JP4092257B2 - Electron beam apparatus and pattern evaluation method using the electron beam apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for performing pattern evaluation of a sample having a pattern with a minimum line width of 0.1 μm or less, for example, a substrate with high throughput, and a pattern evaluation method using the apparatus.
[0002]
[Prior art]
Conventionally, when a pattern formed on a substrate is evaluated using an electron beam, proposals have been made to use a multi-beam in order to improve throughput. These proposals can be broadly divided into two methods: a method using a multi-beam having a plurality of optical axes and a method of forming a multi-beam at a position equidistant from one optical axis.
[0003]
[Problems to be solved by the invention]
However, the former method using a multi-beam having a plurality of optical axes is expensive because it requires a large number of electron optical lens barrels, and can be arranged on a single substrate, for example, a single wafer. There was a problem that the number of optical axes could not be increased too much.
[0004]
On the other hand, in the latter method of forming a multi-beam at a position equidistant from one optical axis, the beam is formed at a position away from the optical axis, so the aberration of the optical system increases, and the brightness of the beam increases. There is a problem that you can not. Therefore, when trying to place many beams very close to the optical axis, secondary electrons generated from each scanning point of the multi-beam overlap each other due to aberration of the optical system, and can be detected independently to prevent this. If the beam interval is increased, many multi-beams cannot be produced. Further, the conventional electron beam apparatus has a drawback that the length of the secondary optical system becomes long.
[0005]
More specifically, as shown in FIG. 5, in the conventional apparatus, the objective lens 4 for focusing the primary electron beam 2 after passing through the reduction lens 1 on the sample surface 3 is one stage. At this time, when the landing energy of the primary electrons is large, the excitation voltage of the objective lens 4 becomes large, and the secondary electrons 5 form a secondary electron image 6 immediately above the objective lens 4 as shown in the figure, and the half angle of the aperture If αi is large and the aperture of the lens 8 of the secondary optical system 7 is not increased, the lens will go out of the lens. For this reason, it is necessary to increase the diameter of the lens 8, which causes a disadvantage that aberrations increase. In addition, since the lens of the secondary optical system cannot be very close to the objective lens, if the object point of this lens is far, it is necessary to further distant the image point in order to make it a magnifying lens. There is a disadvantage that lengthens. In the figure, reference numeral 8 denotes an E × B separator that separates secondary electrons from the primary optical system.
[0006]
The present invention has been made in view of the above-described conventional problems, and can be manufactured at a low cost, and can reduce many aberrations in the vicinity of one optical axis by reducing the aberration of the optical system for detecting secondary electrons. An object is to provide an electron beam apparatus capable of forming a multi-beam and a pattern evaluation method using the apparatus.
[0007]
[Means for Solving the Problems]
In order to solve the above-described problem, a first electron beam apparatus according to a reference example of the present invention separates an electron beam emitted from an electron gun at a plurality of apertures, reduces the image of the aperture to two stages, and multibeams. Is formed on the sample, the sample is scanned, the secondary electrons from the sample are expanded by the objective lens, and the secondary electrons are separated from the primary beam by the E × B separator. The second stage lens for reducing the image of the aperture comprises a two-stage lens, and forms an enlarged image of secondary electrons in the middle of the two-stage lens. Features.
[0008]
In the electron beam apparatus according to the reference example, the objective lens for focusing the primary electron beam on the sample surface is composed of two stages of lenses, and the excitation voltage of the objective lens can be reduced, so that the secondary electron image is located far from the objective lens. In other words, since an image point can be formed in front of the second lens of the secondary optical system, the aperture of the lens of the secondary optical system can be small and the aberration can be reduced. In addition, by creating the secondary electron image in front of the second lens of the secondary optical system, it is not necessary to lengthen the image point of the lens, and the length of the secondary optical system can be shortened.
[0009]
In order to solve the above problem, the second electron beam apparatus according to the reference example of the present invention reduces the electron beam that has passed through the plurality of apertures with a two-stage lens to form a multi-beam, and scans the sample. Then, the secondary electrons emitted from the sample are enlarged by the first lens, and the distance between the secondary electrons is enlarged. After passing through this lens, the secondary electrons are separated from the primary optical system by the E × B separator and made incident on the secondary optical system. The electron beam apparatus further expands the interval between the secondary electrons with at least one stage lens and detects each of a plurality of beams with a secondary electron detector, and the deflector that scans the sample is electrostatic It is a deflector.
