US 20060146427 A1
A method for improving imaging properties of two or more optical elements comprises the step of determining for at least one of the two optical elements a polarisation-dependent perturbation. In a further step a polarisation-independent perturbation is determined for at least one of the two optical elements. Then a target position for the at least one movable optical element is calculated such that, in the target position, the total perturbation of the at least two optical elements which is made up of the polarisation-dependent perturbations and polarisation-independent perturbations of the two optical elements, is minimized. Finally the at least one movable optical element is moved the to the calculated target position.
1. An optical system, comprising:
a) at least two optical elements, at least one of which being a movable optical element,
b) a computer for calculating a target position for the at least one movable optical element such that, in the target position, the total perturbation of the at least two optical elements which is made up of polarization-dependent perturbations and polarization-independent perturbations of the at least two optical elements, is minimized;
c) an actuator for moving the at least one movable optical element to the target position calculated by the computer.
2. The optical system of
3. The optical system of
4. The optical system of
5. The optical system of
6. The optical system of
7. The optical system of
8. The optical system of
9. A photolithographic exposure apparatus comprising the optical system of
10. A photolithographic exposure apparatus, comprising at least two optical elements being in a relative position such that the total perturbation of the at least two optical elements which is made up of polarization-dependent perturbations and polarization-independent perturbations is minimized.
This application is a continuation application of U.S. patent application Ser. No. 11/185,066, filed Jul. 20, 2005, which is a continuation application of U.S. patent application Ser. No. 10/708,098 filed on Apr. 13, 2004 which is a continuation of and claims priority to International Application PCT/EP02/11030, with an international filing date of Oct. 2, 2002, which claims priority to German patent application number 101 62 796.3, filed on Dec. 20, 2001, upon which the present application also claims priority, all of which are incorporated herein by reference.
The invention relates to a method for improving the imaging properties of at least two optical elements. More particularly, the invention relates to such a method in which the relative position of the optical elements is mutually adjusted in order to improve the optical imaging. The invention further relates to a photolithographic fabrication method.
From EP 1 063 684 A1 it is known to determine the birefringence distribution of individual lenses inside a projection lens of a projection exposure system as a polarisation-dependent perturbation. The lenses are then selected and arranged inside the projection lens so as to obtain a total birefringence whose magnitude is less than a predetermined limit value for each optical path through the projection lens. The total birefringence is in this case made up of the sum of all the birefringences of the individual lenses being analysed. Such a method is helpful when lenses need to be rejected on the basis of an intolerable birefringence distribution, but in practice does not always lead to specification values being achieved for the imaging properties of the optical elements.
Another optimisation method is known from the specialist article “The development of microlithographic high-performance optics”, Int. J. of Optoelec., 1989, 545. When optimising the imaging properties of optical systems having optical elements which are made of crystalline materials, this method provides satisfactory results only if the crystalline materials are specially selected and the optical elements are mounted without stress. Such measures are expensive.
It is therefore a first object of the present invention to refine an optimisation method of the type mentioned in the introduction, so that a total imaging error made up of the imaging errors of the individual optical components can be further reduced for most practical applications.
This object is achieved according to the invention by a method having the features mentioned in claim 1.
The method according to the invention is based on the following facts:
As a rule, polarisation-dependent and polarisation-independent perturbations contribute to the total perturbation. Polarisation-dependent perturbations can be subdivided into: intrinsically present polarisation-dependent perturbations, such as the intrinsic birefringence, i.e. that which occurs even in a homogeneous and stress-free material; polarisation-dependent perturbations occurring because of external effects, such as stress birefringence; and polarisation-dependent perturbations occurring because of material inhomogeneities, such as birefringence due to crystal defects, especially due to the formation of domains in the material.
As a rule, previous determination methods for determining the imaging errors of optical elements have been restricted to polarisation-independent perturbations, since it was assumed that conventional optical materials have polarisation-dependent perturbations only in exceptional cases. These polarisation-dependent perturbations have previously been accommodated without including them in a target-position calculation. This was done, as mentioned above, by material selection or special mounting.
It is known from the Internet publication “Preliminary determination of an intrinsic birefringence in CaF2” by J. H. Burnett, G. L. Shirley and Z. H. Levine, NIST Gaithersburg Md. 20899 USA (posted on 7.5.01), however, that single CaF2 crystals also have non-stress-induced, i.e. intrinsic birefringence. This applies, for example, to ray propagation in the (110) crystal direction. For ray propagation in the (100) crystal direction and in the (111) crystal direction, however, CaF2 does not have any intrinsic birefringence. The birefringence that occurs is therefore dependent on the ray direction. It cannot be eliminated either by material selection or by stress-free mounting of an optical element.
Since CaF2 and other crystalline materials with intrinsic birefringence are being used increasingly as optical materials, particularly in conjunction with UV light sources, the neglect of polarisation-dependent perturbations is leading to imaging errors which are not picked up in the known optimisation methods.
Polarisation-dependent perturbations cause light rays with orthogonal polarisations to be imaged at different positions. Polarisation effects can furthermore cause individual polarisation components to experience different imaging errors.
Although the aforementioned EP 1 063 684 A1 takes into account a polarisation-dependent perturbation, namely the birefringence, it ignores other perturbations in the scope of optimising the mutual arrangement of the optical components, so that there may some be error contributions to the total imaging error which are avoidable.
According to the invention, both the polarisation-dependent perturbations and the polarisation-independent perturbations are taken into account in the target-position calculation. In this way, the optical elements can be modelled precisely and fully in respect of their imaging properties.
