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, electrical connectors were placed in the inner layers instead of the top layer. The VCSEL was assembled and connected to ground using 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 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
Sensor packaging is known to have a great influence on sensor performance. In some applications (e.g. gyroscopes) it is important to induce as little stress as possible in the sensor but at the same time the sensor must stay in its predetermined calibrated position. In a study performed at Imego different adhesives have been tested by measuring the capacitance changes in a gyroscope that are translated into displacement of sensor structures. Soft room temperature curing systems seem promising.
Stress can be a result of 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 partially controlled by use of 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 circuit board, preventing 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 on thermal cycling.
Silicon to silicon bonding with intermediate glass layer A new concept for joining two silicon wafers has been developed at Imego. The glass paste developed by DuPont based on of Corning 7070 glass was screenprinted on one of the wafers, polished and then joined to the other wafer using the anodic bonding concept. This technique results in a hermetic seal that is suitable for packaging different kinds of sensors.
A current study in our group aims at investigating lower cost methods for interconnections, packaging, and assembly, for example, by using anodic bonding in combination with LTCC (low temperature co-fired ceramic). 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 due to its good dielectric material characteristics, its conductors’ electrical properties (screen printed) and high packaging density.Recently a new LTCC material has been 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 a 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.
The prospect of using LTCC as a substrate allows less expensive chip/wafer scale packages, which in turn can become an essential factor in bringing MEMS products to a mass market thanks to the lower total cost of the system.