WO2007014662A1

WO2007014662A1 - Optical system for creating a line focus scanning system using such optical system and method for laser processing of a substrate

Info

Publication number: WO2007014662A1
Application number: PCT/EP2006/007233
Authority: WO
Inventors: Alois Herkommer; Holger Muenz; Holger Kierey; Michel Le Maire; Bernhard Weigl
Original assignee: Carl Zeiss Laser Optics Gmbh
Priority date: 2005-08-02
Filing date: 2006-07-22
Publication date: 2007-02-08
Also published as: KR20080039449A; JP2009503593A

Abstract

An optical system for creating a line focus on a surface of a substrate from an input light beam, comprises a light source emitting the input light beam propagating in a propagation direction, the input light beam having an extension in a first dimension transverse to the propagation direction and an extension in a second dimension transverse to the first dimension and to the propagation direction, at least one beam expanding optical element for expanding the input light beam in the first dimension, at least one focusing optical element which is curved in the second dimension so that the focusing optical element focuses the input light beam in the second dimension to the line focus on the surface, and at least one correcting optical element for flattening the line focus such that the line focus is straight and lies in a flat focus plane on the surface over at least approximately the full length of the line focus.

Description

OPTICAL SYSTEM FOR CREATING A LINE FOCUS.

SCANNING SYSTEM USING SUCH OPTICAL SYSTEM.

AND METHOD FOR LASER PROCESSING OF A SUBSTRATE

BACKGROUND OF THE INVENTION

The invention relates to an optical system for creating a line focus of a light beam on a substrate.

The invention further relates to a scanning system for producing a scanning beam focus on a substrate.

Still further, the present invention relates to a method for laser processing of a substrate. The present invention is useful, for example, in annealing of large substrates, in the field of laser induced crystallisation of substrates, in the field of flat panel display or organic light emitting diode (OLED) display manufacturing processes.

The term "line focus" used in the present description and in the claims is understood to be a beam shape which has a large aspect ratio between the two lateral dimensions of the beam orthogonal to the propagation direction of the beam. Such an aspect ratio can be, in particular, larger than 100.

Optical systems according to the prior art, for example as disclosed in US 5,721,416, which create a line focus or a line beam, as it is, for example, used in silicon annealing of large substrates, use refractive optical systems comprising cylindrical lenses as focusing optical elements.

In current optical systems, the typical ratio between the length of the line focus which is, for example, 200 mm, and the width of the line focus which is, for example, 1 mm, is of the order of 100-200. For some applications it is, however, desirable to have a very thin (<0.05 mm) and long (>300 mm) line focus, such that the ratio between the length and the width of the line focus is increased to 600- 10,000.

However, as the width of the line focus is reduced, aberrations of the refractive cylindrical system, especially the so-called bow-tie error, are observed, which need to be compensated. In the following, these shortcomings will be explained in more detail with respect to an optical system for creating a line focus according to the prior art.

Figures 1 through 3 show an optical system for creating a line focus of a light beam on the surface of a substrate according to the prior art. Laser sources usually deliver an input light beam 10 of a diameter D with a more or less quadratic or round cross-section. In order to create a long and thin line beam or line focus from the input light beam 10, the optical system is required to expand the light beam 10 in one dimension (called below x-direction) transverse to the direction of propagation of the light beam 10, and to focus the input light beam 10 in a second dimension (called below y-direction) transverse to the direction of propagation (called below z-direction) as well transverse to the first dimension, as illustrated in Figures 1 through 3. Fig. 1 shows the light beam 10 in the yz-plane, Fig. 2 in the xz-plane and Fig. 3 is a perspective view of the light beam 10. An optical system as shown in Figures 1 through 3 is, for example, described in US 5,721,416 mentioned above.

Focusing is carried out in the yz-plane, while the beam expanding is performed in the xz-plane.

Focusing of the input light beam 10 is usually done by a cylindrical lens 12. The minimum achievable line width W is related to the f-number f# of the lens 12. The minimum (diffraction limited) line width W is given by the product W = 2 β • λ where λ is the operation wave length of the input light beam 10. A small /#<20 is, therefore, required for very thin line foci. The focal length f_x in x-direction of the lens 12 follows from the input beam diameter D to be f_x = D • /#. For example, an input beam 10 of a diameter D = 20 mm requires a focal length of about f_x<400 mm. The distance between the lens 12 and the substrate onto which the line focus is to be formed, is, therefore, typically limited to small distances.

