What Is Reactive Ion Etching?

Reactive ion etching technology is a dry etching technology with strong anisotropy and high selectivity. It uses a molecular gas plasma to etch in a vacuum system. It uses an ion-induced chemical reaction to achieve anisotropic etching, that is, it uses ion energy to form the surface of the etched layer to form an easily etched damage layer. And promote chemical reactions, ions can also remove surface products to expose the role of a clean etched surface. However, this etching technique cannot obtain a high selection ratio, which has a large damage to the surface, is contaminated, and it is difficult to form a finer pattern ^[1] .

The plasma etching process includes the following six steps. Separation: Gas is separated into chemically reactive elements by plasma; Diffusion: These elements diffuse and adsorb to the surface of the silicon wafer; Surface diffusion: After reaching the surface, move around; Reaction: Reaction with the film on the surface of the silicon wafer; Desorption: Reaction The product is desorbed and leaves the surface of the silicon wafer; discharge: discharge out of the reaction chamber ^[1]

FIG. 1 is a schematic diagram of a reactive ion etching system. Under normal circumstances, the entire vacuum wall of the reactive ion etching machine is grounded. As the anode, the cathode is the power electrode, and the ground shield on the side of the cathode prevents the power electrode from being sputtered. The substrate to be etched is placed on the power electrode. The corrosive gas fills the entire reaction chamber according to a certain working pressure and matching ratio. For the corrosive gas in the reaction chamber, plus a high-frequency electric field greater than the gas breakdown threshold, under the action of a strong electric field, stray electrons accelerated by the high-frequency electric field collide randomly with gas molecules or atoms. At a certain level, random collisions become inelastic collisions, resulting in secondary electron emission, and they further collide with gas molecules, continuously exciting or ionizing gas molecules. This intense collision causes ionization and recombination. When the generation and disappearance of electrons reach equilibrium, the discharge can be continuously maintained. Ions, electrons, and free radicals (free atoms, molecules, or atomic groups) generated by inelastic collisions are also known as plasmas, and have strong chemical activity. They can chemically react with the atoms on the surface of the etched sample to form volatilization. Substance, to achieve the purpose of corroding the surface of the sample. At the same time, because the direction of the electric field near the cathode is perpendicular to the surface of the cathode, high-energy ions shoot perpendicular to the surface of the sample under a certain working pressure for physical bombardment, making the reactive ion etching anisotropic ^[2]

A typical (parallel plate) RIE system includes a cylindrical vacuum chamber with a wafer tray at the bottom of the chamber. The wafer tray is electrically isolated from the rest of the chamber. Gas enters through a small inlet at the top of the chamber and exits the vacuum pump system through the bottom. The type and amount of gas used depends on the etching process; for example, sulfur hexafluoride is commonly used to etch silicon. By adjusting the gas flow rate and / or adjusting the vent, the gas pressure is typically maintained in a range between a few millitorr and several hundred millitorr.

There are other types of RIE systems, including inductively coupled plasma (ICP) RIE. In this type of system, an RF-powered magnetic field is used to generate the plasma. Although the etch profile tends to be more isotropic, very high plasma densities can be achieved.

A combination of parallel plates and inductively coupled plasma RIE is possible. In this system, ICP is used as a high-density ion source, which increases the etch rate, while a separate RF bias is applied to the substrate (silicon wafer) to generate a directional electric field near the substrate to achieve more isotropic Anisotropic etching contour.

Plasma is initiated in the system by applying a strong RF (radio frequency) electromagnetic field to a wafer platter. This field is usually set to a frequency of 13.56 MHz and is applied at several hundred watts. The oscillating electric field ionizes gas molecules by stripping electrons, thereby generating a plasma ^[3] .

During each cycle of the field, the electrons accelerate up and down the chamber, sometimes hitting the upper wall of the chamber and the wafer disc. At the same time, larger mass ions move relatively less in response to the RF electric field. When electrons are absorbed into the walls of the chamber, they are simply sent to the ground without changing the electronic state of the system. However, the electrons deposited on the wafer disc cause the disc to accumulate charges due to its DC isolation. This charge build-up produces a large negative voltage on the disc, usually on the order of several hundred volts. Due to the higher positive ion concentration compared to free electrons, the plasma itself generates a slightly positive charge.

Due to the large voltage difference, the positive ions tend to drift towards the wafer disc, where they collide with the sample to be etched. Ions chemically react with materials on the surface of the sample, but some materials can also be knocked out (sputtered) by transferring some kinetic energy. Due to the largely vertical transfer of reactive ions, reactive ion etching can produce very anisotropic etching profiles, as opposed to the typical isotropic profiles of wet chemical etching.

Etching conditions in a RIE system are largely dependent on many process parameters, such as pressure, gas flow, and RF power. An improved version of RIE is deep reactive ion etching for mining deep features.