JP3101114B2 - Scanning electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope which is optimally used in a semiconductor manufacturing process, for example, for inspecting a pattern formed on a silicon wafer.

[0002]

2. Description of the Related Art In a semiconductor manufacturing process, a first pattern (base pattern) is formed on a substrate such as a silicon wafer, and a resist pattern is formed on the base pattern. It is important to form a resist pattern accurately on this underlayer pattern in semiconductor manufacturing. Usually, in a manufacturing process, the overlapping accuracy of the two patterns is inspected.

In the inspection of the overlapping accuracy, since the resist is almost transparent to light, the edges of the resist and the base pattern are determined from the video signal from the TV camera attached to the optical microscope and the signal from the scanning laser microscope. It is widely performed to detect and obtain a shift amount between a base and a pattern thereon.

[0004]

In recent years, patterns formed in the semiconductor manufacturing process have been miniaturized without limit. As a result, in the inspection of an optical pattern, the edge of the pattern is sharply sharpened due to its wavelength limit. It is becoming difficult to detect. From such a background, it is conceivable to use a scanning electron microscope capable of obtaining high resolution for this inspection.

However, a resist having a thickness of about 1 μm is an opaque substance with respect to an electron beam having an energy of several kV or less. The reflected electron image must be observed by irradiating an electron beam having energy. On the other hand, when an electron beam having such a high energy is used, it is difficult to obtain a secondary electron image of the surface with high resolution because the resist is charged up.
Therefore, observing such a sample with a scanning electron microscope is expected to cause the following problems.

First, when observing the resist pattern on the surface, a low acceleration voltage of about 1 kV must be set, and when observing the underlying pattern, it must be switched to a high acceleration voltage of about 20 kV.

When the acceleration voltage is switched, the acceleration voltage,
Since it takes some time for the lens current and the like to stabilize within a predetermined allowable range, the image of the upper resist pattern and the image of the base pattern must be once taken into a storage device and then combined as one image. That is, it is not possible to superimpose and display the upper and lower layer pattern images on one image display device in real time while alternately switching the acceleration voltage at high speed.

[0008] Accelerating voltage (energy of electron beam)
Is switched, the degree of influence of the deflection field generated by the asymmetry of the electron lens or the geomagnetic field on the electron beam changes, causing a field of view shift.

Due to the problems described above, an inspection apparatus for a pattern formed in two layers by a scanning electron microscope or an apparatus using the technique has not yet been realized. The present invention
The present invention has been made in view of such a point, and an object thereof is to realize a scanning electron microscope capable of performing an inspection of a superposition accuracy of a pattern formed in two layers with high resolution and accuracy.

[0010]

The scanning electron microscope according to the present invention comprises a first electron beam irradiation system for generating a relatively high energy electron beam and a second electron beam irradiation system for generating a relatively low energy electron beam. Electron beam irradiation system,
A deflection system for advancing the electron beam from the electron beam irradiation system and the electron beam from the second electron beam irradiation system along substantially the same optical axis, and the first electron beam irradiation passing through the deflection system Lens means for focusing both the electron beam from the system and the electron beam from the second electron beam irradiation system on the sample with the same lens intensity, a reflected electron detector for detecting reflected electrons from the sample, 2 from the sample
A secondary electron detector for detecting secondary electrons, a means for scanning an electron beam from the first electron beam irradiation system and an electron beam from the second electron beam irradiation system in a specific visual field on the sample, The image based on the reflected electron signal detected based on the scanning of the electron beam from the electron beam irradiation system on the sample, and the image detected based on the scanning on the sample of the electron beam from the second electron beam irradiation system Display means for substantially superimposing and displaying an image based on the secondary electron signal.