[0010]
In order to reduce the aberration of the secondary optical system and form many multi-beams near one optical axis, when scanning the multi-beams, the central part and the peripheral part of the scanning field of view are far away from the optical axis. Without incident on the first lens of the secondary optical system. When the deflector that scans the sample is an electromagnetic deflector, the secondary electron that has returned is, for example, in the phase of deflecting the primary beam to the left by this deflector. As a result, the secondary electrons move away from the optical axis and enter the lens at a position away from the optical axis by the first lens of the secondary optical system, so that the aberration becomes large. However, when the scanning deflector is an electrostatic deflector as in the present invention, the secondary electrons are deflected in the same direction as the primary beam and return to the optical axis direction. Since the lens is incident on the lens at a position close to the optical axis, the aberration of the secondary optical system is reduced.
[0011]
In order to solve the above problems, an electron beam apparatus according to the present invention is an electron beam apparatus in which an electron beam emitted from an electron gun having a single emission region is incident on a multi-aperture to form a multi-beam. Conditions required for the multi-beam from the convergence half angle αi at the sample, the emittance E of the electron gun, and the radius rm from which 90% or more of the on-axis luminance is obtained :
rm ≦ E / αi
It is arranged inside a circle that satisfies the following conditions .
[0012]
In the electron beam apparatus of the present invention, it is not necessary to arrange the multi-beams on the circumference as in the prior art, and the minimum distance between the beams can be formed in the circle so as to be larger than the resolution of the secondary optical system. This can increase the number of beams.
[0013]
Further, in order to solve the above problem, a pattern evaluation method according to a reference example of the present invention is a method for evaluating a pattern formed on a sample,
a. Irradiating a plurality of apertures with an electron beam emitted from an electron gun;
b. Reducing the electron beam separated by the aperture in two or more steps, focusing on the sample, and scanning;
c. A step of enlarging a distance between secondary electrons emitted from a scanning point of the sample by an objective lens and separating from a primary electron optical system by an E × B separator;
d. Detecting the separated secondary electrons with a secondary electron detector,
The second stage reduction for reducing the aperture image is performed by an objective lens, and the first enlarged image of secondary electrons is formed behind the E × B separator.
[0014]
Further, in order to solve the above problems, a pattern evaluation method according to a reference example of the present invention is a method for evaluating a pattern formed on a sample,
a. Scanning the sample by reducing the electron beam that has passed through the plurality of apertures with a two-stage lens;
b. Enlarging the distance between the secondary electrons emitted from the scanning point of the sample by the first lens;
c. Separating secondary electrons from the primary optical system with an E × B separator after passing through the lens;
d. Enlarging the space between the secondary electrons with at least one lens after separation from the primary optical system;
e. Independently detecting a plurality of secondary electron groups with a secondary electron detector;
f. Forming an image with the detected signal and performing pattern evaluation;
The plurality of electron beams are arranged in a circle having a radius at which the aberrations of the primary electron beam and the secondary optical system are not more than predetermined values under the condition that the beam convergence half angle on the sample surface is determined. And
[0015]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings.
[0016]
FIG. 1 shows an electron optical system according to an embodiment of the present invention, and includes a primary optical system 10, a secondary optical system 30, and a detection system 40.
