WO2006066687A1

WO2006066687A1 - Optical system for creating a line focus, a scanning system for producing a scanning beam focus and a method for laser processing of a substrate

Info

Publication number: WO2006066687A1
Application number: PCT/EP2005/012658
Authority: WO
Inventors: Alois Herkommer; Holger MÜNZ
Original assignee: Carl Zeiss Laser Optics Gmbh
Priority date: 2004-12-22
Filing date: 2005-11-26
Publication date: 2006-06-29
Also published as: TW200641394A

Abstract

An optical system for producing a line focus of an input light beam comprises a light source emitting the input light beam propagating in a propagation direction, the 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, and at least a first mirror which is curved in the second dimension so that the light beam is focused in the second dimension to the line focus.

Description

OPTICAL SYSTEM FOR CREATING A LINE FOCUS , A SCANNING SYSTEM FOR

PRODUCING A SCANNING BEAM FOCUS AND A 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.

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

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 crystallization of substrates , in the field of flat panel display or organic light emitting diode ( OLED ) display manufacturing processes . Optical systems according to the prior art , which create a line focus or 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, limit the width of the line focus or need to be compensated by complicated optical surfaces or complicated optical systems . Moreover, as the optical power through the system is increased, for example beyond 100 W, refractive systems exhibit disturbing thermal optical effects , for example a change of the refractive index of the focusing element and of the shape with increasing temperature .

With reference to figs . 1 through 3 which show an optical system for creating a line focus of a light beam according to the prior art, the afore-mentioned situation is explained in more detail .

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 from the light beam 10 , the optical system is required to expand the light beam 10 in one dimension ( below called x-direction) , and to focus the beam in the orthogonal direction ( below called y- direction ) as illustrated in Figures 1 through 3 where Figure 1 shows the light beam 10 in the yz-plane, Figure 2 in the xz- plane and Figure 3 is a perspective view of the light beam. An optical system as shown in Figures 1 through 3 is , for example , described in US 5 , 721 , 416.

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

Focusing of the 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 = 2f# • λ where λ is the operation wavelength of the input light beam 10. A small f# < 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# = D • f# . 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 irradiated, is , therefore , typically limited.

The expansion of the light beam 10 in the x-direction as shown in Figure 2 is usually created by a diverging optical element 14. For example , the expanding element 14 can be a negative cylindrical lens , an array of cylindrical lens , a diffractive optical element , or a 1-dimensional diffuser . 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 . Figure 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 in the positive z-direction .

This , in turn , implies that the light rays 16 at the edge of the light beam 10 , i . e . the margin rays 16 and 18 ( cf . Figure 3 ) are incident on the focusing element 12 under a non-zero angle ω ( cf . Figure 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 ' after 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 ( ω + θ ) = n ' ^■ sin ( ω ' + θ ' ) As a consequence , the plane in which the beam 10 focuses will not be flat, but curved . This is not desired, since the substrate S typically is flat . Therefore, the beam 10 will be out of focus at the edges of the beam on the substrate S . This 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 .

Another general disadvantage of refractive optical systems for creating a line focus is that the choice of transparent materials is usually limited, especially at wavelengths < 350 nm. The available materials usually have a finite expansion coefficient and a finite thermal change of the index of refraction . Both effects can lead to image degradation as the optics is heated up by high optical powers . Another aspect is that the materials need to be stable under these high optical powers .

Therefore , there is still the need for an optical system for creating a line focus which overcomes the afore-mentioned drawbacks .

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 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 an optical system for creating a line focus from an input light beam which can be operated at wavelengths in a wavelength range which is chosen such that the absorption coefficient of the substrate material on which the line focus is to be irradiated is high .

It is another object of the present invention to provide an optical system for creating a line focus from an input light beam which can be operated at high optical powers .

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

It is still another object of the present invention to provide a method for laser processing a substrate with a light beam focused to 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 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 a first mirror which is curved in said second dimension so that said mirror focusing said input light beam in said second dimension to said line focus on said substrate . According to another aspect of the present invention , an optical system for creating a line focus from an input light beam is provided, wherein said line focus is telecentric , 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 a first mirror which is curved in said second dimension so that said input light beam is focused in said second dimension to said line focus ,

at least one second mirror which is curved in said first dimension for obtaining telecentricity in said line focus .

