MULTIPURPOSE GASEOUS DETECTOR DEVICE FOR ELECTRON MICROSCOPES

Abstract: A scanning electron microscope which detects photons produced by contact between radiation emitted from the surface of a sample located on a sample platform means (6) and gas molecules of a gaseous medium which surrounds the sample. The invention also provides a method for microscopically imaging the surface of a sample through the use of gaseous photon detection.

Description: MULTIPURPOSE GASEOUS DETECTOR DEVICE FOR ELECTRON MICROSCOPES Background of the Invention Scanning electron microscopes and generally instruments employing an electron beam (probe) operate in vacuum (pressure less than about 0.0001 bar) and the speci¬ mens examined by such instruments are also placed in vacuum. Scanning a sample within a vacuum presents many problems. Many biological specimens cannot sur- vive in vacuum. Wet specimens can experience evapora¬ tion of their fluid content before an accurate image can be obtained. Nonconducting samples can accumulate a surface charge which obscures the details of the sample's surface and lowers the resolution of the image obtained. An environmental scanning electron microscope (ESEM) which allows the examination of specimens in a gaseous environment is described in U.S. Patent No. 4,596,928. However, the predominant detection mode in the ESEM has utilized various scintillator detectors to detect backscattered electrons. Additionally, an ESEM detection system has been described wherein the ionization of the gaseous environment is used as the detection means for all ionizing signals (Danilatos, Micron. Microsc. Acta Li:307-318, 1983). On ob ect of the. present invention is to pro ide a more general and πrnLϋgHrpose means for environmental s ca nni ng el ectroit πticrσseegg Suπunarv of the Invention The present invention provides a scanning electron microscope for cathodoluminescenct detection of specimens which comprises a vacuum envelope having a pressure limiting aperture. An electron beam source is located within the vacuum envelope and is capable of emitting an electron beam. Pocusing means are located within the vacuum envelope and are capable of directing an electron beam emitted by the electron beam source through the pressure limiting aperture. Electron beam scanning means are also located within the vacuum envelope and are capable of scanning an electron beam emitted by the electron beam source across the diameter of the pressure limiting aperture. A sample platform means is disposed outside the vacuum envelope and is capable of maintaining a sample in registration with the pressure limiting aperture such that a surface of the sample may be exposed to an electron beam emitted from the electron beam source and directed through the pressure limiting aperture so as to cause radiation to be emitted from the sample. The scanning electron microscope of the present invention further comprises gas containment means capable of maintaining the sample platform means enveloped in a gaseous medium so as to allow radiation emitted from a sample located on the sample platform means and exposed to an electron beam emitted from the electron beam source to come into contact with gas molecules of the gaseous medium and cause the gas molecules to emit photons. Detection means are provided which are capable of detecting photons emitted from the gas molecules of the gaseous medium. The present invention also provides a method for micro¬ scopically imaging the surface of a sample which comprises surrounding the sample with gas molecules and scanning the surface of the sample with an electron beam having sufficient energy so as to cause radiation to be emitted from the surface of the sample. Photons which are emitted from the gas molecules which come into contact with radiation emitted from the surface of the sample are then detected, the photons being emitted from the gas molecules in an amount proportional to the amount of radiation emitted from the surface of the sample. Images of the sample are then formed based on the number of photons detected. Brief Description of the Figure Fig. 1 is a schematic cross-sectional view of a device which embodies the present invention in a particular form. Detailed Description of the Invention The present invention provides a scanning electron microscope. Referring in more particularity to Figure 1, the invention comprises a vacuum envelope 1 having a pressure limiting aperture 2. An electron beam source 3 is located within the vacuum envelope and is capable of emitting an electron beam. Focusing means 4 are located within the vacuum envelope and are capable of directing an electron beam emitted by the electron beam source through the pressure limiting aperture. Elec¬ tron beam scanning means 5 are also located within the vacuum envelope and are capable of scanning an electron beam emitted by the electron beam source across the diameter of the pressure limiting aperture. A sample pi a -form means 6 is disposed outside the vacuum envelope and is capable of maintaining a sample in registration with the pressure limiting aperture such that a surface of the sample may be exposed to an electron beam emitted from the electron beam source and directed through the pressure limiting aperture so as to cause radiation to be emitted from the sample. Within this application, "radiation" emitted from a sample means electrons or photons emitted from the sam l e . The scanning electron microscope of the present invention further comprises a gas containment means 7' capable of maintaining the sample platform means enveloped in a gaseous medium so as to allow radiation emitted from a sample located on the sample platform means and exposed to an electron beam emitted from the electron beam source to come into contact with gas molecules , of the gaseous medium and cause the gas molecules to emit photons. Detection means 8 are provided which are capable of detecting photons emit¬ ted from the gas molecules of the gaseous medium. In' one embodiment of the invention, the wavelength of the photons is within the range from about lxlO "11 meters to about 4x10-8 rae ters. Preferably within this embodiment of the invention the detection means is a scintillation counter or a lithium drifted silicon detector. In another embodiment of the invention, the wavelength of the photons is within the range from about 4xl0" 8 meters- to about 7x10-7 meters . Preferably within this embodiment of the invention the detection means is a photomultiplier tube or a photodiode. In yet another embodiment of the invention, tne wave¬ length of the photons is within the range from about 7 l0~ 7 meters to aoout 2xl0" 4 meters. Preferably with¬ in this embodiment of- the invention the detection means is a photomultiplier tube or a photodiode. The gaseous medium may comprise a single gas or a mixture of gases. In one embodiment of the invention the gaseous medium comprises nitrogen. In another embodiment of the invention the gaseous medium comprises helium. The present invention also provides a method for micro¬ scopically imaging the surface of a sample which comprises surrounding the sample with gas molecules and scanning the surface of the sample with an electron beam having sufficient energy so as to cause radiation to be emitted from the surface of the sample. Photons which are emitted from gas molecules which come into contact with radiation emitted from the surface of the sample are then detected, the photons being emitted from the gas in an amount proportional to tne amount of radiation emitted from the surface of the sample. Images of the sample are then formed based on the number of photons detected. In one emoodiment of the invention, the wavelength of the photons is within the range from aoout 1x10-11 meters to about 4xl0 ~8 meters. Preferably within this embodiment of the invention the detection means is a scintillation counter or a lithium drifted silicon detector. In another embodiment of the invention, the wavelength of the photons is within the range from about 4xl0~ 8 meters to about 7xiθ-7 me ters. Preferably within this embodiment of the invention the detection means is a' photomultiplier tube or a photodiode. In yet a further embodiment of the invention, the wave¬ length of the photons is within the range from aoouc 7xl0 ~7 meters to about 2xl0~ 4 meters. Preferably with¬ in this embodiment of the invention the detection means is a photomultiplier tube or a photodiode. The gaseous medium may comprise a single gas or a mixture of gases. In one emoodiment of the invention, the gaseous medium comprises nitrogen. In yet another embodiment of the invention, the gaseous medium comprises helium.

