GB2454860A

GB2454860A - An optical system for creating a target intensity distribution

Info

Publication number: GB2454860A
Application number: GB0703049A
Authority: GB
Inventors: Holger Muenz
Original assignee: Carl Zeiss Laser Optics GmbH
Current assignee: Carl Zeiss Laser Optics GmbH
Priority date: 2005-05-27
Filing date: 2007-01-30
Publication date: 2009-05-27
Also published as: GB0703049D0

Abstract

The invention concerns an optical system for creating a target intensity distribution in a target field 12 from an input light beam 14 having an input intensity distribution. The system has at least one first optical element, e.g. an array of lenslets 16a to 16 d, for sub-dividing input light beam in at least one dimension into a plurality of sub-beams 18a to 18d. The at least one first optical element is adjustable, e.g. by tilting (see figure 4) with respect to a propagation direction of the input light beam in order to create an aberration in at least one of said sub-beams. There is at least one second optical element, e.g. a condenser lens 20, for at least partially overlapping the sub-beams in the target in order to create the required target intensity distribution.

Description

OPTICAL SYSTEM AND METHOD FOR CREATING A TARG ET INTENSITY

DISTRIBUTION IN A TARGET FIELD FROM AN INPUT LIGHT BEAM

BACKGROUND OI fl-IE INVENTEON

The present invcntion relates to an optical system for creating a target intensity disiribution in a target field from an input light beam having an input intensity distribution.

The invention further relates to a method lbr creating a targct intensity distribution in a target field from an input light beam having an input intensity distribution.

In some optical applications, for example in laser annealing of semi-conductor material, for example silicon, by irradiating the material with laser light, it is often necessary or at least a certaitrgeritensityThtributThii which1 for

field cai

of recrystallisation oUarnorphous silicon films, it can be advantageous, ilihe target intensity distribution in the target field, i.e. on the amorphous silicon film to be treated, is not exactly homogenous, but has a certain slope.

For the purpose of creating a certain desired target intensity distribution in a target field from an input light beam having an input intensity distribution, optical systems and methods of several kinds have been proposed Those optical systems, which are used to generate a homogeneous target intensity distribution arc often rekrred to as optical homogenizers. A common set-up of such optical homogenizers uses one oi two lenslet arrays together with a condenser lens to generate a homogenous intensity distribution in the lbcal plane of the condenser lens. The lenslet arrays can be one dimensional and comprise cylindrical lenslels to homogenize the input light beam in only one dimension. Nevertheless, two-dimensional lenslet arrays having spherical lenslets can be provtded to homogenize the input light beam in both dimensions of the input light beam transversc to the dii ection of propagation of the light beam at the same time. Instead of using two-dimensional homogenizing lenslet arrays having spherical lenslets, crossed cylindrical lens let arrays arc also known, for example from US 2003/0202251 Al.

In the following, the physical l)riflCiPles of a homogenizer are briefly described for the case of a one-dimensional homogenizer.

The incoming input light beam is sub-divided into a plurality of sub-beams by the cylindrical lenslet array. The number of sub-beams corresponds to the number of lerislets of the lenslet array. The sub- beams created by the lenslet array are imaged by a condenser lens to the target field such that the sub-beams at least partially overlap in the target field In the target field, the single intensity distributions of the sub-beams are added up, thus creating a more or less homogeneous intensity distribution of the light in the target field In general, the known homogenizers are able to reduce variations in the input Intensity distribution of the input light beam. However, if the input intensity distribution of the input light beam exhibits a general slope, then this slope is translated into a slope of the intensity distribution in the target field which is reduced with respect to the slope of the input intensity distribution by a factor which is equal to the number of lenslets. For some applications, however, fir example in laser material processing, the resulting variation of the intensity distribution of the processing light in the target field can still be too high In order to create a target intensity distribution liom an input light beam exhibiting a certain slope, DE 10049 557 Al discloses an optical system which comprises a thiñd..an opticiFiiertr arrangen15 hkhls id béth -honiOgenizer-when-seen--in-the propagation-direction of the input -light-beamii system inverts the intensity distribution of the input light beam in at least one sub-aperture of the input light beam. The inversion of the input intensity distribution of at least one sub-aperture of the input light beam creates a target intensity distribution after homogenizing the partially inverted input light beam which exhibits a slope. The extent of the slope of the target intensity distribution can be adjusted by selecting the number of optical inverters and the number of sub-apertures which are inverted by the inverter arrangements, accordingly.

