JP2006172790A - Method and device of measuring surface charge distribution or surface potential distribution - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device to measure charge distribution or electric potential distribution occurring on the surface of dielectrics in a high resolution of micron order. <P>SOLUTION: This device is provided with a charged particle generating means 14 that generates charged particle beams for irradiating a test piece 23 having a surface charge distribution or a surface electric potential distribution, a scanning means 22 to scan on the test piece 23 by the charged particle beams from the charged particle generating means, a lens means 24 that converges the charged particle beams on the test piece 23, and a signal detecting means 13 that detects charged particles obtained from the surface of the test piece 23 by the irradiation of the charged particle beams by the scanning means 22. The lens means has at least one electrostatic lens means 24. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method and apparatus for measuring charge distribution or potential distribution on a sample surface.

  As a method for observing an electrostatic latent image with an electron beam, there is a method of detecting secondary electrons by electron beam irradiation (for example, Patent Documents 1 and 2). In this method, the sample on which the electrostatic latent image is formed is limited to an LSI chip or a sample that can store and hold the electrostatic latent image. That is, a normal photoconductor that causes dark decay cannot be measured. Since a normal dielectric can hold a charge semipermanently, even if measurement is performed over time after forming a charge distribution, the measurement result is not affected. However, in the case of a photoconductor, since the resistance value is not infinite, the charge cannot be held for a long time, dark decay occurs, and the surface potential decreases with time. The time that the photoconductor can hold the charge is at most several tens of seconds even in the dark room. Therefore, even if an attempt is made to observe with an electron microscope (SEM) after charging and exposure, the electrostatic latent image disappears in the preparation stage.

In view of this, the present inventor has proposed a measurement method capable of measuring an electrostatic latent image even with a photoconductor sample having dark decay (for example, Patent Documents 3 to 6). In the photoreceptor sample, if there is a potential distribution or a charge distribution on the sample surface, an electric field distribution corresponding to the surface charge distribution is formed in the space. In the region where the charge density is relatively high, the secondary electrons reach the detector because the secondary electrons generated by the irradiation of the charged particle beam generate an electric field strength that acts in the direction away from the sample. On the other hand, in the region where is relatively low, an electric field strength is generated so that a force in the direction of pulling back to the sample acts, so that the generated secondary electrons are pulled back by this electric field and the amount reaching the detector is reduced. Therefore, a contrast image corresponding to the surface charge distribution can be detected.
JP-A-3-29867 Japanese Patent Laid-Open No. 3-49143 JP 2003-295696 A JP 2003-305881 A Japanese Patent Laid-Open No. 2004-233261 JP 2004-251800 A

  In a normal SEM, it is common to use a magnetic lens with good aberration correction as an objective lens. In the case of sample observation by a normal SEM, since the surface potential of the sample is 0 V or close to 0 V, the generated secondary electrons can reach the detector by the drawing voltage without any problem.

  However, if there is a potential or electric charge on the surface of the sample and the magnitude of the potential is so large that it cannot be ignored compared to the energy of the electrons, it is relatively affected by the Coulomb repulsive force. This means that the force to push back to the electron optical system side is increased, and thus the component of the emitted electrons that returns to the exit opening of the electromagnetic objective lens of the electron optical system may increase.

  Originally, if secondary electrons or primary inverted electrons that should reach the detector return to the exit aperture of the electromagnetic objective lens of the electron optical system, they cannot be detected by the detector, and there is a region with a high charge density. It may be misunderstood that the region has a low charge density, which may cause noise. Therefore, the S / N ratio can be improved by suppressing the phenomenon of electron return to the exit opening of the electron optical system, thereby enabling measurement of charge distribution or potential distribution with high resolution.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus for measuring charge distribution or potential distribution generated on the surface of a dielectric with high resolution on the order of microns, which has been extremely difficult with the prior art. An object of the present invention is to provide an apparatus for measuring the above electrostatic latent image with high resolution.

  It is well known that the surface charges described here are strictly scattered in the sample. For this reason, the surface charge refers to a state in which the charge distribution state is largely distributed on the surface as compared with the thickness direction. Further, the charge includes not only electrons but also ions.

  Further, there may be a state in which there is a conductive portion on the surface and a voltage is applied to the conductive portion, thereby causing a potential distribution on the sample surface or its vicinity.

  According to the first aspect of the present invention, a sample having a surface charge distribution or a surface potential distribution is scanned with a charged particle beam, and the charge distribution or potential distribution of the sample is detected based on a detection signal obtained by scanning the charged particle beam. In the method for measuring a state, an electrostatic lens for focusing the charged particle beam on the sample is used in the optical system of the charged particle beam.

  According to a second aspect of the present invention, charged particle generating means for generating a charged particle beam for irradiating a sample having a surface charge distribution or a surface potential distribution, and scanning the sample with the charged particle beam from the charged particle generating means. Scanning means, lens means for focusing the charged particle beam on the sample, and signal detection means for detecting charged particles obtained from the surface of the sample by irradiation of the charged particle beam by the scanning means, The lens means has at least one electrostatic lens means.

  The invention described in claim 3 is the invention described in claim 2, wherein the electrostatic lens means is an objective lens of the optical system of the charged particle beam.

