JP2010211973A - Charged particle beam device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a charged particle beam device for measuring correct sample surface potential or adjusting a focus in accordance with the potential of the sample surface without the irradiation of charged particle beams on the sample. <P>SOLUTION: The charged particle beam device includes a charged particle source, an objective lens which focuses the charged particle beams emitted from the charged particle source and irradiates the beams on the sample, a negative potential applying power source which forms an electric field for decelerating charged particle beams reaching the sample by the application of a negative voltage, and a control device which achieves the focus adjustment of the charged particle beams by controlling the negative potential applying power source. The control device determines the sample potential on the basis of solid state property data and the scanning state of the charged particle beams of the sample, and controls the negative potential applying power source on the basis of the surface potential. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a charged particle beam apparatus, and more particularly to a charged particle beam apparatus having a focus adjustment function.

  With the recent high integration and miniaturization of semiconductor devices, the importance of evaluating these complicated pattern shapes and dimensions is increasing. A scanning electron microscope (SEM) is mainly used for inspection and dimension measurement of the semiconductor device pattern. The SEM converges the accelerated charged particle beam on the sample by adjusting the stage voltage for setting the objective lens and the sample, and detects secondary electrons generated when the converged charged particle beam is scanned two-dimensionally. By detecting with a detector, a two-dimensional image of the sample surface is obtained. In order to acquire a clear image, it is very important to converge (focus) the charged particle beam on the sample, and an SEM apparatus having an autofocus function is also common.

  When the sample is charged, the focus position changes according to the amount of charge. Therefore, when autofocusing is performed on a charged sample, it is necessary to increase the adjustment range of the excitation current and stage voltage of the objective lens, resulting in a problem that the autofocus time becomes long. In addition, when several SEM images are acquired on the same wafer, since the entire surface of the wafer is charged by the history up to that point, the SEM image is out of focus, and in order to acquire a clear image, Auto focus must be performed frequently.

  As a solution to such a problem of defocus due to sample charging, it is considered to know the sample surface potential (charge amount). In Patent Document 1, the sample is charged by a capacitance meter in the process of transporting the sample to the sample chamber. A technique for measuring an amount, Patent Document 2, discloses a technique for irradiating a beam while changing a retarding voltage, and a technique for estimating a charge amount based on a detected amount of secondary electrons. Patent Document 3 describes a focusing method in which the energy of an electron beam is changed in a plurality of stages, a signal obtained in each stage is scored, and the beam energy is adjusted based on the analysis of the score. Further, Patent Document 4 describes that a scan image without charge-up is formed by applying a sample voltage at which the peak of the output waveform signal of the secondary electron detector is detected to the maximum to the sample.

International Publication No. WO2003 / 007330 (corresponding US Pat. No. 6,946,656) JP 2007-257969 (corresponding US publication US 2007/0221845) JP 2001-236915 A (corresponding US Pat. No. 6,521,891) JP-A-4-229541

  Although the method of Patent Document 1 can measure the amount of charge before the measurement, strictly speaking, it is a potential measurement in a wide region including a region where the measurement is actually performed, and therefore, is different from the potential of the portion irradiated with the electron beam. There is a case. Even if the surface charge of the sample is measured by detecting the energy of secondary electrons obtained by scanning the electron beam, the surface charge is measured unless the sample is actually irradiated with the beam. Therefore, it takes a considerable amount of time to measure the surface charge. The techniques disclosed in Patent Documents 2, 3, and 4 are also the same. Unless the electron beam is actually irradiated, an appropriate sample voltage value or the like cannot be detected. Therefore, a step for obtaining an appropriate sample potential and focus condition is required.

  As described above, although autofocus is important for acquiring a clear SEM image, it is difficult to accurately grasp the sample surface potential (charge amount) at the time of image acquisition. It was necessary.

  A charged particle beam apparatus for the purpose of accurate measurement of the sample surface potential or focus adjustment according to the potential of the sample surface without irradiating the sample with a charged particle beam will be described below.