This optical system and method requires additional lenslets in front of the homogenizer and is not very flexible in use, because the target intensity distribution has to be adjusted by adding or removing optical elements in the input light beam path.

DE 199 15 000 Al discloses an adjustable aperture which is inserted in an intermediate plane between the condenser lens of the homogenizer and the field plane of the target field.

The adjustable aperture is used to mask out different parts of the sub-beams propagating from different lenslets of the homogenizer and thus to generate a target intensity distribution The drawback of ibis known optical system and method is that some of the energy of the input light beam is lost due to the masking out by means of the adjustable aperture Another optical system for creating a target intensity distribution is disclosed in US 2003/020225 1 Al mentioned above. In this document, it is proposed to vary the width of the cylindrical lenses forming the one or two homogenizers in a certain range and to chose the number of the lenses such that one lens or lenslet is provided for a certain length of the input laser beam perpendicular to the propagation direction thereof. Again, this known optical system is very complex and not very flexible in use, because it requires exchanging the lenses or lenslets of the homogenizer in order to adjust the desired target intensity distribution of the light beam.

All the known optical systems and methods for creating a target intensity distribution suffer from one or more of the drawbacks discussed above. The drawbacks are a little flexibility of these systems in creating an adjustable target intensity distribution, the requirement of additional optical elements and/or a loss of energy of the light beam in the

target field

Thus, there is still a iieed for an optical system and for a method for creating a target intensity distribution in a target field from an input light beam having an input intensity distribution which do not suffer from the drawbacks mentioned before.

SUMMARY OF THE INVENTION JEã

intensity distribution in a target field from an input light beam having an input intensity distribution which renders it possible to adjust the target intensity distribution.

It is another object of the present Invention to provide an optical system for creating a target intensity distribution in a target field from an input light beam having an input intensity distribution which renders it possible to adjust the target intensity distribution without additional optical elements.

It is a further object of the present invention to provide an optical system for creating a target intensity distribution, by which the target intensity distribution can be adjusted in time saving and less expensive manner.

It is a further object of the present invention to provide an optical system for creating a target intensity distribution in a target field from an input light beam having an input intensity distribution by which the target intensity distribution can be adjusted without a loss of transmitted eneigy in the light beam It is another object to provide a method for creating a target intensity distribution in a target field from an input light beam having an input intensity distribution which achieves one or more of the objects mentioned before According to the present!nve!thon, an optca! system for creating a target intensity distribution in a target field from an input light beam having an input intensity distribution is provided, comprising at least one first optical element for sub-dividing said input light beam into a plurality of sub-beams, said at least one first optical element being adjustable with respect to a propagation direction of said input light beam in order to create an aberration in at least one of said sub-beams, and at least one second optical element for at least partially overlapping said sub-beams in said target field in order to create said target intensity distribution According to another aspect of the present invention, a method for creating a target intensity distribution in a target field from an input light beam having an input intensity distribution is provided, comprising sub-dividing said input light beam in at least one dimension into a plurality of sub-beams and creating an aberration in at least one of said sub-beams, at least partially overlapping said sub-beams in said target field in order to create said target intensity distribution.

The principle of the method according to the present invention is to create an aberration in at least one or all of the sub-beams originating from the input light beam. The type and the distributionirrthe-target-fl-eld-accordjng to the user's -needs-In the optical system according to the invention, the aberration in the at least one sub-beam is created by an adjustability of the at least one first optical clement which sub-divides the input light beam into the sub-beams with respect to the propagation direction of the input light beam, preferably in direction of at least that dimension in which the sub-beams are created.

In order to create a target intensity distribution with a certain slope, the aberration created in the sub-beam or in the sub-beams preferably is coma, which can be created by the optical system according to the invention by tilting the at least one first optical elements with respect to the propagation direction of the input light beam.

The optical system and the method according to the invention render it possible to create a target intensity distribution in the target field without energy losses in the light beam, without the nccd for iiirther optical elernenis and in easily controllable and time-saving and not expensive manner.

Further features and advantages will become apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will he described hereafter in detail with reference to the accompanying drawings, in which Fig. I shows an embodiment of an optical system for creating a target intensity distribution From an input light beam; Fig. 2 shows an input intensity distribution of an input light beam; Fig. 3 shows an output intensity distribution ola light beam after passing an optical system for creating a target intensity distribution of the prior art; Fig. 4 shows the optical system in Fig. 1 in an operation state different from the operation state in Fig. 1; shows toi tyd1sJiiibiitipnsqnecfwhicLbeingcmatecftyjhe Qptica! distribution being created by the operating state of the opticaist according to Fig. 4, Fig. 6 another embodiment of an optical system for creating a target intensity distribution in a fist operating state; Fig. 7 the optical system in Fig. 6 in a second operating state; Fig 8 still another embodiment of an optical system for creating a target intensity distribution in a first operating state; and Fig. 9 the optical system of Fig. 8 in a second operating state.

