English
Dry etching equipment refers to equipment that performs etching without using wet chemicals. It primarily directs a gaseous etchant from the upper electrode with microscopic through-holes into the chamber. The electric field generated by the upper and lower electrodes within the chamber ionizes the gaseous etchant, causing it to react with the material on the wafer that needs to be etched, forming volatile substances. These substances are then evacuated from the reaction chamber, completing the etching process.
The dry etching reaction is completed in the process chamber, where the main components are silicon parts. These include silicon exhaust rings, silicon outer rings, silicon showerheads, silicon focus rings, silicon shielding rings, and others.
Image source: DSTC Prospectus
In the chamber of dry etching equipment, the silicon wafer cassette is generally placed into a silicon focus ring, combined and positioned as the positive electrode below the etching equipment. The silicon plate, with densely packed through-holes located above the etching equipment chamber, acts as the negative electrode, supported by a silicon outer ring that also supports the other related components. The upper and lower electrodes are in direct contact with the plasma, which erodes the silicon electrodes while etching the wafer. The lower electrode (focus ring) gradually thins during the etching process and must be replaced once it reduces to a certain thickness. The uniformly distributed holes in the upper electrode (showerhead) corrode due to plasma, causing deviations in the hole size. Once deviation exceeds limits, replacement is necessary, typically every 2-4 weeks of usage.
Name of Silicon Part | Illustration | Explanation |
Showerhead |  | In dry etching, when plasma is etching the wafer, it is necessary to maintain uniform speed and impact across all points. During the etching process, the upper electrode is energized to form an electromagnetic field that accelerates and confines the plasma, and the evenly distributed holes on the upper electrode surface are used to split the process gas. |
Focus Ring, Silicon Ring |  | Under the dry etching process, the lower electrode primarily serves as the component bearing the silicon wafer. It is also energized to form an electromagnetic field that accelerates and confines the plasma (adjusting the plasma sheath and optimizing the direction of plasma bombardment). |
Silicon Outer Ring |  | It serves as a connecting part between the exhaust ring and the showerhead, suffering less plasma erosion. |
Silicon Exhaust Ring |  | As a gas channel, it provides a passage for the entry of process gases/carriers and the evacuation of exhaust gases after the reaction. |
Here, the role of the silicon focus ring (lower electrode) is specifically explained. The silicon focus ring is used to control the thickness of the plasma sheath, thereby optimizing the uniformity of ion bombardment. The plasma sheath is a non-electrically neutral region between the plasma and the container wall, a very important and special region in plasma. Plasma is composed of equal numbers of positive ions and electrons. Because electrons are faster than ions, they reach the container wall first. The relative container wall becomes positively charged, and the sheath electric field accelerates the ions within the plasma (positive and negative attraction), giving the ions very high energy. This high-energy ion stream can be used for coating, etching, and sputtering.
The impedance of the wafer affects the thickness of the plasma sheath (lower impedance, thicker sheath). The impedance at the center of the wafer is different from the edge, resulting in an uneven thickness of the plasma sheath at the edge. An uneven plasma sheath accelerates ions while also causing a shift in the position of ion bombardment, reducing etching precision. Therefore, a focus ring is needed to control the thickness of the plasma sheath to optimize the ion bombardment direction and improve etching precision.
Image source: Research on Superconducting Magnet for Czochralski Silicon Monocrystalline Furnace
In the selection of materials for etching equipment components, silicon is not the only choice. Component materials need to meet three criteria: high purity, chemical composition consistent with the material to be etched/reaction gases, and strong plasma corrosion resistance (long service life). It's challenging for a single material to meet all the requirements of etching equipment, hence different materials are used based on specific needs.
Comparison of material selection for etching equipment components:
| Material Name | Characteristics | Applications in Reaction Chamber Components |
| Quartz (SiO2) | High purity, low cost, good insulation, poor resistance to fluorine plasma corrosion | Reaction chamber insulation top plate, wafer peripheral focus ring, reaction chamber window, other electrical insulation components, other insulation components |
| Monocrystalline Silicon (Si) | High purity, low cost, good thermal conductivity, poor resistance to fluorine/chlorine plasma corrosion | Electrode plate on the reaction chamber, focusing ring around the circular plate, plasma isolation plate, etc |
Polysilicon (Poly Si) | Compared to monocrystalline silicon, it has lower purity, lower price, and poorer corrosion resistance, but it is easy to adjust the conductivity | Circle periphery focusing ring, etc |
Silicon Carbide (Sic) | High purity, high price, good thermal conductivity, stronger resistance to fluoride/chlorine plasma corrosion than silicon and quartz | Electrode plate on the reaction chamber, focusing ring around the circular plate, etc |
Alumina (Al2O3) | Adjustable purity, high price, good thermal conductivity, and strong corrosion resistance; But it is prone to react with fluorinated plasma, forming aluminum fluoride pollution | Insulation top plate of reaction chamber, gas nozzle, surface of circular electrostatic suction cup, other insulation components, etc |
Aluminum Nitride (Al3N4) | Adjustable purity, very high price, higher thermal conductivity than alumina, strong corrosion resistance | Insulation top plate of reaction chamber, gas nozzle, surface of circular electrostatic suction cup, other insulation and thermal conductive components, etc |
Yttrium Oxide (Y2O3) | Adjustable purity, low cost of coating, strong corrosion resistance, often applied in the form of surface coatings due to poor solid thermal conductivity | Insulation top plate of reaction chamber, reaction chamber wall, lining and plasma isolation plate coating, etc |
Aluminum anodizing (Anodized Al) | Low cost, weaker resistance to plasma corrosion than ceramics, but stronger than quartz and silicon | Electrode surface, reaction chamber wall, and lining surface, etc |
Aluminium (Al) | Low cost, strong conductivity, easy processing, low purity, poor resistance to chlorine corrosion | Reaction chamber, lower electrode and electrostatic suction cup base, other conductive components, etc |
Take the focusing ring around the wafer as an example. If quartz material is used, its high purity is most favorable for obtaining low metal contamination, but it corrodes and consumes too quickly in the plasma of fluoride gas, and its life is too short, which not only increases the cost, but also shortens the on-line rate of the device because of the need for replacement and shutdown; if ceramics are selected, the life is sufficiently long, but because it is in the region of bombardment of high-energy ions, the aluminum escaping from sputtering reacts with the fluorine in the plasma to form non Volatile fluoride (such as aluminum fluoride, etc.), if it can not be extracted and deposited on the surface of the device at the edge of the wafer or photoresist, it will prevent the removal of the generation and photoresist in subsequent steps, affecting the product qualification rate. More suitable materials are monocrystalline silicon or silicon carbide, however, from research conducted by most silicon carbide semiconductor companies, monocrystalline silicon is cheaper but shorter life, silicon carbide parts are more expensive but slightly longer life, need to be based on the actual situation of the trade-offs, such as high equipment utilization, its online rate is more important, should be used in the silicon carbide; if the loss of this component is not too high percentage of the cost of the loss of monocrystalline silicon should be used.
