Microlithographic apparatus and method of changing an optical wavefront in such an apparatus

Abstract

A microlithographic apparatus comprises an optical wavefront manipulator. The latter includes an optical element and a gas-tight cavity that is partly confined by the optical element or contains it. A gas inlet device directs a gas jet towards the optical element. The location, where the gas jet impinges on the optical element after it has passed through the cavity, is variable in response to a control signal supplied by a control unit. A gas outlet is in fluid connection with the vacuum pump so that, upon operation of the vacuum pump, the pressure within the cavity is less than 10 mbar even if the gas jet passes through the cavity.

Claims

1. An apparatus, comprising: a vacuum pump; an illumination system configured to produce projection light during operation of the illumination system to illuminate a mask with the projection light; a projection objective configured to use the projection light to image an illuminated portion of the mask onto a light-sensitive material; an optical wavefront manipulator, comprising: an optical element; a gas-tight cavity, the gas-tight cavity partly confined by the optical element or containing the optical element, the gas-tight cavity configured so that, during operation of the apparatus, the gas-tight cavity is in a light path of the projection light; a gas inlet device configured to direct a gas jet toward the optical element during use of the apparatus, a location where the gas jet impinges on the optical element after it has passed through the cavity being variable in response to a control signal supplied by a control unit; and a gas outlet in fluid connection with the vacuum pump so that, during operation of the vacuum pump, a pressure within the gas tight cavity is less than 10 mbar even when the gas jet passes through the gas-tight cavity; and a lens outside the gas-tight cavity and through which the projection light passes during operation of the apparatus, wherein the lens is in the projection objective, and the apparatus is a microlithographic apparatus.

2. The apparatus of claim 1, wherein the gas inlet device comprises a nozzle and an actuator configured to change: a) a position in response to the control signal; and/or b) an orientation of the nozzle in response to the control signal.

3. The apparatus of claim 1, wherein: the gas inlet device comprises a plurality of nozzles and a plurality of flow rate control devices; each flow rate control device is associated with one of the nozzles; and each flow rate control device is configured to control the amount of gas that passes through the flow rate control device during operation of the gas inlet device.

4. The apparatus of claim 1, wherein the gas inlet device is configured to emit at least one supersonic free jet during operation of the gas inlet device.

5. The apparatus of claim 4, wherein the pressure within the cavity is less than 10.sup.3 mbar even if the gas jet passes through the cavity during operation of the apparatus.

6. The apparatus of claim 1, wherein the gas inlet device comprises a Laval nozzle.

7. The apparatus of claim 1, wherein the pressure within the cavity is less than 10.sup.3 mbar even if the gas jet passes through the cavity during operation of the apparatus.

8. The apparatus of claim 1, wherein the apparatus is a projection exposure apparatus which comprises a light source configured to produce a train of successive light pulses during operation of the light source.

9. The apparatus of claim 8, wherein the gas inlet device is configured so that, during operation of the gas inlet device, the gas inlet device produces a train of successive gas jets interleaved with the train of light pulses produced by the light source so that no light pulse impinges on any of the gas jets.

10. The apparatus of claim 1, wherein the apparatus comprises an objective configured to image a mask onto a surface during operation of the apparatus, and the optical element is a refractive optical element in the objective.

11. The apparatus of claim 1, wherein the optical element is a mask, and the apparatus comprises an objective configured to image the mask onto a surface during operation of the apparatus.

12. The apparatus of claim 1, wherein the optical wavefront manipulator is in the projection objective.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a schematic perspective view of a projection exposure apparatus in accordance with the present invention;

(3) FIG. 2 is a schematic meridional section through the apparatus shown in FIG. 1;

(4) FIG. 3 is an enlarged and highly schematic meridional section through the optical wavefront manipulator contained apparatus of FIG. 2;

(5) FIG. 4 shows the optical wavefront manipulator of FIG. 3 at a later time when only one gas jet impinges at a different location on a glass plate;

(6) FIG. 5 shows, in a meridional section similar to FIGS. 3 and 4, the optical wavefront manipulator in a state when no light pulse propagates through the manipulator;

(7) FIG. 6 shows, in a meridional section similar to FIGS. 3 and 4, the optical wavefront manipulator in a state when a light pulse propagates through the manipulator;

(8) FIG. 7 is a schematic meridional section through the apparatus shown in FIG. 1 according to another embodiment of the optical wavefront manipulator in which the gas jets are directed on the mask to be imaged;

(9) FIG. 8 is a flow diagram which summarizes important aspects of a method in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

I.

