GB2079050A

GB2079050A - Schottky contacts

Info

Publication number: GB2079050A
Application number: GB8119415A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1980-06-27
Filing date: 1981-06-24
Publication date: 1982-01-13
Also published as: JPS5732681A; FR2485809A1; GB2079050B; DE3124239A1; FR2485809B1; NL8103021A

Abstract

The long term diffusion of Si into an Al contact is reduced by interposing a layer 132 of a transition metal between a thin Al contact layer 131 and a thicker Al top layer 133. The intermediate layer 132 forms a barrier to prevent diffusion of Al between layers 133 and 131 so that the quantity of Al available for reaction with the substrate 10 is small. The Al may contain (eg 2%) Cu, and the intermediate layer may be W, Ti or Cr (eg W 2% Ti). The contact structure forms a Schottky diode which may be used in integrated logic circuits, the same type of contact structure also forming ohmic contacts to suitably doped regions. <IMAGE>

Description

SPECIFICATION Semiconductor devices and their manufacture The invention relates to a semiconductor device having a Schottky diode, comprising a semiconductor body having at least a surfaceadjoining silicon region, a first conductive layer which is situated on the silicon region and which, with the silicon region, forms the rectifying junction of the Schottky diode, a second layer of at least one transition metal provided on the first layer, and a layer which is situated thereon and which consists at least substantially of aluminium.

A semiconductor device as described above is known from Journal of Applied Physics, Vol. 50, No. 11, November 1979, pp.

6923-6926.

Transition metals are to be understood to mean herein as usual the group of transition elements from the periodic table having atomic numbers 21-30, 39-48, 57-80 and 89, see, for example, "Handbook of Chemistry and Physics, 49th edition, the Chemical Rubber Co., Cleveland Ohio, p. B-3.

The invention relates in particular but not exclusively to integrated circuits having Schottky diodes, in particular logic integrated circuits, in which in general more than one of these diodes are present in said circuits and favourably influence the switching speeds.

The first Schottky diodes which were realised in integrated circuits were formed by deposition of aluminium on comparatively weakly doped silicon surfaces. The manufacture is simple, economical and presents the great advantage that it can easily be incorporated in the general process of providing contacts on the surface of a semiconductor crystal by deposition of aluminium. As is known, aluminium is in fact the best suitable material for this purpose, and that especially due to its low resistivity and its strikingly good adhesion both on silicon and on the dielectric layers used for insulating and passivating, but also due to its deformability so that it can absorb the mechanical load more easily than other metals or alloys.

Unfortunately, aluminium also has a great disadvantage: it dissolves silicon via a diffusion reaction in the solid state. This phenomenon is known in all applications of contacts of aluminium or of alloys with a large aluminium content on silicon; it is expressed in a gradual shift in time of the original interface between the two materials in the direction of the silicon. The surface of the silicon below the aluminium seems to be dotted with very small pits. The effect is particularly unfavourable for a Schottky diode in which the junction is present immediately at the surface of the silicon. It gives rise to a rapid degradation of the characteristics of the diode, even to the complete destruction of the said junction which obtains the character of an ohmic contact.

So in most of the cases one has renounced the use of aluminium for the manufacture of metal-semiconductor diodes. The Schottky diodes which are nowadays used in integrated circuits have a structure analogous to that of the diodes according to French Patent Specification No. 1,591,489.

According to this Patent Specification the Schottky junction is formed between a layer of platimum silicide and silicon. In order to form the said junction, platimum is deposited on an uncovered zone of a silicon substrate, which zone is bounded by regions covered with an insulating material.

In order to avoid these disadvantages, Schottky diodes have been manufactured in which instead of aluminium a layer of a silicide of metals, for example platinum, molybdenum and cobalt, is used as a Schottky contact on which a metal, for example gold or aluminium, is provided for contacting. In the latter case, in order to prevent diffusion of aluminium through the silicide, a thin intermediated layer of, for example, tungsten or titanium-tungsten may be provided between the aluminium and the silicide, as is described in the above publication in Applied Physics Letters. For the formation of the silicide layer first the metal, for example platinum, is deposited on an uncovered zone of silicon substrate, which zone is bounded by regions covered with an insulating material.

The metal is then partly converted into silicide by a thermal treatment. The metal not bound is then removed by etching. Finally the metal silicide is first covered with the intermediate layer and then with an aluminium contact layer. In this manner a diode is obtained the electrical characteristics and stability and the life of which are much better than those of aluminium-silicon diodes.

