Introduction to plasma etching machine

Introduction to plasma etching machine:

There are many kinds of plasma etching machines, and the designs of different etching machine manufacturers are also different. Since plasma maintains the plasma shape of etched gas through external energy input, different energy input modes and the design of machine structure will have great influence on the performance and application of plasma. The following is a description of several Plasma etching machines that are frequently used in vLSI production. They are Capacitively CoupledPlasma (CCP), Inductively CoupledPlasma (ICP& TransformerCoupled Plasma (TCP), and Electron Cyclotron Resonance Plasma (Electron Cyclotron Resonance), ECR, Remote Plasma and Plasma BevelEtch. The first three etching machines are named after the way of plasma generation, while the last two machines achieve different etching effects mainly through special structural design. The remote plasma etching machine filters out the charged particles of plasma and etches the materials to be etched with free radicals. This reaction is pure chemical reaction and belongs to isotropic etching. The plasma edge etching machine is specially designed to clean and etch the edge area of the wafer only through the reaction chamber structure, which has a very good effect on reducing the number of defects and improving the yield.

1. Capacitive coupling plasma machine:

High-frequency electric fields were applied to two parallel plate capacitors. The initial electrons in the reaction chamber obtained energy under the action of rf electric fields, and the etching gas was ionized by bombardment, generating more electrons, ions and neutral free radical particles to form a dynamically balanced low-temperature plasma. Under the action of the radio frequency electric field, the self-bias voltage perpendicular to the direction of the wafer will be formed, which enables the ions to obtain relatively large bombardment energy. In the initial development stage of capacitance-coupled plasma machine, there is only one RF power supply, and the variation of RF power will affect both plasma density and ion bombardment energy, so the controllability of single-frequency capacitance-coupled plasma is not satisfactory.

Multi-frequency capacitive Coupled Plasma makes the performance of capacitive Coupled Plasma Etchers greatly improved by introducing multi-frequency external power source. For multi-frequency applied electric field, high frequency electric field mainly plays the role of controlling plasma density, and low frequency electric field mainly plays the role of controlling ion surface strike energy. At present, the mainstream capacitively coupled plasma etching machines in semiconductor industry are all dual-frequency and multi-frequency capacitively coupled plasma etching machines. Another characteristic of capacitive coupled etching machines is that the two electrical areas are different. For capacitor-coupled plasmas, the electrode with smaller area will get higher potential difference due to self-bias.

2. Inductively coupled plasma machine:

Inductively coupled plasma machine is a method by applying rf voltage to the electromagnetic coil outside the reaction chamber. In the reaction chamber, the rapidly changing induced magnetic field will generate an induced electric field in the chamber, so that the initial electrons can obtain energy and then generate a low-temperature plasma. The electrons in the inductively coupled plasma revolve around the magnetic field lines, and the free path is larger than that in capacitive coupled plasma, and the plasma can be excited at lower pressure. The plasma density is about two orders of magnitude higher than that of capacitive coupling plasma, and the ionization rate can reach 1% ~ 5%. The direct current potential and ion bombardment energy of plasma is about 20 ~ 40V. Compared with capacitive coupled plasma; The ion flux and energy of inductively coupled plasma can be controlled independently. To better control ion bombardment energy, another RF power supply is typically capacitively coupled to a substrate wafer. In the process of inductive discharge, the coil and the capacitive driven substrate will generate capacitive coupling components, that is, in the process of plasma generation, the voltage difference will be generated by the external power supply. This is not conducive to independent control of plasma density and energy. Therefore, a layer of electrostatic shielding is generally added between the coil and the plasma to filter out the capacitive coupling components of the coil without affecting the inductive coupling. The coil layout has a great influence on the performance of the machine, and the design of induction coils from different manufacturers often varies greatly. The main coil layout structure has the aromatic structure and the cylindrical structure.

