Silicon Components for Dry Etching

Dry etching equipment uses no wet chemicals for etching. It primarily introduces a gaseous etchant into the chamber through an upper electrode with tiny through-holes. The electric field generated by the upper and lower electrodes ionizes the gaseous etchant, which then reacts with the material to be etched on the wafer, producing volatile substances. These volatile substances are then extracted from the reaction chamber, completing the etching process.

The dry etching reaction takes place within a process chamber, which primarily consists of silicon components, including a silicon exhaust ring, a silicon outer ring, a silicon showerhead, a silicon focus ring, and a silicon shield ring.

In a dry etching chamber, a silicon wafer is typically placed within a silicon focus ring. This combination serves as the positive electrode, positioned below the etching chamber. A silicon disk with densely packed tiny through-holes, located above the chamber, serves as the negative electrode. A silicon outer ring supports the upper electrode and other related components. The upper and lower electrodes are in direct contact with the plasma. As the plasma etches the silicon wafer, it also wears away the upper and lower silicon electrodes. The lower electrode (focusing ring) gradually thins during the etching process, requiring replacement when the thickness reaches a certain level. Furthermore, the uniformly distributed holes in the upper electrode (showerhead) are corroded by the plasma, causing variations in hole size. Once these variations reach a certain level, they need to be replaced. Typically, a replacement cycle is required every 2-4 weeks of use.

This section specifically explains the role of the silicon focusing ring (lower electrode). It controls the thickness of the plasma sheath, thereby optimizing the uniformity of ion bombardment. The plasma sheath, the non-neutral region between the plasma and the vessel wall, is a crucial and unique region within the plasma. Plasma consists of equal numbers of positive ions and electrons. Because electrons travel faster than ions, they reach the vessel wall first. The plasma is positively charged relative to the vessel wall. The sheath electric field accelerates ions within the plasma (positive-negative attraction), imparting high energy to the ions. This high-energy ion flux enables coating, etching, and sputtering.

The impedance of the wafer affects the thickness of the plasma sheath (the lower the impedance, the thicker the sheath). The impedance at the center of the wafer is different from that at the edge, resulting in uneven plasma sheath thickness at the edge. This uneven plasma sheath accelerates ions but also deflects the ion bombardment point, reducing etching accuracy. Therefore, a focusing ring is needed to control the plasma sheath thickness, thereby optimizing the ion bombardment direction and improving etching accuracy.

Taking the focus ring around the wafer as an example, while quartz, with its high purity, is optimal for achieving low metal contamination, it corrodes rapidly in fluoride gas plasma, resulting in a short lifespan. This not only increases costs but also requires downtime due to replacement, reducing the equipment's uptime. Ceramic, while having a sufficiently long lifespan, is exposed to high-energy ion bombardment. Sputtered aluminum reacts with fluorine in the plasma to form non-volatile fluorides (such as aluminum fluoride). If these cannot be removed and deposited on the device surface or photoresist at the wafer edge, they hinder subsequent removal of the generated fluorides and photoresist, impacting product yield. More suitable materials are single-crystal silicon or silicon carbide. However, single-crystal silicon is inexpensive but has a short lifespan, while silicon carbide is more expensive but has a slightly longer lifespan. The trade-off between these two options depends on the specific circumstances. For example, if equipment utilization is high and uptime is critical, silicon carbide should be used. If the component's wear and tear costs are not too high, single-crystal silicon should be used.

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