Reactive-ion etching

Plasma etching is a material-, plasma-assisted, gas- chemical dry etching method, which is used on a large scale, particularly in the semiconductor, microsystems and display technology.

The term plasma is a more colloquial term, specifically Chemical -dry - etching process ( CDE) is therefore usually meant.

Operation

Plasma etching of the material removal ( " etching " ) by a chemical reaction. Therefore, it is very material selectively in general isotropic and due to the chemical character. The process of plasma etching can not be the plasma-assisted reactive ion etching ( engl. reactive ion etching, RIE), a chemical- physical process, or other dry etching to be confused. The etching process in these processes is primarily physically, this leads to a certain preferred direction in the etching attack, therefore, the method anisotropy in material removal on.

Process Description

In plasma, in a vacuum reactor (English etch tool), which is filled to a pressure of a few millibar with an etching gas, a high frequency or a microwave electrodeless discharge ( 27.2 MHz and 2.45 GHz) to be ignited and a highly reactive generated ätzaktives plasma. In practice, the activation is carried out either in the etching chamber itself (English direct plasma ) or in a pre-chamber or in the supply line (English remote plasma ). The latter procedure is particularly chosen when, can be dispensed with an anisotropy as for cleaning records.

As the etching gas are, for example perfluorocarbons ( perfluorocarbons PFCs ), such as carbon tetrafluoride (CF4 ), hexafluorethane ( C2F6 ), Perfluoropropane ( C3F8 ), perfluorobutadiene ( C4F6 ) and unsaturated PFCs, perfluorinated aromatics, heteroaromatics, etc.

Etching gases are often added a few percent of the oxygen to increase the etch rate. Depending on the etching medium is achieved thereby a higher yield of ätzaktiven species or controls in parallel with the carbonaceous etching gases etch polymer formation takes place. Examples are the admixture of oxygen to carbon tetrafluoride or nitrogen trifluoride ( NF3 ) - Promoting Ätzgaszerfalls by CO, CO2 and NOx formation. More recently, etching gases are used, oxygen-containing molecule as the component. As preferred forms energetically excited oxygen in the oxygen -containing plasma, the Ätzgaszerfall but is also promoted by energy transfer from the excited oxygen to the etching gas. Other efficient energy transfer agents are, for example, argon, xenon, and nitrogen. Such energy transfer reactions are known in atmospheric chemistry for many decades and studied in some detail. In the semiconductor energy transfer reactions are currently used more unconscious.

Among the inorganic etch gases are, in particular sulfur hexafluoride, nitrogen, (III ) fluoride, boron trichloride, chlorine, hydrogen bromide and oxygen as well as to call. Mixtures of different ätzaktiven gases are common. So you can, for example, mix in an etching gas, a gas, the heavy ion forms (for example BCl3, Cl2 mixtures). It thereby achieves some very significant improvement in the anisotropy of the etching reaction.

The most important criterion in the choice of etching gas is its ability to form the solid caustic to a volatile reaction product. The fluorine or chlorine, the etching gases used, contain - generally Therefore, in etching of structures based on silicon - silicon, silicon oxide and silicon nitride are not only the base materials of each micro- electronic component, but also the most commonly used material in the micro-engineering. The reaction product of the etching reaction arise volatile SiF4 or SiCl4. Because of the high vapor pressure of SiF4 predominantly fluorine-containing gases are used for " silicon etching " in practice.

For the etching of aluminum, which is used as a wiring material, is used, inter alia, hydrogen bromide ( HBr) ( AlBr3 formation). Tungsten, a microprocessor in the area consistently encountered conductor material is etched with fluorine-containing gases. When etching is volatile WF6 forms.

During the etching of copper, a modern conductor material, then either at the moment, in the absence of a suitable gaseous Trockenätzmediums, on wet-chemical method returns. The etching reaction here is based on the tendency of copper to form soluble amine complexes.

If organic materials such as etched and ashed eg photoresists, using oxygen. Etching are here CO or CO2.

The substances used for etching is consistently generated by industrial-scale products.

So many of the PFCs used for etching are taken directly to the production of plastics. They are used here either as monomers or as a by- product in monomer synthesis. The sake of completeness it should be mentioned that the PFCs used as an etching gas are produced entirely of chlorofluorocarbon compounds ( FCC or CFCs).

