JP2007280614A - Reflection image forming electron microscope and defect inspecting device using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a defect inspecting technology capable of observing or inspecting a sample by distinguishing a contrast resulting from a shape from another contrast resulting from a potential condition. <P>SOLUTION: In this reflection image forming electron microscope, a defect inspecting device has a preliminary charge control device 40 to control electrostatic charge of a sample surface by charging the sample surface by irradiation of a planar electron beam, and a means to separate the contrast resulting from the shape defect of the sample from another contrast resulting from a potential defect from a mirror electron image obtained, by structuring it so as to control the potential of the sample depending on the kinds of objective defect of the sample. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a defect inspection technique, and in particular, a reflection imaging electron microscope such as a mirror electron microscope for observing a surface state of a sample (semiconductor sample or the like), and pattern defects formed on the sample using the same. The present invention relates to a defect inspection apparatus for inspecting foreign matters and the like.

  In the process of manufacturing a semiconductor device, as a method of detecting a defect in a circuit pattern formed on a wafer by comparative inspection of an image, a minute etching residue that becomes below the resolution of an optical microscope by irradiating a sample with an electron beam, It is possible to detect shape defects such as minute pattern defects and electrical defects such as non-opening defects of minute conduction holes.

  Here, in a method using a scanning electron microscope that scans a sample with a dotted electron beam, there is a limit to obtain a practical inspection speed. Therefore, a semiconductor beam is irradiated with a rectangular electron beam. Thus, an apparatus for inspecting at high speed by a so-called projection method has been proposed, in which secondary electrons, backscattered electrons, or electrons reflected without being irradiated onto the wafer by forming a reverse electric field are imaged by a lens (for example, Patent Documents 1, 2, and 3).

JP 7-249393 A JP-A-10-197462 JP 2003-202217 A

    However, the projection method using secondary electrons or mirror electrons has the following problems.

  An apparatus for enlarging and projecting secondary electrons and backscattered electrons as detection electrons is called a low energy electron microscope. In this method, it is possible to irradiate an electron beam with a larger current at a time than in the SEM (scanning electron microscope) method, and it is possible to obtain an image at a time. However, the emission angle distribution of secondary electrons is spread over a wide angle, and the energy is also spread with about 1 to 10 eV. When such an electron is imaged to form a magnified image of the sample, it can be easily determined that sufficient resolution cannot be obtained unless most of the secondary electrons are cut (reference document: “ LSI Testing Symposium / 1999 Proceedings, P142 ”FIG. 6). According to the relationship between the negative sample applied voltage for accelerating the secondary electrons emitted from the sample and the imaging resolution of the secondary electrons, the resolution is approximately 0.2 μm when the sample applied voltage is −5 kV.

  Not all of the emitted secondary electrons can be used for image formation. For example, in the calculation in the above document, a beam having an opening angle of 1.1 mrad or less is used on the image plane after passing through the objective lens. . The total number of secondary electrons within the range of the opening angle is about 10%. Further, although the energy width of the secondary electrons used for imaging is calculated at 1 eV, the energy width of the emitted secondary electrons is actually emitted with a width of several eV or more. The base on the energy side exists up to about 50 eV. Of the secondary electrons having such a wide energy distribution, when only those having an energy width of at most 1 eV are extracted, the number is further reduced to a fraction.

  In this way, even if an image is formed all at once using secondary electrons obtained by irradiating a sample with a large current using an electron beam as an area beam, the percentage of electrons that can actually contribute to image formation is low. Therefore, it is difficult to secure the S / N ratio, and it is impossible to shorten the inspection time as much as expected. Even when backscattered electrons are used for image formation, backscattered electrons can only be emitted by two orders of magnitude less than the irradiation beam current, and it is difficult to achieve both high resolution and high speed as in the case of secondary electrons. is there.

  An apparatus for enlarging and projecting mirror electrons reflected instead of hitting a sample immediately before the sample instead of secondary electrons or backscattered electrons is called a mirror electron microscope. By using this mirror electron to detect a potential or shape disturbance caused by the defect, the defect can be detected. When the pattern is convex or negatively charged, the equipotential surface formed immediately above the sample acts like a convex mirror lens on the incident electrons, and the pattern is depressed or positively charged from the surroundings. In some cases, the equipotential surface formed immediately above the sample acts as a concave mirror lens for incident electrons. As described above, the orbits of the mirror electrons slightly change depending on the lens formed immediately above the sample, but most of these mirror electrons can be used for image formation by adjusting the focus condition of the imaging lens. That is, if mirror electrons are used, an image with a high S / N ratio can be obtained, and shortening of the inspection time can be expected.

  However, for example, even if the sample is negatively charged or convex, the equipotential surface directly above the sample has a convex shape, so it cannot be determined from the mirror electron image whether there is a potential defect or foreign matter is mixed in. The problem that has arisen.

  The present invention has been made by paying attention to the above-described points, and provides a defect inspection technique that makes it possible to separately detect a contrast caused by a shape and a contrast caused by a potential state from a mirror electronic image. For the purpose.

  The object of the present invention can be achieved by the following method.

  Although the strain on the equipotential surface due to the shape cannot be changed, the strain on the equipotential surface due to charging can be changed depending on the charging conditions.

  For example, in the case of a sample having a structure having a convex shape defect and a pattern potential defect, if the pattern potential in FIG. The equipotential surface has a convex shape. For example, if the focus condition of the imaging system is an overfocus condition in which the focus is above the sample, as shown in FIG. If the under-focus condition for focusing below the sample is used, both defects have a brighter contrast condition as shown in FIG. 4C. Therefore, it is possible to distinguish shape defects and potential defects from the brightness of the pattern. Have difficulty.

