The deposition of a thin film is a series of processes involving the adsorption of atoms, the diffusion of adsorbed atoms on the surface, and the aggregation at appropriate positions to gradually form a thin film and grow. The quality of thin film generation and accurate detection of film thickness are of great importance in semiconductor manufacturing.
The deposition methods of thin films in semiconductor technology are classified as follows:
Chemical Vapor Deposition (CVD)
The reaction gas undergoes a chemical reaction, and the product is deposited on the surface of the chip.
Physical Vapor Deposition (PVD)
Evaporation
The process of depositing a thin film by utilizing the saturated vapor pressure of the deposited material at high temperature (near melting point).
Sputtering
Using ion bombardment on the electrode to deposit a thin film with particles (such as atoms) of the substrate in the gas phase.
Chemical Vapor Deposition (CVD)
How to grow and stack other thin film materials such as poly silicon, silicon nitride, tungsten, or copper on a silicon wafer using high-temperature furnace tubes for silicon dioxide layer growth? Basically, high-temperature furnace tubes are still used, but due to different chemical deposition processes, there are different working temperatures, pressures, and reaction gases, 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". Due to the exponential function of chemical reactions with temperature, chemical reactions are rapidly completed at high temperatures. For chemical vapor deposition, increasing the process temperature makes it easier to control the deposition rate or process repeatability.
There are several drawbacks to high-temperature processes:
1. The electricity cost required for high-temperature process environments is relatively high.
2. If the process temperature is higher than the previous one after arranging the sequence, it may damage the deposited materials. 3. Thin films grown at high temperatures will generate residual stress due to varying degrees of thermal expansion and contraction between the substrate and the film after cooling to room temperature.
So, low process temperature is still one of the goals pursued by chemical vapor deposition, which increases the problems and difficulties faced in process technology.
Following the development process of chemical vapor deposition, we will briefly introduce "atmospheric pressure chemical vapor deposition", "low-pressure chemical vapor deposition", and "plasma assisted chemical vapor deposition":
1. Atmospheric Pressure CVD (APCVD)
The previously developed CVD system operated under one atmosphere pressure and had a similar appearance to an oxidation furnace tube. To achieve uniform flow of chemical vapor from the upstream of the furnace tube towards the silicon crystal for growth material, the qualitative explanation for why it deposits on the surface of the silicon crystal can be simply given by the boundary layer theory:
When a viscous chemical vapor horizontally blows over a silicon chip, the silicon chip, like the furnace wall, is a solid boundary. Due to the significant change in velocity in the boundary layer about 1mm near the chip surface (from the outer edge of the boundary layer vapor velocity to zero on the chip surface velocity), a drag force is applied to hold back the chemical vapor molecules; At the same time, due to the surface temperature of the silicon chip being 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 chemical vapor molecules that are held back to complete the dissociation and precipitation of the thin film material on the chip surface. So basically, chemical vapor deposition is the application of the natural "transport phenomenon".
The atmospheric pressure chemical vapor deposition rate is quite fast, but the texture of the growing film is relatively loose. In addition, if the wafer is not placed horizontally (which takes up too much space), the thickness uniformity of the film will be poor.
2. Low Pressure Chemical Vapor Deposition (LPCVD)
To carry out mass production of 50 or more wafer batches, it is necessary to vertically and densely place the wafers in the furnace tube on the wafer boat, which obviously leads to the problem of thickness uniformity of the deposited thin film; Because the assumption of the boundary layer problem on a flat plate 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 reverse pressure gradient will bring downstream chemical vapor back upstream, causing chaos.
Reducing the pressure of the chemical vapor environment is a feasible way to solve the problem of thickness uniformity when it is inevitable to place the wafer vertically on the wafer boat. According to the observation of Reynolds number, which defines the viscous flow characteristics, the dynamic viscosity coefficient ν decreases with pressure reduction, and the Reynolds number increases sharply, causing the chemical vapor flow to enter turbulent flow from laminar flow. Turbulent flow is not easy to separate, as it is an orderly flow in a chaotic state. Therefore, although the chemical vapor becomes thinner and slows down the deposition rate, there is still no separation or backflow phenomenon after passing through dozens of heavy wafers, while maintaining 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 speed is not fast enough. In order to lower the sedimentation temperature first, it is necessary to find another energy source for chemical deposition. Due to the necessity of low pressure for thickness uniformity, the development of low pressure environment plasma energy assistance (plasma can only exist at 10~0.001 Torr) precisely compensates for the problem of insufficient energy supply in low temperature environments, resulting in a deposition rate higher than LPCVD.
The operating principles of PECVD and RIE machines are similar, with the former using plasma to assist deposition and the latter using 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 can be divided into two types based on principles: evaporation and sputtering. PVD basically requires vacuum pumping: the former evaporates metal in an environment of 10-6-10-7 Torr; The latter requires the removal of residual air in the chamber before exciting the plasma, also to the extent of 10-6~10-7 Torr. A typical mechanical vacuum pump can only achieve a vacuum degree of 10-3 Torr, and then needs to be connected in series with a high vacuum pump (the mechanical pump is used as the pre stage pump that comes into contact with the atmosphere), such as a diffusion pump, turbo pump, or cryogenic pump, in order to achieve a vacuum degree of 10-6~10-7 Torr. Of course, different vacuum pumps involve different principles of pressure gauges, pipeline designs, and prices.
1. There is a difference in heating methods for vapor deposition, which can be divided into two types of machines: thermal coating and E-gun evaporator. The former is relatively easy in principle, which is to directly hang the metal to be melted and evaporated in the form of a wire on a heated tungsten wire. Once heated and melted, due to the surface tension of the liquid, it will climb onto the heated tungsten wire and slowly evaporate around (including the wafer). Due to the limited heat resistance and adhesion space of the heated tungsten wire to the molten metal, it is only used for low melting point metal plating, such as aluminum, and has a limited evaporation thickness.
The electron gun type vapor deposition machine uses an electron beam for heating, and the melted 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 begins to slowly evaporate around (including the wafer). The electron gun type vapor deposition machine can evaporate metals with high melting points and has relatively unrestricted thickness.
The vapor deposition method basically has the disadvantage of poor step coverage, which means that on surfaces with severe undulations, the evaporated metal breaks discontinuously. In addition, there is also a problem of uniform thickness in large-area plating of multiple wafers. Therefore, the chip carrier is equipped with a mechanism for rotation, 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 like throwing a stone into a puddle, it splashes out a lot of mud. Sputtering uses argon plasma to rapidly impact the target material, causing the material near the surface of the target to splash out and fall onto the wafer. Due to the fact that the target material is a whole surface rather than a single point subjected to bombardment, the sprayed material may also fill the dead corners of the chip surface steps, without the problem of broken wires, discontinuities, or so-called step coating.
Sputtering is also divided into two types based on the energy source of plasma excitation: direct current (DC) and radio frequency (RF). Basically, both types of sputtering machines can deposit metal thin films. But the latter is particularly suitable for non-metallic thin films, such as piezoelectric or magnetic materials, with intelligent coating characteristics such as insulation, high melting point, complex composition, and sensitivity to stacking methods.
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