Risato Kobayashi

New heterogeneous multi-chip module integration technology using self-assembly method

We have newly proposed heterogeneous multi-chip module integration technologies in which MEMS and LSI chips are mounted on Si or flexible substrates using a self-assembly method. A large numbers of chips were precisely and simultaneously self-assembled and bonded onto the substrates with high alignment accuracy of approximately 400 nm. Thick MEMS and LSI chips with a thickness of more than 100 mum were electrically connected by unique lateral interconnections formed crossing over chip edges with large step height. We evaluated fundamental electrical characteristics using daisy chains formed crossing over test chips which were face-up bonded onto the substrates by the self-assembly. We obtained excellent characteristics in these daisy chains. In addition, RF test chips with amplitude shift keying (ASK) demodulator and signal processing circuits were self-assembled onto the substrates and electrically connected by the lateral interconnections. We confirmed that these test chips work well.

3D heterogeneous opto-electronic integration technology for system-on-silicon (SOS)

We proposed 3D heterogeneous opto-electronic integration technology for system-on-silicon (SOS). In order to realize 3D opto-electronic integrated system-on-silicon (SOS), we developed novel heterogeneous integration technology of LSI, MEMS and optoelectronic devices by implementing 3D heterogeneous opto-electronic multi-chip module composed with LSI, passives, MEMS and optoelectronic devices. The electrical interposer mounted with amplitude shift keying (ASK) LSI, LC filter and pressure-sensing MEMS chips and the optical interposer embedded with vertical-cavity surface-emitting laser (VCSEL) and photodiode (PD) chips are precisely bonded to form 3D opto-electronic multi-chip module. Opto-electronic devices are electrically connected via through-silicon vias (TSVs) which were formed into the interposers. Micro-fluidic channels are formed into the interposer by wafer direct bonding technique. 3D heterogeneous opto-electronic multi-chip module is successfully implemented for the first time.

Self-Assembly of Chip-Size Components with Cavity Structures: High-Precision Alignment and Direct Bonding without Thermal Compression for Hetero Integration

New surface mounting and packaging technologies, using self-assembly with chips having cavity structures, were investigated for three-dimensional (3D) and hetero integration of complementary metal-oxide semiconductors (CMOS) and microelectromechanical systems (MEMS). By the surface tension of small droplets of 0.5 wt% hydrogen fluoride (HF) aqueous solution, the cavity chips, with a side length of 3 mm, were precisely aligned to hydrophilic bonding regions on the surface of plateaus formed on Si substrates. The plateaus have micro-channels to readily evaporate and fully remove the liquid from the cavities. The average alignment accuracy of the chips with a 1 mm square cavity was found to be 0.4 mm. The alignment accuracy depends, not only on the area of the bonding regions on the substrates and the length of chip periphery without the widths of channels in the plateaus, but also the area wetted by the liquid on the bonding regions. The precisely aligned chips were then directly bonded to the substrates at room temperature without thermal compression, resulting in a high shear bonding strength of more than 10 MPa.

Comparison of electrode materials for the use of retinal prosthesis

Retinal implants may provide vision for people suffering from photoreceptor degeneration caused by different eye diseases. Electrode size in retinal implant should be decreased in order to increase the resolution provided by the implant. We defined electric properties of five different electrode materials (Au, Ir-b, Ti, TiN, Pt-b) widely used in retinal prostheses. The comparison of different electrode materials requires that the electrical properties of different materials are defined using exactly the same measurement conditions and devices. Existing studies about electrode material properties are often made using slightly different measurement parameters or electrode processing conditions making the comparison between different materials difficult. Here, the electrochemical characterization included cyclic voltammetry and electrochemical impedance spectroscopy. Ir-b and Pt-b had greater charge injection capacity than other materials. The fabricated material samples showed that in this experiment the electrode diameter larger than 200 μm should be used to suppress irreversible reaction of stimulus electrodes with the needed stimulus currents. Thus, either we have to find novel electrode materials or surface treatment methods to decrease the electrode area providing increased electrode and pixel number of the prosthesis or we have to show that stimulus currents smaller than 40 μA are enough to induce phosphenes.

Evaluation of Platinum-Black Stimulus Electrode Array for Electrical Stimulation of Retinal Cells in Retinal Prosthesis System

A retinal prosthesis system with a three-dimensionally (3D) stacked LSI chip has been proposed. We fabricated a new implantable stimulus electrode array deposited with Platinum-black (Pt-b) on a polyimide-based flexible printed circuit (FPC) for the electrical stimulation of the retinal cells. Impedance measurement of the Pt-b electrode–electrolyte interface in a saline solution was performed and the Pt-b electrode realized a very low impedance. The power consumption at the electrode array when retinal cells were stimulated by a stimulus current was evaluated. The power consumption of the Pt-b stimulus electrode array was 91% lower than that of a previously fabricated Al stimulus electrode array due to a convexo-concave surface. In the cytotoxicity test (CT), we confirmed that Pt implantation induced no cellular degeneration of the rat retina. In the animal experiments, electrically evoked potential (EEP) was successfully recorded using Japanese white rabbits. These results indicate that electrical stimulation using the Pt-b stimulus electrode array can restore visual sensation.

