MASK PATTERN DEFECT DETECTION APPARATUS

Abstract: In accordance with the present invention, there is pro­ vided a pattern defect detecting apparatus using a scanning and transmission electron microscope, comprising an electron gun (1) for accelerating an electron beam with high energy enough to transmit it through a sample and for radiating the accelerated electron beam (20), a condenser lens (2) for fo­ cusing the electron beam generated by said electron gun, a beam deflection coil (4) for deflecting the electron beam fo­ cused by said condenser lens, an objective lens (3) for further focusing the electron beam deflected by said beam deflection coil onto a fixed spot, an XY stage (8) for disposing the sample so as to be opposed to said objective lens, said XY stage be­ ing movable in X and Y directions in a step and repeat manner, a sample chamber (9) for housing the XY stage in vacuum, said sample chamber (9) including at least the outlet of the electron beam of said objective lens, an electron beam detector (7) for detecting electron beams transmitted through said sample, said electron beam detector being fixed to a stationary mem­ ber such as said chamber or a lens barrel, and defect detecting means (15, 16, 17) for scanning the electron beam by using said beam deflection coil for each step and repeat operation of said XY stage, for comparing a video signal obtained from said electron beam detector with a reference pattern read out from memory means (40), and for thereby detecting a defect of the sample.

Description: The present invention relates to a mask pattern defect detection apparatus using a scanning transmission electron microscope (STEM) optimally suited for inspecting a circuit pattern or the like formed on a mask for fabricating semiconductor devices or the like, and in particular for inspecting the circuit pattern of masks used for X-ray lithography. Scanning transmission electron microscopes (STEM) of the prior art are described in JP-A-55-151757 and in "Microbeam Analysis" written by the 141-th committee (Microbeam Analysis) of Japan Association for Advancement of Science and Technology, published by Asakura Bookstore on June 1, 1985, pp. 199 to 206, for example. With these STEMs of the prior art, however, defects of the circuit pattern of an X-ray mask or the like having a comparatively large size could not be detected accurately. It is the object of the present invention to provide a mask pattern detection apparatus including a scanning transmission electron microscope capable of efficiently detecting defects of a circuit pattern on a mask such as e.g. an X-ray mask. The above object is achieved according to claim 1. The dependent claims relate to prefered embodiments. The mask pattern defect detecting apparatus of the present invention includes a scanning transmission electron microscope and comprises an electron gun for accelerating an electron beam with high energy enough to transmit it through a mask sample and for radiating the accelerated electron beam, a condenser lens for focusing the electron beam generated by the electron gun, a beam deflection coil for deflecting the electron beam focused by the condenser lens, an objective lens for further focusing the electron beam deflected by the beam deflection coil onto a fixed spot, an XY stage for disposing the mask so as to be opposed to the objective lens, the XY stage being movable in X and Y directions in a "step-and-repeat" manner, two laser length measuring instruments 8b for measuring the displacement of the XY stage 8 , a sample chamber for housing the XY stage in vacuum, the sample chamber including at least the outlet of the electron beam of the objective lens, an electron beam detector for detecting electron beams transmitted through the mask, the electron beam detector being fixed to a stationary member such as the sample chamber or a lens barrel, and defect detecting means for scanning the electron beam by using the beam deflection coil for each step-and-repeat operation of the XY stage, for comparing a video signal obtained from the electron beam detector with a reference pattern read out from memory means, and for thereby detecting a defect of the mask. In particular, the mask sample is so placed on the XY stage moving in a step-and-repeat manner as to be opposed to the objective lens. In order to detect electron beams transmitted through the sample, the electron beam detector is so attached to the stationary member such as the sample chamber or lens barrel as to be stationary with respect to the objective lens. The above described XY stage is repetitively advanced by a predetermined distance and stopped, i.e., the XY stage steps and repeats. Whenever the XY stage is stopped, the electron beam is deflected by the beam deflection coil, and the video signal derived from the electron beam detector is compared with the reference pattern read out from the memory. It is checked with respect to the entire face of the mask sample by defect detecting