  On the other hand, as shown in FIG. 5A, when the potential of the pattern is a positive potential (a relatively positive potential relative to other than the pattern portion) from around, the equipotential surface on the pattern has a concave shape. Therefore, for example, when the focus condition of the imaging system is set to the overfocus condition, as shown in FIG. 5B, the convex defects are darker than the surroundings and the potential defects are brighter than the surroundings. If the focus condition is set to the under focus condition, it can be seen that, as shown in FIG. 5C, the convex defect changes to a brighter contrast and the potential defect changes to a darker contrast. Therefore, if the sample insulator is precharged so as to be positively charged before acquiring the mirror electron image, the convex defect and the potential defect can be separated. Furthermore, by setting the focus condition of the imaging system to overfocus or underfocus, the image of the defect of interest can be adjusted to a bright contrast.

  Further, for example, for a sample having a structure having a concave shape defect and a pattern potential defect, as shown in FIG. 7A, when the pattern potential is more positive than the surrounding, the concave shape defect The equipotential surface on the defect of the insulator becomes concave, and for example, if the focus condition of the imaging system is set to the overfocus condition, as shown in FIG. If the focus condition of the imaging system is an under focus condition, as shown in FIG. 7C, both defects become dark contrast conditions, and it is difficult to distinguish between a shape defect and a potential defect.

  On the other hand, as shown in FIG. 6A, the equipotential surface on the negative potential pattern is convex when the potential of the pattern is a negative potential (relatively negative with respect to other than the pattern portion). Therefore, for example, if the focus condition of the imaging system is set to the overfocus condition, as shown in FIG. 6B, the concave defect becomes brighter and the potential defect becomes darker than the surrounding. When the under-focus condition is set to the under-focus condition, as shown in FIG. 6C, it is understood that the concave defect is darker than the surrounding and the potential defect is changed to a brighter contrast. Therefore, if the sample insulator is precharged so as to be negatively charged before acquiring the mirror electron image, the concave defect and the potential defect can be separated. Furthermore, by setting the focus condition of the imaging system to overfocus or underfocus, the image of the defect of interest can be adjusted to a bright contrast.

  The object of the present invention can also be achieved by the following method. The strain on the equipotential surface due to charging largely depends on the charging voltage, but the strain on the equipotential surface due to the shape does not change with the charging voltage. Therefore, two images with different charging conditions are acquired, and it can be determined that the portion where the contrast has changed is caused by charging.

  FIG. 9A shows a sample having a convex shape defect and a pattern potential defect. FIG. 9A shows a case where the pattern potential (Vs) is set to a negative potential of 0.5 V from the surroundings by using precharge means. FIG. 10A shows the potential distribution when the pattern potential is set to a negative potential of 2.0 V from the surroundings using the precharge means (preliminary charging device). The distortion of the equipotential surface immediately above the sample varies depending on the potential state of the pattern, and the virtual object surface from the sample changes. That is, in the convex pattern, the equipotential surface immediately above the pattern is hardly deformed regardless of the precharge condition, but in the potential defect pattern, the equipotential surface immediately above the pattern changes. For example, when the focus position of the imaging lens is adjusted to the convex pattern shown in FIGS. 8 and 9, the respective mirror electronic images are as shown in FIGS. 8B and 9B. In the potential defect pattern of 8 (b), the defocus condition is set, and the brightness of the pattern is changed. For example, as shown in FIG. 9C, the potential defect portion is obtained by obtaining a difference image between the two images. Only can be extracted.

  The precharging means can be realized by arranging a precharging device separately from the electron microscope column and moving the sample to the precharging device and performing precharging before acquiring the inspection image. As the two types of charging conditions, two types of charging conditions are set with the preliminary charging device and the corresponding two types of images are compared, or comparison is made between an uncharged image and an image charged with the preliminary charging device. Alternatively, two types of images having different elapsed times after charging by the preliminary charging device, that is, different charging relaxation may be acquired and compared.

  Alternatively, an initial electron beam irradiation image may be compared with an image after a certain irradiation time has elapsed without using a preliminary charging device. When an electron beam is irradiated onto an insulating sample with little current leakage, the surface potential of the sample rises to a negative potential at which electrons do not hit the sample. That is, as shown in FIG. 10 (a), if there is a skirt on the high energy side of the irradiation electron beam, the potential of the insulator portion of the sample is not charged at the potential Vs in the initial stage of irradiation, but if irradiation continues, As shown in FIG. 10B, the potential rises to a potential at which the electrons on the highest energy side of the tail do not hit. Therefore, a change in the charged state of the insulator portion can be detected by comparing the irradiation initial stage and the progress image. In general, in order to obtain an image with a high SN, signals from the same place are added a plurality of times to obtain an image. Therefore, a change in the charged state can be detected by comparing an initial image and an late image. it can.

  In FIG. 10 (a), the mirror electron image in a state where the sample is not charged is an image reflecting only the unevenness of the sample, and the virtual object surface from the convex pattern moves downward. When the focus of the image system is adjusted to the lower side of the sample, an image in which the convex portion of the sample becomes bright as shown in FIG. 10B is obtained. In the latter stage of irradiation, the equipotential surface directly above the sample is distorted due to the negative charging of the insulator, resulting in a potential distribution as shown in FIG. 11A. Therefore, when the imaging lens is aligned below the sample, as shown in FIG. A bright image is obtained in the convex portion and the negatively charged portion of the sample. Therefore, by comparing the images at the initial stage and the latter stage of the irradiation, for example, as shown in FIG. 11C, the part where the contrast is not changed by acquiring the difference image is caused by the shape, and the part where the contrast is changed. It can be determined that the contrast is caused by charging.

  As described above, if pre-processing is performed so that the contrast between the shape defect and the potential defect is different, or if a means for comparing two images with different charged states is provided, the contrast caused by the shape and the potential state are caused. It becomes possible to observe or inspect while distinguishing the contrast.

