US2408287A

US2408287A - Electron microscope

Info

Publication number: US2408287A
Application number: US520610A
Authority: US
Inventors: Charles H Bachman
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1944-02-01
Filing date: 1944-02-01
Publication date: 1946-09-24
Anticipated expiration: 1963-09-24
Also published as: GB588119A; BE480178A; FR948342A

Description

Sept. 24, 1946. c. H. BACHMAN ELECTRON MICROSCOPE Filed Feb. 1, 1944 Inventor: Charles H. Bachman, 6 1

Hus Attorneg.

'magnified object. 1 7

:It is an .iobject of my invention to provide im- "proved' means by which very thin fluorescent Patented Sept. 24, 1946 Charles II.

General New York S PAT ENT- OFFICE Bachman, Scotia,'- N-.Y., assignor to I Electric Company, I a corpor tion of Application February 1, 194 1, $erial Nor 5 Claims. (Cl. 250-495) The present invention relates to electron optical apparatus particularly adapted for themiOlU- scopic examination of electron pervious objects. It is .an'object of the invention to provide a new and improved fluorescent screen for an electron microscope. v p f It, is known that objects may be microscopically examined by the rays of an electron beam which transra'diates the object, the beam' passing throughithe object and in'this passage being providedwith thelch'aracteristics thereof. Such an "electron beam may be refracted by appropriately conditioned fields and the beam may be impinged upon a fluorescent screen; an image being constructed on. the screen, in accordance withthe characteristics given to 'the'bea'm in its passage through the object. It that there are two fundamental limitations in the use of fluorescent screenin electron microscopy where an electron beamjis impin ed upon and passes through the screen; These limitations are thecsize of thejphosphor or fluorescent parhasbeen observed.

' apertured electrode in the appended claims. The invention itself together with further objects and advantages thereof'may best be understoodby reference to the following description-taken in connection with the acco'mpanying' drawing in which Fig. 1 represents diagrammatically an end section of an electron optical system'employing my invention and Fig. 2 is a cross-sectional view showing'the fluorescent"scre'en supporting arrangement of the invention.

, Referring to Figs. 1" and 2 jointly, there is shown an electron microscope 'comprising an elongatedvacuum-t'ight container H] of a tubular metal construction; Atone end of thecontainer, there isi'provided a glass insulator I l which serves to support an electron source in" the :form of a filamentary cathode-121 The cathode I2 is surrounded by a tubularmetal member'lfi which confines the" emitted electrons to a "narrow beam and is cooperatively positioned with 'res'pect'to an M which is in contact with I the metal envelope part l0.- The opposite end of ticles and the physical thickness'of the screen.

The limitations of course are of consequence only when optical magnification of an image on the screen is desired In this'case, as;the image on a fluorescent screen enlarged to magnifications of the order of, diameters, thethickness 0f the screen,that is, the number of layers'of phosphor or fluorescentparticles causes the edges of the image to become blurred or fuzzy. Onthe other hand,'the sizeof the individual phosphor particles can beso controlled that this factor does not enter 'as a limitation of magnification until enlargements of the order of several hundreddiameters are reached. Improved magnification may be attained, therefore; if asufliciently thin fluorescent to incomplete coverage of a screen surface delescreen, preferably consisting of: a single layer of 1 teriously afiecting satisfactoryimaging of the screens may be successfullyemplo'yed in electron optical studies or .objects.

'"An important feature of the invention consists inran apparatus arrangement by crescent screens used in electron the screen structure'and with to eliminate visual effects.

portingmeans indicated generally at .with -the object supporting likewise employ 'means (notshown) forholding 'transradiate's' the-object, may

scope; the beam theen'velo'pe I0 is sealed by a glass window I5.

' A-fluorescentviewing screen" I 6 is positioned with"- in thejtubelflin front of the window [5. Inthe'normal use of the'apparatus, the envelope l0 and the apertured electrode I4 are maintained at "g'round-potentialand the cathode is maintained at a high negative potential, for example; being connected to 'a potential source "l1 "so that electrons the cathode are promoted axially of the container-with'the objector producing a visible image on the fluorescent screen l8.

Between the cathode I2 and the image-reproducing'screen lt there is provided object supl9.c In the intended use of the apparatus, this means is to-be employed as a support for an object to be transradiated *by an electron beam, the beam being passed-through the object and in the passage assuming the characteristics of the object. A supporting structure may be provided in connection means l9 and may appropriate clamps or other an object with its surface, in exposed position to the electron beam20. I v V In order that the electron beam, after modification in accordance with the structural character of the object under investigation as the beam be caused to provisible image ofthe object in the intended use of the micro- 20, after passing vide an enlarged accordance with through the object, is caused to pass through an electron lens system of known character. In the drawing, the lens system is illustrated as of the electrostatic type, although a lens system of the magnetic type may be employed. The lens system comprises a series of three diaphragms, numbered 22-24, inclusive. The

diaphragms

22 and 24 are maintained at the potential of the casing H1, that is, at ground potential and the intermediate diaphragm biased to cathode potential through a connection 25 which is brought into the microscope enclosure through an insulating bushing 26. Because of the resultant difierence in potential between the diaphragm 23 and diaphragms" 22 and 2, lens fields are established between these diaphragms which have the eifect of refracting the electron beam in a manner analogous to the refraction of a light beam by an optical lens. Additional lens fields may be employed in the electron microscope where so desired.