  To achieve the above object, a charged particle source, an objective lens that focuses and irradiates the sample with the charged particle beam emitted from the charged particle source, and the charged particle beam that reaches the sample by applying a negative voltage are decelerated. In a charged particle beam apparatus comprising a negative voltage application power source that forms an electric field to be controlled and a control device that controls the negative voltage application power source to adjust the focus of the charged particle beam, the control device is a physical property of the sample. The charged particle beam apparatus is characterized in that the sample potential is obtained based on data and scanning conditions of the charged particle beam, and the negative voltage application power source is controlled based on the surface potential.

  According to the above configuration, it is possible to reduce the time required for autofocus.

The schematic block diagram of a scanning electron microscope. The flowchart explaining the process which performs the autofocus by retarding focus after estimating a surface potential. 7 is a flowchart for explaining a process of automatically selecting (setting) an autofocus point from a plurality of autofocus point candidates. 7 is a flowchart for explaining a process for determining whether autofocus is necessary based on an estimated charge amount. The flowchart explaining the process which performs the multiple times scanning of the same measurement point, and estimation of surface potential in parallel. The flowchart explaining the process which determines the measurement order of a several measurement point. The flowchart explaining the process which determines the measurement order of a several measurement point. The figure explaining the focus adjustment at the time of acquiring the SEM image of the measuring object inside a sample.

  In this embodiment, a charged particle source, a detector for detecting secondary charged particles generated by scanning a charged particle beam emitted from the charged particle source, a stage for setting a measurement sample, In a charged particle beam apparatus including a power source for applying a voltage to a stage, the apparatus includes a device for estimating a sample surface potential during scanning of a charged particle beam based on a physical property value and a dimension of a sample to be measured, and using the estimated value, A charged particle beam apparatus characterized by controlling the stage potential will be described. One of the main purposes of this embodiment is to narrow the adjustment range at the time of autofocus and to shorten the autofocus time. That is, the autofocus time and the measurement time can be shortened by controlling the stage potential using the sample surface potential during the charged particle beam scanning estimated based on the physical property value of the sample to be measured.

  Hereinafter, a charged particle beam apparatus that performs focus adjustment based on a calculated value or an estimated value of a sample surface potential will be described with reference to the drawings. In the embodiment described below, a scanning electron microscope (SEM), which is one type of charged particle beam apparatus, will be described as an example. However, the present invention is not limited to this. For example, a focused ion beam (Focused) is used. It can also be applied to Ion Beam equipment.

  FIG. 1 is a schematic configuration diagram of a scanning electron microscope. The electron beam 14 emitted from the electron gun 1 is accelerated by the acceleration electrode 2, focused by the condenser lens 3 and the objective lens 6, and irradiated on the sample 7.

  The electron beam 14 is scanned one-dimensionally or two-dimensionally on the sample 7 by the scanning deflector 5. Electrons such as secondary electrons or backscattered electrons emitted from the scanning area are detected by the detector 4 and stored in a storage medium (frame memory or the like) (not shown). Although not shown in FIG. 1, each component is controlled by a control device that controls each component of the scanning electron microscope. In addition, a negative voltage application power source 9 is provided for applying a negative voltage to the sample via a sample stage 8 that moves the sample at least in the XY direction (when the ideal optical axis of the electron beam is Z). . In this embodiment, an example in which the sample 7 is directly placed on the sample stage 8 will be described. However, after placing the sample on the sample holder for transporting the sample, the sample is placed through the sample holder. You may make it apply a negative voltage to.

  The negative voltage application power source 9 usually reduces the energy that reaches the sample 7 of the electron beam 14, thereby increasing the resolution by passing the electron beam through the objective lens at a high acceleration, and lowering the energy reaching the sample. In this embodiment, it is used for adjusting the focal point of the electron beam 14. This method is called retarding focus. The sample potential estimation apparatus 10 also functions as a control apparatus that performs focus adjustment by controlling the negative voltage application power source 9.

  The sample potential estimation device 10 is necessary from the design data 11, device specification data 12, and scanning condition data 13 stored in a storage medium of a data management device (not shown) connected to the outside or an electron microscope device. It is configured so that various data can be read out. The design data is stored in GDS or OASIS format, and data such as physical property values such as the dielectric constant and conductivity of the sample are also stored. The device specification data mainly stores data unique to the device such as the distance between the sample and the objective lens (working distance) and the distance between the facing electrode facing the sample and the sample. Further, the scanning condition data 13 stores information on beam conditions that can be set by the user, such as beam acceleration energy, magnification, beam current, and voltage applied to the sample.

