Research On Key Technology Of Molecular Beam Epitaxy Source Furnace

The source furnace is one of the most important key components of molecular beam epitaxy equipment.It has a decisive influence on the equipment and the epitaxial film.The performance of the source furnace and the control of the beam flow are of great significance to the thickness uniformity,the surface roughness,the crystal quality and the interface quality of the epitaxial film.For the micro-scale simulation of molecular beam epitaxy,it is necessary to simulate the characteristics of molecular beam epitaxy more realistically.In terms of the research of film thickness uniformity,the calculation model including the geometric configuration of molecular beam equipment and source beam angular distribution has not been established and the multi-parameter optimization method to optimize overall thickness uniformity is not used.The micro-scale simulation,film thickness uniformity,temperature characteristics and reliability related to the source furnace are studied in this dissertation.The main contents and conclusion are as follows:（1）The molecular dynamics is used to simulate the influence of the incident angle of the source furnace particles,the incident speed of the source furnace particles,the rotation speed of the substrate,the temperature of the substrate and the N:Ga beam ratio on the surface morphology,roughness and crystal quality of the epitaxial film.The results show that the surface roughness reaches the optimum at the source furnace incident angle of 45°and the wurtzite component in the epitaxial film increases with the increase of the incident angle of the source furnace particles.When the N:Ga beam ratio is 0.8,the surface roughness and surface morphology of the epitaxial film are better than N:Ga beam ratio of 1.2,and when the N:Ga beam ratio is 1,as the substrate temperature increases,the surface roughness of the epitaxial film will decrease significantly.When the N:Ga beam ratio is 1,the quality of the crystal is better than the beam ratios of 0.8 and 1.2 in overall trend.The rotation of the substrate can effectively reduce the surface roughness of the epitaxial film.The surface roughness of the epitaxial film reaches the optimum when the substrate rotation speed is 2r/ML and the increasing rotating speed could promote the crystal quality.（2）The angular distribution of the source furnace is one of the main factors affecting the thickness distribution of molecular beam epitaxial films.The angular distribution of the source furnace is described by a polynomial with a cosine function as the basis function.A calculation model for the thickness uniformity of the epitaxial film is established,and the influence of each parameter on the thickness uniformity is obtained.The Monte Carlo method is used to calculate the relationship between the shape of the crucible inner wall of the source furnace and the beam angular distribution of the source furnace.The artificial neural network optimization method is used to calculate the optimal inner wall shape of the crucible according to the target beam angular distribution.Finally,the optimized inner wall of the crucible was used to carry out the evaporation simulation experiment according to the simulation results.In the experiment,the thickness of the epitaxial film deposited on the 4-inch substrate was measured.The epitaxial film thickness uniformity of instantaneous evaporation is about 2.5%,and the epitaxial film thickness uniformity of cumulative evaporation is about 3.75%.In addition,the validity of Monte Carlo calculation is verified by the evaporation of the crucible perpendicular to the substrate.（3）A thermal evaporation source was designed.The molecular flow method is used to calculate the relationship between the evaporation surface temperature of the source furnace and the incident flux of molecules on the substrate.Taking the growth of Al N as an example for calculation,when the epitaxial film growth rate is 1μm/h,the molecular incident flux is about 1.33×10¹⁹/m²s,and the corresponding evaporation material surface temperature is about 1421 K.In addition,the moving mesh is used to simulate the rotation of the substrate during the epitaxy process,and the uniformity of the epitaxial film on the rotating substrate is 2.89%.The influence of the source furnace temperature fluctuation on the source beam intensity stability is studied.The saturated vapor pressure formula and the free molecular flow method are used to calculate the temperature pulse disturbance.At the evaporation temperature of1421 K,when the temperature fluctuation amplitude is 1 K,with the corresponding temperature fluctuation rate of 0.07%,the fluctuation rate of incident flux of molecules on the substrate is 1.77%.That is,for the evaporation of Al element,the change rate of beam flux is about 25 times the rate of temperature change.The materials of the parts of the source furnace are selected.The temperature equation of the source furnace is established,and the temperature dynamic response time constant of the source furnace at different temperatures is solved.（4）For the common epitaxial film growth rate of 1μm/h,in order to avoid particle collision scattering affecting the beam angular distribution,the residual gas pressure should be lower than 10^-3Pa according to the distance from the source furnace to the substrate.In order to keep the concentration of impurity elements less than 10¹⁴cm^-3,the residual gas pressure needs to be maintained at 3.4×10^-8Pa,which means that the growth chamber is required to be at an ultra-high vacuum level.According to the simulation results of source furnace heated from room temperature to 873 K using evaporation material of aluminum and crucible material of PBN,when the cone angle of the crucible is greater than 1°,the evaporation material in the source furnace slips significantly and the crucible stress is significantly reduced.The key parts of the Al source furnace are simulated and analyzed at the evaporation surface temperature of 1421 K,and the temperature distribution and stress distribution of each part are obtained.The simulation results show that the shielding cover composed of multilayer tantalum foil has good radiation heat insulation capacity.Source furnace structure design and material selection are reasonable and reliable.By designing and installing a water-cooled flange on the growth chamber and placing a liquid nitrogen cooling screen in the growth chamber,the wall temperature of the growth chamber can be effectively controlled at about 343 K.