JP7629087B2 - Device for generating a defined laser line on a work surface - Google Patents

Description

The present invention relates to an apparatus for generating a defined laser line on a work surface, the apparatus comprising a laser source configured to generate an original beam and an optical assembly configured to receive the original beam and convert it into an illumination beam incident on the work surface, the illumination beam defining a beam direction intersecting the work surface, the illumination beam having a beam profile having a major axis with a major beam width and a minor axis with a minor beam width perpendicular to the beam direction, the optical assembly having a number of optical elements for focusing the beam profile to a defined position within the area of the work surface, the defined position being displaced by a drift length as a result of heating of the optical elements depending on the operating power and/or operating time of the laser source.

Such a device is disclosed, for example, in Patent Document 1.

Such devices generate defined line-shaped laser radiation at defined positions on the working surface, in particular for processing a workpiece. The workpiece can be, for example, a plastic material on a glass plate used as carrier material. The plastic material can in particular be a film for the production thereon of organic light-emitting diodes, so-called OLEDs, and/or thin-film transistors. OLED-films are increasingly used as displays for smartphones, tablet PCs, televisions and other devices with screen displays. After the production of the electronic components, the film must be peeled off from the glass substrate. This is advantageously done by performing the laser radiation in the form of a thin laser line moving at a defined speed relative to the glass plate, which can loosen the adhesive bond of the film through the glass plate. Such an application is often called LLO in practice, or Laser Lift Off.

Another application of irradiating a workpiece with a defined laser line is the line-by-line melting of amorphous silicon on a carrier plate. Here too, the laser line is moved at a defined speed relative to the surface of the workpiece. By melting, the relatively cheap amorphous silicon can be converted into higher quality polycrystalline silicon. Such applications are often referred to in practice as Solid State Laser Annealing, or SLA.

For such applications, the laser line on the working surface must be as long as possible in one direction in order to cover the working surface as widely as possible, and must be very short in the other direction in order to provide the required energy density for the respective process. Long and narrow laser lines parallel to the working surface are therefore desirable. The direction in which the laser line extends is often called the major axis of the so-called beam profile, and the thickness of the line is often called the minor axis. As a rule, the laser line must have a defined intensity profile in both axes. For example, the laser line should have an intensity profile that is as rectangular as possible or, in some cases, trapezoidal in the major axis. The latter may be advantageous if several such laser lines are combined to form a longer overall line. In the minor axis, a rectangular intensity profile (so-called top hat profile), a Gaussian profile, or another intensity profile is desirable, depending on the application.

JP 2003-233963 A discloses an apparatus of the above-mentioned type with numerous details regarding the optical elements of the optical assembly. The laser source generates a laser original beam that is very widely expanded by the optical assembly in a first spatial direction to obtain a major axis. In a second spatial direction perpendicular thereto, the laser beam is focused to obtain a minor axis. The first and second spatial directions are typically perpendicular to the beam direction in which the laser beam impinges on the working surface. The optical assembly of the apparatus of JP 2003-233963 A comprises a collimator for collimating the laser original beam, a beam converter, a homogenizer, and a focusing step. The beam converter receives the collimated original beam and expands it in the major axis direction. It is also possible, in principle, for the beam converter to receive several laser original beams from several laser sources and combine them into a common expanded laser beam with a higher power. The homogenizer generates the desired beam profile, for example a top hat profile, in the major and minor axes, respectively. The focusing step focuses the laser beam thus converted at a defined position in the area of the working surface.

The above-mentioned US Pat. No. 5,399,433 discloses an optical assembly for generating an illumination line by means of a telescopic assembly having an optical power in relation to the short axis. The telescopic assembly comprises a first lens group and a second lens group, which are movable relative to each other along the optical axis. The control unit controls the operation so that the intensity of the illumination line and its half-width, i.e. the line width at 50% of the intensity (Full Width at Half Maximum, FWHM), remain as constant as possible in time while the laser source generates the laser beam. It has been found that the properties of the optical assembly can change during the generation of the laser beam. In particular, heating of the optical elements by irradiation with the laser beam can result in so-called thermal lenses, which change the optical properties of the assembly. US Pat. No. 5,399,433 proposes to compensate for or at least reduce the resulting change in the focal position by a displacement of the telescopic lenses relative to each other. A disadvantage of this solution is the mechanical effort required for the position adjustment of the telescopic lenses. That actuation can cause wear and/or lead to misalignment of the optical assembly.