  The sample potential estimation device 10 includes charging information formed on the sample surface when the sample is irradiated with an electron beam, or charging-related information, sample information (design data 11), device-specific conditions (device specification data 12), And based on the setting conditions (scanning condition data 13) of the apparatus.

  In addition, the sample potential estimation apparatus 10 mainly performs charged particle trajectory calculation and potential calculation after charging. Charged particles are subjected to Lorentz force in an electromagnetic field. Therefore, by obtaining the initial velocity of the charged particles and the magnitude of the electric field and magnetic field in the measurement space from the design data 11, the apparatus specification data 12, and the scanning condition data 13, for example, if it is a primary electron, the electron gun 1 reaches the sample 7. It is possible to calculate the trajectory up to and the position and speed of collision with the sample 7. Further, the primary electrons colliding with the sample 7 are scattered in the sample and emit secondary electrons 15 and the like to cause charging. Therefore, by calculating the potential in consideration of the movement of charged particles accompanying the scattering in the sample and the electron emission process, the sample surface potential, that is, the charge amount generated by the irradiation of the electron beam 14 can be estimated.

  Some of the generated secondary electrons 15 do not reach the secondary electron detector 4 and reach the sample 7 again. Since the secondary electrons 15 that have reached the sample again are charged, the trajectory of the secondary electrons 15 is calculated in the same manner as the electron beam 14, and the potential is also calculated. Can be estimated.

  An example of a specific method for obtaining the surface potential will be described below. In this example, the sample surface charge is obtained by simulation. First, the calculation range is determined according to the measurement location. As the range, an observation region (Field Of View: FOV) + α is selected in the sample surface direction, a space region β above the surface, and a region γ below the surface in the vertical direction.

  Here, α, β, and γ are arbitrary setting items, and in this embodiment, α: 5 μm, β: 2 μm, and γ: 2 μm. This is a region necessary for tracking the behavior (such as surface return) of secondary electrons and backscattered electrons emitted from the sample by irradiating primary electrons with α and β.

  In consideration of the behavior of electrons emitted from the sample, it is desirable to secure α about 10 times the observation region. Further, the sample surface is often not flat, and there are buried wirings and the like. That is, it is necessary to capture the sample structure three-dimensionally. For example, in a semiconductor device, different materials are stacked and another material is embedded inside, and the charged state varies depending on the material, film thickness, and shape.

  In this embodiment, since the structure is defined and simulated based on the three-dimensional information and the pattern configuration outside the FOV, the influence of the groundwork and the state inside the sample are also taken into consideration. Furthermore, since the penetration depth of electrons differs depending on the energy of the primary electrons to be irradiated, it is necessary to set γ in consideration of these. In addition, secondary electrons emitted from the sample diffuse in the space above the sample, but the electric field in the space changes as the sample is charged. For this reason, it is necessary to consider an area of about several μm in the space portion.

  Next, measurement (scanning) conditions (apparatus conditions) are set. Here, the condition designated by the user includes a magnification at the time of observation. Several types of scanning order, current amount, and primary electron acceleration voltage conditions for observation at a specified magnification are prepared, and a simulation is performed under each condition.

  For physical properties of each material, if possible, scan only one line or plane before simulation, and obtain the time change (electron and hole mobility) and yield from the experimental results (change in luminance). May be.

  Even if the same material is used, the influence of charging may be different when the supply source is changed. However, by preparing parameters from the above-described experiment, it is possible to reduce variations among materials.

  As a simulation method, any one of a finite element method (FEM), a boundary element method (BEM), and a Monte Carlo method (MC) is used. Here, an example of FEM will be described. First, the electric field calculation is performed on the range set as described above, and the electric field of each region is prepared. In the obtained electric field, primary electrons are generated and irradiated on the sample surface under specified conditions (angle, energy).

