METHOD FOR DEPOSITING A LAYER OPTICAL ELEMENT, AND OPTICAL ASSEMBLY FOR THE DUV WAVELENGTH RANGE

Abstract

A method for depositing a layer (2) of a coating which is reflective or anti-reflective to DUV radiation onto a surface (3a) of a substrate (3) for a DUV optical element includes: transferring a coating material (M) into the gas phase in a coating source (4), moving the substrate relative to the coating source along a predetermined movement path (5), and varying a coating rate (RB) and/or a rotation speed (?(t)) of a spin axis (7) of the substrate during the movement along the movement path. A covering element (6) is arranged between the coating source (4) and the surface and covers the surface at least partially during the movement of the substrate. Also disclosed is an optical element for the DUV wavelength range, with a substrate and a reflective or anti-reflective coating (B) applied to the substrate, having at least one layer deposited by the disclosed method.

Claims

1. A method for depositing at least one layer of a coating which is reflective or anti-reflective to radiation in the deep ultraviolet (DUV) wavelength range onto a surface to be coated of a substrate for an optical element for the DUV wavelength range, comprising: transferring a coating material into the gas phase in a coating source, wherein the coating material in the coating source is transferred into the gas phase by thermal evaporation, moving the substrate relative to the coating source along a predetermined movement path, wherein the substrate additionally rotates around a spin axis during the movement along the movement path and wherein a covering element is arranged between the coating source and the surface to be coated, which covering element covers the surface to be coated at least partially during the movement of the substrate along the movement path, varying a rotation speed of the spin axis of the substrate during the movement of the substrate along the movement path, wherein the coating material, which is deposited on the surface to be coated, is an oxide coating material or a fluoride coating material.

2. The method as claimed in claim 1, wherein a coating rate during the movement of the substrate along the movement path deviates by no more than 10% from an average coating rate.

3. The method as claimed in claim 1, wherein average coating rates in successive time intervals with a time period that is less by a factor of 50 to 500 than a period duration of the rotation of the substrate around the spin axis deviate by no more than 10% from one another.

4. The method as claimed in claim 1, wherein the coating source has a cover, which is moved between a first position shadowing the coating material and a second position not shadowing the coating material to vary the coating rate.

5. The method as claimed in claim 1, further comprising: measuring a layer thickness profile of the deposited layer, determining a deviation between the measured layer thickness profile and a desired layer thickness profile, adapting a specification for the variation of the rotation speed during the movement of the substrate along the movement path in dependence on the deviation of the measured layer thickness profile from the desired layer thickness profile.

6. The method as claimed in claim 1, further comprising: shifting the substrate along a rectilinear movement path during the movement relative to the coating source.

7. An optical element for the deep ultraviolet (DUV) wavelength range, comprising: a substrate, and a reflective or anti-reflective coating applied to the substrate, having at least one layer deposited by the method as claimed in claim 1.

8. An optical arrangement for the DUV wavelength range, comprising: at least one optical element as claimed in claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Exemplary embodiments are shown in the schematic drawing and are explained in the following description. In the figures:

[0041] FIGS. 1A,B show schematic illustrations of a coating apparatus having a coating source in the form of a sputter source for depositing a layer onto a surface of a substrate in two angular positions of the substrate with respect to a spin axis,

[0042] FIGS. 2A,B show schematic illustrations analogous to FIGS. 1A,B, with a coating source in the form of a thermal evaporator, which has a cover for varying the coating rate,

[0043] FIG. 3 shows a schematic illustration of a trial-and-error method for optimizing a specification of the coating rate and/or the rotation speed of the rotation of the substrate around the spin axis,

[0044] FIG. 4 shows a schematic illustration of an optical arrangement for the DUV wavelength range in the form of a DUV lithography apparatus, and

[0045] FIG. 5 shows a schematic illustration of an optical arrangement for the DUV wavelength range in the form of a wafer inspection system.

DETAILED DESCRIPTION

[0046] In the description of the drawings that follows, identical reference signs are used for components that are the same or analogous or have the same or analogous function.

