Cold-field-emitter electron gun with self-cleaning extractor using reversed e-beam current

An e-beam device includes a cold-field emission source to emit electrons and an extractor electrode to be positively biased with respect to the cold-field emission source to extract the electrons from the cold-field emission source. The extractor electrode has a first opening for the electrons. The e-beam device also includes a mirror electrode with a second opening for the electrons. The mirror electrode is configurable to be positively biased with respect to the extractor electrode during a first mode of operation and to be negatively biased with respect to the extractor electrode during a second mode of operation. The extractor electrode is disposed between the cold-field emission source and the mirror electrode. The e-beam device further includes an anode to be positively biased with respect to the extractor electrode and the cold-field emission source. The mirror electrode is disposed between the extractor electrode and the anode.

TECHNICAL FIELD

This disclosure relates to electron-beam (“e-beam”) devices, and more specifically to cleaning the extractor electrode of the electron gun in an e-beam device.

BACKGROUND

A cold-field emission source could be a suitable choice for high-throughput and high-spatial-resolution e-beam applications. The use of a cold-field emission source in such applications is challenged, however, by residual gas pressure, which can contaminate the cold-field emission source, shorten its lifetime, and introduce emission noise. The residual gas pressure of a typical extreme ultra-high vacuum environment (≈1e-11 torr) is created by the outgassing of vacuum-chamber components. One such component is the extractor electrode (sometimes simply referred to as the extractor). Once the cold-field emission source has been installed and nominal vacuum achieved, the extractor electrode is baked out to reduce subsequent outgassing. But bakeout alone is not sufficient to completely desorb molecules from surfaces of the extractor electrode near the cold-field emission source.

SUMMARY

Accordingly, there is a need for methods and systems for cleaning the extractor electrode.

In some embodiments, an e-beam device includes a cold-field emission source to emit electrons and an extractor electrode to be positively biased with respect to the cold-field emission source to extract the electrons from the cold-field emission source. The extractor electrode has a first opening for the electrons. The e-beam device also includes a mirror electrode with a second opening for the electrons. The mirror electrode is configurable to be positively biased with respect to the extractor electrode during a first mode of operation and to be negatively biased with respect to the extractor electrode during a second mode of operation. The extractor electrode is disposed between the cold-field emission source and the mirror electrode. The e-beam device further includes an anode to be positively biased with respect to the extractor electrode and the cold-field emission source. The mirror electrode is disposed between the extractor electrode and the anode.

In some embodiments, a method includes providing an e-beam device that includes a cold-field emission source, an extractor electrode with a first opening for electrons from the cold-field emission source, a mirror electrode with a second opening for the electrons, and an anode. The extractor electrode is disposed between the cold-field emission source and the mirror electrode. The mirror electrode is disposed between the extractor electrode and the anode. The method also includes cleaning the extractor electrode, which includes positively biasing the extractor electrode with respect to the cold-field emission source and the mirror electrode. The method further includes, after cleaning the extractor electrode, using the e-beam device. Using the e-beam device includes positively biasing the extractor electrode with respect to the cold-field emission source and positively biasing the mirror electrode and the anode with respect to the extractor electrode.

Like reference numerals refer to corresponding parts throughout the drawings and specification.

DETAILED DESCRIPTION

Cold-field emission, which is sometimes simply called field emission, refers to the emission of electrons from a pointed emitter in the presence of a negative potential (e.g., of several kilovolts) relative to a nearby electrode. Cold-field emission may occur at room temperature (which is considered cold compared to thermionic emission) and may result in an e-beam with an electron density several orders of magnitude higher than an e-beam produced through thermionic emission.

