Hercules Pereira Neves

Pseudo-Two-Dimensional Model for Double-Gate Tunnel FETs Considering the Junctions Depletion Regions

This paper presents a pseudo-2-D surface potential model for the double-gate tunnel field-effect transistor (DG-TFET). Analytical expressions are derived for the 2-D potential, electric field, and generation rate, and used to numerically extract the tunneling current. The model predicts the device characteristics for a large range of parameters and allows gaining insight on the device physics. The depletion regions induced inside the source and drain are included in the solution, and we show that these regions become critical when scaling the device length. The fringing field effect from the gates on these regions is also included. The validity of the model is tested for devices scaled to 10-nm length with SiO2 and high-¿ dielectrics by comparison to 2-D finite-element simulations.

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.

Development of Modular Multifunctional Probe Arrays for Cerebral Applications

Recordings from the brain have been used for decades to investigate the activity of individual neurons. However, the complex interaction between electrical and chemical signals with respect to short and long term changes of morphology and information transfer is still poorly understood. We introduce a new modular approach for multifunctional probe arrays for cerebral applications that will enable the addressing of fundamental questions in neuroscience. Our approach allows the individual assembly of multiple probes with customized architecture into three-dimensional arrays to address specific brain regions, including sulci of highly folded cortices such as those of humans. In this paper, we introduce the system approach that allows the integration of recording and stimulation electrodes, biosensors, microfluidics and integrated electronics, all sharing a common backbone. We present the first prototypes of multichannel electrodes, flexible ribbon cables, a backbone platform and the first telemetry unit.

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.

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.

A Low-Voltage Large-Displacement Large-Force Inchworm Actuator

Inchworm microactuators are popular in micropositioning applications for their long ranges. However, until now, they could not be considered for applications such as in vivo biomedical applications because of their high input voltages. This paper reports on the modeling, design, fabrication, and testing of a new family of pull-in-based electrostatic inchworm microactuators which provides a solution to this problem. Actuators with only 7-V operating voltage are achieved with a plusmn18-mum total range and a plusmn30-muN output force. Larger operating voltage (16 V) actuators show even better results in force (plusmn110 muN) and range (plusmn35 mum). The actuator has an in-plane angular deflection conversion which provides a force-displacement tradeoff and allows us to set step sizes varying from few nanometers to few micrometers with a minor change in design. In this paper, we designed 1- and 4-mum step-size devices. The actuator step size may change during the operation because of the slipping of the shuttle and the beam bending; however, our model successfully explains the reasons. One of our actuator prototypes has survived more than 25 million cycles without performance deterioration. The device is fabricated using the silicon-on-insulator-based multiuser MEMS process.

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.

Scaling the Suspended-Gate FET: Impact of Dielectric Charging and Roughness

Suspended gate field-effect transistors (SG-FETs) with switching gates are interesting as digital logic switches because of their high I on/I off current ratio and their infinite subthreshold slope. However, the limits of scalability of the SG-FETs are still unclear. This paper investigates two effects that could limit scaling: the dielectric charging and the dielectric roughness. To do so, a surface-potential-based model for suspended gate transistors with a mechanically switching gate is presented and validated using experimental data. Devices fabricated in a standard complimentary metal-oxide-semiconductor process are used for the model assessment. The model reproduces the effect of a fixed charge and the effect of a nonideal contact of the gate after pull-in. We show that, at the device dimensions required to follow the International Technology Roadmap for Semiconductors, these effects will be critical.