WO2006100217A1

WO2006100217A1 - Photon source comprising an electron cyclotron resonance multicharged ion plasma source

Info

Publication number: WO2006100217A1
Application number: PCT/EP2006/060862
Authority: WO
Inventors: Marc Delaunay; Denis Hitz
Original assignee: Commissariat A L'energie Atomique
Priority date: 2005-03-21
Filing date: 2006-03-20
Publication date: 2006-09-28
Also published as: FR2883410A1

Abstract

The invention relates to a photon source comprising an electron cyclotron resonance (ECR) multicharged ion plasma source which comprises a cylindrical plasma vacuum chamber (CH) , an injection guide (GD) for injecting microwaves into the chamber, a device (I) for injecting a gas into the chamber, a pumping system (P) to extract ionised gas resulting from the action of microwaves on the gas (g) , and a cylindrical magnetic structure (1, 2, 3a, 3b, 4) that surrounds the chamber (CH) and that produces at least two closed surfaces (S) in line along the chamber axis and on which the value of the magnetic field is equal to the value of the electron cyclotron resonance field (ECR) , the photons being extracted through an opening (02) in line along the axis of the chamber .

Description

PHOTON SOURCE COMPRISING AN ELECTRON CYCLOTRON RESONANCE MULTICHARGED ION PLASMA SOURCE

Technical domain and prior art

This invention relates to a photon source and more particularly a photon source comprising an electron cyclotron resonance (ECR) multicharged ion plasma source, more commonly called an ECR source. For example, one application of the photon source according to the invention is the production of EUV (Extreme Ultra-Violet) photons that can be used for lithography.

Different light sources are used for EUV lithography, for example such as laser plasmas (LPP) , synchrotron light, discharge sources (Z-pinch, hollow cathode, capillary source) . Depending on the case, these EUV sources have the following drawbacks: pulsed operation and inadequate power for some lasers; production of debris harmful to lenses (mirrors) ; high cost (lasers, synchrotron) ; severe pumping; mediocre reproducibility and life of the source. A new type of EUV light source has been proposed recently (see reference [I]). This is a light source based on de-excitation of multicharged ions. The divulged light source comprises an electron cyclotron resonance (ECR) multicharged ion plasma source. The plasma source includes a cavity in which a magnetic resonance field is distributed around a non-closed surface that intercepts the walls of the cavity. The light source produces photons with a wavelength of 13.5 nm from de-excitation of Xe¹⁰⁺ ions. Photons with this wavelength can advantageously be used to make etchings smaller than 65 nm (see abstract reference [I]) .

Compared with the light sources mentioned above, the use of an ECR source has many advantages:

- continuous and stable operation;

- no debris at output; - no wear (usage time very long due to the lack of a filament or cathode) ;

- low pressure (ICT⁵ - ICT⁴ mbars) to limit the size of pumps and vibrations, if any;

- low cost, if the magnetic structure is made of permanent magnets.

Despite the advantages mentioned above, one major problem of such a photon source that produces photons from an ECR source is its very low emission power. As will become clear below, the invention does not have this disadvantage.

Production of multicharged ions in an ECR source has been described in many patents and articles .

Reference [2] describes production of an ECR source made entirely of permanent magnets producing a strong flux of Xe¹⁰⁺ ions at extraction to create an ion beam.

Gas is ionised step by step by electron collision to obtain Xe^q+ type multicharged ions. We obtain:

Xe^¹⁺ + e^" → Xe^q+ + e^" + e^" It is found that the following conditions need to be set up in order to obtain Xe^q+ ions: a) microwave power at frequencies typically between 2.45 GHz and 50 GHz is injected into a vacuum cavity in a magnetic structure, producing ionisation of a gas or a metallic vapour, for example Xenon, that is also injected into the cavity at a pressure of between ICT⁵ mbars and ICT⁴ mbars, b) the Xe^q+ ion production term Rq is written:

where n_e is the electron density, n_q__! is the density of Xe^<crl)+ ions and <V_e σ_q-i,_q> is the product of the ionisation cross section due to electron collision, and the electron velocity taking account of their energy distribution, c) the energy of incident electrons must be sufficient to make the ionisation (an energy of 40OeV is optimal for obtaining Xe¹⁰⁺ ions (Xe⁹⁺ + e^~ → Xe¹⁰⁺ +e^~+e^~) ) , d) energy is transferred from the injected microwave power into electrons in the plasma at a location in the magnetic field B_res such that the electron cyclotron resonance condition is satisfied, in other words the angular frequency G0_HF of the high frequency wave is equal to the angular cyclotron frequency of the electron:

GOHF = OϋcE = q_e B_res / m_e

(for example for f= 10 GHz, B_res = 0.36 T and for f = 14 GHz, B_res=0.5T). The result is that a particular magnetic configuration is necessary to obtain given ions in an ECR source.

In this respect, French patents FR 2475798 and FR 2640411 also disclose that the resonance magnetic field is distributed around a closed surface inside the plasma chamber on which it has an approximately constant value B_res, to increase the magnetic confinement and consequently the life-time of ions and electrons. This closed surface, called the resonance surface, has no contact with the walls and is itself located inside a surface on which the magnetic field is equal to a value Bd equal to approximately twice the value B_res (Bd≥2xB_res) . This magnetic configuration with a closed resonance surface is created by bringing an axial magnetic mirror, for example produced by two solenoids or two rings of permanent magnets, adjacent to a multipole structure, for example a hexapole composed of six alternating pole magnets, the resulting magnetic field being the vector sum of the axial and radial contributions.

Presentation of the invention The invention relates to a photon source comprising an electron cyclotron resonance (ECR) multicharged ion plasma source. The photon source comprises :

- a cylindrical plasma vacuum chamber with axis AA, provided with an opening through which photons are extracted from the chamber, - at least one microwave injection guide to inject microwaves into the chamber,

- a gas injection device to inject gas into the chamber, - a pumping system to pump electrically neutral particles present in the chamber, and

- a cylindrical magnetic structure that surrounds the chamber and that produces a magnetic field inside the chamber, distributed on at least two closed surfaces in line along the AA axis, on which the value of the magnetic field is approximately equal to the value of the electron cyclotron resonance field, these surfaces being separated from each other, with no contact with the walls of the chamber and located inside an additional surface with no contact with the chamber, on which the magnetic field is equal to an approximately constant value greater than or equal to twice the value (B_RCE) of the electron cyclotron resonance

) .

According to another characteristic of the invention, the magnetic structure comprises two hollow magnetic cylinders with transverse magnetisation, a first hollow cylinder being magnetised in the direction opposite to the magnetisation of the second hollow cylinder, N hollow multipole magnetic cylinders where N is a number greater than or equal to two, and N-I hollow magnetic cylinders with longitudinal magnetisation, the two hollow magnetic cylinders with transverse magnetisation, the N hollow multipole magnetic cylinders and the N-I hollow magnetic cylinders with longitudinal magnetisation being in line along the AA axis of the chamber, the N hollow multipole magnetic cylinders and the N-I hollow magnetic cylinders with longitudinal magnetisation being placed between the two hollow cylinders with transverse magnetisation, a hollow multipole magnetic cylinder alternating with a hollow cylinder with longitudinal magnetisation, a first hollow multipole magnetic cylinder being adjacent to a first hollow magnetic cylinder with transverse magnetisation and a second hollow multipole magnetic cylinder being adjacent to the second hollow magnetic cylinder with transverse magnetisation . According to yet another characteristic of the invention, the N hollow multipole magnetic cylinders are Halbach type hexapoles.

According to yet another characteristic of the invention, the N hollow multipole magnetic cylinders are four-pole cylinders.

According to yet another characteristic of the invention, the N hollow multipole magnetic cylinders are six-pole cylinders.

According to yet another characteristic of the invention, the N hollow multipole magnetic cylinders are eight-pole cylinders .

According to yet another characteristic of the invention, the N hollow multipole magnetic cylinders are twelve-pole cylinders . According to yet another characteristic of the invention, an additional hollow magnetic cylinder with longitudinal magnetisation surrounds substantially all of the assembly composed of the N multipole hollow magnetic cylinders and the N-I hollow magnetic cylinders with longitudinal magnetisation.

