Abstract:
A field effect transistor includes two channel connection regions, a control region with at least two control sections, an active region that is formed as a projection of a mono crystalline substrate disposed between the channel connection regions and between two control region sections, and insulating regions that are electrically insulating and are disposed between the control region sections and the active region. The projection is isolated from the substrate at a base by an insulating material that is electrically insulating. The insulating material ends laterally at the projection in the mono crystalline substrate.

Description:
RELATED APPLICATIONS 
     This application is a Divisional of U.S. application Ser. No. 10/575,916, filed Dec. 22, 2006, which is a National Stage Entry of PCT/EP04/52333, filed Sep. 28, 2004, which claims priority to German patent application No. 103 48 007.2, filed Oct. 15, 2003. 
    
    
     BACKGROUND 
     The invention relates to a patterning method. In particular, this invention relates to a patterning method and field effect transistors. 
     BRIEF SUMMARY 
     The method is intended to enable the smallest possible minimum feature sizes to be produced, that is to say feature sizes of less than one hundred nanometers or even less than fifty nanometers. 
     In order to produce feature sizes that are less than half a wavelength of electromagnetic waves used in a lithography method, the following methods can be used inter alia:
         spacer technique, in which a layer is deposited on a step and is subsequently etched isotropically,   so-called phase shift masks, which exploit interference effects, and   so-called trimming, in which a structure is etched isotropically in order to reduce its dimensions.       

     It is an object of the invention to specify a simple patterning method which makes it possible to produce in particular minimum dimensions of less than half a photolithographic wavelength, in particular of less than one hundred nanometers or less than fifty nanometers. Moreover, the method is intended to afford the possibility, in particular, of also increasing the mechanical loadability of structures having a minimum feature size. Moreover, the intention is to specify field effect transistors, in particular a double-fin field effect transistor. 
     The object referring to the method is achieved by means of a method having the features specified in patent claim  1 . Developments are specified in the subclaims. 
     The invention is based on the consideration that all the known methods are associated with disadvantages. Thus, the spacer technique leads to rounded spacers which adversely affect the dimensional accuracy of a structure etched with the spacers. Phase shift masks are very expensive in comparison with photomasks that do not exploit interference effects. Trimming leads to comparatively rough structures that are not dimensionally accurate on account of inhomogeneous etching conditions. 
     In the case of the method according to the invention, the following method steps are performed without restriction by the order specified:
         application of an auxiliary layer to a carrier material, the auxiliary layer being either an inorganic layer or an organic layer, in particular a resist layer,   patterning of the auxiliary layer and of the carrier material with production of a cutout,   expansion of the cutout in the region of the auxiliary layer, the cutout, in the region of the carrier material, not being expanded or not being expanded to as great an extent as in the region of the auxiliary layer,   filling of the expanded cutout with a filling material,   preferably complete removal of the auxiliary layer after filling,   patterning of the carrier material using the filling material and with production of at least one further cutout, the filling material being an inorganic material or an organic material, e.g. a resist.       

     A further patterning method is thus specified which permits very small and very dimensionally accurate structures to be produced in a simple fashion. In one refinement, the expansion of the cutout is carried out by means of an etching-back step, which is also referred to as a pull-back step. The expansion gives rise to a cutout having a T-shaped cross section. Consequently, the filling material introduced into the cutout also has a T-shaped cross section, that is to say a cross section which expands symmetrically toward one end. 
     The expansion can also be delimited to a part of the edge of the cutout by means of an additional masking step, in particular by means of an additional photolithographic method, so that therefore, in particular, only one structure is produced per cutout. It is often not a cause of disturbance, however, if two or more than two structures arise per cutout, so that an additional masking step is not required. In particular, the additional masking step can be avoided by a suitable choice of the dimensions of the cutout. 
     In one development, the auxiliary layer and the carrier material are patterned in order to produce the cutout by means of a photolithographic method. The photolithography limits the smallest lateral dimension of the cutout and hence the dimensions between the structures to be produced. This is acceptable, however, since in many cases the distances between structures are considerably greater than the minimum feature size of the structures themselves. If the auxiliary layer is a resist layer, then intermediate layers are used in order to remove firstly only the upper resist layer but not the auxiliary layer. 