  When primary electrons reach the sample surface, secondary electrons are generated in different states depending on the acceleration energy of the primary electrons, the incident angle, and the shape and material (sample information) of the surface on which the electrons have reached. As the number of secondary electrons to be generated, energy, and angle, values known in the literature are used as parameters. Since the secondary electrons generated from the surface are scattered again in the vacuum according to the energy and angle at the time of emission, the secondary electron trajectory is tracked in the vacuum. In addition, since electrons that have entered the sample diffuse in the sample and then jump out into the vacuum, trajectory tracking is performed in consideration of this point.

  When the sample surface is reached again due to surface charging or the like, secondary electron emission, reflection, or surface adhesion occurs depending on the energy, angle, and arrival location (shape, material) of the electrons. Such redeposition causes charge transfer on the sample.

  When adhering to the surface, the surface charge is changed and the next primary electron is irradiated. In addition, when it comes out of the calculation structure without reaching the surface, or when it reaches the electrode or the like, it returns to the irradiation of the next primary electron. Here, secondary electrons that satisfy certain conditions (for example, secondary electrons reaching a height of several μm from the surface) are counted and stored in a memory or a hard disk. Electrons that satisfy this condition can be handled as electrons detected in actual measurement, and the count of electrons can be made to correspond to the brightness of the image. In this case, the electronic count number and the luminance information may be stored in association with each other, and the luminance information corresponding to the calculated count number may be read.

  When all the primary electrons for one pixel have been irradiated, the advection diffusion of the charge accumulated in the surface and the sample is simulated. The charge accumulated on the sample surface and inside moves for each material on the surface and inside depending on the surrounding electric field state. The timing of the advection diffusion calculation here can be changed such as every electron irradiation, every pixel, every 10 pixels, or every line. Next, the irradiation point of the primary electrons is moved, and after the electric field calculation, the primary pixels are irradiated to the next pixel.

  In the above example, the electric field calculation is performed using FEM, but it can be replaced with BEM. In addition, when considering the influence of internal material charging in a three-dimensional structure, it is necessary to study the diffusion of electrons in the sample. By using MC, it is possible to study the diffusion in the sample after the primary electrons reach the surface.

  Note that the following should be considered as the charging phenomenon.

  First, it is considered that the charging phenomenon is classified into the size of the region to which the charge adheres, and it is considered that it is divided into a global charge of about several tens of millimeters, a local charge of about 100 μm, and a micro charge of 10 μm or less. .

(A) Global electrification Mainly, the charge accumulated in the pre-processes of measurement and inspection by SEM, or secondary electrons are returned to the sample by the electric field formed by the facing electrode placed on the sample. Among the charges formed by reattachment, the charges are formed in a relatively wide range. The spatial change is moderate, and it can be obtained from the voltage value that is in focus by changing the retarding voltage directly measured by the electrostatic potentiometer. The optical magnification can be estimated from the objective lens current after autofocus, and the length measurement value can be corrected.

(B) Local charging The influence of return electrons at the time of observing surrounding devices and the electrons returned by the magnetic field and the electrodes are related, and the astigmatism (the point that is not detected as a result of being detected due to the bending of the electron trajectory) is affected. The rate of change of deflection magnification is detected, and the influence of local charging is corrected based on this. Although it can be measured by using an energy filter, it is difficult to specify the spatial distribution due to the following supercharging of the micro charge.

(C) Microcharging Sample charging that occurs on a device structure or pixel scale on a small scale such as an LSI pattern. Causes of image measurement and pattern contour extraction accuracy may decrease due to image distortion or contrast anomalies caused by bending the trajectory of primary electrons or secondary electrons escaped from the sample due to sample charging created by electrons It becomes. Beam scanning further complicates the charging phenomenon. When the pixel is irradiated with electrons, the portion from which the secondary electrons are missing is positively charged, and the secondary electrons return to the sample. While the irradiation position is positively charged, its periphery is negatively charged due to return electrons. When moving to the next pixel, it is affected by the adjacent positive charge and the peripheral negative charge. When moving to another scan line, it is affected by the charging of the already irradiated location. At that time, since the surrounding charging changes with time, it is affected not only by space change but also time change.