[0017]
In the primary optical system, the electron gun 11 operates the LaB6 cathode 12 under space charge limiting conditions to reduce shot noise. The anode 13 is composed of three electrodes, and at the same time as performing the lens operation, it is possible to achieve a high luminance condition or a high emittance condition by changing the voltage applied to the first anode. The axial alignment with the anode is performed by mechanically moving the cathode and Wehnelt in the XY direction or the θ direction. The axis of the beam emitted from the electron gun is aligned by the axis alignment deflector 14. The multi-beam is formed by providing the multi-aperture plate 15 within an angle that falls within 90% of the on-axis luminance of the beam diverging from the electron gun 11, and dividing the beam. The multi-beams that have passed through the multi-aperture plate 15 are focused by the condenser lens 16 to form a crossover at the NA aperture 17. The size of the crossover is larger than the NA opening size and is partially cut at the NA opening. A multi-beam that has been cut to an aperture angle at which aberration is allowed at the NA aperture is first reduced by the reduction lens 18, and further reduced by the first objective lens 19 and the second objective lens 20, and the multi-beam is combined with the sample surface. To burn. At this time, a reduced image is not formed between the first objective lens 9 and the second objective lens 20, and the beam diverging from each point of the multi-aperture plate 15 is shown in 22 between the two objective lenses. Thus, it can be a convergent beam, a parallel beam, or a slightly divergent beam (in the illustrated example, a convergent beam). For example, when the adjustment is made so that the beam is slightly convergent, the excitation voltage of the second objective lens 20 decreases, the image point of the secondary electrons becomes far, and the axial chromatic aberration of the secondary electrons decreases, but the excitation voltage decreases. As a result of the lowering, that of the primary beam becomes larger, and conversely, if it is adjusted slightly to a diverging beam, the excitation voltage of the second objective lens 20 rises, and the relation is reversed.
[0018]
However, in the present invention, the objective lens is not made up of one stage as in the prior art, but is made up of two stages 19 and 20 as shown in the figure, thereby reducing the excitation voltage of the second objective lens 20, so that the secondary electron image 31 is obtained. Is formed at a position far from the objective lens 20 (near the first objective lens 19 in the illustrated example), whereby the secondary electron image 32 separated from the primary electron beam by the E × B separator 23 is secondary. Since it is made in front of the second lens 33 of the optical system, the diameter of the lens of the secondary optical system can be reduced, and thereby the aberration of the secondary optical system can be reduced. In addition, by creating a secondary electron image in front of the second lens of the secondary optical system in this way, there is no need to lengthen the image point when the secondary optical system is a magnifying lens. Can be shortened.
[0019]
It is clear from simulation that the secondary electron image 32 is formed in front of the second lens 33 of the secondary optical system 30 by the first objective lens 19 when the primary beam has the above-described imaging relationship. It has become. In this case, the E × B separator 23 is arranged slightly above the first objective lens 19. An electromagnetic deflector 29 called pre-EB is provided immediately below the reduction lens 18, and the angle α is deflected by the E × B separator 23, and the primary beam passes through the center of the objective lens 19. As a result, the trajectory of the beam that has traveled on the optical axis is as shown at 24. Between the two objective lenses 19 and 20, there are two-stage deflectors 25 and 26 for scanning on the sample, and by superimposing a direct current for axial alignment on these deflectors, the primary beam is made to be the second objective. Although it is possible to align the lens 20 with the axis, in order to increase the angle at which the secondary beam is separated from the primary beam as will be described later, the primary beam is placed on the sample 21 as shown in the figure. Incident light is incident at a position 27 slightly away from the optical axis. As a result, the secondary electrons 34 emitted from a position slightly shifted to the right side in the drawing from the optical axis are attracted by the electric field of the second objective lens 20 and travel in parallel with the optical axis, and left by the second objective lens 20. Is bent to the right by the first objective lens 19, deflected to the right by 3α or more by the E × B separator 23, and enters the secondary optical system 30. Here, the deflection angle α of the primary beam by the E × B separator 23 is caused by the difference of 2α deflected to the left by the electromagnetic deflector of the E × B separator and further α deflected to the right by the electrostatic deflector. The deflection chromatic aberration can be made almost zero. For example, even if α is as small as about 8 °, refraction at the second objective lens 20, refraction at the first objective lens 19, and secondary electrons have considerably lower energy than primary electrons (for example, primary While the electron beam has an energy of 500 eV on the sample surface, the secondary electron beam has an energy of several eV), and the secondary electron 36 is bent by 30 ° or more, so the secondary optical system can be designed easily. be able to. In the illustrated example, the secondary electron image 31 is formed between the second objective lens 20 and the first objective lens 19.
