# Patent

## Title
Charged particle beam microscope with minicolumn

## Abstract

One embodiment of the present invention is an electron microscope that includes: (a) a main vacuum chamber housing a stage therein and connected to a vacuum pump; (b) a load lock for loading a specimen into said main vacuum chamber; (c) a minicolumn non-translatably positioned inside said main chamber; and (d) a vacuum pump situated inside the main vacuum chamber and external to and connected to the minicolumn.

## Background
This is a continuation of a patent application entitled “Charged Particle Beam Microscope with Minicolumn” having Ser. No. 09/162,103 that was filed on Sep. 28, 1998 now U.S. Pat. No. 6,740,889.


TECHNICAL FIELD OF THE INVENTION
The invention relates to charged particle beam microscopes and, particularly, to arrangements for equipping such a microscope with a minicolumn.
BACKGROUND OF THE INVENTION
Charged particle beam microscopes, such as an electron microscope, are well known in the art, and are widely used during the manufacture of semiconductor wafers. For ease of discussion, the remaining disclosure makes reference to electrons as the charged particles; however, it should be appreciated that the discussion is equally applicable to other charged particles. The elements of a conventional electron microscope which are of particular relevance here are depicted in FIG. 1. Specifically, a vacuum chamber 10 houses an x-y stage 20 upon which the wafer 40 is placed by a robot (not shown). The chamber 10 is evacuated via outlet 70. The wafer 40 is introduced into the chamber 10 via a load lock 30 so as to avoid having to evacuate the chamber 10 each time a wafer is loaded
An electron column 50 is hermetically attached to the chamber 10. The column 50 houses the electron source and all the necessary electron optics (not shown). The column 50 is evacuated via outlet 60. The diameter of a conventional column is roughly 6–10 inches, while its height is roughly 15–30 inches. The conventional column is capable of providing an electron beam of sufficiently small diameter for wafer and reticle inspection, review and metrology.
One disadvantage of the prior art design is that whenever the column requires a repair which necessitates its removal from the chamber or breaking the vacuum in the column, the vacuum of the chamber is also broken. Breaking the vacuum in the chamber necessarily means that the microscope will be out of service for several hours. Another disadvantage is the requirement for separate vacuum systems for the column and the chamber, which increases the complexity and price of the system, while adversely affecting its reliability and stability.
Recently, a new type of column has been developed, and is generally referred to as a “minicolumn.” A cross section of a minicolumn investigated by the current inventors is depicted in FIG. 2. In FIG. 2, element 200 is the electron source (preferably a shottky emitter), 210 is an aperture (suppressor), and 220 generally designates the lens arrangement. More specifically, lens arrangement 220 comprises three lenses 230 made of conductive material and insulating spacers 240 interposed between the lenses 230. Ordered from the emitter, the lenses 230 comprise an extraction lens, a focusing lens, and an acceleration lens, respectively.
Notably, the diameter and height of such a column is measured in single-digit centimeters. More specifically, the diameter of the lens arrangement depicted in FIG. 2 is on the order of 3 centimeters, while its height is on the order of 1 centimeter. While this column is remarkably smaller than the conventional column, it provides an electron beam which has small diameter and was determined by the present inventors to be suitable for use in electron microscopes. Further information regarding the study of the minicolumn is presented in an article entitled “Novel high brightness miniature electron gun for high current e-beam applications” by F. Burstert, D. Winkler and B. Lischke, Microelectronic Engineering 31 (1996) pp. 95–100; and in an article entitled “Miniature electrostatic lens for generation of a low-voltage high current electron probe,” by C.-D. Bubeck, A. Fleischmann, G. Knell, R. Y. Lutsch, E. Plies and D. Winkler, Proceedings of the Charged Particle Optics Conference, Apr. 14–17, 1998.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides arrangements for installing minicolumns onto charged particle microscopes, especially electron microscopes, while providing synergistic advantages over prior art column arrangements. That is, the disclosed arrangements provide advantages in addition to the advantages of the minicolumn per se.
According to one set of embodiments of the invention, a second load lock is provided on the microscope's vacuum chamber. The second load lock is used to introduce the minicolumn into the chamber without having to break the vacuum in the chamber. Thus, a technician can replace the minicolumn without having to break the vacuum in the chamber.
According to another set of embodiments, the minicolumn is situated inside the microscope's vacuum chamber. While this arrangement necessitates breaking the vacuum for each minicolumn service, it is still advantageous in that there is no need for a separate vacuum system for the column. This is advantageous especially if more than one minicolumn is used inside the chamber.
Another advantage of the invention is that it provides arrangements for more than one minicolumn per microscope. Such arrangements are especially advantageous for taking multiple perspectives simultaneously or increasing the throughput.