[0011]

A scanning electron microscope according to the present invention irradiates a sample with an electron beam having a relatively high energy and an electron beam having a relatively low energy, and detects the electron beam based on scanning of the electron beam having a relatively high energy on the sample. The image based on the reflected electron signal and the image based on the secondary electron signal detected based on the scanning of the sample with the relatively low energy electron beam are displayed substantially superimposed.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a scanning electron microscope according to one embodiment of the present invention. Reference numeral 1 denotes a high-acceleration electron beam irradiation system including an electron gun 2 for generating a high-energy electron beam and a focusing lens 3. Reference numeral 4 denotes a low acceleration electron beam irradiation system including an electron gun 5 for generating a low energy electron beam and a focusing lens 6. In this embodiment, for example, assuming a silicon wafer in a semiconductor manufacturing process as a sample to be inspected, about 20 kV is selected as the energy of the higher electron beam and about 1 kV is selected as the lower energy.

Reference numeral 7 denotes a deflector, for example, a sector-type magnetic deflector is used. The electron beam from the magnetic field type deflector 7 is narrowly focused by the objective lens 8 and irradiates the sample 9. The electron beam applied to the sample 9 is deflected by a scanning deflector 10 so that a desired region on the sample is scanned by the electron beam.
A scanning signal from a low-acceleration scanning signal generator 11 and a scanning signal from a high-acceleration scanning signal generator 12 are switched and supplied to a scanning deflector 10 by a switch 13.

Secondary electrons generated from the sample 9 upon irradiation of the sample 9 with the electron beam are detected by a secondary electron detector 14 disposed above the objective lens 8. Also,
The backscattered electrons generated from the sample 9 are detected by a backscattered electron detector 15 arranged on the upper part of the sample 9. The detection signals of the secondary electron detector 14 and the backscattered electron detector 15 are switched by a switch 16 and supplied to a cathode ray tube 17.

As the secondary electron detector 14, a multi-channel plate or a combination of a scintillator and a photomultiplier tube is used, and as the reflected electron detector 15, a multi-channel plate on a ring is used. The switch 13 and the switch 16 are switched in conjunction with each other, and the blanking unit 18 in the high-acceleration electron beam irradiation system 1 and the blanking unit 19 in the low-acceleration electron beam irradiation system 4 in conjunction with the switching. Is controlled.

The two electron beam irradiation systems 1 and 4 are arranged as follows. That is, it is now assumed that electrons having a positive charge along the optical axis of an imaging system to be described later and having the same energy as each of the two electron beam irradiation systems are incident from the lower side of the deflector 7. ,
The trajectory after passing through the deflector 7 is obtained by calculation. Thereafter, the optical axes of the electron beam irradiation systems 1 and 4 are arranged on this orbit. Further, although not shown, the two irradiation systems have means for tilting and moving horizontally with respect to the axis of each electron beam. The operation of such a configuration will now be described.

First, the operation of the imaging systems such as the electron beam irradiation systems 1 and 4 and the objective lens 8 will be described with reference to FIG. The electron beams generated by the high-acceleration electron beam irradiation system 1 and the low-acceleration electron beam irradiation system 4 are both deflected by the deflector 7 in accordance with their energies, and after passing through the deflector 7, Both are adjusted so as to travel along the optical axis of the objective lens 8. This adjustment is performed using the tilt and horizontal movement means provided in each of the high-acceleration electron beam irradiation system 1 and the low-acceleration electron beam irradiation system 4. More specifically, in a state where the changeover switches 13 and 16 are fixed for low acceleration, the horizontal movement and tilting means of the low acceleration electron beam irradiation system 4 so that the brightness of the image on the cathode ray tube 17 is maximized. To adjust. Next, with the changeover switches 13 and 16 fixed for high acceleration, the cathode ray tube 1
The horizontal movement and tilting means of the electron beam irradiation system 1 for high acceleration are adjusted so that the brightness of the image on 7 becomes maximum.

The electron beams generated from the respective electron beam irradiation systems are focused by the respective focusing lenses 3 and 6 so as to form a crossover at a point a below the deflector 7. In the figure, the solid line (a) shows the trajectory of the high energy electron beam, and the dotted line (b) schematically shows the trajectory of the low energy electron beam. After passing the point a, the high energy electron beam (a) is again focused (focused) at the point d on the surface of the sample 9 by the objective lens 8.

