DE102011007828A1 - Extreme UV lithography system for use in vacuum housing for bringing perfection to imaging characteristics of projection system, has filling gases and beryllium ions provided in optical path of radiation and including optical element effect - Google Patents

Abstract

The system (1) has an extreme UV (EUV) light source (5) e.g. plasma source, producing a pulsed, EUV radiation (6a). A monochromator (8), reflective optical elements (9, 10, 13, 14) and a photomask (11) are arranged in a radiation forming system (2), an illumination system (3) and a projection system (4). Filling gases (15) e.g. molecular hydrogen, and beryllium ions are provided in an optical path (6) of the EUV radiation in the systems. The filling gas include an effect of an optical element e.g. lens. An atom trap (16) and a penning trap non-contactly spatially localize the filling gases.

Description

Background of the invention

The invention relates to an EUV lithography system with an EUV light source and at least one vacuum chamber in which at least one optical element is arranged.

The central task of lithography equipment or lithography objective development is to perfect the imaging properties of a projection system while maximizing the energy yield. This requires a perfect interplay of light source, lighting, mask design, optics design, layer design and mechanical construction.

Remaining static and operational dynamic residual errors are eliminated, for example, with mechanical manipulators, interchangeable lens elements or controllable facet mirrors. However, these established correction methods are also associated with great constructive effort and have their limitations.

Object of the invention

The object of the invention is to provide an EUV lithography system in which alternative methods for influencing the propagation of EUV radiation and in particular alternative correction methods are used.

Subject of the invention

This object is achieved by an EUV lithography system of the aforementioned type, in which a filling gas located in the beam path of the EUV light source has the effect of an optical element.

It is known to embed EUV lithography equipment in vacuum housings and to clean the mirror surfaces of the EUV mirror z. B. to inject molecular / atomic hydrogen as a filling gas, which is to reduce carbonaceous impurities on the surfaces. So far, the space between the optical elements (EUV mirrors) and the filling gases present therein but no optical effect is assigned. Filling gases are used only for cleaning purposes of the optical surfaces.

With the motto "There's plenty of room at the bottom," Richard Feynman launched nanotechnology in 1959. In this sense, there is " much optical path between the mirrors", which can be elicited by manipulating the optical properties of the highly diluted gas optical effect.

For example, it is known that gaseous nitrogen present in plasma light sources for EUV lithography equipment is excited by the EUV radiation and fluoresces uniformly in all directions. Therefore, the EUV light beam propagating through the gaseous nitrogen is absorbed, scattered, phase-delayed in the forward direction and changes its geometric optical path as in a "gradient index" (GRIN) medium.

The basic parameters of a laser-pumped mercury plasma radiation source are on the input side z. Nominally 100 W and behind the reticle (mask) 16 W, using EUV radiation at a wavelength of 13.5 nm. This time averaged power is pulsed at a repetition rate of 50-100 kHz. The pulse duration is between 20 and 2000 ns (FWHM - "Full Width at Half Maximum"). However, not only time-averaged power is relevant for optical transitions in atoms, but also the temporal field strength course. The residual gas pressure in an EUV lithography system is currently about 0.1 mbar. The gas equation (p V = N k _B T) shows that this corresponds to a density of n = 2.4 10 ¹⁵ 1 / cm ³ , which is compatible with the effects described below.

In one embodiment, the EUV lithography system includes a trap for non-contact spatial location of the inflation gas, e.g. B. by electromagnetic forces or by laser radiation.

