Arno Aarts

Fabrication technology for silicon-based microprobe arrays used in acute and sub-chronic neural recording

This work presents a new fabrication technology for silicon-based neural probe devices and their assembly into two-dimensional (2D) as well as three-dimensional (3D) microprobe arrays for neural recording. The fabrication is based on robust double-sided deep reactive ion etching of standard silicon wafers and allows full 3D control of the probe geometry. Wafer level electroplating of gold pads was performed to improve the 3D assembly into a platform. Lithography-based probe-tracking features for quality management were introduced. Probes for two different assembly methods, namely direct bonding to a flexible micro-cable and platform-based out-of-plane interconnection, were produced. Systems for acute and sub-chronic recordings were assembled and characterized. Recordings from rats demonstrated the recording capability of these devices.

An interconnect for out-of-plane assembled biomedical probe arrays

An out-of-plane interconnect system has been developed for biomedical microprobes. With this type of interconnect, different microelectromechanical system (MEMS) structures can be electrically and mechanically connected perpendicular to a backbone. The probes and backbone are processed separately which enables a modular approach. The MEMS structures are inserted into cavities (which act as sockets) and the electrical contact is established by means of overhanging gold clips which are bent and squeezed between the cavity wall and the MEMS upon insertion. The assembly is done using a flip-chip bonder. Prior to the deposition of the overhanging contact blades, an extreme planarization technique is employed for the fabrication of the gold clips. Topographies of 200 µm have been planarized using benzocyclobutene as a sacrificial material. Test structures have been made and assembled perpendicular to the substrate to perform a contact resistance measurement.

Two-Dimensional Multi-Channel Neural Probes With Electronic Depth Control

This paper presents multi-electrode arrays for in vivo neural recording applications incorporating the principle of electronic depth control (EDC), i.e., the electronic selection of recording sites along slender probe shafts independently for multiple channels. Two-dimensional (2D) arrays were realized using a commercial 0.5- μm complementary-metal-oxide-semiconductor (CMOS) process for the EDC circuits combined with post-CMOS micromachining to pattern the comb-like probes and the corresponding electrode metallization. A dedicated CMOS integrated front-end circuit was developed for pre-amplification and multiplexing of the neural signals recorded using these probes.

A floating 3D silicon microprobe array for neural drug delivery compatible with electrical recording

This paper reports on the design, fabrication, assembly and characterization of a three-dimensional silicon-based floating microprobe array for localized drug delivery to be applied in neuroscience research. The microprobe array is composed of a silicon platform into which up to four silicon probe combs with needle-like probe shafts can be inserted. Two dedicated positions in the array allow the integration of combs for drug delivery. The implemented comb variants feature 8 mm long probe shafts with two individually addressable microchannels incorporated in a single shaft or distributed to two shafts. Liquid supply to the array is realized by a highly flexible 250 µm thick multi-lumen microfluidic cable made from polydimethylsiloxane (PDMS). The specific design concept of the slim-base platform enables floating implantation of the array in the small space between brain and skull. In turn, the flexible cable mechanically decouples the array from any microfluidic interface rigidly fixed to the skull. After assembly of the array, full functionality is demonstrated and characterized at infusion rates from 1 to 5 µL min−1. Further, the effect of a parylene-C coating on the water vapour and osmotic liquid water transport through the PDMS cable walls is experimentally evaluated by determining the respective transmission rates including the water vapour permeability of the used PDMS type.

Development of vertical and tapered via etch for 3D through wafer interconnect technology

Two types of dry silicon etch techniques are developed to cover two different areas of demand for interconnect technology: one for high aspect ratio (AR) vertical vias and one for tapered vias. Various sizes of vertical vias and trenches with diameters/widths ranging from 1-100 mum with an AR up to 50 are realized using Bosch deep reactive ion etch (DRIE) process. A linear model is applied to describe and to give physical insight in the aspect ratio dependant etch (ARDE) effect. The feasibility of the vertical vias as electrical interconnect is shown by isolating them from the substrate by silicon oxide and then filling with polysilicon. The tapered vias are typically post-processed on fabricated device wafers, making it inherently a more generic approach where diameter size can be large and low AR can be tolerated. Vias with a depth of ~100 mum and a diameter of ~50mum at the bottom (though larger at top) are realized. Varying various etch parameters, slope angles of 70deg-80deg are realized to allow for conformal deposition of dielectric/seed materials on the sidewalls and to allow lithography within the via. Reactive ion etch (RIE) is used to fabricate sloped vias by simultaneously applying etch and passivation gasses. Negative angles on the via top and sidewall roughness are observed that introduce conformal coating problems and increased leakage currents. Smoothening techniques using maskless wet and dry silicon etching are investigated to overcome these problems.

