X-RAY SOURCE

Abstract: An X-ray source has a target anode (66) which has a conical apex portion (68) having a comparatively small angle (a1) and a conical base portion (70) having a larger angle (a2). The electron beam is, in use, directed at an inner surface region of the apex portion. The small angle of the apex portion increases the area which is impinged by the electrons, thus facilitating cooling and permitting a higher intensity to be achieved, and also reduces the apparent source size, thereby improving resolution. The apex and base portions may be of different materials (e.g. palladium and nickel or Ni/Cu alloy respectively) so that unwanted X-rays due to stray electrons bombarding the base portion are of such a wavelength as to be comparatively innocuous.

Description: X-RAY SOURCE Background of the Invention This invention relates to sources for generating X-rays. X-ray generators are used in a variety of applications of practical importance. One significant area in which such sources are empl oyed is the field of X-ray l thography. An illustrative X-ray lithographic system used to make structures such as very-large-scale-integrated semiconductor devices is described in an article by M.P. Lepselter entitled "Scal ng the Micron Barrier With X-rays," Technical Digest 1980 IEDM, page 42. An advantageous water-cooled X-ray source for inclusion in such a system is specified by J .R. Mal donado, M.E. Poulsen, T.E. Saunders, F. Vratny and A. Zacharias in "X-Ray Lithography Source Using a Stationary .. Solid Pd Target," Journal Vacuum Science Technology, Volume 16 , page 1942 (1979) . Such a source is also described in U.S. Patent No. 4,258,262. ' In an X-ray lithographic system of the proximity printing type, it is well known that the magnitude of the penumbra is deleteriously affected by the fact that the source of X-rays used to irradiate a mask in the system is in practice not an ideal point source but has instead a finite size. It is further known that increasing the penumbra decreases the resolution capabil ities of the system. Hence, as the drive towards a submicron (for example, < 0.5 micrometres) capabil ity for such an X-ray system continues, various approaches are being explored to decrease the size of the penumbra. One effective way of doing so is to reduce the diameter of the X-ray source included in the system. Additionally, efforts have been directed by workers at trying to increase the power capabilities of X-ray sources. It was recognized that such efforts, if successful , had the potential for decreasing the time required to expose resist-coated wafers in X-ray lithographic systems, thereby increasing the throughput. Alternatively, in such a higher-power system, the penumbra can be reduced while throughput remains unaffected. This is done by increasing the X-ray source-to-wafer distance while at the same time increasing source power to maintain a constant exposure time. Summary of the Invention With the invention as claimed the resolution and/or throughput properties of an X-ray system can be enhanced, since the smaller angle of the apex portion exposes a larger area of target to the electron beam and at the same time reduces the apparent source diameter. Brief Description of the Drawing An embodiment of the invention will now be described by way of example with reference to the accompanying drawing, in which: FIG. 1 is a schematic representation of a specific illustrative X-ray lithographic system that includes a source made in accordance with the present invention; FIG. 2 shows a standard conical target of a known type; and FIG. 3 depicts a specific illustrative double-angle conical target that embodies the invention. Detailed Description In a generalized schematic way, FIG. 1 of the drawing shows the major components of an X-ray lithographic system. An electron gun 10 accelerates a beam of electrons, designated by dot-dash lines 12, towards a portion of the inside surface of an anode. The structure of the anode 14 will be described in detail below with reference to FIG. 3. In response to bombardment by electrons, the anode 14 emits X-rays which propagates downwards in FIG. 1, centered about longitudinal axis 16, through a beryllium window 18 to irradiate the upper surface of a conventional X-ray mask member 20 mounted in a cylindrical exposure chamber 22. By way of a specific example, the chamber 22 is shown open at the bottom end thereof and contains therein a helium atmosphere at a pressure slightly in excess of atmospheric pressure. Helium gas is introduced into the chamber 22 via an inlet tube 26. X-rays directed at the mask member 20 are designated by reference numeral 24. The mask is shown positioned in spaced-apart relationship with respect to a substrate 28, for example a wafer member, whose top surface is coated with a layer of a standard X-ray-sensitive resist material. In turn, the resist-coated substrate is mounted on a conventional work table 30. The anode 14 shown in the drawing is mounted