means whether defects are present or not. Thereby, it is possible to carry out defect detection for a comparatively large sample such as a mask on the basis of the electron beam transmission. In accordance with the present invention, defects can be detected on the basis of the electron beam transmission. In case the sample is an X-ray mask used for an X-ray lithography process, therefore, the requirement that a foreign substance which does not transmit X-rays must be judged defective and a foreign substance transmitting X-rays and hence posing no hindrance must not be judged defective is satisfied. In accordance with the present invention, the acceleration voltage of the electron beam radiated from the electron gun is defined to be within the range of 150 to 350 kV (preferably 200 to 300 kV). Even if the mask surface is covered by a guard film such as polyimide like in the case of an X-ray mask, therefore, the circuit pattern made up of e.g, gold or tantalum and the guard film are not damaged. As a result, the pictures of the circuit pattern can be accurately detected by the electron beam detector, and defects can be detected with high reliability. In accordance with a prefered embodiment of the present invention, electron beams scattered by the pattern of the mask are taken into one electron beam detector together with the electron beams scattered by the substrate portion. Since the contrast is thus improved, defects can be accurately detected without erroneously detecting regular circuit patterns or the like. In the following, the invention will be further explained with reference to the drawings. Fig. 1 is a configuration diagram of an embodiment of the present invention. Fig. 2 is a configuration diagram of an embodiment of an electron beam detector. Fig. 3 is a configuration diagram of an embodiment of an electron beam detector using a channel plate. Fig. 4 is a configuration diagram of an embodiment of an electron beam detector using an electron beam absorbing material. Fig. 5 is a sectional view of an X-ray lithography mask. Fig. 6 shows an example of a pattern on a mask. Fig. 7 shows a detected signal of reflected electrons or scecondary electrons of a mask. Fig. 8 shows the detected signal of the circuit pattern obtained by converting the detected signal of Fig. 7 into binary values. Fig. 9 shows a detected signal of electrons transmitted through a mask. Fig. 10 shows the detected signal of the circuit pattern obtained by converting the detected signal of Fig. 9 into binary values. Fig. 11 shows a detected signal of a circuit pattern obtained when there are no defects. Fig. 12 shows the output signal of a comparator circuit. Fig. 13 shows the output signal of defect judging means. Figs. 14 and 15 show the concrete configuration of the comparator means and the defect judging means, respectively. Fig. 16 concretely shows an electron beam detector and its periphery. Fig. 17 shows the relationship between the electron beam scattering angle in an X-ray mask and the electron beam intensity. Fig. 18 is a generalized view of Fig. 17. The following explanation relates to prefered embodiments. An embodiment of the present invention will now be described with reference to Fig. 1. An electron gun 1 has an electron beam accelerator for accelerating an electron beam by an acceleration voltage of 150 to 350 kV (200 to 300 kV for optimum results) and radiates an accelerated electron beam 20. A condenser lens group 2 focuses the electron beam 20 accelerated and radiated by the electron gun 1. A beam deflection coil 4 deflects the electron beam focused by the condenser lens group 2 so that a range of approximately 0.14 x 0.14 mm on a mask 5 to be inspected may be scanned. An objective lens 3 is disposed so as to focus the electron beam deflected by the beam deflection coil 4 onto a uniform spot even if the electron beam is deflected on the mask 5 to be inspected. A sample stand 6 carries a mask 5 to be inspected having a size of approximately (20 to 50) x (20 to 50) mm so that the mask 5 may be opposed to the objective lens 3 via a minute gap formed between them. An electron beam detector 7 is composed of a combination of a fluorescent plate 21 and a photomultiplier 22 as shown in Fig. 2, for example. The electron beam detector 7 is attached to the objective lens 3, a sample chamber 9 or the like so that the electron receiving section may stand still on the rear face of the mask 5 to be inspected. In particular, the electron beam detector 7 is provided with an electron receiving face having such a size that the detector may receive electron beams transmitted through the inspected mask 5 together with electron beams scattered by the inspected mask 5. That is to say, the electron receiving face of the electron beam detector 7 has such a size as to accommodate nearly up to a scattering angle ϑ where the substrate portion of the inspected mask 5 and the pattern section thereof are detected with nearly equal electron intensities. An XY