According to yet another characteristic of the invention, the gas input into the chamber is Xenon at a pressure of between ICT⁵ mbars and ICT⁴ mbars . In an ECR multicharged ion plasma source, the maximum density of high energy electrons and multicharged ions is located on or close to the resonance surface (equimodulus surface B=B_res) . Therefore the location of the maximum emission of photons is also on or close to the resonance surface B_res .

According to the invention, the increase in the flux of photons along a particular axis is obtained by increasing the number of resonance surfaces along this axis. Unlike a conventional multicharged ion source that comprises a single resonance surface, the multicharged ion source according to the invention thus comprises a plurality of resonance surfaces approximately in line along a given axis and that very significantly increase the power emitted along this axis.

Within the scope of the invention, the resonance surfaces of the magnetic field may be surfaces that are not closed inside the chamber and that intercept the walls of the chamber or closed surfaces with no contact with the walls of the chamber. However, according to the preferred embodiment of the invention, the resonance surfaces of the magnetic field are closed surfaces with no contact with the walls, because the lives of ions and electrons are then very significantly extended.

The power obtained along the axis of the chamber is advantageously approximately proportional to the number of closed resonance surfaces present in the chamber.

Brief description of the drawings

Other characteristics and advantages of the invention will become clear after reading a preferred embodiment described with reference to the attached Figures, among which:

- Figure 1 shows an example of an EUV photon source according to the invention,

- Figure 2 shows a sectional view of a first ring of magnets that participates in the photon source shown in Figure 1;

- Figure 3 shows a sectional view of a second ring of magnets that participates in the photon source shown in Figure 1; - Figures 4a, 4b show examples of sectional views of hexapole magnetic structures that participate in photon source shown in Figure 1;

- Figure 5 shows a sectional view of a magnetic structure with longitudinal magnetisation that participates in the photon source shown in Figure 1; - Figure 6 shows an improvement to the photon source shown in Figure 1;

- Figure 7 shows a sectional view of a magnetic structure with longitudinal magnetisation that participates in the photon source according to the improvement shown in Figure 6.

The same marks denote the same elements on all figures .

Detailed description of embodiments of the invention

Figure 1 shows an example photon source according to the invention.

The photon source comprises an electron cyclotron resonance (ECR) multicharged ion plasma source. The electron cyclotron resonance multicharged ion plasma source comprises a cylindrical plasma vacuum chamber CH, a wave guide GD through which microwaves are injected, a device I for injection of a gas g, a pumping device P, and a magnetic structure 1, 2, 3a, 3b, 4. An opening 02 is formed in the chamber CH to extract photons. Preferably, the opening 02 is approximately in line along the axis AA of the chamber.

The microwave injection guide GD injects microwaves into the chamber CH. The guide GD is provided with an air/vacuum seal window (not shown on the Figure) . For example, the pressure at which the gas g is injected into the chamber CH through the device I may be between 10^~5 mbars and 10^~4 mbars. The pumping system P evacuates electrically neutral particles present in the chamber. For example, the pumping flow may be 300 litres per second.

The cylindrical magnetic structure 1, 2, 3a, 3b, 4 surrounds the chamber CH and produces a magnetic field inside the chamber distributed on a succession of closed surfaces S on which the value of the magnetic field is approximately equal to the value B_RCE of the ECR resonance field, these closed surfaces S being approximately in line along the axis AA of the chamber, independent of each other, with no contact with the walls of the chamber CH and located inside a closed surface Σ on which the value of the magnetic field is approximately constant, equal to or greater than twice the value B_RCE of the resonance

) .

