Nathan Jackson

Influence of aluminum nitride crystal orientation on MEMS energy harvesting device performance

Aluminum nitride (AlN) is a widely researched piezoelectric material due to its CMOS compatibility. One of the most common applications for AlN is in the area of vibrational energy harvesting. The piezoelectric quality of AlN is related to the crystal orientation of the film and optimal conditions are obtained when AlN is c-axis aligned with a (0 0 2) orientation. AlN can be a challenging material to integrate into a fabrication process due to orientation dependency of the fabrication process. This paper reports on the effects of non-(0 0 2) oriented AlN peaks on an energy harvesting MEMS cantilever structure. Results show that FWHM values of the AlN films from different wafers were approximately the same 8.5°, 8.7°, and 9°, however wafer 1 had additional peaks at (1 0 2) and (1 0 3), which significantly affected the piezoelectric constants and the amount of power generated. The measured d31 value for the wafers were 2.04, 1.97, and 0.84 pm V−1, and the power generated was 0.67, 0.64, and 0.24 µW respectively. These values show that non-peaks of AlN can cause a significant decrease in the piezoelectric constant, which causes significant decrease in the ability to generate power from an AlN film.

Flexible-CMOS and biocompatible piezoelectric AlN material for MEMS applications

The development of a CMOS compatible flexible piezoelectric material is desired for numerous applications and in particular for biomedical MEMS devices. Aluminum nitride (AlN) is the most commonly used CMOS compatible piezoelectric material, which is typically deposited on Si in order to enhance the c-axis (002) crystal orientation which gives AlN its high piezoelectric properties. This paper reports on the successful deposition of AlN on polyimide (PI-2611) material. The AlN deposited has a FWHM (002) value of 5.1 and a piezoelectric d33 value of 1.12 pm V 1 , and SEM images show high quality columnar grains. The highly crystalline AlN material is due to the semi-crystalline properties of the polyimide film used. Cytotoxicity testing showed the AlN/polyimide material to be non-toxic to 3T3 cells and primary neurons. Surface properties of the AlN/polyimide film were evaluated as they have a significant effect on the adhesion of cells to the film. The results show neurons adhering to the AlN surface. The results of this paper show the characterization of a new flexible-CMOS and biocompatible AlN/polyimide material for MEMS devices with improved crystallinity and piezoelectric properties. (Some figures may appear in colour only in the online journal)

Flexible Chip-Scale Package and Interconnect for Implantable MEMS Movable Microelectrodes for the Brain

We report here a novel approach called microelectromechanical systems (MEMS) microflex interconnect (MMFI) technology for packaging a new generation of bioMEMS devices that involve movable microelectrodes implanted in brain tissue. MMFI addresses the need for the following: (1) operating space for movable parts and (2) flexible interconnects for mechanical isolation. We fabricated a thin polyimide substrate with embedded bond pads, vias, and conducting traces for the interconnect with a backside dry etch, so that the flexible substrate can act as a thin-film cap for the MEMS package. A double-gold-stud-bump rivet-bonding mechanism was used to form electrical connections to the chip and also to provide a spacing of approximately 15-20 mum for the movable parts. The MMFI approach achieved a chip-scale package that is lightweight and biocompatible and has flexible interconnects and no underfill. Reliability tests demonstrated minimal increases of 0.35, 0.23, and 0.15 mOmega in mean contact resistances under high humidity, thermal cycling, and thermal shock conditions, respectively. High-temperature tests resulted in increases of > 90 and ~ 4.2 mOmega in resistance when aluminum and gold bond pads were used, respectively. The mean time to failure was estimated to be at least one year under physiological conditions. We conclude that MMFI technology is a feasible and reliable approach for packaging and interconnecting bioMEMS devices.

Energy Harvesting from Train-Induced Response in Bridges

The integration of large infrastructure with energy-harvesting systems is a growing field with potentially new and important applications. The possibility of energy harvesting from ambient vibratio ...

Long-Term Neural Recordings Using MEMS Based Movable Microelectrodes in the Brain

One of the critical requirements of the emerging class of neural prosthetic devices is to maintain good quality neural recordings over long time periods. We report here a novel MEMS (Micro Electro Mechanical Systems) based technology that can move microelectrodes in the event of deterioration in neural signal to sample a new set of neurons. Microscale electro-thermal actuators are used to controllably move microelectrodes post-implantation in steps of approximately 9 μm. In this study, a total of 12 movable microelectrode chips were individually implanted in adult rats. Two of the twelve movable microelectrode chips were not moved over a period of 3 weeks and were treated as control experiments. During the first 3 weeks of implantation, moving the microelectrodes led to an improvement in the average signal to noise ratio (SNR) from 14.61 ± 5.21 dB before movement to 18.13 ± 4.99 dB after movement across all microelectrodes and all days. However, the average root-mean-square values of noise amplitudes were similar at 2.98 ± 1.22 μV and 3.01 ± 1.16 μV before and after microelectrode movement. Beyond 3 weeks, the primary observed failure mode was biological rejection of the PMMA (dental cement) based skull mount resulting in the device loosening and eventually falling from the skull. Additionally, the average SNR for functioning devices beyond 3 weeks was 11.88 ± 2.02 dB before microelectrode movement and was significantly different (p < 0.01) from the average SNR of 13.34 ± 0.919 dB after movement. The results of this study demonstrate that MEMS based technologies can move microelectrodes in rodent brains in long-term experiments resulting in improvements in signal quality. Further improvements in packaging and surgical techniques will potentially enable movable microelectrodes to record cortical neuronal activity in chronic experiments.

