The present invention relates to a method of monitoring processes used in the manufacture of semiconductor devices, such as a lithography process and an etching process, and a system therefore; and, more specifically, the invention relates to a method of monitoring the manufacture of a semiconductor device so as to monitor a state of processing in each process from an image obtained by imaging a pattern produced on a substrate that was processed in each treatment process, and a system therefor.
FIG. 2 is a view showing the flow of processing in a conventional lithography process. The lithography process consists of a photo process in which a resist pattern is formed by exposure and development, and an etching process in which the resist pattern is transferred onto a film that is the subject of the processing.
A thin film of a material to be processed is first grown on a substrate of a semiconductor wafer. Then, a resist pattern is produced on this thin film by: (a) coating the film with a resist that acts as a photosensitizer to a predetermined thickness; (b) exposing a mask pattern thereto using exposure equipment; and (c) developing the resist. The resist pattern thus formed is subjected to (d), (f) to (e), (g) to (f) and (h) to (g) dimension measurement by a scanning electron microscope equipped with a measuring function (critical dimension SEM) etc. and, if the dimensions fail to satisfy predetermined the resist pattern is peeled off, and the pattern is reformed after the exposure is altered. It is often the case that the amount of increment/decrement of the exposure is determined by the operator based on his/her experience and intuition.
Next, the resist pattern is transferred by (f) etching the thin film that has been grown below the resist using the resist pattern formed thereon as a mask, to form a circuit pattern. Currently, many minute patterns of semiconductors are processed by dry etching with the use of a plasma. In such a case, (g) the resist is removed, and subsequently, as with the resist pattern, (h) the formed circuit pattern is measured to find its dimensions using a critical dimension SEM, etc. In the dry etching process, since the re-processing of the wafer cannot be executed, if an abnormality is found in the wafer, continuing the manufacture of the wafer is stopped, and an investigation of the cause of the abnormality and possible countervailing measures therefor are performed. If the pattern is normally produced, the film growth process, the photo process, and the etching process are repeated to build up a multilayer circuit.
In order to achieve an excellent pattern shape through use of these manufacturing processes, both the resist pattern formation process and the etching processes must be executed properly. FIG. 3 shows one example of the relationship between the resist pattern and the film pattern after etching (source; “Electron Beam Testing Handbook,” Japan Society for the Promotion of Science, the 132nd Committee of “Applications of Charged Particles to Industries,” Research Material No. 98, p. 255). There is a fixed relationship between the shape of the resist pattern and the shape of the film pattern, assuming that the etching conditions for the film are the same; and, to obtain a film pattern having a predetermined shape, the resist pattern must have a predetermined shape as well.
When a new process is going to be set along with other occasions, “a condition-finding operation” to find the focus and exposure that yield the optimal resist pattern shape is conducted in such a way that, as seen in FIG. 4, a wafer, on which a pattern is exposed and developed with the focus and the exposure being varied, respectively, for each shot (one time of an exposure unit), is produced (usually such a wafer is called a “focus exposure matrix (FEM),” and hereinafter referred to as the “FEM wafer”), and the dimensions of the resist pattern of each shot are measured and the finished state is inspected.
Incidentally, Japanese Patent Application Laid-open No. H11-288879 discloses a system for supporting the condition-finding operation. Through this operation, the exposure and focus conditions that allow wider manufacturing margins are determined, and a product wafer is exposed using these conditions. However, due to diverse process variations (such as a variation in the photosensitivity of the resist, variation in the film thickness of an anti-refection film below the resist, drift in the exposure equipment), among product wafers, some wafers are found which have a resist pattern deviating from the predetermined shape to be obtained under the conditions determined by the condition-finding operation. It is the object of the dimension measurement at the step (d) in FIG. 2 that detects these defective wafers. In the conventional processing, a change in the resist shape caused by process variations is intended to be compensated by correction of the exposure.