The present invention provides an optical system and a method for creating a target intensity distribution in a Larget field from an input light beam having an input intensity distribution In general, the method according to the invention comprises sub-dividing the input light beam in at least one dimension into a plurality of sub-beams, and further, creating an aberration in at least one of said sub-beams so that a desired target intensity distribution is created in the target field when the sub-beams are at least partially overlapped in the target lA Al., I tl.

The optical system according to the invention, in general comprises at least one first optical element for sub-dividing the input light beam in at least one dimension into a plurality of sub-beams, and in order to create the aberration in at least one of said sub-beams, the at least one first optical element is adjustable with respect to the propagation direction of the input light beam.

In a preferred case, the aberration induced in at least one of the sub-beams is coma, which can be induced by tilting the at least one first optical element with respect to the propagation direction of the input light beam in direction of at least one dimension, transverse to the propagation direction, preferably in that dimension, in which the input light beam is sub-divided into the sub-beams.

With reference to Fig. I, an optical system generally labelled with reference numeral 10 is shown, which is suitable to create a target intensity distribution in a target field 12 from an input light beam 14 having an input intensity distribution.

An exemplary input intensity distribution of the input light beam 14 is shown in Fig. 2, wherein the relative intensity of the light versus the position across the input light beam 14 is shown. The input intensity distribution according to Fig. 2 exhibits a general slope, i.e. the intensity of the input light beam 14 decreases, for example from margin 14a to margin l4b (or vice versa) of the input light beam 14 The optical system 10 comprises a first optical element 16 for sub-dividing the input light beam 14 into a plurality of sub-beams 1 8a through I 8d, for example four sub-beams in the present case.

The first optical element 16 comprises an array of cylindrical lenslets l6a through l6d, for example four lenslets in the present case. The number of lenslets is not critical for the present invention, and, thus, can be smaller or larger than four. The number of sub-beams 18a through 18d corresponds to the number ollenslets 16a through l6d.

The lenslets 16a through 16d are cylindrical, i.e. each lenslet 16a through 16d has a curvature in one dimension (y-axis in Fig 1) and no curvature in the second dimension (x-dimension). The z-axis is oriented in the propagation direction of the input light beam 14.

It is to be understood that the shape of the lenslet can slightly deviate from an exactly cylindrical shape in order to further improve the target intensity distribution, for example by reducing spherical aberration.

Instead of cylindrical Jcnslcts 16a through 16d which sub-divides the input light beam 14 in the y-direction only, the first optical element 16 could also comprise spherical or aspherical lenslets, which would sub-divide the input light beam 14 in the x-directioii as well as in the y-direction. For the sake of simplicity, it is assumed that the lenslets I 6a through I 6d arc cylindrical in the present case.

The optical system 10 further comprises at least one second optical element 20, which is a condenser lens, for at least partially overlapping the sub-beams I 8a through I 8d in the target field 12. The at least partial overlapping of the sub-beams 1 8a through 1 8d in the target field 12 leads to a homogenization of the intensity distribution of the light in the target field 1 2, thus creating a target intensity distribution which is more or less homogeneous.

Up to here, the set-up of the optical system lOis common technique and also referred to as an optical homogenizer.

Although variations of the intensity in the input light beam 14 can be reduced by the first optical_clement 16 and the condenser lens 20 such that the target intensity distribution in 1up1puT irItITsrty-tIistribution-s1io-wir in Fig--2-wi II still be translated- into a-slope of thè-itensity distribution in the target field 12 as shown in Fig. 3, so that the target intensity distribution of the light beam in the tal get field 12 has a residual inhomogeneity which may be undesired in certain applications, for example in laser material processing, i.e. the variation of the intensity distribution shown in Fig. 3 can still be too high.

At this point, the present invention solves this problem with very simple means, as shown in Fig. 4.