  ADVANTAGE OF THE INVENTION According to this invention, the defect inspection technique which makes it possible to isolate | separate and detect the contrast resulting from a shape and the contrast resulting from an electric potential state from a mirror electronic image can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Example 1
FIG. 1 shows a configuration for explaining the operation of the first embodiment of the present invention. The E × B deflector 4 is disposed in the vicinity of the imaging surface of the reflected electron beam 302 including the mirror electron beam as a beam separator for separating the irradiation electron beam 301 and the reflected electron beam 302. The optical axis 106 of the vertical imaging system with the optical axis 105 and the wafer 7 of the illumination system, and intersect at an angle of theta _IN together. An E × B deflector 4 serving as a beam separator is disposed between the condenser lens 3 and the objective lens 5, and the irradiated electron beam 301 emitted from the electron source 1 is transferred to the wafer by the E × B deflector 4. 7 is deflected to an optical axis perpendicular to 7. The irradiation electron beam 301 deflected by the E × B deflector 4 is focused on the vicinity of the focal plane 303 of the objective lens by the condenser lens 3 via the condenser lens power source 33, and irradiated on the sample 7 with the substantially parallel irradiation electron beam. can do. In the figure, 2 is an electron gun lens, and 32 is an electron gun lens power source.

A sample (wafer in this example) 7 has a negative potential substantially equal to the acceleration voltage V ₀ applied to the electron source 1 by the electron source application power source 31 through the stage 8 holding the sample 7 from the sample application power source 37. Applied. A positive voltage in the range of several kV to several tens of kV is applied to the sample 7 by the circular electrode application power source 36 to the circular hole electrode 6 facing the sample. Most of the planar irradiation electron beam 301 is pulled back immediately before it collides with the sample 7 due to the deceleration electric field between them to become mirror electrons, and again has the objective and direction with the direction and intensity reflecting the shape, potential, magnetic field, etc. of the sample 7. The light enters the lens 5.

  The reflected electron beam 302 by the mirror electrons is magnified by the objective lens 5 and forms a mirror projection image 304 in the vicinity of the E × B deflector 4. The E × B deflector 4 acts on the reflected electron beam under Wien conditions. That is, for the reflected electron beam 302, the E × B deflector 4 does not have a deflecting action, and the mirror electron image is formed and projected in the vicinity of the E × B deflector 4, so that the E × B deflection is performed. Almost no deflection aberration due to the device 4 occurs. The image of the reflected electron beam 302 projected by the objective lens 5 is projected by the intermediate lens 13 and the projection lens 14, and an enlarged mirror electron image is formed on the scintillator 15. The mirror electronic image is converted into an optical image by the scintillator 15, projected onto the CCD camera 17 by the optical lens 16 or the optical fiber bundle, and the mirror image converted into an electric signal by the CCD camera 17 is displayed on the monitor 22.

  Depending on irradiation conditions, the reflected electron beam 302 may include backscattered electrons in which electrons colliding with the sample are scattered backward, secondary electrons generated secondary from the sample, etc. in addition to the mirror electrons. Because of the variation in the emission direction of backscattered electrons and secondary electrons, the electrons incident on the scintillator are limited to electrons emitted almost vertically, so the ratio of backscattered electrons and secondary electrons to the mirror electrons is small, and the image It does not affect the contrast under normal conditions. If the image contains a large proportion of backscattered electrons and secondary electrons, this electron beam diffraction image is projected by the electron beam diffraction image surface formed on the focal plane of the objective lens 5 or the intermediate lens 13. The ratio of backscattered electrons and secondary electrons can be adjusted by inserting a limiting aperture that restricts the angle of the reflected electron beam on the surface.

A cross section of the E × B deflector 4 viewed from the direction perpendicular to the optical axis has an octupole electromagnetic pole structure shown in FIG. 12, and each electromagnetic pole 51 is made of a magnetic material such as permalloy. Each electromagnetic pole 51 operates as an electrode when a potential is applied thereto, and operates as a magnetic pole by passing an exciting current through a coil 53 wound N times around a bobbin 52 of each electromagnetic pole 51. In the voltage distribution shown in FIG. 12, when the voltage V _X is applied to each electromagnetic pole by the E × B deflector power supply 34, the electrons are deflected in the x direction. When current I _{Y is} passed through each coil with current distribution as shown in FIG. 13, electrons moving from the back side to the front side of the paper in FIG. 13 are positive in the x direction, and electrons moving from the front side to the back side of the paper are It is deflected in the negative x direction. The voltage and current distribution of each electrode is optimized so that a uniform electromagnetic field is generated by electromagnetic field calculation in which a potential or magnetic potential is applied to the actual electromagnetic pole shape. For example, α = 0.414 in the figure. Is set to

FIG. 14 is a cross-sectional view including the optical axis of the E × B deflector 4. When the E × B deflector is used as a beam separator, the crossing angle θ _IN between the optical axis 105 of the irradiation system and the optical axis 106 of the imaging system is approximately 30 considering the arrangement relationship in which the two optical systems do not interfere with each other. It is necessary to take a degree. In order to prevent the irradiation electron beam 301 from hitting the electromagnetic pole even if it is deflected by approximately 30 °, the diameter of the opening must be made larger than the length of the electromagnetic pole. Therefore, the shape of the electromagnetic pole 51 is a conical shape that spreads substantially along the electron trajectory. In addition, shield electromagnetic poles 54 are provided above and below the electromagnetic poles to suppress the bleeding of the electromagnetic field, and the electric field and the magnetic field act in the same space so that the Wien condition is always satisfied in the space. It was made to operate as a B deflector.

  Although the above operation has been described with respect to the deflection direction in the x direction of the E × B deflector 4, the voltage or current of the deflection component in the y direction is supplied to the eight electromagnetic poles for correction of the deflection component in the y direction. This can be done in the same procedure. For example, when leakage of magnetic flux from the surrounding magnetic lens occurs in the E × B deflector, such a rotation correction that superimposes the electromagnetic field in the y direction is necessary.

