What is 'crystal' of crystal 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 DAISHIKU CORP. (KDS) 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 KDS’ 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, KDS 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.
<explanation of term>
| 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 the 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. |