     In another development, the filling material is planarized prior to the repeated patterning, e.g. by means of a CMP method, in order to obtain a dimensionally accurate filling structure and hence a dimensionally accurate subsequent patterning. Instead of planarization, however, it is also possible to use other methods, e.g. selective filling with selective oxidation. 
     In another development of the method according to the invention, the carrier material contains a hard mask layer, which is patterned with the aid of the filling material. The mask layer then serves e.g. for its part for the patterning of a substrate, e.g. for the production of a semiconductor circuit, in particular for the production of gate electrodes, e.g. made from polycrystalline silicon, made from metal or made from a layer sequence of metal and polycrystalline silicon. However, the hard mask layer is also used for the production of a so-called stencil mask, that is to say a mask which is used subsequently to carry out a photolithographic method, e.g. electron projection lithography, with an imaging scale of 1:1. Hard mask layers are more resistant to etching attacks in comparison with a resist. 
     Situated between the mask layer and the auxiliary layer is e.g. only an intermediate layer which is thin in comparison with the mask layer or the auxiliary layer and the thickness of which is e.g. less than one third of the thinner of the two layers. The intermediate layer serves e.g. for better mechanical adhesion or for taking up mechanical stresses. 
     In an alternative development, the carrier material contains a semiconductor material, in particular a monocrystalline semiconductor material, from which a semiconductor circuit or a mask is fabricated. The cutout then e.g. already defines one side area of a fin for a finFET. 
     In a next development, at least one layer is deposited or grown in the expanded cutout prior to filling, in particular an electrically insulating layer for producing a gate dielectric and an electrically conductive layer for producing a gate electrode of a field effect transistor. The layer that is introduced into the cutout can, for its part, be patterned by the method according to the invention, so that short gate lengths are produced in a simple manner. 
     In a next development, a further cutout situated alongside the cutout filled with the filling material is filled with a further filling material before the filling material used for patterning is removed. The filling material serving for patterning is not removed until after the further cutout has been filled, so that even thin structures between the two cutouts are laterally supported at any time. Therefore, the structures can neither tip over nor incline laterally. 
     In an alternative development, the filling material is only partially removed from the cutout, one part of the bottom of the cutout being uncovered and another part of the bottom of the cutout remaining covered with filling material. The remainder of the filling material serves as a mechanical support and is not removed until after further method steps have been carried out, e.g. after the deposition of at least one further layer or after carrying out an oxidation. As an alternative, the remainder of the filling material remains in an integrated circuit arrangement. 
     In another development, a semiconductor material is oxidized in the region between the cutout and the further cutout in order to improve the electrical properties of a transistor, in particular in order to avoid parasitic capacitances under drain and/or source contact pads. The oxidation is preferably carried out prior to the removal of the filling material from the cutout or prior to the complete removal of the filling material from the cutout and preferably after the production of an oxidation protective layer on at least one side wall of the further cutout. The filling material thus supports the structures under which the oxide grows. In the case of complete insulation of the fin by the oxide produced during the underoxidation, an SOI structure arises which is simple to produce and leads to components having outstanding electrical properties. 
     Small minimum feature sizes are required in particular in the case of field effect transistors. Therefore, the invention relates in further aspects to field effect transistors such as can be produced in particular by the method according to the invention or one of its developments, so that the technical effects explained above are applicable in this case. In comparison with a single-fin field effect transistor, a double-fin field effect transistor according to the invention is distinguished by improved and new electrical properties that open up new application possibilities. Field effect transistors having more than two fins per transistor, e.g. having three, four or five fins, are also produced. 
     In another field effect transistor according to the invention, the fin is e.g. thermally underoxidized and preferably completely electrically insulated from the silicon wafer. Such transistors can be produced in a simple manner if, with regard to the mechanical stability, the fin is always supported on a wall, e.g. by the filling material or by a further filling material which is applied after patterning with the aid of the filling material contained in the cutout. 