  In FIG. 13, the number of turns of each coil is equal to N. However, the currents I and N may be changed in a range where the relationship NI with the current I flowing through the magnetic poles is constant.

  In order to detect such defects with high sensitivity, the present invention includes a preliminary charging control device 40 that irradiates an electron beam dedicated to charging control in advance before acquiring an inspection image. If the inspection is performed after the wafer 7 is charged to a predetermined potential in advance by this apparatus, not only a shape defect but also an electrical defect such as a conduction failure portion can be detected. Hereinafter, this operation and configuration will be described.

FIG. 15 is a diagram for explaining the operating principle of the preliminary charging control device 40. The electron source 41 is an electron source that emits a high-current electron beam from a surface having a certain extent (several hundred μm to several tens of mm). For example, an electron source in which carbon nanotubes are bundled, a tungsten filament thermoelectron source, a LaB ₆ electron source, or the like can be used. An electron beam 43 is emitted from the electron source 41 by applying a voltage extraction voltage to the extraction grid 42 by the extraction grid power supply 48. The electron beam passes through the control grid 44 and irradiates the insulating film 46. Secondary electrons 45 are emitted by this electron beam irradiation. The secondary electrons have an energy of about 2 eV with respect to the surface potential of the insulating film 46. If the surface of the insulating film is equivalent to the potential of the substrate 47, the irradiation energy of the electron beam is the voltage of the acceleration power supply 49, and this acceleration voltage is set to a value that makes the secondary electron emission efficiency 1 or more. In an insulating film material for a general semiconductor device, for example, an acceleration voltage of about 500 V is preferable. Under this condition, since the secondary electron emission efficiency of the insulating material is greater than 1, the surface of the insulating film is positively charged.

Since the control grid power supply 50 is connected to the control grid 44 so that an arbitrary positive or negative voltage can be applied, the potential of the insulating film surface becomes more positive than the set potential of the control grid 44, When secondary electrons are pulled back to the surface of the insulating film, the positive charging of the surface of the insulating film stops. For example, when the oxide film is made of SiO ₂ , the charging potential on the surface of the insulating film is stabilized at a potential about 2V higher than the potential of the control grid. The reason why it is not equal to the potential of the control grid is that secondary electrons have energy. Based on the principle described above, the potential on the surface of the insulating film 46 can be controlled by the potential of the control grid 44.

In this embodiment, a Schottky electron source such as a Zr / O / W type that can obtain an irradiation electron beam with a large current and a high luminance is used as the electron source 1. FIG. 16 shows the relationship between the energy distribution of the electron beam and the energy level in a Zr / O / W type Schottky electron source. The Schottky emission electrons are emitted with the potential barrier lowering than the vacuum level due to the extraction electric field, and the energy of the emission electrons is distributed in the vicinity of the vacuum level of the electron source. Usually, in order for electrons emitted from the energy at the vacuum level of the electron source to approach the sample immediately before the sample set to the sample application voltage V ₀ by the sample application power source 37, the electron source application voltage V ₀ and the sample application voltage Vs are used. _It is sufficient that the potential difference of ₀ is substantially equal to the work function difference ΔVw between the electron source and the sample. Here, when the potential of the insulator is controlled by applying the control grid, the insulator voltage Vi is generally controlled to a voltage of Vi = Vg + 2 volts with respect to the control grid potential Vg. In order for the electrons emitted from the energy to approach to just before the insulating film, it is necessary to reset the electron source applied voltage to increase Vi or to decrease the sample applied voltage to Vi.

Strictly speaking, it is necessary to consider the work function difference ΔVw ₂ between the electron source and the control grid and the energy distribution of the electron beam emitted from the electron source. Electronic maximum energy eVmax of the electron source is greater than the vacuum level as [Delta] V _1, the potential difference between the vacuum level energy eVp corresponding to the maximum value of the energy distribution of the electron source differ by [Delta] V _2. The user may determine the sample applied voltage Vs ₀ that the electron beam with the maximum energy approaches just before the insulating film so that Vs ₀ = V ₀ −ΔVw ₂ + ΔV ₁ −Vi. The value of ΔV ₁ may be automatically set according to the operating condition of the electron source from a measured value database of measured values of the radius of curvature of the electron source, heating temperature, extraction voltage, and the like as parameters. The user may set the preset value. Or you may add correction values, such as the Seebeck effect produced by the contact between metals.

  On the operation screen on the monitor 25, the charging voltage Vi of the insulator can be set. The charging voltage input information is input by the user through the input interface 26. From the value of the charging voltage Vi input by the user, the control voltage Vg = Vi−2 to be applied to the control grid 44 of the preliminary charging control device through the control grid power supply 50 can be set at the time of preliminary charging. Further, the difference of 2 volts between the grid control voltage and the charging voltage can be rewritten on the scanning screen by the user by actually measuring the charging voltage with a potential measuring device such as AFM.

In addition, when acquiring a mirror image, a sample application voltage Vs ₀ = V ₀ −ΔVw ₂ + ΔV ₁ −Vi is set so as to reflect directly on the surface of the insulator, but the reflection surface can also be adjusted as follows. .

The mirror electron beam reflecting surface can be operated by setting the height H of the mirror electron beam reflecting surface at which the electron beam with the maximum energy value approaches just before the sample to 0 or by inputting the height specified by the user. Displayed on the screen. The relationship between the fluctuation ΔH in the height H of the mirror electron beam reflecting surface and the fluctuation value ΔV _{0-S in} V ₀ -Vs is expressed by ΔH = ΔV _0-S / E. When designated, ΔV _0-S is controlled to change in conjunction with the input value of ΔH. Further, in conjunction with the change in the electric field strength E = (Va−Vs) / L between the circular hole electrode and the sample with respect to the change in the sample applied voltage Vs, the circular hole electrode voltage Va, and the circular hole electrode-sample distance L, ΔH = ΔV _0−S / E is also corrected and set.

