TEM sample holder with cryogenic cooling and broadband RF irradiation

Abstract

A TEM sample holder enables simultaneous cooling and RF irradiation of a sample. The sample is suspended in a hole that penetrates through a sample stage formed by a dielectric plate having a lower metallic ground layer and an upper metallic lead. The sample stage is supported by an evacuated hollow tube extending from a cryogenic chamber, such as a liquid nitrogen or helium Dewar. A coaxial conductor extends from an ambient connector through the cryogenic chamber and hollow tube to the sample stage, a center conductor and surrounding metallic shield thereof being in thermal and electrical communication with the metallic lead and metallic ground layer respectively of the sample stage, and the metallic shield being is in direct thermal communication with the cryogenic chamber. The coaxial conductor thereby enables simultaneous cooling and RF irradiation of the sample during TEM measurements. Embodiments include a temperature sensor and heater.

Claims

1. A sample holder for a Transmission Electron Microscope (TEM), the sample holder being configured to provide simultaneous cooling and RF irradiation to a sample while the sample is being exposed to electrons applied thereto by the TEM, the sample holder comprising: a sample stage comprising: a flat dielectric plate; a metallic ground layer applied to a lower side of the dielectric plate; at least one metallic lead applied to an upper side of the dielectric plate; and a sample hole penetrating through the flat dielectric plate and ground layer, the sample hole being configured to surround a TEM sample while electrons are applied to the TEM sample by the TEM, the sample being in RF and thermal communication with the metallic ground layer and metallic lead; a cryogenic chamber comprising a cryogenic container that is configured to contain a cryogenic liquid; an evacuated hollow tube extending from the cryogenic chamber to the sample stage; and a coaxial conductor extending from an ambient connector through the cryogenic chamber and hollow tube to the sample stage, the coaxial conductor comprising: a metallic central conductor; and a metallic shield encircling the central conductor, the central conductor and the shield being separated by an annular gap; wherein: the metallic shield is in electrical and thermal communication with the ground layer and the central conductor is in electrical and thermal communication with the at least one metallic lead; and the metallic shield is in thermal communication with the cryogen that is contained within the cryogenic chamber; the coaxial conductor being thereby able to simultaneously cool the sample and conduct RF energy to the sample while the electrons are being applied to the sample by the TEM.

2. The sample holder of claim 1, wherein the metallic shield is in physical contact with a wall of a tunnel that extends through the cryogenic container.

3. The sample holder of claim 1, wherein a diameter of the metallic central conductor is greater than 0.01 inches.

4. The sample holder of claim 1, wherein a diameter of the metallic shield is greater than 0.05 inches.

5. The sample holder of claim 1, wherein a thickness of the metallic shield is great than 0.01 inches.

6. The sample holder of claim 1, wherein the cryogenic chamber is a liquid nitrogen Dewar.

7. The sample holder of claim 1, wherein the cryogenic chamber is a liquid helium Dewar.

8. The sample holder of claim 1, further comprising a recessed shelf surrounding the sample hole.

9. The sample holder of claim 1, wherein the gap between the metallic central conductor and the metallic shield is evacuated.

10. The sample holder of claim 1, wherein the gap between the metallic central conductor and the metallic shield is filled with a dielectric material.

11. The sample holder of claim 1, wherein the hollow tube and the sample stage are joined by a joining element, the joining element comprising a metallic outer portion that interconnects the metallic shield with the ground layer, and a central passage surrounded by a dielectric insulator through which the metallic central conductor extends from the hollow tube to electrical and thermal connection with the at least one metallic lead.

12. The sample holder of claim 1, wherein the at least one metallic lead is a strip line lead.

13. The sample holder of claim 1, further comprising a temperature sensor configured to sense a temperature of the sample.