The curve V shows the variation of the magnetic field along the AA axis, inside the chamber CH. This variation is apparently approximately sinusoidal, and its mathematical expression is then written:

B(z)# B_min+B_! (1+sin (z) ) (1), where - z is a variable that sets parameters for the displacement along the AA axis, and ~~ B_min<B_RCE ^<B_max=B_min+ (2 x Bi) ; Bl#0 The diameter of the plasma chamber CH must be relatively large, limited by the inside diameter of the magnetic structure that produces the closed surfaces S globally included in the equimodulus surface Σ. The chamber CH is a multimode circular cavity for electromagnetic waves. Consequently, if the chamber diameter is less than the cutoff wavelength λ_c of the fundamental propagation mode in the cavity (mode TEn) , no wave can propagate into the chamber .

The following table gives the value of cutoff wavelengths λ_c for four successive propagation modes, including the fundamental propagation mode TEn:

As a non-limitative example, in the case in which the chamber diameter is equal to 4.4 cm, the wave can thus propagate according to the TE₁₁, TM₀₁, TE₂₁ modes and be absorbed by resonance on the different surfaces S (there is no reflection of the wave at the entry to the cavity) . Strictly speaking, the propagation conditions mentioned above are satisfied for propagation in a vacuum or in air. The wave propagates differently in the presence of plasma. However, experience shows that if the condition according to which the diameter of the chamber is greater than the cutoff wavelength of the fundamental mode (λ_c (TE₁₁)) is satisfied, then electromagnetic waves will propagate in the chamber, and these waves will be absorbed by resonance, thus creating plasma. According to the preferred embodiment of the invention, the plasma chamber CH is a hollow cylinder, for example made of non-magnetic stainless steel, or aluminium or copper, with a double outer wall through which a cooling liquid can circulate. As a non-limitative example, the outside diameter of the chamber may be equal to 4.80 cm, the inside diameter 4.40 cm and the length 61 cm.

Microwaves injected into the chamber CH through the injection guide GD may for example be emitted by a variable frequency emitter centred on a frequency of 10 GHz, the frequency being adjusted by optimisation of the photon flux produced. The microwave power injected into the chamber may for example be equal to 1 kW. As a non-limitative example, the waveguide GD may inject microwaves into the chamber through an opening 01 on the opposite side of the opening 02 through which photons are extracted from the chamber. According to other embodiments of the invention, the microwaves may be introduced at different locations at the same time, for example at each closed resonance surface.

The cylindrical magnetic structure that surrounds the chamber CH may for example be composed of two hollow magnetic cylinders with transverse magnetisation 1 and 2, a set of hollow multipole magnetic cylinders 3a, 3b and a set of hollow magnetic cylinders with longitudinal magnetisation 4.

A magnetic cylinder with transverse magnetisation means a magnetic cylinder for which the magnetisation intersects the axis of the cylinder perpendicular to this axis. Figures 2 and 3 show such hollow cylinders . Figure 2 shows a hollow magnetic cylinder with transverse magnetisation for which the magnetisation ml is in the direction towards the axis of the cylinder while Figure 3 shows a hollow magnetic cylinder for which the magnetisation m2 is in the direction towards outwards .

Cylinders 1 and 2 are placed at the two ends of the chamber. Cylinder 1 may for example be placed on the side of an opening 01 through which microwaves are input into the chamber and cylinder 2 may be placed on the side of the opening 02 through which photons are extracted from the chamber. Advantageously, the magnetic field lines diverge from the AA axis at cylinders 1 and 2. The result is that the electrons and ions are deviated from the AA axis output from the photon source, so that the photon source can emit almost only photons. Cylinders 1 and 2 preferably have the same dimensions. For example, the length of cylinders 1 and 2 may be equal to 11 cm, the outside diameter (Dl, D2) 30 cm and the inside diameter (dl, d2) 5 cm.

A multipole magnetic cylinder means a cylinder with 2P alternating poles uniformly distributed around a circumference, P magnets with North-South orientation alternating regularly with P magnets with South-North orientation. For example, the multipole magnetic cylinders may have four poles

(P=2), six poles (P=3) , eight poles (P=4) or twelve poles (P=6) . Figures 4a and 4b show sectional views of six-pole cylinders. The six-pole cylinder 3a shown in Figure 4a is a magnetic hexapole that comprises six magnets uniformly distributed around a circumference, a

North/South oriented magnet alternating with a South/North oriented magnet.

