JP3959063B2 - Alignment method and apparatus for charged particle beam column - Google Patents

Description

Field of Invention

  The present invention relates generally to charged particle columns, and more particularly to a method and apparatus for generating an image for aligning a charged particle beam column. More particularly, the present invention relates to automatic alignment of charged particle beams and automatic correction of aberrations.

Background of the Invention

  In order to construct and explore samples within the nanometer scale, there is an increasing demand in the industry for technologies such as microelectronics, micromechanics and biotechnology. Exploration or construction on such a small scale is often done using an electron beam, which is generated and focused in a charged particle beam device such as an electron microscope or an electron beam pattern generator. Since the charged particle beam has a short wavelength, it provides superior spatial resolution compared to, for example, a photon beam.

  However, the spatial resolution that can be achieved based on wavelengths such as, for example, less than 0.01 nm is limited by the inherent aberrations and misalignment of the beam of charged particles, resulting in reduced resolution.

  For example, in a scanning electron microscope (SEM), the beam is focused on a small spot around 10 nm in size. Scanning with a beam is performed from above the sample. Therefore, the resolution of the obtained image is limited by the beam diameter in the plane of the sample surface.

  The beam diameter is independent of the alignment of the beam, for example, but may be limited by aberrations such as chromatic aberration depending on the energy dispersion of the electron beam. Furthermore, there is also spherical aberration that occurs when the aperture of the imaging lens is not zero. However, aberrations may be worsened or introduced by beam misalignment with respect to the optical axis of the individual coupling elements. Therefore, one of the most important parameters for optimizing the resolution and thus the image quality is the alignment of the optical column. In order to achieve a high spatial resolution, the distortion must be very small, so it is necessary to periodically align the beam with respect to the individual optical elements.

  Conventionally, an operator has to perform alignment of a charged particle column. Therefore, the operator adjusts each signal given to the alignment correction device based on the measured image. One drawback of this approach is that the decision is left to the operator. Therefore, inaccuracies and differences due to the operator arise. Furthermore, manual adjustment is time consuming and this is a disadvantage for on-line inspection equipment that requires particularly high equipment throughput.

  US Pat. No. 5,627,373 describes a method for automatically aligning an electron beam axis with an objective lens in a scanning electron microscope. Accordingly, an image of the sample at the first and second points in the focusing range of the objective lens is measured. For each image, an indication signal for the position of the straight ruler in the field of view of the microscope is generated. After detecting the lateral movement of the image from the two signals and automatically adjusting the alignment, this procedure is repeated in the orthogonal direction. The complete operating procedure is repeated until the image shift caused by misalignment is below a predetermined threshold.

  Further, a method for automatically correcting electron beam astigmatism in a scanning electron microscope is suggested in US Pat. No. 5,627,373. For astigmatism correction, boundary portions are sampled at intervals of 30 degrees around the entire circumference of the sample. The axis of the beam distortion is specified based on an instruction signal indicating the sharpness between samples. Distortion is adjusted and improved along these axes over time.

  US Pat. No. 6,025,600 teaches a method for calculating and correcting astigmatism errors in charged particle beam devices. Images are collected during the cleaning of the objective lens setting of the charged particle device. Image features in different directions, such as straight lines within the astigmatism correction target, are analyzed. Optimal sharpness or best focus value is obtained as a function of objective setting. Appropriate changes to the settings of the astigmatism correction device are calculated by taking in a linear combination of optimal sharpness values associated with image features in different directions.

  US Pat. No. 6,067,167 describes a scheme for realizing automatic adjustment of an electron optical system in an electron optical device such as a scanning electron microscope, in which the refractive index of the objective lens is A predetermined number of images continuously obtained by an electro-optical device are stored at a focus that is continuously adjusted to change. The amount of movement of the sample image is calculated. Therefore, it is determined whether or not adjustment is necessary based on the calculated movement amount, and adjustment is performed if necessary. Furthermore, a scheme for realizing astigmatism correction in a charged particle beam optical system of an electron optical device such as a scanning electron microscope is also disclosed.

  However, there are additional problems associated with the alignment of charged particle beam columns, and the improvements provided in the prior art require further completeness, especially when considering on-line inspection equipment or on-line beam writers.

Summary of the Invention

  The present invention is intended to overcome some of the problems described above. According to one aspect of the present invention, there is provided a rapid image generation method for alignment of a charged particle beam column as defined in independent claim 1, and a charged particle beam apparatus as defined in independent claim 32. Provided.

  Further advantages, features, aspects and details of the invention are apparent from the dependent claims, the description and the attached drawings.

  According to the present invention, there is an advantage that rapid and efficient automatic alignment of the column can be performed. Therefore, the alignment of the charged particle beam can be performed for each wafer or each part of the wafer. Therefore, the above-mentioned drawbacks related to the prior art can be overcome. For example, in the prior art, it has been suggested to adjust each misalignment repeatedly. Therefore, even when the beam of charged particles is automatically adjusted, an operator's action of adjusting the apparatus is imitated. Although imitation of the operator's iterative procedure may improve the alignment time to some extent, the time required to adjust the charged particle device is still in the range of 5-10 seconds. This is particularly true because the prior art teaches changing the focus by changing the current in the objective lens. However, objective lenses are often constructed in the form of magnetic or magnetic electrostatic lenses. Thus, the self-inductance of the coil impedes the rapid change of focus and limits the further time needed to adjust the beam of charged particles.

  According to a further aspect of the present invention, there is provided a method for generating an image that is used to automatically optimize the imaging quality of a charged particle column. The method introduces chromatic aberration in the objective lens by changing the potential affecting the beam of charged particles, resulting in a change in focus, and creating a set of images during beam defocusing. Is included.