  The reflected electron beam 303 mainly composed of mirror electrons reflected immediately before the sample is enlarged and projected on the scintillator 15 by using the imaging lens system, that is, the objective lens 5, the intermediate lens 13 and the projection lens 14. The operating conditions of this imaging system are stored in advance in the control unit 24, and if the user sets the magnification or the field of view on the sample, each lens setting value for the just focus is set on the sample. Operate. In addition, the focus condition is corrected in conjunction with the setting of the insulator charging voltage Vi. In addition, with respect to just focus, over focus that focuses on the top of the sample and under focus that focuses on the bottom of the sample can also be set on the operation screen by the user, for example, the height of the focus position relative to the sample. It is configured as follows. Here, in order to perform defocusing, it is common to change the focal length of the objective lens 5 using the objective lens power source 35, but the objective lens 5 also exerts a lens action on the irradiation electron beam 301. For example, the focus may be blurred by changing the focal length of the intermediate lens 13. Even if the focal length of the intermediate lens 13 is changed, the irradiation electron beam 301 is not affected. Therefore, there is an advantage that the imaging lens can be adjusted independently of the irradiation lens. On the operation screen, the user can specify a lens to be changed under underfocus and overfocus conditions.

  For example, when inspecting foreign matter on a wafer pattern, the equipotential surface caused by the foreign matter is deformed into a convex shape. Therefore, if the focus condition of the imaging system is set to the underfocus side, a contrast that makes the foreign matter brighter is obtained. It is done. Here, if the pre-charging condition is set so that the pattern portion forming the wafer sample has a positive potential relative to other than the pattern portion, the equipotential surface immediately above the pattern is deformed into a concave shape, and under focus Under the conditions, a contrast that darkens the pattern is obtained, and it is possible to inspect only the foreign matter. For example, in the case of inspection of a plug pattern in which polysilicon or the like is embedded between insulating films, if the insulating film is precharged so as to be negatively charged, the polysilicon pattern has a relatively positive potential relative to the insulating film. Therefore, the equipotential surface immediately above the pattern is distorted into a concave shape and can be observed separately from the foreign matter.

  The order of preliminary charging can also be selected by the user on the operation screen on the monitor 25. That is, it is possible to select whether to acquire an image after charging the entire surface of the wafer sample, or to acquire an image after charging every line in the stage movement X direction.

  The user selects an image acquisition range in advance, but the preliminary charging range can be set independently of the image acquisition range.

  In order to inspect the entire wafer, the stage 8 moves step-and-repeat in the x direction, for example. The preliminary charging control device 40 is arranged at a position where the inspection area can be irradiated before the inspection. For example, the preliminary charging control device and the irradiation system optical axis are aligned in the X-axis direction. Furthermore, by arranging two preliminary charging control devices 40 across the irradiation optical axis in the X-axis direction, preliminary charging can be accommodated in the positive and negative movement directions of the X-axis, and the charged state can be controlled before and after the inspection. I did it.

  The mirror electronic image converted into the optical image by the scintillator 15 is projected onto the CCD camera 17 by the optical lens 16 or the optical fiber bundle, converted into an electric signal, and sent to the image processing unit 103.

  The image processing unit 103 includes an image signal storage unit 18, a calculation unit 20, a defect determination unit 21, and a storage unit 23. The image storage unit 18 stores a mirror electronic image, and the calculation unit 20 detects a place where the brightness is higher than the set level or lower than the set level. The result is determined as a defect by the defect determination unit 21, and the coordinates are stored in the storage unit 23. The defect determination unit 21 determines the type of defect based on information such as a focus condition and a charging condition. For example, if positive charge information is input as the pattern charging condition and under focus information is input as the focus condition, a place having a brightness higher than the set level is determined as a convex defect, and a place lower than the set level is determined as a potential defect. The captured image signal is displayed on the monitor 22 as an image.

  As described above, it is possible to distinguish and inspect the shape defect and the potential defect by adjusting the preliminary charging condition according to the type of the target defect.

(Example 2)
FIG. 2 shows a configuration of the second embodiment of the present invention, and a shape defect and a potential defect are identified by comparing two images having different charged states.

  In the sample chamber 102, a wafer 7 is placed on a sample moving stage 8 that can move in a two-dimensional (X, Y) direction, and most of the electron beam is applied to the wafer 7 by a sample applying power source 37 as described above. A negative potential that does not collide with the wafer 7 is applied. A stage position measuring device 38 is attached to the sample moving stage 8 to accurately measure the stage position in real time. This is for acquiring images while continuously moving the stage 8. For example, a laser interferometer is used for the stage position measuring device 38.

  Next, the settling time of the sample moving stage 8 will be described. If the movement of the stage 8 is a step-and-repeat method, the settling time of the stage 8 needs to be on the order of msec. Therefore, even if the image acquisition time is shortened by improving the image S / N ratio, it takes time to move the stage. The inspection time cannot be shortened. Therefore, the moving method of the stage 8 is a continuous moving method in which the stage is always moving at a substantially constant speed. This eliminates the restriction of the inspection time due to the stage settling time.

  The irradiation region or irradiation position of the electron beam is constantly monitored by a stage position measuring device 38 provided on the stage 8, a sample height measuring device (not shown), or the like. The monitor information is grasped in detail by the control unit 24 as the amount of positional deviation and is accurately corrected. As a result, accurate alignment required for pattern comparison inspection can be performed at high speed and with high accuracy.

