Method for delineating a conducting element disposed on an insulating layer, device and transistor thus obtained

A conducting layer is deposited on an insulating layer disposed on a substrate. A mask is formed on at least one area of the conducting layer, thus delineating in the conducting layer at least one complementary area not covered by the mask. The complementary areas of the conducting layer are rendered insulating by oxidation. Oxidation can comprise oxygen implantation and/or thermal oxidation. The material of the conducting layer and the oxygen can form a volatile oxide evaporating partly or totally. The conducting layer is preferably formed by first and second conducting layers. Thus, oxidation can be performed, after the mask has been removed, so that the surface of the second conducting layer is oxidized on the side walls and on the front face.

BACKGROUND OF THE INVENTION

The invention relates to a method for delineating a conducting element disposed on an insulating layer, comprising deposition of a conducting layer on the front face of the insulating layer disposed on a substrate, formation of a mask on at least one area of the conducting layer designed to form the conducting element, so as to delineate in the conducting layer at least one complementary area not covered by the mask, the complementary areas of the conducting layer being rendered insulating by oxidation.

STATE OF THE ART

Microelectronic devices often comprise conducting elements1(FIG. 3) separated from a substrate4by a very thin insulating layer2. For example, the gate of metal oxide semi-conductor (MOS) transistors of different natures, in particular made of metal, is separated from the semi-conducting substrate by an insulating layer the thickness whereof may be about a few nanometers. A typical fabrication method of such a conducting element is illustrated inFIGS. 1 to 3. Formation of the conducting element1is achieved by deposition of a layer of conducting material3on an insulating layer2, disposed on a substrate4, and delineation by etching of the layer of conducting material3through a photoresist mask5that is then removed. The mask is formed on an area6of the conducting layer3designed to form the conducting element1, thus delineating, in the conducting layer and insulating layer, complementary areas7not covered by the mask5. However, etching may damage (for example deform or oxidize) the complementary areas7of the insulating layer2and of the substrate4, which is all the more difficult to prevent the smaller the thickness of the insulating layer2. It is a fact that selective etching of the conducting material3with respect to the material of the insulating layer2certainly enables the etching to be stopped before the substrate4is reached. However selective etching is difficult to achieve. For example, titanium nitride (TiN) etching is typically performed by fluorohydrocarbon-based (CHxFy) processes. The same processes are used for etching of oxides, in particular silica (SiO2). The selectivity of etching of the insulating layer with respect to the TiN is therefore very low and damage to the oxide, or even piercing of the insulating layer and damage to the underlying substrate, is inevitable.

In certain known processes, the substrate4can be oxidized or deformed at the end of etching through the insulating layer2. This oxidation can be disadvantageous, in particular in the case of a Silicon on Insulator (SOI) substrate comprising a very thin active layer the resistance whereof is thus greatly increased.

The document JP2002 134,544 describes a method for delineating a metal electrode. A metal layer is formed on an insulating layer disposed on a semi-conducting substrate. A photoresist mask is formed on an area of the metal layer. The metal layer is transformed by oxygen ion implantation in an insulating oxide layer in the area not covered by the photoresist mask. A metal electrode surrounded by an oxide layer is thus formed.

OBJECT OF THE INVENTION

The object of the invention is to remedy these shortcomings and, in particular, to delineate a conducting element disposed on an insulating layer without damaging the insulating layer and the substrate, so as to preserve the resistance characteristics of the device.

According to the invention, this object is achieved by the accompanying claims.

According to a first alternative embodiment of the invention, the conducting layer is formed by first and second conducting layers, the method comprising etching of the second conducting layer by means of the mask, oxidation being performed after the mask has been removed, so that the surface of the second conducting layer is oxidized on the side walls and on the front face and that the complementary areas of the first conducting layer are oxidized over the whole thickness of the first conducting layer.

According to a second alternative embodiment of the invention, the method comprises stabilizing and evaporating annealing so that the material of the conducting layer and the oxygen arising from oxidation form a volatile oxide, the conducting layer evaporating at least partly.

DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 4shows stacking of a semi-conducting substrate4(for example Si, Ge, SiGe), of an insulating layer2and of a conducting layer3. A mask5is disposed on the front face, on the area6of the conducting layer3designed to form the conducting element, thus delineating, in the conducting layer, complementary areas7not covered by the mask5. The mask5can be made of photoresist or formed by a bilayer (a layer of organic photoresist and a mineral sacrificial layer called “hard mask”). In the particular embodiment represented inFIGS. 5 and 6, in order to delineate a conducting element, the complementary areas7of the conducting layer3are rendered insulating by thermal oxidation. As represented inFIG. 5, during oxidation, the material of the conducting layer3and the oxygen form a volatile oxide so that the complementary areas7of the conducting layer3evaporate partly during oxidation. The residual complementary areas7of the conducting layer3are oxidized over their whole thickness, whereas the area6of the conducting layer is protected by the mask5. The material of the conducting layer is chosen among materials the oxide whereof is insulating so that the complementary areas7are no longer conducting after oxidation. Then the mask5is removed (FIG. 6).

InFIG. 7, the conducting layer3is formed by superposed first and second conducting layers3aand3b. The mask5is formed above the layers3aand3b. The second conducting layer3bcan be etched before oxidizing of the layer3a. As represented inFIG. 8, when the complementary areas7of the second conducting layer3bare removed by etching, only the area6bof the second conducting layer3bis kept.

