METHOD FOR MACHINING A WORKPIECE IN THE PRODUCTION OF AN OPTICAL ELEMENT

Abstract

A method for the zonal polishing of a workpiece includes using a polishing tool to guide a structured polishing pad over the surface of workpiece to remove material from the workpiece. A structured polishing pad includes a structuring adapted to the movement of a polishing tool.

Claims

1. A method, comprising: using a polishing tool to guide a structured polishing pad over a surface of a workpiece to remove material from the workpiece, thereby zonally polishing the workpiece, wherein the structured polishing pad comprises a structure adapted to movement of the polishing tool.

2. The method of claim 1, further comprising rotating the structured polishing pad over the surface of the workpiece.

3. The method of claim 2, further comprising eccentrically moving the structured polishing pad over the surface of the workpiece.

4. The method of claim 1, further comprising eccentrically moving the structured polishing pad over the surface of the workpiece.

5. The method of claim 1, further comprising, before zonally polishing the workpiece, adapting the structure of the structured polishing pad to the movement of the polishing tool over the surface of the workpiece.

6. The method of claim 1, further comprising, after zonally polishing the workpiece, correcting and smoothing the surface of the workpiece.

7. The method of claim 6, further comprising, after correcting and smoothing the surface of the workpiece, producing an optical element from the workpiece.

8. The method of claim 7, wherein the optical element comprises a reflective surface supported by the workpiece.

9. The method of claim 1, further comprising, before zonally polishing the workpiece, zonally lapping the workpiece.

10. The method of claim 9, wherein zonal lapping comprises using an effective area of the tool that is less than 20% of an area of the surface of the workpiece that is to be processed.

11. The method of claim 1, wherein zonal polishing comprises using an effective area of the tool that is less than 20% of an area of the surface of the workpiece that is to be processed.

12. The method of claim 1, wherein a rate of material removal from the workpiece is approximately constant.

13. A polishing pad, comprising: a body comprising a primary structuring comprising a spiral shape comprising a plurality of spiral arms.

14. The polishing pad of claim 13, wherein the spiral arms comprise mutually deviating opening angles.

15. The polishing pad of claim 13, further comprising a secondary structuring which comprises at least one member selected from the group consisting of symmetrical channels, rotationally symmetrical channels, and rotationally asymmetrical channels.

16. A polishing pad, comprising: a body having a structure, wherein at least one of the following holds: the structuring is irregular; the structure is asymmetrical; the structuring is regular; and the structuring is symmetrical.

17. The polishing pad of claim 16, wherein the structuring comprises a checkered pattern that is regular and/or symmetrical.

18. The polishing pad of claim 16, wherein the structuring comprises channels and ridges that are regular and/or symmetrical.

19. The polishing pad of claim 18, wherein: the ridges have a width of approximately 1 mm to approximately 5 mm; the channels have a width of approximately 0.3 mm to approximately 5 mm; and the channels have a depth of at least approximately 100 m.

20. The polishing pad of claim 19, wherein the depth of the channels is at most 500 m.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0042] Various exemplary embodiments are explained in more detail below with reference to the figures. The figures and the relative sizes of the elements illustrated in the figures in relation to one another should not be regarded as to scale. Rather, individual elements may be illustrated with exaggerated size or size reduction in order to enable better illustration and for the sake of better understanding.

[0043] FIG. 1A shows a schematic illustration of a rotary tool.

[0044] FIG. 1B shows a schematic illustration of the zonal machining according to the disclosure with the use of a rotary tool.

[0045] FIG. 1C shows a schematic illustration of a rotary tool in operation.

[0046] FIG. 2A shows a schematic illustration of an eccentric tool.

[0047] FIG. 2B shows a schematic illustration of the zonal machining according to the disclosure with the use of an eccentric tool.

[0048] FIG. 3A shows a polishing pad without structuring from the prior art.

[0049] FIG. 3B shows a polishing pad with structuring according to the disclosure.

[0050] FIG. 3C shows a polishing pad with primary and secondary structuring according to the disclosure.

[0051] FIG. 3D shows the removal rates for the polishing pads in accordance with FIGS. 3A, 3B and 3C.

[0052] FIG. 4 shows a polishing pad with primary and secondary structuring according to the disclosure.

[0053] FIG. 5A shows a polishing pad with structuring according to the disclosure.

[0054] FIG. 5B shows the detailed structure in the case of the polishing pad from FIG. 5A.

[0055] FIG. 5C shows the removal rate for the polishing pad from FIG. 5A in comparison with a polishing pad without structuring.