Power Supply System Using Electromagnetic Induction for Three-Dimensionally Stacked Retinal Prosthesis Chip

We have proposed a new retinal prosthesis system consisting of a three-dimensionally (3-D) stacked retinal prosthesis chip, a flexible cable with an electrode array stimulus, and a power supply system for the retinal prosthesis chip. Electromagnetic induction with a primary coil and a secondary coil was employed as the power supply system. The power was transmitted to the retinal prosthesis chip through an RF/DC voltage conversion chip that converted AC voltage into DC voltage. The 3-D stacked retinal prosthesis chip operates with a DC supply voltage of 3.3 V, and the secondary coil requires a transmitted voltage that is 1 V higher than the DC supply voltage. In order to receive a sufficient supply voltage, several parameters such as external supply voltage and transmission frequency were optimized. A peak RF voltage of 4.5 V was obtained when we employed a primary coil with 50 turns, a secondary coil with 20 turns, an external supply voltage of 4.1 V, and a frequency of 3 MHz. We also successfully fabricated Schottky barrier diodes to rectify the RF voltage received by the secondary coil. The fabricated Schottky barrier diodes have a high breakdown voltage of 4.6 V, which is sufficient for our 3-D stacked retinal prosthesis chip.

Development of Si neural probe with optical waveguide for highly accurate optical stimulation of neuron

Lots of researchers take great interests in brain science. In this area, measurement devices of the brain have been developed by number of groups, and played an important role. Recently, for neuroengineering applications such as optogenetics, optical stimulation devices which consisted of optical fibers have been reported, and used to deliver light to neurons expressing light sensitive channel proteins. However, accurate stimulation of neurons could not be achieved by using these devices because of optical fibers with a diameter of over 100 μm. Here, we have proposed a novel Si neural probe with micromachined optical waveguide for multiple and precise optical stimulations of neurons. SiN film was employed as the optical waveguide core due to its optical transmission characteristics. Both the light propagation in the optical waveguide and controllability of output patterns of the light were clearly confirmed by optical experiments using a blue laser. In vitro experiments of optical stimulation of neurons using the fabricated Si neural probe were performed. A CA1 area of a thin hippocampal slice obtained from the brain of a transgenic rat expressing Channelrhodopsin-2 (ChR2) was employed. We stimulated neurons optically using the Si neural probe and successfully observed increases of firing rates in neurons accordingly with the light exposure.

Development of Si Double-Sided Microelectrode for Platform of Brain Signal Processing System

We have proposed a new implantable neural recording system, which we call the brain signal processing system (BSPS). In this system, LSI chips such as amplifiers, analog-to-digital converters, and multiplexers are integrated on the Si microelectrode array. To analyze the brain functions or to develop medical treatments for brain disorders, a high-density recording of action potentials is strongly required. To realize high-density recording of action potentials, we propose a novel Si double-sided microelectrode that has recording sites on both front and back sides. The back-side recording sites are connected to a recording apparatus by wire bonding through Si via holes. We fabricated the carefully designed Si double-sided microelectrode and evaluated the electrical characteristics of the Si microelectrode. The front- and back-side recording sites had impedance values of 2.5 and 2.7 MΩ at 1 kHz, respectively, which indicated that both recording sites have equivalent characteristics. An in vitro experiment of neuronal action potential recording using the fabricated Si double-sided microelectrode was performed. The CA1 areas of 400-µm-thick hippocampal slices obtained from the brains of guinea pigs were employed, and we successfully recorded neuronal action potentials from the recording sites of both sides.

Development of Si Neural Probe with Microfluidic Channel Fabricated Using Wafer Direct Bonding

This paper reports the development of a novel Si neural probe with a microfluidic channel to deliver drugs into neural tissue. We fabricated this Si neural probe using wafer direct bonding. To confirm the fluidic capability of the fabricated Si probe, we demonstrated the ejection of liquid from the microfluidic channel using a syringe pump. We confirmed that the Si probe had sufficient bonding strength to eject liquid. In addition, we investigated the pressure drops in the microfluidic channel. From the results, we observed a linear relationship between the flow rate and the pressure drop. Since this result agreed well with the calculated values, we confirmed that the microfluidic channel was successfully formed by wafer direct bonding.

Development of double-sided Si neural probe with microfluidic channels using wafer direct bonding technique

We have proposed the intelligent Si neural probe system which can realize high density and multifunctional recording of neuronal behaviors. In this device, LSI chips such as amplifiers, A/D converters, and multiplexers are integrated on the intelligent Si neural probe. In this paper, we report the development of a novel Si neural probe with microfluidic channels which is the key part of the intelligent Si neural probe system. The Si neural probe has microfluidic channels fabricated using a wafer bonding technique to deliver drugs into the brain when neuronal action potentials are recorded. Furthermore, our Si neural probe has recording sites on both front- and back-side of Si to realize high density recording. We fabricated the carefully-designed Si neural probe, and evaluated characteristics of microfluidic channels. From the liquid ejection test, we confirmed that there was no void at bonding interfaces. We observed the liner relationship between the flow rate and the pressure drop, and the relationship was identical to that from the calculation, which indicated that the microfluidic channel was successfully formed. In addition, we fabricated the Si neural probe for in vivo neural recording. Both front- and back-side recording sites of the fabricated Si neural probe had impedance values of 1.5 MΩ and 1.2 MΩ at 1 kHz, respectively, which indicated that both recording sites had equivalent characteristics. The neuronal action potentials in motor area of Japanese macaques brain were successfully recorded by using the fabricated Si neural probe.