The six-pole cylinder 3b shown in Figure 4b is a Halbach type hexapole that also comprises six magnets uniformly distributed around a circumference, a magnet with North/South orientation alternating with a magnet with South/North orientation, intermediate magnets being used to set up transition magnetisation between a magnet with North/South orientation and the magnet with South/North orientation following it or preceding it. A magnetic cylinder with longitudinal magnetisation means a magnetic cylinder for which the magnetisation is parallel to the axis of the cylinder at all points. Figure 5 shows the sectional view of a magnetic cylinder with longitudinal magnetisation 4 for which the magnetisation m3 is parallel to the axis of the cylinder.

As a non-limitative example, the magnetic structure of the photon source shown in Figure 1 comprises five hollow six-pole magnetic cylinders 3a, 3b and four hollow magnetic cylinders with longitudinal magnetisation 4 between the two hollow magnetic cylinders with transverse magnetisation 1, 2 located at the two ends of the chamber, the four hollow magnetic cylinders with longitudinal magnetisation 4 being intermediate between the five six-pole magnetic cylinders at uniform spacings . Cylinders 1 and 2 are each in contact with a Halbach or other type of magnetic hexapole.

As a non-limitative example, the dimensions of cylinders 3a, 3b and 4 are as follows: a) Cylinder 3a:

- outside diameter D3a = 15.8 cm,

- inside diameter d3a = 5 cm,

- length = 8 cm. b) Cylinder 3b: - outside diameter D3b = 15.8 cm,

- inside diameter d3b = 5 cm,

- length = 3 cm. c) Cylinder 4 :

- outside diameter D4 = 15.8 cm, - inside diameter d4 = 5 cm,

- length = 3 cm.

More generally, the magnetic structure of the photon source according to the invention comprises N hollow multipole magnetic cylinders between the two magnetic hollow cylinders with transverse magnetisation 1 and 2 where N is a number greater than or equal to 2, and N-I hollow magnetic cylinders with longitudinal magnetisation intermediate between the N hexapole hollow magnetic cylinders at uniform spacings . The number of closed surfaces S is then equal to N. Advantageously, the larger the number N, the larger the number of photons that will be collected along the axis AA of the chamber CH.

For example, the material from which the magnetic cylinders are made may be an Iron-Neodymium- Boron alloy. Also as a non-limitative example, the value of the remanent field Br is equal to approximately 1.33 Tesla and the coercivity at 2O⁰C is equal to approximately 1360 kA/m.

Figure 6 shows an improvement to the photon source shown in Figure 1. In addition to the elements shown in Figure 1, the magnetic structure of the photon source comprises an additional hollow magnetic cylinder 5 with longitudinal magnetisation that approximately surrounds the set of magnetic cylinders 3a, 3b and 4. Figure 7 shows a sectional view of a hollow magnetic cylinder 5 with longitudinal magnetisation m4. Attachment means are then provided to fix the cylinder 5 on at least one of the magnetic cylinders 3a, 3b or 4. Thus, the additional cylinder 5 may comprise projections on its inside face so that it can be fixed to at least one hollow magnetic cylinder 6 with longitudinal magnetisation, for which the outside diameter is then small (for example outside diameter 12 cm instead of 15.8 cm as mentioned above) . The additional cylinder 5 and one of the cylinders 4 may also form a unique single piece structure. The cylinder 5 can advantageously increase the intensity of the axial field at the centre of the chamber. As a non- limitative example, the dimensions of the additional hollow longitudinal magnetisation cylinder 5 may be as follows :

- outside diameter D5 = 28 cm,

- inside diameter d5 = 16 cm, - length = 26 cm. As a non-limitative example, the magnetic characteristics obtained within the scope of the invention for a photon source conforming with Figure 6 are given below: - five elementary closed resonance surfaces S are present in the chamber CH, on which the magnetic field B_res is equal to approximately 0.36T, an equimodulus surface Σ on which the field is equal to approximately 0.72T (2x0.36T) surrounding the five elementary closed surfaces S,

- the expression of the magnetic field B(z) along the AA axis of the chamber CH is written as follows

(see the previous equation (1) ) :

B(z) = B_min + Bl (l+sin(z)), where B_min= 0.31T, B_res= 0.36T, B_max= 0.6T, B₁=O.145 T

- the intensity of the maximum magnetic field obtained at the ends of the chamber is equal to approximately 0.81T.

