Mono-crystalline Quartz

Quartz used in the manufacture of frequency control products is mono crystalline of an asymmetric hexagonal form. Chemically, Quartz is Silicon Dioxide, SiO2 occurring naturally as the most bundant mineral on earth, constituting approximately 14% of the earth’s surface.

The significance of mono crystalline quartz in the modern electronics industry is the result of its combined properties of piezoelectricity, high mechanical and chemical stability, very high Q at resonance and modern low cost methods of producing extremely high levels of purity in synthetic material.

Quartz is now indispensable as the principal material for controlling frequency in electronic equipment and is only surpassed for long term accuracy by primary atomic standards such as Caesium and Rubidium.

Nevertheless the recent development of mems, micro electro mechanical systems, and nems, nano-electro mechanical systems, is set to revolutionize the frequency control market with the integration of simple clocks into the silicon substrates used for IC fabrication.

These miniature devices may inevitably replace all simple clocks providing added reliability at lower cost and where minimum timing accuracy is a requirement.

In its basic chemical form silicon dioxide cannot be used for frequency control and must be of the mono crystalline structure in which it exhibits usable piezoelectric qualities due to its asymmetric form. Piezoelectricity (Greek Piezein ‘to press’) in mono crystalline quartz was discovered by the Curie brothers at the Sorbonne, Paris 1880.

However it was not until 1917 that this property was utilized in a practical application when professor Langevin in France and A.M. Nicolson at Western Electric independently designed sonar transceivers for the detection of submarines at sea.

Nicolson later went on to file a number of patents for applications using both quartz and Rochelle Salt. This latter material responded strongly to sound waves and electrical stimulus and was incorporated by Nicolson into designs for Microphones, Loudspeakers and Phonograph pick-ups. While Nicolson had proposed the use of Piezo electric materials for controlling the frequency of a vacuum tube oscillator it was Dr. Walter Cady of the Wesleyan University who filed the first patents for crystal controlled oscillators in 1923.

Prof. G. W. Pierce of Harvard University carried out further work on crystal oscillator development at about this time. Pierce’s main achievement was the design of a crystal controlled oscillator using only one vacuum tube and no tuned circuits other than the crystal itself.

During the early 1920’s crystal oscillator development and radio technology progressed steadily side by side. The major applications for crystal oscillators during these early days was for use as time standards and it was not until around 1926 that crystal oscillators were used to control the frequency of a radio transmitter. This was done at radio station WEAF in New York which was owned by AT and T.

Bell Telephone Labs who were part of AT&T and along with The Marconi Company in the U.K. and S.E.L. Germany achieved many significant developments in crystal technology during the 1930’s. In 1934 Messrs. Lack and Willard at Bell Labs discovered the AT Cut and BT Cut crystals which gave the communications industry vastly improved frequency vs temperature performance crystals.

Improved sealing and production techniques along with the discovery of a new family of Stress Compensated cuts are among some of the advances that have been made during the last decade together with the more recent inverted mesa process and miniaturisation of crystals and oscillators.

Piezoelectric materials exhibit a directionally related electric charge when subjected to pressure and conversely the application of an electric charge causes a directionally related force to be generated within the material. The application of an alternating electric field will cause the material to vibrate and subsequently mechanically resonate. The frequency of any mechanical resonance is determined by the physical dimensions of the material, the ‘cut angle’ with respect to the crystalline axis of the original mono crystalline crystal, the ambient temperature and any modifying effects of associated mechanical or electrical components.

The properties of crystallised quartz include its high chemical and mechanical stability and a low temperature coefficient, resulting in a small change in resonant frequency for any change in ambient temperature, together with a very high Q at resonance. It occurs naturally and all early experimental work was carried out using natural crystallised quartz.

However, naturally occurring crystallised quartz suffers from inclusions of impurities, bubbles, cracks and twinning, which reduce its value for use in frequency control as these reduce the Q factor. Therefore the production of synthetic quartz was established in order to produce a pure form of crystalline quartz free from twinning and impurities.

Synthetic quartz is produced in an autoclave from a saturated solution of Si O2 at approximately 400°C and at a pressure of 1000Kg/cm2 to produce a super saturated solution.

The process of manufacturing synthetic quartz is known as the hydrothermal method in which prepared seed plates of pre-orientated mono crystalline quartz are suspended in the saturated solution and by reducing the temperature of the solution the growth of large crystals is obtained under laboratory controlled conditions thus minimising impurities and maximising the useful volume of material.

Growth rates of the synthetic material are in the order of 1mm per day or less to achieve a maximum purity. Quartz resonators for use in electronic circuits are produced by cutting crystalline quartz into wafers(or blanks), plating electrodes onto each side of the wafer and enclosing the resonator into a suitable holder. The dimensions of the quartz wafer essentially determine the resonator frequency although this is also affected by the size and thickness of the electrodes and the associated electrical circuitry.

The orientation of the wafer ‘cut’ to the crystalline optical axis is critical in order to achieve accuracy of the resonant frequency and a necessary low temperature coefficient of frequency for the final resonator unit. The ‘cut’ will produce frequency/ temperature characteristics which are either second order (quadratic) or third order (ternary) and therefore the characteristics will exhibit single or double turn over points.