3. Electron cyclotron resonance plasma machine:

Electron cyclotron resonance plasma etching machine USES high frequency microwave to produce plasma. In a magnetic field, electrons have a radius of rotation much smaller than that of the ion, so they are bound by the magnetic field and rotate around the magnetic field line. Ions, on the other hand, are not significantly affected and move independently. The electron cyclotron frequency is determined by the magnetic field strength. For a particular applied high-power microwave, when the frequency of the microwave is consistent with that of the electron cyclotron, the electron will resonate, thus obtaining the microwave energy transmitted by the magnetic field. With a fixed microwave frequency, the magnetic field lines in the reaction chamber diverge from top to bottom. The intensity of the magnetic field decreases accordingly, and the position of the plasma is fixed when the distribution of the magnetic field intensity covers the resonant magnetic field intensity. For microwave energy with a frequency of 2.45ghz, the magnetic field strength of electron cyclotron resonance is 875G(Gauss). Microwave energy and magnetic field intensity are two important parameters for electron cyclotron resonance plasma etching chamber. The microwave energy can determine the plasma density and adjust the magnetic field strength, that is, adjust the position of the electron resonance area with magnetic field strength of 875G, and then adjust the distance between the plasma production area and the wafer. The energy distribution and incidence Angle distribution of ions can be changed. Low pressure is one of the development directions of plasma. At a lower pressure, the collisions of ions before they are bombarded on the wafer will be reduced, and then the scattering collisions will be reduced. Therefore, the ion incident Angle can be optimized to obtain relatively accurate etching results.

4. Remote plasma etching machine:

The main components of plasma include unionized gas molecules, charged ions, electrons and various free radicals. In the traditional graphic transfer - related etching process, various components in the band ionization bodies are very important. Some processes require only the removal or selective etching of exposed material from the surface of the wafer, rather than the physical bombardment and directional etching caused by charged particles. The remote plasma etching machine can meet the needs of these processes. Plasma generation and etching reaction of remote plasma etching machine are done in different Chambers. The reaction gas enters the plasma excitation chamber, ionizes under the action of an applied electric field or microwave to produce a plasma, and then enters the etched chamber through a pipe or a specific filtration device. As charged particles are transported, they are filtered out of the duct wall or by a specific device. Neutral free radicals may enter the reaction chamber to react with the wafer to be etched. Since there are no charged particles, the entire reaction does not produce damage associated with charged particles. Related applications are very wide, such as photoresist ashing, polysilicon backcutting and other processes.

5. Plasma edge etching machine:

Plasma edge etching refers to the use of plasma etching to remove the wafer edge places do not need the film, plays a role in reducing the number of defects, improve the yield. As the technology nodes extend to 20nm and more advanced process nodes in accordance with Moore's law, the impact of defects related to the edges and sides of the wafer on the yield becomes particularly prominent. In the manufacturing process of VLsI, the complex interactions among film deposition, photoligraphy, etching and chemical-mechanical grinding are easy to cause unstable film accumulation on the edge of wafer. These unstable films may fall off in subsequent processes, affecting subsequent exposure, etching or filling processes and resulting in yield loss. After many processes of deposition, lithography, etching, and chemical-mechanical grinding, the edge regions of the wafer form a complex, unstable membrane structure. After the contact hole etching, the silicon oxide film in the edge area falls off during the metal filling process and falls onto the surface of the wafer, which directly leads to the metal loss in the contact hole after the chemical mechanical grinding, resulting in device failure. In the process of forming metal connection in the latter section, the residual metal filler in the edge area may also cause arcing problems in the plasma-related process, which may lead to the scrapping of the entire wafer. Therefore, it is necessary to control the edge areas in the device manufacturing process. Removing these thin films accumulated at the edge of the wafer can reduce defects and yield loss in the production process.

There are three methods to clean the wafer's outer edge and bevel: grinding and cleaning the outer edge and bevel added in the process of chemical mechanical grinding; Wet etching and cleaning; Plasma edge etching. Plasma edge etching has some prominent features, such as precise control of edge etching area, more types of etching gas can be used to process a variety of thin films, and various adjustable parameters can control the influence on the front layer.

The plasma edge etcher protects most of the wafer by covering the upper and lower parts, while the outer edges and sides are exposed to the plasma. There is no physical contact between the covering device and the wafer, and the distance is usually controlled between 0.3 and 0.5mm. The size of the covering device can be selected according to the needs of the process. Plasma edge etching machines can have different etching gas combinations depending on the material being cleaned. To remove the polymer, oxygen or nitrogen based plasma is needed; for the dielectric layer, fluorine containing plasma such as CF4/SF6 is mainly required; for the metal layer such as titanium, tantalum, aluminum and tungsten, etched gases containing chlorine are required, such as boron chloride and chlorine gas.

Plasma edge etching can improve many defects related to thin film deposition in edge regions. Of course, from the perspective of process integration, the influence of edge etching on subsequent processes should be considered and comprehensively evaluated.