  According to a fourth aspect of the present invention, in the third aspect of the present invention, the electrostatic objective lens is a decelerating electrostatic objective lens.

  The invention according to claim 5 is the invention according to claim 3, wherein the charged particle generating means has an electron gun for a charged particle beam for irradiating the sample, and the scanning means is the electrostatic objective. A deflecting means is provided between the lens and the electron gun for scanning a charged particle beam from the electron gun.

  The invention according to claim 6 is characterized in that an exit opening of the charged particle beam optical system has an electric field vector in a direction in which electrons from the sample are pulled back to the sample side.

  The invention according to claim 7 is characterized in that, in the invention according to claim 2, a member for generating emitted electrons at the time of an electron collision is arranged in the vicinity of the periphery of the exit opening of the optical system of the charged particle beam. To do.

  The invention according to an eighth aspect is the invention according to the second aspect, wherein the signal detection means includes, among charged particles incident on the sample surface by two-dimensional scanning of the surface of the sample by the scanning means. A detection signal is obtained by detecting charged particles whose normal velocity component on the sample surface of the incident velocity vector is inverted.

  According to a ninth aspect of the present invention, there is provided the measurement apparatus according to any one of the second to eighth aspects of the invention, and the charged particles are substantially uniformly charged on the photosensitive member by irradiating the photosensitive member with charged particles as the sample. Generating means and optical system means for selectively exposing the photoconductor charged by the generating means, and scanning the surface of the photoconductor with an electron beam by the measuring device after exposure by the optical system means The electrostatic latent image distribution on the surface of the photosensitive member is measured by a detection signal obtained by the scanning.

The invention according to claim 10 is the measurement apparatus according to any one of claims 2 to 9, wherein the electric field strength in the thickness direction of the sample is 30 V / μm or more and 40 V / μm or less. When the sample is irradiated with charges of 10 ⁻⁸ coulomb / mm ² or more under the above conditions, it is a latent image carrier made of a sample having an insulation resistance region of 99% or more.

  According to the first aspect of the present invention, it is possible to prevent the charged particles from returning to the charged particle optical system by using the electrostatic lens as the lens for focusing the charged particle beam in the charged particle optical system. Thus, it is possible to measure the surface charge distribution or the surface potential distribution, which has been difficult in the past, with high accuracy.

  According to the second aspect of the present invention, it is possible to prevent the charged particles from returning to the charged particle optical system by using at least one electrostatic lens means as the lens means for focusing the charged particle beam. Thus, it is possible to provide an apparatus for measuring surface charge distribution or surface potential distribution with high accuracy, which has been difficult in the past.

  According to the third aspect of the present invention, the detection sensitivity of secondary electrons can be increased by configuring the objective lens with electrostatic lens means.

  According to the fourth aspect of the present invention, a negative voltage is applied to the electrostatic objective lens to form a decelerating electrostatic objective lens, whereby the equipotential surface oozes out, that is, a charged particle optical system lens barrel. Since the acting force in the direction in which the light is emitted from the device acts, detection electrons to be directed to the signal detection means can be prevented from entering the charged particle optical system.

  According to the fifth aspect of the present invention, the deflecting means for scanning the charged particle beam is disposed between the electrostatic objective lens means and the electron gun, so that the exit aperture of the electrostatic objective lens and the charged particle beam is provided. As a result, it is possible to prevent the detected charged particles from entering the charged particle optical system.

  According to the sixth aspect of the present invention, for example, by placing a decelerating electrostatic objective lens in the vicinity of the exit opening of the charged particle beam, an electric field vector in a direction in which electrons are pulled back to the sample side is provided in the exit opening. The detection electrons can be prevented from entering the charged particle optical system.

  According to the seventh aspect of the present invention, by arranging a member that generates emitted electrons at the time of an electron collision in the vicinity of the periphery of the exit opening of the charged particle beam from which the charged particle beam is emitted from the charged particle optical system. It is possible to detect electrons well.

  According to the invention described in claim 8, the charged particle beam is scanned under the condition that the velocity vector in the sample vertical direction of the charged particle incident on the sample is reversed, and the inverted particle is measured. Thus, it is possible to provide an apparatus capable of measuring potential distribution, which has been extremely difficult in the past, with high resolution on the order of microns.

  In the case of primary inversion particles, the distribution of energy in the direction of the electron orbit is very small. Therefore, in the conventional method, the rate of secondary electrons entering the exit aperture becomes very high. The present invention using an electrostatic objective lens is particularly effective in that it can be blocked.

  According to the ninth aspect of the present invention, by providing the charging means and the exposure means necessary for forming the electrostatic latent image, real-time measurement becomes possible, and the electrostatic charge of the photoreceptor whose surface charge amount attenuates with time. It becomes possible to measure the latent image with a high resolution on the order of microns.

  In addition, by measuring the electrostatic latent image on the photoconductor and feeding back the information to the design of the image forming apparatus, the process quality of each process for image formation is improved. Stable and energy saving can be realized.

  According to the tenth aspect of the present invention, a normal location and electrical breakdown are generated by detecting an electronic signal of secondary electrons or tertiary electrons obtained by irradiation of charged particles. It is possible to identify the location. In addition, by identifying the charge leak location and evaluating the quality, it is possible to provide a photoreceptor that achieves high durability and high image quality.

