GB2038541A - Thermionic cathode and method of making same - Google Patents

Abstract

A thermionic cathode comprises an electron emissive tip (1) of lanthanum hexaboride, tip supporting heaters (2, 3) in close contact with respective sides of the tip and supporting the tip, and elastic electroconductive members (4, 5) in pressure contact with the outer surfaces of the tip supporting heaters for supplying an electric current to them. The tip supporting heaters are obtained by cutting a carbonaceous material having high anisotropy due to its layered structure obtained by hot pressing a synthetic resin such as acrylonitrile resin so as to form a pair of parallel planes perpendicular to the pressing direction when hot-pressed, and abrasion- polishing the parallel planes. The tip supporting heaters are brought into close contact with the tip and the elastic electro conductive members at the resulting smooth surfaces thereof by screws (8, 9). The cathode has a prolonged lifetime. <IMAGE>

Description

SPECIFICATION Thermionic cathode and method of making same This invention relates to thermionic cathodes and methods of their manufacture.

The present invention is particularly concerned with a therm ionic emission cathode using, as cathode tips, materials of calcium hexaboride type such as lanthanum hexaboride for emitting electrons, and preferably using highly oriented (anisotropic) carbonaceous materials which tightly hold such a cathode emission tip between suitable electroconductive members and at the same time supply thermal energy sufficient for emission of electrons as a heater.

The term "tip supporting heater" used hereafter refers to a member comprising a carbonaceous material and which supports or tightly holds the cathode emission tip between electroconductive members and concurrently heats the emission tip at working temperature.

Rare earth borides, in particular lanthanum hexaboride, have properties which make them highly suitable as a cathode tip material. The characteristic which has prevented the wide-range use of lanthanum hexaboride as an emitter material is its highly reactive nature at high operating temperatures. Most supporting members will react with lanthanum hexaboride to cause deterioration of the supporting members. Therefore, the lifetime of cathodes is disadvantageously short. In order to eliminate this disadvantage, various proposals have heretofore been made for therm ionic structures of this type.

U.S. Patent 4,054,946 discloses a cathode device which comprises holding a single LaB6 crystal tip with two pieces of vitreous carbon and supporting the pieces by molybdenum jaws.

However, vitreous carbon or glassy carbon materials are extremely rigid and prone to break into glassy fragments so that it is difficult freely to process them into a desired shape and size.

Further, U.S. Patent 4,068,145 discloses a device comprising a heating member using pyrolytic graphite or boron carbide as a means for heating an emitter tip which is designed so as to nullify an undesired shift of the emitter due to thermal deformation by supporting the heating member with elastic electroconductive members. However, this device consumes a considerable amount of electric power when the heater is in a heated condition, and temperature change is liable to occur even though a constant current is continuously supplied. Therefore, the stability is not satisfactory. Furthermore, the device becomes unavoidably large in scale and thus it was impossible to apply this device to conventional tungsten-made hairpin cathode devices.

The invention is defined in the appended claims, to which reference should now be made.

The invention will be described in more detail, by way of example, with reference to the drawings, in which: Figure 1 is an elevational view of apparatus embodying the present invention.

Figure 2 is a detailed elevational view of essential parts of the apparatus of Fig. 1.

Figure 3 is a graph showing the temperature change of a tip with the passage of time.

Figure 4 is a graph showing the change in probe current (measured with a Faraday cup).

The carbonaceous material used in the cathode apparatus illustrated has a layered structure and thus possesses anistoropy, and can relatively easily be prepared. Further when this material is employed as a heater and supporter or holder of a cathode tip, there are the advantages that the emission current becomes constant in a shortened time period, heating is effected with little power, and the temperature after heating (i.e. the working temperature) is stably maintained, as compared to the prior art system using the aforesaid pyrolytic graphite, glassy carbon or vitreous carbon.

