In plasma etching, a distinction is made between etching removal due to a chemical reaction ( chemical dry etching process (CDE)) and physical removal of the surface due to ion bombardment.
With chemical plasma etching, the material is removed ("etching") through a chemical reaction. This is why it is generally isotropic and, due to its chemical character, also very material-selective. When physical plasma etching, and plasma enhanced ion etching (Engl. Reactive ion etching , RIE) called, is a physical process. With this method, a certain preferential direction can arise in the etching attack, therefore the methods may show anisotropy in the material removal. With physical plasma etching, non-reactive ions are generated in the plasma (e.g. Ar +). An applied electric field accelerates these ions onto a surface and thus removes parts of the surface. This process is typically used to remove the natural oxide on silicon wafers.
During plasma etching , a high-frequency or electrodeless microwave discharge (27.2 MHz or 2.45 GHz) is ignited in a vacuum reactor ( etch tool ) that is filled with an etching gas up to a pressure of a few millibars, making it a highly reactive one , etch-active plasma generated. In practice, activation takes place either in the etching chamber itself ( direct plasma ) or in an antechamber or in the supply line ( remote plasma ). The latter procedure is particularly chosen when, as with cleaning etching, anisotropy can be dispensed with.
For example, perfluorinated hydrocarbons ( perfluorocarbons , PFCs) such as tetrafluoromethane (CF 4 ), hexafluoroethane (C 2 F 6 ), perfluoropropane (C 3 F 8 ), perfluorobutadiene (C 4 F 6 ) and unsaturated PFCs, perfluorinated aromatics are suitable as etching gas , Heteroaromatics etc.
A few percent oxygen is often added to the etching gases in order to increase the etching rate. Depending on the etching medium, this results in a higher yield of etching-active species or controls the polymer formation that takes place parallel to the etching process in the case of carbon-containing etching gases. Examples are the admixing of oxygen to tetrafluoromethane or nitrogen trifluoride (NF 3 ) - promotion of the etching gas decomposition through CO, CO 2 or NO x formation. Recently, etching gases have also been used which contain oxygen as a part of the molecule. Since energetically excited oxygen is preferably formed in the oxygen-containing plasma, the etching gas decay is also promoted by energy transfer from the excited oxygen to the etching gas. Other efficient energy carriers are, for example, argon, xenon and nitrogen. Such energy transfer reactions have been known in atmospheric chemistry for many decades and have been studied in great detail. In the semiconductor sector, energy transfer reactions are currently used rather unconsciously.
Among the inorganic etch gases are in particular sulfur hexafluoride , nitrogen (III) fluoride , boron trichloride, chlorine , chlorine and hydrogen bromide , and oxygen may be mentioned. Mixtures of different caustic gases are also common. So you can z. B. mix a gas with an etching gas that forms heavy ions (for example BCl 3 , Cl 2 mixtures). In this way, a sometimes very significant improvement in the anisotropy of the etching reaction is achieved.
The most important criterion when selecting the etching gas is its ability to form a highly volatile reaction product with the solid to be etched. Therefore, when etching structures based on silicon - silicon, silicon oxide and silicon nitride are not only the basic materials of every microelectronic component, but also the most widely used materials in microtechnology - etching gases that contain fluorine or chlorine are used. The reaction product of the etching reaction is volatile SiCl 4 or SiF 4 . Because of the high SiF 4 vapor pressure, fluorine-containing gases are predominantly used in practice for "silicon etching".
For etching aluminum, which is used as a conductor track material, u used. a. Hydrogen bromide (HBr) (AlBr 3 formation). Tungsten , a conductor track material commonly found in the microprocessor area, is etched with fluorine-containing gases. Volatile WF 6 is formed during etching .
When etching copper , a modern conductor track material, in the absence of a suitable gaseous dry etching medium, wet chemical processes are currently used. The etching reaction is based on the tendency of copper to form soluble amine complexes.
Should organic materials such as B. photoresists are etched or ashed, one uses oxygen. Etching products here are CO or CO 2 .
The substances used for etching are all products produced on an industrial scale.
Many of the PFCs used for etching come directly from plastics production. They are either used here as monomers or are a by-product of monomer synthesis. For the sake of completeness, it should be mentioned that the PFCs used as etching gas are consistently generated from chlorofluorocarbons (FCC or CFC).
Boron trichloride, chlorine, hydrogen chloride and hydrogen bromide are traditional basic chemicals in the chemical industry and some of them are produced on a very large scale.
