US2448594A - High resolution microscopy - Google Patents

Description

Sept. 7,. 1948.

mi /A J. HILLIER ET AL HIGH RESOLUTION MICRSCOPY Filed May 1, 1947 l ,p f4

f rf W I y i za| s All- I 72 a` Y (torneg Patented Sept. 7, 1948 D S i 2,448,594

vHIGH RESOLUTION'MICROS'COPY James Hillier, `Granbury, and- Richard" F.1'B'aker,

Princeton," N.` J.,"assignorsf1.to Radio1.' Corporation 'of America, a. :corporation -offDelawa'i-e .Application May 1, 1947', Seria1"NoL'745;3`40

(Cl.'250-`J49.5)

-17 :claims 1 'C'Il'iis I `invention Irelates 'to improvements in methods'fof y'raising the "magnication' `limits of electro-opticalgsystems Tand more particularly to utilizing a lhigh voltage 'ion beam 'to :obtain an image lin a preliminaryrsta'ge of"'magnication `which has suiicient resolutionto ibe furthenmagnied tothe presentrlimits possible with-the usual types'bf electron microscope.

"':t'It haspreviously beenproposed to use beam-sof ions' such. as protons to obtain' highly'` magnied images'pfminute objects because' Vof rthe possibilityof greaterresolving powerthan with the use ofr a focussed beam of electrons. Thergreater massof ions resultsin their having a much shorterwaveleng'th'zthanelectronsjof the same energy. This permits thezusejof" smaller aperturesthan in.tliepresentl lens systems used. in Velectron microscopes; and -thusthe obtaining of .greater resolving' power. Another advantage in .using ions isinthegreatly improved contrast obtainable not only because offlthe smaller apertures thatcan ybeused but also because'of'the greaterscattering the ions under'goon passing through, thespecimen. the other hand, `the difficulty with present ion sources in obtaining a` suiciently large' concentration of ions to utilizetheabovementioned advantages has preventedtheadoption 'of this type .oflelectro-optical system. y

.The principal obj ect of the presentinvention .is to. provide amethod whereby the high resolving power "of an'. ion .beam 'maybe `used` to .forman image having relatively low magnification, which image' is in: a `iormiwhich may then be further magnified using anelectron microscope.

Another Vobject of the invention is 'to provide asystem 'of ma'gncation' inwhich the total possible ma'gnifying! poweris' higher than` with use ofgthe electron microscope alone.

'A further'oybject of thev inventionisto provideA a novel ...method of ,forming av magnified .image replica .otanobject usinganion :beam 'and a sensitiveilhn. l

.iAnotherf-object .ottheinvention is tov provide ari-tions image"v `,by 1 aA method of'` translating .ion intensities into corresponding'variationsrin` thickness in a.lm. e

y,rimotheriobi ect fofi thefinventionris't to" .provide anion imageof high 'contrasten a'recordinglfllm.

"7 These andotherobjects will' beirnore apparenti and `theirrventiorr will bebetter understood from the 'following #specification and Vthe drawings :of

which: v

T'Fig; 131s" a'longitudinal 'crossesectional viewv of aILiOmopticalsyStem WhichA may. be.,.used toI ob`"' ,.:12 i tain anionima'gel'of attype-which'f-may-Ibe-iitilized iri-thelnventl'on, F

Fig. 1A is a diagrammaticvi'ewfoipart'bfthe system-of'Fig; lfshowing typicalfpotential differ- 'enceslof` an' Iion gun rzaii'id.fpart of the accelerating system,

*ft-Fig.' 2ais `a cross-'sectional View' through iapre paredifilmsonelayer of whicris amaterialesenf sitive tofio'nic-fbomba'dment l 'Figi 3-is ai similar viewhaving-parts greatly*esg.V aggerated illustrating 'i t-he t ectslof'fbombarding the-f1mfshownin Fig'.` Zevvitheiorr-'beams of*varyv ingint'enSIteS,

FFigif 4 is 'another similar view v showing-ithisfl'ilm 'after sui-tableifdevelopment, fand if IReferringl to" Figi 1, there-is shown@ `aisys'tem which: incldes, in general,y 'arifion sourceL `2, *of thetype'proposed-'Lbyfvon Ardenneiin combination with*anelectrostaticacceleratingsysteilaeendensing andfocussingelensesystem V6;' Falsoofthe electrostatic-type;endian anode plate 8' carrying a "sensitive: nlm-l urto record anion image.

