Abstract:
A method for manufacturing a strained channel MOS transistor including the steps of: forming, at the surface of a semiconductor substrate, a MOS transistor comprising source and drain regions and an insulated sacrificial gate which partly extends over insulation areas surrounding the transistor; forming a layer of a dielectric material having its upper surface level with the upper surface of the sacrificial gate; removing the sacrificial gate; etching at least an upper portion of the exposed insulation areas to form trenches therein; filling the trenches with a material capable of applying a strain to the substrate; and forming, in the space left free by the sacrificial gate, an insulated MOS transistor gate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 10/57253, filed on Sep. 13, 2010, entitled METHOD FOR MANUFACTURING A STRAINED CHANNEL MOS TRANSISTOR, which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing strained-channel MOS transistors. 
     2. Discussion of the Related Art 
     Integrated MOS transistors are formed at the surface of semiconductor substrates. They include an insulated gate formed on the surface of the substrate and source and drain regions formed in the substrate, on either side of the gate. 
     In a semiconductor substrate, the carriers involved in N-channel or P-channel transistors do not move at the same speed. To improve the mobility of carriers, it is known to form different channel regions according to the type of carriers used. Especially, the mobility of carriers in P-channel MOS transistors is known to be greater when the channel is made of silicon-germanium rather than of silicon. For N-channel MOS transistors, a silicon substrate is better adapted. It can thus be provided to form, on a same structure, silicon-germanium channel regions for P-channel MOS transistors and silicon channel regions for N-channel MOS transistors. 
     It is also known that, on a same substrate, the mobility of some carriers can be improved by local application of a strain on the channel of the concerned transistors. This is especially described in publication “Electron Mobility Model for Strained-Si Devices”, by Dhar et al., TED 52 (2005). 
     Many methods have been provided to apply a local strain on the channel of given transistors. It has especially been provided to modify or to replace the material forming the source and drain regions so that the modified or replacement material applies a strain along the length of the transistor channel. 
     However, known methods are generally relatively complex to implement and necessitate a significant number of additional manufacturing steps with respect to conventional MOS transistor manufacturing methods. 
     SUMMARY OF THE INVENTION 
     An embodiment provides a relatively simple method for forming strained-channel MOS transistors, which implies but few additional steps with respect to known methods. 
     Another embodiment provides a method compatible with the forming of MOS transistor gates at the surface of a same substrate in different materials. 
     An embodiment provides a method for manufacturing a strained channel MOS transistor, comprising the steps of: (a) forming, at the surface of a semiconductor substrate, a MOS transistor comprising source and drain regions and an insulated sacrificial gate which partly extends over insulation areas surrounding the transistor; (b) forming a layer of a dielectric material having its upper surface level with the upper surface of the sacrificial gate; (c) removing the sacrificial gate; (d) etching at least an upper portion of the exposed insulation areas to form trenches therein; (e) filling the trenches with a material capable of straining the substrate; and (f) forming, in the space left free by the sacrificial gate, an insulated MOS transistor gate. 
     According to an embodiment, step (e) is carried out by depositing a material capable of straining the substrate over the entire structure, and by then etching the excess material located outside of the trenches. 
     According to an embodiment, the trenches extend all the way to the semiconductor substrate, or all the way to a semiconductor-on-insulator structure support, and the filling of the trenches at step (e) is performed by epitaxy of the material capable of applying a strain from the bottom of the trenches. 
     According to an embodiment, the substrate is a solid substrate. 
     According to an embodiment, the substrate is a substrate of semiconductor-on-insulator type. 
     According to an embodiment, the material capable of straining the substrate is silicon nitride, silicon-germanium, or a material having a structure capable of reflowing after an anneal. 
     According to an embodiment, step (f) is preceded or followed by a step of removal of the dielectric material layer. 
     An embodiment provides an integrated circuit comprising at least one MOS transistor formed by the above method. 
     The foregoing and other objects, features, and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  respectively are a widthwise and a lengthwise cross-section view, and a simplified top view of a MOS transistor formed by a method according to an embodiment; 
         FIGS. 2A to 7A , respectively  2 B to  7 B, are cross-section views across the width of a transistor, respectively along the transistor length, illustrating results of steps of a method according to an embodiment; and 
         FIG. 8A , respectively  8 B, is a cross-section view across the width of a MOS transistor, respectively along its length, resulting from a variation of a method according to an embodiment. 
     
    
    
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
     DETAILED DESCRIPTION 
     In the drawings which will be described hereafter, a single MOS transistor is shown at the surface of a substrate. It should be noted that, conventionally, many MOS transistors are formed at the surface of a same substrate, for example, in association with other electronic components. 
     Further, in the following description, when a material applying a strain on the adjacent semiconductor substrate will be mentioned, the considered strain may be a tensile strain or a compressive strain, according to the desired application and to the desired carrier mobility. 
