Quartz Crystal Manufacture

- quartz crystal resonators undergo a number of processes in their manufacture. These are key to elements of their operation.

Quartz crystals offer a cheap yet very high performance resonant element for use in many forms of electronic circuit.

Many aspects of their performance and costs reflect on the advanced manufacturing techniques used for these components.

Made from silicon - quartz is a crystalline form of silicon - the manufacture of quartz crystals shares many basic concepts with that of semiconductor manufacture, although there are also many differences as well.


Quartz crystal manufacturing process

There are several stages to the manufacture of quartz crystals. For basic high volume devices, the stages required for producing high precision items may be reduced or excluded.

Essentially the manufacture of the quartz crystal starts with the production of the raw crystals. These complete crystals pass though stages to produce the basic blanks, which are further processed to create the crystals that operate on the right frequency. Finally they are encapsulated and then tested to ensure that they work as required.

The basic manufacturing process starts with the introduction of the raw silicon, and then progresses from that point in a logical fashion.


Growing the crystal

Although quartz does occur in its crystalline form naturally, the crystals are not to the quality required for quartz crystal resonator. Accordingly the crystals need to be grown artificially under highly controlled conditions. The synthetic crystal is grown using a seed crystal at around 350°C and under very high pressure: around 1000 atmospheres. This is done in a specifically manufactured crystal growing furnace and takes around 45 to 90 days.

Once the crystal has been grown, it is removed from the furnace and the faces are accurately ground to enable the X, Y, and Z axes to be accurately defined and to allow the crystal to be accurately held. As the properties of the various crystal cuts are defined by the angles, these must be accurately maintained to ensure the final crystal has the right properties.


Crystal cutting and lapping

Once the crystal has been formed and then had its axes ground, the next stages involve making the crystal blanks.

To create the blanks, the first stage is to cut the quartz crystal into small flat 'plates' of the correct dimensions. This needs to be undertaken using high precision cutters that are able to cut at exactly the right angles needed. These slices through the crystal need to be made a slightly thicker than that required fort he finished crystal.

Once the first cuts have been made, these large slicks can then be cut into smaller blanks. These are then lapped to bring them closer to the required thickness.

Also as the blank is brought closer to its ultimate thickness, so the lapping needs to become more precise and much finer lapping paste is used.

As the lapping process is abrasive, it damages the crystal lattice on the surface of the crystal. This not only lets in gas molecules and other impurities later that will cause ageing, but it also directly causes changes with time.


Final dimensioning and etching

Once the lapping processes have been undertaken the surfaces of the crystal are chemically etched. This provides a very much finer finish to the crystal and provides less opportunity for chemical ingress later. However the surface of the crystal needs to be very carefully washed after the chemical etching to remove all traces of external chemicals.

The chemical etching also reduces the dimensions a little further, sot his acts as the last stage of reducing the blank to its required dimensions.


Electrodes

Once the crystal blank has been manufactured to the required tolerances and finish, the electrode base is added.

The finished blank is placed into an evaporation mask and the silver or gold vapour is deposited into the surface of the crystal - these form the electrodes to the crystal. This is undertaken in a high vacuum deposition chamber.

The crystal blank then has leads attached to the deposited electrodes. It is then mounted on its holder.


Final adjustment and sealing

With the electrodes attached the final adjustment of the crystal is made. This can be achieved by adding additional electrode material.

The last stage of the crystal manufacture is to seal the crystal in its holder. This is normally evacuated or filled with nitrogen. With nitrogen being inert it prevents the ingress of reactive materials that might otherwise cause ageing of the crystal. A vacuum may also be similarly used.


Prior to the final despatch, a final inspection is often carried to out to ensure that crystal operation and characteristics meet the required specification. The level of inspection will obviously be determined by the cost and final specification limits. It may be undertaken on a batch basis, or for more exacting high precision elements it is more likely to be undertaken on an individual basis.

In some instances the crystal may be run at an elevated temperature to take out the first effects of ageing. As ageing occurs at its fastest rate immediately after manufacture, often high precision crystal units, or crystal ovens may be aged for around 40 or 90 days before being despatched.

By Ian Poole


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