HK40011731B - Method for producing a 3d structure by means of laser lithography with a modified exposure dose at edge portions, and corresponding computer program product - Google Patents

Description

Method for manufacturing 3D structures by laser lithography and corresponding computer program product

Technical Field

The present invention relates to a method for producing a three-dimensional monolithic structure by means of laser lithography, and to a computer program product suitable for use in said method.

Background

Such methods find particular use in the manufacture of micro-or nanostructures in areas where high precision and design freedom are required for the structures to be manufactured. In the field of laser lithography, in particular for so-called laser direct writing, it is known to manufacture a desired overall structure by writing in succession a series of partial structures that are completely complementary to one another to the desired structure. Typically, the overall structure is written in layers or slices.

The individual part structures are usually determined mathematically from the known overall structure. For this purpose, it is known to computationally record the overall structure in a grid structure and divide it into partial structures. The problem arises that the partial structure is determined again in the grid structure, so that the exact course of the overall structure cannot be reproduced exactly. In addition, laser lithography inherently has certain resolution limitations, particularly given by the spatial extent of the focal region. As a result, a stepped surface profile is produced in structures composed of different partial structures. This so-called "step" effect makes it problematic to produce monolithic structures with continuous surface evolution. Another problem is that when making very densely packed structures, a so-called "tenting" effect may occur. In this case, certain structural regions may be undesirably enlarged or expanded due to the fact that an excessive amount of energy is input into the lithographic material when the structural details are written next to one another. In general, in the known laser lithography methods, undesirable deviations can occur between the structure to be written and the structure actually produced by the combined partial structure.

In the prior art, various methods are known to solve the mentioned problems and errors. In particular, increased accuracy can be achieved by breaking it down into smaller partial structures. However, this process is time consuming as it also increases the number of partial structures to be generated. It is also conceivable to screen the overall structure more finely only in regions of strong contour variation. This increases the amount of calculation when generating the partial structure and is therefore also time-consuming.

Stereolithography is known from us patent No.4,575,330A1 or us patent 5,247,180a, in which a desired structure of a partial structure is built up in blocks or layers in a bath of liquid lithographic material by local exposure with a writing beam. In this case, the writing beam cures the structural region by local exposure in a layer in each case directly on the bath surface of the photolithographic material. The structure is then built up in layers by stepping down the carrier substrate in a bath of photolithographic material. It is not possible with this method to directly build the desired structure in the volume of lithographic material.

Disclosure of Invention

It is an object of the present invention to compensate for undesired errors in the above-described laser lithography method and to provide greater design freedom for the structures to be manufactured. In addition, the object is to shorten the processing time as much as possible.

This object is achieved by the method according to the invention. This is a laser lithography method, in particular by so-called laser direct writing of a laser lithography apparatus in a volume of lithography material or in a volume filled with lithography material. The overall structure is written into the lithographic material by defining at least one partial structure (i.e. by "writing" into the lithographic material with the laser lithographic apparatus), wherein the at least one partial structure at least approximates the desired overall structure.

To write a partial structure, an exposure dose is irradiated into the lithographic material in a spatially resolved manner in the focal region of the laser writing beam using two-photon absorption or, in general, multi-photon absorption. As a result, the lithographic material is locally modified and structured in this way. A development step may follow the final definition of the structure, for example to trigger areas in the lithographic material that have no effect on the overall structure or to cure the overall structure.

In order to achieve this object, it is proposed in particular to vary the exposure dose in at least one of those edge portions which directly adjoin (and are limited to the outer surface itself) the outer surface of the overall structure to be produced, compared with the other regions of the partial structure.

To create the partial structure, the focal area of the laser writing beam is shifted through the photolithographic material, thereby introducing an exposure dose. For shifting the focus range, the laser writing beam may be deflected in a controlled manner, for example by means of a beam guiding device. However, it is also conceivable to displace the lithographic material or the substrate with the lithographic material in a controlled manner relative to the laser writing beam by means of a positioning device. These two concepts can also be used together for shifting. The exposure dose is in particular the volumetric dose of the radiation energy. When the focus area is shifted to the corresponding edge portion, the exposure dose is changed according to the present invention.