  As described above, the alignment time can be shortened by defocusing the beam of charged particles by the introduced chromatic aberration using a potential that affects the beam energy. The introduction of chromatic aberration in the content of the method is considered to change the chromatic aberration. Therefore, the focal length is changed based on the change of the introduced chromatic aberration.

  Preferably, measuring the lateral movement of the image caused by defocusing and aligning the charged particle beam with respect to the optical axis based on the measured image shift is included.

  It is beneficial to carry out further steps according to the invention. The sharpness of the image set is evaluated to determine the beam energy corresponding to the desired focus. Thus, by setting the beam energy to a desired value, it is possible to automatically adjust the focus on the surface of the sample.

  Furthermore, it is preferred to correct the beam distortion by evaluating the sharpness of the image set in at least two different directions to obtain at least one correction signal for an astigmatism correction device for correcting the beam distortion.

  According to the above two advantages, the set of images can be used to adjust several parameters that optimize the shooting quality using a set of shot images. In this way, the charged particle beam can be aligned within about 0.5-1 second, taking into account the time for calculation in addition to the measurement time. Therefore, autofocusing and at least automatic alignment can be easily performed on all parts of all wafers with respect to the optical axis, and steady imaging conditions can be obtained.

  According to a preferred embodiment of the invention, the sensitivity of shifting the beam of charged particles with respect to the optical axis is calibrated at least once, and the calibration is performed based on an image shift of at least one set of images generated during calibration. .

  Furthermore, it is preferred to calibrate the sensitivity to correct the beam distortion of the beam of charged particles at least once, and the calibration is based on an assessment of the sharpness of at least one set of images generated during calibration.

  All procedures that align the position of the beam with respect to the optical axis and correct beam distortion are beneficial in two directions. More preferably, the above procedure is used independently for the two directions. This is useful when the two directions are substantially orthogonal or at an angle of 45 °.

  In accordance with a further aspect of the present invention, an automatic image generation method for aligning a charged particle beam column is provided, which method introduces a change in focus by introducing chromatic aberration by changing the energy of the charged particle beam. Obtaining an image set while varying the energy of the beam of charged particles, evaluating the sharpness of the image set in at least two different directions, and providing a correction signal to the astigmatism correction device. Correcting beam distortion.

  According to a further aspect of the invention, there is provided an automatic image generation method for aligning a charged particle beam column, the method comprising changing the focus by introducing chromatic aberration by changing the energy of the charged particle beam. Obtaining an image set while changing the energy of the charged particle beam, measuring the image shift, and determining the position of the charged particle beam with respect to the optical axis based on the measured image shift. Correcting.

  According to a further aspect of the present invention, there is also provided a method for automatically aligning a charged particle beam with the optical axis of a charged particle beam column, the method comprising measuring the energy of the charged particle beam in the objective lens. At least two of the steps: introducing chromatic aberration according to a change in potential to affect; scanning the sample with a charged particle beam while changing the energy of the beam of charged particles; generating an image set; Measuring an image shift between different images; calculating a correction signal based on the image shift; and applying the correction signal to the deflecting unit to align the beam with the optical axis.

  The invention also relates to an apparatus for carrying out the above disclosed method, comprising an apparatus portion for carrying out each of the described method steps. The above method steps may be performed by hardware elements, or may be performed by a computer program with appropriate software, or any combination of the two, or other It may be carried out in any manner. The invention further relates to a method of operating the described device. This includes method steps for performing all functions of the apparatus.

Description of preferred embodiments)

  Some of the above and other more detailed aspects of the invention are described in the following description and partially illustrated with reference to the drawings.

  First of all, one skilled in the art should recognize that the present invention can be used with any charged particle device. However, for convenience, the present invention will be described with respect to implementation in a scanning electron microscope (SEM). One skilled in the art should recognize that all considerations relating to voltage and potential are referred to in this text relative rather than absolute. For example, accelerating the beam by connecting the cathode to “earth” and applying 3 kV to the sample is equivalent to applying −3 kV to the cathode and placing the sample on ground. Thus, for convenience, some discussion is provided in terms of specific voltages, but it should be understood that the reference is relative potential.

  A block diagram of the electron microscope is schematically shown in FIG. The electron microscope 100 includes an electron gun 103 that emits an electron beam 101 extracted by an anode 104. The objective lens 112 focuses the electron beam on the sample surface 105a. In order to obtain an image from the entire field of view, the beam is scanned on the sample using the scanning deflection unit 102. Beam alignment with respect to the aperture 106 or the optical axis 113 can be achieved by deflectors 108 and 111, respectively. As the deflection unit coil, an electrostatic module in the form of a charged plate or a combination of a coil and an electrostatic deflector can be used. An image is formed in a scanning electron microscope (SEM) by detecting backscattered electrons or secondary electrons with the detector 16 and synchronizing the detection signal with scanning of the electron beam. Thus, during the measurement, the measurement data is generated to generate an image or one or several frames. In general, data is stored at least in intermediate or buffer memory and will be evaluated in further steps. Alternatively, the images may be stored on any suitable storage device and evaluated after some or all of the images are generated. Suitable storage devices include, for example, a hard disk, a permanent recording device, or a RAM.

  A second product of great importance for sample inspection or imaging is secondary electrons that deviate from the sample 105 at various angles at relatively low energy (3-50 eV). These secondary electrons reach the detector 16 and are detected. By scanning the electron beam 101 on the sample 105 and displaying / recording the output of the detector 16, an image of the surface of the sample 105a is formed.

  Various parts of the apparatus include a corresponding supply device, high voltage supply device 21, gun alignment deflection control device 22, aperture alignment deflection control device 23, scanning coil supply device 27, objective lens supply device 24, astigmatism correction device control ( And a current supply) device 28, a sample voltage supply device 25, and a sample stage supply device 26, which are controlled by a parameter adjusting device 31. The parameter adjustment device 31 is connected to a standard setting device 35 and an analysis and / or synchronization control device 32 that provides the parameter adjustment device 31 with a basic parameter set.

