METHOD FOR MEASURING DIMENSIONS OF FINE PATTERN

Abstract: A method of measuring the dimensions of a fine pattern, comprising: detecting (step 705) a surface area and a profile of a displayed pattern image, detecting (707) the center of gravity of the pattern image, calculating (709) an equivalent diameter of a circular pattern having the same surface area as the pattern image, determining (7l3) a mean value of pattern lengths of lines which pass through the center of gravity of the pattern image and which inter­ sect the pattern image at two intersecting points, and comparing (7l5) the mean value with the equivalent diameter of the pattern image. The pattern lengths are defined as distances between the two intersecting points.

Description: The present invention relates to a method for measuring dimensions of a fine pattern of, for example, integrated circuits (IC's) or semiconductors, etc. More precisely, the present invention relates to a method for measuring the dimensions of a fine pattern or patterns displayed on a display unit, such as a monitor TV by an optical image forming system. In the manufacture of IC's or LSI's etc., it is very important to control the dimensions of fine patterns, for example, contact holes or through holes, etc., formed on a wafer of an IC or LSI, etc., in order to ensure that they conform to design dimensions, and thus produce high quality products. The fine patterns, such as contact holes, are usually reproduced on an enlarged scale on a screen of a monitor TV by an optical image forming system. Namely, enlarged dimensions of the fine patterns are optically displayed on the TV screen. In view of the technical difficulties in a process of manufacturing fine circular patterns (e.g. contact holes) using a so-called CAD (computer-aided design) system, the fine patterns are usually designed to be rectangular- or square-shaped, as is well known. In practice, however, circular patterns are formed on the wafer, etc., in place of the intended angular patterns, mainly because of a limited resolving power of the optical system used, or a limited etching precision or development, during the wafer manufacturing process. Namely, it is impossible or next to impossible to exactly reproduce an angular fine pattern as designed. It should, however, be noted that such an inevitable phenomenon, i.e., formation of the circular patterns contrary to be design, does not in itself cause serious impairment of the quality of the products, since the shape per se of the contact holes is not important, to the establishing of a reliable electrical connection through the contact holes. When such circular shape fine patterns are displayed on the monitor screen by the optical image forming system, slightly collapsed or deformed circular images appear on the monitor screen, depending on a deviation of the optical axis of the optical system, or a deviation of a light beam emission from a light source or the like, causing the peripheral edges of the patterns to be blurred or uneven in particular, thus resulting in a formation of substantially elliptically shaped pattern images on the monitor screen. Namely, the circular patterns on the wafer are reproduced as elliptical patterns on the screen. This often occurs in practice. The deviation or deformation of the pattern images on the monitor screen, which is mainly caused by a failure or inaccuracy of the positioning or adjustment of the optical image forming system, as mentioned above, results in incorrect dimensional measurement of the patterns. An embodiment of the present invention may, therefore, provide a precise and reliable measuring method which is free from the aforementioned drawbacks of the prior art. According to the present invention, there is provided a method for measuring a fine pattern, comprising detecting a surface area and a profile of a displayed pattern image, determining a center of gravity of the displayed pattern image, determining an equivalent diameter of a circular pattern having the same surface area as the displayed pattern image, determining a mean value of pattern lengths of lines which pass through the center of gravity of the displayed pattern image and which intersect the displayed pattern image at two intersecting points, and comparing the obtained mean value with the equivalent diameter of the displayed pattern image. The pattern lengths are conveniently defined as distances between the two intersecting points. Reference is made, by way of example, to the accompanying drawings, in which: Fig. l is a schematic view of an optical image forming system applicable to the present invention; Fig. 2 is a sectional view of a wafer having contact holes as fine patterns; Fig. 3 is a schematic view of a circular pattern and elliptical patterns for explaining the deformation of the circular pattern; Fig. 4 is a schematic view of a fine pattern for explaining the measurement of dimensions of pattern lengths; Fig. 5 is an enlarged view of a part of an elliptical pattern image on picture elements of a CCD image sensor applicable to the present invention Fig. 6 shows four different lines passing the center of gravity of the pattern image for explaining the detection of pattern lengths; Fig. 7 is a flow chart of measuring steps method embodying the present invention; and, Fig. 8 is a