  In addition, there are two types of electrification, in which the charge spreads relatively quickly on the surface of the sample and in other cases, the electrons are injected deep into the sample and the charge does not disappear for several minutes. As described above, micro-charging is a complicated phenomenon even by handling a flat sample. To elucidate, it is necessary to use a quantitative analysis tool in addition to the measurement means. In order to calculate the electric field by obtaining the potential distribution, the Poisson equation is discretized using the finite element method.

    Δφ = −ρ / ε (1)

  Here, φ is a potential, ρ is a charge density, and ε is a dielectric constant. First, φ is expressed as follows using a shape function Ni (x, y, z).

    φ = ΣNiφi (2)

  A residual R is defined by combining the following δφ (x) as a weighting function.

δφ (x) = ΣδφiNi (3)
R = εΔφ + ρ (4)

  The integral formula I using the weight function is defined as follows.

    I = ∫Rδφ (x) dV (5)

  Substituting Equations (3) and (4) into the above equation and performing partial integration gives the following equation.

I = ∫ (εΔφδφ (x) + ρδφ (x)) dV
= ∫ (ε ▽ φ) · nδφ (x) dS-∫ε ▽ φi ▽ iδφ (x) dV + ∫ρδφ (x) dV
... (6)

  Further, when the matrix is displayed, the following equation is obtained.

I = {δφ} T∫ [N] T [N] {(ε ▽ φ) n} dS + {δφ} T∫ [B] T [E] [B] dV {φ}
+ {Δφ} T∫ [N] T [N] {ρ} dV (7)

Here, [E] is a unit matrix, [N] is a matrix representation of a shape function, [B] is a derivative of [N],
{dφ (x) / dx} = [B] {φ} (8)
There is a relationship. When ∂I / ∂ {δφ} = 0, ∫ [N] T [N] {(εεφ) n} dS + {δφ} T∫ [B] T [E] [B] dV {φ}
= −∫ [N] T [N] {ρ} dv (9)
It becomes. This is the finite element formula to be solved.

Based on the above formula,
∫ [N] T [N] {(ε ▽ φ) n} dS ′ + {δφ} T∫ [B] T [E] [B] dV ′ {φ}
= −∫ [N] T [N] {ρ} dV ′ (10)
Get. Further, calculation is performed by replacing the integral part with Gauss-Legendre integral.

    ∫ [N] T [N] {ρ} dV ′ = ΣiΣjΣk [N] T [N] ρ (ξi, ηj, ζk) wiwjwk (11)

  Here, ξi, ηj, ζk are the coordinate values of the sampling points in the element, and wiwjwk is a weighting factor. The trajectory of charged particles (electrons, ions) is tracked using the following equation of motion.

    mdv / dt = q (E + v × B), dx / dt = v (12)

Using this, a four-stage fourth-order Runge-Kutta method is used to determine the temporal change in the position and velocity (x, v) of the charged particles. Particles move anywhere in space, but in order to find the Coulomb force and Lorentz force acting on them, electric and magnetic fields are interpolated with the particle position. For the efficiency of reflected electrons and secondary electrons generated when the electrons collide with the sample, experimental values of the yield versus energy curve are used. The following empirical formula is used for the angle dependence of the incident electrons. If the angle formed with the normal of the sample surface is φ, the reflected electron yield η and secondary electron yield are η = B (η (φ = 0) / B) cosφ, δ = δ (φ = 0) / cosφ (( 13)
It is expressed. When one electron enters the sample, the sample is charged by (1-η−δ) by subtraction. The secondary electron emission angle θ follows the 'cos rule'. The following empirical formula is used for reflected electrons.

    δ (θ) ∝cosθ, η (θ) ∝cos [(π / 2) (φ−θ) / (π / 2−φ)] (14)

  Here, φ is the incident angle of primary electrons. In addition, the charge phenomenon is complicated by the fact that the charge accumulated in the sample changes temporally and spatially. Charging caused by electron irradiation spreads by movement due to an electric field and diffusion due to a density gradient. Therefore, carrier movement is described using a macro model in which carrier density moves by advection and diffusion as follows.

