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2000 Plasma Sources Sci. Technol. 9 441

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Plasma Sources Sci.Technol.9(2000)441–454.Printed in the UK PII:S0963-0252(00)17858-2 Plasma generation and plasma

sources

H Conrads and M Schmidt

INP Greifswald,Institut f¨u r Niedertemperatur-Plasmaphysik e.V.,

Friedrich Ludwig-Jahn-Str.19,D-17489Greifswald,Germany

Received1October1999,in?nal form27September2000

Abstract.This paper reviews the most commonly used methods for the generation of

plasmas with special emphasis on non-thermal,low-temperature plasmas for technological

applications.We also discuss various technical realizations of plasma sources for selected

applications.This paper is further limited to the discussion of plasma generation methods

that employ electric?elds.The various plasmas described include dc glow discharges,either

operated continuously(CW)or pulsed,capacitively and inductively coupled rf discharges,

helicon discharges,and microwave discharges.Various examples of technical realizations of

plasmas in closed structures(cavities),in open structures(surfatron,planar plasma source),

and in magnetic?elds(electron cyclotron resonance sources)are discussed in detail.Finally,

we mention dielectric barrier discharges as convenient sources of non-thermal plasmas at

high pressures(up to atmospheric pressure)and beam-produced plasmas.It is the main

objective of this paper to give an overview of the wide range of diverse plasma generation

methods and plasma sources and highlight the broad spectrum of plasma properties which,in

turn,lead to a wide range of diverse technological and technical applications.

Introduction

Plasmas are generated by supplying energy to a neutral gas causing the formation of charge carriers(?gure1)[1–3]. Electrons and ions are produced in the gas phase when electrons or photons with suf?cient energy collide with the neutral atoms and molecules in the feed gas(electron-impact ionization or photoionization).There are various ways to supply the necessary energy for plasma generation to a neutral gas.One possibility is to supply thermal energy,for example in?ames,where exothermic chemical reactions of the molecules are used as the prime energy source.Adiabatic compression of the gas is also capable of gas heating up to the point of plasma generation.Yet another way to supply energy to a gas reservoir is via energetic beams that moderate in a gas volume.Beams of neutral particles have the added advantage of being unperturbed by electric and magnetic?elds.Neutral beams are primarily used for sustaining plasmas or for plasma heating in fusion devices.The generation of a plasma by using beams of charged particles,especially electrons,and by using beams of photons will be discussed towards the end of this paper.

The most commonly used method of generating and sustaining a low-temperature plasma for technological and technical application is by applying an electric?eld to a neutral gas.Any volume of a neutral gas always contains a few electrons and ions that are formed,for example,as the result of the interaction of cosmic rays or radioactive radiation with the gas.These free charge carriers are accelerated by the electric?eld and new charged particles may be created when these charge carriers collide with atoms and molecules in the gas or with the surfaces of

the electrodes.This leads to an

Figure1.Principles of plasma generation. avalanche of charged particles that is eventually balanced by charge carrier losses,so that a steady-state plasma develops.

The dimensions of a plasma source are determined largely by the particular applications for which the plasma is intended.Hence plasma sources for the production of inte-grated circuits or for coatings in the glass industry,on the one hand,and plasma sources for the remediation of exhaust gases

0963-0252/00/040441+14$30.00?2000IOP Publishing Ltd441

H Conrads and M Schmidt

and?ue gases from power stations,on the other hand,require very different designs.There are distinct differences not only in the physical shape of various plasma sources,but also in the temporal behaviour of the plasmas that are generated in different sources.First,the time constants of the particular process determine the duration of the energy coupling to the plasma.These time constants cover a range of several or-ders of magnitude depending on the nature of the atoms and molecules used in the feed gas.Second,the characteristic features of the electric coupling,especially in electrode-less sources,determine the suitable frequency range of the excit-ing electric?eld.The technically suitable frequency ranges are further restricted by the need to comply with regulatory guidelines(which may vary from country to country)and in some cases also by design limitations for the required power sources.We mention that power ampli?cation by squeezing more and more energy into shorter and shorter pulses has proved to be an effective way to increase the instantaneous power applied to a plasma without raising the operating costs signi?cantly.In all technical applications,almost all of the above considerations are interrelated and not independent of one another.Another important aspect is the different state of technical maturity of the various plasma generation meth-ods and plasma sources.Tables1and2summarize some selected plasma applications together with the required tech-nical features,which are crucial for the appropriate choice of a plasma source for a given application.

In the following sections we review the underlying principles that govern the generation of a plasma using electric?elds as well as beams of electrons and photons. Several examples of commercial plasma sources and reactors will be discussed in some detail with special emphasis on non-thermal,low-pressure plasmas and plasma sources. 1.Plasma production using electric?elds

The most widely used method for plasma generation utilizes the electrical breakdown of a neutral gas in the presence of an external electric?eld.Charge carriers accelerated in the electric?eld couple their energy into the plasma via collisions with other particles.Electrons retain most of their energy in elastic collisions with atoms and molecules because of their small mass and transfer their energy primarily in inelastic collisions.Discharges are classi?ed as dc discharges,ac dis-charges,or pulsed discharges on the basis of the temporal behaviour of the sustaining electric?eld.The spatial and tem-poral characteristics of a plasma depend to a large degree on the particular application for which the plasma will be used.

1.1.Dc discharges

Non-thermal plasmas in dc discharges are generally created in closed discharge vessels using interior electrodes.Different types of discharges and plasmas can be obtained depending on the applied voltage and the discharge current(?gure2) [1,4].The Townsend discharge is a self-sustained discharge characterized by a low discharge current.The transition to a sub-normal glow discharge and to a normal glow discharge is marked by a decrease in the voltage and an increase in the current.An abnormal glow discharge develops as the current

Table1.Applications of plasma sources. Surface modi?cation

Etching

structuring(microelectronics,micromechanics)

cleaning(assembly lines)

Functionalization

hydrophilization

hydrophobization

graftability

adhesability

printability

Interstitial modi?cation

diffusion(bonding)

implantation(hardening)

Deposition

change of properties

mechanical(tribology)

chemical(corrosion protection)

electrical(integrated circuits)

optical(antire?ecting coating)

architecturing

crystallographics(lateral diamonds)

morphologic(scaffolds for cells)

Volume-related transformation

Energy conversion

electrical energy→electromagnetic radiation

particular populations of bounded electronic states

tailoring of the population of free electrons in phase space

luminiscent lamps

high-pressure metal vapour lamps

gas lasers

excimer radiation sources

electrical energy→nuclear energy

fusion of DT

Plasma chemistry

transforming into speci?c compounds

production of precursors

production of excimers

clean-up of gases

odours

?ue gases,diesel exhaust

Carrier functions

Electrical current

circuit breakers

spark gap switches

Heat

welding/cutting arcs

plasma spray

thermoelectric drivers

Particle sources

Electrons

Ions

Neutrals

Table2.Technical features of plasma sources.

Costs

Reliability

Size

Coupling of energy

Conditioning of energy

Ef?ciency

Spatial distribution of plasma parameters(homogeneity)

Positioning of plasma

Controlling boundary conditions

Through-put

Safety

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Figure2.The dependence of voltage upon current for various kinds of dc discharges.(Ne,1.3mbar,?at copper electrodes10cm2, electrode distance50cm),after[4].

is increased even further.Finally,at very high currents, the discharge undergoes an irreversible transition down into an arc(glow-to-arc transition).The operation of a glow discharge requires a resistor in series with the discharge to prevent the transition into an arc.Alternatively,the discharge can be interrupted for a short period of time before the glow-to-arc transition occurs and before it is re-ignited again.

Low-pressure,normal glow discharges between planar electrodes in a cylindrical glass tube exhibit characteristic luminous structures(?gure3)[4].The brightest part of the discharge is the negative glow,which is separated from the cathode by the cathode dark space(‘Crookes’or‘Hittorf dark space’).The cathode dark space is a region of the discharge where the electrical potential drops drastically(cathode fall). The negative glow is separated from the cathode dark space by a well de?ned boundary and it is followed by a diffuse region in the direction towards the anode.The negative glow,where the electric?eld is close to zero,and the positive column are separated by the‘Faraday dark space’.The homogenous or striated(standing or moving striations)positive column stretches all the way to the anode,which may be covered by a characteristic anode glow.

The variations of plasma parameters along the length of the discharge tube are shown schematically in?gure3.The microscopic processes in such a discharge can be described as follows.A positive ion from the negative glow is accel-erated by the electric?eld in the cathode fall and directed towards the cathode surface.The collision of the energetic ion with the surface produces secondary electrons,which are subsequently accelerated in the cathode fall to comparatively high energies.These energetic electrons transfer most of their energy to heavy particles(atoms,molecules)in inelastic col-lisions(excitation,dissociation,and ionization,which also creates additional charge carriers),which occur primarily in the cathode fall and in the negative glow region.The cathode regions of the discharge play a crucial role in sustaining the glow discharge.The positive column is formed only in the presence of a long,narrow discharge gap with charge carrier losses to the wall.In the homogenous positive column,a constant longitudinal electrical?eld is maintained.The elec-trons gain energy in this?eld and form an electron energy distribution with an appreciable number of energetic elec-trons for the formation of a suf?ciently large number of ions and electrons to balance the charge carrier losses to the

wall.Figure3.Variation of light intensity,electric?eld and electric potential along the length of a dc low-pressure glow discharge with plane electrodes,after[4].

An important process with technological applications, that occurs in the cathode region of the dc discharge,is the etching of the cathode material and the deposition of a thin ?lm on a separate substrate by cathode sputtering[1].This process is usually carried out at low pressure(10?1–10Pa) in order to avoid the re-deposition of the sputtered material on the cathode.If additional magnetic?elds are present (magnetron),the ef?ciency of this process can be increased signi?cantly.The Lorentz force causes a circular motion of the electrons e and ions i with the cyclotron frequency[1,2]

ωce/i=

eB

m e/i(1) where e,m e,and m i denote the charge and masses of the electrons and ions,and B refers to the magnetic?eld B.The radius of this motion is given by

r c=

m e v

eB(2) where v is the velocity component perpendicular to B.

