JP2012068196A - Pattern measurement method and pattern measurement device - Google Patents

Abstract

When measuring a pattern dimension of a mask used as a semiconductor circuit original, a sidewall angle is also measured simultaneously with the pattern dimension.
In a pulse height discriminator 9 included in a mechanism for counting secondary electrons emitted from the surface of a substrate, an SEM image is limited by limiting the counting amount of the number of electrons entering the detector within a predetermined time interval. The correlation between the width of the white band generated on the pattern sidewall and the pattern sidewall angle can be emphasized, and the pattern sidewall angle can be measured nondestructively from the correlation between the pattern sidewall angle and the width of the white band obtained in advance. .
[Selection] Figure 1

Description

  The present invention relates to a method and an apparatus for measuring a dimension and a shape of a pattern at the time of manufacturing a semiconductor device and a photomask used when manufacturing these by a lithography technique. More specifically, the present invention relates to a method and apparatus for measuring the size and shape of a pattern by irradiating an accelerated electron beam onto a pattern on a substrate while measuring the amount of secondary electrons emitted from the surface.

Semiconductor integrated circuits are being miniaturized and highly integrated in order to improve performance and productivity, and also with regard to lithography technology for forming circuit patterns, technological development for forming finer patterns with high accuracy Is underway. Along with this, a technique for measuring the dimension and shape of a pattern is also required with higher accuracy.
The pattern size of a semiconductor device or a photomask used for manufacturing the semiconductor device is less than 100 nm, and it is necessary to stably form a pattern of less than 30 nm as a next-generation device. As a method for measuring the size and shape of a specific position of such a fine pattern, a scanning electron microscope (hereinafter abbreviated as CD-SEM) specially designed for dimension measurement is used.

  CD-SEM emits accelerated primary electrons emitted from an electron source onto a pattern formed on a substrate while scanning two-dimensionally by a deflector, and emits secondary electrons and backscattering emitted from the surface. Detect electrons. The scanned area is divided into vertical and horizontal directions, the divided areas are used as pixels, the secondary electrons detected in each area or the amount of backscattered electrons is used as the luminance of the pixels, and secondary electron images and backscattered electron images (hereinafter, referred to as “secondary electron images”). , Generically referred to as SEM image).

  The contrast of the SEM image is formed by the material of the object to be measured and the unevenness of the surface. The amount of secondary electron emission per primary electron is called secondary electron emission efficiency, and the contrast by the material of the SEM image is obtained by the fact that the secondary electron emission efficiency differs depending on the material. The secondary electron emission efficiency varies depending on the acceleration voltage of the primary electrons.

On the other hand, the contrast formed by the irregularities on the surface can be understood using approximate expressions relating to the penetration depth of primary electrons and the escape depth of secondary electrons. According to Non-Patent Document 1 below, the distribution of primary electrons incident on the solid surface is a radius centered around the depth of X _D from the surface using the maximum incident depth R and the diffusion depth X _D. Can be expressed as R-X _D spheres. FIG. 2 schematically represents this in two dimensions. For example, the maximum incident depth R when the energy of incident electrons is 1500 V can be calculated as 62 nm for silicon and 13 nm for tantalum.

It can be considered that the amount of secondary electrons generated in the inelastic scattering process of primary electrons incident on the solid is proportional to the amount of energy loss of the primary electrons. The energy when the primary electrons of the SEM are incident on the sample surface is 500 to 2000 V, whereas the energy of the secondary electrons to be generated is mostly as low as 0 to 50 eV.
For this reason, it is considered that the depth of secondary electrons that can escape from the surface is considerably smaller than the maximum incident depth of the primary electrons. According to the following non-patent document 2, the escape depth of secondary electrons is 2.7 nm for silicon and 0.7 nm for tantalum. Therefore, the escape depth of the secondary electrons is about 1/20 of the maximum incident depth of the primary electrons.

Therefore, it can be considered that the contrast formed by the unevenness of the sample surface is related to the overlap between the sample surface and the sphere having the radius R−X _D. The larger the area of the sphere outside the sample, that is, the area indicated by the oblique lines in FIG. 2, the higher the secondary electron intensity. FIG. 3 schematically represents this in two dimensions. Compared with a flat portion as shown in FIG. 3A, the end portion of the groove structure as shown in FIG. 3B has a smaller area and a smaller amount of secondary electrons. Conversely, in the slope portion as shown in FIG. 3C and FIG. 3D, the area is large and the amount of secondary electrons generated is also large.