  The reflected electron beam 303 mainly composed of mirror electrons reflected immediately before the sample is enlarged and projected on the scintillator 15 by using the imaging lens system, that is, the objective lens 5, the intermediate lens 13 and the projection lens 14. This mirror electronic image is converted into an optical image by the scintillator 15, and this mirror electronic image is converted into an optical image by the scintillator 15 by the optical lens 16 or the optical fiber bundle and projected onto the CCD element 17 by the optical lens 16 or the optical fiber bundle. . The image signal converted into an electrical image signal by the CCD element 17 is taken into the image processing unit 103. That is, it is stored in the storage unit 18 (or 19) as an electron beam image signal of an area region corresponding to the electron beam irradiation position that has received a command from the control unit 24.

  In this embodiment, a time accumulation type CCD sensor is used as an element for converting a sample surface image into an electric signal. This element is called a TDI sensor and is generally used in an optical inspection apparatus. The rest is the same as in the case of the first embodiment described above. The operation concept of this TDI sensor will be described below.

  In the TDI sensor, the electric charge generated according to the intensity of the light received in each light receiving area is moved to the line in the x direction, and at the same time, the electric charge generated according to the intensity of the light received at the destination is changed. It works to add up sequentially. Then, when the final line of the light receiving surface is reached, it is output to the outside as an electrical signal. Accordingly, by making the movement speed of the charge in the x direction equal to the movement speed of the image on the light receiving surface in the x direction, the signals during the movement of the image on the sensor are integrated and output.

  In this embodiment, for example, the signal reading is divided into 128 channels, and the reading speed is set to 4M lines / second by reading the signals in parallel. The size of the light receiving area was 64 pixels in the x direction and 2048 pixels in the y direction. For example, if the x-direction length of one line corresponds to 50 nm on the sample surface and the y-direction length is about 100 μm, an image with a length of 50 nm and a width of 100 μm is output at a speed of 4 M / sec. The continuous moving speed of the stage is also set to the same speed (50 nm / 250 nsec = 200 mm / sec). As described above, the x-direction movement of the inspection region is performed by moving the stage 8.

  The user can select a mode for acquiring an image under two kinds of charging conditions on the operation screen on the monitor 25. When the user selects this mode, the voltage for charging the insulator under the first precharging condition and the second precharging condition can be selected. Alternatively, image acquisition without preliminary charging can be selected as one charging condition.

  When the user sets the charging conditions and starts the inspection, as a first step, the inspection area is first moved to the preliminary charging device 40 to perform the first preliminary charging. In the case of performing a pattern inspection between adjacent chips A and B having the same design pattern formed on the surface of the semiconductor wafer 7, first, an electron beam image signal for a region to be inspected in the chip A is captured. And stored in the storage unit 18. Next, an image signal for the inspection region corresponding to the above in the adjacent chip B is captured and stored in the storage unit 19, and at the same time, compared with the stored image signal in the storage unit 18. Further, an image signal for the corresponding inspection area in the next chip C is acquired and stored in the storage unit 18 by overwriting, and at the same time, the storage for the inspection area in the chip B in the storage unit 19 is stored. Compare with image signal. By repeating such an operation, the patterns are compared while sequentially storing the image signals for the corresponding inspected areas in all the inspected chips.

  Both image signals stored in the storage units 18 and 19 are respectively taken into the calculation unit 20, where various statistics (specifically, average values of image density) are calculated based on the already determined defect determination conditions. , Statistics such as variance), difference values between neighboring pixels, and the like are calculated. Both image signals that have been subjected to these processes are transferred into the defect determination unit 21 and are compared as, for example, comparison means, and a difference signal between the two image signals is extracted. These difference signals are compared with the defect determination conditions that have already been obtained and stored to determine the defect, and the image signal of the pattern area determined to be a defect is stored in the image storage unit 23.

  Next, after the inspection area is moved again to the preliminary charging device 40 to perform the second preliminary charging, an electron beam image signal is acquired and stored in the image storage unit 27. The image stored in the image storage unit 60 may be a previous image in the inspection area, but an image of only the pattern area determined to be defective in the inspection after the first preliminary charging may be acquired. Next, the defect type determination unit 28 compares the image signal of the pattern area determined to be defective in the first preliminary charging with the image signal after the second preliminary charging in the same area, and the defect type determination unit 28 determines the defect type. judge. For example, when the difference signal between the two image signals is acquired and the brightness of the defective part does not change with reference to a certain threshold level, the defect due to the shape, the brightness of the defective part changes Is determined as a defect caused by a potential difference, and the determination information is stored in the storage unit 29.

  Alternatively, the pattern comparison inspection may be performed again after the second preliminary charging. In this case, only the defect extracted under both conditions may be extracted from the defect extracted after the first preliminary charging and the defect extracted after the first preliminary charging. You may extract including the extracted defect. After the defect extraction, the defect type determination unit 28 compares the image signal after the first preliminary charging and the image signal after the second preliminary charging in the pattern area determined to be a defect. Determine the defect type. For example, when the defect type determination unit 28 obtains a difference signal between the two image signals and the brightness of the defect portion does not change with reference to a certain level threshold, the defect due to the shape, the defect portion When the brightness changes, it is determined that the defect is caused by the potential difference, and the determination information is stored in the storage unit 29.

  The interval between the first and second preliminary charges by the preliminary charging device 40 is performed for the entire wafer surface or for each line of the stage movement direction on the wafer. The preliminary charging control device 40 is arranged at a position where the inspection area can be irradiated before the inspection. For example, the preliminary charging control device and the irradiation system optical axis are aligned in the X-axis direction. Here, by arranging two preliminary charging control devices 40 across the irradiation system optical axis in the X-axis direction, the second preliminary charging can be performed immediately after the image acquisition, and the end portion in the X-axis direction. Even when the inspection is performed while turning back and moving in the direction opposite to the X axis, preliminary charging immediately before the image acquisition is possible.

  As described above, it is possible to distinguish and inspect shape defects and potential defects by comparing two charged images.

(Example 3)
FIG. 3 shows a configuration of a third embodiment of the present invention, and a shape defect and a potential defect are identified by comparing an image in the early stage of irradiation with an image in the latter stage of irradiation.