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Figure 4.The bucket plasma source:K n cathodes,I B arc current,U B arc voltage and I F heating current of the cathode [5].The magnetic ?eld of the permanent magnets increases the pathway and hence the ionization ef?ciency of the electrons.A sketch of the plasma box and principal electric scheme.

A magnetic ?eld perpendicular to the electric ?eld increases the pathlength of the electrons and ensures a suf?ciently high ionization rate [2].The con?nement of the secondary electrons by the magnetic ?eld to a region near the cathode results in a high plasma density and an increased discharge current at relatively low discharge voltages.

If the planar cathode is replaced by a hollow cylinder with a diameter of nearly the length of the negative glow,the negative glow is observed inside the cathode cylinder.This results in a signi?cant increase in the discharge current.The high ef?ciency of the so-called hollow cathode is due (i)to the ‘pendulum electrons’,which are trapped in the negative glow between the retarding cathode sheaths,and (ii)to additional electron emission caused by the impact of photons and metastables on the cathode surface.The cathode temperature increases with increasing discharge current.If this temperature is suf?ciently high for thermionic electron emission,the glow discharge changes to an arc discharge characterized by a high current and a comparatively low voltage.Similar conditions are observed in discharges that use cathode materials with low work functions,such as oxide cathodes,or discharges with externally heated cathodes.Oxide cathodes are used,for example,in luminescent lamps.

The ‘bucket source’[5](?gure 4)is an example of a plasma source with heated cathodes.A dense plasma is generated by using an array of low-voltage arcs with heated cathodes in conjunction with additional external magnetic ?elds.Such a source has been jointly developed by JET,ASDEX,and TEXTOR as an ion donor for an ion accelerator (100A,60kV)in fusion research.The ‘bucket source’is a large plasma source with cross-sectional area of 0.2×0.4m 2and consists of a ‘bucket’made of copper,24insulated heated tungsten ?laments,and a ‘checker-board’of permanent magnets on the outside.The gas pressure is about 10?2mbar.A voltage U B is applied between the ?laments and the bucket forcing a high dc arc current I B to ?ow between the cathode ?lament and the bucket which acts as the anode.The ionization is sustained by the high current in conjunction

with magnetic cusp-?elds in the vicinity of the bucket walls produced by the checker-board magnets,which increases the pathlength of the electrons before they impinge on the wall.The result is a plasma that is characterized by a spatially very ?at ion distribution up to the walls of the bucket.Electron temperatures and densities reach values that are typical for arcs:T ～5eV and N ～1012particles per cm 3.In the most recent variant of this source,the arc ?laments are replaced by rf antennae feeding 13MHz power to the bucket in order to sustain the arc current.The plasma parameters are close to those of ‘?lament sources’.Although this kind of source has only been used in nuclear fusion devices up to now,such sources have the potential to be used in the etching of large lateral structures because of the abundance of low-energy ions in such a source.1.2.Pulsed dc discharges

In addition to the continuous dc discharge,pulsed dc discharges are also used in plasma-technological applications.Pulsed sources have the following advantages:?operation at higher power;

?additional performance control by a variable duty cycle of active plasma regime and plasma afterglow;

?while variations in the neutral gas composition between the plasma boundary and the plasma centre (due to plasma chemical reactions)may cause,for example,inhomogeneous thin ?lm deposition in a continuous dc plasma,pulsed operation in conjunction with rapid gas exchange between pulses can prevent or minimize such effects.

The plasma focus [6]is a special type of a high-power pulsed plasma.Figure 5shows a schematic diagram of such a device.The device consists of two coaxial electrodes separated by a hat-shaped insulator at one end,while the other end is open.The space between the electrodes is ?lled with the feed gas.A capacitor bank C is charged via the resistor R 1when the switch S is open.The resistor R 2

444

Plasma generation and plasma

sources Figure5.Schematic drawing of the plasma focus formation during run down(a)and the pinching period(b).Shown are a cross section of the cylindrical inner and outer electrodes,the hat shaped insulator,the plasma sheath j with the magnetic?eld lines B e and the principle electric scheme[6].

facilitates easy ignition of the plasma when the switch S is closed.An umbrella-like plasma is formed as shown in ?gure5(a).In the early stage of the formation of the plasma focus,a plasma sheath is formed as shown in?gure5(b). The dense plasma focus is a result of an m=0plasma instability,which is driven by an azimuthal magnetic?eld. This?eld increases as the current is forced to?ow through an increasingly smaller annular plasma structure.There is a one-to-one correspondence between the maximum current that can be achieved and the power density in the?nal plasma state.This?nal state is called the plasma focus.A coaxial line is used as an inductive storage device for the capacitor bank which is discharged in order to deliver the required high current.During the early stage,the plasma acts as a short circuit between the conductors.Subsequently,it is pushed to the open end of the line by the magnetic?eld of the discharge current.The travel time of the plasma has to equal one quarter of the time it takes to discharge the capacitor. Thus,the m=0plasma instability can start at the maximum value of the current for a given circuit con?guration.The high power density in the plasma focus causes the resistivity to increase by more than a factor of100due to electrostatic ?uctuations.The inductively stored energy is coupled to the plasma very effectively.Ion and electron beams are generated by the plasma focus which exceed power densities of TW cm?2.The nuclei of light ions fuse and a rich spectrum of electromagnetic radiation is emitted.Applications of the plasma focus range from pulsed neutron sources for on-line analysis of volatile components in coal to radiation sources for lithography and microscopy in the range of soft x-rays.

1.3.Rf and microwave discharges

Discharges excited and sustained by high-frequency electromagnetic?elds are of increasing interest for technical and industrial applications.The power absorption[1,2,7] P abs per volume V by a plasma in a high-frequency?eld

is Figure6.Schematic drawing of a deposition apparatus with a capacitively coupled rf discharge.The rf power is transmitted to the electrodes by a matching network.The substrat can be on the powered or grounded electrode,after[11].

given by

P abs

V

=1

2

n e

e2

m eν

ν2

ν2+ω2

E20(3)

where n e is the electron density,e and m e are the electron charge and mass,νis the electron–neutral collision frequency,andωrefers to the angular frequency of the electromagnetic?eld whose amplitude is E0.In the presence of a magnetic?eld B perpendicular to the electric?eld,the power absorption changes to

P abs

V

=1

4

e2

mν

ν2

ν2+(ω?ωc)2+

ν2

ν2+(ω+ωc)2

E20(4)

whereωc denotes the electron cyclotron frequency. Electromagnetic waves with frequencies below the electron

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H Conrads and M

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Figure7.Schematic representation of the rf plasma jet system for plasma processing.The actual process depends on the working gas mixture and system parameters.The powered rf electrode with the nozzle is negatively charged because of the self-bias.In the lower part of the?gure are presented examples of deposition and etch pro?les.Plasma chemistry dominated deposition and etching is maximum in the centre because of the highest concentration of radicals in this part of the plasma.The source of the

sputtered/evaporated material is the nozzle[15].

plasma frequency

ωe=

e20n e

ε0m e

1/2

(5)

(hereε0denotes the permittivity of vacuum)will be re?ected. Therefore,the electron density corresponding to the electron plasma frequency is called the cut-off density.However, the skin effect enables the penetration of the wave into the plasma to some extent.The power absorption is limited to the dimension of the skin sheath of thicknessδs.Forν ωthe skin depth is given by[8]

δs=√

2c

ε0m eν

e2n eω

1/2

.(6)

In a non-thermal plasma with n e=1010cm?3andν= 109s?1the above relation yields a skin depth of0.25m and0.02m,respectively,for frequencies of13.56MHz and 2.45GHz.

Rf discharges usually operate in the frequency range f=ω/2π 1–100MHz.The corresponding wavelengths (λ=300–3m)are large compared to the dimensions of the plasma reactor.For microwaves the most commonly used wavelength is12.24cm,corresponding to a frequency of2.45GHz.This wavelength is roughly comparable to the dimensions of a typical microwave reactor.For lower frequencies,the ions accelerated in the?eld move towards the electrodes and produce secondary electrons,similar to what happens in a dc discharge.As the frequency increases, the ions and subsequently also the electrons can no longer reach the electrode surface during the acceleration phase of the exciting external?eld.

The power coupling in rf discharges can be accomplished in different ways,as:

?capacitively coupled discharges,‘E’discharges;?inductively coupled discharges,‘H’discharges[9].

1.3.1.Capacitively coupled discharge.The vessel of

a capacitively coupled discharge[8–13]may have interior circular disc-shaped parallel electrodes which are separated by a distance of a few centimetres.They may be in contact with the discharge or they be insulated from it by a dielectric. In the case of insulating chamber walls,outer electrodes, i.e.electrodes on the outside of the vessel,are sometimes used.Gas pressures are typically in the range1–103Pa.A conventional rf system for sustaining a discharge consists of a generator,usually combined with an impedance matching network,and the reactor with the electrodes.The generator type has to be licensed in terms of the frequency band for commercial use.A matching network is necessary to match the impedance of the generator to that of the discharge.In this case,the power transfer from the generator to the discharge is at peak ef?ciency and the re?ected rf power is minimized (?gure6)[11].The electrodes in the rf discharge are covered by sheath regions,which are similar to the cathode dark space in a dc glow discharge.The space between the electrodes is?lled with the bulk plasma.For moderate pressures, capacitively coupled rf discharges exist in two forms,the αand theγmode[12,13].Theαmode is characterized by lower currents and a positive voltage–current characteristic, whereas theγmode corresponds to higher currents and a partially negative V–I characteristic.The sheath regions in front of the electrodes are quite different in the two modes. Electrical conductivity and charge carrier concentration in theαsheath are very small in contrast to theγsheath.The αdischarge shows a weak luminous region in the centre of the gap between the electrodes with maximum glow intensity near the electrodes.In theγmode,the emission is generally much more intense.The‘positive column’at the gap centre is separated from the bright‘negative glows’by‘Faraday dark spaces’.Rf discharges at intermediate pressures are used, for example,in CO2lasers.The denotationsαandγmodes are caused by the Townsend’s?rst ionization coef?cientαfor the avalanching of charge carriers in the volume and the γcoef?cient for the releasing of secondary electrons from a target surface by incident positive ions,respectively.

