Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-12-12
2004-03-09
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S055000, C355S077000
Reexamination Certificate
active
06704091
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure apparatus and an exposure method.
2. Description of the Related Art
During a photolithography process, which is one of the manufacturing processes implemented to manufacture semiconductor devices, a reduction projection exposure apparatus (stepper) is utilized. In keeping with the miniaturization of semiconductor devices achieved in recent years, rigorous efforts are made to reduce the wavelength of the exposing light and to achieve a higher NA (numerical aperture) in the stepper.
The relationship between the resolving power R of the stepper and the depth of focus (DOF) is expressed through a Rayleigh formulae presented below.
R=k
1
*&lgr;/NA
DOF=k
2
*&lgr;/(
NA
)
2
As these formulae clearly indicate, the focal depth DOF becomes reduced if the wavelength &lgr; of the exposing light is reduced or the numerical aperture NA is increased to improve the resolving power R. Since the focal depth DOF of the stepper is now achieved in the same order as that of the surface level variation of the semiconductor device, it is necessary to take measures with regard to the defocusing of the exposing light manifesting, in particular, in an area that is not level (an indented area or a projected area), in order to maintain a specific degree of dimensional accuracy for the transfer pattern.
The so-called leveling technology achieved by moving the wafer vertically in correspondence to the surface level variation or even by tilting the exposure surface in some cases during the exposure operation is normally adopted to address the problem discussed above. A surface level variation may be detected by a focus sensor provided in the stepper to measure the distance between the surface of the semiconductor device and a reference position (e.g., the exposing light source) and it may then be expressed with a three-dimensional coordinate system.
In addition, an exposure method achieved by dividing the entire wafer into a plurality of unit areas (hereafter referred to as “shots”) and irradiating slit light on each shot while scanning the slit light is usually adopted in the exposure process in recent years. The range over which the slit light is irradiated is normally approximately {fraction (1/10)} of the entire shot and, by scanning this light, the entire shot becomes exposed.
Adoption of such a scanning method in the exposure process may prove problematic, particularly if non-level portions attributable to the circuit structure are systematically present within the individual shots. If, during the exposure operation, the focal length is adjusted at such a non-level portion, which often manifests a drastic difference in the height relative to the surrounding area, the likelihood of the exposing light becoming defocused over a wide range at the surrounding area increases.
Since the results of measurement performed by the focus sensor are directly utilized in the wafer leveling operation during the scanning exposure process in the related art, a defocus area is formed over a wide range if the measurement point at which the focus sensor performs the measurement hits upon a non-level position, which will result in poorer dimensional accuracy of the transfer pattern.
SUMMARY OF THE INVENTION
An object of the present invention, which has been completed by addressing the problems discussed above, is to provide an exposure apparatus and an exposure method that make it possible to minimize the range over which the exposing light is defocused even when an non-level portion is included within each shot.
In order to achieve the object described above, in a first aspect of the present invention, an exposure method achieved by sequentially exposing a plurality of unit areas set in advance at the surface of a wafer is provided.
A first feature that characterizes the exposure method according to the present invention is that a pre-exposure process is implemented prior to exposure processing implemented on individual unit areas. During this pre-exposure process, one or a plurality of measurement target unit areas are selected from the unit areas and a “virtual surface” and “adjustment values” are calculated in correspondence to the three-dimensional coordinates indicating the position of the surface in each measurement target unit area.
In a first exposure method according to the present invention, one of the plurality of unit areas is set as a measurement target unit area in a first pre-exposure step, the three-dimensional coordinates of a plurality of measurement points at the surface in the measurement target unit area are determined in a second pre-exposure step and a virtual surface approximating the surface in the measurement target unit area is ascertained by using the three-dimensional coordinates of each measurement point in a third pre-exposure step. It is desirable to calculate the virtual surface through, for instance, the methods of least squares.
In a fourth pre-exposure step implemented next, the surface in the measurement target unit area is scanned by a plurality of sensors and the extents to which the individual scanning areas within the measurement target unit area scanned by the sensors are deviation from the virtual surface along a direction in which a specific coordinate axis extends are determined. The extents of the positional deviation manifesting at the respective scanning areas are stored in memory as adjustment values for the individual sensors, each corresponding to one of the scanning areas at, for instance, the controller of the exposure apparatus. It is to be noted that if a Z axis represents the optical axis of the exposing light, the extents of positional deviation can be expressed as a difference in the Z coordinate value.
When these pre-exposure steps are completed, a sensor selection step is implemented. In the first exposure method according to the present invention, at least two sensors among a plurality of sensors are selected. In a second exposure method according to the present invention, at least two sensors with their adjustment values having the smallest absolute values are selected from a plurality of sensors.
According to the present invention, an exposure unit area setting step is implemented. In this step, one of the plurality of unit areas is set as the exposure unit area to undergo the exposure processing. This step may be implemented any time prior to the exposure step. For instance, a plurality of unit areas may be set on the wafer, all the unit areas may be set as an exposure unit areas and the order in which they are to be exposed may be specified before the pre-exposure process.
In the second exposure method according to the present invention, a virtual surface and adjustment values are obtained through steps which are different from those implemented in the first exposure method according to the present invention. Namely, one of the plurality of unit areas is set as a measurement target unit area in a first pre-exposure step, and then in a second pre-exposure step, the surface in the measurement target unit area is scanned by a plurality of sensors and the three-dimensional coordinates of the individual scanning areas within the measurement target unit area having been scanned by the sensors are ascertained. In a third pre-exposure step, a virtual surface approximating the surface in the measurement target unit area is ascertained by using the three-dimensional coordinates of the scanning areas.
In a fourth pre-exposure step implemented next, the extents to which the individual scanning areas within the measurement target unit area having been scanned by the sensors in the second pre-exposure step are deviation relative to the virtual surface along a direction in which a specific coordinate axis extends are determined. The extents of the positional deviation manifested at the scanning areas are stored in memory as adjustment values for the individual sensors each corresponding to one of the scanning areas.
In addition, in a third exposure method accor
Mathews Alan A.
Oki Electric Industry, Ltd.
Volentine & Francos, PLLC
LandOfFree
Exposure apparatus and exposure method 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 apparatus and exposure method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Exposure apparatus and exposure method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3218034