The more manufacturing methods used in physics and electronics are:
1) Vacuum evaporation method 2) Sputtering method 3) Vapor phase epitaxy method.
Film thickness, the measured film thickness value, is important for studying the physical properties of thin films. The thickness is generally between 10-7~10-5cm.
Vacuum evaporation method
Vacuum Necessity:
1) Prevent air molecules from reacting with the evaporation source at high temperature, forming compounds and deteriorating the evaporation source;
2) Prevent the collision of evaporated substance molecules with air molecules and prevent them from reaching the surface of the substrate, and generate compounds on the way or condense before reaching the substrate due to collisions between evaporated molecules;
3) Prevent air molecules from being mixed into the film as impurities or form compounds in the film during the process of forming a film on the substrate.
How to calculate the vacuum required for evaporation?
The movement of gas molecules: the average velocity of gas molecules: v=(8kT/ π m) 1/2 , it can be concluded that the larger the mass number of gas molecules, the smaller the average velocity of the molecules.
Molecular flow intensity is represented by J, then the number of molecules passing through a unit area per unit time can be expressed as: J=(kT/2 π m)1/2=1/4(nV)
n is the number of molecules per unit area, and v is the average velocity of the molecules.
Preparation: For general solid matter, the number of molecules per unit area (1cm 2 ) is about 10 14 . 1Pa=1N/m 2 ; 1atm, that is, the pressure generated by a Hg column with standard atmospheric pressure=760mm=101325Pa; 1Bar=the pressure generated by a mercury column of 750mm; Torr=the pressure generated by a Hg column of 1mm;
Vacuum can be roughly divided into four categories: low vacuum (100-0.1Pa), medium vacuum (0.1-10-4Pa), high vacuum (10-5-10-7Pa), and ultra-high vacuum (<10-7Pa). The corresponding gas density decreases with the 4-5 power, but the corresponding gas molecular mean free path increases with the 4-5 power.
Collision probabilities and mean free paths for evaporated molecules:
When a molecule moves in the remaining gas molecules, it will collide with gas molecules. After a long enough time, average a large number of molecules, and the average number of collisions Z per unit time can be obtained. The average distance λ that a molecule can travel without collision is the mean free path. It can be deduced from the formula that λ is actually the distance traveled when the number of molecules is reduced to the initial value 1/e.
The relationship between λ and molecular size: λ=1/[nr'π(r+r')] 2 Among them, nr' is the concentration of remaining gas, r' is the radius of remaining gas, and r is the radius of evaporated gas. In addition, nr' can also be expressed by pressure, and because the molecular radius is generally about 1.5 Λ , the mean free path at room temperature is about λ≈10-2/Pt (the unit of λ is cm, and Pt is the pressure in Torr. ). Therefore, if λ is to be between 10 1 ~10 2 cm, Pt should be between 10 -5 ~10 -4 Torr. The rate at which a thin film is deposited on a substrate by evaporation is roughly equal to the deposition of an evaporation source with a thickness of one atomic layer per second. When the pressure of the remaining gas is 10-6Torr, the molecules of the remaining gas reach the substrate in almost the same number as the atoms of the evaporation source substance, but these molecules do not necessarily enter the formed film, but 10-5Torr becomes the measure of the remaining gas an indicator of impact. The size of the evaporation device (vacuum chamber) is generally in the lambda range.
Form of evaporated substance:
Mass analyzers and mass spectrometry are used to study how substances escape when they evaporate. The shape of the molecule when it evaporates is related to the formation process of the film and the mechanism of the film.
It is concluded that almost all alkali metals, noble metals and transition metals escape in the form of single atoms.
When evaporating semiconductors and metals, although they also escape as single atoms, they mostly escape as aggregates of two or more atoms. This phenomenon is related to the affinity of substances. For example, when evaporating Sb, Sb4 is the most, and also mixed with Sb2 and a small amount of Sb monomer. When evaporating As, it is mostly AS4 and As2; when evaporating P, it is mostly P4 and P2; when evaporating Bi and Te, it contains more Bi2 and Te2; when evaporating C, Ge, Se and Si, the spectrum of the aggregate is also observed; The situation is more complicated when evaporating alloys and compounds.
When evaporating GaAs at low temperature, Ga does not evaporate, but As2 evaporates; when evaporating CdS, due to the complete decomposition of the material, Cd and S, S2, S3, and S4 can be observed; when evaporating CdTe, it decomposes into Cd and Te2; When, the decomposed O 2 can often be seen .
Evaporation chamber:
There are evaporation source, substrate and evaporation space in the evaporation chamber. In addition, as accessories, there are baffles (to control the evaporation molecular flow), Film Thickness Gauge (to control the film thickness and to monitor the growth rate of the film), (ultra) high vacuum gauge (to measure the vacuum change in the exhaust and the evaporation time). The pressure of the remaining gas), the substrate temperature regulator (to control the shape and crystallinity of the film). There are also cold traps and baffles in the exhaust system (to prevent the backflow of oil vapor).
There are two main types of evaporation source heating: resistance heating and electron beam heating.
