Method for growing lithium tantalate crystals

Abstract

The present invention relates to the technical field of piezoelectric crystals, and in particular relates to a method for growing lithium tantalate crystals. The method comprises the following steps: a) raw material addition, involving: crushing and grinding recycled crystal raw materials to be added, covering an added polycrystalline powder with same, and finally, filling in gaps among crystal fragments with polycrystalline powder; b) material melting and temperature control, involving: after the raw materials are heated, observing a temperature variation curve, adjusting the temperature to be 5-10°C higher than the value of the first wave peak of the temperature curve, and keeping the temperature stable for 1-2 h, thereby completing the melting of the raw materials; c) crystal growth, involving: allowing seed crystals to descend until same come into contact with a molten liquid, maintaining the temperature after necking of the seed crystals, starting automatic growth of crystals, observing changes in the ridges of the crystals, and remelting and remolding furcate and relatively thin ridges; and d) tailing of crystals, involving: when the increment of the weight of the crystals tends to decrease and the speed at which the weight of the crystals increase is lower than 10 g / min, performing cutting and tailing, and detaching the crystals from a liquid surface at a rate of 30-50 mm / min.

Description

A method for growing lithium tantalate crystals Technical Field

[0001] The present invention relates to the field of piezoelectric crystals, and specifically relates to a method for growing lithium tantalate crystals. Background Art

[0002] In recent years, the widespread use of high-frequency and multi-band communications in wireless communications, particularly mobile phones, has significantly increased demand for surface acoustic wave (SAW) filters, leading to higher quality requirements. With the continuous advancement of SAW technology, the application areas of SAW devices have continued to expand, including applications in mobile phones, multimedia data broadcasting systems (such as VOD), automotive electronics, wireless LAN, and digital television, increasing market demand for SAW devices. Furthermore, the miniaturization of communications equipment requires increasingly precise manufacturing of individual components, which in turn drives the miniaturization of SAW filters.

[0003] During the growth of lithium tantalate crystals, due to the melting point of lithium tantalate crystals at 1650°C, the temperature required to melt the lithium tantalate raw materials is high, and a high-melting-point crucible is generally required. However, the high cost of high-melting-point iridium crucibles limits the industrial development of lithium tantalate crystal growth. At the same time, iridium oxidation causes coloration of lithium tantalate crystals, and bubble defects during the crystal growth process also seriously affect the use of lithium tantalate crystals in high-frequency and multi-band applications. Secondly, when using a platinum crucible with a lower melting point and using traditional methods to seed the crystals, if the filling method is improper or the temperature judgment is inaccurate, it is easy to cause the crucible to deform or break down, affecting the crystal quality. To solve this problem, the patent with announcement number CN114775057B proposes the use of platinum-iridium crucibles to grow lithium tantalate crystals. This patent uses the change in seed crystal weight to determine the melting of the raw materials, and at the same time improves the crystal quality by adjusting the pulling speed of shoulder growth and equal diameter growth to control the crystal crystallization rate. The method is significantly different from that of the present application. The invention patent with announcement number CN105696078B discloses a method for preparing lithium tantalate single crystals. The patent produces near-stoichiometric lithium tantalate crystals with high purity and uniform composition by precisely controlling the raw material ratios, process steps, and heating methods, thereby improving the electro-optic coefficient and nonlinear optical coefficient. However, this technical solution uses an iridium crucible, and its process control method is significantly different from that of the present application.

[0004] In summary, the prior art lacks a method for growing lithium tantalate crystals that is difficult to achieve both low cost and high crystal quality.

[0005] Summary of the Invention

[0006] The object of the present invention is to provide a method for growing high-quality lithium tantalate crystals using a platinum crucible.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A method for growing lithium tantalate crystals comprises the following steps:

[0009] a) Raw material filling: First, lithium tantalate polycrystalline powder is spread flat on the bottom of the platinum crucible, and then the recycled crystal raw materials to be added are crushed and polished. The recycled crystals are polished to a particle size of less than 30mm and covered on the filled polycrystalline powder. Finally, the polycrystalline powder is used to fill the gaps between the crystal fragments;

[0010] b) Material temperature control: first increase the temperature to about 1500-1550℃ at a rate of 100-200℃ / h, then slowly increase the temperature at 30-50℃ / h, observe the temperature change curve in the furnace, and adjust the temperature to 5-10℃ higher than the peak value of the first temperature curve. After stable heat preservation for 1-2h, the raw material melting is completed;

[0011] c) Crystal growth: Lower the seed crystal into contact with the melt, shrink the seed crystal by 2-3mm, keep warm for 1h-1.5h, and then enter the crystal automatic growth phase. Observe the changes in the crystal edge line when the crystal shoulder is released. If the crystal edge line is bifurcated or thin, perform crystal remelting and reshaping.