On the other hand, an electron beam having a low energy (b)
Is focused once at point b by the magnetic field of the objective lens 8 after passing point a, then diverges again, focuses again at point c, diverges again, and focuses third time at point d on the surface of the sample 9 (Focus). The energy (acceleration voltage) of the two electron beams so that the focus point of the high-energy electron beam and the n-th focus point of the low-energy electron beam coincide with each other under the same excitation intensity of the objective lens 8. Then, the excitation intensity of the lens is selected by design. In practice, fine tuning of the accelerating voltage of one, fine tuning of the excitation current of the focusing lens, or special auxiliary lens in the path of the electron beam to obtain a more precise focus match It may be provided.

FIG. 3 is a diagram for explaining the operation of detecting reflected electrons generated by irradiating the sample 9 with an electron beam of high energy. The sample 9 has a lower layer 19 and a surface layer 20 provided on a substrate 18. I have. The high-energy electron beam applied to the sample 9 penetrates the surface layer 20 of the sample and reaches the lower layer 19. Here, a part is reflected, and the surface layer 20 is again reflected.
Is scattered upward through the Most of the scattered backscattered electrons are detected by the backscattered electron detector 15. At this time, part of the secondary electron signal is reflected electron detector 1
It is desirable to apply a negative potential of several tens of volts to the backscattered electron detector 15 in order to prevent the incident light from being incident on the detector 5 and being detected.

FIG. 4 is a diagram for explaining a detection operation of secondary electrons generated by irradiating the sample 9 with a low energy electron beam. Most of the low energy electron beam applied to the sample 9 is absorbed in the surface layer 20. At that time, secondary electrons having low energy are generated on the surface of the surface layer 20. The secondary electrons are captured by the strong magnetic field of the objective lens 8, rise in the objective lens 8 while drawing a spiral trajectory, diverge at a position where the magnetic field has disappeared, enter the secondary electron detector 14 and are detected. You.

In FIG. 1, signals detected by the backscattered electron detector 15 and the secondary electron detector 14 are guided to a cathode ray tube 17 via a detection signal switch 16 and used as a luminance modulation signal. On the other hand, 2
In order to obtain a two-dimensional image, the electron beam incident on the sample 9 is scanned in a raster shape by the scanning deflector 10.
From the low-acceleration scanning signal generator 11 and the high-acceleration scanning signal generator 12, the electron beams of the respective energies are scanned on the surface of the sample 9 exactly in the same field of view with the same amplitude (same magnification). Signal is being generated.

The adjustment of the scanning signal can be performed, for example, by the following method. That is, a metal sample which does not charge up due to an electron beam and obtains both a secondary electron signal and a reflected electron signal is prepared as a sample 9, and a predetermined region of the sample is irradiated with electrons from two irradiation systems 1 and 4. Scan by beam. An image is displayed on the cathode ray tube 17 by the reflected electron signal and the secondary electron signal obtained by scanning the sample. Then, the low-acceleration scanning signal generator 11 and the high-acceleration scanning signal generation are performed so that the image formed by the reflected electron signal displayed on the cathode ray tube 17 and the image formed by the secondary electron signal completely match. Fine adjustment of the amplitude (gain of the amplifier) of each scanning signal output by the detector 12 and the horizontal movement of the irradiation system.

The scanning signal adjusted as described above is
The signals are alternately switched by a scanning signal switching switch and applied to the scanning deflector 10. At this time, the detection signal switching switch 16 is switched in conjunction with the switch 13, and the low-acceleration scanning signal from the low-acceleration scanning signal generator 11 is applied to the scanning deflector 8. In the meantime, while the signal from the secondary electron detector 14 is applied and the scanning signal for high acceleration from the scanning signal generator 12 for high acceleration is applied, the signal from the reflected electron detector 15 is
Is led to. The cathode ray tube 17 is connected to the scanning signal generator 15,
Since the screen is operated in synchronization with the same frequency as the screen 16, the screen of the surface layer of the sample 9 formed by the secondary electrons and the image of the lower layer formed by the reflected electrons are alternately displayed. By making the switching of the switches 13 and 16 faster, both images are substantially superimposed and displayed on the screen of the cathode ray tube 17 due to an afterimage effect, so that it is possible to appropriately observe the overlapping state of the surface layer and the lower layer image. Can be.

