US3919553A - Integrated device for controlling charging artifacts in scanning electron microscopes - Google Patents

Integrated device for controlling charging artifacts in scanning electron microscopes Download PDF

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US3919553A
US3919553A US460555A US46055574A US3919553A US 3919553 A US3919553 A US 3919553A US 460555 A US460555 A US 460555A US 46055574 A US46055574 A US 46055574A US 3919553 A US3919553 A US 3919553A
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specimen
vapor
vaporization chamber
stub
coupled
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Arthur L Cohen
Gerald E Garner
Jr Raymond G E Steever
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Research Corp
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Research Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/026—Means for avoiding or neutralising unwanted electrical charges on tube components
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/20—Means for supporting or positioning the object or the material; Means for adjusting diaphragms or lenses associated with the support

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  • This invention relates generally to electron microscope control apparatuses and more particularly to a device for controlling charging interference and image contrast in scanning electron microscopes, and to the defined as those parts of an output image signal which are out of accord with the expected intensity or spatial and temporal distribution of the electrons received by an electron microscope collector.
  • charging artifacts which may also be described as charging interference or image interference due to specimen charging, may be readily understood by reference to FIGS. 1 and 2 which are both electron photomicrographs of a single specimen of untreated moth tongue, both showing the specimen magnified 600 times.
  • the photomicrograph of FIG. 1 demonstrates the occurrence of charging artifacts.
  • a large portion of the moth tongue is shown as being greatly over-exposed to the extent that no image contrast exists along the central portion of the specimen, so that all detail and consequently, all useful image information is lost in this area.
  • the photomicrograph of FIG. 2 shows the same specimen as in FIG. I, and was made in the same manner as the photomicrograph of FIG. I, with the exception of the fact that the present invention was used in making the photomicrograph of FIG. 2 to prevent the development of charging artifacts and to properly adjust the image contrast. As is apparent from inspection of these figures, the specimen image quality is greatly improved in FIG. 2.
  • the charging artifacts illustrated in FIG. 1 are caused by the accumulation of an electrical charge on the specimen surface due to the impingement of an electron beam on the specimen as it is examined under a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • the Lane apparatus includes a reservoir and a heater apparatus for heating a liquid or solid and producing a vapor which is directed through small channels directly at a specimen mounted on a stub.
  • the apparatus disclosed in the Lane paper does not, however, disclose any type of device for controlling image contrast, nor does it suggest the use of vapor in the region of a specimen to control charging artifacts.
  • the device disclosed in the Lane paper includes some similarities to the structure of the apparatus of the present invention, the Lane apparatus does not suggest the purpose or mode of operation of the present invention, and does not suggest or imply that image improvement or image control in electron microscopes can be achieved using the device disclosed therein. Accordingly, the present invention is believed to provide a substantial step forward in the art of SEM image control and image enhancement.
  • one object of this invention is to provide a novel apparatus for controlling charging artifacts in electron microscopes.
  • Yet another object of this invention is the provision of a novel apparatus for controlling image contrast in electron microscopes.
  • a still further object of the present invention is the provision of a novel method for reducing charging interference in electron microscopes.
  • a still further object of the present invention is the provision of a novel method of controlling image contrast in electronmicroscopes by controlling the potential of an apparatus in the specimen zone.
  • Yet another object of the present invention is the provision of a novel apparatus for producing a vapor in the region of a specimen mounted in an evacuated electron microscope housing.
  • Another object of the present invention is the provision of a novel method for controlling charging artifacts by generating a vapor in the region of a specimen in an evacuated environment.
  • a still further object of the present invention is the provision of a novel method for controlling image contrast in electron microscopes by varying the potential of a conductive shield structure positioned in the vicin ity of a specimen.
  • a stub holder assembly including a vaporization chamber having a heater attached thereto, and having a deflection shield positioned around the sample stub.
  • a suitable chemical is placed in the vaporization chamber and is vaporized by the application of heat to the vaporization chamber.
  • the resulting vapors are channeled via the deflection shield to the vicinity surrounding a sample mounted on the stub for controlling charging artifacts.
  • a potential source is coupled to the deflection shield for adjusting the bias voltage thereof whereby image contrast may be varied.
  • FIG. I is a photomicrograph taken at a 600 power magnification showing charging artifacts on a sample consisting of an untreated moth tongue;
  • FIG. 2 is a photomicrograph identical to that of FIG. 1 but showing the improved contrast and absence of charging artifacts when the present invention is used to control the image quality;
  • FIG. 3 is a schematic and block diagram showing the relationship of the present invention to a scanning electron beam microscope system
  • FIG. 4 is a cut-away side view of the apparatus of the present invention showing in detail the components thereof;
  • FIG. 5 is a graphical representation of output signal level as a function of deflection shield voltage using the apparatus of the present invention.
  • FIG. 6 is a graphical representation of output signal level as a function of vaporization chamber temperature using the apparatus of the present invention.
  • the device 10 of the present invention which may be referred to as an integrated device for controlling charging artifacts and image contrast in an SEM, is shown mounted to a stage 12 of a conventional scanning electron microscope.
  • a sample 14 is mounted to a conventional stub 16 held by the device 10.
  • the stub 16 is oriented at an angle of approximately 45, for example, to an impinging electron beam 18 emanating from a conventional SEM beam generating and focusing system 20.
  • the electron beam 18 passes through a cut-away portion of a deflection shield 22 to scan the sample 14.
  • Electrons emitted from the sample 14 are directed toward a conventional SEM collector 24, the output of which is applied to a conventional SEM imaging system 26.
  • the device 10 includes a central body portion 28, preferably formed of a material which has good heat conducting properties, such as brass, copper or the like.
  • the central body portion 28 partially encloses a vaporization chamber 30 wherein a suitable material, such as chloral hydrate is placed when the device 10 is being operated.
  • a plurality of apertures 32 are drilled around the periphery of central body portion 28 to provide clear exhaust channels for vapors generated within the vaporization chamber 30.
  • the central body portion 28 includes a lower appendage 34 which fits into a suitably sized aperture in a heat sink 36.