[0012] d) Crystal finishing: When the increase in crystal weight shows a downward trend and the rate of increase in crystal weight is lower than 10g / min, cutting and finishing are carried out. After leaving the liquid surface at a rate of 30-50mm / min, the crystal growth is completed.

[0013] In step a), the melting point of the platinum crucible is 1750°C, which is close to the temperature of the lithium tantalate material, 1650°C. During the raw material processing, the platinum crucible will become soft. If it directly contacts large pieces of recycled crystals, the crucible wall will be under great pressure, deforming or even cracking. Therefore, the recycled crystals must be crushed and the sharp corners must be rounded. At the same time, lithium tantalate polycrystalline powder is added to the bottom and sides of the crucible to act as a buffer.

[0014] Among them, in step b), due to the low melting point of the platinum crucible, when the temperature of the material is increased, the temperature is likely to rise too high, causing the crucible to break down. By monitoring the temperature curve of the material, when the bottom polycrystalline material begins to melt, the raw material melts and absorbs heat, the temperature drops, and the temperature curve in the furnace will have a first obvious peak. The peak temperature range generally appears between 1620-1650°C. This temperature is the lowest temperature point that can achieve the melt. When the temperature continues to rise, the temperature will rise again. The temperature is controlled to be 5-10°C higher than the peak value of the first temperature curve to achieve complete melting of the raw material, which can prevent the temperature from rising too high and achieve rapid melt temperature judgment.

[0015] During the crystal growth process of the Czochralski method, due to the characteristics of lithium tantalate crystals, when the crystal is shouldered, four ridges will generally appear on the shoulder surface of the crystal. If the ridges are bifurcated or thin, it will cause the formation of polycrystalline, increase crystal defects, and even cause crystal cracking. In step c), the ridge changes are monitored in real time by a CCD camera, and the crystal quality of the crystal can be judged by the ridges. If the ridges are bifurcated or thin, they should be melted back and reshaped in time to ensure the crystal quality. If the ridges are bifurcated, the crystal is melted back to 5-10mm back from the bifurcation point of the ridges, and the bifurcation point is melted away and the shoulder is grown again; if the ridges are thin, all the crystals are directly melted back, and the seed crystal is sunk 3-5mm and the temperature is controlled again to induce the crystal. The thin ridges are caused by the temperature change during the contact of the seed crystal with the liquid surface during the induction of the seed crystal, which causes the seed crystal to be damaged. Sinking the seed crystal and melting the damaged part can solve the problem of subsequent crystal cracking caused by the thin ridges.

[0016] At the same time, the parameters of the lower furnace growth can be adjusted according to the monitored edge bifurcation data. The specific method is as follows: if the edge bifurcation point appears within 1 / 3 of the crystal shoulder surface, the distance between the crucible and the highest point of the heating coil is reduced by 5-10mm; if it appears at 1 / 3-2 / 3 of the crystal shoulder surface, the distance between the crucible and the highest point of the heating coil is reduced by 10-15mm; if it appears at 2 / 3-3 / 3 of the crystal shoulder surface, the distance between the crucible and the highest point of the heating coil is reduced by 15-20mm. This is because the bifurcation of the edge is caused by an excessively large temperature gradient on the growth liquid surface. By adjusting the relative positions of the heating coil and the crucible, the temperature gradient distribution on the growth liquid surface can be adjusted to ensure stable and continuous growth of the crystal, while avoiding the generation of polycrystalline due to edge bifurcation.