    ∂n / ∂t + ▽ j = 0, j = μnE−D ▽ n (15)

  Here, n is the carrier density (electrons, holes), j is the current density, μ is the mobility, and D is the diffusion coefficient. The first term of the current density j is an advection term, and if it is formulated as it is, instability and vibration will occur. Therefore, it is stabilized using the upwind difference corresponding to the sign of μnE. Since μ and D differ from sample to sample, it is necessary to estimate the parameters for each material from the experimental results. These parameters can be registered in advance in the design data. The above-described sample potential estimation apparatus 10 stores a program for performing the above-described calculation, and the program is mainly composed of the following modules (a), (b), and (c).

(A) Pre-processing unit (Pre):
System creation, boundary condition input and beam / scanning condition input A three-dimensional system is created, and boundary conditions and initial conditions are input. Physical property data and secondary electron emission data are prepared for each material. Input the beam current value, energy value, spot diameter, number of pixels, scanning order, etc. as a file.

(B) Charge calculation unit:
Tracking part of charge distribution by electric field and orbit calculation and temporal and spatial change by charge transfer calculation. Calculate electric field, trajectory and charge transfer loops for each pixel. It is assumed that the particle traces to the N-order electron, and the electric field-orbit calculation is repeated for that number, and then charge transfer is obtained. In the meantime, the charge density of the sample part changes spatially and temporally, and thereafter the trajectory of the irradiated and generated electrons changes. At that time, the arrival position of electrons, potential distribution, and the like are recorded. N = 1 is an incident electron, and N ≧ 2 is an electron from the sample surface.

(C) Post-processing section (Post):
When a series of output loops such as image output and potential distribution by contouring of the secondary electron distribution is completed, the recorded electron orbit data and the like are output for each condition. For example, an SEM image can be obtained by counting the number of secondary electrons satisfying the condition for each pixel or for each region including a plurality of pixels and arranging luminance information reflecting the amount of secondary electrons. Using this program, it is possible to analyze the change in charge distribution and electron trajectory due to the beam, scanning conditions, etc., and investigate the effect of charging on the SEM image.

  In the sample potential estimation apparatus 10, the surface charge is estimated using the method as described above. It should be noted that a highly accurate charging simulation can be performed by using the above-described advanced simulation, but there is also a method for performing it more simply. Charging (local charging) generated by electron beam scanning is determined by the secondary electron emission efficiency δ of the electron beam and the amount of return electrons. That is, it is determined by the amount of electrons entering the scanning region (Field Of View: FOV) when scanning with the electron beam and the amount of electrons emitted from the region. If there are more electrons entering the sample than electrons leaving, the sample is negatively charged, and vice versa. By calculating the charge amount for the FOV, the charge of the FOV can be obtained. As an example, the charge amount of the sample may be monitored from (electron amount of electron beam−outgoing electron amount + returned electron amount). The return electrons do not always return to the FOV. For example, by multiplying the amount of return electrons by a coefficient obtained from (return electrons returning to the FOV / total amount of return electrons), the return electron amount is returned to the FOV. The amount of returned electrons may be calculated. This coefficient may be an actual measurement value or may be obtained from the above-described secondary electron trajectory simulation.

  The secondary electron emission efficiency δ varies depending on the sample material and the energy of the electron beam reaching the sample (acceleration voltage of the electron beam−voltage applied to the sample (retarding voltage)). The amount of electrons emitted may be obtained by acquiring material information and acquiring at least information on the amount of incident current and the reached energy from the scanning condition data 13. Further, the relationship between the amount of incident current, the amount of energy reached, and the sample material and the amount of emitted electrons may be stored in a database or function, and the amount of emitted electrons may be read according to the input of conditions.

  Further, as described above, the amount of return electrons that returns to the FOV also changes depending on the pattern shape, surrounding conditions, and the like. Therefore, a weighting coefficient for each piece of information is prepared and used for charge amount calculation as necessary. Anyway.