  In this application, the expression “charged particle beam column” refers to any kind of apparatus in which a beam of charged particles is aligned. An electron microscope, an electron beam writing device, or a corresponding device using ions. The “aperture” used in the present application may be a beam-defining aperture or may be an aperture integrated in a separate and different vacuum chamber. Preferably, however, the expression aperture refers to the final aperture of the charged particle beam column. For example, in certain embodiments, unless otherwise noted, the beam deflection field and beam deflection section are assumed to be electrostatic, magnetic or magneto-electrostatic. Such a device can be realized in the form of a bias plate, a coil or a combination thereof. Measurement of dispersion from the sample or secondary fine particles can be performed using a detector in the form of a scintillator connected to an optical amplifier tube or the like. This does not limit the present invention as the signal measurement method generally does not affect the philosophy of the present invention.

  Further, in this application, the term frame is used as a single scan of the field of view of the charged particle beam device. As a result, an image is obtained. In general, however, an image can be obtained, for example, by averaging two or more frames. Further, a final image may be obtained using a filter function or the like.

  Further, in the present application, changing the energy of the charged particle beam includes changing any potential in the charged particle beam column or the energy that the charged particle gives to the sample, which affects the velocity of the charged particles in the objective lens. Is included. In this way, the acceleration voltage in the gun can be changed. Furthermore, the energy of the beam can also be changed by changing any other potential that affects the focusing characteristics of the objective lens, such as an aperture potential, a sample potential or a beam boost potential. The change in beam energy in this application is similar to the expression used for periodic defocusing using lens current, as known from operator-assisted alignment, "wobble" or "swing motion" (Wobbling) ".

  Further, in this application, the term “evaluate” also refers to the calculation of an algorithm used by a computer, for example.

  Therefore, the aperture alignment deflection units 110 and 111 are used to align the electron beam 101 with the optical axis 113. The expression “aperture alignment” is used because the electron beam 101 can be aligned with the optical axis by mechanically moving the aperture instead. For the embodiments described herein, two deflectors 110 and 111 are used. Therefore, the beam tilt introduced by the first deflection unit 110 can be corrected by the second deflection unit 111. With this double deflection device, the electron beam can be shifted in one direction without introducing a beam tilt of the electron beam with respect to the optical axis. In this way, the beam can be shifted on the optical axis. FIG. 1 shows this deflection device for one direction. The electron beam is shifted on the optical axis with respect to the x direction, as shown in FIG.

  It should be understood that the present invention is a two-dimensional beam shift with respect to aligning the electron beam to the optical axis. Thus, it is beneficial to perform the alignment described above in the second direction. By adjusting both the x and y directions, the electron beam is aligned with the optical axis. This second direction is at least different from the first direction. The second direction is preferably orthogonal to the first direction. Thus, it is preferable to perform the alignment procedure in the x direction also in the y direction. Since the two directions are independent, the alignment steps can be performed independently of each other unless beam tilt is taken into account. For the alignment in the y direction, it is advantageous to arrange the third and fourth alignment deflectors so that the deflection corresponding to the second direction can be realized.

  One embodiment for aligning the electron beam 101 with the electron microscope 100 shown in FIG. 1 will be described with respect to FIGS. 2a and 2b. The objective lens 112 focuses the electron beam 101 on the sample. The electron beam 101 is previously formed by a diaphragm and a condenser lens. Since the objective lens 112 serves primarily to achieve a final spot size that limits resolution, the beam must be accurately aligned with the objective lens. In general, the objective lens defines the optical axis.

  The principle of whether the beam is correctly aligned will be better understood in connection with FIG. 2a. On the left side, the contour of the electron beam 101a aligned in contrast to the optical axis 113 is drawn. If defocusing is deliberately induced, the focal point moves in a direction perpendicular to the sample plane. When measurement is performed during defocusing, the impression of each obtained image is as follows. Image features will be drawn in and out of focus. However, since the focus of the electron beam 101a changes only in the direction perpendicular to the sample surface, the image is not laterally moved. Only image blurring or corresponding image features are visible. Based on the sharpness evaluation described in more detail below, the focus that gives the best imaging results can be calculated.

  On the other hand, as shown for the electron beam 101b on the right side of FIG. 2a, when the beam alignment is not good, the focal point moves not only in the direction perpendicular to the sample surface but also in the direction parallel to the surface. . Thus, in the case of a continuous image, in addition to image blur or feature, lateral movement / shift of the image is also seen. This is further illustrated in FIG. A well-focused image feature 502 within the field of view 501 corresponds to the focus condition of FIG. 5a. If defocusing to the focal point 503b is achieved, the image features will shift with respect to the field of view and will be blurred due to defocusing. Thus, the image feature looks similar to, for example, 502b. On the other hand, for a focus according to 503c, the image feature is moved laterally in the opposite direction as shown for 502c. Again, this lateral movement occurs in addition to blurring.

  In accordance with the present invention, defocusing that causes lateral movement of the image is obtained by a change in beam energy, in other words, by any potential change that affects the energy of the electron beam in the objective lens. According to one embodiment, the change in beam energy is provided using the acceleration voltage of the electron gun 103. The power supply used for the acceleration voltage must be able to provide a stable voltage between zero and 100, 1000 or even tens of thousands of volts. The change in acceleration voltage, in other words, the modulation range is from zero to several hundred volts. One criterion given for the acceleration voltage is stability. Since it is more difficult to provide a stable power source that can be modulated from at least 10 Hz to several hundred Hz or 1 kHz, a second power source that provides a stable power source for high voltages up to several tens of kV and has a modulation range of several hundred volts It is beneficial to provide power. In this way, stable acceleration voltage modulation can be provided.

