Photocopying – Projection printing and copying cameras – Focus or magnification control
Reexamination Certificate
1997-02-12
2001-10-16
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Focus or magnification control
C355S053000, C355S067000, C355S071000
Reexamination Certificate
active
06304317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure apparatus used for forming fine patterns in, for example, semiconductor integrated circuits, liquid crystal displays, etc. More particularly, the present invention relates to a projection exposure apparatus having a mechanism for maintaining the image-forming performance of its projection optical system in a favorable condition.
2. Description of the Related Art
A photolithography process for forming a circuit pattern in a semiconductor device or the like uses a projection exposure apparatus (e.g., stepper) in which a pattern formed on a mask (a reticle) is transferred to a photosensitive substrate (e.g., a semiconductor wafer, glass plate, etc.), which has been coated with a photoresist, through a projection optical system. The projection optical system of such a projection exposure apparatus is incorporated in the apparatus after high-level optical designing, careful selection of a vitreous material, superfine processing of the vitreous material, and precise assembly adjustment. The present semiconductor manufacturing process mainly uses a stepper in which a reticle (or a photomask, etc.) is irradiated with the i-line (wave-length: 365 nm) of a mercury-vapor lamp as illuminating light, and light passing through a circuit pattern on the reticle is passed through a projection optical system to form an image of the circuit pattern on a wafer (or a glass plate, etc.), which has been coated with a photoresist. An excimer stepper that employs an excimer laser (KrF laser of wavelength 248 nm) as an illuminating light source has also been used for evaluation or research purposes.
With the steady increase of the degree of integration of VLSI and other similar devices, various methods have been developed for projection exposure apparatuses in order to perform transfer of finer patterns, such as optimization of illuminating conditions, new schemes of exposure method, etc. For example, there has been proposed a method of improving the resolution and DOF (Depth of Focus) by previously obtaining the most suitable combination of a coherence factor of the illuminating optical system (i.e., value: the ratio of the numerical aperture (N.A.) of the illuminating optical system to the numerical aperture of the projection optical system) and the numerical aperture of the projection optical system for each specific pattern line width, and selecting the most suitable combination for each pattern line width.
Among projection exposure apparatuses which are presently put to practical use, those which are designed for the i-line include a projection optical system having a numerical aperture (NA) of about 0.6. In general, for the same wavelength of illuminating light used, as the numerical aperture of the projection optical system is increased, the resolution improves correspondingly. However, as the numerical aperture NA increases, the focal depth DOF becomes shallower in proportion to &lgr;/NA
2
, where &lgr; is the wavelength of illuminating light.
Incidentally, the resolution can be improved by increasing the image-side numerical aperture NAw (cf. the object-side numerical aperture NAr) of the projection optical system. That is, the resolution can be improved by increasing the pupil diameter of the projection optical system and also increasing the effective aperture of an optical element, e.g., lens, which constitutes the projection optical system. However, the focal depth DOF decreases in inverse proportion to the square of the numeral aperature NAw. Accordingly, even if a projection optical system of high numerical aperature can be produced, the required focal depth cannot be obtained; this is a considerable problem practical use.
Assuming that the wavelength of illuminating light is 365 nm of the i-line and the numerical aperature NAw is 0.6, the focal depth DOF decreases to about 1 &mgr;m (±0.5 &mgr;m) in total range. Accordingly, a resolution failure occurs in a portion whenever the surface unevenness or the curvature is greater than DOF within one shot area (which is about 20 by 20 mm to 30 by 30 mm square) on the wafer.
In order to cope with these problems, the following various methods have been devised:
First, super-high resolution techniques, e.g., an annular zone illuminating method, modified light source method, phase shift method, etc., have been proposed. Among them, the annular zone illuminating method is a technique whereby the light intensity distribution of an illuminating light beam in a pupil plane of an illuminating optical system or a plane neighboring it is regulated to an annular zone shape, and a reticle pattern is illuminated with such an illuminating light beam, as disclosed in Japanese Patent Application Public Disclosure (KOKAI) No. Sho 61-91662. The modified light source method (also known as SHRINC method or inclined illuminating method) is a technique whereby the light intensity distribution of an illuminating light beam in a pupil plane of an luminating optical system or a plane neighboring it is made maximum at least at one position that is a predetermined amount off from the optical axis of the illuminating optical system, and thus the illuminating light beam is applied to a reticle pattern at a predetermined angle of inclination, as disclosed in Japanese Patent Application Public Disclosure (KOKAI) Nos. Hel 04-101148, Hel 04-180612, Hel 04-225358, Hel 04-180613 and Hei 04-225514.
In regard to these problems, super-high resolution techniques have been proposed, for example, a phase shift method such as that disclosed in Japanese Patent Application Post-Exam Publication No. Sho 62-50811, and a SHRINC (Super High Resolution by Illumination Control) method disclosed, for example, in W
0
92/03842, Japanese Patent Application Disclosure (KOKAI) No. Hei 04-180612 and Japanese Patent Application Disclosure (KOKAI) No. Hei 04-1801613 (corresponding to U.S. Ser. No. 791,138 filed on Nov. 13, 1991).
The phase shift method is carried out by using a phase shift reticle having a phase shifter (e.g., a dielectric thin film) whereby the phase of light passing through a specific one of light-transmitting portions of a circuit pattern formed on the reticle is shifted by &pgr;[grad] with respect to the phase of light passing through another light-transmitting portion, as disclosed. The use of such a phase shift reticle enables the resolution to be improved in comparison to the use of an ordinary reticle (i.e., a conventional reticle composed only of a light-transmitting portion and a light-blocking portion) for a predetermined pattern. It should be noted that typical phase shift reticles include a spatial frequency modification type (Japanese Patent Application Publication No. Sho 62-50811), a half-tone type (Japanese Patent Application Public Disclosure (KOKAI) No. Hei 04-162039), a shifter shielding type, and an edge enhancement type.
However, none of the above-described methods are effective for all reticle patterns, that is, all line widths and configurations. Therefore, it is necessary to select an illuminating method and conditions which are most suitable for each reticle or reticle pattern. Accordingly, the projection exposure apparatus needs to have a structure which enables illuminating conditions (v value and other conditions) in the illuminating optical system to be varied. For example, when the phase shift method is used, it is necessary to optimize the a value of the illuminating optical system.
Further, with the above-described methods, advantages such as an improvement in the resolution and an increase in the focal depth can be effectively obtained when a circuit pattern to be transferred is a periodic pattern having a relatively high density. However, substantially no effect can be obtained for discrete patterns (isolated patterns) such as those called “contact hole patterns”.
To enlarge the apparent focal depth for isolated patterns, e.g., contact hole patterns, an exposure method has been proposed, for example, in U.S. Pat. No. 4,869,999,
Shiraishi Naomasa
Taniguchi Tetsuo
Adams Russell
Armstrong Westerman Hattori McLeland & Naughton LLP
Nguyen Hung Henry
Nikon Corporation
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