Method of making a transistor

The invention relates to a method for manufacturing a transistor comprising the preparation of a stack of layers of the semiconductor on insulator type comprising at least one substrate on which an insulating layer and an initial semiconductor layer are successively disposed. The method includes the formation of at least one oxide pad extending from a top face of the insulating layer, the formation of an additional layer made from semiconductor material covering the oxide pad and intended to form a channel for the transistor, the formation of a gate stack above the oxide pad, and the formation of a source and drain on either side of the gate stack.

TECHNICAL FIELD OF THE INVENTION

The present invention concerns in general transistors of the metal oxide semiconductor field effect (MOSFET) type and more particularly the reduction of parasitic elements limiting the operating speed thereof.

PRIOR ART

In a MOSFET transistor a current is made to flow between a “source” electrode and a “drain” electrode under the control of a control “gate”, which creates a conduction channel between source and drain as soon as a sufficient voltage is applied thereto. The maximum switching speed of a MOSFET transistor depends on the speed with which it is possible to effectively establish the conduction current and make it disappear. It depends among other things on many physical parameters particular to the materials used, such as for example the mobility of the carriers of the semiconductor material used and the levels and type of doping of the various regions defining the electrodes. The switching speed also depends largely on the geometry and structure of the transistors. In particular, by means of the parasitic elements that are inevitably introduced by the practical implementation of these devices. Two particularly critical parasitic elements are firstly the serial access resistance of the source and drain electrodes and the stray capacitance between gate and source or drain on the other hand. These parasitic elements form a time constant that helps to limit the maximum switching speed of any transistor.

FIG. 1shows a view in cross-section of a MOSFET transistor100characteristic of the prior art. The integrated circuits can be produced from elaborate substrates of the so-called SOI type, the acronym for silicon on insulator and more generally semiconductor on insulator. InFIG. 1there is the original SOI substrate110that consists of an initial substrate112, usually a homogeneous wafer of silicon, and a buried oxide layer114that provides the isolation of the components that will be manufactured in the fine surface semiconductor layer116present on the buried layer. The surface layer116usually consists of monocrystalline silicon. The isolation of each of the transistors100is computed by the production of lateral isolation trenches referred to STI, the acronym for shallow trench isolation. They reach the buried oxide layer so as to enclose each of the transistors in a continuous layer of oxide. These trenches, which are not necessary for an understanding of the invention, are not shown.

FIG. 2summarises the main standard steps of manufacturing a MOSFET transistor representing the prior art. The first step810consists of producing, from the original SOI substrate110, the STI isolation trenches mentioned above, which will provide the complete isolation of each of the transistors100. At the following global step820, the stacking is carried out of layers and the patterns that will constitute the gate stack160of each transistor comprising two essential layers: the thin gate oxide122and the control gate124proper. The latter is generally made from conductive polycrystalline silicon that fulfils the role of the metal in the MOS structure of the transistor or a stack of layers consisting of a metal and polysilicon. At the following step830, a first layer of spacers130is produced on the flanks of each gate pattern. The spacers, made from silicon nitride, and the gate itself, will serve to protect the channel180during a first operation of implantation of the surface layer of silicon that will follow. It should be noted here that the spacers and the sources and drains are produced without implementing any photolithography operation. As mentioned above, a first implantation840of the source and drain regions140in the surface layer116, generally made from monocrystalline silicon, is next effected. It should be noted that the sources and drains are identical; they have the same physical and geometric structure. Only their interconnections with other transistors and the voltages that will be applied in the electronic device in the course of implementation will be able to make it possible to finally attribute the role of source or drain to one or other of the electrodes. In the following description of the invention the source and drain electrodes are therefore undifferentiated and referenced by the same acronym “S/D”140.

The following step850consists of increasing the thickness142of the source and drain (S/D) regions. An operation the main purpose of which is to reduce the access resistances 145 of these electrodes. The thickening of the S/D regions is conventionally done by epitaxial growth from the underlying layer, that is to say the layer116of monocrystalline silicon of the original SOI substrate. It is therefore a case of reducing one of the parasitic elements mentioned previously that limit the switching speed and the performances of the transistors. As it is found that the access resistance is inversely proportional to the thickness of the S/Ds, it should be noted here that there is every advantage, in order to significantly reduce this parasitic element, in increasing the protrusion of the S/Ds. This type of transistor and method is often referred to as “RSD”, the acronym for “raised source drain”.