The so-called‘self-bias’is a characteristic feature of this type of plasma.The‘self-bias’is a negative dc potential that develops between the plasma and the powered electrode as a consequence of(i)the use of a coupling capacitor between the rf generator and the powered electrode and(ii)the use of appropriately shaped areas of the(smaller)powered electrode and the(larger)grounded electrode.This feature can assume that the currents from the plasma to both electrodes must be equal.The higher current density at the small electrode demands a higher voltage between the plasma and electrode. In other plasma devices,the application of an additional rf bias to the sample holder produces a self-bias with higher ion energies.

In a capacitively coupled rf discharge,the electron density is in the range n e=109–1010cm?3and densities of up to1011cm?3are possible at higher frequencies[14]. The ion energy near the powered electrode can reach energies of a few hundred electron-volts due to the self-bias.Such

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Plasma generation and plasma

sources

Figure8.Diagrams of various ICP reactors:helical coupler(a),helical resonator(b),spiral coupler(c),immersed coupler(d),and transformer-coupled plasma(e).The Faraday shield in devices(a)and(b)avoids capacitively coupling from the coil to the plasma.The permanent magnets(c)and(d)con?ne the plasma,enhance the uniformity,and increase the plasma density[16].

discharges are successfully applied to thin-?lm deposition and plasma etching as well as to the sputtering of insulating materials.

A hollow cathode effect can also be observed for rf discharges and can be used for the design of plasma reactors. Such a scheme has been used in a supersonic rf plasma jet, which was successfully tested in thin-?lm deposition and etching experiments(?gure7)[15].1.3.2.Inductively coupled discharge.An inductively coupled plasma(ICP)[8,9,16]is excited by an electric?eld generated by a transformer from a rf current in a conductor. The changing magnetic?eld of this conductor induces an electric?eld in which the plasma electrons are accelerated. Various ICP reactors are shown in?gure8.The current-carrying coil or wire can either be outside or inside the plasma volume.The coil is formed as a helix(?gures8(a)and(b))

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H Conrads and M

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Figure9.NLD plasma source,after[17].The top and bottom coil generate a magnetic?eld con?guration with the neutral loop(separatrix) with vanishing magnetic?eld.The middle coil determines the radius of the neutral loop.The rf power(13.56MHz)is applied to a one-turn antenna coil for plasma excitation.

or as a spiral(?gure8(c)).It is also possible to use a ring-

shaped plasma volume as a single-turn secondary winding

of a transformer(?gure8(e)).The effect of the electric

?eld of the wire can be shielded resulting in a suppression

of capacitive coupling.The neutral loop discharge(NLD)

source,which was developed for plasma processing of large

wafers for the production of microelectronics[17],is an

interesting variant of the ICP source technology for low

processing pressures(～0.1Pa).A neutral magnetic loop

is generated by a set of coils as indicated in?gure9.If the

upper coil and the lower coil had a balanced current and the

same windings,the neutral loop would be positioned exactly

at the location of the middle coil.The magnetic?eld of

the middle coil,however,squeezes the neutral line inward.

As the current increases,the diameter of the neutral loop

gets smaller(separatrix).Free electrons undergo a periodic

oscillation in the electric?eld,when the magnetic?eld is

turned off.An additional force is added in the presence of

the magnetic separatrix

m e d 2s

d t =e

E+

d s

d t

×B

(7)

and becomes signi?cant when?B=0.The periodically oscillating trajectory of the free electrons in the rf?eld starts to meander.This results in a signi?cant energy gain in the vicinity of the separatrix,which has the largest magnetic?eld gradient in all directions except for the direction tangential to the separatrix circle.The gain in energy is substantial.It was reported[17]that the etch rate of a NLD source can be three times higher than that of a conventional ICP source under otherwise identical conditions in the low-pressure regime. The gain in electron energy by the NLD source enables the production of species that are three times more etch active. As the dimensions of the etched structures become smaller, fast electrons and associated VUV radiation are becoming increasingly undesirable.Since the radius of the separatrix can be varied by the current in the centre coil,the line-of-sight between the separatrix and the substrate can be blocked by a diaphragm,if the radius of the vacuum vessel is not too small.Such a diaphragm is shown schematically in?gure9.

ICPs can achieve high electron densities(n e= 1012cm?3)at low ion energies.Several applications are reported such as thin-?lm deposition,plasma etching,and ion sources in mass spectrometric analysis[18].

The helicon discharge is a special type of the inductively coupled discharge[14,19].The plasma is usually generated in a cylindrical vacuum vessel in a longitudinal homogeneous magnetic?eld at100–300G[14]or higher[20].The electromagnetic energy is transferred to the plasma source with frequencies between1and50MHz,usually with 13.56MHz for processing plasmas[8].Helicon waves are generated in the plasma column by specially-shaped antennas.The damping of this wave can be explained by collisional theory alone[20],but collisionless(Landau) damping of helicon waves has also been discussed[8]. This type of discharge achieves electron densities of up to 1012–1013cm?3in the0.1Pa pressure range.A schematic diagram of an experimental apparatus is shown in?gure10 together with experimental results for the plasma density and the electron temperature[20].The high ef?ciency of the helicon discharge(B=800G,?gure10(a))in comparison with the ICP source(B=0,?gure10(b))is obvious.Several antenna constructions[21]and a schematic diagram of a technical reactor[22]are presented in?gures11and12.

1.4.Microwave discharges

Plasma generation using microwaves is widely employed in many applications[7,23–28].Characteristic features of microwaves are the wavelength,which is comparable to the dimensions of the plasma apparatus(2.45GHz:λ= 12.24cm),and the short period of the exciting microwave ?eld.The amplitude of the oscillations of the electrons in the microwave?eld is very small.For an excitation frequency f=2.45GHz and an amplitude E0=500V cm?1it is 3.5×10?3cm.The power absorption(equation(3))depends on the electron–neutral collision frequency,i.e.on the gas pressure and the gas composition.The absorption ef?ciency in a2.45GHz discharge is high for He in the region between 103and104Pa,whereas the maximum ef?ciency for Ar is reached for200Pa.However,microwave discharges can

448

Plasma generation and plasma

sources

Figure 10.Helicon discharge apparatus with right-hand helical antenna and a quartz tube (length 1.6m,diameter 5cm)with the magnetic coils.Experimental results of electron temperature and plasma density as a function of distance from the antenna (shaded lines)for a 800G helicon discharge (a)and a 0G inductively coupled (ICP)discharge (b)in Ar,15mTorr.The electron density in the helicon discharge is considerably higher than in the ICP discharge [20].

be operated at higher pressures as well,even at atmospheric pressure.The corresponding cut-off density of the electrons (equation (5))at 2.45GHz is about 1011cm ?3.Waves of this frequency can penetrate into plasmas with higher densities only up to the thickness of the skin sheath (equation (6)),which equals a few centimetres under these conditions.The microwave power absorption inside the skin sheath transfers energy into the plasma via waves with a frequency below the cut-off frequency.A microwave plasma reactor consists in principle of a microwave power supply,a circulator,the applicator,and the plasma load.The transmission lines are rectangular waveguides or,at lower powers,coaxial cables.The applicator should optimize the energy transfer into the plasma and minimize the power re?ection.The circulator protects the power supply from re?ected power.

Various types of microwave reactors have been described.Marec and Leprince [24]distinguished three types:discharges produced in closed structures,in open structures,and in resonance structures with a magnetic ?eld.In closed structures,the plasma chamber is surrounded by metallic walls.Resonant cavities of high quality with their high electric ?eld allow an easy ignition of discharges,even at higher pressures.Examples for discharges in open structures are microwave torches,slow wave structures,and surfatrons.Electron cyclotron resonance (ECR)plasmas are a typical example of a microwave plasma in magnetic ?elds.Methods for the coupling of waveguides to discharge tubes are presented in ?gure 13[25].Figure 13(a)presents a closed con?guration.The discharge tube is located at the point of maximum electric ?eld,the distance between tube and stubs is λ/4.The slow wave structure is an open-type con?guration which is shown in ?gure 13(b).This principle can also be used for the excitation of rf plasmas.The excitation of surface waves [24]is another way of generating plasmas by microwaves.The essential elements of a surface-wave plasma source are shown in ?gure 14.The surface wave propagates along the boundary between the plasma column and the dielectric vessel.The wave energy is absorbed by the plasma.A technical application of this type of plasma excitation is realized in a surfatron.

The slot antenna (SLAN)plasma source [7]transfers the microwave energy from a ring cavity through equidistantly positioned resonant coupling slots into the plasma chamber which is made of quartz (?gure 15).As a plasma of

449

H Conrads and M

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Figure 11.Antennae for helicon discharges.Half-wavelength,m =1antenna con?gurations:Nagoya type III (A),Boswell type (B),Shoji (helical)type (C)

[21].

Figure 12.Schematic diagram of m =0helicon plasma

experimental apparatus for SiO 2etching.The magnetic coils generate the magnetic ?eld for the helicon wave to propagate in the plasma.The magnet arrays on the wall of the diffusion chamber prevent plasma losses at the wall,after [22].

high conductivity,this con?guration can be treated as a coaxial waveguide with standing waves.For lower conductivity (caused by lower power),travelling wave modes can be https://www.wendangku.net/doc/301361308.html,rge volume plasmas with electron concentrations of up to 1012cm ?3in broad pressure ranges can be created based on this principle.

Large-area planar microwave plasmas can be generated by coupling the microwave ?eld from a rectangular waveguide into the plasma volume [26,27].This scheme and a sketch of the microwave applicator are presented in ?gure 16.It consists of two waveguides.The ?rst one is connected to the microwave power source and closed by a matched load.The second one is connected to

matched

Figure 13.Methods of coupling of microwaves to discharge tubes

for generation of microwave discharges:coupling with a rectangular waveguide (a)and by a slow wave structure (b),after

[25].

Figure 14.Elements of a surface-wave plasma source,after [24].The surface wave propagates on the plasma surface in the dielectric discharge tube.

loads on both ends.Travelling waves exist in this interface waveguide only.Microwave energy is coupled from one waveguide to the other using discrete,adjustable coupling elements distributed along the waveguides.The plasma which is separated from the second waveguide by a quartz window acts as the fourth wall for this waveguide.An adjustment of the coupling elements creates a homogeneous planar plasma.Plasmas with a length of 1.2m have been produced.The generation of a planar plasma of 30×30cm 2was possible by using an array of such waveguide arrangements.Substrate surfaces are modi?ed either in the active plasma zone or in the remote plasma outside of the active zone depending on the position of the sample holder.