       FIGS. 1A ,  1 B, and  1 C illustrate, respectively in cross-section view along two perpendicular directions at the surface of a substrate and in simplified top view, a MOS transistor M obtained by a method according to an embodiment. The top view of  FIG. 1C  is simplified in that it does not show the insulated gate of the transistor. Conventionally, the channel distance between the source and drain regions is called the transistor length (channel length, L in  FIG. 1C ) and the dimension in a direction, at the substrate surface, perpendicular to its length is called the transistor width (l in  FIG. 1C ). It should be noted that, in the drawings, the length of the shown transistor is shorter than its width. The opposite is also possible. 
     Transistor M is formed at the surface of a semiconductor substrate  10 . More specifically, transistor M is formed in an active area A formed of a portion of semiconductor substrate  10  surrounded with an insulation region  22 , formed at the substrate surface. As an example, insulation regions  22  may be “STI”-type (Shallow Trench Isolation) regions. The transistor comprises, at the surface of active area A, an insulated gate  12  comprising a lower insulating layer  14  topped with a layer of a conductive material  16 , for example, doped polysilicon. 
     Spacers  18  are formed all around insulated gate  12 . Source and drain regions  20  are formed in semiconductor substrate  10 , at the surface thereof, on either side of gate  12  along the length of transistor M. Insulating regions  22  are, in the shown example, in contact with source and drain regions  20 . 
     Across the width of transistor gate  12  ( FIG. 1A ), on either side of insulated gate  12 , insulation regions  22  which enable to insulate the different MOS transistors formed at the surface of a same substrate from one another extend. Under transistor gate  12 , in contact with insulation regions  22 , regions  24  of a material capable of straining the adjacent substrate are formed. Especially, material  24  enables to apply a strain on the region of substrate  10  formed under transistor gate  12 , at the level of the channel of transistor M. 
     In the shown example, portions  24  of a material capable of straining the adjacent substrate do not extend all the way to the bottom of insulation regions  22 . It should be noted that portions  24  may also extend all the way to the bottom of insulation regions  22 . 
     The method described hereafter enables to form, on a same semiconductor substrate, MOS transistors such as that of  FIGS. 1A and 1B , but also conventional MOS transistors, in which the semiconductor material forming the channel portion of the transistor is not strained. To achieve this, it will be enough for the conventional transistors to be masked on manufacturing of the strained-channel transistors. It may also be provided to foam strained-channel and unstrained-channel transistor gates in different materials, selectively etchable with respect to each other. The method described hereafter is also compatible with the forming of MOS transistor gates of different structures or materials at the surface of a same substrate. 
       FIGS. 2A to 7A , respectively  2 B to  7 B, are cross-section views across the width of a transistor, respectively along the transistor length, illustrating results of steps of a method according to an embodiment of the present invention. 
     At the step illustrated in  FIGS. 2A and 2B , a completed conventional MOS transistor is formed at the surface of a semiconductor substrate  10 . This transistor comprises an insulated gate  30  comprising a lower insulating layer  32  and an upper layer  34  of a conductive material such as doped polysilicon. Spacers  18  are formed all around insulated gate  30 . The source and drain regions are formed, on either side of spacers  18  along the transistor length, in semiconductor substrate  10 , at its surface, and may be silicided. 
     The transistor is formed in an active area defined in substrate  10  by insulation regions  36 . Thus, across the transistor width, on either side of the channel region formed under the insulated gate, insulating regions  36  extend at its surface. As an example, insulating regions  36  may be STI-type (Shallow Trench Isolation) regions, for example, made of silicon oxide. Conventionally, insulated gate  30  extends slightly at the surface of insulating regions  36 . 
     At the step illustrated in  FIGS. 3A and 3B , a layer of insulating material  38 , selectively etchable over the material foaming insulation regions  36 , is formed on the structure to cover the entire device. The surface of insulating layer  38  is at the same level as the surface of insulated gate  30 : the surface of layer  38  is flush with the surface of gate  30 . 
     To form insulating layer  38 , it is for example possible to perform a full plate deposition of an insulating material, followed by a chem.-mech. polishing (CMP) or an etching to expose the upper surface of insulated gate  30 . As an example, if insulating regions  36  are made of silicon oxide, insulating layer  38  may be made of silicon nitride selectively etchable over silicon oxide. Other materials may of course be envisaged. 
     At the step illustrated in  FIGS. 4A and 4B , one or several etchings have been performed to eliminate conductive material  34  and insulating material  32  of insulated gate  30 . This enables to form, within dielectric material  38 , a trench  40  at the location of insulated gate  30 . 
     At the step illustrated in  FIGS. 5A and 5B , an anisotropic or isotropic etching is performed to eliminate a portion of insulation areas  36  corresponding to the portion which is not protected by dielectric material  38 . This etching provides trenches  42  in insulation regions  36 . It should be noted that the etching performed to obtain the device of  FIGS. 5A and 5B  may be the same as that used, at the step of  FIGS. 4A and 4B , to remove the insulating material from insulated gate  30 , if this etching is compatible with the material of insulation regions  36 . 