The other standard operations consist of a step860of producing a second layer of spacers150. This second layer of spacers serves to limit laterally, around the gate pattern, the siliconising regions that provide good electrical contact with the silicon of the raised S/Ds140. The siliconising123and the metal contacts170are formed at step880. Prior to this, at step870, a second implantation of the S/Ds is performed, which provides a doping of these regions. As for the S/Ds, it should be noted that a siliconising of the top part of the gate stack160made from polycrystalline silicon is also carried out so that it is also possible to establish good electrical contact on this electrode. For reasons of clarity the gate contact and its siliconised region are however not shown inFIG. 1.

The other parasitic element mentioned previously is the capacitance190between the S/Ds and the gate. The spacers constitute the dielectric of this capacitance. The stray capacitance due to the spacers tends to increase proportionally to the increasing thickness142of the raised epitaxial layer of the S/Ds140. It is thus found that, in the standard method of implementing MOSFETs that has just been briefly described, the conditions for optimisation of the main parasitic elements that limit their switching speed are perfectly antagonistic since, in order to reduce one, the access resistance of the source and drain140, it is necessary to increase the protrusion of the latter, which causes an increase in the other parasitic element, that is to say the capacitance190between the gate and the S/Ds140.

In order to compensate for the increase in the stray capacitance between the gate and the S/Ds140, various techniques have been proposed. A first method consists of using spacers, the material of which has a lower permittivity than that of the silicon nitride currently used. For example, replacing nitride with silicon oxide significantly reduces the stray capacitance.

Replacing the nitride does however significantly complicate the standard method for manufacturing MOSFET transistors.

The standard method may also be modified by making provision for directly using spacers made from silicon oxide. This material does however lend itself much less well than nitride to the production of spacers and there does not currently exist any industrially reliable method that allows direct replacement of nitride.

One method that has been experimented with consists of the fact that the epitaxial growth, which makes it possible to obtain a raised source and drain in order to reduce the access resistance to these electrodes, is implemented while limiting the lateral growth of this epitaxy in order to move the S/D regions140away from the gate and therefore to reduce stray capacitance190while increasing the dielectric thickness between these regions. This so-called “faceted epitaxy” technique is however particularly difficult to master. The thickness of silicon deposited during the growth of a faceted epitaxy is very sensitive to the environment. Thus there are disparities in thickness between wide and narrow devices that may cause malfunctioning such as total siliconising and the appearance of leakage current that serious impairs the reliability of the devices. Disparities in thickness between the regions where the density of patterns is great and those where it is light are also found.

From this brief presentation of the known methods for producing MOSFET transistors, it is clear that there do not exist any simple and reliable methods that make it possible to reduce both the access resistance of the source and drain electrodes and the stray capacitance between these electrodes and the control gate.

The objective of the present invention is to propose a solution that meets at least some of these constraints. The other objects, features and advantages of the present invention will emerge from an examination of the following description and the accompanying drawings. Naturally other advantages may be incorporated.

SUMMARY OF THE INVENTION

According to one embodiment, the present invention proposes a method for manufacturing a transistor comprising the preparation of a stack of SOI layers of the semiconductor on insulator type comprising at least one substrate on which an insulating layer and an initial semiconductor layer are successively disposed, the method comprising:the formation of at least one oxide pad extending from a top face of the insulating layer, the formation of at least one oxide pad comprising the following steps:the formation of at least one cavity in the initial semiconductor layer by etching;the formation of a layer of oxide on the surface of said stack of layers produced so that, in the bottom of the cavities, the film of oxide formed extends at least as far as the top face of the insulating layer; thus the film of oxide and the initial insulating layer form an uninterrupted electrically insulating layer;a partial removal of the film of oxide so as to bare the initial semiconductor layer outside the cavity and to keep at least part of the film of oxide in the cavity in order to leave in place the oxide pad extending from the insulating layer;the formation of an additional layer made from semiconductor material covering the initial semiconductor layer and the oxide pad and intended to form a channel for the transistor;the formation of a gate stack above the oxide pad;the formation of a source and drain on either side of the gate stack.

The invention thus proposes a particularly simple method for forming a source and drain situated at least partly under the gate, by forming pads of oxide on the insulating layer (typically a buried oxide layer) in order to form a continuous insulating layer, forming a channel on the oxide pads and forming a gate on each of these oxide pads.

Both the source and drain extend to a major extent at least on either side of the oxide pad forming a projecting relief and under the gate. Thus the transistor has a very special structure wherein the source and/or drain are reversed compared with known structures, that is to say they are situated under the gate. This structure reduces the stray capacitance between the gate and the source and drain. Furthermore it makes it possible to thicken the source and drain in a direction perpendicular to the plane of the substrate, which has the effect of reducing the access resistance of the electrodes of the source and drain.

Also advantageously, the invention makes it possible to make the height of the gate independent with respect to the height of the source and drain, which in particular makes it possible to reduce the height of the gate.