The expansion of the input light beam 10 in the x-direction as shown in Fig. 2 is usually created by a beam diverging optical element or beam expanding element 14. For example, the expanding element 14 can be a negative cylindrical lens, an array of cylindrical lenses, a diffractive optical element, or a one-dimensional dif- fuser. Instead of expanding the full input beam, the beam can also be deflected by a fast steering mirror (e.g. a galvo-mirror) such that the line focus can be scanned as a function of time. However, in time average, the scanning mirror would provide the same function as the element 14.

All these afore-mentioned elements are easier to manufacture and introduce less aberrations, if the introduced angles ω (cf. Fig. 2) are small (of the order of a few degrees). In order to nevertheless achieve a long line focus F with a length L of multiple times the input beam diameter D, the distance Δz between the expanding element 14 and the substrate S is typically large, since L = D + 2 » Δz « tan (ω). As a consequence, the expanding or diverging element 14 is located in front of the focusing element 12 when seen in the direction of propagation of the light beam 10 (positive z-direction).

This, in turn, implies that the light rays at the edge of the input light beam 10 in x-direction, i.e. the margin rays 16 and 18 (cf. Fig. 3) are incident on the focusing element 12 under a non-zero angle ω (cf. Fig. 3) in the x-direction. If a cylindrical lens, like lens 12, however, is used under an incident angle ω orthogonal to the focusing direction which is the y-direction, i.e. angle θ in Figure 3, the back focal length, which is related to the resulting refractive angle θ' behind the lens 12, will be dependent on ω. This is due to the non-linearity in the sin function, which governs the law of refraction:

n • sin (i)=n' • sin (i')_> where i, respectively V₁ is the total incidence angle and thus a combination of ω and θ, respectively ω' and θ.

As a consequence, the plane in which the input light beam 10 focuses will not be flat, but curved. This is not desired, since the substrate S typically is flat. In particular, the sharp focus will be closer to the lens 12 for the margin rays 16, 18 of the input light beam 10 than the focus for the center rays. In other words, the input light beam 10 will be out of focus at the edges of the beam on the substrate S in x- direction. This situation is depicted in Fig. 5. The generated aberration is usually called a bow-tie error, since the beam footprint which is shown in Figure 4 on the substrate S has the shape of a bow-tie.

Therefore, there is a need for an optical system for creating a line focus which overcomes the afore-mentioned drawbacks, which in particular does not exhibit the bow-tie error.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical system for creating a line focus from an input light beam on a surface of a substrate which is as free of aberrations as possible, which, in particular, does not exhibit the bow-tie error.

It is another object of the present invention to provide a scanning system for producing a scanning beam focus on a surface of a substrate, which achieves the object mentioned before.

It is still another object of the present invention to provide a method for laser processing a substrate with a line beam or a line focus which is as free as possible of aberrations.

These and other objects are achieved according to a first aspect of the present invention by an optical system for creating a line focus on a surface of a substrate from an input light beam, comprising a light source emitting said input light beam propagating in a propagation direction, said input light beam having an extension in a first dimension transverse to the propagation direction and an extension in a second dimension transverse to said first dimension and to said propagation direction, at least one beam expanding optical element for expanding said input light beam in said first dimension, at least one focusing optical element which is curved in said second dimension so that said focusing optical element focuses said input light beam in said second dimension to said line focus on said surface, and at least one correcting optical element for flattening said line focus such that said line focus is straight and lies in a flat focus plane on said surface over at least approximately the full length of said line focus.

According to another aspect of the present invention, a scanning system for producing a scanning beam focus on a substrate is provided, comprising a light source emitting an input light beam propagating in a propagation direction, said input light beam having an extension in a first dimension transverse to the propagation direction and an extension in a second dimension transverse to said first dimension and to said propagation direction, at least one beam expanding optical element for expanding said input light beam in said first dimension, at least one focusing optical element which is curved in said second dimension so that said focusing optical element focuses said input light beam in said second dimension to said line focus on said flat surface, and at least one correcting optical element for flattening said line focus such that said line focus is straight and lies in a flat focus plane on said surface over at least approximately the full length of said line focus.

