1. synthesis of diamond films by chemical vapor deposition
As early as 1950s and 1960s, scientists in the United States and the former Soviet Union successively realized the research and development of polycrystalline diamond films deposited by low-pressure chemical vapor deposition. Although the sedimentation rate was very low at that time, it was undoubtedly a groundbreaking measure. Since 1980s, scientists have successfully developed a variety of CVD diamond polycrystalline films preparation methods, such as hot filament CVD, microwave plasma CVD, DC plasma CVD, laser plasma CVD, plasma enhanced PECVD and so on. With the maturity of synthesis technology, the growth rate, deposition area and structural properties of diamond films have gradually reached the applicable level.
The principle of synthesizing polycrystalline diamond films by 1.CVD method
Chemical vapor deposition (CVD) is a method to generate gas from low molecular hydrocarbons (methane CH4, acetylene C2H2, benzene C6H6, etc.). ) is mixed with hydrogen (part of Al2O3 is oxygenated), hydrocarbons dissociate under certain temperature and pressure conditions, and carbon ions are generated in the plasma state, and then the carbon ions grow polycrystalline diamond on diamond or non-diamond (Si, SiO2 _ 2, Al _ 2O _ 3, Si C, Cu, etc.). ) substrate guided by electric field. CVD method for growing diamond films on diamond substrates is also called epitaxial growth method. Someone has successfully grown a film with a thickness of 20? On the (100) plane, CH4 and H2 are used as raw materials. M, the epitaxial growth surface of the epitaxial layer is smooth, the crystal quality is high, and the growth rate is 0.6? However, the quality of crystals epitaxially grown on diamond (11) and (11) surfaces is poor. This shows that the quality of diamond homoepitaxial layer is directly related to the crystal plane orientation of diamond substrate.
2. Process conditions of plasma enhanced PECVD.
Plasma enhanced PECVD is one of the most widely used methods to synthesize diamond films at present, and its reaction device is shown in Figure 4- 1-28.
Fig. 4-1-28 schematic diagram of diamond films synthesized by PECVD.
Plasma enhanced chemical deposition (PECVD) process requires the use of energy devices to ionize the input gas and generate charged particles of carbon-rich plasma gas. Hydrocarbon gas usually adopts methane and hydrogen, and its volume ratio is (0.1~1): (0.9 ~ 9); The reaction temperature is 700 ~1000℃ and the pressure is (0.7 ~ 2) ×104 pa. Under the above technological conditions, hydrocarbon gas particles are decomposed and carbon atoms are deposited on the matrix material, forming synthetic diamond films.
Application of 3.3. Synthesis of diamond films by CVD.
According to reports, diamond films synthesized by chemical vapor deposition are widely used in industry, such as coating on mechanical parts to increase wear resistance and lubricity; When used in electronic products, it can improve the heat dissipation effect; Can be used to make super computer chips, the best filter; Used in optical products, it can enhance the perspective effect and protect the lens; In medicine, it can be used as the interface of artificial joint, the valve plate of artificial heart and the best protective film against acid, alkali and radiation. It can be used as a radome for missiles in the military; In daily life, it can be used for non-stick pan, audio diaphragm, razor blade protective film, barcode machine protective film and so on.
At present, the application of diamond films synthesized by CVD method in gem industry mainly includes the following aspects:
1) diamond film is plated on all facets of synthetic diamond, which makes it have some properties of natural diamond.
2) Coating a colored diamond film on the surface of natural diamonds to change the appearance color of faceted diamonds and imitate colored diamonds.
3) Coating diamond film on the surface of the cut diamond can increase the weight of the finished diamond.
4) Plating diamond film on the surface of gemstones with low hardness to enhance their wear resistance. For example, apophyllite or kyanite is treated with diamond film and patented in Germany.
5) Artificial diamond film technology can be used to coat opal surface to prevent it from dehydration and cracking.