According to still another aspect of the present invention, an optical system for creating a line focus from an input light beam is provided, wherein said line focus is telecentric and said line focus is straight, 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 a first mirror which is curved in said second dimension so that said input light beam is focused in said second dimension to said line focus on said substrate,

at least one second mirror which is curved in said first dimension for obtaining telecentricity in said line focus ,

at least one third mirror which is curved in said first dimension for compensating a line bending of said line focus introduced by said second mirror .

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 :

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

Figure 2 is a plane view in the xz-plane of the optical system of Figure 1 ;

Figure 3 is a perspective view of the optical system of Figure 1 and 2 ;

Figure 4 is a beam footprint of the line focus on a substrate as created by the optical system of Figures 1-3 ; Figure 5 is a beam footprint of a line focus on a substrate obtainable by the present invention;

Figure 6 is a perspective view of a first embodiment of an optical system for creating a line focus according to the invention;

Figure 7 is a perspective view of another embodiment of an optical system for creating a line focus of a light beam on a substrate according to the invention ;

Figure 8 is a plan view in the xz-plane of the optical system in Figure 7 ;

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

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

Figure 11 is a beam footprint of the light beam on the substrate as created by the optical system in Figures 9 and 10 ; and

Figure 12 is a perspective view of still another embodiment of an optical system for creating a line focus of a light beam on a substrate according to the invention . DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, preferred embodiments of an optical system for creating a line focus of a light beam are described which have in common that the refracting focusing element 12 according to Figures 1 through 3 is replaced by at least one reflecting optical element, i . e . at least one mirror which is curved in a dimension transverse to the propagation direction of the light beam and transverse to a dimension in which the light beam is expanded .

As far as the term " line focus" is used in the following, generally a beam shape is mentioned which has a 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 mirror" is used in the following description , this term is used for simplicity and includes all forms of 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 mirror" in principle includes also mirrors which have a aspherical curvature . In the present invention, particular shapes of the curvature of the "cylindrical mirror" are parabolic , elliptical or aspherical shapes , which can be described by a polynomial or a conic surface . The advantage of the use of a reflective focusing element instead of a refractive focusing element is that a reflective focusing element is intrinsically free of the bow-tie error . The general reason for this is that in contrast to refractive systems , which are governed by the law of refraction in which the sin function is a non-linear function, the refraction angle θ ' is changed as the orthogonal input angle ω changes . In contrast, a reflective focusing element is governed by the law of reflection , which is a linear function :

(ω + θ) = - (ω ' + θ ' )

Therefore , the reflected angle θ ' is not dependent on the orthogonal incidence angle ω. Therefore , reflective systems are free of the bow-tie aberration, such that the beam footprint on the substrate can be made an equally wide line as illustrated in Figure 5.

An additional advantage of the use of a reflective focusing element instead of a refractive focusing element is that low expansion materials and high power reflective coatings can be used on the mirrors such that thermal problems are greatly reduced .

Now referring to Figure 6 , an optical system 20 for creating a line focus F of a light beam 22 for irradiating onto a substrate S is shown .

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

The optical system 20 further comprises a beam expanding optical element 26 for expanding the light beam 22 in the first dimension , i . e . the x-direction . The beam expanding optical element 26 can be chosen from the group comprising at least one negative cylindrical lens , an array of cylindrical lenses , at least one diftractive element, a 1-dimensional diffuser . A time-averaged expansion can also be obtained if element 26 is a fast steering or scanning mirror, e . g . a galvo-mirror, as explained in the introductory portion with respect to element 14.

The optical system 20 further comprises a mirror 28 which is curved in the second dimension , i . e . the y-direction in order to focus the light beam 22 after the expansion thereof to the line focus F on the substrate S . The preferred shape of the curvature of the mirror 28 in the y-direction is a parabolic shape or elliptical shape for the divergent light beam 22 according to the invention.

The mirror 28 is tilted about an axis 30 parallel to the first dimension ( x-direction ) with respect to the propagation direction ( z-direction ) of the light beam 22 , in order to avoid a blocking of the light beam 22 by the substrate S .

The light source 24 is a laser light source, for example an excimer laser . The wavelength λ of the light of the light beam 22 is in the range of about 300 nm to about 360 nm.