The present invention is particularly suitable for substrates of the semiconductor on insulator type, such as elaborate substrates of the silicon on insulator (SOI) type with a thin buried oxide layer (buried insulating oxide, BOX). This is because, by virtue of the invention, the thickness of the initial insulating layer is preserved, no etching of the initial BOX being necessary for obtaining a buried source and drain. The advantage of the invention is therefore being compatible with a substrate integrating a thin or thick BOX layer.

The drawings are given by way of examples and are not limitative of the invention. They constitute outline schematic representations intended to facilitate understanding of the invention and are not necessarily to the scale of practical applications. In particular the relative thicknesses of the various layers and films are not representative of reality.

DETAILED DESCRIPTION OF THE INVENTION

Before beginning a detailed review of embodiments of the invention, optional features are stated below, which may optionally be used in association or alternatively:

Advantageously, the formation of the oxide pad is effected so that the width L of the oxide pad is greater than or equal to the width Lg of the gate stack.

Advantageously, the gate stack is formed by lithography and the oxide pad is formed so that: L≥Lg+2×Des, L being the width L of the oxide pad, Lg being the width of the gate stack and Des being the maximum imprecision of misalignment of the lithography. This makes it possible to ensure that the gate is always fully situated above the oxide pad without projecting beyond the ends of said oxide pad despite any unintentional misalignment of the lithography in order to form the gate.

These widths are taken in a direction parallel to the plane of the substrate and in the direction of movement of the electrons in the channel, that is to say on the figures in the horizontal direction contained in the plane of the cross-section depicted.

Advantageously, the method comprises a step of forming spacers on the flanks of the gate stack and the formation of the oxide pad is effected so that: Lg+2×Lsp≥L, L being the width of the oxide pad, Lg being the width of the gate stack and Lsp being the width of the spacer layer.

Advantageously, the partial removal of the oxide film effected so as to bare the initial semiconductor layer outside the cavity and to preserve at least part of the oxide film in the cavity comprises a polishing step of the mechanical and chemical type (CMP, the acronym for “chemical-mechanical planarisation”) with stoppage on the initial semiconductor layer.

The additional layer made from semiconductor material covering the oxide pad and intended to form a channel for the transistor is a layer of crystalline semiconductor material. Preferably, it is made from monocrystalline semiconductor material.

Advantageously, the formation of an additional layer of semiconductor material covering the oxide pad and intended to form a channel for the transistor comprises the transfer of a layer of crystalline semiconductor material onto the oxide pad. Preferably, the transfer of the layer of semiconductor material is effected by a method comprising the detachment of a thin film of monocrystalline semiconductor material by rupture of a thicker layer at a weakened region. A person skilled in the art can refer for this purpose to the numerous known methods of transferring a layer by molecular bonding.

According to another embodiment, the initial semiconductor layer is crystalline, preferably monocrystalline, and the formation of an additional layer of semiconductor material covering the oxide pad and intended to form a channel for the transistor comprises:the deposition of a layer of amorphous semiconductor material on the initial semiconductor layer and the oxide pad,heat treatment carried out so as to crystallise the layer of amorphous semiconductor material by solid-phase epitaxy from the initial semiconductor layer.

According to one embodiment, the formation of a source and drain comprises the doping of source regions on either side of the gate stack. This embodiment is particularly advantageous in the case where the thickness of the channel is so thin that it becomes difficult to produce a source and drain by epitaxy. Furthermore, this embodiment is particularly advantageous if it is wished for the semiconductor material of the source and drain to be the same as that of the channel.

Alternatively, the formation of a source and drain comprises: etching of at least the additional layer of semiconductor material so as to remove at least a portion of the thickness of the additional layer of semiconductor material that is not situated under the gate stack and so as to keep at least part of the additional layer of semiconductor material at the transistor channel; a growth of the source and drain regions by epitaxy from the semiconductor material forming the channel. Thus the etching re-forms a cavity on each side of the gate stack and the growth by epitaxy forms the source and drain regions in these re-formed cavities.

Preferably, the formation of a source and drain also comprises the doping of the source and drain regions during the step of growth of the source and drain regions by epitaxy.

This embodiment is particularly advantageous when it is wished for the material in the source and drain regions to be different from that of the channel. Preferably, the materials used for the channel on the one hand and for the source and drain regions have, for example, mesh parameters that differ by less than 1% and preferably by less than 0.5% in the plane parallel to the face of the substrate.

