Patent Description:
A scanning electron microscope is an instrument for imaging a sample by scanning it with an electron beam and detecting secondary electrons or backscattered electrons emanating from the sample. A scanning electron microscope generally has its interior maintained in a high vacuum. Low-vacuum scanning electron microscopes have attracted attention in which a sample can be observed in a sample chamber which is placed in a low vacuum environment while keeping the electron gun and electron optical system in a high vacuum environment as set forth, for example, in patent document <NUM>. In such a low-vacuum scanning electron microscope, various samples including insulators, aqueous samples, and samples giving off large amounts of gas can be observed.

Objective lenses for use in scanning electron microscopes come in various types. For example, patent document <NUM> discloses a semi-in-lens objective lens having inner and outer polepieces. A sample is placed in a magnetic field which leaks from between the inner and outer polepieces. In the semi-in-lens objective lens, electrons released from the sample are absorbed up into the objective lens and guided upwards along the optical axis by the magnetic field leaking from the objective lens. Therefore, in the objective lens of this type, electrons released from the sample are detected by a detector inserted in a through-hole extending through the objective lens.

Patent document <NUM> discloses a scanning electron microscope wherein the secondary electrons are guided upward by electric field of an accelerating electrode into the objective lens. The secondary electrons entering the objective lens are prevented by an electric field formed by a decelerating electrode from proceeding to the top part of the objective lens.

However, in the scanning electron microscope equipped with a semi-in-lens objective lens, the objective lens is provided with the through-hole as described above. Therefore, the interior space of the objective lens is in communication with the sample chamber via the through-hole. Accordingly, when the sample chamber is evacuated to a low vacuum, even the electron gun chamber and the electron beam path inside of the objective lens which need a high vacuum are placed in a low vacuum environment.

One aspect of a scanning electron microscope associated with the present invention comprises: an electron gun for emitting an electron beam; an objective lens for focusing the emitted electron beam onto a sample; and a sample chamber in which the sample is housed. The objective lens includes: an inner polepiece; an outer polepiece disposed outside the inner polepiece and facing the sample chamber; at least one through-hole extending through the inner and outer polepieces; and at least one cover member that closes off the through-hole. The objective lens causes leakage of magnetic field from an aperture between the inner and outer polepieces toward the sample. The sample chamber has a degree of vacuum lower than that in an inner space that forms an electron beam path inside the inner polepiece. One opening of the through-hole is formed in the inner surface of the inner polepiece, the inner surface defining an inner space through which the electron beam passes, and the other opening of the through-hole is formed in the outer surface of the outer polepiece, the outer surface facing the sample chamber. Electrodes are mounted in the inner space, and wherein said cover member has voltage supply terminals for supplying voltages to the electrodes.

In this scanning electron microscope, the through-hole going through the inner and outer magnetic polepieces is plugged up by the cover member and, therefore, if the degree of vacuum in the sample chamber is made lower than that in the space inside the inner polepiece, this inner space can be maintained in a high vacuum. Consequently, the sample can be observed while placing the sample chamber in a low vacuum.

One aspect of an objective lens associated with the present invention is for use in a scanning electron microscope and comprises an inner polepiece, an outer polepiece disposed outside the inner polepiece, at least one through-hole extending through the inner and outer polepieces, and at least one cover member that closes off the through-hole. There is leakage of magnetic field toward a sample from an aperture between the inner and outer polepieces. One opening of the through-hole is formed in the inner surface of the inner polepiece the inner surface defining an inner space through which the electron beam is arranged to pass, The other opening of the through-hole is formed in the outer surface of the outer polepiece. Electrodes are mounted in the inner space and the cover member has voltage supply terminals for supplying voltages to the electrodes. In this objective lens, the through-hole extending through the inner and outer polepieces is closed off by the cover member and, therefore, if the degree of vacuum in the sample chamber is made lower than that in the space inside of the inner polepiece, this inner space can be maintained in a high vacuum. Consequently, the sample can be observed while placing the sample chamber in a low vacuum.