In the currently used type of 3D laser writing, it is particularly advantageous to introduce the exposure dose by multiphoton absorption. For this purpose, the lithographic material is preferably designed and the laser writing beam is tuned to the lithographic material in such a way that: it is only possible to change the lithographic material by absorbing multiple photons (e.g. local polymerisation). For this purpose, for example, the wavelength of the laser writing beam (and thus the magnitude of the allocated quantum energy) may be selected such that the energy input required to change the lithographic material is only achieved by absorbing two or more quanta simultaneously. The probability of this process is intensity dependent and increases significantly in the focal area compared to the rest of the writing beam. Basic considerations indicate that the probability of absorbing two or more quanta may depend on the square or higher power of the radiation intensity. In contrast, the probability of a linear absorption process has a different intensity dependence, in particular with a lower power of the radiation intensity. Since attenuation (e.g. according to beer's law) occurs when the laser writing beam penetrates into the lithographic material, writing deep below the liquid level of the lithographic material in the focus region using a linear absorption process would be problematic, since due to attenuation itself there is not necessarily the highest probability of absorption even when focusing below the surface in the focus region. In contrast, the mechanism of multiphoton absorption makes it possible to introduce the required exposure dose into the interior of a volume of lithographic material and to locally modify the lithographic material. Therefore, as known in the art, no equipment is required for stepwise lowering of the support structure in the lithographic material bath.

The lithographic material is locally chemically and/or physically altered, e.g., cured or polymerized, by the radiation exposure dose. The size of the changed regions or "voxels" in the lithographic material depends on the exposure dose, so by changing the exposure dose the spatial extent of the corresponding structural region or "voxel" can be changed. By appropriately dimensioning the exposure dose variation, the size of the written area can be adjusted such that the desired missing areas between the overall structure and the partial structure are compensated.

This makes it possible to compensate for the errors described at the outset, which may occur when the overall structure approaches one or more partial structures. Other problems typically encountered in laser lithography can also be eliminated by the above steps. For example, when a negative photoresist is used, undesirable shrink effects may occur. Negative tone photoresists are cured in the areas exposed to radiant energy. The shrinking effect is caused, for example, by the fact that: the lithographic material occupies less space in the cured state than in the original (e.g., viscous) state. This can be compensated for by a targeted change of the exposure dose.

However, the variation of the exposure dose also makes it possible to directly generate the desired surface structure in a targeted manner. In this case, the exposure dose in the edge portion, which then provides the surface of the overall structure, varies according to the defined pattern. This leads to the following fact: the voxels written in the edge portion have different expansions and thus contribute to the desired surface structuring.

Preferably, the exposure dose is changed only in the edge portion. In other parts, in particular in the inner regions of the partial structure, preferably no change in the exposure dose occurs. Thereby, the arithmetic operations required for controlling the laser lithography apparatus can be simplified.

The described method makes it possible to approximate the overall structure by a partial structure with a relatively coarse screening. At the same time, short processing times can be achieved.

In the present context, the term "lithographic material" essentially refers to a substance whose chemical and/or physical material properties can be altered by irradiation with, for example, a laser writing beam, which is referred to as a photoresist. Depending on the nature of the change induced by the writing beam, the lithographic material may be formed in so-called negative photoresists, in which the radiation causes local hardening or solubility in the developing medium, and in so-called positive photoresists, in which the radiation locally increases solubility in the developing medium.

According to an advantageous embodiment, the overall structure is defined by defining in sequence a plurality of partial structures, which together approximate the overall structure. In this case, it is provided in particular that the exposure dose is changed only in those partial structures which adjoin the outer surface of the overall structure to be produced. In this case, the exposure dose is changed again in the respective partial structure compared with the remaining partial structure in those edge portions directly adjoining the outer surface.

Depending on the desired overall structure, it is also possible to design the partial structure completely as an edge portion, i.e. completely adjoining the outer surface.

The partial structures are for example layered, so that the overall structure is approximated by a plurality of partial structures which are adjacent to one another in a layered manner (overlap one another). In principle, however, the partial structures can also have different shapes. For example, the entire structure may be divided into several line-shaped neighboring regions, or into so-called voxels. For partial structures even complex shapes are advantageous.

The exposure dose in the edge portion can be basically changed in various ways.

It is conceivable to increase the exposure dose in the edge portion compared to the remaining partial structure. This step makes it possible to compensate for the "step effect" described above. In this regard, by increasing the exposure dose, the marginal space regions effectively acquire a greater volume and, due to the grid-like approximation of the overall structure, may fill in unwanted steps or missing regions. In this way, it is also possible to approximate a monolithic structure with continuously extending and curved or arched surfaces.