in a circular opening on the bottom of a cylinder 32 whose upper portion is secured to the upper surface of a cylindrical vacuum chamber 38. Illustratively, the pressure within the chamber 38 is maintained in the range of 1.3xl0" 7 to 1.3x10-6 Pa (χo-9 t0 10-8 Torr). Advantageously, the chamber 38 is constructed to include two spaced apart walls that form between them a cooling jacket 40. Cooling of the chamber 38 is accomplished simply by circulating tap water through the jacket 40 via respective inlet and outlet pipes 42 and 44. The structure and operation of the electron gun 10 represented in FIG. 1 herein are described in detial in U.S. Patent No. 4,258,262 of J.R. Maldonado. Illustratively, the gun 10 comprises an annular dispenser-type tungsten cathode. By way of example, the gun 10 bombards the anode 14 with 25 kilo-electron-volt electrons. In addition, as described in the Maldonado patent, cooling of the anode 14 is carried out by directing a fluid such as water over the top surface of the anode in a precisely controlled manner. As described therein, this is done by positioning a so-called diverter 46 to encompass a portion of the anode 14. Fluid is delivered to the diverter by means of an inlet pipe 48. Cooling fluid is directed downward over the top surface of the anode 14 of FIG. 1 via a tube 49 that constitutes an extension of the inlet pipe 48 within chamber 54. The bottom end of the tube 49 is designed to fit into a cylindrically shaped recess portion formed in the top of the diverter 46. Fluid directed through the diverter 46 then flows via an annular gap formed between the diverter and the bottom inside surface of the cylinder 32 upwards through multiple passageways formed in the diverter 46. The fluid then flows upwards through the main interior chamber 54 of the cylinder 32 and through an outlet pipe 56. Further detail s concerning the diverter 46 and specific il lustrative operating characteristics of the overall system represented in the drawing herein are contained in the aforecited Mal donado patent. As described therein, a substantial ly uniform and turbulent fl ow of water characterised by nucl eate boil ing is establ ished in the immediate vicinity of the surface of the target anode to be cooled. In one advantageous system design that includes the target anode 14 (FIG. 1 ) , the X-ray wavelength chosen for resist exposure was the .436 nm (4.36 -Angstrom-unit) pal l adium l ine. Avai l able resist materials have hi gh sensitivity to X-rays of this wavelength. Additional ly, as is known, this wavelength is short enough to allow exposure to be made in helium at atmospheric pressure and to all ow the use of high-strength mask substrates. A conventional stationary water-cooled target anode 15 i s shown in FIG. 2. The anode 15 comprises a hol l ow cone made of pure or substantial ly pure pall adium having a wall thickness t in the range of 200-to-350 micrometres. As indicated, an annul ar beam of electrons represented by arrows 57 to 60 enters the open or base portion of the conical anode 15 and impinges upon an annul ar inner-surface region to cause X-rays to be emitted therefrom. The emitted X-rays are propagated downward in FIG. 2 in the direction of arrow 62. From bel ow, the X-ray source appears to be a spot. In one il lustrative case, the diameter of this apparent source is approximately 2 mil l imetres. In the course of propagating upward inside the anode 15 of FIG. 2, the converging electron beam exhibits a waist or cross-over region 17. After the waist 17 , the beam diverges before impacting the inner surface of the anode 15. As set forth in U.S. Patent No. 4,439,870 of M.E. Poul sen, F. Vratny and A. Zacharias, it is highly desirable to avoid diffusion of hydrogen species into the outside or water-cooled surface of a pal l adium anode. To avoid such diffusion in the conventional anode 15 of FIG. 2 or in the anode shown in FIG. 3, a limited-depth hydrogen-barrier l ayer is advantageously formed within the anode extending from the outside surface thereof, as specified in detail in U.S. Patent 4,439,870. In the standard conical anode 15 shown in FIG. 2, the entire inner surface of the base and apex portions thereof is disposed at the same angle with respect to longitudinal axis 64. In one such illustrative single-angle anode as heretofore made, the angle a ^ (FIG. 2) was approximately 12.5 degrees. In that particular anode, the inner surface area of the annular region impacted by electrons was about 14.5 square millimetres. The apparent diameter of the resulting source of X-rays was approximately 2 millimetres. The height jn and the diameter _d of the bottom opening of that particular anode were about 1.5 and 0.6 centimetres, respectively. In a conical target anode of the FIG. 2 or FIG. 3 type, there are two regions that are relatively poorly cooled by the high-velocity fluid directed along the outer surface thereof. These poorly cooled surface regions are in the vicinity of the apex and of the base