METHOD OF CONTACTING SEMICONDUCTOR CATHODES AND OF MANUFACTURING AN ELECTRON TUBE PROVIDED WITH SUCH A CATHODE

Abstract: The invention relates to a contact (9) for a semiconductor cathode consisting of one of the metals Ag, Au Cu (11) and one of the metals Ta, Ti, V (10). Such a contact does not exhibit any degradation when the cathode, after mounting in a vacuum tube, is heated several times to approximately 850°C for cleaning purposes.

Description: The invention relates to a method of manufacturing, notably contacting a semiconductor cathode having a surface zone of a first conductivity type in a semiconductor region at least partially surrounding the surface zone. The invention also relates to a method of manufacturing an electron tube comprising such a semiconductor cathode. Finally the invention relates to a semiconductor cathode and an electron tube manufactured by means of the said methods. The method according to the invention is particularly but not exclusively suitable for semiconductor cathodes of what is commonly referred to as the reverse biased junction type as described, inter alia , in the Netherlands Patent Application No. 7905470 in the name of the Applicant. As described in the said Patent Application the emitting surface is coated with a layer of material decreasing the electron work function, preferably a mono-atomic layer of pure caesium in order to obtain a satisfactory efficiency. To this end the emitting surface must be cleaned in advance. This cleaning operation, which is also desirable when the layer of material decreasing the work function is not provided, is carried out by heating the semiconductor cathode after it has been mounted in the electron tube and after evacuation of the electron tube to a temperature which is sufficiently high (approximately 850°C) to remove all unwanted elements from the emitting surface. This heating temperature is generally so high that contacts conventionally used in the semiconductor technology such as, for example, aluminium, gold and silver contacts, provided by means of soldering, ultrasonic bonding or thermocompression are not resistant thereto, inter alia , because eutectic alloys or (in silicon cathodes) silicides are produced or material is attacked by melting or evaporation. Such problems notably occur if the depth of the surface zone is approximately 5µm or less; due to the said phenomena for example, short circuit may be produced between this zone and the surrounding semiconductor region. When using contacts of materials melting at higher temperatures such as, for example, tantalum contacts provided by means of laser welding, such problems do not occur but the weld may become unreliable due to crack formation. A method according to the invention in which the said problems are avoided as much as possible is characterized in that the surface zone is provided with a contact comprising at least one layer of a first metal from the group of tantalum, titanium and vanadium and one layer of a second metal from the group of gold, silver and copper and in that the contact is obtained by means of a thermal treatment. In this application thermal treatment is understood to mean conventional bonding techniques at elevated temperatures such as, for example, thermocompression, resistance welding, laser welding, etc. A preferred embodiment of the invention is characterized in that the layer of the second material is directly provided on the semiconductor surface and has a thickness which is at most 0.25 times the depth of the surface zone of the first conductivity type. A semiconductor cathode obtained by means of this method can be heated after mounting in an electron tube to temperatures of between 800°C and 950°C without the said short-circuit occurring because the thickness of the second metal layer is so thin that the formation of possible eutectic compounds and/or silicides is limited to a thin upper layer of the surface zone of the first conductivity type. In practice it is found that contacting of silicon semiconductor cathodes remains intact with out any degradation, even in the case of heating several times to temperatures which are far above the eutectic temperature of silicon and the second metal. Particularly, the combination of tantalum and silver was found to yield very stable contacts, notably if they were provided by means of thermocompression. The cathode obtained by this method can subsequently be introduced in an electron tube by means of a method in which the semiconductor cathode is heated to a temperature of between 800°C and 950°C after the semiconductor cathode has been mounted in the electron tube and this tube has been sealed. The semiconductor surface cleaned by means of this thermal treatment has a substantially uniform emission behaviour. In addition a material decreasing the work function, preferably a mono-atomic layer of caesium can be precipitated without any difficulty on such a clean surface. The invention will now be described in greater detail with reference to an embodiment and the drawing in which Figure 1 is a diagrammatic plan view of a semiconductor cathode provided with a contact obtained by a method according to the invention; Figure 2 diagrammatically shows a cross-section taken on the line II-II in Figure 1 and Figure 3 diagrammatically shows an electron tube manufactured by means of a method according to the invention. The semiconductor cathode 1 (Figures 1, 2) has a p-type substrate 2 of silicon with an n-type zone having a depth of approximately 5 micrometers on a surface 3. This is a semiconductor cathode of what is commonly referred to as the "reverse biased junction" type. For a detailed description of the operation of such a semiconductor cathode reference is made to the above-cited Netherlands Patent Application No. 7905470. The actual electron-emitting region is present at the area of the circular emission region 5 in Figure 1 where the surface can be coated with a mono-atomic layer of caesium in order to increase the emission efficiency. This layer of caesium is provided after the cathode is mounted on the end wall 7 of the electron tube 6 (Figure 3) and the electron tube 6 is evacuated. The other elements of the electron tube 6 such as, for example, deflection units etc. are omitted in Figure 3 as well as a caesium source for providing the mono-atomic layer of caesium. Before the layer of caesium can be provided, the surface 3 must first be cleaned at the area of the emitting region 5; this is effected by heating the cathode 1 to approximately 850°C, for example, by means of a heating resistor. As described in the opening paragraph the connection wires 9 according to the invention are manufactured from a first layer 10 of tantalum which melts at a high temperature and a second layer 11 of silver which melts at a much lower temperature, the silver layer in this embodiment having a thickness of approximately 1 micrometre. Since this layer is thin with respect to the depth of the surface zone 6, a contact is obtained which is found to be satisfactorily resistant to the high temperatures in subsequent steps for manufacturing the electron tube, notably cleaning of the emitting surface. The silver-tantalum connection wires 9 are obtained by precipitating a thin layer of silver on a tantalum foil whereafter the connection wires or tapes are formed therefrom by means of cutting. The double layer of silver-tantalum is subsequently secured to the surface 3 at the area of the semiconductor zone 4 by means of thermocompression. The connection wires 9 are passed outwards through lead-throughs in the end wall 7, as well as a connection wire 12 for contacting the substrate 2. After the cathode is thus secured, the tube 6 is vacuum-exhausted or filled with an inert gas and subsequently sealed. Subsequently the cathode is heated to approximately 850°C by means of a heating resistor for cleaning the emitting surface. Due to the small thickness of the silver layer 11 with respect to that of the n-type zone 4 there is no degradation of the pn-junction 8. Finally a mono-atomic layer of caesium is provided in a conventional manner on the emitting surface from a caesium reservoir not shown. An electron tube according to the invention is then obtained. The invention is of course not limited to the embodiment shown but several variations are possible within the scope of the invention. For example, a layer of tantalum of approximately -.2µm may be provided in advance on the surface 3, which layer covers the underlying semiconductor body. In that case the silver layer 11 may have a larger thickness. Although the embodiment refers to a pn-junction 9, a pin structure may be alternatively used instead of a pn-structure for the semiconductor cathode. In addition the surface 3 may be provided with an insulating layer on which acceleration electrodes may be provided, if necessary, around the emitting region 5 as described in the Netherlands Patent Application No. 7905470.

