Quartz is highly pure, so it has a high working and melting temperature. In addition, there are many other features of quartz wafers, such as high corrosion resistance or low dielectric loss, etc. These special properties make it the prototype for quartz wafers, the perfect materials for semiconductors. In the processing of semiconductors, the demands of high energy and high temperature scare other materials away. Owing to the most stable properties that quartz wafers have, they are the optimal choice under the harsh condition.
As mentioned above, the purity, thermal and optical properties make quartz wafers superior to other glass materials. When they are applied for processing the semiconductors, they can free the semiconductors from easily corroding or wearing.
Quartz wafers come in different sizes and thicknesses, and allows for side polished, either one or both sides. The formation of the quartz wafer is the product of generating a single quartz crystal through hydrothermal synthesis.
When it comes to quartz wafer, it should be derived from the prototype or the fundamental form of this material. In the beginning, the quartz is presented as ingots, which are the one type of mineral bricks. Through hydrothermal synthesis, the quartz ingots are melted and generated into the finer form, which are the quartz rods. Under the stable properties the quartz rods have, they are sliced into smaller parts, which make the quartz wafers.
So, how these sliced parts of quartz becomes suitable materials of semiconductors? The answer lies in the special properties given from the fundamental form, which are the properties of quartz.
1. High thermal conductivity
Quartz is a naturally occurring substance that owns high thermal conductivity in the original state. In other words, it is good at heat transmission between two different substances.
2. Corrosion resistance
This substance is also resistant to corrosion when coming in contact with chemical disturbance. This means that it remains the optimal condition even if it is under harsh environments.
3. Optical transmission
Quartz also has good optical transmission, which refers to its excellent UV transparency, and this is an important factor in the application of semiconductor.
4. High stability
In the practical perspective, when under high temperature and chemical disturbance, quartz still remains stable condition. It is the high stability that makes quartz the ideal materials for semiconductors as they are sliced into wafers.
Fused quartz and fused silica are two substances made with different principles but have similar properties and relevant applications. The former is a naturally occurring substance, while the latter is a synthetic material instead. Despite of the different forming processes, they have similar properties such as the abilities to withstand high temperature, corrosion, or chemical effects.
In addition to the varied forming principles, there is another difference between the fused quartz and fused silica. The foundation of non-crystalline glass makes the latter better in the transmission of UV spectrum than the former. Besides, considering the composition of the substances, the fused silica has higher OH content than the fused quartz as well.
Quartz is a single crystal material with various crystal faces, and each has its own periodic arrangement of silicon and oxygen atoms. Quartz wafers are usually cut from single crystals parallel to these crystal planes, and their orientation is determined by the X, Y, and Z-axis. Moreover, there are also other orientations that are not easily expressible, such as AT-cut and ST-cut, which refer to crystal planes that are inclined towards the direction of the main crystal. Let’s take a closer look at some of the important procedures involved.
During the slicing process, the wire saw cuts many quartz rods at once. Before being installed on the saw, the X-ray orientation of the quartz rod is very precise and can produce a precise cutting angle. Usually, twelve rods are cut in the machine at a time, and up to 2000 individual wafers with very good thickness uniformity can be produced. The typical sawing process takes about 3 to 4 hours to complete. After the slicing is completed, an X-ray diffraction goniometer is used to measure the sample wafer to check the cutting angle. With the help of a large grinder, the thickness of the quartz wafer can be reduced to a specified thickness while finishing.
Then, the polished quartz wafers are bonded together to form a strong wafer stack. The paper pile can then be ground up through a rounding operation, or it can be cut into four or nine smaller paper piles for subsequent rounding. Similarly, the precise wafer stack designed for the SM blank is cut into many small stacks and then processed to a prescribed size. The template can be used to grind the flat plate directly on the rounded quartz blank, or it can be smoothed afterward by a flat grinder.
The frequency-temperature characteristics of crystal resonators mainly depend on the cutting direction. The X-ray machine can measure the cut corners on round blanks very accurately. The automatic angle sorting machine sorts the AT-cut blanks into 15-second angle groups. There is also a semi-automatic X-ray machine that can classify the θ angle on the blanks cut by dual rotation into five seconds.
Crystals with frequencies below 20 MHz that operate under the third or fifth overtones usually require at least one surface to be convex to enhance the Q of the desired operating mode. The contoured form concentrates the mode vibration to the central part of the quartz blank, thereby reducing edge effects. When it comes to forming blanks, two processes are most common. One is to grind the blanks in the long blanks into some cheaper biconvex or inclined crystal specifications. Centrifugal barrel grinding is used to increase the speed of forming SM parisons. The second is to contour and chamfer the lens through a precise diopter bowl.
Thick crystal blanks and lenses, especially crystals with double rotation cutting, can be manually sorted by a crystal impedance meter with a frequency counter or a special device composed of a frequency card and a network analyzer. The conventional automatic frequency classification is performed on almost all-round AT-cut blanks and SM blanks.
The installation of the crystal resonator is carried out by inserting the blank between the ribbons of the holder and then cleaning the parts in the stainless steel tray. The components are then reloaded into the ribbon tray, which is designated for electroplating operations. Finally, a small amount of conductive glue will be applied between the mounting structure and the blank. Controlled cement curing will establish a permanent fixation between the blank and the bracket. The crystal structure uses a heavy metal-free process.
Electrodes are formed on opposite surfaces of the blank by vacuum evaporation, thereby forming a resonator. The applied mask will define the shape and size of the electrode. Most manufacturers use direct electroplating to generate frequencies while depositing electrodes on both sides of the blank. The blank has been installed with the bracket and fixed on the bracket with conductive glue. This method is very economical and suitable for less stringent specifications. It can achieve high throughput, so the product price is low. A big advantage of this method is that metal deposition can be stopped precisely when the specified frequency endpoint is reached. After continuous baking, the subsequent final plating calibration is performed on only one blank surface.
The packaging of the quartz resonator is performed only by the resistance welding method. Resistance welding is carried out in a sealed chamber filled with very pure nitrogen and a small amount of helium. Helium is necessary for leak testing of sealed units. After sealing and leak testing, the parameters and temperature performance of the resonator are measured. In addition, accelerated life tests were performed on the crystals, including temperature cycling and temperature baking. All crystals undergo final inspection and testing before leaving the factory.
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