Main methods
There are many ways to grow single crystals, there are mainly four methods:
The hydrothermal method crystals grow from solution under high temperature and high pressure, and the container is high pressure. Kettle (Figure 1). Generally, the temperature difference method is used, that is, the raw materials are placed at the bottom of the autoclave at a higher temperature, and the seed crystals are located at the upper part of the autoclave at a lower temperature (if the temperature coefficient of solubility is negative, the opposite is true), and the container is filled with solvent. The raw material is dissolved in the solvent under high temperature and high pressure. Due to the convection of the temperature difference, the solution reaches the supersaturation at the seed crystal site and causes the seed crystal to grow. The circulation of the solution promotes the continuous dissolution of the raw materials and the continuous growth of crystals. The main purpose of this method is to grow crystals. Generally speaking, many oxide single crystals can use this method.
Zone melting method in a relatively long solid raw material, a short melting zone moves slowly, so that the solute in the raw material is redistributed during the crystallization process (Figure 2). The number, size and moving direction of the melting zone can be controlled. When a molten zone passes through the ingot, there are two liquid-solid interfaces. The solidification interface repels some solutes and absorbs others. This method was invented by WG Pufan in the United States as early as 1952. It has a wide range of applications. It is mainly used to purify metals, semiconductors, organic and inorganic compounds; in addition to purifying crystals, it can also make certain impurities evenly distributed throughout the entire In the crystal.
The epitaxial growth method also known as epitaxial growth, refers to the continuous growth of two crystal surfaces to form an oriented growth interface. Generally speaking, a crystal surface provides a preferential location from the structure, so that the second crystal phase is attached to it. There are two main methods of epitaxy: one is gas phase epitaxy, and the other is liquid phase epitaxy. Most of the epitaxy is to epitaxial a layer of film from a crystal substrate, therefore, according to the nature of the substrate and the film can be divided into homogeneous and heterogeneous epitaxy.
Sublimation method is the basic method of growing crystals from the vapor phase. The raw materials are sublimated and crystallized in the tube, heated at the high temperature end of the tube, and then sublimated into a gas phase, and then transported to the other end with a lower temperature, where they condense and nucleate and grow. This method is divided into two methods: open tube and closed tube.
Process flow
The specific process of CZ growth includes several stages such as charging and melting, welding, necking, shoulder setting, shoulder turning, equal diameter growth and finishing. . 1. The charging, melting, charging, and melting stages are the first stage of the CZ growth process. This stage seems very simple, but the correct operation of this stage is often related to the success or failure of the growth process. Most accidents that cause significant losses (such as crucible rupture) occurred or originated at this stage.
2. Fusion of seed crystal and molten silicon
When the silicon material is completely melted, the heating power is adjusted to control the temperature of the melt. Under normal circumstances, there are two sensors to monitor the temperature of the melt surface and the graphite cylinder of the heater insulation cover. In the case of little change in the thermal field and crystal pulling process, the temperature reading of the previous furnace can be used as a reference to set Seeding temperature. Adjust the gas flow, pressure, crucible position, crystal rotation, and crucible rotation according to the process requirements. After the silicon material is completely melted, the melt must have a certain stabilization time to reach the melt temperature and the stability of the melt flow. The larger the charge, the longer the time required. After the melt is stable, lower the seed crystal to a distance of 3~5mm from the liquid surface to preheat the grain to reduce the temperature difference between the seed and the molten silicon, thereby reducing the generation of the seed crystal when the seed crystal contacts the molten silicon Thermal stress. After preheating, the seed crystals are lowered to the surface of the melt to allow them to fully contact. This process is called welding. During the welding process, pay attention to the phenomenon that occurs to judge whether the temperature of the molten silicon surface is appropriate. At the appropriate temperature, the interface will gradually produce the meniscus at the solid-liquid-gas three-phase junction after welding. The halo (usually called the "aperture") gradually changes from part of the halo to a complete circular halo. Too high temperature will melt the seed crystal, and if the temperature is too low, the meniscus halo will not appear or even grow too much. Crystal. Skilled operators can judge whether the temperature of the melt is appropriate according to the width and brightness of the meniscus halo.
3. Neck lead
Although the seed crystals are made of dislocation-free silicon single crystals [16-19], when the seed crystal is inserted into the melt, it is affected by the temperature difference between the seed crystal and the molten silicon. The resulting thermal stress and surface tension can produce dislocations. Therefore, after the welding, the application of the thin necking process, that is, the Dash technology, can make the dislocations disappear and establish a dislocation-free growth state.
See section 7.2 for the principle of Dash's dislocation-free growth technology. The slip plane of dislocations in a silicon single crystal with a diamond structure is the {111} plane. When growing in [l00], [lll] and [ll0] crystal directions, the minimum included angles between the slip surface and the growth axis are 36.16°, 19.28° and 0°, respectively. Dislocations extend along the slip plane and produce slippage. Therefore, dislocations must extend and slip to the crystal surface and disappear. The [100] crystal orientation is the easiest to grow, followed by the [111] crystal orientation, and [ll0] In the case of crystal orientation growth, if there is only an extension effect, dislocations will penetrate the entire crystal. The neck-neck process usually uses high pulling speed to reduce the crystal diameter to about 3mm. Under this condition, the thermal stress during the cooling process is very small and no new dislocations will be generated. High drawing speed can form oversaturation point defects. Under this condition, even [ll0] crystal orientation growth dislocations propagate to the crystal surface by climbing. It has been found in practice that the thick and short necks of heavily doped antimony crystals can eliminate dislocations, which may be achieved through the climbing mechanism. On the premise that the seed crystal can bear the weight of the ingot, the neck should be as long as possible, and the ratio of diameter should generally reach 1:10.