Inorganic fluorine compounds that are used as etching gas are often created by direct conversion of the elements in a one-step reaction. Typical examples are sulfur hexafluoride (SF 6 ) and nitrogen trifluoride (NF 3 ). Sulfur hexafluoride is also widely used as an insulating gas, but is problematic because of its high stability and its high potential as a greenhouse gas . Nitrogen trifluoride (NF 3 ), also a large-scale product with a very high global warming potential , is used exclusively as an etching gas.
The fluorine produced in large quantities can also be used as an etching gas. In all plasma etching processes that use fluorine-containing etching media, elemental fluorine inevitably occurs as a product of recombination processes and therefore takes part in practically all etching processes as an active component. The low F – F dissociation energy means that fluorine can be used as a plasma-free thermal etching gas at a moderate temperature.
Fluorine can also be used for plasma etching. If the etching process is also carried out at an elevated temperature, shaded and narrow reactor areas such as openings and channels z. B. effortlessly clean the showerhead . In contrast to pure plasma etching, volume recombination losses and wall loss reactions do not play a role.
In contrast to all other etching gases, fluorine is not a greenhouse gas . Fluorine can also be removed very easily from the exhaust gas flow by dry absorption. The expensive exhaust gas treatment systems required for other corrosive gases, which usually lead to the secondary formation of new environmentally relevant substances, are thus eliminated.
The etching gas used is either preferably undiluted fluorine or a fluorine-noble gas mixture.
All of the inorganic fluorine compounds described above are used in the semiconductor and display industries and in the manufacture of solar cells, in particular for cleaning etching. Sometimes used in increasing amounts.
SF 6 , a very old etching gas, is also used in microtechnology in the RIE process .
Within the 'activation zone' of the etching reactor, a highly reactive plasma is generated from the etching gas, which without activation would usually not develop an etching effect . In addition to neutral gas particles, free electrons , ions of various degrees of ionization , radicals and electronically excited molecules occur in the plasma . Which species occur in which concentration depends on the chemical nature of the etching gas. As a clue here z. B. serve the ionizing energy of the etching gas.
In addition to the reactive species above, the discharge zone also generates short-wave UV radiation . Here, too, the emitted frequency depends on the nature of the etching gas. The UV radiation can contribute significantly to the etching process.
Depending on the type of process, one is interested in plasma etching either in the atoms and radicals generated by the plasma or in atoms and ions.
In plasma etching, ions are used in a rather profane way as the "sand of a sandblasting blower". The permanent “sandblasting” of the grounded substrate with ions causes its surface to be mechanically “torn open” and made accessible to chemical attack by etch-active species. Special structures can be created by covering certain areas of the substrate surface with a photoresist .
Practically all particles within the plasma zone are in constant interaction with one another. Many of the reactions that take place (ion-molecule reactions, recombinations, etc.) are strongly exothermic and cause the high temperature of the plasma zone. The high temperature in turn favors the particle interaction and has the effect that the net yield of reactive particles that are available for the etching process always remains low. In a similar way, too high a reactor pressure counteracts the concentration of etch-active species.
The walls of the plasma reactor and the walls of reactor internals are also another sink for the reactive particles. The material properties of the recombination surfaces and the flow conditions in the reactor play an important role. Using complicated or narrow gas inlets, e.g. B. so-called showerheads should therefore be avoided, as they significantly reduce the amount of plasma-activated etching gas available in the reactor chamber.
Since many etching gases are used in diluted form, the "neutral" etching gas component must also be taken into account. The unselective excitation in the plasma zone inevitably leads to the formation of long-lived electronically excited species which can influence the etching process by activating gas impurities.
However, excited species are also formed during the etching process. So z. B. in NF 3 plasma inevitably large amounts of electronically excited molecular nitrogen. The same applies to oxygen plasmas and many other etching media.
However, another problem in etching processes is process control. In the plasma zone, all of the above processes take place almost simultaneously. However, since in practice gas flow reactors are used throughout, the processes are mapped onto the reactor interior in a time-resolved manner . The very complex dynamics of the etching process, which is already kinetically extremely complex, means that the smallest parameter changes - for example changes in gas flow and reactor pressure, a change in the discharge strength or the field strength in the reactor, a change in the reactor geometry - can fundamentally change the conditions in the reactor. Running a plasma etching process therefore not only requires experience in systems engineering, but also and especially deeper insight into the chemical kinetics of the processes taking place. Once set and optimized, individual process parameters should not be changed if possible. Reactor conversions are also not advisable.
