MICROLITHOGRAPHIC PROJECTION EXPOSURE APPARATUS, AND MICROLITHOGRAPHIC PROJECTION EXPOSURE APPARATUS

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a method for modifying a polarization distribution in a microlithographic projection exposure apparatus, and to a microlithographic projection exposure apparatus .

Prior Art

The invention relates to an illumination device of a microlithographic projection exposure apparatus.

Microlithography is used for producing microstructured components such as integrated circuits or LCDs, for example. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection objective. In this case, the image of a mask ( = reticle) illuminated by means of the illumination device is projected, by means of the projection objective, onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective, in order to transfer the mask structure to the light-sensitive coating of the substrate.

It is known that, in the microlithography process, the imaging contrast can be significantly improved if the mask is illuminated with linearly polarized light, wherein the preferred direction of this polarization is expediently parallel to the longitudinal direction of the grating structures present on the mask. In this case, an as far as possible constant polarization distribution in the entire reticle field is also desirable for a constant imaging contrast and hence a defect-free imaging of the grating structures.

In this case, however, the problem occurs that a polarization distribution originally present (generally linear polarization upon entry into the illumination system) can be altered to different extents by polarization- influencing effects (e.g. stress birefringence induced by mount components in the material of the optical components such as e.g. lenses, polarization- influencing effects of dielectric layers, etc.).

US 2007/0146676 Al disclose, inter alia, arranging in a pupil plane of the illumination device, or in the vicinity thereof, a polarization manipulator for converting the polarization state, which has a plurality of variable optical rotator elements that can be used to rotate the polarization direction of impinging, linearly polarized light with a variably adjustable rotation angle. The variable rotation angle or polarization state provided by said rotator elements can be set in accordance with the measurement result supplied by a device for measuring the polarization state.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for modifying a polarization distribution in a microlithographic projection exposure apparatus, and a microlithographic projection exposure apparatus, which enable a greater homogeneity of the polarization state to be achieved in a predetermined plane.

A method for modifying a polarization distribution in a microlithographic projection exposure apparatus, wherein the projection exposure apparatus has an illumination device and a projection objective, and wherein the illumination device has an optical axis and a correction arrangement having a lambda/4 plate arranged rotatably about the optical axis and/or a lambda/2 plate arranged rotatably about the optical axis, has the following steps:

a) determining a polarization distribution in a predetermined plane of the projection exposure apparatus ; and b) rotating the lambda/4 plate and/or the lambda/2 plate about the optical axis in such a way that a local variation of the polarization distribution is reduced after the rotating in comparison with the state before the rotating.

The present invention is based on the concept of setting a defined polarization state (in particular at the entrance into the illumination device) in a targeted manner with regard to ellipticity and/or polarization direction, by means of a correction arrangement comprising a lambda/4 plate and/or a lambda/2 plate, in a manner dependent on the previously determined polarization distribution in such a way as to achieve an improvement in the homogeneity of the polarization distribution in the relevant plane.

In this case, the invention is based on the insight, in particular, that such a polarization state set in a targeted manner by means of the correction arrangement, even when it deviates with regard to the polarization direction (adjustable by the lambda/2 plate) and/or with regard to the ellipticity (adjustable by the lambda/4 plate) from a polarization state that is to be striven for in principle with regard to the structure to be imaged (e.g. a state with exactly linearly polarized light having a polarization direction in the x direction or in the y direction) , can lead to an improvement in the homogeneity of the polarization distribution in the relevant predetermined plane .

A value which is referred to hereinafter as IPS_PV value and defined below can serve for quantitatively describing the homogeneity of the polarization distribution in the relevant predetermined plane. The degree of realization of a desired polarization state at a specific location is referred to as "IPS value", and its averaging over the scanning direction is referred to as "scanned IPS value". In this case, IPS is the abbreviation of "Intensity in Preferred State", and the IPS value specifies the energetic ratio of the light intensity in the desired polarization state (which can be measured e.g. for a desired linear polarization state by means of an ideal polarizer whose transmission direction is set in the desired direction) to the total intensity. In this case, the desired polarization state can also be e.g. a state with circular polarization (in which case circular light should also be used as desired polarization) . The IPS_PV value (PV = " Peak-to-Value" ) of a distribution of the scanned IPS value is defined as the difference between maximum and minimum IPS value (PV=IPS _max -IPS _m in) and is referred to hereinafter as IPS_PV value.