The polarisation-dependent perturbation according to claim 2 takes into account the effect of internal stresses in the optical materials. These internal stresses may, for instance, have been frozen in the material during the production process, or may occur because of the mechanical mounting (frame) of the optical element. Taking the stress birefringence into account improves the optimisation of the imaging properties even for optical elements which do not have any intrinsic stress birefringence.
Determining the position of at least one crystal axis according to claim 3 can obviate further measurement of polarisation-dependent perturbations in the most favourable case, if there are no other polarisation-dependent perturbations, since the intrinsic birefringence can be calculated following determination of the crystal axis position.
A degree of freedom in movement which is relatively straightforward to achieve, since it does not involve significant intervention in the mounting of the optical element, is the rotatability of the at least one optical element according to claim 4.
The effects of displacing a linearly displaceable optical element according to claim 5 on the imaging properties of the at least two optical elements allow precise predictions, for example by means of optical design programs, which facilitates calculation of the target position.
Centring errors, in particular, can be compensated for by a displaceable optical element according to claim 6.
A tiltable optical element according to claim 7, for example, allows alignment of the crystal axes of the optical element relative to the optical axis of an overall optical system, which includes the at least two optical elements.
The effect of determining the polarisation-dependent perturbation according to claim 8 is that the contributions to the stress birefringence from the frame are also taken into account in the determination of the polarisation-dependent perturbation. This increases the precision of the optimisation method.
It is also an object of the present invention to provide a photolithographic fabrication method with improved optical quality.
This object is achieved according to the invention by a method having the features mentioned in claim 9. The advantages of the fabrication method derive from the aforementioned advantages of the optimisation method.
At an exposure wavelength according to claim 10, many optical materials have polarisation-dependent perturbations which affect the imaging properties of optical elements more strongly than, for example, when they are exposed to visible light. The optimisation method according to the invention is therefore very effective with exposure to wavelengths of less than 200 nm.
An exemplary embodiment of the invention will be explained in more detail below with reference to the drawing, in which:
A projection exposure system denoted overall by 1 in
A light source 3, for example an F2 laser with a wavelength of 157 nm, generates a projection light beam 4 for this purpose. It passes first through illumination optics 5 for shaping, and subsequently through the mask 2. A projection lens 6 images the structure present on the mask 2 onto the wafer.
A position-sensitive sensor 11 is provided in order to analyse perturbations which affect the imaging properties of the projection lens 6. It is displaceable transversely to the optical axis of the projection lens 6, between a measurement position represented in
The lens 9 and the optical plate 10 are made from single crystals of CaF2, which has a cubic crystal symmetry. For production, these optical elements 9, 10 are cut from crystal blocks and polished.
Such a crystal block 15 for the lens 9 is represented by way of example in
The optical plate 10, which is represented separately in
The ray direction of a light ray 18 of the projection light beam 4 is defined by an aperture angle θ and an azimuth angle α
In the following description, the optical components 9, 10 are oriented so that the (100) crystal direction coincides with the z axis and the projection of the (101) crystal direction onto the xy plane coincides with the x axis.
As the maximum intrinsic birefringence (ray propagation e.g. in the (110) crystal direction, i.e. θ equal to 45 degrees, α equal to 90 degrees), a value of (11.0+/−0.4) nm/cm was measured at a wavelength of 156.1 nm for CaF2.
At the azimuth angles for which intrinsic birefringence occurs (cf.
Besides these intrinsic contributions to the birefringence, the lens 9 and the optical plate 10 have additional stress birefringence contributions depending on their installation situation in the projection lens 6, which are added to the intrinsic birefringence. Further birefringence contributions may, for example, be due to crystal defects, in particular the formation of domains. There may even be non-intrinsic birefringence contributions in optical materials which do not have any intrinsic birefringence.
A method for improving the imaging properties of the projection lens 6 is carried out as follows:
First, the optical perturbations of all the optical elements of the projection lens 6 are determined individually. Such measurement methods for determining the aforementioned birefringence contributions as an example of polarisation-dependent perturbations, on the one hand, and polarisation-independent perturbations, on the other hand, are known to the person skilled in the art. To this end, for example, as indicated by the sensor 11 in
As an alternative or in addition, the individual optical elements of the projection lens 6 may be analysed independently of one another with the aid of known measurement methods. In this case, care should be taken to simulate the installation situation of the optical elements in the projection lens 6 as precisely as possible during this independent analysis, so as to prevent the installation of the optical elements in the projection exposure system 1 from giving rise to additional perturbation contributions, which impair the optimisation of the imaging properties of the projection lens 6.
The determination of the birefringence contributions may, for example, comprise determination of the position of the crystal axes of the optical elements to be analysed, when crystalline materials are involved.
The measurement results are evaluated by the computer 14. It determines the respective perturbation contributions of the individual optical elements of the projection lens, and assigns these contributions to the individual polarisation-dependent and polarisation-independent perturbations. The computer 14 subsequently calculates and optimises a target function (merit function). This target function takes in the dependencies of the perturbation contributions of all the optical elements on the degrees of freedom in movement of these optical elements (rotation, inclination, centring).
In the exemplary embodiment which is represented, this calculation is carried out for the optical components 9 and 10.
As was mentioned above, the lens 9 is rotatable relative to the optical plate 10 about the optical axis. For the lens 9 and the optical plate 10, their respective contributions to the polarisation-dependent and polarisation-independent perturbations are available after the perturbation contributions have been analysed. Besides the perturbations of the lens 9 and the optical plate 10, the merit function also contains the dependency of the perturbation contributions of the lens 9 on its rotation about the optical axis.
The merit function is subsequently optimised by varying of the degrees of freedom in movement of the mobile parts of the projection lens 6. In the embodiment according to
Finally, the mobile optical elements are brought into the target position which has been determined. In the embodiment according to