[0056] FIG. 6 shows a flow diagram for elucidating one possible embodiment of the method according to the disclosure.

[0057] FIG. 7 shows a schematic illustration of an optical element.

[0058] FIG. 8 shows a schematic illustration of a construction of a microlithographic projection exposure apparatus designed for operation in the EUV.

[0059] FIG. 9 shows a schematic illustration of a construction of a microlithographic projection exposure apparatus designed for operation in the DUV.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0060] FIGS. 1A-1B show a schematic illustration of a rotary tool 100. The tool carrier 106, which carries a polishing pad (not shown in FIG. 1A), rotates about the rotation axis 104. A polishing agent is guided onto the surface to be polished via a polishing agent supply 102 consisting of tubes fixed outside the tool.

[0061] FIG. 1B shows a schematic illustration 120 of the zonal machining according to the disclosure with the use of a rotary tool. The polishing machining according to the disclosure is carried out as zonal workpiece machinings. Here in each case the size of the tool is significantly smaller than the size of the workpiece, wherein the area of the tool can typically occupy less than 10% of the surface 114 to be polished of the workpiece 140. For complete coverage of the zonally machined surface 114 of the workpiece 140 an excursion section 110 is used, which is additionally used by the rotary tool 100 and the area of which has to be swept over by the rotary tool 100 in addition to the surface 114 that is actually to be polished. The effective area 112 of the rotary tool 100, that is to say the area on which material is removed, when the tool is not guided over the workpiece, is illustrated schematically in FIG. 1B. The shape of the polishing pad is illustrated as circular in the present case, but can also have other shapes, such as, for example, a rectangular, square or irregular shape.

[0062] FIG. 1C shows a schematic illustration of a rotary tool 100 in operation. The surface 114 to be polished of the workpiece 140 is embodied as a freeform surface. A tool carrier 136 carries a polishing pad 130. A polishing agent 132 is situated between the surface 114 to be polished and the polishing pad 130. The view in FIG. 1C is applicable to all the zonal tools shown in the present patent application.

[0063] FIG. 2A shows a schematic illustration of an eccentric tool 200. In the case of the eccentric movement, the tool maintains its orientation or its alignment with respect to the workpiece surface. The tool carrier 206 moves about the rotation axis 204. The polishing agent is guided onto the surface to be polished via a polishing agent supply 202 consisting of tubes fixed outside the tool.

[0064] FIG. 2B shows a schematic illustration 220 of the zonal machining according to the disclosure with the use of an eccentric tool 200. Here in each case the size of the tool is significantly smaller than the size of the workpiece, wherein the area of the tool can typically occupy less than 10% of the surface 214 to be polished of the workpiece 140. For complete coverage of the zonally machined surface 214 of the workpiece 140 an excursion section 210 is used, which is additionally used by the eccentric tool 200 and the area of which has to be swept over by the eccentric tool 200 in addition to the surface 214 that is actually to be polished. Owing to the eccentric movementfor the same area of the polishing pada larger excursion section 210 is used in the case of the eccentric tool 200 than in the case of the rotary tool 100. The effective area 212 of the eccentric tool 200, or area on which material is removed, when the tool is not guided over the workpiece, is illustrated schematically in FIG. 2B. The graphical illustration of the eccentric tool in operation corresponds to the illustration of the rotary tool in operation as shown in FIG. 1C. A separate illustration has therefore been dispensed with.

[0065] FIG. 3A shows a polishing pad 312 without structuring from the prior art. The rotary tool 100, the polishing pad 312 of which is not structured, has a removal rate that varies significantly during machining (see FIG. 3D), and also dried-on polishing agent residues 316 that are visible on the polishing pad 312 after machining. These effects, which are attributed to an inadequate supply of polishing agent to the center of the tool, which is caused by the rapid rotation of the rotary tool 100 about the rotation axis 318 can be reduced via a suitable structuring of the polishing pad.

[0066] FIG. 3B shows a polishing pad 322 having a structuring according to the disclosure in a spiral shape 320. The spiral here is designed such that during the rotary movement in the direction of rotation 314 (=in the counterclockwise direction) of the rotary tool 100, the polishing agent 132 is forced into the center 318 of the polishing pad 322. Drying-on of the polishing agent 132 can be reduced as a result. The opening angles 317, 319 of the spiral arms deviate from one another in order to produce a certain asymmetry. The asymmetry is intended to reduce the fine structures introduced by the polishing pad 322 itself on the polished surface 114, 214 of the workpiece 140.