  The features of the present invention will be described in detail below with reference to the illustrated embodiments.

  FIG. 1 shows an embodiment of a measuring apparatus suitable for carrying out the measuring method according to the present invention.

  The measuring apparatus 10 according to the present invention includes a casing 10a, a charged particle irradiation unit 11 that is accommodated in a vacuum in the casing, and irradiates a charged particle beam, a sample setting unit 12, a primary inverted charged particle, and a secondary And signal detection means 13 comprising a charged particle trap for detecting electrons and the like.

  As used herein, charged particles refer to particles that are affected by an electric or magnetic field, such as an electron beam or an ion beam. Hereinafter, an example in which an electron beam is irradiated will be described.

  The charged particle irradiation unit, that is, the electron beam irradiation unit 11 includes an electron gun 14 which is an electron beam generating unit, a lead electrode 15 for generating a strong electric field and emitting electrons from the tip of the emitter by the action of the electric field, The electron beam is emitted from the electron gun 14 to the electron optical system 17 through the extraction electrode 15 and the acceleration electrode 16. In the illustrated example, the electron optical system 17 that receives the emitted electron beam includes a condenser lens (electrostatic lens) 18 that focuses the electron beam and a beam blanking electrode that turns the electron beam on and off. A beam blanker 19, an aperture 20 that is a movable diaphragm for controlling the irradiation current of the electron beam, a sting meter 21 for astigmatism correction, and a deflection electrode for scanning the electron beam that has passed through the sting meter And an objective lens 24 composed of an electrostatic lens for condensing the electron beam that has passed through the scanning lens onto the sample 23 on the sample setting unit 12. A driving power supply (not shown) is connected to each optical system element such as the lens. In addition, an emission electron generating member 27 having an opening 27a aligned with the opening 25 is disposed on the side of the sample placement portion 12 in the vicinity of the beam emitting opening member 26 in which the emission opening 25 of the electron optical system 17 is formed. .

  In the example shown in FIG. 1, electrostatic lenses are used for the electron lenses 18 and 24 of the electron optical system 17 of the measuring apparatus 10, but a magnetic lens similar to the conventional one can be used for the condenser lens 18.

  The electrostatic lens and the magnetic lens will be described. The electronic lens includes an electrostatic lens and a magnetic lens. A magnetic lens is a lens that focuses an electron beam by a magnetic field, and bends the electron beam by a magnetic field generated by passing a current through a solenoid magnet wound with a coil. Changing the current to the coil changes the generated magnetic field, changing the focal length and magnification. Sometimes called an electromagnetic lens or a magnetic lens, the above-described conventional apparatus uses this magnetic lens as an objective lens.

  On the other hand, the electrostatic lens here refers to a lens that focuses an electron beam by an electrostatic field. There is a disadvantage that the aberration is larger than that of a magnetic lens, and as a scanning electron microscope, a magnetic lens is generally used as an objective lens. However, in the present invention for measuring a potential distribution, an electrostatic lens is used.

  The objective lens constituted by this electrostatic lens, that is, the electrostatic objective lens 24, basically comprises three conductive discs 24a, 24b, and 24c each having an opening formed at the center through which the beam passes. The upper and lower disks 24a and 24c are grounded. By applying a voltage to the central disk 24b, an electric field is generated, and when an electron beam passes, an electric force is received to change the course of electrons, thereby forming an electrostatic lens.

  The electrostatic objective lens 24 can be disposed in the vicinity of the beam exit aperture 25 of the electron optical system 17 by disposing the deflection electrode, that is, the scanning lens 22 between the electrostatic objective lens 24 and the electron gun 14. .

  Regardless of whether the voltage of the central disk 24b of the electrostatic objective lens is a positive voltage or a negative voltage, an effect as a convex lens is brought about. When a positive voltage is applied, incident electrons are accelerated, so that an accelerated electrostatic objective lens is formed. When a negative voltage is applied, incident electrons are decelerated, so that a decelerated electrostatic objective lens is obtained. For example, when the acceleration voltage Vb is 1 kV, a voltage of about −200 to −700 V is applied to the central disk 24 b in the deceleration objective lens, although it varies depending on conditions such as the working distance. In the case of an acceleration type objective lens, a voltage of about +1000 to 2000 V is applied to the central disk 24b. In general, an accelerating objective lens has a smaller aberration, but secondary electrons may be pulled. Therefore, when a positive voltage is applied, a force acts in the direction in which the electrons enter the electron optical system barrel, and when a negative voltage is applied, a force is exerted in the direction from which the electron optical system barrel emits (the direction in which the equipotential surface oozes out). Work. Accordingly, the electrostatic objective lens that applies a negative voltage to the disc 24b of the electrostatic objective lens 24 has an effect of suppressing the entry of electrons into the electron optical system exit aperture 25.

  In the case of an ion beam, a liquid metal ion gun or the like is used instead of the electron gun 14. In this case, if the charged particles incident on the sample 23 are positive ions, the electrostatic objective lens 24 works in reverse to the positive ions. It becomes an electric objective lens.

  A detector such as a scintillator or a photomultiplier tube is used as the charged particle trap constituting the signal detection means 13 for detecting secondary electrons, primary inversion electrons, and the like. Usually, a scintillator captures charged particles when a high voltage of about 10 kV is applied to the scintillator.