  As already described, the first aperture alignment deflection unit 110 does not shift the beam toward the optical axis 113, but deflects the beam and introduces an inclination to the beam. That is, for example, the angle of the beam with respect to the optical axis changes due to deflection. This beam deflection is at least partially compensated by the second aperture alignment deflection unit 111. Preferably, the second aperture alignment deflection unit 111 tilts the beam in the opposite direction at the same angle as the first aperture alignment deflection unit. This can be achieved, for example, by electrically connecting them so that deflection fields having two similar deflection parts and having the same absolute value have opposite directions. However, the present invention is not limited to the similar first and second aperture alignment deflection units 110 and 111. The difference in deflection angle with respect to the same signal given to the deflection unit can also be made uniform by a correction factor or the like.

  In accordance with the present invention, focusing and defocusing is realized by a change in the energy of the electron beam 101, in other words, by a change in the potential that affects the energy of the beam in the objective lens 112, whereby chromatic aberration is achieved by the objective lens. Introduced in. This is advantageous over defocusing using objective lens current. Therefore, the focus change is not limited to the self-inductance in the lens. Furthermore, no hysteresis occurs in the objective lens.

  Independent of specific embodiments, for various aspects of the invention, the generated image set preferably includes at least 20 images, and the generated image set preferably includes at least 80 images. Generally speaking. In this way, it is possible to more accurately determine the shooting performance for various focal positions.

  Further embodiments use images or frames generated to correct the distortion of the electron beam 101, respectively. The electron beam deviates from an ideal circular cross section, for example due to astigmatism in the objective lens. This astigmatism can be compensated by an astigmatism correction device. In describing astigmatism, the use of the term x or y direction does not necessarily imply that the two directions are orthogonal to each other. Furthermore, the x and y directions preferably refer to those rotated by 45 degrees with respect to astigmatism correction. The principle of astigmatism alignment will be better understood with respect to FIGS.

  When astigmatism occurs, the point-to-point object 801 is not ideally shot because the focal point 809 in the first image plane 803 is not the same as the focal point 811 in the second image plane 805. Thus, in order to find the point of best focus 807, a compromise must be made. For this reason, the diameter of the image 808 at the point of optimum focus is increased. In the case of defocusing, the image of the point-to-point charged particle beam is already elliptical, not circular. The direction of the ellipse depends on whether the image plane is in front of or behind the point of the optimum focus 807. Furthermore, when the image is out of focus, the resolutions in the two directions defined by the first and second image planes are different from each other.

  In the electron beam apparatus, beam distortion is corrected by an astigmatism correction apparatus. This correction is preferably performed every time alignment is performed because point-to-point imaging changes depending on the alignment of the electron beam 101 in the electron beam column. The distortion in the x direction is corrected for the coil set 902. The coil set is preferably arranged in the form of a quadrupole. In general, coils are used to form quadrupoles. However, a combination of a coil and an electrostatic plate can also be used. In order to perform correction in a direction independent of the x direction, it is preferable to use a second quadrupole structure as the coil set 903. Thus, the beam distortion can be corrected by the octupole structure formed as described above in each direction. However, it is also possible to use only a quadrupole astigmatism correction device that can be rotated and adjusted in each direction of distortion of the beam 101. As shown in FIG. 4, the beam does not necessarily pass through the center 901 of the astigmatism correction device. Further, when an electric current is applied to the coil, an electric field is generated to shift the electron beam 101. This meaning is described below.

  In general, the change in potential affecting the beam energy in an objective lens in the form of a lamp is preferably used to define the lamp by its initial value, amplitude and resolution of the potential affecting the beam energy. Therefore, the resolution of the potential affecting the beam energy in the objective lens is preferably optimized by defining the total number of frames or by defining the lamp amplitude. In this way, a sufficient ramp can be generated automatically if sufficient results are not obtained by automatic ramp generation. A sufficient lamp is a lamp that can perform an automatic alignment process. The resolution of the potential affecting the beam energy is assumed to be the potential difference between two consecutive frames.

  In general, the following should be considered for all value changes in the form of a lamp. For example, it is preferred that an individual frame is generated during a single lamp cycle given a change in potential that affects the beam energy in the objective lens in the form of a lamp. Such lamps can be beneficially linear or stepped. In this way, it is possible to synchronize the change in the potential that affects the beam energy and the generation of the image. This synchronization is also beneficially used to average the individual frames generated in the corresponding image conditions.

  With respect to the above, the image set is preferably generated to obtain a pseudo-continuous image shift. It is further preferred that the image shift of the two image tubes of the image set is less than 10 pixels, preferably less than 5 pixels, more preferably less than 2 pixels. In this way, for example, the preferred algorithms for measuring the lateral movement of the image or the characteristics of the image are more accurate, respectively, and errors are less likely to occur in the image processing routine. The preferred algorithm for measuring image shift is based on image processing. Preferably, the image shift is measured using pattern tracking, particularly recursive pattern recognition.

  According to one embodiment, it is preferred that the change in one of the potentials affecting the beam energy of the electron beam in the objective lens and thereby introducing beam defocusing is applied in the form of a lamp. . This variation is preferably variable in the vicinity of the focal length corresponding to the surface of the sample. The ramp as shown in FIG. 5 b is defined by the amplitude of ramp 906 and the initial value of ramp 905. A lamp such as that shown in FIG. 5b has a constant first derivative, so that it is easy to synchronize the change in beam energy with the generation of the frame / image. The duration of the ramp is between 0.5 seconds and 1 second. Within this time, 45-90 or more frames are generated. For example, when applying 3TV mode, a new frame can be generated about every 11 milliseconds. Since synchronization is performed, each frame and each frame number 907 can be matched to the beam energy. On the other hand, the beam energy can be correlated with the focal length with respect to the surface of the sample. By assigning the amount of defocusing or the beam energy of the electron beam to the frame number, several alignment procedures and some corrections described below can be performed.