Theeion source-,viridicated `more generally*at*` 2", -is enclosed in a corona-pro'of chamber 2 Landincludesafsource "ofn gasflil :tof which islconnected'a; deiivery -tube fl l"v endingin af *lf-shaped Vnatale |18 having'openings *I 8a and "l-Bb iri the' ends brittle 'crosspiece )off-the-` 1'- and "alsoY anropening :il c'ffin the center of-the crosspiece. Adjae'enttheopenings in` the n'ends of the fcrcss'pie'ce are* electron sourcesand 22comprising cathodes Vwhich may be 'of' tungsten andf'whichare suppliedwi-tha-po tential by means of a battery "24. ""tl?artially"sur--V rounding theses electron --sources` Vare curvelttY refleeting `electrodes 26 and -`2 8*' which are aisofcon-l nected 'to' battery 24. lOutside *of *the T-rei'l-ectifng electrodes Aare :permanent magnet"poieapieeesssn and 32 `opposite in polarity'fromieafch other'.

In atypical'ionsourcefof this type',-and-as-lllus trated in Fig: -.-1A, the potentials off-various ,parts ofthe system may al1 be referredV to that 'ofithe cathcdes. Thexdlivery tube--may-be=at apotem. tial of '1'00"V v.L morepositive than fthe'cathodes and'lthe"reilectinglelectrodes areat azsoxnewhat highernegative potential than-thecathodes The particular typebf ion'source used-if howeverfdoes not constitute a partnfthe presentfinventiorrand other=ionsources'maybe' substituted.

-'Theoperation of `the abovedescribedf-ion source islas `follows.A `Gras; which rmay/behydrogenf is" intro'ducedinto the deliveryftubeli from thefgas theaxisofthe electronbeam. i"Eledtronsparegiva- 3 en olf by the heated cathodes 2li and 22 and are repelled by the reflecting electrodes 26 and 28, which are charged somewhat higher negatively than the electron sources 20 and 22. The magnet poles 30 and 32 establish a transverse magnetic field which collimates the electrons in a plane perpendicular tojthe axis of the ion beam. The electrons "thus are made to oscillate between the cathodes and traverse the gas stream many times before their energy is entirely dissipated. In travelling through the gas stream, these electrons collide with gas molecules and presumably knock electrons out of their outer shells, causing them to be transformed into positively charged ions. These ions are removed by a transverse electrical field and are then drawn off through anaperture 34, which may be maintained at a potential of the for the recording film. The film may be composed of a thin backing member Illa of collodion (i. e., cellulose nitrate) or of any other film forming synthetic resin, coated with a layer IOb of a substan-ce such as gum arabic which has been. treated with potassium dichromate. More commonly, this is kn-own as bichromated gum arabic. It has -been found that bichromated gum arabic 4may be hardened by impact of ions such that the portions which have been bombarded are changed chemically and become substantially insoluble in water. It has also been found that the depth of the hardening action depends upon the. intensity of the bombardment. The invenorder of 300 v. more negative than the delivery nozzle, that is to say, about 200 v. more negative than the cathode.: They are then projected into an accelerating system 4 which will now be described in more detail.

'Ill'ie` accelerating system includes an aperture 36 andl la tubular electrode 38 which may be maintained at ya potential of the order of '700 v. more negative than the cathodee. These electrodes are connected to the positive side ci a yhigh voltage source orf the order ofBOO kv. The aperture 36 serves to remove the ions completely from the vicinity of the ion-source and also gives them some initial acceleration. The tubular electrode 38 exerts ra concentrating or -collimating effect. The ions then pass through the tubular electrode 40 which is at ground potential.A This is, in effect, 300 kv. ymore negative than the preceding part of the system and provides a strong accelerating field for the positively charged ions.

From the accelerating system the ions next pass t-o a, conventional type of single stage microscopesystem 6 having electrostatic lenses. Such a. system is `more completely described in the book Electron Optics and the Electron Microscope, Zworykin, et al., New York, John Wiley and Sons, Inc., 1945,(pp. Bil-98).. In the form illustrate-d, the ions rst pass through a condensing lens 42 .having two outer apertures connected to ground vand -a central aperture connected to a source of positive high voltage. Here, .the ion beam is condensed to a smaller diameter before it isimpinged upon the object 44 being studied.

The object unde-robservation is held in position on a. specimen stage 46 which maybe of any ofthe conventional types-shown in the alcovem'entioned book by Zworykin, et al., pages l85-195.

