BRIEF DESCRIPTION THE DRAWINGS
FIG. 1 is a plan view of a diffractive optical element with grid sections according to an exemplary embodiment of the invention.
FIG. 1A is an enlarged plan view of a section of the plan view of FIG. 1 showing the grid sections of the diffractive optical element.
FIGS. 2A to 2C are plan views of the grid sections of a diffractive optical element according to other exemplary embodiments of the invention.
FIG. 3 illustrates schematically an optical projection system with a diffractive optical element positioned at an exemplary position according to an embodiment of the invention.
FIG. 4 illustrates schematically an optical projection system with a diffractive optical element at another exemplary position according to a further embodiment of the invention.
FIG. 5 is a cross-sectional view of a photomask and a diffractive optical element according to an embodiment of the invention with the diffractive optical element fixed on the photomask.
FIGS. 6A to 6C show cross-sectional views of a diffractive optical element according to further embodiments of the invention.
FIG. 7 illustrates schematically the effect of a diffractive optical element according to an embodiment of the invention.
FIGS. 7A and 7B show the illumination source distribution in an optical projection system before and after the diffractive optical element according to an embodiment of the invention.
FIG. 8 shows a plan view on exemplary mask pattern elements.
FIGS. 9 to 11A show plan views on grid sections of a diffractive optical element with different grating parameters and absorption properties according to embodiments of the invention and corresponding resist pattern elements obtained from the mask pattern elements of FIG. 8.
Corresponding numerals in the different figures refer to corresponding parts and features unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily in all respects drawn to scale.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a diffractive optical element 20 according to an exemplary embodiment of the present invention. The diffractive optical element 20 may be part of a mask arrangement according to a first aspect of the invention or part of an optical projection system according to another aspect of the invention. The diffractive optical element 20 includes an active region 240 and an edge region 25. In the edge region 25 a suspension mechanism (not shown) may be provided to fix the diffractive optical element 20 in a predetermined position with respect to a corresponding photomask or scanner optics (not shown). The active region 240 comprises a plurality of grid sections 24. Each grid section 24 corresponds to a mask section of the corresponding photomask (not shown), wherein each mask section comprises mask pattern elements, respectively.
FIG. 1A illustrates an enlarged section of the active region 240 with a plurality of grid sections 24a to 24i. Each grid section 24a to 24i comprises a grating and an absorbing element having defined grating parameters and absorption properties, respectively. The grating parameters and absorption properties of each grid section 24a to 24i correspond to the desired dimension correction of resist pattern elements obtained from the respective mask pattern elements. A first mask section comprises a first mask pattern element and a second mask section comprises a second mask pattern element, wherein the first and the second mask pattern elements have essentially the same shape and size. Differently stated, both mask pattern elements are elements of the same type, like elements for trench openings, contact vias or landing pads of electronic devices, for instance, that have slightly different dimensions. The mask pattern elements may be of any type of element, but typically the dimensions of these elements and their homogeneity across the imaging field or/and the wafer are critical and have to be in a defined range in order to achieve high yield and performance of electronic devices.
If the dimensions of the resist pattern elements obtained from the mask pattern elements of the first and second mask section deviate from the desired dimensions and the deviation may not be compensated for both mask sections by changing the projection parameters, like illumination source distribution, numerical aperture or exposure dose for example, a locally restricted change of projection parameters for one or both mask sections is desired.
The deviations may be caused by different dimensions of the mask pattern elements. Furthermore, different dimensions of resist pattern elements may be obtained even from mask pattern elements having the same dimensions due to deviations in the projection of the mask pattern elements onto the photoresist, wherein the deviations are caused by defects or production imperfections of the illumination optic and/or projection lens mechanisms of the whole projection system. Furthermore, the deviations of the dimensions of the resist pattern elements caused by imperfections of the projection system may not be the same for both dimensions of two-dimensional pattern elements. Differently stated, the length and the width of the resist pattern elements may vary from the desired dimensions in a different ratio.
A local correction of the dimensions of the resist pattern elements may be achieved by locally changing illumination source distribution through a corresponding grid section in the diffractive optical element. Therefore, each grid section has defined grating parameters, like the width and the period of the grating lines or their orientation for example, and defined absorption properties. The grating parameters and absorption properties may be defined by the shape and the orientation of the grating elements, by the thickness and the optical properties (e.g., refractive index, absorption coefficient) of the material of the gratings, and by the absorption element as well. The grid sections may comprise linear gratings, differently-shaped grating elements, semi-transparent phase-shifting elements, transparent elements or two-dimensional gratings. Thus, the length or the width of a resist pattern element may be corrected independently from one another or the length and the width may be corrected in a defined ratio such that resist pattern elements obtained from mask pattern elements of different mask sections have the same specified (predetermined) dimensions.
