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Monolithic Crystal Oscillator

IP.com Disclosure Number: IPCOM000088827D
Original Publication Date: 1977-Aug-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 49K

Publishing Venue

IBM

Related People

Katyl, RH: AUTHOR

Abstract

Quartz crystal oscillators have long been a part of electronics, and are widely used because of their well-known stability. For example, they form the time base for many digital wristwatches, and set the frequencies for almost all communications systems. However, it has not been possible to integrate them onto a monolithic silicon chip. They remain a discrete circuit element encased, usually, in a separate package.

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Monolithic Crystal Oscillator

Quartz crystal oscillators have long been a part of electronics, and are widely used because of their well-known stability. For example, they form the time base for many digital wristwatches, and set the frequencies for almost all communications systems. However, it has not been possible to integrate them onto a monolithic silicon chip. They remain a discrete circuit element encased, usually, in a separate package.

Shown above is a device which can be fabricated on a silicon chip along with the other electronics and which possesses an electrically excitable sharp mechanical resonance in a practical frequency range. Such a structure, when connected to an on-chip amplifier, could be used as follows: 1. Crystal oscillators for receivers and transmitters. 2. Time bases for clocks, counters, computers, etc. 3. Several oscillators could be placed on a single chip for scanner receivers, or digitally controlled crystal oscillators. 4. They could be used passively for filters and FM discriminators.

A cross section of the device construction is shown in Fig. 1, and Fig. 2 is a plan view. The main feature of the device is a thin silicon membrane 10 (about
2.5 microns thick). A structure such as this as well as the special etching technique required were recently described [*]. Silicon is not a piezoelectrically active material. A film of zinc oxide 12 sputtered adjacent to the membrane serves to provide the piezoelectric coupling. The aluminum metallization 15 forms the electrical field within the zinc oxide to provide the mechanical stress. Zinc oxide transducers are used in surface acoustic wave devices. A layer of silicon dioxide 20 is located between layers 10 and 12.

The silicon membrane forms a mechanical assembly known as a "clamped thin plate". It vibrates much in the manner as a drum head, that is, in and out in a direction perpendicular to the equilibrium plane of the membrane. The exact calculation of the resonant frequencies of this structure is complicated by the anisotropic elastic constants of silicon and the corners of a square structure. However, an estimate can be obtained from the formula for a clamped circular plate whose diameter equals one side of the square membrane.

For example, a 2.5-micron membrane, 100 microns in di...