Oficina de Gestió de Monuments
1. Funcions de l’Oficina
The planar-coil inductive plasma source shown on figure 16.17 has been used to an increasing extent for plasma processing of microelectronic wafers and other applications. This trend has arisen partly in response to concerns about using electromagnets, and difficulties with the uniformity of effect possible with ECR plasma sources. The physical processes in this source have been discussed in sections 11.4 and 11.5 of Volume 1, and in the journal literature (see Dai and Wu 1995, Li et al 1995, Ventzek et al 1994, Wainman et al 1995).
16.2.3.1 Source Configuration
The planar-coil inductive plasma source illustrated in figure 16.17 is energized by 13.56 MHz RF power applied to a planar helical coil. The coupling between the RF power supply and the plasma is achieved by currents induced in a disc at the upper surface of the plasma, below the planar helical coil. As illustrated in figure 16.17, the formation and heating of the plasma occurs in an interaction
layer of thickness δ, which is approximately equal to the plasma skin depth,
discussed in section 11.3 of Volume 1. The plasma generated in this interaction layer can reach densities of several times 1018electrons/m3, in the low-pressure
Figure 16.17. A plasma source for etching based on the inductive planar helical coil
configuration.
regime at neutral gas pressures below 1.3 Pa (10 mTorr). This plasma convects below the interaction layer to process wafers or other workpieces on the baseplate. The baseplate may have a provision to bias the workpiece, a feature sometimes desirable for microelectronic etching or other plasma-processing operations.
To avoid losses at the cylindrical sidewall of the inductive plasma source shown in figure 16.17, multipolar permanent magnets, discussed in section 3.4.3 of Volume 1, may be mounted on the boundary. The magnetic field magnetizes the electron population only, but is sufficient to significantly reduce plasma bombardment and recombination on surfaces other than the workpiece. These permanent magnets do not pose the reliability or power consumption problems associated with the electromagnets required for ECR plasma sources.
16.2.3.2 Plasma Parameters
The neutral gas pressure at which the planar-coil inductively coupled plasma source can be operated ranges from less than 0.13 Pa (1 mTorr) for etching of microelectronic wafers, up to values in the intermediate-pressure regime for deposition or tasks other than microelectronic plasma processing. The electron kinetic temperature in planar-coil inductive plasma sources can range as high as 5 to 10 eV, and their electron number density can range from values as low as 1016 electrons/m3, to values above 1018 electrons/m3. The power level of the helical coil inductive plasma source illustrated in figure 16.17 can range from a few hundred watts to several kilowatts in microelectronic plasma-processing applications. The electron number density and power input of the planar-coil inductive plasma source is about the same as that of ECR microwave sources.
Its electron number density is about a factor of 10 higher than that generated by parallel-plate RF glow discharge plasma sources.
16.2.3.3 Independent Operating Variables
The active-species flux, duration of exposure, and other effects on plasma processing are controlled by the same easily adjusted parameters as other plasma sources used for processing. For etching applications, argon gas is used as the carrier gas because it is inexpensive, and its electrical characteristics produce a stable, easily controlled discharge. The gas flow rate and working gas pressure are linked in most vacuum systems, and in microelectronic etching applications these are adjusted to produce a satisfactory compromise between low etching rates at low pressure, and undercutting the mask at higher pressure.
The electron number density and active-species production rate are proportional to the power input to the plasma. To achieve the desired plasma-processing effects in acceptable durations, the power level at which microelectronic wafers are treated in planar-coil inductive plasma sources is on the order of kilowatts. Although the efficiency of inductive coupling is only weakly related to the operating frequency above the critical sheath frequency, nearly all planar-coil inductive plasma sources are operated at 13.56 MHz to avoid regulatory problems with RFI.
16.2.3.4 Performance Issues
The flat-coil inductive plasma source is capable of operating over approximately the same range of neutral gas pressures and electron number densities as ECR plasma sources. However, the flat-coil sources do not require electromagnets, and they possess what appears at the time of writing to be a superior capability to achieve uniform plasma densities and processing effects across workpieces up to 30 cm in diameter. By adjusting the radial spacing between individual turns of the planar helical coil shown at the top of the plasma source in figure 16.17, and by adjusting the frequency, the power input, and the operating pressure of the plasma, it should be possible to achieve a virtually flat radial profile of electron number density across the plasma diameter. Such a flat profile should translate into uniformity of processing effect over large diameter wafers.
The axisymmetric coils with which most inductively coupled plasmas are generated usually produce an axisymmetric plasma, which may not be uniform enough across its diameter to provide acceptably uniform processing of a workpiece. In such a case, motional averaging of the workpiece may be required, using one of the methods discussed in section 18.4 of this volume. As noted in chapter 11 of Volume 1, the cross section of the inductive coil need not be circular, and can be formed into rectangular, triangular, and other geometric cross sections to improve depositional uniformity on workpieces with corresponding shapes.
