J. Ohta

Effect of lower growth temperature on C incorporation in GeC epilayers on grown by MBE

Abstract Effect of lower growth temperature T s on C incorporation to substitutional sites in Ge 1−x C x / Si (0 0 1) grown by molecular beam epitaxy was investigated. To enhance the non-equilibrium growth condition, the temperature T s was lowered from 600°C down to 300°C. The C incorporation into substitutional sites of GeC epilayers was very sensitive to T s . X-ray diffraction (XRD) measurement indicated that the substitutional C composition x increased with decrease in T s from 600°C to 400°C. At T s ⩽350°C, the estimation of x by the XRD analysis was impossible because of polycrystallization. The Raman shift measurement enables to estimate x for T s ⩽350°C, as consequently larger x than that grown at T s =400°C was verified. The enhancement of non-equilibrium growth condition by decreasing T s was important to increase x .

Light-controlled retinal stimulator for subretinal implantation

We present a retinal stimulator embedded a light-controlled function for subretinal implantation. The stimulator is based on a multiple-microchip architecture which we have been developed to realize over 1000 stimulus electrodes. Each microchip is controlled retinal stimulation by the intensity of light. The fabricated stimulator is evaluated by implanting a rabbit eye and stimulating retinal cells. The EEP signals were successfully obtained.

A 16/spl times/16-pixel pulse-frequency-modulation based image sensor for retinal prosthesis

We have designed and fabricated a 16/spl times/16-pixel retinal prosthesis device using a pulse-frequency-modulation (PFM) photosensor. This chip is a prototype for demonstrating applications in in-vitro electrophysiological experiments. Each pixel has a bonding pad on which a stimulus electrode is formed by a Pt/Au stacked biocompatible bump electrode. We have evaluated the PFM retinal implant chip by in-vitro electrophysiological experiments using detached frog retina, and confirmed that it can electrically stimulate the retinal cells effectively.

A high-precision CMOS biophotometry sensor with noise cancellation and two-step A/D conversion

Fluorescence biophotometry measurements require wide dynamic range (DR) and high sensitivity laboratory apparatus. Indeed, it is often very challenging to accurately resolve the small fluorescence variations in presence of high background tissue autofluorescence. There is a great need for smaller detectors combining high linearity, high sensitivity, and high-energy efficiency. This paper presents a new high-dynamic range CMOS photodetector embedding a photosensor and a high-precision two-step analog-to-digital converter (ADC) with a noise cancellation scheme. In this system, a 16-bit two-step ADC sucessivley uses an integrating ADC and a successive approximation register (SAR) ADC enabling wide dynamic range and high energy-efficiency photocurrent quantization. Noise cancellation is achieved through a SAR digital-to-analog (DAC) capacitor bank to store and subtract the low-frequency noise from the output of a capacitive transimpedance amplifier (CTIA) throughout each data conversion. The 6-most significant bits are resolved through the integrating ADC, while the 10-least significant bits are extracted by the SAR ADC. The two-step data converter uses a hardware sharing scheme to decrease the chip size and to improve energy-efficiency. The proposed optoelectronic detector is implemented in a 0.18-µm CMOS technology, consuming 60 µW from a 3.3-V supply voltage while achieving a DR of 94 dB, a minimum detectable current of 200-ƒArms, at 1-kS/s sampling rate. The proposed biosensor presents a FOM of 1.46 pJ/conv. which is among the best reported performance among similar systems.

Improvement of a pulse frequency modulation based photosensor for retinal prosthesis

This paper describes improvements in output waveform of pulse frequency modulation (PFM) photosensor to be effectively adapted for retinal prosthesis. A 32/spl times/32-pixel PFM photosensor array chip is fabricated using 0.6 /spl mu/m CMOS technology and demonstrated the improved functions. Also the output pulse pattern is displayed with 32/spl times/32-LED array to visualize the pulse output waveform. The next version of a PFM vision chip is designing for in vivo experiment, in which in-pixel image processing functions in pulse domain are employed.