The ions `pass through the object 44 and are deflected. They are then brought to a sharp focus by the electrostatic -objective lens 48 similar in construction to the condenser lens 42 but of shorter focal length. More than one lens similar tothe objective lens 48 may be used, if desired, in order to get diiferent degrees of magnification.Y n

The image formed by the lsystem above described may be observed directly by allowing the focussed ion beam to impinge on a fluorescent screen 50 carried on .a prism 52 attached to a viewer consisting of the `usual objective 54 and eyepiece 52 lens combinations.`

According to the invention, however, the viewer may be withdrawn from the path of the ion beam and the image may now be recorded on a, sensitive lm l0 such asshcwn in Fig. 2, this film being placed onastage 8 which is maintained at anfadjustable'. positive potential. The potential is adjusted to give maximum sensitivity tion is .ibyno means limited to the case of bi- .chromalted gum arabic.

Bichrornated gelatin, alsolbichlromated resins, land gums other-than gum arabic may be used. Although there has been illustrated a substance Which is hardened. in depth .and which, when developed, has a surface varying in depth, it is also possible to use a substance which varies in density due to bombardment and in Whi-ch the depth or thickness remains vthe same after developing as it was prior to developing.

In the present instance, an image is produced in the 'following manner. The ion beam strikes the speci-men -44 and emerges on the opposite side with its component parts changed in intensity proportional to the varying thickness of the cor'- responding parts of the specimen through which it haspassed. This ion stream of varying intensity in', lone part oi its cross section compared to another now is directed onto the backing mem'- ber Illa of the lm I0, as indicated in Fig..2, for a brief period of time. The ions readily pass through the :backing layer laxiand penetrate the sensitive layerv lb for a distance depending upon their velocity. l In penetrating the sensitive layer,

they induce a chemical change in the nature of a hardening action. The layer l0b is thus hardened in a variable manner, as illustrated by the layer lllc in Eig. 3the non-uniformities corresponding to the degrees of opaqueness to ions of the various parts of the specimen although in reversed proportion. That is, the thicker parts of the hardened lrn correspond to the more iontra-nsparent parts of the specimen.

The thickness `of the sensitive layer 10b, before bombarding, is not closely critical but is preferably made :about 3000 lan-d the ionic velocity is adjusted by yadjusting the electrodev potentials such that the ions will just pass through ythe layer if not obstructed.

'Ilhe exposed film is next developed by Washingoif the unexp-osed or unhardened portions in Warm water leaving a film having an irregular surface configuration .as illustrated in Fig. 4. In this iilm the thi-cker portions correspond to the parts of the specimen through which the ions were able to pass readily While the thinner portions correspond to those parts of thespe'cimen relatively more opaque to the passage of the ions.

The replica is thus seen to be in the nature of 'a the surface film portion 10c, the metal may-be.

evapora-ted in a direction other than n-ormalto the plane of the film backing, thus introducing a shading eifect. This type of evaporation'and' details :appear las 120043001 Vdetails 'inthe Vimage replica after100-fold-magnication. Suc-h details now be resolved bya conventional electron microscope. The total useful magnification possible with -thei-mproved technique of the present `invention is Anovv of the yorder of 1,000,000.

Another modification of the present invention using a different type of sensitive lm will now be described. A very thin film of a salt such as magnesium chloride of the order of from a feW molecules up to 3000 in thickness is deposited on a thin backing sheet of a synthetic resinous plastic by the Well known evaporation-in-vacuo technique, such as described in U. S. Patent 2,338,497. The prepared film is placed on the object stage 8 of the ion microscope and subjected t bombardment by high energy protons which have passed through a specimen as described in connection with the previous example. The protons reduce the magnesium chloride to metallic magnesium and hydrogen chloride gas is evolved in small quantities and Withdrawn through the vacuum system. The degree of reduction to metal at a given point on the film depends upon the energy level of the protons striking it and the more complete the reduction at a given point the higher Will be the relative transparency to electrons since some of the atoms in the film will have been removed. The film thus prepared is then placed on the specimen stage of an electron microscope and viewed at high magnifications. Many other metallic halide salts capable of reduction by the action of protons may be used in this form of the process.

In order to make it clear that useful magnifications of the order of a million are not possible with any known arrangement using electron microscopes alone, it is pointed out that it would not be feasible to first obtain a replica of 100 diameters With an electron microscope having low magnifying power and then subsequently magnify this replica at 10,000 diameters. The inherent limitations of the electron microscope, due to its use of a bombarding medium which has a Wavelength about 40 times as large as that of a proton of the same energy, prevent the obtaining of suiiicient resolution to continue raising the magnification indefinitely. With the present types of lenses, and because of difficulties With aberrations which make it impractical to attain the limit of magnification theoretically possible using an electron optical system, no smaller detail can be observed at extremely high magnifications than at 200,000 diameters. For this reason, the same results Would not be possible using successive enlarging steps with only electron optical systems.

