1. Field of the Invention
The present invention relates to a method of monitoring, using the electron-beam image of a resist pattern, whether or not the exposure has been performed under an appropriate exposure condition at the time of forming the resist pattern on a semiconductor board at the lithography step in the manufacture of a semiconductor device. Also, it relates to a method of manufacturing the semiconductor device. In particular, it relates to a technology for controlling the exposure process so as to maintain the correct exposure condition, and a method of manufacturing the semiconductor device by using this condition.
2. Description of the Related Art
FIG. 27 illustrates a flow of the conventional lithography step.
At first, a resist, i.e., a photosensitive material, is coated on a board such as a semiconductor wafer with a predetermined thickness. Then, the downsize exposure of a mask pattern to the resist is performed, using an exposure apparatus (step 2050). After that, the downsize-exposed resist is developed (step 2051), thereby forming a resist pattern. Next, the size checking of the formed resist pattern is carried out, using a scanning electron microscope equipped with a size-measuring function (i.e., length-measuring SEM) (step 2052). The conventional processing contents performed using the length-measuring SEM are as follows, for example: After acquiring the electron-beam image of the region including a portion whose size-accuracy is to be strictly managed (step 2053), the resist-pattern size is measured (step 2054). Next, a judgement is made as to whether or not the size will satisfy a criterion (step 2055). If the size does not satisfy the criterion, the exposure quantity provided by the exposure apparatus is modified (step 2056, the correction quantity to the exposure quantity is represented by ΔE). In the case of, e.g., a positive-type resist, the exposure quantity is increased if the resist-pattern size is too large, while the exposure quantity is decreased if the resist-pattern width is too small. Also, in many cases, the increased or decreased quantity of the exposure quantity has been determined based on the experience and the intuition of a worker in charge.
FIG. 28 illustrates the relationship between the resist pattern and a film pattern after being etched (cited from “Electron Beam Testing Handbook”, p. 255, Japan Society for the Promotion of Science, 98th research material of 132nd Committee on Application of Charged Particle Beam to Industry). If the etching conditions are identical, there exists a certain fixed relationship between the configuration of the resist pattern and that of the after-etched film pattern. Accordingly, in order to obtain the film pattern having a predetermined configuration, the resist pattern is also required to have the predetermined configuration. Also, when starting to manufacture a new product-type of semiconductor device or the like, prior to the introduction of a product wafer, there is performed “a condition-submitting work” for finding out a focal-point position and an exposure quantity that make it possible to acquire a predetermined resist-pattern configuration. This finding-out is carried out as follows: At first, a wafer illustrated in FIG. 12A is formed on which a pattern is pasted by being baked under a condition that the focal-point position and the exposure quantity are changed on each shot (i.e., onetime exposure unit) basis (the wafer of this kind is commonly referred to as “FEM (: Focus & Exposure Matrix) sample”). Next, in addition to the execution of the size measurement of the resist pattern on each shot basis, the wafer is cut off so as to inspect its cross-section configuration or the like. Incidentally, JP-A-11-288879 has disclosed a system for supporting the condition-submitting work. This work determines an exposure quantity (E0) and a focal-point position (F0) that allow variation margins to be taken more widely in the exposure quantity and the focal-point position. Then, based on these conditions, the exposure to the product wafer is carried out. There exist, however, some cases where a probability is increased that the resist pattern having the specification-satisfying configuration cannot be acquired. This is because various process variations (e.g., a change in the photosensitivity of the resist, a film-thickness variation in an anti-reflection film under the resist, and drifts in the respective types of sensors of the exposure apparatus) decrease the variation margins under the conditions (E0, F0) determined in the condition-submitting work. Detecting these process variations is a role to be played by the above-described size measurement (i.e., the step 2). In the prior arts, the attempt has been made to compensate, by the correction to the exposure quantity, for the size change in the resist-pattern configuration caused by the process variations.