Said etching of at least the additional layer of semiconductor material is an anisotropic etching where the favoured etching direction is perpendicular to the plane of the substrate. Said etching may be highly anisotropic, that is to say, for a dimension etched in the direction perpendicular to the plane of the substrate, a zero or much lower dimension will be etched in a direction parallel to the plane of the substrate. Said etching may also be only slightly anisotropic, that is to say, for a dimension etched in the direction perpendicular to the plane of the substrate, a smaller dimension will be etched in a direction parallel to the plane of the substrate. Alternatively, the etching is isotropic.

According to a first embodiment, said etching of at least the additional layer of semiconductor material removes, on either side of the gate stack, the entire thickness of the additional layer of semiconductor material and the entire thickness of the initial semiconductor layer. Preferably, the etching is stopped on the insulating layer. Thus the etching bares the insulating layer on either side of the gate stack. The formation of the source and drain is effected by epitaxy from the semiconductor material remaining in place. This epitaxy is preferentially directed downwards, that is to say from the channel and in the direction of the substrate.

In a second embodiment, said etching preserves, on either side of the gate stack, a film formed by at least a portion of the thickness of the initial semiconductor layer and optionally by a portion of the additional layer of semiconductor material. Thus the etching is partial in that it does not bare the insulating layer on either side of the gate stack. Advantageously, in this case, the epitaxy is initiated by the lateral ends of the channel and/or by the film of semiconductor material left in place, on either side of the gate stack, on the insulating layer at the end of the etching. Typically only a top portion of the assembly formed by the additional layer of semiconductor material and the initial semiconductor layer is removed. Said top portion removed by anisotropic etching typically has a thickness of between 1 nm and 20 nm.

According to a first embodiment, the formation of at least one cavity is effected by etching only part of the thickness of the initial semiconductor layer in the bottom of the cavity so as to preserve an uninterrupted semiconductor layer on the surface of the insulating layer. Thus the insulating layer is not bared in the bottom of the cavities. Advantageously, this makes it possible to stop the etching before reaching the insulating layer, which reduces the etching time.

Preferably, the formation of an oxide layer on the surface of said stack of layers of the semiconductor on insulator type is done by oxidation of the initial semiconductor layer in order to form a continuous oxide film on the uncovered surface of the initial semiconductor layer. This embodiment makes it possible to obtain an oxide of very good quality. It also offers very good continuation of the insulation between the insulating layer and the oxide pad.

According to a second embodiment, the formation of at least one cavity is effected by etching the whole of the thickness of the initial semiconductor layer in the bottom of the cavities. Thus the insulating layer is bared in the bottom of the cavities.

According to one embodiment, the formation of a source and drain is effected so as to form the source and drain at least partly in the cavity.

According to one embodiment, the formation of a source and drain is effected so as to form the source and drain at least partly on either side of the oxide pad.

Preferably, the formation of an oxide layer on the surface of said stack of layers of the semiconductor on insulator type can be done by a deposition of oxide over the entire surface of said stack of layers. Preferably, the thickness of the deposition of oxide is at least equal to the depth of the cavity.

Preferably, the formation of at least one cavity in the initial semiconductor layer comprises, prior to the partial etching, a lithography step for protecting with a mask the regions that will not be etched.

Preferably, the formation of at least one cavity in the initial semiconductor layer by etching of the initial semiconductor layer comprises, prior to the etching, a step of oxidation of the initial semiconductor layer in the regions to be etched and over a thickness corresponding to the thickness to be etched. Advantageously, the oxide etching kinetics is very much superior to that of the semiconductor material. The oxidation step thus makes it possible better to control the etched thickness and therefore the depth of the cavities. Alternatively, no oxidation step is performed prior to the etching. The etching is then performed after the definition by lithography of the patterns defining the cavities to be etched.

Preferably, the etching of the cavities is effected by reactive ion etching (RIE).

Advantageously, the active layer is a layer of semiconductor material based on silicon or germanium.

The initial semiconductor layer, also referred to as the active layer of the semiconductor on insulator stack, is a layer of silicon, silicon-germanium, or materials of the elements in columns III/V of the periodic classification of elements.

The additional layer of semiconductor material is a layer of silicon, germanium, silicon-germanium or materials of the elements in columns III/V of the periodic classification of elements.

Preferably, provision is made for the initial semiconductor layer and the additional layer of semiconductor material to be identical materials.

Alternatively, materials that are different but exist in a compatible crystalline form can be provided. Thus the materials used have, for example, mesh parameters that differ by less than 1% and preferably less than 0.5% in the plane parallel to the face of the substrate. This is particularly advantageous for facilitating the formation of a crystalline and preferably monocrystalline channel by epitaxy for example.

Preferably, the initial insulating layer is a buried oxide layer in the stack of layers of the semiconductor on insulator type.

Preferably, the initial semiconductor layer is the active layer in the stack of layers of the semiconductor on insulator type.