detailed flow chart of the detec­ tion of pattern lengths shown in Fig. 7. Figure 2 shows an IC wafer l0 having contact holes ll and l3, to be measured. The wafer l0 has an insulation layer l5 thereon, and having a contact hole or contact holes ll in which an electrode l7 is formed. On the insulation layer l5 and the electrode l7 is formed another insulation layer l9 having a contact hole or contact holes l3 in which an electrode 2l is formed, to establish an electrical connection between the two electrodes l7 and 2l through the contact holes ll and l3. The contact holes ll and l3 are fine circular patterns, as mentioned above. When the dimensions, particularly the diameters of the circular patterns ll and l3, are displayed on a screen 5l of a monitor TV 50 (Fig. 1), the displayed pattern l00 (Fig. 1) is not a true circle but is slightly deformed, i.e., an elliptical pattern, as mentioned before. Figure l shows an optical image forming system in which a light beam from a light source (not shown) is focused on the wafer l0 having the insulation layer l5 having the contact hole ll to be measured, by means of a focusing lens 3l. When the contact hole(s) ll are measured, measuring is effected before the insulation layer l9 is formed on the insulation layer l5 and on the electrode l7. The light beams reflected by the wafer l0 and the insulation layer l5 are displayed on a charge-­ coupled device image sensor (CCD) image sensor) 35 by an imaging lens 33. The CCD image sensor 35, produces electrical charges when a light beam is focused thereon, to create image signals. The output signal of the CCD image sensor 35 is fed to a control unit 40 which feeds the image signals to the monitor TV 50, so that a pattern image l00 is displayed on the screen 5l of the TV 50 in accordance with the fine pattern formed on the wafer l0. Then, the pattern l00 displayed on the monitor screen 5l is measured by a measuring method embodying the present invention. As mentioned before, the circular pattern Pl formed on the wafer l0 is displayed as an elliptical image patterns P2 and P3 on the monitor screen, as shown in Fig. 3. Consequently, the actually measured dimensions (diameters) Ll and L2 are different from the real diameter L of the circular pattern Pl. Accordingly, it can not be determined whether the dimensions of the fine pattern Pl formed on the wafer l0 are identical to a design value. The inventors of the present invention have discovered that, when pattern dimensions which are calculated from the surface area of the pattern displayed on the monitor screen are substantially identical to mathematical mean values of pattern dimensions which are measured along lines passing through the center of gravity of the pattern, the pattern image displayed on the monitor screen substantially correctly represents the actual pattern formed on the wafer. Figure 7 shows a flow chart of the process carried out in the control unit 40 (Fig. l), in a method embody­ ing the invention. In step 70l, the image data from the CCD image sensor 35 are fetched into the control unit (computer) 40, and then at step 703, a surface area A of the pattern image is displayed on the monitor screen 5l, and is detected by the CCD image sensor 35 at step 705. The CCD image sensor 35 has a large number of photoelectric devices 35a (Fig. 5) which are in a matrix arrangement and which are given predetermined addresses (i, j) in an X-Y coordinate. The surface area of the pattern image displayed on the monitor screen can be obtained by counting the number of photoelectric devices (picture elements) 35a activated when the light beam impinges thereon. The picture elements 35a are designed so that when, for example, the light beam impinges on more than half the surface area of each picture element 35a, that element 35a issues image signals (image data). The accuracy of the detection of the surface area of the pattern image displayed on the monitor screen can be easily increased by increasing the density of the matrix arrangement of the picture elements 35a. The outline (profile) of the pattern image displayed on the monitor screen can be also determined by the picture elements 35a, which are activated when the light beam impinges on more than half the surface area thereof. At step 707, the center of gravity G(i G , j G ) of the pattern image is detected. The center of gravity G can be easily mathematically obtained by the following equations: wherein a i,j are image data which is "l" when the picture elements 35a are activated, i.e., when the light beam impinges on more than half the surface area of the picture elements 35a, and which is "0" when the picture elements 35a are not activated, i.e. when the light beam does not impinge at all on the picture elements 35a or the light beam impinges on less than half the surface area thereof, and wherein n = the number of picture elements 35a along the X-axis (ordinate) and the Y-axis (abscissa) in the coordinate (Fig. 5). In the illust­ rated embodiment, the number of picture elements 35a is identical in the ordinate and in the abscissa. For example, n = 2¹⁰ = l024. Alternatively, a difference in the number of the picture elements 35a between in the ordinate and in the abscissa can be made. The origin O of the coordinate is represented by (0,0) in Fig. 5. Subsequently, at step 709, an equivalent diameter D A of the generally elliptical pattern image Pl or P2 displayed on the screen is calculated. The equivalent diameter D A is a diameter of an equivalent circle having the same surface area A as the pattern image measured at step 705. The equivalent diameter D A can be easily obtained from the following equation: D A =2√A/π ...