Miniature crystalline quartz electromechanical structures

Abstract

Three-dimensional miniature structures may be fabricated from crystalline quartz using photolithographic processes. Crystalline quartz has an-istropic etching properties in buffered HF solutions and etch rates can vary by factors of 200 along different crystal axes. Although the prevalent application of this technology has been for miniature frequency-control timebases, other applications are beginning to emerge. Sensors for force, strain, acceleration, temperature, pressure and gas density have been developed as well as miniature actuators, precision springs and flow-control devices, to name a few. This paper will focus upon the use of precision quartz resonators in sensing applications. The primary intent is to demonstrate the unique characteristics of crystalline quartz and promote its use for new fields of application.

Miniature quartz resonators that change frequency with a single predominant physical effect are typically used in sensor applications. Thin metal film electrodes deposited on the surface of the miniature quartz structure couple electrical field energy to strain energy in the bulk of the quartz structure through the piezoelectric effect. The quartz crystal unit controls the frequency of an oscillator circuit, which is designed to excite the sensor's resonant mode. The resonant mode to which coupling takes place depends upon the electrode configuration, the shape of the quartz structure and the mechanical and piezoelectric properties of the quartz for the orientation of the crystal axes. Each of these effects must be well understood through a variety of analytical techniques prior to the development of a quartz resonator. In addition, the range over which the desired frequency of the quartz resonator changes in sensor applications indicates that the mode spectrum of the sensor and the surrounding structure must be well understood to avoid interfering modes. Piezoelectric coupling, fabrication techniques and the design of quartz resonators for sensing applications will each be discussed at length.