[0017] In step d), when the increase in crystal weight shows a downward trend and the rate of increase in crystal weight is less than 10 g / min, it is determined that the remaining amount of material at the crucible bottom is very small, indicating that crystallization is nearing completion. At this point, the optimal cutting weight is confirmed and suitable for final cutting. Since platinum crucibles soften at high temperatures, if too much material remains in the crucible bottom, it will inevitably cause deformation of the crucible bottom due to thermal expansion and contraction during cooling. Furthermore, due to the deposition of impurities in the raw materials, the crystal quality of the crystals will be reduced during continuous crystal growth.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) Platinum crucibles are used instead of iridium crucibles to grow lithium tantalate crystals. On the one hand, the absorption of crystal impurities caused by iridium oxidation during the crystal growth process is avoided, thereby improving the crystal quality. On the other hand, due to the low melting point of platinum crucibles, the risks of deformation and breakdown of platinum crucibles are overcome during the crystal growth process by optimizing the processes of raw material addition, chemical temperature control, crystal growth and adjustment. This improves the quality of crystal growth while also reducing production costs.

[0020] (2) This process monitors the temperature changes during the crystal growth process through the temperature curve, confirms the target temperature adjustment temperature according to the temperature curve, and realizes convenient and controllable process temperature adjustment, avoiding the risk of crucible breakdown caused by excessive temperature rise;

[0021] (3) This process uses a CCD camera to monitor the changes in the crystal edge line during the shouldering process in real time. If the crystal edge line is bifurcated or thin, the crystal is melted and reshaped, and the corresponding parameters are adjusted immediately to ensure the quality of the output crystal. At the same time, according to the analysis of the edge line, the crystal edge line is adjusted to ensure the quality of the output crystal.

[0022] Line deviation data is used to adjust key growth parameters such as the crucible and heating coil in the lower furnace to achieve continuous high-quality crystal growth and improve production efficiency;

[0023] (4) This process confirms the optimal cutting point for the end of the crystal by the change in crystal weight, ensuring that the bottom material of the crucible is crystallized as completely as possible each time, reducing the impact of impurity deposition of the bottom raw material on the overall crystal quality, ensuring the consistency of the crystal Curie temperature, and at the same time significantly reducing the deformation of the platinum crucible caused by thermal expansion and contraction of the residual crucible bottom material during the cooling process. DETAILED DESCRIPTION

[0024] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0025] Example 1

[0026] (a) Raw material filling: First, grind the recycled lithium tantalate crystals into small pieces with a particle size of less than 30 mm and grind off the sharp corners. Then, load the lithium tantalate polycrystalline raw material powder into a platinum crucible, fill the bottom of the crucible with a filling thickness of 25 mm, and then fill the center with the polished lithium tantalate block raw material. At the same time, continue to fill the gaps at the edges with lithium tantalate polycrystalline raw material powder and fill and compact them to ensure that the block raw material does not contact the crucible wall to prevent the high-temperature block from sticking to the crucible and causing deformation. After the filling is completed, set up the hot field;

[0027] (b) Material temperature control: First, increase the temperature at a rate of 150°C / h to approximately 1500°C, then slowly increase the temperature at 50°C / h. Observe the temperature curve and adjust the temperature to 10°C above the peak of the first temperature curve. After maintaining the temperature for 2 hours, the raw materials are melted.

[0028] (c) Crystal growth: The seed crystal was lowered into contact with the melt, and the seed crystal was necked 3 mm. The crystal was kept warm for 1.5 h, and then the crystal began to grow automatically. The edge line of the crystal during the shouldering process was monitored by a CCD camera. If a bifurcation of the edge line was found, the position of the bifurcation was recorded. If the bifurcation point was found to be within 1 / 3 of the shouldering surface of the crystal, the automatic growth was terminated, and the solution was heated and melted to the bifurcation point of the edge line. After stabilization, the automatic growth was resumed.

[0029] (d) Crystal tailing: When the increase in crystal weight shows a downward trend and the rate of increase in crystal weight is less than 10 g / min, the crystal is cut and tailed, and separated from the liquid surface at a rate of 50 mm / min.

[0030] The platinum crucible used for growth in this embodiment did not deform as a whole, and no breakdown or cracks occurred. At the same time, the XRD half-peak width of the crystal was within 15', and the head-tail Curie temperature of the crystal was 603±1°.