The first optical element 16 which sub-divides the input light beam 14 into the plurality of sub-beams 18a through 18d is adjustable with respect to the propagation direction of the input light beam 14 in order to create an aberration in at least one of the sub-beams 18a through 18d. In particular, the first optical element 16 is tiltable with respect to the propagation direction of'the input light beam 14 in the y-direction, i.e. the tilt axis is parallel to the x-direction. Now the input light beam 14 is no longer incident at right angles onto the first optical element 16, i.e. the lenslets 16a through l6d, but is obliquely incident on the first optical element 16, thus creating an aberration, in particular a coma in the sub-beams 18a through 18d.

The induced coma in the sub-beams I 8a through I 8d leads to a slope of the target intensity distribution in the target field 12 which varies with tilt angle. Thus by adjusting the tilt angle 3, it is possible to obtain a target Intensity distribution in the target field 12 exhibiting a predetermined slope Fig. 5 shows two intensity distributions in the target field 12, wherein the solid line corresponds to the intensity distribution in Fig. 3, and the broken line shows the intensity distribution in the target field 12, when the first optical element 16 has been tilted by a certain tilt angle.

In the embodiment shown in Fig. 4, it is provided that the sub-elements 16a through 16d of the first optical element 16 are adjustable. in particular tiltablc jointly together, i.e. about a common tilt axis 22.

The tilt axis 22, the position of which is merely exemplary in Fig. 4, is preferably chosen such that the target field 12 is not displaced by the tilting of the first optical element 16.

Tilting of the first optical element 16 can be performed manually or automatically, wherein in the latter case an intensity measuring device 24 is provided which measures the intensity distribution of the light in the target field 12, and an adjusting device 26 which adjusts the first optical element 16 in dependence on the intensity distribution measured by the iluiiii, ]h parti.iiIfá 15f the ihfeniIdiMibutiowbyadjisti EhiiIt angk accordingly.

It is to be understood that a desired slope of the intensity distribution in the target field 12 may not only be zero as shown in Fig. 5 by the broken line, but a desired slope can also be different from zero in dependence on the use of the light. In general, the desired slope can be adjusted by adjusting the tilt angle, until the desired slope of the intensity distribution

in the target field 12 is obtained

Fig. 6 and Fig. 7 show another embodiment of an optical system 40, which differs from the optical system 10 with respect to the following: The optical system 40 according to Fig. 6 comprises a first optical element 46, a second optical element 50 which correspond to the first optical element 1 6 and the second optical element 20 of optical system 1 0.

Differently from the optical system 10, sub-elements 46a through 46d of the first optical element 46 are adjustable, in the present case tiltable, individually, i.e. independent of one another. Such a configuration is particularly uselul, if sub-beams 48a through 48d are to be directed to different field positions in the tal get field 42, where it can be necessary to adjust the slope of the intensity distribution in the target field 42 for each sub-beam or group of sub-beams 48a through 48d separately. Thus, the advantage of the optical system over the optical system 10 is its still enhanced flexibility in adjusting the desired target

intensity distribution in the target field 42.

While in Fig. 6 all sub-elements 46a through 46d have been tilted by the same tilt angle, Fig. 7 shows an operating state of the optical system 40 in which the sub-elements 46a and 46d have been tilted, while the sub-elements 46b and 46c have not been tilted. It is to be understood, that the operating state of the optical system 40 in Fig. 7 is merely exemplary, and a large number of different adjustments of the sub-element 46a through 46d can be made in order to create the desired target intensity distribution in the target field 42.

Fig. 8 shows another embodiment of an optical system 60, which differs from the optical systems 10 and 40 by the leature that it comprises two first optical elements 66 and 67 for splitting the input light beam 64 into a plurality of sub-beams 68a through 68d. The configuration shown in Fig. 8 corresponds to common homogenizers which usc two cylindrical lenslet arrays for homogenizing the input light beam 64 in one dimension. An optical system using two lenslet arrays for homogenizing is, for example, described in DE 49557A1.

According to the present invention, the two first optical elements 66 and 67 are tillable Pmpa U je1Mllhji thm 64 as shQwn in Fig. 9.In angle, in order to keep the distance between the single sub-elements 66a through 66d and the sub-elements 67a through 67d and thus the image field size of the target field 12 for all sub-elements 66a through 66d and 67a through 67d the same.

Further, the present invention is not limited to those optical systems, which homogenize the input light beam in one dimension, but the principles of the present invention can be applied to those optical systems, which homogenize the input light beam in two dimensions. An optical system which homogenizes an input light beam in two dimensions is disclosed in US 2003/020225 1 Al, the disclosure of which with respect to the two-dimensional homogenizer comprising crossed cylindrical lenslet arrays is incorporated herein by reference.