Preferably, the transistor is a transistor of the MOSFET (field effect transistor) type.

In the context of the present invention, the term “on”, “surmounts” or “underlying” does not necessarily mean “in contact with”. Thus, for example, the deposition of a first layer on a second layer does not necessarily mean that the two layers are directly in contact with each other but means that the first layer at least partially covers the second layer while being either directly in contact therewith or being separated therefrom by another layer or another element.

In the context of the present invention, width means dimensions taken in a direction parallel to the plane of the bottom face of a substrate112and in the direction of movement of the electrons in the channel. Thickness means a dimension taken in a direction perpendicular to the plane of the bottom face of the substrate112. Thus, in the figures illustrated, the width L of an oxide pad311, the width Lg of a gate stack160, and the width Lsp of the spacers410are taken in a horizontal direction and the thicknesses are taken in a vertical direction.

FIG. 3summarises the main steps of manufacturing a transistor200according to an example embodiment of the invention. These steps will be described with more detail with reference toFIGS. 4ato 12b. Only two transistors200will be illustrated in these figures in order to facilitate understanding of the invention. Naturally a much higher number of transistors may be effected during the same method.

The starting structure of the method according to the invention is a structure of the semiconductor on insulator type, for example of the SOI type. More precisely, the structure comprises a substrate112, an insulating layer114(thin or thick) of buried oxide and an initial semiconductor layer312also referred to as a thin layer and forming the active layer of the stack of the SOI type. Advantageously, the thickness of the insulating layer114does not constitute a constraint for implementing the invention.

Step210consists of forming a cavity315for each transistor200to be produced. This cavity315can be produced either by etching only a portion of the thickness of the initial semiconductor layer312in the bottom of the cavities315(as illustrated inFIG. 4a) or by etching the whole of the thickness of the initial semiconductor layer312in the bottom of the cavities315(as illustrated inFIG. 4b).

In the embodiment of the partial etching, the depth of the cavity315is defined so as to leave in place semiconductor material in the bottom of the cavities315, as illustrated inFIG. 4a. Preferably, a thickness of the initial semiconductor layer312of around one to several nm (nanometers) is preserved in the bottom of the cavity315. For example, in order to obtain a pad rising by 15 nm (that is to say a step difference of 15 nm) above the insulating layer114of the SOI stack, it is possible to leave a thickness of semiconductor material of around 6 nm in the bottom of the cavities315. The height of the oxide pad311formed subsequently is referenced316inFIG. 6described below. The semiconductor material is for example silicon, germanium or silicon-germanium.

In the embodiment of the total etching of the regions of the initial semiconductor layer312intended to form the cavities315(as illustrated inFIG. 4b), an etching is carried out in order to remove the entire thickness of the initial semiconductor layer312in the bottom of the cavities315. Thus no more material of the initial semiconductor layer312remains in the bottom of the cavities315; in other words, the top surface of the insulating layer114at the cavities315is bared.

The height of the oxide pad311formed subsequently is referenced316inFIG. 6described below. The semiconductor material is for example silicon, germanium or silicon-germanium.

In order to produce the cavities315by either partial or total etching of the regions of the initial semiconductor layer312intended to form the cavities315, a lithography step is previously carried out in order to form a mask covering the regions that it is not wished to etch, that is to say the regions outside the cavities315.

The width of the oxide pad311is dependent on the width of the cavity315. It may be that the lithographies used firstly to form the cavities315and secondly used to produce the gate stack316have misalignment. In the context of the present invention, the gate stack160must be situated above the oxide pad311without projecting beyond the ends of said oxide pad311. The cavity315is therefore produced so that its width L is greater than the width Lg of the gate stack160. Advantageously, the following formula is applied to calculate the sizing of the oxide pad311:
L≥Lg+2×lithographic misalignment

According to a first embodiment, the etching of the initial semiconductor layer312is carried out directly through the mask defining the patterns of the cavities.

According to another embodiment, the etching of the initial semiconductor layer312is carried out after an oxidation of the initial semiconductor layer312through the mask defining the patterns of the cavities. This oxidation step thus oxidises the areas not protected by the mask. This oxidation step makes it possible to very precisely control the thickness to be etched, taking advantage of the selectivity to etching of the oxide with respect to the semiconductor material. The oxidation thickness then directly defines the thickness of the regions of the initial semiconductor layer312to be etched subsequently. It should be noted that the oxidation tends to increase the size of the patterns to be etched and it is therefore necessary to take it into account during the step of lithography of the cavities.

In these two embodiments, the etching is preferably a reactive-ion etching (RIE). It is preferably anisotropic, directed perpendicular to the plane of the substrate112.