  In the embodiment, a time accumulation type CCD sensor (TDI) is used as the CCD element 17 for converting the sample surface image into an electrical signal. The signal reading of the CCD sensor was divided into 128 channels and read out in parallel, so that the reading speed was 4M lines / second. The size of the light receiving area was 64 pixels in the x direction and 2048 pixels in the y direction. The length of one line in the x direction corresponds to 50 nm on the sample surface, and the length in the y direction corresponds to about 100 μm. At this time, since an image having a length of 50 nm and a width of 100 μm is output at a speed of 4 M / sec, the continuous moving speed of the stage is set to the same speed (50 nm / 250 nsec = 200 mm / sec). As described above, the x-direction movement of the inspection area is performed by moving the stage 8.

  On the operation screen on the monitor 25, the number of lines to be added by TDI is displayed. In the normal mode, a maximum of 64 lines can be selected. When the defect type determination mode is selected, a screen for selecting the line range of the initial captured image of the irradiation among 64 lines and the screen for selecting the line range of the captured image in the later stage of irradiation are displayed. The user inputs the line range input information through the input interface 26. For example, among the 64 lines, the first half of irradiation can be selected as the first half 32 lines, and the second half of the irradiation can be selected as the second half 32 lines.

  In the case of performing a pattern inspection between adjacent chips A and B having the same design pattern formed on the surface of the semiconductor wafer 7, first, an electron beam image signal for a region to be inspected in the chip A is captured. The accumulated images for 64 lines are stored in the storage unit 18, and at the same time, the first 32 lines are stored in the storage unit 61 and the latter 32 lines are stored in the storage unit 62. Next, an image signal for the region to be inspected corresponding to the above in the adjacent chip B is captured, and an integrated image for 64 lines is stored in the storage unit 19, and at the same time, the first 32 lines are stored in the storage unit 63. , The latter 32 lines are stored in the storage unit 64. The image stored in the storage unit 19 is simultaneously compared with the stored image signal in the storage unit 18. Further, an image signal for the corresponding inspection area in the next chip C is acquired, and the accumulated image for 64 lines is overwritten and stored in the storage unit 18, and at the same time, the image in the chip B in the storage unit 19 is simultaneously stored. The stored image signal for the inspection area is compared. Such an operation is repeated, and comparison is performed while sequentially storing image signals for corresponding inspection regions in all inspection chips.

  Both image signals stored in the storage units 18 and 19 are respectively taken into the calculation unit 20, where various statistics (specifically, average values of image density) are calculated based on the already determined defect determination conditions. , Statistics such as variance), difference values between neighboring pixels, and the like are calculated. Both image signals subjected to these processes are transferred to the defect determination unit 21 and compared there to extract a difference signal between the two image signals. These difference signals are compared with the defect determination conditions that have already been obtained and stored to determine the defect, and the image signal of the pattern area determined to be a defect is stored in the image storage unit 23. For the image signal of the pattern area determined to be defective, images for the first half 32 lines and the second half 32 lines are stored in the storage unit 61 and storage unit 62 or in the storage unit 63 and storage unit 64.

  The defect determination unit 21 compares the first half image and the second half image of the pattern area determined to be defective. For example, when the difference signal between the two image signals is acquired and the brightness of the defective part does not change with reference to a certain threshold level, the defect due to the shape, the brightness of the defective part changes Is determined as a defect caused by a potential difference, and the determination information is stored in the storage unit 29.

  In this way, by comparing the image at the initial stage of irradiation with the image at the latter stage of irradiation, it is possible to acquire images with different charged states without moving the stage, so that shape defects and potential defects can be distinguished and inspected with high throughput. Become.

  As described above in detail, according to the present invention, it is possible to provide a defect inspection technique that enables observation or inspection by distinguishing between the contrast caused by the shape and the contrast caused by the potential state. In a series of processes for manufacturing a semiconductor device or the like, it is effective when applied to a defect inspection of a sample.

The figure explaining the structure of the 1st Example of this invention. The figure explaining the structure of the 2nd Example of this invention. The figure explaining the structure of the 3rd Example of this invention. The figure which shows equipotential surface distortion and an image in case there exists a pattern of a convex surface shape and a negative electric potential state. The figure which shows equipotential surface distortion and an image in case there exists a pattern of a convex surface shape and a positive potential state. The figure which shows the equipotential surface distortion and image in case there exists a concave surface shape and the pattern of a negative electric potential state. The figure which shows the equipotential surface distortion and image in case there exists a concave surface shape and the pattern of a positive potential state. The figure which shows an equipotential surface distortion and image in case there exists a pattern of a convex surface shape and a negative potential state of -0.5V. The figure which shows the equipotential surface distortion and image in case there exists a pattern of a convex surface shape and a negative potential state of -2.0V. The figure which shows the equipotential surface distortion and image of an irradiation initial stage in case there exists a pattern of a convex surface shape and a negative electric potential state. The figure which shows the equipotential surface distortion and image of the latter stage of irradiation in case there exists a pattern of a convex surface shape and a negative electric potential state. The figure explaining the voltage distribution of the x direction deflection | deviation of an 8-pole type ExB deflector. The figure explaining the electric current distribution of the x direction deflection | deviation of an 8 pole type ExB deflector. Sectional drawing of an 8-pole type ExB deflector. The figure explaining the structure of the preliminary charging device in this invention. The figure explaining the relationship between the energy distribution of a Schottky electron source, and an energy level.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electron source, 2 ... Electron gun lens, 3 ... Condenser lens, 4 ... ExB deflector, 5 ... Objective lens, 6 ... Circular electrode, 7 ... Sample, 8 ... Stage, 9 ... Energy filter, 10 ... E × B deflector, 11 ... Restriction aperture, 12 ... Second condenser lens, 13 ... Intermediate lens, 14 ... Projection lens, 15 ... Scintillator, 16 ... Optical fiber bundle, 17 ... CCD camera, 18 ... Image storage unit, 19 ... Image storage unit, 20 ... calculation unit, 21 ... defect determination unit, 22 ... monitor, 23 ... storage unit, 24 ... control unit, 25 ... monitor, 26 ... input interface, 27 ... stage position measuring device, 28 ... height measurement 30 ... Stage control system, 31 ... Electron source application power supply, 32 ... Electron gun lens application power supply, 33 ... Condenser lens power supply, 34 ... ExB deflector power supply, 35 ... Objective lens power supply, 36 ... Round hole electrode Power source, 37 ... Sample application power source, 38 ... Stage position measuring device, 39 ... Height measuring device, 40 ... Preliminary charging control device, 41 ... Electron source, 42 ... Drawer grid, 43 ... Electron beam, 44 ... Control grid, 45 ... Secondary electrons, 46 ... Insulating film, 47 ... Substrate, 48 ... Extraction grid power supply, 49 ... Acceleration power supply, 50 ... Control grid power supply, 51 ... Electromagnetic pole, 52 ... Bobbin, 53 ... Coil, 54 ... Shield, 101 DESCRIPTION OF SYMBOLS ... Electron optical system, 102 ... Sample chamber, 103 ... Image processing part, 301 ... Irradiation electron beam, 302 ... Reflection electron beam, 303 ... Objective lens focal plane.