of the anode where the circulating fluid experiences turbulence due to abrupt changes in direction in the fluid passageways. Fluid in those regions is relatively stagnant and exhibits a relatively low velocity. Hence, any significant amount of electron-beam power falling on inner surfaces of the anode opposite the poorly cooled regions may induce boiling in those regions. In turn, vapour may collect in those regions and thereby impede the desired high-velocity fluid flow along the outer surface of the anode. As a consequence, poor overall cooling of the target may result and burn-out thereof may occur in the region where the main portion of the electron beam impinges. As seen from FIG. 2, the main portion of the electron beam impacts the inner surface of the target anode 15 in a region that is relatively close to the aforementioned poorly cooled apex region. Accordingly, it is particularly desirable that the extent of this poorly cooled region from the apex of the anode be made as short as possible. In any case, the poorly cooled region should preferably not overlap to any appreciable extent the beginning of the outer surface region that is directly opposite the region of maximum electron-beam impact. Otherwise, the probabil ity of achieving high-power operation of such a target over extended periods of time is significantly diminished. In FIG. 3 the anode 66 includes an upper or apex portion 68 and a l ower or base portion 70. The inner surface of the portion 68 is disposed at an angle ai with respect to longitudinal axis 72 , and the inner surface of the portion 70 is disposed at an angle a2 with respect to reference line 74 that is paral lel to the axis 72. The angle ai is less than- the angle a_> Moreover, the angle ai is typical ly less than the angle a ^ of the standard anode shown in FIG. 2. By way of exampl e, the hei ght Ji of the anode 66 , the diameter ji of the bottom opening thereof and the anode thickess Jt shown in FIG. 3 are the same as the corresponding dimensions of the anode 15 of FIG. 2. The height hi of the apex portion 68 (FIG. 3) is about 6.25 mil l imetres, the height h2 of the base portion 70 is approximately 8.75 mill imetres, ai is approximately 8.5 degrees and &2 is about 15 degrees. The anode 66 of FIG. 3 comprises pal ladium, with a l imited-depth hydrogen-barrier layer, as described in the previously mentioned U.S. Patent No. 4,439,870. Further, the system util ized to cool the anode 66 includes a diverter 76 simil ar to the di verters described in U.S. Patent No. 4,258,262 and shown in FIG. 2 herei n. Several significant advantages stem from the fact that the apex angle ai shown in FIG. 3 is rel atively smal l . Fi rst, the extent of the change in direction in the fluid passageway of the diverter 76 in the immediate vici nity of the apex of the anode 66 is thereby reduced. As a result, the distance required along the outer surface of the anode for the perturbed fl ow to restabilize is al so reduced. Thus, the high-velocity fluid flow is re-establ ished and effective cool ing resumed at a point that is a relatively short distance from the apex of the anode 66. Consequentlv, the area of the apex portion of the anode that may be safely impacted by electrons is rel atively l arge. In turn, this rel axes the constraints on and facilitates the overall design of a system that includes an anode of the type depicted in FIG. 3. The fact that the inner surface of the apex portion of the anode 66 of FIG. 3 is relatively steep is the basis for other important advantages. Thus, for a specified input electron beam included at a given angle, the beam power per unit area of the impacted inner surface of the anode 66 is less than that of an anode with a less steep apex portion such as the standard anode 15 depicted in FIG. 2. For the particular case wherein (FIG. 2) is 12.5 degrees and ai (FIG. 3) is 8.5 degrees, the beam power per impacted unit area is less for the FIG. 3 structure by a factor determined by the ratio sin 17°:sin 25° or by approximately 0.7. Significantly, this translates into an increase of about 45 percent in the electron beam power that a given anode cooling system can handle. Accordingly, an improved high-throughput lithographic system exhibiting a long lifetime characteristic is thereby made feasible. Another advantage of the relatively steeply incl ned apex port on 68 (FIG. 3) is that the diameter of the apparent X-ray source is decreased compared to a larger-angle anode. Thus, again for the particular case wherein ^ (FIG. 2) is 12.5 degrees and ai (FIG. 3) is 8.5 degrees, the source diameter as viewed from below is reduced for the FIG. 3 structure by a factor approximately determined by the ratio tan 17° tan 25° or about 0.7. Thus, a 2-millimetre-diameter source can be converted into a 1.4-m llimetre-diameter source in an X-rav lithographic system by replacing the conventional anode 15 (FIG. 2) by the anode 66 (FIG.3). Significantly, the penumbra is thereby reduced and the resolution