Means are provided for optical magnification of the image formed on the fluorescent screen I6 and this means is represented conventionally as an o tical lens 29 throu h which the beam of vis ble radiation 28. projected throu h window I from fluorescent screen I6. is caused to p ss.

T e fluorescent screen I 5 shown in plan View in Fig. 2 comprises a frame member 30 wh ch Inav b i her circular or recta'n nlar in sha e and which su orts a trans are t window 3| u on hich is deposited a t n coatin o a suit able fluo escent material 32. Preferab the coat ng 32 consists of a minimum number of lavers of s ll particles Of'a su table hos hor. The frame 30 is sus ended within member In by means of a s t of swine members 33 which provirie a good cond ctive connection, between screen l6 and tu e H). To one ed e f the rame 30. there is attached a rod m mber 35 which extends throu h an o ening 35 in tube and passes throu h a bellows device 31. the bellows 31 providing a seal across opening 36. The member 35 a a nnr irm exten in pwterio-rlv of the e]- lows 31 by wh ch motion may be im arted to screen [6 to produce lateral movement of this screen.

In or er to prevent any motion im arted to screen I6 bv actuating rod 35 f om oducing axi l mo ion of the screen to aiTect either the electronic or optical focus. a plurality of guide members 39 are attached to the inner surface of co tainer Ill. The uide members 39 may be prov ded with slots or fin ers at o e of their ends and the screen I 6 may b restrained by these fingers from axial movement and yet perm tted to move in a plane perpendicular to the axis of the container I0.

Bv the screen supporting arrangement described, the screen 16 may be moved laterally and the eifect of any kind of screen irregularities, such as holes, dirt specks, and similar defects in the phosphor coating on screen l6 which would normally produce distortion in optically enlarged images on the fluorescent screen I 6 is eliminated, Since the electron beam 20 always bombards the same points in space, as the screen I6 is shifted in a plane perpendicular to the beam, the effect of smoothing out all the irregularities of the above-mentioned type in the screen structure is obtained. So long as the motion is confined to a lateral direction and there is not suflicient axial motion to afl'ect either electronic or optical focus, no ill effects in the re- 23 may be insulatingly supported and is 'of an electrical coil 40 control switch 42. An armature 43 may be provided at the lower. end of rod 36 to by coil 40'.

When the lens .system 29 is replaced by a camera for photographing the image produced on screen IS, the need for eliminating flicker effect is no longer present so that at such times the frequency of vibration of screen [6 may be reduced and the screen may be moved manually during the exposure time.

By the present invention, in an electron microscope of the type described above, a fluorescent screen may be employed. which is made up of a single layer of fluorescent particles; Such a screen usually is not suitable for electron microscopic examinations, since the coverage of the screen under such conditions is only about 60% complete. In the normal microscope, therefore, more effective coverage of the screen surface is obtained by adding additional layers of fluorescent material. In a microscope, however, in which lateral shaking of the screen is provided, a single layer of fluorescent particles may be employed and the shaking produces the effect of a completely covered very thin screen having re duced light output because of the decreased number of fluorescent particles.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination, in an electron microscope, of a viewing screen coated with a thin layer of fluorescent material and means for producing lateral motion of said screen to reduce blurring of images produced on said screen.

2. An electron microscop comprising an evacuated container having a fluorescent screen positioned therein, means for projecting a beam of electrons upon said screen, means for supporting an object to be examined in the path of said beam, and means for vibrating said screen in a plane substantially transverse to said beam to reduce blurring of the image of said object produced on said screen.

3. An electron microscope system comprising an evacuated container having a transparent wall, a fluorescent screen supported within said container in front of said wall, means for probe vibrated screen, means external of said container for producing optical enlargement of the' image on said screen, and means for vibrating said screen in a plane substantially transverse to said beam to reduce blurring of said enlarged image due to said beam to reduce 5. The combination, in an electron microscope having a vacuum-tight container, of a fluorescent screen resiliently supported within said container, means for projecting an electron beam on an image thereon, means said image, and means for in a plane transverse to said screen to produce for optically enlarging vibrating said screen image.

CHARLES H. BACHIVLAN.

blurring in said enlarged