BRIEF DESCRIPTION OF THE FIGURE
FIG. 1 depicts relevant elements of a conventional electron microscope.
FIG. 2 depicts a lens arrangement of a minicolumn investigated by the present inventors.
FIG. 3A depicts an embodiment of an isolation valve arrangement in a closed position, while FIG. 3B depicts the arrangement of FIG. 3A in an open position.
FIG. 4A depicts another embodiment of an isolation valve in a closed position, and FIG. 4B depicts the embodiment of FIG. 4A in an open position.
FIG. 5A depicts an embodiment of a minicolumn within the microscope chamber, while FIG. 5B depicts an embodiment of a plurality of minicolumns arranged inside the chamber at different angles.
FIGS. 6A and 6B depict embodiments using a turntable stage for reduced footprint.
FIG. 7A depicts an arrangement of an arm having a plurality of minicolumns situated at different angles that is advantageous for defect review, while FIG. 7B depicts an arm having a plurality of minicolumns at a single angle that is advantageous for sector-wise inspection.



DETAILED DESCRIPTION
FIGS. 3A and 3B depict a first embodiment of an isolation valve for the minicolumn 300. Specifically, the vacuum chamber 310 is equipped with a valve 320 capable of hermetically sealing chamber 320 in the closed position. Minicolumn 300 is position inside a mini-environment chamber 330, which can be evacuated via outlet 335. In the exemplified embodiment, the mini-environment chamber 330 has collapsible walls 340, which are actuated by bellows 345. However, it should be appreciated that other solutions having rigid walls with means for elevating and lowering the columns are also workable.
During maintenance, the mini-environment chamber 330 is in its closed position. In the closed position, exemplified in FIG. 3A, the bellows 345 are extended so as to raise the walls 340 to an upright position. In this upright position, the minicolumn is extruded from the chamber 310 and valve 320 is closed to maintain the vacuum level inside the chamber 310. When maintenance is completed, the mini-environment chamber 330 can be evacuated via outlet 335 and, when the evacuation is completed, the valve 320 can be opened and the minicolumn lowered to the chamber 310.
Specifically, FIG. 3B exemplifies the situation during operation of the microscope. When the mini-environment chamber 330 has been evacuated and the valve 320 opened, the bellows collapse the walls 340 so as to introduce the minicolumn into the chamber 310, close to the stage 315. Unless the minicolumn malfunctions, there is no need to revert to the position shown in FIG. 3A, and the microscope can be maintained in the position shown in FIG. 3B. However, if the minicolumn requires maintenance or replacement, the bellows 345 are extended to raise the walls 340 and extrude the minicolumn 300 from chamber 310; the valve 320 is closed; and the mini-environment chamber 330 is brought to atmospheric pressure via outlet 335.
Another embodiment for isolation valve is depicted in FIGS. 4A and 4B. Minicolumn 400 is situated inside a mini-environment chamber 430 that is open at its bottom to chamber 410. Mini-environment chamber 430 has an outlet 435 which, in this example, is connected to the outlet 445 via vacuum valve 440. Thus, mini-environment chamber 430 and chamber 410 can be connected to the same vacuum pump (not shown). However, it should be appreciated that outlet 435 can be connected independently to a separate vacuum pump. Isolation valve 450 is pivoted on shaft 455, which is capable of elevation motion, i.e., in the Z direction.
During operation (depicted in FIG. 4B), isolation valve 450 is swiveled away from the opening of mini-environment chamber 430, and the shaft 455 is in its uppermost position so as to place the isolation valve 450 out of the working area of the microscope. Preferably, stage 415 is equipped with actuators for Z motion so that during operation the distance between the minicolumn and the specimen can be adjusted for proper imaging. Such stages are well known in the art and will not be described here. If the outlet arrangement depicted in FIG. 3B is used, then during operation valve 440 can be maintained open so that vacuum pump operation maintains vacuum in both chambers 410 and 430.
When access to the minicolumn is required, the stage is lowered and the valve 450 is brought to its closed position. For that operation, preferably the shaft 455 is lowered to its lowest position, the valve 450 is swiveled to its close position and the shaft 455 is elevated sufficiently to cause a hermetic seal between the valve 450 and the opening of the mini-environment chamber 430. Then valve 440 can be moved to the open position so that mini-environment chamber 430 is bought to atmospheric pressure. Then the back plate 460 can be removed for access to the minicolumn. Preferably, the minicolumn itself is secured to the back plate 460 so that it is removed together with the back plate 460.
FIG. 5A depicts an arrangement of a minicolumn enclosed within the microscope chamber. Specifically, minicolumn 500 is positioned completely inside the chamber 510 so that no separate evacuation is necessary for the minicolumn 500. Preferably, the stage 515 is capable of elevation motion to control the distance between the minicolumn 500 and the specimen. Here again, it is preferred that the minicolumn be attached to a back plate 560, so that removal of the back plate 560 would remove the minicolumn 500 as well. Such an arrangement is particularly useful for metrology, such as for critical dimension (CD) measurement microscopes. Also exemplified in FIG. 5A is an in-chamber integrated vacuum pump, which controls the vacuum inside the minicolumn 500.