FIGS.1A and1Bare cross-sectional side views of an e-beam device100that includes a cold-field emission source102, an extractor electrode104, a mirror electrode116, and an anode120, arranged in that order, in accordance with some embodiments. The cold-field emission source102may also be referred to as a field emission source, field emitter, or cold-field emitter. The extractor electrode104is disposed between the cold-field emission source102and the mirror electrode116, and the mirror electrode116is disposed between the extractor electrode104and the anode120. The cold-field emission source102has a sharpened tip pointed toward the anode120. In this arrangement, the cold-field emission source102acts as a cathode. These components are disposed in a vacuum chamber122. The vacuum chamber122may provide an ultra-high vacuum (UHV). (UHV is a standard, well-known technical term that refers to vacuums with a pressure on the order of 10−9torr or lower.) In some embodiments, the e-beam device100is an electron microscope (e.g., a scanning electron microscope (SEM)).

FIG.1Ashows the e-beam device100configured for a first mode of operation in accordance with some embodiments. The first mode of operation is a normal operating mode. A first voltage −Vbeamis applied to the cold-field emission source102. A second voltage +Vext-Vbeamis applied to the extractor electrode104to positively bias the extractor electrode104with respect to the cold-field emission source102. In one example, Vbeamis 10 kV and Vextis 3 kV, such that −10 kV is applied to the cold-field emission source102and −7 kV is applied to the extractor electrode104. Other examples are possible. For example, the voltage difference between the extractor electrode104and the cold-field emission source102(i.e., Vext) may be 3 kV-5 kV. The anode120and mirror electrode116are positively biased with respect to the cold-field emission source102and the extractor electrode104. For example, the anode120and mirror electrode116are grounded, with the mirror electrode116being electrically connected to the anode120.

With the components biased in this manner, the cold-field emission source102emits electrons114: the extractor electrode104extracts electrons114from the cold-field emission source102. The cold-field emission source102and the extractor electrode104thus act as an electron gun for the e-beam device100. Some electrons114-1pass through an opening112in the extractor electrode104(e.g., in the center of the extractor electrode104). The electrons114-1also pass through an opening118in the mirror electrode116(e.g., in the center of the mirror electrode116). The opening112in the extractor electrode104is situated beneath the cold-field emission source102. The opening118in the mirror electrode116is situated beneath the opening112in the extractor electrode104. In some embodiments, the opening118is wider than the opening112(e.g., such that substantially all electrons114-1that pass through the opening112also pass through the opening118). In some embodiments, the extractor electrode104and/or the mirror electrode116are radially symmetric about an axis that extends through the cold-field emission source102, the opening112(e.g., the center of the opening112), and/or the opening118(e.g., the center of the opening118).

Other electrons114-2emitted by the cold-field emission source102miss the opening112and impact a portion106of the top surface of the extractor electrode104that surrounds the opening112. The top surface of the extractor electrode104is the surface that faces the cold-field emission source102. The current from the electrons114-2impacting the portion106of the top surface cleans chemisorbed radicals (i.e., desorbs the radicals) from the portion106by means of e-beam Desorption Induced by Electronic Transitions (DIET). Other surfaces of the extractor electrode104, including the bottom surface110(i.e., the surface facing the mirror electrode116) and the portion108of the top surface beyond (e.g., surrounding) the portion106, are impacted by a much smaller electron current than the portion106. The portion108and the bottom surface110therefore tend to have a chemisorbed monolayer that will release positive ions upon occasional electron impact through induced desorption. The released positive ions will follow the reversed electron trajectory and reach the cold-field emission source102, reducing emission lifetime and creating emission noise.

To clean and anneal a portion of the bottom surface110near the opening112, including the inner bore of the extractor electrode104, as well as to clean and anneal the portion106of the top surface, the electric field in the region between the extractor electrode104and the mirror electrode116is reversed with respect to the normal operating mode. Reversing the electric field in this region causes the e-beam (i.e., the electrons114-1) to turn around and impact a portion124(FIG.1B) of the bottom surface110(including the portion defining the inner bore of the extractor electrode104), thereby desorbing radicals from the monolayer on the portion of the bottom surface110. (The bottom surface110faces the mirror electrode116). This process may be performed after baking out the extractor electrode104and/or flashing the cold-field emission source102. The ions emitted by this desorption will follow the reversed field and be directed away from the cold-field emission source102.