Single neuronal recordings using surface micromachined polysilicon microelectrodes

Bulk micromachining techniques of silicon have been used successfully in the past several years to microfabricate microelectrodes for monitoring single neurons in acute and chronic experiments. In this study we report for the first time a novel surface micromachining technique to microfabricate a very thin polysilicon microelectrode that can be used for monitoring single-unit activity in the central nervous system. The microelectrodes are 3 mm long and 50 microm x 3.75 microm in cross-section. Excellent signal to noise ratios in the order of 25-35 dB were obtained while recording neuronal action potentials. The microelectrodes successfully penetrated the brains after a microincision of the dura mater. Chronic implantation of the microprobe for up to 33 days produced only minor gliosis. Since the polysilicon shank acts as a conductor, additional processing steps involved in laying conductor lines on silicon substrates are avoided. Further, surface micromachining allows for fabricating extremely thin microelectrodes which could result in decreased inflammatory responses. We conclude that the polysilicon microelectrode reported here could be a complementary approach to bulk-micromachined silicon microelectrodes for chronic monitoring of single neurons in the central nervous system.

Artificial dural sealant that allows multiple penetrations of implantable brain probes.

This study reports extensive characterization of the silicone gel (3-4680, Dow Corning, Midland, MI), for potential use as an artificial dural sealant in long-term electrophysiological experiments in neurophysiology. Dural sealants are important to preserve the integrity of the intracranial space after a craniotomy and in prolonging the lifetime and functionality of implanted brain probes. In this study, we report results of our tests on a commercially available silicone gel with unique properties that make it an ideal dural substitute. The substitute is transparent, elastic, easy to apply, and has re-sealing capabilities, which makes it desirable for applications where multiple penetrations by the brain probe is desirable over an extended period of time. Cytotoxicity tests (for up to 10 days) with fibroblasts and in vivo tests (for 12 weeks) show that the gel is non-toxic and does not produce any significant neuronal degeneration when applied to the rodent cortex even after 12 weeks. In vivo humidity testing showed no sign of CSF leakage for up to 6 weeks. The gel also allows silicon microprobes to penetrate with forces less than 0.5 mN, and a 200-microm diameter stainless steel microprobe with a blunt tip to penetrate with a force less than 2.5 mN. The force dependency on the velocity of penetration and thickness of the gel was also quantified and empirically modeled. The above results demonstrate that the silicone gel (3-4680) can be a viable dural substitute in long-term electrophysiology of the brain.

Ultrasound induced increase in excitability of single neurons

The aim of this study was to carefully assess the level of modulation in electrical excitability of single neurons with the application of high frequency ultrasound. High frequency tone bursts of ultrasound have been shown to dramatically increase the spike frequency of primary hippocampal neurons in culture. In addition, these ultrasonic bursts also induce silent or still developing neurons to fire. Results indicate that the increase in excitability is largely mediated by mechanical effects and not thermal effects of ultrasound. Future studies on culture models exposed to varying ultrasound protocols may provide insight into the feasibility of using ultrasound as a means for neurostimulation studies conducted on brain slice and in vivo models.

Early onset of electrical activity in developing neurons cultured on carbon nanotube immobilized microelectrodes

In this study, we test the hypothesis that increased surface roughness due to carbon nanotubes (CNTs) enhances neuronal adhesion and consequently electrical excitability of single neurons. Neurons are grown on CNT modified microelectrode arrays (MEAs). Multi-unit activity was seen as early as 4 days after seeding compared to 7 days in control cultures on microelectrodes without CNTs. The results overall, show earlier onset and higher level of electrical activity in neurons seeded on CNT modified MEAs compared to non-modified control MEAs. We conclude that CNTs on microelectrodes enhance electrical excitability of single neurons in culture.

A diaphragm based piezoelectric AlN film quality test structure

Aluminum nitride (AlN) is becoming a commonly used piezoelectric material for various applications due to its compatibility with CMOS processing. However, the piezoelectric properties of AlN are highly dependent on the deposition process and the underlying layers, and typically require several test structures in order to determine the quality of the film. This paper highlights a MEMS based diaphragm test structure which allows various types of material characterization to be tested, in order to determine the quality of the AlN film on a bulk micromachined device wafer.