     In one development of the field effect transistor, a projection for the active region of the transistor has sidewalls of different lengths, the difference being greater than one nanometer, greater than three nanometers or greater than five nanometers. This affords the possibilities of implementing the etching stop during the first patterning and during the second patterning with greater tolerances. Moreover, an underoxidation of the projections is facilitated. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Exemplary embodiments of the invention are explained below with reference to the accompanying drawings, in which: 
         FIGS. 1A to 1D  show production stages of an exemplary embodiment for the production of a hard mask or for the direct patterning of a semiconductor substrate, 
         FIGS. 2A to 2D  show production stages of an exemplary embodiment for producing a double-fin field effect transistor with and without underoxidation, and 
         FIGS. 3A and 3B  show further production stages for producing the double-fin field effect transistor. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A to 1D  show production stages of an exemplary embodiment for the production of a hard mask or for the direct patterning of a semiconductor substrate. The production of the hard mask is explained first. 
     A hard mask layer  12  is applied on a semiconductor substrate  10 , e.g. on a silicon wafer, the thickness of which hard mask layer depends e.g. on the height of a structure that is to be produced later using the completed hard mask. By way of example, the thickness of the hard mask layer  12  matches the height of a gate or the height of a fin for a finFET. In the exemplary embodiment, the thickness of the hard mask layer  12  is e.g. 40 nanometers. 
     An auxiliary layer  14  is then applied to the hard mask layer  12 , said auxiliary layer comprising a different material than the hard mask layer  12 . By way of example, the hard mask layer  12  comprises TEOS (tetraethyl orthosilicate) and the auxiliary layer  14  comprises silicon nitride or some other nitride. In an alternative exemplary embodiment, by contrast, the hard mask layer  12  comprises a nitride and the auxiliary layer  14  comprises TEOS. Optionally, a thin intermediate layer or an intermediate layer sequence is applied after the application of the hard mask layer  12 . The auxiliary layer  14  is then applied to the intermediate layer or the intermediate layer sequence. In the exemplary embodiment, the auxiliary layer  14  has a thickness in the range of fifty nanometers to one hundred nanometers. 
     A resist layer  16 , e.g. photoresist, is subsequently applied to the auxiliary layer  14 , a thin antireflection layer, by way of example, optionally having been applied beforehand. The resist layer  16  is subsequently irradiated, in particular exposed, in accordance with a pattern and developed. The lithography method is noncritical since minimum feature sizes to be produced are greater than one hundred nanometers or at least greater than fifty nanometers. A pattern is e.g. a rectangular area with optional extensions for later source and drain contact pads. 
     The auxiliary layer  14  is subsequently patterned in accordance with the resist layer  16  by means of an anisotropic etching method, e.g. by means of reactive ion etching (RIE), a cutout  18  being produced. The cutout  18  is subsequently likewise extended right into the hard mask layer  12  by means of an anisotropic etching method, so that the bottom of the cutout reaches as far as the semiconductor substrate  10 . The semiconductor substrate  10  serves e.g. as an etching stop. Preferably, the etching conditions remain the same during the etching of the cutout  18  and etching is effected without any interruption. Afterward, in one exemplary embodiment, the residues of the resist layer  16  that have remained on the prepatterned auxiliary layer  14  are removed. In a further exemplary embodiment, the resist structures remain on the auxiliary layer  14 . During the etching of the cutout  12 , in another exemplary embodiment, the auxiliary layer  14  is also used as a mask if the resist layer  16  has already been removed on account of the depth of the cutout  18 . 