The preferred embodiments of the present invention are hereinafter described in detail with reference to the accompanying drawings. It is to be noted that the preferred embodiments described below are not intended to unduly restrict the scope and spirit of the present invention delineated by the appended claims and that not all the configurations described below are the essential constituent components of the present invention.

A scanning electron microscope associated with one embodiment of the present invention is first described by referring to <FIG>, which shows the configuration of this scanning electron microscope, <NUM>. The microscope <NUM> is an apparatus for imaging a sample S by scanning it with an electron beam (electron probe) and detecting secondary electrons or backscattered electrons emanating from the sample S. As shown, the electron microscope <NUM> includes an electron optical column <NUM>, a sample chamber <NUM>, an electron gun <NUM>, condenser lenses <NUM>, scan coils <NUM>, an objective lens <NUM>, a detector <NUM>, a tertiary electron generator <NUM>, valves <NUM>, <NUM>, <NUM>, a vacuum pumping system <NUM>, and vacuum pumping equipment <NUM>.

The electron gun <NUM> emits the electron beam, for example, by accelerating electrons, which are released from a cathode, by means of an anode. The electron gun <NUM> is housed in an electron gun chamber <NUM> that is present within the electron optical column <NUM>.

The condenser lenses <NUM> condense the electron beam released from the electron gun <NUM> and can control the diameter and current of the electron probe. The objective lens <NUM> focuses the electron beam into the electron probe. The objective lens <NUM> will be described in detail later.

The electron probe formed by the condenser lenses <NUM> and the objective lens <NUM> is deflected in two dimensions by the scan coils <NUM>. Thus, the sample S can be scanned with the electron probe.

The condenser lenses <NUM>, scan coils <NUM>, and objective lens <NUM> together constitute an electron optical system operating to form the electron probe with which the sample S is scanned. The electron optical system may include lenses, apertures, and other optical elements, as well as the condenser lenses <NUM>, scan coils <NUM>, and objective lens <NUM>.

The detector <NUM> detects electrons produced from the sample S in response to irradiation with the electron beam. For example, the detector <NUM> is a secondary electron detector for detecting secondary electrons produced from the sample S. The detector <NUM> includes, for example, a corona ring to which a high voltage is applied, a scintillator on which electrons impinge, and a photomultiplier tube for converting light emerging from the scintillator into an electron signal.

The tertiary electron generator <NUM> includes a metal plate against which the electrons released from the sample S collide. In the scanning electron microscope <NUM>, high-energy electrons (secondary electrons) released from the sample S by irradiation with an electron beam (primary electrons) are caused to collide against the metal plate, producing tertiary electrons which can be detected by the detector <NUM>.

The sample S is received in the sample chamber <NUM> with which the valves <NUM>, <NUM>, and <NUM> are connected. The valve <NUM> is connected with the vacuum pumping system <NUM> via an exhaust tube <NUM>. The sample chamber <NUM> is evacuated to a vacuum by opening the valve <NUM>. A space including the electron gun chamber <NUM> and a space forming an electron beam path in the electron optical column <NUM> as well as the sample chamber <NUM> can be evacuated to a high vacuum by the vacuum pumping system <NUM>. The valve <NUM> is connected with the vacuum pumping equipment <NUM> which consists, for example, of a rotary pump. The valve <NUM> is a leak valve for introducing gas into the sample chamber <NUM>. The degree of vacuum (pressure) in the sample chamber <NUM> can be regulated to a desired level by adjusting the degree of opening of the leak valve <NUM>.

In the scanning electron microscope <NUM>, the internal pressure of the sample chamber <NUM> is set to a low vacuum of from on the order of Pa to hundreds of Pa, and the sample S can be observed. When the sample chamber <NUM> is evacuated to a low vacuum, the valve <NUM> is closed and the valves <NUM> and <NUM> are controlled.

<FIG> is a schematic cross-sectional view of the objective lens <NUM>. <FIG> is a schematic view of the objective lens <NUM> as viewed from the sample chamber side. As shown in <FIG> and <FIG>, the objective lens <NUM> includes an inner polepiece <NUM>, an outer polepiece <NUM>, a coil <NUM>, a first cover member 410a, a second cover member 410b, a third cover member 410c, and a fourth cover member 410d. The objective lens <NUM> is of the semi-in-lens type and causes leakage of magnetic field toward the sample S from an opening <NUM> between the inner polepiece <NUM> and the outer polepiece <NUM>.