[0047] FIGS. 1A,B show a coating apparatus 1 during the deposition of a layer 2 onto a substrate 3. The coating apparatus 1 has a coating source 4, which is shown in the form of a small square in FIGS. 1A,B. In the example shown, the coating source 4 is a sputter source having a coating material M in the form of a sputtering target. The coating material M in the form of the sputtering target is transferred into the gas phase in the coating source 4 by bombarding the sputtering target with high-energy ions. The coating material M transferred into the gas phase passes from the coating source 4 to a surface 3a to be coated of the substrate 3 and is deposited in the form of a layer 2 on the surface 3a to be coated.

[0048] The substrate 3 is shifted during the deposition of the layer 2 in the coating apparatus 1 with the aid of a movement device (not depicted) along a specified movement path 5, which is rectilinear in the example shown, wherein the surface 3a to be coated is partially covered or shadowed by a covering element 6 in the form of a stop during the movement along the movement path 5. The effect of the shadowing caused by the covering element 6 is indicated in FIGS. 1A,B by two arrows, which symbolize two trajectories of the coating material M, of which the first terminates at the covering element 6 and the second terminates at the surface 3a to be coated.

[0049] At the left-hand end of the movement path 5 of the substrate 3 shown in FIGS. 1A,B, there is no longer a direct line of sight between the coating source 4 and the surface 3a to be coated due to the covering element 6. At the right-hand end of the movement path 5 shown in FIGS. 1A,B, the entire surface 3a to be coated is no longer shadowed by the covering element 6. The substrate 3 is shifted during the deposition of the layer 2 from the left end of the rectilinear movement path 5 to the right end of the rectilinear movement path 5 and in so doing crosses the coating source 4, more precisely an opening in the coating source 4, from which the coating material M emerges. The translational movement of the substrate 3 takes place with a translation speed v(t), which can be kept constant or can be varied during the movement along the movement path 5.

[0050] In addition to the translational movement of the substrate 3 relative to the coating source 4 along the rectilinear movement path 5, the substrate 3 also rotates around a spin axis 7 of the substrate 3 during the movement along the movement path 5. In the example shown, in which the surface 3a to be coated of the substrate 3 is rotationally symmetric to the spin axis 7, a rotationally symmetric layer thickness profile of the layer 2 applied to the surface 3a to be coated can be produced during a rotation of the substrate 3 at a constant angular velocity ?(t) during the entire movement of the substrate 3 along the movement path 5. In the event that a coating rate RB of the coating material M emerging from the coating source 4 is also kept constant, the thickness or the layer thickness profile d(r, ?) of the layer 2 in the azimuthal direction along the surface 3a to be coated is constant, i.e. the thickness d(r, ?) does not depend on the azimuth angle ? but only on the distance r from the spin axis 7.

[0051] FIG. 1A shows a snapshot of the deposition process at a first time t.sub.1, FIG. 1B shows a snapshot of the deposition process at a second, later time t 2. Between the two times t.sub.1, t 2 shown in FIG. 1A and FIG. 1B, the substrate 3 was rotated around the spin axis 7 by 180?, so that a first and second point P1, P2 on the surface 3a to be coated, which have the same radial distance from the spin axis 7 and which are diametrically opposite each other, are reversed.

[0052] As can be seen in FIG. 1B, the thickness d(r, ?) of the deposited layer 2 at point P1 is greater than at point P2, i.e. the applied layer 2 has a non-rotationally symmetric layer thickness distribution d(r, ?). Such a non-rotationally symmetric layer thickness distribution d(r, ?) can be produced by varying the rotation speed ?(t) of the substrate 3 around the spin axis 7 and/or the coating rate RB during the movement of the substrate 3 along the rectilinear movement path. For example, for this purpose, the rotation speed ?(t.sub.2) at the second time t.sub.2 can be selected to be smaller than the rotation speed ?(t.sub.1) at the first time t.sub.1, so that more coating material M is deposited at the first point P1 of the surface 3a to be coated and the local thickness of the layer 2 at the first point P1 increases accordingly, as indicated in FIG. 1B.

[0053] Alternatively or in addition, the coating rate R.sub.B(t) can be varied during the movement along the movement path 5, for example, the coating rate R.sub.B(t.sub.1) at the first time t.sub.1 can be selected to be smaller than the coating rate R.sub.B(t.sub.2) at the second time t.sub.2, as a result of which the thickness of the layer 2 at the first point P1 of the surface 3a to be coated likewise increases compared with the thickness of the layer 2 at the second point P2, as indicated in FIG. 1B.