  The emission electron generating member 27 disposed around the beam emission opening 25 is a metal member such as aluminum, copper, or gold that easily emits secondary electrons when electrons collide with the member, and is grounded. It is desirable.

  FIG. 1 shows an example in which a dedicated conductive member that easily generates electrons is arranged as the emitted electron generating member 27 in addition to the beam emitting aperture member 26. However, the beam emitting aperture member 26, the emitted electron generating member 27, Can be shared as shown in FIG.

  The operation of the measuring apparatus 10 according to the present invention will be described below.

  When the electron beam collides with the sample 23 disposed on the sample setting unit 12 from the charged particle irradiation unit 11 through the emission opening 25 of the electron optical system 17, secondary electrons are generated from the sample. If there is a potential distribution or a charge distribution on the surface of the sample 23 that is irradiated with the electron beam, an electric field distribution corresponding to the surface charge distribution is formed in the space. In a region where the charge density is relatively high, an electric field strength is generated such that a force in a direction away from the sample 23 acts on the secondary electrons by irradiation with the charged particle beam. Therefore, secondary electrons from the region having a high charge density reach the detector 13. On the other hand, in the region where the charge density is relatively low, an electric field strength is generated so that a force in the direction of pulling back to the sample 23 is generated. Therefore, the secondary electrons generated in the region where the charge density is low The amount reaching the detector 13 is reduced. Therefore, as is well known in the art, a contrast image corresponding to the surface charge distribution on the sample 23 can be detected by signal processing from the signal detection means 13 as a detector.

  The relationship between the charge density and the movement of electrons will be described with reference to FIG. FIG. 2A illustrates the potential distribution in the space between the detector 13 which is a charged particle trap and the sample 23 in an explanatory diagram with contour lines. The surface of the sample 23 is uniformly charged to a negative polarity except for a portion where the potential is attenuated by light attenuation, and a positive potential is applied to the detector 13, which is indicated by a solid line. In the potential contour line group, the potential increases as it approaches the detector 13 from the surface of the sample 23.

  Accordingly, the secondary electrons el1 and el2 generated at the points Q1 and Q2 in the drawing, which are parts of the sample 23 that are uniformly charged to the negative polarity, are attracted to the positive potential of the detector 13, and the arrows G1 and G2 And is captured by the detector 13.

  On the other hand, in FIG. 2A, a point Q3 is a portion where the negative potential is attenuated by light irradiation, and the arrangement of potential contour lines is indicated by a broken line in the vicinity of the point Q3. The closer to, the higher the potential. In other words, the secondary electron el3 generated in the vicinity of the point Q3 is acted on by the electric force restrained on the sample 23 side as indicated by the arrow G3. For this reason, the secondary electron el3 is trapped in the potential hole indicated by the broken line potential contour and does not move toward the detector 13. FIG. 2B schematically shows the above-described potential holes.

  When there is a potential or charge on the surface of the sample 23 and the magnitude of the potential is not negligible compared to the energy of electrons, for example, when the surface potential is about minus several hundreds of volts to minus 1,000 volts, An electric field larger than the electric field strength due to the pull-in voltage of the particle trap 13 is generated, and 2 of the total emitted electrons from the sample 23 including secondary electrons generated in the sample 23 are directed to the emission opening 25 which is an electron optical system opening. The proportion of secondary electrons increases.

  Here, in the measuring apparatus 10 according to the present invention, as described above, when a negative voltage is applied to the electrostatic objective lens 24, as shown in FIG. 3, the electron optical system which is an electron optical system barrel is used. An equipotential surface 28 is generated in the direction of springing out from the 17 injection openings 25 toward the sample 23. For this reason, an electric field vector 29 is generated on the electron gun 14 side in the vicinity of the beam exit aperture 25, so that electrons do not easily enter the electron optical system aperture 25. As a result, the detected electrons proceed so as to avoid the electron optical system opening 25 and collide with the emitted electron generating member 27 which is a peripheral conductive portion, so that new emitted electrons (n-th order electrons) are generated there. Since the n-order electrons are separated from the sample 23 and have a small energy of several tens of eV, they are drawn into the detector 13.

  Therefore, the electrostatic objective lens 24 to which a negative voltage is applied has an effect of suppressing the entry of electrons such as secondary electrons into the electron optical system aperture 25.

  Therefore, according to the measuring apparatus 10 according to the present invention, the electrostatic objective lens 24 can suppress the phenomenon of electrons returning to the exit aperture 25 of the electron optical system 17, thereby improving the S / N ratio. Thus, the charge distribution on the sample 23 can be measured with high resolution.

  When a dedicated conductive member that easily generates electrons is disposed as the emitted electron generating member 27 separately from the beam emitting aperture member 26, as shown in FIG. 4, the emitted electron generating member 27 includes the detector 13 that emits electrons. So that it can be easily inclined to the detector 13 side.

  In the example shown in FIG. 4, as in the example shown in FIG. 6 to be described later, the sample mounting portion 12 has a laminated structure in which an insulator 12b is interposed between a pair of conductors 12a and 12c. An adjustable applied voltage Vg is applied to one conductor 12a on which 23 is mounted, and the other conductor 12b is grounded (GND).

