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survol de l'entreprise plasma gravure de plasma nettoyage décapsulation PECVD Si3N4 Si02 Extrémité
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Francais survol de l'entreprise plasma gravure de plasma nettoyage décapsulation PECVD Si3N4 Si02 Extrémité |
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Plasma is often called the fourth state of matter. It is different from the other three, in that it contains free disassociated electrons and ions in a balanced steady-state condition. This, by definition, is a plasma. A plasma contains in a disassociated state: free radicals, ions, electrons and unexcited molecules. The ratio of the ions to the rest of the molecules constitutes the "ion density" of the plasma. The plasma has a net charge of zero, hence equal numbers of positive and negative charges. For practical use, a plasma is simply a method for turning nonreactive molecules into electrically charged reactive molecules. Many safe inert gases when broken up yield extremely reactive by-products. Chlorofluorocarbons CFCs, for instance, are extremely long lived inert compounds but when broken up they yield relatively large concentrations of fluorine and chlorine (some of the most reactive compounds known). Since this process is controlled directly from the application of an electric field, the reactivity and directionality of the process can be controlled by the applied power. This flexibility and control is what makes plasma processing useful.
Plasma is made by introducing energy into
matter. This is accomplished in many ways, such as: through heat,
radiation and (as in our case) an electric field. The ionization rate
determines the plasma's electron energy. Regular plasmas exhibit
ionization rates of 0.001%, but high-density plasmas or HDP have an
ionization rate of approximately 1%. The following is a simple
illustration of ionization where a neutral
molecule or atom collides with an electron to produce an ion and
another electron.
e- + A → A+ +
2e-
When a molecule or atom is exited to a higher energy state by the
collision with an electron it stays in that energy state for a short
period and then returns to its natural relaxed state. When that
happens, energy is emitted in the form of a photon. Since different
atoms or molecules emit light at different wavelength, different gases
display distinctive plasma glow colors. This makes the use of
spectrometers for end point
detection a very useful one as wavelength peaks are used to determine
when a certain layer has been removed.
e- + A → A* + e-
A* → A + hν (photon)
The plasmas used in semiconductor processing are very specific in
nature. Processing semiconductor devices requires a relatively low
temperature plasma. To create this low temperature plasma, the plasma
has to be created through the application of an electric field to
conductive gases. Fortunately, gases are electrically conductive at
easily achieved, moderately low pressures (on the order of 1 Torr).
Plasma processing was introduced to the Semiconductor Industry in the 60s. The first systems were of the "Barrel" type and were typically used for stripping photoresist. Previous to this time, wet chemical solvents were used. These solvents were potentially carcinogenic and expensive to dispose of. Plasma processing, on the other hand, is far more effective in removing positive resist, uses orders of magnitude less chemicals, and is therefore far more kind to the environment.
The first barrels systems were inductively coupled and consist of a quartz bell jar with a coil wrapped around it, turned on its side. Since the quartz chambers etch in fluorinated gas, these systems are usually only used to strip photoresist with oxygen. Later versions of barrel systems were capacitively coupled and consist of a cylindrical aluminum chamber with an inner concentric cathode. Since the anode in capacitively coupled barrels is the barrel wall itself, these barrels can be made of aluminum; and because the aluminum is inert to the fluorinated etch gas, these systems can be used for etching. Despite the fact this is an older technology and they etch isotropically (due to the chamber geometry), barrel systems are very versatile and are still widely used in the industry today.
Parallel Plate reactors are by definition capacitively coupled, they are either bottom or top powered. Etching in a top powered reactor is referred to as "Plasma mode" etching or "PE mode" and etching a bottom powered reactor it is referred to as "Reactive Ion Etching". This is a bit of a misnomer, because both systems are generically "plasma etchers". The first widely used Parallel Plate Reactor was the "Reinburg" reactor, which was developed at Texas Instruments in 1972. There are many variants of this design, but they are all basically a bottom powered electrode system, large enough to accommodate twenty five, 100 mm wafers. The first fully automated, single wafer parallel plate system was introduced to semiconductor production lines in 1979, by Tegal corporation. Because of the superior process results that single wafer systems offer, almost all production etch systems are of this configuration.