The absorbing elements of the grid sections of the diffractive optical element may include a two-dimensional grating (checkerboard-like gratings) or statistically distributed absorbing structures. The grating parameters of the respective two-dimensional gratings or the shape and the density of the absorbing structures are defined such that desired absorbing properties of the absorbing element of each grid section are achieved.
If only one dimension of a first resist pattern element differs from the corresponding dimension of a second resist pattern element and the dimensions of the first resist pattern element has specified, predetermined dimensions, the second grid section of the diffractive optical element may comprise a second grating. The absorption properties of the second absorbing element may be equal to that of the absorption properties of the first absorbing element comprised in the first grid section of the diffractive optical element.
Referring again to FIG. 1, the active region 240 of the diffractive optical element 20 may comprise a plurality of grid sections 24 having the same shape and the same size. The grid sections 24 may also have different shape and size. Exemplarily, the size of the grid sections 24 may be in the range of about (5×5) μm2 to about (500×500) μm2. In one embodiment, the size is about (100×100) μm2.
Referring again to FIG. 1A, the grid sections 24a to 24i have gratings 26 with different grating parameters, while the absorption properties of the grid sections are essentially the same. Each grating 26 has a pattern of parallel opaque, transparent and/or semitransparent grating lines. The grating lines fill the whole area of grid sections 24a to 24i in this example. The orientation of the grating 26 of each grid section 24a to 24i is the same in this example.
FIG. 2A illustrates a plan view of a section of a diffractive optical element 20 according to a further embodiment of the invention. Each grid section 24a to 24i has the same shape and size and comprises a grating 26 arranged in a central region and a non-grating region 28. Each grating 26 has the same size. The non-grating regions 28 are transparent. Each grid section 24a to 24i has predetermined grating parameters and absorption properties depending on the actual dimensions of the respectively corresponding mask sections of the photomask.
FIG. 2B illustrates a plan view of a section of a diffractive optical element 20 according to a further embodiment of the invention. Each grid section 24a to 24i has the same shape and size, wherein the gratings 26 and the respective non-grating regions 28 may have different sizes. The grid sections 24 comprise gratings 26 arranged in central regions of the grid sections 24, respectively, and transparent non-grating regions 28. The size of the gratings 26 differs from grid section 24a to 24i to grid section 24a to 24i. Thus, the size of the respective transparent region 28 differs for each grid section 24a to 24i resulting in different absorption properties. Each grid section 24a to 24i has defined grating parameters and absorption properties depending on the corresponding mask sections of the photomask.
FIG. 2C illustrates a plan view of a section of a diffractive optical element 20 according to a further embodiment of the invention. Each grid section 24a to 24i has the same shape and size. The grid sections 24 comprise gratings 26 arranged in central regions of the grid sections 24 respectively, and non-grating regions 28. The non-grating regions 28 comprise absorbing structures 27. The absorbing structures 27 are, by way of example, absorbing dots with a size of (1×1) μm2 to (2×2) μm2. The absorbing dots are homogeneously and (statistically) randomly distributed within the non-grating region 28 with a predetermined average density. The gratings 26 of respective grid sections 24a to 24d have different sizes. Thus the size of the non-grating regions 28 differs for grid sections 24a to 24d. Each grid section 24a to 24d has defined grating parameters and absorption properties depending on the corresponding mask sections of the photomask. Furthermore, the average density and the size of the absorbing structures 27 differ for each grid section 24, thus varying the absorption properties of the respective grid section 24.
FIG. 3 illustrates an optical projection system according to an exemplary embodiment of the invention. The optical projection system comprises a light source 1, an illumination optic 2 defining the illumination source distribution and the polarization characteristics of the illumination light beam 100, a photomask 10 comprising a transparent mask substrate 11 and mask pattern elements 12 and a corresponding diffractive optical element 20 comprising a transparent element substrate 21 and grid pattern elements 22. The optical projection system comprises further a projection lens 3 for projecting the mask pattern elements 12 onto a photoresist layer 5 that covers a semiconductor wafer 4. The diffractive optical element 20 is positioned in an intermediate projection plane 13 of photomask 10 between illumination optic 2 and photomask 10, wherein a further lens (not shown) is positioned between diffractive optical element 20 and photomask 10.
Intermediate projection plane 13 is an optical conjugate plane to a plane of a conventional pellicle having a distance of 100 μm to 10 mm to the plane of mask pattern elements 12 of photomask 10 and being positioned between the plane of mask pattern elements 12 and illumination optic 2. Grid pattern elements 22 are projected in focus into this plane.