According to still another aspect of the present invention, a method for laser processing of a substrate is provided, using a light source emitting an input light beam propagating in a propagation direction, said input light beam having an extension in a first dimension transverse to the propagation direction and an extension in a second dimension transverse to said first dimension and to said propagation direction, at least one beam expanding optical element for expanding said input light beam in said first dimension, at least one focusing optical element which is curved in said second dimension so that said focusing optical element focuses said input light beam in said second dimension to said line focus on said flat surface, and at least one correcting optical element for flattening said line focus such that said line focus is straight and lies in a flat focus plane on said surface over at least approximately the full length of said line focus.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are shown in the drawings and will be described hereinafter in more detail with reference to the drawings. In the drawings:

Fig. 1 is a plan view in the yz-plane of an optical system for creating a line focus according to the prior art;

Fig. 2 is a plan view in the xz-plane of the optical system of Fig. 1;

Fig. 3 is a perspective view of the optical system of Figures 1 and 2;

Fig. 4 is a beam footprint of the line focus on a surface of a substrate as created by the optical system of Figures 1 through 3;

Fig. 5 is a graph showing the focus line in the xz-plane as created by the optical system in Figures 1 through 3;

Fig. 6 is a plan view in the xz-plane of an optical system according to a first embodiment of the present invention;

Fig. 7 is a beam footprint of the line focus on a surface of a substrate obtained by the optical system of Fig. 6; Fig. 8 is a graph in the xz-plane of the line focus created by the optical system in Fig. 6;

Fig. 9 is a plan view in the xz-plane of an optical system according to another embodiment of the present invention;

Fig. 10 is a perspective view of the optical system in Fig. 9;

Fig. 11 is a plan view in the yz-plane of another embodiment of an optical system for illustrating another aspect of the present invention;

Fig. 12 is a graph showing the line focus in the xz-plane obtainable by the optical system of Fig. 11;

Fig. 13 is a plan view in the yz-plane of an optical system according to another embodiment of the present invention which is improved with respect to the optical system of Fig. 11; and

Fig. 14 is a plan view in the xz-plane of the optical system in Fig. 13.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, preferred embodiments of an optical system for creating a line focus of a light beam on a substrate are described.

An optical system for creating a line focus on a surface of a substrate from an input light beam according to the present invention generally comprises a light source which emits the input light beam. The input light beam propagates in a propagation direction, wherein the input light beam has an extension in a first dimension transverse to the propagation direction and an extension in a second dimension transverse to the first dimension and to the propagation direction. At least one beam expanding optical element is provided for expanding the input light beam in the first dimension.

Further, at least one focusing optical element which is curved in the second dimension is provided so that the focusing optical element focuses the input light beam in the second dimension to the line focus or line beam on the surface of the substrate.

The embodiments of an optical system according to the present invention, which will be described hereinafter in more detail, have in common that at least one correcting optical element is provided in order to flatten the line focus such that the line focus is straight and lies in a flat focus plane on the surface of the substrate over at least approximately the full length of the line focus.

In a first type of embodiments, the at least one correcting optical element is positioned between the at least one focusing element and the surface of the substrate and has a negative power in order to compensate the aberrations in the line focus which are caused by the oblique incidence of the input light beam onto the focusing element. Preferably, the at least one correcting optical element is the last optical element in front of the surface of the substrate seen in the direction of propagation of the input light beam in this case. It is to be understood that a correcting optical "element" can principally be only a surface, in particular the light exit surface of the focusing lens.

The correcting optical element can have the negative power in the first dimension or the second dimension or in both, the first dimension and the second dimension if the need arises.

In another type of embodiments, the at least one correcting optical element is a collimating optical element which is positioned in front of the at least one focusing optical element and collimates the input light beam such that the input light beam is incident on the at least one focusing optical element at an at least approximately right angle. As illustrated above with reference to Figures 1 through 5, the bow-tie error is caused by an oblique incident of the input light beam onto and through the focusing optical element. By collimating the input light beam before the input light beam is incident on the focusing optical element, such oblique incidence is avoided, and thus aberrations like the bow-tie error are avoided, too.

In this case, the at least one correcting optical element can be designed to col- limate the input light beam in the first dimension or the second dimension or, preferably, in the first and the second dimension, if the need arises so that the input light beam is incident at right angles in the first dimension as well as in the second dimension.