Pattern dimension measurement by CD-SEM uses the phenomenon that the brightness of the SEM image increases at the pattern edge as described above, and the pattern is determined from the interval of the white area (hereinafter referred to as white band) on the SEM image. The dimensions are being measured.
Under conditions where the acceleration voltage is low as used in CD-SEM, the white band width is typically about 10 to 15 nm. Therefore, in order to measure the pattern dimension with high accuracy, it is important which part of the white band is set as the pattern end. For example, a relative value with respect to the peak of the brightness value of the white band is set as a threshold value, or the position of the inflection point of the original profile is obtained from the peak position obtained by differentiating the brightness profile, and is used as the pattern end position. Methods are used.

  It can be seen from the above description that the width of the white band depends on the material forming the pattern, but also changes depending on the side wall angle of the pattern. That is, when the side wall angle of the pattern is close to vertical, the white band width is reduced, and when the side wall angle of the pattern is small, so-called taper shape, the white band width is increased.

K. Kanaya et al., Journal of Physics D, 5, 43 (1972) T. Lin et al., Surface and Interface Analysis, 37, 895 (2005) H. Hakii et al., Proceedings of SPIE, 7379, 737922 (2009) S. Babin et al., Physics Procedia, 1,305 (2008)

  In general, the photomask pattern is manufactured so that the side wall angle is as close to vertical as possible, but some variation occurs depending on the shape of the resist pattern, etching conditions, and the like. In recent years, the pattern dimension has become finer, the numerical aperture of the projection optical system of the exposure apparatus has increased, and the influence of the pattern side wall angle on the transfer dimension has increased. In addition, in extreme ultraviolet light (EUV) masks that are being developed as next-generation lithography, light is incident obliquely on the substrate, and therefore, control of the pattern side wall angle is an important issue.

The most straightforward method for measuring the pattern side wall angle is to observe the exposed pattern cross section with an electron microscope by cutting the substrate or processing it with a focused ion beam. It has been broken.
However, since this method destroys the photomask, it cannot be used as a photomask product inspection method. Non-destructive methods for measuring the pattern cross-sectional shape include a method using an atomic force microscope (hereinafter abbreviated as AFM) and an optical method such as scatterometry.

In the method using AFM, the shape to be measured is greatly influenced by the shape of the probe tip. Further, when the probe is excited perpendicular to the substrate surface, it is difficult to accurately measure the pattern side wall angle close to the vertical. Also, the method of exciting in parallel to the substrate is difficult to apply when the pattern groove is narrow or shallow.
Further, when using an optical technique typified by scatterometry, the complex refractive index of the pattern forming material needs to be accurately known. Although there is a method for obtaining the complex refractive index using spectroscopic ellipsometry or the like, it is a fitting using a model and it is difficult to avoid ambiguity.

  Non-Patent Document 3 discloses a method for measuring the pattern side wall angle of an EUV mask using a CD-SEM for such a problem. This method uses the fact that the change amount of the white band width when the probe current value of the SEM is changed shows a correlation with the pattern side wall angle, and has an advantage that the pattern side wall angle close to the vertical can be measured nondestructively. When the SEM probe current is changed, there is a problem that a certain amount of time is required for the measurement apparatus to be in a stable steady state.

  The pattern measurement method of the present invention uses an apparatus that has an electron beam source, a beam blanking mechanism, and a beam deflection mechanism, and can irradiate a specified amount of electrons to a specified position on the substrate to be measured. As a mechanism for detecting secondary electrons emitted from the surface, a scintillator that converts secondary electrons into light energy, a photomultiplier that multiplies the light energy into an electrical signal, and a secondary that has been multiplied into the electrical signal. It has a wave height discriminator for counting the number of pulses of an electronic signal, and the wave height discriminator is characterized in that the number of electrons entering the detector within a certain time interval is limited and processed.

  The pattern measurement apparatus of the present invention has an electron beam source, a beam blanking mechanism, and a beam deflection mechanism, and can irradiate a prescribed amount of electrons to a prescribed position on the substrate to be measured from the substrate surface. As a mechanism for detecting the emitted secondary electrons, a scintillator that converts the secondary electrons into light energy, a photomultiplier that multiplies the light energy into an electrical signal, and a secondary electron signal that has been multiplied into the electrical signal. The pulse height discriminator has a function of limiting the count amount of the number of electrons entering the detector within a predetermined time interval.

  According to the pattern measurement method and the pattern measurement apparatus of the present invention, it is possible to emphasize the correlation between the width of the white band generated on the pattern side wall of the CD-SEM image and the pattern side wall angle. The pattern side wall angle can be measured nondestructively from the correlation of the widths of the patterns, and the pattern can be measured stably without greatly changing the conditions of the electron source.

It is a block diagram of the pattern measuring device which is one Embodiment of this invention. It is explanatory drawing which represented scattering of the electron in a solid typically. It is explanatory drawing showing the relationship between pattern cross-sectional shape and secondary electron intensity.