The optical system 20 is the most simple configuration of a system for creating a line focus of the light beam 22.

Figure 7 shows another embodiment of an optical system 40 for creating a line focus on a substrate S . Those parts of the optical system 40 which are identical or similar to the corresponding parts of the optical system 20 are referenced with the same reference numerals .

The optical system 40 differs from the optical system 20 in that a second mirror 42 is provided in the path of the light beam 22 which folds the light beam 22 in order to direct the light beam 22 into a desired direction . The second mirror or folding mirror 42 is , for example , a plane mirror without any curvature . While the second mirror 42 is arranged before the focusing mirror 28 in the embodiment shown in Figure 7 , it could be also envisaged to arrange the folding mirror 42 behind the focusing mirror 28 in the direction of the propagation of the light beam 22. Further , it can be envisaged to provide two or more such folding mirrors in the path of the light beam 22.

Figure 8 is a plan view of the optical system 40 in Figure 7. From Figure 8 , it can be taken that the focused light beam 22 is incident on the substrate S under an angle ω with the normal of the surface of the substrate S which is not zero . In other words, the focused light beam 22 is not orthogonally incident on the substrate S . In some applications of an optical system for creating a line focus it is , however, desired or even required that the beam is orthogonally incident on the substrate S .

In an optical system according to another embodiment which is shown in Figures 9 and 10 , the angle ω of incidence of the focused light beam 22 onto the substrate S is corrected to zero, i . e . the focused light beam 22 is orthogonally incident on the substrate S . Such an optical system is also referred to as a telecentric system.

Telecentricity of the optical system 60 in Figures 9 and 10 is achieved by exchanging the folding mirror 42 of the optical system 40 by a folding mirror 62 which is curved in the first dimension, i . e . the x-direction .

The mirror 62 of the optical system 60 provides two functions , namely to fold the light beam 22 and to provide telecentricity of the system 60. However, it could be envisaged, to distribute the two functions of folding and telecentricity to two separate mirrors as the need arises .

The optical system 60 in Figures 9 and 10 does not create a straight line focus F, but a bended line focus as shown in Figure 11. The bending arises from the fact that the telecentricity mirror 62 is tilted in the yz-plane , while it is curved in the x-direction . This causes a local beam rotation, which creates the line bending according to Figure 11. Figure 12 shows an optical system 80 for creating a line focus which avoids the afore-mentioned line focus bending.

To this end, the optical system 80 comprises a mirror 82 which is also curved in the first dimension (x-direction) which compensates a line bending of the line focus on the substrate S which is introduced by the mirror 62. The optical system 80 , therefore , comprises three mirrors, the focusing mirror 28 , and the telecentricity mirrors 62 and 82 which also act as folding mirrors . The tilt angle of the mirrors 28 , 62 and 82 is chosen such that the line focus on the substrate S is straight.

The optical systems 20 , 40 , 60 , 80 are or can be part of or themselves scanning systems for producing a scanning beam focus . "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 anamorphous silicon or semiconductor film, for laser induced crystallization of semiconductor films , for plat panel or OLED, in display manufacturing, or for any other kind of laser material processing .

According to the application of the optical system as described hereinbefore , the wavelength of the light emitted by the light source is chosen such that the absorption coefficient of the substrate S for this wavelength is high . In particular, the laser source 22 can be a high power excimer laser or other high power light source .

Therefore , what is claimed , is :

Claims

1. An optical system for creating a line focus 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 a first mirror which is curved in said second dimension so that said mirror focuses said input light beam in said second dimension to said line focus .

2. The optical system of claim 1 , further comprising at least one beam expanding optical element for expanding said input light beam in said first dimension .

3. The optical system of claim 1 or 2 , further comprising at least one second mirror which is curved in said first dimension for obtaining telecentricity in said line focus .

4. The optical system of claim 1 or 2 , further comprising at least one second mirror for folding said input light beam.

5. The optical system of claim 1 or 2 , further comprising a second mirror which is curved in said first dimension and at least one third mirror which is curved in said first dimension for compensating a line bending of said line focus introduced by said second mirror .

