US3385949A - Ion gun apparatus for treatment of surfaces with beams of very small size - Google Patents

Description

May 28, 1968 HEIL 3, .949

ION GUN APPARATUS FOR T TMENT OF SURFACES WITH BEAMS OF VERY SMALL SIZE Filed Oct 215, 1964 Hons Hei I,

INVENTOR.

. ATTORNEY.

QLHRUH gym United States Patent f AESTRACT ()F THE DISCLOSURE An electron beam machining and etching apparatus used in the formation of miniaturized electronic circuits. The apparatus includes an electron beam gun having a cathode hairpin Wire and an alkali metal vapor environment. A focuser is mounted at the gun outlet for transmission therethrou-gh of a stream of electrons. The focuser includes a thin sheet material with a concentric limiting aperture of one mil diameter and a converging lens element having an aperture of mils diameter. A series of electron lens elements are positioned in alignment with the cathode and focuser. A micromanipulator is positioned below the focuser and adjacent a target to be bombarded with the stream of electrons. The focuser and target are electrically connected such that an accelerating electric field is created therebetween. The micrornanipulator is made to scan a desired area whereby the bombarding current is controlled in the gun so that an intended pattern results.

This invention relates to apparatus for generating a beam of ions or electrons, directing and post focusing the beam of ions or electrons to a fine spot and precise and reproducible micromanipulation of a target to be subjected to the action of the focused beam of ions or electrons.

Ion beam sputtering and electron beam machining and etching operations can be used in the formation of miniaturized electronic circuits. Maximization of the geometrical accuracy of the ion or electron beam is desirable to produce the finest structures having sharp, reproducible contours. For example, in electron beam machining and etching to produce fine patterns out of a metal film, the degree of resolution obtainable is partially due to the fineness of the electron beam employed. Similarly, in doping semiconductor materials by implanting ions therein and in ion bombardment of such materials to facilitate the making of electrical contact thereto in making miniaturized electronic circuits extremely fine beams of ions are desirable. Similarly, in the making of masks through which patterns of metal films are obtained by evaporation, one is limited in fineness by the sharpness of the edge of the mask where it touches the surface. It is, therefore, desirable to make such masks with sharper edges. Apparatus of this type is, of course, applicable to many other situations involving high resolution and accurate, precise, and reproducible positioning of target materials for ion or electron bombardment.

Accordingly, it is an important object of this invention to provide an apparatus for producing a very fine beam of electrons or ions.

It is another object of this invention to provide an apparatus for post-focusing of a beam of ions or electrons to a very fine spot.

Additional objects of this invention will become apparent from the following description, which is given primarily for purposes of illustration, and not limitation.

Broadly stated in general terms, the objects of this invention are attained by providing apparatus for ion and electron bombardment of surfaces which comprises an ion and electron gun, a focuser which includes a concentric, communicating hole of about 1 mil serving as 3,385,9 .39 Eatentecl May 28, 1968 a limiting aperture, and a lens in the form of a hole of a diameter of about 5 mils, and a micromanipulator or micrometer cross slide stage for accurate and precise positioning of a target for bombardment by ions or electrons.

A more detailed description of a specific embodiment of the invention is given below with reference to the accompanying drawing, wherein:

FIG. 1 is a vertical sectional view showing the gun, focuser and a portion of the micromanipulator;

FIG. 2 is a vertical sectional view, drawn to an enlarged scale, showing the focuser; and

FIG. 3 is a partial vertical sectional detail view, drawn to a still larger scale, showing details of the lens and limiting aperture of the focuser.

The apparatus according to the present invention includes an ion or electron gun 10, an electron optical beam focuser 11, and a mioromanipulator 12. Although any of the well known types of ion guns may be employed, an embodiment of a gun based on the phenomenon of surface ionization of alkaline metal vapors is described in detail. The gun 10 is of the conventional cathode ray electron tube gun type, the individual lens elements being joined and held in their position by glass rods along the structure. The cathode assembly, however, differs.

Instead of a customary fiat oxide coated cathode, a tungsten hair pin cathode 13, mounted from a glass stem is used. In addition, all the voltages applied to the electrodes 14 and 15, and measured with reference to cathode 13, are applied with either positive or negative polarity; e.g., for operation with ions; the ultimate gun voltage is several krilovolts negative, cathode 13 is positive, and the control grid 13a is more positive than the cathode. According to known electron optical principles, the paths of ions throughout the gun for the case of ion operation are identical to the paths of electrons for the case of electron operation of the gun, since only electrostatic fields are used to deflect the charge carriers.

