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Application for Plasma Etching and Ashing

Plasma ashing

The process of plasma ashing, plasma stripping or micro-incineration is usually restricted to the total removal of organic matter by an oxygen plasma; the products being carbon oxides and water vapour, which are volatile and pumped away by the vacuum system. Historically, the first application was for the removal of photoresist in the microelectronics industry. Photoresist is composed of organic compounds, essentially consisting of carbon, plus hydrogen and oxygen. Exposure to an oxygen plasma eventually removes all the photoresist as volatiles leaving no residues, unless there are inorganic contaminants in the photoresist. The shows process is therefore totally dry and is also a means of concentrating inorganic contaminants in organic materials.

Asbestos and man-made mineral fibre (MMMF) detection

This is a major application for RF plasma systems. The exact protocols vary from country to country, but in the UK the following is a summary of the recommended method. The technique requires that a sample of filtered air (or water) is collected from a known volume on MF series filter (mixed cellulose acetate and nitrate). The filter is then transferred to a microscope slide, treated with a clearing solution, dried and then transferred to the plasma unit. The asbestos fibres are then exposed by partial ashing of the filter by treatment in an oxygen plasma for typically seven minutes. This removes surface layers of the filter leaving the asbestos fibres exposed but still attached to the collapsed filter matrix. The fibres can be counted and identified by light microscopy, light contrast microscopy, SEM and EDX. Some SEM and EDX protocols require complete ashing of the filter.

Coal ashing

Small samples can be ground and distributed in a Petri dish to produce maximum surface area. The coal dust can be ashed by an oxygen plasma at low temperatures, compared to the extremely high temperatures generally used in a muffle furnace for this process. Volatile elements, such as selenium, are retained and therefore a more accurate calorific and ash value can be produced. Problems in estimating the required values are a result of the structure of coal which includes organic materials (and hence convertible material) together with inert inorganic materials in the same overall matrix.
Plasma chemistry is a surface reaction, so methods such as the ashing of coal require the exposure of new surfaces. For this reason physical stirring of the sample is recommended every 1-2 hours. The complete ashing of a one-gram sample is typically completed in 12-24 hours.

Detection of metals in blood

Plasma ashing as a pre-treatment for atomic absorption analysis (AAS) is another well-established application. In this case one is normally looking for metals such as lead, cadmium, zinc and mercury in trace quantities in organic materials such as vegetables, dairy products or animal tissue. A specific example involves the treatment of multiple samples of human blood exposed to a CF4/02 plasma. The organic materials in this application can be removed in 15 minutes, leaving only the metallic contaminants to be analysed for cadmium.

Ashing of biological material, food stuffs etc.

Plasma ashing has also been successfully used to ash materials as varied as post-mortem lung tissue (for asbestos), bread (to determine type and distribution or iron) and samples of prepared food (for asbestos and man made mineral fibres). Samples need to be dried prior to ashing and their size kept to a minimum.

Organic and inorganic composites

Similar problems are encountered in composite materials such as paints, vehicle tyres and brake linings, contaminated oils and the application of clays onto paper.
In paints, the organic binder can be removed to leave the inorganic pigment in its original distribution.
In paper, the clay platelet distribution and adhesion can be investigated after ashing of the paper and binder.
Similarly epoxy composite materials can be investigated.

Plasma etching

Plasma ashing and plasma etching rely on the same basic principles. Plasma Etching is usually confined to the semiconductor industry, and more often than not, uses carbon tetrafluoride (with oxygen) as the plasma process gas. Probably the most frequent application is the etching of silicon, silicon oxides, and silicon nitrate, as well as glass passivation layers.
Failure analysis of integrated circuits is also an important application of plasma processing. Oxygen gas is used to remove epoxy encapsulates, CF4/02 is also used to remove glass filters in the encapsulants and so uncover devices which have failed. Inspection by methods, such as SEM, is then possible.
When etching, it should always be remembered that not only the required surface will be removed. Careful choice of gas is made so that preferential etching of the required surface is attained.
Plasma etching is a chemical process. The RF discharge generates species which then react with the material being etched to form a volatile product. The resulting products are swept away by the gas flow. Since reactive species are being formed, the reactant gas is chosen to give the highest concentration of the etching species. For example, CF4 and CF4/02 mixtures produce very reactive fluorine and CF3 radicals and ions. Similarly, other gases and volatile compounds have been investigated and used to etch a wide variety of materials. These include CCl4, CFCl3, C2Cl6, C2F6, SF6, SIF4, and mixtures of these gases with H2, O2, Ar, He, CO2, CO, N2 etc.

