LARGE-FIELD UNIT-MAGNIFICATION PROJECTION OPTICAL SYSTEM

Technical Field

The present invention relates to optical projection systems and in particular to a large-field unit-magnification projection optical system.

Background of the invention

At present, photolithography is widely employed in sub-micron resolution integrated circuit (IC) manufacturing. With an increasing degree in gold-bumping and other wafer-level IC packaging technologies, there has been an increasing demand for the projection optical system having a large depth of focus, a high throughput, and a relatively low (i.e., a few microns) resolution. Projection photolithography has been developed and applied gradually.

The present invention described below is related to the optical system described in U.S. Pat. No. 6,879,383 (hereinafter, "the '383 patent") issued on April 12, 2005 to Mercado and assigned to Ultratech, Inc.

FIG. 1 is a cross-sectional diagram of an example optical system 100 according to the '383 patent.

The invention described in the '383 patent is a large field, broad spectral band, color-corrected, anastigmatic projection optical system that projects an image of a pattern formed on a reticle onto a substrate.

The term "large field" means a field having a rectangular dimension of about 50mm X 100mm or greater. Also, the term "broad spectral band" refers a spectral band that includes the g, h, i, spectral lines of mercury (i.e., 436nm, 405nm, 365nm). Optical system 100 includes, along an axis Al, a concave spherical mirror M with an associated aperture stop AS2. The mirror M includes an aperture AP on the optical axis. The aperture AP may be used, for example, to introduce light into the optical system for aligning of an object (e.g., a mask) with its image or inspecting the object. The optical system 100 is essentially symmetrical relative to an aperture stop AS2 so that the system 100 is initially corrected for coma, distortion, and lateral chromatic aberration. All of the spherical surfaces in optical system 100 are nearly

concentric.

Optical system 100 further includes a field corrector lens group G with positive refractive power arranged along axis Al adjacent to and spaced apart from mirror M. The lens group G includes a positive lens element Ll with surfaces S3 and S4. The positive lens Ll is plano-convex with surface S4 being convex and surface S3 being flat. The lens group G further includes adjacent lens element Ll and towards mirror M at least two negative lens L2 and L3. Lens L2 has surfaces S5 and S6 and Lens L3 has surfaces S7 and S8. Negative lenses L2 and L3 are meniscus type, with S6 and S8 being convex. ^' Adjacent lens group G is a first prism PA with surfaces SlA and S2A, and a second prism PB with surfaces SlB and S2B. The surface SlA faces an object plane OP2 and the surface SlB faces an image plane IP2. The object plane OP2 and the image plane IP2 are spaced apart from respective flat surfaces SlA and SlB by respective gaps WDA and WDB representing working distances. There is complete symmetry with respect to the aperture stop AS2, WDA=WDB. Although prisms PA and PB are not included in field corrector lens group G, these prisms play a role in the aberration correction, including chromatic aberration correction.

However, there is a certain angle between the object plane and the image plane. Because the object plane is not parallel to the image plane, there are some difficulties in photolithography machine design.

Summary of invention

It is an object of the present invention to provide a large-field, broad-spectral band, color-corrected, anastigmatic projection optical system that an object plane is parallel to an image plane.

In order to achieve the aforementioned object, the present invention is set forth as follows: the optical system includes an optical axis, a spherical concave reflection mirror; a lens group with positive refracting power arranged adjacent the mirror with an air space therebetween. The lens group includes a first plano-convex lens having a convex surface facing the mirror and a flat surface facing away from the mirror, a negative meniscus lens adjacent the plano-convex lens and having a convex surface

facing the mirror and a concave surface facing the first plano-convex lens, a positive lens adjacent the negative meniscus lens having a concave surface facing the negative meniscus lens and a convex surface facing the mirror, a negative double-convex lens spaced apart far from the positive lens and having a concave surface facing the positive lens and a concave surface facing the mirror, and a second plano-convex lens having a flat surface facing the negative double-concave lens and a convex surface facing the mirror.

The optical system further includes a pair of prisms each having respective first and second surface. The second surfaces are arranged adjacent the flat surface of the plano-convex lens element on opposite sides of the optical axis and the first surfaces are arranged adjacent object planes and image planes, respectively. Each lens in the lens group and the pair of prisms provide chromatic aberration correction in a spectral region that contains at least g, h and i-line wavelengths.

The optical system has at least one of a square field having a size of at least 44mm X 44mm, and a numerical aperture of 0.18 or greater.

The lens elements in the lens group are spherical.

The object plane is parallel to the image plane.