However, it is also conceivable to reduce the exposure dose in the edge portion compared to the remaining partial structure. By this step, for example, the effects of overexposure, which can occur in small written structures written in rapid succession, can be counteracted. In particular, the so-called "doming effect" which leads to an undesired expansion of the written area due to heat accumulation in closely adjacent and rapidly successively written partial structures can be counteracted.

The exposure dose can be locally changed, for example by changing the radiation power of the laser writing beam. Thus, the intensity of the laser writing beam can be varied, for example by an acousto-optic modulator.

In another aspect, writing of the partial structure is accomplished by scanning a focal region through the photolithographic material. The exposure dose can then be varied by changing the speed of displacement of the focal zone accordingly. As a result, the dwell time of the focal area in a certain region of space is effectively changed and thus the exposure dose of the radiation (both spatially and temporally averaged) is changed.

For example, writing of the partial structure is performed such that the focal region is shifted through the lithographic material along a scan curve having a plurality of closely adjacent curve segments. To change the effective exposure dose, a waiting time may then be waited during which no exposure dose is irradiated between traversing two consecutive and immediately adjacent curve portions. In particular, the laser writing beam is disabled during the waiting time. In the region where the curve portions extend adjacent to each other, the effective exposure dose depends on how long the waiting time is. Thus, by applying a prolonged or shortened waiting time between the curves in the edge portion, the effective (in particular time-averaged) exposure dose can be varied in the edge portion compared to the remaining inner structure.

The exposure dose can also be effectively changed by the following procedure: the immediately adjacent curved portions have an average distance between each other in the edge portions that is different from the average distance in the other portions of the respective local structure. If the curved portions are, on average, close to each other, an increased exposure dose is effectively introduced, both spatially and temporally. If the curve portions are further spaced apart on average, the effective exposure dose is reduced.

When writing partial structures, the exposure dose may also be effectively varied such that the focus region traverses a scan curve through the lithographic material and the scan curve is traversed at least two or more times within the edge portion.

Thus, in general, the exposure dose can be varied by controlling the radiation output of the laser writing beam on the one hand and by varying the exposure pattern on the other hand.

The laser writing beam can basically be designed as a continuous wave laser or as a pulsed laser. Pulsed lasers are characterized by, among other things, an intrinsic pulse rate (e.g., MHz range) and an intrinsic pulse length (e.g., femtosecond range). In order to vary the effective exposure dose (averaged over time), the laser writing beam may additionally be modulated in time, for example by an acousto-optic modulator, and thus modulation pulses may be generated. The modulated pulses in turn have a modulated pulse length and a modulated pulse rate. As described above, a pulsed laser writing beam or a continuous wave laser may be shifted through the lithographic material along a scan curve.

For the determination of the partial structures, the overall structure is preferably selected by calculation, in particular decomposed into voxels or "voxels", and the partial structures are each formed from a group of preferably consecutive voxels. In this respect, the partial structure is composed of a set of voxels (voxels) of similar or identical shape.

For example, the laser write beam is formed as a series of modulated laser pulses having a modulated pulse rate and a modulated pulse length. It may be provided that the voxels are written using modulated laser pulses in each case. However, a voxel may also be defined by a plurality of modulated pulses. The effective (in particular time-averaged) exposure dose may be varied, for example, by varying the modulation pulse rate and/or the modulation pulse length. In this case, the modulation pulse rate particularly denotes the number of modulation pulses per time unit and the modulation pulse length particularly denotes the radiation duration of each pulse. The pulsing of the laser writing beam may be achieved, for example, by temporally modulating the intensity of the laser light, for example, by an acousto-optic modulator. As described above, the pulsed laser writing beam may be displaced through the lithographic material along a scan curve.

In principle, the overall structure is preferably broken down into one or more partial structures by software engineering and a computer. In this regard, a data set (e.g., CAD data) representing the overall structure is provided and stored, as appropriate, in a data carrier or volatile storage device. Whereby the further data set is determined by the computer. In particular, a further data set representing the partial structure or partial structures is determined. Preferably, software is used to perform a non-overlapping decomposition of the whole structure into partial structures. However, it is also advantageous to provide overlapping regions between the different partial structures. In the overlap region, a smooth transition can be achieved in the described manner by changing the exposure dose. In general, a data set representing an overall structure is provided in a computing device having a processor and memory and stored in the memory. Thereby, further data records are determined by the computing device and also stored in the memory. Preferably, when determining the further data set, a grid accuracy is used by which the overall structure is approximated. As explained, steps and/or missing regions may occur during decomposition with the aforementioned grid precision. If the overall structure has a continuous surface course with curvature and curvature, this can lead to the problems explained at the outset.

