Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2000-09-05
2002-06-18
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S296000, C430S942000
Reexamination Certificate
active
06406820
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure method and apparatus, a method of making an exposure apparatus, a device, and a device manufacturing method. More particularly, the present invention relates to an exposure method and apparatus which are used when a circuit device such as a semiconductor device or liquid crystal display device is manufactured in a lithography process, a method of making an exposure apparatus, and a device and the method of manufacturing the device using the exposure method and apparatus.
2. Description of the Related Art
Currently, at the sites which manufacture semiconductor devices, circuit devices with minimum line width of about 0.3 to 0.35 &mgr;m (64 M(Mega)bit D-RAMs and the like) are mass-produced by using reduction projection exposure apparatus, so-called steppers, using the i line from mercury lamps that has a wavelength of 365 nm as an illumination light. At the same time, the introduction of exposure apparatus, which are designed to mass-produce circuit devices of the next generation that have an integration degree equivalent to those in the class of 256 Mbit and 1 G(Giga)bit D-RAMs and minimum line widths of 0.25 &mgr;m, has begun.
As the exposure apparatus for manufacturing circuit devices of the next generation, a scanning exposure apparatus based on the step-and-scan method is being developed. This apparatus uses an ultraviolet pulse laser beam with a wavelength of 248 nm from a KrF excimer laser source or an ultraviolet pulse laser beam with a wavelength of 193 nm from an ArF excimer laser source as an illumination light. A scanning exposure apparatus then linearly scans a mask or a reticle (to be generically referred to as a “reticle” hereinafter) on which a circuit pattern is drawn and a wafer serving as a photosensitive substrate, relatively to the projection field of a reduction projection optical system. This allows the transfer of the entire pattern within a shot area on the wafer by repeating the inter-shot stepping operation and the scanning exposure operation.
There is no doubt that the integration degree of semiconductor devices will further increase and will shift from 1 Gbit to 4 Gbit in the future. In this case, the device rule will become about 0.1 &mgr;m, i.e., 100 nm L/S, which gives rise to various technical issues when an exposure apparatus uses an ultraviolet pulse laser beam having a wavelength of 193 nm. The resolution of an exposure apparatus which, in turn, indicates a device rule (a practical minimum line width), is generally expressed using the following formula (1), using an exposure wavelength &lgr; and the numerical aperture N.A. of a projection optical system:
(resolution)=
k&lgr;/N.A.
(1)
where k, in this case, is called a k factor and is a positive constant equal to or less than 1, which varies depending on the characteristics of the resist used.
As is obvious from equation (1), in order to increase the resolution, it is extremely effective to decrease the wavelength &lgr;. For this reason, recently, an EUV exposure apparatus using light in the soft X-ray region of 5 to 15 nm in wavelength (in this specification, this light will also be referred to as “EUV (Extreme Ultraviolet) light”) as the exposure light has been developed. Such an EUV exposure apparatus has recently attracted a great deal of attention as a promising candidate for an exposure apparatus of the generation after the next, having a minimum line width of 100 nm.
In the EUV exposure apparatus, a reflection type reticle is generally used. This reflection type reticle is obliquely irradiated with illumination light, and light reflected by the reticle surface is projected on a wafer through a projection optical system. As a consequence, a pattern, which is irradiated with the illumination light in an illumination area on the reticle, is transferred onto the wafer. This EUV exposure apparatus employs a scanning exposure method, in which a ring-shaped illumination area is set on the reticle, and the reticle and wafer are scanned relative to the projection optical system, thereby sequentially transferring the entire pattern on the reticle onto the wafer through the projection optical system.
At the wavelength (5 to 15 nm) of light used in the EUV exposure apparatus, no material can efficiently transmit light without any absorption. Inevitably, a reflection type reticle must be used. In addition, since it is difficult to form a beam splitter, the reticle must be obliquely irradiated with illumination light.
For this reason, the projection optical system becomes non-telecentric on the reticle side. As a consequence, the displacement of the reticle along the optical axis appears on the wafer as a magnification change in the longitudinal direction of a ring-shaped exposure area (an area on the wafer which corresponds to the above ring-shaped illumination area on the reticle), and as a positional change in the transversal direction.
This technique will be described in more detail with numerical values. Assuming that a projection optical system having a resolution of 100 nm L/S is designed by using EUV light having a wavelength of 13 nm as exposure light, then Equation (1) can be rearranged into the following equation (2).
N.A.=k&lgr;
/(resolution) (2)
If, for example, k=0.8, then the necessary N.A. to obtain a resolution of 100 nm L/S is N.A.=0.104 is approximately equal to 0.1. The N.A. is a value at the wafer side, and apparently, differs from that at the reticle side.
Assuming that the projection magnification of the projection optical system is ¼, which is generally used in a conventional deep ultraviolet exposure apparatus (DUV exposure apparatus) with the i line, g line, KrF excimer laser, or ArF excimer laser as exposure light, when the N.A. at the wafer side is 0.1, then the N.A. at the reticle side is 0.025, being ¼ that of the wafer side. This means, that illumination light applied to the reticle has a divergence angle of about ±25 mrad with respect to a principal ray. In order to prevent incident light and reflected light from overlapping, the minimum incident angle must be at least 25 mrad or more.
For example, referring to
FIG. 17
, if an incident angle &thgr; (=outgoing angle &thgr;) is 50 mrad, a transverse shift &egr; of a circuit pattern drawn on a reticle R with respect to a Z-direction displacement &Dgr;Z of the pattern surface of the reticle R (also referred to as “the Z-direction displacement of a reticle” as needed) can be given as:
&egr;=&Dgr;Z tan&thgr; (3)
As is obvious from the equation (3), when, for example, the reticle R is displaced by 1 &mgr;m in the vertical direction (Z direction) as in
FIG. 17
, the transverse shift of an image on the reticle pattern surface becomes about 50 nm, and the image shift of 12.5 nm being ¼ the transverse shift, occurs on the wafer. The allowable overlay error in the semiconductor process of a device rule of 100 nm L/S is said to be 30 nm or less, thus an overlay error as large as 12.5 nm caused by a displacement of a reticle in the Z direction alone poses a serious problem. This is because overlay errors of about 10 nm can be caused by other factors, e.g., alignment accuracy of a reticle and wafer, wafer stage alignment accuracy including stepping accuracy, or the distortion of the projection optical system.
As described previously, when EUV light having a wavelength of 5 to 15 nm is used in the EUV exposure apparatus, no material can efficiently transmit in this bandwidth without any absorption. Inevitably, an allreflection optical system formed by only several mirrors (reflection optical elements) must be used as a projection optical system, which makes it difficult to control projection magnification and causes a serious problem.
Projection magnification is generally controlled in conventional deep ultraviolet exposure apparatus (DUV exposure apparatus) which uses KrF excimer lasers and the like as a light source by (1) changing
Nikon Corporation
Young Christopher G.
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