Patent Application
20110063597
The disclosure relates to an optical system for a microlithographic projection exposure apparatus, and to a microlithographic exposure method.
Microlithographic projection exposure apparatuses are used for the production of microstructured components such as, for example, integrated circuits or LCDs. Such a projection exposure apparatus has an illumination device and a projection objective. In the microlithography process, the image of a mask (=reticle) illuminated with the aid of the illumination device is projected, via the projection objective, onto a substrate (e.g. a silicon wafer) that is coated with a light-sensitive layer (photoresist) and is arranged in the image plane of the projection objective, in order to transfer the mask structure to the light-sensitive coating of the substrate.
US 2004/0262500 A1 discloses a method and an apparatus for the image-resolved polarimetry of a beam pencil generated by a pulsed radiation source (e.g., an excimer laser), e.g., of a microlithographic projection exposure apparatus, wherein two photoelastic modulators (PEM) that are excited at different oscillation frequencies and a polarization element e.g. in the form of a polarization beam splitter are positioned in the beam path, the radiation source is driven for emission of radiation pulses in a manner dependent on the oscillation state of the first and/or the second PEM, and the radiation coming from the polarization element is detected in image-resolved fashion via a detector.
The abovementioned photoelastic modulators (PEM) are optical components which are produced from a material exhibiting stress birefringence in such a way that an excitation of the PEM to effect acoustic oscillations leads to a periodically varying mechanical stress and thus to a temporally varying retardation. “Retardation” denotes the difference in the optical paths of two orthogonal (mutually perpendicular) polarization states. Photoelastic modulators (PEM) of this type are known in the prior art, e.g., U.S. Pat. No. 5,886,810 A1 or U.S. Pat. No. 5,744,721 A1, and can be produced and sold for use at wavelengths of visible light through to the VUV range (approximately 130 nm), e.g., by the company Hinds Instruments Inc., Hillsboro, Oreg. (USA).
In the operation of a microlithographic projection exposure apparatus it is often desirable to set defined illumination settings, that is to say intensity distributions in a pupil plane of the illumination device, in a targeted manner. Apart from the use of diffractive optical elements (so-called DOEs), the use of mirror arrangements is also known for this purpose, e.g., from WO 2005/026843 A2. Such mirror arrangements include a multiplicity of micromirrors that can be set independently of one another.
EP 1 879 071 A2 discloses an illumination optical unit for a microlithographic projection exposure apparatus which has two separate optical assemblies which are different from one another for setting at least two different illumination settings or for rapidly changing between such illumination settings, a coupling-out element being arranged in the light path upstream of the assemblies and a coupling-in element being arranged in the light path downstream of the assemblies. In this case, the coupling-out element can also have a plurality of individual mirrors arranged on a rotationally drivable mirror carrier, in which case, with the mirror carrier rotating, the illumination light is either reflected by one of the individual mirrors or transmitted between the individual mirrors.
The disclosure provides an optical system for a microlithographic projection exposure apparatus and a microlithographic exposure method by which an increased flexibility is afforded with regard to the intensity and polarization distributions that can be set in the projection exposure apparatus.
An optical system according to the disclosure for a microlithographic projection exposure apparatus includes:
The polarization state altering device includes at least one element out of the group of photoelastic modulator, Pockels cell, Kerr cell, and rotatable polarization-changing plate. A polarization-changing plate is described in WO 2005/069081. Such plate acts as a polarization state altering device when it is rotated about an axis, e.g. about any symmetry axis. Fast polarization altering devices with switching or altering times down to 1 ns are Pockels or Kerr cells which are known per se from laser physics.
The photoelastic modulator can be subjected to a temporally varying retardation via suitable (e.g. acoustic) excitation in a manner known per se, which retardation may in turn be temporally correlated with the pulsed light, such that individual (e.g. successive) pulses of the pulsed light are subjected in each case to a defined retardation and hence to a defined alteration of their polarization state. This alteration can also be set differently for individual pulses. According to the present disclosure the photoelastic modulator also includes acoustic-optical modulators in which not necessarily standing waves of density variations are generated within the modulator material. Also the other exemplary polarization state altering devices mentioned above can be synchronized or correlated accordingly with the light pulses.