References on the helicon plasma source include Lieberman et al (1996), Smith (1995), and Stevens et al (1995).
16.2.4.1 Source Characteristics
The helicon plasma source is illustrated in figure 16.18. It is a cylindrical plasma generated in a magnetic field, and is driven by the RF coil arrangement shown. The helicon plasma source is similar in operation to the Rotamak plasma source discussed in section 11.5.2 of Volume 1. In order for the RF power to be resonantly coupled to the plasma, the electrons must be magnetized, and confined throughout the plasma volume by a magnetic induction in the range from 5 to 50 mT. These inductions are generally beyond the reach of permanent magnets and must be generated by external electromagnets if large volumes of plasma are required.
16.2.4.2 Plasma Parameters
The helicon plasma source can operate at pressures below 1.3 Pa (10 mTorr), and number densities as high as 1019electrons/m3have been reported. The RF used to energize the source is typically 13.56 MHz, and the coupling to the plasma is both inductive and resonant. Power levels of the helicon plasma source range from a few watts to more than a kilowatt in research applications.
16.2.4.3 Performance Issues
The plasma in a helicon source can operate in the steady state at high electron number density and at low gas pressures. However, its operational requirement for a large volume of magnetic field requiring electromagnets is a serious operational disadvantage. In addition, it is not clear whether helicon plasma sources can provide the radial uniformity of plasma density and active-species fluxes required for application to production-line microelectronic plasma processing that have been demonstrated by other sources.
Figure 16.18. A plasma source for plasma etching based on helicon RF excitation in an
axisymmetric magnetic field.
16.3
HIGH-VACUUM PLASMA SOURCES
Other than plasma sputtering and a few niche applications requiring long mean free paths, few high-throughput plasma-processing applications are operated under high vacuum, at pressures below 0.013 Pa (∼10−4 Torr). In this regime,
the mean free paths of all species are greater than the dimensions of the plasma and workpiece. This is desirable for microelectronic etching applications because it is consistent with vertical etching and good pattern transfer. It is also desirable for evaporative or sputter deposition of workpieces, since vaporized or sputtered atoms will travel directly from the source or target to the workpiece without scattering collisions.
The long mean free paths associated with the high-vacuum regime also have important disadvantages for some plasma-processing applications. Low gas density in the high-vacuum regime reduces the probability of inelastic collisions of plasma electrons that produce active species, and reduces the flux and concentration of active species that result from such collisions. In unmagnetized plasma sources the long mean free paths associated with low pressures make it difficult to conserve electrons, to prevent wall recombination, and to maintain a high enough electron number density to maintain an adequate flux of active species to complete a plasma processing task within an acceptable duration.
Some of the sources available for plasma-processing operations in the high- vacuum regime are discussed in Volume 1, and include Penning discharges (section 6.6 of Volume 1), hollow cathodes (see section 5.4 of Volume 1, and
Anonymous 1986), vacuum arcs (see section 10.3.4 of Volume 1, and Lafferty 1980), and a variety of other specialty sources originally developed for research purposes. These sources, and the high-vacuum regime itself, have associated problems when used for industrial production. The high-vacuum regime requires expensive vacuum pumping equipment, and as the operating pressure drops below 0.013 Pa (10−4Torr), the pump-out time required to reach these pressures lengthens, adding to processing time.
In order to create plasmas of sufficient density to provide adequate fluxes or concentrations of active species in the high-vacuum regime, it is generally necessary to magnetize at least the electron population as is done in ECR microwave, helicon, and Penning sources. This need for a magnetic field normally requires electromagnets, with power requirements that can greatly exceed that of the plasma source itself. The electromagnets also have reliability problems associated with an additional subsystem not needed for the unmagnetized plasma sources operated at higher pressure.
16.4
SUMMARY OF PLASMA SOURCE PARAMETERS
The DC, RF, and flat-coil inductive plasma sources have electron number densities proportional not only to the RF power, but also to background pressure. Because the ECR plasma is generated by a resonant process, its electron number density, while roughly proportional to the power input below the critical density, is not directly proportional to the power input above it. The electron number density in ECR plasma sources is decoupled from the neutral gas pressure, allowing a constant electron number density to be maintained over a wide range of neutral pressures. Such plasma sources allow an additional degree of control over the flux of active species.