  • the heat sink 36 is preferably formed of copper or a similar material having very good heat conducting properties.
  • the lower appendage 34 may be press-fitted, threaded or otherwise affixed to the heat sink 36 as long as the appendage 34 and the heat sink are in close engagement to permit an unimpeded flow of heat energy therebetween.
  • the heat sink 36 is secured by means of plurality of bolts 38, or other suitable fasteners, to a pair of heat transfer and mounting plates 40.
  • a stage mounting screw 42 is coupled to the heat transfer and mounting plates 40 to permit the device 10 to be secured to the stage 12 of a SEM, as shown in FIG. 3.
  • thermoelectric device 44 is secured between the heat sink 36 and the heat transfer and mounting plates 40 by the tension applied to bolts 38.
  • the thermoelecr. tric device 44 is preferably a conventional, commerically available Peltier heater, and is coupled to a suit, able reversible polarity DC supply.
  • the Peltier heater operates according to the well known Peltier effect whereby heat is produced or absorbed at the junction of two metals when current is passed through the junction. More particularly, heat generated by a current flowing through the junction in one direction will be absorbed when the current is reversed.
  • This type of heat source is particularly suitable in the environment of the present invention since the Peltier heater is arranged such that when the upper portion thereof generates heat, the lower portion is cooled.
  • heat may be supplied to the heat sink 36 but not to the microscope stage 12 through the heat transfer plates 40.
  • This provides a substantial advantage in allowing rapid temperature changes to be made simply by reversing the polarity of the DC supply 46. Rapid temperature changes could not be made if other types of conventional heaters, such as filament heaters, were used, since such heaters have the effect of supplying the same quantity of heat to the heat transfer plates 40 and the microscope stage 12 as is supplied to the heat sink 36. Accordingly, when such conventional non-directional heat sources are used, the microscope stage is heated to a high degree and requires a considerable time period to cool, preventing rapid temperature changes at the location of the vaporization chamber 30.
  • the Peltier heater can also, of course, be used to cool rather than heat the heat sink 36 and the vaporization chamber 30.
  • central body portion 28 includes a cylindrical aperture defined by inwardly curved side walls 48.
  • the shape of the side walls slightly restricts the flow of vapor from the vaporization chamber 31) and serves to partially enclose the upper portion of the vaporization chamber.
  • An ionization ring 50 constructed of an insulating material such as Teflon (TM) is press-fitted to the upper portion of side walls 48 of central body portion 28.
  • the ionization ring 50 includes two fine wires 51 formed of non-corrosive metal and positioned within suitable grooves 52 formed around the inner periphery thereof.
  • the wires 51 may, for example, be formed of a gold palladium alloy, or any other suitably non-corrosive metal or alloy.
  • the wires 51 are coupled to a conventional DC voltage supply capable of generating a continuously variable static output potential of from O to 5 kv.
  • the wires 51 which are preferably placed approximately 3 millimeters apart and located just below the top surface of the stub 16 are intended to supply a potential for ionizing vapor molecules rising from the vaporization chamber 30, as will be explained in greater detail subsequently.
  • the apparatus of the present invention functions adequately without the use of the ionization wires 51, and thus they may be omitted from the ionization ring 50, or the device 10 may be operated with the variable voltage supply 54 switched off.
  • the ionization ring 50 functions solely as an insulator to insulate the deflection shield 22 from the body of the device, allowing it to maintain any desired applied charge, as will be apparent to those skilled in the art.
  • the previously mentioned deflection shield 22 is press-fitted to an upper surface of the ionization ring 50.
  • the deflection shield 22 is preferably constructed of a highly conductive material having an inner diameter of approximately 1.3 centimeters and having a partially cut-away side wall structure, as shown in FIG. 3, to permit the SEM beam 18 to reach the sample 14 placed on the surface of stub 16.
  • the deflection shield 22 is externally connected to a continuously variable, preferably well filtered DC power supply 56 which is preferably capable of producing a continuously variable output potential of from O to 900 volts.
  • the shape of the deflection shield prevents line-of-sight paths between the sample 14 and the collector 24, thus eliminating the possibility of high voltage discharge light which might occur in the ionization system of the present invention.
  • the main purpose of the deflection shield is to channel vapors rising from the vaporization chamber 30 into the area occupied by the sample 14, as will be explained in greater detail subsequently.
  • a stub holder 58 is positioned within the opening defined by the deflection shield 22, the ionization ring 50 and the vaporization chamber 30.
  • the stub holder 58 includes a body 60 having a threaded mounting and adjusting screw 62 extending downwardly therefrom for cooperating with a threaded aperture in the central body portion 28 of device 10.
  • a coil spring 64 is positioned around the mounting and adjusting screw 62 for preventing inadvertent rotation of the stub holder 58.
  • the body 60 of the stub holder 58 includes an axial aperture 66 for accomodating a mounting leg 68 of stub 16.
  • a detent ball 71 is movably mounted in a side aperture 70 of the body 60, and is biased into engagement with the mounting legs 68 by means of a resilient ring 74, which may be a conventional elastic O-ring, for example.
  • a vapor flow control flange 76 is formed integral with the upper portion of body 60 of the stub holder 58.
  • the vapor flow control flange 76 includes an inner surface which is sloped away from the stub 16 and also includes an outer surface 78 which is angled toward the wall of deflection shield 22.
  • the stub holder 58 is designed to secure one Cambridge pin type stub, for example recessed into the conical, flanged depression defined by the vapor flow control flange 76.
  • a small vapor gap 80 is formed between the outer surface 78 of the vapor flow control flange 76 and the inner periphery of the ionization ring 50.
  • the vapor gap 80 can be varied from a very thin circular slit to a significant gap depending upon the extent to which the mounting and adjusting screw 62 is threaded into its cooperating aperture.
  • the stub holder is also preferably electrically grounded, as indicated at 82.
  • a thermistor probe 84 is preferably mounted in the side wall 48 of the central body portion 28 to monitor the temperature of the vaporization chamber 30.
  • the output of the thermistor probe 84 is coupled to a conventional temperature indicator 86 to provide a convenient readout of the vaporization chamber temperature.