However, these advantages are obtained only at the expense of a considerable increase of the costprice of the manufactured products, said increase being related on the one hand to the complexity of the manufacturing process which comprises a special step for etching the metal, and on the other hand to the use of comparatively expensive metals. This metal is often attacked only by very agressive etchants, for example aqua regia, which no protective lacquer can withstand.

The excess of metal has to be removed before other metallic layers are deposited which cannot easily be protected.

One of the objects of the invention is to provide a semiconductor device having a Schottky diode of a new structure which can be cheaper and simpler to manufacture than a Schottky diode having a metal silicide contact layer and the electrical properties of which can be better and stabler than those of the known aluminium-silicon diodes.

The invention is inter alia based on the recognition of the fact that for this purpose aluminium may be used as a Schottky contact if it is ensured that the quantitiy of aluminium which can dissolve the underlying silicon is minimized .

A semiconductor device having the features specified in the opening paragraph is characterized according to the invention in that the first layer also consists substantially of aluminium and has a thickness of at least 1 Onm and at most 100 nm.

The Schottky junction is present between the thin sub-layer of aluminium and the silicon. The presence of these two materials gives rise to the dissolution of the silicon by the aluminium as described above. In the present case, however, this reaction discontinues rapidly in that the aluminium is available only in a restricted quantity. In fact, only the aluminium of the thin sub-layer takes part in the dissolution. The intermediate layer consisting of a transition metal restricts in known manner any migration of aluminium atoms from the top layer to the underlying silicon and thus prevents said aluminium from taking part in the dissolution.

The thin aluminium layer is rapidly saturated with silicon and this the more rapidly according as it is thinner. Due to the restricted availability of one of the elements, and extend and the duration of the dissolving process are reduced. The degradations of the junctions are very much restricted. As a result of this, upon investigation, the silicon surface shows only very few very shallow pits after a few hundred hours in operation.

Applicants have found that Schottky diodes constructed according to the invention have a striking stability of their characteristics in time. Consequently, the use of aluminium may be preferred to that of other rarer metals.

This is a great advantage both from a technological point of view and from an economical point of view, in particular with respect to the manufacture of integrated circuits.

From a technological point of view it is avoided on the one hand that a heating (formation of metal silicide) has to be carried out immediately after the deposition of the aluminium. On the other hand, whereas upon using metal silicides first a deposition step and then an etching step have to be carried out (deposition and then etching of the remaining metal after the formation of the metal silicide, then deposition and etching of the other composing metals of the electrode), the layer composed of aluminium, a transition metal (for example tungsten or titanium or an alloy of these two elements) and again aluminium can be deposited in one step and then be etched in one step.

From an economical point of view the advantages are obvious both with respect to the price of the materials and with respect to the simplification of the manufacturing process.

It is to be noted that the deposited composed layer of aluminium-transition metal-aluminium is also very suitable for the formation of Schottky junctions on low-doped silicon as well as for the formation elsewhere on the substrate of Ohmic contacts on highly doped silicon. This deposited composite layer is therefore excellently suitable for integrated circuits.

A very suitable method of manufacturing the semiconductor device according to the invention is characterized in that without removing the semiconductor body from the apparatus, the said first, second and third layers are successively deposited on a silicon region, that said layers are then brought in the form of the desired electrode configuration by etching, and that finally the assembly is subjected to one single temperature treatment at approx- imately 430 C.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a diagrammatic cross-sectional view of a semiconductor device according to the invention, and Figure 2 is a diagrammatic cross-sectional view of an integrated circuit having a Schottky diode in accordance with the invention.

The drawings are not drawn to scale, in particular in the direction of thickness. Corresponding parts are referred to by the same reference numerals.

The metal-to-semiconductor diode as shown in Fig. 1 is formed on a silicon region 10, for example of the conductivity type, which is low-doped (from 1015 to 1017 atoms/cm3) and forms a first electrode of said diode, in this example the cathode. A composite layer 13 consisting of a first layer 131, a second layer 132 and a third layer 1 33 is formed on the silicon via an aperture 11 which is provided in an insulating layer 12 of, for example, silicon dioxide, which composite layer 13 forms the second electrode of the diode, in this example the anode. The first layer 131 with the region 10 forms the Schottky junction, the second layer 132 consists of one or more transition metals and the third layer 133 consists substantially of aluminium.