FIG. 5illustrates the structure obtained at the end of the step220of formation of a continuous electrically insulating oxide layer on the surface of the structure obtained after the implementation of the previous step210. According to the structure obtained at the end of step210(illustrated inFIGS. 4a, 4b), the step220can be performed either by oxidation of the initial semiconductor layer312, or by a deposition of oxide over the entire surface of the structure obtained at the end of step210.

In the case where a thickness of the initial semiconductor layer312is preserved in the bottom of the cavity315(as illustrated inFIG. 4a), step220consists of effecting a thermal oxidation in order to form a continuous oxide film on the uncovered surface of the initial semiconductor layer312. The mask or the layer of resin used to form the cavities315will previously have been removed.

The oxidation parameters, in particular the duration of oxidation, are to be defined so as to oxidise the whole of the initial semiconductor layer312at the bottom of the cavity315. Thus the entire initial semiconductor layer312present under the cavities315is oxidised. The oxide film formed in the bottom of the cavities315therefore extends as far as the insulating layer114. It forms with the latter an uninterrupted layer of oxide. Thus the entire thickness317is formed by an insulating material, i.e. oxide. On the other hand, the initial semiconductor layer312is divided into several parts because of the presence of the oxide pads311.

The oxidation may increase the size of the cavities in the initial semiconductor layer312, which is not a problem in itself. It is then simply necessary to anticipate this as from step210in order to calculate the width of the patterns of the mask and cavities315to be etched before oxidation.

Alternatively, in the case where the bottom of the cavities315is bared (as illustrated inFIG. 4b), step220may also be performed, as illustrated above, by oxidation of the initial semiconductor layer312. In another embodiment, step220is performed by a deposition of oxide over the entire surface of the structure obtained after the implementation of step210. In this case, the deposited thickness of the oxide deposition is preferably at least equal to the depth of the cavity315.

FIG. 6illustrates the structure obtained at the end of the step230of baring portions of the initial semiconductor layer312, disposed between the cavities315. This step230consists of removing the oxidised regions on top of the initial semiconductor layer312by carrying out a polishing, such as a chemical and mechanical polishing CMP. The CMP polishing stops on the initial semiconductor layer312. Thus, after CMP, there is no longer any oxide on top of the initial semiconductor layer312. Only the oxide pads311formed at the cavities315remain. Preferably, the thickness316is equal to that of the initial semiconductor layer312which remains after CMP.

NextFIGS. 7ato 7cillustrate the structure obtained at the end of the step240of formation of a layer of semiconductor material covering the oxide pads311.

More precisely, step240comprises the steps310and/or320of formation of an additional layer of semiconductor material313on the oxide pads311, this additional layer of semiconductor material313being intended to form the channel180of each transistor200. It is also intended to form an uninterrupted semiconductor layer146with the initial semiconductor layer312. Two embodiments are presented hereinafter.FIG. 7cillustrates the structure obtained at the end of each of these embodiments.

Step310, illustrated inFIG. 7a, consists of depositing an additional layer313of semiconductor material, advantageously crystalline, made from silicon for example. The thickness of the additional layer313is preferably between 5 and 20 nm. Advantageously, this additional layer313is transferred onto the stack comprising the oxide pads311and the initial semiconductor layer312from another structure, such as a donor substrate. To reflect this transfer, it is possible for example to use the SMART CUT technique developed by the company SOITEC. Typically, the additional layer313is a thin film that is detached from a donor substrate while previously having created in this donor substrate a weakened area, for example by ion implantation.

The portions of the initial semiconductor layer312and the additional layer of semiconductor material313form an interrupted semiconductor layer146, as illustrated inFIG. 7c.

Preferably, the initial semiconductor layer312and the additional layer of semiconductor material313are monocrystalline.

Optionally, the thickness of the semiconductor layer146is adjusted by carrying out a CMP polishing. Advantageously, the thickness of the channel180formed by the portion of the additional layer313situated above the oxide pad311is thus adjusted.

This embodiment is particularly advantageous for obtaining a monocrystalline channel without defect in the crystalline structure, including at the middle of the channel180. The channel180is therefore homogeneous and has a crystalline structure without defects or with a limited number of defects.

According to the other method, the uninterrupted semiconductor layer146is re-formed by solid-phase epitaxy. The semiconductor material as deposited from the additional layer313is then amorphous (for example amorphous silicon). This additional layer313is deposited, at a temperature of 400° C. for example, on the face having the oxide pads311and the initial semiconductor layer312.