The ignition of microwave discharges in the low-pressure regime with a low collision frequency and thus low power absorption can be aided by a magnetic ?eld B where the electrons rotate with the electron cyclotron frequency ωc in a magnetic ?eld (equation (1)).If the cyclotron frequency equals the microwave frequency,the power absorption reaches a maximum (equation (4)).For a magnetic ?eld of

450

Plasma generation and plasma

sources

Figure15.Microwave applicator of the SLAN plasma source.The resonant annular cavity enables a homogenous microwave power distribution,the microwave power is radiated by slot antennas into the application space with the plasma

[7].

Figure16.Scheme of a planar microwave plasma source with a modi?ed slotted waveguide microwave?eld applicator consisting of two rectangular waveguides in a T-shaped con?guration,after[26].

875G,the electron cyclotron frequency becomes2.45GHz and the rotational movement of the electrons is in resonance with the microwaves of2.45GHz.The mean free path of the electrons between collisions should be larger than the radius r c(equation(2)).

Figure17[28]shows an example of an ECR plasma source.The plasma is excited in the upper part of the apparatus and the magnetic?eld is generated by the magnetic coils.The remote plasma generated in this device can be used for thin-?lm deposition applications.ECR plasma sources work in the pressure range1to10?3Pa[2].Typical values of electron temperatures and ion energies at the substrate are near5eV and between10and25eV,respectively[1].In such plasmas,collisions between the atoms,molecules,and ions are reduced and the generation of particles in the plasma volume(dusty plasmas)is avoided.1.5.Dielectric barrier discharges

The silent or dielectric barrier discharge[29,30]which operates at higher pressures(0.1–10bar)is a special type of ac or rf discharge.In1857,Siemens[31]used this type of discharge for the generation of ozone from air or oxygen.Today,these silent discharge ozonizers are effective tools and a large number of ozone installations are being used worldwide for water treatment.The silent discharge is generated between two electrodes with a dielectric barrier in between(?gure18).The gas-?lled gap is small(typically a few millinetres).A voltage of1–100kV with frequencies of50Hz–1MHz is necessary to sustain these discharges. The streamer breakdown mechanism leads to the formation of a large number of?laments(diameter～0.1mm).The current is limited by the dielectric materials between the

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H Conrads and M

Schmidt

Figure17.Scheme of a ECR plasma source for thin-?lm deposition[28].

electrodes.The charge carriers streaming from the plasma to the dielectric remain on the surface of the dielectric and compensate the external electric?eld.Therefore,the lifetime of the?laments is very short(1–10ns).The current density in the?laments is100–1000A cm?2,the electron density is1014–1015cm?3,and typical electron energies are in the range1–10eV.This non-thermal plasma is also used to pump CO2lasers and to generate excimer radiation in the UV and VUV spectral regions.Other applications include the production of methanol from methane/oxygen mixtures,various thin-?lm deposition processes[32,33], and the remediation of exhaust gases.These discharges are also being used for plasma displays.The advantage of the dielectric barrier discharge over other discharges is the option to work with a non-thermal plasma at atmospheric pressure and the comparatively straightforward scale-up to larger dimensions.

2.Plasma production using beams

Plasma generation using beams is most frequently accomplished by the use of electron beams and laser beams.

A beam-produced plasma discharge[34]is sustained,for example,by the interaction of an electron beam with a gaseous medium.Collective effects produce turbulent plasma oscillations with high amplitudes.The heating of the plasma electrons in this turbulent?eld is suf?cient to sustain the beam-produced discharge plasma.The energy transfer is very effective as up to70%of the beam energy can be transferred to the plasma.It is possible to create plasmas with high degrees of ionization in low-pressure environments.The plasma properties may be controlled by the electron beam current,the acceleration voltage,the gas pressure,and by the shape of the beam.Electron-beam generated plasmas are being used for large-area material processing[35].A spatially?at plasma with a processing area of about1m2and a thickness of about1cm is sustained by an electron beam con?ned by a magnetic?eld of100–200G.The beam source is a hollow cathode discharge.For process gas pressures between1and100Pa,plasmas with electron densities up to 1012cm?3and electron temperatures of about1eV can be obtained.

The interaction of laser beams with matter has many facets that cannot be discussed in detail here.As the electric ?eld near a given surface or in a given gas exceeds a particular value,molecules and atoms begin to lose electrons and a plasma can be generated.The required power and duration of the laser irradiation strongly depends on the speci?c application.Two examples are discussed further.The cutting of Al or Cu by dc plasmas results in unwanted seams,because too much energy is deposited in the liquid phase of the material.A short,intensive laser pulse ignites a plasma in air.Hence the power transfer from the laser to the metal is hindered.Only special conditions regarding the wavelength, the power,and the pulse shape achieve adequate power coupling to the metal and a suf?ciently high power density in the cutting groove to vaporize the molten material[36].

The remote chemical analysis of solids requires a?rst laser pulse to vaporize the material and a second laser pulse of different wavelength and power to excite a particular atomic or molecular states in the resulting cloud for the actual analysis.Examples for the application of such an analysis procedure are rapidly moving goods,hazardous materials,or melts at high temperatures.

https://www.wendangku.net/doc/301361308.html,parison of various plasma excitation mechanisms

Each of the various plasma sources discussed above has its own peculiarities,advantages,and disadvantages.The choice of the proper source for the speci?c task requires the study of the characteristics of the various plasmas.We can only give a brief summary of the various plasma sources. The dc discharge has the advantage that the microscopic pro-cesses are rather well known and understood and that such plasmas can be diagnosed in great detail.Interior electrodes are required and the possibility of reactions with reactive and corrosive gases must be considered in certain applications. The ion energy at the cathode is usually comparatively high. Power sources are well developed and widely available.By contrast,rf discharges can operate with insulated or external electrodes,i.e.they are electrode-less.Therefore,reactive processes with metal electrodes can be avoided.The life-time of devices with electrode-less rf discharges is long.Rf sources can be operated over a wide range of pressures.Low gas pressure is possible in discharges using magnetic?elds (helicon discharges).The ion energy can be controlled over a wide range.The microscopic processes in rf discharges are rather complex.Diagnostics tools are well developed, but sometimes dif?cult to use due to interference by the rf sustaining voltage.Microwave discharges also operate with-out electrodes.Plasmas of high density can be generated in the pressure range from10Pa up to atmospheric pressures. The plasma excitation at very low pressures(less1Pa)is ef-fective in microwave-excited and magnetic-?eld-supported ECR discharges.The ion energy in the microwave plasma is generally low and can be controlled by additional dc?elds or

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sources

https://www.wendangku.net/doc/301361308.html,mon dielectric barrier discharge con?gurations with one or two dielectric barriers[30].

Table3.Plasma sources and their parameters.

Pressure n e T e

Physics(mbar)(cm?3)(eV)Bias Application

Dc glow10?3–100

cathode region100Yes Sputtering,deposition,surface elementary negative glow10120.1No Chemistry,radiation

positive column10111–10No Radiation

hollow cathode10?2–80010120.1No Radiation,chemistry

magnetron10?3Yes Sputtering

Arc,hot cathode

external heating low voltage110110.1No Radiation

internal heating100010130.1No Radiation welding

Focus10keV Radiation

Rf capacitive

low pressure10?3–10?110111–10Yes Processing,sputtering

moderate pressure10?1–1010111–10No Processing,deposition

hollow cathode110120.1No Processing,radiation

magnetron10?3Yes Sputtering

Rf inductive10?3–1010121No Processing,etching

helicon10?4–10?210131No Processing

MW

closed structure100010123No Chemistry

SLAN100010115No Processing

open structure

surfatron100010125No Processing

planar10010112No Processing

ECR10?310125No Processing

Electron beam

BPD10?2–110121No Processing

Dielectric barrier discharge100010145No Ozone,processing chemisry

an rf bias voltage.Suitable power supplies are readily avail-able.Plasma generation in the low-pressure range(<100Pa) by electron beams involves more or less complicated beam sources.The shape of the plasma may by controlled by the shape of the exciting beam.The dielectric barrier discharge operates at or near atmospheric pressure.The plasma that is produced is highly non-thermal.Various applications in the area of surface treatment,cleaning,and modi?cation are feasible,because no vacuum environment is required.

Plasma sources are usually operated with static electric or alternating electromagnetic?elds.An additional static magnetic?eld performs two tasks.Firstly,the plasma con?nement is enhanced by limiting the diffusion of charged particles perpendicular to the magnetic?eld.Secondly,the power absorption is enhanced by increasing the electron–neutral collision rate due to the longer trajectories of the electrons in the plasma at low pressures.The magnetic?elds also open new absorption channels,caused by,for example, ECR resonances or helicon waves.The installation of static magnetic?elds can be cheap,if permanent magnets are used.

4.Summary

The generation and maintenance of a plasma is one of the main challenges in plasma technology.Plasma parameters,such as densities,temperatures,potentials, chemical composition,?ows,pulse shaping,the position relative to the target,and additional bias potentials,etc,have to be designed speci?cally for a given application.The choice of the plasma source and its particular design depend on the

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speci?c requirements of each application.In this paper,we discussed only a small selection of plasma sources.Table3 lists some of the sources along with their parameters and their possible applications.These sources cover a wide range of plasma parameters and linear dimensions because most of the plasma sources are not scaleable over a large range of parameters,i.e.there are no simple,known formulae according to which power densities,homogeneity of the plasma parameters,or linear dimensions etc can be predicted for sources not yet built and diagnosed.Furthermore,the state of maturity of the various plasma sources discussed in this paper is quite different from source to source.Many opportunities remain for further research and development of plasma sources in order to meet the demands of the various diverse plasma technological applications. Acknowledgments

Helpful discussions with K Becker,Stevens Institute of Technology,Hoboken,NJ,and our colleagues R Foest and L Mechold are gratefully acknowledged.

