Patent ID: 12255043

DESCRIPTION OF THE DISCLOSURE

First Embodiment

Prior to explaining a scanning electron microscope according to a first embodiment, a typical scanning electron microscope is explained with reference toFIG.1.FIG.1shows a typical scanning electron microscope100. The scanning electron microscope100is an apparatus for irradiating a specimen with an electron beam and detecting reflected electrons released from the specimen to generate an image.

The scanning electron microscope100includes an electron gun12which generates electron beams, a group of lenses including a focusing lens14, a deflection lens16, and an objective lens18, a group of coils including a deflection coil and a scanning coil, and a detector20which detects the reflected electrons released from a specimen22. The electron gun12, the focusing lens14, the deflection lens16, the objective lens18, the detector20, and the specimen22are arranged in that order. The electron gun12, the group of lenses, the group of coils, and the detector20may be implemented using well-known components.

The specimen22is placed within a specimen chamber. The specimen chamber has a stage on which the specimen22is placed. The stage may be movable in three dimensions (within a plane and in a height direction) along an x axis, a y axis, and a z axis, or may be movable in two-dimensions (within the plane).

The scanning electron microscope100further includes components which are not illustrated inFIG.1, such as a replacement chamber for replacing specimens22to be placed within the specimen chamber, and a vacuum pumping system for reducing pressure in the inside of the specimen chamber, for example.

The electron beam generated by the electron gun12is irradiated onto the specimen22. The electron beam is focused through the group of lenses including the focusing lens14, the deflection lens16, and the objective lens18onto the surface of the specimen22, and is scanned on the specimen22by the scanning coil. Reflected electrons and secondary electrons are released from the specimen22irradiated with the electron beam.

The detector20is disposed above the specimen22(i.e., on an electron gun12side), to detect the reflected electrons released from the specimen22. For example, the detector20may be a semiconductor detector. The reflected electrons released from the specimen22are incident upon the detector20and detected therein. Upon detection of the reflected electrons, the detector20generates an electric signal which is amplified for generation of an image of the reflected electrons. For example, the electric signal is input into an image processor, and the image processor generates the image based on the electric signal. The image is displayed, for example, on a display.

With reference toFIG.2, a release angle of the reflected electron is explained.FIG.2is a cross sectional view of the detector20and the specimen22. The detector20has an annular shape, for example. Specifically, a circular through hole20ais defined in the center of the detector20.

A release angle θ is defined with reference to the surface of the specimen22. Angles close to an angle of a line parallel to the surface of the specimen22are referred to as low angles, whereas angles close to an angle of an optical axis along which the electron beams propagate are referred to as high angles.

Electrons scattered in an extremely shallow area close to the surface of the specimen22must be detected in order to detect a shape of a step corresponding to several atomic layers. A group of electrons released at the low angle relative to the surface of the specimen22, for example, contains a high percentage of electrons scattered at positions close to the surface of the specimen22(i.e., at shallow positions).

FIG.3shows a relationship between release angles of the reflected electrons and scattered depths of the electrons. InFIG.3, the horizontal axis represents the release angles of the reflected electrons, while the vertical axis represents average values of the scattered depths of the electrons. The specimen22is composed of gallium nitride (GaN), and an acceleration voltage of primary electrons is 3 kV. A value plotted on the vertical axis is an average of scattered depths corresponding to each release angle. The average value is calculated using Monte Carlo simulation CASINO (v2.4.8.1).

As shown inFIG.3, as release angles of the reflected electrons approach the lower angles, the scattered depths become shallower. Therefore, when the image is generated using the reflected electrons released at lower angles, a slight change in the shape of the surface of the specimen22can be identified in the image. In general, however, the number of reflected electrons released at higher angles is great, while the number of reflected electrons released at the lower angles is small. For this reason, when the specimen22is observed under ordinary measurement conditions, information about the reflected electrons released at the lower angles is buried in information about those released at the higher angles. Consequently, it becomes problematically difficult to observe the slight change in the shape of the surface of the specimen22.

To overcome the problematic difficulty, a method for shortening a distance between the detector20and the specimen22may be employed. Assuming that the reflected electrons released from the specimen22travel in straight lines, angles of detectable reflected electrons are shifted toward lower angles by shortening the distance between the detector20and the specimen22. For example, the distance between the detector20and the specimen22is reduced by moving the specimen22toward the detector20, or may be reduced by moving the detector20toward the specimen22.

Alternatively, deceleration optics may be employed. In the scanning electron microscope, a relatively high voltage of minus several kilovolts (a specimen bias voltage) may be applied to the specimen22in some cases, to create an electric field between the specimen22and the objective lens18. The electric field acts as a deceleration field on primary electrons. In this way, a chromatic aberration is improved and energy of incident electrons is reduced, which can cause the electrons to be scattered within a limited range, leading to an improved resolution of the SEM image.

On the other hand, the electric field generated by means of the deceleration optics acts, as an acceleration field, on the reflected electrons released from the specimen22. For this reason, the reflected electrons released from the specimen22are accelerated toward the objective lens18, which causes travel directions of the reflected electrons to deflect toward a high angle side. As a result, low-angle reflected electrons, which would pass through a space between the specimen22and the detector20under usual conditions, are directed into the detector20. The acceleration field also functions to increase kinetic energy of the reflected electrons.

With reference toFIGS.4A and4B, a trajectory of the reflected electron that is changed in response to application of the specimen bias voltage is explained.FIGS.4A and4Bare cross sectional views showing the detector20and the specimen22. When no specimen bias voltage is applied to the specimen22, a reflected electron24released at a low angle from the specimen22travels, as shown inFIG.4A, through the space between the detector20and the specimen22, and is therefore not detected by the detector20. On the other hand, when the specimen bias voltage is applied to the specimen22, a reflected electron26released from the specimen22is deflected toward the high angle side as shown inFIG.4B, which causes the reflected electron26to enter the detector20.

In a semiconductor detector which is commonly used as the detector20, electrons incident onto the semiconductor detector generate carriers, and an electric signal can be extracted when the carriers travel toward a P type semiconductor and an N type semiconductor. As kinetic energy of the electron incident upon the semiconductor detector becomes higher, a greater number of carriers are generated, and magnitude of the electric signal is accordingly increased. Consequently, electrons can be detected with efficiency, and an SN value is accordingly improved. In this regard, the deceleration optics can be considered an efficient technique for acquiring the reflected electrons whose release angles are low.

However, application of the specimen bias voltage may not always lead to efficient acquisition of reflected electrons having low angles.

For example, electrons can be greatly decelerated prior to reaching the specimen22by reducing a difference between an electron beam acceleration voltage and the specimen bias voltage, in order to decrease energy of electrons incident onto the specimen22to 1 keV or lower. However, decreased energy of incident electrons results in decreased energy of the reflected electrons. When the specimen bias voltage is excessively high relative to release energy of the reflected electrons, the trajectory of the reflected electron is greatly deflected toward the high angle side. In general, the detector20has the through hole20afor allowing the primary electron to pass therethrough. The reflected electrons which are greatly deflected to the high angle side will pass through the through hole20awithout being received by the detector20.

A simulated result of trajectories of the reflected electrons is shown inFIG.5.FIG.5is a cross sectional view showing the detector20and the specimen22. Conditions for simulation are indicated below.Diameter of the through hole20a:4 mmSpecimen bias voltage: −2 kVDistance between the detector20and the specimen22: 3 mmElectron beam acceleration voltage: 2.1 kVUnder the above conditions, because energy of the electrons at the time of entering the specimen22is 100 eV, maximum energy of the reflected electrons released from the specimen22is 100 eV.

Assuming that reflected electrons having energy of 100 eV are released at angles in a range between 0 degrees and 90 degrees from the surface of the specimen22, trajectories of the reflected electrons within an electric field are simulated.

The simulation result reveals, as illustrated inFIG.5, that most of the reflected electrons pass through the through hole20a, without being caught by the detector20.

As such, application of the specimen bias voltage to the specimen22does not always enable detection of the reflected electrons having the low angles. Optimum observation conditions exist for effectively detecting the reflected electrons released at the low angles, the optimum observation conditions including conditions for the specimen bias voltage, the shape of the detector20, the distance between the detector20and the specimen22, and the primary electron acceleration voltage.

<Optimum Observation Conditions>

Hereinafter, the optimum observation conditions for effectively detecting the reflected electrons released at the low angles will be described.

For example, when no specimen bias voltage is applied to the specimen22, and no strong magnetic field is created around the detector20, the reflected electrons travel in a straight line while maintaining their released angles. A range of angles of the reflected electrons detectable by the detector20can be calculated based on the shape of the detector20and the distance between the detector20and the specimen22.

However, when the specimen bias voltage is applied to the specimen22, an electric field is created between the detector20and the specimen22, which causes the reflected electrons to deflect through the electric field. Because of this, it becomes impossible to know at which angle the detected reflected electron was released. As a result, it cannot be easily determined which observation condition is suitable for effectively detecting the reflected electrons released at the low angles.

This issue is explained below with reference toFIGS.6and7.FIGS.6and7are cross sectional views showing the detector20and the specimen22.

It is assumed that reflected electrons are deflected due to the presence of the electric field, and a reflected electron28released at the release angle of 0 degrees is incident upon the detector20at a position close to the center of a detection surface (a surface facing the specimen22) of the detector20as shown inFIG.6. Because there are no reflected electrons released at an angle smaller than 0 degrees, a portion of the detection surface of the detector20that is located outward of the position close to the center (i.e., the position at which the reflected electron28released at 0 degrees is incident) receives no reflected electrons, and is not utilized for acquisition of a signal.

On the other hand, when the observation condition is established so as to cause a reflected electron30released at 0 degrees to be incident upon the outermost position on the circumference of the detection surface of the detector20as shown inFIG.7, the entire area of the detection surface of the detector20can be utilized for detecting the reflected electrons. This yields a favorable result that the reflected electrons released at the lower angles can be detected with efficiency. To achieve this, specific conditions must be defined for the specimen bias voltage, the shape of the detector20, the distance between the detector20and the specimen22, and the primary electron acceleration voltage, respectively. The conditions are computed, for example, through a simulation.

Because a trajectory of a charged particle in an electromagnetic field can be computed by means of a simulation, optimum observation conditions for effectively detecting the reflected electrons of the low angles can be computed by performing simulations while changing observation conditions. However, performing a simulation under each of the observation conditions is time-consuming work. To perform the simulation, shapes of components in the scanning electron microscope must be known. However, it is extremely difficult for a user to know all of the shapes of the components for computing the observation conditions.