The inner polepiece <NUM> is disposed inside the outer polepiece <NUM>. A space <NUM> inside of the inner polepiece <NUM> forms an electron beam path. The objective lens <NUM> has an optical axis OA passing through the inner space <NUM>. The electron beam passes through the inner space <NUM> along the optical axis OA and impinges on the sample S. The outer polepiece <NUM> is disposed outside the inner polepiece <NUM> and has an outside surface <NUM> that faces the sample chamber <NUM>.

The inner magnetic polepiece <NUM> and the outer magnetic polepiece <NUM> together create a magnetic path. The inner and outer polepieces <NUM>, <NUM> cooperate to confine lines of magnetic force created by the coil <NUM> and to cause leakage of magnetic field toward the sample S from the opening or aperture <NUM> between the inner polepiece <NUM> and the outer polepiece <NUM>. Therefore, in the scanning electron microscope <NUM>, the sample S is placed in the magnetic field leaking from the objective lens <NUM>.

The coil <NUM> is disposed in a space surrounded by the inner polepiece <NUM> and outer polepiece <NUM> and produces a magnetic field. The coil <NUM> is wound along a circle centered at the optical axis OA. When the coil <NUM> is electrically energized, magnetic fluxes are created around the inner polepiece <NUM> and outer polepiece <NUM>, causing leakage of magnetic field from the opening <NUM>.

An acceleration electrode <NUM> is mounted at the front end of the objective lens <NUM>. The acceleration electrode <NUM> is attached to the front end of the inner polepiece <NUM> via an insulation layer (not shown) and provided with a through-hole for passage of the electron beam.

The acceleration electrode <NUM> produces an electric field such that electrons emitted from the sample S are thereby urged into the inner space <NUM>. In the scanning electron microscope <NUM>, the electrons emitted from the sample S are constrained in the magnetic field leaking from the objective lens <NUM> and guided into the inner space <NUM> through the through-hole of the acceleration electrode <NUM>, i.e., guided by the acceleration electrode <NUM>.

The acceleration electrode <NUM> is disposed between the inner space <NUM> and the sample chamber <NUM> and functions also as an orifice for maintaining a differential pressure between the inner space <NUM> and the sample chamber <NUM>. Therefore, the sample chamber <NUM> can be evacuated to a low vacuum while maintaining the inner space <NUM> in a high vacuum.

A deceleration electrode <NUM> is disposed in the inner space <NUM> and placed above the detector <NUM>, i.e., closer to the electron gun <NUM>. The deceleration electrode <NUM> prevents the electrons guided into the inner space <NUM> by the acceleration electrode <NUM> from traveling upwardly. In the scanning electron microscope <NUM>, the electrons released from the sample S are guided to the detector <NUM> by the acceleration electrode <NUM> and deceleration electrode <NUM>. There may be other electrodes (not shown) in the inner space <NUM> such that the electrons emitted from the sample S are guided to the detector <NUM>.

The objective lens <NUM> is provided with a plurality of through-holes extending through both inner polepiece <NUM> and outer polepiece <NUM> as shown in <FIG>. In the illustrated example, the through-holes are a first through-hole 408a, a second through-hole 408b, a third through-hole 408c, and a fourth through-hole 408d.

The first through-hole 408a interconnects an inner surface <NUM> of the inner polepiece <NUM> and the outer surface <NUM> of the outer polepiece <NUM>. That is, one opening of the first through-hole 408a is formed in the inner surface <NUM> of the inner polepiece <NUM>. The other opening of the through-hole 408a is formed in the outer surface <NUM> of the outer polepiece <NUM>. The inner surface <NUM> of the inner polepiece <NUM> defines the inner space <NUM> through which the electron beam passes. The outer surface <NUM> of the outer polepiece <NUM> faces the sample chamber <NUM>.