[0054] The coating material M, which is applied to the surface 3a to be coated of the substrate 3 is an oxide or a fluoride material in the example shown in FIGS. 1A,B. Oxides or fluorides are used for the production of reflective or anti-reflective coatings for optical elements for operation at wavelengths in the DUV wavelength range, as are described further below in connection with FIG. 4 and FIG. 5.

[0055] In order to deposit such an oxide or fluoride material on the surface 3a to be coated of the substrate 3, a reactive DC sputtering method is carried out in an oxygen gas atmosphere 8 (or alternatively in a fluorine gas atmosphere) in the coating apparatus 1 in the example shown in FIG. 1A. In this case, the coating material M is an electrically conductive sputtering target, e.g. Si, Al, Hf or Ti. The material of the sputtering target is usually removed with the aid of noble gas ions, which are accelerated from a plasma onto the target surface due to an electric potential applied to the sputtering target, and transferred into the gas phase. In this case, the gaseous coating material M, which emerges from the coating source 4, reacts with the oxygen in the oxygen gas atmosphere 8 and forms a corresponding oxide material, e.g. SiO.sub.2, Al.sub.2O.sub.3, HfO.sub.2 or TiO.sub.2, which is deposited on the surface 3a to be coated of the substrate 3. For the deposition of layers 2 from fluoride materials which are substantially absorption-free in the DUV wavelength range, such as MgF.sub.2 or LaF.sub.3, the corresponding sputtering targets of Mg or La are provided as coating material M in the coating source 4 and transferred in the fluorine gas atmosphere 8 described further above in the coating apparatus 1 into the corresponding fluoride material MgF.sub.2, LaF.sub.2, which is deposited on the surface 3a to be coated.

[0056] As an alternative to the use of electrically conductive sputtering targets, electrically insulating sputtering targets can be used as coating material M in the coating source 1. In this case, the coating source 4 is formed to perform a pulsed sputtering method, a high-frequency sputtering method or an ion beam sputtering method, wherein the ion beam is produced by a separate ion beam source (not depicted). The coating material M may be, for example, ceramic sputtering targets, e.g. in the form of SiO.sub.2, Al.sub.2O.sub.3, HfO.sub.2 or TiO.sub.2.

[0057] FIGS. 2A,B show a coating apparatus 1, which is formed analogously to the coating apparatus 1 shown in FIGS. 1A,B. The coating apparatus 1 shown in FIGS. 2A,B differs from the coating apparatus 1 shown in FIGS. 1A,B substantially in that the coating source 4 is a thermal evaporator source, rather than a sputter source 4 as in FIGS. 1A,B.

[0058] With the coating source 4 being in the form of the thermal evaporator source, the coating material M is transferred into the gas phase by thermal evaporation. For this purpose, the coating source 4 may include, for example, an electron beam evaporator or an electrical resistance heater. In thermal evaporation processes, a controlled variation of the coating rate R.sub.B is very limited, since the thermal noise of the evaporation rate and the inertia of the evaporation rate are many times higher when the evaporation performance changes than is the case with the removal rate of the sputtering target in sputtering processes.

[0059] For producing a free-form coating with a layer thickness d(r, ?) of the deposited layer 2, which varies in the azimuthal direction ?, it is therefore generally advantageous if the coating rate R.sub.B or the evaporation rate of the coating source 4 is kept as constant as possible and, for producing a layer 2 with a non-rotationally symmetric thickness profile d(r, ?), the rotation speed ?(t) of the substrate 3 is varied during the movement of the substrate 3 along the rectilinear movement path 5.

[0060] A substantially constant coating rate R.sub.B is understood to mean that, during the movement of the substrate 3 along the movement path 5, the coating rate R.sub.B(t) does not deviate by more than 10% from an average coating rate R.sub.B,M during the movement of the substrate 3 along the movement path 5, i.e.: 0.9 R.sub.B,M<R.sub.B(t)<1.1 R.sub.B,M. The average coating rate R.sub.B,M can be determined and regulated using one or more stationary sensors.

