Radiation imagery chemistry: process – composition – or product th – Plural exposure steps
Reexamination Certificate
1999-09-08
2001-05-29
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Plural exposure steps
C430S022000, C430S030000, C430S311000, C355S077000
Reexamination Certificate
active
06238851
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an exposure method in a photolithography process for producing, for example, semiconductor devices or liquid crystal displays, wherein mask patterns are overlaid on one another on a substrate, such as a wafer, using two exposure apparatuses having respective exposure fields of different sizes. The exposure method of the present invention is especially advantageous when applied to exposure by the mix-and-match method in which exposure of a critical layer is carried out by a projection exposure apparatus of a scanning exposure type.
In factories for producing semiconductor devices, such as ultra LSIs, from the viewpoint of improving the throughput (productivity) of the production process, there has been an increasing tendency to use different exposure apparatuses for carrying out exposure of different layers on a wafer in a process for producing a single type of semiconductor devices. As a method for effecting exposure of different layers on a wafer by using different exposure apparatuses, there can be mentioned a so-called mix-and-match method disclosed in, for example, Unexamined Japanese Patent Application Public Disclosure No. 62-90931, in which exposure of a critical layer which requires high resolution is effected by a first one-shot exposure type projection exposure apparatus (stepper) having a high reduction ratio, and exposure of a rough layer which does not require high resolution as compared to the critical layer is effected by a second one-shot exposure type projection exposure apparatus (stepper) having a low reduction ratio.
In the mix-and-match method, for example, when the size of the exposure field of the second projection exposure apparatus is twice that of the first projection exposure apparatus in both vertical and horizontal directions, the throughput of the exposure process for the rough layer is four times that for the critical layer. However, when the size of exposure field (i.e., the size of shot area) is different between the first projection exposure apparatus and the second projection exposure apparatus, it is difficult to achieve a satisfactorily high overlay accuracy. Therefore, in the conventional mix-and-match method in which exposure of the critical layer is effected by a projection exposure apparatus with a small exposure field, which has high resolution and small distortion, and exposure of the rough layer is effected by a projection exposure apparatus with a large exposure field, which has relatively low resolution and relatively large distortion but offers a high throughput, various alignment methods have been attempted to achieve a high overlay accuracy.
As mentioned above, in the conventional mix-and-match method, exposure of the rough layer is effected using a stepper having a large exposure field, in order to increase the throughput.
Recently, in producing semiconductor devices, it is desired to not only improve the resolution of the stepper for transferring a mask pattern having a small line width but also increase a unit chip size. For improving the resolution of the stepper, the conventional stepper utilizing an i-line of a mercury lamp (wavelength: 365 nm) as exposure light is unsatisfactory. Because light having a short wavelength is effective for achieving high resolution, an excimer laser beam having a wavelength in an ultraviolet range, such as a KrF excimer laser beam (wavelength: 365 nm) and an ArF excimer laser beam (wavelength: 193 nm), is recently used as exposure light, instead of the i-line.
On the other hand, when it is attempted to increase the size of a projection optical system of the stepper for increasing the size of the exposure field, it is difficult to design and produce a projection optical system for one-shot exposure having a large size. Further, when the excimer laser beam having a wavelength in an ultraviolet range is used, vitreous materials which can be used for lenses for the projection optical system are limited to quartz or fluorite, so that it is difficult to suppress various aberrations in a large image-forming area within predetermined allowable ranges, leading to difficulty in increasing the size of the projection optical system.
In such a situation, for effecting exposure of high resolution using a projection exposure apparatus having a large exposure field (i.e., a large shot area) without increasing the size of a projection optical system of the projection exposure apparatus, it has been proposed to employ a step-and-scan type projection exposure apparatus in which a reticle and a wafer are synchronously moved in scanning directions at respective predetermined speeds, relative to a projection optical system, to thereby transfer a pattern on the reticle to each of shot areas on the wafer. When exposure is effected by a scanning exposure type projection exposure apparatus, such as the step-and-scan type projection exposure apparatus, a magnitude of distortion in the exposure field is suppressed by about 40% as compared to the one-shot exposure type projection exposure apparatus, as disclosed in Unexamined Japanese Patent Application Public Disclosure No. 6-349702.
Recently, it has been strongly desired to achieve a high overlay accuracy with respect to exposure of the rough layer. Therefore, in exposure by the mix-and-match method, it has been attempted to effect exposure of the critical layer by the scanning exposure type projection exposure apparatus with a large exposure field utilizing an excimer laser beam, which has high resolution and small distortion, and effect exposure of the rough layer by the one-shot exposure type projection exposure apparatus utilizing the i-line, which has conventionally been used for exposure of the critical layer.
In this case, however, exposure of the critical layer is effected by a projection exposure apparatus having a large exposure field and exposure of the rough layer is effected by a projection exposure apparatus having a small exposure field. Therefore, various alignment methods in the conventional mix-and-match method in which exposure of the critical layer is effected by a projection exposure apparatus having a small exposure field and exposure of the rough layer is effected by a projection exposure apparatus having a large exposure field are unsatisfactory for achieving a high overlay accuracy.
In addition, from the viewpoint of efficiency, the exposure field of the scanning exposure type projection exposure apparatus has a rectangular form which is long in the scanning direction and the exposure field of the one-shot exposure type projection exposure apparatus has, for example, a square form, so that the length in the scanning direction of the exposure field of the scanning exposure type projection exposure apparatus may be a multiple of a non-integer (e.g., a half-integer, such as {fraction (3/2)}) of the width of the exposure field of the one-shot exposure type projection exposure apparatus. When the size of the exposure field for exposure of the critical layer is a multiple of a non-integer of the size of the exposure field for exposure of the rough layer, there is a shot area in the rough layer which extends over a plurality of shot areas in the critical layer, leading to difficulty in achieving a high overlay accuracy.
BRIEF SUMMARY OF THE INVENTION
In view of the above, the present invention has been made. It is an object of the present invention to provide an exposure method for effecting exposure by the mix-and-match method utilizing two exposure apparatuses having respective exposure fields of different sizes, in which a high overlay accuracy can be achieved, even when, as compared to the exposure field for first exposure, the exposure field for subsequent exposure is small.
It is another object of the present invention to provide an exposure method for effecting exposure by the mix-and-match method, in which a high overlay accuracy can be achieved, even when the length in a predetermined direction of the exposure field for first exposure is a multiple of a non-integer of that of the e
McPherson John A.
Nikon Corporation
Oliff & Berridg,e PLC
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