[0020]
As described above, in the present invention, the objective lens has two stages 19 and 20, so that the primary beam passes through the center of the lens even when the E × B separator 23 is placed at a position that is not a conjugate point with the sample. Therefore, low aberration can be achieved. Further, detection of secondary electrons is facilitated by increasing the angle at which the secondary beam is separated from the primary beam.
[0021]
FIG. 2 is a view showing a direction perpendicular to the paper surface of FIG. 1, that is, an XZ plane. The trajectory of secondary electrons when the two scanning deflectors 25 and 26 are scanning as indicated by solid lines is indicated by dotted lines. That is, electrical scanning is performed in the X direction, but the beam trajectory at that time becomes a solid line and is always deflected so as to pass through the center of the objective lens 20. The secondary electrons are deflected by the deflector 26 so as to take any trajectory between the two dotted lines shown.
[0022]
In the present invention, since the two deflectors 25 and 26 are both electrostatic deflectors, when the primary beam is bent to the left by the second deflector 25, the secondary electrons are also bent to the left, and the E × B separator. This is convenient because it is returned in the direction of the optical axis so as to be incident on the central portion of 23. (If the second deflector 26 is an electromagnetic deflector, it is inconvenient because the secondary electrons are deflected in a direction away from the optical axis.) Also, the first deflector 25 as shown in FIG. The secondary electrons are advantageously deflected in the direction of the optical axis or in a direction parallel to the optical axis. After the secondary electrons enter the secondary optical system, the secondary electrons can be adjusted so that they pass through the center of the lens when scanning the primary beam by a deflector for secondary electrons (not shown). There is no problem.
[0023]
Next, consider how far a multi-beam having the required resolution can be placed on the sample from the optical axis.
[0024]
The beam convergence half angle αi (mrad) on the sample surface is determined from the condition that the aberration satisfies the specification. That is, when the beam convergence half angle αi is increased, the aberration is increased. Conversely, when αi is decreased, the beam current is decreased, and the optimum value of αi is determined by a trade-off between the two. The specification value of the aberration can be determined as, for example, a beam blur of 110 nmφ or less under the condition for resolving a 100 nm pattern.
[0025]
On the other hand, the electron beam emitted from the electron gun has a value of emittance E (mrad · μm) as an important characteristic as well as luminance. For example, when the multi-aperture plate is provided within an angle that is within 90% of the on-axis luminance as in the above-described embodiment, the emittance E has a crossover diameter d (half-value width, μm) and luminance generated by the electron gun. It is represented by the product of the emission direction θ (mrad) falling to 90% of the on-axis luminance. That is, E = θ · d
The emittance E value is stored at any position on the optical axis. Therefore, if the beam convergence half-angle on the sample surface is αi, E = rm · αi where rm is the radius at which multibeams are arranged to obtain an intensity of 90% or more of the on-axis luminance.
What is necessary is just to arrange | position in the circle | round | yen of the radius of rm which satisfy | fills.
[0026]
When αi is determined, if the beam position r is increased, the aberrations of the primary electron beam and the secondary electron beam increase due to coma, curvature of field, astigmatism, lateral chromatic aberration, and the like. Since r satisfying the specification values of these aberrations is determined, the beam needs to be arranged in a circle smaller than this r.
[0027]
The right side of FIG. 2 shows an example of the arrangement of multi-beams and their scanning directions. Conventionally, the number of beams is small because multi-beams are arranged on the circumference, but in the present invention, the primary beam is arranged inside a circle that satisfies the condition of rm ≦ E / αi. Can be increased. Of course, the minimum beam interval needs to be larger than the resolution of the secondary optical system, but in the present invention, since the aberration of the secondary optical system can be reduced, a single multi-beam within the above rm. Many can be formed near the optical axis.
[0028]
FIG. 3 shows an application of the electron beam apparatus shown in the above embodiment to the evaluation of a wafer in a semiconductor device manufacturing process.
[0029]
An example of the device manufacturing process will be described with reference to the flowchart of FIG.
This manufacturing process example includes the following main processes.
