Introduction to Cutting Process of Xtal Blanks

When defining specifications for specific crystals for RF circuit design or other applications, it is often necessary to define cut quartz crystals. Terms including AT cut, CT cut, and SC cut appear in quartz crystal, and choosing the right cut can have a significant impact on performance.

Basic knowledge of crystal cutting

Quartz crystals have a complex structure, but all crystals are the same. There are countless ways of cutting relative to the X, Y, and Z axes of a blank crystal. Quartz crystals are anisotropic like other piezoelectric materials. This means that many of its properties (including mechanical, electrical, and optical properties) depend on the spindle. Therefore, the way the quartz blank is cut from the spindle crystal defines many characteristics of the final quartz crystal resonator. Bending mode of the main mode, temperature coefficient, aging characteristics, frequency stability, Q value, etc.

The so-called various "cuts" have different names, some of which are popular in some applications and others are widely used in others. With the introduction of new measures in recent years, some reduction measures have also been eliminated. Some basic cuts include cutting along the axis and then marking the cut according to the plane perpendicular to the cut. These cuts are rarely used today because other cuts have been found to provide better levels of performance for modern applications. Among the cuts used in today's RF and clock applications, At BT and SC cuts are the most important.

Development of quartz crystal cutting

Quartz crystals were widely used in transmitters in the 1920s and early 1930s. Compared with the use of LC oscillators, they have significant improvements in stability. Even if the stability of the LC oscillator is optimized, quartz crystal is obviously better. The early stage can be significantly improved, and even become zero at a certain temperature.

In order to develop these electronic components, we have carried out a lot of research. Since their functions are limited compared with the functions we have today, it will take some time to develop them.

In 1934, lac and Willard who worked in Bell Labs developed at cut quartz crystal and published a paper in the Bell Labs journal in July 1934: "some improvement of quartz crystal circuit components". Their other development is BT cut crystal, which also has many good properties, but it has not been widely used.

So far, at cut quartz crystal is the most widely used type, although SC cut quartz crystal began to appear in the 1970s for crystal ovens, etc. 

It was first proposed by Dr. Holland in 1974. In 1975, E. EER NISSE of the US Army Signal Corps published a paper predicting the performance of resonators φ=  22.5 ° and θ=- The 34.3 ° coordinate will produce the mechanical stress of the resonator with low-frequency change, which gives rise to the name SC of stress compensation.

Main crystal cutting process

An infinite number of crystal cuts can be defined. However, some of them define particularly useful attributes and assign specific names to these cuts. 

AT CUT:at cut of quartz crystal is usually used for frequencies between 0.5 and 300MHz, and has a thickness cutting mode of vibration.

It is the most widely used cutting, especially suitable for electronic instruments requiring oscillators to operate in the range of 500KHz to 300MHz, although the upper limit is increasing with the development of technology.

BT CUT: This is another kind of cutting similar to at cut of quartz crystal, which vibrates in thickness cutting mode and is usually used for frequencies from 0.5 to 200MHz.

It uses different angles: 49 ° to the Z axis. It has repeatable characteristics with a frequency constant of 2.536 MHz / mm. However, the temperature stability characteristics are not as good as at cut quartz crystal, but due to its high frequency constant, it can be more easily used for higher frequency operation.

GT CUT: GT cutting for quartz crystals is usually used at a frequency of about 0.1 to 2.5MHz and uses the width expansion mode of vibration. 

It is cut at an angle of 51 ° 7 ', and since the two vibration modes with different temperature coefficients cancel each other, the temperature coefficient is almost zero between + 25 and + 75 ° C.

IT CUT: This cutting uses the thickness cutting mode and is used for frequencies between about 0.5 and 200 MHz.

This crystal cutting is very similar to SC. However, for crystal ovens that need to work in the temperature range of 80-90 ° C, this option can overcome the difficulty of using SC at these temperatures.The highest turning point of it incision is between 85 and 105 ° C, but it is different from the mechanical stress sensitivity of SC.

SC CUT: This crystal cutting is used for frequencies between about 0.5 and 3200mhz.This cutting was developed in the late 1970s, especially for precision crystal ovens, but it does require a more complex manufacturing process because of the subsequent need for double angle rotation and precision grinding at the same time.

XY CUT: This cutting is essentially a crystal cutting format for low-frequency applications, usually between 5 and 100kHz.It uses the long bend mode. 

Crystal cutting is widely used in a low frequency with a common frequency of 32.768 kHz.Its advantage is that the frequency is very small and cheaper than other low-frequency crystal types. In addition, it has low impedance and a low C0 / C1 ratio.

AT quartz crystal cutting

At cut quartz crystal is the most widely used cutting method, especially for electronic instruments, radio systems, microprocessor clocks, and many other applications that require oscillators to operate in the MHz range. With the development of technology, the upper limit is higher and higher. However, the highest frequencies usually operate in overtone mode because the crystal becomes very thin at high frequencies. One of the advantages of this quartz cutting is the temperature coefficient. At 26 ° C, this value becomes zero and even either side is relatively flat, especially compared with other cuts.

BT quartz crystal cutting

BT crystal cutting and at crystal cutting are introduced almost at the same time. The thickness of the cut crystal is about 50% thicker than that of the at cut quartz crystal.

However, the thicker the crystal is, the thicker it is at higher frequencies and therefore the stronger it is. It can be expected that the crystal will become thinner as the frequency increases. However, its disadvantage is that its temperature characteristic is worse than that of at cut quartz crystal, which is caused by a higher frequency constant. 

Even so, BT cut crystals are still used in higher frequency applications, especially when it is necessary to work at the fundamental frequency rather than overtone.

SC quartz crystal cutting

The SC used to describe this type of quartz crystal cutting represents "stress compensation".It is specially developed for use in precision crystal ovens where some key requirements are insensitive to thermal and mechanical stresses. Another important factor is that SC cut crystals have good phase noise and aging characteristics.Oven controlled crystal oscillator OCXO usually requires characteristics.

This cut rotates the reference axis twice: 35 ° 15 'and 21 ° 54'. It not only has excellent aging and stability performance, but also has excellent phase noise performance, but it has higher ESR and is more vulnerable to resonance. 

One of the difficulties of SC cutting is to create difficulties in the manufacturing process because the requirement to use composite angles in SC cutting increases the cost of measuring angles, which are then maintained in the subsequent grinding and polishing process. The tolerance of SC cutting is very small. They usually require tolerance of ± 10ppm, while the tolerance for at cut quartz crystal is ± 30ppm.