14. The sample of claim 13, wherein the temperature sensor comprises a microstrip line on the dielectric plate.

15. The sample of claim 1, further comprising a heater configured to control a temperature of the sample.

16. The sample of claim 15, wherein the heater comprises a microstrip line on the dielectric plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A is a perspective view of a prior art cryogenic TEM sample holder;

(2) FIG. 1B is a cross-sectional view of the prior art cryogenic TEM sample holder of FIG. 1A;

(3) FIG. 2 is a perspective view, drawn to scale, of an embodiment of the present invention;

(4) FIG. 3 is side view, drawn to scale, of the embodiment of FIG. 2;

(5) FIG. 4 is a perspective view from above, drawn to scale, of the sample stage portion of the embodiment of FIG. 2;

(6) FIG. 5 is a perspective view from below, drawn to scale, of the sample stage portion of the embodiment of FIG. 2;

(7) FIG. 6 is a top view, drawn to scale, of the sample stage portion of the embodiment of FIG. 2;

(8) FIG. 7 is a cross-sectional side view, drawn to scale, of the embodiment of FIG. 2;

(9) FIG. 8 is a cross-sectional side view, drawn to scale, of the cryogenic chamber portion of the embodiment of FIG. 2;

(10) FIG. 9 is a cross-sectional side view, drawn to scale, of the sample stage portion of the embodiment of FIG. 2; and

(11) FIG. 10 is a top view of a sample stage in an embodiment that includes a temperature sensor and heater.

DETAILED DESCRIPTION

(12) The present invention is a TEM sample holder that is able to apply radio frequency energy to a sample within a TEM while at the same time cryogenically cooling the sample. It should be noted that, unless otherwise required by context, the terms “radio frequency” and “RF” are used herein to refer to all electronic frequencies from zero Hz to 10 GHz.

(13) With reference to FIGS. 2 and 3, according to the present invention a TEM sample holder includes a specimen stage 200 that is supported within the TEM vacuum space by a sample positioner 202. The sample positioner 202 comprises a cryogenic chamber 204 and an evacuated hollow tube 206 extending from the cryogenic chamber 204 to the specimen stage 200. In the illustrated embodiment, the hollow tube 206 is connected to the sample stage 200 by a joining section 210 that provides electrical and thermal interconnection as described in more detail below.

(14) With reference to FIGS. 4 and 5, The specimen stage 200 comprises a flat dielectric plate 400 that is covered on a lower side thereof by a solid ground plane 500, while at least one RF lead 402 is provided on an upper side thereof. In the illustrated embodiment, the central lead 402 is provided down the center of the flat dielectric plate 400, while a pair of ground leads 404 extend along right and left sides of the flat dielectric plate 400. The thickness of dielectric slab, the width of central lead 402, and the space between the center lead 402 and the ground leads 404, are chosen according to calculated values that provide a 50-ohm microstrip line extending from the coaxial conductor 700 to the sample.

(15) The flat dielectric plate 400 of the specimen stage 200 is penetrated by an imaging hole 406 within which a sample can be positioned. In the illustrated embodiment, the imaging hole 406 is surrounded by a recessed shelf 408 that can support margins of the sample, thereby holding the sample in place over the imaging hole 404. Electrical and thermal communication is provided between the underlying ground plane 500 and the sample, and between the electrical lead 402 and the sample, thereby providing both thermal and RF communication with the sample.

(16) With reference to the cross-sectional illustrations of FIGS. 7 through 9, a vacuum coaxial conductor 700 extends from an external, ambient RF connector 208 through the cryogenic chamber 204 and the hollow tube 206 to the specimen stage 200. The coaxial conductor 700 includes a relatively thick central wire or rod 702 that is coaxially surrounded by a relatively thick tubular metallic shield 704, the rod 702 being separated from the shield 704 by an annular vacuum space or by a dielectric insulator such as Kapton® 706. In embodiments, the central wire or rod has a diameter of between 0.01 inches and 0.05 inches, and the metallic shield has a diameter of between 0.05 and 0.2 inches, with a thickness of between 0.01 and 0.02 inches.

