Referring to the drawings, embodiments of the invention will be described in detail below.
The scanning electron microscope shown in
The electro-optical system 3 includes an electron source 10, an electron beam withdrawing electrode 11, a capacitor lens 12, a deflector for blanking 13, a scanning deflector 15, a diaphragm 14, an objective 16, a lens for converging a secondary signal 69, a reflector 17 and an ExB deflector 18. In the detecting system 7, a detector 20 is arranged on the upside of the objective 16 in the chamber 2. A signal output from the detector 20 is amplified in a preamplifier 21 installed outside the chamber 2 and is converted to digital data by an A/D converter 22. In the electro-optical system in this embodiment, the ExB deflector 18 makes the crossover of primary electron beams. Therefore, the lens for converging a secondary signal 69 is arranged between the ExB deflector 18 and the scanning deflector 15.
The electrification control system includes an electrification control electrode 65 arranged opposite to the stage, an electrification control electrode controller 66 and an electrification control power source 67.
The detecting system 7 includes the detector 20 in the evacuated chamber 2, the preamplifier 21, the A/D converter 22, an electric-to-optical converter 23, an optical fiber 24, a photoelectric converter 25, a high voltage power supply 26, a preamplifier driving power source 27, an A/D converter driving power source 28 and a reverse bias power source 29, which except the detector are located outside the chamber 2. The detector 20, the preamplifier 21, the A/D converter 22, the electric-to-optical converter 23, the preamplifier driving power source 27 and the A/D converter driving power source 28 are kept at positive potential by the high voltage power supply 26.
The sample housing 8 includes a sample stage 30, an X stage 31, a Y stage 32, a wafer holder 33, a length measuring machine for a position monitor 34 and an optical height gauge 35.
The optical microscope 4 is located in the vicinity of the electro-optical system 3 in the chamber 2, they are installed in positions apart by an extent in which they have no effect on each other, and distance between the electro-optical system 3 and the optical microscope 4 is already known. The X stage 31 or the Y stage 32 is reciprocated in the already known distance between electro-optical system 3 and the optical microscope 4. The optical microscope 4 is configured by a light source 40, an optical lens 41 and a CCD camera 42.
An instruction and a condition for operating each unit are output from the controller 6. The controller 6 is provided with a database in which control parameters of each unit, for example, the electro-optical system 3, the X stage 31, the Y stage 32 and others and their operational conditions are stored. The database is configured by hardware such as a nonvolatile storage that stores predetermined information, an arithmetic unit that processes the information stored in the storage and a memory that temporarily stores the information processed by the arithmetic unit. The storage in the database stores conditions such as acceleration voltage when an electron beam is generated, electron beam deflected width, deflection velocity, signal input timing to the detector, sample stage traveling speed and the setting of the lens for converging a secondary electron, the conditions are selected according to a purpose, and the control of each unit is executed. A user of the apparatus may also select the conditions for control stored in the database manually via a user interface, an operational condition is set in the controller 6 beforehand, and the apparatus may be also operated according to the setting.
The controller 6 monitors displacement in a position and height based upon signals from the length measuring machine for the position monitor 34 and the optical height gauge 35 using a correction control circuit 43, generates a correction signal based upon the result, and sends the correction signal to a lens power source 45 and a scanning signal generator 44 so that an electron beam always irradiates a right position.
To acquire an image of the wafer 9, an electron beam 19 converged to be thin is radiated onto the wafer 9, a secondary electron or a reflected electron or both 51 are generated, an image of the surface of the wafer 9 is acquired by detecting these in synchronization with the scanning of the electron beam 19, if necessary, the movement of the stages 31, 32.
For the electron source 10, a diffusive supply type thermal-field-emission electron source is used. As stable electron beam current can be secured by using the electron source 10, compared with a tungsten (W) filament electron source and a cold-emission electron source for example which are conventional types, a potential contrast image hardly varied in brightness is acquired. The electron beam 19 is withdrawn from the electron source 10 by applying voltage between the electron source 10 and the electron beam withdrawing electrode 11. The electron beam 19 is accelerated by applying high negative potential to the electron source 10.
