Lab-on-a-chip devices

This paper describes the main protocols that are used for fabricating microfluidic devices from glass and silicon. Methods for micropatterning glass and silicon are surveyed, and their limitations are discussed. Bonding methods that can be used for joining these materials are summarized and key process parameters are indicated. The paper also outlines techniques for forming electrical connections between microfluidic devices and external circuits. A framework is proposed for the synthesis of a complete glass/silicon device fabrication flow.

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A practical guide for the fabrication of microfluidic devices using glass and silicon

Surface and interface characteristics of substrates are critical for reliable wafer bonding. Understanding the elemental and compositional states of surfaces after various processing conditions is necessary when bonding dissimilar materials. Therefore, we investigated the elemental and compositional states of silicon (Si), silicon dioxide (SiO2) and glass surfaces exposed to oxygen reactive ion etching (O2 RIE) plasma followed by storage in controlled humidity and/or ambient atmospheric conditions to understand the chemical mechanisms in the direct wafer bonding. High-resolution X-ray Photoelectron Spectroscopy (XPS) spectra of O2 RIE treated Si, SiO2 and glass showed the presence of Si(-O)2 resulting in highly reactive surfaces. A considerable shift in the binding energies of Si(-O)2, Si(-O)4 and Si(-OH)x were observed only in Si due to plasma oxidation of the surface. The humidity and ambient storage of plasma activated Si and SiO2 increased Si(-OH)x due to enhanced sorption of hydroxyls. The amounts of Si(-O)2 and Si(-OH)x of Si varied in different humidity storage conditions which are attributed to crystal-orientation dependent surface morphology and oxidation. The O2 RIE plasma induced high surface reactivity and humidity induced Si(-OH)x can play an important role in the hydrophilic wafer bonding with low temperature heating. © 2013 The Electrochemical Society. [DOI: 10.1149/2.007312jss] All rights reserved.

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Oxygen Plasma and Humidity Dependent Surface Analysis of Silicon, Silicon Dioxide and Glass for Direct Wafer Bonding

Thisreviewcoversthekeypapersrelatingtothetheory,techniques and quantification of hydrophilically activated silicon wafer bonding. This begins with a review of the history and development of the art. Bond strength characterization is then reviewed followed first by models of physical deformation and then by plasma and radical activation techniques. In the interests of clarity and succinctness this review is not an attempt to exhaustively catalog all direct bonding wafer literature. Excluded are hydrophobic and UHV bonding, and the many papers applying bonding techniques to differing combinations of wafer materials. This paper concludes with a summery of the state of the art of direct wafer bonding and a summation of the current wafer-bonding model derived from the papers reviewed.

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A Review of Hydrophilic Silicon Wafer Bonding

For heterogeneous integration in many More-than-Moore applications, surface preparation is the key step to realizing well-bonded multiple substrates for electronics, photonics, fluidics and/or mechanical components without a degradation in performance. Therefore, it is critical to understand how various processing and environmental conditions affect their surface properties. In this paper, we investigate the effects of oxygen plasma and humidity on some key surface properties such as the water contact angle, roughness and hardness of three materials: silicon (Si), silicon dioxide (SiO2) and glass, and their impact on bondability. The low surface roughness, high surface reactivity and high hydrophilicity of Si, SiO2 and glass at lower activation times can result in better bondability. Although, the surface reactivity of plasma-ambient-humidity-treated Si and SiO2 is considerably reduced, their reduction of roughness and increase of hydrophilicity may enable good bonding at low temperature heating due to augmented hydroxyl groups. The decrease of hardness of Si and SiO2 with increased activation time is attributed to higher surface roughness and the formation of amorphous layers of Si. While contact angle and surface roughness results show a correlation with bondability, the role of hardness on bondability requires further investigation.

