JP4401200B2 - Measuring method using a scanning electron microscope - Google Patents

Description

  The present invention relates to a length measurement method using a scanning electron microscope suitable for length measurement and inspection of a semiconductor device.

Recent scanning electron microscopes (SEMs), particularly length measurement SEMs, have a true length measurement value, and the length measurement value does not vary or offset depending on the sample configuration and film type. It is needed. Conventionally, as a dimension correction means for a length measurement SEM, a pitch pattern is created using a material (for example, Al, W, etc.) with less charge on Si, and the length measurement value is adjusted to the pitch pattern. The length measurement correction value of the length measurement SEM was obtained.

  Further, in recent measurement SEMs, the acceleration voltage of incident electrons on the primary side is increased to improve the resolution of secondary electrons, and a negative voltage is applied to the sample side to decelerate the acceleration voltage of incident electrons. An electric field method (hereinafter referred to as a retarding method) is employed. Specifically, compared to the conventional method (sample-side voltage 0), the retarding method increases the acceleration voltage of electrons emitted from the electron gun section by “+ Vr” and applies an acceleration voltage of “−Vr” to the sample. As a result, the electrons emitted from the electron gun section with the acceleration voltage of “V + Vr” are decelerated by the voltage of “−Vr” applied to the sample side after passing through the objective lens, and finally the acceleration voltage “V” is configured to be incident on the sample. The effects of retarding include that the chromatic aberration of the lens can be reduced and that the collection efficiency of secondary electrons is improved. The former effect is that the incident electrons on the primary side pass under the objective lens at the acceleration voltage “V + Vr”, and the latter effect is detected by the electric field of “−Vr” in the secondary electrons generated from the sample. This is because it is accelerated in the direction of the vessel.

  However, with respect to a scanning electron microscope that employs the retarding method as described above, the acceleration voltage of the electron beam incident on the sample (hereinafter also referred to as Landing Voltage) varies depending on the voltage applied to the sample side. When the sample to be observed is covered with an insulating film, or when an insulating film is attached to the surface, the landing voltage changes and the magnification (Field of view) shifts. Since the dimension correction of the length measurement SEM is performed under the condition of constant Landing Voltage and constant magnification, there arises a problem that if one of the values changes, the measured dimension value changes. That is, when the Landing Voltage changes, a dimensional error occurs, causing a problem that the measured dimension is not a true value.

  Further, according to the invention described in Patent Document 1 (Japanese Patent Laid-Open No. 2000-337846), although the above problem can be solved, other methods are also demanded.

JP 2000-337846 A

  An object of this invention is to provide the length measuring method using the scanning electron microscope which can perform a measurement with high precision.

  As a result of intensive studies to solve the above problems, the present inventor has come up with various aspects of the invention described below.

In the measurement method using the engagement Ru run査型electron microscope to the present invention, the electron when the electron at a predetermined incident voltage is incident on the sample in the case of changing the intensity of the electric field of the electron incident side of the electron detector A reference electric field intensity at which the amount of electrons emitted from the sample detected by the detector exhibits a peak is obtained in advance. Then, while changing the intensity of the electric field, the sample on which the pattern to be measured is irradiated with an electron beam, and the amount of electrons emitted from the sample is detected by the electron detector. Next, an error in incident voltage is obtained based on the difference between the electric field intensity at which the amount of electrons shows a peak and the reference electric field intensity, and a potential corresponding to the error in the incident voltage is superimposed and applied. Determine the gun potential and sample stage potential. Next, the sample is measured under the potential of the electron gun and the potential of the sample stage.

  In the present invention, it is possible to obtain a true dimension value by compensating for a length measurement error caused by a change in potential on the surface of the sample.

