Takayuki Takao

Fabrication of Back-Side Illuminated Complementary Metal Oxide Semiconductor Image Sensor Using Compliant Bump

We fabricated a back-side illuminated (BSI) complementary metal oxide semiconductor (CMOS) image sensor in which a very-thin BSI photodiode array chip was stacked on a CMOS read-out circuit chip by compliant bumps. Cone-shaped bumps made of Au were prepared as the compliant bumps. The base diameter was 10–12 µm and the height was 9–10 µm. To fabricate the BSI CMOS image sensor, we developed a novel thin-chip assembly process. The key features of the process are as follows: preparation of a photodiode array wafer and a CMOS read-out circuit wafer, Au cone bump formation, bonding to support glass, thinning of the photodiode array wafer to 21 µm, through silicon via (TSV) formation using Cu electroplating, formation of back-side electrodes, transfer of the photodiode array wafer to a polymer support tape, dicing of the photodiode array wafer, separation of support tape, formation of Ni–Au bumps, dicing of CMOS read-out circuit wafer, and three-dimensional (3D) chip-stacking. The BSI CMOS image sensor thus fabricated has the following specifications: number of active pixels is 16,384 (128 ×128), photodiode size is approximately 18 µm square, photodiode pitch is 24 µm, and fill factor is approximately 55%. No defects were observed in the obtained image frames.

Thin-Layer Ablation of Metals and Silicon by Femtosecond Laser Pulses for Application to Surface Analysis

For the purpose of thin-layer surface analysis, comparison of laser ablation by 24 ns ArF laser pulses at 193 nm and that by 150 fs Ti:sapphire laser pulses at 780 nm for samples ranging from metals to semiconductors is reported. In contrast to nanosecond laser pulses, the ablation threshold for femtosecond laser pulses could be reduced significantly. In particular, the threshold for a Si sample was reduced to about one order of magnitude, and an ablation rate of 0.48 nm/shot was obtained at 5% above the threshold fluence. The root-mean-square (RMS) growth rate in the roughness of the ablated surface was 0.05 nm/shot. These results show the advantages of femtosecond laser ablation for trace element analysis of a wide range of materials, coupled with laser-induced fluorescence (LIF) spectroscopy.

Operation of rhodamine 6G dye laser in water solution

The ethanolic solution of Rhodamine 6G is used for dye lasers in the atomic vapor laser isotope separation (AVLIS) system. However, the water solution is preferable from the standpoint of easy treatment and safety. In this paper, we systematically investigated the laser efficiency of a Rhodamine 6G dye laser pumped by a frequency-doubled Nd:YAG laser, for various water-based solutions with surface-active agents and alcohols. By adding an appropriate amount of surface-active agent (Triton X or Emulgen) or alcohol (ethanol or propanol), we obtained almost the same level of laser efficiency as in the pure ethanolic solution. The improvement in the laser efficiency with these additives was attained by the deaggregation of nonradiative dimers. It was also confirmed that the water-based solution showed better performance against thermal distortion than the ethanolic solution, and was suitable for copper vapor laser (CVL) pumping.

Room-temperature hermetic sealing by ultrasonic bonding with Au compliant rim

We newly introduce a compliant rim to realize hermetic sealing of electronic components at low temperature. The compliant rim easily deforms under pressing load owing to its cone-shaped cross section and, therefore, intermetallic bonding can be performed at low temperature. We demonstrate the room-temperature vacuum sealing using the compliant rim made of Au with the aid of ultrasonic vibration of submicron amplitude. A test vehicle fabricated using silicon and glass showed that the air leak rate of the room-temperature sealing was well below 1 ? 10?12 Pa?m3/s, which is sufficiently low for use in vacuum packaging.

Nanometer-Scale Depth Resolution and Sensitive Surface Analysis Using Laser Ablation Atomic Fluorescence Spectroscopy

An extremely high-resolution, nanometer-resolution, solid-surface trace element detection has been demonstrated. Nanometer-thinned pulsed laser ablation was combined with extremely sensitive laser-induced fluorescence spectroscopy, and depth resolution of 3.6 nm was experimentally demonstrated for the first time on sodium detection in polymeric samples. An extremely high absolute detection limit of 25.2 fg was also obtained, and a theoretical calculation program was also generated to analyze the results.

Room-temperature vacuum packaging using ultrasonic bonding with Cu compliant rim

In this study, we demonstrate room-temperature vacuum sealing by combining Cu compliant rim with ultrasonic assist.

Compliant bump technology for back-side illuminated CMOS image sensor

We have developed a compliant bump technology for 3D chip stacking with the same number of inter-chip connections as that in a VGA (video graphic array, 640 × 480). Using this technology together with a through-Si via (TSV) technology, we demonstrate a prototype of back-side illuminated CMOS image sensor, in which a very-thin rear-illuminated photodiode array is electrically connected to the CMOS readout circuit at a pixel level.

Influence of Irradiation Wavelength on Atomic Emission Spectroscopy Using Laser Ablation

In this study, the influence of laser wavelengths on atomic emission spectroscopy using laser ablation for samples (glass, polymer, Si, and Cu) has been investigated using 1064 nm and 266 nm pulses of a Q-switched Nd:YAG laser. The threshold fluence is decreased and the emission intensity is increased for UV laser ablation, compared with IR laser ablation. Simultaneous irradiation by both IR and UV pulses has also been found in decreasing the threshold and increasing the emission intensity.

Analysis of trace element in solids by using laser ablation atomic fluorescence spectroscopy

Summary form only given. We have proposed a very sensitive detection technique of trace element in samples such as polymers, metals and semiconductors. It is called laser ablation atomic fluorescence (LAAF) spectroscopy, which combines laser ablation process to atomize the sample surface and laser-induced fluorescence (LIF) spectroscopy to make a quantitative analysis of the atoms in the ablated plume. LIF provides selective excitation of the trace element to avoid interference due to the resonant excitation with the probe laser. In this study, we demonstrated the availability of LAAF spectroscopy for the nano-meter scale analysis of the sample surface.

Active control of the ablation plume for laser ablation atomic fluorescence spectroscopy

We have developed an extreme sensitive trace element detection technique that has been labeled Laser ablation atomic fluorescence (LAAF) spectroscopy, and applied to a nanometer-scale solid surface analysis. The absolute weight of the detection limit of 870 ag (10-18g) and high depth resolution of 3.6 nm had been demonstrated in trace sodium detection of polymethylmethacrylate. As a laser ablation was used in the LAAF spectroscopy, the behavior of the ablation plume is a very important factor for high sensitivity. So, we tried to control the plume by a buffer gas and an assist mask for more sensitive analysis. The diffusion velocity of the ablated particles was modified in collision with the gas molecules. Furthermore, it was found that the form of the plume was changed by the mask. Thus, improvement of the detection sensitivity of the LAAF is expected using this approach.