Hereby, the electron beam 19 advances in a direction of the sample stage 30 with energy equivalent to the potential, is converged by the capacitor lens 12, is further converged to be thin by the objective 16, and irradiates the wafer 9 mounted on the X and Y stages 31, 32 on the sample stage 30. The scanning signal generator 44 that generates a scanning signal and a blanking signal is connected to the deflector for blanking 13, and each lens power source 45 is connected to the capacitor lens 12 and the objective 16. Negative voltage (retarding voltage) can be applied to the wafer 9 by a retarding power source 36. A primary electron beam is decelerated by adjusting the voltage of the retarding power source 36 and electron beam irradiation energy to the wafer can be adjusted to an optimum value without changing the potential of the electron source 10.
A secondary electron or a reflected electron or both 51 generated by radiating the electron beam 19 onto the wafer 9 are accelerated by negative voltage applied to the wafer 9. The lens for converging a secondary signal 69 is arranged in a position which the primary electron beam crosses or in a position close to the cross position on the upside of the wafer 9 and hereby, the divergence of the secondary electrons or the reflected electrons or both 51 respectively accelerated is adjusted by the lens 69. For the lens for converging a secondary signal, either of a magnetic field type lens or an electrostatic type lens can be used. The magnetic field type lens and the electrostatic type lens may be also combined.
A controller 70 that controls the lens for converging a secondary signal 69 can vary in interlock with negative voltage applied to the sample and optical conditions of a primary electron beam including a set condition of the electrification control electrode 65. Besides, the ExB deflector 18 is arranged and a secondary electron or a reflected electron or both 51 respectively accelerated are deflected in a predetermined direction. An amount of deflection can be adjusted by the intensity of voltage and a magnetic field applied to the ExB deflector (Wien filter) 18. This electromagnetic field can be varied in interlock with negative voltage applied to the sample. The divergence and a traveling direction of secondary electrons or reflected electrons or both 51 are adjusted by the lens 69 and the ExB deflector 18, and the electron or both collide with the reflector 17 on a predetermined condition. When the secondary electron or the reflected electron or both 51 respectively accelerated collide with the reflector 17, a second secondary electron or a second reflected electron or both 52 are generated from the reflector 17.
The second secondary electron and a back scattered electron 52 respectively generated by the collision with the reflector 17 are led to the detector 20 by an attractive electric field. The detector 20 detects the second secondary electron or the second reflected electron or both 52 generated when the secondary electron or the reflected electron or both 51 generated while the electron beam 19 irradiates the wafer 9 are accelerated afterward and collide with the reflector 17 at the scanning timing of the electron beam 19. A signal output from the detector 20 is amplified by the preamplifier 21 installed outside the chamber 2 and is converted to digital data by the A/D converter 22. The A/D converter 22 converts the analog signal detected by the detector 20 to a digital signal immediately after the analog signal is amplified by the preamplifier 21 and transmits it to the image processor 5. As the converter digitizes the detected analog signal immediately after the detection and transmits it, a high-speed and high-SN ratio signal can be acquired. For the detector 20, a semiconductor detector may be also used.
The wafer 9 is mounted on the upside of the X and Y stages 31, 32 and in inspection, either of a method of making the X and Y stages 31, 32 at a standstill and dimensionally scanning the electron beam 19 or a method of continuously moving the Y and Y stages 31, 32 in a Y (longitudinal) direction at fixed speed in inspection and scanning the electron beam 19 straight in an X (lateral) direction can be selected. When a certain specific relatively small area is inspected, the former method of making the stages at a standstill and inspecting is effective and when a relatively large area is inspected, the latter method of continuously moving the stages at fixed speed and inspecting is effective. When the electron beam 19 is required to be blanked, the electron beam 19 is deflected by the deflector for blanking 13 and can be controlled so that the electron beam does not pass the diaphragm 14.
