Projection optical system with diffractive optical element

Details Projection optical system with diffractive optical element Projection optical system with diffractive optical element

2001-03-14

2003-07-29

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With field curvature shaping

C359S566000

Reexamination Certificate

active

06600606

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
This invention relates to an optical system having a diffractive optical element. More particularly, the invention concerns an optical system suitably usable in a semiconductor manufacturing apparatus, for example, for printing, by projection exposure, a device pattern formed on a reticle or a mask (hereinafter, mask) on different locations on a wafer in accordance with a step-and-repeat method or a step-and-scan method, to produce various devices having a pattern of submicron or quarter-micron size or smaller, such as ICs, LSIs, CCDs or liquid crystal panels, for example.
Most optical systems, including projection optical systems, of semiconductor manufacturing apparatuses are constituted by dioptric systems only. Recently, however, many proposals have been made to an optical system using a diffractive optical element (DOE). Examples of such diffractive optical elements are a phase type diffractive optical element or an amplitude type diffractive optical element, known as a Fresnel zone plate. In the amplitude type, a portion of light is blocked by the optical element and, therefore, it is undesirable with respect to the efficiency of light utilization. In the phase type diffractive optical element, on the other hand, it is known that, if it is assuredly manufactured to provide an idealistic phase change, a diffraction efficiency of 100% is attainable. Particularly, those called a surface relief type are used in many cases in an ordinary optical system. In this type, a structure is defined in the depth direction of the element substrate, by which a phase change corresponding to the position on the element surface is applied to the light passing therethrough. The depth which is normally required is of a wavelength order, and the thickness of the element can be made small. Further, various phase changes can be accomplished by changing the position of the structure. Thus, for ordinary dioptric systems, an effect such as attainable by forming an aspherical surface can be attained widely. The function for describing phase changes applied to light in accordance with the position on the element surface is called a phase function.
Another feature of diffractive optical elements is that a color dispersion appears inversely to that of a dioptric system. Based on this feature, chromatic aberration produced by a dioptric system can be corrected by use of a diffractive optical element.
Due to these features, diffractive optical elements may be suitably used in a projection optical system for a semiconductor manufacturing apparatus. Conventionally, the light used with such an optical system has a wavelength of i-line (&lgr;=356 nm). As for such a wavelength, there are plural glass materials having a sufficient transmission factor and, for this reason, correction of chromatic aberration is attainable with a combination of dioptric optical elements. On the other hand, as regards an ultraviolet region of a currently used KrF excimer laser (&lgr;=248 nm) or a next-generation ArF excimer laser (&lgr;=139 nm), for example, glass materials having a sufficient transmission factor are only SiO
2
and F
2
. Further, as for an F
2
laser (&lgr;=157 nm), only CaF
2
is available. Although the bandwidth of an emission spectrum of a laser light source is narrow, the imaging performance required for a projection optical system of a semiconductor manufacturing apparatus is extraordinarily high. Therefore, with an optical system constituted by dioptric systems only, there arises a problem of chromatic aberration. For this reason, a strict condition that the bandwidth must be not greater than 1 pm is additionally applied to the light source, and this necessitates a structure for narrowing the bandwidth. Further, the number of lenses required for sufficiently reducing the wavefront aberration of the optical system becomes larger, and this leads to an increase of the lens whole thickness and an increase of the surfaces where an anti-reflection film should be applied. As a result, the transmission factor of the optical system as a whole becomes lower. This means that the absorption of exposure light by the lens system as a whole increases, and it is undesirable also with respect to the aberration (exposure aberration) produced with the exposure.
Use of a diffractive optical element may be effective for problems of an increase in total lens thickness or lens surfaces or large aberration correction.
Although the advantages of diffractive optical elements themselves are known in the art, many proposals for such optical systems (e.g., Japanese Laid-Open Patent Application, Laid-Open No. 331941/1994) are made recently just after a binary optics element (BOE) is proposed. Details of such a binary optics element are discussed in G. J. Swanson, Technical Report 854, MIT Lincoln Laboratory, Aug. 14, 1989, or G. J. Swanson, Technical Report 914, MIT Lincoln Laboratory, Mar. 1, 1999, for example.
Conventionally, from the machining precision and the like, it is very difficult to directly produce an idealistic shape (blazed shape) required for a diffractive optical element, that is, a shape necessary for correctly depicting the phase function. In the case of binary optics, however, a blazed shape is not directly produced, but it is approximated by use of a step-like shape. Such a step-like shape can be produced precisely in a very fine structure, through a lithographic process and by use of a stepper as an exposure apparatus.
Now, a description will be made with reference to an idealistic lens for converging parallel light to a single point. In order that parallel light (plane wave) incident on a lens is converged to a single point, a phase function such as follows may be given:
ø(
r
)=−2&pgr;|(
r
2
+f
2
)
½
−f|/&lgr;
  (1)
where f is the focal length, &lgr; is the wavelength of light used, and r is the distance from an arbitrary origin.
In the diffractive optical element, the fact that light has a period of 2&pgr; with respect to the phase is used. First, the value of r=R
m
with which the value of a phase function ø(r) becomes equal to a multiple of 2&pgr;, by an integer, is calculated (wherein m is an integer not less than 0, and, while taking R
O
=0, the counting is done sequentially from the origin toward the outside), and a phase function ø′(r) having a multiple of 2&pgr; added is prepared so that in the period [R
m
, R
m+1
] the value of ø(r) comes into range [0, 2&pgr;]. An optical element having its surface shaped to satisfy this phase function ø′(r) is thus an idealistic lens based on a diffractive optical element.
FIGS. 1A and 1B
are schematic view of such a surface shape. The ring interval T
m
may be defined as T
m
=R
m+1
−R
m
. The ring interval is relatively large at the central portion (r to 0) and, depending on the difference in m, the difference in ring interval is large. On the other hand, at the peripheral portion, the ring interval is approximately regular even if the value of m differs and, therefore, it can be considered to be a regular interval grating.
Reference numeral
101
denotes the surface shape at the central portion, and it is a blazed shape which completely describes the phase function. Here, the shape
101
is a portion of a curved surface. On the other hand, reference numeral
102
denotes a blazed shape at the peripheral portion. It can be considered to be approximately a plane.
FIGS. 2A and 2B
are schematic views of a surface shape, wherein an idealistic lens is manufactured as binary optics. This shape is provided by approximating the blazed shape of
FIG. 1
by a step-like shape. Here, the step difference (height) of the steps may be determined so that the phase is sampled with regular intervals. Namely, if the depth in the case of the blazed shape is D and the number of approximated steps is N, each step has a height D/N. Since the step difference (height) is made constant, the width of each

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