With this in view, an equation to easily find the optimum observation conditions is found in this embodiment. Specifically, on the assumption that the detection surface of the detector20and the surface of the specimen22are parallel plate electrodes, a quantity of deflection of an electron is found based on an amount of force that is exerted on the electron existing between the parallel plate electrodes when a voltage is applied between the parallel plate electrodes; i.e., the detection surface of the detector20and the surface of the specimen22. In an actual scanning electron microscope, because the detection surface of the detector20and the surface of the specimen22are not infinitely parallel or flat, a resultant value obtained from the equation contains an error. Nevertheless, the resultant values may be exploited as material for determining whether or not a found observation condition is the optimum observation condition (i.e., the condition for enabling effective acquisition of the reflected electrons released at the low angles).

FIG.8shows parallel plate electrodes corresponding to the detector20and the specimen22. It should be noted that inFIG.8, one of the parallel plate electrodes corresponding to the detector20is identified with reference numeral (20), the same as that of the detector20, and the other one of the parallel plate electrodes corresponding to the specimen22is identified with reference numeral (22), the same as that of the specimen22.

Voltage applied to the specimen22is defined as Vb, and a distance between the detector20and the specimen22is defined as g. Further, an (r, z) coordinate system is defined. The origin (r=0, z=0) of the coordinate system is set to the center of the specimen22. Coordinate component r is a radius component, and coordinate component z is a height component. The detector20is placed at a position of z=g with reference to the position of the specimen22.

Maximum kinetic energy of the reflected electron released from the specimen22is defined as Va, and the release angle of the reflected electron is defined as 0. The angle θ is defined relative to the specimen22.

Coordinates (r, θ) of the trajectory of the reflected electron released from the specimen22are expressed as indicated in the following equation (1).

[Equation⁢1]z=1g⁡(1+cos⁢θ)⁢-Vb2⁢Va⁢r2+r⁢tan⁢θ(1)

The energy Vapossessed by the reflected electron at the time of release from the specimen22usually has a value equal to that of energy (also referred to as landing energy) possessed by the primary electron immediately before entering the specimen22.

FIG.9shows the trajectories determined using the above equation (1). InFIG.9, the horizontal axis represents the radius component r (mm), and the vertical axis represents the height component z (mm). InFIG.9, the trajectories are plotted every 5 degrees in a range of release angles between 0° and 45°. For example, when the height z=2 mm, reflected electrons released at a release angle of 45° or lower can be detected with the detector20having an outer radius r of approximately 4 mm (having the through hole20awhose radius is approximately 1.5 mm). In other words, when the detector20having the outer radius r of approximately 4 mm is used, the reflected electrons having the release angle of 45° or lower can be detected by placing the detector20substantially at a position where z=2 mm. It should be noted that the energy of the reflected electron is 2 keV, and the specimen bias voltage is −2 kV.

The coordinate z obtained by equation (1) for a location where the electron finally impinges on the detector20is equal to g. Then, g is substituted for z in equation (1), and equation (1) is converted, under conditions that 0≤g, 0≤Va, Vb≤0, 0≤0≤90, into the following equation (2) for obtaining a value of the coordinate component r.

[Equation⁢2]r=g⁢Va(1+cos⁢2⁢θ)⁢(-tan⁢θ+tan2⁢θ+2(1+cos⁢2⁢θ)⁢-VbVa)-Vb(2)

Equation (2) yields the coordinate component r of the position at which the electron having the kinetic energy Vaand the release angle θ impinges on the detection surface of the detector20when the voltage Vbis applied to the specimen22, and the detector20is placed at the distance g from the specimen22. When the calculated coordinate component r is found to be within the range of the detection surface, it can be determined that the reflected electron released at the release angle θ corresponding to the calculated coordinate component r can be detected by the detector20.

<Effective Detection of Low-Angle Reflected Electron>

Using equation (2), the observation condition for enabling effective detection of the low angle reflected electron is determined.

As shown inFIG.7, the observation condition under which the reflected election having the release angle of θ=0 impinges on the outer circumferential edge of the detection surface of the detector20is the optimum observation condition for enabling effective detection of reflected electrons released at the low angles. Substituting θ=0 in equation (2), the following equation (3) is obtained.

[Equation⁢3]r=-2⁢g⁢Va⁢-VbVaVb(3)

When two out of the parameters of incident energy Vaof the primary electron, the specimen bias voltage Vb, and the distance g between the detector20and the specimen22are specified, the remaining one of the parameters can be calculated. The below described equations (4), (5), and (6) are equations for calculating the parameters, in which equation (4) is an equation for calculating the incident energy Va, equation (5) is an equation for calculating the specimen bias voltage Vb, and equation (6) is an equation for calculating the distance g.

[Equation⁢4]Va=-r2⁢Vb4⁢g2(4)[Equation⁢5]Vb=-4⁢g2⁢Var2(5)[Equation⁢6]g=r⁢-VbVa2(6)

For example, in a case of detecting reflected electrons of low angles with the detector20having the detection surface whose diameter is 10 mm, when it is desired to perform observations under conditions that r=0.005 m, Va=2000, and Vb=−2000, the distance g of approximately 2.5 mm can be found as one of the optimum conditions by substituting the above values of r, Va, and Vbinto equation (6). In this way, the observation conditions for enabling effective detection of the reflected electrons released at the low angles can be calculated.

For example, the user may set the calculated parameters in the scanning electron microscope. Alternatively, the user may select two parameters out of the parameters Va, Vb, and g, and the remaining parameter may be automatically calculated using the corresponding one of equations (4) to (6) and the two parameters set by the user, and automatically set in the scanning electron microscope. For example, a controller51, which will be described below, may calculate the remaining parameter.

It is assumed, for example, that the user specifies 2000 as Vaand 3 mm as a working distance of the objective lens18. When the detection surface of the detector20is placed at a distance 0.5 mm from the objective lens18, g=2.5 mm. Then, these values are substituted into equation (5) to calculate the specimen bias voltage Vb, and the calculated specimen bias voltage Vbis automatically set. As a result, the observation conditions for enabling effective detection of the reflected electrons released at the low angles are automatically established in the scanning electron microscope.

The above-described method is not limited to application to detection of the reflected electrons of low angles, and there is no limitation to the range of release angles applicable to the above-described method. Further, there is no limitation to detection of electrons applicable to the method, such as a limitation that electrons to be detected must be released at a particular release angle or lower.

<Adjustment on High Angle Side>

Depending on specimens, an object to be detected may not necessarily be limited to the reflected electrons released at low angles, and it may be beneficial in terms of observation of the SEM image that reflected electrons whose release angles are high and whose signal quantities are great are included in the object to be detected. In such a case, for example, an observation condition for enabling detection of reflected electrons released at a release angle of 45° or lower may be specified. Using the above-described equation (2), the parameter r is calculated to find an incident position of a reflected electron released at the release angle θ, the incident position being on a rim of the through hole20ain the detector20(i.e., an inner circumference edge of the detection surface). Then, the through hole20ahaving the radius r is defined in the detector20, which enables the detector20to detect reflected electrons released at the release angle θ or lower.FIG.10shows a reflected electron32that is released at the release angle θ and incident on the inner circumference edge of the detection surface.

For example, substituting θ=45° into the equation (2) yields equation (7) shown below.

[Equation⁢7]r=-gVa(-1+1+2⁢-VbVa)Vb(7)

When an inner radius r of the detection surface (i.e., the radius of the through hole20a) is defined as r=0.0015 m, the incident energy Vaof the primary electron is defined as Va=2000, and the specimen bias voltage Vbis defined as Vb=2000, equation (7) is expressed with the distance g between the detector20and the specimen22as shown in the following equation (8).

[Equation⁢8]0.0015=-2⁢0⁢0⁢0⁢g⁡(-1+1+2)-2⁢0⁢0⁢0(8)
From equation (8), g=0.002049 m is obtained.

Then, the distance g between the detector20and the specimen22is set to approximately 2 mm. In this way, the detector20is configured to be able to detect the reflected electrons released at 45° or lower.

For example, when the user specifies two parameters from among the parameters of the acceleration voltage Va, the specimen bias voltage Vb, and the distance g between the detector20and the specimen22, and further specifies an angle range of the reflected electrons which the user desires to detect (for example, with upper and lower limits of the range), other parameters required for detecting the reflected electrons under the specified conditions may be automatically established.

This process is described in detail with reference toFIG.11.FIG.11is a cross section view showing the detector20and the specimen22. Reference sign r1represents a radius of the outer circumference of the detector20(the outer radius), and reference sign r2represents a radius of the inner circumference of the detector20(i.e., the radius of the through hole20a). Reference signs34aand34brepresent reflected electrons released from the specimen22. The reflected electron34ais incident upon a position close to the inner circumference of the detector20, and the reflected electron34bis incident upon a position close to the outer circumference of the detector20.

In a case where the user specifies the release angle to an angle θ or lower, substituting r=r1and values of observation conditions specified by the user into equation (2), the remaining parameter which is not specified by the user is calculated. For example, when the user specifies the specimen bias voltage Vband the distance g between the detector20and the specimen22, the acceleration voltage Vais obtained from equation (2). Then, using the specified and obtained parameters, the detector20can be configured to detect reflected electrons released at angles in the range desired by the user.

<Display of Detectable Angle Range>

An angle range of the reflected electrons detectable by the detector20may be calculated based the observation conditions, and presented on a display. This allows the user to recognize the angle range of reflected electrons being detected. As a result, the user can determine with reference to the displayed angle range whether or not reflected electrons are detected under the optimum observation conditions.

From the observation conditions, the incident energy Vaof the primary electron and the specimen bias voltage Vbare determined. The distance between the detector20and the specimen22is determined from an attached position of the detector20and the working distance (WD) of the objective lens18. The above-described equation (2) can be used for finding an angle θ1in a state where the parameter r matches the outer radius r1of the detector20substituted and an angle θ2in a state where the parameter r matches the inner radius r2of the detector20. For example, a value of 0 is substituted into equation (2) while changing the value in 0.1° increments in a range from 0° to 90°, to calculate r. Then, when a value closest to the outer radius r1is obtained for r by equation (2), the value of 0 substituted herein can be used as an approximate value of θ1. Similarly, when a value closest to the inner radius r2is obtained for r by the equation (2), the value of θ2 substituted herein can be used as an approximate value of θ2. After the values of θ1 and θ2 are obtained, information indicating, for example, a “Detectable Angle Range from θ1° to θ2°” is presented on the display. The below-described controller51operates the display to show the information indicating the detectable angle range.

<Divided Detector>

The reflected electrons of low angles are apt to provide information reflecting the surface shape of the specimen22, although, depending on observation conditions, there may be a situation that the information is not obtained. The reason for occurrence of the situation is explained with reference toFIGS.12to14.FIGS.12and13are cross sectional views showing a part of the specimen22, andFIG.14is a perspective view showing the detector20and the part of the specimen22.

As shown inFIGS.12to14, the specimen22has a step riser22aon the surface (for example, a region forming a wall on the surface). For example, the surface of the specimen22has a stepwise shape whose height is gradually increased in one direction, and step risers22aare accordingly formed on the surface of the specimen22.

When the step riser22ais formed on the surface of the specimen22, a quantity of reflected electrons detected by the detector20varies depending on released directions of the low-angle reflected electrons.