In order to solve this problem, it is proposed in particular to determine and possibly store further data records (by means of a computing device) from the data set. The further data record represents the deviation of the overall and overall structure of the partial structure or partial structures. This makes it possible to determine the variation in exposure dose from the local deviation in the edge portion. If a step occurs in the edge portion, for example due to screening, which step is not present in the actual overall structure, this will result in a sudden increase in the deviation. The exposure dose can then be increased within this range to complete the step.

A commonly used laser lithography apparatus comprises an optical system (beam directing means, focusing means, lenses, etc.). Such optical systems may be subject to imaging errors, such as field curvature, aberrations or astigmatism. Imaging errors may also lead to undesired deviations between the desired overall structure and the actual structure approximated by the at least one partial structure. With the described method, deviations can be corrected. For example, imaging errors can be determined in relation to the position by calibration measurements and stored in the form of a feature data set. The variation of the exposure dose can then be determined in a position-dependent manner from the characteristic data set in order to minimize the deviation.

The function determining the exposure dose from the deviation may have different characteristics. For example, the use of a monotonically increasing function may be considered. As explained, unwanted missing regions can thus be filled in. However, it may also be advantageous to use a local monotonically decreasing function, for example to compensate for the effects of overexposure. Even more complex functions may be advantageous.

Another method of varying the exposure dose is to first provide a data set representing the outer surface of the overall structure and store it in memory, with the calculator determining the local slope and/or curvature of the outer surface. The change of the exposure dose is then preferably performed in dependence on the local slope and/or curvature of the region of the outer surface adjoining the respective edge portion. In other words, the exposure dose is varied, in particular due to the curvature, slope or change in curvature of the surface of the overall structure, by which method at least one partial structure deviates considerably from the desired overall structure. Again, a monotonically decreasing or monotonically increasing function may in principle be used to reflect the dependence of the slope and/or curvature on the exposure dose.

As explained, the present invention is preferably implemented by a computer. In this respect, the control apparatus for a laser lithography apparatus preferably comprises a computer on which a computer program is executable which executes a control function for a laser lithography apparatus according to the above-described method.

The object mentioned in the introduction is therefore also achieved by a computer program product according to the invention. Furthermore, the object is achieved by a data carrier on which the respective computer program is stored and/or by a data stream representing the respective computer program (e.g. downloadable from the internet).

Drawings

The invention will be explained in more detail below with reference to the drawings.

In the figure:

FIG. 1 shows a schematic view of an overall structure with a complex outer surface formed, wherein the structure is to be fabricated by laser lithography;

FIG. 2 shows a schematic view of a curved region and an approximate partial structure of an exemplary exterior surface;

FIG. 3 shows a schematic to illustrate an unwanted step configuration;

fig. 4 is a schematic view for explaining a variation in exposure dose in an edge portion;

fig. 5 is a schematic view for explaining an alternative step of changing the exposure dose in the edge portion;

FIG. 6 shows a schematic illustrating exposure dose variation;

FIG. 7 shows a schematic view of a structure to be fabricated with a structured outer surface;

fig. 8 shows a schematic diagram for explaining a structure manufactured by exposure variation of exposure dose;

FIG. 9 illustrates another example of applying exposure changes in a perspective plan view;

fig. 10 shows a cross section through the structure according to fig. 9.

Detailed Description

In the figures and the following description, the same reference numerals are used in each case for identical or corresponding features.

Fig. 1 shows a monolithic structure to be written in a volume of a lithographic material by 3D laser lithography. Such structures typically have structural features with measurements in the micrometer or nanometer range. The dimensions of the overall structure may be, for example, in the micrometer range, the millimeter range, or the centimeter range.

The overall structure is designated by reference numeral 10 and has an outer surface 12, which outer surface 12 has a course with different slopes and possibly also curves. As described above, these areas may present problems in writing. The illustrated form is merely an example. The advantages described herein may also be achieved by shapes and curves having curvature, among others.

For the purpose of lithographic manufacturing, the overall structure 10 or a data record representing the structure is decomposed into, for example, a plurality of partial structures 14 or corresponding data sets. This is typically computerized.