  Further, even if the electrons reach the outside of the FOV, if they reach the vicinity of the FOV, there is a possibility of affecting the focus condition. Therefore, the above coefficient can also be a value that takes the situation into consideration. If the amount of electrons reaching the FOV is obtained, the charge amount corresponding to the amount of electrons can be obtained. The charge amount is not necessarily expressed as a voltage value, and may be expressed as a value (electron amount or similar value) different from the voltage value, for example. The charge amount of the FOV portion can be obtained by adding the value related to the charge amount thus obtained to, for example, the global charge amount. Since global charging can be measured even before introduction into the sample chamber of a scanning electron microscope, for example, measurement of global charging is performed at the time of sample introduction, and the measurement result and the amount of charge obtained by simulation are added. If the focus adjustment described later is performed based on the addition, it is possible to reduce the time required for autofocus.

  As described above, estimating the surface potential of the sample 7 is particularly effective in focus adjustment (autofocus). With reference to FIG. 2, a process for performing autofocus by retarding focus after estimating the surface potential will be described. As shown in FIG. 2, the sample surface potential at the autofocus point is estimated in advance, and autofocus is performed based on the value. By applying this procedure, the amplitude of the stage potential at the time of autofocus can be reduced, so that the time required for autofocus can be shortened.

  In addition, since the point on which autofocus is performed may be other than the measurement point, there are many candidate locations on the sample. Therefore, as shown in FIG. 3, when the autofocus is performed on each autofocus point candidate, the surface potential of the autofocus point and the measurement point is estimated, and the position where the measurement point is not charged is the autofocus point. You may provide the procedure to determine. Furthermore, in FIG. 3, the position at which the measurement point is not charged is set as the autofocus point, but the purpose of the SEM measurement is to obtain an image that can be used for inspection and dimension measurement on a measurement target such as a semiconductor pattern. Therefore, the amount of charge does not need to be minimum as long as an image with a desired resolution can be acquired. Therefore, a threshold value may be set for the charge amount or the surface potential in accordance with the desired resolution, and a point not exceeding this threshold value may be set as the autofocus point.

  In the present embodiment, in the sample potential estimation apparatus 10 shown in FIG. 1, the primary electrons calculate the trajectory from the electron gun 1 to the sample 7, but it is only necessary to know the position and velocity at which the primary electron collides with the sample. Therefore, it is not always necessary to track the trajectory from the electron gun 1, and the position of the primary electrons may be measured or assumed between the electron gun 1 and the sample 7, and the trajectory of the primary electrons may be calculated from that point. Further, although the accuracy of estimating the surface potential is reduced, the position and velocity at which the primary electrons collide with the sample are assumed from the design data 11, the device specification data 12 and the scanning condition data 13 without tracking the trajectory. Therefore, the potential calculation may be performed in consideration of the movement of charged particles accompanying the scattering in the sample and the electron emission process.

  It should be noted that the relationship between the value related to surface charging and the retarding voltage may be made into a function or a database, and the retarding voltage may be derived based on the calculation of the charge amount or simulation.

  In the first embodiment, the estimated value of the surface potential is applied to autofocus, but may be applied to measurement. FIG. 4 shows a procedure for scanning the same measurement point a plurality of times. In order to obtain a clearer SEM image, the same measurement point is scanned many times to acquire an image for multiple frames. At that time, measurement is performed every time an image for one frame is acquired as shown in FIG. The surface potential of the point is estimated, and it is confirmed whether or not the value exceeds the threshold value of the surface potential at which the desired resolution is obtained. If it exceeds, a procedure of performing autofocus again can acquire a high-resolution SEM image for the measurement point. Furthermore, as shown in FIG. 5, the actual measurement and the estimation of the surface potential are performed in parallel, so that the measurement time can be further increased. Note that the autofocus procedure shown in FIGS. 4 and 5 may be the conventional autofocus procedure or the autofocus procedure shown in the first embodiment. If it is desired to shorten the measurement time as compared with the high-resolution image, the measurement may be terminated instead of performing the autofocus when the charge estimation value exceeds the threshold value.