Requirements of the resistance heating method: the evaporation source material should be a low vapor pressure substance with a high melting point that does not melt even at high temperatures. W, Ta or Mo can be used to make a net shape or a straight plate shape, and the evaporation source material is placed on it for heating. The vapor pressure of the evaporating material is much higher than that of the evaporating substance, and it is less desirable to form an alloy with the evaporating source, lowering the melting point of the evaporating source. For example, Fe, Al, Ge and Ni are representative materials that are easy to form alloys. Au is not easy to form an alloy, but it is easy to form an alloy with Ta. In addition, the indirect heating method can also be selected, that is, the evaporation source substance is placed in a heat-resistant container with small holes, and the container is heated from the outside to evaporate it. If the temperature of the container is uniform, the thermal equilibrium state during evaporation can be achieved. At this time, the molecular flow intensity is determined by the temperature, which is called Knudzen type evaporation source. Corresponding to this, the evaporation in non-equilibrium state like heating resistance wire is called Wolf Muir type evaporation source.
Electron beam heating method: local high-temperature heating can be realized, and high-purity thin films can be easily evaporated. A magnetic field can also be used to focus the electron beam and deflect it onto the rake.
Special evaporation:
For thin-film resistors or magnetic thin-film devices, alloy or compound materials are often used, and if the general evaporation method is used, the stoichiometric ratio of the obtained thin film is different from that of the raw material.
Evaporation of alloys: Ural's law can be used by analogy: Pa+n=[Pa/(Na+Nb)]Pa where Pa is the vapor pressure of solvent a, Na and Nb are the moles of solvent a and solute b respectively, and the vapor of the solution Pressure can be deduced from this formula. In addition, if the alloy is regarded as a solution, and the one with more content is regarded as a solvent, the vapor pressure of the alloy can be estimated qualitatively.
What is the state of the thin film obtained by evaporating the alloy in the usual way? Experiments on iron-nickel alloys (2:1) (analyzing the iron-nickel content of the film deposited on each square centimeter per second by X-ray analysis) showed that: the iron-rich state at the beginning, after the change in the surface of the film into rich nickel.
In order to obtain a thin film with the same composition ratio as the evaporation source, two new evaporation methods have emerged: 1) instantaneous evaporation method 2) dual source evaporation method
Instantaneous evaporation method: The evaporation material is made into fine particles, which fall on the evaporation source little by little, and the fine particles are evaporated in an instant. It is suitable for ternary or quaternary alloys, but the evaporation rate is difficult to control.
Evaporation of inorganic compounds: If SiO fine particles are evaporated to make SiO thin films, the Si content of the obtained thin films is excessive.
Inorganic compounds that are closer to the raw materials of the films obtained by evaporation by general methods include: MgO, BeO, Al2O3, CoO2, MgF2 and ZnS. For NiO, SiO and TiO2, the content of O in the film is obviously insufficient; for CdS, there is an excess of Cd.
Therefore, in order to prepare compound thin films, the reaction evaporation method, the three-temperature method and the electron beam evaporation method have been studied.
反应蒸发法：将活性气体导入真空室，使活性气体的分子、原子和蒸发源逸出来的蒸发原子、分子在基片上反应以得到需要的化合物薄膜。如SiO，就是鼓入10-2~10-5Torr左右压强的干燥O2。若控制好蒸发速度和基片的温度，可得到接近SiO2成分比的薄膜。还有AlN是NH3；TiC是C2H4。
三温度法：即是双源蒸发法，经常用于制作GaAs薄膜。
溅射法：把离子加速，然后去轰击固体表面，加速的离子与固体表面的原子碰撞，进行动量交换后将原子从固体表面溅出，溅出的原子在基片上淀积成膜。Sputtering。溅射时，多半是让被加速了的正离子去轰击蒸发源阴极，再从阴极溅出原子，所以也称为阴极溅射法。
辉光放电和溅射现象：
在进行阴极放电时，在阴极附近的管壁上常附有电极的金属测层，这是由溅射现象引起的。
辉光放电就是正离子轰击阴极，从阴极发射出次级电子，此电子在克鲁克斯暗区被强电场加速后再冲撞气体原子，使其离化后再被加速，然后再轰击阴极这样一个反复过程。在这个过程中，当离子和电子相结合或是处在被激发状态下的气体原子重新恢复原态时就会发光。 溅射现象具有方向性。
溅射率：就是一个正离子轰击到靶子后溅射下来的原子数。用s表示。
溅射现象的几个特点：
1）用某种离子在固定电压下去轰击不同的靶材，s会随元素周期表的族而变化。反之，靶材种类确定，用不同种类的离子去轰击靶材，s也随元素周期表的族作周期性变化。
2）S随入射离子的能量（即加速电压V）的增加而单调地增加。不过，V有临界值（一般为10V）。在10V以下时，S为零。当电压≥10KV时，由于入射离子会打入靶中，S反而减少了。
3) For a single crystal target, the size of s varies with the orientation of the crystal plane. Therefore, the flying direction of the sputtered atoms does not obey the cosine law, but tends to be along the densest surface of the crystal.
4) For polycrystalline targets, S increases when ions bombard the surface from an oblique direction. Most of the atoms flying out by sputtering are in the same direction as the positive and opposite directions of the ions.
5) The energy of the sputtered atoms is 1~2 orders of magnitude larger than the energy (~0.1eV) of the atoms ejected by vacuum evaporation.
Gas molecules mixed into the film: When the Ni film is made by sputtering, when the sputtering voltage is below 100V, Ar is hardly mixed. However, the number of Ar atoms in the thin film increases sharply when the voltage reaches above 100V.
