There are three main types of dry etching:
• Physical dry etching: accelerates the physical wear of the wafer surface by particles.
• Chemical dry etching: Gas reacts chemically with the wafer surface.
• Chemical-physical dry etching: a physical etching process with chemical properties.
1. Ion beam etching
Ion beam etching is a physical dry etching process. Argon ions are irradiated onto the surface in an ion beam of approximately 1 to 3 keV. Due to the energy of the ions, they bombard the surface material. The wafer is inserted into the ion beam perpendicularly or at an angle, and the etching process is absolutely anisotropic. Selectivity is low because it is indistinguishable across layers. Gases and the etched material are expelled by a vacuum pump; however, because the reaction products are not gaseous, particles can deposit on the wafer or chamber walls.
To avoid particles, a second gas is introduced into the chamber. This gas reacts with argon ions, inducing a physicochemical etching process. Some of the gas reacts with the surface, but also with the abraded particles to form gaseous byproducts. Almost all materials can be etched using this method. Due to vertical radiation, wear on the vertical walls is very low (high anisotropy). However, due to low selectivity and low etching rate, this process is rarely used in modern semiconductor manufacturing.
2. Plasma etching
Plasma etching is an absolutely chemical etching process (chemical dry etching). Its advantage is that the wafer surface is not damaged by accelerated ions. Due to the movable particles of the etching gas, the etching profile is isotropic, making this method suitable for removing entire film layers (e.g., back-side cleaning after thermal oxidation).
One type of reactor used for plasma etching is a downstream reactor. This involves igniting the plasma at a high frequency of 2.45 GHz through impact ionization, separating the ionized area from the wafer.
In the gas discharge region, various particles, including free radicals, are present due to the impact. Free radicals are neutral atoms or molecules with unsaturated electrons, and are therefore highly reactive. As a neutral gas, tetrafluoromethane (CF4) is introduced into the gas discharge region and separates into CF2 and fluorine molecules (F2). Similarly, fluorine can be separated from CF4 by adding oxygen (O2).
2 CF4 + O2 ---> 2 COF2 + 2 F2
Fluorine molecules can be split into two separate fluorine atoms through the energy of the gas discharge region: each fluorine atom is a fluorine radical, as each atom has seven valence electrons and aims for an inert gas configuration. In addition to the neutral radicals, there are several partially charged particles (CF+4, CF+3, CF+2, ...). All particles, radicals, etc., then enter the etching chamber through a ceramic tube. Charged particles can be blocked from the etching chamber by an extraction grating or recombine along their way to form neutral molecules. Fluorine radicals also partially recombine, but enough to reach the etching chamber, react on the wafer surface, and cause chemical etching. Other neutral particles are not part of the etching process and are depleted along with the reaction products.
Examples of thin films that can be etched in plasma etching: • Silicon: Si + 4F ---> SiF4 • Silicon dioxide: SiO2 + 4F ---> SiF4 + O2 • Silicon nitride: Si3N4 + 12F ---> 3SiF4 + 2N2 3. Reactive Ion Etching Etching characteristics: Selectivity, etching profile, etching rate, uniformity, and repeatability can all be very precisely controlled in reactive ion etching (RIE). Isotropic etching profiles as well as anisotropic ones are possible. Therefore, RIE is a chemical physical etching process and is the most important process in semiconductor manufacturing for constructing a wide variety of thin films. In the process chamber, the wafer is placed on a high-frequency electrode (HF electrode). Plasma is generated by impact ionization, in which free electrons and positively charged ions appear. If the HF electrode is at a positive voltage, free electrons accumulate on it and cannot leave the electrode again due to their electron affinity. Therefore, the electrode is charged to -1000 V (bias voltage). Slow ions cannot follow the rapidly alternating field to move toward the negatively charged electrode.
If the mean free path of the ions is high, the particles impact the wafer surface at an almost perpendicular angle. Therefore, the material is ejected from the surface by accelerated ions (physical etching), and some particles also chemically react with the surface. The lateral sidewalls are unaffected, so there is no wear and the etch profile remains anisotropic. The selectivity is not too small, but it is not too large due to the physical etching process. Furthermore, the wafer surface is damaged by accelerated ions and must be cured by thermal annealing. The chemical part of the etching process is accomplished by the reaction of free radicals with the surface and the physically milled material, so it does not redeposit onto the wafer or chamber walls as in ion beam etching. By increasing the pressure in the etching chamber, the mean free path of the particles decreases. Therefore, there are more collisions, and the particles travel in different directions. This results in less directional etching, and the etching process acquires more chemical properties. Selectivity increases, and the etch profile becomes more isotropic. Anisotropic etch profiles are achieved through passivation of the sidewalls during silicon etching. Therefore, oxygen in the etching chamber reacts with the milled silicon to form silicon dioxide, which is deposited on the vertical sidewalls. Due to ion bombardment, the oxide film on the horizontal region is removed, allowing the lateral etching process to continue.
The etching rate depends on pressure, high-frequency generator power, process gas, actual gas flow rate, and wafer temperature. Anisotropy increases with increasing high-frequency power, decreasing pressure, and decreasing temperature. The uniformity of the etching process depends on the gas, the distance between the two electrodes, and the electrode material. If the distance is too small, the plasma cannot be dispersed uniformly, resulting in inhomogeneity. Increasing the electrode distance reduces the etching rate because the plasma is distributed over an expanded volume. For electrodes, carbon has proven to be the preferred material. Because fluorine and chlorine gases also attack carbon, the electrodes produce a uniform strained plasma, thus the wafer edges are affected in the same way as the wafer center.
Selectivity and etching rate depend heavily on the process gas. For silicon and silicon compounds, fluorine and chlorine are primarily used.
Etching processes are not limited to a single gas, gas mixture, or fixed process parameters. For example, native oxides on polysilicon can be removed first at a high etch rate and with low selectivity, followed by etching of the polysilicon with higher selectivity relative to the underlying layers.