The tip supporting heater illustrated can be prepared using a highly oriented carbonaceous moulding having a layered strutcure. The moulding is obtained by hot-pressing in a graphite die a solid moulding of a resin (condensation product or polymer), such as acrylonitile resin or the like, which does not melt under heating and is easily converted into graphite, under a pressure of 100 to 500 kg/cm2 at temperatures of 1 600 to 3000"C. in a non-oxidizing atmosphere. The moulding thus obtained has a layered structure in which a plane perpendicular to the c-axis of the graphite microcrystals is extremely highly oriented in the direction perpendicular to the pressing direction. Thereafter, the hot press moulding (ingot) is cut into a suitable size at the section perpendicular to the pressing direction to obtain a carbonaceous block.The carbonaceous block so obtained is generally in the form of a cube or rectangular prism. In such a case, the block is cut into small pieces in the direction perpendicular to the pressing direction of the aforesaid hot press and in the direction parallel to the pressing direction of the hot press.

Preparation Example Commercially available furane resin was moulded and hardened using commercially available aniline sulphate in a conventional manner to obtain a resin moulding of 200 mm diameter and 30 mm length. This resin moulding was charged into a graphite die of 700 mm outer diameter, 200 mm inner diameter and 700 mm height. While applying thereto a pressure of 200 kg/cm2 in the longitudal direction of the resin moulding, the resin moulding was heated to 2200"C and kept at the same temperature for 30 mins. Thereafter, the moulding was cooled to obtain a carbonaceous moulding of 200 mm diameter and a thickness of about 6 mm. A cubic block of 50 X 50 X 50 mm was cut out of the carbonaceous moulding thus obtained in such a manner that a pair of planes facing each other were perpendicular to the pressing direction of the hot press.This block showed a density of 2.1 0 g/cm3, a specific resistance of 5 x 103 El cm in the pressing direction and 3 x 102 2 cm in the perpendicular direction, and a thermal conductivity of 3 X 10-3 C.G.S. in the pressing direction and 1 X 10-i C.G.S in the perpendicular direction.

The cathode apparatus including the aforesaid highly oriented carbonaceous material will be described with reference to the drawings.

Reference number (1) denotes a lanthanum hexaboride cathode tip. Reference numerals (2) (3) each denotes a tip supporting heater for tightly holding the tip in close contact with both sides of the tip and positioned such that the side of the heater in close contact with the tip is perpendicular to the c-axis of graphite microcrystals, i.e., perpendicular to the pressing direction when hot-pressed. For this reason, the heater exhibits a high resistance and low thermal conductivity in the direction of current flow. Reference numerals (4)(5) denote resilient or elastic electroconductive supports imparting a holding or tightening force to the heater. The elastic electroconductive supports are fixed to a base (10) and connected with lead terminals (1 i)(1 2) penetrating into the base. The elastic supports are formed into a forked shape at their base portion.Screws (8)(9) for controlling the pressing force are engaged in outer supports (6)(7).

The ends of the respective screws are brought into contact with the outer surface of the elastic supports (4)(5). The screws for controlling a pressing force are necessary for preventing the tip and heater from thermal deformation or thermal distortion upon heating and for keeping the location of the top of the tip at the right position.

The above cathode apparatus is connected with a constant current source. When a fixed current is applied to the cathode apparatus, the heaters are rapidly heated so that the tip is heated to and maintained at its working temperature.

The heater material can easily be cut and processed into a desired shape and size. If the tip supporting heaters are made into the shape of a truncated pyramid as shown in Fig. 2, the contact area with the tip can be reduced and the contact area with the elastic supports can be made relatively large. By doing so, undesirable heating of the elastic electroconductive supports can be prevented and the tip can be heated more stably.

The cathode illustrated involves advantages-that the heating power is less, the time period for elevating the temperature of the tip, i.e., the time period until the tip reaches its working temperature, is shortened, and chemical as well as physical stability is attained at high temperatures.

In order to demonstrate such effects, the cathode device described and illustrated mounted to a scanning electron microscope (SEM) was compared with the cathode using highly anisotropic pyrolytic graphite, similarly mounted to an SEM. The results are shown in the table below.