DE 102018200078 A1 International Publication No. 2018/019374

In view of the above, it is an object of the present invention to provide an apparatus of the above type which helps to maintain the working surface within the working range of the apparatus in an alternative manner.

According to one aspect of the invention, there is presented herein an apparatus of the above type, in which the short axis beam width of the beam profile varies along the beam direction, thereby defining a useful process window, and the optical assembly is configured to produce a useful process window having a depth of field in the beam direction that is greater than the drift length.

The novel device makes it possible to dispense with mechanical adjustments of the optical assembly or optical elements that result in focusing of the beam profile in the minor axis direction. Preferably, the optical elements have an optical power with respect to the minor axis of the beam profile, focus the beam profile at a defined position in the area of the working surface and therefore have a fixed distance relative to each other. In a preferred embodiment, the optical elements are fixed, respectively. This reduces mechanical wear and the risk of the optical assembly being displaced by mechanical movements. Instead, the novel device is based on the idea of intentionally increasing the depth of field of the assembly in the beam direction (hereinafter referred to as the longitudinal direction), so that the defined position at which the beam profile is focused remains within the depth of field even if it is displaced as a result of thermal lensing. In other words, the novel device intentionally allows for a focus drift due to heating of the optical elements, which depends on the operating power and/or operating time of the laser source. However, the optical assembly is designed to intentionally reduce the beam quality, especially in the minor axis direction, so that the beam profile remains within the process window even if the focus position drifts. Instead of mechanical tracking, optical assemblies are specifically designed to increase depth of field by adding optical elements and/or by taking advantage of the illumination up of the optical elements and the associated aberrations.

The novel device therefore has an optical assembly which favourably influences the relationship between the depth of field and the focus shift. This increases the process window of the device compared to prior art devices. Mechanical tracking and the associated disadvantages are avoided. The above-mentioned problems are accordingly completely solved.

In a preferred embodiment, the short axis beam width along the beam direction has a maximum value, the process window has a front end and a rear end in the beam direction, and the short axis beam widths at the front end and rear end are each up to 10% less than the maximum value, preferably up to 5% less than the maximum value, and particularly preferably 1% less than the maximum value.

In this embodiment, the optical assembly is configured to generate a beam profile in which a process window having specified parameters is formed. In particular, in this embodiment, the short axis beam width can be determined as a half-width, i.e., as the difference between two intensity values of the beam profile in the short axis, which has 50% of the maximum intensity of the beam profile at the corresponding position along the beam direction (Full Width at Half Maximum, FWHM). Alternatively, in this embodiment, the short axis beam width can be specified as the difference between two intensity values of the beam profile in the short axis, which has 90% of the maximum intensity of the beam profile at the corresponding position along the beam direction (Full Width at 90% of Maximum). This value provides a beneficial increase in depth of field while maintaining a level of beam quality in the short axis suitable for typical LLO and SLA applications.

In a further embodiment, the illumination beam has a beam waist in the beam direction, and the optical assembly is configured to generate the beam waist in the region of the work surface.

Every real laser beam has a so-called beam waist. This is the position in the divergence or beam direction of the laser beam where the laser beam has the smallest diameter or radius. In other words, the diameter or radius of the real laser light varies in the beam direction. With increasing distance from the beam waist, the beam diameter increases. The spread of the beam waist in the beam direction can be quantified, for example, using the so-called Rayleigh length. The Rayleigh length is the distance from the center of the waist _w0 at which the radius of the beam (usually measured in the electric field) increases by a factor of √2, i.e. the following applies:

ここで、ｚ_Ｒはレイリー長、ｗ（ｚ）は短軸におけるビーム半径である。この形態においては、光学アセンブリは、ビームウエストを作業面の領域、好ましくは作業面内に位置させるように構成される。本形態は、短軸方向におけるビームプロファイルがガウシアンプロファイルである場合に特に有利である。それは、ワークの加工位置において効率よく高エネルギー密度を実現することができる。

where _zR is the Rayleigh length and w(z) is the beam radius in the minor axis. In this configuration, the optical assembly is configured to position the beam waist in the region of the working surface, preferably within the working surface. This configuration is particularly advantageous when the beam profile in the minor axis direction is a Gaussian profile. It can efficiently achieve a high energy density at the processing position of the workpiece.

