Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2000-11-15
2004-05-04
Bruce, David V. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S559300, C355S053000
Reexamination Certificate
active
06730925
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus used in photolithographic processes for manufacturing of semiconductor device elements, image pickup elements (CCD), liquid crystal display elements, plasma display elements and thin film magnetic heads. The method is particularly suitable for controlling light exposure of an exposure beam from a pulse light source used in scanning type projection exposure apparatus based on the step-and-scan method. This invention is based on a Japanese Patent Application, First Publication, Hei 11-326192, the content of which is incorporated herein by reference.
2. Description of the Related Art
One of the basic functions of the projection exposure apparatus for manufacturing of semiconductor device elements, for example, is to control the integrated exposure level on the object to be exposed within a suitable range, at each point in each shot region of a wafer (or glass plate) that has been coated with a photoresist coating (photo-sensitive material). Conventionally, regardless of whether a continuous light source such as mercury lamp or a pulsed light source such as excimer laser light source is used to expose the wafer, exposure control for a static exposure type apparatus such as the conventional stepper is based on the so-called “cutoff control”, in which the exposure is continued until the integrated exposure on the wafer measured indirectly by the detector, comprised by an integrator sensor disposed in the illumination optical system, exceeds a specific exposure value (target exposure level) corresponding to a critical level.
When the exposure light source emits pulses of laser light, because individual pulses have different levels of light energy, it has been a practice to assure reproducibility of precision in exposure control by applying more than a certain minimum number of exposure pulses. In such a case, because the minimum exposure energy level is low for a high sensitivity photoresist material, it is necessary to place a light reducing member in the optical path to reduce the power of the laser pulses uniformly so as to assure delivery of pulses exceeding the minimum number of exposure pulses.
Further, for scanning exposure type apparatus based on the step-and-scan method, which has been in use in recent years, a conventional method (open-level exposure control method) is used, in which the exposure light (exposure beam) containing pulses of laser light is simply accumulated by integrating the light energy. In such a method, it is necessary to adjust the pulse energy so that a linear control can be applied to obtain a desired degree of exposure control as computed from the following relation. In other words, the pulses must be counted in whole numbers.
(target exposure level)=(number of pulses)×(average energy per pulse)
where a value for the average energy per pulse is to be obtained by the integrator sensor immediately prior to an exposure event.
In contrast, as disclosed in a recent Japanese Patent Application, First Publication, Hei 6-252022 and in a corresponding U.S. Pat. No. 5,627,627, the pulse energy of individual pulses is controlled by determining the values of individual pulse energy in real-time during the exposure process so that an integrated energy level of prior pulses can be used to determine a target value for the pulse energy of a next group of pulses. This per-pulse exposure level control method enables to minimize the scatter in the integrated exposure levels compared with the open-level exposure control method.
As outlined above, although there have been proposals for various types of exposure level control method, when it is necessary to change the exposure level over a relatively wide range of transmittance, all of these methods require the use of a specific light reducing member to lower the power of the exposure light (either pulsed or continuous). In such a process, it is necessary to mechanically switch the optical filters having different light transmittance characteristics in the light reducing member, and, immediately after changing the transmittance, to perform test emission of light source to measure the power (exposure energy) of the exposure light transmitted through the filter, and to re-adjust the exposure conditions to be consistent with the measured values of the existing power (exposure pulses per one point on the wafer if the exposure light consists of pulses).
It should be noted that, deviations in the line widths of the circuit patterns formed on the wafer are caused by variations in the thickness of the coating applied on the wafer in the course of applying the photoresist coating, and by a related phenomenon of non-uniformity of standing waves within the photoresist coating, as well as non-uniformity in developing the patterns. Such errors in line widths cause errors in the line widths of the circuit patterns on each layer of integrated circuit fabricated on the wafer. Therefore, as the density of circuit integration of semiconductor devices increases further, there is a danger of lower yield of final product caused by such errors in the line width. To correct such line widths errors in the photoresist pattern, it is necessary to conduct a series of test exposures. For example, several evaluation wafers are prepared by applying a photoresist coating and performing test printing by varying the integrated exposure level over the coated wafer by a given amount. After developing the photoresist pattern, line widths of resist patterns in each shot region are measured so that the exposure level that produced a line width nearest to the design value can be chosen as the correct exposure level for that shot region. The distribution of target exposure level thus obtained is roughly concentric about the center of the wafer, for example, so that it is considered practical to divide the entire shot region into a number of sub-regions and to determine a proper exposure level for a group of sub-regions. Also, it may be considered that the variation in the target exposure level in various sub-regions is about ±10% with respect to the average value of the exposure level.
Therefore, when different exposure levels are assigned to a plurality of sub-regions in the wafer, use of the conventional light reducing member leads to the necessity of performing exposure testing whenever the exposure power is changed in the course of successive exposures of various shot regions over the wafer. Such a procedure leads to increasing the time necessary to process each wafer and lowering the throughput of photolithographic process.
SUMMARY OF THE INVENTION
It is therefore a first object of the present invention to provide an exposure method and an exposure apparatus to enable to prevent the reduction of the throughput, without decreasing the control of exposure level precision, when exposing a plurality of regions (or sub-regions) defined on a wafer at different target exposure levels.
It is a second object of the present invention to provide an exposure method and an exposure apparatus based on the scanning exposure method using pulses of laser light to prevent the reduction of the throughput and the loss of precision in exposure level control when exposing sub-regions on a wafer at different target exposure levels.
It is a third object of the present invention to provide an exposure method and an exposure apparatus to enable to quickly determine target exposure levels in a plurality of regions defined on a substrate base such as a wafer.
It is a further object of the present invention to provide a manufacturing method for high precision devices based on the present exposure method.
A first method is for exposing a pattern formed on a first object (
11
) onto a sub-divided region defined on a second object (
14
), and exposing the pattern successively on a plurality of divided regions (
31
(
2
,
1
))~(
31
(
5
,
6
)) defined on the second object so as to replicate the pattern in each of the divided regions, by mo
Bruce David V.
Nikon Corporation
Oliff & Berridg,e PLC
Song Hoon
LandOfFree
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