  Next, FIG. 5 shows a measurement model for measuring the potential distribution profile with higher accuracy. The acceleration voltage of the electron beam is Vb, and the potential of the sample 23 is Vp (<0). However, when the incident charged particles are electrons or negative ions, Vp <0. However, when the incident charged particles are positive ions, Vp> 0.

  Incident electrons initially move toward the sample 23 at a speed corresponding to the acceleration voltage Vb, but as the sample 23 approaches the surface of the sample 23, the speed changes due to the influence of Coulomb repulsion of the sample charge. In this case, the following phenomenon generally occurs.

  In the case of Vb> −Vp (FIG. 5A), the electrons reach the sample 23 although the speed is reduced.

  In the case of Vb <−Vp (FIG. 5B), the speed of the irradiated electrons is affected by the potential potential of the sample 23, and gradually decelerates. The irradiation electrons travel in the direction and reach the detector 13 without reaching the sample 23. Here, the case of primary inversion charged particles, particularly electrons, will be referred to as primary inversion electrons.

  Therefore, the potential potential distribution Vp (x) of the sample 23 can be measured by scanning the surface of the sample 23 with the incident electrons and detecting the inverted electrons with the detector 13.

  When a positive potential (Vp> 0) is given to the sample 23, positive ions such as gallium and protons may be incident.

  That is, when the sample is scanned with the acceleration voltage Vb of the charged particle under the condition satisfying the following expression (1), there is a state in which the velocity vector in the sample vertical direction of the incident charged particle is inverted, and the inversion is performed. By detecting primary inversion charged particles, the surface potential distribution of the sample can be measured.

Acceleration voltage Vb <Max | Vp (x) | (1)
Therefore, according to the measuring apparatus 10 according to the present invention, the surface of the sample 23 is scanned with the incident electrons, and these inverted electrons are detected by the detector 13, whereby the potential potential distribution Vp (x) of the sample 23 is accurately determined. It becomes possible to measure.

  As shown in FIG. 6, similarly to the example shown in FIG. 4, the sample placement unit 12 can have a laminated structure in which an insulator 12b is interposed between a pair of conductors 12a and 12c. An adjustable applied voltage Vg is applied to one conductor 12a on which the sample 23 is placed, and the other conductor 12b is grounded (GND). Since the voltage can be applied to the lower portion of the sample 23 through the one conductor 12a by using the sample mounting portion 12 having such a laminated structure, a bias is applied to the surface potential distribution Vp of the sample 23. The bias component becomes an adjustment component by adjusting the voltage Vg applied to the conductor 12a.

  FIG. 7A shows an example of the surface potential distribution Vs (x) generated by the charge distribution on the surface of the sample 23. Here, for the sake of convenience, the surface potential distribution Vs (x) refers to the surface potential distribution when the opposite surface of the dielectric sample having the charge distribution, that is, the back surface is grounded (GND).

  As is apparent from the characteristic line of the graph in FIG. 7A, the center (X = 0) potential is about −520 V, and the potential increases in the negative direction from the center toward the outside, and the radius from the center is 0. The potential in the peripheral region exceeding 1 mm is about -830V.

  FIGS. 7B and 7C are images obtained by the detection signal of the detector 13 when the sample 23 is scanned two-dimensionally. A white portion indicates a large detection amount, and a black portion indicates a small detection amount.

  When the electron acceleration voltage Vb was 600 V, the measurement result shown in FIG. 7B was obtained. In this case, the boundary between the white portion and the black portion having a difference in the detection signal amount is indicated by a contour line with Vs (x) = − 600 V as a threshold level potential (Vth). Therefore, this contrast image is represented by Vth = −600 V. It can be expressed as a contrast image. FIG. 7 (c) shows the measurement result obtained at Vb = 750V. When the acceleration voltage Vb is 750 V, the incident electron velocity is higher than that in the case of Vb = 600 V shown in FIG. 7B, so that the possibility of reaching the sample 23 is increased, and the velocity of the incident electrons is reversed. As a result of the reduction of the area, the black part increases. Therefore, a contrast image of Vth = −750 V shown in FIG. 7C was obtained. In other words, the threshold level potential indicating the boundary and the value obtained by inverting the sign of the electron acceleration voltage are equal (Vth = −Vb).

  When Vth = −Vb does not hold due to the influence of the surrounding electric field environment and the sample potential state, the electrostatic field environment of the electron optical system 17 and the detector 13 is calculated in advance and corrected based on it. As a result, high-precision measurement of the actual potential distribution can be realized.

  Since the secondary electrons generated when the primary electrons collide with the sample 23 are distributed in the energy and emission direction, only a part of the secondary electrons enter the emission opening 25 and cannot reach the detector 13. However, in the case of primary inversion electrons, the distribution of energy in the electron trajectory direction is very small. Therefore, in the conventional method, the ratio of secondary electrons entering the exit aperture 25 is very high. In this respect, the present invention using the electrostatic objective lens 24 that reliably prevents entry into the exit opening 25 is particularly effective.

  When the charged particles have a positive charge, it is desirable to make the pull-in voltage of the detector 13 negative when detecting the primary inversion charged particles in order to obtain the surface potential distribution.

  FIG. 8 shows an apparatus 100 that implements a method for forming and measuring an electrostatic latent image on a photoreceptor sample. The same functional parts as those of the measuring apparatus 10 shown in FIG. 1 are denoted by the same reference numerals.