In the above-described embodiment, since two electron beams having different energies coexist in the imaging system at the same time, the reflected electron image formed by the high energy electron beam and the low energy electron beam are generated. In some cases, backscattered electron images of different magnifications appear in duplicate. Of course, the same phenomenon may occur for the secondary electron image. In order to solve this problem, each irradiation system 1, 4
The blanking means 18 and 19 arranged in the
The operation is performed in conjunction with the switching of the scanning signals by 3 and 16, so that electron beams having different energies are alternately incident on the imaging system.

FIG. 5 shows an embodiment for quantitatively determining the degree of overlap between the upper layer pattern and the lower layer pattern using two types of video signals. In the figure, reference numeral 21 denotes a signal processing device;
Reference numeral 22 denotes an electronic computer. Detection signal switch 16
Is passed to the signal processing device 21, where it is converted into a digital signal, and then the secondary electron signal and the reflected electron signal are respectively stored as digital image signals in dedicated frame memories. The electronic computer 22
The data can be read by specifying an arbitrary area in the frame memory.

FIG. 6A shows an image display device (cathode ray tube).
17 shows an example of the image displayed on the upper part, in which b shows the pattern of the lower layer of the sample obtained from the backscattered electron signal, and b shows the pattern of the surface layer of the sample obtained from the secondary electron signal. ing. FIG. 6B shows a cross section of the sample. Reference numeral 18 denotes a substrate, and 19 denotes a lower layer pattern (for example, a wiring pattern) formed thereon.
Reference numeral 20 denotes a resist pattern formed on the surface layer, for example, to form a through hole on a wiring pattern.

The frame memory stores the image data A and B shown in FIG. 6A in dedicated memories. When the data in the area C shown by hatching in FIG. 6A is read into the electronic computer 22 and added in the vertical direction, a signal waveform as shown in FIG. From the side of the electronic signal, FIG.
The signal waveforms shown in FIG. The position coordinates P _{1 of the} point where the signal takes the minimum value by the electronic computer 22,
Find P ₂ , Q ₁ , Q ₂ . In the figure, O is the starting point of the waveform, and indicates the coordinate origin in the horizontal direction. From the values of P ₁ , P ₂ , Q ₁ , and Q ₂ , the displacement δ between the two patterns is calculated by the following equation.

Δ = (P ₁ + P ₂ ) / 2− (Q ₁ + Q ₂ ) / 2 In this way, the degree of overlap between the upper layer pattern A and the lower layer pattern B is quantitatively measured.

FIG. 7 shows another embodiment of the present invention.
In the embodiment of FIG. 1, the trajectories of two electron beams having different energies are configured to be matched by adjusting a horizontal moving device provided in each of the irradiation systems 1 and 4. The embodiment shown in FIG. 7 employs a method in place of the horizontal moving device. In FIG. 7, 31 is a correction deflection signal generation circuit, 32 is a correction deflection signal storage device, and 33 is a knob for adjusting the magnitude of the correction deflection signal. These 31, 32, and 33 are provided for a low acceleration voltage. For a high acceleration, a correction deflection signal generation circuit 34, a correction deflection signal storage device 35, and a knob 36 for adjusting the magnitude of the correction deflection signal are provided. Is provided. In addition,
The signal from the correction deflection signal generation circuit 31 and the signal from the scanning signal generator 11 for low acceleration are added by the adder 37 and then supplied to the scanning deflector 10 via the switch 13. Further, the signal from the correction deflection signal generation circuit 34 and the signal from the scanning signal generator 12 for high acceleration are added by the adder 38 and then supplied to the scanning deflector 10 via the switch 13.