The name "Reactive Ion Etching" is misleading. It really should be called "Ion Assisted Etching". The percentage of ions in a plasma is very small, and if they were the only species participating, the overall etch rate would also be very small. The actual mechanism is a three step process:
Reactive Ion Etch processes were, until the late 80's, the only production plasma processes that produced an anisotropic etch. This phenomenon is a result of the chamber geometry, plasma physics and operating pressures of the systems. In general, two things define Reactive Ion Etching, a bottom powered reactor and an operating pressure below 100 mTorr. This is also misleading, because these are only two contributing factors in achieving an anisotropic etch.
Within the last few years, there have been many new types of reactors introduced into the industry. These are Triode, ECR (or downstream microwave) and ICP reactors. We call these "Hybrid" reactors, because they introduce power into the reactor through a secondary source. This is advantageous, because you can add power to the plasma without coupling it through the sample. These types of reactors or secondary power sources excite the plasma to produce high density plasma (HDP). Hybrid reactors, therefore, have much lower charge damage, sputtering and operating temperature; and they have higher etch rate and selectivity.
In conventional RIE the plasma density is limited by the method of coupling RF energy into the plasma. This limits the rates at which certain materials can be etched or deposited. This problem becomes particularly acute at reduced pressures where, with the efficiency of an RIE, the plasma density can become prohibitively low. Conventional RIE plasma sources typically achieve a plasma density in the 0.5-5.0 x 1010 cm-3 range. Inherent in these sources is a decrease of plasma density (and hence etch rate) with pressure. The utilization of an inductively coupled plasma source allows for higher plasma densities as power is transferred into the bulk plasma via the magnetic field resultant from inductively coupling. Processing at lower pressure has a number of significant benefits. It allows for tight control of anisotropy in high aspect ratio structures and reduces microloading effect. Inductively coupled plasma sources have potential advantages over other high density sources, such as ECR, including: simplicity of design, less stringent requirements on operating pressures, no requirement for magnetic field and RF power supplied at lower frequencies than microwave systems. To summarize, the addition of Trion Technology/s ICP source will result in improved etch rates and profile control, improved uniformity, greatly increased selectivity and a dramatic reduction in radiation damage and contamination from RIE sputtering. Plasma etching applications benefiting from this technology are oxide, deep silicon trench, polyimide, polysilicon and active devices that are effected by radiation. ICP is also used frequently in failure analysis.
The external plasma can also be generated by microwave excitation. In this case, a Magnetron source is used for the generation of microwaves at 2,45 GHz and a magnet is used to deflect the electrons generated by the inserted energy in a circular path. If the microwave frequency and the magnetic flow are adjusted correctly, electron cyclotron resonance (ECR) can be reached. This method results in a spirally trajectory of the electrons and continuous increase of kinetic energy. The number of crushes between particles and therefore also plasma density increases. The main advantages of this technology compared to ICP generated plasma is the higher plasma density and the low self bias and charging of the substrate. The result is a very effective and damage free plasma, which can be used very well for plasma ashing of photo resist, where anisotropy does not need to be controlled so exactly and high aspect ratios are not so important. Finally, microwave excitation is also little bit cheaper than ICP technology. All these arguments make the microwave plasma source the tool of choice for photo resist stripping.
By using special gases like silan, diethylsilane, ammonia, TEOS or nitrous oxide and proper adjustment of plasma enegy and gas flow, the parallel plate reactor and ICP technology can be also used for the plasma enhanced chemical vapor deposition (PECVD) of dielctric layers like silicon oxide, nitride, oxynitride or amorphos or poly-silicon.
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