For the length measuring machine for the position monitor 34, a length measuring meter depending upon laser interference is used in this embodiment. Positions of the X stage 31 and the Y stage 32 can be monitored at real time and are transferred to the controller 6. Besides, the data of the number of revolutions of each motor of the X stage 31, the Y stage 32 and the wafer holder 33 is also similarly transferred to the controller 6 from each driver, the controller 6 can precisely grasp an area and a position irradiated by the electron beam 19 based upon these data, and if necessary, the displacement of the position irradiated by the electron beam 19 is corrected by the correction control circuit 43 at real time. The area irradiated by the electron beam can be stored every wafer.
For the optical height gauge 35, optical measurement equipment according to a method of measuring except an electron beam, for example laser interference measurement equipment and reflected light type measurement equipment that measures variation depending upon a position of reflected light are used to enable measuring the height of the wafer 9 mounted on the upside of the X and Y stages 31, 32 at real time. In this embodiment, a method of irradiating the wafer 9 by white light radiated from a light source 37, instructing the position detection monitor to detect a position of its reflected light and calculating an amount of the variation of the height based upon the variation of the position is used. The focal distance of the objective 16 for converging the electron beam 19 is dynamically corrected based upon data measured by the optical height gauge 35 so that the electron beam 19 always focused on an inspected area can be radiated. Besides, the warpage of the wafer 9 and the asymmetry of the height are measured before the electron beam is radiated and a condition for correction every inspection area of the objective 16 may be also set based upon the data.
The image processor 5 includes an image storage 46, an information processor 46 and a monitor 50. The information processor 48 is provided with a memory storing software that acquires the dimensional distribution information of a secondary signal emitted from an arbitrary area on the inspected sample from a signal output from the detecting system 7 and calculates potential charged on the surface of the inspected sample based upon the dimensional distribution information and software that processes the dimensional distribution information and inspects a defect of the inspected sample, and processing for detecting charged potential and inspecting a defect is executed. For the dimensional distribution information, image data in desired power or pixel data of the corresponding image can be used. For the size of a picture element and the size of a field of view of image data, data in arbitrary size can be used. Though the following is not shown, an information input unit for an apparatus user to set and input required information to a control system of the apparatus is provided to the monitor 50, and the monitor 50 and the information input unit configure a user interface of the apparatus. A picture signal of the wafer 9 detected by the detector 20 is amplified in the preamplifier 21, is converted to an optical signal in the electric-to-optical converter 23 after the picture signal is digitized in the A/D converter 22, is transmitted via the optical fiber 24, and is stored in the image storage 46 after the picture signal is converted to an electric signal in the photoelectric converter 25 again. A condition on which the electron beam irradiates and various detection conditions of the detecting system in image formation are set in setting detection conditions beforehand, are filed, and are registered in a database in the controller 6.
Next, operation executed by the controller 6 for controlling the lens for converging a secondary signal 69 will be described in detail. First, suppose that shading occurs in an image sensed on a certain electro-optical condition. An arithmetic unit in the information processor 48 analyzes the signal strength of picture elements in a shading area in the image and estimates an extent of the shading. For example, when a value of the signal strength of picture elements in the whole specific area on an image in a certain field of view is larger than a predetermined threshold, it is judged that shading occurs. The arithmetic unit estimates an extent of caused shading based upon information such as a rate for which a shading caused area accounts in the field of view and a maximum value of the signal strength of the picture elements in the shading caused area and transmits the information of the extent of the shading to the controller 6.
In the meantime, a storage in the controller 6 stores a correction table correlating the information of an extent of shading and a control condition of the lens for converging a secondary signal 69. For the information of an extent of shading stored in the correction table, maximum picture element signal strength in a shading area, the area of the shading caused area, and a rate for which the shading caused area accounts in a sensed field image (for example, the ratio of the number of all picture elements in a field image of predetermined size in predetermined power and the number of picture elements in the shading caused area in the corresponding field image) can be given. For the condition for controlling the lens for converging a secondary signal 69, a value of voltage applied to an electrode can be given when the lens 69 is an electrostatic lens and a value of current for exciting a coil can be given when the lens is an electromagnetic lens.