On the other hand, the processed shape of the pattern that is required to be obtained in the etching process depends on the product classes of the device concerned and its processes. For example, in an element isolation process, it is often the practice that a part of the pattern is tapered to improve the embedding performance, and a corner part thereof is rounded to prevent the electric field from concentrating at the corner part, as shown in FIG. 41(a). Further, in the gate wiring process shown in FIG. 41(b), since the wiring width (gate length) of a gate oxide film part at the bottom of the wiring is important, dimensional accuracy at the bottom of the thin film is especially required. Moreover, these shapes in conformity with the object must be processed uniformly over the whole wafer surface. Because the etching utilizes chemical reactions, the equipment and gasses to be used are different in accordance with the material that is being subject to processing.
Control of a tilt angle of a side wall of the pattern, as shown in FIG. 41(a), is carried out by establishing a balance between generation of a side-wall protective coating made with a byproduct that is being generated in the processing and the etching; therefore, it is largely affected by the flow rates of the process gasses being used, the ratio of these flow rates, their pressures, etc. Besides these parameters, the etching time, the electric power of the plasma discharge, the bias electric power applied to the sample, the wafer temperature, etc. mutually affect, in a complicated manner, not only the side-wall shape, but also the etching rate, the dimensional difference with respect to the resist pattern, the shape of the corner part, etc Therefore, the processing condition determined at the time when a new product is intended to be manufactured and the like depends on the experience and craftsmanship of experts to a large degree. Moreover, it is difficult to check the shape with high precision, and so a shape check is carried out by cross-sectional observation in most cases.
Even when processing conditions are fixed by the determined condition, it is often the case that a desired shape cannot be obtained due to drift in the property of the equipment etc. Especially, regarding a process variation resulting from adhesion of byproducts generated during the process onto the interior of a chamber, the wear of parts, etc., countervailing measures have been devised by the practice of periodic cleaning and replacement of parts. However, since the process is affected by a lot of parameters, as described above, a change in the pattern shape caused by these variations cannot be avoided. Detecting such a change is the purpose of the post-etching dimension measurement step (g) of FIG. 2. However, among conventional processes, there is no effective means of quantitatively evaluating the slope angle, the shape of a corner part, etc. of the pattern, as shown in FIG. 5, in an invasive manner.
In the above-mentioned discussion of the conventional processing, there are problems, as will be described in the following items (1)-(3). (1) There are cases where an abnormality in the shape of the pattern cannot be detected with dimension values alone. (2) There are cases where the countervailing measures taken against the formation of a shape abnormality in the pattern cannot be devised with the dimension values alone. (3) Information that indicates the process variations quantitatively and that is required to monitor the lithography process and maintain the process stably cannot be obtained.
Hereafter, how the above-mentioned problems come into being will be described.
FIG. 5 and FIG. 6 show possible variations of a change in the cross section of a resist pattern. FIG. 5 schematically shows the change in the resist cross section in the case where the exposure produced by exposure equipment is fixed and the focus is altered. The focus position is moved in a positive direction for the curves (a), (b), (c), (d), and (e) in this order, Further, FIG. 6 schematically shows the change in the resist cross section in the case where the focus of the exposure equipment is fixed and the exposure is altered. The exposure is increased for the curves (a), (b), (c), (d), and (e) in this order.
Since it is common for the dimension measurement on the critical dimension SEM to be performed by using a line profile of a secondary electron image, first a relationship between the cross section and the line profile of the secondary electron intensity will be introduced, as described in the “Electron Beam Testing Handbook,” Japan Society for the Promotion of Science, the 132nd Committee of “Applications of Charged Particles to Industries,” Research Material No. 98, p. 261.