On account of the combination according to the disclosure of a polarization state altering device like, e.g., the photoelastic modulator firstly with a mirror arrangement having a plurality of mirror elements that are adjustable independently of one another, secondly, the possibility is afforded, combined with a changeover of the polarization state that is achieved via the polarization state altering device like, e.g., the photoelastic modulator, of performing an adjustment of the mirror elements that is coordinated therewith precisely such that, via the mirror arrangement, the entire light entering into the illumination device is directed, in a manner dependent on the polarization state currently set by the polarization state altering device like, e.g., the photoelastic modulator, into a region of the pupil plane which is in each case “appropriate” or suitable for generating a polarized illumination setting respectively sought, in which case, in particular, loss of light can be substantially or completely avoided.
In this case, the use of a polarization state altering device like a photoelastic modulator, a Pockels cell or a Kerr cell for generating an (in particular pulse-resolved) variation of the polarization state has the further advantage that the use of movable (e.g. rotating) optical components can be dispensed with, thereby also avoiding a stress birefringence that is induced in such components on account of e.g. centrifugal forces that occur, and an undesirable influencing of the polarization distribution that accompanies the stress birefringence.
In accordance with one embodiment, the polarization state altering device like, e.g., the photoelastic modulator is arranged upstream of the mirror arrangement in the light propagation direction.
In accordance with one embodiment, at least two illumination settings which are different from one another can be set by the alteration of an angular distribution of the light reflected by the mirror arrangement and/or by variation of the retardation generated in the polarization state altering device like, e.g., the photoelastic modulator. In this case, polarization state altering device like, e.g., photoelastic modulator and mirror arrangement can be operated in particular independently of one another, such that the alteration of an angular distribution of the light reflected by the mirror arrangement can be set independently of a polarization state of the light that is set by the polarization state altering device like e.g. the photoelastic modulator.
In accordance with one embodiment, provision is made of a driving unit for driving an adjustment of mirror elements of the mirror arrangement, the adjustment being temporally correlated with the excitation of the photoelastic modulator to effect mechanical oscillations.
In accordance with one embodiment, over all of the illumination settings that can be set, the ratio of the total intensity of the light contributing to the respective illumination setting to the intensity of the light entering into the photoelastic modulator varies by less than 20%, particularly less than 10%, more particularly less than 5%. In accordance with another approach, also upon variation of the illumination setting over all of the illumination settings that can be set, a wafer arranged in the wafer plane of the projection exposure apparatus is exposed with an intensity that varies by less than 20%.
In accordance with one embodiment, for each of the illumination settings that can be set, the total intensity of the light contributing to the respective illumination setting is at least 80%, particularly at least 90%, more particularly at least 95%, of the intensity of the light upon entering into the photoelastic modulator. This consideration disregards intensity losses owing to the presence of optical elements which do not contribute to the variation of the illumination setting, that is to say to the change of the angular distribution and/or of the polarization state, and can occur in particular between the photoelastic modulator and the mirror arrangement, such that for example intensity losses owing to absorption in lens materials are disregarded in this consideration.
In accordance with a further aspect, the disclosure relates to an optical system for a microlithographic projection exposure apparatus, including:
In this case, illumination settings that are regarded as differing from one another in terms of their polarization state include both illumination settings for which identical regions of the pupil plane are illuminated with light of different polarization states and illumination settings for which light of different polarization states is directed into mutually different regions of the pupil plane.
Furthermore, the wording “without exchanging one or more optical elements” should be understood to mean that all the optical elements remain in the beam path both during the exposure and between the exposure steps, in particular no additional elements being introduced into the beam path either.
