Surface Activated Bonding of Glass Using Aluminum Oxide Intermediate Layer for Microchannel Fabrication
Authors: Takeuchi Kai , Mu Fengwen , Matsumoto Yoshiie , Kim Beomjoon , Suga Tadatomo 
Published: December 2020

With a huge demand from the society on a more precise and rapid biological molecule detection in a reduced amount of biosamples, biomedical engineering technology has been in rapid development, resulting in a wide range of biosensors using microfluidic systems. For microfluidic structure, glass is still the main material owing to its chemical stability, cost efficiency, and optical characteristics. In order to seal the fluidic channel based on glass materials, glass substrates should be bonded to cover the channel grooves. Conventionally, glass materials are bonded by anodic bonding or hydrophilic bonding at high temperature process, in which the bonding surfaces are bonded via covalent bonds. However, these bondings of glass at high temperature lead to residual stress at the bonding interface and damage to the device on the substrate, especially the biomolecule sensors using enzymes. In addition to the process temperature, the bonding interface should be transparent as a wide range of biosensors utilizes optical measurements to detect biomarkers.
For this reason, a room temperature bonding of glass is desirable for the microchannel sealing. As a room temperature direct bonding method of glass, modified surface activated bonding (mSAB) using Si intermediate layer has been reported to achieve a strong bond of glass via Si layers and Fe nano adhesion layers [1]. Although this approach enables the glass bonding at room temperature, the bonding interface is covered with the deposited Si layers, resulting in deteriorated optical characteristics at the bonding interface. Therefore, mSAB using Si intermediate layer is not suitable for the optical measurement of biomolecules in the microchannels.
Recently, we reported a new SAB using aluminum oxide (AlO) intermediate layer instead of Si [2]. AlO has excellent optical characteristics such as low absorbance and refraction index uniformity. From the aspect of bonding, thin AlO membrane by ALD has been reported as intermediate layer for hydrophilic bonding [3]. Furthermore, it has been also reported that bulk sapphire is bonded by standard SAB process, where the sapphire surfaces are activated by Ar ion beam irradiation and bonded through amorphous-like AlO structure [4].
In this study, we apply mSAB using AlO intermediate layer for glass substrate and Si substrate with microchannel patterning. The bonding is evaluated from the perspective of the channel sealing.
In order to evaluate the sealing by the proposed bonding method, microfluidic channels were realized on a 4 inch and 525 µm thick Si wafer. The patterning consists of 64x64 inlet ports 100 µm diameter, 46-142 µm wide microchannels, 4x4 mm2 cavity, and capillary pump. For the fabrication of the microfluidic components, inductively coupled plasma reactive ion etching (ICP-RIE) was employed. For the sealing of the microchannels on the Si wafer, mSAB using AlO intermediate layer was conducted. The substrates are activated by Ar ion beam and the AlO layers are deposited by ion beam sputtering, followed by contact and press. After sealing by the wafer bonding, through-glass holes were fabricated by sand blast etching to enable flow in the channels.
In order to evaluate the sealing of the microfluidic system by the proposed bonding method, first, scanning acoustic tomography (SAT) was conducted. As shown in Figure, the whole structure is sealed without significant voids.
Additionally, we also evaluated the sealing of the microfluidic structure by a flow test. A droplet of phosphate buffered-saline (PBS) as a model liquid of biosample was put on the inlet ports. As a result, PBS flow was observed through the microchannels and capillary pump without leakage. Therefore, it can be said that the proposed bonding method successfully sealed the microfluidic system and the bonding interface kept its transparency.
From these result, it is indicated that the developed bonding method is suitable for the microchannel device fabrication by realizing a transparent bonding interface at room temperature.

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Figure 1