  In a general length measurement SEM that does not use a retarding method, the length measurement error due to charge-up of the sample surface due to incident electron irradiation is sufficiently corrected to obtain a true value regardless of the sample configuration and type. Can be obtained. In other words, when the sample is charged up, the surface potential of the sample changes, and as a result, a magnification shift occurs due to an error in the landing voltage of the incident beam, and an offset value is entered in the measurement value. This offset value is compensated. This solves this problem. As a result, a true value can always be obtained even if the surface potential of the sample changes due to electron beam irradiation.

  Further, in the length measuring SEM employing the retarding method, the problem of magnification shift due to the setting error of the Landing Voltage, which has been a problem until now, is solved. That is, conventionally, there may be a dimensional error depending on the sample to be measured, such as when an insulating film is attached to the back surface of the sample or when the interlayer structure of the portion irradiated with the electron beam is changed. However, this problem is solved and a true value can always be obtained.

  According to the present invention, a true dimension value can be obtained by correcting an error in length measurement caused by a change in potential of the surface of the sample. That is, highly accurate measurement can be performed regardless of the configuration and type of the sample.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings.

  First, the SEM used in the embodiment of the present invention will be described. FIG. 1 is a schematic diagram showing the configuration of the SEM.

  The SEM used in this embodiment is the same as that generally used. Specifically, a stage (sample stage) 2 on which a wafer 1 as a sample is placed is provided, and an objective lens 3, an aperture 5 and a condenser lens 6 are arranged in that order from directly above. A gun (electron gun) 7 that emits electrons toward the sample on the stage 2 is disposed above the condenser lens 6. Further, a detector (electron detector, scintillator) 4 is disposed above the stage 2 at a position away from the path of electrons emitted from the gun 7. The SEM control unit is provided with a storage device for storing a database (not shown).

  In the SEM configured as described above, generally, the potential of the gun 7 is set to 3000V, and the potential of the stage 2 is set to -2400V. For this reason, the incident voltage of electrons to the wafer 1 is 600V.

(First embodiment)
Next, a first embodiment of the present invention will be described. In the first embodiment, the length of a part of the pattern formed on the wafer 1 is measured using the SEM configured as described above.

  In the first embodiment, a database used for SEM correction is created in advance, and SEM correction is performed while referring to the database for length measurement. FIG. 2 is a flowchart illustrating a database creation method according to the first embodiment, and FIG. 3 is a flowchart illustrating a length measurement method according to the first embodiment of the present invention.

  In creating the database, first, a reference wafer on which a line-and-space pattern or the like having a constant pitch is formed is loaded onto the SEM (step DS1), and then moved to a measurement point (step DS2).

  Next, the pitch of the pattern formed on the reference wafer is measured for each of a plurality of types of incident voltages, for example, 400 V, 500 V, 600 V, 700 V, and 800 V (step DS3).

  Then, a magnification correction coefficient is calculated for each incident voltage based on the measurement result of the pitch, and a relationship between the incident voltage and the magnification correction coefficient for each incident voltage is obtained (step DS4). This relationship is, for example, as shown in FIG.

  Then, the data thus obtained is transferred to the SEM database (step DS5).

  Next, the amount of electrons emitted from the wafer 1 and flying is detected while changing the intensity of the electric field on the electron incident side of the detector (scintillator) 4 while fixing the incident voltage (step DS6). As a result, the relationship between the electric field and the amount (intensity) of electrons as shown in FIG. 5 is obtained. Next, the electric field intensity (reference electric field intensity, peak point) at which the secondary electron peak appears is specified from the curve as shown in FIG. 5 (step DS7).

  Such a relationship is obtained for each of various incident voltages, for example, 400V, 500V, 600V, 700V, and 800V.

  Then, the data thus obtained, for example, the peak point for each incident voltage is transferred to the SEM database (step DS8).

  Next, a length measurement method using the database created as described above will be described.

  First, a wafer on which a pattern to be measured is formed is loaded onto the SEM (step S1), and global alignment is performed (step S2). Next, the visual field is adjusted, and the measurement location is moved into the visual field (step S3).