Although the invention has been described in connection with the use of positively charged hydrogen ions, it is possible to use other ionic forms as Well. For example, the two orbital elecverition. the :repli-ca,- Y

oroscopo wl'lich4 may he .ofthetyne de trous 4:of fthenheliunirratdmsmay be .stripped -Ol leaving alpha particlescwliich. mayalso` be "used for'lbombardment.;

There has thus boerin-described anvv improved 1 method fofzfutilizing. electrical v.optical .system .to

obtainwimages ofi lmuch, greater magnification than that hitherto attained, Without yloss inyfresolutionr-` Thishasbeen accomplished by evolving a ,techlliquefembining `an. ion and an electron lopjticalrsystem.With..theformation of an intermediate. imageimthe Afor-m yof.a replica. The ap.- paratus, described. isY only illustrative.` Other formsof-,ion sourcesand other accelerating systemsoouldfbeiused in, the production of the ion imagerfor example.` And in the .further magniication step, either an electrostatic or la Amagnetic typeeleotron. optical. system may be. used although..for,highest magnications the latter is preierlfedw:

We claim`l as ,our invention:y

LA 4method of.,for.1ning ,a highly magnified :image-Dfi a specimen .comprising first. subjecting said specimenvto a` .beam of ions in an ionoptical system in a manner such as to form an ion image of said specimen at relatively low magnication, recording said ion image on a film adapted to exhibit subsequent opacity to the passage of electrons therethrough varying proportionally to the intensity of the ion beam to which it was previously subjected, and utilizing said recorded image as the object in an electron optical system in a manner such that said recorded image is further magnified.

2. The method of claim 1 in which said ion image is enlarged not more than diameters compared to the size of said specimen.

3. The method of claim 1 in which said ions are positively charged.

4. The method of claim 1 in Which said ions are positively charged hydrogen nuclei.

5. The method of claim 1 in which said ions are positively charged helium nuclei.

6. The method of claim 1 in which said ions are positively charged hydrogen nuclei and in which said recording lm is of a type Which translates varying intensity of ion bombardment into proportionally varying film thicknesses upon being developed.

7. The method of claim 1 in which said ions are positively charged hydrogen nuclei and in which said recording nlm is of a type which translates varying intensity of ion bombardment into proportionally Varying film thicknesses upon being developed, said lm thicknesses appearing as correspondingly varying configurations on a surface of said lm.

8. The method of claim 1 in which said recording film is of a type which translates Varying intensity of ion bombardment into proportionally varying film thicknesses upon being developed, said film thicknesses appearing as correspondingly varying configurations on a surface of said film, and said method including the step of imparting further contrast to said lm by evaporating a layer of metal over said surface configurations.

9. The method of claim 1 in which said recording lm is of a type which translates varying intensity of ion bombardment into proportionally varying film thicknesses upon Abeing developed, said film thicknesses appearing as correspondingly varying configurations on a surface of said film, and said method including the step of evaporating a layer of metal over said surface configurations, said metal being evaporated in a di- 7 rection other than normal to the plane of the backing layer of said sensitive film.

10. The method of claim 1 in Which said iilm is bichromated gum arabic.

11. The method of claim 1 in Which said film is a layer of bichromated gum arabic about 3000 in thickness.

12. A method of forming a highly magnified image of a specimen comprising subjecting said specimen to a beam of positively charged hydrogen nuclei in an ion optical system in a manner such as to form a magnified ion image of said specimen, focussing said ion image on the backing layer side of a lm comprising a backing layer of a synthetic resinous material and a sensitive layer of bichromated gum arabic whereby said sensitive layer is hardened to depths varying With the intensity of the impinging ions, developing said sensitive layer by removing the unhardened portions thereof to form an image replica, and subjecting said image replica to a beam of electrons in an electron optical system in a manner such as to further magnify said image.

13. The method of recording an ion image of an object subjected to bombardment by ions in an ion optical system comprising focussing said ionimage on a lm of sensitive material capable of being hardened to depths varying with the intensity of ionic bombardment and developing said film to remove the unexposed portions and thereby form a surface of irregular conguration.

14. The method of claim 13 in which said ions are positively charged hydrogen nuclei.

15. The method of claim 14 in which said sensitive material is bichromated gum arabic.

16. The method of claim 13 in which additional contrast is imparted to said lm after developing by evaporating a thin metal lm over said surface.

17. The method of claim 1 in which said illm is a thin layer of a metallic salt capable of being reduced to a metal by the action of protons.

JAMES HILLIER. RICHARD F. BAKER.

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