FIG.1Bshows the e-beam device100configured for a second mode of operation (i.e., a mode for cleaning the extractor electrode104) in accordance with some embodiments. As in the first mode ofFIG.1A, the extractor electrode104is positively biased with respect to the cold-field emission source102. For example, the first voltage −Vbeamis applied to the cold-field emission source102and the second voltage +Vext-Vbeamis applied to the extractor electrode104, to extract electrons114from the cold-field emission source102(i.e., to cause the cold-field emission source102to emit electrons). Also as in the first mode, the anode120may be positively biased with respect to the extractor electrode104and the cold-field emission source102. For example, the anode120is grounded. The mirror electrode116, however, is configured to be negatively biased with respect to the extractor electrode104. In some embodiments, the mirror electrode116is electrically connected to the cold-field emission source102, as shown inFIG.1B. For example, the first voltage −Vbeamis applied to both the cold-field emission source102and the mirror electrode116, while the second voltage +Vext-Vbeamis applied to the extractor electrode104.

The mirror electrode116is thus configurable to be positively biased with respect to the extractor electrode104during the first mode of operation and to be negatively biased with respect to the extractor electrode104during the second mode of operation. In some embodiments, the mirror electrode116is configurable to be electrically connected to the anode120during the first mode of operation (as shown inFIG.1A) and to be electrically connected to the cold-field emission source102during the second mode of operation (as shown inFIG.1B).

Negatively biasing the mirror electrode116sufficiently (e.g., by at least Vext) with respect to the extractor electrode104directs the electrons114-1that pass through the opening112back toward the extractor electrode104, where they impact a portion124of the bottom surface, thus cleaning and annealing the portion124. The mirror electrode116effectively acts as a mirror that reflects the electrons114-1back to the extractor electrode104, resulting in self-cleaning of the extractor electrode104.

FIG.2is a cross-sectional side view of an e-beam device200that is an example of the e-beam device100(FIGS.1A-1B) in accordance with some embodiments. The e-beam device200includes a switch202that is used to configure (i.e., bias) the mirror electrode116. The switch202is used to electrically connect the mirror electrode116to the anode120during the first mode of operation (i.e., during the normal operating mode) and to the cold-field emission source102during the second mode of operation (i.e., during the extractor-electrode cleaning mode). The mirror electrode116, cold-field emission source102, and anode120are electrically connected to respective terminals of the switch202. The switch202is controlled by a control signal204. In the first mode of operation, the control signal204causes the switch202to connect the terminal for the mirror electrode116to the terminal for the anode120. In the second mode of operation, the control signal204causes the switch202to connect the terminal for the mirror electrode116to the terminal for the cold-field emission source102. In some embodiments, the control signal204may be asserted for the first mode of operation and de-asserted for the second mode of operation, or vice-versa.

FIG.3is a flowchart illustrating a method300of operating an e-beam device in accordance with some embodiments. In the method300, an e-beam device (e.g., device100,FIGS.1A-1B; device200,FIG.2) is provided (302) that includes a cold-field emission source (e.g., cold-field emission source102), an extractor electrode (e.g., extractor electrode104) with a first opening (e.g., opening112) for electrons from the cold-field emission source, a mirror electrode (e.g., mirror electrode116) with a second opening (e.g., opening118) for the electrons, and an anode (e.g., anode120). The extractor electrode is disposed between the cold-field emission source and the mirror electrode. The mirror electrode is disposed between the extractor electrode and the anode. In some embodiments, the first opening is situated beneath the cold-field emission source and the second opening is situated beneath the first opening. The second opening may be wider than the first opening. The extractor electrode and the mirror electrode may be radially symmetric about an axis through the cold-field emission source, the first opening (e.g., the center of the first opening), and/or the second opening (e.g., the center of the second opening).

To reduce contamination and achieve a high vacuum, the extractor electrode is baked out (304) and the cold-field emission source is flashed (304). Bakeout may be performed at approximately 200° C., by direct heating of the extractor electrode through a resistor. Flashing refers to briefly (e.g., for a period of several seconds) heating the cold-field emission source to clean its surface. Bakeout and flashing are performed before turning on the e-beam.

