Abstract:
A charged particle beam apparatus includes a charged particle source which generates a charged particle beam, a condenser lens which converges the charged particle beam, a deflector which deflects the charged particle beam to scan a sample with the charged particle beam, an objective lens which converges the charged particle beam on the surface of the sample, a sample position imaginary variation detection part which detects an imaginary variation of a sample position caused by variation of the focal position of the charged particle beam due to variation in the potential of the sample, and a sample position imaginary variation compensation part which compensates for the detected imaginary variation of the sample position.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims benefit of priority under 35USC §119 to Japanese patent application No. 2001-291223, filed on Sep. 25, 2001, the contents of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a charged particle beam apparatus, a pattern measuring method and a pattern drawing method. 
     2. Description of the Prior Art 
     A charged particle beam apparatus is widely used in a semiconductor fabricating process as a scanning electron microscope which observes a pattern formed on a semiconductor wafer and an electron beam lithography apparatus which draws a pattern on a semiconductor wafer. In using such a charged particle beam apparatus, prior to observation and drawing, the focal position of a charged particle beam may be fine-adjusted to decide a measuring position and a patterning position. In a prior art, a height detector of an optical type or an electrostatic capacity type is used to detect a height of a sample surface, and a calibration parameter of the detected sample height and the focal position of a charged particle beam is calculated to set a focus control current or a focus control voltage so that the charged particle beam is focused on the sample surface, thereby coarse-adjusting the focal position. 
     As an example of a prior art apparatus and method which coarse-adjust a focal position, there is Japanese patent application Laid-Open No. 1999-149895. Japanese patent application Laid-Open No. 1999-149895 discloses a height detector which detects the position of a lattice-like light flux reflected from the surface of a sample by projecting the lattice-like light flux onto the sample from above the sample in a slanting direction to measure the height of the surface of the sample from a change of the position; focus control means which converges an electron beam on the surface of the sample based on the measured height of the surface of the sample; and deflection control means which calibrates image distortion including a magnification error of an electron beam image caused based on the focus control. 
     A charged particle beam apparatus typically has a sample stage which supports and/or fixes a sample. To stabilize the incident voltage of a charged particle beam onto the sample, a ground or a constant voltage is applied to the sample stage. This holds the potential of the sample constant. 
     However, there is a case in which a suitable voltage cannot be applied to the sample since the periphery of the sample is covered by an insulator film or a case in which the irradiated region can be electrically charged by irradiation of the charged particle beam so that the potential of the sample can be varied locally. In such cases, magnification variation becomes a problem. This point will be described by taking an electron beam apparatus as an example of a charged particle beam apparatus with reference to FIGS. 6 to  7 B. In the following drawings, like parts are indicated by the same reference numerals and repetitive descriptions thereof are suitably omitted. 
     FIG. 6 is a block diagram for explaining problems in the prior art electron beam apparatus. The drawing shows that deviation between a sample surface position obtained by a sample height detector and an electron beam focal position results in a magnification error. Electron beam EB produced from an electron beam generation source  10  is converged by a condenser lens  12 , which is then deflected by two deflectors  14   a,    14   b  to be focused so as to focus on the surface of a sample  20  by an objective lens  16 . The potential of the sample  20  is typically constant. The electron beam EB is focused on the surface of the sample. The sample surface position obtained from the height detector of an optical type or an electrostatic capacity type is thus matched with the electron beam focal position (The sample position is referred to as Z 0 ). When the sample potential is varied, however, the incident voltage onto a sample  20  is changed due to the influence. There occurs deviation between the sample surface position Z 0 , and the electron beam focal position (The electron beam focal position is referred to as Z 1 ). Specifically, along with the variation in the sample potential, the sample position is changed imaginarily. The focus control current of the objective lens  16  is varied from the original current value I 0  to I 1  corresponding to the imaginary position. In accordance with this, the deflection width of the electron beam EB is varied from the original width W 0  to width W 1 . Image distortion including a magnification error is thus caused as long as the magnification compensation of an electron beam image is not properly performed, even if the focal position of the electron beam EB can be detected in this state. FIGS. 7A and 7B show distortion of the electron beam image caused by the deviation of the sample position. When the electron beam image is used to measure the size of pattern P, the case in which a voltage is applied properly to the sample  20  (Ra) and the case in which a voltage is not applied properly to the sample  20  (Rb) are largely different in the size of the irradiated region of the electron beam EB, as shown in FIG.  7 A. As a result the magnification of the electron beam image obtained from a secondary electron detector  62  and a SEM image signal processing part  64  (see FIG. 6) is varied. That is, as is apparent from the contrast of image Im (Z 0 ) with image Im (Z 1 ) shown in FIG. 7B, the size of the pattern P displayed on the screen is varied. As a result, the measured value is changed so that sufficient measurement reproducibility cannot be obtained. 