[0031] Example 2

[0032] (a) Thermal field adjustment: According to Example 1, the bifurcation point appears within 1 / 3 of the crystal shoulder surface. The bottom support of the crucible is raised by 10 mm to adjust the thermal field temperature gradient distribution.

[0033] (b) Raw material filling: First, the recycled lithium tantalate crystals are ground into small pieces with a particle size of less than 30 mm and sharp corners are ground off. Then, the lithium tantalate polycrystalline raw material powder is loaded into a platinum crucible. The bottom of the crucible is filled to a thickness of 25 mm. Then, the polished lithium tantalate block raw material is filled in the center. At the same time, the lithium tantalate polycrystalline raw material powder is continued to be filled and compacted in the gaps at the edges to ensure that the block raw material does not contact the crucible wall to prevent the high-temperature block from sticking to the crucible and causing deformation. After the filling is completed, the hot field is set up;

[0034] (c) Material temperature control: First, increase the temperature at a rate of 150°C / h to approximately 1500°C, then slowly increase the temperature at 50°C / h. Observe the temperature curve and adjust the temperature to 10°C above the peak of the first temperature curve. After maintaining the temperature for 2 hours, the raw materials are melted.

[0035] (d) Crystal growth: The seed crystal was lowered into contact with the melt, and the seed crystal was necked 3 mm. After holding for 1.5 h, the crystal was cut into the automatic growth stage. The edge lines of the crystal during the shouldering process were monitored by a CCD camera. The crystals grown in this furnace did not show edge bifurcation.

[0036] (e) Crystal tailing: When the increase in crystal weight shows a downward trend and the rate of increase in crystal weight is less than 10 g / min, the crystal is cut and tailed, and separated from the liquid surface at a rate of 50 mm / min.

[0037] The platinum crucible used for growth in this embodiment did not deform as a whole, and no breakdown or cracking occurred. At the same time, the XRD half-peak width of the crystal was 12′, the head and tail Curie temperature of the crystal was 603±1°, the crystal crystallization quality was good, and no ridge bifurcation occurred during the crystal growth process, thereby improving production efficiency. To highlight the beneficial effects of the present invention, the following comparative example experiments were also conducted.

[0038] Comparative Example 1

[0039] (a) Raw material filling: The lithium tantalate polycrystalline raw material powder to be filled is loaded into the platinum crucible and compacted, and then the lithium tantalate recycled crystals to be filled are loaded into the platinum crucible. After the filling is completed, the hot field is set up;

[0040] (b) Material temperature control: same as in Example 1;

[0041] (c) Crystal growth: same as in Example 1;

[0042] (d) Crystal finishing: same as Example 1.

[0043] After the growth of this comparative example, the platinum crucible was oval and the crystal was distorted. It was judged that the lithium tantalate crystal tipped over during the heating process and hit the platinum crucible, causing deformation. At the same time, due to the deformation of the crucible, the crystal heating uniformity deteriorated, causing the crystal to be distorted.

[0044] Comparative Example 2

[0045] (a) Raw material addition: same as in Example 1;

[0046] (b) Temperature control of chemical materials: first increase the temperature at a rate of 150°C / h to about 1500°C, then slowly increase the temperature at 50°C / h, observe the changes in convection, adjust the temperature so that temperature convection occurs on the liquid surface, and maintain a stable temperature for 2 hours;

[0047] (c) Crystal growth: same as in Example 1;

[0048] (d) Crystal finishing: same as Example 1.

[0049] The platinum crucible used for growth in this comparative example did not deform as a whole, and the crystal did not crack, but small cracks occurred on the side of the crucible. It was judged that the temperature rose too high during the heating process, approaching the melting point of platinum.

[0050] Comparative Example 3

[0051] (a) Raw material addition: same as in Example 1;

[0052] (b) Material temperature control: same as in Example 1;

[0053] (c) Crystal Growth: Although the problem of ridge bifurcation was found during the crystal growth process in Example 1, the problem of ridge bifurcation in Example 1 was not analyzed, nor was the relative position of the heating coil and the crucible properly adjusted. Instead, the seed crystal was directly lowered into contact with the melt, the seed crystal was necked 3 mm, and the temperature was kept for 1.5 hours before the crystal was directly allowed to grow automatically.