Lastly, provision is made for the presence of an alkali metal vapor, such as cesium, either within the entire volume of the gun, or preferably at the tip of the hairpin 13. This is provided for by means of pellets containing cesium chromate and silicons which are mounted close to the wall of the gun, such as indicated at 16, in such a fashion that they can be heated from outside the gun by an RF heater.

In this manner, ion gun 10 furnishes an essentially parallel beam of ions of high current density. Gun 10 also allows, by simple several of all voltages, the provision of an electron beam having properties resembling those of the ion beam.

At the bottom of gun 1b is an electron optical beam focuser 11 into which the ion beam enters through two pairs of deflection plates 17 which align the beam into the focuser. Focuser 11 contains one aperture 18 of the smallest size hole which can conveniently be drilled, e.g. 1 mil. The gun volume is sealed against the space below focuser 11, which space contains the target 19, except for small hole 18 in the focuser. In this manner the migration of alkali metal vapors from the gun space to the target space in minimized.

The manufacture of focuser 11 achieved by brazing a thin sheet 20, of metal such as nickel, into the bottom of focuser 11. Smaller hole 18 and larger hole 21 are formed by concentric drilling through sheet 20 and bottom 22, respectively. First the larger hole 21, the lens, is drilled, and then the smaller hole 18, the limiting aperture, is drilled. Since concentricity is important for a very sharp concentration of the beam, the drilling of both holes is accomplished with the same tool. The spark drilling method was used.

In operation, the focuser 11 is maintained at a desired potential. The two holes 18 and 21 are maintained at the same potential and to the surface immediately below the lower hole 21 is applied a constant electric field of such direction as to accelerate the ions passing through this aperture. The upper hole 18, of about 1 mil diameter, serves as a limiting aperture; being in a field free space, there is no lens action at its position. The lower hole 21, which has a diameter of about 5 mils, constitutes a convergent lens for the ion beam.

The electric field at the surface of the target results from the potential difference maintained between focuser l1 and target 19. Thus, the closer the target, the larger the field. Within the range of use of aperture lens 21 its focal length is independent of its bore diameter. It also is independent of the spacing of the accelerating electrode so long as the voltage ratio is maintained.

Under ideal conditions of parallel ion beam arrival, homogeneous extracting field and negligible space charge forces at the focus, the voltage ratio of focuser to target, is 1 to 9 for best focus. Thus, target 19 should be at about 9 times more negative potential than focuser 11. The spacing and parallelism between focuser 11 and target 19 are not critical.

Micromanipulator 12 is a micrometer cross slide stage including accurate electrical feelers 22 and 23 used to measure very small travel and to indicate the repetition of a certain position of target 19. The reproducibility of the mechanism is accurate to better than 1 micron. The movements of manipulator 12 can, of course, be programmed to follow a line pattern which is desired.

Alternately, one may make use of the property of the gun that the current directed to the target can be readily controlled by the potential applied to the control grid, of the gun. In this regime of operation, one scans the the target under the gun in television scan fashion, or one scans a straight line which is displaced at the end of the scanned line in a perpendicular direction to the line in small steps. At the same time and synchronous with the scan the gun current is turned on whenever one desires to affect the target surface and remains currentless in the times between. The image thus reproduced may be read from a special drawing by a flying spot scanner or by a curve tracer, devices which are known in the art of television or of X-Y recorders.

The table 24 of micromanipulator 12 supports target holder 25. A knob 26 is connected to a shutter 27 for operation of the shutter outside evacuated space 28, which is pumped by diffusion through conduit 29. Gun is mounted upon a glass plate 30 and focuser 11 is con nected to the bottom of the gun by internal threads at 31.

When operating with an electron beam, with reversed polarity of potentials applied to gun 10, the generated electrons are focused as described above in the case of the ion beam. Contrary to the ion beam, the electron beam it post focused to a fine spot is generally not capable of heating the target material to a great extent, and generally is not suitable for treatment of the target materials or for machining it to high degrees of fineness as in the ion beam. However, it is used for diagnostic purposes.

Using the electron gun for diagnosis implies that at the point of electron impact the electric field is altered in such a way that the slow, secondary electrons released by the primary beam are collected at another electrode or are made to enter a secondary multiplier. A signal derived from the secondary current is then displayed on the screen of a display tube in sychronism with the scanning movement of the target. This permits orientation within the microscopically small area and the localization of the components of the circuit.