     In the example shown in  FIGS. 5A and 5B , trenches  42  do not extend all the way to the bottom of insulation regions  36 . It should be noted that these trenches may also extend to the bottom of insulation regions  36 . 
     At the step illustrated in  FIGS. 6A and 6B , trenches  42  have been filled with a material  24  capable of straining the adjacent substrate, and especially the channel region formed of semiconductor substrate  10  located between straining material regions  24 . Material  24  may be formed in several ways. For example, it may be provided to deposit this material over the entire structure, then to etch it so that its upper surface is at the same level, or substantially at the same level, as the upper surface of substrate  10 . 
     It may also be provided, if trenches  42  extend all the way to the bottom of insulating regions  36 , to form regions  24  by performing an epitaxy of a semiconductor material capable of straining the adjacent substrate from substrate  10 . 
     The material of regions  24  may for example be silicon-germanium (which may be undergo an epitaxy), silicon nitride (Si 3 N 4 ), or any other material, for example a material having a structure capable of reflowing after the application of an anneal, for example, a boron- or phosphorus-doped oxide annealed at a temperature greater than 850° C., such as described in publication “A 3-D BPSG Flow Simulation with Temperature and Impurity Concentration Dependent Viscosity Model”, by Unimoto et al., IEEE 1991. 
     At the step illustrated in  FIGS. 7A and 7B , a new insulated MOS transistor gate  12  has been formed in trench  40 . Gate  12  comprises a lower insulating layer  14  having a conductive layer  16 , for example, made of doped polysilicon, formed at its surface. This gate may be formed in many ways, for example, by depositing all the materials forming the gate in the form of thin layers on the contour of the structure of  FIGS. 6A and 6B , and thus in trench  40 , and then by planarizing the structure (case shown in  FIGS. 7A and 7B ). The method of etching a sacrificial gate and reforming a MOS transistor gate used herein is especially described in publication “75 nm Damascene Metal Gate and High-k Integration for Advanced CMOS Devices” by B. Guillaumot et al., IEDM 2002, pages 355-358. 
     A subsequent step of removal of dielectric material  38  provides the structure of MOS transistor M of  FIGS. 1A and 1B . This step may be followed by a step of forming of a silicide on the source and drain areas, if necessary. 
       FIGS. 8A and 8B  illustrate a result of a variation of the above-described method enabling to form a MOS transistor M′. 
     In this variation, substrate  10  is not a solid substrate, but a substrate of semiconductor-on-insulator type (SOI). In this configuration, substrate  10  is a semiconductor layer which is formed at the surface of an insulating layer  45 , which itself extends on a semiconductor support  44 . 
     Insulation regions  22  of the transistors, defining the active area of the transistor, extend in substrate  10 , through insulating layer  45 , and into underlying substrate  44 . Regions  24 ′ of a material capable of straining the channel of transistor M′ are formed across the width of insulating material  22 . Insulated gate  14 - 16  of transistor M′ is similar to that of transistor M of  FIGS. 7A and 7B . 
     The forming of regions  24 ′ of a material capable of straining the channel of transistor M′, through insulating layer  45 , enables to form these regions  24 ′ by performing an epitaxy from the semiconductor material of support  44 . 
     To form the structure of  FIGS. 8A and 8B , operations similar to those described in relation with  FIGS. 2A to 7A  and  2 B to  7 B are carried out, except that the etching of the steps of  FIGS. 5A and 5B  crosses buried insulating layer  45 . 
     Advantageously, the methods discussed herein enable to form, at the surface of a same substrate, strained-channel transistors and unstrained channel transistors. To achieve this, the method described herein will be carried out after having masked all the transistors which should not have a strained channel. 
     Further, advantageously, the step of etching the sacrificial layer shown in  FIGS. 4A and 4B  is a step currently used to form, on a same substrate, insulated MOS transistor gates of different materials, for example, with an insulating layer made of a material of strong or low dielectric constant or having a conductive layer formed of a stack of several materials, for example. 
     Further, the fact for the source and drain regions of the transistors to be formed prior to the forming of final insulated gates  12  enables to avoid for these insulated gates to be submitted to too strong a thermal processing. Indeed, the source and drain regions of the transistors are generally formed by applying strong thermal processings, that is, a high temperature, to the structure. The method discussed herein thus enables forming transistor gates made of materials which cannot stand strong thermal stress. 
     The method discussed herein thus enables to obtain, at the surface of a same substrate, various types of MOS transistors having channels ensuring an optimum mobility of the carriers traveling through them. This method has the advantage of only requiring two additional steps with respect to conventional methods of replacement of a sacrificial insulated gate with a new gate, that is, the step of etching of trenches  42 , which may be carried out at the same time as the step of suppression of insulating portion  32  of the sacrificial gate if the etching is compatible, and the step of filling of these cavities. 
     Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, the various etch and deposition steps may be carried out by any known method. Further, according to the considered type of carriers and to the semiconductor channel material, it will be within the abilities of those skilled in the art to determine the material to be formed in regions  24  so that a strain capable of optimizing the carrier mobility is applied on the channel. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.