As shown inFIG.12, for example, reflected electrons36which are released in a rising direction of the stepwise region may be blocked by the step riser22a. Such a blocked reflected electron36is not detected by the detector20, which causes a decrease in the quantity of the signal in the vicinity of the step riser22a.

On the other hand, as shown inFIG.13, reflected electrons38which are released in a descending direction of the stepwise region are not blocked by the step riser22aand accordingly are detected by the detector20. Further, electrons released from the step riser22aare joined to the reflected electrons38. Therefore, as compared with the situation illustrated inFIG.12, the quantity of signal is increased.

As described above, the signal reflecting the surface shape of the specimen22has directional properties. For this reason, when reflected electrons are detected by means of the detector20having the annular shape shown inFIG.14, information about an uneven shape of the surface of the specimen22is cancelled, and an image representing the uneven shape cannot be obtained.

To address this issue, a detector40shown inFIG.15is used in the first embodiment.FIG.15is a plan view of the detector40according to the first embodiment. The detector40is formed in an annular shape with a through hole42defined at the center of the detector40. The detector40is divided at a parting line44into two pieces of detectors40aand40bwhich are symmetric with respect to the parting line44. A detection surface of the detector40aand a detection surface of the detector40bare of the same size and shape. The detectors40aand40bmay be physically separated, or may not be physically separated.

The detectors40aand40bare placed at positions which are 180-degree rotational symmetric with respect to an optical axis along which the electron beam travels (i.e., the center of the through hole42). Accordingly, the detection surface of the detector40aand the detection surface of the detector40bare arranged in 180-degree rotational symmetry about the optical axis of the electron beam. In this way, the detector40is configured to detect the reflected electrons on two detection surfaces arranged at 180-degree rotational symmetric positions (i.e., the detection surface of the detector40aand the detection surface of the detector40b).

Hereinafter, a quantity of signal from reflected electrons which are detected by the detector40ais referred to as a signal quantity IA, while a quantity of signal from reflected electrons which are detected by the detector40bis referred to as a signal quantity IB.

Directions are explained with reference toFIG.16.FIG.16is a perspective view showing the detector40and a part of the specimen22.

An extending direction of the parting line44for dividing the detector40into the detector40aand the detector40bis defined as an X direction. A rising direction in which the height of the stepwise region is increased on the surface of the specimen22(i.e., a stepping up direction of the step risers22a) is defined as a Y direction. Alternatively, a descending direction of the stepwise region may be defined as the Y direction.

As shown inFIG.16, for example, the detector40and the specimen22are disposed in an orientational relationship such that the X direction is perpendicular to the Y direction; i.e., the extending direction of the parting line44of the detector40is perpendicular to the stepping up or down direction of the step risers22a, and the reflected electrons are detected in the detectors40aand40b.

Detection of the reflected electrons in the above-described state of the orientational relationship is explained with reference toFIGS.17A and17B.FIGS.17A and17Bare cross sectional view showing a part of the specimen22.

As shown inFIG.17A, reflected electrons38released in the descending direction of the stepwise region travel without being blocked by the step riser22a. Because the detector40ais designed to detect the reflected electrons38released in this direction, the signal quantity IAis increased due to the presence of the step riser22a.

On the other hand, as shown inFIG.17B, reflected electrons38released in the rising direction of the stepwise region are blocked by the step riser22a. Because the detector40bis designed to detect the reflected electrons38released in this direction, the signal quantity IBis decreased due to the presence of the step riser22a.

Accordingly, in a region around the step riser22a, a difference between the signal quantity IAand the signal quantity IBis conspicuous. Meanwhile, in regions other than the region around the step riser22a(i.e., flat regions), because there is no hinderance to reflected electrons, the difference between the signal quantity IAand the signal quantity IBis inconspicuous.

Because of such a phenomenon, the difference between the signal quantity IAand the signal quantity IBcan be utilized for generating an image in which the step riser22ais enhanced.

<Enhanced Representation of Step Riser>

The image in which the step riser22ais enhanced can be obtained by means of the difference between the signal quantity IAand the signal quantity IBas described above, while in the image generated according to the above-described method, the step riser22amay appear excessively bright or dark depending on the relationship between the extending direction of the parting line44of the detector44and the rising direction of the stepwise region. In such a case, it might be difficult for the user to recognize a portion of the image corresponding to the step riser22a. To avoid this, it is necessary to perform a process to adjust brightness of the step riser22a. Specifically, it may be considered, for example, that an absolute difference ISbetween the signal quantity IAand the signal quantity IBis calculated as shown in the below-described equation (9), and an image is generated using the calculated absolute difference IS.

[Equation 9]
IS(x,y)=|IA(x,y)−IB(x,y)|  (9)
In equation (9), variables x and y represent positions on the surface of the specimen22.

In the region around the step riser22a, because the difference between the signal quantity IAand the signal quantity IBis great, the step riser22acan be brightened in the image by generating the image using the absolute difference IS. On the other hand, when it is desired to darken the region around the step22b, a value of the absolute difference ISadded with a minus sign can be used for generating the image.

An SEM image shown inFIG.18is generated based on the absolute difference ISobtained in a state where the X direction is perpendicular to the Y direction. In this state, the extending direction of the parting line44of the detector40is perpendicular to the rising direction of the stepwise region, and the step riser22ais brightly represented.

In addition, a filtering process may be applied to the acquired SEM image.FIG.19shows an image obtained by applying a Gaussian filter to the SEM image shown inFIG.18. As shown inFIG.19, the step riser22ais further enhanced by applying the filter.

Meanwhile, in a case where the specimen22is inclined relative to the detector40; i.e., the detection surface of the detector40is not positioned in parallel with the surface of the specimen22, use of equation (9) may not lead to enhanced representation of the step riser22a. In this case, for example, an average value IAmeanof the signal quantity IAand an average value IBmeanof the signal quantity IBmay be calculated, and an image may be generated based on an absolute difference between a value of (IA(x, y)−IAmean) and a value of (IB(x, y)−IBmean). In this case, a signal difference resulting from the inclination is compensated, which renders the step riser22aeasily recognizable.

Also depending on the relationship between the X direction and the Y direction; i.e., the relationship between the extending direction of the parting line44of the detector40and the rising direction of the stepwise region, the step riser22amay be unrecognizably represented. For example, in a case where the extending direction of the parting line44of the detector40(the X direction) is aligned with the rising direction of the stepwise region (the Y direction) as shown inFIG.20, because almost no difference is found between the signal quality IAand the signal quality IB; i.e., the absolute difference ISis small, the step riser22adoes not appear in the resulting SEM image as shown inFIG.21.

To circumvent such a problem, it may be considered to rotate a stage on which the specimen22is placed while observing an image being generated based on the absolute value IS, until the specimen22is adjusted to an orientation where the step riser22ais represented in an emphasized manner. Manual operation to adjust the orientation of the specimen22every time the specimen22is replaced with another one is a time-consuming task for the user. The operation to adjust the orientation may be automatically performed. A process of automatically performing the operation to adjust the orientation is described below.

<Rotation of Specimen Stage>

For example, it is necessary that the SEM image be acquired while the stage for the specimen22is rotated, to automatically determine whether the step riser22ais visible in the SEM image. When standard deviations (of pixel values in the SEM images) are respectively calculated for the SEM image illustrated inFIG.18(in which the step riser22ais represented in an enhanced manner) and the SEM image illustrated inFIG.21(in which the step riser22ais not represented in the enhanced manner), a greater standard deviation is obtained for the SEM image illustrated inFIG.18. Because of this, whether or not the relationship between the extending direction of the parting line44of the detector40and the rising direction of the stepwise region satisfies a condition which is optimum for representing the step riser22ain the enhanced manner can be determined by calculating the standard deviations for SEM images which are acquired while rotating the stage, and evaluating the calculated standard deviations.

Specifically, the specimen22, such as a wafer, is placed on the stage in such a manner that the surface of the specimen22can be observed even though the stage is rotated. The electron beam is focused on the surface of the specimen22, and the stage retaining the specimen22is rotated by a predetermined angle, for example, in a range from 0° to 90°, and intermittently moved to positions separated by the predetermined angle. At each of the rotationally moved positions, the absolute difference ISis calculated from the signal quantities IAand IB, and an image is generated based on the absolute difference IS. In this way, images are acquired at the rotationally moved positions, and the standard deviation is calculated for each of the images. Then, a position at which a maximum value of the standard deviation is obtained is identified from the rotationally moved positions. The image in which the step riser22ais enhanced can be obtained by setting the stage to the identified position. It should be noted that in addition to or instead of rotating the stage with the specimen22, the detector40may be rotated. The angular relationship between the X direction and the Y direction may be changed by at least one of the specimen22and the detector40.

<3D Reconstruction of Surface Structure>

The image representing the step riser22ain the enhanced manner can be obtained by using the absolute difference ISdefined by equation (9), while the rising direction of the stepwise region (the Y direction) on the surface of the specimen22cannot be identified. In other words, it is impossible to identify which one of flat portions across the step riser22ais located at a position higher than the other one of the flat portions.

For identification of the rising direction of the stepwise region, a scanning electron microscope10according to the first embodiment is configured to reconstruct a three-dimensional shape of the surface of the specimen22using signals obtained from the detector40for acquiring a shape of the stepwise region including the step risers22a.

To this end, the following equation (10) is defined.

[Equation⁢10]Is=IA-IBIA+IB(10)

As indicated in equation (10), the difference between the signal quantity IAand the signal quantity IBis normalized using the total signal quantity (IA+IB). When a normalized difference ISobtained by the equation (10) has a value of 0, the normalized difference ISindicates that a flat surface including no projections or depressions is present in a region around the position at which the electron beam is incident on the specimen22. The normalized difference IShaving values other than 0 indicates that projections or depressions are present is the region. When the region including projections or depressions has the stepwise shape, values of the normalized difference ISvary depending on a distance between the incident position of the electron beam and the step riser22aand the height of the step riser22a. In addition, the rising direction of the stepwise region can be found based on whether the normalized difference IShas a positive value or a negative value. In other words, a value corresponding to a change in height of the surface can be obtained. Values representing the surface shape of the specimen22can be obtained by integrating the normalized difference IS.

Hereinafter, a structure of an apparatus for acquiring the surface shape and directions associated with the apparatus are described.

FIG.22shows components of the scanning electron microscope10according to the first embodiment. The scanning electron microscope10is configured to irradiate the specimen22with an electron beam and detect reflected electrons released from the specimen22for generating an image.