stage 8 has a movable range in X and Y directions which is at least the size of the inspected mask 5. The XY stage 8 moves and stops at intervals of approximately 0.12 mm in the "step-and-repeat" manner. The XY table 8 is so formed that a sample stand 6 may be mounted on the upper face thereof, and the electron receiving section of the electron ray detector 7 may be inserted between the upper face of the XY stage 8 and the rear face of the inspected mask 5. A sample chamber 9 is connected to a vacuum source for evacuation and is configured to house the above described objective lens 3 at the upper portion thereof. The sample cahmber 9 is also configured to house the sample stand 6 and the XY stage 8 therein. In particular, a drive source 8a for moving the XY stage 8 is preferably disposed outside the sample chamber 9 in order not to pollute the vacuum of the sample chamber 9. However, it is evident that the drive source 8a may be disposed within the sample chamber provided that the drive source 8a does not exhaust oil or the like. Two laser length measuring instruments 8b are disposed on side walls of the sample chamber 9 for the X and Y directions, respectively, of the XY stage 8 (one of the laser length measuring instruments 8b is not illustrated). The two laser length measuring instruments 8b are configured to measure the displacements of the XY stage 8 in the X and Y directions, respectively, with high precision. A deflection control unit 10 controls the beam deflection coil 4 to scan the electron beam. An amplifier 11 amplifies a video signal supplied from the electron beam detector 7. A CRT 12 displays the image of a local region (a region scanned by the beam deflection coil 4) of the inspected mask by running a video signal (a luminance signal) supplied from the above described amplifier 11, on the basis of a deflection control signal having a sawtooth waveform supplied from the above described deflection control unit 10. A binary decision circuit 13 converts the video signal supplied from the above described amplifier 11 into a binary-valued picture element signal by converting the video signal into a binary value by means of a predetermined threshold and sampling the video signal at a predetermined repetition period. On the basis of a readout directive signal provided by a timing controller 19 for each region scanned by the beam deflection coil 4, a reference pattern data readout unit 14 reads out reference pattern data stored in memory means 40 such as a disk (reference pattern data being created based on design data, for example). On the basis of a signal indicating the coordinate position synchronized to the deflection control signal, a reference pattern generator 15 generates a reference pattern by using binary-valued picture element signals. A comparator means 16 compares the binary-valued picture element signal with the reference pattern under the condition that there is no relative positional displacement between them. On the basis of a signal indicating the noncoincidence supplied from the above described comparator means, defect judging means 17 judge whether there is a defect or not. That is to say, the defect judging means 17 put out a defect signal provided that there is a defect on the inspected mask 5. On the basis of the displacement data of the XY table measured by the laser measuring instrument 8b, an XY stage control unit controls the XY stage 8 so as to advance and stop (or step and repeat) it in X and Y directions at predetermined intervals, say, 0.12 mm. A timing controller 19 supplies a directive signal for making the XY stage 8 step and repeat to the XY stage control unit 18 and supplies a deflection directive signal to the deflection control unit 10 after the XY stage 8 has stopped. In addition, the timing controller 19 is configured to supply the reference pattern generator 15 with a signal which is synchronized to the deflection directive signal supplied to the deflection control unit 10. The electron beam 20 generated by the electron gun 1 is so focused as to form a spot on the inspected mask 5 by the condenser lens group 2 and the objective lens 3. In the present embodiment, focusing is carried out magnetically. However, the focusing may also be carried out electrostatically. This holds true for the deflection system as well. The acceleration voltage (150 to 350 kV) of the electrons is high enough to transmit electrons through the mask 5, and the size of the spot is smaller than the size of the defect to be detected. By means of the deflection coil 4 driven by the deflection control unit 10, the electron beam 20 scans a region of approximately 0.14 x 0.14 mm on the mask 5 in a two-dimensional manner. Electrons transmitted through the mask 5 and scattered are detected by the electron beam detector 7 to generate an output signal depending upon the amount of transmitted electrons. The sample stand 6 is so fixed on the XY stage 8 as to assure a space for disposing a transmitted electron beam