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454

等离子体实验

一、等离子体-物质第四态 如果给物质施加显著的高温或通过加速电子、加速离子等给物质加上能量，中性的物质就会被离解成电子、离子和自由基。不断地从外部施加能量，物质被离解成阴、阳荷电粒子的状态称为等离子体。将物质的状态按从低能到高能的顺序排列依次为固体、液体、气体,等离子体。 等离子体是宇宙中物质存在的一种状态,称为物质第四态.其中含有电子、离子、激发态粒子、亚稳态粒子、光子等,既有导电性又可用磁场控制,而且能为化学反应提供丰富的活性粒子,总体上是电中性的导电气体。自然界中，等离子体普遍存在，地球大气外层的电离层、太阳日冕、恒星内部、稀薄的星云和星际气体都存在等离子体，地球上自然存在的等离子体虽不多见，但在宇宙中却是物质存在的主要形式，估计宇宙中有99%以上的物质以等离子体的形式存在。 二、等离子体的产生 获得等离子体的方法和途径是多种多样的。通常把在电场作用下气体被击穿而导电的物理现象称之为气体放电，如此产生的电离气体叫做气体放电等离子体。人们对气体放电的研究己有相当长的一段历史，目前世界各国有很多研究者正从各个方面研究和发展气体放电。现代气体放电的研究大致可分为两个发展时期:第一个时期是1930年左右，人们从理论上集中对各种气体放电的性质进行了分析和研究，Langmuir首次提出等离子体(plasma)的概念[1] Tonks L, Langmuir I. Oscillations in ionized gases. Phys.Rev., 1929, 33

(2):195-210，即由电子、离子和中性原子组成的宏观上保持电中性的电离物质;第二个时期是1950年左右，人们对受控热核反应的研究。近年来，随着微电子、激光、材料的合成与改性等高新技术的发展，气体放电得到了越来越广泛的研究与应用。运用气体放电获得等离子体是一种直接、有效的方法。迄今为止，人们在实验室和生产实践中产生了各式各样的气体放电形式。按工作气压的不同，气体放电可分为低气压放电和高气压放电;按激励电场频率的不同，可分为直流放电、低频放电、高频放电和微波放电;按放电形式及形成机制可分为汤森放电、辉光放电、弧光放电、电晕放电和介质阻挡放电等。 在等离子体发展的不同阶段和从不同的研究角度，它的分类方法也不同，下面介绍按温度分类的等离子体[２]（见下表）

【WO2019199648A1】具有分离窗口的微波等离子体源【专利】

( (51)International Patent Classification:Morgan Hill,California95037(US).TANAKA,Tsuto- H01J37/32(2006.01)mu;744Nicholson Avenue,Santa Clara,California95051 (US).GARACHTCHENKO,Alexander;2011California (21)International Application Number: PCT/US2019/026289St.,Apt.10A,Mountain View,California94040(US).XIA, Yanjun;6214Joaquin Murieta Ave.,Unit B,Newark,Cal? (22)International Filing Date:ifornia94560(US).RAMASAMY,Balamurugan;Door 08April2019(08.04.2019)No.:302Sy.No.165/1,Manjula Apartments,Near KSVK School,Late Chairman Krishnappa Layout Immadihalli, (25)Filing Language:English Whitefield,560066Bangalore(IN). SHAH,Kartik;18908 (26)Publication Language:English Bellgrove Circle,Saratoga,California95070(US). (30)Priority Data:(74)Agent:WRIGHT,Jonathan B.;Servilla Whitney LLC,33 62/655,74610April2018(10.04.2018)U S Wood Avenue South,Suite830,Iselin,New Jersey08830 (US). (71)Applicant:APPLIED MATERIALS,INC.[US/US]; 3050Bowers Avenue,Santa Clara,California95054(US).(81)Designated States(unless otherwise indicated,for every kind o f national protection av ailable).AE,AG,AL,AM, (72)Inventors:CHANDRASEKAR,Siva;Plot.No.270,Sri AO,AT,AU,AZ,BA,BB,BG,BH,BN,BR,BW,BY,BZ, Meenakshi Nagar,Bagalur Road,635109Hosur(IN).CA,CH,CL,CN,CO,CR,CU,CZ,DE,DJ,DK,DM,DO, TRUONG,Quoc;4035Canyon Crest Rd.W.,San Ra?DZ,EC,EE,EG,ES,FI,GB,GD,GE,GH,GM,GT,HN, mon,California94582(US).DZILNO,Dmitry;712Glen?HR,HU,ID,IL,IN,IR,IS,JO,JP,KE,KG,KH,KN,KP, coe Court,Sunnyvale,California94087(US).SHERVE-KR,KW,KZ,LA,LC,LK,LR,LS,LU,LY,MA,MD,ME, GAR,Avinash;180Alicante Dr.,#323,San Jose,Cali?MG,MK,MN,MW,MX,MY,MZ,NA,NG,NI,NO,NZ, fornia95134(US).KUDELA,Jozef;18222Bautista Cir.,OM,PA,PE,PG,PH,PL,PT,QA,RO,RS,RU,RW,SA, (54)Title:MICROWAVE PLASMA SOURCE WITH SPLIT WINDOW FIG.2 (57)Abstract:Plasma source assemblies,gas distribution assemblies including the plasma source assembly and methods of generating plasma are described.The plasma source assemblies include a powered electrode with a ground electrode adjacent a first side,a first dielectric adjacent a second side of the powered electrode and at least one second dielectric adjacent the first dielectric on a side opposite the first dielectric.The sum of the thicknesses of the first dielectric and each of the second dielectrics is in the range of about10mm to about17mm. [Continued on next page]

等离子体概述

一、等离子体概述 物质有几个状态？学过初中物理的会很快回答固态、液态、气态。其实，等离子态是物质存在的又一种聚集态，称为物质的第四态。它是由大量的自由电子和离子组成，整体上呈现电中性的电离气体。 在一定条件下，物质的各态之间是可以相互转化的，当有足够的能量施予固体，使得粒子的平均动能超过粒子在晶格中的结合能，晶体被破坏，固体变成液体。若向液体施加足够的能量，使粒子的结合键破坏，液体就变成了气体。若对气体分子施加足够的能量，使电子脱离分子或原子的束缚成为自由电子，失去电子的原子成为带正电的离子时，中性气体就变成了等离子体。物质的状态对应了物质中粒子的有序程度，等离子内物质中的粒子有序程度是最差的。相应的，等离子体内的粒子具有较高的能量、较高的温度。实际上，宇宙中99.9%的物质处于等离子态，它是宇宙中物质存在的普遍形式，不过地球上，等离子体多是人造的。 人工如何造出等离子体呢？从上面的论述可以看出，等离子体的能量是很高的，任何物质加热到足够高的温度，都会成为电离态，形成等离子体。在太阳和恒星的内部，都存在着大量的高温产生的等离子体。太阳和恒星的热辐射和紫外辐射能使星际空间的稀薄气体产生电离，形成等离子体，如地球上空的电离层就是这样来的。各种直流、交流、脉冲放电等均可用来产生等离子体。利用激光也可以产生等离子体。 等离子体如何描述？温度。等离子体有两种状态：平衡状态和非平衡状态。等离子体中的带电粒子之间存在库伦力的作用，但是此作用力远小于粒子运动的热运动能。当讨论处于热平衡状态的等离子体时，常将等离子体当做理想气体处理，而忽略粒子间的相互作用。在热平衡状态下，粒子能量服从麦克斯韦分布。每个粒子的平均动能32 E kT =。对于处于非平衡状态下的等离子体，一般认为不同粒子成分各自处于热平衡态，分别用e T 、i T 、n T 表示电子气、离子气和中性气体的温度，并表示各自的平均动能。可以用动力学温度E T （eV ）表示等离子体的温度，E T 的单位是能量单位，由粒子的动能公式可得 2133222 E E mv kT T ===，E T 就是粒子的等效能量kT 值（1eV 的能量温度，相应的开氏绝对温度为1T k ==11600K ）。 温度是描述等离子体能量的，还有其它的一些概念来表述。（1）高温等离子体，低温等离子体，冷等离子体。高温等离子体也是完全电离体，温度68 10~10K ，核反应、恒星的等离子体是这类。低温等离子体是部分电离体， 463410~10,310~310e i T K T K ==??，电弧等离子体、燃烧等离子体是这种。冷等离子体是410,e i T K T >约等于室温的等离子体。 （2）电离度。强电离等离子体指电离度η>10-4的等离子体，弱电离等离子体η<10-4。η是电离度，0=n n n η+，n 是两种异电荷粒子中任何一种密度，0n 为中性粒子密度。粒子密度是表示单位体积中所含粒子的数目。（3）稠密等离子体和稀薄等离子体。具体区分度不详。