     As shown in  FIG. 1B , a step of isotropically etching back the auxiliary layer  14  is then carried out, the auxiliary layer  14  being thinned direction-independently and selectively with respect to the hard mask layer  12  to form an auxiliary layer  14   b . In this case, the resist layer  16  acts as protection of the auxiliary layer  14  and prevents this layer from being thinned if the resist layer  16  is still present. The resist layer  16  is subsequently removed in this case. The auxiliary layer  14  is always etched back laterally, however. In this case, the cutout  18  is extended in the region of the auxiliary layer  14   b  to form a cutout  18   b . In the region of the hard mask layer  12 , by contrast, the dimensions of the cutout  18   b  are unchanged in comparison with the cutout  18 . At the level of the boundary between hard mask layer  12  and patterned auxiliary layer  14   b , an area  20  lying approximately parallel to the bottom of the cutout  18  arises in the cutout  18 , from which area the auxiliary layer is removed during the etching-back. By way of example, this process effects etching-back by less than fifty nanometers or by less than twenty nanometers, so that the area  20  also has a corresponding minimum dimensioning. The cutout  18   b  has a T-shaped cross section on account of the expansion. The magnitude of the etching-back thus determines the minimum feature size to be produced. 
     As is likewise illustrated in  FIG. 1B , the extended cutout  18   b  is subsequently filled with a filling material  22 , which differs with regard to its material composition both from the material of the hard mask layer  12  and from the material of the thinned auxiliary layer  14   b . By way of example, silicon carbide or polycrystalline silicon is used as the filling material. After the cutout  18   b  has been filled, a planarization step is carried out, during which the auxiliary layer  14   b  serves as a stop layer. By way of example, planarization is effected by means of a CMP method (chemical mechanical polishing) or a whole-area etching process. 
     The residues of the thinned auxiliary layer  14   b  are then removed selectively with respect to the hard mask layer  12  and selectively with respect to the filling material  22  by means of a wet-chemical or dry-chemical etching method. Projections  24  of the filling material  22  remain above the area  20 , which projections cover a part of the hard mask layer  12  in regions at which the hard mask is to be formed. 
     As is illustrated in  FIG. 10 , the projections  24  subsequently serve as a mask during the patterning of the hard mask layer  12  e.g. by means of an anisotropic etching method. During the patterning of the hard mask layer  12 , the semiconductor substrate  10 , by way of example, serves as an etching stop. Hard mask regions  26  arise below the projections  24 . 
     As is shown in  FIG. 1D , the filling material  20  is subsequently removed, e.g. by means of a dry-chemical etching process or by means of a wet-chemical etching process. As a result, the hard mask regions  26  stand free and can serve for the patterning of the semiconductor substrate  10 . The hard mask regions  26  are situated close to one another and have a minimum dimension A of the fin widths which is sublithographic and, in particular, lies in the range of 5 nanometers to 50 nanometers. 
     A finFET can then be produced according to a customary method. The next step for producing the finFET consists in producing the fin of the finFET. 
     If a planar field effect transistor is intended to be produced with the aid of the hard mask regions  26 , then instead of the semiconductor substrate  10  use is made of a substrate which contains e.g. a polycrystalline silicon layer and a dielectric as gate stack, which is patterned with the aid of the hard mask regions  26  to form gate electrodes. 
     An alternative exemplary embodiment involves carrying out a method having the same method steps as explained above with reference to  FIGS. 1A to 1D . However, semiconductor substrate is present in place of the hard mask layer  12 , see dashed lines  28 . However, the cutout  18  is etched in a time-controlled manner. The etching step using the filling material as a mask is also carried out in a time-controlled manner. Sidewalls of the hard mask regions  26  that have different heights can thereby arise, see e.g.  FIG. 2D . With regard to greater tolerances during the time-controlled etching, however, height differences of several nanometers are acceptable since e.g. the electrical properties of a FET are impaired only insignificantly on account of the asymmetry that arises. 
       FIGS. 2A to 2D  show production stages of an exemplary embodiment for producing a double-fin field effect transistor with and without underoxidation. An exemplary embodiment without underoxidation is explained first. 
     An auxiliary layer  14   c , for example an oxide layer, in particular a silicon oxide layer, or a nitride layer, in particular a silicon nitride layer, is applied to a semiconductor substrate  10   c  with or without interim deposition of a thin intermediate layer or a thin intermediate layer sequence. The auxiliary layer  14   c  is patterned with the aid of a resist layer  16   c  in a photolithographic method, a cutout  18   c  being produced. The cutout  18   c  is extended right into the semiconductor substrate  10   c  using the patterned resist layer  16   c  and optionally using the auxiliary layer  14   c  as a mask. Residues of the resist layer  16   c  that are still present, if appropriate, may subsequently be removed. An etching-back step is effected, during which the auxiliary layer  14   c  becomes a thinned auxiliary layer  14   d , which covers a smaller area than the auxiliary layer  14   c  because areas  20   c  of the semiconductor substrate are uncovered in the upper part of an extended cutout  18   d . With regard to the details, reference is made to the explanations concerning  FIG. 1A . 