According to the present invention, tilling of the lenslct arrays is provided in order to improve these known optical systems as described above. I0

Further, while the enibodirncnis above have been described with respect to lenslct arrays lbr the first optical element, such lenslet arrays can alco be replaced by arrays of cylindrical mirrors which are tillable or rotatable with respect to the propagation direction of the input light beam, accordingly, in order to create an aberration in at least one of the sub-beams.

The optical systems described herebcforc can he advantageously used in laser materials processing and micro-chininr' and especially in the laser annealing of silicon using excirner lasers or solid stale lasers Further, the tilting technique described above is not limited to cylindrical lenslets or mirrors, hut can also be used with hexagonal or rectangular or otherwise shaped arrayc of spherical lenses

Claims

II

Claims I. An optical system for creating a target intensity distribution in a target field from an input light beam having an input intensity distribution, comprising at least one first optical element for sub-dividing said input light beam in at least one dimension into a plurality of sub-beams, said at least one first optical element being adjustable with respect to a propagciLion direction of said input light beam in order to create an aberration in at least one of said sub-beams, at least one second optical element for at least partially overlapping said sub-beams in said target in ordcr to create said target intensity distribution.

2. The optical system of claim I, wherein said aberration is coma.

3. The optical system of claim 1, wherein said at least one first optical clement is tiltable with respect to said propagation direction of said input light beam.

4. The optical system of claim 1, wherein said at least one optical element comprises a plurality of sub-elements, and at least some of said sub-elements are adjustable jointly together.

5. The optical system of claim 4, wherein said at least some of said sub-elements are onjlt axis. -_____ ______ 6. The optical system of claim 5, wherein said one tilt axis is chosen such that said target field is at least approximately not displaced by the tilting of said at least one first optical element.

7. The optical system of claim 1, wherein said at least one first optical element comprises a plurality of sub-elements, and at least some of said sub-elements are individually adjustable independent from one another.

8. The optical system of claim 7, wherein said at least some of said sub-elements are individually tiltable independent from one another about individual tilt axes. 7-,

9 The optical system of claim 8, wherein said individual tilt axes arc chosen such that said target field is at least approximately not displaced by the tilting of said sub-elements.

10. The optical system of claim I, wherein said at least one first optical element for sub-dividing said input light beam is configured to sub-divide said input light beam in a first dimension, said system further comprising at least one further first optical element for sub-dividing said input light beam in a second dimension transverse to said first dimension.

11. The optical system of claim 1, wherein said at least one first optical element for sub-dividing said input light beam is configured to sub-divide said input light beam in a first dimension and in a second dimension orthogonal to said first dimension into said sub-beams 12. The optical system of claim 1, wherein said at least one first optical element is a refractive optical element.

13. The optical system of claim 1, wherein said at least one first optical element is an array of cylindrical, spherical or aspherical lenslets.

14. The optical system of claim 1, wherein said at least one first optical element is a reflective optical element.

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arrif5lindricál herical or ahcrical rniñörs 16. The optical system of claim 1, further comprising an intensity measuring device for measuring an intensity distribution in said target field, and an adjusting device for adjusting said at least one first optical element in dependence on said measured

intensity distribution in said target field.

17. A method for creating a target intensity distribution in a target field from an input light beam having an input intensity distribution, comprising sub-dividing said input light beam in at least one dimension into a plurality of sub-beams and creating an aberration in at least one of said sub-beams, at least partially overlapping said sub-beams in said target field in order to create said target intensity distribution 18 The method of claim 17, wherein said aberration crcatcd in said sub-beams is coma 19. The method of claim 17, wherein said sub-dividing of said input light beam into a plurality of sub-beams is performed by means of at least one first optical element, and creating au aberration in said sub-beams is performed by adjusting said at least one first optical clement with tespect a propagation direction of said input light beam.

20. The method of claim 19, wherein said adjusting of said at least one first optical element comprises tilting said at least one first optical element with respect to said propagation direction of said input light beam.

21. l'he method of claim 19, wherein said at least one first optical element comprises a plurality of sub-elements, and said adj usting comprises adjusting said sub-elements jointly together.

22. The method of claim 20, wherein said at least one first optical element comprises a plurality of sub-elements, and said adjusting comprises adjusting said sub-elements individually independent from one another.

23. The method of claim 18, further comprising measuring an intensity distribution in said target field, and adjusting said at least one first optical element in dependence on said measured intensity distribution in said target field.