Hereinafter, an embodiment of a pattern measuring method and a pattern measuring apparatus according to the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram of a pattern measuring apparatus according to the present embodiment. Electrons emitted from the electron beam source 1 are accelerated to a predetermined energy by the acceleration electrode. In the case of CD-SEM, the energy of this electron is typically 500 eV to 2000 eV. This is because the secondary electron yield is larger than 1 in this energy range, and the influence of charging of the sample is reduced.

  The electrons emitted from the electron beam source enter between the blanking electrodes of the blanking mechanism (beam blanking mechanism) 2, and the electrons do not reach the sample when a voltage is applied between the electrodes. Electrons can be guided to the sample surface by changing the electrode voltage for a predetermined time. The timing for changing the electrode voltage is controlled by a pulse generator 3 that receives a signal from a control / measurement computer (PC in the figure) 4.

The electrons that have passed through the blanking electrode enter between the deflection electrodes of the XY-deflector (beam deflection mechanism) 5, and the incident position of the electrons on the surface of the sample 6 can be changed by this electrode voltage. This electrode voltage is controlled by a deflection circuit of the XY-deflector 5 that receives a signal from the control / measurement computer 4.
The electrons incident on the surface of the sample 6 in this manner repeatedly scatter within the sample 6, and some of the electrons jump out of the sample 6 as backscattered electrons, and gradually by inelastic scattering accompanied by generation of secondary electrons. In some cases, the energy is lost and remains in the sample 6 or flows through the sample 6 and flows out of the sample 6 through the sample holder. Such a scattering process is a very fast phenomenon in femtosecond units.

  Some of the secondary electrons jump out of the sample 6 and enter the detector. When the secondary electrons reach the scintillator 7, the energy of the electrons is converted into fluorescent light energy. Although the fluorescence lifetime varies greatly depending on the type of scintillator 7, a plastic scintillator with a short fluorescence lifetime is suitable for measuring such a high-speed phenomenon. In that case, the fluorescence lifetime is about 2 to 3 nanoseconds.

  The light emitted from the scintillator 7 is weak, but can be changed to a sufficiently large electric signal using the photomultiplier tube 8. As the photomultiplier tube 8, various types such as a dynode type and a microchannel plate type can be selected. However, it is preferable to use a photomultiplier tube having a high-speed response with a short pulse width of the obtained electric signal. As the high-speed responsive photomultiplier tube 8, for example, one having a pulse width of about 3 nanoseconds can be used.

  The signal from the photomultiplier tube 8 enters a wave height discriminator (discriminator) 9. The main function of the wave height discriminator 9 is to remove the dark current and weak noise of the photomultiplier tube 8 and strong noise caused by cosmic rays, etc., and to count the number of pulses of the secondary electron signal emitted from the sample 6. is there. The count of the wave height discriminator 9 is analyzed by the multichannel analyzer 10, and the analysis result is output from the control / measurement computer 4.

  As the wave height discriminator 9, there is one having a function of blocking the next pulse within a set fixed time after a pulse is input. In this case, a plurality of pulses that have entered within a set fixed time can be regarded as one signal. Even when a wave height discriminator without such a function is used, an equivalent function can be realized by adding a circuit capable of setting a time constant in front.

  When the probe current of the SEM is 5 pA, the electron incidence interval is 32 nanoseconds on average, which is one digit longer than the time characteristic of the device. Therefore, the device can count as a value for each incident primary electron. However, when a plurality of secondary electrons generated from one primary electron reach the detector, the time difference is considered to be in picosecond units at the maximum, and the peak discriminator 9 does not recognize it as an independent peak.

As a result, the number of secondary electrons counted by the detector is counted down to be smaller than the actual detection amount. Such a phenomenon becomes prominent when a large amount of secondary electrons on the pattern sidewall are emitted. In addition, when the probe current is increased, the primary electron incidence interval is shortened, and this count-down becomes more remarkable.
The present inventor has found that such a counting-down phenomenon can be effectively used as a means for estimating the side wall angle of the pattern from the white band width. The principle will be described below.

  In general, an SEM image used for pattern measurement is recorded as 8 bits per pixel, that is, 256 gray scales. Therefore, the contrast of the image is adjusted so that the count value of the secondary electrons falls within this gray scale range. Thereafter, an averaging process is performed on a plurality of pixels in the vertical direction with respect to the pattern, and a luminance profile is generated in a direction crossing the pattern edge.

In the luminance profile, a large amount of secondary electrons are emitted from the pattern edge, so that the luminance value becomes high. When the pattern end is not vertical but has a tapered shape, the amount of secondary electrons emitted from the pattern side wall is large, so that the width of the white band becomes large.
If the change amount of the white bandwidth can be measured with high accuracy, the side wall angle of the pattern can be measured. However, not only the width of the white band but also the peak value increases as the side wall angle changes. Such a phenomenon can be confirmed by simulation, and is also disclosed in Non-Patent Document 4, for example.

