Compact light source for metrology applications in the EUV range

Abstract

A compact light source based on electron beam accelerator technology includes a storage ring, a booster ring, a linear accelerator and an undulator for providing light having the characteristics for actinic mask inspection at 13.5 nm. The booster ring and the storage ring are located at different levels in a concentric top view arrangement in order to keep the required floor space small and to reduce interference effects. Quasi-continuous injection by enhanced top-up injection leads to high intensity stability and combats lifetime reductions due to elastic beam gas scattering and Touschek scattering. Injection into the storage ring and extraction from the booster ring are performed diagonal in the plane which is defined by the parallel straight section orbits of the booster ring and the storage ring. For the top-up injection from the booster ring into the storage ring two antisymmetrically arranged Lambertson septa are used.

Claims

1. A compact light source based on electron beam accelerator technology, the compact light source comprising: a storage ring being a compact multi-bend magnet structure configured to generate a small emittance leading to high brilliance and large coherent content of the light; a booster ring disposed at a different level from said storage ring in a concentric top view arrangement in order to keep a required floor space small and to reduce interference effects; a linear accelerator and an undulator for providing light having the characteristics for actinic mask inspection at 13.5 nm; and two antisymmetrically arranged Lambertson septa for a top-up injection from said booster ring into said storage ring; wherein an intensity of an electron beam is maintained down to a level of 10.sup.3 and wherein quasi-continuous injection, respectively enhanced top-up injection is implemented to reach a high intensity stability and to combat lifetime reductions due to elastic beam gas scattering and Touschek scattering; wherein injection into said storage ring and extraction from said booster ring are effected diagonally in a plane defined by parallel straight section orbits of said booster ring and said storage ring.

2. The compact light source according to claim 1, wherein said booster ring and said storage ring are concentrically arranged with small lateral displacement to facilitate a beam transfer and larger vertical displacement to reduce interference effects.

3. The compact light source according to claim 1, which comprises a multipole kicker for an enhanced top-up injection into said storage ring to avoid a gap in a ring filling, in order to reduce a bunch current and to achieve a required high intensity and position stability.

4. The compact light source according to claim 1, wherein: said storage ring, said booster ring and said linear accelerator are disposed in a 3-dimensional arrangement within a footprint of approximately 50 m.sup.2 in total and forming a racetrack design with two long straight sections; said storage ring and said booster ring having multi-functional magnets and wherein a compact dispersion suppressing beam transfer from said booster ring to said storage ring is effected with two antisymmetrically arranged Lambertson septa, and by performing the injection into said storage ring by a single nonlinear kicker only.

5. The compact light source according to claim 1, wherein: a) said storage ring is disposed to receive accelerated electrons from said booster ring via enhanced top-up injection, keeping a beam intensity stable to a level of 10.sup.3 and combatting lifetime reductions caused by the low energy storage ring combined with said low gap undulator, wherein an electron energy of the electron beam in said storage ring ranges from 200 to 500 MeV and a current of the electron beam ranges from a lower value to 200 mA; b) said booster ring is configured for enhanced top-up injection receiving the accelerated electrons via an injection pathway from said linear accelerator; c) said booster and storage rings are concentrically arranged, with only a slight lateral displacement in order to facilitate the beam transfer and a large vertical displacement in order to minimize an interference effect of the cycling booster on the electron beam in said storage ring and enabling an extremely compact source without compromising a beam stability and machine reliability; d) said low gap undulator is integrated in the storage ring, said undulator having an undulator period of 8 to 24 mm and a length of a large multiple of the undulator period.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) Preferred embodiments of the present invention are hereinafter described with reference to the attached drawings which depict in:

(2) FIG. 1 as an example the variation of the beam current as a function of the electron energy for an undulator with 200 periods of 16 mm length;

(3) FIG. 2 the related magnetic field for the same range of electron energy;

(4) FIG. 3 schematically the baseline design of a compact light source for providing light having the characteristics for actinic mask inspection; and

(5) FIG. 4 3D-integration view of the compact light according to FIG. 3.

DESCRIPTION OF THE INVENTION

(6) For a better understanding of the technical background, the photon beam requirements for actinic mask inspection with CDI are explained first.