Claims: What is claimed is: 1. A scanning electron microscope which comprises: a) a vacuum envelope having a pressure limiting aperture; b) an electron beam source located within the vacuum envelope and capable of emitting an electron beam; c) focusing means located within the vacuum enve- • lope and capable of directing an electron beam emitted by the electron beam source through the pressure limiting aperture; d) electron beam scanning means located within the vacuum envelope and capable of scanning an electron beam emitted by the electron beam source across the diameter of the pressure limiting aperture; e) sample platform means, disposed outside the vacuum envelope, capable of maintaining a sample in registration with the pressure lim- iting aperture such that a surface of the sample may be exposed to an electron beam emitted from the electron beam source and directed through the pressure limiting aper- ture so as to cause radiation to be emitted from the sample; f) gas containment means capable of maintaining the sample platform means enveloped in a gas¬ eous medium so as to allow radiation emitted from a sample located on the sample platform means and exposed to an electron beam emitted from the electron beam source to come into contact with gas molecules of the gaseous medium and cause the gas molecules to emit photons; and g) detection means capable of detecting photons emitted from the gas molecules of the gaseous medium. 2. A device of claim 1 wherein the wavelength of the photons is within the range from about 1 x 10 "11 meters to about 4 x 10~8 meters. 3. A device of claim 2 wherein the detection means is a scintillation counter. 4. A device of claim 2 wherein the detection means is a lithium drifted silicon detector. 5. A device of in claim 1 wherein the wavelength of the photons is within the range from about 4 x lo~ 8 meters to about 7 x 10"" 7 meters. 6. A device of claim 5 wherein the detection means is a photomultiplier tube. 7. A device of claim 5 wherein the detection means is a photodiode. 8. A device of claim 1 wherein the wavelength of the photons is within the range from about 7 x 10" 7 meters to about 2 x 10-4 me ters. 9. A device of claim 8 wherein the detection means is a photomultiplier tube. 10. A device of claim 8 wherein the detection means is a photodiode. 11. A device of claim 1 wherein the gaseous medium comprises a single gas. 12. A device of claim 1 wherein the gaseous medium comprises a mixture of gases. 13. A device of claim 1 wherein the gaseous medium comprises nitrogen. 14. A device of claim 1 wherein the gaseous medium comprises helium. 15. A method for microscopically imaging the surface of a sample which comprises: a) surrounding the sample with gas molecules; b) scanning the surface of the sample with an electron beam having sufficient energy so as to cause radiation to be emitted from the surface of the sample; c) detecting photons emitted from gas molecules which come into contact with radiation e itted from the surface of the sample, the photons being emitted from the gas molecules in an amount proportional to the amount of radiation emitted from the surface of the sample; and d) forming images of the sample based on the number of photons detected. 16. A method of claim 15 wherein the wavelength of the photons is within the range from about 1 x 10" 11 meters to about 4 x 10-8 rae ters. 17. A method of claim 15 wherein the wavelength of the photons is within the range from about 4 x 10-8 meters to about 7 x 10~7 meters. 18. A method of claim 15 wherein the wavelength of the photons is within the range from about 7 x 10~ 7 meters to about 2 x 10-4 meters. 19. A method of claim 15 wherein the gaseous medium comprises a single gas. 20. A method of claim 15 wherein the gaseous medium comprises a mixture of gases. 21. A method of claim 15 wherein the gaseous medium 5 compri_s.es. nitrogen. 22. A method af. cl im 15 wherein the gaseous medium cnmnriaes.:heliuπr. iσ 15 0 5 0 5

Inventors: DANILATOS GERASIMOS D, DANILATOS, GERASIMOS, D.
Applicants: ELECTROSCAN CORP, ELECTRO-SCAN CORPORATION
Application year: 1987
Main classification: H01J37
IPC codes: H01J37