For this reason, the brightness value of the white band is reduced as a whole when adjusting the contrast to the gray scale, and the increase in the white band width is canceled.
However, when the count-down occurs, the higher the luminance value, the higher the probability that the count-down occurs, so that the change in the peak value accompanying the change in the side wall angle is suppressed. Therefore, it is possible to retain a change in white band width to some extent even after contrast adjustment.

The count-off probability can be adjusted by the block time of the wave height discriminator 9 described in the above device configuration or the setting of the time constant setting circuit provided therewith. Therefore, a correlation occurs in the white band width when the block time or the time constant is changed.
This pattern is measured for patterns with different side wall angles, and a table can be created by comparing with the values obtained by directly measuring the side wall angle with an electron microscope after sample breakage or FIB processing. The angle can be measured.

In addition, when cross-sectional observation is performed to create this table, if the dimensions of the upper and lower portions of the tapered pattern sidewall are measured, the upper and lower dimensions of the pattern edge can be accurately measured together with the pattern sidewall angle. It can be measured.
Thus, according to the present invention, it is possible to accurately measure the side wall angle and the pattern dimension of the pattern with high accuracy without destruction.

  Embodiments of the present invention will be further described using examples.

Forty pairs of 4.2 nm thick silicon films and 2.8 nm molybdenum films are alternately stacked on a quartz glass substrate, and 4 nm thick silicon films, 4 nm thick zirconium silicate films, and tantalum are mainly formed thereon. An EUV mask blank having an absorption layer having a thickness of 70 nm as a component and an antireflection layer having a thickness of 20 nm mainly composed of tantalum oxide was prepared.
The zirconium silicate film was formed by reactive binary sputtering using a target mainly composed of silicon, a target mainly composed of zirconium, and a mixed gas of argon and oxygen.

A resist was coated on the antireflection layer, exposed using an electron beam exposure apparatus, and developed after baking to form a resist pattern. The resist remaining on the surface after etching the antireflection layer and the absorption layer by dry etching was removed to obtain an EUV mask.
When the EUV mask pattern was measured with the apparatus of this example, the white band width tended to increase as the time constant of the measurement circuit was increased from 20 nanoseconds to 200 nanoseconds.

Further, an EUV mask in which a pattern in which the side wall angle was changed by changing the pattern forming conditions of the EUV mask was prepared by the same process as described above.
For these EUV masks, the difference in white band width was measured when the time constant was 20 nanoseconds and 200 nanoseconds.
Thereafter, the EUV mask pattern was subjected to FIB processing, and the side wall angle was measured from the cross-sectional direction using an electron microscope. As a result, it was confirmed that the sidewall angle of these masks was in the range of 75 to 90 degrees.

  When the relationship between the difference in white band width and the side wall angle of the mask pattern observed with an electron microscope was examined, the change in white band width when the time constant was changed became smaller as the side wall angle approached 90 degrees. It was confirmed that there was a correlation.

  By using the pattern measurement method and the pattern measurement apparatus of the present invention, it is possible to perform accurate dimension measurement in consideration of the influence of the pattern side wall angle, and it is possible to manufacture a semiconductor or the like with high accuracy.

DESCRIPTION OF SYMBOLS 1 ... Electron beam source 2 ... Blanking mechanism 3 ... Pulse generator 4 ... Control / measurement computer 5 ... XY-deflector 6 ... Sample 7 ... Scintillator 8 ... Photomultiplier tube 9 ... Wave height discriminator 10 ... Multi-channel analyzer

Claims (2)

  Secondary electrons emitted from the substrate surface using an apparatus that has an electron beam source, a beam blanking mechanism, and a beam deflection mechanism and can irradiate a specified amount of electrons to a specified position on the substrate to be measured Is a scintillator that converts secondary electrons into light energy, a photomultiplier tube that multiplies the light energy into an electric signal, and counts the number of pulses of the secondary electron signal multiplied into the electric signal. A pattern measuring method comprising: a wave height discriminator, wherein the wave height discriminator performs processing while limiting a count amount of the number of electrons entering the detector within a predetermined time interval.   A mechanism that has an electron beam source, a beam blanking mechanism, and a beam deflection mechanism, and can irradiate a specified amount of electrons to a specified position on the substrate to be measured and detect secondary electrons emitted from the substrate surface A scintillator that converts secondary electrons into light energy, a photomultiplier that multiplies the light energy into an electrical signal, and a pulse height discriminator that counts the number of pulses of the secondary electron signal multiplied into the electrical signal. And the wave height discriminator has a function of limiting a counting amount of the number of electrons entering the detector within a predetermined time interval.

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