JP2002530833A - Detector array for efficient secondary electron collection in microcolumns - Google Patents

Abstract

(57) Abstract: A structure and method for detecting secondary electrons and backscattered electrons in a microcolumn. The secondary electron detector and the backscattered electron detector, which are installed together upstream of the Einzel (objective) lens of the microcolumn, provide a highly efficient axially symmetric electron detector, shorten the column length, and operate further The distance becomes shorter. The secondary electron detector is provided between the deflection system and the Einzel lens, between the suppression plate and the Einzel lens, or between the deflection system and the aperture limiting aperture. A backscattered electron detector is provided between the field aperture and the deflection system and is integrated into the aperture. The secondary electron extractor located between the sample and the Einzel lens further enhances the spatial resolution caused by surface imperfections or local surface potential on the sample surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an electron beam microcolumn, and more particularly, to a microelectron beam column provided with a detector for secondary electrons and backscattered electrons.

BACKGROUND OF THE INVENTION In the late 1980's, an electron beam microcolumn based on micromachined electron optical components and a field emission source operating by an alignment method using a scanning tunneling microscope (STM) was first introduced. Was done. Electron beam microcolumns are used to form precisely focused electron beams, have high beam currents, have small physical size, and have the advantage of low cost and very high resolution, and are also used in e-beam lithography and other applications. Can be used in a wide range of applications. Regarding microcolumns, Chang, T.M. Et al., “Electron-Beam Microcolumns for Lithography and Rethography
lated Applications) '', Journal of Vacuum Science Technology Bulletin 14 (6)
pp.3774-3781, Nov./Dec. 1996; By Kratschmer and others
Experimental Evaluation of 20 × 20mm Footprint Micro Column (Experimental Evalu
ation of a 20 × 20mm Footprint Microcolumn), Journal of Vacuum Science Te
Chnology Bulletin 14 (6), pp. 3792-96, Nov./Dec. 1996, both of which are hereby incorporated by reference herein.

[0003] A microcolumn may be used as a general scanning electron microscope (SEM). Conventional SEMs detect secondary electrons (SE) and generate an image thereof. Generally, secondary electrons are of low energy and provide information about the contour of the sample. The contrast of the sample material is high-energy backscattered electrons (BS
E).

When electrons from an electron beam source, such as a microlens system, strike a surface with sufficient energy, secondary electrons are emitted from the surface. The direction and extent to which secondary electrons are emitted depends greatly on the shape of the surface. High energy backscattered electrons are likewise emitted, depending on the properties of the surface material.

FIG. 1 (Prior Art) shows a side view of a conventional microcolumn detection technique. The main components are: (a) an electron source 105 consisting of a cathode with one or more electrodes for extracting the emitted electrons and accelerating them to a desired energy; and (b) for forming a focused beam. The objective lens 130, which is usually an Einzel lens, and the (c) deflection plate 150 perform beam scanning. The secondary electron detector 150 is provided between the last electrode of the Einzel lens 130 and the sample 160.

A primary electron 170 is extracted from the electron source 105, passes through the limited aperture 110, is accelerated to a final beam voltage of 1 keV, and is sampled by the Einzel lens 130.
Refocused at 60. When a periodic voltage is applied to the deflecting plate 120, the focused primary beam 170 is swept across the sample 160 to produce secondary electrons (SE) 180. Secondary electrons 180 emitted from the sample surface are emitted in a wide-angle conical shape having a cosine distribution. Only a small portion of the secondary electrons 180 in the outer emission cone (shown as the shaded area) falls on the area of the SE detector 150. The collected secondary electrons 180 are used to create a secondary electron image.

The amount of detected secondary electrons can be increased by increasing the distance w between the SE detector 150 and the sample 160. FIG. 2 (prior art) shows that the SE detector 2
5 shows an example in which the distance w between the sample 50 and the sample 260 is increased. For example, the interval s is 0.1 mm
If the working distance w is 1 mm and the inner diameter of the SE detector 250 is 1.5 mm, only the secondary electrons 280 emitted at an angle larger than 83 ° are emitted from the SE detector 2.
50, resulting in a detector efficiency of less than 2%. However, as the spacing s is increased to 1 mm, the detector efficiency increases to 39%. However, in order to increase the interval s, it is necessary to increase the working distance w to the Einzel lens 230.
Spatial resolution decreases due to an increase in aberration.