  Although not shown, the structure of the photoconductor sample 123 on the grounded sample setting unit 12 includes a charge generation layer (CGL) and a charge transport layer (CTL) on a conductive support, as is well known. It is constructed by sequentially laminating. When the photoreceptor sample 123 is exposed in a state where the surface of the charge transport layer (CTL) is charged by electric charge, light is absorbed by the charge generation material (CGM) constituting the charge generation layer (CGL). The This light absorption generates a pair of positive and negative charge carriers in the charge generation layer (CGL). One of the pair of carriers is injected into the charge transport layer (CTL) by the electric field, and the other is moved to the conductive support by the electric field. Carriers injected into the charge transport layer (CTL) move in the charge transport layer (CTL) to the surface of the charge transport layer (CTL) by an electric field, and charge on the surface of the charge transport layer (CTL), that is, the surface of the photoreceptor. This surface charge is eliminated by combining with. As a result, a charge distribution, that is, an electrostatic latent image is formed on the surface of the photoreceptor sample 123 in relation to the charge remaining on the photoreceptor surface.

  In order to uniformly charge the surface of the photoconductor sample 123 having such characteristics, the photoconductor sample 123 is arranged on the sample setting unit 12 of the apparatus 100 according to the present invention, and the charged particle irradiation unit 11 supplies the electrons. An electron beam is irradiated through the optical system 17.

  The charged particle irradiation system including the charged particle irradiation unit 11 and the electron optical system 17 of the apparatus 100 is basically the same as that of the measurement apparatus 10 shown in FIG. 1, but for simplification of the drawing, FIG. In the example shown in FIG. 8, the extraction electrode 15 and the acceleration electrode 16 of the charged particle irradiation unit 11 are omitted, and a beam monitor 101 for observing an electron beam is provided at the output unit of the electron gun 14 instead. Although the emitted electron generating member 27 is omitted, the emitted electron generating member 27 can be provided integrally with or separately from the beam emitting aperture member 26.

  The acceleration voltage Vb of the electron beam for irradiating the photoconductor sample 123 from the charged particle irradiation unit 11 is set to an acceleration voltage higher than the acceleration voltage at which the secondary electron emission ratio δ becomes 1, whereby the photoconductor sample 123 is applied. Since the amount of incident electrons exceeds the amount of emitted electrons emitted from the photoconductor sample, electrons can be accumulated in the sample. Accordingly, by uniformly scanning the surface of the photoconductor sample 123 using an electron beam accelerated by an acceleration voltage Vb at which the secondary electron emission ratio δ is 1, the surface of the photoconductor sample 123 is uniformly minus. Can be charged. By appropriately setting the acceleration voltage and the irradiation time, a desired charging potential can be formed on the surface of the photoreceptor sample 123.

  In order to charge the photoconductor sample 123, contact charging, injection charging, and ion irradiation charging can be used as another charging means.

  Next, selective exposure is performed on the photoconductor sample 123 whose surface is uniformly charged by an exposure optical system. The exposure optical system is adjusted so as to form a desired beam diameter and beam profile.

  The exposure unit 30 constituting the exposure optical system is housed in a casing 10a that can seal the charged particle irradiation unit 11, the sample setting unit 12, the detector 13, and the electron optical system 17 under vacuum. The exposure unit 30 includes a light source 31 such as a laser diode (LD) that emits a wavelength at which the photosensitive member exhibits sensitivity, a collimate lens 32 that converts the divergent light from the light source into a parallel light beam, and the aperture of the parallel light beam. And an imaging lens group 34 a, 34 b, 34 c for converging the parallel light flux that has passed through the aperture onto the photoconductor sample 123, and a desired beam on the photoconductor sample 123. Diameter and beam profiles can be generated. Further, by controlling the light source 31 by the LD control unit 35, it is possible to irradiate the photoconductor sample 123 with an appropriate exposure time and exposure energy. For example, in order to form a line pattern by this irradiation, a scanning mechanism using a galvano mirror or a polygon mirror may be attached to the optical system of the exposure unit 30. By adding a scanning mechanism, an arbitrary latent image pattern including a line pattern can be formed in the generatrix direction of a cylindrical photoreceptor such as a photosensitive drum. That is, the shape of the photoconductor sample 123 may be a cylindrical curved surface as described above in addition to a flat surface.

  An electrostatic latent image can be formed on the surface of the photoconductor sample 123 by selective exposure of the photoconductor sample 123 by the exposure unit 30 described above.

  A detector for detecting secondary electrons or primary inversion electrons obtained by irradiating the electrostatic latent image on the photoreceptor sample 123 thus obtained by electron beam irradiation from the electron optical system 17 in the same manner as described above. The detection signal from 13 can be measured. In this electrostatic latent image measurement, the acceleration voltage Vb of the electron beam is set to an acceleration voltage at which the secondary electron emission ratio δ is equal to 1.

  In the apparatus 100 shown in FIG. 8, the detection signal from the detector 13 is output to the signal processing unit 37 through the detection unit 36, is subjected to predetermined processing by the signal processing unit 37, and is output to the measurement result output unit 38. . Further, a residual charge removing light source 40 such as a light emitting diode for removing the charge remaining on the surface of the photoconductor sample 123 after the measurement is completed is disposed in the casing 10a, and an LED for controlling the light source 40 The control unit 41, the charged particle control unit 41 that controls the scanning lens 22, the sample stage control unit 42 that controls the movement of the sample setting unit 12, and the LD control unit 35 described above operate under the control of the host computer 43.