detector 7 between the mask 5 and the XY stage 8. Further, the upper face of the sample stand 6 adjoining the sample is provided with a hole to transmit the electron beam. The size of a portion of the electron beam detector 7 whereto the electron beam is applied is wider than the region over which the electron beam is deflected by the deflection coil 4 and has such an aperture that transmitted electrons may be detected by the electron beam detector 7 in whichever direction the electron beam is deflected. The size of the hole provided in the upper face of the sample stand 6 is sufficiently large in the same way. In the electron beam path, the lens barrel and the sample chamber 9 are kept at sufficiently high vacuum in order not to prevent the passage of electrons. Further, a power source 2a for focusing the electron beam may also be configured to be controllable by the deflection control unit 10. The electron beam detector 7 is composed of a combination of a fluorescent plate 21 and a photomultiplier tube 22 (Fig.2). Electrons transmitted through the mask 5 strike against the fluorescent plate to generate photons 23 therefrom. The photons 23 are applied to the photomultiplier tube 22 to be converted into a current, resulting in a detected signal of mask pattern. In the present embodiment, a photomultiplier tube 22 is used as means for detecting photons 23 generated from the fluorescent plate 21. As long as a sufficient sensitivity is obtained, however, photodetector devices such as photodiodes or CdS may be used instead. Further, a channel plate 24 may be used instead of the fluorescent plate 21 as shown in Fig. 3. When electrons are applied to the channel plate 24, a large number of electrons are generated within the channel plate by the electron multiplication function. And these electrons are generated in a fluorescent plate located at the output end. Even if an extremely small number of electrons are applied to the channel plate, therefore, it is possible to make the fluorescent plate brightly luminous. Accordingly, it is possible to reduce the intensity of the electron beam and to minimize the damage caused by the electron beam and the change in the pattern dimensions caused by the temperature rise. Another example of the electron beam detector is shown in Fig. 4. This electron beam detector is composed of a metal plate 26 covered by an electron absorbing material 25 which easily absorbs and hardly reflects electrons, made of carbon, for example. The electron beam detector is electrically insulated as a whole. An electric wire 27 is pulled out from one position of the metal plate to ground the metal plate to the lens barrel. If an electron beam is transmitted through the mask to strike against the electron absorbing material 25, the electron beam is absorbed to flow to the lens barrel via the metal plate and the electrical wire 27 in the form of a current. When the electron beam is scanning over a membrane, a large amount of electrons are transmitted and absorbed into the plate, resulting in a larger current flowing through the electric wire 27. When the electron beam is scanned over the circuit pattern, a small amount of electrons are transmitted, resulting in less current flowing through the electric wire 27. Therefore, the circuit pattern can be detected by detecting the value of the current flowing through the electric wire. At first, the timing controller 19 provides at its output a directive signal to make the XY stage 8 step and repeat at a predetermined interval via the XY stage control unit 18. After the inspected mask 5 has been stopped, the timing controller 19 supplies a deflection start signal to the deflection control unit 10. The beam deflection coil 4 is thus activated to deflect the electron beam spot focused to a fixed point in the X and Y directions. As a result, the inspected mask 5 is scanned. The electron beam detector 7 receives electrons transmitted through the inspected mask 5 together with those scattered within a particular angle ϑ and converts them into a video signal to be put out. After the output signal of the electron beam detector 7 has been amplified to a suitable level by the amplifier 11, it is converted into a binary-valued picture element signal. For a portion transmitting the X-rays, a binary-valued picture element signal "1" is obtained. For a portion which does not transmit the X-rays, namely, a circuit pattern portion, a binary-valued picture element signal "0" is obtained. On the basis of a directive signal supplied from the timing controller 19, the reference pattern data readout unit 14 reads out the reference pattern data of the region now deflected by the deflection coil 4 out of the reference pattern data of the entire mask stored in the memory means 40. The reference pattern data thus read out are put out to the reference pattern generator 15 in synchronism with the above described deflection. On the basis of deflection coordinates directed by the timing controller 19, the