Crystal Industry Innovation
In the physical sense, a crystal is a set of atoms,molecules,or ions arranged in a definite geometric pattern in three dimensions.The ideal crystal is made up of unit cells which are the smallest parallel-epipeds that can be fitted together to form the crystal.Each unit cell of a given crystal is identical to every other unit cell of the crystal in that each one contains the same set of atoms,molecules or ions,arranged in exactly the same way.
Quartz crystal resonators (often called “crystals”) are widely used in frequency control applications because of their unequalled combination of high Q, stability, small size and low cost. Many different substances have been investigated as possible resonator materials, but for many years quartz resonators have been preferred in satisfying needs for precise frequency control. Compared to other resonators, for example, LC circuits, mechanical resonators such as tuning forks, and piezoelectric ceramic resonators based or other single-crystal materials, the quartz resonator has a unique combination of properties.
First, the material properties of single-crystal quartz are extremely stable with time, temperature, and other environmental changes, as well as highly repeatable from one specimen to another. The acoustic loss or internal friction of quartz is very low, leading directly to one of the key properties of a quartz resonator, its extremely high Q factor.
The intrinsic Q of quartz is about 107 at 1 MHz. Mounted resonators typically have Q factors ranging from tens of thousands to several hundred thousand, which is orders of magnitude better than the best LC circuits.
The second key property of the quartz resonator is its stability with respect to temperature variation. Depending on the shape and orientation of the crystal blank, many different modes of vibration can be used and it is possible to control the frequency-temperature characteristics of the quartz resonator to within close limits by an appropriate choice. The most commonly used type of resonator is the AT-cut, where the quartz blank is in the form of thin plate cut as an angle of about 35°15' to the optic axis of the crystal.
The third essential characteristic of the quartz resonator is related to the stability of its mechanical properties. Short and long term stabilities manifested in frequency drifts of only a few parts per million per year are readily available from commercial units. Precision crystal units manufactured under closely controlled conditions are second only to atomic clocks in the frequency stability and precision achieved.
The above is a schematic diagram of a cultured quartz crystal grown from a Y-oriented seed crystal for use in fabricating AT-cut resonators. A seed crystal establishes the initial crystal orientation and encourages growth in the Y direction at the expense of the Z-axis. Seed crystals are carefully selected to avoid defects which might propagate as the crystal grows. The position of the seed crystal is indicated. The lines sloping left from the x-axis mark the saw cut position for AT plates, the line sloping to the right indicates the BT-cut. In practice, these angles are very critical and are precisely determined using Bragg x-ray diffraction.