A 3D slim-base probe array for in vivo recorded neuron activity

This paper introduces the first experimental results of a new implantable slim-base three-dimensional (3D) probe array for cerebral applications. The probes are assembled perpendicularly into the slim-base readout platform where electrical and mechanical connections are achieved simultaneously. A new type of micromachined interconnect has been developed to establish electrical connection using extreme planarization techniques. Due to the modular approach of the platform, probe arrays of different dimensions and functionality can be assembled. The platform is only several hundred microns thick which is highly relevant for chronic experiments in which the probe array should be able to float on top of the brain. Preliminary tests were carried out with the implantation of a probe array into the auditory cortex of a rat.

Validating silicon polytrodes with paired juxtacellular recordings: method and dataset

Recording in vivo from the same neuron with two different methods is difficult. It requires blindly moving each probe to within ∼100 μm of one another and for this reason such “dual-recordings” are rare. However, comparing the signals measured by different techniques is necessary to understand what they measure. We developed a method to precisely align the axes of two manipulators and used it to gather a “ground truth” dataset for dense extracellular polytrodes.

The NeuroProbes project: A concept for electronic depth control

The European project NeuroProbes has introduced a new methodology to allow the fine positioning of electrodes within an implantable probe with respect to individual neurons. In this approach, probes are built with a very large number of electrodes which are electronically selectable. This feature is implemented thanks to the modular approach adopted in NeuroProbes, which will allow the implementation of integrated electronics both along the probe shaft and on the array backbone.

Integration of silicon-based neural probes and micro-drive arrays for chronic recording of large populations of neurons in behaving animals.

OBJECTIVE Understanding how neuronal assemblies underlie cognitive function is a fundamental question in system neuroscience. It poses the technical challenge to monitor the activity of populations of neurons, potentially widely separated, in relation to behaviour. In this paper, we present a new system which aims at simultaneously recording from a large population of neurons from multiple separated brain regions in freely behaving animals. APPROACH The concept of the new device is to combine the benefits of two existing electrophysiological techniques, i.e. the flexibility and modularity of micro-drive arrays and the high sampling ability of electrode-dense silicon probes. MAIN RESULTS Newly engineered long bendable silicon probes were integrated into a micro-drive array. The resulting device can carry up to 16 independently movable silicon probes, each carrying 16 recording sites. Populations of neurons were recorded simultaneously in multiple cortical and/or hippocampal sites in two freely behaving implanted rats. SIGNIFICANCE Current approaches to monitor neuronal activity either allow to flexibly record from multiple widely separated brain regions (micro-drive arrays) but with a limited sampling density or to provide denser sampling at the expense of a flexible placement in multiple brain regions (neural probes). By combining these two approaches and their benefits, we present an alternative solution for flexible and simultaneous recordings from widely distributed populations of neurons in freely behaving rats.

Two-dimensional multi-channel neural probes with electronic depth control

Multi-electrode arrays for in vivo neural recording are presented incorporating the principle of electronic depth control, i.e. an electronic selection of electrode locations along the probe shaft independently for multiple channels. Two-dimensional (2D) arrays are realized using a commercial CMOS process for the electronic circuits combined with post-CMOS micromachining for shaping the probes and electrode metallization. These 2D arrays can be further assembled into 3D arrays. Two-dimensional arrays with IrO x metal finish show electrode impedances between 100 KΩ and 1 MΩ. In vivo tests demonstrate the capability to simultaneously record multi-unit activity in addition to local field potentials on all 32 available output channels of the probe combs. Electronic steering enabled some of the electrodes to record from cortical and others to record from thalamic sites in the rat. This new device significantly increases the amount of useful information that can be obtained from a single experiment.