The technology to capture, hold and cool non-contact neutral and ionized atomic ensembles in vacuum chambers with electromagnetic forces from a background gas is today a well-developed technique that has been widely used for many years. These ideas were of such fundamental importance that they were honored with, among others, three Nobel Prizes in physics:
1989 for the development of ion traps: N. Ramsey, H. Dehmelt, H. Paul. "For the separated oscillatory field method and its use in the hydrogen maser and other atomic clocks / for the development of the iontrap technique". Nobel Price for physics. 1989 ,
1997 for the capture and cooling of the kinetic motion of atoms with light: S. Chu, C. Cohen-Tannudji, B. Phillips. "For development of methods to cool and trap atoms with laser light". Nobel Price for physics. 1997 , such as
2001 for the Bose-Einstein condensation of gases: E. Cornell, C. Wiemann, W. Ketterle. "For the achievement of the Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates ". Nobel Prize in Physics, 2001 ,

Various possibilities are shown below to enable a spatial localization of gaseous objects, to place them in the beam path at suitable locations, if necessary to hold them without contact by means of electromagnetic forces, and, if necessary, to cool their mechanical movement.

In one embodiment, the trap is formed as an atomic trap or as an ion trap. With an ion trap, single ions trapped, ionic plasmas or ionic Wigner crystals can be generated. In an atomic trap, trapped neutral, atomic ensembles can be spatially localized in localized electromagnetic, laser-generated traps (MOT magneto-optical trap, optical dipole trap) or optical, periodic grids (standing laser waves), and in this way z. B. Bose-Einstein condensates are generated.

In one embodiment, the filling gas serves to refract the EUV radiation and in particular has the optical effect of a lens or a waveguide. The imaging optical properties of passive, linear, gaseous media are from everyday life z. In the form of streaks or mirages and can also be used in connection with highly diluted gases in vacuum chambers. The refraction of EUV radiation at the filling gas can also be used to realize waveguides or beam splitters. It goes without saying that by deliberately changing the spatial distribution of the filling gas (for example by temporally varying the electromagnetic fields used for localization), an active lens effect can also be achieved, ie. H. The imaging properties of the lens can change over time and z. B. be adapted to avoid aberrations or reduce.

In one embodiment, the filling gas serves to diffract the EUV radiation and has the optical effect of a grating. This can be achieved by using an ion plasma, e.g. In the form of beryllium ions, in an ion trap, e.g. B. in a Penning trap, is cooled so far that the ion plasma passes into a crystalline phase and forms a Wigner crystal.

If one places such a trap in the intermediate image of a projection lens, then the overlap with the pulsed EUV radiation is maximal.

In a further embodiment, the filling gas serves to scatter the EUV radiation and has the optical effect of a diffuser. Complementary to the imaging property of an inhomogeneous gas is on the illumination side, i. H. before the mask, the incoherent scattering effect of localized gases relevant. In contrast to the refractive index of solids i. A. have a spectrally broad, structureless dispersion and absorption behavior, are found in gases many narrow, well-isolated resonance lines. Depending on the bandwidth of the EUV pulse, these can be excited resonantly in the area of Doppler, burst or power broadening and in turn emit spontaneously, incoherently in all directions of the dipole lobe. This property can be used to use the gas as a diffuser lens to improve the homogeneity of the illumination field and pupil illumination. It goes without saying that a strongly diluted gas, which in particular fills the relevant installation space homogeneously, can also be introduced into the vacuum chamber in order to serve as an absorber for the EUV radiation.

The above-described filling gases, which have an optical effect, can thus be used, on the one hand, to achieve incoherent, more uniform illumination in the field and pupil in front of the mask, and on the other hand to make coherent small wavefront corrections behind the mask in the projection objective.

In a further embodiment, the EUV lithography system has a field generator for generating (possibly time-variable) electromagnetic fields. The field generator can serve for the spatial localization of the gas atoms or gas ions. In addition to the localization of the gaseous matter of course, the field generator can also serve to control or influence the optical properties of the localized gaseous matter by additional electromagnetic fields. These may be, for example, modifications of the linear dispersion relation, non-linear dispersive or active amplifying effects.

Further features and advantages of the invention will become apparent from the following description of embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be realized individually for themselves or for several in any combination in a variant of the invention.

drawing

Embodiments are illustrated in the schematic drawing and will be explained in the following description. It shows:

1 a schematic representation of an EUV lithography system with a filling gas with optical effect,

2 a schematic representation of a Penning trap for the EUV lithography of 1 , and

3 a schematic representation of a gaseous diffuser lens for incoherent illumination of a mask.

