Semiconductor device manufacturing: process – Including control responsive to sensed condition – Optical characteristic sensed
Reexamination Certificate
2000-04-27
2001-05-08
Powell, William (Department: 1765)
Semiconductor device manufacturing: process
Including control responsive to sensed condition
Optical characteristic sensed
C216S059000, C216S084000, C438S014000, C438S689000, C438S745000
Reexamination Certificate
active
06228661
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to a method to determine the swing curve which will be used in photolithography of semiconductor technology. More particularly, the present invention relates to a method for determining a dark-to-clear exposure dose in a swing curve.
B. Description of the Related Art
Currently, the integration degree of large-scale integrated circuits is proceeding higher at a speed of a generation per three years. This high integration degree is gradually achieved along with the development of the fine processing technology, especially the technique of photolithography. In the semiconductor process, the main function of the process of photolithography is to define patterns and doping area on individual thin films formed on a semiconductor substrate.
Generally, the process of photolithography for a semiconductor wafer includes the following steps. At first, a photoresist film is coated on the surface of the semiconductor wafer. Next, using a mask with a pattern and an ultraviolet light as an exposing light source, the photoresist film is exposed such that the pattern is transferred from the mask to the photoresist film as a latent pattern. After the exposure, the photoresist film is developed to remove the exposed portion of the photoresist film.
In the process of photolithography, the object of the exposure step is to make the photoresist film absorb appropriate and sufficient exposure dose so as to perform the photochemical transformation. The factors for controlling the exposure dose are the intensity of the exposing light and the time period of the exposure. Furthermore, the magnitude of the exposure dose required to complete the photochemical transformation depends on both the thickness of the exposed photoresist film and the pattern size needed to be transferred.
In the semiconductor process, after the parameters of the process for the etching step is decided and set, it is usually necessary to determine or adjust the thickness of the photoresist film coated on the surface of the thin film. As for the process of photolithography, a swing curve as shown in
FIG. 1
is typically adapted for determining the required magnitude of the exposure dose corresponding to a specific thickness of the photoresist film. Referring to
FIG. 1
, the swing curve is similar to a sine wave and depicted in terms of the magnitude of the exposure dose with respect to the thickness of the photoresist film. The magnitude of the exposure dose decides the degree of the photochemical transformation of the photoresist film. More specifically, the magnitude of the exposure dose is positively proportional to the degree of the photochemical transformation of the photoresist film.
As a method for obtaining the above-mentioned swing curve, a dark-to-clear exposure process has been disclosed. Hereafter, the conventional method will be described in detail with reference to FIGS.
2
(A) and
2
(B). FIG.
2
(A) is a cross-sectional view showing the conventional method for obtaining the above-mentioned swing curve. Referring to FIG.
2
(A), at the beginning of the dark-to-clear exposure process, a photoresist film
2
with a given thickness
3
is coated on a silicon dummy wafer
1
. Subsequently, using a mask
4
without a pattern, the photoresist film
2
is exposed by means of step and repeat method using an exposing light
5
. In other words, the surface of the photoresist film
2
is sectioned to a plurality of rectangular chips and repeatedly exposed from one chip to another. FIG.
2
(B) is a plane view showing the photoresist film coated on the silicon dummy wafer shown in FIG.
2
(A). Referring to FIG.
2
(B), each of the chips
6
is exposed by different magnitudes of the exposure dose. After the completion of exposing all the chips
6
, the photoresist film
2
is developed to remove the exposed portions. Next, based on the color appearing due to the light reflected from the surface of the silicon dummy wafer
1
, each of the chips
6
is inspected by using human eyes to determine whether the photoresist film
2
coated thereon is completely removed. After the inspection, the magnitude of the exposure dose resulting in the complete removal of the photoresist film
2
is recorded.
Using the same steps as mentioned above,
24
silicon dummy wafers are sequentially exposed. It should be noted that the thicknesses of the photoresist films coated on the 24 silicon dummy wafers are different from each other. By this manner, the swing curve representing the relationship between the thickness of the photoresist film and the magnitude of the exposure dose for completely removing the photoresist film is obtained. However, for the thickness of the photoresist film at the order of &mgr;m, the above-mentioned conventional method using the human eyes to inspect whether the removing of the photoresist is complete or not is very easy to make error. Therefore, it difficult to obtain an accurate swing curve for the &mgr;m order's semiconductor technology.
SUMMARY OF THE INVENTION
In view of the above problem, it is therefore an object of the present invention to provide a method to determine a dark-to-clear exposure dose for a swing curve. By the present invention, it is easy to accurately determine the relationship between the thickness of the photoresist film and the corresponding magnitude of the exposure dose.
The embodiment according to the present invention is to modify the conventional method for determining a dark-to-clear exposure dose in a swing curve by replacing the conventional mask without pattern with that having a critical dimension bar's pattern. Furthermore, the inspection by the human eyes to determine whether the removing is complete is replaced with that performed by a scanning electron microscope. Therefore, an accurate swing curve is obtained for the &mgr;m order's semiconductor technology.
According to the present invention, the method comprises the following steps:
First, photoresist films with different thicknesses are coated on 20 to 40, preferably 25, silicon dummy wafers, respectively. Using a mask with a critical dimension bar's pattern, each of chips of the silicon dummy wafers is exposed by different exposure doses such that the pattern is transferred on each of the chips. After all the silicon dummy wafers are developed, each of the chips of the silicon dummy wafers is inspected by using a scanning electron microscope. For each of the silicon dummy wafers, the exposure dose resulting in completely removing the photoresist film on the region between the adjacent critical dimension bars of the pattern by developing is recorded. In terms of the thickness of the photoresist film as the abscissa and the exposure dose resulting in completely removing the photoresist film on the region between the adjacent critical dimension bars of the pattern as the ordinate, a swing curve is depicted. Finally, the exposure dose corresponding to a peak point or valley point of the swing curve is chosen depending on the subsequent process of etching.
REFERENCES:
patent: 5690785 (1997-11-01), Nakaya
patent: 5747201 (1998-05-01), Nakayama et al.
Darby & Darby
Mosel Vitelic Inc.
Powell William
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