Fig. 2 shows how the combined partial structure 14 approximates the desired overall structure 10. For this purpose, only a cross-section of the overall structure 10 is shown in fig. 2. The illustrated decomposition into hierarchical partial structures 14 is exemplary. In principle, the partial structure 14 can be freely selected.

Since the decomposition into partial structures 14 is usually carried out with a certain degree of screening for practical reasons, there is a missing region 16 between the desired outer surface 12 of the overall structure 10 and a certain number of assembled partial structures 14, wherein the outer surface 12 has an offset 18 from the partial structures 14. In principle, these deviations 18 only occur in the edge portions 20 of the partial structure 14, since these edge portions 20 abut the desired outer surface 12 of the overall structure 10 to be obtained. In particular, the partial structure 14 arranged at the top in fig. 2 can be considered as a whole as an edge portion 20, since this partial structure 14 completely adjoins the outer surface 12.

As shown in fig. 3, the writing of the partial structure is performed such that a focal area 26 of a laser writing beam 28 of a corresponding laser lithography apparatus (not shown) is directed through a volume of lithography material (surrounding the overall structure). In the focus region 26, an exposure dose is introduced into the lithographic material.

In one possible embodiment of the method, the partial structures 14 in the photolithographic material are written by a focal region 26, which focal region 26 passes through a scanning curve 22 (in fig. 2 and 3, the scanning curve 22 is shown in part) through the respective partial structure 14.

For example, the partial structure 14 may be written by a focal region which passes through the scan curve 22 and emits a sequence of laser pulses with a defined pulse rate and pulse length. As a result, along the scan curve 22, a series of voxels 24 or voxels 24 forming part of the structure 14 is defined. The voxels 24 are similar or identical in shape. As mentioned above, the size of the voxels 24 is related to the exposure dose introduced into the lithographic material.

If the individual partial structures 14 are written spatially averaged with locally identical exposure doses, the result is an image plotted in fig. 3. In particular, each voxel 24 will have the same size. As a result, this will lead to the errors explained in fig. 2 (missing region 16, offset 18).

To overcome this, according to the method, the exposure dose in the edge portion 20 of the partial structure 14 is changed such that it deviates from the exposure dose in the inner region of the partial structure 14.

In the example of fig. 4, the exposure dose is increased in a voxel 24 '(or voxel 24') of the partial structure 14 which lies within the edge portion 20 of the partial structure 14. As a result, voxel 24' has a larger spatial extent. As a result, the missing regions 16 or deviations 18 may be reduced and the surface 12 may be better approximated.

Depending on the course of the outer surface 12, a better approximation of the partial structure 14 can also be achieved by reducing the exposure dose in the edge portion 20 (see fig. 5).

Fig. 6 shows an example of this: by steadily reducing the exposure dose in the partial structure 14 as the partial structure 14 approaches the outer surface 12, the slope can be approximated with high accuracy. The surface 12 extends in particular obliquely with respect to the direction of extension of the partial structure 14.

However, the general idea of the method according to the invention is not limited to the fact that: the exposure dose in the edge portion 20 may be only reduced or only increased. Basically, it is a general aspect of the present invention to locally vary the exposure dose in the edge portion 20 adjacent to or defining the outer surface 12 according to a defined pattern to produce a desired pattern or to reproduce the pattern as good as possible (if the overall component structure).

In this respect, an advantageous field of application of the invention also consists in producing the desired overall structure on the outer surface by variation of the exposure dose in the edge portions 20 of the respective partial structures 14. This is drawn in an exemplary manner in fig. 7, fig. 7 showing a lens body 30 to be written in a lithographic material. The outer surface 12 of the lens body 30 has a complex structure. As discussed, the lens body 30 is preferably written by sequentially writing a plurality of partial structures 14 approximating the lens body 30. To obtain the contoured outer surface 12, the exposure dose is varied locally in the edge portion 20 of the partial structure 14 as required. The relief on the outer surface 12 is obtained, for example, by introducing an increased exposure dose in the edge region and thus creating an increased said volume of space in the photolithographic material.

For example, FIG. 8 illustrates the fabrication of a structure whose outer surface 12 represents the profile of a Fresnel zone plate or Fresnel lens. The exposure dose used to generate each voxel or voxel 24 is selectively varied in such a way as to approximate the outer surface 12 approximation as accurately as possible.