General Construction of Projection Exposure Apparatus

(10) FIG. 1 is a perspective and simplified view of a projection exposure apparatus 10 in accordance with the present invention. The apparatus 10 comprises an illumination system 12 containing a light source LS which produces projection light having a central wave-length of 193 nm. The projection light illuminates a field 14 on a mask 16 containing a pattern 18 of fine features 19. In this embodiment the illuminated field 14 has a rectangular shape. However, other shapes of the illuminated field 14, for example ring segments, and also other operating wavelengths, for example 157 nm or 248 nm, are contemplated as well.

(11) A projection objective 20 having an optical axis OA and containing a plurality of lenses L1 to L4 images the pattern 18 within the illuminated field 14 onto a light sensitive layer 22, for example a photoresist, which is supported by a substrate 24. The substrate 24, which may be formed by a silicon wafer, is arranged on a wafer stage (not shown in FIG. 1) such that a top surface of the light sensitive layer 22 is precisely located in an image plane of the projection objective 20. The mask 16 is positioned via a mask stage (not shown in FIG. 1) in an object plane of the projection objective 20. Since the latter has a magnification with ||<1, a minified image 18 of the pattern 18 within the illuminated field 14 is projected onto the light sensitive layer 22.

(12) During the projection the mask 16 and the substrate 24 move along a scan direction which corresponds to the Y direction indicated in FIG. 1. The illuminated field 14 then scans over the mask 16 so that patterned areas larger than the illuminated field 14 can be continuously imaged. The ratio between the velocities of the substrate 24 and the mask 16 is equal to the magnification of the projection objective 20. If the projection objective 20 does not invert the image (>0), the mask 16 and the substrate 24 move along the same direction, as this is indicated in FIG. 1 by arrows A1 and A2. However, the present invention may also be used with catadioptric projection objectives 20 having off-axis object and image fields, and with apparatus of the step-and-scan type in which the mask 16 and the substrate 24 do not move during the projection.

(13) FIG. 2 is a schematic meridional section through the apparatus 10 shown in FIG. 1. In this section also the mask stage denoted by 26, which supports and moves the mask 16 in the object plane 28 of the projection objective 20, and the wafer stage denoted by 32, which supports and moves the substrate 24 in the image plane 30 of the projection objective 20, are schematically illustrated.

(14) Inside the projection objective 20 two manipulators M1 and M2 are arranged that are configured to individually displace the lenses L1 and L2, respectively, along an optical axis OA of the projection objective 20.

(15) In this embodiment the projection objective 20 has an intermediate image plane 34. A first pupil plane 36 is located between the object plane 28 and the intermediate image plane 34, and a second pupil plane 38 is located between the intermediate image plane 34 and the image plane 30 of the projection objective 20. In the first and second pupil plane 36, 38 all light rays converging or diverging under the same angle from any of the field planes, i.e. the object plane 28, the intermediate image plane 34 and the image plane 30, pass through the same point, as this is illustrated in FIG. 2.

(16) In the first pupil plane 36 an optical wavefront manipulator 42 for correcting, or more generally changing, wavefront deformations is arranged. The optical wavefront manipulator 42 will be described in more detail in the following section.

II.

Wavefront Manipulator

(17) FIG. 3 is an enlarged and highly schematic meridional section through the optical wavefront manipulator 42 shown in FIG. 2.

(18) The optical wavefront manipulator 42 comprises two optical elements that are formed in this embodiment by a first plate 44 and a second plate 46. Each plate 44, 46 has two plane and parallel surfaces and a square contour. The two plates 44, 46 confine, together with a housing structure 48, a gas-tight cavity 50. Seals 52 are provided at the interfaces between the plates 44, 46 on the one hand and the housing structure 48 on the other hand so as to ensure that virtually no exchange of gas particles occurs between the cavity 50 and the outer atmosphere 54 surrounding the optical wavefront manipulator 42. This outer atmosphere may consist of a purge gas such as N.sub.2 or an inert gas at room temperature (22 C.) and standard pressure (1 bar), for example. The projection objective 20 comprises a conditioning unit (not shown) that maintains the atmosphere 54 inside the projection objective 20 at these conditions.

(19) The plates 44, 46 are transparent for projection light indicated by arrows 56 in FIG. 3. Fused silica is a suitable material for the plates 44, 46 since it has a small coefficient of thermal conduction which, as it will turn out further below, has a favorable impact on the function of the optical wavefront manipulator 42.