The scanning electron microscope10includes, as in the case of the scanning electron microscope100illustrated inFIG.1, the electron gun12which generates the electron beam, the group of lenses including the focusing lens14, the deflection lens16, and the objective lens18, and the group of coils including the deflection coil and the scanning coil. Here, the electron gun12, the group of lenses, and the group of coils constitute an exemplary electron beam irradiation apparatus which irradiates the specimen22with the electron beam while scanning the electron beam on the specimen22. Further, the scanning electron microscope10includes, in place of the detector20, the detector40illustrated inFIG.15. The electron gun12, the focusing lens14, the deflection lens16, the objective lens18, the detector40, and the specimen22are arranged in that order. The scanning electron microscope10is able to apply a negative voltage of several kilovolts to the specimen22. The scanning electron microscope10may include a secondary electron detector which detects secondary electrons generated in the specimen22.

The specimen22is placed in the specimen chamber. In the specimen changer, a stage is installed, and the specimen22is placed on the stage. The stage may be movable in three dimensions (within a plane and in a height direction) along an x axis, a y axis, and a z axis, or may be movable in two dimensions (within the plane). The stage is further configured to be rotatable. The stage may include an arrangement capable of inclining the specimen22while maintaining the specimen22in position.

The scanning electron microscope10further includes the replacement chamber for replacing specimens22to be placed within the specimen chamber, the vacuum pumping system for reducing pressure in the inside of the specimen chamber, and other components, which are not illustrated inFIG.22.

The scanning electron microscope10includes a signal acquiring unit46, a signal processor48, the display50, and the controller51.

The electron beam generated by the electron gun12is irradiated onto the specimen22. The electron beam is focused through the group of lenses including the focusing lens14, the deflection lens16, and the objective lens18onto the surface of the specimen22, and scanned thereon by the scanning coil. The reflected electrons and the secondary electrons are released from the specimen22through irradiation with the electron beam.

The detector40is disposed above the specimen22(i.e., on an electron gun12side), and configured to detect the reflected electrons released from the specimen22. The detector40is, for example, a semiconductor detector. The reflected electrons released from the specimen22are incident upon the detector40and detected therein. Specifically, the reflected electrons are detected by either the detector40aor the detector40b, depending on the release angles of the reflected electrons. Electric signals are generated upon detection of the reflected electrons, and subsequently acquired as data of scanned positions of the electron beam and the signal quantity by the signal acquiring unit46. The data acquired by the signal acquiring unit46is output to the signal processor48.

The signal processor48generates the image based on the data transmitted from the signal acquiring unit46. The generated image is displayed on the display50. For example, the signal processor48calculates the shape of the surface of the specimen22, and causes the display50to display the calculated result.

The signal processor48performs a calculation for obtaining the standard deviation of the image and the calculation according to equation (10). The signal processor48may be configured to perform the calculation according to equation (9).

The controller51controls each component in the scanning electron microscope10. For example, the controller51controls rotation of the stage on which the specimen22is placed. For example, an arrangement including a motor, for example, for rotating the stage is installed in the scanning electron microscope10, and the controller51controls rotation of the motor to control rotation of the stage. In this way, the controller51controls rotation of the specimen22. For example, the scanning electron microscope10irradiates the specimen22with the electron beam while rotating the specimen22to obtain the SEM image. Further, the controller51performs calculations according to equations (1) to (8), and causes the display50to display the above-described “Detectable Angle Range from θ1° to θ2°”.

Apparatuses, such as the signal acquiring unit46, the signal processor48, and the controller51, may be cooperatively implemented by hardware and software, for example. Specifically, the apparatuses are equipped with one or more processors, such as a CPU. The functions of the apparatuses are implemented when programs stored in an unillustrated storage are read and executed by the one or more processors. For example, the apparatuses may be composed of a personal computer (PC). Alternatively, components in the apparatuses may be implemented by hardware resources, such as a processor, an electronic circuit, and an ASICs (Application Specific Integrated Circuit). For such implementation, a device, such as a memory, may be utilized. Further alternatively, the components in the apparatuses may be implemented by a DSP (Digital Signal Processor), an FPGA (Field Programmable Gate Array), and the like.

The signal processor48performs a process in accordance with a flowchart shown inFIG.23. Initially, the signal processor48acquires data from the signal acquiring unit46(S01) and calculates the signal difference (S02) using an equation (11), which will be described below. Then, the signal processor48extracts terraces (S03), performs a labeling process (S04), and calculates an average value on a label-by-label basis (S05). The terraces refer to regions other than the step risers22a(i.e., flat regions) on the surface of the specimen22. Details of the labeling process and the calculation of the average value will be explained below. Further, the signal processor48acquires a height line profile (S06) and performs leveling of the line on the label-by-label basis (S07). The height line profile and the leveling will be described in detail further below. Then, the signal processor48compensates for the height line profile (S08) and displays the image of a three-dimensional shape (S09).

Hereinafter, each process step performed by the signal processor48is described in detail.

The specimen22is set in the scanning electron microscope10, and the observation conditions for enabling observation of step risers22aformed on the surface of the specimen22are established. That is, the observation conditions are established such that the X direction is defined to be perpendicular to the Y direction for enabling acquisition of the SEM image illustrated inFIG.18(in which the step risers22aare clearly shown). Specifically, as described in the Rotation of Specimen Stage section, the scanning electron microscope10irradiates the surface of the specimen22with the electron beam, and the controller51causes the specimen stage to be rotated by the predetermined angle in the range between 0° and 90° and moved to the positions separated by the predetermined angle. In this way, the signal quantities IAand IBand the difference ISare obtained at each of the rotationally moved positions. Images are generated based on the values of the difference IS, the standard deviation is calculated for each of the images, and the stage is set to the position at which the maximum standard deviation is obtained. In this state, the image in which the step risers22aare enhanced will be obtained.

Then, the scanning electron microscope10detects the reflected electrons from the specimen22while scanning the electron beam to acquire the signal. The signal acquiring unit46acquires the signal quantity IA(x, y) and the signal quantity IB(x, y) at each scanned position (x, y) of the electric beam. In other words, the signal quantities IA(x, y) and IB(x, y) are acquired by the signal acquiring unit46at each position (x, y) where the electron beam is incident onto the specimen22.

The signal processor48calculates the signal difference IS(x, y) between the signal quantities IAand IBusing the following equation (11).

[Equation⁢11]Is(x,y)=IA(x,y)-IB(x,y)IA(x,y)+IB(x,y)(11)

FIG.24shows an example of a map of the signal difference IS(x, y). In the map of FIG.24, values of the signal difference IS(x, y) are indicated in connection with scanned positions (x, y). Reference letters x and y inFIG.24correspond to the variables x and y of the scanned position (x, y) on the specimen22.

<Height Calculation>

Next, the signal processor48calculates, based on the signal difference IS(x, y), a height of the surface of the specimen22at each scanned position (x, y). It should be noted that the height at each scanned position (x, y) is a height which is defined relative to a predetermined reference height.

For example, the signal processor48calculates a height with respect to each of rows r=0, 1, . . . , by integrating the differences ISwith respect to the x direction for each of the rows y=0, 1, . . . according to the following equation (12).

[Equation 12]
Z(x)=k∫IS(x)dx(12)

In equation (12), a letter k represents a coefficient used for correcting the height, and the coefficient k varies depending on the observation conditions. The coefficient k is further explained below.

For example, a height of the surface of a specimen that has already been measured and known by another height measurement means (such as, for example, a scanning probe microscope (SPM)) is calculated in accordance with equation (12) taking the coefficient k as k=1. A height calculated here will be different from the know height obtained by the other height measurement means. Then, a value of the coefficient k with which equation (12) yields a height equal to the known height measured by the other height measurement means is sought while substituting different values for the coefficient k. Then, under the same observation conditions, a specimen whose height is unknown can be measured, and the height of the specimen can be calculated in accordance with equation (12) using the found value of the coefficient k. In this way, calculation of the unknown height of the specimen can be achieved.

Meanwhile, the step and terrace structure of the surface of the semiconductor wafer is formed by crystal constituting the semiconductor wafer. Therefore, the semiconductor wafer has a feature that the step riser has a height reflecting a crystal lattice when the crystal has no distortion. When it is known that a step riser having a height equivalent to one atomic layer is present in the field of view, the feature can be used for determining the value of the coefficient k in such a manner that a height of a lowest step riser matches equal to a height of one atomic layer. In this way, a true height is calculated.

FIG.25shows heights which are calculated based on the signal difference IS(x, y) shown inFIG.24in accordance with equation (12) taking the coefficient k as k=0.2. The values of (x, y) inFIG.25correspond to the scanned positions (x, y) on the specimen22.

<Height Correction>

The height Z(x, y) includes an error. The error is explained with reference toFIG.26.FIG.26shows the three-dimensional shape of the surface of the specimen22. Heights of the surface are represented with shades of gray. The heights shown inFIG.26are calculated by equation (12). The surface of the specimen22has a stepwise shape whose height is increased from left to right in the x direction, while the stepwise shape remains at the same height in the y direction. However, projections and depressions are formed in the y direction, and an actual stepwise shape is not precisely represented. Hereinafter, a method for correcting the height including an error to calculate the actual stepwise shape is described.

In the step and terrace structure on the semiconductor wafer, the terrace (flat) regions are flat on an atomic level. Using this feature, the signal processor48extracts a flat region and levels heights of the flat region. In this way, the heights are corrected. A process of correcting height is explained in detail below.

<Extraction of Flat Region>

The signal processor48determines, based on a value of the signal difference IS(x, y) at each scanned position, whether or not the surface of the specimen22is flat at the scanned position. When the signal difference IS(x, y) has a value of 0, the reflected electrons are evenly incident on the detectors40aand40b. Therefore, a scanned position at which the signal difference IS(x, y) of 0 is obtained can be regarded as a flat region. In practice, because a signal may include noise, the signal processor48specifies a threshold value, and determines from the absolute value of the signal difference IS(x, y) at a scanned position that the scanned position is in the flat region when the absolute difference IS(x, y) obtained for the scanned position is equal to or smaller than the threshold value.

For example, from the values of the signal difference IS(x, y) shown inFIG.24, the signal processor48extracts, as flat regions, scanned positions (x, y) having the signal difference IS(x, y) whose absolute value is equal to or smaller than the threshold value (of 0.2 in this example).FIG.27shows the result of extraction. InFIG.27, an area consisting of scanned positions assigned a value “1” corresponds to the flat region, and an area consisting of scanned positions assigned a value “0” corresponds to a region where the absolute value of the signal difference IS(x, y) exceeds the threshold value.

In areas extracted as flat regions, an area composed of continuously adjoining scanned positions having the value “1” is basically considered to be of the same height. Meanwhile, a scanned position (x=0, y=2) and a scanned position (x=9, y=4) are independent without being adjoined to any other scanned positions having the value “1”. These independent scanned positions have no counterpart positions having heights to be equalized together. Then, the signal processor48filters out the independent scanned positions and reassigns the value “0” to the filtered independent scanned positions.

<Labeling Process>

Next, the signal processor48defines the area composed of the continuously adjoining scanned positions having the value “1” as one group and performs labeling on defined groups. Specifically, after defining groups based on the area in which the value “1” continues, the signal processor48assigns a unique label to each of the groups. For this labeling process, any well-known labeling technique may be employed.

