Method and apparatus for measuring pattern dimension

Abstract: A pattern dimension measuring method for measuring the dimension of a measuring portion of a pattern of a specimen (10) placed on a specimen stage (1d) by controlling a deflector (1b) of a scan type electron microscope (1) capable of setting a desired inclination angle of one of the specimen stage (1d) and a lens barrel, applying an electron beam to the measuring portion of the specimen (10), and image processing a secondary electron signal from the measuring portion, the method comprising: a first step of calculating the distance between top edges of the measuring portion of the pattern by image processing the secondary electron signal when the electron beam is applied to the measuring portion at an inclination angle of zero; a second step of obtaining the number of pixels at a taper portion of the measuring portion of the pattern by image processing the secondary electron signal when the electron beam is applied to the measuring portion at a first predetermined inclination angle which allows to observe the bottom edges; a third step of obtaining the number of pixels at the taper portion by image processing the secondary electron signal when the electron beam is applied to the measuring portion at a second predetermined inclination angle different from the first inclination angle which allows to observe the bottom edges; a fourth step of calculating the taper angle and height of the pattern in accordance with the numbers of pixels of the taper portion obtained at the second and third steps and the first and second predetermined inclination angles; and a fifth step of calculating the distance between the bottom edges of the pattern and the difference ratio between the top edge and bottom edge distances in accordance with the results calculated at the fourth step.

Description: The present invention relates to a method of and an apparatus for measuring a pattern dimension using an electron beam. EP-A-0 345 772 discloses a method of and an apparatus for measuring the measuring portion of a pattern of a specimen placed on a rotating specimen stage by controlling a deflector of a scan type electron microscope (SEM) capable of setting a desired inclination angle of one of the specimen stage and a lens barrel, applying an electron beam to the measuring portion of the specimen, and image processing a secondary electron signal from the measuring portion, said method and apparatus comprising: image processing means for processing a secondary electron signal generated from the measuring portion of the specimen placed on said specimen stage upon application of an electron beam to said measuring portion; a computer for calculating the taper angle and depth or height of said pattern and the profile of a taper portion in accordance with an output from said image processing means when said electron beam is applied to said measuring portion at a predetermined inclination angle. The electron beam is scanned onto said pattern with said scanning electron microscope in the directions perpendicular and parallel to the longitudinal direction of said pattern. The specimen shown in this document may have a "usual" taper, that is a top width (distance of the top edges) which is smaller than the bottom width (distance of the bottom edges), such that the SEM may observe the bottom edges when an electron beam is applied to the object from above. US-A-4 725 730 discloses a scanning electron microscope (SEM) for automatically measuring the height and the width of a step-shaped object having a "usual" taper, as defined above. The method and system described in this document comprises the steps of means for detecting the images of the step-shaped object from at least two directions so as to form pictures corresponding to the images detected from different directions, determining the distances between the upper and lower edges of a step in respective pictures; determining the angles of inclination of the edge lines with respect to the axis of the pictures, determining the angles of inclination of the standard plane fixed to the step formed object with respect to the viewing directions, and determining, as significant factors of the shape to be determined, the height and width of the step-shaped object. For facilitating to understand the present invention the technical background thereof will be briefly outlined hereinafter. In a conventional technique for measuring a pattern dimension using a secondary electron signal generated upon application of an electron beam to a pattern to be measured, various automatic measuring algorithms have been proposed for a pattern of a normal taper shape, such as a linear approximation method, a curve approximation method, a threshold method, a maximum inclination method, a difference method and the like. Such algorithms are selectively used depending upon the material and shape of a pattern and the material of an underlying layer. For a pattern of a reversed taper shape such as a line pattern having a larger top dimension than a bottom dimension or a space pattern having a smaller top dimension than a bottom dimension, the distance between bottom edges has been determined from the distance between top edges or from the cross section obtained by cutting a specimen, because the bottom edges cannot be observed when a beam is applied vertically. An automatic measuring algorithm conventionally used can detect with high precision the distance between bottom edges of a normal taper shape pattern. However, for a reversed taper shape pattern, the distance between bottom edges cannot be measured if a beam is applied vertically. Manual measurement also allows measurement of only the distance between top edges. A reversed taper shape pattern is found, for example, in a negative type electron beam sensitive resist pattern, an Al-Si wiring pattern and the like. In the case of an Al-Si wiring pattern of a reversed taper shape, as the distance between adjacent patterns becomes short, the probability of generation of a "blowhole" increases during a manufacturing process for an interlayer insulator film. It is therefore necessary to quantitatively check the degree of reversed taper. It is therefore required to obtain not only the distance between top edges of a reversed taper shape pattern, but also the distance between bottom edges and a difference ratio (dimension reduction ratio) between the top edge and bottom edge distances. It is possible to measure the degree of reversed taper by cutting a specimen and observing its cross section. However, it is necessary to break the specimen. In addition, there is a problem that it is difficult to quantitatively observe the cut section of a pattern if it is not exactly vertically cut relative to the pattern. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of and an apparatus for measuring a pattern dimension capable of measuring the dimension and a cross sectional area of a pattern of a reversed taper shape without breaking a specimen. According to one aspect of the present invention, there is provided a pattern dimension measuring method according to claim 1. According to the pattern dimension measuring method of the first invention, the distance between top edges of a pattern to be measured is calculated by a first step, and the number of pixels of a taper portion of the pattern is obtained for first and second predetermined inclination angles at second and third steps, respectively. In accordance with the numbers of pixels of the taper portion of the pattern obtained at the second and third steps and the first and second predetermined inclination angles, the taper angle and height of the pattern are calculated at a fourth step. In accordance with the calculated results at the fourth step, the distance between bottom edges of the pattern to be measured and the difference ratio between the top edge and bottom edge distances are calculated at a fifth step. In accordance with a change in strength of a secondary electron signal from the taper portion, the profile of the taper portion is obtained at a sixth step. In accordance with the distance between bottom edges and the taper portion profile obtained at the fifth and sixth steps, the cross sectional area of the pattern to be measured is calculated at a seventh step. In the above manner, without breaking a specimen, it is possible to correctly measure the dimension and cross sectional area of a fine pattern of a reversed taper shape. According to another aspect of the present invention, there is provided a pattern dimension measuring apparatus according to claim 5. According to the pattern dimension measuring apparatus of the second invention, a first calculation means calculates the width or distance between top edges of a pattern to be measured, in accordance with an output from an image processing means when an electron beam is applied at a zero inclination angle. A second calculation means calculates the taper angle, height, bottom edge width or distance of the pattern and a dimension reduction ratio between the top edge and bottom edge distances in accordance with an output from the image processing means when an electron beam is applied at first and second predetermined inclination angles. In accordance with an output from the second calculation means, a third calculation means calculates the cross sectional area of the pattern. In the above manner, without breaking a specimen, the dimension and cross sectional area of a fine pattern of a reversed taper shape can be correctly measured. As described above, according to the present invention, the dimension and cross sectional area of a fine pattern of a reversed taper shape can be correctly measured without breaking a specimen. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a block diagram showing the structure of a pattern dimension measuring apparatus according to an embodiment of this invention; Fig. 2 is a flowchart explaining the operation of the embodiment; Figs. 3 to 5 are graphs showing signal waveforms obtained by the image processing means of this invention; Fig. 6 is a graph showing a cross section of a pattern measured with the pattern dimension measuring apparatus shown in Fig. 1; and Fig. 7 shows a cross section of a pattern of a reversed taper shape. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows the structure of a pattern dimension measuring apparatus according to an embodiment of this invention. The pattern dimension measuring apparatus of this embodiment is constructed of a scanning electron microscope 1 (hereinafter called an SEM) including an electron optical column 1a, deflector 1b, detector 1c, and stage 1d for holding a specimen 10, an image processing means 2 for receiving a video signal from the detector 1c and performing various image processing, a computer 3 for controlling the image processing means and the stage and for performing calculations for measurement of dimensions and shapes, an image monitor 4, and a stage controller 5. The structure and operation of this embodiment will be described with reference to Figs. 2 to 5 by taking as an example the case where a pattern of a reversed taper shape shown in Fig. 7 is measured. The pattern shown in Fig. 7 is assumed that the right and left of the cross section thereof are symmetrical relative to the central axis. First, a specimen 10 is placed on the stage 1d and the inclination angle ϑ of the stage 1d is set to zero (perpendicular to the incident beam direction) by using the stage controller 5 (at step F21 of Fig. 2). After the measurement magnification M is set, the image processing means 2 sends deflection control signals for SEM 1 to the deflector 1b to thereby scan the electron beam, the deflection control signals including an Horizontal Scan (HS) signal, Vertical Scan (VS) signal, Horizontal Blank (HB) signal, Vertical Blank (VB) signal, and Control (CONT) signal (step F22A). The scanned electron beam impinges to the specimen 10 placed on the stage 1d so that secondary electrons are emitted from the specimen 10 which are then detected by the detector 1c. A detection output (secondary electron signal) from the detector 1c is sent to the image processing means 2 whereat it is sampled at a predetermined sampling timing and A/D converted (step F22B), and thereafter subjected to a mean addition process, a spatial filtering process and a linear image emphasizing process, the processed results being stored in a frame memory of the image processing means 2 having 256 tonal steps, and the signal waveforms being displayed on the image monitor 4 (step F22C). The processes from the step F22A to F22C are collectively called an image data input process hereinafter where applicable. In accordance with the signal waveforms thus obtained and shown in Fig. 3, there is obtained the number P LT of pixels at the top portion of the pattern, and the top portion width or distance L T is calculated by the computer 3 (step F23). Next, by using the stage controller 5, the inclination angle ϑ of the stage 1d of SEM 1 is set to a predetermined inclination angle ϑ₁ to allow observing the bottom edges (step F24), and thereafter, the above-described image input process is executed (step F25). In accordance with the obtained signal waveforms shown in Fig. 4, the number P₁ of pixels corresponding to the taper portion width of the pattern is obtained (step F26). Thereafter, by using the stage controller 5, the inclination angle ϑ of the stage 1d is set to a predetermined inclination angle ϑ₂ (different from ϑ₁) to allow observing the bottom edges (step F27), and thereafter the similar image input process is executed (step F28). In accordance with the obtained signal waveforms shown in Fig. 5, the number P₂ of pixels corresponding to the same taper portion width is obtained (step F29). In accordance with the obtained pixel numbers P₁ and P₂, the taper angle , and height H (film thickness) are calculated by the computer 3 using the following equations (1) and (2) derived from the stereoscopic principle (step F30): where M represents a measurement magnification, and C represents a length on the image monitor 4 of one pixel at the measurement magnification M. In accordance with the calculated taper angle , height H, and top portion width or distance L T , the bottom portion width or distance L B of the pattern is calculated by the computer 3 using the following equation (3) (step F31): L B = L T - 2·H·tan(-90) Similarly, a dimension reduction ratio F between the top edge and bottom edge widths or distances is calculated by the following equation (4) (step F33): If the taper portion can be approximated by a straight line, the cross sectional area S of the pattern of the reversed taper shape is calculated by the computer using the following equation (5): If the taper portion cannot be approximated by a straight line, first the height h (x i ) of the taper portion at the distance x i from the bottom edge is approximated using the corresponding secondary electron signal S (x i ) from the following equation (6) (step F33): where n represents the number of pixels corresponding to the taper portion width. From the relation between x i and h(x i ), the distance X (h i ) from the bottom edge at the height h i can be obtained. Using X (h i ), the width D (h i ) of the pattern at the height h i is calculated using the following equation (7): D (h i ) = L B + 2·X(h i ) The cross sectional area S of the pattern is therefore calculated using the following equation (8) (step F34): where Δh represents the increment of height per one pixel at the height h i . The measurement results of a pattern of a reversed taper shape obtained using the pattern dimension measuring apparatus of this embodiment are shown in Fig. 6. The measurement results indicated that the taper angle  = 92.9 degrees, height H = 1.04 µm, top width or distance L T = 0.93 µm, and bottom width or distance L B = 0.82 µm. The measurement results of the pattern dimension obtained from the cross section photograph of the cut pattern indicated that the taper angle  = 93 degrees, height H = 1.03 µm, top width or distance L T = 0.94 µm, and bottom width or distance L B = 0.83 µm. It is thus possible to correctly measure the dimension and cross sectional area of a pattern of a reversed taper shape by using the pattern dimension measuring apparatus of this embodiment. In the above embodiment, the description is made assuming that taper angles in right and left sides of the pattern cross section are the same. However, in general, taper angles in both sides are different. For such a pattern, the measurement is performed as follows: For the case where the rotating stage is not used, after determining the taper shape at one side using the first and second predetermined angle, the stage is inclined in the opposite angular direction to obtain third and fourth predetermined inclination angles ϑ₃ and ϑ₄. Of course, these angles are selected under the condition that they allow observing the bottom edges. The tape shape (height, taper angle, profile and so on) at the other side is calculated using the angles ϑ₃ and ϑ₄, and the number of pixels P3 and P4 obtained by the same method as described above. Then the bottom dimension is calculated using the taper shapes at both sides and the top dimension. For the case where the rotating stage is used, the stage is roated after obtaining P1 at the first predetermined inclination angle ϑ₁. If the bottom edge can be observed after rotating the stage for 180 degrees, there is no need to change the inclination angle of the stage, and the third predetermined inclination angle ϑ₃ at this time is the same as the first predetermined inclination angle ϑ₁. Similarly, the fourth predetermined inclination angle ϑ₄ can be selected the same as the second predetermined inclination angle ϑ₂ by rotating the stage.

