Area Selective Atomic Layer Deposition for the Use on Active Implants: An Overview of Available Process Technology

Abstract

Area-selective atomic layer deposition (ASD) is a bottom-up process that is of particular importance in the semiconductor industry, as it prevents edge defects and avoids cost-intensive lithography steps. This approach not only offers immense potential for the manufacture of active implants but can also be used to improve them. This review paper presents various processes that can be used for this purpose. It also identifies aspects that shall be considered when implementing such a process for medical applications. For example, the inherent selectivity can be used to produce new biosensors, the passivated ASD can be used to encapsulate polymer-based implants, and the activated ASD can be used to improve electrode performance. Finally, the aspects that shall be considered in a coating for active implants are highlighted. ASD therefore offers great potential for use on active implants.

Keywords: active implantable medical devices; area selective atomic layer deposition; coating technologies; encapsulation strategies; surface reactions.

Figure 1

a) Examples of neural implants. Deep brain stimulation is used for the therapy of Parkinson's disease and essential tremor, brain‐machine interfaces are used for the control of external devices, the cochlear implant as well as retinal prostheses are used for the restoration of hearing and vision, respectively. Vagus nerve stimulation is used for the treatment of epilepsy, the pacemaker for the treatment of cardiac arrhythmia, and spinal cord stimulation for the treatment of chronic pain. The grasp prosthesis is used for gripping movement and sacral nerve stimulation is used to treat incontinence. The drop foot stimulator restores the gait pattern. b) Example of the “Brainboard”, a flexible implant to record activities in the brain. Reproduced with permission from NMI Reutlingen, Germany. c) Example of a Chip for subretinal electrical stimulation manufactured by the Retina Implant AG.

Figure 2

Schematic cross section of a neurotechnical interface with possible ASD processes. a) Sidewall passivation of the electrodes using self‐assembled monolayer (SAM) or small molecule inhibitor (SMI). By using these molecules, the layer growth on the electrode surface is prevented so that a layer is only deposited on the polymer (e.g. polyimide). b) Utilization of the catalytic effect of certain noble metals as electrode material. Platinum dissociates oxygen on the surface, creating an active surface that allows selective coating. c) Enlargement of the electrode surface by topographically selective ALD. Using ion implantation, the horizontal surface is passivated by fluorine compounds so that ALD growth can only take place on vertical surfaces.

Figure 3

Advantages of area selective atomic layer deposition compared to conventional sputter coating processes for the production of high‐performance electrodes using IrO_x.

Figure 4

Schematic representation of the reaction mechanisms during the ALD process and fundamental reaction for achieving the area‐selective atomic layer deposition process.

Figure 5

Comparison between SAM molecule and an SMI. The SMI has no backbone. Adapted with permission^{[ 97 ]} 2021, Journal of Vacuum Science & Technology A.