In general, to increase the detector efficiency, a bias voltage is applied to the surface of the SE detector 250 to attract some of the secondary electrons that would have been lost. This bias voltage has little effect on the focusing of primary beam 270, even when shield 240 is incorporated. The applied bias does not significantly affect the collection efficiency of the backscattered electrons. However, the higher the bias voltage, the larger the aberration, and the lower the spatial resolution.

Therefore, there is a need for a microcolumn structure that increases the detection amount of secondary electrons that enhances the spatial resolution by increasing the signal-to-noise ratio and shortening the working distance.

A backscattered electron (BSE) detector is an optical device that provides information about the material contrast of a sample. Conventionally, a single detector is used to detect both secondary and backscattered electrons, and the SE / BSE detector is generally mounted directly on the bottom of the objective.

To obtain the best geometric collection efficiency for BSE detection, the BSE detector needs to be mounted as high as possible above the sample. However, by raising the BSE detector, it is necessary to raise the objective lens as well. As described above, the longer the working distance to the Einzel lens, the lower the spatial resolution. Furthermore, because the secondary electrons are of low energy, if the SE detector is placed too far from the sample, many electrons will be lost before reaching the SE detector.

Therefore, there is a need for a microcolumn structure for detecting high-energy backscattered electrons with high efficiency.

According to the present invention, there is provided a structure for detecting secondary electrons and backscattered electrons and a method thereof. In the present invention, a pre-Einzel lens secondary electron detector (that is, provided upstream of the Einzel lens with respect to the electron beam direction) and a pre-Einzel lens backscattered electron detector separate the SE detector from the SE detector. Thus, a highly efficient axially symmetric combination of electron detectors is provided, which reduces the column length and the working distance.

In one embodiment, the SE detector is located upstream of the Einzel lens between the deflection system and the Einzel lens. In one embodiment, the SE detector is located upstream of the Einzel lens between the suppression plate and the Einzel lens. The shield for the Einzel lens faces upwardly facing the emission source.
In another embodiment, the SE detector is located upstream of the Einzel lens between the deflection system and the field limiting aperture. In yet another embodiment, an SE detector is located upstream of the Einzel lens between the field aperture and the deflection system, and further upstream of the Einzel lens. In another embodiment, the SE extractor is located proximate to the sample surface.

FIG. 3 shows a secondary electron detection array located upstream (relative to the electron beam) of the Einzel lens of the microcolumn 300. Micro column 3 described here
It should be understood that 00 also includes a conventional support housing structure (not shown) for supporting and surrounding the components shown in FIG. The sample 360 is held by the fixed support 365, and this fixed support is also a part of the microcolumn structure.

The SE electrode 3 is located at a position above the first electrode of the objective lens 330 by a distance d.
50 are provided. The point that the SE detector 350 is arranged at a fixed distance above the objective lens is different from the conventional microcolumn in which a detector is provided between the sample and the objective lens.

The objective lens 330 is usually an electrostatic Einzel lens having the same potential, but is not limited to this. Objective lens 330 may also be, for example, an immersion lens. When an immersion lens is used for the objective lens 330, a certain potential is applied to the last electrode of the objective lens instead of the ground potential. As a result, the objective lens 3
An electric field exists between the last 30 electrodes and the sample 360, which is normally grounded. The electric field between the last electrode and the sample 360 is used to attract or repel secondary electrons emitted from the sample 360. The sample 360 emits secondary electrons when the electron beam modified by the objective lens 330 is directed to the sample 360. ("
The terms "Einzel lens" and "objective lens" are used interchangeably herein. ) Design and working distance w of specific Einzel lens
Is selected based on the distance d.

The Einzel lens 330 is a very powerful electron optical lens for secondary electrons having an energy of several eV to several tens eV. Thereby, the secondary electrons 380 are strongly focused and exit the Einzel lens 330 in a wide-angle cone shape. A large amount of the emitted secondary electrons 380 reach the active area of the SE detector 350. In this arrangement, only secondary electrons emitted at very small angles are not captured. Therefore, the detector efficiency is high, resulting in a good signal to noise ratio. A further improvement is made by applying a small bias voltage to the detector surface, which attracts secondary electrons that would have missed secondary electron detector 350.