  Further, as an application of the present invention, as will be described below, the charge leak portion of the photoreceptor can be specified and the quality can be evaluated.

  As described above, by charging the surface of the sample, electric field strength can be applied in the thickness direction of the sample. The electric field strength is desirably 10 V / μm or more. If the electric field strength is extremely small, electrostatic breakdown does not occur, and if the sample causes such damage, the entire sample may cause electrostatic breakdown, and as a result, it may be difficult to evaluate. Therefore, it is possible to evaluate the quality of a sample that does not cause electrostatic breakdown with an electric field strength of about 10 V / μm.

  An example of evaluating the insulation resistance of a photoreceptor as a latent image carrier as an insulator sample will be described below.

  Assuming that the film thickness d of the photosensitive member is 30 μm and the charging potential V is −900 V, the absolute value E of the electric field strength applied in the thickness direction of the photosensitive member is expressed by the following equation (2).

E = V / D = 30V / μm (2)
The charge leak occurs from the weakest part of the photoconductor, and the holes generated by the charge leak reach the surface of the photoconductor. Then, the positive holes cancel out the negative charges on the surface of the photoreceptor, and a charge distribution occurs on the surface as described above.

  This charge distribution is scanned with the electron beam from the charged particle irradiation unit 11 and the electron optical system 17 and the secondary electrons or n-order electrons are detected by the detector 13, thereby electrostatic discharge of the photosensitive member is prevented. It is possible to specify the generated charge leak location on the micron order.

The total amount of charge applied to the photoreceptor sample is preferably 10 ⁻⁸ C / mm ² or more when considered per unit area. The total charge Cm is determined by the irradiation current A, the irradiation area S, and the irradiation time t as shown in the following equation (3).

Cm = A × T / S (3)
For example, the irradiation current can be 5 × 10 ⁻¹⁰ A, the irradiation area can be 1 mm ² , and the irradiation time can be 20 seconds. The irradiation time should be longer in proportion to the increase in irradiation area. If the amount of irradiation current is increased, a shorter time is required. When the total charge amount irradiated to the sample is increased by extending the irradiation time or the like, the portion of the charge leak becomes conspicuous accordingly.

For example, when the irradiation current is 10 ⁻⁹ A and the irradiation area is 1 mm ² and irradiation is performed for 5 minutes, the total charge amount is 3 × 10 ⁻⁷ C / mm ² , and although it takes some time, there is a photosensitivity in which an abnormal part exists. If there is a body, the difference from the normal product becomes clear.

The withstand voltage required for the photoreceptor is usually 30 V / μm or more, and if it is high, about 40 V / μm is required. Under this condition, the charge leakage region is desirably 1% or less. If it is 1% or less, it is known that the output image is not noticeable as background stains. Therefore, as a preferable condition of the latent image carrier, when the withstand voltage is 30 V / μm or more and 40 V / μm or less, the sample is irradiated with electrons of 10 ⁻⁸ C / mm ² or more. It can be said that 99% or more of the insulation-resistant regions are not destroyed.

FIG. 9 shows the measurement results when the irradiation current A is 10 ⁻⁹ A, the irradiation area S is 1.47 mm ² , and the latent image carrier is charged under the irradiation time t of 4 minutes. In this case, the irradiated charge density Cm is expressed by the following equation (4).

Cm = At / S = 1.6 × 10 ⁻⁷ C / mm ² (4)
It can be said that the smaller the area ratio of the region where the charge leakage occurs, the higher the insulation resistance. FIG. 9A is an image result showing an example in which the charge leakage area ratio is 1.2%, and since the insulation-resistant region is 88.8%, it can be said that the sample is a bad sample in which background contamination is likely to occur. . On the other hand, FIG. 9B is an image result showing an example in which the charge leak area ratio is 0.4%, and it can be said that the hole size is small and the product is good.

  Thus, the insulation resistance of a sample can be evaluated by using the apparatus 10 or 100 according to the present invention.

FIG. 10 shows a flow for calculating the charge leak area for evaluating the insulation resistance of the photoreceptor sample. As shown in FIG. 10, for example, in the measuring device 10 of FIG. 1 or the measuring device 100 shown in FIG. 8, the electron beam from the charged particle irradiation unit 11 passes through the electron optical system 17 to the samples 12 and 123 of the sample setting unit 12. Irradiate and scan the surface to uniformly charge the surface with a predetermined charge amount (step S1)
Thereafter, in order to observe the location of dielectric breakdown due to charging in step S1, the surface is scanned with an electron beam that passes through the electron optical system 17 from the charged particle irradiation unit 11, and secondary electrons by this scanning are detected by the detector 13. (Step S2).

  The detection signal detected by the detector 13 is subjected to a two-dimensional mapping process by the signal processing unit 37 (step S3).

  The signal processing unit 37 further performs binarization processing on the information subjected to the mapping processing (step S4). Subsequently, the signal processing unit 37 identifies the location where the charge leak has occurred from the binarized data (step S5), calculates the area of the identified leak location (step S6), and The result is output to the measurement result output unit 38 (step S7), and the result is displayed on the output unit.