Claims: 1. A method of manufacturing, notably contacting a semiconductor cathode having a surface zone of a first conductivity type in a semiconductor region at least partially surrounding the surface zone, characterized in that the surface zone is provided with a contact comprising at least one layer of a first metal from the group of tantalum, titanium, vanadium and one layer of a second metal from the group of gold, silver, copper and in that the contact is obtained by means of a thermal treatment. 2. A method as claimed in Claim 1, characterized in that the layer of the second material is directly provided on the semiconductor surface and has a thickness which is at most 0.25 times the depth of the surface zone of the first conductivity type. 3. A method as claimed in Claim 1 or 2, characterized in that the first metal is tantalum and the second metal is silver. 4. A method as claimed in Claim 1, 2 or 3, characterized in that the thermal treatment consists of thermocompression or laser welding. 5. A method as claimed in any one of Claims 1 to 4, characterized in that the semiconductor material is silicon. 6. A semiconductor cathode manufactured by means of a method as claimed in any one of the preceding Claims. 7. A method of manufacturing an electron tube, characterized in that a semiconductor cathode manufactured by means of a method as claimed in any one of Claims 1 to 5 is provided in an electron tube and in that the semiconductor cathode is heated to a temperature of between 800°C and 950°C after sealing the electron tube. 8. A method as claimed in Claim 7, characterized in that the surface of the semiconductor cathode is coated with a material decreasing the electron work function. 9. A method as claimed in Claim 8, characterized in that a mono-atomic layer of caesium is provided as a material decreasing the electron work function. 10. An electron tube manufactured by means of methods as claimed in any one of Claims 7 to 9.

Inventors: PETERS, JACOBUS MARIA PETERS, STOFFELS, JACOBUS, VAN ESDONK, JOHANNES
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H01J29
IPC codes: H01J1, H01J9, H01J29