(20) In the embodiment shown the plates 44, 46 have identical dimensions. However, the plates 44, 46 may also have different dimensions, and other refractive optical elements such as lenses may be used instead of the plates 44, 46 as optical elements. In a projection objective 20 of the catadioptric type it may also be envisaged to replace one of the plates 44, 46, and in particular the second plate 46, by a mirror.

(21) The housing structure 48 confining and surrounding the cavity 50 accommodates a gas inlet device denoted generally by 58, and two gas outlets 64, 66 each being in fluid connection with a vacuum pump 60, 62 which are arranged outside the projection objective 20 and are vibrationally isolated therefrom.

(22) The gas inlet device 58 is configured to direct one or more gas jets towards the first plate 44. To this end the gas inlet device 58 comprises a plurality of inlet units. In the meridional section shown in FIG. 3, only two of these inlet units are shown and denoted by 72a and 72b. Further inlet units may be arranged at other circumferential positions of the optical wavefront manipulator 42.

(23) The first inlet unit 72a comprises a pressure chamber 74a, a valve 76a, a movable Laval nozzle 78a and a flexible tube 80a which connects the valve 76a to the Laval nozzle 78a. The Laval nozzle 78a is mounted in an articulated manner and is connected to an actuator 82a which is configured to change the orientation of the Laval nozzle 78a in response to a control signal received from a control unit 84. The latter communicates with an overall system control 85, which is represented in FIG. 2 as personal computer. As schematically illustrated by a double arrow in FIG. 3, the Laval nozzle 78a is thus able to rotate around at least one rotational axis so that the direction of a gas jet 86a emitted by the Laval nozzle 78a can be varied with the help of the actuator 82a.

(24) The second inlet unit 72b has an identical construction. It thus comprises a pressure chamber 74b, a valve 76b, a Laval nozzle 78b, a flexible tube 80b and an actuator 82b for changing the orientation of the Laval nozzle 78b.

(25) The pressure chambers 74a, 74b of the inlet units 72a and 72b, respectively, are connected to a common gas feed unit 87 which serves to provide a gas, for example an inert gas such as argon, having a predetermined temperature (for example 22 C.) and a certain pressure, for example a pressure between 50 mbar and 5 bar.

III.

Function

(26) In the following it will be described how the optical wavefront manipulator 42 may be operated to change the optical wavefront of projection light 56 passing through it.

(27) Before projection light 56 is allowed to pass through the optical wavefront manipulator 42, a vacuum is created in the cavity 50 with the help of the vacuum pumps 60, 62. The pressure inside the cavity 50 should be reduced to less than 10 mbar and preferably less than 10.sup.3 mbar. Then the valves 76a, 76b are opened so that gas contained in the pressure chambers 74a, 74b is allowed to enter the cavity 50. Due to the great pressure difference between the pressure chambers 74a, 74b on the one hand and the vacuum in the cavity 50, gas flowing through the Laval nozzles 78a, 78b is supersonically expanded to produce the free gas jets 86a, 86b. As a result of this supersonic expansion, the collision frequency between the gas particles is greatly reduced, while the velocity of the gas particles is increased significantly. The thermal movement of the gas particles is also reduced so that the temperature of the gas jets 86a, 86b may be in the order of a few K only. Since all gas particles move with approximately the same (high) velocity, but do not collide with each other, the gas jets 86a, 86b have a very small diameter which does not increase significantly along the propagation direction. The diameter of the gas jets 86a, 86b is mainly determined by the inner diameter of the Laval nozzles 78a, 78b and may be as small as 1 mm.

(28) The gas jets 86a, 86b emitted from the Laval nozzles 78a, 78b pass through the cavity 50 and then impinge on the inner surface 90 of the first plate 44. There the gas particles interact with the atoms at the inner surface 90 of the first plate 44. This interaction results in a heat exchange between the gas particles and the first plate 44 at the locations where the gas jets impinge on the first plate 44. Whether the gas jets 86a, 86b have a heating or cooling effect at these locations depends on various parameters, including the composition of the gas, the properties of the inner surface 90 of the first plate 44, the pressure difference between the pressure chambers 74a, 74b and the cavity 50, and also on the angle under which the gas jets 86a, 86b impinge on the inner surface 90 of the first plate 44.

(29) The gas jets impinge, after reflection at the inner surface 90 of the first plate 44, on one of the lateral surfaces confining the cavity 50, and preferably enter directly one of the gas outlets 64, 66 connected to the vacuum pumps 60, 62. Generally it should be avoided that the gas jets 86a, 86b impinge, after interaction with the first plate 44, on the second plate 46 because this may result in an additional, but this time undesired, exchange of heat between the gas jets 86a, 86b and the second plate 46.