In the prior arts, in order to detect and take a countermeasure against the process variations, the following method has been employed: The size value of a line width or the like is inspected, using the length-measuring SEM. Then, if the size value does not satisfy a criterion, the exposure quantity is corrected. This method, however, is accompanied by the following 1st to 3rd problems:
As the first problem, the length-measuring SEM usually observes, from directly above, the resist-pattern configuration formed on the semiconductor board. Also, the length-measuring SEM measures the size value of the edge configuration of the resist pattern that has appeared on the electron-beam image acquired in this state. There exists, however, a variation in the resist-pattern configuration which cannot be measured by conventional measuring techniques. Concretely, this variation means a variation in the resist-pattern's edge configuration caused by the variation in the focal-point position at the time of the exposure. The conventional measuring techniques find it impossible to detect this variation. The cross-section configuration of the resist pattern formed on the semiconductor board is a substantially trapezoidal configuration. Concerning the signal intensity of secondary electrons detected by the scanning electron microscope as a signal generated from the sample, the signal intensity from the inclined portion is stronger than the one from the flat portion. As a result, as illustrated in FIG. 29A, the signal waveform becomes a one that has the peaks at the locations corresponding to the edges of the trapezoid. In the size measurement by the length-measuring SEM, as illustrated in, e.g., FIG. 29B, the following method or the like is employed: Straight lines are applied to the outer-side portions and the base portions of the peaks so as to determine each intersection point of the respective 2 straight lines, then defining, as the line width, the distance between the right and left intersection points. FIG. 30 is a graph for indicating how the line width will change if the exposure quantity and the focal-point position are changed, where the focal-point position is set up in the transverse-axis and the line width is plotted on each exposure-quantity basis (i.e., e 0 to e 8). The exposure quantity lies in the range of e 0<e 1< . . . <e 8. Moreover, there exists a relationship that the line width becomes smaller as the exposure quantity grows larger (, which is in the case of a positive-type resist. This relationship becomes opposite in the case of a negative-type).
Consequently, inspecting the line width makes it possible to detect the variation in the exposure quantity. As is apparent from the same graph, however, the line width does not change greatly with respect to the change in the focal-point position. In particular, in proximity to the correct exposure quantity (i.e., e 4), the line width scarcely changes if the focal-point position is changed. Accordingly, the inspection of the line width does not make it possible to detect the variation in the focal-point position. Meanwhile, even if the line width does not change, the cross-section configuration of the resist pattern changes if the focal-point position is changed as is illustrated in FIG. 30B. As described earlier, the change in the cross-section configuration of the resist pattern exerts the influence on the film pattern after being etched. As a consequence, the use of the conventional measuring techniques, which are incapable of detecting the variation in the focal-point position, may result in even a danger of producing a configuration failure of the after-etched film pattern in large quantities.
As the second problem, there exists a problem that the correction of the exposure quantity alone, naturally, finds it impossible to deal with the case where the focal-point position has been deviated. In the case of, e.g., the situation A in FIG. 30A, since the line width is larger than the normal line width, the processing of increasing the exposure quantity will be carried out based on the measurement result of the line width. This processing, however, makes no correction to the deviation in the focal-point position, thereby simply bringing about the situation B in FIG. 30A and never implementing the normal cross-section configuration of the resist pattern. This, accordingly, may again result in even the danger of producing the configuration failure of the after-etched film pattern in large quantities.
As the third problem, in the above-described prior arts, there also exists a problem of being unable to acquire information for indicating the process variations in a quantitative manner. Here, this information is needed in order to maintain the normal exposure process. In accompaniment with the microminiaturization of a semiconductor pattern in recent years, the variation allowable range of the exposure quantity and that of the focal-point position have become exceedingly small. For example, it is requested that, when the pattern rule is smaller than 180 nm, the variation range of the exposure quantity is controlled to become smaller than ±10% of the size value, and that of the focal-point position is controlled to become smaller than ±0.2 to 0.3 μm. The implementation of these variation allowable ranges requires the information for indicating the process variations in a quantitative manner, i.e., an accurate quantification of the variation quantities. This accurate quantification is such that the deviation in the exposure quantity is equal to such-and-such mJ, and the deviation in the focal-point position is equal to such-and-such μm. In the prior arts, the detection of the deviation in the focal-point position is not performed at all. Moreover, it cannot help saying that the detection of the deviation in the exposure quantity is inaccurate. The reason for this is as follows: Despite the fact that, in general, the variation in the focal-point position also changes the line width, the size variation quantity caused by the variation in the focal-point position has also been compensated for by the adjustment of the exposure quantity. Consequently, the use of the prior arts finds it impossible to maintain the normal exposure process.