Illumination – Light fiber – rod – or pipe
Reexamination Certificate
2001-12-20
2004-05-11
Husar, Stephen (Department: 2875)
Illumination
Light fiber, rod, or pipe
C362S560000, C362S556000, C362S035000, C385S146000, C385S901000
Reexamination Certificate
active
06733165
ABSTRACT:
The following disclosure is based on German Patent Application No. 100 65 198.4, filed on Dec. 20, 2000, which is incorporated into this application by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an optical integrator for an illumination device for a microlithographic projection exposure system, as well as an illumination device equipped with the optical integrator.
2. Description of the Related Art
The performance of projection exposure systems for the microlithographic production of semiconductor components is essentially determined by the imaging properties of the projection system. In addition, the image quality and the achievable wafer flow rate with a wafer stepper or wafer scanner are also essentially determined by the properties of the illumination device upstream of the projection system. This must be capable to provide as large a quantity of light as possible in the case of homogeneous distribution of intensity in a way that is precisely adjusted to the downstream optical system.
The adjustment to the subsequent system is also essentially determined by the telecentric alignment at the exit of the illumination device. A high degree of homogeneity of the illumination falling on the photo mask can be achieved by mixing the light coming from a light source in the illumination device with the aid of so-called optical integrators or light mixing elements. In addition to the optical integrators working with honeycomb condensers, those optical integrators with a rod, which consists of a material transparent for the light of a light source of the illumination device and, which is essentially penetrated by radiation in its longitudinal direction (z direction), have also gained acceptance. The rod is also described as a glass rod in the following, although it does not only consist of a glass-type material, for example synthetic quartz glass, but instead can also consist of a crystalline material, calcium fluoride for example. The rod has an entrance surface optically facing towards the light source of the illumination device, in which the light of the light source is incident, as well as an opposing exit surface, which can form an intermediate field plane of the illumination device.
As the cross-section form of the rod is intended to be adjusted to the form of the surface to be illuminated, the rod cross-section of the rods considered here is rectangular with an aspect ratio between width (or x direction) and height (or y direction) deviating from the value of 1. In the glass rod the light passing through is totally reflected many times on the lateral boundary surfaces, as in a kaleidoscope, which allows an almost perfect mixture of non-homogeneous parts of light to be achieved. Thus, the exit surface of the rod is reproduced as an almost uniform illuminated field on the photo mask. Illumination devices, which use that type of rectangular rod as an optical integrator, are disclosed for example in German Patent Applications DE 44 21 053, DE 195 20 563 or DE 199 12 464.
It has become known that the distribution of the light energy at the exit of such rod-shaped optical integrators exhibits an undesired asymmetry in the angular space. This asymmetry is described in the following as (energetic) ellipticity of the pupil and can be described for every image location or point (x, y) of the exit surface of the rod. The description of the ellipticity is based on the consideration that light energy is emitted via a specific angular distribution from every image location, i.e. not only in the z direction (equivalent to the longitudinal direction of the rod) but also with components diagonal to the z direction. Whereas the energy density for all directions should be identical within the angular distribution, in real systems a deviation from the symmetry i.e. a deformation of the distribution of energy is observed, in which the light intensity in areas at the distance from the x axis is lower than that in those areas that are just in the same distance from the y axis. This can lead to an undesired irregularity of the illumination at the wafer.
It is an object of the invention to provide an optical integrator of the type mentioned above, which in comparison with conventional optical integrators allows improved homogeneity of the illumination, in particular with regard to angle-dependent intensity inhomogeneities.
SUMMARY OF THE INVENTION
To resolve this problem, according to one formulation, the invention proposes an optical integrator for an illumination device for a microlithographic projection exposure system. The optical integrator includes:
a rod made of a material transparent for the light of a light source;
the rod having an entrance surface optically facing towards the light source and an opposing exit surface;
the rod further having an essentially rectangular cross-section having a width and a height perpendicular to the width, an aspect ratio between the width and the height of the cross section deviating from unity; and
the optical integrator having compensation means for compensating direction-dependent total reflection losses of the rod.
Another solution is an illumination device including such an optical integrator. Advantageous further embodiments are specified in the dependent claims. The verbatim of all claims is incorporated by reference into the subject matter of the description.
An optical integrator according to the invention is characterized in that compensation means are provided for the compensation of direction-dependent total reflection losses of the rod. This proposal is based on the knowledge that the reflection of light beams on the broad and narrow lateral surfaces of the glass rod is not total, but rather incomplete, even in the case of optimally prepared lateral surfaces. The cause of this can for example be a roughness of the reflecting surfaces, resulting in light beams no longer being situated locally in the angle range of the total reflection and part of the light intensity being uncoupled accordingly. It is also possible that in the area of the glass rod surfaces impurity atoms are embedded in the rod material, with the result that the refractive index at the edge of the rod does not correspond to that of the material in the interior. This can contribute to a partial uncoupling of the light. Absorption effects in the surface area can also reduce the intensity of the total reflected light.
As the glass rod in the case of the optical integrators considered here is rectangular due to its construction, the number of the reflections in the case of the passage of light on the lateral surfaces is perceptibly different. Light, the plane of reflection of which is essentially oriented parallel to the wider side of the rod and which is reflected on the narrow sides, is perceptibly less frequently reflected in the mean than light which is predominantly reflected in planes, which essentially run in the longitudinal direction of the rod parallel to or at an acute angle to the narrow sides of the rod, i.e. close to the y direction. Due to the incomplete total reflection in the case of real systems, intensity losses result at the rod exit, the amount of which is essentially dependent on the number of reflections during the passage of light, as well as on the degree of total reflection losses per reflection process. This results in higher losses in the angle range with large y values in the case of essentially identical surface properties of the wide and narrow sides than in the case of x values of the same magnitude. A compensation for these disadvantageous effects and, thus, a reduction of the ellipticity of the pupil, can be achieved with the aid of suitable compensation means, wherein the products, given by the number of reflections multiplied by the total reflection loss per reflection, in both the x direction and y direction are aligned with each other.
This can for example be achieved by targetedly reducing the specific total reflectance of the narrow lateral surfaces, at which fewer reflections occur, so that the tota
Hoegee Jan
Koehler Jess
Tas Marnix Aldert
Van Der Lei Sijbe Abraham
Van Der Veen Paul
Carl Zeiss Semiconductor Manufacturing Technologies AG
Husar Stephen
Payne Sharon
Sughrue & Mion, PLLC
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