Detailed characterization of anodic bonding process between glass and thin-film coated silicon substrates

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Abstract

Anodic bonding between Si-based and glass substrates has been characterized in detail. The effects of magnitude of the applied voltage, surface properties (coating of Si substrate), and surface cleanliness (pre-bonding cleaning procedure) on the time required for complete bonding were thoroughly studied. First, the generic bonding time versus applied voltage plot was found to be concave in shape (viewed from the origin). For bonding between p-type Si substrate and Corning 7740 glass pre-cleaned with acetone, the time required was cut down from 38 to 4 min if the applied voltage was increased from 200 to 500 V. Second, the bonding time required for five Si-based substrates in ascending order was determined to be Si (p-type), polysilicon, silicon nitride, silicon oxide and then Si (n-type). Third, the bonding between p-type Si substrate, pre-cleaned with H2SO4–H2O2 and HF, and Corning 7740 glass was completed within 1 min, which was much faster than that pre-cleaned with acetone (4 min). Finally, from bonding point of view, Corning 7740 glass was superior to Corning 7059 glass and Fisher slide due to its thermal coefficient of expansion matching with the underlying Si substrate and the presence of significant amount of sodium ions in the glass.

Introduction

Over the past decades, various bonding techniques have been used extensively in the integrated circuit (IC) industry for the packaging of pressure sensors and accelerometers as well as the construction of complex microchemical reactor [1]. These techniques can be categorized into direct, intermediate layer, and anodic bondings. Direct bonding places stringent requirements on the cleanliness and flatness of the surfaces to be bonded and it requires a high temperature annealing to establish high strength bonding. Intermediate layer bonding resembles direct bonding except that an additional layer is deposited to lower the annealing temperature of the bonding process. Anodic bonding is gaining significance in micro-Total Analysis Systems (μTAS) and miniaturized biological reactors because of its promising use in sealing Si and glass-based microfluidic devices under an applied voltage of 100–1000 V [1], [2], [3], [4]. It offers advantages of high bond strength at moderate or low processing temperature (∼200°C–400°C).

Deterministic parameters for anodic bonding include magnitude of applied DC voltage, temperature, nature of surfaces to be bonded, and bonding time. Research groups in this field have done a lot of work investigating the effects of applied voltage and temperature on the bondability and the final strength of the bonded pieces such as the tensile strength [2], [3] and interfacial fracture toughness [4]. Nevertheless, to our knowledge, little attention was paid to characterize the time required for a complete bonding at different operating conditions. There are a number of factors influencing the bond strength and the bonding time required in a typical Si-glass anodic bonding process. Among them, the cleanliness [5] and chemical nature of the to-be bonded surfaces are essential. Often, glass favourable for bonding exhibits a close thermal coefficient of expansion (TCE) to that of the underlying Si. However, from device fabrication point of view, such glass may not be easily patterned and etched, or vice versa.

In this work, anodic bonding process for thin-film coated Si substrates and glass materials is investigated in detail. First, the time needed for a complete bonding (thereafter termed as bonding performance) at different applied voltages is evaluated. Second, the bondability between thin-film coated Si substrates and Corning 7740 glass is investigated. Third, the effect of surface pre-treatment and cleanliness on the bonding performance is discussed. Finally, bonding quality and performance between p-type Si substrate and different glasses are studied.

Section snippets

Experimental setup

The bonding system comprised of a hot plate, a DC power supply (HP6035A, USA) together with a pair of Pt electrodes, and ceramic bars. Pre-cleaned Si and glass substrates were sandwiched between the pair of Pt electrodes. Then, the ceramic bars and screws were used to fix the bonding pair including the electrodes. The same number of turns was applied to the screws to avoid the undesired load effect [4]. The assembly was heated to 300°C and a DC voltage was applied to the electrodes, ensuring a

Effect of applied voltage

Anodic bonding between Si and Corning 7740 glass, both pre-cleaned with acetone, was investigated. Fig. 2 shows the plot of the bonding time required at different applied voltage conditions. As can be seen from the figure, the required bonding time drops significantly as the applied voltage increases from 200 to 500 V. This can be explained with respect to the bonding mechanism discussed below [1], [6].

At an elevated temperature, Na+ ions in the glass become so mobile that they are attracted

Conclusions

The effects of voltage, surface properties, and surface cleanliness on the bonding time required for anodic bonding between Si-based substrates and glass have been addressed. It was found that the magnitude of the applied voltage had great influence on the bonding time. For bonding between p-type Si substrate (pre-cleaned with acetone) and Corning 7740 glass, the bonding time required at an applied voltage of 500 V (4 min) was only 10% of that at 200 V (38 min). Besides, bonding with p-type Si

Acknowledgements

The authors gratefully acknowledge the funding support of the Biotechnology Research Institute of the Hong Kong University of Science and Technology as well as the Industry Department of the Hong Kong Special Administrative Region. The authors thank K.H. Kwok for his technical assistance throughout the project.

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