Thus, regardless of the specific embodiment, it is preferable that defocusing takes less than 1 second, preferably less than 0.5 seconds. Therefore, the total alignment time can be shortened by 5 to 10 times compared to the state of the art.

  Defining the change in beam energy in the form of a lamp has a further advantage. Different frames generated during one ramp period are analyzed for several characteristics. Therefore, the best beam energy value is determined. In general, the best beam energy value for any property is an extreme value. If extreme values are not observable, it may be necessary to continue the ramp. Defining the lamp using the amplitude and the initial value facilitates continuation of the electron beam energy in the form of these further lamps.

  According to one embodiment, the beam energy associated with the optimized focus can be determined. Thus, measurements performed during a single lamp cycle can be used for an autofocus device. Accordingly, the sharpness of the obtained image is evaluated by one of the following procedures.

  Initially, correlations between specific regions of successive frames or successive frames containing individual image features can be calculated. For strong defocusing, blurring occurs. Thus, there is a lot of statistically distributed noise. For this reason, the correlation between two continuous frames or regions of consecutive frames is reduced. In the case of small defocusing, there is almost no blurring. This is due in particular to the fact that the electron beam has a so-called DOF (depth of focus) which is the length around the best focus in the direction of the beam radio wave where the beam diameter does not change significantly. Thus, two consecutive frames show little statistical noise and are nearly identical. Therefore, the correlation is increased. From the correlation, a so-called focus mark can be directly estimated. Thus, the higher the correlation or focus mark, respectively, the better the beam will be focused on the sample. The above evaluation can be automated. Thereby, an autofocus routine is obtained. In practice, however, there are several problems that must be overcome. For example, the image may shift from one shooting frame to the next shooting frame in the image set. This lateral movement, discussed in more detail below, prevents an accurate assessment of correlation-based sharpness. This effect may be negligible if the lateral movement of the image is within 2-3 pixels. If the shift exceeds 2-3 pixels, the image shift must be corrected. Thus, the above procedure can be improved by measuring the lateral movement of the image. The shift of the image or the feature of the image can be detected by using a pattern recognition tool described later. Once the feature shift is known, the image can be corrected for the calculated shift before calculating the correlation. In this way, the reliability of the focus mark as defined above can be increased.

  Second, a statistical evaluation of sharpness can be used. Therefore, the contrast is evaluated not only for individual features of the image but also for the entire field of view. This is done by calculating the histogram of the induced image. To obtain sharpness independent of any direction of the image, the average value of the directional derivatives in the x and y directions can be used. This evaluation is based on the assumption that sharper images with less blur show greater contrast at the edges of all types of image features. In addition to edge contrast, noise also affects the histogram. From the above, it can be expected that the histogram shows a region with noise and a region with contrast of the edge of the image feature. If the contrast is good due to good focusing, the region with edge contrast is more important than a blurred image with low contrast. Using the above behavior to evaluate sharpness, the focus mark can be obtained again. Since the entire image is statistically evaluated, field shifts can be ignored as long as sufficient image features evaluated are randomly distributed throughout the sample. Furthermore, the histogram for the first derivative, i.e. the number of sharp edges, can be normalized to the average contrast of the individual frames.

  As an example, the device stores an image of 90 frames. The image processor then calculates the average of each of five consecutive image frames and calculates the fiber score for each of the two directions. Thus, there are 18 ratings in each direction. Thus, for the present invention, in order to calculate the correction signal based on the sharpness score, it is necessary to use at least two sharpness scores for each direction. More preferably, a sharpness score of at least 10 is used.

  It should be understood that the present invention is not limited to the two examples described above with regard to sharpness assessment. Other further sharpness evaluation routines may be applied to the present invention.

  As described above and as described above with respect to FIGS. 2a and 2b, according to a further embodiment, if the electron beam is deviated with respect to the optical axis, the potential affecting beam energy is changed. An image shift occurs between successive frames, thus resulting in electron beam defocusing with respect to the sample surface. The dependence of this image shift on, for example, a change in acceleration voltage shows a substantially linear behavior. If the observed image shift is known, the beam can be aligned to the optical axis based on calibration.

  Therefore, the image shift can be measured as follows. The generated frame is sent to the frame grabber and supplied to the image processor. The image processor selects one or more features of the image. Therefore, edges with high contrast and different directions with respect to the field of view are selected. The pattern recognition algorithm calculates the number of pixels of the image feature shift between successive frames according to the image feature. In general, pattern recognition can be performed using one of several possibilities, such as a template image, classification, a neural network or a correction score for other pattern recognition routines. Another option is to use recursive pattern tracking. Recursive pattern recognition is beneficial because the shift between successive images is generally limited to a few pixels. If the shift from frame number k to frame number k + 1 is, for example, less than 10 pixels in each direction of the image, the reliability of the pattern tracking routine can be improved. As a further option, individual frame features can be compared and correlated with features of one initial frame. Such an initial frame may have the best focus mark. If the correlation between the initial frame and the individual frame is worse than a certain threshold, the individual frame can be excluded from further calculations. In this way, it is possible to avoid an image with excessive blurring from reducing the accuracy of calculation. As a result, the image shift of each image in the generated image set can be obtained.

  In order to be able to non-repetitively align the electron beam with the optical axis, calibration must be used. Such calibration may be performed once during manufacture. However, it is preferable to periodically update the calibration to increase accuracy. The time interval from calibration update to update is likely to depend largely on the operation and environmental conditions using the SEM. The calibration may be updated once a month, for example.