In one embodiment, the diffractive optical element 20 is maintained at a mechanical system (not shown), which is used to replace a first diffractive optical element 20 corresponding to a first photomask 10 by a second diffractive optical element 20 corresponding to a second photomask 10. Furthermore, the mechanical system carrying the diffractive optical element 20 moves corresponding to the motion of photomask 10 during the projection of mask pattern elements 12 into photoresist 5.
FIG. 4 illustrates an optical projection system according to another embodiment of the invention. The optical projection system comprises a light source 1, an illumination optic 2 defining the illumination source distribution and the polarization characteristics of an illumination light beam 100, a photomask 10 comprising a transparent mask substrate 11 and mask pattern elements 12, and a corresponding diffractive optical element 20 comprising a transparent element substrate 21 and grid pattern elements 22. The optical projection system further comprises a projection lens 3 for projecting the mask pattern elements 12 onto a photoresist layer 5 that covers a semiconductor wafer 4. A mounting frame 29 fixes the diffractive optical element 20 on that side of photomask 10 facing illumination optic 2.
FIG. 5 is a cross-sectional view of a diffractive optical element 20 and a corresponding photomask 10 being part of an optical projection system as shown in FIG. 4. A mounting frame 29 fixes the diffractive optical element 20 on a mask substrate 11 of photomask 10. Mask pattern elements 12 of photomask 10 may be disposed on that side of transparent mask substrate 11 of photomask 10 that faces a projection lens 3 as shown in FIG. 4. Grid pattern elements 22 of diffractive optical element 20 are disposed on that side of a transparent element substrate 21 of diffractive optical element 20 that faces photomask 10. Nevertheless, grid pattern elements 22 may be formed on the other side of diffractive optical element 20. Furthermore, grid pattern elements 22 may be formed within transparent element substrate 21 of diffractive optical element 20.
As shown in FIG. 6A, a diffractive optical element 20 comprises a transparent element substrate 21 and a grid layer 220 disposed on a surface of the element substrate 21. Within grid layer 220 grid pattern elements 22 comprising gratings and/or absorbing structures are formed. The material of grid layer 220 may be arbitrarily selected from materials that influence the illumination light beam in a predetermined way. For instance, MoSi or another semitransparent material or an opaque material like Cr may be used. Furthermore, transparent or semitransparent phase-shifting materials or layer stacks comprising one or more of the above-mentioned materials may be used. Grid pattern elements 22 of each grid section 24a to 24d, shown in FIG. 6A, form a grating with grating parameters and absorption properties such that resist pattern elements obtained from mask pattern elements in mask sections corresponding to the grid sections 24a to 24d of diffractive optical element 20 have predetermined dimensions.
As shown in FIG. 6B, an antireflective coating (ARC) layer 23 may be provided on both sides of a transparent element substrate 21 of a diffractive optical element 20. A grid layer 220 comprising grid pattern elements 22 is disposed on ARC layer 23. ARC layer 23 may also be disposed only on one side of element substrate 21.
The use of one or more ARC layers 23 may cause additional dimension deviations of resist pattern elements obtained from mask pattern elements 12. Process imperfections may cause local thickness variations of ARC layer 23 across the active area of diffractive optical element 20. The thickness of ARC layer 23 corresponds to the transmission efficiency of ARC layer 23 and thus influences the projection of mask pattern elements 12 onto a photoresist layer.
According to an exemplary embodiment, the correction of dimension deviations of resist pattern elements caused by variations in the thickness of ARC layer 23 is incorporated into the correction of dimension deviations caused by dimension variations of mask pattern elements 12 or caused by local deviations of the projection system. Dimension deviations caused by ARC layer 23 may be corrected by absorbing structures 27 comprised in grid sections 24 of diffractive optical element 20. The distribution and density of absorbing structures 27 of each grid section 24 corresponds to the required correction of transmission efficiency in respective sections of ARC layer 23. Thus the absorption properties of each grid section 24 are defined such that they correspond to respective mask sections of photomask 10 and respective layer sections of ARC layer 23.
According to another embodiment, as shown in FIG. 6C, a diffractive optical element 20 comprises a first grid layer 220 and a second grid layer 221. First grid layer 220 comprises grid pattern elements 22. The grating parameters and the absorption properties of the gratings and/or absorbing structures forming grid pattern elements 22 of grid layer 220 for each grid section 24 are defined such that they correct dimension deviations caused by dimension variations of mask pattern elements 12 or caused by local deviations of the projection system. Second grid layer 221 is disposed on first grid layer 220 as shown in FIG. 6C, but may also be disposed beneath first grid layer 220. Second grid layer 221 comprises grid pattern elements 222. Grid pattern elements 222 are absorbing structures having absorption properties for each grid section 24 defined such that they correct dimension deviations caused by variations in the thickness of ARC layer 23.