Electron microscopes can also be used for review of locations on wafers which are suspected of having defects thereupon. In such application, it is particularly useful to be able to scan the suspect area at different angles. A particularly elegant way of doing so using a conventional column is described in U.S. Pat. No. 5,329,125 to Feuerbaum. In that patent, a system is disclosed which is capable of placing the column at any tilt between 0–45 degrees, without breaking the vacuum inside the column or the microscope chamber. Thus, one can take a picture at 0 tilt, and then tilt the column to a desired position and take another picture for added information. Notably, pictures taken at a tilt tend to have more topographic information than those taken without tilt.
FIG. 5B exemplifies a system having a plurality of minicolumns, and particularly suitable for an electron microscope review station. As shown in the Figure, a first minicolumn 500 is situated inside the chamber at zero tilt. A second minicolumn 520 is positioned at a first tilt θ and a third minicolumn is positioned at a second tilt φ. In the preferred embodiment, the tilt angles θ and φ are fixed and different from each other. Preferably, the tilts are fixed at 30 and 60 degrees, respectively, or 30 and 45 degrees, respectively. However, as shown in FIG. 5B, the tilts can be variable by, for example, pivoting the columns 520 and 525 about pivots 530 and 535, respectively.
It is well known that chamber size directly affects the quality of the vacuum maintained within the chamber, and consequently, can affect the reliability and “cleanliness” of the equipment. Additionally, large chambers require large footprint, which is of paramount consideration for fabrication plants, wherein clean room real estate is at a premium. However, x-y stages generally require large chambers since they require motion space that is at least twice the size of the largest specimen to be inspected. Considering that the semiconductor industry is moving towards 300 mm wafers, an x-y stage for such wafers can dictate a very large footprint.
FIGS. 6A and 6B depict embodiments which are particularly advantageous for reducing the footprint of the microscope. Specifically, FIGS. 6A and 6B depict a minicolumn 600 attached to an arm 620, which is situated inside the chamber 610. Rather than a x-y stage, a turntable stage 615 is used. In FIG. 6A, the arm 620 is pivoted about pivot 625, while in FIG. 6B the arm is attached to a linear carriage 635. In both cases, the arms 620 are capable of moving the minicolumn 600 through the entire radius of the stage 615. Through the rotational motion of the turntable stage 615, and the motion of the arm 630 (whether radial or linear), every location on the specimen can be reached in polar (r,θ) coordinates.
As noted above, it is desirable to be able to obtain images of the same spot using tilt. FIG. 7A depicts a turntable arrangement similar to that depicted in FIG. 6B, except that the arm 720 carries two minicolumns 700 and 705. In the exemplified embodiment, minicolumn 700 is situated with zero tilt, while minicolumn 705 is situated with a fixed tilt, preferably of 30 or 45 degrees. However, it should be appreciated that more than two minicolumns can be provided, and that the tilt can be variable rather than fixed.
It is also well known to use electron microscopes to inspect wafers and reticles for defects. An exemplary system is disclosed in U.S. Pat. No. 5,502,306 to Meisburger et al. That system uses a single conventional column to scan the entire wafer/reticle for defects. The system is sold under the name of SEMSpec by KLA of San Jose Calif. and is known to have a very slow throughput.
FIG. 7B depicts an arm 740 structured to support a plurality of columns 745 positioned with zero tilt. The arm 740 is attached to a linear carriage 755. Such an arm can be installed in a chamber having a turntable stage for inspecting an entire wafer for defects. Specifically, the wafer is divided into concentric sectors corresponding to the number of minicolumns 745 attached to arm 740. Thus, as the wafer is rotated, the carriage 755 need travel only a length equal to the radial length of one sector. During such motion, each minicolumn 745 would scan its corresponding sector, thereby covering the entire wafer. Of course, a small overlap may be provided to ensure complete coverage.
While the invention has been described with reference to particular embodiments thereof, it should be appreciated that other embodiments and modification can be implemented without departing from the spirit and scope of the invention as defined by the appended claims.

## Claims

1. An electron microscope, comprising:
a main vacuum chamber housing a stage therein and connected to a vacuum pump;
a load lock for loading a specimen into said main vacuum chamber;
a minicolumn non-translatably positioned inside said main chamber; and
a vacuum pump situated inside the main vacuum chamber and external to and connected to the minicolumn.

2. An electron microscope comprising:
a main vacuum chamber connected to a vacuum pump and housing;
a turntable stage for holding a semiconductor wafer thereon; and
a plurality of minicolumns;
wherein imaging of every point on the wafer by the minicolumns is enabled by the turntable rotating the wafer.

3. The electron microscope of claim 2, wherein each of the minicolumns comprises a plurality of lenses, all of which are electrostatic.