  Calibration of the sensitivity of the aperture alignment, that is, alignment of the electron beam with respect to the optical axis is performed as follows. First, manually align the SEM as accurately as possible. Therefore, if calibration has been performed before, the operation can be supported by automatic alignment using the old calibration. The next step is performed at least twice, preferably four times. Thus, at each time, a known exact misalignment of the beam is deliberately introduced. Finally, a set of calibration frames is generated for each misalignment, giving a known change in the beam energy of the electron beam. This is done, for example, by modulating the acceleration voltage. The image processor analyzes this set of images and each known observed shift against misalignment is used as a reference measurement. Based on known beam energy changes, known misalignments, and measured image shifts, the sensitivity of the device can be calibrated separately in the x and y directions. The conversion factor for calibration is calculated using, for example, a linear polynomial regression model or any other suitable function. Thus, among the fitting routines, especially least squares fitting or absolute value minimization for the fitting curve can be used. Preferably, a linear calibration function is used for calibration.

Using calibration, the signal I to be applied to the alignment deflection unit can be determined in order to align the beam in the adjustment step according to the following.

したがって、Ｃ_{ｘ（ｙ），ｓｃｎｓ}は、較正から得られる感度換算係数であり、ΔｘおよびΔｙはそれぞれの方向における画像シフトである。

Therefore, C _{x (y) and scns} are sensitivity conversion coefficients obtained from calibration, and Δx and Δy are image shifts in the respective directions.

If there is a beam tilt with respect to the optical axis in any direction relative to the original alignment, the above equation can be developed. A small beam tilt that is not corrected results in an image shift in the y direction and vice versa, independent of the alignment deflection in the x direction. Thus, the above equation can be read as follows:

毎日、あるいはウェハの異なる部位で新しい測定を始める前ごとに、電子ビームを光軸に対してアライメントするために、様々なビームエネルギの間に生成された画像セットを、ビームシフトに関して分析することができる。以前に較正された感度換算係数Ｃ_{ｉ，ｓｃｎｓ}と、測定されたビームシフトΔｘおよびΔｙと、ビームエネルギの変化ΔＶ_ａｃｃとから、ｘ方向のための絞りアライメント偏向部１１０および１１１に与えられることになる信号ｌ_ｘ（ｎｅｗ）と、ｙ方向のための更なる絞りアライメント偏向部に与えられることになる信号ｌ_ｙ（ｎｅｗ）とが算出される。このようにして、全く反復を必要とせずに、電子ビーム１０１を光軸にアライメントすることができる。しかしながら、画像シフトΔｘ、Δｙの信号補正からの依存性の直線性が、大きなミスアライメントに対しては有効でないので、随意で更なるアライメントステップを行うこともできる。

The image set generated during various beam energies can be analyzed for beam shifts to align the electron beam with respect to the optical axis daily or before starting a new measurement at a different part of the wafer. it can. From previously calibrated sensitivity conversion factors C _{i, scns} , measured beam shifts Δx and Δy, and beam energy changes ΔV _acc , to be given to the aperture alignment deflection units 110 and 111 for the x direction. A signal l _x (new) and a signal l _y (new) to be provided to a further aperture alignment deflection unit for the y direction. In this way, the electron beam 101 can be aligned with the optical axis without requiring any repetition. However, since the linearity of the dependency of the image shifts Δx, Δy from the signal correction is not effective for large misalignments, further alignment steps can optionally be performed.

  In general, independent of the particular embodiment, the calibration described above eliminates the need for initializing repetitive adjustments originally performed by the operator. For this reason, there is essentially no need to repeat. In this way, a quick adjustment of charged particles or rams can be performed. Furthermore, if iteration is performed, higher accuracy can be achieved with fewer iteration steps. Furthermore, it is preferable to calibrate the charged particle beam device for different settings. Therefore, a correction that is affected by any kind of device parameter can be taken into account, so that a more accurate calibration can be obtained.

  Furthermore, it can be generally said that the correction of the position of the charged particle beam with respect to the optical axis is preferably performed by moving the diaphragm. More preferably, the correction of the position of the charged particle beam with respect to the optical axis is obtained by deflecting the beam of charged particles. In this way, the beam can be automatically aligned with the optical axis of the charged particle device.

  Furthermore, independent of the specific embodiment, the following applies: If the beam of charged particles is deflected in one deflector, the beam is further tilted. Thereby, the two degrees of freedom affecting the alignment of the charged particle beam are paired with each other. This pairing complicates beam alignment. In order to avoid this, it is preferable to use a second deflector for redirecting the beam in the original direction for the above method. More preferably, the beam is redirected to propagate along the original direction.

  In a further embodiment, the image distortion generated while changing the beam energy can be used to evaluate beam distortion. In this manner, astigmatism of the objective lens can be corrected using the astigmatism correction device as described above with reference to FIG.

  As described above with respect to FIG. 3, astigmatism moves the focal point away from the surface of the sample, resulting in direction-dependent image blur. As shown in FIG. 3, the cross section of the beam for different focal positions changes into an ellipse with a specific directionality. Compared with the above-described sharpness evaluation, if the sharpness of individual images (at least two images) of the image set is analyzed depending on the direction, the strength and directionality of astigmatism can be evaluated.

  The directionality of the sharpness can be specified according to one of the following algorithms. These algorithms are based on the sharpness assessment described above for autofocus. Successive frames can be compared and different features with edges with different orientations can be compared. Since the beam having astigmatism has an elliptical shape, the sharpness in directions orthogonal to each other is different. Therefore, sharpness is analyzed in at least two orthogonal directions. However, it is beneficial to analyze the sharpness in four directions while rotating at an angle of 45 °. The intensity of astigmatism is detected by the difference in focus marks for two different defocusing amounts. When the histogram of the first derivative is used for sharpness evaluation, the first derivative can be calculated for the direction to be evaluated. Again, at least two orthogonal directions must be evaluated. However, preferably four directions rotated 45 ° are evaluated.