In the embodiment of FIG. 7, while observing the secondary electron image and the reflected electron image from the standard sample in which the shift amount of the pattern between the upper layer and the lower layer is known, the knob 33
Or, adjust 36 to make the two optical axes coincide. Thereafter, the magnitude of the correction deflection signal at this time is stored in the correction deflection signal storage devices 32 and 35, respectively, and the output from this storage device is thereafter stored in the correction deflection signal generation circuits 31 and 3.
Use while inputting to 4. Correction deflection signal generation circuit 3
1 and 34 are added to the output signal of the low-acceleration scanning signal generator 11 and the output signal of the high-acceleration scanning signal generator 12, respectively.
Through the scanning deflector 10. In this way, the optical axes of the two beams with different energies can be perfectly matched. It is a convenient device to store a plurality of correction signal amounts corresponding to various different conditions in a storage device and to switch and use them according to changes in conditions.

FIG. 8 shows another embodiment of the present invention.
This is an example in which a correction deflector 40 is separately provided in addition to the main deflector 10. In this case, the correction deflection signal generation circuit 3
The correction deflecting signals from the deflecting devices 1 and 34 are supplied to the correcting deflector 40 via the changeover switch 41. The changeover switch 41 is switched in conjunction with the changeover switch 13.

FIG. 9 shows still another embodiment of the present invention. In this embodiment, the same parts as those of the embodiment of FIG. 1 are denoted by the same reference numerals and the description thereof will be omitted. In this embodiment, FIG.
A Wien filter 50 is provided instead of the sector type deflection system 7 in the embodiment. Vienna filter 50
Is a deflector with a deflecting magnetic field and a deflecting electric field arranged orthogonally.
For the electron beam having a specific energy, the deflection force by the magnetic field and the deflection force by the electric field are configured to act equally in the opposite direction, and the electron beam having the specific energy goes straight, It acts as a deflector for electron beams with different energies. In this embodiment, the electron beam emitted from the electron beam irradiation system 1 for high acceleration is adjusted so as to go straight through the Wien filter 50. As a result, the electron beam irradiation system 1 for high acceleration emits light of the imaging system. It is arranged on the axis, that is, on the axis of rotational symmetry of the objective lens 8. As a result, since one of the irradiation systems can be arranged on the rotationally symmetric axis of the lens of the imaging system, the optical axis can be easily aligned and the design of the apparatus can be simplified.

FIG. 10 shows another embodiment of the present invention. In this embodiment, the cathode ray tube 1 in the embodiment of FIG.
7 and a color display 5 as an image display device.
5, the secondary electron signals are displayed in different colors. That is, the signal from the secondary electron detector 14 is supplied to the red signal input terminal 57 of the color display 55 by the switch 56, and
A signal from the backscattered electron detector 15 is supplied to a green signal input terminal 58 of the color display 55 by a switch 56. As a result, the outline of the upper layer pattern and the outline of the lower layer pattern are displayed in different colors (red and green), so that it is easy to determine which is the upper layer and which is the lower layer, thereby improving the work efficiency. Can be.

FIG. 11 also shows an embodiment of the present invention, in which a video signal from a backscattered electron detector and a video signal from a secondary electron detector are guided to independent image processing devices 60 and 61, respectively, and recursive filters are provided. Processing and contour emphasis processing are performed. As a result, the SN ratio of the image can be improved, the contour can be displayed more clearly, and the measurement accuracy can be further improved.

Although one embodiment of the present invention has been described in detail, the present invention is not limited to this embodiment. For example, a reflected electron image and a secondary electron image are alternately supplied to a cathode ray tube, and both images are superimposed and displayed by an afterimage effect. However, two types of images are stored in a frame memory and stored. The two image signals may be added and displayed on an image display device such as a cathode ray tube.