According to the invention, therefore, the correction arrangement is not utilized for instance for setting the pupil plane of the illumination device to maximum performance with regard to the polarization, rather the IPS_PV value is manipulated in the predetermined plane by way of a targeted setting of the correction arrangement comprising lambda/2 plate and lambda/4 plate, said targeted setting being effected in a manner dependent on the previously determined polarization distribution .

In accordance with one embodiment, the rotating is effected in such a way that, in the local distribution of a parameter (IPS value) that is characteristic of the degree of realization of a specific polarization state in the predetermined plane, the difference between the maximum value and the minimum value of said parameter (IPS_PV value) is reduced by at least 10%, preferably by at least 25%, more preferably by at least 50%, in comparison with the state before the rotating.

In accordance with one embodiment, the illumination device has a device for changing the angle distribution of light passing through the illumination device, wherein the correction arrangement is arranged upstream of said device in the light propagation direction for changing the angle distribution. In particular, the correction arrangement is preferably situated in a beam feeding unit of the illumination device. The optical element for changing the angle distribution can be a diffractive optical element (DOE) or else a mirror arrangement comprising a multiplicity of micromirrors (that are adjustable in particular independently of one another), as is known e.g. from WO 2005/026843 A2.

A positioning of the correction arrangement upstream of the device for changing the angle distribution is expedient in so far as the correction arrangement only sets a global offset with regard to ellipticity and/or polarization direction without introducing an angle or field dependence. Setting an offset here means setting a defined polarization state in particular at the entrance into the illumination device, wherein said polarization state deviates depending on the determined polarization distribution with regard to the polarization direction (by deviating from the desired polarization direction, e.g. x direction or y direction) or with regard to the ellipticity (by deviating e.g. from a desired, exactly linear polarization state) from the polarization state that is to be striven for in principle with regard to the mask structure to be imaged.

In accordance with one embodiment, a further retardation element (in particular a second lambda/2 plate) is introduced into the beam path in such a way that it extends only over a partial region of the light beam cross section of light passing through. In this case, in particular, a region of the light beam cross section of light passing through which is covered by said further retardation element or the second lambda/2 plate can be set in a manner dependent on the measurement result of the polarization measuring device.

Such a further retardation element extending only over a partial region of the light beam cross section can improve the efficacy of the method according to the invention in situations in which an offset set by means of the correction arrangement, owing to an unfavorable profile of the ellipticity or the polarization direction, can admittedly be achieved in specific partial regions of the relevant plane (e.g. reticle plane or wafer plane) only at the expense of a deterioration in other partial regions without further measures.

In such cases, the further retardation element or the second lambda/2 plate makes it possible, in the last-mentioned partial regions, to invert the chirality of an existing ellipticity of the polarization state (such that, e.g. on account of the use of the second lambda/2 plate, the polarization states in one field region correspond to those in another field region with regard to the chirality of the ellipticity) and, if appropriate, to achieve a mirroring of the orientation of the polarization at the optical axis of the lambda/2 plate, such that the correction concept according to the invention by means of rotating the lambda/4 plate and/or the (first) lambda/2 plate of the correction arrangement can then advantageously be applied to the polarization distribution provided by the additional lambda/2 plate.

In this case, the invention is not restricted to the manipulation of the local variation of the polarization state in a field plane, but can also advantageously be employed if a pronounced inhomogeneity with regard to the retardation and/or with regard to the rotation of the polarization direction is present in a pupil plane. In this case, the use of an additional retardation element - or a second lambda/2 plate - partially covering the optically active region of a plane perpendicular to the optical axis is advantageous if, e.g. in the pupil plane, a positive birefringence is present in a first region of the light beam cross section and a negative birefringence is present in a second region of the light beam cross section. In this case, too, the use of the further retardation element or the second lambda/2 plate is expedient in order partially to cover the region having the initially "incorrect" chirality and in this way to provide the "correct" chirality in this region. The correction concept according to the invention can then be applied to the resultant IPS distribution in the pupil plane, in which concept, through suitable rotation of the lambda/4 plate and/or of the lambda/2 plate of the correction arrangement, a partial compensation of those polarization states which cause minimum IPS values is brought about with regard to the ellipticity and/or polarization direction.

