Takahiro Nishimoto

Studies on SiO2-SiO2 bonding with hydrofluoric acid. Room temperature and low stress bonding technique for MEMS

Abstract Studies on SiO 2 –SiO 2 bonding with hydrofluoric acid (HF) are described. This method has a remarkable feature that bonding can be obtained at room temperature. Advantages of this method are low thermal damage, low residual stress and simplicity of the bonding process, which are expected for the packaging and assembly of micro-electro-mechanical systems (MEMS). The bond characteristics were measured under different bonding conditions of HF concentration, applied pressure, another chemicals for bonding and so on. The bond strength depends on the applied pressure during bonding. To achieve reliable bonding, HF concentration of higher than 0.5 wt.% and a large applied pressure of 1.3 MPa are required. The bonding is also observed using KOH solution in stead of HF. Transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), radioactive isotope (RI) analysis and electron probe micro analysis (EPMA) were applied to evaluate the bonded interface. The results of these analysis indicated that an interlayer of a silicon oxide complex including hydrogen and fluorine atoms is formed between bonded SiO 2 to SiO 2 . The thickness of the interlayer depends strongly on the applied pressure during bonding. Large bond strength is obtained when the interlayer is thin. The bonding mechanism is expected when the SiO 2 at both surfaces is dissolved in HF solution, and that the interlayer, which is a binding layer, is formed between substrates by resolidification of dissolved silicon dioxide. Formation of the interlayer plays very important roles for the characteristics of HF-bonding.

Fabrication of quartz microchips with optical slit and development of a linear imaging UV detector for microchip electrophoresis systems

We have developed quartz microchips for electrophoresis and a linear imaging UV detector along with the microchip. The microchips have an optical slit, which cut off the stray light in order to improve the sensitivity of UV absorption detection on the chip, at the bonding interface. They have been successfully fabricated on synthesized quartz glass substrates using the hydrofluoric acid (HF) solution bonding method. The signal level of UV absorption detection was effectively improved by applying microchips with the “on‐chip” optical slit. It is also possible to improve the signal‐to‐noise ratio by repetitive scanning of linear photodiode array located along the separation channel, and signal averaging during elimination of the potential. Furthermore, the analysis may be performed until the separation of the target component is complete, because the real‐time migration pattern of each component in the sample can be seen just as in a slab‐gel electrophoresis, thus enabling a shorter analysis time.

Condition optimization, reliability evaluation of SiO2-SiO2 HF bonding and its application for UV detection micro flow cell

Abstract In order to apply SiO2–SiO2 bonding with hydrofluoric acid (HF bonding) for micro-electro-mechanical systems (MEMS) fabrication, the optimal bonding conditions were examined under different temperature, HF concentration and bonding time. The necessary HF concentration and the necessary time for bonding are reduced by elevating the bonding temperature. The time for bonding was reduced from 24 h at room temperature to 30 min at 80°C, 60 min at 60°C under the conditions of 0.5 wt.% HF concentration and 1.3 MPa applied pressure. The bonding time is comparable to that of anodic bonding. Reliability of the HF bonding was confirmed by the results of temperature cyclic tests and thermal shock tests. A long term stability of the bonded sample was also evaluated by helium (He) leak detection. The measured He leak rate was less than 2.0×10−9 atm cm3/s which is much smaller than that calculated value through component materials of the sample. A novel quartz UV detection micro flow cell for chemical analysis having Si shade structure was fabricated by the HF bonding. The absorbance unit for UV absorption detection of the cell was improved remarkably.

Influences of electroosmotic flows in nanopillar chips on DNA separation: Experimental results and numerical simulations

The various potential factors affecting the performance of nanopillar chips on DNA separation were investigated from the viewpoints of both numerical calculations and actual experiments. To yield higher performance and replace the conventional DNA separation techniques such as microchip electrophoresis, the phenomenon specific to the nanopillar chips should be deeply understood. In this paper, although various factors affecting the performance of the nanopillar chips are considered, we focused on the effect of electroosmotic flow, which is particularly noticeable in quartz-made nanopillar chips. High-resolution separation of DNA was realized when an electroosmotic flow was suppressed by simply using a higher concentration of buffer, but DNA separation failed in the presence of an electroosmotic flow. It was confirmed from the numerical simulations and the direct observations that the deformation of DNA band during the injection process was induced by electroosmotic flow and consequently led to a poor resolution.

