Fabrication of micro fluidic cavities using Si-to-glass anodic bonding

TL;DR

This paper presents a novel method for fabricating large, deep microfluidic cavities using silicon-glass anodic bonding without the need for posts, enhancing reliability and optical access for superfluid experiments.

Contribution

The authors introduce a post-free silicon-glass anodic bonding technique for microfluidic cavities, improving fabrication reliability and enabling optical and pressure access for superfluid research.

Findings

Cavities of ~1.08 μm depth and up to 11 mm size were successfully fabricated.

Pressurizing cavities above 30 bar causes glass failure, not bond separation.

The fabrication process yields smooth surfaces and low-friction joints suitable for sensitive measurements.

Abstract

We demonstrate the fabrication of $\sim$1.08 $\mu$m deep microfluidic cavities with characteristic size as large as 7 mm $\times$ 11 mm or 11 mm diameter, using a silicon$-$glass anodic bonding technique that does not require posts to act as separators to define cavity height. Since the phase diagram of $^3$He is significantly altered under confinement, posts might act as pinning centers for phase boundaries. The previous generation of cavities relied on full wafer-bonding which is more prone to failure and requires dicing post-bonding, whereas the these cavities are made by bonding a pre-cut piece of Hoya SD-2 glass to a patterned piece of silicon in which the cavity is defined by etching. Anodic bonding was carried out at 425 $^{\circ}$C with 200 V, and we observe that pressurizing the cavity to failure ($>$ 30 bar pressure) results in glass breaking, rather than the glass-silicon bond separation. In this article, we discuss the detailed fabrication of the cavity, its edges, and details of the junction between the coin silver fill line and the silicon base of the cavity that enables a low internal-friction joint. This feature is important for mass coupling torsional oscillator experimental assays of the superfluid inertial contribution where a high quality factor ($Q$) improves frequency resolution. The surface preparation that yields well-characterized smooth surfaces to eliminate pinning sites, the use of transparent glass as a cover permitting optical access, low temperature capability and attachment of pressure-capable ports for fluid access may be features that are important in other applications.