For example, if the working distance w = 0.5 to 1 mm and the detector distance d = 0.75, the secondary electrons 380 emitted at an angle greater than 15 to 20 ° are applied to the SE detector 350. Reaching, the detector efficiency is higher than 80%.

The SE detector 350 is a one-stage or two-stage microchannel plate (MCP) which is a conventional commercially available high-gain low-noise continuous dynode type electron multiplier.
) It may consist of a detector. High gains of 10 ⁴ to 10 ⁸ are obtained at operating voltages of 1000 to 3000 V for single-stage or two-stage MCP detectors, respectively. The high intrinsic gain of the detector allows it to be used for signal processing. Further, the MCP detector consists of two parts, one for the MCP and one for
One is an anode-collector electrode machined from an insulator such as, for example, Macor or other ceramics, on which patterned electrodes have been formed by metal vacuum evaporation and electroplating. Thereby, one end is provided at both ends of the input side and the output side of the MCP.
The entire detector assembly operating at 000 V is only 0.8 mm high. SE detector 350 may be a conventional pin or Schottky diode type solid state detector, an Everhart-Thornley scintillator / photomultiplier tube combination, or a channeltron electron multiplier. However, the present invention is not limited to this.

When the working distance w is long, some of the secondary electrons emitted at a large angle are blocked by the inner hole of the electrode of the Einzel lens 330. Einzel lens 3
By appropriately increasing the diameter of the 30 holes, secondary electrons that would have been lost can be recovered.

Further, even if the distance between the sample 360 and the SE detector 350 is increased, the working distance w does not become longer, and the relatively thick SE detector 350 is connected to the Einzel lens 330 and the sample 360. Since they do not occupy the spacing between them, the working distance w is kept to a minimum distance of, for example, less than 0.5 mm. If the working distance w is short, the aberration is reduced. Therefore, the spatial resolution is further enhanced by using the upstream of the Einzel lens arrangement.

FIG. 4 shows an alternative arrangement of FIG. The SE detector 450 is also provided between the Einzel lens 430 and the deflecting plate 420 in this case. However, in this arrangement, the SE detector 450 is located just above the first electrode of the Einzel lens 430 with the shield 449 facing upwards toward the emission source. SE detector 4
A suppression plate 490 is disposed at a distance d above the position 50 and bends secondary electrons behind the SE detector 450.

The arrangement shown in FIG. 4 causes the suppression plate 490 to bend so that secondary electrons near the axis at the rear reach the SE detector 450 with a wider distribution, so that the detection efficiency of secondary electrons near the axis is improved. Is obtained.

FIG. 5 shows a different arrangement of a secondary electron detection system, again with a detector upstream of the Einzel lens. In this example, the SE detector 5 is located at a distance b above the deflection plate 520 which is several mm above the first electrode of the Einzel lens 530.
50 are provided. Based on the design of the particular Einzel lens and the working distance w, a distance b is selected that optimally collects electrons.

Similar to the arrangement described above, the secondary electrons 480 are strongly focused and exit the Einzel lens 530 in a wide-angle cone. Thereafter, the secondary electrons 580 pass through the deflection plate 520. A large amount of emitted secondary electrons 580 reach the active region of the secondary electron detector 550, and only secondary electrons emitted at a very small angle are not captured. Thus, the detector efficiency is increased, resulting in a good signal to noise ratio.

The arrangement shown in FIG. 5 has an advantage that a large field of view can be obtained because the driving voltage of the deflection plate 520 can be reduced for a given field of view.

FIG. 6 shows another arrangement for detecting both secondary and backscattered electrons. Secondary electrons are detected as in the method described above with reference to FIG. Primary electron 670
Backscattered electrons 680 having a cosine distribution in a wide-angle cone shape are emitted from the surface of the sample 660 at an energy close to or equal to the energy of the sample 660. Einzel lens 630 focuses backscattered electrons 680 near the plane where primary electrons 670 are emitted. However, the backscattered electrons 680 are emitted at a fairly wide angle. Backscattered electrons 6 emitted at an angle larger than the convergence angle of the primary electrons (about 0.5 °)
80 may be captured by a BSE detector 690 provided below the irradiation field limiting aperture 610. If the inner hole diameter of the BSE detector 690 is sufficiently small, that is, a few micrometers in diameter, the majority of backscattered electrons are detected in this arrangement.