FIG. 7billustrates the structure obtained during the step320of recrystallisation of the semiconductor material of the amorphous additional layer313, recrystallisation initiated by the crystalline structure of the initial semiconductor layer312. In the case where this material is amorphous silicon, heat treatment is carried out at a preferential temperature of 500° C., so that recrystallisation of the amorphous silicon of the layer313is initiated by the crystalline semiconductor material of the layer312. Thus a crystalline semiconductor layer146is formed from the layer313deposited in the amorphous state and the crystalline layer312, as illustrated inFIG. 7c. Preferably, the layer312is monocrystalline and the semiconductor layer146obtained after recrystallisation of the layer313is also monocrystalline.

FIG. 8illustrates the structure obtained at the end of step250of formation of a gate stack160vertically in line with an oxide pad311. The stack160comprises for example the following layers stacked from the semiconductor layer146: a gate insulation layer, often referred to as a thin gate oxide131or high-k layer, a metal layer132, the gate124, also referred to as the gate electrode, and a hard mask126covering the top surface of the gate124. The invention is not limited this gate stack160and some of the layers mentioned below may not be present or other layers may be added without departing from the scope of the present invention.

As mentioned previously at step210, each gate stack160is deposited in line with an oxide pad311without projecting beyond the ends of said oxide pad311. The gate is not necessarily aligned with the centre of the pad. In order to avoid or reduce any misalignment of the source and drain140with respect to the gate stack160, it is possible to use the following formula mentioned previously in order to define the widths of the gate and cavities: L≥Lg+2×maximum lithographic misalignment. This is because any misalignment of the gate stack160with respect to the oxide pad311has an impact on numerous parameters such as the serial resistance, the electrostatic control of the channel, the stray capacitance, the leakage, the speed, etc.

The gate stack160is produced in a conventional manner, for example by a succession of lithographies and etchings.

FIG. 9illustrates the structure obtained at the end of the step260of formation of protective layers covering the flanks of the gate stack160. These protective layers, normally referred to as spacers410, are situated on either side of the gate stack160, extending mainly perpendicular to the plane of the substrate112and covering the flanks of the gate124at least. They are advantageously made from silicon nitride. The hard mask126and the spacers410protect the gate stack160so that the flanks of the gate stack160are not etched during following operations. In particular, as will emerge in the remainder of the description, the hard mask126and the spacers410are made from material resistant to the cavity etching substances that are used during step520described below.

Particularly advantageously, for positioning the gate stack160vertically in line with the oxide pad311, the following condition is applied:
Lg+2×Lsp≥L,
in which Lsp is the width of the spacers410.

In addition, the maximum misalignment of the gate stack160, e.g. 3 to 4 nm, must be less than the width of the spacers410, e.g. 6 to 8 nm. The ends of the spacers410must be above the very thick regions of the semiconductor material of the semiconductor layer146. Thus the free faces of the spacers410, i.e. those turned towards the outside of the gate stack160, are not situated vertically in line with an oxide pad311. On the other hand, the gate124must be situated vertically in line with the oxide pad311, that is to say whatever its position above the oxide pad, but its flanks must not project beyond the ends of said oxide pad311.

The following step270consists of producing the source and drain regions140and may be implemented by one of the following three embodiments510,520,540respectively illustrated inFIGS. 10a, 10b(step510),FIGS. 11aand 11b(step520) andFIGS. 12aand 12b(step540).

The first embodiment illustrated inFIGS. 10aand 10bis particularly advantageous if it is wished for the material of the source and drain140to be the same as that of the channel180. The second and third embodiments provide firstly an etching of the semiconductor layer146carried out while preserving the latter at least at a region forming the channel180and secondly an epitaxy from this region. This offers the possibility of having a source and drain140the material of which may be different from that of the channel180.

FIG. 10aillustrates the structure obtained at the end of the step510of doping the semiconductor layer146re-formed from the layers312and313and in accordance with one of the embodiments of step240illustrated inFIG. 3. This semiconductor layer146is doped by ion implantation121on either side of the gate124in order to form the source and drain140.

The structure200thus obtained (illustrated inFIG. 10b) has a source and drain140below the gates124. The stray capacitances are therefore reduced whereas the thickness of the source and drain140can be freely adapted to reduce the access resistance to the transistor200.

The first embodiment of formation of the source and drain140has the advantage of being simple and independent of the thickness of the channel180. This makes it possible to form a source and drain140in the case where the channel180is so thin that it becomes difficult to produce a source and drain140by epitaxy.

FIG. 11aillustrates the structure obtained at the end of the step520of etching the whole of the thickness of the semiconductor layer146with the exception of the region situated under the gates124and of defining the channel180. This step520is explained below.