In a further embodiment, the optical assembly has an optical phase element that imprints an axicon-shaped phase front in the short axis onto the illumination beam.

In its proper sense, an axicon is a cone-shaped ground lens that images a point source onto a line along the optical axis or transforms a laser beam into a ring. Axicon is often used in cases of rotational symmetry or in the case of rotationally symmetric elements. However, in this case, the phase element affects the short axis, i.e., it generates an axicon-shaped phase front when the optical assembly is considered about the short axis. The long axis is barely affected. Therefore, no rotational symmetry is implied. The axicon-shaped phase front generated by using the phase element in this configuration results in the laser beam having a ring portion when the beam path is viewed about the short axis perpendicular to the optical axis of the assembly. A portion of the laser light is "scattered out" by the use of the phase element. This portion leads to a relatively simple and inexpensive extension of the depth of field. Accordingly, this configuration allows a very simple and inexpensive implementation of the novel device.

In a further embodiment, the optical phase element comprises a refractive optical element, in particular a prismatic or non-cylindrical polished lens.

This configuration allows a particularly simple and cost-effective implementation of the new device. To reach the new device, in particular the optical assemblies corresponding to the prior art mentioned above can be modified by introducing such refractive optical elements, for example non-cylindrical lenses that are wedge-shaped in side view.

In a further embodiment, the optical phase element includes a diffractive optical element.

In this embodiment, a diffraction effect is used to generate an axicon-shaped phase front in the short axis direction. In some embodiments of this embodiment, the optical phase element includes an irregular diffraction grating. This embodiment allows for implementation with low optical losses and large tolerances regarding the position of the phase element in the beam path of the optical assembly.

In a further embodiment, the optical phase element includes a spatial light modulator (SLM).

A spatial light modulator is a device for imprinting a spatial modulation on light. In particular, a spatial light modulator may include an assembly of micromirrors and/or one or more deformable mirrors. Such light modulators can generate highly individual phase fronts, thereby enabling optimal beam shaping.

In a further embodiment, the optical assembly includes a telescopic assembly including at least two spaced optical elements, the at least two optical elements having optical power with respect to the minor axis of the beam profile, and an optical phase element is disposed in front of the telescopic assembly in the beam direction.

This configuration allows for an increase in depth of field of about two in a very simple and low-cost way. This configuration is particularly advantageous for LLO applications and other applications that have a Gaussian beam profile in the short axis direction, since the effect of the phase element on the beam profile can be easily tolerated in this configuration.

In a further embodiment, the optical assembly includes a telescopic assembly including at least two spaced apart optical elements, the at least two optical elements having optical power with respect to the minor axis of the beam profile, and the optical phase element is disposed between the at least two spaced apart optical elements of the telescopic assembly.

In this embodiment, the optical phase element is advantageously arranged in the far-field plane of the short axis. The far-field can be defined in this embodiment with respect to the exit of the beam transformer and can be realized via an optical system (focusing) or by looking into the plane away from the exit aperture. Typically, the path length is then much larger than the Rayleigh length of the output beam from the beam transformer. This embodiment is particularly suitable for applications with a top-hat beam profile in the short axis, such as SLA applications.

In a further embodiment, the optical assembly includes an objective lens having an aberration at an edge region, and the illumination beam includes a beam portion from the edge region.

In this embodiment, the device advantageously uses an optical assembly that is non-diffraction limited. In this embodiment, the device advantageously exploits aberrations in the edge region of the objective lens. This embodiment has the advantage of reducing local heating of the optical elements due to large area illumination, which results in less thermal drift of the focus position. Furthermore, the depth of field is extended by including the beam portion from the edge region, which in this embodiment has two simultaneous advantageous effects. This embodiment allows the implementation of the novel device with a small number of optical elements, thereby enabling low long-term cost realization. This embodiment is particularly advantageous for SLA applications and other applications with a top-hat beam profile in the short axis.

