Direct bonding and mechanical thinning of pre-etched silicon wafers have been studied for the fabrication of silicon-on-insulator ͑SOI͒ wafers with buried cavities. The thin Si diaphragm over the cavity is deflected downward during the grinding and polishing, as the thinning is carried out without supporting the diaphragm. The deflection causes thickness variation for the Si diaphragm that can also be observed as a hill on the wafer surface after thinning. The results show that the thickness variation of the Si diaphragm increases with increasing cavity size and with decreasing SOI layer thickness. After grinding the measured hill height was about 1.5 m for a 20-m-thick Si diaphragm over a 1 ϫ 1 mm cavity. The hill height was reduced to less than 0.5 m when a small supporting column was placed under the diaphragm. With polishing the hill height was further reduced to Ͻ0.1 m. It appears that mechanical thinning of the bonded wafers with pre-etched cavities is a viable method for various applications.
The design of a complete dc .SQUID with a flux transformer input circuit is discussed. The flux coupling circuits introduce a substantial capacitance across the SQUID and give rise to many resonances which may couple strongly to the SQUID dynamics. Both effects lead to multiple modes in the SQUID dynamics and consequently to excess noise. For a low-noise SQUID with smooth characteristics, our analysis and practical considerations suggest signal coupling via an intermediary transformer. This method allows simultaneous optimization of the SQUID parameters, minimizing the parasitic capacitance, control over the resonances, and good inductance matching to practical magnetometer coils. A model is developed to optimize the structure: it describes the whole circuit with the help of a suitably modified autonomous SQUID, provided that the system is free from multiple modes due to resonances or large parasitic capacitance. Following these design principles, we have built a dc SQUID, primarily for use in biomagnetic research, but also well suited for other applications. The fabrication of the SQUID and the high-quality electronics especially suitable for multiple-SQUID devices is presented. The SQUIDs showed smooth characteristics, and the lowest measured noise of our complete SQUID is 1.3 x 10-6tT])o//x/-~, indicating the success of the design.
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