Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-05-09
2002-08-27
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S072000, C355S075000
Reexamination Certificate
active
06441884
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure apparatus which is used when manufacturing a semiconductor element, a liquid crystal display element, or the like by a photolithographic process.
2. Description of the Related Art
In a photolithographic process for making a semiconductor element or liquid crystal substrate or the like, a projection exposure apparatus is employed to expose a pattern image on an original plate such as a reticle or the like onto a photosensitive substrate such as a wafer or the like. With this projection exposure apparatus, for the reason described below, high accuracy positional adjustment has been necessary between an optical system for illumination and an exposure apparatus main body, in other words between an illumination optical system for illuminating an original plate at a uniform illumination distribution, and an exposure apparatus main body for forming a predetermined circuit pattern on a photosensitive substrate by projecting a pattern provided upon the illuminated original plate onto the photosensitive substrate via a projection optical system. That is, the line width of the circuit pattern formed on the photosensitive substrate is increased or decreased in accordance with the amount of exposure of the pattern image which is formed via the above described projection optical system on the photosensitive substrate. Accordingly, in order to limit wide variation of the line width of the circuit pattern, it is necessary to illuminate the pattern which is provided on the original plate at as even an illumination level as possible, and to keep the unevenness of illumination low. Due to this, the positioning accuracy required between the elements which constitute the illumination optical system has become high.
For the above reason, it is only possible for the various elements of the illumination optical system to be separated at portions with a comparatively low value of positioning required accuracy, and as a result it has been necessary to provide the portions which constitute most of the weight of the illumination optical system and the exposure apparatus main body as one unit. Due to this, it has been necessary to provide a frame in the exposure apparatus main body to support the illumination optical system, and because of this the exposure apparatus main body has become large in size. Further, this has led to weight increase of the exposure apparatus main body.
On the other hand, with the exposure apparatus main body, vibration from the surface of the floor is intercepted by an active vibration prevention system, and further vibration generated when heavy parts such as a wafer stage or the like provided inside the exposure apparatus main body accelerate or decelerate during their movement is also suppressed. The active vibration system comprises air springs (hereinafter termed air mounts) which support the exposure apparatus, voice coil motors (hereinafter termed VCMs) which impart thrusts in predetermined directions to the exposure apparatus main body, acceleration sensors which detect vibrations of the exposure apparatus main body, and a feedback control system for controlling the amounts of air supplied to the air mounts and the thrusts of the VCMs and the like based upon vibrations of the exposure apparatus main body detected by the acceleration sensors, etc. According to the above construction, the active vibration prevention system interrupts the transmission of vibrations from the outside, and suppresses vibrations generated within the exposure apparatus main body.
(The Problem of Load on the Air Mounts)
However, along with the fact that the weight of the exposure apparatus main body is increased as described above, the load on the air mounts also becomes large, and the capacity of the compressed air supply which is ordinarily provided in the workshop in which the projection exposure apparatus is set up has sometimes become insufficient. Further, in the same way, the VCMs are also required to be high thrust ones, and the amount of heat generated when driving the VCMs also increases. This heat generation causes air turbulence, and in some cases the measurement accuracy of interferometers for measurement of the position of the wafer stage etc. has become deteriorated.
(Problem of Vibration Transmission to the Exposure Apparatus Main Body)
Further, in the case that the projection exposure apparatus is a scanning exposure method one, a reticle blind which is provided in the illumination optical system is driven during exposure. In a projection exposure apparatus from which an accuracy on the nanometer level is required it has happened that vibration generated at this time cannot be ignored. This will be explained below.
The reticle blind is a device for preventing non-required light emitted by the illumination light in a range of the original plate outside the pattern portion from impinging upon the photosensitive substrate. And the operations of opening it before the beginning of scanning exposure action and closing it after completion are repeatedly performed. It has happened that during these opening and closing operations, vibration generated from a reticle blind opening and closing drive mechanism which has employed a stepping motor and a ball screw or the like, has exerted an influence upon the accuracy of projection exposure, as will be explained below.
When scanning exposure is being performed, the original plate is moved under a band shaped illumination region formed by the optical system for illumination, in the short direction of the band shaped illumination region. A pattern on the original plate which is illuminated in a band shape is formed is as a band shaped pattern image on the photosensitive substrate by the projection optical system. At this time, by making the photosensitive substrate move in the opposite direction from the movement direction of the original plate, and moreover by making the ratio of the movement speeds of the original plate and of the photographic substrate and the projection ratio of the projection optical system agree, a projection image of the pattern is exposed onto the photographic substrate, as it were like a slit camera.
If during scanning exposure the above described ratio of speeds is not kept constant, distortion is generated in the circuit pattern which is formed upon the photosensitive substrate. In detail, when starting to move both the original plate and the photosensitive substrate they are both accelerated, but exposure is performed after reaching a constant speed. In order to perform this acceleration, it is necessary to start mutually moving the original plate and the photosensitive substrate from portions outside the range of exposure, and at this time the above described reticle blind is kept closed so that unnecessary light should not impinge upon the photosensitive substrate. And the reticle blind is opened just before the arrival of the pattern region of the original plate in the band shaped illumination region which is formed by the optical system for illumination is imminent. Moreover a frame shaped light interception band (so called “chrome”) is formed surrounding the pattern region, and, since the width of this frame shaped light interception band is greater than the dimensions of the short side of the band shaped illumination region, a light interception state is maintained by the above described light interception band after the reticle blind has been completely opened. Scanning scanning exposure starts as the arrival of the head portion of the pattern region of the original plate in the band shaped illumination region becomes imminent. The arrival of the end edge portion of the pattern region of the original plate in the band shaped illumination region becomes imminent just before this scanning exposure is completed, and next the arrival of the frame shaped light interception band (the “chrome”) becomes imminent, and scanning exposure is finished. After this the reticle blind is closed.
As explained above, the
Mori Futoshi
Takahashi Masato
Adams Russell
Nguyen Hung Henry
Nikon Corporation
Oliff & Berridg,e PLC
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