The deposition of thin films is a series of processes involving the adsorption of atoms, the diffusion of adsorbed atoms on the surface, and their coalescence at appropriate positions to gradually form and grow thin films. The generation quality of thin films and the precise detection of film thickness are of great significance in semiconductor manufacturing.
The deposition methods of thin films in semiconductor technology can be classified as follows:
Chemical Vapor Deposition (CVD
The reaction gas undergoes a chemical reaction, and the products deposit on the surface of the wafer.
Physical Vapor Deposition (PVD
Evaporation (Evaporation)
The process of depositing thin films by taking advantage of the saturated vapor pressure of the object to be evaporated at high temperatures (near the melting point).
Sputtering (Sputtering)
By bombarding the spatter (Electrode) with ions (Bombardment), particles (such as atoms) containing the spatter in the gas phase are deposited to form a thin film.
Chemical Vapor Deposition (CVD)
High-temperature furnace tubes are used for the growth of silicon dioxide layers. As for other thin-film materials such as polycrystalline silicon, silicon-nitride, tungsten or copper metals, how can they be grown and stacked onto silicon wafers? Basically, high-temperature furnace tubes are still used. However, due to different chemical deposition processes, there are different working temperatures, pressures and reaction gases, which are collectively referred to as "chemical vapor deposition".
Since it is a chemical reaction, it is inevitable to have two mechanisms: "mass transfer" and "chemical reaction". Since chemical reactions change exponentially with temperature, when the temperature is high, chemical reactions can be completed rapidly. For chemical vapor deposition, increasing the process temperature makes it easier to control the deposition rate or the repeatability of the process.
High-temperature processes have several disadvantages:
1. The electricity cost required for high-temperature process environments is relatively high.
2 If the temperature of the processes arranged later in the sequence is higher than that of the former, it may damage the deposited materials. 3 Films grown at high temperatures, when cooled to room temperature, will generate residual stress due to the different degrees of thermal expansion and contraction between each substrate and the film.
Therefore, low process temperature remains one of the goals pursued in chemical vapor deposition. As a result, the problems and difficulties faced in process technology also increase accordingly.
According to the research and development process of chemical vapor deposition, this paper introduces "atmospheric pressure chemical vapor deposition", "low-pressure chemical vapor deposition" and "plasma-assisted chemical vapor deposition" respectively:
1. Atmospheric Pressure chemical vapor deposition (CVD; APCVD)
The previously developed CVD systems operated under one atmosphere of pressure, and the appearance of the equipment was similar to that of oxidation furnace tubes. The chemical vapor of the material to be grown flows uniformly from the upstream of the furnace tube to the silicon crystal. As for why it deposits on the surface of the silicon crystal, it can be qualitatively explained simply by the boundary layer theory:
When the viscous chemical vapor is horizontally blown over the silicon chip, the silicon chip, like the furnace tube wall, is a solid boundary. Due to the significant change in velocity within the boundary layer approximately 1mm close to the chip surface (the vapor velocity decreases from the outer edge of the boundary layer to zero at the chip surface), a dragging external force is applied, which holds the chemical vapor molecules. At the same time, as the surface temperature of the silicon chip is higher than the vapor temperature at the outer edge of the boundary layer, the chip will release heat to supply the energy required for the trapped chemical vapor molecules to complete the separation of the thin film material on the chip surface. So basically, chemical vapor deposition is the application of nature's "transport phenomena".
Atmospheric pressure chemical vapor deposition is quite fast, but the texture of the grown film is relatively loose. In addition, if the wafers are not placed horizontally (which takes up too much space), the uniformity of the film thickness (thickness uniformity) will be poor.
2 Low-pressure chemical vapor deposition (Low Pressure CVD; LPCVD
To achieve mass production of 50 or more wafers in batches, the wafers in the furnace tube must be vertically and densely placed on the wafer boat, which obviously leads to the problem of uniformity in the thickness of the deposited film. Because the assumption of the flat boundary layer problem is no longer appropriate, the chemical vapor becomes viscous after passing through the wafer
Semiconductor technology - Thin film deposition
The flow field immediately enters a state of separation, and the reversed pressure gradient will bring the chemical vapor downstream back upstream, causing chaos.