reference pattern generator 15 generates the reference pattern signal formed by the binary-valued picture element signals. In the comparator circuit 16, the binary-valued picture element signal obtained from the above-described binary decision circuit 13 is aligned in position and thereafter compared with the defectless reference pattern signal generated by the reference pattern generator 15. The noncoincident portions between them are put out. The defectless reference pattern signal is generated by the reference pattern generator 15 when a line A - A' shown in Fig. 6 is scanned by the electron beam. The detected signal indicating the noncoincidence put out by the comparator circuit 16 at that time is shown in Fig. 12. The defect judging circuit 17 detects only noncoincident portions having a magnitude exceeding the pattern dimension tolerance and judges them defective. Fig. 13 shows an example of a signal representing the defect decision. The scanning of the electron beam, the reference pattern data, and the reference pattern generator 15 are mutually synchronized by the timing controller 19. Accordingly, the defectless pattern detection signal corresponding to the mask pattern actually detected is generated by the reference pattern generator for comparison. After all regions which can be scanned by the electron beam have been inspected, the XY stage control unit 18 drives the XY stage 8 on the basis of the directive signal supplied from the timing controller 19. Thus the XY stage 8 step and repeat. As a result, areas of the mask which have not been inspected yet are scanned by the electron beam for inspection. The movement of the XY stage and the inspection are repeated until the entire surface of the mask has been inspected. The amplified signal of the electron beam detector 7 is supplied to the CRT 12 scanned in synchronism with the deflection control unit 10 and displayed on the CRT as a two-dimensional image. If the XY stage is moved to apply the electron beam to the detected defect after the termination of the inspection and the electron beam is scanned, the two-dimensional image of electrons transmitted through the defect is displayed on the CRT. In the present embodiment, the pattern detection signal of the inspected mask is compared with the defectless pattern detection signal generated from the design data. In case a plurality of identical circuit patterns are present on the mask, however, the detection signal of one circuit pattern may be temporarily stored, and this signal may be reproduced to be used as the reference pattern for comparison in synchronism with the detection of another inspected pattern. Since both patterns are originally the same, noncoincident portions may be judged defective. This method needs a memory for temporarily storing the circuit pattern or a memory unit such as a disk unit. However, the reference pattern readout unit and the reference pattern generator are not required. Therefore, the unit cost can be reduced. In addition, high speed inspection becomes possible without restriction in the inspection speed due to the readout speed of the reference pattern data readout unit. The comparator means 16 and the defect judging means 17 will now be concretely described with reference to Figs. 14 and 15. The comparator means receive signals of a reference picture 51a and signals of an inspected picture 51b which have been aligned in position. A local picture having 3 x 3 picture elements is extracted from the reference picture 51a by shift registers 52a and 52b for delaying the picture by one scanning line and serial-in parallel-out shift registers 53a, 53b and 53c. On the other hand, the inspected image 51b undergoes delay and serial-in to parallel-out conversion by using a similar shift register 52c and a serial-in parallel-out shift register 53d. The output of the shift register 53d is synchronized to the center position of the above described local image. Thereafter, the respective outputs of the shift registers 53a to 53c and the output of the shift register 53d undergo subtraction in subtraction circuits 56a to 56i. This results in a difference picture between the inspected picture 51b and the picture obtained by respectively shifting the reference picture 51a in X and Y directions by ± one picture element. The steps to this point correspond to a processing procedure (1) described later. On the other hand, the output of the shift register 53d and a preset value α undergo addition and subtraction in an addition circuit 54 and a subtraction circuit 55, respectively. The outputs of the addition circuit 54 and the subtraction circuit 55 undergo subtraction respectively in subtraction circuits 56j and 56k with respect to the picture element of the above described local picture having 3 x 3 picture elements. This corresponds to a processing procedure (2) which will be described later. The respective outputs of the above-described subtraction circuits 56a to 56k are supplied to a maximum detection