To further reduce contamination, the extractor electrode is cleaned (306). This cleaning may be performed after bake-out and flashing. Cleaning the extractor electrode includes positively biasing the extractor electrode with respect to the cold-field emission source and the mirror electrode. Positively biasing the extractor electrode with respect to the cold-field emission source extracts electrons from the cold-field emission source, thereby turning on the e-beam. Positively biasing the extractor electrode with respect to the mirror electrode causes the mirror electrode to act as a mirror that reflects electrons back to the bottom surface of the extractor electrode.

In some embodiments, to clean the extractor electrode, the mirror electrode is electrically connected (308) to the cold-field emission source (e.g., as shown inFIG.1B). For example, a switch (e.g., switch202,FIG.2) is configured (310) to electrically connect the mirror electrode to the cold-field emission source. In some embodiments, a first negative voltage (e.g., −Vbeam) is applied (312) to the cold-field emission source and the mirror electrode (e.g., is applied to the cold-field emission source while the mirror electrode is electrically connected to the cold-field emission source), and a second negative voltage (e.g., +Vext-Vbeam) is applied (312) to the extractor electrode. The magnitude of the second negative voltage is less than the magnitude of the first negative voltage (e.g., by the amount Vext). For example, the first negative voltage is −10 kV and the second negative voltage is −7 kV. In other examples, the difference between the second negative voltage and the first negative voltage is 3 kV-5 kV.

After the extractor electrode has been cleaned, the e-beam device is used (314). To use the e-beam device, the extractor electrode is positively biased with respect to the cold-field emission source, and the mirror electrode and the anode are positively biased with respect to the extractor electrode. In some embodiments, the mirror electrode is electrically connected (316) to the anode (e.g., as shown inFIG.1A). For example, the switch (e.g., switch202,FIG.2) is configured (318) to electrically connect the mirror electrode to the anode. The mirror electrode and the anode may be grounded (320) (e.g., the anode may be grounded and the mirror electrode connected to the anode, thereby also grounding the mirror electrode). The first negative voltage (e.g., −Vbeam) may be applied (322) to the cold-field emission source and the second negative voltage (e.g., +Vext-Vbeam) may be applied (322) to the extractor electrode.

The method300improves the cleaning of the surfaces of the extractor electrode by exploiting Desorption Induced by Electronic Transition in the cleaning step306. Performance of the cold-field emission source during use of the e-beam device (i.e., during the step314) is thus improved, as is the lifetime of the cold-field emission source.

FIG.4is a graph400demonstrating the reduction in steady-state outgassing achieved through e-beam irradiation of a surface. To generate the graph400, the anode120in the arrangement ofFIG.1Bwas replaced with a micro-channel plate that acts as an ion detector. The bias402of the ion detector is varied and the ion current404measured by the ion detector is recorded for different values of the bias402. The bias402is relative to the potential of the extractor electrode104. The pressure of the vacuum was 1e-8 torr (i.e., 10−8torr). Down to a bias402of −1000V, the current404is in a regime406that corresponds to high-energy ions generated at the extractor electrode by desorption (i.e., by DIET). In the regime406, the current404is substantially constant and independent of the bias402. As the bias402drops below −1000V and becomes increasingly more negative, the current404enters a regime408in which it becomes more negative: the absolute value of the current increases. The regime408corresponds to lower-energy ions created by electron ionization along the beam path (i.e., free-space ionization); the extra current in the regime408as compared to the regime406is due to ions generated by free-space ionization. The increase in the magnitude of the current404as the bias402becomes more negative indicates that ions created by free-space ionization outnumber ions created by desorption from the extractor electrode104: the surface of the extractor electrode104generates fewer ions than free-space electron ionization. These results show that continuous electron irradiation of the extractor electrode104removes chemisorbed contaminants from the extractor electrode104and minimizes steady-state desorption for the extractor electrode104by preventing a monolayer of chemisorbed contaminants from forming.