For example, the signal processor48performs filtering processing on the extracted result shown inFIG.27, and subsequently performs labeling processing on the filtered extracted result.FIG.28shows an example of the result of the labeling processing. In the example shown inFIG.28, one of the groups is assigned a label ‘1’, and another one of the groups is assigned a label ‘2’. In this way, each of the flat regions is grouped based on the area composed of continuously adjoining scanned positions, and the label is assigned to each of the groups. It should be noted that a label ‘0’ is assigned to the area in which the absolute signal difference IS(x, y) exceeds the threshold value (i.e., the area consisting of the value “0” inFIG.27). Hereinafter, the group assigned the label ‘1’ is referred to as Group 1, and the group assigned the label ‘2’ is referred to as Group 2.

<Hight Correction Process>

Next, a process of equalizing heights on a group-by-group basis is explained.FIG.29shows heights at the scanned position (x, y) identical to those shown inFIG.25.

The signal processor48calculates an average of the heights for each of the groups. Specifically, the signal processor48extracts heights of scanned positions (x, y) belonging to Group 1 assigned the label ‘1’, and calculates an average of the extracted heights of Group 1. Similarly, the signal processor48extracts heights of scanned positions (x, y) belonging to Group 2 assigned the label ‘2’, and calculates an average of the extracted heights of Group 2.

An overall average height Zm1of Group 1 is approximately 14.86, and an overall average height Zm2of Group 2 is approximately 29.22.

The signal processor48corrects heights in each row of y=0, 1, . . . , using the average of the heights.

Firstly, the signal processor48extracts, from scanned positions (x, y) in a row of y=0, those belonging to group 1, and calculates an average of heights of the extracted scanned positions (x, y) as an average height Zm1y0of the extracted scanned positions (x, y) in the row y=0.

Secondly, the signal processor48corrects each of the heights of the scanned positions (x, y) belonging to Group 1 in the row of y=0 in such a manner that the average height Zm1y0obtained from corrected heights in the row y=0 becomes equal to the overall average height Zm1of Group 1. Specifically, the signal processor48corrects each of the heights of the scanned positions (x, y) belonging to Group 1 in the row y=0 as indicated in the following equation (13) where reference sign Z′ (x) represents a corrected height.

[Equation⁢13]Z′(x)=Zm⁢1⁢y⁢0·Z⁡(x)Zm⁢1(13)

The signal processor48performs similar processing on those belonging to Group 2 in the row y=0. That is, the signal processor48corrects each of heights of scanned positions (x, y) belonging to Group 2 in the row y=0 in such a manner that the average height Zm2y0obtained from corrected heights in the row y=0 becomes equal to the overall average height Zm2of Group 2.

FIG.30shows the corrected heights in the row of y=0. When compared toFIG.29, it can be seen inFIG.30that heights of the scanned positions (x, y) are corrected in both Groups 1 and 2.

The signal processor48performs correction of heights in each row of y=1, 2, . . . , in a manner similar to that performed for the row of y=0. As a result, heights of the scanned positions (x, y) are corrected in each of Groups 1 and 2.FIG.31shows the corrected heights, in which the heights of the scanned positions of Groups 1 and 2 are corrected in each of the rows of y=0, 1, 2, . . . , on a group-by-group basis. Specifically, the signal processor48corrects heights of scanned positions belonging to Group 1 in the row y=1 in such a manner that an average height Zm1y1obtained from corrected heights belonging to Group 1 in the row y=1 becomes equal to the overall average height Zm1of Group 1. The same processing is performed for Group 1 in the row y=2 and subsequent rows. The same is applied to Group 2.

As shown inFIG.31, the heights of the scanned positions belonging to the same group being extracted as the same flat region have values close to each other. That is, the heights of the scanned positions belonging to Group 1 have values close to each other, and those belonging to Group 2 also have values close to each other.

As a result of the height correction process, the heights of the flat region are equalized. Hereinafter, a process for correcting heights of a region other than the flat region (mainly a region of the step riser) is explained.

Initially, the signal processor48extracts scanned positions (x, y) located at ends of each of the areas extracted as the flat regions and a scanned position (x, y) which is located at an end of the image and not contained in the flat region. In this example, scanned positions (x, y) where x=0, 3, 7, and 12 are extracted. It should be noted that the scanned positions (x, y) extracted in each row are circled by broken lines inFIG.32.

Then, the signal processor48creates, from the extracted scanned positions (x, y) in each row, a pair of scanned positions (x, y) which sandwich therebetween a region other than the flat region. In the row of y=0, scanned positions (0, 0) and (3, 0) constitute a pair, and scanned positions (7, 0) and (12, 0) constitute another pair. Here, in the pair of scanned positions (x, y), a coordinate x of the left scanned position (x, y) is referred to as a coordinate x1, and a coordinate x of the right scanned position (x, y) is referred to as a coordinate x2. The signal processor48corrects heights of the region other than the flat region using uncorrected heights and corrected heights at these coordinates.

In the pair of the scanned position (0, 0) and the scanned position (3, 0), an uncorrected height Z(x1) of the scanned position (0, 0) (i.e., an uncorrected height at a coordinate x1=0) is 3, and its corrected heigh Z′(x1) is 3. Here, because the scanned position (0, 0) is not contained in the flat region, the corrected height of the scanned position (0, 0) is equal to the uncorrected height thereof. Meanwhile, an uncorrected height Z(x2) of the scanned position (3, 0) (an uncorrected height at a coordinate x=3) is equal to 11, and its corrected height Z′(x2) is equal to 15. The signal processor48uses these values to calculate coefficients a and b used in an equation for correcting heights as shown in the following equations (14) and (15).

[Equation⁢14]a=Z⁡(x⁢1)-Z′(x⁢1)-Z⁡(x⁢2)+Z′(x⁢2)x⁢1-x⁢2(14)[Equation⁢15]b=Z⁡(x⁢1)-Z′(x⁢1)-a·x⁢1(15)

The signal processor48corrects heights using the coefficients a and b in the following equation (16).

[Equation 16]
Z′(x)=Z(x)−(ax+b)  (16)

The above-described correction processing corrects a quantity of inclination of a step (i.e., a step riser) in a state in which the uneven shape of the surface of the specimen22is reflected.

FIG.33shows values of the coefficients a and b in each row of y=0, 1, 2, . . . that are calculated for the region from x=0 to the end (x=3 in row y=0) of Group 1.FIG.34shows values of the coefficients a and b calculated for a region from the end of Group 1 (x=7 in the row y=0) to the end of Group 2 (x=12 in the row y=0).

The signal processor48corrects, in the row y=0, heights of scanned positions (x, y) contained in a range of 0<x<3 using the coefficients a=−1.22 and b=0 according to equation (16), and corrects heights of scanned positions (x, y) contained in a range of 7<x<12 using the coefficients of a=0.02 and b=−3.80 according to equation (16). Then, the signal processor48performs the same correction processing on the subsequent rows y=1, 2, and so on. In this way, the heights of the region other than the flat region are corrected.FIG.35shows corrected heights of all of the scanned positions (x, y).

FIG.36shows a three-dimensional shape of the surface of the specimen22that is obtained after height correction. The three-dimensional shape illustrated inFIG.36is obtained by applying height correction to the three-dimensional shape illustrated inFIG.26.

As is evident from comparison between the three-dimensional shape obtained before height correction (illustrated inFIG.26) and the three-dimensional shape obtained after height correction (illustrated inFIG.36), terrace regions on the surface of the specimen22, which is the semiconductor wafer, are smoothened by the correction, while the step shape is rendered easily recognizable.

It is difficult to place the specimen22and the detector40at positions where the detection surface of the detector40is located precisely in parallel to the flat regions on the surface of the specimen22being the semiconductor wafer. Accordingly, equation (9) does not always yield a value of 0 for the flat region. When the value of 0 is not obtained for the flat region on the surface of the specimen22, the signal processor48corrects the signal quantities IAand IBso as to obtain the value of 0 for the flat region from equation (9).

For example, a flat region is specified by the user, and the signal processor48corrects the signal quantities IAand IBusing values at scanned positions (x, y) corresponding to the specified flat region.

Initially, the signal processor48calculates the absolute difference IS(x, y) using equation (9), generates an image using the calculated absolute difference IS(x, y), and operates the display50to display the generated image. The user identifies the step shape on the surface of the specimen22while viewing the displayed image, and specifies a scanned position (xp, yp) corresponding to the flat region.

The signal processor48corrects the signal quantities IAand IBto signal quantities IA′ and IB′ using signal quantities IA(xp, yp) and IB(xp, yp) at the specified scanned position (xp, yp) according to the following equations (17) and (18).

[Equation⁢17]IA′(x,y)=IA(x,y)IA(xp,yp)⁢IA(xp,yp)+IB(xp,yp)2(17)[Equation⁢18]IB′(x,y)=IB(x,y)IB(xp,yp)⁢IA(xp,yp)+IB(xp,yp)2(18)

The signal processor48calculates a difference IS′ between the corrected signal quantities IA′ and IB′. In this way, a smaller signal difference can be obtained for the flat region.

It should be noted that the user may specify two or more scanned positions (x, y), and the signal processor48may calculate an average of signal quantities IAat the specified two or more scanned positions (x, y) and an average of signal quantities IBat the specified two or more scanned positions (x, y). In this case, the calculated averages are substituted for the signal quantities IA(xp, yp) and IB(xp, yp) in equations (17) and (18), respectively.

According to the above-described first embodiment, the two-piece detector40(consisting of the detectors40aand40b) is used for detecting the reflected electrons released from the specimen22while radiating the electron beam onto the specimen22and rotating the specimen22. As a result, the difference in the signal quantity between the signal from reflected electrons detected by the detector40aand the signal from reflected electrons detected by the detector40bcan be calculated and evaluated to identify the angle of rotation for the specimen22to be set at the position where the image representing enhanced steps can be obtained. Then, the specimen22is rotated by the identified angle of rotation. At the rotated position, an image representing enhanced steps can be obtained by detecting the reflected electrons.

Further, based on the obtained difference in the signal quantity, the heights of the surface of the specimen22can be calculated and corrected (seeFIG.36) through the height correction process including the labeling and extraction of flat regions.

Still further, the range of release angles of the reflected electrons detectable with the detector40can be calculated based on the energy of the electron beam, the bias voltage applied to the specimen, and the distance between the specimen22and the detector40, and the calculated range can be presented to the user.

In addition, when the user selects two parameters from the parameters consisting of the energy of the electron beam, the bias voltage applied to the specimen22, and the distance between the specimen22and the detector40, the remaining parameter needed to detect the reflected electrons released from the specimen22at angles in the presented range of release angles can be automatically calculated. Accordingly, the user does not need to manually specify all of the parameters.