(3) wherein, A is a surface area of the pattern image measured at step 705. Then, at step 7ll, actual lengths D₁ , D₂ , ... D n of lines x₁ , x₂ , ... x n (Fig. 4) which pass the center of gravity G and extend between two extremities at which the lines intersect the profile of the pattern image, are measured. Details of step 7ll for the detection of the pattern length are shown in Fig. 8. In Fig. 8, the detection of only four pattern lengths D₁ , D₂ , D₃ , and D₄ of four lines x₁ , x₂ , x₃ and x₄ , which are spaced at an equiangular distance of 45°, is shown for clarification. The four lines passing the center of gravity G are shown in Fig. 6, in which the line x₁ has a line parallel to the Y-axis (abscissa), and accordingly, has the coordinate represented by (i G , j). namely, i is a constant (i = i G ). The line x₁ is successively shifted by 45° in the counterclockwise direction to assume the positions x₂ , x₃ , and x₄ , respectively. In Fig. 8, first, at step 80l, the line x₁ is determined. The line x₁ intersects the X-axis at a point S₀ , which is represented by (i G , 0). At step 803, if a i,j is "0" the control proceeds to step 805, which counts the abscissa up by l and returns the flow to step 803. At step 803, if a i,j is "l", the control proceeds to step 807, which causes values (i Sl , j Sl ) of a first intersecting point S₁ in which the line x₁ first intersects the profile of the pattern image to be i G and j, respectively. The data a i,j becomes "l" only when the line extends within the profile of the pattern image. Then, at step 809, if the data a i,j is "l", the control proceeds to step 8ll, which counts the abscissa up by l. If a i,j is "0" at step 809, values (i S2 , J S2 ) of a second intersecting point S₂ , at which the line x₁ again intersects the profile of the pattern image, are caused to be i G and j, respectively at step 8l3. the data a i,j becomes "0" when the line x₁ intersects the profile of the pattern image at the second intersecting point S₂. Thus, the pattern length D₁ of the line x₁ is obtained from the distance between the points Pl and P2. Namely at step 8l4, D₁ = j S2 - j Sl . Then, at step 8l5, the line x₂ is determined, which is angularly spaced from the line x₁ by 45°. The line x₂ intersects the X-axis at a point S₀ which is represented by (i G + j G , 0), since the ordinate of the point S₀ is (i G + j G ). Subsequently, if a i,j is "0" at the step 8l7, the control proceeds to step 8l9 which counts both the ordinate and the abscissa down and up by l, respectively and returns the flow to step 8l5. At step 8l5, if a i,j is "l", the control proceeds to step 82l, which causes values (i Sl , j Sl ) of a first intersecting point S₁ in which the line x₂ first inter­ sects the profile of the pattern image to be i and j, respectively. Then, at step 823, if the data a i,j is "l", the control proceeds to step 825, which counts the ordinate and the abscissa down and up by l, respectively. If a i,j is "0" at step 823, values (i S2 , j S2 ) of a second intersecting point S₂ , at which the line x₂ again intersects the profile of the pattern image, are caused to be i and j, respectively at the step 827. Thus, the pattern length D₂ of the line x₂ is obtained form the distance between the points Pl and P2, i.e. from D₂ = (i S1 - i S2 )² + (j S2 - j S1 )², at step 829. Similarly, at step 83l, the third line x₃ is determined, which is angularly spaced from the second line x₂ by 45°. The line x₃ intersects the Y-axis (abscissa) at the point S₀ , which is represented by (0, j G ). If a i,j is "0" at step 833, the control proceeds to step 835 which counts the ordinate up by l and returns the flow to step 833. At step 833, if a i,j is "l", the control proceeds to step 837, which causes values (i Sl , j Sl ) of a first intersecting point S₁ in which the line x₃ first intersents the profile of the pattern image to be i and j G , respectively. Then, at step 839, if the data a i,j is "l", the control proceeds to step 84l which counts the ordinate up by l. If a i,j is "0" at step 839, values (i S2 , j S2 ) of a second intersecting point S₂ , at which the line x₃ again intersects the profile of the pattern image, are caused to be i and j G , respectively at step 843. The data a i,j becomes "0" when the line x₃ intersects the profile of the pattern image at the second inter­ secting point S₂. Thus, the pattern length D₃ of the line x₃ is obtained from the distance between the points Pl and P2, i.e., from D₃ = i S2 - i Sl , at step 845. Finally, at step 847, the fourth line x₄ is determined which is angularly spaced from the third line x₃ by 45°. The line x₄ intersects the X-axis (ordinate) at the starting point S₀ , which is repre­ sented by (i G - j G , 0), subsequently, if a i,j is "0" at step 849, the control proceeds to step 85l which counts the ordinate and the abscissa up by l and returns the flow to step 849. At step 849, if a i,j is "l", the control proceeds to step 853, which causes values (i Sl , j Sl ) of a first intersecting point S₁ in which the line x₄ first intersects the profile of the pattern image to be i and j, respectively. Then, at step 855, if the data a i,j is "l", the control proceeds to step 857, which counts th ordinate and the abscissa up by l. If a i,j is "0" at step 855, values (i S2 , j S2 ) of a second intersecting points S₂, at which the line x₄ again intersects the profile of the pattern image, are caused to be i and j, respectively at step 859. the data a i,j becomes "0" when the line x₄ intersects the profile of the pattern image at the second intersecting point S₂. Thus, the pattern length D₄ of the line x₄ obtained from the distance between the points Pl and P2, i.e., from D₄ = (i S2 - i S1 )² + (j S2 - j S1 )², at step 86l. It will be easily understood that pattern lengths of any lines which pass the center of gravity G of the pattern image can be detected in the same way as des­ cribed above. The detection of the pattern length at step 7ll is completed at step 863. Then, at step 7l3, a mean value D a of the pattern lengths D₁ , D₂ , D₃ , ... D n is detected. The mean value can be obtained from the following equation: Subsequently, at step 7l5, the mean value D a thus obtained is compared with the equivalent diameter D A obtained at step 709. If a difference between D a and D A is below a predetermined limit value T at step 7l7, the control proceeds to step 7l9 which outputs a display "good", which means that the pattern image on the wafer is correctly displayed on the monitor screen by the optical image forming system. On the contrary, if the difference between D a and D A is above the predetermined limit value T, the term "optical error" is displayed at step 72l. When "optical error" is displayed at step 72l, the operator must adjust the optical system. The adjustment of the optical system includes, for example, an adjust­ ment of the optical axis of the light source, an adjust­ ment of lens angles of the lenses 3l and 33 with respect to the light beams incident thereupon, or an adjustment of lens centers of the lenses 3l and 33. The lenses 3l and 33 are supported at one ends by springs 43 and 45 for universal movement. The opposite ends of the lenses 3l and 33 are provided with adjusting screws 32, 34 and 36, 38. The adjusting screws 32 and 36 adjust the lens angles of the lenses 3l and 33 with respect to the light beams incident thereupon. The adjusting screws 34 and 38 adjust the lens centers of the lenses 3l and 33 in the lens axis directions. The lenses 3l and 33 can be provided with additional adjusting screws (not shown) which adjust the lens position in the direction perpendicular to the directions of adjustment by the adjusting screws 32, 34 and 36, 38, respectively. The operator can manually adjust the adjusting screws in accordance with the display at steps 7l9 and 72l. After the adjustment is carried out, the process shown in Fig. 7 is repeated until "good" is finally displayed at step 7l9. Alternatively, it is also possible to automatically adjust the optical system by means of suitable actuators in accordance with the "optical error" display. At step 7l9, if "good" is displayed, either the equivalent diameter D A or the mean value D a can be sued as an actual diameter of the substantially circular pattern formed on the wafer. As can be seen from the above discussion, by a method embodying the invention, the dimensions of fine patterns can be easily and precisely measured. Furthermore, in a method embodying the invention, the optical system reproduces the pattern on the display unit, such as monitor TV, with an increased precision. The present invention is not limited to the embodi­ ments described above and illustrated in the drawings and can be applied to measure dimensions of any fine pattern, for example, that of a mask which forms the contact holes or through holes on the wafer mentioned above.

Claims: 1. A method for measuring dimensions of a fine pattern, comprising: detecting a surface area and a profile of a displayed pattern image, determining a center of gravity of the pattern image, determining an equivalent diameter of a circular pattern having the same surface area as the pattern image, determining a mathematical mean value of pattern lengths of lines which pass through the centre of gravity of the pattern image and which intersect the pattern image at two intersecting points, said pattern lengths being defined as distances between the two intersecting points, and, comparing the mean value with the equiva­ lent diameter of the pattern image. 2. A method according to claim l, wherein said pattern image is displayed on a display unit by means of an optical image forming system. 3. A method according to claim 2, further com­ prising adjusting the optical image forming system in accordance with the result of said comparison of the mean value and the equivalent diameter. 4. A method according to claim 2 or 3, wherein said optical image forming system comprises an optical image sensor having a large number of photoelectric picture elements and wherein the step of detecting the surface area of the pattern image comprises counting the number of picture elements.

Inventors: KOBAYASHI, KENICHI, MATSUI, SHOUGO
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: G01B11
IPC codes: G01B11