Ionic Purity vs Plasma Resistance - What's More Critical for a Plasma Chamber Plastic Material?
Do I dare try to answer the question… Yes, because I am protected by the “it depends” clause. The simple answer is that it does not matter what is in the chamber as long as there is zero chemical erosion of the material and thus it does not find itself dispersed within the chamber. But all plastics are subject to erosion by plasma, whether mechanical by the etching plasma or chemical oxidation due to the breakdown of the chemical covalent bonds as a result of contact with oxygen plasma. So let’s explore how to evaluate materials for ionic purity for plasma chamber applications.
First off, all plastic materials are made through a series of chemical reactions known as polymerization. Many of the steps involve catalysts or intermediates and often produce bye products. These catalysts often contain metal ion components. Most industries do not care about minute quantities of metal ion bye products as materials are often characterized based on physical, thermal and electrical properties. These metal ions are typically found in minute ppm level quantities within the finished plastic itself. Total digestion methodology is used to determine their quantity and the values are not typically published. They are not generally considered foreign contaminants nor are they necessarily part of the backbone of the polymer though they can be. Important to note that similar resins made by different manufacturers will have different levels in terms of ionic purity. For example, all PEEK resins are not the same. What makes a highly pure resin is a virgin resin synthesis that does not require metal catalyst. If the metal is part of the backbone of the polymer, the element is less prone to cause issues in the chamber.
There are 16 elemental metals that the Semiconductor capital equipment industry generally monitors to determine the ionic purity of a material for use in wafer critical applications. They are listed in the table to the right These metal ions are deemed potentially dangerous, especially in high levels to the functional quality of the finished wafer. But again, it depends if the metal ion finds its way onto the wafer itself.
Now how does one determine the effect of the material to introduce metal ions into the chamber in an etch application? One needs to consider the rate of erosion of the plastic material given the parameters of the chamber (electrode power, distance from source, specific plasma) and use this as a factor to adjust for actual contamination potential within the chamber. Let’s explore this.
The below chart (left) displays the ionic purity for the same three common chamber materials however the quantity for each metal ion has been adjusted based on the values of erosion for each material given the listed etch chamber parameters (below right).
So, getting back to the original question, “what is more critical for evaluating plasma chamber materials, ionic purity level or rate of erosion?” In this case if you look at the level of Iron as a result of the total digestion method, than Polyimide is the best candidate:
However, if you adjust for the rate of erosion, which provides a more realistic data point as to what is actually going to end up in the chamber then Semitron MPR-1000 is the best candidate while PEEK is far and away the largest contributor of iron ion in the chamber:
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