Similarly, the second through-hole 408b, third through-hole 408c, and fourth through-hole 408d interconnect the inner surface <NUM> of the inner polepiece <NUM> and the outer surface <NUM> of the outer polepiece <NUM>. The four through-holes 408a, 408b, 408c, and 408d are arranged symmetrically with respect to the optical axis OA and angularly spaced <NUM> degrees from each other about the optical axis OA.

The first through-hole 408a has a central axis which is orthogonal, for example, to the optical axis OA. Similarly, the second, third, and fourth through-holes 408b, 408c, 408d have their respective central axes orthogonal to the optical axis OA. Note that it is not essential that the central axis of the first through-hole 408a be orthogonal to the optical axis OA.

The detector <NUM> is inserted in the first through-hole 408a. The tertiary electron generator <NUM> is inserted in the second through-hole 408b. Voltage supply terminals <NUM> are inserted in the third through-hole 408c. Other voltage supply terminals <NUM> are inserted in the fourth through-hole 408d. The supply terminals <NUM> and <NUM> are used to supply voltages to the acceleration electrode <NUM>, deceleration electrode <NUM>, and other electrodes placed in the inner space <NUM>.

The first through-hole 408a is plugged up by the first cover member 410a, which in turn closes off the opening of the first through-hole 408a that is formed in the outer surface <NUM> of the outer polepiece <NUM>. The first cover member 410a is provided with an insertion hole extending therethrough. The detector <NUM> is inserted in the insertion hole. The first cover member 410a plugs up the gap between the detector <NUM> and the outer polepiece <NUM>.

The second through-hole 408b is plugged up by the second cover member 410b, which in turn closes off the opening of the second through-hole 408b that is formed in the outer surface <NUM> of the outer polepiece <NUM>. The second cover member 410b is provided with an insertion hole extending therethrough. The tertiary electron generator <NUM> is inserted in the insertion hole. The second cover member 410b plugs up the gap between the tertiary electron generator <NUM> and the outer polepiece <NUM>.

The third through-hole 408c is plugged up by the third cover member 410c, which in turn closes off the opening of the third through-hole 408c that is formed in the outer surface <NUM> of the outer polepiece <NUM>. The third cover member 410c has terminals <NUM> capable of supplying voltages to the electrodes in the inner space <NUM> from outside while maintaining the airtightness of the inner space <NUM>. Cables (not shown) which interconnect the terminals <NUM> and electrodes are connected to the electrodes through the third through-hole 408c.

The fourth through-hole 408d is plugged up by the fourth cover member 410d, which in turn closes off the opening of the fourth through-hole 408d that is formed in the outer surface <NUM> of the outer polepiece <NUM>. The fourth cover member 410d has terminals <NUM> capable of supplying voltages to the electrodes in the inner space <NUM> from outside while maintaining the airtightness of the inner space <NUM>. Other cables (not shown) which interconnect the terminals <NUM> and electrodes are connected to the electrodes through the fourth through-hole 408d.

The first cover member 410a is made of a nonmagnetic material (such as resin, e.g., Duracon (TM)). The second cover member 410b, third cover member 410c, and fourth cover member 410d are made of the same material as that of the first cover member 410a.

In the foregoing description, there are four through-holes in the objective lens <NUM>. No restriction is placed on the number of the through-holes formed in the objective lens <NUM>.

In the scanning electron microscope <NUM>, SEM images of the sample S can be obtained while evacuating the sample chamber <NUM> to a low vacuum. In the following description, secondary electrons emitted from the sample S are detected by the detector <NUM>.

In the scanning electron microscope <NUM>, a space forming an electron beam path inside the electron optical column <NUM> which includes the electron gun chamber <NUM> and the space <NUM> inside of the objective lens <NUM> is evacuated to a vacuum by the vacuum pumping system <NUM> via the exhaust tube <NUM>. Therefore, the space inside the electron optical column <NUM> is maintained in a high vacuum. The pressure in the space within the electron optical column <NUM> is approximately <NUM> × <NUM>-<NUM> Pa, for example.