In a further embodiment, the optical assembly includes at least one plano-convex lens having a first planar surface and a second convex surface, the convex surface facing the work surface.

It is particularly advantageous for the plano-convex lens to be arranged at the light outlet of the device. The plano-convex lens may in particular be the last optical element of the optical assembly in the beam direction. Typically, such a plano-convex lens is arranged inverted in a general-purpose device, whereby its plane faces the working surface. The preferred arrangement has the advantage that aberrations from the edge regions of the lens can be more effectively utilized. In some embodiments, the at least one lens may comprise a plurality of individual lenses, at least one of which is a plano-convex lens and arranged in the above-mentioned direction.

It is understood that the features above and those described below may not only be used in the combinations indicated, but may also be used in other combinations or alone without departing from the scope of the present invention.

An embodiment of the invention is shown in the drawings and explained in more detail in the following description.

FIG. 1 is a schematic diagram showing a first embodiment of the novel device; FIG. 1 is a schematic diagram showing a first embodiment of the novel device; 1 is a simplified diagram of a beam profile for explaining a first embodiment and a further embodiment. 1 is a simplified diagram of beam waist and waist length according to some embodiments of the present novel device. FIG. 2 is a schematic diagram showing a second embodiment of the novel device; FIG. 2 is a schematic diagram showing a second embodiment of the novel device; FIG. 2 is a schematic diagram showing a further embodiment of the novel device. FIG. 2 is a schematic diagram showing a further embodiment of the novel device.

In Fig. 1a and Fig. 1b, a first embodiment of the novel device as a whole is designated by the reference number 10. Fig. 1a shows the device 10 in a simplified top view of a laser line 12, which in this embodiment is arranged in the region of a working surface 14. The device 10 comprises a laser source 16, which may be, for example, a solid-state laser generating laser light in the infrared or ultraviolet range. For example, the laser source 16 may comprise a Nd:YAG laser with a wavelength in the range of 1030 nm. In further embodiments, the laser source 16 may comprise a diode laser, an excimer laser or a solid-state laser generating laser light with a wavelength between 300 nm and 350 nm, 500 nm and 530 nm or 900 nm and 1070 nm.

Figure 1b shows the device 10 from the side, i.e. a view into the minor axis of the laser line 12. In the following, the beam direction of the laser beam is indicated by the coordinate axis z. The laser line 12 extends in the x-direction and the line width is seen in the y-direction. Accordingly, in the following, the x-axis indicates the major axis of the beam profile and the y-axis indicates the minor axis of the beam profile.

The laser source 16 generates a laser source beam 18, which is further transformed by an optical assembly 20 into an illumination beam 22 having a beam direction 23. The optical assembly 20 includes, in this embodiment, a beam guiding unit 24 and a beam converter and homogenizer. The beam guiding unit may include, in some embodiments, a collimator for collimating the laser source beam 18. The beam converter and homogenizer are collectively indicated in this embodiment by the reference numeral 26. The beam converter and homogenizer 26 expands the (collimated) laser source beam 18 along the x-axis to generate the major axis of the laser line 12. The beam converter and homogenizer 26 further generates a transversal beam profile 28, as shown by a simplified diagram in FIG. 2. As shown, the beam profile 28 has a major axis 30 with a major axis beam width 31 in the x-direction and a minor axis 32 with a minor axis beam width 33 in the y-direction. In FIG. 2, the intensity I of the laser light is shown on the ordinate axis, with the minor beam width 33 shown as an example of the full width at half maximum (FWHM). The beam profile may be a Gaussian or top-hat profile with finite edge steepness, unlike the trapezoidal intensity profile shown in the simplified form in FIG. 2.