(7) A verification of the principle of mask inspection using CDI has been performed at the XIL-II beamline at the SLS (Swiss Light Source at Paul Scherrer Institute, 5232 Villigen PSI, Switzerland). The photon beam requirements for an actinic mask inspection tool based on CDI are collected in Tab. 1. It has to be noted that these values are rough estimations. A more precise estimate of the requirements needs a conceptual design of the complete system with its optics, measurement methods, reconstruction algorithms and detector specifications. Moreover, a very likely scenario is that a single source serves multiple tools simultaneously. Currently, the best option could be to use a single undulator and distribute the beam with beam splitters.

(8) TABLE-US-00001 TABLE 1 Photon beam requirements for actinic mask inspection with CDI on the mask level Parameter Unit Value Wavelength nm 13.5 Central cone power mW >10 Brightness kW/mm.sup.2/sr >10 Beam stability 10.sup.3 Spot size m 10-100 Bandwidth (temporal coherence) % 2-0.1

(9) Based on the requirements for actinic mask inspection with CDI at a wavelength of 13.5 nm a first optimization of the source parametersundulator and compact storage ring were performed. The calculations are based on the flux requirement of 1.310.sup.15 photons per second in 0.1% bandwidth.

(10) The relevant relations for the compact light source are:

(11) $\begin{array}{cc}=\frac{{}_u}{2{}^2}\left(1+\frac{K^2}{2}\right)& \left(1\right)\\ =\frac{E\left[\mathrm{MeV}\right]}{0.511}& \left(2\right)\\ \frac{\stackrel{.}{N}}{0.1\%\mathrm{BW}}=n_0=1.43{\mathrm{.10}}^{11}{N}_{u}I\left[\mathrm{mA}\right]\frac{K^2}{1+K^2\text{/}2}& \left(3\right)\\ K=0.934.{}_u\left[\mathrm{cm}\right]B_u\left[T\right]& \left(4\right)\end{array}$
wherein stands for the wavelength of the emitted light; .sub.u is the period length of the undulator, is the Lorentz factor as defined by (2), n.sub.0 is the number of photons per second in 0.1% of the bandwidth as defined by (3) and K is the undulator parameter as defined by (4). N.sub.u stands for the number of undulator periods, while I is the current of the electron beam.

(12) FIG. 1 shows the variation of the beam current as a function of the electron energy if conditions (1) and (3) are fulfilled, for an undulator period length .sub.u of 16 mm, which has been chosen as conservative value. If K approaches 0, the beam current I goes to infinity in order to fulfill condition (1). But at a rather modest distance from this pole a reasonable current can be reached. For the considerations here the energy was chosen as 430 MeV. There is not much gain in current reduction above this energy limit.

(13) FIG. 2 shows the related magnetic field B for the same range of electron energy (as in FIG. 1).

(14) In conclusion: For the development of the source concept, an undulator period length of 16 mm has been chosen. All the other parameters are a consequence of this choice. The energy of the compact storage ring results in 430 MeV and the undulator field in 0.42 T.

(15) There are some technical limits for undulators with short period lengths and high fields. An undulator period length of 16 mm is at the limit for what can be conventionally reached today. An even shorter period length would have the advantage of lower beam energy as it is evident from equation (1) but requires on the other hand higher undulator field strengths to achieve a reasonable large K parameter (4). And if the K parameter is too low, higher beam currents are needed to reach the required flux defined by equation (3).

(16) Cryo undulators would allow even shorter period lengths combined with higher fields but they add a complexity which would affect the reliability and are therefore not considered here.

(17) The required number of photons can be reached with 150 mA beam current. This is sufficiently low in order to avoid harmful collective effects. In conclusion, the energy of 430 MeV is reasonably small to allow a compact storage ring. The field of 0.42 T for the undulator is well within the actual standards. The K value is 0.63 and consequently small enough to not enhance the higher harmonics.

(18) The selected parameters of the undulator and the electron beam are summarized in Tab.2.

(19) TABLE-US-00002 TABLE 2 Undulator and electron beam parameters Resonance wavelength [nm] 13.5 Undulator length [cm] 320 Undulator period length [mm] 16 Undulator magnetic field [T] 0.42 K-value 0.63 Energy [MeV] 430 Beam current [mA] 150

(20) CDI methods ask for a high intensity stability of the electron beam which makes top-up injection mandatory. An enhanced top-up injection or quasi-continuous injection becomes necessary in order to combat lifetime reductions due to elastic beam-gas scattering and Touschek scattering. Both are strongly enhanced by the low storage ring energy combined with the small undulator gap.