The invention furthermore relates to a microlithographic projection exposure apparatus, wherein the projection exposure apparatus has an illumination device and a projection objective, and wherein the illumination device has an optical axis, having a correction arrangement having a lambda/4 plate arranged rotatably about the optical axis and/or a lambda/2 plate arranged rotatably about the optical axis, a polarization measuring device for determining a polarization distribution in a predetermined plane of the projection exposure apparatus, and a further retardation element, wherein a region of the light beam cross section of light passing through which is covered by said further retardation element is variably adjustable in a manner dependent on the measurement result of the polarization measuring device.

With regard to advantages and preferred configurations of the apparatus, reference is made to the above explanations in connection with the method according to the invention.

The invention furthermore relates to a method for microlithographically producing microstructured components .

Further configurations of the invention can be gathered from the description and the dependent claims.

The invention is explained in more detail below on the basis of exemplary embodiments illustrated in the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

In the figures:

figure 1 shows a schematic illustration of a microlithographic projection exposure apparatus in accordance with an embodiment in which the method according to the invention is realized;

figures 2-3 show diagrams each illustrating the field-dependent profile of the scanned IPS values, before and respectively after carrying out a correction by means of the method according to the invention;

figure 4 shows a schematic illustration of a microlithographic projection exposure apparatus in accordance with a further embodiment in which the method according to the invention is realized;

figures 5-6 show diagrams each illustrating further field-dependent profiles of the scanned IPS values, before and respectively after carrying out a correction by means of the method according to the invention; and

figure 7 shows a schematic illustration of a measuring device known per se for determining the polarization state, this measuring device being used in the projection exposure apparatus from figure 1 or figure 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with figure 1, the microlithographic projection exposure apparatus comprises an illumination device and a projection objective 150. The illumination device serves for illuminating a structure-bearing mask (reticle) 140 with light from a light source unit 101, which comprises for example an ArF laser for an operating wavelength of 193 nm and a beam shaping optical unit 102, which produces a parallel light beam. The illumination device furthermore has a deflection mirror 103.

The illumination device furthermore has a group comprising a lambda/4 plate 104, a lambda/2 plate 105 and a depolarizer 106. In this case, the lambda/4 plate 104 and the lamba/2 plate 105 are utilized as a correction arrangement in the context of the present invention, the function of said correction arrangement being described in more detail below. The lambda/4 plate 104 and the lambda/2 plate 105 are fitted rotatably about the optical axis OA of the illumination device. The depolarizer 106 is fitted in such a way that it can be removed from the beam path and, although it is provided in the exemplary embodiments shown in figure 1 and figure 4, it is removed from the beam path in each case in the embodiments of the invention described subsequently and is optional, in principle.

The group comprising lambda/4 plate 104, lambda/2 plate 105 and depolarizer 106 is followed by a diffractive optical element 107, which, as indicated, can be removed from the beam path, a zoom unit 108, a further diffractive optical element 109 (interchangeable with e.g. a further diffractive optical element 109a) and a further zoom unit 110, downstream of which follows a microlens array 111. Instead of the diffractive optical elements 107 and 109, for setting a desired angle characteristic, it is also possible to provide a mirror arrangement comprising a multiplicity of micromirrors (that are adjustable in particular independently of one another), as known e.g. from WO 2005/026843 A2.

In accordance with the exemplary embodiment from figure 1, in the beam path there furthermore follows a unit 112 (optional at this point) for monitoring the polarization state with a beam splitter 112a, a converging lens 113, a diaphragm unit 114 with downstream imaging system 115 and a deflection mirror 116, downstream of which an imaging onto the structure-bearing mask (reticle) 140 arranged in a field plane takes place. The structure-bearing mask 140 is imaged onto a substrate 160 - or a wafer - provided with a light-sensitive layer by means of the projection objective 150.