In further preferred embodiments to be described hereinafter, the optical system can be designed to create an intermediate focus between the substrate and the light source. The reason for preferably providing an intermediate focus in such an optical system is that the input light beam emitted by the light source often is not collimated, but convergent or divergent. The input light beam is often not diffraction limited, but may have some intrinsic beam divergence. Focusing such a non- diffraction limited beam will not result in the minimum (diffraction limited) line width as given by the product W = 2 • β • λ, but will be larger due to the limited beam quality. It is therefore desirable to focus the beam at an intermediate position and to use a mask (aperture) which is arranged in the region of the intermediate focus for creating a sharp edge of the input light beam in at least one of the first dimension and the second dimension.

Such an aperture (mask) can have a slit of rectangular shape, for example.

In an optical system, creating an intermediate focus in the second dimension, the input light beam will be obliquely incident onto the focusing optical element which focuses the input light beam onto the substrate in the second dimension leading again to aberrations in form of a bow-tie error.

Hence, in order to solve this problem, the further focusing optical element, which produces the intermediate focus, is assigned a further collimating optical element arranged behind the intermediate focus (seen in the direction of propagation of the input light beam) for collimating the input light beam in the second dimension. Thus, the input light beam is incident on the focusing optical element which focuses the input light beam onto the surface of the substrate, at at least approximately right angles also in the second dimension thus avoiding oblique incidences and aberrations caused by such oblique incidences.

Preferably, the further focusing optical element which produces the intermediate focus and the further collimating optical element can be arranged to form an afocal telescope, for example in form of an anamorphic Kepler telescope. "Afocal" means that the focal length of the further focusing optical element and the further collimating optical element are different from each other which difference can be used to form a one-dimensional beam expander or compressor and therefore may at the same time be used to expand or compress the input laser beam in order to adapt the input light beam size to the required size for the optical system.

In all the types of embodiments described before, the at least one beam expanding optical element can be a fast steering mirror, e.g. a galvo-mirror, or, also preferably, a negative cylindrical lens, an array of cylindrical lenses, a diffractive element, or a one-dimensional diffuser.

Further, the at least one focusing optical element can be a cylindrical lens, and the further focusing optical element mentioned before can also be a cylindrical lens. The optical system according to the present invention can use exclusively refractive elements, or a combination of refractive and reflective elements. The refractive elements can comprise calcium fluoride (CaFa).

Further, the input light beam can comprise light of a wave length which is chosen such that the absorption coefficient of the substrate to be treated with the line focus, for this wavelength is high.

The substrate itself can comprise a semiconductor film, or can comprise amorphous silicon, for example.

The light source used in the optical systems as described before preferably is a high power laser providing optical power of more than 50 w. Preferably, the light source is an excimer laser.

In those cases described before, where correcting the bow-tie error is based on the principal to have the input light beam incident on the focusing optical element which focuses the input light beam onto the surface of the substrate at right angles, the line focus is, as a further advantage of the optical system of the present invention, telecentric on the surface of the substrate as it is often desired. "Telecentric" means that the light is incident on the surface of the substrate at right angles, too.

As far as the term "line focus" has been used above or will be used in the following, generally a beam shape is mentioned which has large aspect ratio between the two lateral dimensions of the light beam transverse to the propagation direction of the light beam. In particular, the afore-mentioned aspect ratio is larger than 100.

As far as the term "cylindrical optical element" is used in the following description, this term is used for simplicity and includes all forms of optical elements (lenses and mirrors) which have some curved surface which is not necessarily spherical, in one dimension transverse to the propagation direction of the light beam, and no or substantially no curvature in the other dimension which is transverse to the first dimension and to the propagation direction of the light beam. The term "cylindrical optical element" in principle includes also optical elements (lenses and mirrors) which have an aspherical curvature. In the present invention, particular shapes of the curvature of the "cylindrical optical element" are parabolic, elliptical, or aspherical shapes, which can be described by a polynomial or a conic surface.

Now referring to Fig. 6, an optical system 20 for creating a line focus F of an input light beam 22 on a surface S of a substrate is shown.

The input light beam 22 is emitted by a light source 24 which emits the input light beam 22 in a propagation direction which is the z-direction in Fig. 6. The input light beam 22 has an extension in the first dimension which is the x-direction in Fig. 6 and which is transverse to the propagation direction of the light beam 22, and an extension in a second dimension which is the y-direction in Fig. 6, and which is transverse to the first dimension and to the propagation direction of the input light beam 22. The shape of the cross section of the input light beam 22 as emitted by the light source 24 can be quadratic, but could also have a different shape, for example a circular, oval, rectangular or any other shape.