Under this condition, by closing the valve <NUM> connected to the exhaust tube <NUM> and controlling the valves <NUM> and <NUM>, the degree of vacuum or pressure in the sample chamber <NUM> is adjusted or evacuated to a low vacuum. For example, the valve <NUM> is opened, and the degree of opening of the valve <NUM> is adjusted while evacuating the sample chamber <NUM> to a vacuum by means of the vacuum pumping equipment <NUM>. Air or inert gas is admitted into the sample chamber <NUM>, and the degree of vacuum in the sample chamber <NUM> is controlled. Consequently, the degree of vacuum in the sample chamber <NUM> is made lower than that in the inner space of the electron optical column <NUM>. The pressure in the sample chamber <NUM> is from on the order of Pa to hundreds of Pa, for example.

The acceleration electrode <NUM> serving as an orifice is disposed between the inner space <NUM> and the sample chamber <NUM>. This makes it possible to maintain the differential pressure between the inner space <NUM> and the sample chamber <NUM> while passing the electron beam. Consequently, the sample chamber <NUM> can be maintained in a low vacuum.

An electron beam is emitted from the electron gun <NUM> while placing the interior space of the electron optical column <NUM> in a high vacuum and at the same time placing the sample chamber <NUM> in a low vacuum. An electron probe is formed by the condenser lenses <NUM> and objective lens <NUM>. The sample S is scanned with the electron probe by deflecting the electron beam using the scan coils <NUM>. The electron beam emanating from the electron gun <NUM> received in the electron gun chamber <NUM> passes through both the inner space <NUM> of the objective lens <NUM> and the aperture in the acceleration electrode <NUM> and impinges on the sample S.

<FIG> schematically illustrates the manner in which secondary electrons SE emitted from the sample S are detected by the detector <NUM>. The irradiation of the sample S with the electron beam emits the secondary electrons SE from the sample S. The secondary electrons SE are collected on the optical axis OA by a magnetic field M created by the objective lens <NUM>. The acceleration electrode <NUM> produces an electric field E1 which causes the collected electrons SE on the optical axis OA to pass into the inner space <NUM> of the objective lens <NUM> through the orifice in the acceleration electrode <NUM>. In the inner space <NUM>, the secondary electrons SE are hindered from traveling upwards by the electric field E2 set up by the deceleration electrode <NUM>.

The detector <NUM> has a corona ring which produces an electric field E3. In the objective lens <NUM>, the secondary electrons SE are attracted to the electric field E3 and collide against the scintillator, producing light which is converted into an electrical signal by a photomultiplier tube. In this way, the secondary electrons SE produced from the sample S can be detected. An SEM image can be derived by scanning the sample S with the electron probe and detecting the intensity of secondary electrons at each beam impact point.

In the scanning electron microscope <NUM>, the objective lens <NUM> includes the inner polepiece <NUM>, the outer polepiece <NUM> disposed outside the inner polepiece <NUM> and facing the sample chamber <NUM>, the first through-hole 408a extending through both inner polepiece <NUM> and outer polepiece <NUM>, and the first cover member 410a plugging up the first through-hole 408a. The objective lens <NUM> causes leakage of the magnetic field M from the opening <NUM> between the inner polepiece <NUM> and the outer polepiece <NUM> toward the sample S. The degree of vacuum in the sample chamber <NUM> is lower than that in the inner space <NUM> inside of the inner polepiece <NUM> which forms an electron beam path.

In this way, in the scanning electron microscope <NUM>, the first through-pole 408a running through both the inner polepiece <NUM> and outer polepiece <NUM> is plugged up by the first cover member 410a and, therefore, if the degree of vacuum in the sample chamber <NUM> is made lower than that in the inner space <NUM>, the inner space <NUM> can be maintained in a high vacuum. Consequently, the sample S can be observed under conditions where the sample chamber <NUM> is placed in a low vacuum and where the inner space <NUM> and the interior space of the electron optical column <NUM> including the electron gun chamber <NUM> are placed in a high vacuum.

The scanning electron microscope <NUM> includes the orifice formed by the acceleration electrode <NUM> mounted between the inner space <NUM> and the sample chamber <NUM>. Therefore, in the microscope <NUM>, the differential pressure between the inner space <NUM> and the sample chamber <NUM> can be maintained while passing the electron beam.