Second Embodiment

Hereinafter, a scanning electron microscope according to a second embodiment will be explained. In the second embodiment, a four-piece detector50is used in place of the two-piece detector40.

The use of the detector40according to the first embodiment involves adjusting the relationship between the direction of the parting line44of the detector40(the X direction) and the rising direction of the stepwise region on the surface of the specimen22(the Y direction), and thus rotating the specimen stage to adjust the relationship. When a detector52according to the second embodiment is used, the image in which step risers are emphasized can be generated without the necessity of adjusting the relationship between a direction of a parting line of the detector52and the rising direction of the stepwise region (the Y direction) on the surface of the specimen22, and thus without the necessity of rotating the specimen stage.

Hereinafter, the detector52according to the second embodiment is explained with reference toFIG.37.FIG.37shows a plan view of the detector52.

The detector52is formed in an annular shape with a through hole52ein the center thereof. A detection surface of the detector52is divided into four detectors by parting lines52fand52gwhich are perpendicular to each other, and the detector52has detection surfaces52a,52b,52c, and52d. The parting line52fseparates the detection surfaces52aand52bfrom the detection surface52dand52c, respectively, and the parting line52gseparates the detection surfaces52aand52dfrom the detection surfaces52band52c, respectively. The detection surfaces52a,52b,52c, and52dare placed in rotation symmetric positions with respect to the optical axis of the electron beam (i.e., the center of the through hole52e) that is taken as an axis of rotation. For example, the detection surfaces52a,52b,52c, and52dare arranged in that order in a circumferential direction of the detector52, and the four detectors are placed at positions which are 90-degree rotational symmetric with respect to the optical axis taken as the axis of rotation. It should be noted that the detection surfaces may or may not be physically spaced from each other. In the example shown inFIG.37, the detection surfaces52a,52b,52c, and52dare of the same shape and size.

Hereinafter, quantities of signals from reflected electrons detected on the detection surfaces52a,52b,52c, and52dare referred to as signal quantities Ia, Ib, Ic, and Id, respectively.

Using the detector52configured as described above, the image in which the step risers are enhanced can be generated without the necessity of adjusting the relationship between the direction of the parting line52for52gin the detector52and the rising direction of the stepwise region on the surface of the specimen22. Meanwhile, as in the case of the first embodiment, the reflected electrons may be detected while rotating the specimen stage, to find an angle of rotation and rotate the specimen stage by the found angle so as to be set at the position where the greatest signal difference is obtained. In this case, because a range of the angle of rotation necessitated by the detector52is narrower than a range of the angle of rotation necessitated by the two-piece detector40, the angle of rotation can be adjusted within a shorter time.

Referring toFIGS.38and39, the reason why the image in which the step risers are emphasized can be generated without adjusting the relationship between the direction of the parting line of the detector52and the rising direction of the stepwise region (the Y direction) on the surface of the specimen22is explained below.FIGS.38and39are perspective views showing the detector52and a part of the specimen22.

As shown inFIG.38, the extending direction of one of the parting lines (the parting line52fin the example shown inFIG.38) of the detector52is perpendicular to the rising direction of the stepwise region (the Y direction) on the surface of the specimen22. In other words, the other of the parting lines (the parting line52gin the example shown inFIG.38) is parallel to the Y direction. In this case, the signal processor48divides the detection surfaces of the detector52into detection surface pairs consisting of a pair of detection surfaces located on an upper step side of the step riser22aand a pair of detection surfaces located on a lower step side of the step riser22a, calculates a total signal quantity of each of the pairs, and calculates a signal difference between the total signal quantity of the upper pair and the total signal quantity of the lower pair. Then, the signal processor48generates an image representing the calculated signal difference and causes the display50to display the generated image.

In the example shown inFIG.38, the detection surfaces52aand52blocated on the upper step side constitute one pair, and the detection surfaces52cand52dlocated on the lower step side constitute another pair. The total signal quantity of the upper pair is (Ia+Ib), and the signal quantity of the lower pair is (Ic+Id).

On the other hand, when the parting line52for52gof the detector52is not perpendicular to the Y direction or not in parallel with the Y direction as shown inFIG.39, the signal processor48calculates a signal difference between signal quantities detected by two detection surfaces of the detector52which are disposed on 180-degree rotational symmetric positions with respect to the center of the through hole52etaken as the axis of rotation. For example, the signal processor48uses the signal quantity Ibof the detection surface52band the signal quantity Idof the detection surface52dto calculate the signal difference, generates an image representing the calculated signal difference, and causes the display50to display the image.

As described above, whichever direction the rising direction of the stepwise region (the Y direction) extends relative to the parting line52for52gin the detector52, the signal difference can be calculated, and a three-dimensional shape of the surface of the specimen22can accordingly be obtained.

When the detector52is arranged with one of the parting lines52fand52gbeing perpendicular to the Y direction as shown inFIG.38, a detection surface area capable of detecting the reflected electrons inFIG.38is double a detection surface area in positional arrangement shown inFIG.39, which can help improve the SN value of the signal. Specifically, in positional arrangement shown inFIG.38, the total signal quantity (Ia+Ib) and the total signal quantity (Ic+Id) are used to calculate the signal difference, whereas in positional arrangement shown inFIG.39, the signal quantity Iband the signal quantity Idare used to calculate the signal difference. Therefore, in the positional arrangement shown inFIG.38, the signal difference can be calculated using the doubled signal quantity relative to the signal quantity used in the positional arrangement shown inFIG.39.

<Display of Emphasized Step Riser>

Even though the four-piece detector52is used, the step risers22acan also be represented in the enhanced manner. For example, a maximum value among the signal quantities Ia, Ib, Ic, and Idis defined as a maximum signal quantity Smax, and a minimum value among the signal quantities Ia, Ib, Ic, and Idis defined as a minimum signal quantity Smin. The signal processor48finds the maximum and minimum signal quantities Smaxand Sminfor each of the scanned positions (x, y), calculates a difference IS(x, y) between the maximum and minimum signal quantities Smaxand Smin, and generates an image representing the calculated difference IS(x, y). In this way, the image in which the step risers22aare clearly represented can be obtained.

The maximum signal quantity Smaxis defined as shown in the below-described equation (19), the minimum signal quantity Sminis defined as shown in the below-described equation (20), and the difference IS(x, y) is defined as shown in the below-described equation (21).

[Equation 19]
smax(x,y)=max(Ia(x,y),Ib(x,y),Ic(x,y),Id(x,y))  (19)
[Equation 20]
smin(x,y)=min(Ia(x,y),Ib(x,y),Ic(x,y),Ic(x,y),Id(x,y))  (20)
[Equation 21]
IS(x,y)=smax(x,y)−smin(x,y)  (21)

Equations other than the equation (21) may be employed. For example, the signal processor48may calculate the signal difference as shown in the following equations (22) and (23) and generate an image representing the signal difference.

[Equation 22]
s1(x,y)=|(Ia+Ib)−(Ic+Id)|  (22)
[Equation 23]
s2(x,y)=|(Ia+Id)−(Ib+Ic)|  (23)

For example, the signal processor48selects either one of the signal difference S1(x, y) or S2(x, y) having a greater value, and generates an image based on the selected signal difference S1(x, y) or S2(x, y).

<3D Reconstruction>

The signal processor48is able to reconstruct a three-dimensional shape similar to that obtained in the first embodiment, using, for example, a combination 1 of the total signal quantity (Ia+Ib) and the total signal quantity (Ic+Id) or a combination 2 of the total signal quantity (Ia+Id) and the total signal quantity (Ib+Ic).

In a case of using the combination 1, the signal processor48substitutes (Ia+Ib) for IAand (Ic+Id) for IBin equation (11) to calculate the difference IS, and generates an image representing the difference IS. In this way, the image representing the three-dimensional shape of the surface of the specimen22can be obtained.

In a case of using the combination 2, the signal processor48substitutes (Ia+Id) for IAand (Ib+Ic) for IBin the equation (11) to calculate the difference IS, and generates an image representing the difference IS. In this way, the image representing the three-dimensional shape of the surface of the specimen22can be obtained.

Alternatively, the signal processor48may select a pair of the detection surfaces which are 180-degree rotational symmetric with respect to the center of the through hole52etaken as the axis of rotation, and calculate the difference ISin the signal quantity between the detection surfaces in the selected pair.

In a case of using a combination of the signal quantity Iaand the signal quantity Ib, the signal processor48substitutes the signal quantity Iafor IAand the signal quantity Icfor IBin equation (11), calculates the difference IS, and generates the image representing the difference IS. In this way, the image representing the three-dimensional shape of the surface of the specimen22can be obtained.

In a case of using a combination of the signal quantity Iband the signal quantity Id, the signal processor48substitutes the signal quantity Ibfor IAand the signal quantity Idfor IBin equation (11), calculates the difference ISusing substituted equation (11), and generates the image representing the difference IS. In this way, the image representing the three-dimensional shape of the surface of the specimen22can be obtained.

In the first embodiment, the step riser22amay not be clearly represented, depending on the positional relationship between the extending direction of the parting line44of the detector40and the Y direction. As opposed to this, the use of the four-piece detector52enables, as described with reference toFIG.39, calculation of the three-dimensional shape of the surface of the specimen22regardless of the positional relationship between the extending direction of the parting line of the detector52and the rising direction of the stepwise region.

When four signal quantities (the signal quantities Ia, Ib, Ic, and Id) are utilized, a method other than the above-described method may be employed for generating the image representing the signal difference.

For example, a signal difference Ishin the horizontal direction and a signal difference Isvin the vertical direction may be used. In this case, equation (11) is converted into the following equations (24) and (25).

[Equation⁢24]Is⁢h=Ia+Ib-(Ic+Id)Ia+Ib+Ic+Id(24)[Equation⁢25]Is⁢v=Ia+Id-(Ib+Ic)Ia+Ib+Ic+Id(25)

In addition, a height Zhin the horizonal direction and a height Zvin the vertical direction are calculated by the following equations (26) and (27).

[Equation 26]
Zh(x,y)=k∫Ish(x,y)dx(26)
[Equation 27]
Zp(x,y)=k∫Isv(x,y)dy(27)

The calculated heights include errors. Height correction can be performed to obtain more accurate values of the heights. As in the case of the first embodiment, the signal processor48extracts a flat region and performs height correction using the signal quantity of the flat region.

For example, when the detector52is placed with the detection surfaced being in parallel to the surface of the specimen22, the signal quantities Ia, Ib, Ic, and Idobtained from the flat region have almost equal values. For this reason, when a difference between the maximum value and the minimum value in the signal quantities Ia, Ib, Ic, and Idis equal to or smaller than the threshold value, the signal processor48determines that scanned positions from which the signal quantities Ia, Ib, Ic, and Idare obtained are in the flat region.