The calibration of the current to be applied to the astigmatism correction device can be calculated in analog to the calibration of the alignment process. In the first approximation, the linear dependence of the current to be applied can be inferred from the defocus amount in each direction. The defocus amount with respect to one direction can be defined by a numerical focus mark. Thereby, the focus mark fm is calculated for one direction. The difference between the focus marks of the two frames having the distance ΔV _acc is a measure of the amount of astigmatism. Further, as can be seen in FIG. 3, the two orthogonal directions are correlated with each other. Thus, as described above, the x and y directions are rotated with respect to the difference by an angle of 45 °. From calibration using linear regression,

に従う関数が得られる。較正の間、感度係数Ａ_{ｘ，ｙ，ｓｃｎｓ}が特定される。上記の数式はアライメントの間に、ｘ非点補正装置９０２およびｙ非点補正装置９０３に対する非点補正装置電流を補正するために用いることができる。

A function that follows is obtained. During calibration, the sensitivity coefficients A _{x, y, scns} are identified. The above formula can be used to correct the astigmatism corrector current for the x astigmatism corrector 902 and y astigmatism corrector 903 during alignment.

In summary, according to some aspects and embodiments of the present invention, the acceleration voltage or any potential that affects the energy of the electron beam, for example in the form of a lamp or any other well-defined manner, is changed. Image sets can be generated. Such a measurement takes only less than a second, and preferably takes less than a half second. An image set is a sufficiently large number of images, for example, exceeding 20 images. From these generated images, an autofocus routine, electron beam alignment with respect to the optical axis, and correction for electron beam column astigmatism can be calculated. It can be used remaining _^1/2 seconds to computation required. Within 2 seconds, focusing, aperture alignment, and astigmatism correction can be performed. This time is several times faster than the alignment time used in the prior art charged particle column alignment method. Therefore, the method of the present invention can be used for column adjustment before measuring each part of the wafer. Thereby, even if the wafer shows a strong surface topology, it is possible to perform measurement with high accuracy over a long period of time.

  A flowchart of a method according to some embodiments is shown in FIG. From the SEM workstation, i.e., the user interface, the operator initiates an autofocus routine or automatic wobble calibration with or without wobble. See step 601. The focus manager software shown at 602 sends setup parameters (see 604) and image parameters (see 605) to the microscope electronic cage (MEC) and image processing (IP) computer. The MEC sends parameters for the acceleration voltage ramp to the VBEAM section (see 605). The VBEAM section serves to generate a linear ramp of electron beam energy according to its input parameters. In parallel with each other, the IP and VBEAM begin their operation and the image signal is collected by the IP, thereby obtaining an image set with a beam energy ramp effect. The IP analyzes the generated frame according to aspects of the above-described embodiment. If autofocus fails, an error is sent to the workstation. Possible errors include, for example, focus marks that are below a certain threshold. This represents a serious problem with image quality. Further possible examples include focus marks for a wide range of beam energies that cannot be fully distinguished. In addition, the IP 606 calculates the image displacement alignment in the x and y directions. In failure such as insufficient number of edges in the field of view or insufficient frame contrast, or other conditions, it is sent to the failed workstation. If the alignment evaluation procedure is successful, the respective image shift x and y alignments are sent to the workstation. The workstation calculates a new focus (beam energy) and a value for the aperture alignment deflector for the MEC. One MEC provides each value in a column.

  Therefore, the autofocus and aperture alignment of the apparatus can be adjusted within 2 seconds. Further evaluation of beam distortion does not require any further measurements and can be included in the procedure of FIG. 6 with little time.

1 is a block diagram of a charged particle beam device suitable for implementing various aspects of the present invention. It is the figure which showed the principle of the aperture alignment explaining how a misalignment beam introduce | transduces a lateral movement to the produced | generated image. It is the figure which showed the principle of the aperture alignment explaining how a misalignment beam introduce | transduces a lateral movement to the produced | generated image. It is explanatory drawing of the principle of astigmatism. FIG. 2 is a block diagram of two quadrupole astigmatism correction devices for two directions integrated in an octupole astigmatism correction apparatus. It is explanatory drawing of the principle of 2 lamp | ramp analysis for evaluating the sharpness of an image. It is explanatory drawing of the principle of 2 lamp | ramp analysis for evaluating the sharpness of an image. 4 is a flowchart of a process according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 16 ... Detector, 21 ... Voltage supply source, 32 ... Control part, 102 ... Scanning deflection part, 103 ... Supply source, 105 ... Sample, 110 ... 1st aperture alignment deflection part, 111 ... 2nd aperture alignment deflection part 112 ... Objective lens.