  • Chloral hydrate was selected as the preferred compound since it is normally a solid, and thus convenient to handle, and possesses a volatility such that at ambient temperature its vapor pressure is low enough to permit satisfactory pump down of the SEM vacuum system. However, once the temperature is raised sufficiently, this compound volatilizes sufficiently for its gaseous ions to conduct away charges. It is noted that the decomposition products of chloral hydrate may be highly toxic, and thus a foreline trap is advisable in the SEM vacuum system.
  • the crystals thus wrapped are placed into the vaporization chamber 30 and the stub holder 58 is subsequentially screwed into place above the crystals until the vapor gap is approximately 0.5 millimeters wide.
  • the stub 16 with the specimen 14 attached thereto is then inserted into the stub holder 58, and the deflection shield 22 is then snapped on. It is assumed at this point that the device 10 is mounted to the SEM stage 16, and thus evacuation of the microscope chamber is begun at this time.
  • the reversible polarity DC supply 46 is switched on and the polarity is selected so that heat is applied from the thermoelectric device 44 to the heat sink 36.
  • the vaporization chamber 30 may be heated to a suitable operating temperature, such as 80C.
  • the temperature indicator 86 is used to monitor the temperature of the vaporization chamber, as will be apparent to those skilled in the art.
  • Heating of the chloral hydrate crystals contained in the vaporization chamber 30 causes the crystals to vaporize, with the result that the emitted vapors rise toward the deflection shield through the apertures 32 around the peripheryof the vaporization chamber 30.
  • a near vacuum exists in the region of the device 10, and. thus the vapors rise in a virtually straight line. Accordingly the rising vapor molecules will impinge upon the angled outer surface 78 of the vapor flow controlflange and will be directed along a line parallel to the angle of the surface 78 into the side wall of deflection shield 22.
  • the deflection shield will deflect these molecules back into the region occupied by the sample 14 so that the vapor molecules may engage the surface of the sample 14 and neutralize any charge developing onthe surface of the sample 14.
  • variable power supply 54 may also be energized to supply an ionizing potential to the wires 51 for the purpose of ionizing vapor molecules as they rise through the ionization ring 50.
  • the ionized molecules have a somewhat greater ability to neutralize charges developing on the sample 14.
  • the apparatus of the present invention can be operated without ionization of the rising vapor molecules.
  • the contrast of the output information appearing on the imaging system 26 may be varied by adjusting the output of voltage supply 56 to change the bias potential of deflection shield 22. It has been found that potentials in the range between 0 and 00 volts are most effective in adjusting image contrast.
  • FIG. 5 which is a graphical representation of output signal level (vertical axis) a curve 90 represents background signal level.
  • the distance between curves88 and 90 represents'the image contrast in the output signah FIG. illustrates that the image contrast is sharply reducedwhen negative potentials are applied to the deflection shie ld22 and ismore gradually reduced when positive potentials are applied.
  • any biasing of the deflection shield causes a drop in the signal level which must be compensated for in the video amplification of the microscope imaging system.
  • deflection shield biasing tends to reduce the output signal level, vapor presence appears to raise the signal level somewhat as will be apparent by reference to FIG. 6.
  • FIG. 6 is a graphical representation of output signal level (right hand vertical axis) versus vaporization chamber temperature (horizontal axis).
  • the vapor pressure of chloral hydrate is also represented. More particularly the vapor pressure of chloral hydrate is represented by a curve 92 in FIG. 6, while a curve 94 represents background signal level, a curve 96 represents object signal level in the presence of ionized chloral hydrate molecules and a curve 98 represents the object signal level in the presence of un-ionized chloral hydrate molecules.
  • FIGS. 5 and 6 show data conditions for a non-charging sample.
  • chloral hydrate was previously mentioned as the'preferred compound for use with the present invention, it should be noted that other compounds such as Freon TF (TM), paradichlorobenzene and other organic halogen compounds may be also conveniently used.
  • the vapor may be introduced into the chamber 30 from a reservoir external to the main body of the device.
  • the vapor may be purchased from a commercial producer and introduced into the chamber 30 from a suitable tank.
  • a method for controlling electron beam specimen charging in an electron microscope comprising the steps of:
  • step of introducing further comprises the step of:
  • step of introducing includes the step of:
  • step of ionizing includes the step of:
  • An apparatus for controlling charging artifacts and for controlling image contrast in electron microscopes comprising:
  • heat control means thermally coupled to said body means for controlling the temperature of said vaporization chamber, specimen holding means coupled to said body means for supporting a specimen in said apparatus; and, electrode means secured to and electrically insulated from said body means for supplying an image contrast control potential to a region adjacent said specimen holding means.
  • thermoelectric device for heating and cooling said vaporization chamber. 10. An apparatus as in claim 9, wherein said thermoelectric device comprises:
  • thermoelectric device which produces or absorbs heat in accordance with the Peltier effect.
  • temperature indicating means coupled to said body means for monitoring temperatures within said vaporization chamber.
  • variable direct current voltage supply means adapted to be coupled to said electrode means for applying an adjustable contrast control voltage thereto.
  • ionization means coupled between said body means and said electrode means for applying an ionizing potential to vapors emanating from said vaporization chamber.
  • said ionization means comprises:
  • variable voltage supply means coupled to said conductors for applying an adjustable ionizing potential thereto.
  • said specimen holding means comprises:
  • specimen holding means further comprises:
  • threaded means for fastening said specimen holding means to said body means and for adjusting the position of said vapor flow control flange whereby vapor flow from said vaporization chamber may be regulated.
  • detent means resiliently mounted in said structure for engaging said stub mounting leg.
  • vapor flow control means including said electrode for directing vapor generated in said vaporization chamber into a region adjacent said specimen holdingme'ans.
  • An apparatus for controlling charging artifacts and for controlling image contrast in electron microscopes comprising:
  • specimen holding means coupled to said body means for supporting a specimen in said apparatus; and, electrode means secured to said body means for supplying an image contrast control potential to a region adjacent said specimen holding means.