     To properly apply a voltage to the sample  20 , there is a method in which removes the insulator film of the periphery of the sample  20 . However, the sample can be scratched and dust can be produced. In consideration of these, this method is lacking in practicality. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a charged particle beam apparatus comprising: a charged particle source which generates a charged particle beam; a condenser lens which converges the charged particle beam; a deflector which deflects the charged particle beam to scan a sample with the charged particle beam; an objective lens which converges the charged particle beam on the surface of the sample; a sample position imaginary variation detection part which detects an imaginary variation of a sample position in height caused by variation of the focal position of the charged particle beam due to variation in the potential of the sample; and a sample position imaginary variation compensation part which compensates for the detected imaginary variation of the sample position. 
     According to a second aspect of the present invention, there is provided a pattern measuring method using a charged particle beam apparatus, the charged particle beam apparatus comprising, a charged particle source which generates a charged particle beam, a condenser lens which converges the charged particle beam, deflector which deflects the charged particle beam for scanning a sample with the charged particle beam, an objective lens which converges the charged particle beam on the surface of the sample, and a measuring part which measures a pattern formed on the surface of the sample on the basis of a secondary charged particle or a reflective charged particle which are produced from the surface of the sample by irradiation of the charged particle beam, the method comprises: detecting an imaginary variation of a sample position caused by variation of the focal position of the charged particle beam due to variation in the potential of the sample; and compensating for the imaginary variation of the sample position. 
     According to a third aspect of the present invention, there is provided a method for writing a pattern on a surface of a sample using a charged particle beam apparatus, the charged particle beam apparatus comprising a charged particle source which generates a charged particle beam, a condenser lens which converges the charged particle beam, a deflector which deflects the charged particle beam to scan the sample with the charged particle beam, and an objective lens which converges the charged particle beam on the surface of the sample, the method comprises: adjusting a focal position of the charged particle beam by adjusting a current or a voltage given to the objective lens to search the focal position of the charged particle beam and obtaining a focus control current value or a focus control voltage value for the objective lens when the charged particle beam is focused on the surface of the sample, as a first focus control current value or a first focus control voltage value, respectively; detecting the height of the surface of the sample; calculating a focus control current value or a focus control voltage value given to the objective lens when a focal position of the charged particle beam corresponds to the detected height of the surface of the sample, as a second current value or a second voltage value, respectively; calculating a focus control current difference which is a difference between the first focus control current value and the second focus control current value or a focus control voltage difference which is a difference between the first focus control voltage value and the second focus control voltage value; calculating a magnification variation of the charged particle beam on the basis of the focus control current difference or the focus control voltage differential; and generating a deflection control signal as a compensation signal which compensates for a control signal to the deflectors corresponding to the calculated magnification variation; and writing a pattern on the sample with the charged particle beam while compensating for the deflection amount of the deflectors on the basis of the deflection control signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the schematic construction of a first embodiment of a charged particle beam apparatus according to the present invention; 
     FIG. 2 is a flowchart showing the schematic steps of a first embodiment of a pattern measuring method according to the present invention; 
     FIG. 3 is a flowchart showing the schematic steps of an embodiment of a pattern drawing method according to the present invention; 
     FIG. 4 is a block diagram showing the schematic construction of a second embodiment of a charged particle beam apparatus according to the present invention; 
     FIG. 5 is a flowchart showing the schematic steps of a second embodiment of a pattern measuring method according to the present invention; 
     FIG. 6 is a block diagram for explaining problems in a prior art electron beam apparatus; and 
     FIGS. 7A and 7B are drawings showing distortion of an electron beam image caused by the prior art. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Some embodiments of the present invention will be described below with reference to the drawings. The following embodiments will be described by taking electron beam apparatuses using an electron beam as a charged particle beam. The present invention is not limited to these electron beam apparatuses, and can be naturally applied to an ion beam apparatus using an ion beam as a charged particle beam. 