The projection exposure apparatus 100 furthermore has a polarization measuring device 170 for determining the polarization state of the light arriving in the wafer plane, and a control device 180 for driving (explained in even more detail below) the correction arrangement comprising the lambda/4 plate 104 and the lamba/2 plate 105 in a manner dependent on the polarization state determined by the polarization measuring device 170. As an alternative or in addition, the polarization state can also be determined in the reticle plane provided for accommodating the mask (reticle) 140.

In accordance with figure 1, the polarization measuring device 170 supplies information about the polarization state and hence the IPS distribution in the wafer plane PP, such that the IPS profile in the wafer plane can correspondingly be set or readjusted by means of the feedback via the control device 180 and the correction arrangement comprising the lambda/4 plate 104 and the lambda/2 plate 105.

One possible construction of the polarization measuring device 170 is disclosed in US 2007/014667 Al and is illustrated in figure 7 taken from this published patent application. Accordingly, the polarization measuring device 170 has a perforated plate 702 provided with a hole (pinhole) 701, wherein light that has passed through the hole 701 passes through a converging lens 703 and, after reflection at a deflection mirror 704, passes through a lens system 705. In the light propagation direction there follow a lambda/4 plate 706, which can be rotated about the optical axis by means of a setting device 710, and a polarization beam splitter cube 707, downstream of which is arranged a two-dimensional CCD sensor arrangement 709 provided with a sensor layer 708, wherein the light intensity of the light impinging on the CCD sensor arrangement 709 can be varied by rotating the lambda/4 plate 706, and wherein the polarization state of the light can be deduced from the measurement signal of the CCD sensor arrangement 709.

An explanation is given below, referring to figure 2 and figure 3, of how a setting of the correction arrangement comprising lambda/4 plate 104 and the lambda/2 plate 105 in a manner dependent on the measurement result supplied by the polarization measuring device 170 is effected according to the invention. In this case, typical values of the rotation angle of the lamba/4 plate 104 and/or of the lambda/2 plate 105 may lie merely by way of example (in each case relative to a basic setting in which they are operated in the characteristic state) in the range of approximately -20° to +20°, in particular at approximately -10° to +10° .

Figure 2a shows a measured field profile of the scanned IPS values, in which the scanned IPS value rises from a value of approximately scan_IPS = 0.8 at the left-hand field edge up to a value of approximately scan_IPS = 1.0 at the right-hand field edge. Polarization states 201 which can lead to such a profile are indicated schematically above the diagram of figure 2a, and likewise in the further diagrams of figures 2b, 3a-b, 5a-b and 6a-b.

Accordingly, in accordance with figure 2a, in the state before correction, a polarization state 201 with linearly polarized light having exactly the desired direction (e.g. y direction) is present at the right-hand field edge, a polarization state 203 with relatively highly elliptically polarized light is present at the left-hand field edge, and a polarization state 202 with light significantly more weakly elliptically polarized in comparison with the left-hand field edge is present in the field center.

The profile of the IPS value in accordance with figure 2a in dependence on the field position (that is to say from left to right in the diagram) corresponds to a quadratic dependence of the IPS value on the ellipticity. If it is assumed, for example, that the linearly polarized light obtained at the right-hand field edge (that is to say on the right in the diagram of figure 2a) is obtained without any influence of birefringence, that is to say on account of a retardation of 0 nm, and the comparatively weakly elliptically polarized light corresponding to an IPS loss of 5% arises in the field center on account of a retardation of 2 nm, the IPS loss at the left-hand field edge, where relatively highly elliptically polarized light arises on account of a retardation of 4 nm, has not double but quadruple the value compared with the right-hand field edge. In other words, the curve for the IPS profile falls relatively rapidly even though the disturbance rises only linearly.

According to the invention, as is illustrated in figure 2b, the correction arrangement comprising lambda/4 plate and lambda/2 plate is then set in such a way that, in the field center, the elliptical polarization state that was previously present is just cancelled out precisely by an opposite ellipticity, that is to say that a retardation of opposite sign for compensation in the field center is set at the entrance into the illumination device. Upon entry into the illumination device, in accordance with the example from figure 2b, a polarization state with elliptical polarization is thus set in a targeted manner in order to bias the ellipticity of the polarization state which is brought about by the system for the field center, which is effected by means of suitable rotation of the lambda/4 plate 104 of the correction arrangement about the optical axis OA.