capabilities of the system accordingly enhanced. In practice, the maximum steepness of the inner surface of the apex portion 68 (FIG. 3) is determined by the configuration of the incident electron beam. More specifically, the diameter of the opening at the base of the apex portion 68 must be at least as large as the diameter w_ of the electron beam at its waist or cross-over region. The anode 66 shown in FIG. 3 cannot in practice simply consist of a relatively short apex portion 68 having the steep angle ai. This is because the turbulent effects caused by the change in direction near the bottom of the associated diverter must occur sufficiently far away from the electron-impacted region of the anode to insure that reliable cooling in the impacted region is not significantly affected. In actual systems, the height of the anode 66 will generally be at least about 1.5 centimetres. Additionally, the anode 66 (FIG. 3) cannot in practice simply consist of a relatively long apex portion 68 with the angle aj. The outline of the lower portion of such a steep single-angle anode is represented in FIG. 3 by dotted lines 78 and 80. As indicated, such a single-angle anode would not provide a sufficients large opening at its base to allow the incident electron beam to enter the interior of the anode without impacting the base of the structure. Thus the lower or base portion 70 of the anode 66 shown in FIG. 3 is angled out sufficiently to allow the entirety of the incident electron beam to enter the anode without striking the base portion 70 to any appreciable extent. This minimizes the likelihood of burn-out of the anode, for the reasons discussed earlier above, and also insures that virtually all of the available incident electrons end up impacting the target region of interest in the apex portion 68. In general, significant angling out of the opening of the base portion 70 (FIG. 3) beyond that needed to clear the incident electron beam should be avoided. This is so because as the angle a2 increases substantially beyond that represented in the drawing, the amount of turbulence introduced in the cooling fluid flow also increases substantially. And this turbulence, if excessive, may extend into and deleteriously affect the cooling action in the critical impacted region of the apex portion 68. The electron beam shown in FIG. 3 is represented in idealized form. In practice, some electrons traverse paths outside the beam extents depicted in the drawing. Accordingly, even in the structure shown in FIG. 3, some electrons strike the inner surface of the base portion 70 and cause unwanted X-rays to be emitted therefrom. In turn, these X-rays may irradiate portions of a resist-coated wafer that are not intended to be irradiated. Generation of these unwanted X-rays in a target anode can be el imi nated or at least substantial ly reduced by making the base portion 70 of the anode 66 of FIG. 3 of a material that emits X-rays whose wavelengths lie outside the sensitivity ranges of typical X-ray resists or above the cutoff wavelength of the beryl l ium window 18 (FIG. 1) . Thus, by way of example, it is advantageous to make the base portion 70 (FIG. 3) out of nickel or a nickel -copper all oy such as Monel alloy. As before, the apex portion 68 remains, il lustratively, made of pal l adium. Any unwanted X-rays emitted from the base portion of such a composite anode are largely outside the sensitivity range of typical X-ray resists used in a palladium-based system. To make such a composite anode having the two different materials are, for example, first brazed or wel ded together in stock form. Then the two-material stock is machined in a conventional way to form a double-angle target of the type depicted in FIG. 3. The geometry of an anode as shown in FIG. 3 can often in practice be tail ored to optimal ly match the confi guration of the beam provided by a particul arly advantageous electron source.

Claims: CLAIMS 1. An X-ray source having a general ly conical target (66 ) comprising X-ray emissive material for producing X-rays in response to electron bombardment of an inner surface region of the target characterised in that the target comprises an apex portion (68) which includes the said inner surface region and a base portion (70) , the angle between the inner surface of the target and the longitudinal axis (72) being greater 2) in the base portion than in the apex portion (ai ) . 2. A source as cl aimed in cl aim 1 wherein the inner surface of the apex portion (68) is of a different material from the inner surface of the base portion (70) . 3. A source as cl aimed in claim 2 wherein at least the inner surface of the apex portion (68) is of pal l adium and at least the inner surface of the base portion (70) is of nickel or a nickel -copper al l oy.

Inventors: GEORGIOU GEORGE E, POULSEN MARTIN ELLINGSEN, GEORGIOU, GEORGE, E., POULSEN, MARTIN, ELLINGSEN
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1985
Main classification: H01J35
IPC codes: H01J35