(30) The locally restricted exchange of heat between the gas jets 86a, 86b and the first plate 44 modifies the temperature distribution in the first plate 44. This modified temperature distribution is directly associated, via the dependence dn/dT of the refractive index n of the first plate 44 on the temperature T, with a modified distribution of the refractive index n. Hence also the phase distribution of an optical wavefront passing through the first plate 44 is modified. In this manner undesired deformations of the optical wavefront can be reduced or changed in such a manner that the wavefront deformations can be reduced more easily with other manipulators provided in the projection objective 20, for example the manipulators M1 and M2 that displace the lenses L1, L2 along the optical axis OA.

(31) It should be noted that, although the heat exchange between the gas jets 86a, 86b and the first plate 44 may be small, it will usually suffice to change the temperature in the first plate 44 locally. This is due to the fact that the cavity 50 does not contain any substantial amounts of gas that could help to remove heat from or add heat to the first plate 44 by heat conduction or convection. Thus heat at the inner surface 90 of the first plate 44 can only be removed by radiation or thermal conduction inside the first plate 44. However, at least for certain materials such as fused silica, the coefficient of thermal conduction is quite small. Therefore a modification of the temperature distribution in the first plate 44 produced by the gas jets 86a, 86b is maintained over a relatively long time. Put another way, even a small heat exchange suffices to produce a substantially stable temperature distribution in the first plate 44.

(32) Various parameters, in particular the throughput through the valves 76a, 76b and the orientation of the Laval nozzles 78a, 78b, can be used to vary the effect of the optical wavefront manipulator 42 on the optical wavefront. FIG. 4 illustrates the case in which the valve 86b has been closed and the Laval 1 nozzle 78a has been tilted so that only one gas jet 86a passes impinges on the first plate 44, but now at a different location as compared to the configuration shown in FIG. 3. By moving the gas jets 86a, 86b continuously or intermittently over the inner surface 90 of the first plate 44, almost any temperature distribution within a certain temperature range can be produced.

(33) In order to be able to direct gas jets 86a, 86b to almost any arbitrary location on the first plate 44, it may be envisaged to use actuators that are capable to displace the Laval nozzles 78a, 78b also translationally, i.e. along straight or curved lines.

(34) In this embodiment the second plate 46 does not contribute to the correction of wavefront deformations. As a matter of course, additional inlet units may be provided that direct gas jets on the second plate 46. If very sophisticated control schemes are applied, it is even possible to direct the gas jets 86a, 86b on the first plate 44 such that they are reflected towards the second plate so that each gas jet 86a, 86b interacts twice. The desired modification of the optical wavefront has then to be distributed among the two plates 44, 46, but with the additional constraint that the temperature distribution in the second plate 46 cannot be determined independent from the temperature distribution in the first plate 44.

(35) The density of the gas particles in the gas jets 84a, 84b may be so small that the refractive index of the gas jets 86a, 86b does not differ substantially from those portions of the cavity 50 through which no gas jet passes. Thus the gas jets 86a, 86b as such have virtually no impact on the optical wavefront of the projection light.

(36) However, if, for whatever reason, all possible interactions of the projection light 56 with the gas jets 86a, 86b shall be prevented, the latter may be produced only during times at which no projection light 56 passes through the optical wavefront manipulator 42, as this is shown in FIG. 5. Since the gas jets 86a, 86b propagate with ultrasonic velocities and are thus very fast, even the short intervals between successive scan operations will suffice to emit a short gas jet pulse by the Laval nozzles 78a, 78b and to remove the gas jets from the cavity 50 after they have interacted with the first plate 44, but before projection light (see FIG. 6) again passes through the cavity 50. If the valves 76a, 76b are controlled such that the gas jet pulses are sufficiently short, it is even possible to emit the gas jet pulses during the time intervals between successive light pulses of a single scan cycle. This exploits that fact that usually the light source LS contained in a projection exposure apparatus emits a train of light pulses with a pulse frequency of a few kHz. Then a train of successive gas jets 86a, 86b can be interleaved with the train of light pulses produced by the light source LA such that no light pulse impinges on any of the gas jets 86a, 86b. The conditions shown in FIGS. 5 and 6 then alternate very quickly and synchronous with the pulse frequency.

IV.

Correction Method

(37) In the following it will be described how the optical wavefront manipulator 42 may be used to correct wavefront deformations.