(21) FIG. 3 schematically shows schematically a top-view on a compact light source 2 for providing light having the characteristics for actinic mask inspection at 13.5 nm. Of course, by adapting the design of the specific components the emitted light can have other dominant wavelengths. The compact light source 2 comprises a storage ring SR, a concentric booster synchrotron BO and a linear pre-accelerator LI. In FIG. 3 also included is a schematic side view of a booster extraction scheme 4 and a storage ring injection scheme 6 with two antisymmetrically arranged Lambertson septa YEX, YIN. YEX marks an extraction septum, YIN an injection septum, KEX represents an extraction kicker and KIN a nonlinear injection kicker. FIG. 4 schematically shows a 3D-view of the compact light source 2 with the storage ring SR, the booster synchrotron BO and the linear pre-accelerator LI with transfer lines TL, an undulator UN and acceleration cavities CY.

(22) The design of the booster synchroton BO follows the racetrack shape of the storage ring SR. Since the required floor space should be minimum, the booster synchroton BO as shown in FIG. 3 and FIG. 4 is placed concentrically below the storage ring SR with minimum lateral spacing in order to facilitate the beam transfer and large vertical spacing in order to maximize the separation between the booster synchroton BO and the storage ring SR. This will alleviate the electromagnetic disturbances of the cycling booster synchroton BO on the electron beam in the storage ring SR.

(23) The tilted extraction and injection systems 4, 6 are built up by two antisymmetrically arranged Lambertson septa YEX, YIN that are connecting the two straight sections of the booster synchroton BO and the storage ring SR. The electron beam is horizontally displaced in both septa YEX, YIN and gets deflected vertically. From the storage ring injection septum YIN it is guided with a small slope to the multipole injection kicker KIN where it is captured inside the storage ring acceptance.

(24) The innovative features of this compact light source 2 presented above, especially the combination of all of them, have never been applied to a compact low energy storage ring based light source. For the solution presented here, all intrinsic problems of such a complex system have been solved.

(25) For the undulator UN, permanent magnet material Dy enhanced NdFeB was selected which provides a remanent field of B.sub.r=1.25 T. With an enhanced materialcompared to the U15 undulator at the SLS (block height from 16.5 to 26.5 mm and pole width from 20 to 30 mm)a field of B=0.47 T can be reached with 8.5 mm gap and B=0.42 T with 9 mm. Tab. 3 below summarizes the major beam parameters, the source parameters and the light characteristics.

(26) TABLE-US-00003 TABLE 3 Beam parameters, source parameters and light characteristics of COSAMI (Compact EUV Source for Actinic Mask Inspection) for actinic mask inspection. Beam parameters: Beam energy MeV 430 Beam current mA 150 Horizontal emittance.sup.+) nm 9.2 Emittance coupling 0.01 U-optics parameters: .sub.x/.sub.y m/m 0.43/1.17 .sub.x/.sub.x m/rad 79.1/116.4 .sub.y/.sub.y m/rad 8.3/11.2 Source parameters: U-length m 3.2 Period length mm 16.0 Number of periods N.sub.u 200 Peak field T 0.42 K-value 0.624 Light characteristics: Resonance wavelength nm 13.5 Diffractive emittance nm 1.07 Diffractive beam sizes: .sub.r/.sub.r m/urad 23.4/45.9 Central cone power mW 103.1 Flux ph/s/0.1% BW 1.28 10.sup.15 Brilliance ph/s/mm.sup.2/mrad.sup.2/0.1% BW 2.64 10.sup.18 Coherent Brilliance ph/s/mm.sup.2/mrad.sup.2/0.1% BW 2.82 10.sup.19 Coherent fraction % 9.4 .sup.+)Intra-Beam-Scattering blow up included

REFERENCES

(27) [1] A. Wrulich et al, Feasibility Study for COSAMIa Compact EUV Source for Actinic Mask Inspection with coherent diffraction imaging methods [2] A. Streun, OPA, http://ados.web.psi.ch/opa/ [3] A. Streun: COSAMI lattices: ring, booster and transfer line, Internal note, PSI Jun. 28, 2016.