About Quartz crystal

What is 'crystal' used on quartz device?

Crystal is a monocrystal made of Si and O2. Amethyst, citrine and rose quartz used for accessories are also crystals. These crystals are discolored by metal oxide. Crystal is filled with mysterious beauty and owns a property of piezoelectric effect — the generation of electric polarization when given mechanical stimulus which causes deformation. On the other hand, if given electrical polarization, it is mechanically deformed in terms of property, which is called inverse piezoelectric effect. The crystal device is an electric part manufactured by crystal.
The crystal device made by Renlux Crystal is used in familiar electronic equipment in our daily life such as AV equipment (including DVD and digital television), automotive electronics and game machines. Electric equipment is controlled by information transmission carried out by computer and RLC’ crystal devices play a very important role in transmitting this information. In another words, a crystal device, such as a crystal resonator will keep oscillating the specified signal and plays the same role as a heart sending blood into body. That is why it is considered one of the most important electronic mechanism.

How is it turned into electric device?

In the following section, this question will be answered in reference to crystal resonators.

(1) How to develop synthetic quartz crystal

Gigantic pressure vessel called autoclave is filled with alkaline solution and its upper half is seed quartz (plate of pure crystal with less flaw in crystal arrangement) and its lower half is raw material of natural crystals called lasca. Then re-crystalization is generated under high temperature and high pressure conditions with 300- 400 deg.C and 1000- 1500 atmospheric pressure. This is how synthetic quartz crystal is made. The development stage differs from crystal to crystal, usually taking a couple of months. Since the quality of synthetic quartz crystal is the biggest determinant in deciding a good or bad crystal resonator, RLC employs the most stringent control system so that the synthetic quartz crystal with highest level quality can be produced. It is free from impurities so that it can be used for electronics parts. At this stage, synthetic quartz crystal is still heavy and big, so it may be hard to imagine how it is processed into a crystal resonator, which is smaller than a grain of rice.

(2) Processing into a crystal blank

Crystal is colorless and clear substance and seems to have no directional property whichever part is used because its axis is invisible. However, crystal does have directional property and it is therefore necessary to identify the crystal axis to make it functional as an electronics component. This process is called the lumbered quartz bar processing. Depending on the purpose, cutting is carried out at required angles measured against this crystal axis. The crystal resonator is an electronics part designed to oscillate in a stable manner. At high frequencies, the oscillating element is in inverse proportion to thickness of crystal, meaning the higher frequency is, the thinner crystal is, and vice versa. By applying this characteristic, crystal is cut and smoothed until a targeted frequency (thickness) and size (outer diameter dimension) are available. This process is critical because the property of a crystal resonator is almost decided. The work process requires very fine precise control up to parts per million. In addition, crystal is hard and likely for a brittle fracture to occur, so it is incredible to see it bent with ease. But if the processing continues and crystal becomes very thin, it could be bent as shown in the photo.

(3) Frequency adjustment and cleaning

The crystal blank, after being cut and ground with grinding material, has distortions with small cracks and stains on the surface, which together is called the affected layer. In order to remove such distortion and stain, chemicals are used to smooth out and clean up the surface of the crystal. At this point, the fine-tuning of the frequency is conducted as well. Up until this point, some of the crystal blanks turn out to be smaller than a grain of rice and thinner than paper. This change is really a big surprise.

(4) Making on electrode

By the processes so far, targeted frequency and size have been accomplished. However, it is impossible to have oscillation of an electric signal because the electric connection can not be made. So this comes in where the metal film of gold, silver or other metals are glued onto the crystal blank by the method of base plating. If current is supplied to this metal film, the crystal blank oscillates and frequency is available. This is the application of an inverse piezoelectric effect.

(5) Adhering and sealing of crystal blank

In order to connect the electrode-formed crystal blank to a circuit board, crystal blank is stuck to the ceramics package by using a conductive adhesive agent. Here you begin to see something like a crystal resonator. After ensuring that it is firmly adhered and can withstand falls and vibrations, frequency is checked up to the level of ppm (×10-6) and sealed with a cap under the atmosphere of vacuum and nitrogen. This allows crystal blanks to be protected against air and dust, thus making crystal resonator more stable over a long period of time.

(6) Inspection and packing

A strict final check is conducted as to whether the targeted specifications are fulfilled or not and are only packed when crystal resonators are approved.

(7) Delivery

It is essential to make crystal resonators precise. After going through these long processes, crystal resonators are ready for customer’s delivery.

Affected layer	Micro cracks on layers of crystal blanks, which is caused by the lapping or cutting process.
Autoclave	A sealed vessel made from special iron, that withstands high pressure and heat. The largest size is 14 m.
Base-plating	A process of applying coatings of metal layers on surface of crystal wafers. There are two main methods: vacuum deposition and sputtering. The vacuum deposition melts metals in the chambers under a vacuum state. The sputtering method occurs by bombarding the surface of the sputtering target with gaseous ions.
Brittle Fracture	A material which gets fractured by a rapid progress of crack. An opposite phenomenon of ductile fracture.
Cap (Lid)	A ceramic or a metal cover, which protects the crystal blank bonded on the package.
Conductive adhesive agent	An adhesive agent which becomes electrically conductive when cured. It is used to electrically connect the crystal wafer onto the package.
Inverse Piezoelectric Effect	The phenomenon of a piezoelectric material where certain crystals deform when they are exposed to electrical field. Lippmann predicted its presence, and P. Curie and J. Curie proved it.
Lasca	Natural quartz, used as a raw material for growing synthetic quartz.
Lumbered quartz bar processing	A process which grinds the reference surfaces (crystallographic axis) of synthetic quartz bars.
Package	A ceramic or a metal container for bonding crystal blank. The container also protects the crystal blank.
Piezoelectric Effect	The phenomenon of a piezoelectric material where certain crystals generate a voltage in response to applied mechanical stress. This was discovered by P. Curie and J. Curie in 1880.
Seed Quartz	A highly pure crystal stick or a plate used as a crystal nucleus for growing synthetic quartz bars. This crystal stick/plate serves as the seed for the recrystallization process.

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