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  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Sampling And Sample Adjustment (AREA)

Abstract

A device is disclosed which permits neutralization of specimen charging in electron microscopes so that charging interference is eliminated from output image information. The device also enables contrast adjustments to be made in the specimen zone rather than in the output signal processing apparatus. The device includes an apparatus for generating a vapor in the specimen region and further includes a vapor deflection shield structure which carries an adjustable bias voltage for permitting contrast control. The method of operating the device of the present invention in conjunction with a scanning electron microscope is also disclosed.

Description

United States Patent Cohen et al.
1451 Nov. 11, 1975 0 DC SUPPLY [5 INTEGRATED DEVICE FOR 857.253 11/1952 Germany 250/511 CONTROLLING CHARGING ARTIFACTS IN SCANNING ELECTRON MICROSCOPES Primary E.\'uminer.lames W. Lawrence [75] Inventors: Arthur L. Cohen, Pullman, Wash; 4535mm EMWII'IIW-T Grigsby Gerald Gamer, Santa Monica, Armrney, Age/1r. or F1rmOblon. Fisher. Spn'ak. Califi; Raymond G. E. Steever. Jr., Mccleuand & Miller Pullman. Wash.
[73] Assignee: Research Corporation, New York, [57] ABSTRACT NY. {22] Filed: Apr 12, 1974 A devlce is disclosed which perm1ts neutrahzation of speclmen charging 1n electron microscopes so that [21] Appl. No.: 460,555 charging interference is eliminated from output image information. The device also enables contrast adjust- [52 US. (:1. 250/311; 250/306 m f the rather 9 2 the output signal processing apparatus. The deuce in- [51] Int. Cl. GOlN 23/00 eludes ,m uppamtus for oenemting a vapor in the Spec [58] Field of Search 250/306. 3ll. 310. 307 I I H lmen region and further Includes a \apor deflectlon shield structure which carries an adjustable bias volt- [56] References Cited age for perm1ttmg contrast control. The method of op UNITED STATES PATENTS crating the device of the present invention in conjunc- Z.890.342 6/1959 Columbe .1 250/306 tion with a scanning electron microscope is also dis 3.505.52l 4/1970 Wegmann et al 250/3 ll l d FOREIGN PATENTS OR APPLICATIONS 0 Cl 6 D F 4624460 7/1971 Japan 250/310 rawmg gums DICATOR REVERSIBLE POLARITY DC SUPPLY US. Patent Nov. 11, 1975 Sheet10f3 3,919,553
US. Patent Nov. 11,1975 Sheet 2 of3 3,919,553
01o i900V 22 H6. 4 DC SUPPLY II,
| 80 76 H l6 5) v 7 L) h 50 86 VARIABLE 0c TEMPERATURE VOLTAGE SUPPLY I INDICATOR 71 7 1 as H8 32/ 6o 70 32 f 8" 82 3 1 28 as 36 as POLARITY 0c SUPPLY w TL 1 SEN BEAM GENERATING AND FOCUSING SYSTEM US. Patent Nv.11, 1975 Sheet3of3 3,919,553
l \L[ 50 I00 200 300 u 900 DEFLECTION SHIELD VOLTAGE mmammmmm moms TEMPERATURE (C) INTEGRATED DEVICE FOR CONTROLLING CHARGING ARTIFACTS IN SCANNING ELECTRON MICROSCOPES BACKGROUND OF THE INVENTION 1. Field of the Invention:
- This invention relates generally to electron microscope control apparatuses and more particularly to a device for controlling charging interference and image contrast in scanning electron microscopes, and to the defined as those parts of an output image signal which are out of accord with the expected intensity or spatial and temporal distribution of the electrons received by an electron microscope collector. The significance of charging artifacts, which may also be described as charging interference or image interference due to specimen charging, may be readily understood by reference to FIGS. 1 and 2 which are both electron photomicrographs of a single specimen of untreated moth tongue, both showing the specimen magnified 600 times.
I The photomicrograph of FIG. 1 demonstrates the occurrence of charging artifacts. In particular, a large portion of the moth tongue is shown as being greatly over-exposed to the extent that no image contrast exists along the central portion of the specimen, so that all detail and consequently, all useful image information is lost in this area. The photomicrograph of FIG. 2 shows the same specimen as in FIG. I, and was made in the same manner as the photomicrograph of FIG. I, with the exception of the fact that the present invention was used in making the photomicrograph of FIG. 2 to prevent the development of charging artifacts and to properly adjust the image contrast. As is apparent from inspection of these figures, the specimen image quality is greatly improved in FIG. 2.
The charging artifacts illustrated in FIG. 1 are caused by the accumulation of an electrical charge on the specimen surface due to the impingement of an electron beam on the specimen as it is examined under a scanning electron microscope (SEM). The conductivity of the specimen, the existence of neighboring charged surfaces, the energy of the impinging electron beam and the exposure parameters of the specimen to the electron beam are significant factors in determining the extent to which charging artifacts are likely to'occur.
Devices for reliably controlling charging artifacts and for controlling SEM image contrast in the region of the specimen are believed to be essentially unknown in the prior art. However, at least one device having some similarity to the apparatus of the present invention is known. This device is called an environmental control stage and was described in a paper by W. C. Lane included in the Proceedings of the Third Annual Scanning Electron Microscope Symposium, IIT Research Institute, Chicago, Illinois. The device described in this publication is designed to form a water vapor zone in the area of a wet specimen, that is, a specimen whose properties are best maintained'in a wet or moist environment. The Lane apparatus includes a reservoir and a heater apparatus for heating a liquid or solid and producing a vapor which is directed through small channels directly at a specimen mounted on a stub.
The apparatus disclosed in the Lane paper does not, however, disclose any type of device for controlling image contrast, nor does it suggest the use of vapor in the region of a specimen to control charging artifacts. Thus, while the device disclosed in the Lane paper includes some similarities to the structure of the apparatus of the present invention, the Lane apparatus does not suggest the purpose or mode of operation of the present invention, and does not suggest or imply that image improvement or image control in electron microscopes can be achieved using the device disclosed therein. Accordingly, the present invention is believed to provide a substantial step forward in the art of SEM image control and image enhancement.
SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a novel apparatus for controlling charging artifacts in electron microscopes.
Yet another object of this invention is the provision of a novel apparatus for controlling image contrast in electron microscopes.
A still further object of the present invention is the provision of a novel method for reducing charging interference in electron microscopes.
A still further object of the present invention is the provision of a novel method of controlling image contrast in electronmicroscopes by controlling the potential of an apparatus in the specimen zone.
Yet another object of the present invention is the provision of a novel apparatus for producing a vapor in the region of a specimen mounted in an evacuated electron microscope housing.
Another object of the present invention is the provision of a novel method for controlling charging artifacts by generating a vapor in the region of a specimen in an evacuated environment.
A still further object of the present invention is the provision of a novel method for controlling image contrast in electron microscopes by varying the potential of a conductive shield structure positioned in the vicin ity of a specimen.
Briefly, these and other objects of the present invention are achieved by providing a stub holder assembly including a vaporization chamber having a heater attached thereto, and having a deflection shield positioned around the sample stub. A suitable chemical is placed in the vaporization chamber and is vaporized by the application of heat to the vaporization chamber. The resulting vapors are channeled via the deflection shield to the vicinity surrounding a sample mounted on the stub for controlling charging artifacts. A potential source is coupled to the deflection shield for adjusting the bias voltage thereof whereby image contrast may be varied.
BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
FIG. I is a photomicrograph taken at a 600 power magnification showing charging artifacts on a sample consisting of an untreated moth tongue;
FIG. 2 is a photomicrograph identical to that of FIG. 1 but showing the improved contrast and absence of charging artifacts when the present invention is used to control the image quality;
FIG. 3 is a schematic and block diagram showing the relationship of the present invention to a scanning electron beam microscope system;
FIG. 4 is a cut-away side view of the apparatus of the present invention showing in detail the components thereof;
FIG. 5 is a graphical representation of output signal level as a function of deflection shield voltage using the apparatus of the present invention; and,
FIG. 6 is a graphical representation of output signal level as a function of vaporization chamber temperature using the apparatus of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 3 thereof, the relationship of the apparatus of the present invention to a conventional scanning electron microscope system is illustrated schematically. The device 10 of the present invention, which may be referred to as an integrated device for controlling charging artifacts and image contrast in an SEM, is shown mounted to a stage 12 of a conventional scanning electron microscope. A sample 14 is mounted to a conventional stub 16 held by the device 10. The stub 16 is oriented at an angle of approximately 45, for example, to an impinging electron beam 18 emanating from a conventional SEM beam generating and focusing system 20. The electron beam 18 passes through a cut-away portion of a deflection shield 22 to scan the sample 14. Electrons emitted from the sample 14 are directed toward a conventional SEM collector 24, the output of which is applied to a conventional SEM imaging system 26.
Having thus established the relationship of the device 10 of the present invention to a conventional SEM system, attention is now directed to FIG. 4 wherein the device 10 of the present invention is illustrated in greater detail. The device 10 includes a central body portion 28, preferably formed of a material which has good heat conducting properties, such as brass, copper or the like. The central body portion 28 partially encloses a vaporization chamber 30 wherein a suitable material, such as chloral hydrate is placed when the device 10 is being operated. A plurality of apertures 32, shown by dashed lines, are drilled around the periphery of central body portion 28 to provide clear exhaust channels for vapors generated within the vaporization chamber 30.
The central body portion 28 includes a lower appendage 34 which fits into a suitably sized aperture in a heat sink 36. The heat sink 36 is preferably formed of copper or a similar material having very good heat conducting properties. The lower appendage 34 may be press-fitted, threaded or otherwise affixed to the heat sink 36 as long as the appendage 34 and the heat sink are in close engagement to permit an unimpeded flow of heat energy therebetween.
The heat sink 36 is secured by means of plurality of bolts 38, or other suitable fasteners, to a pair of heat transfer and mounting plates 40. A stage mounting screw 42 is coupled to the heat transfer and mounting plates 40 to permit the device 10 to be secured to the stage 12 of a SEM, as shown in FIG. 3.
A thermoelectric device 44 is secured between the heat sink 36 and the heat transfer and mounting plates 40 by the tension applied to bolts 38. The thermoelecr. tric device 44 is preferably a conventional, commerically available Peltier heater, and is coupled to a suit, able reversible polarity DC supply. As will be apparent to those skilled in the art, the Peltier heater operates according to the well known Peltier effect whereby heat is produced or absorbed at the junction of two metals when current is passed through the junction. More particularly, heat generated by a current flowing through the junction in one direction will be absorbed when the current is reversed. This type of heat source is particularly suitable in the environment of the present invention since the Peltier heater is arranged such that when the upper portion thereof generates heat, the lower portion is cooled. Thus heat may be supplied to the heat sink 36 but not to the microscope stage 12 through the heat transfer plates 40. This provides a substantial advantage in allowing rapid temperature changes to be made simply by reversing the polarity of the DC supply 46. Rapid temperature changes could not be made if other types of conventional heaters, such as filament heaters, were used, since such heaters have the effect of supplying the same quantity of heat to the heat transfer plates 40 and the microscope stage 12 as is supplied to the heat sink 36. Accordingly, when such conventional non-directional heat sources are used, the microscope stage is heated to a high degree and requires a considerable time period to cool, preventing rapid temperature changes at the location of the vaporization chamber 30. The Peltier heater can also, of course, be used to cool rather than heat the heat sink 36 and the vaporization chamber 30.
The upper end of central body portion 28 includes a cylindrical aperture defined by inwardly curved side walls 48. The shape of the side walls slightly restricts the flow of vapor from the vaporization chamber 31) and serves to partially enclose the upper portion of the vaporization chamber.