高密度等离子体化学气相淀积工艺简介

高密度等离子体化学气相淀积工艺简介 随着半导体技术的飞速发展，半导体器件特征尺寸的显著减小，相应地也对芯 片制造工艺提出了更高的要求，其中一个具有挑战性的难题就是绝缘介质在各 个薄膜层之间均匀无孔的填充以提供充分有效的隔离保护，包括浅槽隔离 （STI ），金属前绝缘层（PMD ，金属层间绝缘层（IMD ）等等。本文所介绍的 高密度等离子体化学气相淀积（HDP CVD 工艺自20世纪90年代中期开始被先 进的芯片工厂采用以来，以其卓越的填孔能力，稳定的淀积质量，可靠的电学 特性等诸多优点而迅速成为 0.25微米以下先进工艺的主流。图1所示即为在 超大规模集成电路中HDP CVDL 艺的典型应用。 PASSIVATION PECVD SIN PETEOS Hi D?p RW U$G HOP- USC CMP Hi Dep RqU USG Hi D 呻 Rot* 吒G 、 HDP-PSC^?S 4BCFC 刖一^ Th^nnoJ Ox-匚扌 CUP // 图1 HDP GVD 工艺在趙大規摟集成电路中的典型应用 HDP CVD 勺工艺原理 在HDP CVDE 艺问世之前，大多数芯片厂普遍采用等离子体增强化学气相 沉积（PE CVD ）进行绝缘介质的填充。这种工艺对于大于 0.8微米的间隙具有良 好的填孔效果，然而对于小于0.8微米的间隙，用单步PE CVDT 艺填充具有高 的深宽比（定义为间隙的深度和宽度的比值）的间隙时会在其中部产生夹断 （pinch-off ） 和空洞（图 2）。 / HDP USG Z CMP

f?]2PECVD工艺填孑l中产生的夹斯和空洞 为了解决这一难题，淀积-刻蚀-淀积工艺被用以填充0.5微 米至0.8微米的间隙，也就是说，在初始淀积完成部分填孔尚未发生夹断时紧跟着进行刻蚀工艺以重新打开间隙入口，之后再次淀积以完成对整个间隙的填充（图 3 ）。 显而易见，为了填充越小的间隙，越来越多的工艺循环需要被执行，在不断降低产量的同时也显著增加了芯片成本，而且由于本身工艺的局限性，即便采用循环工艺，PE CVD寸于小于0.5微米的间隙还是无能为力.其他一些传统CVD 工艺，如常压CVD（APCVD）亚常压CVD（SACVD虽然可以提供对小至0.25微米的间隙的无孔填充，但这些缺乏等离子体辅助淀积产生的膜会依赖下层表面而显示出不同的淀积特性，另外还有低密度和吸潮性等缺点，需要PE CVD增加上保 护层和下保护层，或者进行后淀积处理（如退火回流等）。这些工序的加入同样提高了生产成本，增加了整个工艺流程的步骤和复杂性。 HDPCVDX艺正是在探索如何同时满足对高深宽比间隙的填充和控制生产成本的过程中诞生的，它的突破创新之处就在于在同一个反应腔中同步地进行淀积和刻蚀的工艺（图4）。具体来说，在常见的HDP CV制程中，淀积工艺通常是由SiH4和02的反应来实现，而蚀刻工艺通常是由Ar和Q的溅射来完成。

微波等离子体

微波等离子体 ●微波等离子体反应器特点： 微波：为交流能量（信号），通过波导传输，每一种波导 具有一定的特征阻抗 （射频传输线理论） 等离子体的反应器：本质上是具有一定阻抗的负载。 微波等离子体工作要求：波导特征阻抗=等离子体负载 阻抗。 微波反射波能量将至最低。 ●微波等离子体反应器发展： 小尺寸共振腔---->表面波长细等离子体--->大面 积（体积）表面波等离子体。 ●微波等离子体反应器结构： ⊙单模谐振腔 谐振腔尺寸: λ λ= R,（谐振条件） =d 阻抗匹配: 好，可以不设置附加匹配。 激励电场 单模（单一本征模） 方向：图中电场沿轴向。 状态：驻波

缺点：体积小（？） 电场不均匀-----〉等离子体空间均匀性差。应用：放电灯，光谱分析。

⊙多模腔 谐振腔尺寸: λ λ>> R;（非谐振） >>d 阻抗匹配: 差，需要附加匹配。 优点：电场较均匀-----〉等离子体空间均匀性好。 ⊙表面波等离子体（surface microwave plasma,SWP）源 尺寸: λ = R（谐振条件），轴向尺寸没有限制阻抗匹配: 需要设置附加匹配。 激励电场 单模或多模（单一本征模） 状态：行波 优点：大体积，细长 缺点：面积小 应用：气体反应（甲烷--->乙炔），有害气体处理

侧视图

多管SWP源

●大面积/体积SWP源 两种方式：（a）顶面馈入;(b)侧面馈入 三种典型装置：（a）日本平面狭缝（顶面）耦合； (b) 德国环状狭缝（侧面）耦合； （c）法国改进型表面波导（侧面）耦合美国： 中国（中国科大、合肥等离子体物理所----> 德国版）●日本顶面狭缝（重点） （1）两种加热模式

等离子体的描述方法

等离子体的动力论和流体描述 等离子体既然是与电磁场做相互作用，首先看电磁场对等离子体的影响。我们对带电粒子的单粒子运动的理论已经有了一些认识，但对于等离子体是如何影响电磁场的，还需要有所了解。从Maxwell方程组可以看到，主要是电荷分布和电流分布（以及边界条件）决定了电磁场。而电荷分布与等离子体各个带电成分的密度分布有关。如果没有新的复合和电离过程，密度分布满足连续性方程。 对流体进行描述，考察各个物理量随着时间的变化，常用的是欧拉法，即考察固定的地点上物理量随时间的变化，另外一种方法是拉格朗日法，是考察固定的物质上的物理量随时间的变化。因为物质是移动的，因此不但随时间变化，也随空间变化。我们分别就这两种方法，考察等离子体的连续性方程。 连续性方程 假设等离子体没有产生（电离）、没有消失（复合），一块等离子体的数量会保持不变。

()0d n V dt ?= 这里是随体微分，即拉格朗日法描述流体。为了了解体积元的变化，先看看流体中一段长度元的变化。 21 =-l r r 经过时间t ?之后，新的位置为 ()2 1221121()()()()t t t t '''=-=+?--?=+-?=+???l r r r v r r v r l v r v r l l v 即d dt =??l l v ，应用这个结果，考 察一个小体积元V x y z ?=???，因而， d x dt x ??=??x v ，取x 分量，x v d x x dt x ??=??，因此， ()()()0d dn d x d y d z n V V n y z x z x y dt dt dt dt dt dn n V dt ????=?+??+??+??=+???=v 电流分布不但与等离子体各个带电成分的密度分布有关，而且与它们的运动速度有关。动力论的描述使用分布函数f(t, x , v )，不但包含密度信息，也包含了带电粒子的速度信息。这是在相空间中的密度分布，类似r

高密度等离子体化学气相淀积工艺简介

高密度等离子体化学气相淀积工艺简介 随着半导体技术的飞速发展，半导体器件特征尺寸的显著减小，相应地也对芯片制造工艺提出了更高的要求，其中一个具有挑战性的难题就是绝缘介质在各个薄膜层之间均匀无孔的填充以提供充分有效的隔离保护，包括浅槽隔离（STI)，金属前绝缘层（PMD），金属层间绝缘层（IMD）等等。本文所介绍的高密度等离子体化学气相淀积（HDP CVD)工艺自20世纪90年代中期开始被先进的芯片工厂采用以来，以其卓越的填孔能力，稳定的淀积质量，可靠的电学特性等诸多优点而迅速成为0.25微米以下先进工艺的主流。图1所示即为在超大规模集成电路中HDP CVD工艺的典型应用。 HDP CVD的工艺原理 在HDP CVD工艺问世之前，大多数芯片厂普遍采用等离子体增强化学气相沉积(PE CVD)进行绝缘介质的填充。这种工艺对于大于0.8微米的间隙具有良好的填孔效果，然而对于小于0.8微米的间隙，用单步PE CVD工艺填充具有高的深宽比(定义为间隙的深度和宽度的比值)的间隙时会在其中部产生夹断(pinch-off)和空洞(图2)。

为了解决这一难题，淀积-刻蚀-淀积工艺被用以填充0.5微 米至0.8微米的间隙,也就是说，在初始淀积完成部分填孔尚未发生夹断时紧跟着进行刻蚀工艺以重新打开间隙入口，之后再次淀积以完成对整个间隙的填充（图3）。 显而易见，为了填充越小的间隙,越来越多的工艺循环需要被执行,在不断降低产量的同时也显著增加了芯片成本，而且由于本身工艺的局限性，即便采用循环工艺，PE CVD对于小于0.5微米的间隙还是无能为力.其他一些传统CVD 工艺,如常压CVD(APCVD)和亚常压CVD(SACVD)虽然可以提供对小至0.25微米的间隙的无孔填充,但这些缺乏等离子体辅助淀积产生的膜会依赖下层表面而显示出不同的淀积特性，另外还有低密度和吸潮性等缺点，需要PE CVD增加上保护层和下保护层，或者进行后淀积处理(如退火回流等)。这些工序的加入同样提高了生产成本,增加了整个工艺流程的步骤和复杂性。 HDP CVD工艺正是在探索如何同时满足对高深宽比间隙的填充和控制生产成本的过程中诞生的，它的突破创新之处就在于在同一个反应腔中同步地进行淀积和刻蚀的工艺(图4)。具体来说,在常见的HDP CVD制程中，淀积工艺通常是 由SiH 4和 O 2 的反应来实现，而蚀刻工艺通常是由Ar 和O 2 的溅射来完成。

等离子体刻蚀..

等离子体刻蚀 ●集成电路的发展 1958年：第一个锗集成电路 1961年：集成8个元件 目前：集成20亿个元件 对比： 第一台计算机（EN IAC，1946），18000 只电子管, 重达30 吨, 占 地180 平方米, 耗电150 千瓦。奔II芯片：7.5百万个晶体管 ●集成电路发展的基本规律 穆尔法则：硅集成电路单位面积上的晶体管数，每18个月翻一番，特征尺寸下降一半。 集成度随时间的增长： 特征长度随时间的下降：

集成电路制造与等离子体刻蚀 集成电路本质：微小晶体管，MOS场效应管的集成 微小晶体管，MOS场的制作：硅片上微结构制作----槽、孔早期工艺：化学液体腐蚀----湿法工艺 5微米以上 缺点: (a)腐蚀性残液----->降低器件稳定性、寿命 (b)各向同性 (c)耗水量大（why） (d)环境污染

随着特征尺寸的下降，湿法工艺不能满足要求，寻求新的工艺----> 等离子体干法刻蚀，在1969引入半导体加工，在70年代开始广泛应用。

等离子体刻蚀过程、原理： 4

刻蚀三个阶段 (1) 刻蚀物质的吸附、反应 (2) 挥发性产物的形成; (3) 产物的脱附, 氯等离子体刻蚀硅反应过程 Cl2→Cl+Cl Si（表面）+2Cl→SiCl2 SiCl2+ 2Cl→SiC l4（why） CF4等离子体刻蚀SiO2反应过程 离子轰击作用 三种主要作用 （1）化学增强物理溅射(Chemical en2hanced physical sputtering) 例如,含氟的等离子体在硅表面形成的SiF x 基与元素 Si 相比,其键合能比较低,因而在离子轰击时具有较高 的溅射几率, （2）晶格损伤诱导化学反应(damage - induced chemical reaction) 离子轰击产生的晶格损伤使基片表面与气体物质的反 应速率增大 （3）化学溅射(chemical sputtering) 活性离子轰击引起一种化学反应,使其先形成弱束缚的 分子,然后从表面脱附。 其他作用 ?加速反应物的脱附 ---> 提高刻蚀反应速度 ?控制附加沉积物---> 提高刻蚀的各向异性