In 1 is schematically an EUV lithography system 1 shown which from a beam-forming system 2 , a lighting system 3 and a projection system 4 housed in separate vacuum housings or vacuum chambers and sequentially in one of an EUV light source 5 of the beam-forming system 2 outgoing beam path 6 that of the EUV light source 5 generated, pulsed EUV radiation 6a are arranged. The vacuum chambers 2 . 3 . 4 is associated with a (not shown) vacuum pump, which is for generating a residual gas pressure of z. B. 0.1 mbar is used.

As an EUV light source 5 For example, a plasma source can be used. The radiation emerging from this in the wavelength range between about 5 nm and about 20 nm is first in a collimator 7 bundled. With the help of a subsequent monochromator 8th is filtered out by varying the angle of incidence, as indicated by a double arrow, the desired operating wavelength. In the aforementioned wavelength range are the collimator 7 and the monochromator 8th Usually designed as reflective optical elements, wherein at least the monochromator 8th does not have a multi-layer system on its optical surface in order to reflect the broadest possible wavelength range.

The in the beam-forming system 2 radiation treated in terms of wavelength and spatial distribution is introduced into the lighting system 3 introduced, which a first and second reflective optical element 9 . 10 having. The two reflective optical elements 9 . 10 direct the radiation onto a photomask 11 as another reflective optical element having a structure formed by the projection system 4 on a smaller scale on a wafer 12 is shown. These are in the projection system 4 a third and fourth reflective optical element 13 . 14 intended. The reflective optical elements 9 . 10 . 11 . 12 . 13 . 14 each have an optical surface 9a . 10a . 11a . 12a . 13a . 14a on, in the beam path 6 the EUV lithography system 1 is arranged.

There are various possibilities today, such as X-rays or EUV radiation 6a at a wavelength of about 13.5 nm. Basically, this wavelength corresponds to a photon energy with (1 / 2π) h ω = 91.8 eV and is a multiple of the Rydberg energy of 13.61 eV, which indicates the ionization threshold of atomic hydrogen. Particularly prominent are large-scale research facilities, such as the free-electron laser FLASH in Hamburg [1], which has been in operation for around two years and provides coherent radiation pulses. This z. B. currently the ionization paths of many gases, such. B. xenon [2] examined at the highest possible power densities. There are also other incoherent sources of synchrotron radiation at accelerator rings or, as in the case of EBIT at Lawrence Livermore National Laboratory [3], who study X-ray emission from plasmas.

In addition to these large-scale research facilities, which achieve the maximum achievable achievements with the greatest effort, there are also "table-top" experiments today that generate EUV radiation with relatively little effort [4]. This is done through "higher harmonics generation" using pulsed optical laser radiation and generates coherent soft and hard X-rays of moderate power [5].

Use of filling gases with optical effect

The above and possibly further sources can be used in the EUV lithography system 1 be used simultaneously to the plasma light source 5 the gaseous medium in the vacuum chambers 2 . 3 . 4 or specifically introduced there to inject filling gases and this u. U. also to use quantum interference effects.

Furthermore, the dispersions produced in gaseous media are highly controllable. Therefore, one can use a partial beam of EUV radiation 6a the EUV light source 5 , use the above-mentioned alternative or additional EUV light sources or optical multiphoton transitions to capture the trapped gases in the projection system 4 simultaneously with the EUV exposure beam 6a to drive. This leads by means of quantum interference to overshoots, active amplification or complete extinction of the refractive indices. In this way, the propagation velocity of a visible light pulse in a gaseous medium was delayed in time and even reduced to a complete stop (down to 0 m / s): "stopping light" [6], [7].