An ionization ring 50 constructed of an insulating material such as Teflon (TM) is press-fitted to the upper portion of side walls 48 of central body portion 28. The ionization ring 50 includes two fine wires 51 formed of non-corrosive metal and positioned within suitable grooves 52 formed around the inner periphery thereof. The wires 51 may, for example, be formed of a gold palladium alloy, or any other suitably non-corrosive metal or alloy. The wires 51 are coupled to a conventional DC voltage supply capable of generating a continuously variable static output potential of from O to 5 kv. The wires 51, which are preferably placed approximately 3 millimeters apart and located just below the top surface of the stub 16 are intended to supply a potential for ionizing vapor molecules rising from the vaporization chamber 30, as will be explained in greater detail subsequently. However, the apparatus of the present invention functions adequately without the use of the ionization wires 51, and thus they may be omitted from the ionization ring 50, or the device 10 may be operated with the variable voltage supply 54 switched off. When the ionization wires 51 are omitted, or when no power is supplied to them, the ionization ring 50 functions solely as an insulator to insulate the deflection shield 22 from the body of the device, allowing it to maintain any desired applied charge, as will be apparent to those skilled in the art.
The previously mentioned deflection shield 22 is press-fitted to an upper surface of the ionization ring 50. The deflection shield 22 is preferably constructed of a highly conductive material having an inner diameter of approximately 1.3 centimeters and having a partially cut-away side wall structure, as shown in FIG. 3, to permit the SEM beam 18 to reach the sample 14 placed on the surface of stub 16. The deflection shield 22 is externally connected to a continuously variable, preferably well filtered DC power supply 56 which is preferably capable of producing a continuously variable output potential of from O to 900 volts. The shape of the deflection shield prevents line-of-sight paths between the sample 14 and the collector 24, thus eliminating the possibility of high voltage discharge light which might occur in the ionization system of the present invention. The main purpose of the deflection shield, however, is to channel vapors rising from the vaporization chamber 30 into the area occupied by the sample 14, as will be explained in greater detail subsequently.
A stub holder 58 is positioned within the opening defined by the deflection shield 22, the ionization ring 50 and the vaporization chamber 30. The stub holder 58 includes a body 60 having a threaded mounting and adjusting screw 62 extending downwardly therefrom for cooperating with a threaded aperture in the central body portion 28 of device 10. A coil spring 64 is positioned around the mounting and adjusting screw 62 for preventing inadvertent rotation of the stub holder 58.
The body 60 of the stub holder 58 includes an axial aperture 66 for accomodating a mounting leg 68 of stub 16. A detent ball 71 is movably mounted in a side aperture 70 of the body 60, and is biased into engagement with the mounting legs 68 by means of a resilient ring 74, which may be a conventional elastic O-ring, for example.
A vapor flow control flange 76 is formed integral with the upper portion of body 60 of the stub holder 58. The vapor flow control flange 76 includes an inner surface which is sloped away from the stub 16 and also includes an outer surface 78 which is angled toward the wall of deflection shield 22.
As will be apparent from FIG. 4, the stub holder 58 is designed to secure one Cambridge pin type stub, for example recessed into the conical, flanged depression defined by the vapor flow control flange 76. A small vapor gap 80 is formed between the outer surface 78 of the vapor flow control flange 76 and the inner periphery of the ionization ring 50. The vapor gap 80 can be varied from a very thin circular slit to a significant gap depending upon the extent to which the mounting and adjusting screw 62 is threaded into its cooperating aperture. The stub holder is also preferably electrically grounded, as indicated at 82. A thermistor probe 84 is preferably mounted in the side wall 48 of the central body portion 28 to monitor the temperature of the vaporization chamber 30. The output of the thermistor probe 84 is coupled to a conventional temperature indicator 86 to provide a convenient readout of the vaporization chamber temperature.
Having described in detail the structure of the present invention, the method of operation thereof will be more fully explained. A suitable chemical compound was first selected for generation of the desired vapor. Chloral hydrate was selected as the preferred compound since it is normally a solid, and thus convenient to handle, and possesses a volatility such that at ambient temperature its vapor pressure is low enough to permit satisfactory pump down of the SEM vacuum system. However, once the temperature is raised sufficiently, this compound volatilizes sufficiently for its gaseous ions to conduct away charges. It is noted that the decomposition products of chloral hydrate may be highly toxic, and thus a foreline trap is advisable in the SEM vacuum system.
To reduce the initial volatization of the chloral hydrate, three or four crystals of the material are first preferably wrapped in Teflon tape in which several puncture holes are subsequen'tially made to permit vapors to escape. This arrangement permits a suitable vacuum to be achieved in the SEM system at a more rapid rate, although it is not crucial to the operation of the present invention.
The crystals thus wrapped are placed into the vaporization chamber 30 and the stub holder 58 is subsequentially screwed into place above the crystals until the vapor gap is approximately 0.5 millimeters wide. The stub 16 with the specimen 14 attached thereto is then inserted into the stub holder 58, and the deflection shield 22 is then snapped on. It is assumed at this point that the device 10 is mounted to the SEM stage 16, and thus evacuation of the microscope chamber is begun at this time.
When the microscope chamber is fully evacuated, the reversible polarity DC supply 46 is switched on and the polarity is selected so that heat is applied from the thermoelectric device 44 to the heat sink 36. In this manner the vaporization chamber 30 may be heated to a suitable operating temperature, such as 80C. The temperature indicator 86 is used to monitor the temperature of the vaporization chamber, as will be apparent to those skilled in the art.
Heating of the chloral hydrate crystals contained in the vaporization chamber 30 causes the crystals to vaporize, with the result that the emitted vapors rise toward the deflection shield through the apertures 32 around the peripheryof the vaporization chamber 30. It will be noted that a near vacuum exists in the region of the device 10, and. thus the vapors rise in a virtually straight line. Accordingly the rising vapor molecules will impinge upon the angled outer surface 78 of the vapor flow controlflange and will be directed along a line parallel to the angle of the surface 78 into the side wall of deflection shield 22. The deflection shield will deflect these molecules back into the region occupied by the sample 14 so that the vapor molecules may engage the surface of the sample 14 and neutralize any charge developing onthe surface of the sample 14.
The variable power supply 54 may also be energized to supply an ionizing potential to the wires 51 for the purpose of ionizing vapor molecules as they rise through the ionization ring 50. The ionized molecules have a somewhat greater ability to neutralize charges developing on the sample 14. However, the apparatus of the present invention can be operated without ionization of the rising vapor molecules.