In another alternative embodiment, the method comprises stabilizing and evaporating annealing after oxygen implantation using ion or plasma implantation techniques. Implantation is for example performed by oxygen ion acceleration or by a reactive ion etching (RIE) process.FIG. 9illustrates evaporation of the oxidized complementary areas7of the first conducting layer, the area6aof the first conducting layer3abeing protected by the mask5. During annealing, the material of the first conducting layer3aand the implanted oxygen form a volatile oxide and the oxidized complementary areas7of the conducting layer3aevaporate. The conducting element1is then formed by superposition of the residual part (area6b) of the layer3band by the non-oxidized part (area6a) of the layer3a. According to the annealing time and the implanted oxygen dose, the complementary areas evaporate partly (FIG. 9) or totally (FIG. 10). Removal of the mask5can be performed after annealing if the mask is mineral. In the case of a photoresist mask, it can be removed beforehand.

For application of the method, with evaporation, the material of the first conducting layer3ais preferably taken from the group comprising tungsten, molybdenum, nickel and cobalt, and the material of the second conducting layer3bis polycrystalline silicon, a metal nitride or a metal silicide containing for example tungsten, tantalum or molybdenum (WSix, MoSix, TaSix).

For example, using a first conducting layer3amade of tungsten, the oxygen atoms are implanted in the tungsten crystal in a metastable state, for example on interstitial sites. A tungsten oxide then forms during stabilizing annealing. The WOxtype oxide (x being comprised between 1 and 3) is volatile and evaporates. Typically this phenomenon can be obtained above 200° C. In the case of this technique, lateral oxygen diffusion is almost eliminated and the peripheral oxidation of the area6aof the first conducting layer3aunder the area6bof the second conducting layer3b, represented inFIG. 11, is very low.

In another development of the method with evaporation, represented inFIGS. 11 and 12, a volatile oxide is formed by thermal oxidation from the material of the conducting layer3and from the oxygen. InFIG. 11, the conducting layer3is formed by a first conducting layer3aand an etched second conducting layer3b. After the photoresist mask5has been removed, thermal oxidation can be performed in a furnace, for example at a temperature of more than 200° C. for tungsten. In this case, a volatile oxide of the tungsten WO3is formed and evaporates.FIGS. 11 and 12illustrate this method respectively during evaporation and after complete evaporation. This method fosters diffusion of the oxygen atoms in the conducting material and the periphery of the area6aof the first conducting layer3ais oxidized under the area6bof the second conducting layer3b. This peripheral area thus also evaporates and a device is obtained the area6bof the second conducting layer3bwhereof is salient at the periphery of the area6aof the first conducting layer3a. The area6aof the first conducting layer3ais thus reduced. In order to limit reduction of the area6aand damage to the substrate4, the thermal oxidation can be stopped as soon as the second conducting layer has evaporated or just before. The complementary areas7rendered insulating can then preferably present a thickness at least equal to one atomic layer. As represented inFIGS. 11 and 12, the material of the second conducting layer3bis oxidized at the surface on the side walls and on the front face.

The gate electrode of a transistor can be achieved by the method described above. In this case, the substrate4is formed by an active layer of semi-conducting material, for example homogeneous silicon or silicon on insulator (SOI). The method according to the invention enables the gate electrode to be delineated preventing deformation of the areas of the substrate corresponding to the complementary areas7and preventing diffusion of the oxidizing species in the active layer or in the insulating layer between the gate electrode and the active layer. Fabricating the gate electrode by means of two superposed layers3aand3bpresents several advantages. This in particular makes it possible to reduce the stresses exerted by the conducting material on the insulator, to mask source and drain implantations made after the gate electrode has been achieved, to ensure a contact with the interconnections, to prevent any oxidation of the gate material subsequent to fabrication of the gate electrode and to protect the gate material from self-aligned metallization (siliconizing) of the source and drain.

In another alternative embodiment of the invention, the mask5is removed (FIG. 13) after etching of the second conducting layer3b(FIG. 8). The complementary areas7of the first conducting layer3aare then oxidized by oxygen implantation, under suitable temperature and pressure conditions, or by thermal oxidation. In this case, represented inFIG. 14, the material of the second conducting layer3bis oxidized at the surface both on its side walls and on its front face, whereas the complementary areas7of the first conducting layer3aare oxidized over the whole thickness of the first conducting layer3a. Diffusion of the atoms in the materials at high temperature, in particular in the case of thermal oxidation, can also lead to peripheral oxidation of the area6aof the first conducting layer3aunder the second conducting layer3b, as represented inFIG. 14. The first conducting layer3ais preferably made of TiN and the second conducting layer3bis made of polycrystalline silicon. Thus, an oxynitride TiOxNyforms when oxidation is performed.

In the case of an oxygen implantation, a thermal stabilization of the metastable state of the layer comprising oxygen implanted by annealing in an inert atmosphere, for example an argon atmosphere, is preferably added. In this case, as in the case of thermal oxidation, the complementary areas7of the conducting layer3can form a solid oxide in which the oxygen atoms and the atoms of the conducting material are integrated in a single crystalline network, the oxygen atoms replacing for example the atoms of the conducting material. Thus, the conducting element1is formed by the non-insulating, in particular non-oxidized, parts of the conducting layer, whereas the areas rendered insulating form a lateral barrier of the conducting element.

The invention is not limited to the embodiments represented, in particular oxidation can be performed either thermally or by oxygen implantation, after the mask has been removed. Moreover, formation of a volatile oxide, before or after the mask is removed, can be achieved by thermal oxidation or by oxygen implantation using a single conducting layer or two superposed conducting layers.