     (1) First Embodiment of Charged Particle Beam Apparatus 
     FIG. 1 is a block diagram showing the schematic construction of a first embodiment of a charged particle beam apparatus according to the present invention. An electron beam apparatus  1  shown in the drawing has, in addition to the construction of the electron beam apparatus shown in FIG. 6, a sample height detector  18 , a height detector control part  22 , a focus control current calculation part  24 , a focus control current control part  26 , a focal position detection part  28 , a focus control current differential calculation part  32 , a magnification variation calculation part  34 , a deflection control part  36 , and a pattern measuring part  42 . 
     The sample height detector  18  is constructed by a height detector of an optical type or an electrostatic capacity type and detects position Z 0  of the surface of a sample  20  upon reception of a command signal from the height detection control part  22 . The focus control current calculation part  24  receives information of the sample surface position Z 0  obtained by the sample height detector  18  and calculates focus control current (exciting current) I 0  of an objective lens  16  at the position Z 0  on basis of the relation between a previously prepared sample surface position and the focus control current of the objective lens  16 . The focal position detection part  28  supplies a control signal to the focus control current control part  26  to detect focal position Z 1  of an electron beam EB while changing the focus control current, and sets focus control current I 1  in which the objective lens  16  focuses the electron beam EB at the position Z 1 . The focus control current differential calculation part  32  calculates difference ΔI between the focus control current I 0  and the focus control current I 1 . The magnification variation calculation part  34  receives information of the difference ΔI from the focus control current differential calculation part  32  to calculate magnification variation ΔMag using a previously prepared calibration parameter. The calibration parameter is obtained from the relation between the focus control current value and the deflection amount. The deflection control part  36  receives information of the magnification variation ΔMag from the focus control current differential calculation part  34  and generates deflection control signal Scd corresponding to the variation ΔMag to supply it to deflectors  14   a ,  14   b , compensating for the deflection amount. 
     The charged particle beam apparatus  1  of this embodiment thus compensates for the deflection amount of the deflectors  14   a ,  14   b  corresponding to the magnification error due to the imaginary variation of the sample position caused by the variation in potential of the sample. Therefore, for example in measuring a pattern size, stable and high-precision measurement can be realized. In addition, when the objective lens  16  is an electrostatic lens, the charged particle beam apparatus can be adapted by controlling a lens applied voltage in place of the exciting current. This point is the same in the later-described embodiment. 
     (2) First Embodiment of Pattern Measuring Method 
     A pattern measuring method using the electron beam apparatus  1  shown in FIG. 1 will be described as the first embodiment of a pattern measuring method according to the present invention with reference to FIG.  2 . 
     FIG. 2 is a flowchart showing the schematic steps of the pattern measuring method of this embodiment. 
     First, the sample surface position Z 0  is detected by the sample height detector  18  (step S 1 ). 
     Next, the exciting current I 0  of the objective lens  16  at the sample surface position Z 0  is calculated by the focus control current calculation part  24  on the based of the relation between the sample surface position and the focus control current of the objective lens  16  (step S 2 ). 
     The focal position Z 1  of the electron beam EB is then detected by the focal position detection part  28  while the focus control current of the objective lens  16  is being changed (step S 3 ), the exciting current I 1  of the objective lens  16  at the position Z 1  being calculated (step S 4 ). 
     Next, the focus control current I 0  is compared with the focus control current I 1  by the focus control current differential calculation part  32  (step S 5 ). In the case of I 1 =I 0 , it can be determined that no magnification error is caused. While the exciting current (I 1 =I 0 ) is fed to the objective lens  16 , the electron beam EB is irradiated to obtain an electron beam image (step S 9 ). A pattern is measured (step S 10 ) and the measured result is outputted (step S 11 ). 