The contrast of the output information appearing on the imaging system 26 (FIG. 3) may be varied by adjusting the output of voltage supply 56 to change the bias potential of deflection shield 22. It has been found that potentials in the range between 0 and 00 volts are most effective in adjusting image contrast. In this regard attention is directed to FIG. 5 which is a graphical representation of output signal level (vertical axis) a curve 90 represents background signal level. The distance between curves88 and 90 represents'the image contrast in the output signah FIG. illustrates that the image contrast is sharply reducedwhen negative potentials are applied to the deflection shie ld22 and ismore gradually reduced when positive potentials are applied. Such control over image contrastis useful in all situations in which normal imagecontrast is too great for convenient interpretationor recording of the output image. In some cases, for example,-image contrast extends beyond the range of sensitivity of conventional photographic recording films so that a moderate reduction in contrast results in improved final images. In other situations the contrast between two portions of a specimen image is so great that the entire specimen image cannot be clearly observed or recorded. In such situations the contrast reducing capability of the present invention greatly improves the clarity of the output image.
It is further illustrated in FIG. 5 that any biasing of the deflection shield causes a drop in the signal level which must be compensated for in the video amplification of the microscope imaging system. However, while deflection shield biasing tends to reduce the output signal level, vapor presence appears to raise the signal level somewhat as will be apparent by reference to FIG. 6.
FIG. 6 is a graphical representation of output signal level (right hand vertical axis) versus vaporization chamber temperature (horizontal axis). The vapor pressure of chloral hydrate is also represented. More particularly the vapor pressure of chloral hydrate is represented by a curve 92 in FIG. 6, while a curve 94 represents background signal level, a curve 96 represents object signal level in the presence of ionized chloral hydrate molecules and a curve 98 represents the object signal level in the presence of un-ionized chloral hydrate molecules. FIGS. 5 and 6 show data conditions for a non-charging sample.
It will be seen from a comparison of FIG. 5 and 6 that the presenceof vapor tends to enhance the signal level i to at least partially compensate the decrease in the signal level due to the deflection shield biasing.
Although chloral hydrate was previously mentioned as the'preferred compound for use with the present invention, it should be noted that other compounds such as Freon TF (TM), paradichlorobenzene and other organic halogen compounds may be also conveniently used.
It should also be noted that while the preferred embodiment of the invention includes a chamber within the device 10 for generating the described charge-neutralizing vapor, the vapor may be introduced into the chamber 30 from a reservoir external to the main body of the device. For example, the vapor may be purchased from a commercial producer and introduced into the chamber 30 from a suitable tank.
It is believed clear that the teachings set forth herein identify to those skilled in the art a method and apparatus for both controlling charging artifacts and controlling image contrast in ,the specimen zone which will substantially enhance and improve theperformance of conventional scanning electron microscopes,
Obviously, numerous modifications and variations of the present invention are possible in lightof the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
What is claimed as new and desired to be secured by letters patent of the United States is:
l. A method for controlling electron beam specimen charging in an electron microscope comprising the steps of:
introducing a charge-neutralizing vapor into said electron microscope, directing said vapor into a region within said microscope at which said specimen is normally positioned; and, neutralizing charges'accumulated on said specimen due to impingement of said electron beam on said sample by contacting said sample with said vapor. 2. A method as in claim 1, wherein said step of introducing includes the step of:
generating said vapor within said electron microscope. 3. A method as in claim 1, wherein said step of introducing further comprises the step of:
heating a substance which normally exists in a nonvaporous state until vaporization occurs. 4. A method as in claim 1, wherein said step of introducing includes the step of:
vaporizing chloral hydrate crystals. 5. A method as in claim 4, further comprising the step of:
placing said chloral hydrate in an evacuated chamber within said electron microscope prior to said step of heating. 6. A method as in claim 1, further comprising the step of:
ionizing said vapor prior to said step of neutralizing. 7. A method as in claim 6, wherein said step of ionizing includes the step of:
passing said vapor by a pair of charged conductors. 8. An apparatus for controlling charging artifacts and for controlling image contrast in electron microscopes comprising:
body means in said apparatus for defining a vaporization chamber, heat control means thermally coupled to said body means for controlling the temperature of said vaporization chamber, specimen holding means coupled to said body means for supporting a specimen in said apparatus; and, electrode means secured to and electrically insulated from said body means for supplying an image contrast control potential to a region adjacent said specimen holding means. 9. An apparatus as in claim 8, wherein said heat control means comprises:
a thermoelectric device for heating and cooling said vaporization chamber. 10. An apparatus as in claim 9, wherein said thermoelectric device comprises:
a device which produces or absorbs heat in accordance with the Peltier effect. 11. An apparatus as in claim 9, further comprising: heat sink means mounted between said thermoelectric device and said body means for storing heat; and, temperature indicating means coupled to said body means for monitoring temperatures within said vaporization chamber.
9 12. An apparatus as in claim 8, further comprising: variable direct current voltage supply means adapted to be coupled to said electrode means for applying an adjustable contrast control voltage thereto. 13. An apparatus as in claim 8, further comprising: ionization means coupled between said body means and said electrode means for applying an ionizing potential to vapors emanating from said vaporization chamber. 14. An apparatus as in claim 13, wherein said ionization means comprises:
a ring of insulating material having a plurality of conductors mounted to an interior surface thereof. 15. An apparatus as in claim 14, further comprising: variable voltage supply means coupled to said conductors for applying an adjustable ionizing potential thereto. 16. An apparatus as in claim 8, wherein said specimen holding means comprises:
stub holder means for holding a specimen stub; and vapor flow control flange means integral with said stub holder means. i 17. An apparatus as in claim 16, wherein said specimen holding means further comprises:
threaded means for fastening said specimen holding means to said body means and for adjusting the position of said vapor flow control flange whereby vapor flow from said vaporization chamber may be regulated.
18. An apparatus as in claim 16, wherein said stub holder means comprises:
a structure having an axial aperture therein for re ceiving a stub mounting leg; and
detent means resiliently mounted in said structure for engaging said stub mounting leg.