The disclosure furthermore relates to a microlithographic exposure method.
Further configurations of the disclosure can be obtained from the description and the claims.
The disclosure is explained in more detail below on the basis of exemplary embodiments illustrated in the accompanying figures, in which:
a-6b show exemplary illumination settings that can be set using an optical system according to the disclosure.
Firstly, with reference to
According to the disclosure, part of the illumination device 10 is, in particular, a mirror arrangement 200, as is explained in more detail below with reference to
A polarization state altering device could be at least one element out of the group of photoelastic modulator, Pockels cell, Kerr cell, and rotatable polarization-changing plate. A polarization-changing plate is described in WO 2005/069081, e.g., in
In the following detailed description of the disclosure the effect of the polarization state altering device is described by the example of a photolelastic modulator, which alters the polarization state according to the pressure performed on the photoelastic modulator, or more general, according to any force subjecting shear, strain or distension to at least parts of the material of the photoelastic modulator.
For the example of a Pockels cell as a polarization state altering device an electric field is applied at the Pockels cell. For the example of a Kerr cell a magnetic field or an electric field is used. Any other polarization state altering device based on an electro-optical principle (based e.g. on Pockels- and/or Stark-effect) and/or magneto-optical principle (based e.g. on Faraday and/or Cotton-Mouton-effect) can be used.
For the example of a polarization-changing plate as described in WO 2005/069081 there is no need for an external electric or magnetic field, pressure or force acting on the optical element to achieve the polarization altering effect. In this case the polarization altering effect is achieved by a rotation of the polarization-changing plate.
The illumination settings and the advantages as described below with the example of a photoelastic modulator acting as a polarization state altering device can also be achieved by using the other above mentioned polarization state altering devices. Therefore the embodiments described below are not limited to the operation of a photoelastic modulator only. Also a combination of several of the above mentioned polarization state altering devices parallel or in sequence according to the light beam path can be used to achieve the illumination settings and the advantages mentioned below.
The PEM 100 as one example of a polarization state altering device 100 in
In accordance with
For elucidating the interaction according to the disclosure of the PEM 100 with the mirror arrangement 200 situated in the illumination device 10, firstly a description is given hereinafter of how an “electronic switch-over” of the polarization state of light passing through the PEM 100 can be achieved by the PEM 100.
The light source unit 1 can generate for example a pulse at a point in time at which the retardation in the PEM 100 is precisely zero. Furthermore, the light source unit 1 can also generate a pulse at a point in time at which the retardation in the PEM 100 amounts to half the operating wavelength, that is to say λ/2. The PEM 100 therefore acts on the latter pulse as a lambda/2 plate, such that the polarization direction of the pulse upon emerging from the PEM 100 is rotated by 90° with respect to its polarization direction upon entering into the PEM 100. Depending on the instantaneous retardation value set in the PEM 100, in the example described the PEM 100 therefore either leaves the polarization direction of the light impinging on the PEM 100 unchanged or it rotates the polarization direction by an angle of 90°.
The PEM 100 is typically operated with a frequency of a few 10 kHz, such that the period duration of the excited oscillation of the PEM 100 is long in comparison with the pulse duration of the light source unit 1, which may typically be approximately 10 nanoseconds. Consequently, a quasi-static retardation acts on the light from the light source unit 1 in the PEM 100 during the duration of an individual pulse. Furthermore, the above-described variation of the polarization state set by the PEM 100 can be effected on the timescale of the pulse duration of frequency of the light source unit 1, that is to say that the changeover of the polarization state e.g. via rotation of the polarization direction by 90° can be performed in a targeted manner for specific pulses, in particular also between directly successive pulses from the light source unit 1. In the example described above, the two pulses described are oriented orthogonally with respect to one another in terms of their polarization direction when emerging from the PEM 100.