4. Shoulder-releasing
After the neck-inducing stage is completed, the diameter must be enlarged to the target diameter. When the neck-neck grows to a sufficient length and reaches a certain pulling rate, the pulling speed can be reduced to carry out shoulder-releasing. Almost all crystal pulling processes use the flat shoulder process, that is, the shoulder angle is close to 180°. This method reduces the loss of raw material at the head of the ingot.
5. Shoulder turning
When crystal growth changes from the diameter enlargement stage to the equal diameter growth stage, shoulder turning is required. When the shoulder diameter is close to the predetermined target, the pulling speed is increased and the crystal gradually enters the equal diameter growth. In order to keep the position of the liquid level unchanged, start the pot raising when turning the shoulder or after turning the shoulder, generally use a proper pot and make it change with the crystal rise. When shoulders are placed, the diameter of the meniscus increases quickly, and there is almost no meniscus halo. During the shoulder turning, the meniscus halo gradually appears, the width increases, and the brightness becomes larger. The crystal pulling operator should be able to follow the meniscus halo Width and brightness, accurately judge the change of diameter, and adjust the pulling speed in time to ensure that the shoulder is smooth, the crystal diameter is uniform and reaches the target value. In principle, it is also possible to increase the temperature of the melt to achieve the shoulder rotation, but the increase in temperature will enhance the heat convection in the melt, reduce the stability of the melt, and prone to dislocations (broken buds). Therefore, the process is adopted Quick-turn shoulder technology to increase the pulling speed.
6. Equal diameter growth
When the crystal basically achieves equal diameter growth and reaches the target diameter, automatic diameter control can be implemented.
In the equal-diameter growth stage, it is not only necessary to control the diameter of the crystal, but more importantly, to maintain the dislocation-free growth of the crystal. There is always thermal stress in the crystal. Practice shows that the isothermal surface of the crystal cannot maintain an absolute plane during the growth process. As long as the isothermal surface is not a plane, there will be a radial temperature gradient, forming thermal stress, and axial temperature distribution in the crystal. Often in the form of an exponential function, thermal stress is inevitably generated. When these thermal stresses exceed the critical stress of silicon, dislocations will occur in the crystal. Therefore, it is necessary to control the radial temperature gradient and the axial temperature gradient not to be too large, so that the thermal stress does not exceed the critical stress of silicon, and to meet such conditions to maintain dislocation-free growth.
On the other hand, the refractory solid particles contained in the polycrystal, the furnace dust (the particles formed by cooling in the furnace atmosphere after the SiO in the melt in the crucible volatilizes), the crucible When they move to the growth interface, they will cause dislocations (often called broken buds). The reason is that they are non-uniform nucleation crystal nuclei, and the other is that they become the source of dislocations. Adjusting the structure of the thermal field and the initial position of the crucible in the thermal field can change the temperature gradient in the crystal. Adjusting the flow and pressure of the shielding gas, and adjusting the flow direction of the gas, can take away the volatile SiO and harmful impurities CO gas, prevent the fall of the furnace dust, which is conducive to the growth of dislocation-free single crystals, and it also changes the temperature gradient in the crystal. Role.
The judgment of the dislocation-free state differs depending on the crystal orientation of the crystal. Generally, it can be determined by the growth stripes (usually called bracts) and facets (usually called flat edges and ribs) on the outer surface of the ingot. Line) to judge. When growing, there are six ridges appearing in the shoulder-releasing stage, three main ridges, three auxiliary ridges, and bracts and three flat ridges on the ingot at the isocrystalline stage. The meniscus is caused by the appearance of small planes on the growth interface. There are obvious straight sections on the face halo. When the growth crystal direction is aligned, the three small planes should be equal in size and form an angle of 120° with each other. However, in actual growth, due to the deviation of the growth direction, the small planes may be large or small, and some may even disappear. When growing in the direction, there are four ridges and no bracts. When growing without dislocations, the four ridges should be continuous on the entire crystal. As long as one of the ridges disappears or is discontinuous, it means that a dislocation (broken bud) has occurred.
The treatment after dislocation occurs depends on the situation. The treatment method is different. When the length of the ingot is not long, it should be melted back and then re-pulled; when the ingot exceeds a certain length, the crucible When there is still a lot of molten material, you can lift the crystal ingot, take it out after cooling, and then pull out the next crystal ingot; when there is not much melt in the crucible, either lift the crystal or continue to pull it down, and it will break. The bud part is used as reheating material. The crystal pulling personnel should adjust the crystal pulling process parameters to avoid dislocations as much as possible.
The "bud silk" mentioned here is essentially a rotating surface stripe. In Section 4.2.5, we have discussed the growth rate fluctuations in the axial direction (in the pulling direction) produced by the rotation of the crystal and the resulting rotation under the condition that the rotation axis of the crystal is inconsistent with the symmetry axis of the temperature field. Impurity streaks. Let us discuss the results of the fluctuations in the growth rate of the crystal in the radial direction (perpendicular to the pulling direction) under the same conditions.
7. Closing
The function of closing is to prevent dislocations from delaying backward. In the crystal pulling process, when the dislocation-free growth state is interrupted or the crystal pulling is completed and the crystal suddenly leaves the liquid surface, the dislocation-free crystal that has been grown is subjected to thermal shock, and its thermal stress often exceeds the critical stress of silicon. At this time, dislocations will be generated and will be extended back to the crystal whose temperature is still at the lowest temperature of parametric deformation (Figure 4.20), forming a dislocation row and a star-shaped structure.