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The effects of oxygen plasma and humidity on surface roughness, water contact angle and hardness of silicon, silicon dioxide and glass

The findings of this study report the bonding of glass/glass wafers by using the surface activated bonding (SAB) method at room temperature (RT) without heating. In order to bond, the glass wafers were activated by a sequential plasma activation process, in which the wafers were cleaned with reactive ion etching (RIE) oxygen radio frequency (rf) plasma and nitrogen radical microwave (MW) plasma one after another and then contacted under hand-applied pressure followed by cold rolling under 20 kg load in atmospheric air. High bonding strength for glass/glass was achieved. Paramount influence of N 2 radical MW plasma on the adhesion enhancement of silicon/silicon bonding motivated the investigation of the N 2 radical MW plasma relationship with the bonding strength of glass/glass. A considerable influence of N 2 pressure on the bonding strength was not observed except in N 2 gas pressure of 30 Pa, which might be due to the debonding between glue and fixture used for tensile pulling test. No significant effect of OH density of glass wafers on the bonding strength was found below 400 °C. The result was evident from 400 °C and it was about twofold higher at 600 °C than that of RT to 400 °C. This result indicated that the sequential process bonding mechanism was consisting of long bridges of hydrogen bonding by water molecules. Significant environmental influence on the bonding strength was found and which could be correlated with OH molecules of glass wafers.

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Room temperature wafer level glass/glass bonding

A sequential plasma activation process consisting of oxygen reactive ion etching (RIE) plasma and nitrogen radical plasma was applied for microfluidics packaging at room temperature. Si/glass and glass/glass wafers were activated by the oxygen RIE plasma followed by nitrogen microwave radicals. Then, the activated wafers were brought into contact in atmospheric pressure air with hand-applied pressure where they remained for 24 h. The wafers were bonded throughout the entire area and the bonding strength of the interface was as strong as the parents bulk wafers without any post-annealing process or wet chemical cleaning steps. Bonding strength considerably increased with the nitrogen radical treatment after oxygen RIE activation prior to bonding. Chemical reliability tests showed that the bonded interfaces of Si/Si could significantly withstand exposure to various microfluidics chemicals. Si/glass and glass/glass cavities formed by the sequential plasma activation process indicated hermetic sealing behavior. SiOx Ny was observed in the sequentially plasma-treated glass wafer, and it is attributed to binding of nitrogen with Si and oxygen and the implantation of N2 radical in the wafer. High bonding strength observed is attributed to a diffusion of absorbing water onto the wafer surfaces and a reaction between silicon oxynitride layers on the mating wafers. T-shape microfluidic channels were fabricated on glass wafers by bulk micromachining and the sequential plasma-activated bonding process at room temperature

Room-temperature microfluidics packaging using sequential plasma activation process

A sequential plasma activation process consisting of oxygen reactive ion etching (RIE) plasma and nitrogen radical activation was applied for microfluidics packaging at room temperature. Si/glass and glass/glass wafers were activated by the oxygen RIE plasma and nitrogen microwave radicals one after another. The activated wafers by the two-step process were brought into contact in air followed by keeping them in air for 24 h. The wafers were bonded throughout the entire area and the bonding strength of the interface was as strong as silicon and glass bulk without any post-annealing process and wet chemical cleaning steps. Bonding strength considerably increased with the nitrogen radical treatment after oxygen RIE activation prior to bonding. Chemical reliability tests showed that the bonded interfaces were significantly worked with various chemicals. Si/glass and glass/glass cavities formed by the sequential plasma activation process indicated hermetic sealing behavior. High bonding strength was thought to be due to a diffusion of absorbing water into wafer surface and a reaction between silicon oxynitride layers on the mating wafers. An amorphous layer of 7 nm thick was found at the Si/Si interface. Activation with N/sub 2/ radical for 1200 s after O/sub 2/ RIE plasma treatment for 60 s generated a new phase in the Si wafers across the amorphous layer, probably due to the implantation effect of N2 radical in Si wafer. T-shape microfluidics channels were fabricated on glass wafers by bulk micromachining and bonded by using the sequential plasma activation process at room temperature.

Satoru Suehara

Papers

Room temperature wafer level glass/glass bonding

Room-temperature microfluidics packaging using sequential plasma activation process

Sequential plasma activation process for microfluidics packaging at room temperature

Low Temperature Glass Wafer Bonding with Sequential Plasma Activation