When the detection surfaces of the detector52are not in parallel to the surface of the specimen22, the user specifies a scanned position in a flat region, for example. The signal processor48calculates a difference in the signal quantity Ia, Ib, Ic, Idbetween the specified scanned position and each of the scanned positions, and determines, when all calculated differences are equal to or smaller than the threshold value, that the specified scanned position is in the flat region.

In this way, flat regions are extracted. The signal processor48performs the labeling processing in a manner similar to the first embodiment. Then, the signal processor48corrects the heights Znand Zvin each of labeled groups, to obtain corrected heights Zh′ and Zv′. The signal processor48calculates an average of the corrected heights Zh′ and Zv′ as indicated in the following equation (28).

[Equation⁢28]Z=Zh′+Zv′2(28)

The calculated average is the height Z of the surface of the specimen22. In this way, the three-dimensional shape of the surface is calculated.

The above-described process is described by way of illustration, and other three-dimensional reconstruction processes may be employed. For example, the terrace region of the semiconductor wafer is flat on an atomic level. Also in the other three-dimensional reconstruction processes, after a flat region is extracted, an accurate step and terrace structure can be obtained by correcting heights of the extracted flat region so as to be equalized.

For example, a method for acquiring a stepwise shape by means of a photometric stereo (PS) method which is conventionally utilized for 3D reconstruction, extracting a flat region from the obtained stepwise shape, and correcting heights may be performed.

Although in the second embodiment the four-piece detector52has been used, a three-piece detector may be used. A three-piece detector54is shown inFIG.40.FIG.40is a perspective view representing the detector54and a part of the specimen22.

The detector54formed in an annular shape has a detection surface which is divided into three pieces of detection surfaces54a,54b, and54c. The detection surfaces54a,54b, and54care placed at 120-degree rotational symmetric positions with respect to the center of a through hole54d(i.e., the optical axis of the electron beam) taken as the axis of rotation.

In the example illustrated inFIG.40, the detection surface54ais located on an upper side of the step riser22aon the surface of the specimen22, and the detection surface54cis located on a lower side of the step riser22a. In this case, the signal processor48calculates a difference between signal quantities from the detection surface54aand the detection surface54c, and generates an image representing the difference. Then, the image in which the step risers22aare enhanced can be obtained.

Specifically, the signal processor48selects two detection surfaces from the detection surfaces54a,54b, and54c, calculates a difference between signal quantities from the selected two detection surfaces, and generates an image representing the difference. For example, the signal processor48may select two detectors in such a manner that a maximum difference in the signal quantity is obtained from the two detectors, and generates an image representing the difference. In this way, the image representing the step riser22ain the enhanced manner can be obtained regardless of the positional relationship between the extending direction of the parting line in the detector54and the rising direction of the stepwise region on the surface of the specimen22.

<Representation Form of Step Riser>

In the first and second embodiments, the signal processor48may represent heights of the surface of the specimen22with colors and change colors depending on the heights in the SEM image. Specifically, the signal processor48represents regions having the same height with the same single color in the SEM image, and changes colors depending on the heights. Alternatively, rather than the SEM image, the signal processor48may generate a two-dimensional image in which heights at scanned positions (x, y) are indicated with colors.

In another example, the signal processor48may represent, in the image in which the step risers are enhanced, a same terrace region with a particular color and another terrace region with another color while changing colors on a terrace region-by-terrace region basis. For example, the signal processor48may apply, in the image in which the step risers are enhanced, colors by means of a color scale for gradually mapping different colors from a low terrace region to a high terrace region. The signal processor48may extract a boundary of each of the step risers from the image in which the step risers are enhanced, and apply colors to extracted boundaries.

The boundary of the step riser in the image generated in accordance with the first embodiment may appear thicker than an actual boundary. This is caused by a phenomenon that the primary electrons are scattered and spread by the specimen22. The signal processor48may find a distribution of signal quantities in a direction vertical to the step riser between different levels represented in the SEM image, and identify a scanned position at which the maximum signal quantity is obtained as an actual position of the step riser. The signal processor48may plot the identified actual position on the image. In this way, the image reflecting the actual position of the step riser is generated and displayed on the display50.

The signal processor48may estimate a point spread function (PSF) from a scattered region of the electrons, and perform deconvolution integration processing on the image in which the step risers are enhanced, to calculate the actual position of the step riser.

In addition, the signal processor48may calculate, from a three-dimensional structure of a semiconductor wafer, a surface roughness which can be used as an index of quality of the semiconductor wafer. For example, the signal processor48may calculate a roughness parameter, such as an arithmetic mean height, and operate the display50to display the parameter.

Further, the signal processor48may evaluate the surface roughness based on an interval between step risers. For example, the signal processor48may calculate a dimension of a terrace in the vertical direction relative to the step riser, calculate a length of each terrace, variations in length, and an average of lengths of terraces, and operate the display50to display the calculated values.

Third Embodiment

Hereinafter, a third embodiment is explained. In the third embodiment, the signal processor48extracts a defect on the surface of a specimen based on the shape of the surface of the specimen.

As described above, the step risers which are observed in the first and second embodiments correspond to a part of a crystal lattice. When the crystal lattice extends without having any defect, the step risers extend without any breaks. However, an image representing an actual specimen may include a break in a line representing the step riser. The break derives from distortion, such as a screw dislocation, of the crystal lattice. Therefore, a location where the screw dislocation occurs can be identified by finding a site where the step riser is broken.

For example, the signal processor48is able to identify the site where the step riser is broken, through image processing for extracting features from an image which is generated based on differences in signal quantity. For the image processing, a corner detection method may be employed, for example.

Hereinafter, a process to extract a break in the step riser is explained with reference toFIG.41.FIG.41shows images56,58, and60which are generated based on the differences in signal quantity.

The image56shows step risers in the enhanced manner. In the image56, white areas represent terraces (i.e., flat regions), and black regions represent steps (i.e., step risers).

The image58is obtained by processing the image56through both noise reduction using, for example, a Gaussian filter, and binarization. As a result of processing the image56, the step risers are clearly differentiated from other regions in the image58. The noise reduction and the binarization may be executed, for example, by the signal processor48.

When a typical corner detection method is applied to the image58, corners of both white and black areas are detected. However, in this embodiment which is directed to detect a break in the steps (i.e., the step risers), the signal processor48extracts only corners of the step risers (i.e., corners of the black areas). The signal processor48performs corner extraction processing on the image58obtained as the binarized image, to extract the corners of the step risers. As a result, a site where the step is broken is extracted, and a position of a screw dislocation is accordingly identified. For example, in the image60, an area in a circle indicated by reference numeral62is identified as the position of the screw dislocation.

<Display of Burgers Vector>

Three-dimensional structure of steps can be identified based on the three-dimensional shape of the steps in which heights are corrected in accordance with the first and second embodiments (for example, the three-dimensional shape illustrated inFIG.36). The signal processor48may identify a winding direction (a clockwise direction or a counterclockwise direction) of the screw dislocation based on the three-dimensional structure of the steps. A height of the dislocation can be identified from a height of a step riser. The signal processor48may identify the height of the dislocation based on the height of a step riser, and calculate a Burgers vector.

For example, the signal processor48may extract a defect from an image in which step risers are enhanced (for example, the image56), calculate a Burger vector of a dislocation, and represent the size and direction of the calculated Burgers vector on the image (such as, for example, the image56). For example, the signal processor48represents a clockwise screw dislocation with a red color and a counterclockwise screw dislocation with a blue color.

FIG.42shows a colored image64. Reference sign64aindicates the counterclockwise screw dislocation, and areas indicated by reference sign64aare represented with the blue color. Reference sign64bindicates the clockwise screw dislocation, and areas indicated by reference sign64bare represented with the red color. Alternatively, the signal processor48may overlay blue markers on the areas indicated by reference sign64ain the image64, and overlay red markers on the areas indicated by reference sign64bin the image64.

In consideration that any distortion of crystal is reflected on the shape of the step riser, the signal processor48may identify a type of a defect from the three-dimensional shape of the steps. The signal processor48may analyze the image representing the enhanced step risers, to evaluate a type and quantity of a distortion.

The signal processor48may use machine learning, deep learning, and other techniques to identify a position, a type, and a quantity of a defect. For example, defects may be visually extracted by a person from the image representing the enhanced step risers, and a computer may learn from the extracted result and create a model for extracting the defects. The signal processor48may use the created model for detecting, from the three-dimensional shape, positions of the defects and the number of defects per a unit area. The signal quantities Ia, Ib, Ic, and Idmay be learned by the computer to create the model for extracting positions of the defects.

Meanwhile, when incident energy of the primary electrons is increased by changing the acceleration voltage for the electron beam, the electrons enter the specimen to a certain depth of the specimen. As a result, it becomes difficult to visually recognize the structure of the surface. On the other hand, because scattering conditions of the electrons change in a portion of the specimen where a defect of crystal is present, it becomes possible to observe a state of the defect which cannot be recognized from the surface structure. However, there is a possibility that no signal difference is obtained from the defect existing inside the specimen in an ordinary SEM image.

With this in view, in the third embodiment, after finding a position of the defect, the acceleration voltage is changed, electron beams are emitted toward a region around the found position of the defect, and signals from the reflected electrons are analyzed for a longer time. In this way, a state of the defect in a depth direction can be detected. For example, there can be observed a phenomenon that a quantity of signal from reflected electrons is increased in a particular direction, while a quantity of signal from reflected electrons is decreased in another direction. The signal processor48identifies, based on such a change in signals, an extending direction of the defect relative to the depth direction of the specimen and the type of the defect.

The change in signals caused by the defect in a deep region of the specimen cannot be easily detected from the ordinary SEM image. However, the position of a dislocation can be identified from the three-dimensional shape of the steps according to the third embodiment. Therefore, even though the change in signals caused by the defect is small, such a small change in signals can be detected with low noise by extending the length of time for acquiring signals from the location of the dislocation. As a result, the defect can be analyzed in detail.

Hereinafter, a method for presenting a quantity of extracted defects is explained.

For example, the signal processor48indicates positions of defects with points and markers on the image in which the step risers are enhanced (for example, the image56). In addition, the signal processor48may change colors used for representing the defects depending on types of the defects. For example, the signal processor48may represent the screw dislocation with red color, an edge dislocation with blue color, and a void dislocation with green color.

Further, the signal processor48may calculate the number of defects per unit area, and operate the display50to display the calculated number of defects.

The signal processor48may divide the image in which the steps are enhanced into two or more areas, and calculate a defect density in each of the two or more areas. The signal processor48may change colors applied to the two or more areas depending on defect densities. For example, the signal processor48may represent areas having high defect densities with red color, and areas having low defect densities with blue color. Alternatively, the signal processor48may further divide the defect densities into finer levels and color the areas depending on the levels of the defect densities.

FIG.43shows an image66in which colors are applied to the areas depending on the defect densities. The image66may be the image which is generated based on the signal difference, the image which represents the step risers in the enhanced manner, or the image which is generated by mapping the scanned positions (x, y) of the electron beams.

