Dry etching technology is an etching process carried out under atmospheric or vacuum conditions. Ions or chemicals in the gas are usually used to remove parts of the material surface. By controlling the mask and etching parameters, arbitrary switching between anisotropic and isotropic etching can be achieved to form the desired pattern or structure. Common dry etching equipment includes reactive ion etching machine (RIE), inductively coupled plasma etching machine (ICP), magnetic neutral line plasma etching machine (NLD), and ion beam etching machine (IBE). The purpose of this article is to analyze the structure of each etching equipment and analyze the advantages and disadvantages of the technology.
RIE
The structure of RIE equipment is usually relatively simple. It generally consists of a radio frequency power supply, anode, cathode, gas source and vacuum pump. Among them, the wafer is located on the cathode, and the radio frequency power supply is connected to the cathode. At the beginning of etching, the radio frequency power supply applies an electric field to the gas, and the gas is ionized into ions, electrons, atoms and molecules. Since electrons move much faster than other ions, an obvious sheath will be formed in the etching cavity, and a self-biased electric field will be formed above the wafer. RIE uses this effect to attract and accelerate ions in the plasma to produce physical and chemical interactions with the etched material, and the by-product gases eventually volatilize and are discharged. However, this self-bias effect causes the different movement speeds of each substance during the etching process, resulting in a mismatch between RIE etching ions and reactive radicals. In the RIE etching process, the “black silicon” phenomenon that often occurs is caused by this principle.
ICP
With the continuous improvement of process requirements, researchers have developed an inductively coupled plasma etching machine (ICP). This equipment overcomes the problem of free radical and ion mismatch. The structure of a commonly used ICP etching machine is shown in Figure 2. Generally, the etching gas is introduced from the top, passes through the RF coil on the side wall of the etching cavity, and is excited into plasma by the electric field applied by the RF coil, forming an obvious glow layer in the cavity; Then, the etching ions in the plasma are absorbed by applying Bias power on the lower electrode. Compared with RIE etching technology, ICP etching chambers generally have lower air pressure and higher density plasma. Etching with higher speed, higher precision and larger aspect ratio can be achieved by controlling the RF coil power, gas flow and Bias power.
The right side of Figure 2 shows the process of ICP etching quartz with C4F8. Among them, the fluorocarbon radicals generated by radio frequency discharge C4F8 will spontaneously form a passivation layer and deposit on the surface. Then under the action of Bias applied to the wafer surface, the positive ions have a high velocity and bombard the passivation layer perpendicular to the wafer surface and etch the quartz, achieving high aspect ratio etching of quartz.
NLD
Traditional ICP etching RF coils will affect the magnetic field in the etching cavity, making the radial plasma density inconsistent and interfering with the movement of etching ions, thus affecting the etching uniformity. The researchers further developed the magnetic neutral loop discharge. The structure of the equipment is shown in Figure 3. It applies three coils to control the magnetic field on the periphery of the ICP etching coil. By controlling the magnitude of the magnetic field of the three coils, an environment with a magnetic center ring strength of 0 can be formed in the etching cavity, thereby producing higher density and uniformly distributed plasma. With Bias control, NLD technology has better etching uniformity and higher etching rate. In addition to commonly used dielectric layers, it is also often used to etch silicon carbide and other etching-resistant materials.
IBE
IBE etching is pure physical bombardment etching and is often used for materials that cannot be etched by fluorine-based or chlorine-based plasma, such as gold, copper, platinum, etc. The structure of a conventional IBE etching machine is shown in Figure 4. The argon gas is ionized by the electrons provided by the filament to form a plasma and an ion source. Then, the ions are ejected uniformly towards the wafer workpiece stage through the electron extraction acceleration system, and finally by bombarding the atoms on the solid surface, the material atoms are sputtered to achieve the purpose of etching.
The main characteristics of IBE are: 1) The highly directional neutral ion beam can control the sidewall profile and optimize the radial uniformity and structural morphology during the etching process. 2) By adjusting the angle of the workpiece table, the side wall angle can be controlled by tilting the wafer and changing the impact direction of the ion beam.