(38) In a first step an aberration of the projection objective 20 is determined. This can be done either by measurements and/or by simulation. Simulation may be performed on the basis of experimental data and has the advantage that the operation of the projection exposure apparatus does not have to be disrupted to perform measurements of the image quality, for example. Simulation will usually be involved also if image enhancement technologies are applied. Determining an aberration by measurement, on the other hand, may be necessary if the aberration shall be determined with the highest possible accuracy. For measuring the aberration, an optical wavefront measuring device 110, for example a Fizeau interferometer, may be inserted into the image plane 30 of the projection objective 20, as this is indicated in FIG. 2 by an arrow 112.

(39) Also a mixed approach, which uses certain measurements as well as simulations, may be used to quickly and accurately determine the aberration. For example, an ideal non-planar wavefront may be computed using image enhancement technologies, and the real optical wavefront is measured.

(40) In a next step the corrective effect which is required to obtain the desired optical wavefront has to be determined. This step may also take into account that not only the optical wavefront manipulator 42, but also other correction systems, for example the manipulators M1, M2 which are configured to displace the lenses L1, L2 along the optical axis OA, are available to reduce the aberration. One approach is to consider all available correction systems in a common optimization process. Singular value decomposition (SVD) or Tikhonov regularization may be used in this respect. Another approach based on Convex Programming is described in WO 2010/034674 A1. In such an optimization process the first plate 44 may be conceptionally divided into a plurality of pixels that correspond to locations where the gas jets 86a, 86b may impinge on the first plate 44.

(41) The optimization process yields a phase variation which is to be generated by the first plate 44. If the projection objective 20 comprises other correction devices (such as the manipulators M1, M2) having a rotationally symmetric impact on the optical wavefront, the phase generation to be generated by the first plate 44 will, at least generally, be rotationally asymmetric in such a way that the optical wavefront becomes rotationally symmetric. This implies that the coefficients of higher order Zernike polynomials used to describe the optical wavefront deformations at least substantially vanish.

(42) Then an algorithm computes the temperature distribution which is required in the first plate 44 to generate the phase variations determined before. In a next step it has to be determined where which amounts of heat which have to be exchanged between the first plate 44 and the gas jets, and how the gas inlet device 58 has to be controlled so as to obtain this heat exchange. This again may be achieved by using an optimization algorithm.

(43) Finally, the control unit 84 controls the valves 76a, 76b and the actuators 82a, 82b so that the gas jets 86a, 86b impinge on the first plate at the locations and for time intervals as determined above.

V.

Alternative Embodiment

(44) FIG. 7 is a schematic meridional section through an apparatus 10 comprising an optical wavefront manipulator 42 according to another embodiment. The cavity 50 is again confined by the two plates 44, 46 and the housing structure 48. In this embodiment, however, the cavity 50 accommodates the mask 16 and the mask stage 26, and the gas jets 86a, 86b are directed not on one of the plates 44, 46 confining the cavity 50, but on one side of the mask 16. Otherwise the configuration of the optical wavefront manipulator 42 is identical to the wavefront manipulator shown in FIG. 3.

(45) Since the mask 16 is accommodated in the cavity 50 in which a pressure of less than 10 mbar, and preferably of less than 10.sup.3 mbar, prevails, the mask 16 cannot stir a substantial amount of gas when it is quickly moved by the mask stage 26, as this is indicated in FIG. 7 by a double arrow. Therefore the mask movements cannot result in quickly changing refractive index distributions (schlieren) in the vicinity of the mask 16, as this may be the case in prior art apparatus in which the mask moves through an N.sub.2 or an inert gas atmosphere at normal pressure. Furthermore, distortion and other field dependent aberrations can be very effectively corrected, because the mask 16 is arranged in a field plane.

(46) As a matter of course, the apparatus 10 may also comprise two wavefront manipulators 42, namely a first one in a pupil plane 36 of the objective 20 as shown in FIG. 2, and a second one in a field plane as shown in FIG. 7. Then both field dependent and field independent wavefront aberrations can be partially corrected or at least changed in such a manner that they can be corrected more easily by other means.

VI.

Important Method Steps

(47) FIG. 8 is a flow diagram which summarizes important aspects of a method of changing an optical wavefront in an objective of a microlithographic apparatus.

(48) In a first step S1 an objective with a gas-tight cavity is provided.

(49) In a second step S2 a vacuum is produced in the cavity.

(50) In a third step S3 a gas is injected into the cavity so as to produce a gas jet that is directed towards an optical element confining the cavity or contained therein.