circuit 57 and a minimum detection circuit 58, respectively. The absolute values of the resulting maximum and minimum values are computed in absolute value circuits 59a and 59b, respectively. The absolute value circuits 59a and 59b send out signs 60a and 60b of the input signal together with the absolute values 61a and 61b of the input signal. A multiplexer 62 is supplied with the absolute value 61a of the maximum value, the absolute value 61b of the minimum value, and a constant 0. Depending upon the sign 60a of the maximum value and the sign 60b of the minimum value similarly supplied, the multiplexer 62 selects and outputs one of the three input values. That is to say, the multiplexer 62 outputs 0 when one of the signs 60a and 60b is positive and the other is negative. The multiplexer 62 outputs the absolute value 61b of the minimum value when both the signs 60a and 60b are positive or both the signs 60a and 60b are negative. The steps described hitherto correspond to a processing procedure (3) which will be described later. The output of the multiplexer 62 is converted into a binary value by a binary decision circuit 63. The defect signal is thus put out in a processing procedure (4) described later). Owing to the above described operation, it becomes possible to allow a minute dimension difference between the reference picture 51a and the inspected picture 51b which does not result in a defect indication, and only true defects can be detected. Fig. 15 is a block diagram for illustrating the entire configuration of a picture processing circuit including the defect detection circuit 16 and 17 shown in Fig. 14. According to Fig. 15, signals of a reference picture 71a and of an inspected picture 71b photographed by using photo-electric conversion devices are at first supplied to edge detection circuits 72a and 72b for attaining the position alignment, respectively. In the edge detection circuits 72a and 72b, the respective input pictures undergo the second derivation to emphasize the pattern edges and are converted into binary values. By using the edge picture thus converted into binary values, a position deviation detection circuit 73 computes the position deviation values 76 and 77 between two patterns to be computed in the X and Y directions. Delay circuits 74a and 74b are buffer memories respectively for holding the reference picture 71a and the inspected picture 71b until the position deviation value between the two patterns has been computed. The position alignment between the two patterns can be attained by shifting in a position alignment circuit 75 the outputs of the delay circuits 74a and 74b by values depending upon the outputs 76 and 77 of the above described position deviation detection circuit 73. The reference picture 51a and the inspected picture 51b which have thus undergone the position alignment are supplied to the defect detection circuit 16 and 17 having the configuration shown in Fig. 14, resulting in the defect detection. The defect detection is attained by carrying out the processing described below after two pictures to be compared have been aligned in position. (1) With respect to the inspected picture (picture signal f(x, y)), the reference picture (picture signal g(x, y)) is shifted in x and y directions by ±1 picture element, respectively. The difference in brightness between the inspected picture and the reference picture (picture signal g(x-i, y-j) i,j=-1,0,1 ) including the shifted picture is computed. By this computation, 9 differential pictures (picture signals D n (x, y) n=1 to 9 ) are obtained. (2) A picture is so produced as to have a picture signal value g(x, y) ± α obtained by adding/subtracting a predetermined value α to/from the picture signal g(x, y) of the reference picture. The difference in brightness between the picture thus produced and the inspected picture (picture signal f(x, y)) is computed. By this computation, two difference pictures (picture signal D n (x, y) n=10,11 ) are obtained. (3) With respect to the picture signal D n (x, y) n=1 to 11 of 11 differential pictures in total derived from (1) and (2), the processing described below is effected to produce the output h(x, y). (a) When max {D(x, y)} n=1 to 11 x min {D(x, y)} n=1 to 11 < 0, h(x, y) = 0 (b) Otherwise, h(x, y) = min {|D(x, y)|} n=1 to 11 . (4) In case of (b), the output h(x, y) is converted into a binary value by using a predetermined threshold TH. The portion represented as h(x, y) ≧ TH is put out as defective. The detection method by using the electron beam detector 7 will now be described concretely with reference to Figs. 16 to 18. The sample stand 6 is fixed on the XY stage 8. Between the inspected mask 5 held on the upper face of the sample stand 6 and the XY stage 8, an exit diaphragm 42 and the electron beam detector 7 are disposed. The sample stand 6 has a hole 6a for passing electrons transmitted through the inspected mask 5. In order to limit the scattering angle ϑ of electrons passed through the exit