Both grid layers 220 and 221 may be formed on one side or on opposite sides of diffractive optical element 20.
Referring to FIG. 7, the effect of a diffractive optical element 20 on the illumination source distribution of the projecting light is explained. A diffractive optical element 20 fixed on a photomask 10 is shown, but the effect is essentially the same if a diffractive optical element 20 is positioned in an intermediate projection plane of the photomask 10 as shown in FIG. 3.
As shown in FIG. 7, the illumination light beam 100 is diffracted by grid pattern elements 22 of the diffractive optical element 20 into a 0-order light beam 100c and in ± (plus/minus) 1st-order light beams 100a and 100b. The diffracted light may include also higher order lights depending on the grating parameters of grid pattern elements 22. The angles of the diffracted light beams 100a and 100b are given by
sin(θ)=λ/P,
wherein λ is the wavelength of the light and P is the period of the grating lines of grid pattern elements 22.
FIG. 7A shows the illumination source distribution 30 of the incident illumination light beam 100 of FIG. 7. Illumination source distribution 30 is defined by illumination optic 2 of FIG. 3 or FIG. 4. By the way of example, a quadruple illumination source distribution 30 is shown having four light regions 31 and a dark region 32.
FIG. 7B shows the resulting corrected illumination source distribution 30′ of the light after passing a grid section with a linear (parallel lines) grating of a diffractive optical element 20. Corrected illumination source distribution 30′ is altered with respect to illumination source distribution 30 as shown in FIG. 7A due to the diffraction of light beam 100. Each light region 31 is spread along a first direction by two light regions 31a and 31b, wherein light region 31a results from the minus 1st-order light beam 100a and light region 31b results from the plus 1st-order light beam 100b. Nevertheless, other corrected illumination source distributions 30′ are possible depending on the grating parameters of grid pattern elements 22. Furthermore, the intensity of the diffracted light may be altered with respect to the intensity of incident illumination beam 100 by tuning the phase and the absorption properties of grid pattern elements 22.
Referring now to FIGS. 8 to 11, the effect of a diffractive optical element 20 on the dimensions of resist pattern elements 52 is explained.
FIG. 8 shows a plan view of a section of a photomask 10 comprising opaque mask pattern elements 12 and a transparent mask substrate 11. The mask pattern elements 12 corresponding to contact structures in a contact layer of high-density array transistors are shown. Photomask 10 may comprise other mask pattern elements 12. The mask pattern elements 12 have a width wm measured in x-direction and a length lm measured in y-direction.
FIGS. 9 to 11 illustrate resist pattern elements 52 that are obtained from the mask pattern elements 12 as shown in FIG. 8. The resist pattern elements 52 may be unexposed regions of a photoresist layer 5 which are surrounded by an exposed region 51. The contours of the respectively corresponding mask pattern elements 12 are shown by the dashed lines. The resist pattern elements 52 have a width wr measured in x-direction and a length lr measured in y-direction.
In FIG. 9, wr is 75 nm and lr is 114.6 nm for example. The resist pattern elements 52 are obtained from a mask section corresponding to a grid section 24a of a diffractive optical element 20, wherein grid section 24a is shown in FIG. 9A. Grid section 24a comprises only a non-grating section 28 being transparent (non-absorbing). Differently stated, the grating parameters of a grating and the absorption properties of an absorbing element of grid section 24a are defined such that they do not change the projection of mask pattern elements 12 onto photoresist layer 5 by an optical projection system. These parameters are chosen since the resist pattern elements 52 have the desired dimensions.
FIG. 10 shows resist pattern elements 52 obtained from a mask section corresponding to a grid section 24b of the diffractive optical element 20 according to FIG. 10A. Grid section 24b comprises a grating 26 with grating lines running along the y-direction. The dimensions of the resist pattern elements 52 are wr=75 nm and lr=125 nm. Thus, the lengths of resist pattern elements 52 are increased with respect to the lengths of resist pattern elements 52 of FIG. 9 due to the diffraction of the projecting light at the grating lines of grid section 24b without changing the widths of corresponding resist pattern elements 52.
FIG. 11 illustrates resist pattern elements 52 obtained from a mask section corresponding to a grid section 24c of the diffractive optical element 20, shown in FIG. 11A. The grid section 24c comprises a grating 26 with grating lines running along the x-direction. The dimensions of the resist pattern elements 52 are wr=75 nm and lr=102 nm. Thus, the lengths of resist pattern elements 52 are decreased with respect to the lengths of resist pattern elements 52 of FIG. 9 due to the diffraction of the projecting light at the grating lines of grid section 24c without changing the widths of corresponding resist pattern elements 52.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and the scope thereof. Accordingly, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
Patent Application
20070287075