The following describes how neutral or ionized atomic ensembles of filling gases in the vacuum chambers 2 . 3 . 4 Captured, held and cooled by electromagnetic energy from a background gas can be to this in the beam path 6 to be placed in suitable places. It is understood that for supplying the filling gases into the vacuum chambers 2 . 3 . 4 Gas supply lines (not shown) and - if necessary - suitable gas discharge lines can be provided. The non-contact filling gases can, for. As passive or active lenses, periodic grids or diffusers are used, as described in more detail below.

1.) Gas lenses and waveguides

The imaging optical properties of passive, linear, gaseous media are well known. In connection with strongly diluted gases in vacuum chambers 2 . 3 . 4 Effects such as "lensing", "wave guiding" or "optical lattices" have recently been intensively investigated, cf. z. B. [8]. For example, Ref. [8] describes the focusing effect of a highly diluted "cigar-shaped" gas on a laser steel consisting of ⁸⁷ Rb with a density of n = 10 ¹² 1 / (cm ³ ). Such a "cigar-shaped" filling gas 16 is in 1 in the projection system 4 in the beam path 6 the EUV light source 5 shown. To produce an ultracold medium (as in [8]) becomes an atom trap 16 used in the form of a magneto-optical trap "magneto-optic trap" MOT, which (not shown) ferromagnets for generating magnetic fields, which has a strong radial confinement of the filling gas 15 allow at the same time (not shown) laser cooling takes place. Due to the special structure, an optically amplifying effect is observed in [8], which leads to the filling gas 15 or the atomic cloud can serve as a convex or concave lens depending on the application.

In the same experiment [8], the waveguide properties of the quasilinear gaseous structure (radially enclosed filling gas 15 ). There it was shown that at (in 1 not shown) different entrance angles in serving as a waveguide filling gas 15 the proportion of coupled into the waveguide radiation can be changed. In particular, the waveguide can be used as a beam splitter due to this property.

The serving as a lens "cigar-shaped" filling gas 15 in the projection system 4 can z. B. serve to coherently mount small wavefront corrections.

2.) Self-organizing ionic lattices in ion traps

Both single highly charged ions [9] and gaseous cold plasmas offer themselves as optical elements for the EUV lithography system 1 at. This is to be understood on the one hand by the internal electronic structure of multiply charged ions, on the other hand by the possibility of transforming cold plasmas into crystalline phases (Wigner crystal).

When all but the last (outermost) shell electron is ruptured from a neutral gas atom, an "alkaline" ion which has an ionization threshold of> 13.6 eV, e.g. Xe ⁶⁺ with about 93 eV [2] or rubidium-like tungsten W ³⁷⁺ [10]. Such an ion again has well-defined spectral lines that can be selectively used in the wavelength range around 13.5 nm.

The ion trap technology knows many different configurations: Paul, Penning or Kingdon traps, which have advantages after application. In 2 you can see a cut through a Penning trap 18 for storing a plasma of beryllium ions 17 , The Penning Trap 18 includes a field generator consisting of a cylindrical coil 19 for generating a magnetic field B and an electrode arrangement 20a -C with a ring electrode 20a as well as with an upper and lower end cap 20b , c, which are at the same potential and which generate an electric field E, which is the beryllium ions 17 trapped in the axial direction. In addition, two crossed laser beams are used 21a . 21b Laser cooling of beryllium ions 17 ,

The Penning Trap 18 generally allows a very good optical access. When placing such a trap in the intermediate image of a projection system, the overlap with the EUV radiation or individual EUV pulses becomes maximum.

If an ultra-cold plasma is generated by the laser cooling, a phase transition of the beryllium ions 17 take place in a solid phase as soon as the Coulomb repulsion exceeds the thermal energy. In the solid phase forms the beryllium plasma 17 a defect-free Wiegner crystal, which can assume different crystalline forms (see [11]). The Beryllium ions forming the Wiegner crystal 17 Thus, a grid or a
diffractive structure for the EUV radiation 6a , z. In the projection system 4 from 1 , form.