At the right-hand field edge it is evident in figure 2b that the previously ideal polarization state 201 was disturbed since a polarization state 204 with slightly elliptically polarized light was obtained after the correction, such that an impediment in the polarization state was obtained in this region. In the case of the polarization state 206 obtained at the left-hand field edge on account of the correction, although ellάpticity is still present, this ellipticity is nevertheless weaker than before the correction. In the field center, a polarization state 205 having exactly the desired polarization direction now results.

For the retardation values specified in the above example, this means that as a result a retardation of 0 nm is produced for the field center, a retardation of +2 nm is produced at the left-hand field edge and a retardation of -2 nm is produced at the right-hand field edge. The circumstance that the profile of the IPS value is a quadratic function of the disturbance has the consequence that a relatively great improvement in the polarization state or in the IPS value is produced at the left-hand field edge, this improvement being opposed by a comparatively less pronounced impediment in the polarization state or in the IPS value at the right-hand field edge. Overall the IPS_PV value was improved by a factor of 4 , namely from a value of 100% - 80% = 20% before the correction to a value after the correction of 100% - 95% = 5%. In particular, the impediment of the IPS value (as a result of the setting of the polarization state 204 that is now slightly elliptical after the correction) obtained in the right-hand section of the curve from figure 2b is significantly less pronounced than the improvement (as a result of the setting of the likewise only weakly elliptical polarization state 206) obtained in the left-hand field region, which was particularly poor beforehand with regard to the IPS value .

A further exemplary embodiment is explained with reference to figures 3a-b, wherein the correction according to the invention here, in contrast to figures 2a-b, is not obtained by setting the phase or the ellipticity of the polarization state, but rather by varying the orientation of the polarization over the field.

The dependence of the IPS value on the orientation of the polarization is also a quadratic function, such that the correction in accordance with figures 3a-b proceeds analogously to that from figures 2a-b, with the difference that here a specific orientation of the polarization state rather than a birefringence or ellipticity is biased.

In accordance with figure 3a, before the correction, a polarization state 301 having exactly the desired polarization direction (e.g. y direction) is present at the right-hand field edge, while this polarization direction is increasingly rotated in the polarization state 302 obtained in the field center and in the polarization state 303 obtained at the left-hand field edge. In the case of the correction according to the invention, in accordance with figure 3b, the rotation of the system for the field center is then biased, such that a polarization state 305 having exactly the desired polarization direction (e.g. y direction) just results after the correction in the field center. Upon entry into the illumination device, in accordance with the example from figure 3, therefore, a polarization state having a polarization direction rotated relative to the polarization direction that is to be striven for in principle with regard to the mask structure to be imaged is set in a targeted manner in order to bias the rotation of the polarization direction which is brought about by the system for the field center, which is effected by means of suitable rotation of the lambda/2 plate 105 of the correction arrangement about the optical axis OA.

The circumstance that the profile of the IPS value is a quadratic function of the disturbance has the consequence here in turn, analogously to figures 2a-b, that a relatively great improvement is produced at the left-hand field edge, with this improvement being opposed by a comparatively less pronounced impediment at the right-hand field edge. Here, too, the IPS_PV value was improved overall by a factor of 4 (from a value of 20% before the correction to a value of approximately 5% after the correction) . In particular, the impediment (as a result of the setting of the polarization state 304 that is now slightly rotated after the correction) obtained in the right-hand section of the curve from figure 3b is significantly less pronounced than the improvement (as a result of the setting of the polarization state 306 that is likewise only weakly rotated) obtained in the left-hand field region, which was poor beforehand with regard to the IPS value.

Generally, it is also possible (by way of a combination of the exemplary embodiments from figures 2a-b and 3a-b) , by means of the correction arrangement according to the invention comprising lambda/4 plate 104 and lambda/2 plate 105, to set both the ellipticity (by means of the lambda/4 plate) and the orientation (by means of the lamba/2 plate) of the polarization state.

