The present invention general to optical arrangements for focusing a laser beam to form a line of light on a substrate. The invention relates in particular to focusing an excimer laser beam as a line of light on a silicon substrate.
There are several laser applications wherein it is necessary to focus laser radiation on a substrate in the form of a line of light.
A common approach to focusing the laser radiation to form a line of light is to focus a laser beam using an anamorphic optical system which has different magnification in two transverse axes perpendicular to each other. This can include an optical system in which the magnification in one of the axes is greater than unity and the magnification in the other axis is less than unity. Typically such an optical system includes at least three refractive optical elements, at least one of which is a cylindrical optical element or an anamorphic optical element. Laser radiation focused by such an optical system is usually incident, generally, normally on the optical elements. The term “generally normally”, here, meaning that the general direction of propagation through the elements is at normal incidence thereto, while a laser beam entering the system, leaving the system and between elements of the system may be collimated, converging, or diverging, depending on the laser delivering the beam and the configuration of the optical elements.
Elements of such an optical system are preferably antireflection coated to reduce Fresnel reflection losses at surfaces of the optical elements. An optical system is typically more expensive the more optical elements are included in the system. Providing optical coatings for the elements adds cost. This is particularly true of optical systems for focusing ultraviolet (UV) laser radiation from excimer lasers. Such lasers can provide radiation at wavelengths less than 200 nanometers (nm) for which optical systems are preferably made from crystalline materials such as calcium fluoride (CaF2). Optical coatings on such crystalline substrates are less durable than comparable coatings on glass substrates or fused silica substrates. Such coatings can also be prone to degradation by the ultraviolet radiation itself.
A general problem encountered in focusing ultraviolet radiation on a silicon (Si) substrate is that the relatively high refractive index (about 3.45 at a wavelength of 200 nm) of the silicon can cause about 30% of the radiation incident thereon at near normal incidence to be lost by Fresnel reflection from the surface of the substrate. There is a need for a method and apparatus for focusing UV radiation on a substrate that does not require a multi-element optical system and that can avoid radiation loss at the substrate due to Fresnel reflections.
The present invention is directed to method and apparatus for projecting a beam from an excimer laser to form a line of light on a substrate, in one aspect the method of the present invention comprises providing a substantially collimated, plane polarized excimer laser beam. The collimated, plane-polarized beam is projected onto the substrate by a cylindrical lens arranged at non-normal incidence to the beam to form the line of light on the substrate beam. Because of the non-normal incidence of the beam on the lens, the line of light has a length greater than the height of the beam. The projected line has a width less than the width of the beam.
By selecting an appropriate incidence angle and polarization-plane alignment of the beam with the lens, total light reflection losses from the lens and the substrate can be minimized. In one example wherein the lens is a calcium fluoride lens and the substrate is a silicon substrate, an incidence angle of about 640 can provide a line length about 2.4 times greater than the beam height and total reflection losses of only about 7%. In another example, wherein the lens is furnished with antireflection coatings, an incidence angle of about 73.8° degrees can provide a line length about 3.6 times the beam height with near zero total reflection losses.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.
Referring now to the drawings
The beam may be received directly from the excimer laser if the laser output has an inherently low divergence (good beam quality). The beam is preferably delivered as a plane-polarized beam with the electric vector (polarization plane) aligned parallel to the long-axis as indicated in
Apparatus 10 includes a plano-convex cylindrical lens 16 having a convex upper surface 18 and a plane lower surface 20. Those skilled in the art will recognize that term “cylindrical”, as used and in the appended claims means that the lens has power in one transverse axis only. Lens 16 has positive power in the short-axis (X-axis) and zero optical power in the long-axis (Y-axis). A substrate 24 is located below lens 16, with an upper surface 26 of the substrate parallel to lower surface 20 of the lens. Surface 26 is preferably located at about a back focal-length of lens 16 from surface 20 of the lens.
Beam 12 is incident on lens 16 at an angle θ to a normal 22 to the lens. As can be seen in
In the arrangement of apparatus 10, beam 12 is plane-polarized, with the electric vector P being parallel to the plane of incidence (and parallel to the long-axis plane, or Y-Z plane, of the beam). This polarization alignment with respect to the lens is usually referred to by practitioners of the art as P-polarization. If the angle of incidence of the beam on the lens is the Brewster angle for the material of the lens, the reflection loss from lens surface will be essentially zero. Similarly, if the angle of incidence of the beam on surface 26 of the substrate is the Brewster angle for the material of the substrate, the reflection loss at the substrate surface will be essentially zero. Usually lens 16 will be made from a UV-transmissive material such as fused silica (SiO2) or calcium fluoride (CaF2). Such materials have a relatively low refractive index. By way of example, CaF2 has a refractive index of about 1.5 at a wavelength of about 200 nm. The Brewster angle for CaF2 at this wavelength is about 56.3°. If substrate 24 were a silicon substrate, the Brewster angle would be about 73.8°, as silicon has a refractive index of about 3.45 at a wavelength of about 200 nm. Clearly, in the arrangement of
Now, the incidence angle relates directly to the stretching factor as discussed above. Accordingly it is useful to analyze losses as a function of stretching factor. Such an analysis is summarized in
It can be seen that in the case where the lens is uncoated, the stretching factor at the lowest total loss is only about 2.4. Even if losses equal to the normal incidence loss from silicon (which would be experienced in prior art multi-element normal incidence projection systems) could be tolerated, the stretching factor could only be increased to 4.6. If lens losses were eliminated by antireflection coatings, then the 4.6 stretching factor could be achieved with total losses of only about 1.5%. For a loss of 7.0% a stretching factor of greater than 6.0 could be achieved. A stretching factor of 10 (at an incidence angle of about 84.3°) could be achieved with total losses less than the 30% loss from a normal incidence reflection from silicon. Generally then, if lens 16 is furnished with antireflection coatings, a range of incidence angles between about 45° and 87° is useful in the apparatus of the present invention. With an uncoated lens a useful range of incidence angles is between about 45° and 75°.
Regarding the durability of antireflection coatings on the lens, it should be noted that, for a fixed power in the beam, the higher the angle of incidence of the beam on the lens, the lower the intensity of radiation incident on the lens. In fact, the intensity would be about inversely proportional to the stretching factor. Because of this, the susceptibility of the coatings to radiation damage would be correspondingly reduced.
If, in any configuration of apparatus 10, the stretching factor available for tolerable losses is not sufficient to provide a line of light of a desired length from a laser-beam, it is possible to “pre-stretch” the laser beam using an afocal long-axis beam expander to increase the long-axis beam height before the beam is incident on lens 16. Such a beam expander may include a long-axis plano-concave cylindrical lens followed by a long-axis plano convex cylindrical lens.
Referring again to
In summary present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.
This application claims priority to U.S. Provisional Application Ser. No. 60/740,476, filed Nov. 29, 2005, the disclosure of which is incorporated herein by reference.
Number | Date | Country | |
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60740476 | Nov 2005 | US |