Thanks to its high thermal conductivity and resistance to abrasion, corrosion, and erosion, silicon carbide (SiC) is a natural choice for semiconductor-equipment components. Its ability to withstand constant and intensive use also has made SiC one of today's most reliable materials. Although SiC can be manufactured in a variety of ways —including hot pressed, reaction bonded, and sintered— the most advantageous is through a process called chemical vapor deposition (CVD).

Types of Silicon Carbide

Table 1 displays the four most common types of silicon carbide: chemical vapor deposition (CVD), hot pressed, reaction bonded, and sintered. As the table illustrates, silicon carbide formed through a CVD process has higher chemical purity, density, and thermal conductivity as well as lower porosity. These aspects enable it to outperform and outlast other types of silicon carbide —as well as quartz, metal, and other ceramics— in the extremely hostile environment of semiconductor manufacturing.

Traditionally, a number of semiconductor components have been fabricated from SiC-coated graphite. In this case, the part is machined from high-purity graphite and subsequently coated with a layer of CVD SiC. Yet the lifetimes of SiC-coated graphite components are limited by a slow attack of the SiC coating, which results in the formation of pinholes in the material and a subsequent rapid attack of the underlying graphite.

Hot-pressed and sintered SiC are two less expensive grades of silicon carbide with similar manufacturing techniques and performance characteristics. However, dimensional changes occur between the green and fired states. In addition, the surface finish of the final product is relatively rough and porous. More significantly for semiconductor manufacturing applications, the additives used in manufacturing hot-pressed and sintered SiC are chemically reactive. They result in corrosion, oxidation, and chemical erosion, which can cause complications.

Another type of SiC used in wafer holders within semiconductor equipment is silicon-infiltrated, reaction-bonded, impervious silicon carbide. Reaction-bonded SiC has a relatively low density (3.00 to 3.15 g cm-3) and high levels of organic impurities. Furthermore, reaction-bonded SiC has a tendency to produce leachable elements. Therefore, this material is too chemically reactive and weak for many semiconductor applications.

The Advantages of CVD SiC

For its part, CVD SiC has traditionally been used in semiconductor processing applications, such as rapid thermal processing (RTP) and oxide etch chamber components. They take advantage of SiC's resistance to thermal shock and erosion by high-energy plasmas.

The chemical-vapor-deposition process produces freestanding monolithic CVD SiC of extremely high purity (99.9995%). The isotropic cubic ß crystal structure provides theoretical density (3.21 g/cc) with no porosity and no micro-cracks. It therefore ensures homogeneity within a production run and reproducibility between batches. Moreover, CVD SiC is much harder than common metals and ceramics. It can be polished to a durable mirror-like finish (<3 Å RMS). A lightweight material (similar to aluminum), CVD SiC also boasts an impressive stiffness-to-weight ratio.

These attributes offer several important performance advantages. With its high resistance to wear and abrasion, CVD SiC is an extremely durable, non-particle-generating material that's well-suited for the ultra-clean environment of semiconductor manufacturing facilities. With their resistance to corrosion, oxidation, and chemical erosion, CVD SiC components also stand up to the plasmas and acids used in semiconductor processing and cleaning. CVD SiC has a low coefficient of thermal expansion (4.5 x 10^-6/°C from 20°C – 400°C) and high thermal conductivity (>=250 W/m-K at 20°C) as well as good performance at high temperatures (up to 1700°C).

Typical Applications

The superior thermal properties of CVD SiC make it an ideal material for rapid-thermal-processing (RTP) applications. During RTP, an intense heat pulse is applied to an individual wafer for a very short period of time. The heat is then turned off and the wafer is rapidly cooled. In a typical process, a wafer may be heated from 20°C to 1100°C in 6 to 7 sec. and then cooled just as rapidly.

Precision-machined CVD SiC “edge rings” are now available that hold the silicon wafer during processing (see Figure 1). The wafer sits inside a recess in the inner diameter of the edge ring. The edge ring doesn't break during RTP because the CVD SiC has higher thermal shock resistance than other conventional ceramic materials, such as alumina and quartz. The high thermal conductivity of CVD SiC also performs the important function of rapidly equalizing the temperature around the outer diameter of the SiC wafer.

Figure 1. Chemical-vapor-deposition SiC “edge rings” can hold the silicon wafer during processing.

In an ideal RTP processing scenario, the wafer would be suspended in air or vacuum to enable uniform heating of the wafer surface. The CVD SiC edge ring is very thin (providing a low thermal mass). Because the ring does not hold significant heat, it is nearly invisible to the heating and cooling process.

High chemical resistance also makes CVD SiC suitable for etch-processing applications. A number of etch-process chambers use CVD SiC gas-distribution plates. Here, the etching gas is distributed through a showerhead that has several thousand small holes into a plasma and subsequently onto the wafer surface. In this application, the advantage of CVD SiC compared to alternative materials is its low reactivity to chlorine- and fluorine-containing etch gases.

In addition, CVD SiC can be used as a material for focus rings in which a voltage is applied onto the rings to focus a plasma that passes through the ring. These rings must have sufficient electrical conductivity to allow the application of voltage. Many of these rings are currently made of silicon. Depending on the process gas in the plasma (e.g., chlorine or fluorine-containing molecules), the silicon focus ring will be chemically attacked. The ring is degraded and its lifetime is reduced unless it is made of CVD SiC, which prevents deterioration.

In summary, CVD SiC is recognized as the premium choice for components in etch, RTP, and epitaxy processing chambers. It is especially suitable for applications that require chemical resistance, high temperature, rapid thermal cycling, and ultra-high purity.