6. The optical system of anyone of claims 1 through 5 , wherein said at least one mirror is tilted about an axis parallel to said first dimension with respect to said propagation direction of said input light beam.

7. The optical system of claim 2 , wherein said at least one beam expanding optical element is a fast steering mirror, e . g . a galvo-mirror .

8. The optical system of claim 2 , 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 .

9. The optical system of anyone of claims 1 through 8 , wherein said input light beam comprises light of a wavelength which is chosen such that the absorption coefficient of a substrate , to which said line focus is irradiated, for said wavelength is high .

10. The optical system of claim 9 , wherein said substrate comprises a semiconductor film.

11. The optical system of claim 9 or 10 , wherein said substrate comprises anamorphous silicon .

12. The optical system of anyone of claims 1 through 11 , wherein said light source is a high power laser providing optical power of more than 50 W.

13. The optical system of anyone of claims 1 through 12 , wherein said light source is an excimer laser .

14. The optical system of anyone of claims 1 through 13 , wherein said first mirror is at least approximately parabolically curved.

15. The optical system of anyone of claims 1 through 13 , wherein said first mirror is at least approximately elip- tically curved .

16. An optical system for creating a line focus from an input light beam, wherein said line focus is telecentric , comprising

at least a mirror which is curved in said second dimension so that said input light beam is focused in said second dimension to said line focus , at least a second mirror which is curved in said first dimension for obtaining telecentricity in said line focus .

17. An optical system for creating a line focus from an input light beam, wherein said line focus is telecentric and said line focus "is straight, comprising

at least a first mirror which is curved in said second dimension so that said input light beam is focused in said second dimension to said line focus on said substrate,

at least a second mirror which is curved in said first dimension for obtaining telecentricity in said line focus ,

at least a third mirror which is curved in said first dimension for compensating a line bending of said line focus introduced by said second mirror .

18. A scanning system for producing a scanning beam focus , 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 propaga- tion direction and an extension in a second dimension transverse to said first dimension and to said propagation direction,

at least one mirror which is curved in said second dimension so that said input light beam is focused in said second dimension to said line focus .

19. The scanning system of claim 18 , further comprising at least one beam expanding optical element for expanding said input light beam in said first dimension .

20. The scanning system of claim 18 or 19 , further comprising at least a second mirror which is curved in said first dimension for obtaining telecentricity in said line focus .

21. The scanning system of claim 18 or 19 , further comprising at least one second mirror for folding said input light beam.

22. The scanning system of claim 18 or 19 , further comprising a second mirror which is curved in said first dimension and at least one third mirror which is curved in said first dimension for compensating a line bending of said line focus introduced by said second mirror .

23. The scanning system of anyone of claims 18 through 22 , wherein said input light beam comprises light of a wavelength which is chosen such that the absorption coefficient of a substrate, to which said line focus is irradiated for said wavelength is high .

24. The scanning system of claim 23 , wherein said substrate comprises a semiconductor film.

25. The scanning system of claim 23 or 24 , wherein said substrate comprises anamorpheous silicon .

26. The scanning system of anyone of claims 18 through 25 , wherein said light source is a high power laser providing optical power of more than 50 W.

27. The scanning system of claim 26 , wherein said light source is an excimer laser .

28. A method for laser processing of a substrate , using a scanning system for creating a scanning beam focus , said scanning system 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 ,

29. The method of claim 28 , said scanning system further comprising at least one beam expanding optical element for expanding said input light beam in said first dimension .

30. The method of claim 28 or 29 , said scanning system further comprising a mechanical means for scanning the line focus over the substrate .

31. The method of claim 28 or 29 , said scanning system further comprising an optical means for scanning the line focus over the substrate .

32. The method of anyone of claims 28 through 31 , wherein said substrate is a silicon substrate .

33. The method of anyone of claims 28 through 31 , wherein said substrate is a semiconductor film.

34. The method of anyone of claims 28 through 33 , wherein said laser processing comprises annealing said substrate .

35. The method of anyone of claims 28 through 33 , wherein said laser processing comprises laser induced crystallization of said substrate .

36. The method of anyone of claims 28 through 35 , wherein said laser processing is carried out in a flat panel display manufacturing process .

37. The method of anyone of claims 28 through 35 , wherein said laser processing is carried out in an organic LED display manufacturing process .