-fired ceramics). This new hybrid technology opens up new possibilities for MEMS packaging. The automotive and telecommunication industries are already using LTCC technology for circuit boards for radio-frequency products, optoelectronics, and sensor packaging because of its favorable dielectric characteristics, the electrical properties of its conductors (screen printed), and its high packaging density.
A new LTCC material has been recently developed by HITK and VIA Electronic (Germany) that has a lower CTE, 3.6 ppm/K, which opens up more promising MEMS applications. The new LTCC material contains alkali ions that allow direct anodic bonding to silicon as opposed to the well-known DuPont 951 material.
When a lower bonding voltage and/or confined electric field are required, an LTCC substrate with metal electrodes embedded in the substrate (placed below the surface) can be used. An LTCC substrate of 2 x 2 cm2 was successfully bonded anodically to a Si wafer at 420° C and 30 V in a few seconds. This configuration containing metal electrodes opens up possibilities for applications containing more critical structures.
Using LTCC as a substrate would reduce the cost of chip/wafer scale packages, which in turn can become an essential factor in bringing MEMS products to a mass market.