The optical system of the present invention

The projection optical system under the large-field and broad-spectral spectrum conditions has good image quality, which requires relatively low resolution and a large depth of focus. In addition, the object plane is parallel to the image plane to reduce risk of photolithography machine design effectively.

Brief description of drawings FIG.1 is a schematic view of an example prior art unit-magnification projection optical system according to the '383 patent;

FIG.2 is a schematic view of an example embodiment of a large-field unit-magnification projection optical system of the present invention;

FIG.3 is a schematic view of a field area of the large-field unit-magnification projection optical system of the present invention.

Detailed Description of Preferred Embodiments

The present invention will be described in detail by reference to the drawings and the preferred embodiments.

FIG.2 is a schematic view of an example embodiment of a large-field unit-magnification projection optical system 200 of the present invention. The optical system 200 includes an axis A2, a concave spherical mirror Ml with an associated aperture stop ASl. The concave spherical mirror Ml has a concave surface 12.

Optical system 200 further includes a lens group with positive refractive power that includes a lens E5 adjacent to and spaced apart from mirror Ml, a negative lens E4 adjacent to the lens E5, a positive lens E3 spaced apart from the negative lens E4, and a negative lens E2 adjacent from the positive lens E3.

The lens E5 is plano-convex with a surface 11 being convex and a surface 10 being flat.

The Lens E4 is negative double-concave with a surface 8 and a surface 9 faced to the mirror Ml.

The positive lens E3 has a surface 6 being concave and a surface 7 being convex. The convex surface 7 faces the negative double-concave lens E4.

The negative lens E2 is meniscus type, which has a surface 4 being concave and a surface 5 being convex. The convex surface 5 faces the positive lens E3. There is a small air space between the negative lens E2 and positive lens E3.

The plano-convex lens El has surface 2 being flat and surface 3 being convex. The convex surface 3 faces to the negative lens E2.

The optical system 200 further includes a first prism Pl with surfaces Al and A2, and a second prism P2 with surfaces Bl and B2. The surface A2 of the first prism Pl and the surface B2 of the second prism P2 are closely contact to the surface 2 of the plano-convex lens El. The surface Al of the first prism Pl faces an object plane OP and spaces apart from the object plane OP by a gap WDl representing working distance. The surface Bl of the second prism P2 faces the image plane IP and spaces apart from the image plane IP by a gap WD2 representing working distance.

In example embodiments where there is complete symmetry with respect to the aperture stop ASl, WD1=WD2.

The lens group with positive refractive power, the first prism Pl and the second prism P2 play a role in the aberration correction, including chromatic aberration correction of the g, h, i, spectral lines. The object plane is parallel to the image plane.

The optical system 200 of the present invention has very good image quality. The monochromatic wavefront error (Wavelength=365nm) is less than 44nm. Thus, the optical system 200 is useful for wafer-level IC packaging applications, such as bumping lithography and the like, that do not require the highest levels of resolution.

The optical system 200 uses Dyson- Wynne catadioptric structure so that the system 200 is initially corrected for coma, distortion, and lateral chromatic aberration. AU of the surfaces in the optical system 200 are spherical surfaces.

Characteristic description of the present invention is described below.

1) Large field. The term "large field" means a field having a rectangular dimension of about 44mm X 44mm.

2) Broad spectral band. Optical source is g, h i, spectral lines of the mercury( i.e. 436nm, 405nm, 365nm respectively).

3) 1 X magnification. The size of the object plane OP is equal to the size of the image plane IP.

4) The object plane is parallel to the image plane IP which is a help to lithography machine design. 5) Dyson- Wynne catadioptric optical system.

Further example embodiments of projection optical system 200 are provided in Tables 1. Because of the symmetry of the optical system 200, the specifications set forth in Tables 1 only include values from the object plane OP to concave mirror Ml.

In Table 1, a positive radius indicates that the center of curvature is at the right side of the surface and a negative radius indicates that the center of curvature is at the left side of the surface. The thickness of an element, or the separation between elements, is the axial distance to the next surface, and all dimensions are in millimeters.

Further, "S#" stands for surface number, "STOP" stands for aperture stop AS, and "REFL" stands for reflection surface. Under the heading "Shape", a flat surface is denoted by "FLT", a spherical surface by "SPH", "CC" by concave surface, and

"CX" by convex surface. Under the heading "Radius", "INF" denotes that the value

of radius is infinity.

TABLE 1

In Table 1, The Materials used in the optical system 200 are fused silica and all optical surfaces are spherical. The field height is 72mm, the numerical aperture is 0.18 and Optical source is g, h i, lines of the mercury lamp(i.e. 436nm, 405nm, 365nm respectively).