In FIG. 7, (A) When the electron beam irradiates the substrate, the intensity of the detected secondary electron signal indicates a constant value governed by the secondary electron emission efficiency of the substrate material. (B) As the beam irradiation point approaches the pattern, the signal strength decreases due to a decrease in the collection efficiency of the secondary electrons, that results from an increase in secondary electrons that collide against a slope of the pattern, among the generated secondary electrons. (C) The secondary electron signal strength marks its minimum at a position that is shifted by a half beam diameter from the bottom edge of the pattern toward the outside. (D) After passing the point C, the intensity increases abruptly with almost linear dependence because of a change in the secondary electron emission efficiency that corresponds to the change in the slope angle of the sample. (E) As the beam irradiation point approaches the vicinity of a top edge, the increase of the signal strength becomes milder because of the difference of collection efficiencies of emission secondary electrons at irradiation points of the slope. (F) The secondary electron signal strength marks its maximum at a position that is shifted by a half beam diameter from a top edge of the pattern toward the outside. (G) After passing the point F, the strength gradually decreases and settles to a constant value that is governed by the secondary electron emission efficiency of the pattern material.
To measure the dimensions from the line profile, it is necessary to detect the edges of the line profile. For an edge detecting technique that is installed on the critical dimension SEM, there are, for example, a method of detecting a largest slope position as the edge, as shown in FIG. 8(a); a method of detecting the edge at a predetermined threshold value, as shown in FIG. 8(b); and, a line approximation method wherein lines are applied to the edge parts and the substrate parts and then intersections of these lines are detected as edges, as shown in FIG. 8(c).
It is the conventional processing of FIG. 2 that detects the edges from the line profile of the secondary electron image of the resist pattern having a cross-section as shown in FIG. 5 and FIG. 6 by means of one of the techniques shown in FIGS. 8(a) to 8(c), measures the dimensions, and judges whether the resist cross section is good or bad according to the results. Assuming that the conventional process uses the maximum slope method of FIG. 8(a) or the threshold method of FIG. 8(b) with the threshold being set to 50%, the dimensions obtained for curves (b) to (e) in FIG. 5 hardly differ from one another; therefore, the conventional process that judges whether the resist cross section is good or bad with dimensional values alone is quite capable of overlooking the change in the shape (→problem (1)).
Alternatively, assuming that the conventional process uses the line approximation method of FIG. 8(c), although overlooking the above-mentioned type of variation would not occur because a dimension comparable to a bottom width of the cross section of a trapezoid is obtained, the shape of curve (b) in FIG. 5 and the shape of curve (a) in FIG. 6, both of which have an almost equal bottom width, cannot be distinguished from each other. If the state of the resist pattern corresponds to curve (b) in FIG. 5, the parameter that should be corrected is the focus. However, in the conventional process, since the exposure is corrected on the basis of a result of the measurement of dimensions, it can hardly be expected that the pattern shape will be improved with correction of the exposure (→problem (2)).
Besides, in order to attain stabilization of the lithography process, it is essential to detect, not large changes, such that measurement results deviate from the standard specification, but a slight change before the variation comes to it, that is, to detect a slight difference between the measurement results and the optimal shape and thereby to sense the sign of the process drift. Then, the results must be used to adjust the exposure conditions for a product to be put into the production line next or to adjust the conditions of the etching process waiting as the next one. However, in the above-mentioned conventional process, because the resist cross section cannot be always monitored accurately, such process control is actually difficult to execute (→problem (3)).
In the passed-over age of large device size (FIG. 9(a)), the effect of the taper of the side-wall upon the post-etching film pattern shape was insignificant. However, since the aspect ratio of the pattern (a ratio of the height dimension to the width dimension of the pattern) has become large (FIG. 9(b)) to accommodate a reduction in device size in recent year the miniaturization has reached a stage where the effect of the taper of the side-wall cannot be ignored. With the conventional process that cannot take into consideration the taper of the side-walls, the lithography process is becoming invalid.
In the foregoing, en example of the conventional photo process has been explained, but also the conventional etching process is subject to similar problems. As shown in FIGS. 41(a) and 41(b), since the targeted shape is different according to the pattern the conventional measurement of dimension cannot provide the necessary shape information. Moreover, since the conventional process cannot detect a process abnormality in the photo process accurately, there is a problem in that, when a problem occurs in the post-etching pattern shape, it is hardly possible to judge which process is responsible for the problem, the photo process or the etching process.