CVD SiC – Chemical Vapor Deposited Silicon Carbide for Semiconductor Applications

Silicon Carbide (SiC) is a natural choice for semiconductor equipment components due to its high thermal conductivity and resistance to abrasion, corrosion and erosion. Its ability to withstand constant and intensive use has made the material one of the most reliable. 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). Morgan Advanced Materials CVD Materials division has developed advanced CVD SiC manufacturing technology to supply high performance CVD SiC components required by the semiconductor industry.

Types of Silicon Carbide

Table 1 displays the four most common types of silicon carbide, which includes chemical vapor deposition (CVD) SiC, hot pressed SiC, reaction bonded SiC, and sintered SiC.

Table 1. Comparison of Silicon carbide manufacturing methods.

CVD SiC

Hot Pressed SiC

Reaction Bonded SiC

Sintered SiC

Purity     

99.999%+

97-99.9%+

99%

99%

Density (g cm-3)

3.21

3.15-3.20

3.00-3.15

3.15

Porosity

negligible

<1%

<1%

1-2%

Thermal Conductivity

(W m-1 K-1)

250-300

100-180

100-150

100-140

Coefficient of Thermal Expansion (°C-1)

4.4 x 10-6

4.5 x 10-6

4.5 x 10-6

4.2 x 10-6

Flexural Strength

(103 psi)

60-70

60-80

40-70

50-60

Elastic Modulus

(106 psi)

65

65

65

60

Chemical Vapor Deposited Silicon Carbide in Semiconductor Applications

As Table 1 illustrates, silicon carbide formed through a CVD process outperforms and outlasts other types of silicon carbide, as well as quartz, metal and ceramic, 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. However, the lifetime of SiC-coated graphite components are limited by a slow attack of the SiC coating, resulting in formation of pinholes in the material and subsequent rapid attack of the underlying graphite.

Monolithic CVD SiC is a far superior choice for semiconductor components because it provides a longer lifespan and a more cost effective proposition.

Hot Pressed and Sintered Silicon Carbide in Semiconductor Applications

Hot pressed SiC and sintered SiC are two less expensive grades of silicon carbide with similar manufacturing techniques and similar performance characteristics. However, dimensional changes occur between the green and fired states and 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, resulting in corrosion, oxidation and chemical erosion, which can cause complications in semiconductor manufacturing applications.

Reaction Bonded Silicon Carbide in Semiconductor Applications

Another type of SiC used in Rapid Thermal Processing (RTP) semiconductor equipment is silicon-infiltrated, reaction-bonded, impervious silicon carbide. Reaction bonded SiC has a relatively low density (3.00-3.15 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 too weak for many semiconductor applications.

Advantages of CVD Silicon Carbide in Semiconductor Processing

CVD SiC has traditionally been used in semiconductor processing applications, such as RTP and oxide etch chamber components that take advantage of SiC’s excellent 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, ensuring homogeneity within a production run and reproducibility between batches. Moreover, CVD SiC is much harder than common metals and ceramics, and can be polished to a durable mirror-like finish (<3 Å RMS). A lightweight material (similar to aluminum), CVD SiC has one of the best stiffness-to-weight ratios available.

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 is ideal for the ultra-clean environment of semiconductor manufacturing facilities. Additionally, with their resistance to corrosion, oxidation and chemical erosion, CVD SiC components stand up to the plasmas and acids used in semiconductor processing and cleaning.

CVD SiC also has a low coefficient of thermal expansion (4.5x10-6 °C from 20°C - 400°C ) and extremely high thermal conductivity (≥ 250 W/m-K at 20°C), as well as good performance at high temperatures (up to 1700°C).

Typical Applications of CVD Silicon Carbide

Rapid Thermal Processing Applications.

The exceptional 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 six to seven seconds and then cooled just as rapidly.

Morgan Advanced Materials CVD Materials division produces revolutionary, precision machined CVD SiC “edge rings” that hold the silicon wafer during processing. The wafer sits inside a recess in the inner diameter of the edge ring. The edge ring does not 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.

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), thus the ring does not hold significant heat, making it nearly invisible to the heating and cooling process.

Plasma Etch Applications

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

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 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.

Summary

CVD SiC components provided by Morgan Advanced Materials CVD Materials division have been developed to provide the highest performance in the semiconductor industry. CVD SiC is recognized as the premium choice for components in Etch, RTP and Epitaxy processing chambers, especially in applications where chemical resistance, high temperature, rapid thermal cycling, and ultra-high purity are system requirements.

This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.

For more information on this source please visit Morgan Advanced Materials.

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