Additional design data is helpful in describing the operation of the apparatus. The spacing between focuser 11 and target 19 is held between and 100 mils. The post-acceleration voltage is 50 kv. with the gun operating at a voltage of 6 kv. Beam post-focus compressions of more than one to ten linearly have been observed in a focuser which did not have a limiting aperture. Area compression ratios of several hundred, and a spot size reduction to one micron are possible.

An alternate design of the ion gun uses a gas discharge of the Penning type or an ion gun of the type usual in mass spectrometers. However, the surface ionization gun has the superior feature of producing monochromatic ions and the chromatic aberrations of the focuser lens are then negligibly small with the consequence that a higher compression ratio and a smaller ion beam in the focus is achievable.

An ion gun based on surface ionization of alkali metal vapors also is useful for the present apparatus, In this case, the vapor flows through a heated porous body of a refractory metal. However, such guns are not easily controlled as to their current, whereas the hair pin gun is readily capable of current modulation analogous to a television picture tube gun.

Regarding the post-acceleration ratio necessary for best post focus, if ion space charges at the image point become important, the ratio is increased beyond the 1:9 figure necessary for a geometrical image and one can accommodate a certain amount of space charge influence. Rays entering the focuser in a slightly convergent fashion require a smaller ratio and vice versa.

Obviously many other modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention can be practiced otherwise than as specifically described.

What is claimed is:

1. A device for identifying minute details of a target surface comprising an apparatus for electron brombardment of surfaces which includes an electron beam gun for generating and transmitting a stream of electrons, a focuser mounted at the gun outlet of said gun for transmission therethrough of said stream of electrons generated by the gun, said focuser including a thin sheet material with a concentric limiting aperture in the form of a hole in the order of 1 mil diameter, and a lens element having a hole therein in the order of 5 mils diameter and serving as a converging lens, a target mounting means positioned in alignment with said gun and focuser for positioning a target thereon for bombardment by the focused stream of electrons, and electrical means connecting said focuser and target for creating an accelerating electrical field therebetween.

2. Apparatus for ion and electron bombardment of target surfaces which comprises an electorn beam gun for generating and transmitting a stream of electrically charged particles, a focuser mounted to the outlet of the gun for transmission therethrough of said stream of electrically charged particles generated by the gun, said focuser including a thin sheet material having a concentric limiting aperture in the form of a hole in the order of 1 mil diameter, and a lens element adjacent thereto having a hole therein in the order of 5 mils diameter and serving as a converging lens for said particles, said lens being bounded by an accelerating electric field between it and the target, and target mounting means positioned adjacent said focuser for accurate positioning of a target for bombardment by the focused stream of charged particles.

3. Apparatus for ion bombardment of target surfaces which comprises an electron beam gun for generating and transmitting a stream of ions, a focuser mounted at the gun outlet for transmission therethrough of said stream of ions generated by the gun, said focuser including a thin sheet material with a concentric limiting aperture in the form of a hole in the order of 1 mil diameter, and a converging lens element in the form of a hole in the order of 5 mils diameter, electric voltage means connecting said focuser and a said target surface, and target mounting means positioned in alignment with said gun and focuser for accurate positioning of a target for bombardment by the focused stream of ions.

4. The structure of claim 2 where said electron beam gun includes a cathode hairpin wire adapted to be electrically heated, a support for a vaporizable alkali metal and a series of electron lens elements in alignment with said cathode and said focuser.

References Cited UNITED STATES PATENTS 2,249,453 7/1942 Boersch et a1. 250-49.5

6 Dimmick 219-121 Fleming 250-495 Steigerwald 219-121 Masuda 250-495 Hetherington 219-69 Steigerwald 219-121 Sibley 219-121 Schleich et al. 219-121 Steigerwald 219-121 RICHARD M. WOOD, Primary Examiner.

W. D. BROOKS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,385,949 May 28, 1968 Hans Heil It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as 1 shown below:

Column 2, line 44, "silicons" should read silicone line 50, "several" should read reversal Column 3, line S9,"materials" should read material same line 59, "in" should read is Column 4, line 34, "1." should read 3. line 35, "brombardment" should read bombardment line 50, "2." should read l. line 51, "electorn" should read electron line 66, "3." should read 2. Column 5, line 4, l i reference numeral "2" should read l Signed and sealed this 4th day of November 1969.

SEAIJ attest:

WILLIAM E. SCHUYLER, J-R.

Edward M. Fletcher, Jr.

Commissioner of Patents attesting Officer

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