Imec has derived a process flow concept
for the packaging of medical implants that meets the requirements for miniaturization,
biocompatibility and safety. The proposed solution is a promising alternative
for the currently used rigid packages that tend to enhance foreign body reactions.
The technological developments within the field of electronics have allowed
for a significant downsizing of active medical devices. In order to take full
advantage of this miniaturization, the housing/packaging of the devices should
keep pace with these developments. Imec now proposes a process flow for packaging
active medical implants allowing for further miniaturization and taking into
account the stringent requirements for biocompatibility and safety. The latter
include hermetic sealing to protect the human body, protection of the device
from seeping of bodily fluids into the implant, biocompatibility (putting requirements
on the chemical composition, shape and mechanical properties of the package)
and suitability for the required standard sterilization technique.
(a) Scheme of biocompatible chip encasulation and (b & c) possible basic concept of an implantable system consisting of thin dies encapsulated with biocompatible bi-directional diffusion barriers. In case (b), only electronics and metallization is embedded, in case (c) a second embedding is used to combine the electronic sub-part of the device with other parts such as a battery.
An active implantable device typically consists of various sub-devices, i.e.
a CMOS chip, a microsystem, a battery, electric components, a drug reservoir,
a sensor, a microfluidic device etc. All sub-devices are electrically connected
with each other and sometimes electrodes are also in direct contact with the
human body.
The first phase of the process flow involves the hermetic encapsulation of
all individual dies. For this step, imec proposes a wafer-level based processing
that is carried out by using conventional cleanroom tools. It is realized by
encapsulating the topside of the chips after (partial) wafer dicing, followed
by mounting the wafer upside down on a carrier for wafer thinning and a second
encapsulation step from the chip backside. The encapsulation layers serve as
biocompatible bi-directional diffusion barriers and can be made from various
materials such as Si (or Ti) oxide, Si (or Ti) nitride, Si carbide etc. A sloped
sawing technique is used to make sloped chip edges, ensuring a better step coverage
of the capping layers. As a proof of concept, the obtained diffusion barrier
properties of a Si oxide/nitride based encapsulation layer is illustrated by
cell culture tests.
The second phase of the packaging consists of the assembly of sub-devices,
the fabrication of interconnects using a biocompatible metallization technique,
and the global embedding in a soft flexible material. This phase is often split
into two parts: firstly, the electronic chips will be embedded resulting in
a kind of interposer, followed by a second global embedding of this interposer
together with other non-Si based subparts (e.g. battery, passives) of the total
system. The global embedding is typically a board-level process since larger
interconnect pitches have to be realized. The fabrication of the interposer-like
package to embed Si dies might be carried out using board-level or wafer-level
technologies. Cost plays an important role in the selection of the technology.
However, imec’s fabrication cost calculations have demonstrated that the
dominant factor in the cost of a biocompatible packaging is given by the metallization
process. Noble metals such as platinum show excellent biocompatibility but their
metallization is very expensive. This second phase of the packaging process,
including the development of an optimized and cost-effective metallization scheme,
is the subject of ongoing research.
The proposed solution is a promising alternative to the existing titanium or
ceramic based packages. Titanium/ceramic packaged implants are large and rigid,
enhancing the foreign body reaction and preventing further miniaturization.
These results have been proposed in more detail during the Electronics System
Integration Technology Conference (ESTC) 2010.