[0067] FIG. 3c shows a polishing pad 332 having a primary structuring in a spiral shape 320 and a secondary structuring in the form of rotationally symmetrical (about the rotation axis 318) channels 334. The channels 334 are intended to result in an improved supply of polishing agent between the spiral arms 320.

[0068] FIG. 3d shows the removal rates for rotary tools 100 having polishing pads in accordance with FIGS. 3A, 3B and 3C. The removal rate 311 with the use of a rotary tool 100 having a polishing pad without structuring 312 varies greatly. The removal rate 321 with the use of a rotary tool 100 having a polishing pad having a structuring in a spiral shape 320 still varies greatly. The removal rate 331 over time with the use of a rotary tool 100 having a polishing pad 332 having a primary structuring in a spiral shape 320 and a secondary structuring in the form of rotationally symmetrical channels 334 is substantially constant. However, an undesired fine structure can be introduced by the rotationally symmetrical channels 334, and it would have to be removed again in subsequent steps. In order to avoid this detour, the disclosure proposes, as illustrated schematically in FIG. 4, using a polishing pad 412 having a primary structuring in a spiral shape 420 and a secondary structuring in the form of asymmetrically arranged channels 422. The avoidance of periodicities and symmetries in the structuring of the polishing pad 412 serves to minimize the fine structure caused by the rotary tool 100 itself. Expressed in yet another way: chaotically arranged channels are intended to minimize the formation of fine structures that are transferred from the polishing pad 412 to the surface 114, 214 of the workpiece 140.

[0069] FIG. 5A shows a polishing pad 512 having a regular and symmetrical structuring 521 according to the disclosure for an eccentric tool 200. The structuring 521 has channels 522 and ridges 524. A checkered pattern is shown in the present example. The shape of the polishing pad 512 is illustrated as circular in the present case, but can also have other shapes, such as, for example, a rectangular, square or irregular shape.

[0070] FIG. 5B shows the detailed structure 521 for the polishing pad 512 from FIG. 5A. The ridges 524 have a width d1 of approximately 1 mm to approximately 5 mm, the channels 522 have a width d2 of approximately 0.3 mm to approximately 1 mm and a depth d3 of at least approximately 100 m and preferably approximately 500 m. This is intended to ensure that the polishing agent 132 remains distributed uniformly below the polishing pad 512 during the polishing process.

[0071] FIG. 5C shows the removal rate 511 for an eccentric tool 200 having the polishing pad 512 from FIG. 5A in comparison with the removal rate 510 for an eccentric tool 200 having a polishing pad without structuring 312. The variation of the removal rate 511 with the use of an eccentric tool having a polishing pad having a regular and symmetrical structuring (checkered pattern) 512 is significantly reduced. The eccentric tool 200 having the polishing pad 512 according to the disclosure can be in operation for longer in comparison with the polishing pad without a structuring 312.

[0072] Furthermore, a method for machining a workpiece 140 or a (mirror) substrate 140 in the production of an optical element 150 is explained with reference to the flow diagram illustrated in FIG. 6.

[0073] In accordance with FIG. 6, a first step 610 firstly involves providing a workpiece blank 140 composed of the raw material or (mirror) substrate material. In a subsequent step 620, the workpiece blank 140 is machined for the contouring of the optical element 150 by grinding, for example, the desired contour of the mirror substrate 140 or of the optical element 150 being produced.

[0074] Afterward, in order to produce the fully polished surface 114, 214 of the workpiece 140 in a manner according to the disclosure, a two-stage process is carried out, in which optional zonal lapping machining (step 630) is combined with zonal polishing machining (step 640). In this case, firstly the surface 114, 214 of the workpiece 140, which surface can be a freeform surface without rotational symmetry or other axes of symmetry, is zonally machined using a lapping tool. The lapping removal can be 15 m, for example, wherein the removal rate can be chosen to be greater than in the subsequent polishing step 640 by a factor of ten. This results in a significant speed advantage during the production of the fully polished surface. As a result of the zonal lapping process, the temporary production of additional depth damage is deliberately accepted. If it is assumed in the above example of a lapping removal of 15 m, for instance, that depth damage having a depth of approximately 30 m is originally present as a result of the preceding grinding process in the workpiece, then firstly a partial reduction of this depth damage already present to e.g. approximately 15 m and additionally further depth damage having a depth of likewise approximately 15 m, for example, arise as the interim result after the zonal lapping process. However, both types of depth damage (i.e. both that already present originally as a result of the grinding process 620 and that added owing to the lapping process 630) can then be eliminated efficiently in the subsequent zonal polishing process 640.