According to the invention, the first layer 131 which is in contact with the said substrate 10 also consists substantially of aluminium and it has a thickness of at least 10 nm and at most 100 nm.

The Schottky junction is present at 14 at the area of the contact between the thin layer 131 and the substrate 10.

In this example the first layer 131 consists either of pure aluminium or of an alloy having a high percentage of aluminium (for example 98% aluminium and 2% copper); the thick ness thereof is in this example approximately 50 nm.

The second layer 132 consists advantageously of an alloy of tungsten and titanium, for example, 90% tungsten and 10% titanium.

This choice is not limiting; metals and metallic alloys, for example, pure tungsten, pure titanium chromium and the allous thereof may also be used. The thickness of the layer 132 is advantageously chosen between 20 and 100 nm.

Finally, the third layer 133 preferably has the same composition as the layer 131. The thickness thereof is, for example, between 0.5 and 3#m.

The semiconductor device described can be manufactured by means of known technologies. The three layers 131, 132 and 133 are advantageously obtained by cathode sputtering, starting from two targets which are successively bombarded without returning the substrate to the ambient atmosphere. The chemical etching of said layers is carried out with phosphoric acid as far as the aluminium is concerned and with hydrogen peroxide as far as the tungsten-titanium is concerned. The thermal treatment during which the passivating layer which consists, for example, of silicon oxide (not shown in the figures) is deposited and during which the contact between the various materials, in particular the aluminium-siiicon contact, are simultaneously stabilized is carried out at a temperature of approximately 430 C.

An integrated circuit having a Schottky diode in accordance with the invention will now be described with reference to Fig. 2.

An integrated circuit is formed on a plate 1 of silicon of the Rtype on the upper surface of which an epitaxial layer 10 of the ntype has been deposited which is low doped. A transistor T and a Schottky diode D of this circuit are shown in Fig. 2 in which said diode is coupled via the cathode to the collector of the transistor and via its anode to the base of the transistor. This is a configuration which is known in logic integrated circuits.

Visible are the Rtype base region 20 and the n±type emitter region 21; the collector region is formed by the epitaxial layer 10 in which an n±zone 22 serves for the formation of contacts for the collector output.

A metal layer which is composed of three layers of aluminium 131, tungsten/titanium 132 and aluminium 133 forms on the three regions 20, 21 and 22 an ohmic contact and on the region 10 a rectifying Schottky junction 14 so that the Schottky diode is parallel across the base-collector junction. The Schottky junction 14 is shown as a solid line in Fig. 2.

It will be obvious from Fig. 2 that all Ohmic contacts of the transistor T together with the rectifying contacts of the diode D can simultaneously be formed both as regards the formation of the metallic layers and as regards the etching of said layers.

Claims

1. A semiconductor device having a Schottky diode, comprising a semiconductor body having at least a surface-adjoining silicon region, a first conductive layer which is present on the silicon region and which, with the silicon region, forms the rectifying junction of the Schottky diode, a second layer of at least one transition metal provided on the first layer, and a third layer which is provided thereon and which consists at least substantially of aluminium, characterized in that the first layer also consists substantially of aluminium and has a thickness of at least 10 nm and at most 100 nm.

2. A semiconductor device as claimed in Claim 1, characterized in that the first layer consists of substantially pure aluminium.

3. A semiconductor device as claimed in Claim 1, characterized in that the first layer consists of an alloy of approximately 98% aluminium and approximately 2% copper.

4. A semiconductor device as claimed in any of the preceding Claims, characterized in that the third layer consists of the same material as the first layer.

5. A semiconductor as claimed in any of the preceding Claims, characterized in that the second layer consists of an alloy of tungsten and titanium.

6. A semiconductor device as claimed in any of the preceding Claims, characterized in that the second layer has a thickness of at least 20 nm and at most 100 nm.

7. A semiconductor device as claimed in any of the preceding Claims, characterized in that the Schottky diode is connected parallel to the collector-base junction of a bipolar transistor.

8. A method of manufacturing a semiconductor device as claimed in any of the preceding Claims, characterized in that without removing the semiconductor body from the apparatus, the said first, second and third layers are successively deposited on a silicon region, that said layers are then brought in the form of the desired electrode configuration by etching, and than finally the assembly is subjected to one single temperature treatment at approx- imately 430 C.

9. A method of manufacturing a semiconductor device substantially as described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

10. A semiconductor device substantially as described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.