The optical system 20 further comprises a beam expanding optical element 26 for expanding the input light beam 22 in the first dimension, i.e. the x-direction. The beam expanding optical element 26 is, for example, a negative cylindrical lens.

The optical system 20 further comprises a focusing optical element 28, which focuses the input light beam 22 after the expansion thereof to a line focus F on the surface S of the substrate. The focusing optical element 28 is, for example, a cylindrical lens with positive power in the Y-direction.

As can be seen in Fig. 6, the input light beam 22 after expansion thereof is incident on the focusing optical element 28 at right angles only in the center portion 30 of the input light beam 22, while the edges or margin rays 32, 34 of the input light beam 22 are obliquely incident on the focusing optical element 28 with increasing inclination from the center portion 30 to the margin rays 32, 34. The oblique incidence of the input light beam 22 in the region of its edges normally gives rise to the beam footprint as shown in Fig. 4 which is the result of the curvature of the focal plane or the line focus F as shown in Fig. 5. In order to compensate for the curvature of the line focus F, a correcting optical element 36 is positioned between the surface S of the substrate and the focusing optical element 28 and which has a negative power in order to straighten the focus line F on the surface S of the substrate as shown in Fig. 7 and 8. In Fig. 8, the x-axis has been chosen to be incident with the surface S of the substrate.

Figures 9 and 10 show another embodiment of an optical system 40 in accordance with the present invention.

Similarly to the embodiment shown in Fig. 6, the optical system 40 comprises a light source 44 emitting an input light beam 42 propagating in the z-direction, a beam expanding optical element 46 expanding the input light beam 42 in the x- direction, and a focusing optical element 48 focusing the expanded input light beam 42 in the y-direction.

The optical system 40 further comprises a correcting optical element 50 configured as a collimating optical element which is arranged in front of the focusing optical element 48 when seen in the direction of the propagation of the input light beam 42. The collimating optical element 50 collimates the expanded input light beam 42 such that the input light beam 42 is incident on the focusing optical element 48 at right angles, over the full extension of the input light beam in x- direction. In this simple embodiment, the collimating optical element 50 collimates the input light beam 42, thus, in the x-direction. The angle ω in Fig. 2 thus is zero, and Snell's Law of refraction no longer depends on ω. The line focus F on the surface S of the substrate, thus, is straight and lies in one plane, so that the beam footprint of the line focus F is the same as shown in Fig. 7.

A further advantage of the optical system 40 is, that the input light beam 42 after being focused is incident on the surface S of the substrate at right angles, too, thus rendering the optical system 40 a telecentric optical system.

The line focus F is a uniform thin line over the full length of the line focus as shown in Figures 7 and 8.

Fig. 11 shows another optical system 60. The optical system 60 comprises a light source 64 emitting an input light beam 62, a beam expanding optical element 66 expanding the input light beam 62 in the x-direction, a focusing optical element 68 focusing the input light beam 62 in the y-direction onto a surface S of a substrate, and a correcting optical element 70 in form of a collimating optical element, which collimates the input light beam 62 after expansion in x-direction thereof in x- direction.

The optical system 60 further comprises a further focusing element 72 which focuses the incoming input light beam 62 in y-direction, in order to create an intermediate focus Fi in an intermediate focus plane. A mask 74 with an aperture 76 is arranged in or nearby the intermediate focus Fi in order to create a sharp edge or a sharp line of the line focus F, as already described above. The aperture 76 can be, for example, configured as a slit of rectangular shape, thus creating a sharp edge of the input light beam 62 in the x-direction and in the y-direction.

The further focusing element 72 focuses the input light beam 62 in the y- direction, as shown in Fig. 11. Due to the intermediate focus Fi, the input light beam 62 has oblique incidence angles in the y-direction (in Fig. 3 called θ) as the input light beam 62 passes the beam expanding optical element 66 and also as the input light beam 62 passes the collimating optical element 70. Therefore, the plane where the input light beam 62 focuses on the surface S of the substrate, will again be curved and not flat, as desired, as depicted in Fig. 12. The effect of line focus bending will not be as important as the effect of an oblique incidence of the input light beam in x-direction as in the cases of Figures 1 through 3, but it will be large enough to be considered as a disturbance of the line focus F.