Claims (40)

A method for automatically generating an image for alignment of a charged particle beam column comprising:
a) changing the focus by introducing chromatic aberration by changing the energy of the beam of charged particles;
b) generating an image set while changing the energy of the beam of charged particles;
Including
Defocusing is completed within 1 second ,
A change in the energy of the beam of charged particles is given in the form of a ramp , and an image set is generated within one ramp period ,
The lamp is defined by its initial value, amplitude, and energy resolution of the beam of charged particles ,
At least two lamps are used, and a first lamp of the at least two lamps has an energy value defined by a first initial value and a first amplitude of the first lamp, and of the at least two lamps The second lamp has an energy value defined by a second initial value and a second amplitude of the second lamp. At least the first initial value and the second initial value are different, and the energy of the beam energy of the charged particles is different. The method, wherein the values are duplicated . c1) evaluating the sharpness of the image set;
c2) setting the beam energy such that the beam energy corresponds to the desired focus;
The method of claim 1, further comprising: d1) evaluating the sharpness of the image set in at least two different directions;
d2) providing a correction signal to the astigmatism correction device so that the astigmatism correction device corrects beam distortion;
The method according to claim 1, further comprising:   The method of claim 3, wherein the sharpness of the image set is evaluated in at least four different directions.   The method of claim 4, wherein the at least four directions are rotated 45 ° with respect to each other.   The method according to claim 1, wherein calibration of the astigmatism correction device is performed.   The method of claim 6, wherein a linear calibration function is used for calibration of the astigmatism correction device. e1) measuring the image shift;
e2) correcting the position of the beam of charged particles with respect to the optical axis based on the measured image shift;
The method according to any one of claims 1 to 7, further comprising:   The method according to claim 8, wherein the deflection unit is calibrated.   The method according to claim 9, wherein a linear calibration function is used for deflection unit calibration.   11. A method according to any one of claims 8 to 10, wherein an image set is generated to obtain a pseudo-continuous image shift.   12. A method according to any one of claims 8 to 11, wherein the image shift between two successive images of the image set is less than 10 pixels.   The method according to any one of claims 8 to 12, wherein the image shift is measured using pattern tracking.   14. A method according to any one of claims 8 to 13, wherein image shift is measured using recursive pattern recognition.   15. A method according to any one of claims 8 to 14, wherein correction of the position of the charged particle beam relative to the optical axis is achieved by moving the aperture.   The method according to claim 8, wherein the correction of the position of the charged particle beam with respect to the optical axis is achieved by deflecting the beam of charged particles using the first deflecting unit.   The method of claim 16, wherein a signal is provided to the second deflector to redirect the beam of charged particles.   The method according to claim 17, wherein the angle of the beam with respect to the optical axis before the first deflector is re-established.   The method according to any one of claims 16 to 18, wherein a coil is used in the first and / or the second deflection part.   The method according to any one of claims 16 to 19, wherein an electrostatic module is used in the first deflection part and / or the second deflection part.   21. A method according to any one of the preceding claims, wherein the generated image set consists of at least 20 images.   The method according to claim 1, wherein the defocusing is completed within 0.5 seconds.   23. A method according to any one of claims 1 to 22, wherein the alignment is completed within 2 seconds.   The method according to any of claims 3 to 7, wherein step d2 is performed in a second direction substantially perpendicular to the first direction or at an angle of 45 degrees.   25. A method according to any of claims 8 to 24, wherein step e2 is performed in a second direction substantially perpendicular to the first direction or at an angle of 45 degrees. 26. The method of claim 25 , wherein the ramp is linear. 27. A method according to any one of claims 25 to 26 , wherein the ramp is synchronized with the generation of the image set. 28. The method of claim 27 , wherein the second set of images is generated during the second ramp and the corresponding images of the image set are averaged. 29. The method of claim 28 , wherein the energy resolution of the beam of charged particles is defined by specifying a total number of frames. 30. A method according to any one of claims 28 to 29 , wherein the energy resolution of the beam of charged particles is defined by specifying the amplitude of the lamp. A source (103) for emitting a charged particle beam;
A scanning deflection unit (102) for scanning the sample (105) with a beam of charged particles;
A first aperture alignment deflector (110) for shifting the beam with respect to the optical axis of the charged particle device;
A second aperture alignment deflector (111) for redirecting the beam with respect to the optical axis of the charged particle device;
An objective lens (112) defining the optical axis of the charged particle device;
A voltage source (21) for charging the energy of the beam of charged particles;
A detector (16) for the imaging frame;
With control unit for synchronizing and shooting frequency of the frame of the voltage supply and (32), defocusing is completed within one second, the charged particle beam device. 32. The apparatus according to claim 31 , wherein the first aperture alignment deflection section (110) and the second aperture alignment deflection section (111) are electrically connected to each other. The apparatus according to any one of claims 31 to 32 , wherein each of the first and second aperture alignment deflection units includes a coil set. 34. The apparatus according to any one of claims 31 to 33 , wherein each of the first and second aperture alignment deflection units comprises an electrostatic deflection module. 35. Apparatus according to any one of claims 31 to 34 , further comprising an astigmatism correction device (900). 36. The apparatus of claim 35 , wherein the astigmatism correction device is an octupole. A method for automatically correcting astigmatism of a charged particle beam column,
a) introducing a chromatic aberration to change the focus by changing a potential that affects the energy of the beam of charged particles in the objective lens;
b) scanning the sample with a beam of charged particles while changing the energy of the beam of charged particles to generate an image set;
c) calculating a sharpness score from at least two images of the image set in at least two different directions to quantify beam distortion;
d) calculating a correction signal based on at least two different scores of the calculated scores;
e) providing a correction signal to the astigmatism correction device so that the astigmatism correction device corrects the beam distortion;
Wherein the defocusing is completed within 1 second. A method for automatically aligning a charged particle beam with an optical axis of a charged particle beam column,
a) introducing a chromatic aberration to change the focus by changing a potential that affects the energy of the beam of charged particles in the objective lens;
b) scanning the sample with a beam of charged particles while changing the energy of the beam of charged particles to generate an image set;
c) measuring an image shift between at least two different images of the image set;
d) calculating a correction signal based on the image shift;
e) providing a correction signal to the deflector so that the beam is aligned with the optical axis;
Wherein the defocusing is completed within 1 second. Energy of the beam is varied by modulation of the acceleration voltage, according to claim 1 to 30, 37 or 38 The method according to any one of. Contains any of the features of claims 2 to 30, The method of claim 37 or 38.

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