[0061] In addition, it is advantageous if average coating rates R.sub.B,M in successive time intervals with a time duration ? that is less by a factor of 50 to 500 than a period duration T of the rotation of the substrate 3 around the spin axis 7 deviate by no more than 10% from one another. The typical duration for one rotation of the substrate 3 is 1 s to 10 s, with the result that the statistical fluctuations of the evaporation or coating rate R.sub.B should not differ significantly from one another with regard to their average values, i.e. the average coating rate R.sub.B,M, in time intervals of 10 ms to 100 ms as otherwise a systematic drift of the evaporation or coating rate R.sub.B is present, which undesirably influences the layer thickness profile resulting at the end of the coating process. Undesirable influences can typically be avoided if the average coating rates R.sub.B,M in respectively two consecutive time intervals with the time specified above deviate by no more than 10% from one another. In order to meet the abovementioned condition, the period duration T of the rotation of the substrate 3 can be specified appropriately or, if necessary, set during the coating process. The average coating rate R.sub.B,M or its fluctuation can be measured, for example, with the aid of the sensors described further above.

[0062] In order to change in a controlled manner the coating rate R.sub.B, i.e. the rate at which the coating source 4 releases the coating material M, despite the problem described further above relating to the insufficiently controllable evaporation rate, the coating source 4 shown in FIGS. 2A,B has a cover 9. The cover 9 shown in FIGS. 2A,B can be used to vary the coating rate R.sub.B between a first position S1 shown in FIG. 2A, in which the cover 9 completely shadows the opening in the coating source 4 and thus the coating material M located in the coating source 4 so that the material can no longer reach the surface 3a to be coated, and a second position S2 shown in FIG. 2B, in which the cover 9 does not shadow the coating material M so that the material can escape unhindered from the coating source 4 and pass to the surface 3a to be coated or to the covering element 6, which partially shadows the surface 3a to be coated.

[0063] In the example shown in FIGS. 2A,B, the cover 9 is quickly rotated around a rotational axis which is arranged laterally next to the coating source 4, but it is also possible to quickly move the cover 9 back and forth between the first position S1 and the second position S2 in a different way. In contrast to what is shown in FIG. 2A, the cover 9 may optionally not completely cover or shadow the coating source 4 in the first position Si, so that some of the coating material M transferred into the gas phase can also reach the surface 3a to be coated in the first position Si.

[0064] With the aid of cover 9, the coating rate R.sub.B of the coating source 4 in the form of the thermal evaporator can be varied in a controlled manner such that in this case, too, due to the variation of the coating rate R.sub.B, there is an additional, well-controllable degree of freedom during the deposition, which enables a free-form coating, i.e. a coating with any arbitrary, non-rotationally symmetric thickness profile d(r, ?) of the deposited layer 2.

[0065] A further advantage of a free-form coating which is carried out in the manner described further above, i.e. by a controlled variation of the coating rate R.sub.B and/or the rotation speed ?(t) of the rotation of the substrate 3 around the spin axis 7, is that this is a time- and cost-saving option for approximating a desired layer thickness profile ds(r, ?) of the deposited layer 2 with the aid of a trial-and-error method, as illustrated below with reference to FIG. 3.

[0066] In the trial-and-error method, a layer 2 is applied as a trial coating onto the substrate 3 in a first step, as described further above in connection with FIGS. 1A,B and with FIGS. 2A,B. During the deposition of the layer 2, a variation of the coating rate R.sub.B(t) and/or the rotation speed ?(t) around the spin axis 7 is specified during the movement of the substrate 3 along the path curve 5, which is to produce a desired layer thickness distribution ds(r, ?) of the deposited layer 2, which is typically a free-form coating.

[0067] In a subsequent step, the (actual) layer thickness profile d(r, ?) of the deposited layer 2 is measured. The measurement of the layer thickness profile d(r, ?) of the layer 2 can be taken, for example, using an interferometric measurement method or in another way. In a subsequent step, a deviation ?d(r, ?) from the specified desired layer thickness profile ds(r, ?) is determined. For example, the deviation ?d(r, ?) can be the difference between the measured (actual) layer thickness profile d(r, ?) and the desired layer thickness profile ds(r, ?) of the deposited layer 2, i.e. ?d(r, ?)=d(r, ?)?d.sub.s(r, ?).

[0068] Depending on the measured deviation ?d(r, ?), a new, improved specification for the time profile of the variation of the coating rate R.sub.B(t) and/or the variation of the rotation speed ?(t) during the movement of the substrate 3 along the movement path 5 is calculated in a subsequent step. The new time profile of the variation of the coating rate R.sub.B(t) or the variation of the rotation speed ?(t) of the substrate 3 is programmed in the coating apparatus 1 or stored as a new specification in the integrated controller. The steps described further above may be repeated one or more times on further trial coatings, in which a layer 2 is applied one or more times onto the same substrate 3 (after removing the layer 2) or onto an identically shaped substrate 3 until the actual layer thickness distribution d(r, ?) is adapted to the desired layer thickness distribution ds(r, ?) as far as the process stability permits.