To process a workpiece (not shown in this embodiment), the beam profile 28 can be moved relative to the work surface 14 together with the device 10, for example in the y-direction. For further details of the device 10, reference is made to the above-mentioned US Pat. No. 6,399,343, which is incorporated by reference in this embodiment. In particular, in some advantageous embodiments, the beam transformer and homogenizer 26 is realized in the manner described in US Pat. For example, the beam transformer and homogenizer 26 may accordingly comprise a transparent and monolithic plate-like element having a front face and a rear face, the front face and the rear face being substantially parallel to each other and arranged at an acute angle (not shown here) with respect to the optical axis 34 of the optical assembly 20. The front face and the rear face may each have a reflective coating. Thereby, the collimated raw beam 18 is coupled obliquely into the plate-like element at the front face and is multiple-reflected at the front face before the beam is fanned out and homogenized at the rear face. The beam transformer and homogenizer 26 may include further optical elements having a number of lenses (not shown in this embodiment) that form a fan-shaped illumination beam, particularly along the longitudinal axis 30. Alternatively, the beam transformer and homogenizer 26 may be implemented by a method as described in the above-mentioned US Pat. No. 6,399,363, which is incorporated by association into this embodiment. Accordingly, the optical assembly may include further optical elements that are not shown in this embodiment for reasons of clarity and that serve particularly for beam shaping along the longitudinal axis.

In this embodiment, the optical assembly 20 includes a telescopic assembly having a first optical element 36 and a second optical element 38. The telescopic assembly has an optical power that primarily affects the minor axis 32 of the beam profile 28. The telescopic assembly is configured to shape the beam profile 28 in the minor axis 32. The laser beam thus transformed illuminates, in this embodiment, an objective lens 40, which then uses the illumination beam 22 to focus the beam profile 28 to a defined location 42 in the area of the work surface 14.

As shown in FIG. 1b, the illumination beam 22 has an object-side beam waist 44 in the beam direction, which in this embodiment is located in the region of the working surface 14 (see FIG. 3). However, it is also possible to locate the beam waist in front of or behind the working surface. The beam waist 44 has a waist length that can be quantified, for example, using the Rayleigh lengths on both sides.

In some embodiments, the waist length may be quantified by a percentage increase 48. That is, the waist length corresponds to the longitudinal distance of those points where the beam diameter in the y direction increases by a specified percentage relative to the minimum beam diameter in the minor axis. The specified percentage may be 10% or less in some embodiments. In some embodiments, the waist length may be equal to the depth of field 46.

The irradiation of the high-energy laser beam heats up the optical elements 36, 38, 40. This heating can result in the formation of thermal lenses. Thermal lenses are formed by the absorption of the laser light into the optical assembly. Local temperature increases can be particularly significant, especially when using laser sources with a power of several kW. The low thermal conductivity of the optical materials generates strong temperature gradients. The resulting refractive index gradient and the thermal expansion of the material act like an additional lens. These thermal lenses result in a displacement of the defined position 42 or the focal position of the irradiation beam 22 by the drift length 50. Displacement of the defined position can occur in particular after switching of the laser source 16, when the optical elements are exposed to the laser beam again only after a longer interruption, but also when the operating power of the laser source 16 is changed, for example because a switch is made from operation at low to high power.

For this reason, the device 10 according to Figs. 1a and 1b is equipped with an optical phase element 52 in the beam path. In the embodiment according to Figs. 1a and 1b, the phase element 52 is arranged before the short-axis-telescopic assembly with the optical elements 36, 38. The phase element 52 imprints an axicon-shaped (but not rotationally symmetric) phase front about the short axis on the laser beam illuminating the objective lens 40. As a result, the laser beam acquires a ring portion transverse to the optical axis 34 of the assembly 20. Moreover, the ring portion projects the short-axis profile several times in succession in the beam direction z on the working surface 14. As a result, the depth of field in the beam direction is intentionally enlarged. In a preferred embodiment, the depth of field 46 is dimensioned to be larger than the drift length 50, as shown in a simplified manner in Fig. 3. In Fig. 3, the depth of field 46 corresponds to the waist length, but this does not have to be the case in all embodiments.