Surface treatment of plastics

A number of applications of plasma involve the surface treatment of plastic materials, prior to a subsequent process.
An example is the treatment of reinforcing fibres that are to be integrated into an epoxy structure. Treatment in an oxygen plasma for say, five minutes at 50-100 watts, increases surface roughness. These pitted fibres enhance adhesion and a good mechanical bond is produced with enhanced rigidity and strength.
Plasma processing of plastics can also convert a hydrophobic surface to a Hydrophilic surface. This type of treatment usually requires short exposure (3-5 minutes) at low power (50 watts). This sort of reaction has been applied to the assembly of ink pens to improve the speed of ink filling and transfer. Other examples include the treatment of electrical wiring so that the insulation can be printed upon with regular inks. Plasma treatment of car bumpers allows simpler and more cost-effective painting schedules and the treatment of textile fibres can improve water retention.
Plasma surface treatment in biomedical applications is expanding rapidly. For example, surface modifications of a polymer to improve blood compatibility. This involves tailoring the polymer surface to minimise blood reaction. Similarly, the internal surfaces of tubing can be modified, allowing pharmaceutical materials to be chemically bonded to the surfaces, thus allowing the drug to be slowly dispensed in a localised area.

Plasma polymerisation

Plasma polymerisation refers to the polymerisation of active species generated in a plasma. For example, the introduction of polysiloxanes on to hard contact lenses improves the hydrophilic nature of the surface. An application in the soft drinks industry using CF₄, to create a fluorinated surface on P.E.T. and polypropylene bottles, making bottles less pervious to carbon dioxide.
A porous surface can be produced on medical equipment, this makes it possible to sterilise the equipment with nitrous oxide whilst remaining impervious to air. Deposition of polymers on to the surfaces of implants is also possible and can help prevent rejection by improved bio-compatibility.

References

An important application of plasma technique are is to improve the wettability and adhesion of polymers for surface coatings, inks and dyes (1). Self-adhesion can also be markedly improved by plasma treatment. (2, 3).
Plasma techniques are widely used in the electronics industry (4) particularly for microelectronics fabrication. Although most of the materials involved in these applications are inorganic, they are of interest to polymer chemists because polymers can be use as resists, insulators or semiconductors. Operations carried out by plasma techniques include photoresist removal (5), etching silicon compounds (6) and deposition of polymer films (7, 8). The chief virtue of plasma techniques in microelectronics fabrication is that it permits automated, multi-step processing of complex devices (9).
Applications for plasma polymerisation have included the production of protective coatings for metals and other reactive surfaces (12), fabrication of reverse-osmosis membranes (13), coatings for optical plastics (14) and the formation of radiation resistant coatings (15).
Last and by no means least, is the application of plasma ashing for the analysis of inorganic materials within an organic matrix. Prime examples include the investigation of asbestos fibres in air and investigation of metal contamination of food.

1. M. Hudis, Techniques and applications of plasma chemistry, Wiley-Interscience, New York, 1974 chap. 3.
2. M. Stradl and D. A. L. Goring, Can, J, Chem. Eng., 53, 427 (1975)
3. D. K. Owens, J. Appl, Polym, Sci., 19, 265, (1975)
4. R. W. Kirk, as (1) chap. 9.
5. S. M. Irving, Kodak Photoresist Seminar, 2, 26, (1968).
6. H. Abe, Y. Sonobe, and T. Enomoto, Jpn. J. Appl. Phys., 12, 154 (1973).
7. P. J. Ozowa, IEEE Trans, Parts Mater., Pachag., PMP-5, 112 (1969)
8. A. Chida, Jpn. Kokai Tokkyo Koho, 78,47, 393, (1978).
9. R. L. Bersin, Kodak Microelectronics seminar proc., San Diego, Calif., October 1-3 (1978).
10. P. Kassenbeck, Bull. Inst. Text, FR. 18, 7 (1963).
11. W. J. Tjhorsen, Text. Res J 38,644, (1968).
12. T. Williams J, Oil Colour Chem Assoc., 48,936 (1965).
13. H. Yasuda, Appl., Polym. Symp., 22,241 (1979).
1. R. M. Kubacki, US Patent 4096315 (1978).
2. Y. Masuda and Y. Nakagawa JPN, Kokai Tokkyo Koho 78, 56, 177, (1978).
15. M. Miyamura, JPN, Kokai Tokkyo Koho, 78, 120, 527 (1878).