19. Apparatus as in claim 8, further comprising:
vapor flow control means including said electrode for directing vapor generated in said vaporization chamber into a region adjacent said specimen holdingme'ans.
20. An apparatus for controlling charging artifacts and for controlling image contrast in electron microscopes comprising:
body means in said apparatus defining a chamber,
means for supplying a charge-neutralizing vapor to said chamber,
specimen holding means coupled to said body means for supporting a specimen in said apparatus; and, electrode means secured to said body means for supplying an image contrast control potential to a region adjacent said specimen holding means.

Claims (20)

1. A method for controlling electron beam specimen charging in an electron microscope comprising the steps of: introducing a charge-neutralizing vapor into said electron microscope, directing said vapor into a region within said microscope at which said specimen is normally positioned; and, neutralizing charges accumulated on said specimen due to impingement of said electron beam on said sample by contacting said sample with said vapor.
2. A method as in claim 1, wherein said step of introducing includes the step of: generating said vapor within said electron microscope.
3. A method as in claim 1, wherein said step of introducing further comprises the step of: heating a substance which normally exists in a non-vaporous state until vaporization occurs.
4. A method as in claim 1, wherein said step of introducing includes the step of: vaporizing chloral hydrate crystals.
5. A method as in claim 4, further comprising the step of: placing said chloral hydrate in an evacuated chamber within said electron microscope prior to said step of heating.
6. A method as in claim 1, further comprising the step of: ionizing said vapor prior to said step of neutralizing.
7. A method as in claim 6, wherein said step of ionizing includes the step of: passing said vapor by a pair of charged conductors.
8. An apparatus for controlling charging artifacts and for controlling image contrast in electron microscopes comprising: body means in said apparatus for defining a vaporization chamber, heat control means thermally coupled to said body means for controlling the temperature of said vaporization chamber, specimen holding means coupled to said body means for supporting a specimen in said apparatus; and, electrode means secured to and electrically insulated from said body means for supplying an image contrast control potential to a region adjacent said specimen holding means.
9. An apparatus as in claim 8, wherein said heat control means comprises: a thermoelectric device for heating and cooling said vaporization chamber.
10. An apparatus as in claim 9, wherein said thermoelectric device comprises: a device which produces or absorbs heat in accordance with the Peltier effect.
11. An apparatus as in claim 9, further comPrising: heat sink means mounted between said thermoelectric device and said body means for storing heat; and, temperature indicating means coupled to said body means for monitoring temperatures within said vaporization chamber.
12. An apparatus as in claim 8, further comprising: variable direct current voltage supply means adapted to be coupled to said electrode means for applying an adjustable contrast control voltage thereto.
13. An apparatus as in claim 8, further comprising: ionization means coupled between said body means and said electrode means for applying an ionizing potential to vapors emanating from said vaporization chamber.
14. An apparatus as in claim 13, wherein said ionization means comprises: a ring of insulating material having a plurality of conductors mounted to an interior surface thereof.
15. An apparatus as in claim 14, further comprising: variable voltage supply means coupled to said conductors for applying an adjustable ionizing potential thereto.
16. An apparatus as in claim 8, wherein said specimen holding means comprises: stub holder means for holding a specimen stub; and vapor flow control flange means integral with said stub holder means.
17. An apparatus as in claim 16, wherein said specimen holding means further comprises: threaded means for fastening said specimen holding means to said body means and for adjusting the position of said vapor flow control flange whereby vapor flow from said vaporization chamber may be regulated.
18. An apparatus as in claim 16, wherein said stub holder means comprises: a structure having an axial aperture therein for receiving a stub mounting leg; and detent means resiliently mounted in said structure for engaging said stub mounting leg.
19. Apparatus as in claim 8, further comprising: vapor flow control means including said electrode for directing vapor generated in said vaporization chamber into a region adjacent said specimen holding means.
20. An apparatus for controlling charging artifacts and for controlling image contrast in electron microscopes comprising: body means in said apparatus defining a chamber, means for supplying a charge-neutralizing vapor to said chamber, specimen holding means coupled to said body means for supporting a specimen in said apparatus; and, electrode means secured to said body means for supplying an image contrast control potential to a region adjacent said specimen holding means.
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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5033834A (en) * 1989-01-30 1991-07-23 Micron Technology, Inc. Microscope specimen mount converter
US6140581A (en) * 1997-12-03 2000-10-31 Mitsubishi Electronics America, Inc. Grounded packaged semiconductor structure and manufacturing method therefor
US6608305B1 (en) 2000-02-29 2003-08-19 National University Of Singapore Selective deposition of a particle beam based on charging characteristics of a sample
US7378664B1 (en) * 2006-05-12 2008-05-27 Thermo Electron Scientific Instruments Llc Deicing of radiation detectors in analytical instruments

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2890342A (en) * 1954-09-29 1959-06-09 Gen Electric System for charge neutralization
US3505521A (en) * 1965-11-25 1970-04-07 Balzers Patent Beteilig Ag Electron emission microscope object manipulator

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2890342A (en) * 1954-09-29 1959-06-09 Gen Electric System for charge neutralization
US3505521A (en) * 1965-11-25 1970-04-07 Balzers Patent Beteilig Ag Electron emission microscope object manipulator

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5033834A (en) * 1989-01-30 1991-07-23 Micron Technology, Inc. Microscope specimen mount converter
US6140581A (en) * 1997-12-03 2000-10-31 Mitsubishi Electronics America, Inc. Grounded packaged semiconductor structure and manufacturing method therefor
US6608305B1 (en) 2000-02-29 2003-08-19 National University Of Singapore Selective deposition of a particle beam based on charging characteristics of a sample
US7378664B1 (en) * 2006-05-12 2008-05-27 Thermo Electron Scientific Instruments Llc Deicing of radiation detectors in analytical instruments
US20080121801A1 (en) * 2006-05-12 2008-05-29 Howard James V Deicing of radiation detectors in analytical instruments
WO2008060326A3 (en) * 2006-05-12 2008-11-20 Thermo Electron Scient Instr Deicing of radiation detectors in analytical instruments

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