When it is inevitable to place the wafer vertically in the crystal boat, reducing the pressure of the chemical vapor environment is a feasible way to solve the problem of thickness uniformity. According to the Reynolds number observation of the defined viscous flow characteristics, the dynamic viscosity coefficient ν decreases as the pressure drops, and the Reynolds number surges, causing the chemical vapor flow to shift from laminar flow to turbulent flow. Turbulent flow is not easy to separate. It is an orderly flow in chaos. Therefore, although the chemical vapor becomes thinner and the deposition rate slows down, after passing through dozens of heavy wafers, there is still no phenomenon of separation and countercurrent, and it retains the advantages of uniform thickness and even dense texture.
3 Plasma Enhanced chemical vapor deposition (PECVD)
Although LPCVD has solved the problem of uniform thickness, the temperature is still too high and the deposition rate is not fast enough. In order to lower the deposition temperature first, another energy source must be sought for chemical deposition. Due to the necessity of low pressure for thickness uniformity, the development of plasma energy assistance in a low-pressure environment (where plasma can only exist at 10 to 0.001 Torr) precisely compensates for the insufficient energy supply in a low-temperature environment, enabling the deposition rate to be higher than that of LPCVD.
The operating principles of PECVD and RIE machines are similar. The former uses plasma to assist in deposition, while the latter uses plasma to perform etching. The difference lies in the use of different plasma gas sources, and the working pressure and temperature are also different.
Physical Vapor Deposition (PVD)
Also known as Metal Deposition, it is divided into two types according to the principle: evaporation (evaporation) and sputtering (sputtering). PVD basically all require vacuuming: the former is steamed with metal in an environment of 10-6 to 10-7 torr; The latter requires that the residual air in the air chamber be removed before the plasma is excited, and it should be done to a degree of 10-6 to 10-7 torr. A common mechanical vacuum pump can only achieve a vacuum degree of 10-3Torr. After that, a high vacuum pump (with the mechanical pump serving as the forepump in contact with the atmosphere) must be connected in series, such as: Only diffusion pumps, turbo pumps, or cryogenic pumps can achieve a vacuum level of 10-6 to 10-7Torr. Of course, different vacuum pumps involve pressure gauges based on different principles, pipeline designs, and prices.
1. Evaporation is divided into two types of machines based on the heating method: thermal coater and E-gun evaporator. The former is relatively easy in principle. It involves directly hanging the metal to be melted and evaporated onto the heating tungsten wire in the form of a wire. Once heated and melted, due to the tension on the liquid surface, it will adhere to the heating tungsten wire and then slowly evaporate to all sides (including the wafer). Due to the limited heat resistance of the heating tungsten wire and the space for the molten metal to adhere, it is only used for low-melting-point metal plating, such as aluminum, and the thickness of the vapor deposition is limited.
The electron gun evaporation machine uses an electron beam for heating, and the molten and evaporated metal particles are all placed in a graphite or tungsten crucible. When the vapor pressure of the metal exceeds the critical limit, it also begins to steam slowly to the periphery (including the wafer). The electron gun type evaporation machine can evaporate metals with relatively high melting points, and the thickness is also relatively unrestricted.
The evaporation method basically has the drawback of poor step coverage, that is to say, on surfaces with more intense undulations, the metal in the evaporation process breaks discontinuously. In addition, there is also the problem of uniform thickness in large-area plating of multiple wafers. For this reason, the chip carrier platform is equipped with a axial-rotation mechanism, which is used to improve the above two problems.
2 Sputtering
Although sputtering is a physical coating method, it has nothing to do with evaporation. Just as throwing a stone into a quagmire will splash out a lot of mud, sputtering uses argon plasma to impact the target material at high speed, thus splashing out the material near the target surface and falling onto the wafer. Since the target material is bombarded as a whole surface rather than a single point, the splashed material may also fill the dead corners of the chip's surface steps, and there will be no problem of discontinuous lines or so-called step coating.
Sputtering is also classified into direct current (DC) and radio frequency (RF) types based on the different energy sources for plasma excitation. Basically, both types of sputtering machines can deposit metal films. However, the latter can be particularly applied to non-metallic films, such as piezoelectric (piezoelectric) or magnetic materials, which possess the coating characteristics of intelligent films such as "insulation, high melting point, complex composition, and considerable sensitivity to stacking methods".
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