Low-temperature Atomic Layer Deposition (ALD) processes for hermetic encapsulation of medical implants

Since a few decades, the use of electronic devices as medical implants is gaining interest. Pacemakers, Deep Brain Stimulators, cochlear implants,.. they all exists some time already, and they are life-saving or improve greatly the quality of life of the patients. Recently many other future electronic implants are under investigation to help people suffering from a variety of medical issues: implants to stimulate certain areas of the brain or the peripheral nerves, retinal implants for people with vision problems, electronic hip or knee implants, etc.  The trend of miniaturization of electronics is obviously an important advantage for implants, but most electronic implants today are still packaged in a rigid and rather large Titanium case. Such a Titanium case is indeed a biocompatible and hermetic device encapsulation, but a much thinner and flexible encapsulation would offer important improvements since a much more ‘bio-mimetic’ device can be made resulting in less reaction of the body upon implantation. In addition, some novel applications would become realistic, such as miniaturized implants in a very narrow location in the body, where no room is available for a larger and rigid Titanium case.

At imec, we are developing alternative ultrathin and flexible encapsulations based om multistacks of polymers and ‘ALD-layers’. ALD-layers are ceramic layers deposited with extreme accuracy using the atomic layer deposition (ALD) technique. If optimised well, these polymers/ALD multistacks are biocompatible, biostable and they are excellent bi-directional diffusion barriers, hence they are ideal candidates for encapsulations of electronic devices. At imec, we have experience in using polymer/ALD stacks deposited at ‘moderate’ process temperatures (120 °C and higher), but for certain applications lower process temperatures (below 100 °C) offer important advantages. Therefore, a novel low temperature ALD process reactor has been purchased, allowing ALD depositions at 80-85° C on Parylene,  a biocompatible polymer that can also be deposited at low temperature.

During this PhD study, the low temperature ALD processes will be evaluated from a fundamental and application point of view:

• We will investigate the structure and properties of the ALD layers, their adhesion, uniformity, and nucleation and growth behaviour on various polymers. The multistacks of these ALD-layers combined with Parylene will be studied for various hermeticity evaluations, leading to the rational design of an optimal low-temperature-ALD/Parylene multistack.
• In addition, we will investigate practical integration aspects to enable the use of these low-temperature multistacks as device encapsulation. An important integration aspect is the realisation of so called ‘hermetic feedthroughs’, these are ‘hermetic apertures’ made in the encapsulation stack to give room to sensors (electrodes, chemical sensors, pressure sensors,..) which need to be in direct contact with the local tissue in order to realize the medical device functionality. It is a challenge to make apertures in an hermetic encapsulation, without creating locally a leakage path for body fluids. Etch techniques, laser patterning, optimum adhesion treatments… will be evaluated to fabricate these hermetic feedthroughs.