REFERENCES

[1] "Permanent magnet ECR source for generation of EUV light", S.K.Hatho et al . , Lithography Int. Sematech, Santa Clara, CA-USA, Source Workshop, February 22 2004.

[2] "An all-permanent magnet ECR ion source for the ORNL MIRF upgrade project", D. Hitz et al . , 16 International Workshop on ECR Ion Sources ECRIS'04; September 26-30 2004 ; Berkeley, USA.

Claims

1 Photon source comprising an electron cyclotron resonance (ECR) multicharged ion plasma source, characterised in that it comprises:

- a cylindrical plasma vacuum chamber (CH) , with axis AA, provided with an opening (02) through which photons are extracted from the chamber,

- at least one microwave injection guide (GD) to inject microwaves into the chamber (CH) ,

- a gas injection device (I) to inject gas (g) into the chamber (CH) ,

- a pumping system (P) to pump electrically neutral particles present in the chamber, and - a cylindrical magnetic structure (1, 2, 3a, 3b, 4) that surrounds the chamber (CH) and that produces a magnetic field inside the chamber, distributed on at least two closed surfaces (S) in line along the AA axis, on which the value of the magnetic field is approximately equal to the value (B_RCE) of the electron cyclotron resonance (ECR) field, these surfaces (S) being separated from each other, with no contact with the walls of the chamber (CH) and located inside an additional surface (Σ) with no contact with the chamber (CH) , on which the magnetic field is equal to an approximately constant value greater than or equal to twice the value (B_RCE) of the electron cyclotron resonance field ( \B\ >2 \B_RCE\ ) . 2. Photon source according to claim 1, in which the magnetic structure (1, 2, 3a, 3b, 4) comprises two hollow magnetic cylinders (1, 2) with transverse magnetisation, the magnetisation (ml) of a first hollow cylinder (1) being in the direction opposite to the magnetisation (m2) of the second hollow cylinder (2), N hollow multipole magnetic cylinders (3a, 3b) , N being a number greater than or equal to 2 and N-I hollow magnetic cylinders with longitudinal magnetisation (4), the two hollow magnetic cylinders with transverse magnetisation (1,

2), the N hollow multipole magnetic cylinders (3a, 3b) and the N-I hollow magnetic cylinders with longitudinal magnetisation (4) being in line along the AA axis of the chamber, the N hollow multipole magnetic cylinders (3a, 3b) and the N-I hollow magnetic cylinders with longitudinal magnetisation

(4) being placed between the two hollow cylinders with transverse magnetisation, a hollow multipole magnetic cylinder alternating with a hollow cylinder with longitudinal magnetisation, a first hollow multipole magnetic cylinder being adjacent to a first hollow magnetic cylinder with transverse magnetisation and a second hollow multipole magnetic cylinder being adjacent to the second hollow magnetic cylinder with transverse magnetisation.

3. Photon source according to claim 2, in which the N hollow multipole magnetic cylinders are Halbach type hexapoles (3b) .

4. Photon source according to claim 2 in which the N hollow multipole magnetic cylinders are four-pole cylinders .

5. Photon source according to claim 2 in which the N hollow multipole magnetic cylinders are six-pole cylinders.

6. Photon source according to claim 2 in which the N hollow multipole magnetic cylinders are eight-pole cylinders .

7. Photon source according to claim 2 in which the N hollow multipole magnetic cylinders are twelve-pole cylinders.

8. Photon source according to claim 2, in which an additional hollow magnetic cylinder with longitudinal magnetisation (5) surrounds substantially all of the assembly composed of the N multipole hollow magnetic cylinders (3a, 3b) and the N-I hollow magnetic cylinders with longitudinal magnetisation (4).

9. Photon source according to claim 1 in which the gas input into the chamber (CH) is Xenon at a pressure of between 10^~5 mbars and 10^~4 mbars .