What can be achieved, then, through suitable adjustment of the mirror elements 200a, 200b, 200c, . . . that is coordinated with the above-described changeover of the polarization state is that the entire light entering into the illumination device 10 is directed by the mirror arrangement 200 into a respectively different region of the pupil plane that respectively “matches” the polarized illumination setting sought, in which case, in particular, loss of light can be substantially or completely avoided. In this case, in order to achieve a switch-over between the corresponding illumination settings, the driving of the mirror elements 200a, 200b, 200c, . . . via the driving unit 205 can be suitably correlated temporally with the excitation of the PEM 100 via the excitation unit 105.
Furthermore, photoelastic modulator 100 and mirror arrangement 200 can also be operated independently of one another, such that the alteration of an angular distribution of the light reflected by the mirror arrangement can be set independently of a polarization state of the light that is set by the photoelastic modulator 100. In this case, for example, even with the setting of the mirror elements 200a, 200b, 200c, . . . remaining the same, only a change in the polarization state can be performed via the PEM 100. Furthermore, what can also be achieved through suitable coordination or triggering of the pulses from the light source unit 1 in a manner dependent on the excitation of the photoelastic modulator 100 is that the pulses emerging from the photoelastic modulator 100 each have the same polarization state, in which case a different deflection for different pulses can be set via the mirror arrangement.
For the description of concrete exemplary embodiments it is assumed below, without restricting the generality, that the light which impinges on the PEM 100 and is generated by the light source unit 1 is polarized linearly in the y-direction relative to the system of coordinates depicted in
Referring to
In this case, a “tangential polarization distribution” is generally understood to mean a polarization distribution in the case of which the oscillation direction of the electric field strength vector runs perpendicular to the radius directed at the optical system axis. A “quasi-tangential polarization distribution” is the term correspondingly employed when the above condition is met approximately or for individual regions in the relevant plane (e.g. pupil plane), as for the regions 311, 312, 321 and 322 in the examples of
In order to set the “quasi-tangentially polarized H dipole setting” from
Furthermore, the arrangement according to the disclosure can also be used as follows for setting a quasi-tangentially polarized quadrupole illumination setting 400, as is illustrated in
The embodiments described above with reference to
In accordance with further embodiments, the setting or excitation of the PEM 100 by the excitation unit 105 can be correlated with the emission from the light source unit 1 and the driving of the mirror arrangement 200 via the driving unit 205 in such a way that illumination settings with left and/or right circularly polarized light are produced or a switch-over between these illumination settings is realized. For this purpose, pulses can pass through the PEM 100 for example in each case at a point in time at which the retardation in the PEM 100 amounts to one quarter of the operating wavelength, that is to say λ/4 (which leads e.g. to left circularly polarized light). Furthermore, pulses can pass through the PEM 100 at a point in time at which the retardation in the PEM 100 is of identical magnitude and opposite sign, that is to say amounts to −λ/4, which leads to right circularly polarized light.
In accordance with further embodiments, the PEM 100 can also interact with the mirror arrangement 200 in such a way that an electronic switch-over is achieved between the illumination settings 510 and 520 shown in
In accordance with further embodiments, the PEM 100 can also interact with the mirror arrangement 200 in such a way that an electronic switch-over is achieved between the illumination settings 610 and 620 shown in
The hatched region 605 once again corresponds to that region in the pupil plane which is not illuminated but which can still be illuminated alongside the illuminated regions. Even though the disclosure 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 disclosure, and the scope of the disclosure is only restricted within the meaning of the accompanying patent claims and the equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 009 601.6 | Feb 2008 | DE | national |
This application is a continuation of, and claims benefit under 35 USC 120 to, international application PCT/EP2009/000854, filed Feb. 6, 2009, which claims benefit of German Application No. 10 2008 009 601.6, filed Feb. 15, 2008 and U.S. Ser. No. 61/028,928, filed Feb. 15, 2008. International application PCT/EP2009/000854 is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61028928 | Feb 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2009/000854 | Feb 2009 | US |
| Child | 12851074 | US |