To use the correction table shown in
For example, suppose that shading occurs in an image for adjustment acquired when the electro-optical system is set. Such an image for adjustment can be acquired when the primary electron beam is focused for example. When the controller 6 detects the occurrence of shading, it transmits a signal telling the occurrence of shading to the information processor 48 and the information processor 48 instructs the monitor 50 to display a request for determining whether the shading is required to be eliminated or not. The request for determining is made by instructing the monitor to display a button and an icon including “ELIMINATE SHADING” and “RESET ELECTRO-OPTICAL SYSTEM” for example via GUI and requesting the user of the apparatus to select the button or the icon. The selection may be also made via an information input unit. Simultaneously, a button or an icon including “UNNECESSARY TO ELIMINATE SHADING” and “UNNECESSARY TO RESET ELECTRO-OPTICAL SYSTEM” for example for confirming the user's will that the elimination or resetting is unnecessary is also displayed via GUI. When the user of the apparatus presses the button that the elimination or resetting is necessary, the information processor 48 transmits information that the button is pressed to the controller 6. When the controller 6 receives the information that the elimination of shading is necessary from the information processor 48, the controller selects a control condition most suitable for eliminating shading of the lens for converging a secondary signal 69 referring to the correction table and transmits the control condition to the secondary signal converging lens controller 70. Hereby, the automatic control of the lens for converging a secondary signal is realized.
In the above-mentioned description, conditions for selecting the control condition of the lens for converging a secondary signal are required to be included in the correction table, however, a correction table storing only conditions for adjusting the lens for converging a secondary signal 69 and the electro-optical system may be also used. In that case, when shading occurs, plural conditions for resetting the electro-optical system such as “MEASURE 1 FOR SHADING”, “MEASURE 2 FOR SHADING”, - - - , “MEASURE N FOR SHADING” are displayed on the monitor 50. The user of the apparatus selects a condition considered most suitable for correcting shading, for example, “MEASURE 2 FOR SHADING” referring to an image on which shading occurs. The information processor 48 transmits information that the user of the apparatus selects the measure 2 for shading to the controller 6, the controller 6 selects an operating condition of the lens for converging a secondary signal corresponding to the measure 2 for shading referring to the correction table, and transmits the selected operating condition to the secondary signal converging lens controller 70. The above-mentioned control method is semiautomatic control and is not complete automatic control; however, even if causality between the information of the extent of shading and the condition for setting the electro-optical system is not definite, the above-mentioned control method has an advantage that the operation of the apparatus can be automized to some extent. Besides, as much data is not required to be stored in the correction table, the above-mentioned control method also has an advantage that a memory of small capacity can be used.
The example that control parameters of the lens for converging a secondary signal 69 are stored in a table format has been described, however, the control parameters and conditions for selecting the corresponding control parameter are not necessarily required to be stored in the table format. The conditions for selecting the corresponding control parameter can be made intricate to an arbitrary extent by combining various parameters.
The example that the image processor 5 and the controller 6 are separate has been described; however, the image processor 5 and the controller 6 may be also configured by the same information processor.
Next, one example of a result of calculating the divergence of secondary electrons emitted from the wafer on the reflector 17 in the configuration of the apparatus in this embodiment when the lens for converging a secondary signal 69 is unoperated (
In the meantime, as shown in
As described above, in the apparatus equivalent to this embodiment, a detection rate of secondary signals emitted from a wafer pattern can be greatly enhanced by providing the lens for converging a secondary signal and a uniform SEM image hardly having shading in the field of view can be acquired. This contributes to the enhancement of the SN ratio of the image and contributes to the enhancement of the sensitivity of inspection and higher precision and higher repeatability of measurement.