The scanning electron microscope <NUM> includes the detector <NUM> disposed in the first through-hole 408a. The first cover member 410a is provided with the insertion hole in which the detector <NUM> is inserted. Therefore, in the scanning electron microscope <NUM>, the detector <NUM> can be placed in the inner space <NUM> while maintaining the differential pressure between the inner space <NUM> and the sample chamber <NUM>.

In the scanning electron microscope <NUM>, the objective lens <NUM> is provided with the plurality of through-holes which are arranged symmetrically with respect to the optical axis OA. In the example shown in <FIG>, there are four through-holes which are arranged at angular intervals of <NUM> degrees about the optical axis OA. Therefore, in the scanning electron microscope <NUM>, the presence of the through-holes can reduce the effect on the electron beam passing through the objective lens <NUM>. As an example, if plural through-holes are arranged asymmetrically, the asymmetry of the objective lens <NUM> may produce astigmatism. Symmetrical arrangement of a plurality of through-holes can produce reduced astigmatism.

In the scanning electron microscope <NUM>, the first cover member 410a is made of a resinous material and, therefore, has no magnetic effect on the electrons released from the sample S. The objective lens <NUM> of the scanning electron microscope <NUM> is of the semi-in-lens type and causes leakage of magnetic field toward the sample S from the opening <NUM> between the inner polepiece <NUM> and the outer polepiece <NUM>. The electrons released from the sample S are constrained by the magnetic field M leaking from the opening <NUM> of the objective lens <NUM>. Therefore, if the first cover member 410a is made of a resin, the electrons released from the sample S do not electrostatically charge the first cover member 410a. Consequently, in the scanning electron microscope <NUM>, a normal SEM image can be obtained while maintaining the differential pressure between the inner space <NUM> and the sample chamber <NUM>. If the first cover member 410a were electrostatically charged, the orbit of the electron beam would be bent, hindering normal SEM imaging.

The effect produced by making the first cover member 410a from a resin has been described. Where the second, third, and fourth cover members 410b, 410c, 410d are made of a resin, the same effect arises.

In the scanning electron microscope <NUM>, the deceleration electrode <NUM> is mounted in the inner space <NUM>. The third cover member 410c is provided with the terminals <NUM> for supplying a voltage to the deceleration electrode <NUM>. Therefore, in the scanning electron microscope <NUM>, a voltage can be supplied to the deceleration electrode <NUM> while maintaining the differential pressure between the inner space <NUM> and the sample chamber <NUM>.

<FIG> shows a modification of the configuration of the scanning electron microscope <NUM>. In the scanning electron microscope of <FIG>, members which are similar in function to their respective counterparts of the above-described scanning electron microscope <NUM> shown in <FIG> are indicated by the same reference numerals as in the above-referenced figures and a detailed description thereof is omitted.

Claim 1:
An objective lens (<NUM>) for use in a scanning electron microscope (<NUM>), comprising:
an inner polepiece (<NUM>);
an outer polepiece (<NUM>) disposed outside the inner polepiece (<NUM>);
at least one through-hole (408a, 408b, 408c, 408d) extending through the inner (<NUM>) and outer polepieces (<NUM>);
at least one cover member (410a, 410b, 410c, 410d) that closes off the through-hole (408a, 408b, 408c, 408d); and
an aperture (<NUM>) which is formed between the inner polepiece (<NUM>) and the outer polepiece (<NUM>) and through which there is leakage of magnetic field toward a sample (S), wherein one opening of the through-hole (408a, 408b, 408c, 408d) is formed in the inner surface (<NUM>) of the inner polepiece (<NUM>), the inner surface (<NUM>) defining an inner space (<NUM>) through which the electron beam is arranged to pass, and the other opening of the through-hole (408a, 408b, 408c, 408d) is formed in the outer surface (<NUM>) of the outer polepiece (<NUM>),
wherein electrodes (<NUM>, <NUM>) are mounted in the inner space (<NUM>), characterized in that said cover member (<NUM>, <NUM>) has voltage supply terminals (<NUM>, <NUM>) for supplying voltages to the electrodes (<NUM>, <NUM>).