  According to the present invention, as described above, it is possible to prevent the charged particles from returning to the charged particle optical system by using the electrostatic lens as the lens for focusing the charged particle beam in the charged particle optical system. Thus, it is possible to measure the surface charge distribution or the surface potential distribution, which has been difficult in the past, with high accuracy.

It is an optical arrangement | positioning figure which shows typically the Example of the measuring apparatus which concerns on this invention. FIG. 2A is an explanatory diagram showing a measurement principle, FIG. 2A is an explanatory diagram showing an electron trajectory on a potential contour line, and FIG. 2B is a graph showing a charge density distribution of a sample. It is explanatory drawing which shows typically the electric potential in the injection opening vicinity of the measuring apparatus shown in FIG. FIG. 4 is a view similar to FIG. 3 showing a modification of the measuring apparatus shown in FIG. 1. 3 shows a measurement model of the measurement apparatus shown in FIG. 3, and FIG. 5A and FIG. 5B are diagrams showing case classification of the relationship between the acceleration voltage Vb of the charged particle and the potential potential Vp of the sample 23, respectively. . FIG. 6 is a view similar to FIG. 3 showing still another modification of the measuring apparatus shown in FIG. 1. FIG. 7A is an explanatory diagram showing the relationship between the potential distribution density and the detection signal, FIG. 7A is a graph showing the potential distribution on the sample surface, and FIGS. 7B and 7C are two-dimensional scans. 5 is a diagram showing contrast images with different threshold potentials of signals obtained by the above method. It is drawing similar to FIG. 1 which shows typically the other Example of the measuring apparatus which concerns on this invention. FIG. 9A and FIG. 9B are diagrams showing the results of charge leak measurement in order to evaluate the insulation resistance of each sample. It is a flowchart which shows the charge leak measurement procedure for insulation resistance evaluation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10,100 Measuring apparatus 11 Charged particle irradiation part 13 (Signal detection means) Detector 14 (Charged particle generation means) Electron gun 17 (Optical system of charged particle beam) Electron optical system 22 (Scanning means) Scanning lens 23, 123 Sample 24 (Focusing lens means) Electrostatic objective lens 25 Ejection aperture 27 Emission electron generating member

Claims (10)

  In the method of scanning a sample having a surface charge distribution or a surface potential distribution with a charged particle beam, and measuring the charge distribution or potential distribution state of the sample based on a detection signal obtained by scanning the charged particle beam, the charging A method of measuring a surface charge distribution or a surface potential distribution, wherein an electrostatic lens for focusing the charged particle beam on the sample is used in a particle beam optical system.   Charged particle generating means for generating a charged particle beam for irradiating a sample having a surface charge distribution or surface potential distribution, scanning means for scanning the sample with a charged particle beam from the charged particle generating means, and the charged particle beam Lens means for focusing the sample on the sample, and signal detection means for detecting charged particles obtained by scanning the charged particle beam by the scanning means, and the lens means has at least one electrostatic lens means. A device for measuring surface charge distribution or surface potential distribution.   3. The surface charge distribution or surface potential distribution measuring device according to claim 2, wherein the electrostatic lens means is an objective lens of an optical system of the charged particle beam.   4. The surface charge distribution or surface potential distribution measuring apparatus according to claim 3, wherein the electrostatic objective lens is a decelerating electrostatic objective lens.   The charged particle generating means has an electron gun for a charged particle beam that irradiates the sample, and the scanning means emits a charged particle beam from the electron gun between the electrostatic objective lens and the electron gun. 4. The surface potential distribution measuring device according to claim 3, further comprising a deflecting unit arranged for scanning.   3. The surface charge distribution or surface potential distribution measuring apparatus according to claim 2, wherein the exit aperture of the charged particle beam optical system has an electric field vector in a direction in which electrons from the sample are pulled back to the sample side. .   3. The apparatus for measuring surface charge distribution or surface potential distribution according to claim 2, wherein a member for generating emitted electrons at the time of electron collision is disposed in the vicinity of the periphery of the exit opening of the charged particle beam optical system.   The signal detection means is a charge in which the normal direction component of the incident velocity vector on the sample surface is reversed among charged particles incident on the sample surface by two-dimensional scanning of the sample surface by the scanning means. 3. The surface charge distribution or surface potential distribution measuring apparatus according to claim 2, wherein a detection signal is obtained by detecting particles.   A measuring apparatus according to any one of claims 2 to 8, means for generating charged charges substantially uniformly on the photosensitive member by irradiating the photosensitive member with charged particles as the sample, and the generating means An optical system means for selectively exposing the charged photoconductor, and after the exposure by the optical system means, the measurement device scans the surface of the photoconductor with an electron beam, and a detection signal obtained by the scanning And measuring the electrostatic latent image distribution on the surface of the photoconductor. Using the measuring apparatus according to any one of claims 2 to 9, a charge is applied to the sample under a condition where the electric field strength applied in the thickness direction of the sample is 30 V / μm or more and 40 V / μm or less. A latent image carrier made of a sample, characterized by having an insulation resistance region of 99% or more when irradiated with 10 ⁻⁸ coulomb / mm ² or more.

Images