Table 1 Tip supporting heaters Tip supporting embodying this Pyrolytic graphite heaters invention (A) heaters (B) Reactivity with No reaction after Substances which might tip operation for 1062 be reaction products hours appeared at the inter face with the tip after operation for 300 hrs.

(observed by an optical microscope) Time until the top of the tip reached 1600"C 8 mins 21 mins Heating current (tip temp 1600"C) 2.5A 2.7A Heating Power (") 7.9W 10.2W Temperature decrease (Fig. 3) small large Stability of prove current (Fig. 4) stable unstable Figs. 3 and 4 show the results obtained by measuring the probe current while observing the temperature of the tip, the curve for the heaters embodying this invention being denoted "This invention" and the curve for the pyrolytic graphite heaters being denoted "PG".

In the system using pyrolytic graphite, the temperature was markedly decreased at an initial stage. In addition, lowering of temperature and decrease in probe current are remarkable with the passage of time. Accordingly, it was impossible to continue this test after 300 hours lapsed.

At the end of the test, the surface of the pyrolytic graphite heater was observed and reaction products with the tip material were recognised there. However, the tip supporting heater embodying the present invention could be used stably even after 1000 hours lapsed. The pyrolytic graphite used in the above test was graphite obtained by heat treatment at 2000"C (PG 2000). In general, it is known that pyrolytic graphite shows high anisotropy with high temperature heat treatment. However, the electric current required for heating the tip using PG 3200 (heat treated at 3200"C) at 1600"C was 4.2 A and the power was 1 2.2 watts. This data indicates that only a degree of anisotropy is not a decisive factor for preference as tip supporting heater materals.The reason why PG 2000 is superior to PG 3200 would appear to be that PG 3200 has higher anisotropy due to the orientation property of crystal faces larger than is PG 2000 but, on the other hand, graphitization is advanced with PG 3200 to reduce its resistance.

Pyrolytic graphite subjected to heat treatment at lower temperatures contains non-crystalline phase and provides poor anisotropy. Therefore, such pyrolytic graphite is not suitable for use as heater materials. Considering the above, it is necessary that the otpimal temperature for heat treatment be chosen when using pyrolytic graphite as heater material. In addition, pyrolytic graphite does not form completely parallel planes even if its surface is subjected to abrasionfinishing. The completely smooth surface of the tip supporting heater embodying the present invention enables the supporting heater to be brought completely into contact with the tip.

Therefore, the tip supporting heater embodying the present invention provides less change in contact resistance and thus less change in current. In addition, the tip supporting heater embodying the present invention enables the tip stably and continuously to emit electron beams of high quality over a long period of time. The apparatus can be used as a cathode in the electron beam lithographic system which is indispensable for manufacturing VLSI (very large scale integrated circuits).

Claims (5)

1. A thermionic cathode comprising an electron emittable tip, tip supporting heaters of a carbonaceous material in contact with both sides of said tip, respectively, and supporting said tip, and resilient electroconductive members, fixed to a base, and which are brought into pressure contact with the outer surfaces of said tip supporting heaters, said tip supporting heaters being an assembly obtained by cutting said carbonaceous material, which is obtained by hot-pressing a moulding of a synthetic resin that does not melt under heating, so as to form a pair of planes parallel to said pressing direction, abrasively-polishing said parallel planes and then bringing the resulting smooth surfaces into close contact with said tip and said resilient electroconductive members.

2. A thermionic cathode according to claim 1, wherein the resilient electroconductive members have screws for controlling the pressing force for pressure contact with said tip.

3. A thermionic cathode according to claim 1 or 2, wherein said parallel planes of the tip supporting heaters are formed such that the area in contact with the resilient electroconductive members is larger than the area in contact with the tip.

4. A thermionic cathode substantially as herein described with reference to the drawings.

5. A method of making a thermionic cathode in accordance with any preceding claim, in which the tip supporting heater is formed by the steps of hot-pressing a moulding of a synthetic resin that does not melt under heating, cutting the carbonaceous material thus obtained so as to form a pair of planes parallel to the pressing direction, and abrasively polishing the said parallel planes.