  Next, electrons flying from the wafer 1 are detected while changing the electric field on the electron incident side of the detector 4 while the incident voltage is fixed at 600 V, for example (step S4).

  Subsequently, it is determined whether or not the secondary electron peak position (electric field) stored in the database matches the secondary electron peak position obtained in step S4 (step S5). At this time, it is not necessary to consider the height of the peak.

  If it is determined in step S5 that the peak positions coincide with each other, the magnification correction coefficient value used in the subsequent length measurement is obtained from the relationship between the incident voltage and the magnification correction coefficient stored in the database. The value of the correction coefficient is determined (step S6).

  On the other hand, if it is determined in step S5 that the peak positions do not match each other as shown by the dotted line in FIG. 6, the amount of shift (movement amount) of the peak position is calculated using the database (step S7). .

  Then, the error of the incident voltage corresponding to the amount of peak movement is obtained with reference to the database (step S8). At this time, since the peak point for each incident voltage is stored in the database, the error of the incident voltage can be obtained based on this.

  Next, the magnification correction coefficient based on the error of the incident voltage is obtained based on the data stored in the database as shown in FIG. 4, that is, the magnification correction coefficient corresponding to the true incident voltage is obtained, and this value is used for the subsequent length measurement. Is determined as the value of the magnification correction coefficient used in (Step S9).

  Thereafter, the lengths of all measurement target portions are measured using the magnification correction coefficient determined in step S6 or S9 (step S10).

  Subsequently, the wafer 1 to be measured is unloaded from the SEM (step S11).

  According to the first embodiment, even when the incident voltage fluctuates due to an insulating film formed on the back surface of the wafer, the magnification correction coefficient corresponding to the actual incident voltage is set. Can be used. Therefore, correct length measurement can always be performed regardless of the state of the wafer.

  Note that when creating the database, instead of changing the intensity of the electric field on the electron incident side of the detector (scintillator) 2, the potential of the electron gun (gun 7) may be changed. In this case, instead of the graph shown in FIG. 5, a graph is obtained in which the horizontal axis represents the potential or incident voltage of the electron gun and the vertical axis represents the electron intensity. However, even in this case, the shape of the graph is similar to that shown in FIG. 5, and a peak of secondary electrons (reference electron gun potential) and a peak of reflected electrons appear. Even when the length measurement is performed, the electric field potential may be changed instead of changing the electric field strength.

  Similarly, instead of changing the strength of the electric field in creating and measuring the database, the potential of the stage 2 may be changed to use the secondary electron peak (reference sample stage potential).

(Second Embodiment)
Next, a second embodiment of the present invention will be described. Also in the second embodiment, the length of a part of the pattern formed on the wafer 1 is measured using the SEM configured as described above. However, the database creation method is different from that of the first embodiment, and accordingly, the length measurement method is slightly different from that of the first embodiment. FIG. 7 is a flowchart showing a database creation method according to the second embodiment, and FIG. 8 is a flowchart showing a length measurement method according to the second embodiment of the present invention.

  In creating the database, first, as in the first embodiment, the reference wafer is loaded into the SEM (step DS1) and then moved to the measurement point (step DS2).

  Next, without determining the relationship between the incident voltage and the magnification correction coefficient, the relationship between the electric field on the electron incident side of the detector 4 and the amount (intensity) of electrons is determined for each incident voltage (steps DS6 to DS8).

  That is, in the second embodiment, the relationship between the incident voltage and the magnification correction coefficient is not necessary.

  Next, a length measurement method using the database created as described above will be described.

  First, in the same manner as in the first embodiment, the process from loading of a measurement target wafer to determination of peak movement is performed (steps S1 to S5).

  If it is determined in step S5 that the peak positions coincide with each other, the value of the incident voltage used in the subsequent length measurement is determined as it is (step S12).