Another application example is outlined in fig. 9 and 10. In this case, the outer surface 12 has a structured area 32 (see fig. 9), wherein the outer surface 12 has a distinct height profile 34. Such a structure may be used, for example, for calibration in microscopes, in particular for atomic force microscopes.

The variation of the exposure dose in the edge portion 20 is carried out in particular according to a function whose input variables are determined by the course of the outer surface of the overall structure 10 and possibly by the result of the decomposition of the overall structure within the screening frame through the partial structure 14. For example, in fig. 2, one of the partial structures 14 has a region 36 directly adjoining the actual outer surface 12. It is conceivable to computationally determine the slope and/or curvature of the outer surface 12 in the region 36. The variation of the exposure dose in the edge portion 20 of the respective partial structure 14 can then be carried out in accordance with the locally present slope and/or curvature in the region 36. As a result, missing regions 16 or deviations 18 can be compensated for, which occurs particularly in those regions of the outer surface 12 having a pronounced slope and/or curvature. It is also conceivable to calculate the deviations 18 caused by the screening. These deviations 18 represent the difference between the desired surface formed by the outer surface 12 and the actual surface defined by the partial structure 14 in the edge portion 20. Then, as described in detail above, the variation of the exposure dose may be performed according to the deviation 18.

Claims (13)

1. A method for producing a three-dimensional monolithic structure (10) by laser lithography in a lithography material,

defining the monolithic structure (10) by defining at least one partial structure (14) such that the at least one partial structure (14) approximates the monolithic structure (10),

for writing the partial structure (14), an exposure dose is irradiated into the lithography material in a focus region (26) of a laser writing beam (28) while exploiting multiphoton absorption,

characterized in that in the at least one partial structure (14) the exposure dose in an edge portion (20) directly adjoining the outer surface (12) of the overall structure (10) to be produced is varied compared to the remaining partial structures (14),

providing a data set representing the outer surface (12) of the overall structure (10), determining a local slope and/or curvature of the outer surface, the change in exposure dose depending on the local slope and/or curvature of a region (36) of the outer surface (12) adjoining the edge portion (20) of the partial structure (14).

2. The method according to claim 1, wherein the monolithic structure (10) is defined by defining a plurality of partial structures (14) in sequence, wherein the partial structures (14) together approximate the monolithic structure (10).

3. The method according to claim 1 or 2, wherein the exposure dose in the edge portion (20) is increased compared to the remaining partial structure (14).

4. The method according to claim 1 or 2, wherein the exposure dose in the edge portion (20) is reduced compared to the remaining partial structure (14).

5. The method according to claim 1 or 2, wherein the radiation power of the laser writing beam (28) is varied in order to vary the exposure dose.

6. Method according to claim 1 or 2, wherein for writing the partial structure (14) the focal area (26) of the laser writing beam (28) is displaced through the lithographic material and the displacement speed of the focal area (26) is varied in order to vary the exposure dose.

7. Method according to claim 1 or 2, wherein, for writing the partial structure (14), the focal region (26) of the laser writing beam (28) traverses a scan curve (22) which traverses the lithography material in a plurality of immediately adjacent curve portions, wherein a waiting time is waited between traversing two successively traversed curve portions during which no exposure dose is irradiated.

8. Method according to claim 1 or 2, wherein, for writing the partial structure (14), the focus region (26) of the laser writing beam (28) passes through a scanning curve (22) which passes through the lithography material in a plurality of immediately adjacent curve portions, wherein the exposure dose in the edge portion (20) of the respective partial structure (14) is varied such that immediately adjacent curve portions have an average distance to each other in the edge portion (20) which is different from an average distance to each other in other portions of the respective partial structure (14).

9. Method according to claim 1 or 2, wherein for writing the partial structure (14) the focus region (26) of the laser writing beam (28) traverses a scan curve (22) which traverses the lithography material, and the exposure dose in the edge portion (20) is changed by traversing the scan curve (22) several times within the edge portion (20).

10. Method according to claim 1 or 2, wherein the partial structure (14) consists of a set of voxels (24) and the laser writing beam (28) is formed as a series of laser pulses radiated at a pulse rate, wherein each laser pulse is used to write a voxel (24) and the exposure dose is varied by varying the pulse rate and/or pulse length.

11. The method of claim 1, wherein the dependence is given by a monotonically increasing function.

12. The method of claim 1, wherein the dependence is given by a monotonically decreasing function.

13. A computer program product comprising instructions which, when executed by a computer, cause the computer to perform the method of any one of claims 1 to 12.