Studies on SiO/sub 2/-SiO/sub 2/ bonding with hydrofluoric acid-room temperature and low stress bonding technique for MEMS

Studies on SiO/sub 2/-SiO/sub 2/ bonding with hydrofluoric acid (HF) are described. This method has a remarkable feature that bonding can be obtained at room temperature. Advantages of this method are low thermal damage, low residual stress and simplicity of the bonding process, which are expected for the packaging and assembly of MEMS. The bond characteristics were measured under different bonding conditions of HF concentration, pressure, chemicals and so on. The bond strength depends on the applied pressure during bonding. HF concentration can be reduced to 0.1%. The bonding is also observed using KOH solution instead of HF. TEM, SIMS, RI and EPMA were applied to evaluate the bonded interface. From the TEM results, an interlayer is formed between SiO/sub 2/-SiO/sub 2/. The thickness of the interlayer depends strongly on the applied pressure during bonding. The SIMS results showed that hydrogen and fluorine partially exist in the interlayer. Considering the result of the RI analysis, surplus HF solution is squeezed out from the interface as the bonding progress. From these results, both surfaces of the SiO/sub 2/ are solved by HF and an interlayer, which is a binding layer, is formed. Formation of the interlayer plays a very important role for the characteristics of HF-bonding.

High pressure electroosmotic pump packed with uniform silica nanospheres

We report a novel electroosmotic pump fabricated on a plastic chip. Electroosmosis is one of electrokinetic phenomena and used as a precise liquid handling technique in some microfluidic devices. Electric double layer (EDL) between a channel wall and an electrolyte plays an important role in producing electroosmotic flow. When the net charge of EDL migrates, it carries the rest of the electrolyte due to the shear viscosity. The use of parallel narrow channels is a pathway to the more effective electroosmotic pumps since we can increase the net charge of EDL. We fabricated a plastic chip that confined uniform silica nanospheres within the channel, and tested its performance. We obtained the maximum flow rate 0.47 /spl mu/L/min and the maximum pressure 72 kPa when applied 3 kV to the electroosmotic pump.

Microfabricated CE Chips with Optical Slit for UV Absorption Detection

We have developed novel μ-CE chips with “on chip” optical slit which cut off the stray light in order to improve the sensitivity of the optical absorption detection on μ-CE chips. Our chips which have an optical slit at the bonding interface have been successfully fabricated on synthesized quartz glass substrates using HF (hydrofluoric acid solution) bonding method [1],[2]. The signal level of UV absorption detection was improved about ten times by applying the “on chip” optical slit compared with that of conventional µ-CE chips.

DNA Size Separation Employing Micro-Fabricated Monolithic Nano-Structure

Electrophoresis features of large DNA in dense pillar array structures were studied. The sieving structures were all-quartz-made, 200 – 500nm in diameter, 5 jim in height, and fabricated by dry etching process employing high selective Ni mask. Electrophoresis velocities of DNA in the structure were measured by a fluorescence microscope. The separation ability of T4 and lambda in the pillar region was dependent on the pillar size. The ability was significantly enhanced at the boundary where the DNA entered into the dense pillar region from free solution region. These results shows the importance of pillar pattern design for the DNA separation, and the possibility of high performance separation by optimizing the pillar size and the boundary effect.

μ-CE Chip Fabricated by Moving Mask Deep X-ray Lithography Technology

The concept of a kind of new micro capillary electrophoresis chip (μ-CE) for DNA analysis is stated. The micro channel array with high aspect ratio is the key part, whose wall should have a slight inclination to ensure the fabrication. It is demonstrated that M2DXL (Moving Mask Deep X-ray Lithography) technology can successfully control the inclination and enables to integrate micro optical components such as micro lens into the chip which improve detection performance greatly.

Microfabricated Chips for Capillary Electrophoresis on Quartz Glass Substrates Using a Bonding with Hydrofluoric Acid

Fabrication method and characteristics of micro capillary electrophoresis (μ-CE) chips fabricated on synthetic fused silica (quartz glass) substrates are described. A novel room temperature bonding method with hydrofluoric acid solution is applied [1], [2]. Advantages of this method are low thermal damage, low residual stress due to the room temperature bonding and simplicity of the bonding process which are expected for the assembly of microfluidic devices. The migration time variations are investigated to confirm the reproducibility of the fabricated μ-CE chips.