A surface-sensitive detector, such as, but not limited to, a metal-semiconductor-metal (MSM) detector, a delta-doped detector, or a PN junction detector, may be included in the field aperture 610. May be captured. The MSM detector has the advantage of being easy to integrate with the Einzel lens assembly element. In a microcolumn, M
The SM detector may be used for BSE detection only with a gain of 200 to 1000.

The detector upstream of the Einzel lens arrangement has the advantage of capturing most of the secondary electrons from a relatively flat surface. However, if the holes or trenches are deep, the secondary electrons emitted at the bottom may be absorbed by the side walls. In order to make secondary electrons easily escape from such holes and trenches, an electrostatic field is required on the surface of the sample. The electrostatic field can be achieved, for example, by using the objective lens in the immersion lens mode as described above. However, the approach of using the objective in the immersion lens mode has the undesirable effect of making the sample an integral part of the objective. Therefore, if there is any surface imperfection or local surface potential on the sample, spatial resolution may be reduced.

In order to minimize the effects of surface imperfections or local surface potential on the sample surface, a sheet-like secondary electron (SE) extractor, as shown in FIG. It is arranged in a close position. SE extractor 735 includes a circular hole having a diameter _dext .
The circular hole of SE extractor 735 is aligned with the column axis. The size of the hole should be large enough that the primary beam scans the sample 760 and the SE can detect upstream away from the sample 760. For microcolumns, d _ext is typically 50-100 μm.

By applying the voltage V _ext to the SE extractor 735, an extraction electric field is formed in the sample 760 to make it easier for electrons to escape from deep holes and trenches. When the same electron V _ext is applied to the last electrode of the objective lens 730, a field-free region is formed between the objective lens 730 and the SE extractor 735. By using the arrangement shown in FIG. 7, the imperfect vision and local potential of any surface on the sample 760 is effectively shielded from the focusing effect of the objective lens 730, thereby reducing the effect of the sample on spatial resolution. Can be suppressed.

Although the present invention has been described with reference to particular embodiments, the above description is only an example of the invention's application and is not intended to be limiting. Various other adaptations and combinations of features of the embodiments disclosed are within the spirit of the invention as defined by the appended claims.

[Brief description of the drawings]

FIG. 1 (Prior Art) shows a prior art secondary electron detection system in which an SE detector is located downstream of the Einzel lens between the Einzel lens and the sample.

FIG. 2 (Prior Art) shows a prior art secondary electron detection system with an increased spacing s and working distance w.

FIG. 3 shows a secondary electron detection system in which an SE detector is arranged between the Einzel lens and the deflection plate upstream of the Einzel lens.

FIG. 4 shows a secondary electron detection system in which the SE detector is located between the Einzel lens and the suppression plate upstream of the Einzel lens.

FIG. 5 shows a secondary electron detection system in which the SE detector is located upstream of the Einzel lens between the Einzel lens and the field limiting aperture.

FIG. 6 shows a BSE detector located upstream of the Einzel lens between the field limiting aperture and the deflection system.

FIG. 7 shows an SE extractor located between the sample and the Einzel lens.

[Explanation of symbols]

 105 Electron source 110 Limited aperture 120 Deflector 130 Objective lens 150 SE detector 160 Sample 170 Primary electron 180 Secondary electron 230 Einzel lens 240 Shield 300 Micro column 365 Fixed support 490 Suppression plate 520 Deflection plate 550 Secondary electron detector 610 Irradiation field limited aperture 680 Backscattered electrons 690 BSE detector

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01J 37/12 H01J 37/12 (72) Inventor Chang, tee. , H. , P. United States, California, Milpitas, Nimitz Lane 1105 (72) Inventor Moulay, Lawrence United States of America, California, Moraga, Buckingham Drive 62 (72) Inventor Kim, Ho-Theo United States of America, California, Milpitas, Berylwood Lane 64 (72) 72) Inventors Lee, Kim, Y. Benchmark Avenue 2672 F-term (Reference) 2G001 AA03 BA07 BA15 CA03 DA01 DA06 DA09 EA04 GA01 GA06 HA13 JA11 5C033 CC01 NN01 NN02 NP01 NP05

