Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-01-06
2002-12-31
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S069000
Reexamination Certificate
active
06501535
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for controlling the amount of exposure of a photosensitive substrate in an exposure apparatus used in a lithography process for producing, for example, semiconductor devices, liquid crystal display devices, image pickup devices (such as charge coupled devices), or thin-film magnetic heads. The invention is suitable for use in full wafer exposure apparatus and, more particularly, for use in exposure control in a scanning projection exposure apparatus of, for example, a scan-and-step type.
The production of semiconductor devices and the like conventionally employs a projection exposure apparatus to project and transfer the pattern of a reticle onto each shot area on a wafer (or a glass plate) coated with a photoresist. As a basic function, the projection exposure apparatus controls the total amount of exposure (total exposure energy) of each point in each shot area on a wafer within an appropriate amount range.
For the exposure control of conventional full wafer projection exposure apparatus, such as steppers, cut-off control is normally performed whether the exposure light source employed is a continuous light source such as an extra-high pressure mercury lamp, or a pulsed laser light source such as an excimer laser light source. In the cut-off control, a portion of the light for exposure of a wafer coated with a photosensitive material is branched and directed to an integrator sensor formed of a photoelectric detector, whereby the amount of exposure of the wafer is indirectly detected. The light emission is continued until the total of exposure amounts detected by the integrator sensor exceeds a predetermined level (critical level) corresponding to a total amount of exposure (hereinafter, referred to as “set amount of exposure”) that is required for the photosensitive material used (in the case of continuous light, a shutter is closed when the critical level is exceeded).
In the case of exposure using a pulsed laser light source as an exposure light source, a desired precision reproducibility in exposure control can be achieved by using at least a certain number of laser light pulses for exposure (hereinafter, the “certain number” will be referred to as “minimum exposure pulse number”) because energy varies from one laser light pulse to another. In the case of a highly sensitive resist for which the set amount of exposure is small, the exposure to at least the minimum exposure pulse number of laser light pulses may become impossible if laser light from the pulsed laser source is directly used for exposure. Therefore, if the set amount of exposure is small, the pulsed laser light must be reduced in intensity by, for example, a light reducing device disposed in the optical path, so that at least the minimum exposure pulse number of laser light pulses can be employed for exposure.
To enable transfer of a pattern of an increased area to a wafer with a high precision without imposing severe requirements on the projection optical system, a step-and-scan projection exposure apparatus has recently been developed that synchronously scans or moves a reticle and a wafer relative to the projection optical system while projecting the images of portions of the pattern of the reticle onto the wafer using the projection optical system, so as to sequentially transfer portions of the pattern of the reticle onto individual shot areas on the wafer by exposure. In such a scanning exposure type apparatus, exposure control regarding a point on a wafer is impossible, and the aforementioned cut-off control cannot be applied. Therefore, for exposure control in scanning exposure type exposure apparatus, the conventional art normally employs a method (open exposure control method) that controls the amount of exposure simply by totalling the light quantity of each illumination pulse, or a method (every pulse exposure amount control method) that controls the energy of every illumination light pulse by measuring the total amount of exposure of an area on a wafer in real time, the area included in a slit-like illumination field (exposed area) extending in the scanning direction, and calculating a target energy value of the next illumination light pulse based on the total amount of exposure.
The former control method requires fine adjustment of pulse energy to satisfy the following equation (1) wherein the number of exposure pulses is an integer in order to achieve a desired linearity in the exposure control:
(set amount of exposure)=(number of pulses)×(average energy per pulse) (1)
In equation (1), the average energy per pulse is a value measured by an integrator sensor immediately before exposure. The latter control method requires fine adjustment of the pulse energy at every emission of a pulse.
A conventional pulsed laser light source used in connection with either of the control methods contains an energy monitor formed by a photoelectric detector, and performs feedback control of the laser electric power source so that the detection result by the energy monitor conforms to an output energy value (central energy value) provided by an external device in order to constantly output light pulses of the same amount of energy. More specifically, the central energy value inputted to the pulsed laser light source is fixed, and the fine modulation of energy of a light pulse is performed using an energy fine modulator.
FIG.
8
(
a
) illustrates a conventional energy fine modulator of a double grating type. In this fine modulator, a stationary grating
41
having light transmitting portions and blocking portions formed at a predetermined pitch and another grating
42
movable in the direction of the grating pitch are arranged one over the other in the optical path of a laser beam LB emitted in a pulsed manner. By shifting the positions of the gratings
41
,
42
relative to each other, the laser beam transmittance can be finely modulated. FIG.
8
(
b
) illustrates another conventional energy fine modulator in which two glass plates
43
,
44
each coated with anti-reflection coating on both sides are arranged in the optical path of a laser beam LB, with the glass plates symmetrically inclined at a variable inclination angle &thgr;. Exploiting a property of the glass plates
43
,
44
that the transmittance varies depending on the incident angle of the laser beam LB, the fine modulator finely adjusts the overall laser beam transmittance by controlling the inclination angle &thgr;.
The conventional energy fine modulators as shown in FIGS.
8
(
a
) and
8
(
b
), however, have drawbacks in that since a mechanical drive is employed for the adjustment of transmittance, high-speed adjustment of transmittance is difficult. Moreover, since neither of the fine modulators is able to achieve a maximum transmittance of 100%, an energy loss results even in an initial maximum transmittance state, thus adversely affecting device efficiency in utilizing the pulsed light.
The double grating type energy fine modulator as shown in FIG.
8
(
a
) has another drawback in that even though the fine modulator is disposed at the light source-side of a fly eye lens provided as an optical integrator, the overlapping effect of the fly eye lens becomes small if the aperture of the illumination diaphragm is small, so that the grating pattern slightly remains as an illumination non-uniformity in an image. A small aperture of the illumination diaphragm means that a so-called a value, that is, a coherence factor, is small.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an exposure control method capable of finely modulating the energy of illumination light and therefore the amount of exposure of a photosensitive substrate at a high speed without inserting a mechanically-driven energy fine modulator for finely modulating the transmittance (light reduction rate) in the optical path of illumination light, and without causing an energy loss along the optical path of illumination light.
It is another obje
Mathews Alan A.
Nikon Corporation
Nixon & Vanderhye P.C.
LandOfFree
Exposure control method and apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Exposure control method and apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Exposure control method and apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2929553