diaphragm 42 below 5° to 25°, the diameter of the exit diaphragm 42 is variable, or several kinds of replaceable diaphragms having different diaphragm diameters are provided, or an exit diaphragm movable in the vertical direction is used. Instead of providing the exit diaphragm 42, an optimum detection angle may be obtained by controlling the size of the detection face of the electron beam detector 7 or the distance between the detection face and the inspected mask 5. As shown in Fig. 16, the electron beam detector 7 has an electron receiving face (scintillator) 43, a light guide 44, and a photomultiplier tube which is not shown. The electron beam 20 is applied to the inspected mask 5 and scattered. When the electron beam has reached the electron receiving face (scintillator) 43 at a scattering angle limited within a detection angle ϑ defined by the exit diaphragm 42, electrons are converted into light. The resultant light is conveyed through the light guide 44 and converted into an electrical signal at the photomultiplier tube. However, the electron beam detector 7 is not limited to this structure. For example, a semiconductor detector for detecting electron beams may also be used. The value of the detection angle ϑ maximizing the detection contrast varies depending upon the acceleration voltage of the electrons and the configuration of the inspected mask 5. However, the optimum value of the detection angle ϑ is approximately 5° to 25°. The reason why the electron beams transmitted through the inspected object within the scattering angle ϑ are taken in by the light receiving face 43 of the electron beam detector 7 will now be described with reference to Figs. 17 and 18 by taking an X-ray mask as an example. Figs. 17A through 17D show examples of the scattering distribution of the X-ray transmitting portion and the X-ray absorbing portion obtained as a result of simulation of electron beam scattering using the Monte-Carlo method. The simulation of Figs. 17A through 17D was carried out under the condition that the acceleration voltage of the incident electrons was 200 kV. The model used as the X-ray lithography mask in Fig. 17A is a BN X-ray mask which includes a membrane composed of BN having a thickness of 2 µm and polyimide having a thickness of 3 µm and which includes a guard film composed of polyimide having a thickness of 2 µm. The model indicated in Fig. 17B is a BN X-ray mask which includes a membrane composed of BN having a thickness of 2 µm and polyimide having a thickness of 3 µm and which includes a guard film composed of polyimide having a thickness of 0.5 µm. The model indicated in Fig. 17C is a SiN X-ray mask which includes a membrane composed of SiN having a thickness of 2 µm, an X-ray absorbing pattern composed of Ta having a thickness of 0.75 µm, and a guard film composed of polyimide having a thickness of 2 µm. The model indicated in Fig. 17D includes a membrane composed of SiN having a thickness of 2 µm, an X-ray absorbing pattern composed of Ta having a thickness of 0.75 µm, and a guard film composed of polyimide having a thickness of 0.5 µm. As evident from Fig. 17, the detection angle maximizing the detected signal for the X-ray transmitting portion of the X-ray lithography mask differs from that for the X-ray absorbing portion because of the difference in the electron beam scattering distribution between both portions. As shown in Fig. 18, the electron beam scattering angle at the intersection A of the scattering distribution curve of the X-ray transmitting portion and the scattering distribution curve of the X-ray absorbing portion is represented by ϑ. The detection contrast becomes largest in a STEM detecting electrons having scattering angles not more than the angle ϑ, namely, in a STEM having a detection angle ϑ. The reason therefor will now be described. Assuming that the detection angle is ϑ, the detected signal of the X-ray transmitting portion is equal to a value obtained by integrating the scattering distribution curve of the X-ray transmitting portion from 0 to ϑ. That is to say, the detected signal of the X-ray transmitting portion is equal to the area of the region surrounded by the curve OBA, the straight line OA', and the straight line AA'. In the same way, the detected signal of the X-ray absorbing portion is equal to a value obtained by integrating the scattering distribution curve of the X-ray absorbing portion from 0 to ϑ, i.e., the area of the region surrounded by the curve OB'A, the straight line OA', and the straight line AA'. Therefore, the detected contrast corresponds to the difference between the detected signal of the X-ray transmitting portion and the detected signal of the X-ray absorbing portion, i.e., the area of the region surrounded by the curves OBA and OB'A. If it is supposed that the detection angle  is smaller than ϑ, the detected contrast corresponds to the area of the region surrounded by the curve OB, the curve OB' and the straight line BB'. This contrast is smaller