[0075] In the zonal polishing 640, a polishing tool 100, 200 guides a structured polishing pad 322, 332, 412, 512, the structuring of which is adapted to the movement of the polishing tool 100, 200 in a material-removing manner over a surface 114, 214 to be polished of the workpiece 140.

[0076] In the case of a rotary tool 100 as polishing tool, the structured polishing pad 322, 332, 412 is guided in a rotary movement over the surface 114 to be polished of the workpiece 140.

[0077] In the case of an eccentric tool 200 as polishing tool, the structured polishing pad 512 is guided in an eccentric movement over the surface 214 to be polished of the workpiece 140.

[0078] Both the optional lapping machining (step 630) and the subsequent polishing machining (step 640) are carried out as zonal workpiece machinings. In this case, the size of the tool is significantly smaller than the size of the workpiece, wherein the area of the tool can typically occupy less than 10% of the workpiece surface. Furthermore, as illustrated in FIGS. 1B and 2B, for complete coverage of the zonally machined area of the workpiece, an excursion section 110, 210 is used, which is additionally used and the area of which has to be swept over by the tool 100, 200 in addition to the surface 114, 214 that is actually to be polished.

[0079] A final step 650 involves correcting and smoothing the surface 114, 214 of the workpiece 140 or of the optical element 150. The final specification of the optical element 150 is thus produced. Besides polishing processes, the step 650 can, for example, also include ion beam figuring processes (IBF).

[0080] FIG. 7 shows a schematic illustration of an optical element 150. The optical element 150 is a mirror in the present example. A layer or a layer system 115 (which, in the case of a mirror, for instance, can include e.g. a reflection layer system composed of molybdenum and silicon layers) is applied on the fully polished surface 114 of the workpiece 140, also referred to as substrate. The substrate 140 is machined using a suitable material-removing (optionally also material-adding) tool, which is referred to for short as tool in the present text. Not only the substrate 140 but also the layer 115 itself can be machined in this way. The workpiece 140 can be produced e.g. from silicon (Si) or quartz glass doped with titanium dioxide (TiO2), with examples of materials that are usable being those sold under the trade names ULE (by Corning Inc.) or Zerodur (by Schott AG).

[0081] FIG. 8 shows a schematic illustration of a construction of a microlithographic projection exposure apparatus designed for operation in the EUV, wherein the present disclosure can be used in the production of an arbitrary optical element of the projection exposure apparatus. However, the disclosure is not restricted to realization in the production of optical elements for operation in the EUV, but rather is also realizable in the production of optical elements (including transmissive elements such as e.g. lens elements) for other operating wavelengths (e.g. in the DUV range or at wavelengths of less than 250 nm).

[0082] In accordance with FIG. 8, an illumination device in a projection exposure apparatus 700 designed for EUV includes a field facet mirror 703 and a pupil facet mirror 704. The light from a light source unit including a plasma light source 701 and a collector mirror 702 is directed onto the field facet mirror 703. A first telescope mirror 705 and a second telescope mirror 706 are arranged in the light path downstream of the pupil facet mirror 704. A deflection mirror 707 is arranged downstream in the light path, the deflection mirror directing the radiation that is incident thereon onto an object field in the object plane of a projection lens including six mirrors 751-756. At the location of the object field, a reflective structure-bearing mask 721 is arranged on a mask stage 720, the mask being imaged with the aid of the projection lens into an image plane in which a substrate 761 coated with a light-sensitive layer (photoresist) is situated on a wafer stage 760.

[0083] FIG. 9 shows a schematic view of a DUV lithography apparatus 800, which includes a beam shaping and illumination system 802 and a projection system 804. In this case, DUV stands for deep ultraviolet and denotes a wavelength of the working light of between 30 and 250 nm.

[0084] The DUV lithography apparatus 800 includes a DUV light source 806. By way of example, an ArF excimer laser that emits radiation 808 in the DUV range at 193 nm, for example, can be provided as the DUV light source 806.

[0085] The beam shaping and illumination system 802 illustrated in FIG. 9 guides the DUV radiation 808 onto a photomask 820. The photomask 820 is embodied as a transmissive optical element and can be arranged outside the systems 802, 804. The photomask 820 has a structure which is imaged onto a wafer 824 or the like in a reduced fashion via the projection system 804.