  On the other hand, when the peak positions do not coincide with each other as shown by the dotted line in FIG. 6 in the determination in step S5, the amount of displacement (movement amount) of the peak position is calculated as in the first embodiment. Then, the error of the incident voltage based on the movement amount is calculated (steps S7 to S8).

  Next, the potential corresponding to the error of the incident voltage is superimposed on the stage 2, and the potential of the stage 2 after the superposition is determined as a potential used in the subsequent length measurement (step S13). As a result, the incident voltage is also changed.

  Thereafter, the lengths of all measurement target portions are measured using the incident voltage determined in step S12 or S13 (step S10).

  Subsequently, the wafer 1 to be measured is unloaded from the SEM (step S11).

  According to the second embodiment, even when the incident voltage fluctuates due to an insulating film formed on the back surface of the wafer, the incident voltage can be corrected to a desired value. it can. Therefore, correct length measurement can always be performed regardless of the state of the wafer.

(Third embodiment)
Next, a third embodiment of the present invention will be described. FIG. 9 is a flowchart showing a length measuring method according to the third embodiment of the present invention.

  In the second embodiment, after calculating the error of the incident voltage (step S8), the corresponding potential is applied to the member on the wafer side, that is, the stage 2 in a superimposed manner. In the third embodiment, however, Then, the corresponding potential is superimposed on the electron gun (gun 7) (step S14).

  Also in the third embodiment, the incident voltage can be appropriately corrected as in the second embodiment, and the same effect as in the second embodiment can be obtained.

  In the first to third embodiments, the database is created using the reference wafer and the magnification correction coefficient or the incident voltage is corrected for each measurement target wafer. However, the database is created using the reference chip. The correction may be performed for each measurement target chip. Further, correction may be performed for each pattern. The correction for each pattern is particularly suitable when an isolated pattern and a dense pattern exist in the chip.

  Note that a pattern used when creating the database, for example, a pattern with a constant pitch, may be formed on the wafer or may be formed on the stage.

  Further, when length measurement is performed with the sample charged up, the magnification for creating the database, that is, the magnification for obtaining the reference electrolysis intensity, the reference sample stage potential, or the reference electron gun potential is 20,000 to It is preferable to set the magnification to 30,000 times, and the magnification when actually measuring the length is higher than the magnification at the time of creating the database.

  On the other hand, when length measurement is completed before the sample is charged up, the magnification at the time of creating the database may be set to a high magnification, and when actually measuring the length, beam blanking may be sandwiched between multiple frames. preferable.

  Hereinafter, various aspects of the present invention will be collectively described as supplementary notes.

(Supplementary note 1) When the intensity of the electric field on the electron incident side of the electron detector is changed, the electrons emitted from the sample detected by the electron detector when the electrons are incident on the sample at a predetermined incident voltage A reference electric field strength at which the amount shows a peak is obtained in advance,
Irradiating an electron beam to a sample on which a pattern to be measured is formed while changing the intensity of the electric field, and detecting the amount of electrons emitted from the sample by the electron detector;
Determining a magnification correction coefficient based on the difference between the electric field intensity at which the amount of electrons exhibits a peak and the reference electric field intensity;
Measuring the sample and correcting the measurement result using the magnification correction coefficient; and
A length measuring method using a scanning electron microscope.

(Supplementary Note 2) Reference electron gun in which the amount of electrons emitted from the sample detected by the electron detector when the electron enters the sample at a predetermined incident voltage when the potential of the electron gun is changed shows a peak Obtain the potential in advance,
Irradiating an electron beam onto a sample on which a pattern to be measured is formed while changing the potential of the electron gun, and detecting the amount of electrons emitted from the sample by the electron detector;
Determining a magnification correction coefficient based on the difference between the electron gun potential and the reference electron gun potential at which the amount of electrons peaks.
Measuring the sample and correcting the measurement result using the magnification correction coefficient; and
A length measuring method using a scanning electron microscope.