In some advantageous embodiments, the depth of field 46 is defined based on the minor axis beam width 33, which varies along the beam direction 23. The minor axis beam width 33 varies along the beam direction and has a maximum value at one point, which in FIG. 3 corresponds to the center of the beam waist as an example. The depth of field defines a useful process window in the beam direction. At the front and rear ends of the process window, the minor axis beam width is 10% less than the maximum value of the minor axis beam width, preferably 5% less than the maximum value, and particularly preferably 1% less than the maximum value. Since the drift length 50 is smaller than the depth of field 46, the beam profile 28 remains within the process window for processing the workpiece even if the focal plane is displaced by the effect of thermal lensing. Placing the phase element 52 in the beam path before the telescopic assembly for the minor axis 32 is particularly advantageous for LLO applications and other applications having a Gaussian-shaped beam profile in the minor axis 32.

In the embodiment shown in Figures 4a and 4b, the phase element 52 is placed between the optical elements 36, 38 of the telescopic assembly for the short axis. In this case, the phase element 52 mainly affects the far field of the short axis 32 of the beam profile 28. Such a placement of the phase element 52 is advantageous for SLA applications and other applications with top-hat beam profiles.

In the embodiments of Figures 1a and 1b and 4a and 4b, the phase element 52 may implement one or more refractive optical elements, one or more diffractive optical elements, and/or a spatial light modulator, respectively.

In the embodiment according to Fig. 5a and Fig. 5b, a dedicated phase element 52 is not necessary. In that embodiment, instead, the aberrations of the optical elements, in particular from the edge region of the objective lens 40, are utilized to achieve a larger depth of field 46. In principle, it is also possible to use a dedicated phase element in addition (not shown here). Advantageously, the optical assembly 20 of this embodiment is not diffraction limited. The fanned laser beam illuminates the objective lens 40 up to the edge region in this embodiment, for example up to the outer 20% of the lens radius in the field of view in the short axis direction. This alone has the advantage that the radiation power of the laser beam is distributed over a larger lens area and local heating of the objective lens 40 is reduced. Thus, in this embodiment, the drift length can be advantageously reduced. Furthermore, the aberrations from the edge region of the objective lens increase the depth of field. The quotient of the drift length 50 to the depth of field 46 is favorably influenced. The resulting deterioration of the beam quality in the short axis direction is acceptable in many applications.

In the preferred embodiment, the objective lens 40 comprises a plano-convex lens with its convex surface 54 facing the work surface 14 while its planar surface 56 facing the telescopic assemblies 36, 38. This objective lens orientation is highly unusual for a general purpose device optical assembly. However, it enhances the effects of spherical aberration from the edge regions of the objective lens 40.

The objective lens design criteria are based on the desired longitudinal displacement: The wavefront aberration W(y _p ) can be converted to longitudinal aberration:

ここで、Δｓ’は長手方向の変位、ｎ’は像側の屈折率、Ｒは基準球の半径、ｙ_ｐは瞳座標である。

where Δs′ is the longitudinal displacement, n′ is the image-side refractive index, R is the radius of the reference sphere, and _yp is the pupil coordinate.

For a cylindrical lens, the wavefront aberration is W _sph (y _p ) ∝ y _p ^4. Thus, the total longitudinal displacement in air is Δs′=aR ² y _p ² , where a is the degree of spherical aberration and depends on the objective lens design.

For Gaussian illumination _yp of an objective lens with intrinsic focal length f,

の熱屈折力が生成される。ここで、αはワークの吸収、κは材料の熱伝導率、Ｌ_ｘは長軸方向のライン長、ｙ_ｐは照射のビーム半径である。ガラスの膨張と同様に屈折率の変化も定数γに集約される。

where α is the absorption of the workpiece, κ is the thermal conductivity of the material, _Lx is the line length in the major axis direction, and _yp is the beam radius of the irradiation. The change in refractive index as well as the expansion of the glass are summarized in the constant γ.

ここで、Ｐ_０はレーザ出力である。熱レンズは、大きな焦点変位を発生させる。

where _P0 is the laser power. Thermal lensing produces large focal shifts.

At the same time, a beam incident on the work plane along the minor axis is characterized by a depth of field that is approximately scaled by the irradiance _yp and the minor axis beam diameter FW at the work plane, i.e.