[0086] The projection system 804 has a plurality of lens elements 828 and/or mirrors 830 for imaging the photomask 820 onto the wafer 824. In this case, individual lens elements 828 and/or mirrors 830 of the projection system 804 can be arranged symmetrically in relation to the optical axis 826 of the projection system 804. It should be noted that the number of lens elements and mirrors of the DUV lithography apparatus 800 is not restricted to the number illustrated. More or fewer lens elements and/or mirrors can also be provided. Furthermore, the mirrors are generally curved on their front side for beam shaping.

[0087] An air gap between the last lens element 828 and the wafer 824 can be replaced by a liquid medium 832 which has a refractive index of >1. The liquid medium 832 can be high-purity water, for example. Such a construction is also referred to as immersion lithography and has an increased photolithographic resolution.

[0088] Even though the disclosure has been described on the basis of specific embodiments, to numerous variations and alternative embodiments will be apparent to the person skilled in the art, for example through combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying for the person skilled in the art that such variations and alternative embodiments are also encompassed by the present disclosure, and the scope of the disclosure is restricted only within the meaning of the appended patent claims and the equivalents thereof.

LIST OF REFERENCE SIGNS

[0089] 100 Rotary tool [0090] 102 Polishing agent supply [0091] 104 Axis of rotation [0092] 106 Tool carrier with polishing pad in the case of the rotary tool [0093] 110 Excursion section with the use of a rotary tool [0094] 112 Effective area of the rotary tool, or area on which material is removed, when the tool is not guided over the workpiece [0095] 114 surface to be polished (e.g. freeform surface) of the workpiece [0096] 115 Reflection layer system (e.g. MoSi layers) [0097] 120 Illustration of the polishing method with zonal machining by the rotary tool [0098] 130 Polishing pad [0099] 132 Polishing agent [0100] 136 Tool carrier [0101] 140 Workpiece=(mirror) substrate [0102] 150 optical element=(mirror) substrate 140 with reflection layer system 115 or lens element [0103] 200 Eccentric tool [0104] 202 Polishing agent supply [0105] 204 Axis of rotation [0106] 206 Tool carrier with polishing pad in the case of the eccentric tool [0107] 210 Excursion section with the use of an eccentric tool [0108] 212 Effective area of the eccentric tool, or area on which material is removed, when the tool is not guided over the workpiece [0109] 214 surface to be polished (e.g. freeform surface) of the workpiece [0110] 220 Illustration of the polishing method with zonal machining by eccentric tool [0111] 311 Removal rate over time with the use of a rotary tool having a polishing pad without structuring [0112] 312 Polishing pad without structuring [0113] 314 Direction of rotation [0114] 316 dried-on polishing agent residues on the polishing pad [0115] 317 first opening angle of the spiral arms [0116] 318 Axis of rotation=center of the polishing pad [0117] 319 second opening angle of the spiral arms [0118] 320 primary structuring in a spiral shape [0119] 321 Removal rate with the use of a rotary tool having a polishing pad having a structuring in a spiral shape [0120] 322 Polishing pad having a structuring in a spiral shape [0121] 331 Removal rate with the use of a rotary tool having a polishing pad having a primary structuring in a spiral shape and a secondary structuring in the form of rotationally symmetrical channels [0122] 332 Polishing pad having a primary structuring in a spiral shape and a secondary structuring in the form of rotationally symmetrical channels [0123] 334 rotationally symmetrical channels [0124] 412 Polishing pad having a primary structuring in a spiral shape and a secondary structuring in the form of asymmetrically arranged channels [0125] 414 Direction of rotation [0126] 418 Axis of rotation [0127] 420 primary structuring in a spiral shape [0128] 422 secondary structuring in the form of asymmetrically arranged channels [0129] 510 Removal rate with the use of an eccentric tool having a polishing pad without structuring [0130] 511 Removal rate with the use of an eccentric tool having a polishing pad having regular and symmetrical structuring (checkered pattern) [0131] 512 Polishing pad having regular and symmetrical structuring (checkered pattern) [0132] 521 Structuring [0133] 522 Channels [0134] 524 Ridges [0135] d1 Width of the ridges [0136] d2 Width of the channels [0137] d3 Depth of the channels [0138] 610, 620, 630, 640, 650 are the partial steps of the method for machining a workpiece in the production of an optical element [0139] 700 EUV projection exposure apparatus [0140] 701 to 760 parts of the EUV projection exposure apparatus [0141] 800 DUV projection exposure apparatus [0142] 802 to 832 parts of the DUV projection exposure apparatus