Boron trichloride, chlorine, hydrogen chloride and hydrogen bromide are traditional basic chemicals in the chemical industry and are manufactured partly in very large scale.

Inorganic fluorine compounds, which are used as an etching gas, often caused by direct reaction of the elements in a single-stage reaction. Typical examples are sulfur hexafluoride ( SF6) and nitrogen trifluoride ( NF3 ). Sulfur hexafluoride is also used as insulating widespread application, but is problematic because of its high stability and its high potential as a greenhouse gas. Nitrogen trifluoride ( NF3 ), also an industrially produced product with very high global warming potential, is used exclusively as an etching gas.

Also the fluorine- technically produced in a large amount can be used as an etching gas. In all plasma etching processes that make use of fluorine-containing etching media, elemental fluorine occurs inevitably as a product of recombination and therefore assumes virtually all etching processes, in part as an active component. The low F-F - dissociation means that fluorine can be used at a moderate temperature as a plasma -free thermal etching.

Fluorine can, however, inserting also for plasma etching. If the etching process also still carried out at elevated temperature, also shaded and narrow reactor areas such as openings and channels can be easily cleaned, for example, the showerhead. Volumenrekombinationsverluste and wall loss reactions do not play a role in contrast to the pure plasma.

In contrast to all other etching gases fluorine is not a greenhouse gas. Also from the exhaust stream of fluorine can be removed by dry absorption very simple. The required with other etching gases expensive exhaust treatment systems that usually lead to the secondary formation of new environmentally relevant substances, thus account.

As an etching gas is used preferably either undiluted fluorine or fluorine - inert gas mixtures.

All inorganic fluorine compounds described above are especially used in the semiconductor, display industry and in the manufacture of solar cells for cleaning records and therefore used in some cases greatly increasing amounts.

SF6, a very old etching gas is also used in microtechnology in RIE - process application.

Within the ' activation zone ' of the etching reactor is a highly reactive plasma from the etchant gas, which would usually cease to have corrosive effects without activation generated. In addition to neutral gas occur in the plasma of free electrons, ions, ionization of diverse, radicals and electronically excited molecules. Which species in which concentration occur depends on the chemical nature of the etching gas. A clue can be used in the example, Ioniersierungsenergie of the etching gas.

In addition to the above reactive species, the discharge zone also generates short-wave UV radiation. Again, the frequency emitted is dependent on the nature of the etching gas. UV radiation can contribute to the etching process significantly.

Depending on the procedure you are interested in plasma etching to either the plasma generated by the atoms and radicals or atoms and ions.

In plasma using ion in quite profane manner as the " sand of a sand blasting machine ". Through the permanent " sandblasting " of the grounded substrate with ions whose surface is mechanically " torn " and made ​​available to the chemical attack ätzaktiver species. Specific structures can be generated by covering certain areas of the substrate surface with a photoresist.

Problems of the method

Virtually all particles within the plasma zone are in constant interaction with each other. Many of the reactions taking place (ion -molecule reactions, recombination, etc.) are strongly exothermic, causing the high temperature of the plasma zone. The high temperature in turn favors the particle interaction and causes the net yield, always remains low in reactive particles that are available for the etching process. Similarly, also counteracts a ätzaktiver to high reactor pressure of the species concentration.

A further depression of the reactive particles are also the walls of the plasma reactor and the walls of the reactor internals. The material properties of the Rekombinationsflächen and the flow conditions play an important role in the reactor. The use of complicated, or narrow inlets, such as so-called showerheads, should therefore be avoided because they reduce the amount of available plasma-activated etching gas into the reactor chamber significantly.

Since many etching gases are used diluted, and the "neutral" is to give Ätzgaskomponente attention. The non-selective excitation in the plasma zone inevitably leads to the formation of long-lived electronically excited species that may affect the etching process through activation of gas impurities.

Excited species are also formed during the etching process. This results eg in NF3 plasma necessarily large amounts electronically excited molecular nitrogen. The same is true for oxygen plasma etching media and many others.

