DOPED SEMICONDUCTOR VIAS TO CONTACTS

Abstract: In the manufacturer of integrated circuits, an undope d wide band-gap semiconductor is used for the insulating layer to isolate the silicon substrate from the metal interconnection pattern. To provide vias through the insulating layer for connection to the source and drain of the transistors of the circuit, the wide band-gap semiconductor is implanted with a dopant selectively in the portion overlying the source and drain for making the implanted portion of low resistivity and of the conductivity type of the source and drain. Preferably, carbon is the wide band-gap semiconductor and nitrogen is the dopant implanted. This invention relates to the fabrication of integrated circuit devices.

Description: This invention relates to a method of fabricating IC devices in which conductive vias to contact areas on substrates are formed by reducing the resistivity of selected areas of a normally insulating film. In the fabrication of integrated circuit devices a conventional way of forming connections between a conductor and a substrate separated by an insulating layer is to form openings in the insulating film and introduce a conductive material, such as doped polycrystalline silicon. In this regard reference is made for example to US-A-4,472,210 and US-A-4,214,918, which describe different methods of producing high conductivity polycrystalline silicon. Other processes have been developed which depend on the conversion in resistivity of selected portions of a film of an insulator overlying a region to which low resistance eectrical connections need to be made, for example, the source and drain regions of a MOS transistor. One such technique is described in US-A-4,511,445 in which increased conductivity regions are obtained by irradiating a film of a carbon-containing material: irradiation of these materials leaves an amorphous like carbon in a highly conducting state. Such conversion avoids the need for removal of selected portions of the insulating film which removal may be difficult and which may leave openings which are difficult to penetrate reliably for making electrical connection the the underlying regions. Another technique previously proposed has been to increase locally the conductivity of the film by the introduction of dopants so that electrical connection can be made to the local portion of the film avoiding the need for making an opening in the film. However, it proves difficult to dope films of insulating materials sufficiently to increase the conductivity to the desired level, because of limitations in available implantation apparatus and the tendency of the substrate to be affected by such heavy doping. It is an object of the invention to facilitate the conversion of an insulating film. To achieve this object, the invention utilizes for the normally insulating film a wide band-gap undoped semiconductive carbon, which has a high resistivity in its intrinsic undoped state, but which can be readily converted to have a relatively low resistivity when implanted with a dopant such as nitrogen. The invention will be better understood from the following more detailed description taken with the drawing which shows a semiconductive device at an intermediate stage of processing in accordance with the invention. The drawing shows in section a semiconductive device, for example, a conventional MOS transistor 10, which comprises a substrate 11, typically monocrystalline silicon which includes spaced source and drain regions 12 and 13, respectively, to each of which a separate low resistance connection needs to be made. Additionally, the transistor includes a gate 15 which overlies the channel region between the source and drain and is insulated therefrom by a thin oxide film 16. Thick field oxide regions 17 are used to isolate the transistor from other portions of the integrated circuit. It is generally necessary to provide an insulating film or layer 18 which extends over the transistor to isolate the substrate and gate from overlying metal layers (not shown) which provide connections to the source drain and gate and interconnections between the circuit elements. Typically, this layer 18 is of a dielectric material such as silicon oxide. Since low resistance connection needs to be made to the source and drain regions, these portions 19 of the layer 18 shown stippled in the drawing, overlying the source and drain either to be opened up or converted to a good conductor. This has proved a difficult problem particularly when the source and drain are relatively shallow and of limited surface area. In accordance with the invention, the layer 18 is made of a semiconductive material which has a relatively wide band gap to be of high resistivity in its intrinsic or undoped state but which can be implanted with dopants to have its resistivity sufficiently reduced in the implanted region that such region acts as a conductor, permitting a low resistance connection therethrough to the source and drain. To this end, there is first deposited a uniform layer 18 of the chosen material, carbon, over the transistor, to a sufficient thickness that it can serve as a satisfactory dielectric layer for isolating the substrate from the overlying metal layers to be deposited over the layer 18. This requires a thickness adequate to avoid pinholes and typically will be 500 nm (5000 Angstroms) thick. The carbon layer is then coated with a layer of a photoresist 20 of the kind normally used for photolithographic patterning. The photoresist is then patterned in the usual fashion to form openings in the photoresist selectively over the stippled regions of layer 18 to be converted to low resistivity by implantation. After the openings are formed, the layer 18 is implanted with dopants of the type to convert the implanted regions to the same conductivity type as the source and drain. Typically, these are N-type for the popular N-channel transistor in which case nitrogen can be implanted to make the implanted carbon N-type. After implantation, and stripping of the remaining photoresist, the transistor is annealed in the usual fashion to activate the implanted ions by substituting them in the carbon lattice. Annealing at about 900 C is typical. A typical doping would be average concentration of about nitrogen 10exp17 -10exp18 ions per CC. Any of known techniques can be used for depositing the carbon. Typically, these involve a radio frequency plasma deposition process using a hydrocarbon gas as the carbon source. Other known processes involve ion beams, ion plating or pulsed plasmas. In some instances, it may prove advantageous first to thin the carbon film selectively where its resistivity is to be reduced to reduce the accelerating voltage which needs to be used to implant the nitrogen ions the full thickness of the film. This can readily be done by coating the film with a positive photoresist, developing the photoresist appropriately, and etching the photoresist in an oxygen plasma such that where the photoresist is removed first and the carbon film exposed, continued etching will thin the carbon film. After thinning the carbon film, the process proceeds as before. The choice of carbon is advantageous for several reasons. It is easily deposited and generally is not entirely amorphous as deposited but is a form of substantially crystalline phase intermediate between graphite and diamond with a definite band-gap which is wide and so made for good insulating properties with little temperature sensitivity. Additionally, carbon has good interface properties with silicon and silicon oxide. Finally, its susceptibility to etching in an oxygen plasma allows etching back planarization by use of a sacrificial photoresist.

Claims: A process of manufacturing an integrated circuit in which a low resistance connection (19) is to be made to a localized region (12; 13) of a semiconductive substrate (10) of one conductivity type underlying a layer which is normally an insulating layer (18) characterised by forming the normally insulating layer of a wide band-gap undoped semiconductive carbon to be of high resistivity, and implanting selectively the portion (19) of the layer (18) overlying the region to be connected with a dopant of the same conductivity type as the localized region. A process according to claim 1 in which the dopant implanted is nitrogen to make the implanted portion n-type.

Inventors: MILLER, ROBERT O.
Applicants: SGS-THOMSON MICROELECTRONICS, INC.
Application year: 1987
Main classification: H01L21
IPC codes: H01L23, H01L21