Another optical system 80 which solves the problems arising with respect to the optical system 60 in Fig. 11 is shown in Figures 13 and 14.

The optical system 80 comprises a light source which emits an input light beam 82, a beam expanding optical element 86 for expanding the input light beam 82 in x-direction, a focusing optical element 88 for focusing the expanded input light beam 82 onto a surface S of a substrate, a collimating optical element 90 for collimating the expanded input light beam 82 in x-direction, a further focusing optical element 92 for creating an intermediate focus Fi in an intermediate focus plane, a mask 94 having an aperture 96 arranged in the intermediate focus Fi.

The further focusing optical element 92 is assigned a further collimating optical element 98 which is arranged behind the intermediate focus Fi when seen in the direction of propagation of the input light beam 82, and before the beam expanding optical element 86. The further collimating optical element 98 collimates the input light beam 82 in y-direction thus avoiding that the input light beam 82 is incident on the focusing optical element 88 at a non-zero angle in y-direction. Thus, the optical system 80 is designed such that for all cylindrical optical elements 86 and 88 the input light beam 82 enters these elements 86 and 88 without non-zero angles in x- and y-direction, i.e. orthogonal to the focusing direction of the respective element 86 and 88, in other words, the input light beam 82 is always collimated in the direction orthogonal to the powered (focused) direction.

The optical system 80 is free of a bow-tie aberration and exhibits a line focus F as shown in Figures 7 and 8.

The further focusing optical element 82 and the further collimating optical element 98 together form an anamorphic Kepler type afocal telescope. If the focal length of the elements 92 and 98 is chosen different from each other, this part of the optical system 80 forms a one-dimensional afocal beam expander or compressor and therefore may at the same time be used to expand or compress the incoming input light beam 82 in order to adapt the out-coming input light beam 82 size to the required size for the optical system 80.

The optical systems 20, 40, 80 are or can be part of or themselves scanning systems for producing a scanning beam focus on a substrate. "Scanning" preferably means that the line focus is scanned or swept over the substrate in a direction perpendicular to the line focus by mechanically moving the substrate relative to the line focus or by optical means for moving the line focus over the substrate.

Such a scanning system is preferably used in a variety of methods for laser processing a substrate, for example, and preferably for laser annealing of an amorphous silicon or semiconductor film, for laser induced crystallization of semiconductor films, for flat panel or OLED, display manufacturing, or for any other kind of laser material processing.

According to the application of the optical system as described hereinbefore, the wave length of the light emitted by the light source is chosen such that the absorption coefficient of the substrate for this wave length is high. In particular, the laser sources 24, 44, 84 can be a high power excimer laser or other high power light source.

Claims

1. An optical system for creating a line focus on a surface of a substrate from an input light beam, comprising

a light source emitting said input light beam propagating in a propagation direction, said input light beam having an extension in a first dimension transverse to the propagation direction and an extension in a second dimension transverse to said first dimension and to said propagation direction,

at least one beam expanding optical element for expanding said input light beam in said first dimension,

at least one focusing optical element which is curved in said second dimension so that said focusing optical element focuses said input light beam in said second dimension to said line focus on said surface, and

at least one correcting optical element for flattening said line focus such that said line focus is straight and lies in a flat focus plane on said surface over at least approximately the full length of said line focus.

2. The optical system of claim 1, wherein said at least one correcting optical element is positioned between said at least one focusing element and said surface and has a negative power.

3. The optical system of claim 2, wherein said at least one correcting optical element has said negative power in at least one of said first dimension and said second dimension.

4. The optical system of claim 1, wherein said at least one correcting optical element is a collimating optical element positioned in front of said at least one focusing optical element and collimating said input light beam in at least one of said first dimension and said second dimension such that said input light beam is incident on said at least one focusing optical element at an at least approximately right angle in at least one of said first dimension and said second dimension.

5. The optical system of claim 1, further comprising at least one further focusing optical element arranged between said at least one focusing optical element and said light source for creating an intermediate focus in at least one of said first dimension and said second dimension.

6. The optical system of claim 5, wherein an aperture is arranged in the region of said intermediate focus for creating a sharp edge of said input light beam in at least one of said first dimension and said second dimension.

7. The optical system of claim 6, wherein said aperture has a slit of rectangular shape.

8. The optical system of claim 5, wherein said further focusing optical element is assigned a further collimating optical element arranged behind said intermediate focus for collimating said input light beam in said second dimension.