ECR等离子体

电子回旋共振等离子体 (Electron CyclotronResonance,ECR) z ECR等离子体源发展历史： （1）微波电源的发展 1921： 磁控管 1939：速调管 （2）二战中微波技术的迅速发展 雷达 （3）微波灶的普及 1960-1970 微波电源价格大幅度下降 （4）1970年代前期：高温核聚变等离子体微波加热 后期：日本，捷克 低温等离子体应用 （5）1980 集成电路芯片刻蚀加工： 低气压高密度等离子体源竞争 ECR,ICP.Helicon. Hitachi, Astex. z ECR等离子体源结构:

z 微波电子回旋共振加热原理 （a）微波ECR 等离子体内的有效电场 B 0 0 ≠()()? ???????+?+++=2222222112~c c c c c eff v v v E E ωωωω [对比] B 0=0 2 2222 ~c c eff v v E E +=ω 特性 电子回旋频率附近，击穿电场显著降低。 实验结果：

回旋运动角频率ωce= eB0/m e =ωwave (b)ECRplasma 中微波传输及吸收的主要特性 ---微波ECR 等离子体为各向异性介质,沿磁场方向传播的TE 波将分为右旋偏振波和左旋偏振波,色散关系为：

n2R=1-(ω2pe/ (ω - ωce)ω) n2L=1-(ω2pe/ (ω + ωce)ω) 右旋波的共振和截止条件为: ωce/ω =1 (共振条件: n R =∞) ω2pe/ω2=1-ωce/ ω(截止条件: n R =0) ----微波不同馈入模式的结果 低场馈入：图中路径a-----> 右旋波在低密度区截止（对应 的临界密度n crit= n c (1 - ωce/ω) ----->低密度 高场输入：图中路径b,没有高密度截止------>高密度运行条件

等离子体加工技术

等离子体加工技术 摘要 随着科学技术的不断发展，工业需求的不断提高，各种高新设备应运而生，然而要加工这些设备就要使用更先进的加工技术。而等离子体加工技术就是一种不断发展的新型加工技术。本文简要介绍了工业用等离子体的分类及等离子体加工技术涉及的科学工程问题。围绕材料添加与去除加工，讨论了等离子体喷涂、增强沉积、离子去除等若干典型加工工艺的技术发展和应用情况，并对一些工艺中出现的现象以及某待深入研究的潜在科学问题进行了举例说明。 关键词：等离些有子体；加工；等离子体喷涂；等离子体聚合 Abstract With the continuous development of science and technology,increasing industrial demand,a variety of high-tech equipment came into being,however, to the processing of these devices is necessary to use more advanced processing technology.The plasma processing technology is a continuous development of new processing technology.This article briefly describes the classification of industrial plasma and plasma processing technology involved in scientific engineering problems.Adding and removing surrounding material processing,Discusses the plasma spraying, enhanced deposition, ion removal, etc. Several typical processing technology development and application,And some of the processes the phenomenon appears to be in-depth study as well as some of the potential scientific issues illustrate. Key words: Plasma；Machining；Plasma spraying；Plasma polymerization 引言 随着科学与工程技术的迅速发展，对新材料、新结构、新工艺的要求日益迫切。人们不仅要对材料的表面性能进行改进，而且还要了解元素（原子）的相互作用，新相的形成，亚稳态、非晶态的形成等机制；对一些结构器件的要求已达到了μm、nm 量级。在实现这些要求的过程中，作为特种加工手段之一的等离子体加工工艺的应用越来越广泛，实际上，等离子体之所以成为现代制造技术的重要手段之一，是由其能量状态决定的。物体由固体到等离子体态的转化过程中，都伴随有足够能量的输入。所以作为一种物质形态的等离子体具有最高的能量状态，为现代材料加工提供了巨大潜力。

高能量高密度脉冲等离子体轰击HDPE薄膜表面微观结构的研究

电子显微学报990607 电子显微学报 JOURNAL OF CHINESE ELECTRON MICROSCOPY SOCIRTY 1999年 第18卷 第6期 vol.18No.6 1999 高能量高密度脉冲等离子体轰击HDPE 薄膜表面微观结构的研究 鲍春莉　杨思泽　刘赤子　徐端夫 摘　要： 本文利用SEM研究了高能量高密度脉冲A1等离子体轰击高密度聚乙烯(HDPE)薄膜引起的表面微观结构变化。结果表明：等离子体单脉冲轰击导致HDPE薄膜表面局域熔化和非晶化；随着脉冲次数的增加，HDPE薄膜表面微观结构发生明显变化。化学分析和红外光谱分析表明HDPE薄膜表面发生了交联反应。 关键词： 等离子体；HDPE薄膜；微观结构；脉冲 分类号： O539；O766 文献标识码： A Surface microstructures of HDPE films bombarded by pulsed high energy density plasma BAO Chun-li* YANG Si-ze LIU Chi-zi XU Duan-fu* (*Institute of Chemistry,the Chinese Academy of Sciences,Beijing 10080,China.) (Institute of Physics,the Chinese Academy of Sciences,Beijing 10080,China.) Abstract: Surface microstructures of HDPE films bombarded by pulsed high energy density plasma were observed using scanning electron microscopy (SEM).Domain melting and amorphous of HDPE surface were induced by Al atoms bombardement with single pulse plasma. With the increase of the pulse times of plasma,the surface microstructures of HDPE films dramatically changed.The crosslinking reaction of HDPE films was investigated by chemical analysis and FTIR measurements.The surface microstructures of HDPE films were controlled by thermal-spike effects and crosslinking reaction. Keywords: plasma; HDPE films; microstructure; pulse 高聚物表面金属化不仅可以提高高分子材料表面硬度和耐磨性［1］，而且可以提高抗腐蚀性和导电［2］等性能。高能量高密度脉冲等离子体枪［3］是一种新的材料表面改性技术。与真空镀膜和CVD等技术相比，该技术的突出特点是在室温条件下等离子体处理与薄膜沉积同时发生。高能量高密度脉冲等离子体枪产生的氩等离子体激发同轴金属电极，电极溅射出金属原子，在加速电磁场的作用下形成混合等离子体，轰击注入到材料表面。该技术对金属材料的表面改性已有多篇报道，而对高分子材料还没有研究，而且等离子体与高聚物表面相互作用的物理过程尚不清楚，因此观察等离file:///E|/qk/dzxwxb/dzxw99/dzxw9906/990607.htm（第 1／5 页）2010-3-22 18:39:51

微波等离子体气相沉积

等离子体合成金刚石 在20世纪80年代初,一种新的方法出现了,那就是微波等离子体化学气相法合成金刚石薄膜（CVD）制备金刚石薄膜，它成本低，质量高，有利于大规模合成利用,且装置简单,能量集中,反应条件易于控制,产物比较纯净,成为当前研究的主要方向和热点。现在该领域的最新进展是用微波化学气相合成法合成纳米级的金刚石薄膜,纳米级金刚石薄膜除了有普通微米级金刚石薄膜的性质外,还具有高光洁度,高韧性,低场放射电压,是具有广阔应用前景的新材料。摩擦系数低,光洁度高,颗粒极细,硬度高,耐磨度高,可广泛应用医疗,交通,航空航天,工业制造领域的涂料,涂层,钻头,更可为微型机电领域带来革命性的飞跃.许多科学家纷纷预言：21世纪将是金刚石的时代。 合成与机理：等离子态是物质的第四态,之所以把等离子体视为物质的又一种基本存在形态，是因为它与固、液、气三态相比无论在组成上还是在性质上均有本质区别。即使与气体之间也有着明显的差异。首先，气体通常是不导电的，等离子体则是一种导电流体而又在整体上保持电中性。其二，组成粒子间的作用力不同，气体分子间不存在净电磁力，而等离子体中的带电粒子间存在库仑力，并由此导致带电粒子群的种种特有的集体运动。第三，作为一个带电粒子系，等离子体的运动行为明显地会受到电磁场的影响和约束。需说明的是，并非任何电离气体都是等离子体。只有当电离度大到一定程度，使带电粒子密度达到所产生的空间电荷足以限制其自身运动时，体系的性质才会从量变到质变，这样的“电离气体”才算转变成等离子体。否则，体

系中虽有少数粒子电离，仍不过是互不相关的各部分的简单加合，而不具备作为物质第四态的典型性质和特征，仍属于气态。按热力学分析只要压力适当，石墨转变成金刚石在低温下并非不能自发进行，问题在于反应速率太低，以致必须提供苛刻的高温高压条件。但若借助非平衡等离子体，情况就不同了。如用微波放电把适当比例的CH4和H2气激发成等离子体，便可在低于1.0133×104Pa，800—900℃条件下以相当快的生长速率（1μm/h）人工合成金刚石薄膜。 依照此原理设计的CVD合成金刚石薄膜的装置都有一共同特性，即使低分子碳烃气体稀释在过量氢气中，在一定电磁能激发产生等离子体，在等离子体中形成局部的高温高压条件,通过适宜的沉积工艺在基片(硅片)上沉积出金刚石薄膜。常用的方法有热丝法、微波法、等离子体炬和燃烧火焰法等。热丝法是利用高温金属丝激发等离子体，装置简单，使用比较方便。但由于金属丝的高温蒸发会将杂质引入金刚石膜中，因此该方法不能制备高纯度的金刚石膜；微波法是利用微波的能量激发等离子体，具有能量利用效率高的优点。同时由于无电极放电，等离子体纯净，是目前高质量、高速率、大面积制备金刚石膜的首选方法；等离子体炬是利用电弧放电产生等离子体，制备的金刚石膜质量高。但由于电弧面积的限制，金刚石膜的面积较小；同时由于电弧点燃及熄灭的热冲击，对金刚石膜的附着力影响很大，设备的磨损大，反应气体的消耗也高；燃烧火焰法是利用乙炔在氧气中燃烧产生的高温激发等离子体，可以在常压下工作，也存在着金刚石膜沉积面积小，不均匀等问题。