At the end of the step of formation of the gate stack160and the preferential step of formation of the spacers410, an etching is carried out in order to remove, on either side of the gate stack160, the entire thickness of the re-formed semiconductor layer146while preserving the entire thickness of this semiconductor layer146under the gate124. This etching is for example a wet etching of the TMAH (tetramethylammonium hydroxide) type. Preferably, it is anisotropic and directed perpendicular to the surface of the substrate112. According to another embodiment, in particular if the spacers are sufficiently wide with respect to the thickness of the layer146, the etching may be only slightly anisotropic or even isotropic. The regions of the semiconductor layer146situated below the gate124are left in place so as to form the channel180.

In the example illustrated inFIG. 11a, the entire semiconductor layer146is removed with the exception of the regions situated under the gates124.

Advantageously, a thickness of the semiconductor layer146is removed under the spacers410. Preferably, the entire thickness of the semiconductor layer146situated under the spacers410is removed. The lateral ends of the channel180are thus bared and the bottom ends of the spacers410form a rim above each lateral end of the channel180. The advantage of this feature for promoting epitaxy directed downwards is detailed hereinafter. Lateral ends of the channel180means the ends that appear in the plane of the figures. The lateral ends of the channel180are indicated by the reference117inFIG. 11a. As mentioned previously, the gate stack160, protected by the spacers410and the hard mask126, is not etched during the etching of the semiconductor layer146.

FIG. 11billustrates the structure obtained at the end of the step530of formation of the source and drain140. An epitaxy initiated by the semiconductor material of the channel180is effected in order to fill the volumes formed on either side of the gate stack160and under the gate124.

The channel180is preferably a monocrystalline semiconductor material.

It is possible for example to use identical materials for on the one hand the channel180formed by the semiconductor layer146and on the other hand the source and drain regions140formed by epitaxy. It is possible for example to use silicon or other semiconductor materials also able to be used in a monocrystalline form, such as germanium (Ge) or alloys of these two materials (Si—Ge).

Alternatively, this embodiment makes it possible to use different materials for the channel180formed by the semiconductor layer146and for the source and drain regions140formed by epitaxy. Thus, for this alternative, it is possible to provide, for the source and drain regions140, a material different from that of the channel180but existing in a crystalline form suited to the crystalline lattice of the channel180, that is to say it has, for example, mesh parameters that differ by less than 1% and preferably less than 0.5% from that of the material of the channel180in the plane parallel to the face of the substrate112.

The epitaxy, from the bared lateral ends of the channel180, is done downwards, preventing the source and drain regions140coming opposite the gate124. In addition, the presence of the spacers410that are on top of the lateral ends of the channel180forming a rim above the latter tends to prevent the epitaxy from rising in the direction of the gate124. The spacers410thus promote the epitaxial growth downwards.

Preferentially, a doping of the source and drain regions140is effected simultaneously with the growth by epitaxy. This type of doping is normally referred to as in situ doping.

FIG. 12aillustrates the structure obtained at the end of the step540of partial etching of the semiconductor layer146with the exception of the regions situated under the gate stack160and definition of the channel180. This step540is explained below.

As illustrated at step520above, the thickness of the semiconductor layer146below the gate stack160is entirely preserved during the performance of step540so as to form the channel180. On the other hand, a film of the semiconductor layer146is left in place on the insulating layer114outside the region situated in line with the gate stack160, as illustrated inFIG. 12a. This film consists at least partly of the layer312and optionally part of the thickness of the layer313.

To this end, an anisotropic etching, preferably a reactive ion etching RIE, is performed in order to remove, on either side of the gate stack160, a portion of the thickness of the semiconductor layer146. This thickness is preferably between 1 nm and 20 nm. In addition, as mentioned previously, the gate stack160, protected by the spacers410and the hard mask126, is not etched during this step540.

FIG. 12billustrates the structure obtained at the end of step530of formation of the source and drain140. The source and drain140are formed by epitaxy. Advantageously, a doping is carried out of the source and drain regions140simultaneously with the growth by epitaxy. This type of doping is normally referred to as in situ doping.

The epitaxy is initiated at least partly by the semiconductor film left in place by partial etching of the layer146. According to another embodiment, the epitaxy is initiated by the lateral ends of the channel. According to another embodiment, the epitaxy is initiated by the lateral ends of the channel and by the semiconductor film left in place by the partial etching of the layer146.

In all the embodiments described above, the thickness of the source and drain regions140can be freely adapted to reduce the access resistance to the transistor200while maintaining these source and drain regions140mainly and preferably entirely under the gate124, thus reducing the stray capacitance.

Particularly advantageously, the source and drain140are auto-aligned with the gate124.

The invention is not limited to the embodiments previously described but extends to any embodiment covered by the claims.