2. Synthesis of diamond single crystal by 2.CVD method.
In recent ten years, chemical vapor deposition synthesis technology has developed rapidly, especially in 2003, CVD technology has made a new breakthrough, which can grow larger single crystal diamond with relatively low cost, and the color and purity can reach a higher level, even cutting and grinding jewelry diamonds with D color level above 1ct and cleanliness level of IF. On May 7th, 2005, Carnegie Geophysical Laboratory in Washington, USA announced at the 10th International Diamond New Technology Conference in Japan and Gem-A Mailtalk of British Gem Association that they could improve the chemical precipitation technology to 100? M/h, high-quality colorless single crystal diamond with a thickness of half an inch 10ct was rapidly grown. However, the details of the synthesis technology were not disclosed.
The principle of CVD synthesis of single crystal diamond is that methane and hydrogen are introduced into the reaction chamber, and carbon is decomposed into atoms by heating wire, microwave, flame, DC arc and other equipment to form plasma in the reaction chamber. The carbon atoms in methane already have a four-bond structure. Under the catalysis of hydrogen, each carbon atom combines with four carbon atoms to form a diamond structure, which gradually precipitates and grows on the prepared "pedestal", and its growth rate is usually one micron to several tens of microns per hour. The growth pedestal can be cut into thin slices with {100} crystal planes by natural or high-temperature and high-pressure synthetic diamond, and then heated by microwave to form a plasma field at the temperature of 800 ~ 1000℃,1/0 ATM (standard atmospheric pressure) =106544.
Diamonds with different thickness or particle size can be synthesized according to needs at high temperature.
Diamond synthesized by CVD is shown in Figure 4- 1-29.
Fig. 4- 1-29 CVD diamond synthesis
Third, synthesize moissanite crystal.
1. Overview
As early as a century ago, synthetic moissanite (SiC) was manufactured and widely used in industry as an abrasive. The growth of SiC single crystals has also been studied for many years. The grown SiC single crystal has two main uses: one is as a semiconductor material, and the other is as a substitute for diamonds in jewelry.
1955, moissanite crystal was grown by sublimation method, which laid the foundation for the development of synthetic moissanite. Although the crystal grown by this method is small in size and irregular in shape, the quality of the grown crystal is very good, so Riley method has always been a method to grow high-quality moissanite single crystals. At the beginning of 1980, Russian Tairov and others improved Riley's method, and used seed crystal sublimation technology (also known as physical vapor transport technology) to grow moissanite large crystals, which effectively avoided spontaneous nucleation and declared the success of controlled growth synthesis by moissanite technology. The color of a faceted gem of this material can be similar to colorless. This synthetic material is produced by Cree Researchinc in Dorham, North Carolina and sold by C3 Company.
Charles&Colvard Ltd, founded in 1995, formerly known as C3 Company, adopted high-tech achievements to solve the color and transparency problems of synthetic moissanite at high temperature and normal pressure, and synthesized large-particle gem-grade synthetic moissanite crystals, which were finely cut and embedded in platinum and K-gold jewelry, and officially introduced to the international market. By 2000, the diameter of synthesized moissanite crystal has reached 100 mm. At present, the annual output of synthetic moissanite can reach more than 70,000 kwh.
2. moissanite single crystal synthesis technology.
Fig. 4- 1-30 Structure diagram of Davis patent synthetic moissanite growth equipment.
1990, Davis improved the Riley method, and its mature technology was patented. The equipment structure diagram of this method is shown in Figure 4- 1-30. In this process, raw material powder used for growing and synthesizing moissanite single crystal passes through a porous graphite tube, is heated and sublimated into a gaseous state, and then directly crystallizes on the seed crystal, thereby growing pear-shaped Si C single crystal. The whole process has both the change of material state and the change of material structure.
The technical requirements of Davis patent are as follows:
1) The particle size of powder should be controlled, and ultrasonic vibration method should be used for filling.
2) The seed crystal and powder should belong to the same polytype, and the orientation of the seed crystal should slightly deviate from the axial direction.
3) Vacuum should be pumped at the initial stage of growth, and then low-pressure argon gas should be introduced.
4) heat-resistant graphite sleeve is used for heating, the temperature in the supply area is 2300℃, and the crystal growth temperature is lower than the supply area temperature 100℃.
5) The rotation of the seed crystal and the adjustment of the position of the growing crystal should be accurate. By this method, gem-grade colored synthetic moissanite crystals with a diameter of 65438±02mm, a thickness of 6mm and a growth period of 6h can be grown. The surface of some synthetic moissanite pears shows triangular pits similar to the surface of diamonds.