(17) The cryogenic chamber 204, which in embodiments is a liquid nitrogen or liquid helium Dewar, includes an interior cryogenic container 708 that is filled with a cryogenic liquid and surrounded by a vacuum space 710. In the illustrated embodiment, the coaxial conductor 700 passes through a tunnel 712 that is provided through the cryogenic container 706 of the cryogenic chamber 204. Within the tunnel 712, the shield 704 of the coaxial conductor 700 is in direct physical contact with the walls of the tunnel 712, such that the coaxial conductor 700 is in direct thermal communication with the cryogenic liquid. Beyond the cryogenic chamber 204, the coaxial conductor 700 is surrounded by the evacuated hollow tube 206, thereby minimizing heat exchange between the coaxial conductor 700 and the environment. The cryogenic chamber 204 and evacuated tube 206, together with the coaxial conductor 700 extending therethrough, function together as a sample positioner 202 that supports and positions the specimen stage 200 within the TEM vacuum space.

(18) The central rod 702 and shield 704 of the coaxial conductor 700 are both made from one or more electrically and thermally conductive metals, such as copper, silver, or gold. As a result of the thickness of the coaxial rod 702 and shield 704, the coaxial conductor 700 is able to function both as an RF conductor and a thermal conductor, whereby the coaxial conductor 700 conveys RF energy to the sample while also simultaneously cryogenically cooling the sample. A strip line conductor 110 would not be able to provide this dual functionality, because the leads of a strip line conductor 110 would be much too thin to efficiently conduct heat away from the sample.

(19) The joining section 210 that joins the hollow tube 206 to the sample stage 200 comprises an outer portion 412 that is made from a thermally conductive metal, such as copper. The outer portion is in strong thermal and electrical communication with the shield 704 of the coaxial conductor 700 and with the ground plane 500 and ground leads 404 of the sample stage 200. Within the joining section 210 the central rod 702 of the coaxial conductor 700 extends into electrical and thermal contact with the central lead 402 of the sample stage 200, while being isolated from the outer portion 412 by a dielectric spacer 414. The joining section 210 thereby places the RF lead 216 of the sample stage 200 into both electrical and thermal communication with the central rod 402 of the coaxial conductor 700, while also placing the ground plane 500 and ground leads 404 into electrical and thermal communication with the surrounding shield 704 of the coaxial conductor 700. In embodiments, the coaxial conductor 700, joining section 210, and specimen stage 200 maintain a 50-ohm RF impedance at frequencies up to 6 GHZ over the entire RF path extending from the external connector 208 to the sample.

(20) With reference to FIG. 10, in embodiments the specimen stage 200 further comprises a temperature sensor 1000 and heater 1002 that are configured to monitor and control the sample temperature. In the illustrated embodiment, both the temperature sensor 1000 and heater 1002 are implemented as “zig-zag” microstrip lines on the dielectric plate 400. In similar embodiments, the temperature sensor 100 and/or heater 1002 have other shape configurations, and are not necessarily provided as microstrip lines.

(21) Electrical communication between the temperature sensor 1000 and/or heater 1002 and an external temperature monitor and controller (not shown) is provided by connecting wires (not shown) that extend through vacuum feedthroughs into the exterior environment. In the illustrated embodiment, both the sensor 1000 and the heater 1002 are resistive. In other embodiments other types of sensors and heaters are used, as are known in the art.

(22) he foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. Each and every page of this submission, and all contents thereon, however characterized, identified, or numbered, is considered a substantive part of this application for all purposes, irrespective of form or placement within the application. This specification is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure.

(23) Although the present application is shown in a limited number of forms, the scope of the invention is not limited to just these forms, but is amenable to various changes and modifications. The disclosure presented herein does not explicitly disclose all possible combinations of features that fall within the scope of the invention. The features disclosed herein for the various embodiments can generally be interchanged and combined into any combinations that are not self-contradictory without departing from the scope of the invention. In particular, the limitations presented in dependent claims below can be combined with their corresponding independent claims in any number and in any order without departing from the scope of this disclosure, unless the dependent claims are logically incompatible with each other.