Increasing the minor axis width FW increases the depth of field. The smaller the illumination, the greater the depth of field. Accordingly, the ratio is:

The power density P ₀ /FW is a constant and is predefined through the process. The focal length f is obtained from the working distance. That is, in the diffraction limit case, the quotient of the focal shift and the depth of field does not depend on the illumination y _p . In contrast, in the embodiment of the novel device, the depth of field does not decrease significantly with illumination. Therefore, the quotient Q is a function of y _p and is no longer independent of it. Thereby, the quotient Q is smaller than 1, preferably much smaller than 1.

12 laser line 14 work surface 16 laser source 18 raw beam 20 optical assembly 22 irradiated beam 23 beam direction 26 homogenizer 28 beam profile 30 major axis 31 major axis beam width 32 minor axis 33 minor axis beam width 36, 38 optical elements (telescopic assembly)
40 Optical element (objective lens)
42 Defined position 44 Beam waist 46 Depth of field 50 Drift length 52 Optical phase element 54 Second convex surface 56 First flat surface

Claims (12)

1. An apparatus for generating a defined laser line (12) on a work surface (14), comprising:
a laser light source (16) configured to generate an original beam (18);
an optical assembly (20) that receives the original beam (18) and converts it into an illumination beam (22) that is incident on the work surface (14);
The illumination beam (22) defines a beam direction (23) that intersects the work surface (14);
the illumination beam (22) has a beam profile (28) having a major axis (30) with a major beam width (31) and a minor axis (32) with a minor beam width (33) perpendicular to the beam direction (23);
the optical assembly (20) having a number of optical elements (36, 38, 40) that focus the beam profile (28) to a defined location (42) within an area of the work surface (14);
The defined position (42) is displaced by a drift length (50) as a result of heating of the optical element (36, 38, 40) depending on the operating power and/or operating time of the laser light source (16),
the short axis beam width (33) of the beam profile (28) varies along the beam direction (23) thereby defining a useful process window;
the optical assembly (20) comprising an optical phase element (52) for imprinting an axicon-shaped phase front onto the illumination beam (22);
The apparatus, wherein the optical assembly (20) is configured to generate the useful process window having a depth of field (46) in a beam direction greater than the drift length (50). the short axis beam width (33) along the beam direction (23) has a maximum value;
the process window has a front end and a rear end in a beam direction;
2. The device of claim 1, wherein the minor axis beam widths (33) at the front and rear ends are each up to 10% less than the maximum value. The device according to claim 1 or 2, characterized in that the optical elements (36, 38, 40) have optical power with respect to the minor axis (32) of the beam profile (28) and are fixed relative to each other. the illumination beam (22) has a beam waist (44) in a beam direction;
The apparatus according to any one of claims 1 to 3, characterized in that the optical assembly (20) is configured to generate the beam waist (44) in the area of the working surface (14). The device according to any one of claims 1 to 4 , characterized in that the optical phase element (52) comprises a refractive optical element, in particular a prismatically polished lens or a non-cylindrically polished lens. The device according to any one of claims 1 to 5 , characterized in that the optical phase element (52) comprises a diffractive optical element. The device according to any one of claims 1 to 6 , characterized in that the optical phase element (52) comprises a spatial light modulator. the optical assembly (20) comprises a telescopic assembly including at least two spaced apart optical elements (36, 38), the at least two optical elements having optical power with respect to the minor axis (32) of the beam profile (28);
The device according to any one of claims 1 to 7 , characterized in that the optical phase element (52) is arranged in front of the telescopic assembly as seen in the beam direction. the optical assembly (20) comprises a telescopic assembly including at least two spaced apart optical elements (36, 38), the at least two optical elements having optical power with respect to the minor axis (32) of the beam profile (28);
The device according to any one of claims 1 to 7 , characterized in that the optical phase element (52) is disposed between the at least two spaced apart optical elements (36, 38) of the telescopic assembly. The optical assembly (20) includes an objective lens (40) having an aberration in an edge region,
The device according to any one of the preceding claims, characterized in that the illumination beam (22) comprises a beam portion from the edge region. The optical assembly (20) includes at least one plano-convex lens (40) having a first planar surface (56) and a second convex surface (54);
A device according to any one of the preceding claims, characterized in that the convex surface (54) is directed towards the working surface ( 14 ). 12. The device according to claim 11 , characterized in that the at least one plano-convex lens (40) is a terminal element in the beam direction of the optical assembly (20).

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