The optical properties of the gaseous matter can also be controlled by additional (external) electromagnetic fields. In this way, modification of the linear dispersion relation, non-linear dispersive or active amplifying effects can be achieved.

3.) Optical lattices for neutral atoms

With the help of standing optical laser waves one can generate periodic, defect-free, optical potential fields in 1, 2 or 3 space dimensions. Polarisable atoms can be loaded into these conservative force fields and now represent a matter-wave lattice for light, which in turn has the optical effect of an optical lattice.

4.) Diffuser lenses

Complementary to the imaging property of an inhomogeneous gas is on the illumination side, ie in the illumination system 3 in front of the mask 11 the incoherent scattering effect of an inhomogeneous gas is relevant. In contrast to the refractive index of solids i. A. have a spectrally broad, structureless dispersion and absorption behavior, are found in gases many narrow, well-isolated resonance lines. Depending on the bandwidth of the EUV pulse, these can be excited resonantly in the range of Doppler, burst or power broadening and in turn emit spontaneously, incoherently in all directions of the dipole lobe. This property can be exploited to create an in 3 illustrated trap arrangement 22 to form a gaseous diffuser lens. By optical light forces emitted by an auxiliary laser (optical trap laser beams) 21a . 21b as well as auxiliary lenses 24a . 24b can be generated, you can fill a gas 23 locate in the intensity center of a Gaussian ray. If now EUV radiation 6a penetrates these trapped atoms, so are due to stimulated and spontaneous scattering processes (see. 3 ) Homogenized field and pupil areas of the geometric beam space and thus improves the homogeneity of the illumination of the illumination field and pupil.

In summary, in the manner described above, the beam propagation of EUV exposure radiation 6a through the controllable, spatial localization of the gases 15 . 17 . 23 with the help of traps 16 . 18 . 22 be formed and thereby the optical effect of optical elements such as lenses and waveguides, gratings and diffusers are generated.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• N. Ramsey, H. Dehmelt, H. Paul. "For the separated oscillatory field method and its use in the hydrogen maser and other atomic clocks / for the development of the iontrap technique". Nobel Price for physics. 1989 [0011]
• S. Chu, C. Cohen-Tannudji, B. Phillips. "For development of methods to cool and trap atoms with laser light". Nobel Price for physics. 1997 [0011]
• E. Cornell, C. Wiemann, W. Ketterle. "For the achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates". Nobel Prize in Physics, 2001 [0011]

Claims (7)

EUV lithography system ( 1 ), comprising: an EUV light source ( 5 ) for generating EUV radiation ( 6a ), and at least one vacuum chamber ( 2 . 3 . 4 ), in which at least one optical element ( 8th . 9 . 10 . 11 . 13 . 14 ) is arranged, characterized by a in the beam path ( 6 ) of EUV radiation ( 6a ) in the vacuum chamber ( 2 . 3 . 4 ) filling gas ( 15 . 17 . 23 ) having the effect of an optical element. EUV lithography apparatus according to claim 1, further comprising: a trap ( 16 . 18 . 22 ) for non-contact, spatial localization of the filling gas ( 15 . 17 . 23 ). EUV lithography system according to claim 2, wherein the trap is an atomic trap ( 16 . 22 ) or as an ion trap ( 18 ) is trained. EUV lithography system according to one of the preceding claims, in which the filling gas ( 15 ) for refraction of the EUV radiation ( 6a ) and has the optical effect of a lens or a waveguide. EUV lithography system according to one of the preceding claims, in which the filling gas ( 17 ) for diffracting the EUV radiation ( 6a ) and has the optical effect of a grid. EUV lithography system according to one of the preceding claims, in which the filling gas ( 23 ) on the scattering of EUV radiation ( 6a ) and has the optical effect of a diffuser. EUV lithography apparatus according to one of the preceding claims, further comprising: a field generator ( 19 . 20a -C) for the generation of electromagnetic fields.

Images