9. The optical system of claim 8, wherein said further focusing optical element and said further collimating optical element are arranged to form an afocal telescope.

10. The optical system of claim 8, wherein said further collimating optical element is arranged before said at least one beam expanding optical element.

11. The optical system of claim 1, wherein said at least one beam expanding optical element is a fast steering mirror, e.g. a galvo-mirror.

12. The optical system of claim 1, wherein said at least one beam expanding element is chosen from the group comprising at least one negative cylindrical lens, an array of cylindrical lenses, at least one diffractive element, a one- dimensional diffuser.

13. The optical system of claim 1, wherein the beam expanding element also homogenizes the beam.

14. The optical system of claim 1, wherein said at least one focusing optical element is a cylindrical lens.

15. The optical system of claim 5, wherein said at least one further focusing optical element is a cylindrical lens.

16. The optical system of claim 1, wherein at least one of said at least one beam expanding optical element, said at least one focusing optical element and said at least one correcting optical element comprises calcium fluoride.

17. The optical system of claim 1, wherein said input light beam comprises light of a wavelength which is chosen such that the absorption coefficient of said substrate for said wavelength is high.

18. The optical system of claim 1, wherein said substrate comprises a semiconductor film.

19. The optical system of claim 1, wherein said substrate comprises amorphous silicon.

20. The optical system of claim 1, wherein said light source is a high power laser providing optical power of more than 50 W.

21. The optical system of claim 1, wherein said light source is an excimer laser.

22. The optical system of claim 1, wherein said line focus is telecentric on said surface of said substrate.

23. A scanning system for producing a scanning beam focus on a substrate, comprising

a light source emitting an input light beam propagating in a propagation direction, said input light beam having an extension in a first dimension transverse to the propagation direction and an extension in a second dimension transverse to said first dimension and to said propagation direction,

at least one focusing optical element which is curved in said second dimension so that said focusing optical element focuses said input light beam in said second dimension to said line focus on said flat surface, and

24. The scanning system of claim 23, wherein said at least one correcting optical element is positioned between said at least one focusing element and said surface and has a negative power.

25. The scanning system of claim 24, wherein said at least one correcting optical element has said negative power in at least one of said first dimension and said second dimension.

26. The scanning system of claim 23, wherein said at least one correcting optical element is a collimating optical element positioned in front of said at least one focusing optical element and collimating said input light beam in at least one of said first dimension and said second dimension such that said input light beam is incident on said at least one focusing optical element at an at least approximately right angle in at least one of said first dimension and said second dimension.

27. The scanning system of claim 23, further comprising at least one further focusing optical element arranged between said at least one focusing optical element and said light source for creating an intermediate focus in said second dimension.

28. The scanning system of claim 27, wherein an aperture is arranged in the region of said intermediate focus for creating a sharp edge of said input light beam in at least one of said first dimension and said second dimension.

29. The scanning system of claim 28, wherein said aperture has a slit of rectangular shape.

30. The scanning system of claim 27, wherein said further focusing optical element is assigned a further collimating optical element arranged behind said intermediate focus for collimating said input light beam in said second dimension.

31. The scanning system of claim 30, wherein said further focusing optical element and said further collimating optical element are arranged to form an afocal telescope.

32. The optical system of claim 30, wherein said further collimating optical element is arranged before said at least one beam expanding optical element.

33. The scanning system of claim 23, further comprising a means for scanning the line focus over the substrate.

34. A method for laser processing of a substrate, using a scanning system for creating a scanning beam focus on a surface of said substrate, said scanning system comprising

35. The method of claim 34, further comprising scanning said line focus over said substrate.

36. The method of claim 35, further comprising mechanically scanning said line focus over said substrate.

37. The method of claim 35, further comprising optically scanning said line focus over said substrate.

38. The method of claim 34, wherein said substrate is a silicon substrate.

39. The method of claim 34, wherein said substrate is a semiconductor film.

40. The method of claim 34, wherein said laser processing comprises annealing said substrate.

41. The method of claim 34, wherein said laser processing comprises laser induced crystallization of said substrate.

42. The method of claim 34, wherein said laser processing is carried out in a flat panel display manufacturing process.

43. The method of claim 34, wherein said laser processing is carried out in an organic LED display manufacturing process.