     On the other hand, in the case of I 1 ≠I 0 , magnification compensation is performed. First, its difference ΔI (I 1 −I 0 ) is calculated by the focus control current differential calculation part  32  (step S 6 ). Then, magnification variation ΔMag (=f Δ(I)) corresponding to the difference ΔI on the basis of the relation between the value of the exciting current and the deflection amount is calculated using the prepared calibration parameter by the magnification variation calculation part  34  (step S 7 ). Deflection control signal Scd corresponding to the magnification variation ΔMag is generated by the deflection control part  36  to be supplied to the deflectors  14   a ,  14   b , thereby the deflection amount is compensated by the magnification variation ΔMag(step S 8 ). When the deflection amount at the focal position is for example W 1 , the deflection amount of the electron beam EB is compensated from the deflection amount W 1  to the deflection amount W 0  corresponding to the sample surface position. As a result, the observation magnification is the same as in the case in which the sample position is not varied imaginarily. Thereafter, while the deflection control signal Scd corresponding to the magnification variation ΔMag is supplied to the deflectors  14   a ,  14   b , the electron beam EB is irradiated to obtain an electron beam image of the pattern by a secondary electron detector  62  and a SEM image signal processing part  64  (step S 9 ). the size of a pattern is measured by the pattern measuring part  42  on the basis of the electron beam image (step S 10 ) and the measured result is outputted (step S 11 ). 
     (3) An Embodiment of a Pattern Drawing Method 
     A pattern drawing method using the electron beam apparatus  1  shown in FIG. 1 wilt be described below as an embodiment of a pattern drawing method according to the present invention with reference to FIG.  3 . 
     FIG. 3 is a flowchart showing the schematic steps of the pattern drawing method of this embodiment. As is apparent from the contrast of it with the flowchart shown in FIG. 2, the steps of the pattern drawing method of this embodiment are substantially the same as steps S 1  to S 8  of the pattern measuring method of the first embodiment except for step S 29 . 
     That is, the sample surface position Z 0  is detected by the sample height detector  18  (step S 21 ). the exciting current I 0  of the objective lens  16  at the sample surface position Z 0  is calculated by the focus control current calculation part  24  on the basis of the relation between the sample surface position and the focus control current of the objective lens  16  (step S 22 ). 
     Next, the focal position Z 1  of the electron beam EB is detected by the focal position detection part  28  while the focus control current of the objective lens  16  is being changed (step S 23 ), the exciting current I 1  of the objective lens  16  at the position Z 1  being calculated (step S 24 ). 
     The focus control current I 0  is compared with the focus control current I 1  by the focus control current differential calculation part  32  (step S 25 ). In the case of I 1 =I 0 , it can be determined that no magnification error is caused. The electron beam EB is then irradiated to draw pattern while the exciting current (I 1 =I 0 ) is given to the objective lens  16  (step S 29 ). 
     In contrast, in the case of I 1 ≠I 0 , magnification compensation is performed. Its difference ΔI (I 1 -I 0 ) is calculate by the focus control current differential calculation part  32  (step S 26 ). Magnification variation ΔMag (=f(ΔI)) corresponding to the difference ΔI is calculated by the magnification variation calculation part  34  from the relation between the value of the exciting current and the deflection amount using the prepared calibration parameter (step S 27 ). Deflection control signal Scd corresponding to the magnification variation ΔMag is generated by the deflection control part  36  to be supplied to the deflectors  14   a,    14   b,  thereby the deflection amount being compensated by the magnification variation ΔMag (step S 28 ). In more specific the deflection amount W 1  at the focal position of the electron beam EB is compensated to the deflection amount W 0  corresponding to the sample surface position. As a result, the drawing magnification is the same as In the case in which the sample position is not varied imaginarily. Thereafter, a pattern is drawn on the surface of the sample at the drawing magnification (step S 29 ). 
     According to this embodiment, the pattern can be drawn stably on the surface of the sample with stable and high precision. 