By lowering the partial pressure of the etching-active gas, the partial pressure of reactive particles and their service life can be increased considerably. The partial pressure reduction can take place here either by targeted reduction of the total reactor pressure or by diluting the etching-active gas with an inert gas .
With a systematic approach, which of course also takes into account that the residence time of the gas in the reactor also changes when the reactor pressure changes, etching processes can be optimized quite easily.
Incidentally, simply increasing the discharge strength usually does not lead to the desired increase in the etching rate because the loss reactions also increase drastically when the discharge strength is increased. Additionally generated etching-active species are immediately lost again.
Increasing the flow rate usually does not lead to success either, since this can drastically change the composition of the active species.
Compared to ions, atoms and radicals have a very long life under the high vacuum conditions of the plasma reactor. Ions, on the other hand, only occur in the plasma zone and its immediate edge areas. For this reason, remote plasma systems are only suitable for isotropic etching processes and are used as a second system for cleaning etching.
The above problem also shows that, in principle, good reproducibility and process transferability is only given when using simple etching media with a clear decay pattern.
Problems that arise later
Problems that occur later in routine operation are mostly due to operating errors, unknowing changes to basic process parameters or the wrong choice of etching medium. The surprising improvements or deteriorations in yield that are occasionally observed in practice are mostly due to the unintentional or accidental change in important process parameters. Since the many commercial plasma systems lack facilities that allow the process chemistry to be monitored, excessive process costs and high emissions are often not detected at all or only very late.
|Etching gas||Medium atm. Lifespan in years||Global warming potential compared to CO 2|
|CF 4 ( tetrafluoromethane )||50,000||7,390|
|C 2 F 6 ( hexafluoroethane )||10,000||12,200|
|C 3 F 8 ( perfluoropropane )||2,600||8,830|
|cC 4 F 8 ( octafluorocyclobutane )||3,200||10,300|
|SF 6 ( sulfur hexafluoride )||3,200||22,800|
|NF 3 ( nitrogen trifluoride )||550||17,200|
In the case of large-scale plasma etching, environmental protection is becoming more and more important, since the gases used in the etching process, their precursors and secondary products, in some cases, are of considerable environmental relevance. The precursors of the C / F etching media used for silicon etching are mostly chlorofluorocarbons (CFCs). Also from the etching media itself can, for. T. cause significant environmental damage.
The degree of potential environmental damage depends primarily on the molecular structure of the etching medium. Saturated fluorocarbon compounds are characterized here by a particularly high stability against atmospheric degradation processes, which is one of the reasons for their high global warming potential .
In large-scale plasma etching, exhaust gas treatment systems are therefore connected downstream of the etching reactor, which destroy excess etching gas. Here, too, it is important to carefully control the remaining emissions, since a large number of new substances inevitably form in the plasma process and in the aftertreatment of the exhaust gases, which can be extremely stable and even exceed the actual etching gas in terms of their environmental relevance. A continuous analysis of the exhaust gas flows, which also covers the decomposition by-products, is therefore essential.
Reactive etching media such as fluorine , boron trichloride , chlorine , hydrogen chloride , hydrogen bromide etc. are less problematic here, since these etching media can be removed from the exhaust gas flows very easily and inexpensively by washing or absorption without leaving any residue.
Areas of application
Plasma etching is used extensively for cleaning production systems in all of the aforementioned areas, where it is an indispensable part of the production process. The largest field of application is the manufacture of TFT flat screens. The process is also used in semiconductor technology.
Plasma - assisted in -situ dry cleaning processes were used very early in semiconductor technology and - with a few exceptions - have replaced the often extremely problematic and very costly wet-chemical cleaning processes in production.
The mass production of memory chips, flat screens, sensors etc. would be inconceivable without the use of plasma-assisted cleaning processes.
System cleaning makes a significant contribution to the production costs per electronic component produced ( integrated circuits , screen or sensor). Naturally, the cleaning costs are particularly noticeable in cost-sensitive products with high quality requirements in production and relatively simple process technology (sensors and memory modules). The conditions are particularly unfavorable for large-scale mass-produced products with high quality sensitivity and low target production costs (TFT flat screens). A cleaning cost share of 30% and more is currently still common here.
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