[0037]

As described above, the scanning electron microscope according to the present invention is capable of obtaining a reflected electron image from the lower layer of the sample obtained by the high acceleration electron beam and a reflection electron image from the surface layer of the sample obtained by the low acceleration electron beam. Since the next electronic image can be superimposed and observed in real time, the inspection speed can be improved. Further, since two types of electron beams having different energies are simultaneously imaged by one imaging system, there is no axis deviation error, and the measurement accuracy can be improved.

Further, since the reflected electron signal and the secondary electron signal are displayed in different colors, it is possible to clearly identify which is the surface layer pattern. Furthermore, since the image processing device is provided independently, it is possible to perform an optimal process on each image or perform a different process. For example, a process can be performed in which one image is a normal image and the other image is a contour image. And
By using the Wien filter as a means for matching the optical axes of the two types of electron beams, one of the electron beam irradiation systems can be arranged on the axis of the lens system, and the electron optical system can be easily adjusted. .

[Brief description of the drawings]

FIG. 1 is a diagram showing a scanning electron microscope according to one embodiment of the present invention.

FIG. 2 is a diagram for explaining operations of an electron beam irradiation system and an imaging system such as an objective lens.

FIG. 3 is a diagram for explaining an operation of detecting backscattered electrons.

FIG. 4 is a diagram for explaining a secondary electron detection operation.

FIG. 5 is a diagram for explaining an embodiment for quantitatively determining the degree of overlap between an upper layer pattern and a lower layer pattern.

FIG. 6 is a diagram for explaining an embodiment for quantitatively determining the degree of overlap between an upper layer pattern and a lower layer pattern.

FIG. 7 is a diagram showing another embodiment of the present invention.

FIG. 8 is a diagram showing another embodiment of the present invention using a main deflector and a correcting deflector.

FIG. 9 is a diagram showing another embodiment of the present invention using a Wien filter for the deflection system.

FIG. 10 is a diagram showing another embodiment of the present invention using a color display.

FIG. 11 is a diagram showing another embodiment of the present invention using an image processing apparatus.

[Explanation of symbols]

 Reference Signs List 1 electron beam irradiation system for high acceleration 4 electron beam irradiation system for low acceleration 7 deflector 8 objective lens 9 sample 10 scanning deflector 11 scanning signal generator for low acceleration 12 scanning signal generator for high acceleration 14 secondary electron detection Detector 15 backscattered electron detector 17 cathode ray tube 18, 19 blanking means

Claims (4)

(57) [Claims] A first electron beam irradiation system for generating an electron beam having a relatively high energy; a second electron beam irradiation system for generating an electron beam having a relatively low energy;
A deflection system for advancing the electron beam from the first electron beam irradiation system and the electron beam from the second electron beam irradiation system along substantially the same optical axis;
Lens means for focusing both the electron beam from the electron beam irradiation system and the electron beam from the second electron beam irradiation system on the sample with the same lens intensity, and reflected electrons for detecting reflected electrons from the sample A detector, a secondary electron detector for detecting secondary electrons from the sample, and an electron beam from the first electron beam irradiation system and an electron beam from the second electron beam irradiation system in a specific field of view on the sample And an image based on the reflected electron signal detected based on the scanning of the electron beam from the first electron beam irradiation system on the sample, and an image on the sample of the electron beam from the second electron beam irradiation system. And a display means for displaying an image based on the secondary electron signal detected based on the scanning substantially superimposed. 2. A configuration in which deflection correction is performed on at least one electron beam of the first electron beam irradiation system and the second electron beam irradiation system, and the optical axes of the two electron beams are made to coincide exactly. The scanning electron microscope according to claim 1. 3. A Wien filter is used to form a deflection system for advancing the electron beam from the first electron beam irradiation system and the electron beam from the second electron beam irradiation system along substantially the same optical axis. 2. The scanning electron microscope according to claim 1, wherein the optical axis of one of the electron beam irradiation systems and the optical axis of the imaging system at the subsequent stage of the Wien filter are made to coincide with each other. 4. The display means is a color display,
2. The scanning electron microscope according to claim 1, wherein the reflected electron image and the secondary electron image are displayed in different colors.