(Supplementary Note 3) Reference sample stand-by voltage in which the amount of electrons emitted from the sample detected by the electron detector when the electron enters the sample at a predetermined incident voltage when the potential of the sample stand is changed shows a peak Find the place in advance,
Irradiating an electron beam to a sample on which a pattern to be measured is formed while changing the potential of the sample stage, and detecting the amount of electrons emitted from the sample by the electron detector;
Determining a magnification correction coefficient based on the difference between the potential of the sample stage and the reference sample stage potential at which the amount of electrons peaks.
Measuring the sample and correcting the measurement result using the magnification correction coefficient; and
A length measuring method using a scanning electron microscope.

(Supplementary Note 4) When the intensity of the electric field on the electron incident side of the electron detector is changed, the electrons emitted from the sample detected by the electron detector when the electrons enter the sample at a predetermined incident voltage A reference electric field strength at which the amount shows a peak is obtained in advance,
Irradiating an electron beam to a sample on which a pattern to be measured is formed while changing the intensity of the electric field, and detecting the amount of electrons emitted from the sample by the electron detector;
Determining the potential of the electron gun and the potential of the sample stage based on the difference between the intensity of the electric field at which the amount of the electrons shows a peak and the reference electric field intensity;
Measuring the sample under the potential of the electron gun and the potential of the sample stage;
A length measuring method using a scanning electron microscope.

  (Supplementary Note 5) The length measurement using the scanning electron microscope according to Supplementary Note 4, wherein the step of determining the potential of the electron gun and the potential of the sample stage includes a step of changing the potential of the sample stage. Method.

  (Supplementary Note 6) The step of determining the potential of the electron gun and the potential of the sample stage has a step of changing the potential of the electron gun, and the scanning electron microscope according to Supplementary Note 4 or 5 is used. Measuring method.

  (Additional remark 7) The magnification at the time of calculating | requiring the said reference electric field strength shall be 20,000 times-30,000 times, and the magnification at the time of measuring the said sample is made higher than the magnification at the time of calculating | requiring the said reference electric field strength. A length measuring method using the scanning electron microscope according to appendix 1, 4, 5 or 6.

  (Additional remark 8) The magnification at the time of calculating | requiring the said reference sample stand electric potential shall be 20,000 times-30,000 times, and the magnification at the time of measuring the said sample should be made higher than the magnification at the time of calculating | requiring the said reference sample stand electric potential. A length measuring method using the scanning electron microscope according to Supplementary Note 2, wherein

  (Additional remark 9) The magnification at the time of calculating | requiring the said reference electron gun electric potential shall be 20,000 times-30,000 times, and the magnification at the time of measuring the said sample is made higher than the magnification at the time of calculating | requiring the said reference electron gun electric potential. A length measuring method using the scanning electron microscope according to appendix 3, wherein

  (Appendix 10) The length measurement method using the scanning electron microscope according to any one of appendices 1 to 6, wherein when measuring the length of the sample, beam blanking is sandwiched for each of a plurality of frames. .

(Supplementary Note 11) When the intensity of the electric field on the electron incident side of the electron detector is changed, the electrons emitted from the sample detected by the electron detector when electrons enter the sample at a predetermined incident voltage A reference electric field strength showing a peak in the amount of
A procedure for irradiating an electron beam to a sample on which a pattern to be measured is formed while changing the intensity of the electric field, and detecting the amount of electrons emitted from the sample by the electron detector;
A procedure for determining a magnification correction coefficient based on the difference between the electric field intensity at which the amount of electrons exhibits a peak and the reference electric field intensity;
A program for length measurement using a scanning electron microscope, characterized in that a computer is executed.