An area indicated by reference sign66ais represented with blue color. An area indicated by reference sign66bis represented with yellow color. An area indicated by reference sign66cis represented with red color. For example, blue areas, such as the area66a, have defect densities higher than or equal to a first threshold value, and yellow areas, such as the area66b, have defect densities lower than the first threshold value and higher than or equal to a second threshold value where the second threshold value is lower than the first threshold value. Red areas, such as the area66c, have defect densities lower than the second threshold value.

The image66is, for example, displayed on the display50. The user can recognize the defect density in each of the regions in the specimen while viewing the image66.

For example, it becomes possible to produce elements in such a manner that an element which is resistant to influence of a defect is formed in a region having a high defect density, while an element which is susceptible to the influence of a defect is formed in a region having a low defect density. In addition, a region where a defect is apt to be created may be detected, and the result of detection may be used as a basis for analyzing a cause of the presence of the defect based on the result of detection or used as a reference for reviewing manufacturing processes of a semiconductor wafer.

Fourth Embodiment

Hereinafter, a fourth embodiment is explained. In the fourth embodiment, position information of a defect that is identified according to the third embodiment is used in an external analytical instrument.

For example, the position information of the defect is sent to a focused ion beam (FIB) apparatus. The focused ion beam apparatus processes a region including a defect indicated by the position information into a specimen applicable to a transmission electron microscope (TEM). The processed specimen is observed in the transmission electron microscope.

For example, the signal processor48defines a reference point which can be used as a mark on the semiconductor wafer, includes additional information, such as information about a direction of the defect with respect to the reference point and information about a distance of the defect from the reference point, into the position information of the defect, and outputs the position information of the defect including the additional information. It should be noted that the direction and the distance of the defect may be identified by any apparatuses other than the signal processor48. The position information including the additional information may be utilized in another defect analysis method. In this case, when the other defect analysis method is performed, the position of the defect can be easily identified in operation to analyze the defect. This can shorten a length of time for analysis.

<Output of Position Information of Defect>

The position information of a defect is further described in detail below.

The reference point functioning as a guide is placed on the surface of the specimen of the semiconductor wafer. When the shape of a semiconductor wafer has a distinctive feature at a certain site, the position of a defect may be identified with respect to the site having the distinctive feature. In this case, the site having the distinctive feature may be used as the reference point. The guide used as the reference point may be a scratch formed on the surface, a mark formed of a pigment, or a grain fixed onto the surface, but is not limited to those described here. Any other marks may be used as the guide, so long as the position of the guide can shared with other apparatuses with reference to the mark.

However, in a case of using a circle mark as the guide, for example, the position of the guide can be shared with other apparatuses, although directions cannot be uniquely identified with reference to the position of the guide. In this case, the position of the defect cannot be shared among other apparatuses. On the other hand, when a mark having a directional shape, such as a letter L shape, for example, is used as the guide, directions can be uniquely defined with reference to the guide, which allows the other apparatuses to share the position of the defect. Even when the guide having the directional shape is used, however, the direction of a defect relative to the guide has a greater error as the distance between the defect and the guide increases. In this regard, two or more guides spaced from each other by an appropriate distance may be advantageously provided. In general, because the precise position of a defect is unknown at a stage of placing the guide, the guide may be placed at any position.

The specimen22provided with guides is set to the scanning electron microscope10, and a region including the guides on the specimen22is photographed by the scanning electron microscope10. As a result, positions of the guides are recognized in the SEM image.

Then, a three-dimensional shape having corrected heights is calculated through the process steps according to the first or second embodiment, and positions of defects are identified through the process steps according to the third embodiment. The signal processor48identifies the positions of the defects, calculates a direction and a distance of each of the defects relative to the guide, and outputs position information of the defects including information of directions and distances.

The position information of the defects is output to a defect analyzer, for example, and shared therein. The defect analyzer obtains, from the position information of the defects, the positions of the defects with reference to the position of the guide to analyze the defects. In this way, the information acquired by the scanning electron microscope10is shared with other apparatuses and used for analyzing the defects, for example.

Hereinafter, specific examples according to the fourth embodiment are described. As one example, a process of identifying the position of a defect, processing a portion of a specimen including the defect to prepare a flake of the portion with the focused ion beam apparatus, and observing the flake with the scanning electron microscope is described.

In this example, a guide is placed at appropriate positions on the surface of the specimen22. Alternatively, positions of defects may be identified in the three-dimensional shape obtained through the process steps according to the third embodiment, one of the defects is selected as an object to be analyzed, and the guide may be placed in the vicinity of the selected one of the defects.FIG.44shows an example of the guides. A guide68inFIG.44consists of a plurality of guides having mutually different shapes. Because directions with reference to the guides can be uniquely determined, the positions of the defects can be easily obtained using the guides68.

The specimen22including the guides is placed in the scanning electron microscope10, and photographed therein. For example, the user adjusts the position of the specimen22and other settings so as to include the guides68in the SEM image while viewing the SEM image.

Then, the process steps according to the first or second embodiment are performed to calculate the three-dimensional shape and the image representing the enhanced steps on the surface of the specimen22and calculate the shape of steps (i.e., shapes of step risers).FIG.45shows an image70representing the calculated shape of the steps. For example, the image70represents the steps in the enhanced manner. The image70further represents the guides68.

The signal processor48identifies positions of defects in the obtained shape of the steps.FIG.46shows the identified defects. Reference sign72indicates the defects. When a particular defect is specified as an object to be analyzed, observation magnification may be increased to include the specified defect and a guide located close to the defect in a field of view. In addition, the direction and distance of the defect with reference to the guide may be recorded to enable acquisition of the position of the defect in a subsequent FIB process. In this way, a positional relationship between the guide and the defect is acquired.

Following this, the specimen22is set in the focused ion beam apparatus, and the guides68are observed therein. The guides68and the defects (indicated by reference sign72) shown in the SEM image are overlaid on a SIM (Scanning Ion Microscope) image acquired by the focused ion beam apparatus. As a result, the positions of the defects can be recognized in the SIM image.FIG.47shows an example of an SIM image74. In the SIM image74, the guides68and the defects (indicated by reference sign72) are represented.

Because an optical system of the scanning electron microscope differs from that of the focused ion beam apparatus, image distortion, such as distortion aberration, may differ between the SEM and SIM images. In this case, an amount of image distortion, such as distortion aberration, may be calculated based on relative positional shifting of the guides between the SEM and SIM images, and the distortion may be compensated based on the calculated result.

When the directions and distances of the defects with reference to the guides are recorded, the positions of the defects may be determined based on orientation and size of the guides.

Then, a portion of the specimen22that includes the positions of the defects obtained as described above is processed and prepared as a specimen applicable to the transmission electron microscope. In this way, the specimen for the transmission electron microscope is produced.

Any defect in a semiconductor wafer influences the performance of a device which is manufactured using the semiconductor wafer. For this reason, identifying a type of the defect in the semiconductor wafer and analyzing a source of the defect have advantageous effects on development of devices having better performance. Hereinafter, a process to analyze the defect with the transmission electron microscope is explained, the process including steps of identifying positions of defects through the process according to the third embodiment, and processing the specimen in the focused ion beam apparatus to create a flake of the specimen.

For elaborated analysis of the defect in the semiconductor wafer, it is necessary that a flake created by slicing a portion of a specimen including a defect be observed in the transmission electron microscope. The flake of the specimen can be created by means of the focused ion beam apparatus, while there are difficulties in acquiring the shape of steps and the position of the defect in an SIM image generated by the focused ion beam apparatus. On the other hand, the position of the defect can be acquired through the process according to the third embodiment.

<Defect Analysis with Cathode Luminescence (CL) Method>

An analyzing apparatus and a processing apparatus may be attached to the scanning electron microscope10. This enables operation to conduct an analysis of a specimen while observing the SEM image, or conduct an analysis of the specimen without removing the specimen from the apparatus after observation of the SEM image. In other words, analyses and processing of the specimen can be performed in the same coordinate system as that of the scanning electron microscope10, which can, in turn, eliminate the necessity of aligning the coordinate systems using guides. Hereinafter, a specific configuration of such a combination of apparatuses is described with reference to an example using a CL method.

The CL method is a method for detecting emission of light from a specimen irradiated with electron beams. For example, when an object to be analyzed is a defect in a semiconductor wafer, a state of dislocation (such as, for example, threading dislocation) can be analyzed. When an apparatus for implementing the CL method is combined with the scanning electron microscope10, both operation to identify the position of a defect and operation to analyze the defect can be performed in a single apparatus.

FIG.48shows an apparatus10A in which the apparatus for implementing the CL method is combined with a scanning electron microscope. In the apparatus10A, the detector40according to the first embodiment is installed. In place of the detector40, the detector52according to the second embodiment may be used.

The apparatus10A further includes a condenser mirror76for acquiring CL and an analyzer78for performing an analysis according to the CL method. As indicated by an arrow80, the condenser mirror76and the detector40are switchingly used. For example, the controller51controls switching between the condenser mirror76and the detector40to place either the condenser mirror76or the detector40in position above the optical axis of the electron beam.

In operation to identify the position of a defect on the surface of the specimen22, the detector40is positioned above the optical axis of the electron beam, and the process according to the third embodiment is performed to acquire the shape of steps and identify the position of the defect. Following this, the condenser mirror76is placed above the optical axis of the electron beam, the electron beam is irradiated onto the defect, and CL light is analyzed by the analyzer78. Typically, the CL light is analyzed while scanning the electron beam on the entire surface of the specimen22, whereas in this method, the CL light is analyzed while selectively irradiating the electron beam onto a region in the vicinity of the defect. In this method, the defect can be analyzed within a shorter length of time.

FIG.49shows another apparatus10B. In the apparatus10B, both the detector40and the condenser mirror76are positioned above the optical axis of the electron beam. For example, the detector40is disposed so as to surround the specimen22, and the condenser mirror76is disposed above the detector40. The apparatus10B further includes the analyzer78, which is not illustrated inFIG.49.

In the apparatus10B, both the SEM image and the CL light can be obtained without switching the detector40and the condenser mirror76. In this way, while acquiring the shape of steps, a state of dislocation can be analyzed with the CL method.

It should be noted that in the apparatus10B, because a bias voltage applied to the specimen22hampers electrons from entering the detector40, no bias voltage is applied to the specimen22. Instead, a detector capable of detecting low-energy electrons is used as the detector40. Alternatively, there may be employed a mechanism for applying a positive voltage to both poles of the detector40to direct accelerated electrons into the detector40.

As another example, the detector40or the detector52may be installed in a focused ion beam apparatus including both an electron beam source and an ion beam source, to perform, with a single apparatus, a series of process steps from finding the position of a defect and creating a flake of a region of the specimen that includes the defect.

In addition, when the positions of the guide and the defect are identified by means of the optical system for the electron beam, a region including a defect which is specified as an object to be analyzed from extracted defects may be automatically processed into a flake.