[0069] In contrast, with a free-form coating based on an optimized shape of a pinhole or honeycomb mask, it is necessary to first calculate an optimized shape of the shadowing mask for each trial coating and to subsequently manufacture and position such a shadowing mask, which results in a long production time, high production costs and high production tolerances (see FIG. 3). When using new, optimized masks for shadowing the substrate 1, manufacturing tolerances in the mask shape and mask optimization in the coating apparatus 1 additionally have a limiting effect on an approximation to the desired layer thickness profile ds(r, ?).

[0070] As described further above, the deposited layer 2 forms part of a reflective or anti-reflective coating B for the DUV wavelength range or the deposited layer 2 itself forms such a reflective or anti-reflective coating B. In the event that the coating B includes a plurality of layers 2, these typically serve to amplify the reflective or anti-reflective effect on the basis of interference effects. The substrate 3 coated with the coating B forms an optical element that can be used in optical arrangements for the DUV wavelength range. These optical arrangements, for example, can be the optical arrangements described below in FIG. 4 and FIG. 5.

[0071] FIG. 4 shows an optical arrangement for the DUV wavelength range in the form of a DUV lithography apparatus 21. The DUV lithography apparatus 21 comprises two optical systems, namely an illumination system 22 and a projection system 23. The DUV lithography apparatus 21 additionally has a radiation source 24, which can be an excimer laser, for example.

[0072] The radiation 25 emitted by the radiation source 24 is conditioned with the aid of the illumination system 22 such that a mask 26, also called a reticle, is illuminated thereby. In the example shown, the illumination system 22 has a housing 32, in which both transmissive and reflective optical elements are arranged. In a representative manner, the illustration shows a transmissive optical element 27, which focuses the radiation 25, and a reflective optical element 28, which deflects the radiation.

[0073] The mask 26 has, on its surface, a structure which is transferred to an optical element 29 to be exposed, for example a wafer, with the aid of the projection system 23 for the purpose of producing semiconductor components. In the example shown, the mask 26 is designed as a transmissive optical element. In alternative embodiments, the mask 26 can also be designed as a reflective optical element.

[0074] The projection system 22 has at least one transmissive optical element in the example shown. The example shown illustrates, in a representative manner, two transmissive optical elements 30, 31, which serve, for example, to reduce the structures on the mask 26 to the size desired for the exposure of the wafer 29.

[0075] Both in the illumination system 22 and in the projection system 23, a wide variety of transmissive, reflective or other optical elements can be combined with one another in an arbitrary, even more complex, manner. Optical arrangements without transmissive optical elements can also be used for DUV lithography.

[0076] FIG. 5 shows an optical arrangement for the DUV wavelength range in the form of a wafer inspection system 41, but it may also be a mask inspection system. The wafer inspection system 41 has an optical system 42 with a radiation source 54, from which radiation 55 is directed onto a wafer 49 using the optical system 42. For this purpose, the radiation 55 is reflected onto the wafer 49 by a concave minor 46. In the case of a mask inspection system, a mask to be examined could be arranged instead of the wafer 49. The radiation reflected, diffracted and/or refracted by the wafer 49 is directed onto a detector 50 for further evaluation by a further concave mirror 48, which is likewise associated with the optical system 42, via a transmissive optical element 47. The wafer inspection system 41 additionally has a housing 52, in which the two mirrors 46, 48 and the transmissive optical element 47 are arranged. The radiation source 54 can be for example exactly one radiation source or a combination of a plurality of individual radiation sources in order to provide a substantially continuous radiation spectrum. In modifications, one or more narrowband radiation sources 54 can also be used.

[0077] At least one of the optical elements 27, 28, 30, 31 of the DUV lithography apparatus 21 shown in FIG. 5 and at least one of the optical elements 46, 47, 48 of the wafer inspection system 41 shown in FIG. 6 are designed here as described further above. Their coatings B thus have at least one layer 2, for example a fluoride or an oxide, which has been deposited with the aid of the method described further above.