There is also a method of instructing a detector 20 to directly detect a secondary signal 51 accelerated from a wafer without using a reflector 17 and instructing a detecting system 7 to image. In this embodiment, the configuration of an apparatus and a setting method when the detector 20 is arranged outside an optical axis of a primary electron beam in such a method of directly detecting the secondary signal will be described. In this embodiment, the same reference numeral is allocated to a unit and others provided with the same function as those in the first embodiment and the description is omitted. In the configuration of the apparatus shown in
The divergence and a traveling direction of the secondary electrons or the reflected electrons or both 51 are adjusted by the lens 69 and the ExB deflector 18, and the electron or both are led to the detector 20 by an attractive electric field generated from the detector 20. The detector 20 detects a secondary electron or a reflected electron or both 51 generated while the electron beam 19 irradiates the wafer 9 at the scanning timing of the electron beam 19. In this embodiment, operating conditions of the ExB deflector 18 and the lens for converging a secondary signal 69 are required to be controlled together. Therefore, in a correction table stored in the information processor 100, the operating condition (a value of current for exciting a coil and a value of voltage applied to an electrode) of the ExB deflector 18 is stored in addition to the operating condition of the lens for converging a secondary signal 69.
As described above, the secondary electron or the reflected electron or both 51 can be detected substantially without loss in a state in which they directly hit the detector 20 by optimizing the settings of the ExB deflector 18 and the lens for converging a secondary signal 69 according to the optical conditions of the primary electron beam, and an SEM image the SN ratio of which is high and which hardly has shading caused by the failure of the detection of the secondary signal in a field of view can be acquired.
There is also a method of installing a detector 20 on a course (an optical axis) of a primary electron beam (a method of directly detecting a secondary signal). In this embodiment, the configuration of an apparatus and a setting method in that case will be described. In this embodiment, the same reference numeral is allocated to a unit and others provided with the same function as that in the first embodiment and the description is omitted.
When the detector 20 is arranged on an optical axis, a course of the secondary electron or the reflected electron or both 51 is required to be devised so that they does not pass a hole so as to secure a detection rate of the electron or both because the detector has the hole to pass the primary electron beam. For example, the divergence of the secondary electrons or the reflected electrons or both 51 is first extended by the lens 69, secondary signals 51 that pass the hole to pass the primary electron beam are reduced, and the secondary electron or the reflected electron or both are led to the detector 20 by an attractive electric field generated from the detector 20. The detector 20 detects the secondary electron or the reflected electron or both 51 generated while the electron beam 19 irradiates the wafer 9 at the scanning timing of the electron beam 19.
As described above, the secondary electron or the reflected electron or both 51 can be detected substantially without loss so that they directly hit the detector 20 by optimizing the setting of the secondary signal converging lens 69 according to optical conditions of the primary electron beam and an SEM image the SN ratio of which is high and which hardly has shading caused by the expected defeat of the detection of a secondary signal in a field of view can be acquired.
There is also a method of adjusting and detecting the divergence of secondary signals 51 accelerated from a wafer by a lens for converging a secondary signal 69 after the secondary signal is separated from the primary electron beam by an ExB deflector (Wien filter) 18. In this embodiment, the configuration of an apparatus and a setting method in that case will be described.
As described in detail above, according to the invention, the failure of the detection of the secondary signal due to the variation of optical conditions of the primary electron beam or the occurrence of an electric field perpendicular to a traveling direction of the primary electron beam in a surface of the sample is minimized, an SEM image the SN ratio of which is high and which hardly has shading in the field of view can be acquired, measurement such as the measurement of dimensions and a configuration of a measured object, the inspection of a defect and review at high precision and at high repeatability is enabled, and the information closer to a truth of a semiconductor device can be acquired.
Besides, the measurement precision and the repeatability of the inspection and measurement apparatus can be enhanced by applying the secondary signal control lens according to the invention to an inspection and measurement apparatus using a charged particle. When the secondary signal control lens according to the invention is applied to a semiconductor inspection apparatus using a charged particle, the failure of an electric characteristic can be detected at high sensitivity.
Number | Date | Country | Kind |
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2006-199072 | Jul 2006 | JP | national |