Claims (36)

[Claims] 1. A microcolumn, comprising: an electron source; an electrostatic objective lens provided downstream of the electron source with respect to an electron beam emitted from the electron source; and an electron beam deflecting the electron beam. A microcolumn including a deflection plate and a secondary electron detector provided upstream of an objective lens with respect to an electron beam. 2. The microcolumn according to claim 1, wherein the objective lens is an equipotential Einzel lens. 3. The microcolumn according to claim 1, wherein the objective lens is an immersion lens. 4. The microcolumn according to claim 1, wherein the secondary electron detector includes a shield. 5. The microcolumn according to claim 4, wherein the secondary electron detector and the shield face downward toward a target of the electron beam. 6. The microcolumn according to claim 4, wherein the secondary electron detector and the shield face upward toward the electron source. 7. The micro column according to claim 1, wherein the secondary electron detector is a micro channel plate detector. 8. The microcolumn according to claim 7, wherein the microcolumn plate detector is a single-stage microchannel plate detector. 9. The microcolumn according to claim 7, wherein the microcolumn plate detector is a two-stage microchannel plate detector. 10. The microcolumn according to claim 1, wherein the secondary electron detector is provided between the objective lens and the deflection plate. 11. The microcolumn according to claim 1, further comprising a suppression plate provided to bend secondary electrons located rearward toward the secondary electron detector. 12. The microcolumn according to claim 11, wherein the secondary electron detector is provided between the objective lens and the suppression plate. 13. The microcolumn according to claim 12, wherein the secondary electron detector includes a shield, and the secondary electron detector and the shield face upward toward the electron source. 14. The microcolumn according to claim 1, wherein the secondary electron detector is provided between the limited aperture and the deflection plate. 15. The microcolumn according to claim 1, further comprising a backscattered electron detector provided upstream of the objective lens. 16. The microcolumn according to claim 15, wherein the backscattered electron detector is provided between the limited aperture and the deflection plate. 17. The microcolumn according to claim 16, wherein the backscattered electron detector is incorporated in the limited aperture. 18. The microcolumn according to claim 17, wherein the backscattered electron detector is a surface-sensitive detector. 19. The microcolumn according to claim 18, wherein the surface sensitive detector is a metal-semiconductor-metal detector. 20. The microcolumn according to claim 18, wherein the surface-sensitive detector is a delta-doped detector. 21. The microcolumn according to claim 1, further comprising a support for the electron beam target. 22. The microcolumn according to claim 1, further comprising a secondary electron extractor provided between the objective lens and a target of the electron beam. 23. A method for detecting secondary electrons in a microcolumn having an objective lens, the method comprising: directing an electron beam focused by the objective lens to a sample; Emitting a plurality of backscattered electrons with electrons; and detecting the secondary electrons at a position upstream of the objective lens with respect to the electron beam. 24. The method of claim 23, further comprising the step of deflecting the electron beam. 25. The method of claim 24, wherein detecting the secondary electrons comprises detecting at a location between the objective lens and the location where the deflection occurs. 26. The method of claim 25, further comprising the step of limiting the width of the electron beam. 27. The method of claim 26, further comprising detecting backscattered electrons at a location between where the limiting occurs and where the deflection occurs. 28. The method of claim 23, further comprising the step of limiting the width of said electron beam. 29. The method of claim 28, further comprising the step of deflecting the electron beam. 30. The method of claim 29, wherein detecting the secondary electrons further comprises detecting at a location between where the limiting occurs and where the deflection occurs. 31. The method of claim 29, further comprising detecting backscattered electrons at a location between where the limiting occurs and where the deflection occurs. 32. The method of claim 23, further comprising applying a bias voltage to said objective lens. 33. The method of claim 23, further comprising the step of bending the secondary electrons in the direction of the location. 34. The method further comprises: disposing a secondary electron extractor between the objective lens and the sample; and forming a non-electric field region between the objective lens and the secondary electron extractor. A method according to claim 23. 35. The step of forming an electric field-free region, forming an extraction electric field in the sample by applying a first voltage to the extractor, and applying a second voltage to a last electrode of the objective lens. 35. The method of claim 34, further comprising applying. 36. The method according to claim 35, wherein the first voltage is equal to the second voltage.