An explanation will now be given below of further exemplary embodiments wherein, unlike in figures 2 and 3, the IPS values are not worse at one field edge than at the other, opposite field edge, rather equally poor IPS values are present at opposite field edges, wherein now a comparatively good or optimum IPS value is present in the field center.

An exemplary construction of a projection exposure apparatus suitable for application of the concept according to the invention in such situations is illustrated in figure 4. This construction substantially corresponds to the construction shown in figure 1, but differs in so far as an additional lambda/2 plate 117 is provided at the location of the reticle plane, wherein a region of the light beam cross section of light passing through which is covered by said second lambda/2 plate 117 is variably adjustable in a manner dependent on the measurement result of the polarization measuring device 170. In accordance with figure 4, the second lambda/2 plate 117 for example just covers half of the optically utilized region of the reticle plane or half of the light beam cross section of the light passing through the illumination device. The optical crystal axis of said lambda/2 plate 117 is oriented parallel to the preferred direction of the polarization or perpendicular thereto.

Figure 5a then shows a situation in which, before the correction, a polarization state 503 with linearly polarized light of exactly the desired polarization direction (e.g. y direction) is present in the field center, whereas elliptically polarized light of opposite chirality is present at the mutually opposite field edges (a polarization state 505 with right elliptically polarized light at the left-hand field edge and a polarization state 501 with left elliptically polarized light at the right-hand field edge) , wherein both polarization states bring about the same IPS loss (of once again approximately 20% in the example) on account of the ellipticity that is equal in magnitude and differs merely in the sign or chirality. Here the further problem then occurs that although a compensation of the ellipticity at the left-hand field edge by means of the lambda/4 plate 104 of the correction arrangement (by setting an ellipticity of opposite chirality) is possible, it would lead to a further impediment at the right-hand field edge, such that as a result the polarization state or the IPS value is improved in one field region only at the expense of an impediment in the other field region.

In order to overcome this problem, according to the invention, as illustrated in figure 5b, by means of the lambda/2 plate 520, which only extends over that part of the light beam cross section which impinges on the right-hand field region, the chirality for the polarization distributions 501, 502 present in the right-hand field region before the correction is then inverted, such that, owing to the use of said lambda/2 plate 520, the polarization states in the right-hand field region correspond to those in the left-hand field region with regard to the chirality of the ellipticity. Consequently, a polarization state 505 with right elliptically polarized light is produced at the left-hand field edge and a polarization state 501' with likewise right elliptically polarized light is produced at the right-hand field edge. This is achieved by virtue of the fact that the region having the initially "incorrect" chirality is covered by the additional lamdbda/2 plate 520 and the "correct" chirality is thus provided in this region as a result. The correction concept analogous to figure 2 is then applied to the IPS distribution obtained by the effect of the lambda/2 plate 520 by virtue of the fact that a partial compensation of those polarization states which cause minimum IPS values (that is to say of the polarization states 505 and 501') is obtained by the setting of the lambda/4 plate 104, wherein, analogously to figure 2, the resultant improvement in the IPS profile outweighs the impediment brought about in other regions (in particular in the field center) .

Figures 6a-b show an analogous exemplary embodiment for the application of the correction concept according to the invention in the case of an IPS profile which is symmetrical about the field center and which is based on a change in the orientation of the polarization which is symmetrical about the field center.

In this case, use is made of the fact that the orientation of the polarization is mirrored at the optical crystal axis of the lambda/2 plate 620, such that initially polarization states 605 and 601' having an identical orientation of the polarization are set at the left-hand field edge and at the right-hand field edge by means of the lambda/2 plate 520. The correction concept analogous to figure 3 is then applied to the resultant IPS distribution by virtue of the fact that a partial compensation of those polarization states which cause minimum IPS values (that is to say of the polarization states 605 and 601') is obtained by the setting of the lambda/4 plate 104 with regard to the orientation of the polarization, wherein, analogously to figure 3, the resultant improvement in the IPS profile outweighs the impediment brought about in other regions (in particular in the field center) .

Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments can be deduced by the person skilled in the art, e.g. by 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 invention, and the scope of the invention is only restricted within the meaning of the accompanying patent claims and the equivalents thereof .