However, a further problem in etching processes is the process control. In the plasma zone all the above processes take place almost simultaneously. Since, however, carried out in practice, continuous gas -flow reactors, the processes are time-resolved imaged on the reactor interior. The complex dynamics of the already kinetically extremely complex etching process requires that the smallest parameter changes - for example, changes in gas flow and reactor pressure, a change in the discharge intensity or field strength in the reactor, a change in the reactor geometry - can change the conditions in the reactor fundamentally. The retraction of a plasma therefore requires not only plant-specific experience, but also and especially a deeper insight into the chemical kinetics of the processes involved. Once adjusted and optimized, individual process parameters as possible should not be changed. Also of reactor conversions is not recommended.

Solutions

By lowering the partial pressure of the gas ätzaktiven can significantly increase the partial pressure of reactive particles and their life. The partial pressure reduction can take place here either by selective reduction of the total reactor pressure or by diluting the ätzaktiven gas with an inert gas.

In a systematic approach, which of course also taken into account that at reactor pressure changes the residence time of the gas in the reactor changed to etching processes can be quite easy to optimize.

By the way, simply increasing the discharge intensity does not usually result therefore to the desired increase of the etch rate, because in increasing the discharge intensity drastically increase the loss reactions. So additionally generated ätzaktive species go again lost.

Also increasing the flow rate usually does not result in success, since this can change the composition of the active species dramatically.

Compared to ions atoms and radicals have a very long life under high-vacuum conditions of the plasma reactor. The other hand, ions occur only in the plasma zone and its immediate surroundings areas. Remote plasma systems are suitable for this reason only for isotropic etching processes or as a second system when cleaning sets for use.

The above problem also shows that in principle apply only when using simple etching media with clear decay pattern good reproducibility and process portability.

Later problems

Problems that occur in the future workloads are usually due to operator error, on unwitting alteration of basic process parameters or wrong choice of the etching medium. The occasionally observed in practice surprising yield improvements or deteriorations are mostly due to the inadvertent or accidental modification of important process parameters. Since in many commercial plasma systems facilities missing, allow for monitoring of the process chemistry, excessive litigation costs and high emissions are often not or only very late detected.

Conservation

The environment also in the industrial scale plasma used to an ever- increasing importance, since the gases used in the etching process, their precursors and derivatives in part to have significant environmental relevance. So are the precursors of the C used for silicon etching / F etching media mostly chlorofluorocarbons (CFCs ). Also from the etching media itself can in some cases pose significant environmental damage.

The measure of the potential environmental impact depends primarily on the molecular structure of the etching medium. Saturated Fluorocarbons are characterized here by a particularly high stability against atmospheric degradation processes of which is responsible for you with high global warming potential.

When large-scale plasma etch reactor, therefore, be the downstream gas treatment units, destroy the excess etching gas. Also important here is the careful control of the remaining emissions, since it comes in the plasma process and in the treatment of exhaust gases inevitably to the formation of a variety of new substances which can be extremely stable and the actual etching gas can still excel in their Umweltrelevenz. A continuous analysis of gas streams, which also covers the collapse of secondary products, is therefore essential.

Reactive etching media such as fluorine, boron trichloride, chlorine, hydrogen chloride, hydrogen bromide, etc. are problematic here, since these can be very simple and inexpensive etching media by washing or absorption residue when removed from waste gas streams.

Applications

Plasma etching is widely used for cleaning of production equipment in all areas mentioned above, and there is an indispensable part of the production process. The main field of application is the production of TFT flat screens. The method is used in the semiconductor technology.

Plasma-assisted in-situ dry cleaning process were used in the semiconductor technology very early on and have in production - with some exceptions - replaced the often extremely problematic and very costly wet-chemical cleaning methods.

The mass production of memory chips, flat panel displays, sensors, etc. would not be possible without the use of plasma -based cleaning methods.

The production cost per produced electronic component (integrated circuits, display or sensor) cleaning the system contributes significantly. Naturally, do the cleaning costs particularly in cost-sensitive products with high quality standards in production and relatively simple process noticeable (sensors and memory devices ). Particularly unfavorable, the ratios for large-scale mass products with high quality sensitivity and low production cost target ( TFT flat screens). A cleaning cost share of 30 % and more is here still common.