VAPOUR EFFUSION SOURCE FOR EPITAXIAL DEPOSITION PLANTS

The present invention relates to equipment for preparing wafers for electronic and electro-optical semiconductor devices, and more particularly to a vapour effusion source for epitaxial deposition plants.

The manufacture of electronic and optoelectronic devices, and more particularly those using materials from groups III-V of the periodic table of the elements, typically requires the deposition of sequences of different single-crystal layers on a starting substrate, in order to obtain appropriate sequences of semiconductor materials.

This deposition, prior to the actual fabrication of individual devices or circuits, must be of the epitaxial type (namely it maintains initial crystal characteristics), and can be performed by various techniques, those using epitaxial growth from liquid phase (LPE), from vapour phase (VPE) or from molecular beam (MBE) being well known.

In this last technique, the materials to be deposited are separately evaporated in an ultrahigh vacuum environment and then directed towards the substrate as a more or less collimated beam. In a variant of this technique, vapours of organo-metallic compounds, such as alkyls of group III metals, are used as vapour sources; at ambient temperature they have a low vapour pressure, in contrast to the solid metals of the same group, namely Gafln/Al. This permits the part of the plant providing the vapour source to be placed outside of the ultrahigh vacuum zone, so that ultrahigh vacuum does not have to be restored whenever the materials to be evaporated are handled. In the vacuum zone, high-temperature furnaces cannot be provided for solid metal evaporation, which entails considerable complexity in the design of plant for handling the metals to avoid possible material pollution. Vapour state sources on the other hand can be introduced into the vacuum zone by a single effusion source, in which they are mixed and directed towards the substrate to be grown, instead of resorting to an effusion source with a furnace for each element.

Conventional effusion sources, such as those depicted at page 96 of the book "Molecular Beam Epitaxy" written by M.A. Herman and H. Sitter, Springer-Verlag, edited by Morton B. Panish, are fabricated by using inlet tubes passing through a vacuum flange. The external tube ends are equipped with means for connection to vapour lines, whilst the internal ends terminate in a zone for mixing the vapours leaving the tubes. This zone is equipped with baffle plates for breaking up the flows from the tubes to enhance vapour mixing, an outlet nozzle to shape appropriately the beam directed towards the substrate on which growth is to occur, and a heater maintaining a constant temperature under the control of an electric thermocouple. The necessity of maintaining a controlled temperature in the effusion source, when the surrounding environment is at the temperature of liquid nitrogen, arises because vapour condensation on the walls, and consequent flow intensity alteration, is to be avoided, whilst avoiding high temperatures which could give rise to vapour decomposition. Temperatures ranging from ambient temperature to 50°C are usually required.

Such effusion sources are rather difficult to implement, since they require vacuum feedthrough connectors for the heating and temperature controlling means, despite the fact that the temperature to be maintained inside the effusion source is rather low.

These disadvantages are addressed by a vapour effusion source for epitaxial deposition plants, provided by the present invention, which permits simple temperature control in the vapour mixing zone without requiring the presence of heating and temperature controlling means in the ultrahigh vacuum environment.

%' •• v ' f" 'fit ft ..j, t r , / It is a particular object of the present invention to provide an effusion source for epitaxial deposition plants in which a molecular vapour beam is directed towards a substrate in which crystal layers are to be grown in ultrahigh vacuum environment, the source comprising a first tube, hermetically closed at one end by means carrying vapour inlet tubes and containing at its other end baffle plates and a nozzle for vapour mixing and molecular beam shaping, wherein a second tube, having a diameter greater than that of the first tube and supplied with a vacuum flange, is coaxially connected to the first tube through a round washer, so as to form a chamber at ambient temperature and pressure between an outer lateral surface of the first tube and an inner lateral surface of the second tube surrounding a zone of the first tube containing the vapour mixing and molecular beam shaping nozzle.

These and other features of the present invention will appear from the following description of a preferred embodiment by way of non-limiting example, with reference to the annexed drawing which is a longitudinal section of the effusion source.

The vapour effusion source comprises two tubes having different lengths and diameters TP and TG, soldered at one end to a round washer KO so as to be maintained coaxial, and provided with vacuum flanges FP and FG respectively at their other ends.

Vapour inlet tubes terminate within the smaller tube TP. The number of these tubes, TA1, TA2 and TA3, is appropriate to the number of vapours to be introduced, usually 4 or more, and they are soldered in an airtight manner to another vacuum flange FA, which perfectly matches with flange FP of the smaller diameter tube TP. Their length is such that they penetrate more than half the length of the smaller diameter tube TP in order to avoid premature mixing of different vapour streams. The outer ends of inlet tubes, which remain outside the apparatus, have flanges Fil, F12, F13 suitable for connection to a vapour source system.

Inside the smaller diameter tube TP, in a vapour mixing zone, is a molecular beam shaping system. It consists of one or more baffle plates SR, for interrupting the direct flow of the vapours and making them follow a tortuous path, and of a nozzle UG, allowing exact definition of the molecular beam shape on the basis of the ratio between its diameter and its length. The entire system is screwed into the smaller diameter tube TP so as to be easily interchanged according to plant requirements and geometry.

Between the outer lateral surface of the tube TP and the inner lateral surface of the tube TG is a gap which remains at ambient temperature and pressure. This is generally sufficient to avoid vapour condensation inside the tube TP. A heating coil RS functioning by electrical resistance or hot liquid circulation can be used to provide and extra heat required. Since this coil is not in the ultrahigh vacuum environment, it can be of any suitable commercially available type. The coil input and output pass through the open end of the larger diameter tube TG, which is shorter than the tube TP.

The above description is by way of non-limiting example only, and variations and modifications are possible within the scope of the appended claims.

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