At present, more than 98% of electronic components are made of monocrystalline silicon. Among them, Czochralski method accounts for about 85%, and the rest is FZ growth method by floating melting method. Single crystal silicon grown by Czochralski method is used to produce low power integrated circuit components. Single crystal silicon grown by FZ method is mainly used for high power electronic components. CZ method is more widely used in semiconductor industry than FZ method, mainly because its high oxygen content provides the advantages of wafer strengthening. Another reason is that Czochralski method is easier to produce large-size monocrystalline silicon rods than FZ method.
At present, CZ method is mainly used in China.
Main equipment of Czochralski method: Czochralski growth furnace
The composition of Czochralski growth furnace can be divided into four parts.
(1) Furnace body: including timely crucible, graphite crucible, heating and insulation elements and furnace wall.
(2) Pulling and rotating mechanism of crystal bar and crucible: including seed crystal chuck, hanging wire and pulling and rotating element.
(3) Atmospheric pressure control: including gas flow control, vacuum system and pressure control valve.
(4) Control system: including detection sensor and computer control system.
Processing technology:
Feeding → melting → necking growth → shoulder growth → equal diameter growth → tail growth.
(1) charging: polysilicon raw materials and impurities are put into a timely crucible, and the types of impurities depend on the N-type or P-type resistance. Impurities include boron, phosphorus, antimony and arsenic.
(2) Melting: After the polycrystalline silicon raw material is added into the timely crucible, the crystal growth furnace must be closed and vacuumized, then high-purity argon gas is filled to keep it within a certain pressure range, and then the graphite heater is turned on and heated to the melting temperature (1420℃) to melt the polycrystalline silicon raw material.
(3) Neck growth: When the temperature of the silicon melt is stable, the seed crystal is slowly immersed in the silicon melt. Due to the thermal stress when the seed crystal is in contact with the silicon melt field, dislocations will occur in the seed crystal, which must be eliminated by shrinkage growth. Necking growth is to quickly pull the seed crystal upward, so that the diameter of the grown seed crystal is reduced to a certain size (4-6mm). Because the dislocation line forms an angle with the growth axis, as long as the necking is long enough, dislocations can grow on the crystal surface, resulting in zero dislocation crystals.
(4) Shoulder growth: After the neck grows, the temperature and pulling speed must be reduced to gradually increase the crystal diameter to the required size.
(5) Equal-diameter growth: after the neck shoulder is grown, the diameter of the crystal bar can be kept between plus and minus 2mm by constantly adjusting the drawing speed and temperature, and this sizing part is called equal-diameter part. The monocrystalline silicon wafer is taken from the equal-diameter part.
(6) Tail growth: If the crystal bar leaves the liquid surface immediately after the equal-diameter part grows, the force will cause dislocation and slip line of the crystal bar. Therefore, in order to avoid this problem, the diameter of the crystal rod must be gradually reduced until it becomes a sharp point and separates from the liquid surface. This process is called tail growth. The grown crystal bar is lifted to the upper furnace chamber to be cooled for a period of time and then taken out, thus completing a growth cycle.
Processing the monocrystalline silicon rod into a monocrystalline silicon polished silicon wafer.
Processing flow:
Single crystal growth → cutting → external diameter rolling → flat edge or V-groove treatment → slicing.
Chamfering → abrasive corrosion → polishing → cleaning → packaging.
Cutting: The purpose is to cut off the head and tail of the monocrystalline silicon rod and the part exceeding the customer's specifications, divide the monocrystalline silicon rod into lengths that can be processed by slicing equipment, and cut the specimen to measure the resistivity and oxygen content of the monocrystalline silicon rod.
Cutting equipment: internal cutting machine or external cutting machine.
Main imported materials for cutting: blades.
Outer diameter grinding: Because the outer diameter surface of monocrystalline silicon rod is uneven and the diameter is larger than the diameter specification specified by the final polished wafer, more accurate diameter can be obtained by outer diameter rolling.
Outside diameter rolling equipment: grinder.
Flat edge or V-groove treatment: refers to orientation and specified treatment, which is used for flat edge or V-shape of monocrystalline silicon in a specific crystallization direction.
Processing equipment: grinder and X-ray diffractometer.
Slicing: refers to cutting single crystal silicon rods into thin wafers with precise geometric dimensions.
Slicing equipment: internal cutting machine or wire cutting machine.
Chamfering: refers to trimming the tax edge of the cut wafer into an arc to prevent the wafer edge from cracking and lattice defects, and to increase the flatness of the epitaxial layer and photoresist layer.
The main equipment for chamfering: chamfering machine
Grinding: Grinding can remove the saw marks and surface damage layers caused by slicing and grinding wheel grinding, effectively improve the curvature, flatness and parallelism of monocrystalline silicon wafer, and reach the specifications that can be handled during polishing.
Grinding equipment: grinding machine (double-sided grinding)
Main raw materials: grinding slurry (mainly composed of alumina, chrome sand and water) and mud.
Corrosion: refers to the damage layer formed on the wafer surface by processing stress after slicing and grinding, and is usually removed by chemical etching.
Corrosion method: (a) Acid corrosion is the most commonly used. The acidic corrosion solution consists of nitric acid (HNO3), hydrofluoric acid (HF) and some buffer acids (CH3COCH, H3PO4).
(b) alkaline corrosion, wherein the alkaline corrosion solution consists of KOH or NaOH and pure water.
Polishing: refers to polishing the wafer with high flatness by improving the micro-defects on the surface of monocrystalline silicon wafer.
Polishing equipment: multi-piece polisher, single-piece polisher.
Polishing method: rough polishing: it is mainly used to remove the damaged layer, and the general removal amount is about10-20um;
Fine polishing: it is mainly used to improve the micro-roughness of wafer surface, and the removal amount is generally below 1um.
Main raw materials: the polishing solution consists of micro-suspended silica gel containing SiO2 _ 2 _ 2 and NaOH (or KOH or NH _ 4OH), which can be divided into rough polishing and fine polishing.
Cleaning: Many steps in the processing of monocrystalline silicon wafer need cleaning, and the cleaning here is mainly the final cleaning after polishing. The purpose of cleaning is to remove all pollution sources on the wafer surface.
Cleaning method: mainly the traditional RCA wet chemical cleaning technology.
Main raw materials: sulfuric acid, hydrogen peroxide, hydrofluoric acid, ammonia water and hydrochloric acid.
(3) Causes of loss
A. polysilicon-monocrystalline silicon rod
In the process of processing polycrystalline silicon into monocrystalline silicon rods, if the bottom material and head and tail material of the heavily doped crucible are lost, they cannot be reused and can only be used as additives in metallurgical industries such as ironmaking and aluminum smelting. If the loss of non-heavily doped crucible bottom material and head and tail material can be used to manufacture low-grade silicon products, this part should be taxed as scrap.
Heavy doping refers to mixing polycrystalline silicon raw materials with nearly saturated impurities (such as boron, phosphorus, antimony and arsenic). The type of impurity depends on the type of resistor (n or p), and the resistor is put into a crucible to be melted.
Heavy additives are mainly used to produce silicon wafers with low resistivity (resistivity < 0.0 1 1 ohm/cm).
Loss: The bottom material of crucible after single crystal drawing is about 15%.
The head and tail materials account for about 20% in the process of single crystal silicon rod forming.
In the single crystal forming process (outer diameter grinding process), because the outer diameter surface of single crystal silicon rod is uneven and the diameter is larger than the diameter specification specified by the final polished wafer, more accurate diameter can be obtained by outer diameter grinding. The loss is about 10%- 13%.
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