     (4) Second Embodiment of Charged Particle Beam Apparatus 
     FIG. 4 is a block diagram showing the schematic construction of a second embodiment of a charged particle beam apparatus according to the present invention. An electron beam apparatus  2  shown in the drawing comprises a measured result compensation part  46  in place of the deflection control part  36  provided in the electron beam apparatus  1  shown in FIG.  1 . The electron beam apparatus  2  also comprises a pattern measuring part  44  connected to the magnification variation calculation part  34  and the measured result compensation part  46 , in place of the pattern measuring part  42 . Other constructions of the electron beam apparatus  2  are substantially the same as the electron beam apparatus  1  shown in FIG.  1 . 
     The pattern measuring part  44  uses the electron beam image obtained from the secondary electron detector  62  and the SEM image signal processing part  64  by irradiation of the electron beam EB to measure the size of a pattern, supplying the measured result to the measured result compensation part  46 . The measured result compensation part  46  compensates for the measured result on the basis of information of the magnification variation ΔMag supplied from the magnification variation calculation part  34 . Although this does not compensate for the observation magnification itself, the same measured result as that of the first embodiment can be obtained. 
     This embodiment solves the measured error due to the imaginary variation of the sample position caused by the variation in potential of the sample. When measuring the pattern size, stable and high-precision measurement can be realized. 
     (5) Second Embodiment of Pattern Measuring Method 
     A pattern size measuring method using the charged particle beam apparatus shown in FIG. 4 will be described below as a second embodiment of the pattern measuring method according to the present invention with reference to FIG.  5 . 
     FIG. 5 is a flowchart showing the schematic steps of the pattern size measuring method of this embodiment. 
     First, the sample surface position Z 0  is detected by the sample height detector  18  (step S 41 ). 
     Next, the exciting current I 0  of the objective lens  16  at the sample surface position Z 0  is calculated by the focus control current calculation part  24  on the basis of the relation between the sample surface position and the focus control current of the objective lens  16  (step S 42 ). 
     The focal position Z 1  of the electron beam EB is detected by the focal position detection part  28  while the focus control current of the objective lens  16  is being changed (step S 43 ), the exciting current I 1  of the objective lens  16  at the position Z 1  being calculated (step S 44 ). 
     Next, the electron beam EB is irradiated to obtain an electron beam image (step S 45 ) and a pattern in the electron beam image is measured by the pattern measuring part  44  (step S 46 ). 
     Then, the focus control current I 0  is compared with the focus control current I 1  by the focus control current differential calculation part  32  (step S 47 ). In the case of I 1 =I 0 , it can be determined that no magnification error is caused. The measured result of the pattern measuring part  44  is outputted as-is (step S 51 ). 
     On the contrary, in the case of I 1 ≠I 0  (step S 47 ), the measured result is compensated by the following steps. A difference ΔI (=I 1 −I 0 ) between the focus control current I 0  and the focus control current I 1  is calculated by the focus control current differential calculation part  32  (step S 48 ). Magnification variation ΔMag (=f (ΔI)) corresponding to the difference ΔI is calculated by the magnification variation calculation part  34  on the basis of the relation between the value of the exciting current and the deflection amount using the prepared calibration parameter to calculate (step S 49 ). Sequentially, the measured result of the pattern size outputted from the pattern measuring part  44  is compensated by the measured result compensation part  46  on the basis of information of the magnification variation ΔMag (step S 50 ), the compensated measured result is then outputted as a final measured result (step S 51 ). Although this does not compensate for the observation magnification itself, the same measured result as that of the first embodiment can be obtained. 
     This embodiment thus compensates for the measured result corresponding to the imaginary variation of the sample position caused by the variation in potential of the sample. Accordingly, stable and high-precision measurement can be realized. 
     Some embodiments of the present invention are described above. However, the present invention is not limited to the above embodiments and can be applied by various modifications within the scope thereof. In the above embodiments, the magnification variation ΔMag is described above as the function of the difference ΔI of the exciting current of the objective lens  16 . Without being limited to this, it is apparent that the present invention can be applied when the magnification variation ΔMag is taken as the function of difference ΔV of a voltage applied to excite the objective lens  16 .