等离子体第一部分

等离子体化工导论讲义 前言 等离子体化工是利用气体放电的方式产生等离子体作为化学性生产手段的一门科学。因其在原理与应用方面都与传统的化学方法有着完全不同的规律而引起广泛的兴趣，自20世纪70年代以来该学科迅速发展，已经成为人们十分关注的新兴科学领域之一。 特别是，近年来低温等离子体技术以迅猛的势头在化工合成、材料制备、环境保护、集成电路制造等许多领域得到研究和应用，使其成为具有全球影响的重要科学与工程。例如：先进的等离子体刻蚀设备已成为21世纪目标为0.1μm线宽的集成电路芯片唯一的选择，利用等离子体增强化学气相沉积方法制备无缺陷、附着力大的高品位薄膜将会使微电子学系统设计发生一场技术革命，低温等离子体对废水和废气的处理正在向实际应用阶段过渡，农作物、微生物利用等离子体正在不断培育出新的品种，利用等离子体技术对大分子链实现嫁接和裁剪、利用等离子体实现煤的洁净和生产多种化工原料的煤化工新技术正在发展。可以说，在不久的将来，低温等离子体技术将在国民经济各个领域产生不可估量的作用。 但是，与应用研究的发展相比，被称为年轻科学的等离子体化学的基础理论研究缓慢而且较薄弱，其理论和方法都未达到成熟的地步。例如，其中的化学反应是经过何种历程进行，活性基团如何产生等等。因此，本课程力求介绍这些方面的一些基础理论、研究方法、最新研究成果以及应用工艺。 课程内容安排： 1、等离子体的基本概念 2、统计物理初步 3、等离子体中的能量传递和等离子体的性质 4、气体放电原理及其产生方法 5、冷等离子体中的化学过程及研究方法 6、热等离子体中的化学过程及研究方法

7、当前等离子体的研究热点 8、等离子体的几种工业应用 学习方法： 1、加强大学物理和物理化学的知识 2、仔细作好课堂笔记，完成规定作业 3、大量阅读参考书和科技文献 第一章等离子体的概念

射频放电和低压高密度等离子体放电

2.6 电晕放电 除了辉光放电之外，还存在另外一种脉冲直流放电，它的阴极时金属丝。在大气压下，阴极表面施加高负电压时，就会产生放电。电晕放电产生是因为在阴极周围产生暗辉光。 负极性电晕放电的机理与直流辉光放电类似，正离子被加速向阴极运动，到达阴极后轰击阴极产生二次电子发射。这些电子被加速进入到等离子体中。这叫做流光。也就 是前面是高能电子后面跟着低能电子。高能电子与重粒子发生非弹性碰撞，例如，造 成离子化，激发，解列。因此，等离子体的根部形成，这会造成在碰撞中产生更大的 分子。因此在应用中，点电子动力学和重粒子动力学有很明显的区别。这两者之间的 区别表现在时间上而不是空间上。在温度和化学性质方面，电晕放电也处于极不平衡 的状态。主要原因是脉冲的作用时间短，如果施加的电压源不是脉冲形式的，那么就 会产生高温，引起热电子发射，并向接近平衡状态的弧光放电过渡。 事实上，除了负极性电晕放电外，也存在正极性电晕放电，其中，金属丝上存在正电压，因此它为阳极。 电晕放电的应用包括废气清洁，油漆中挥发性化合物的处理，水的净化等等。气体或 液体中的尘埃能够通过电子的吸附清除掉，电子吸附后，尘埃带负电，这样就能够从 气体或液体中隔离了。 2.8 低压、高密度等离子体技术 近些年来，很多低压、高密度等离子体放电技术得到应用。它主要是替代容性射频放 电（射频二极管）的蚀刻和皮膜处理应用。确实，射频二极管的电压和电流不能独立 控制，因此，除非施加不同的频率，否则离子冲击通量和冲击能不能单独改变。而且 并不是每次都能施加不同的频率。因此，要产生适度的离子通量，鞘层电压必须具备 很高的数值。由于高冲击能会对施加在电极上的薄片造成不应有的破坏。而且，低离 子通量和高离子能的结合在应用中会导致较窄的加工面积。在射频二极管中有限的离 子通量导致较低的处理比率，较低的处理比率经常会造成多薄片或成批处理，这会产生薄片间再现能力的损失。为了克服这些问题，平均离子冲击能应该独立控制离子和 中性助溶剂。通过在未受驱策的电极上放置薄片并且独立的在具有二次射频源的电极 上施加偏压，可以控制离子冲击能。尽管这些所谓的射频三极管已经应用，但是在低 压下处理比率很低而且产生的飞溅污染物也是个问题。各种磁场增强射频二极管三极 管已经发展来增加等离子体密度和离子通量。然而，如上所述，离子通量不具有良好 的均匀性，这限制了它在等离子体处理应用中的适用性。 新一代的低压高温等离子体的特征正如它的名字一样，具有低压（0.1到10帕）,具有更高的密度（1011-1013cm3）,在同等压强下比电容耦合射频放电具有更高的离子通量。

表面等离子体

LSPs和PSPs的区别 局域表面等离子体（Localized Surface plasmons, LSPs）和传播型表面等离子体（Propagating surface plasmons. PSPs）同属于表面等离子体（SPs）1。 表面等离子体（SP）是存在于金属与电介质截面的自由电子的集体振荡2。SPR是由于入射激光在特殊波长处局域电磁场增强，物理机制是表面增强拉曼散射（Surface-enhanced Raman scattering, SERS）和尖端增强拉曼散射（Tip-enhanced Raman scattering, TERS）。 入射光的电场分量诱导球形金属粒子的表面等离子体共振的原理分析（即图1的解读）3。 当入射光照射到贵金属（如：金、银，见脚注1、3）时，在纳米颗粒表面形成一种振荡电场，纳米颗粒中的自由传导电子在振荡电场的激发下集体振荡，入射光子频率与金属纳米颗粒的自由电子云的集体振动频率相等（入射光波长一定）时，发生局域表面等离子体共振（LSPR）。亦可解释为入射光在球形颗粒表面产生电场分量，电子的共谐振荡与激发其的振荡电场频率相同时发生共振，诱导产生LSPR 3。 对于LSPs而言，颗粒内外近场区域的场强会被极大增强，原因是：纳米粒子的尺寸远小于入射光波长，使得电子被束缚在纳米粒子周围局域振荡，导致场强增大。 对于PSPs（部分文章中称为：SPPs4，金属与介质界面上的电子集体激发振荡的传播型表面电磁波），其表面等离子激元（即TM模式）如上图所示。在SPPs 的情况下，沿金属介质界面，等离子体在X和Y方向上传播，在Z方向上衰减， 1等离激元学[M]. 东南大学出版社, 2014. 2 Zhang Z, Xu P, Yang X, et al. Surface plasmon-driven photocatalysis in ambient, aqueous and high-vacuum monitored by SERS and TERS[J]. Journal of Photochemistry & Photobiology C Photochemistry Reviews, 2016, 27:100-112. 3邵先坤, 郝勇敢, 刘同宣,等. 基于表面等离子体共振效应的Ag(Au)/半导体纳米复合光催化剂的研究进展[J]. 化工进展, 2016, 35(1):131-137. 4王五松, 张利伟, 张冶文. 表面等离子波导及应用[J]. 中国光学, 2015(3):329-339.

火与等离子体

火是物质燃烧产生的光和热。必须有可燃物、燃点、助燃气体（不一定是氧气）并存才能生火。三者缺任何一者就不能生火。 火是很泛的概念，基本包含两大元素：发光（光子的产生）和产热（如氧化、核反应所致）。在生活中，火可以被认为是物质发生某些变化时的表征。很多物质都能在某些特定的变化或说反应中产生光和热，两者共同构成我们所说的“火”。 譬如以蜡烛为例，蜡烛燃烧时当然产生了火。但我们到底该认为谁是火呢？是蜡，还是二氧化碳、水，甚至是炭或蜡分解出的小分子有机物？ 水和二氧化碳是无法独自产生火的，可排除此可能性；我们在蜡烛燃烧时看到黑烟，说明炭还好好的存在着，并未发生反应，所以这种可能性亦不存在，至于其他杂分子，也是燃烧的副产物，既然称为产物，则不会在我们所讨论的反应过程中发生变化了，排除。只剩下蜡了。蜡是火？确实荒谬。不错，蜡本身绝不是火，但火源自蜡，而非上述任何其他物质，这是肯定的。蜡产生了火，而火却不是此反应中的任何反应物或生成物本身！火就是火自己！但火实际上确是一种物质，但又不仅仅是物质。 或许我们也会问“闪电是什么物质？”，有人可能会回答道“闪电是一种现象，不是一种物质”，这样的答复没什么意义。其实这个问题颇值得思考。闪电产生于空气中，更准确地说，是云（以水为主）中。书本告诉我们闪电是电中和所致，但这并不直击问题要害。相信某人说“闪电是一种大自然的现象”没人会反驳，但我提出的闪电与他说的闪电是两个不同的词。我说的是一个物质名词，他说的是一个动名词！举个例子，我说的闪电好比雪snow，而他所说的闪电好比下雪fall of snow OR snowing。对于火的理解，也有相同的理解分歧。但是，我们要清楚一点，任何自然现象都是物质的。客观存在的是物质本身，而其现象只是人脑中的反映，或说人的感知及后继的理性思考。 在火中，光既是物质又是能量，这不难接受。而对于热，大多数人认为热仅仅是能量，但实际上，热辐射作为一种电磁辐射，在量子物理中亦有物质性，其和光的本质是同一的。更深层上，物质与能量是统一的，可等价的。只是当代物理学界倾向于将物质统一于能量——受限的能量。所以火的本质既是同具光波和热辐射的电磁波，是物质，也是同具光能、热能的能量。 电子离开原子核，这个过程就叫做“电离”。这时，物质就变成了由带正电的原子核和带负电的电子组成的，一团均匀的“浆糊”，人们称它离子浆。这些离子浆中正负电荷总量相等，因此又叫等离子体。 火是物质吗?如果是,是什么物质?