Beam steering
In a project funded by the Swedish Foundation for Strategic Research, the
role of Imego 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 making 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, 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
Adhesives
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.
LTCC
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.