  As another embodiment in which the estimated value of the surface potential is applied to the measurement, FIG. 6 shows a procedure for determining the measurement order of a plurality of measurement points. As shown in FIG. 6, the surface potential when scanning is performed for all the measurement points listed, and the measurement point with the smallest amount of charge is set as the first measurement point. Next, the surface potential is estimated when the remaining measurement points are scanned under the initial charging conditions at which the first measurement point is completed, and the measurement point with the smallest amount of charge is set as the next measurement point. By repeating this, it is possible to determine a measurement order that is not easily affected by charging. In FIG. 6, the measurement point with the smallest charge amount is selected for each measurement opportunity. However, as described above, the charge amount needs to be minimum if it is within a range where an image with a desired resolution can be acquired. Therefore, from the start of measurement to the end of measurement at all measurement points, a threshold value is set so that the estimated charge amount at the measurement point at each measurement is minimized or the estimated charge amount at the measurement point is always constant. The measurement order may be determined so as not to exceed.

  By providing the above-described sample potential estimation apparatus 10 with a function of estimating not only the sample surface but also the potential inside the sample, it is possible to inspect and measure the measurement object buried inside the sample. FIG. 8 shows the focus adjustment when the measurement target is buried inside the sample. When an image of the measurement target 16 inside the sample is acquired with high resolution, the primary electron beam 17 is preferably focused on the surface of the measurement target. However, the potential of the measuring object 16 inside the sample cannot be measured directly. Therefore, the above-described sample potential estimation apparatus 10 estimates the relationship between the potential of the sample stage 8 and the potential of the measuring object 16 inside the sample, and the focus adjustment is performed by the negative voltage application power source 9 based on this relationship. Thereby, it is possible to obtain an image with high resolution and to shorten the autofocus time. Although FIG. 8 shows the case where the measurement target is buried inside the sample, the focus adjustment method similar to the present embodiment is applied to the whole structure in which the primary electron beam cannot be directly irradiated to the measurement target. can do.

DESCRIPTION OF SYMBOLS 1 Electron gun 2 Acceleration electrode 3 Condenser lens 4 Secondary electron detector 5 Scanning deflector 6 Objective lens 7 Sample 8 Sample stage 9 Negative voltage application power source 10 Sample potential estimation device 11 Design data 12 Device specification data 13 Scan condition data 14 Electron Beam 15 Secondary electron 16 Measuring object 17 inside sample Focused primary electron beam

Claims (10)

A charged particle source, an objective lens that focuses a charged particle beam emitted from the charged particle source and irradiates the sample, and a negative voltage application that forms an electric field that decelerates the charged particle beam that reaches the sample by applying a negative voltage In the charged particle beam apparatus including a power source and a control device that controls the negative voltage application power source and performs focus adjustment of the charged particle beam,
The control device obtains the sample potential based on physical property data of the sample and scanning conditions of the charged particle beam, and controls the negative voltage application power source based on the surface potential. Wire device. In claim 1,
The charged particle beam apparatus, wherein the control device controls energy of a charged particle beam irradiated on the sample based on the obtained sample potential. In claim 2,
The sample potential is obtained for a plurality of points on the sample, and the charged particle beam is irradiated at a point where the difference between the sample potential and the voltage applied by the negative voltage application power source is the smallest. Charged particle beam device. In claim 2,
The said control apparatus calculates | requires the sample surface potential change by the charged particle beam scanning implemented before the desired measurement, The charged particle beam apparatus characterized by the above-mentioned. In claim 4,
The said control apparatus calculates | requires the surface potential change by scanning the same measurement point in multiple times, and determines the frequency | count of scanning of the said charged particle beam based on the said change value, The charged particle beam apparatus characterized by the above-mentioned. In claim 5,
The charged particle beam device according to claim 1, wherein the control device controls the negative voltage application power source based on the surface potential at the time of final scanning when performing scanning exceeding the determined number of scans. In claim 4,
The control device determines a measurement order of a plurality of measurement points on the basis of the obtained sample potential, and a charged particle beam device characterized in that: In claim 7,
The control device determines a measurement order of a plurality of measurement points so that a maximum value of a difference between a sample potential obtained for each measurement point and a voltage applied by the negative voltage application power source is minimized. Characterized charged particle beam device. In claim 1,
The charged particle beam apparatus characterized in that the control device obtains a potential inside the sample during charged particle beam scanning. In claim 9,
The charged particle beam device according to claim 1, wherein the control device controls the negative voltage application power source according to the obtained potential.

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