than the contrast obtained when the detection angle is ϑ by the area of the region surrounded by the curves AB and AB' and the straight line BB'. Supposing that the detection angle is an angle ψ larger than the above described ϑ, the contrast corresponds to the area obtained by subtracting the area of the region surrounded by the curves AC and AC' and the straight line CC' from the area of the region surrouded by the curves OBA and OB'A. This contrast is smaller than the contrast obtained when the detection angle is ϑ by the area of the region surrounded by the curves AC and AC' and the straight line CC'. From the above described results, it is evident that the contrast becomes largest when the detection angle is ϑ. The value of ϑ varies to some degree depending upon the mask model shown in Fig. 17. However, this value is close to π/16 rad (approximately 11°) in any of the four computed cases (Figs. 17A through 17D). It is considered that the value of ϑ does not vary largely. The value of ϑ becomes smaller when the acceleration voltage of the electrons is higher than 200 kV and becomes larger when the acceleration voltage of the electrons is lower than 200 kV. By making the STEM detection angle 10⁻¹ rad or more, preferably, 5° to 25°, in accordance with the present invention, therefore, the contrast becomes largest as compared to the conventional case of a detection angle of approximately 10⁻⁴ rad, resulting in high contrast detection. In addition, the optimum detection angle becomes a value significantly larger than the conventional value, i.e., 10⁻¹ rad or more. Accordingly, the proportion of the detected electrons to the radiated electrons can be increased. Since the S/N ratio of the detected signal is considered to be proportional to the square root of the number of detected electrons, the S/N ratio of the apparatus of the present invention is improved by a factor of approximately 10 to 100. As a result, the pattern defect inspection of masks can be carried out at high speed. In the circuit pattern inspection of masks used for X-ray lithography processes, the present invention described above makes it possible to prevent erroneous defect detection caused by a foreign substance transmitting X-rays. In addition, the pattern can be detected even if a guard film of polyimide or the like is present on the surface. As a result, the reliability of inspection is improved.

Claims: A mask pattern defect detecting apparatus including a scanning and transmission electron microscope (STEM), comprising: an electron gun (1) for accelerating an electron beam with high energy enough to transmit it through the mask and for radiating the accelerated electron beam (20); a condenser lens (2) for focusing the electron beam generated by the electron gun (1); a beam deflection coil (4) for deflecting the electron beam focused by the condenser lens (2); an objective lens (3) for further focusing the electron beam deflected by the beam deflection coil (4) onto a fixed spot; an XY stage (8) for disposing the mask so as to be opposed to the objective lens (3), the XY stage (8) being movable in X and Y directions in a step-and-repeat manner; two laser length measuring instruments (8b) for measuring the displacement of the XY stage (8); a sample chamber (9) for housing the XY stage (8) in vacuum, including at least the outlet of the electron beam of the objective lens (3); an electron beam detector (7) for detecting electron beams transmitted through the mask the electron beam detector (7) being fixed to a stationary member such as the sample chamber (9) or a lens barrel; and defect detecting means (15, 16, 17) for scanning the electron beam by using the beam deflection coil (4) for each step-and-repeat operation of the XY stage (8), for comparing a video signal obtained from the electron beam detector (7) with a reference pattern read out from memory means (40), and for thereby detecting a defect of the mask. The apparatus according to claim 1, wherein the mask (5) is for X-ray exposure. The apparatus according to claim 1 or 2, wherein the electron beam detector (7) has an electron receiving face (43) for detecting electrons transmitted through the mask (5) and then scattered by the pattern together with electrons transmitted through the mask (5) and then scattered by the substrate. The apparatus according to one of claims 1 to 3, wherein the electron beam detector (7) takes in electrons up to a scattering angle (ϑ) where the intensity of electrons obtained at the pattern portion is nearly equal to the intensity of electrons obtained at the substrate portion. The apparatus according to one of claims 1 to 4, wherein a diaphragm (42) is disposed between the mask (5) and the electron receiving face (43) of the electron beam detector (7).

Inventors: FUSHIMI, SATORU, KOSHISHIBA, HIROYA, KUBOTA, HITOSHI, KUNI, ASAHIRO, NAKAGAWA, YASUO
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: H01J37
IPC codes: G01N23, H01J37, H01L21, G03F1, G01B15, G02B21