(Supplementary Note 12) A reference electron gun in which the amount of electrons emitted from the sample detected by the electron detector when the electron enters the sample at a predetermined incident voltage when the potential of the electron gun is changed shows a peak Obtain the potential in advance,
A procedure for irradiating an electron beam to a sample on which a pattern to be measured is formed while changing the potential of the electron gun, and detecting the amount of electrons emitted from the sample by the electron detector,
A procedure for determining a magnification correction coefficient based on the difference between the electron gun potential and the reference electron gun potential at which the amount of electrons exhibits a peak;
Measuring the sample, and correcting the length measurement result using the magnification correction coefficient;
A program for length measurement using a scanning electron microscope, characterized in that a computer is executed.

(Supplementary note 13) Reference sample stand-by voltage in which the amount of electrons emitted from the sample detected by the electron detector when the electron enters the sample at a predetermined incident voltage when the potential of the sample stand is changed shows a peak Find the place in advance,
A procedure for irradiating an electron beam to a sample on which a pattern to be measured is formed while changing the potential of the sample stage, and detecting the amount of electrons emitted from the sample by the electron detector;
A procedure for determining a magnification correction coefficient based on the difference between the potential of the sample stage and the reference sample stage potential at which the amount of electrons peaks.
Measuring the sample, and correcting the length measurement result using the magnification correction coefficient;
A program for length measurement using a scanning electron microscope, characterized in that a computer is executed.

(Additional remark 14) When the intensity | strength of the electric field of the electron incident side of an electron detector is changed, when an electron injects into a sample with predetermined | prescribed incident voltage, of the electron discharge | released from the said sample detected by the said electron detector A reference electric field strength at which the amount shows a peak is obtained in advance,
A procedure for irradiating an electron beam to a sample on which a pattern to be measured is formed while changing the intensity of the electric field, and detecting the amount of electrons emitted from the sample by the electron detector;
A procedure for determining the potential of the electron gun and the potential of the sample stage, based on the difference between the intensity of the electric field at which the amount of electrons shows a peak and the reference electric field intensity,
A procedure for measuring the sample under the potential of the electron gun and the potential of the sample stage,
A program for length measurement using a scanning electron microscope, characterized in that a computer is executed.

It is a schematic diagram which shows the structure of a scanning electron microscope (SEM). It is a flowchart which shows the creation method of the database in the 1st Embodiment of this invention. It is a flowchart which shows the length measuring method which concerns on the 1st Embodiment of this invention. It is a graph which shows the relationship between an incident voltage and a magnification correction coefficient. It is a graph which shows the relationship between the intensity | strength of an electric field, and the quantity of electrons (the intensity | strength of the signal detected with the detector). It is a graph which shows the fluctuation | variation of the peak of a secondary electron. It is a flowchart which shows the creation method of the database in the 2nd Embodiment of this invention. It is a flowchart which shows the length measuring method which concerns on the 2nd Embodiment of this invention. It is a flowchart which shows the length measuring method which concerns on the 3rd Embodiment of this invention.

Explanation of symbols

1: Wafer 2: Stage 3: Objective lens 4: Detector (scintillator)
5: Aperture 6: Condenser lens 7: Gun

Claims (2)

When the electric field strength on the electron incident side of the electron detector is changed, the amount of electrons emitted from the sample detected by the electron detector when the electrons enter the sample at a predetermined incident voltage has a peak. The reference electric field strength to be shown is obtained in advance,
Irradiating an electron beam to a sample on which a pattern to be measured is formed while changing the intensity of the electric field, and detecting the amount of electrons emitted from the sample by the electron detector;
Based on the difference between the electric field intensity at which the amount of electrons shows a peak and the reference electric field intensity, an error in the incident voltage is obtained, and an electric potential corresponding to the error in the incident voltage is superimposed and applied to the electric gun potential. And determining the potential of the sample stage,
Measuring the sample under the potential of the electron gun and the potential of the sample stage;
A length measuring method using a scanning electron microscope. The electron gun potential and the step of determining the sample stage potential measurement method using a scanning electron microscope according to claim 1, characterized in that it comprises a step of changing the potential of the electron gun.

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