Characteristic values of the neutral gas pressure, electron kinetic temperature, and electron number density of plasma sources used for plasma
processing are listed in table 16.1. These values are approximations only, as the database of well diagnosed industrial plasmas is small, and the wide range of configurations and operating conditions actually used in industrial applications can lead to large variations of these parameters. The four ‘low-pressure’ plasma sources are capable of providing plasma parameters almost identical with each other. If the plasma parameters produced by different plasma sources are identical, then the effects associated with etching, deposition, etc also will be the same (Herschkowitz et al 1996).
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a much larger plasma reactor or production tool, which is the device or apparatus that allows the plasma to process the workpiece in the desired manner. These sources often play a minor role in terms of capital investment and operating costs, but a major role in system reliability, product quality, and engineering manpower requirements.
Once a satisfactory plasma source has been selected for a particular application, it must be incorporated into an appropriate plasma reactor system, or production tool. In this chapter, some of the more widely used plasma reactors are discussed according to their processing application. General references on plasma processing and plasma reactors may be found in Gross et al (1969), Flinn (1971), Hollahan and Bell (1974), Boenig (1988), Lieberman and Lichtenberg (1994), Kohl (1995), Madou (1995), Roth (1995), and Lieberman et al (1996).
Most industrial glow discharges are operated at pressures below 1.3 kPa (10 Torr). It is therefore necessary to treat webs, films, and individual workpieces exposed to such discharges by batch processing, using an apparatus like that shown in figure 17.1. Typical operational steps in plasma processing under vacuum are, with reference to figure 17.1,
(1) Install the feed roll or workpiece.
(2) Close bleed valve, seal off vacuum system. (3) Open gate valve to vacuum pump.
(4) Pump vacuum system below 1.3 kPa (10 Torr). (5) Turn on plasma.
(6) Feed material from supply to take-up roll, or expose individual workpiece for the required duration.
(7) Turn off plasma.
Figure 17.1. Batch processing of fabrics or films in a low-pressure glow discharge plasma.
(8) Close gate valve.
(9) Bleed system up to 1 atm.
(10) Remove take-up roll or workpiece for further processing.
These procedures, and the attendant cost of purchasing and operating vacuum systems, are the principal reasons why low-pressure glow discharges are not more widely used in industry. When such glow discharges are used, it tends to be for the fabrication of high-value items like microelectronic chips or biomedical products, which can be accomplished economically in no other way.
Continuous processing at 1 atm, illustrated in figure 17.2, is much to be
preferred in industrial applications. In this case, either a corona discharge, a dielectric barrier discharge (DBD), or a one atmosphere uniform glow discharge plasma (OAUGDP) reactor can provide active species to treat the surface of a workpiece at the plasma boundary. In such continuous processing, the plasma source is usually within a gas-tight safety/environmental enclosure. Such an enclosure is present to control the working gas if other than air, and/or to prevent the outflow of ozone, UV radiation, or other active species that may cause safety concerns.
It is possible to use continuous processing in vacuum glow discharge plasma reactors by passing a fabric web, fiber or film into and out of the vacuum system through one or more stages of vacuum pumping. An example of such a system
Figure 17.2. Continuous processing of fabric or film through an OAUGDP.
was published by Rakowski (1989), and is illustrated in figure 17.3. In this apparatus, wool toe is fed continuously through a plasma operating at a few hundred Pascals (a few Torr) at speeds, and with results, of commercial interest. This continuous-processing vacuum hybrid has the disadvantages of increased capital and operating costs for the required differential pumping of the vacuum system, as opposed to the smaller capacity vacuum systems that are adequate for batch processing.
17.1
PLASMA REACTORS FOR SURFACE TREATMENT
The industrial plasma treatment of surfaces requires a variety of plasma reactors, the plasma sources for which have been discussed in chapters 15 and 16 of this volume. Additional discussion of plasma reactors for surface treatment may be found in d’Agostino (1990), Coburn (1991), Batenin et al (1992), Roth (1999), Montie et al (2000), and Roth et al (2000). In this section, we survey reactor configurations widely used for plasma surface treatment, and comment on features that qualify them for their applications.
To achieve industrially important effects, plasmas used for surface treatment must have a sufficiently high electron number density to provide useful fluxes of active species, but not so high or energetic as to damage the material being treated. These constraints rule out dark discharges other than coronas for most surface treatment applications, because of their low production rate of active species. These constraints also rule out arc or torch plasmas, which have power densities and active-species flux intensities high enough to damage most exposed materials. Glow discharge plasmas produce the appropriate electron number density and active-species flux for nearly all plasma surface treatment applications.
Figure 17.3. Apparatus for continuous low-pressure glow discharge plasma treatment of
wool developed by Rakowski (1989).
treatment can be classified according to the type of workpiece they process. These workpieces include thin films and webs processed as rolls for batch processing; and individual solid workpieces such as wafers or three-dimensional objects, the