Claims: A pattern dimension measuring method for measuring the dimension of a reversed tapered pattern of a specimen (10), said reversed tapered pattern having a larger top width (L T ) and a smaller bottom width (L B ), so that the bottom edges of said pattern cannot be observed vertically from above, placed on a specimen stage (1d) by controlling a deflector (1b) of a scan type electron microscope (1) capable of setting an inclination angle (Θ) between the specimen stage (1d) and a lens barrel (1a), applying an electron beam to a measuring portion of the specimen (10), and image processing a secondary electron signal from the measuring portion, said method comprising: a first step of calculating the width (L T ) between top edges of said measuring portion of said reversed tapered pattern by image processing said secondary electron signal when said electron beam is applied to said measuring portion at an inclination angle (Θ) of zero; a second step of obtaining the number of pixels (P₁) at a tapered side wall of said measuring portion of said reversed tapered pattern by image processing said secondary electron signal when said electron beam is applied to said measuring portion at a first predetermined inclination angle (Θ₁) which allows to observe the bottom edges; a third step of obtaining the number of pixels (P₂) at said tapered side wall by image processing said secondary electron signal when said electron beam is applied to said measuring portion at a second predetermined inclination angle (Θ₂) different from said first inclination angle (Θ₁) which also allows to observe the bottom edges; a fourth step of calculating an angle  of the tapered side wall and the height (H) of said pattern in accordance with the numbers of pixels (P₁,P₂) of the tapered side wall obtained at said second and third steps and the first and second predetermined inclination angles (Θ₁,Θ₂); and a fifth step of calculating the width (L B ) between the bottom edges of said reversed tapered pattern and a dimension reduction ratio F = L B - L T )/L B , wherein L B is the bottom width and L T is the top width of said pattern, in accordance with the results calculated at said fourth step. A method according to claim 1 further comprising a sixth step of obtaining, if said taper portion of said pattern cannot be approximated by a straight line, the profile of said taper portion, a contour of said tapered wall in accordance with the strength change of said secondary electron signal from said taper portion. A method according to claim 1, wherein if the angles of the tapered side walls at the right and left side of said pattern are different, after a first dimension measurement at one side using the first predetermined inclination angle (Θ₁) and the second predetermined inclination angle (Θ₂), a second dimension measurement at the other side is performed using a third predetermined inclination angle (Θ₃) and a fourth inclination angle (Θ₄) which are obtained by rotation of a rotating stage or change of stage angle in opposite angle direction. A method according to claim 2, further comprising a seventh step of calculating the cross sectional area (S) of said measuring portion of said pattern inn accordance with the bottom width (L B ) and said contour of said tapered wall obtained at said fifth and sixth steps, respectively. A reversed tapered pattern dimension measuring apparatus comprising: a scanning electron microscope (1) capable of setting an inclination angle (Θ) by moving at least one of a specimen stage (1d) placing a specimen (10) and an electron optical column (1a); image processing means (2) for processing a secondary electron signal generated from the measuring portion of the specimen (10) placed on said specimen stage (1d) upon application of an electron beam to said measuring portion; first calculation means (3) for calculating the width (L T ) between top edges of said measuring portion of said reversed tapered pattern in accordance with an output from said image processing means (2) when said electron beam is applied to said measuring portion at an inclination angle (Θ) of zero; second calculation means (3) for calculating an angle () of a tapered side wall, the height (H) of said reversed tapered pattern, the width (L B ) between the bottom edges of said pattern, a ratio (F) of a difference (L B -L T ) between the top edge and bottom edge widths (L T ,L B ) and the bottom width (L B ), and a contour of the tapered wall in accordance with outputs from said processing means when said electron beam is applied to said measuring portion at a first inclination angle (Θ₁) and a second inclination angle (Θ₂), both inclination angles,(Θ₁,Θ₂) allowing to observe the bottom edges of said reversed tapered pattern and being of different value from each other; and third calculation means (3) for calculating the surface area (S) of said pattern in accordance with the outputs from said second calculation means (3).

Inventors: KAGA YASUHIRO, KOMATSU FUMIO, KAGA, YASUHIRO, KOMATSU, FUMIO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: G01B15
IPC codes: G01B15, G01Q80