(1) Wafer manufacturing process for manufacturing a wafer (or preparation process for preparing a wafer) (Step 10)
(2) Mask manufacturing process for manufacturing a mask used for exposure (or mask preparation process for preparing a mask) (Step 11)
(3) Wafer processing process (step 12) for performing necessary processing on wafers
(4) Chip assembly process for cutting chips formed on the wafer one by one and making them operable (Step 13)
(5) Chip inspection process for inspecting the assembled chip (step 14)
Each process is further composed of several sub-processes.
[0030]
Among these main processes, the main process that has a decisive influence on the performance of the semiconductor device is a wafer processing process. In this process, designed circuit patterns are sequentially stacked on a wafer to form a large number of chips that operate as memories and MPUs. This wafer processing step includes the following steps.
(1) A thin film forming process for forming a dielectric thin film to be an insulating layer, a metal thin film for forming a wiring portion or an electrode portion (using CVD, sputtering, etc.)
(2) Oxidation process for oxidizing the formed thin film layer and wafer substrate (3) Lithography process for forming a resist pattern using a mask (reticle) to selectively process the thin film layer and wafer substrate, etc. (4) ▼ Etching process to process thin film layer and substrate according to resist pattern (for example, using dry etching technology)
(5) Ion / impurity implantation diffusion process (6) Resist stripping process (7) Inspection process for inspecting the processed wafer The wafer processing process is repeated for the required number of layers to manufacture a semiconductor device that operates as designed. .
[0031]
The lithography process that forms the core of the wafer processing process is shown in the flowchart of FIG. This lithography process includes the following steps.
(1) Resist coating process for coating a resist on the wafer on which the circuit pattern is formed in the preceding process (Step 20)
(2) Exposure process for exposing resist (step 21)
(3) Development step for developing a resist pattern by developing the exposed resist (step 22)
(4) Annealing process for stabilizing the developed pattern (step 23) Well-known processes are applied to the semiconductor device manufacturing process, wafer processing process, and lithography process described above.
[0032]
When the defect inspection apparatus according to each of the above embodiments of the present invention is used in the wafer inspection process of (7) above, even with a semiconductor device having a fine pattern, it is highly accurate in a state where there is no image defect of the secondary electron image. Since defects can be inspected, product yield can be improved and shipment of defective products can be prevented.
[0033]
The pattern evaluation according to the present invention can be widely applied to the pattern evaluation of samples such as defect inspection, line width measurement, alignment accuracy, potential contrast measurement of samples such as photomasks, reticles, and wafers. .
[0034]
【The invention's effect】
As described above, according to the present invention, since many multi-beams can be formed around one optical axis, pattern evaluation can be performed with high throughput, and a single electron optical column is used. Since it is good, it can be manufactured at low cost.
[Brief description of the drawings]
FIG. 1 is a diagram showing an optical system of an electron beam apparatus according to an embodiment of the present invention.
FIG. 2 is a diagram showing the trajectory of the principal ray of secondary electrons in the case of the scanning method of the present invention, and the upper right shows an example of arrangement of multi-beams and the scanning direction.
FIG. 3 is a flowchart showing a semiconductor device manufacturing process.
4 is a flowchart showing a lithography process in the semiconductor device manufacturing process of FIG. 3;
FIG. 5 is a diagram showing an optical system in a conventional electron beam apparatus.
[Explanation of symbols]
10: primary optical system, 11: electron gun, 15: multi-aperture plate 16: condenser lens, 17: NA aperture, 18: reduction lens 19: first objective lens, 20: second objective lens, 21: sample 22 1: primary beam, 23: E × B separator, 25: first deflector 26: second deflector, 30: secondary optical system, 31, 32: secondary optical system 33: lens of secondary optical system, 34, 35: secondary electrons, 40: detector

Claims (1)

In an electron beam apparatus in which an electron beam emitted from an electron gun having a single emission region is incident on a multi-aperture to form a multi-beam, the multi-beam includes a convergence half angle αi at the sample, an emittance E of the electron gun, and Conditions obtained from the radius rm at which an intensity of 90% or more of the on-axis luminance is obtained :
rm ≦ E / αi
An electron beam apparatus, wherein the electron beam apparatus is disposed inside a circle that satisfies the following conditions .

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