[0054] (d) Crystal finishing: same as Example 1.

[0055] After the growth of this comparative example, no abnormalities occurred in the platinum crucible, but it was found that the crystal edge lines bifurcated during the growth process, and the XRD half-peak width and Curie temperature of the completed crystal were large. It was judged that the relative position of the crucible and the heating coil were not adjusted, resulting in a thermal field gradient problem and the inability to achieve efficient continuous crystal growth.

[0056] Comparative Example 4

[0057] (a) Raw material addition: same as in Example 1;

[0058] (b) Material temperature control: same as in Example 1;

[0059] (c) Crystal growth: same as in Example 1;

[0060] (d) Crystal finishing: When the crystal reaches the target weight, it is directly cut and finished, leaving the liquid surface at a rate of 50 mm / min.

[0061] The bottom of the platinum crucible used for growth in this comparative example was severely deformed. This was judged to be due to excessive excess material at the bottom of the crucible. After cooling, the material solidified and expanded, causing the crucible to deform.

[0062] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for growing lithium tantalate crystals, characterized in that: The following steps are involved: a) Raw material filling: first, spread the lithium tantalate polycrystalline powder on the bottom of the platinum crucible, then crush and grind the recycled crystal raw materials to be added, and cover them on the filled polycrystalline powder, and finally use the polycrystalline powder to fill the gaps between the crystal fragments; b) Temperature control of melting materials: Heat the raw materials and observe the temperature change curve in the furnace. When the polycrystalline material at the bottom begins to melt, the temperature drops due to the heat absorption of the raw materials during melting. The first obvious peak will appear in the temperature curve in the furnace. The peak temperature appears at 1620-1650℃. The temperature is adjusted to 5-10℃ higher than the peak value of the first temperature curve. After stable heat preservation for 1-2h, the raw material melting is completed. c) Crystal growth: lower the seed crystal to contact the melt, shrink the seed crystal by 2mm-3mm, keep warm for 1h-1.5h, and then enter the automatic growth of the crystal. Observe the changes in the crystal edge lines. When the crystal edge lines are bifurcated and thin, the crystal needs to be melted back and reshaped. Specifically, if the edge lines are bifurcated, melt the crystal back to 5-10mm above the bifurcation point of the edge lines, melt the bifurcation point and then grow the shoulder again. At the same time, adjust the thermal field process parameters of the lower furnace crystal growth according to the edge line bifurcation data; if the edge lines are thin, directly melt all the crystals, sink the seed crystal 3-5mm and then control the temperature again to seed the crystal; d) Crystal finishing: When the increase in crystal weight shows a downward trend and the increase rate of crystal weight is lower than 10g / min, cutting and finishing are carried out. After leaving the liquid surface at a rate of 30-50mm / min, the crystal growth is completed.

2. The method for growing lithium tantalate crystals according to claim 1, characterized in that: In the step a), a grinder is used to grind the recycled crystals, and the particle size of the recycled crystals after grinding is within 30 mm.

3. The method for growing lithium tantalate crystals according to claim 1, characterized in that: In the step b), the temperature is first increased to 1500-1550° C. at a heating rate of 100-200° C. / h, and then slowly increased at 30-50° C. / h to observe the temperature change curve.

4. The method for growing lithium tantalate crystals according to claim 1, characterized in that: In the step c), the edge line changes are monitored in real time by a CCD camera.

5. The method for growing lithium tantalate crystals according to claim 1, characterized in that: In the step c), the specific method of adjusting the thermal field process parameters of the lower furnace crystal growth according to the monitored edge data is as follows: if the edge bifurcation point appears within 1 / 3 of the crystal shoulder surface, the distance between the crucible and the highest point of the heating coil is reduced by 5-10mm; if it appears at 1 / 3-2 / 3 of the crystal shoulder surface, the distance between the crucible and the highest point of the heating coil is reduced by 10-15mm; if it appears at 2 / 3-3 / 3 of the crystal shoulder surface, the distance between the crucible and the highest point of the heating coil is reduced by 15-20mm.