Claims (14)

  An irradiation optical system that irradiates the sample with a first electron beam emitted from the first electron source as a planar irradiation electron beam having a two-dimensional extension, and a mirror electron beam that is reflected without colliding with the sample is projected An imaging optical system that forms an enlarged image, means for converting the imaged mirror electron image into an optical image and performing image processing, and a beam separator that separates the irradiation electron beam and the mirror electron beam The reflection imaging electron microscope includes a preliminary charging control device that charges the sample surface by irradiation with a second electron beam and controls charging of the sample surface, and the preliminary charging control device includes: 2. A reflection imaging electron microscope characterized in that two image optical systems are arranged across the optical axis.   The preliminary charging control device has a second electron source that emits a planar second electron beam, and controls the potential applied to the control grid to control the potential of the sample surface. The reflection imaging electron microscope according to claim 1.   The beam separator is composed of an E × B deflector in which an electric field and a magnetic field are orthogonal and superimposed, and the electric field and the magnetic field are deflected with respect to the mirror electron beam while the deflection angle with respect to the irradiation electron beam is kept substantially constant. 3. The reflection imaging electron microscope according to claim 1, wherein a voltage and a current supplied to the deflector are controlled so that a Wien condition in which the action cancels out is achieved.   An irradiation optical system that irradiates the sample with a first electron beam emitted from the first electron source as a planar irradiation electron beam having a two-dimensional extension, and a mirror electron beam that is reflected without colliding with the sample is projected An imaging optical system for forming an image by enlarging, means for imaging the imaged mirror electron beam and performing image processing, and a beam separator for separating the irradiation electron beam and the mirror electron beam In a reflection imaging electron microscope, the sample surface is charged by irradiation with a second electron beam, and a pre-charging control device that controls charging of the sample surface, and according to the type of defect targeted by the sample And a means for separating the contrast caused by the shape defect of the sample and the contrast caused by the potential defect from the acquired mirror electron image. Defect inspection apparatus.   5. The defect according to claim 4, wherein a projection imaging condition of the imaging optical system is set according to a type of defect of the sample and a potential of an insulator to be charged in advance in the sample. Inspection device.   Means for controlling a potential of an insulator in the sample according to the type of defect of the sample, detecting a plurality of mirror electron images having different charged states, and comparing the plurality of mirror electron images; The defect inspection apparatus according to claim 4, wherein the defect inspection apparatus is configured to identify a shape defect and a potential defect of the sample by comparing the plurality of mirror electronic images.   The potential of the insulator is set in advance so that the pattern part forming the sample is relatively positive with respect to other than the pattern part, and the projection imaging condition is set to an underfocus condition, The defect inspection apparatus according to claim 5, wherein a defect on the pattern is inspected.   The potential of the insulator is set in advance so that the pattern portion forming the sample has a relatively negative potential with respect to other than the pattern portion, and the projection imaging condition is set to an underfocus condition or an overfocus condition. The defect inspection apparatus according to claim 5, wherein the defect inspection apparatus sets and inspects a defect on the pattern.   5. The apparatus according to claim 4, wherein two mirror electronic images having different charging conditions are acquired and a shape defect and a potential defect of the sample are identified based on a difference signal between the two image signals. Defect inspection apparatus as described.   The comparison means compares the mirror electron image detected by the initial irradiation of the second electron beam with the mirror electron image of the latter irradiation stage, and based on the comparison, the shape defect and the potential defect of the sample The defect inspection apparatus according to claim 6, wherein:   The preliminary charging control device has a second electron source that emits a planar second electron beam, and controls the potential applied to the control grid to control the potential of the sample surface. The defect inspection apparatus according to any one of claims 4 to 10.   The defect inspection apparatus according to any one of claims 4 to 11, wherein two preliminary charging control devices are arranged across the optical axis of the imaging optical system.   The beam separator is composed of an E × B deflector in which an electric field and a magnetic field are orthogonal and superimposed, and the electric field and the magnetic field are deflected with respect to the mirror electron beam while the deflection angle with respect to the irradiation electron beam is kept substantially constant. The defect inspection apparatus according to any one of claims 4 to 12, wherein a voltage and a current supplied to the deflector are controlled so as to satisfy a Wien condition in which the actions cancel each other. 6. The defect inspection of the sample according to claim 5, wherein a plurality of mirror electronic images having different projection imaging conditions of the imaging optical system are acquired, and defect inspection of the sample is performed based on the plurality of mirror electronic images. Defect inspection equipment.

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