Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-05-27
2002-07-30
Berman, Jack (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C378S034000, C359S366000, C359S859000
Reexamination Certificate
active
06426506
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to an optical system for use with short wavelength radiation in photolithography equipment.
2. Background of the Invention
Photolithography is a well-known manufacturing process used to create devices upon substrates. The process typically involves exposing a patterned mask to collimated radiation thereby producing patterned radiation, which is passed through an optical reduction system. The reduced patterned radiation or mask image is projected onto a substrate coated with photoresist. Radiation exposure changes the properties of the photoresist, allowing subsequent processing.
Exposure tools used in photolithography have two common methods of projecting a patterned mask onto a substrate: “step and repeat” and “step and scan.” The step and repeat method sequentially exposes portions of a substrate to a mask image. The step and repeat optical system has a projection field that is large enough to project at least one die image onto the substrate. After each image exposure, the substrate is repositioned and the process is repeated.
In contrast, the step and scan method scans the mask or reticle onto a wafer substrate over an annular field or a slit field that is the full height of one or more of the dies. Referring to
FIG. 1
, a ring field lithography system
100
for use in the step and scan method is shown. A moving mask
101
is illuminated by a radiation beam
103
, which reflects off the mask
101
and is directed through a reduction ring field optical system
107
. Within the optical system
107
, the image is inverted and the arcuate shaped ring field
109
is projected onto a moving substrate
111
. The arcuate shaped reduced image ring field
109
can only project a portion of the mask
101
, thus the reduced image ring field
109
must scan the complete mask
101
onto the substrate
111
. Because the mask
101
and substrate
111
move synchronously, a sharp image is scanned onto the substrate
111
. The dimensions of the arcuate slit are typically described by a ring field radius and a ring field width.
These step and scan ring field systems have less distortion than step and repeat systems because it is easier to correct distortion over the narrow slit width. Referring to
FIG. 2
, an image is projected by the optical system onto to wafer through an arcuate ring field slit
201
, which is geometrically described by a ring field radius
203
, a ring field width
205
, and a length
207
. Ring field coverage is up to 180° in azimuth
209
.
As manufacturing methods improve, the minimum resolution dimension or critical dimension (CD) that can be achieved decreases, thereby allowing more electronic components or devices to be fabricated within a given area of a substrate. The number of devices that can be fabricated within an area of substrate is known as device density. With existing technology, 0.18 &mgr;m resolution is possible using projection systems designed to operate at either 248 or 193 nanometers. One well-known means of improving the resolution dimension and increasing device density is to use shorter exposure wavelength radiation during photolithography processes. The relationship between resolution (R) and radiation wavelength is described by the formula: R=(K
1
&lgr;)/(NA), wherein R is the resolution dimension, K
1
is a process dependent constant (typically 0.7), &lgr; is the wavelength of the radiation, and NA is the numerical aperture of the optical system at the wafer plane. Either reducing the operating wavelength or an increasing in the numerical aperture will improve the resolution of the system.
Improving the resolution by increasing the numerical aperture (NA) has several drawbacks. The most prevalent drawback is the concomitant loss of depth of focus. The depth of focus determines, in part, the process latitude or the available “process window”. A reduced depth of focus limits the available process window. The relationship between NA and depth of focus (DOF) is described by the formula: DOF=(K
2
&lgr;)/NA
2
, wherein DOF is depth of focus, and K
2
is a process dependent constant (typically close to unity). This simple relationship shows the inverse relationship between DOF and NA. For several reasons, including practical wafer flatness and scanning stage errors, a large depth of focus is on the order of 1.0 micrometers is desirable.
There is a rapid loss in DOF as the NA is increased. It is preferable to use shorter wavelengths combined with a low NA to maximize resolution and available DOF simultaneously. A state-of-the-art extreme ultraviolet (EUV) projection system operating at 13.4 nm achieves a 100 nm resolution (assuming K
1
=0.7) with a depth of focus of 1.0 &mgr;m at a NA of 0.10. This large depth of focus improves process latitude, thus enhancing the “process window”. In contrast, a deep ultraviolet (DUV) wavelength projection lithography optical system operating at a wavelength (&lgr;) of 193 nm can only achieve a minimum critical dimension of 180 nm at a numerical aperture of 0.75, assuming the same value for the process dependent constant K
1
. Further, the depth of focus of the DUV optical system is reduced to 0.34 &mgr;m, resulting in a loss of process latitude that adversely impacts device yield. As the process window shrinks, it becomes more difficult to maintain the CD control required for high-density integrated circuits in commercial production.
To produce integrated circuits with ever smaller critical dimensions and higher device density with sufficient process latitude for volume manufacture, it is necessary to develop projection systems operating at extreme ultraviolet (EUV) wavelengths (from 4 to 20 nm). Radiation at these wavelengths must be focused using mirrors coated with multilayer coatings that have high reflectivity at near normal incidence angles. The reflection of radiation off of a mirror is known as “bounce”.
State of the art EUV imaging systems have relatively low numerical apertures in the range of 0.08 to 0.10 and can resolve features on the order of 100 nm. To extend the resolution of EUV lithography below 100 nm, optical systems having higher numerical apertures are needed. Increasing the numerical aperture of current EUV designs results in a substantial degradation in the residual wavefront error, making these designs unsuitable for projection lithography.
Prior art contains few examples of high (>0.10) numerical aperture designs suitable for EUV lithography. An optical system that is usable in the extreme ultraviolet portion of the spectrum is disclosed in U.S. Pat. No. 5,315,629 to Jewell et al., which is herein incorporated by reference. The '629 patent discloses a four mirror design with a numerical aperture of 0.10 and a ring field width of 0.5 mm. The design has diffraction-limited performance with approximately 10 nanometers of static distortion at the edge of its 0.5 mm ring field. The disclosure states that the numerical aperture of the optical system can be increased to approximately 0.14 without loss in image quality, if the image distortion tolerance is relaxed. The increase in numerical aperture would enable the design to resolve features less than 100 nanometers. However, as this design is re-optimized to minimize the residual wavefront error, the ability to correct distortion over any meaningful ring field width is lost. For example, the residual distortion at a numerical aperture of 0.14 is on the order of 30-40 nm at the edge of the 0.5 mm ring field. This is too much distortion for a practical lithographic projection, even when the effects of scan-averaging are included.
An optical system that is usable in the extreme ultraviolet portion of the spectrum is disclosed in U.S. Pat. No. 5,212,588 to Viswanathan et al., which is herein incorporated by reference. The '588 patent demonstrate a multi-bounce projection system that incorporates two coaxial aspheric mirrors in a 4-bounce arrangement, where mirror M
1
is convex and mirror M
2
is concave. To obtain high resolution imag
Berman Jack
The Regents of the University of California
Thompson Alan H.
Wooldridge John P.
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