     As shown in  FIG. 2B , after the production of the cutout  18   d , first a thin insulating layer  50  that is electrically insulating is produced, e.g. by thermal oxidation or in a deposition method. The insulating layer  50  comprises e.g. silicon dioxide or a material having a relative permittivity of greater than 3.9, the value of the relative permittivity of silicon dioxide. The electrical thickness of the insulating layer  50  is e.g. less than 25 nanometers, e.g. 5 nanometers. 
     Afterward, a thin gate electrode layer  52  is applied to the insulating layer  50 , said gate electrode layer e.g. comprising a metal or containing a metal. As an alternative, the gate electrode layer  52  comprises a highly doped polycrystalline silicon. The thickness of the gate electrode layer  52  is e.g. less than 25 nanometers. 
     After the production of the gate electrode layer  52 , a filling material  22   c , e.g. an electrically conductive material, e.g. doped silicon, or an electrically insulating material, e.g. an oxide, is applied to the gate electrode layer  52 . This is followed by planarization, stopping on the auxiliary layer  14   d . By way of example, planarization is effected by a CMP method. After planarization, the insulating layer  50 , the gate electrode layer  52  and the filling material  22   c  are still present only within the cutout  18   d.    
     After planarization, the auxiliary layer  14   d  is removed selectively with respect to the semiconductor substrate  10   c , with respect to the filling material  22   c , with respect to the gate electrode layer  52  and as far as possible also selectively with respect to the insulating layer  50 . 
     As illustrated in  FIG. 2C , fins  56  are subsequently produced with the aid of projections  54  of the filling material in the cutout  18   d  in an anisotropic etching method. The anisotropic etching gives rise, if appropriate, to a height difference D of, for example, approximately 5 nanometers between the bottom of the cutout  18   d  and substrate areas lying parallel to the bottom of the cutout  18   d  outside the cutout  18   d . The free sides of the fins  56  preferably go deeper into the semiconductor substrate  10   c  than the sides of the fins  56  that delimit the cutout  18   c.    
     As is furthermore illustrated in  FIG. 2C , an insulating layer  60  having the same material composition and the same layer thickness as the insulating layer  50  is produced on the uncovered side areas of the fins  56  and on the uncovered semiconductor substrate  10   c . The two insulating layers  50  and  52  serve as gate dielectric of a double-fin field effect transistor in the region of the fins  56 . 
     A further gate electrode layer  62 , which comprises the same material and has the same thickness as the gate electrode layer  52 , is subsequent applied to the insulating layer  60 . 
     At this point in time, the fins  56  are already surrounded by a gate dielectric  50 ,  60  and by a thin gate electrode  52 ,  62 , which, however, has not yet been patterned. The thin gate electrode  52 ,  62  and the filling material  22   c  serve as a mechanical support for the ultrathin fins  56 . 
     As illustrated in  FIG. 2D , in the exemplary embodiment, gate material  70 , e.g. doped silicon, in particular polycrystalline silicon, is applied prior to the removal of the filling material  22   c  made of oxide. Planarization is then effected, the filling material  22   c  serving as a stop. Only afterward is the filling material  22   c  removed and replaced by gate material  72 , e.g. by polycrystalline silicon. Accordingly, the fins  56  are always adequately mechanically protected. 
     As illustrated in  FIG. 2D , the gate material  70 ,  72  and the gate electrode layers  52 ,  62  are then patterned. By way of example, a photolithographic method and/or a spacer technique is used for this purpose. A hard mask  74  is optionally used. A step of trimming the hard mask  74  is furthermore optionally carried out in order to reduce the gate length. As an alternative, an electron beam lithography may be used. 
     In another exemplary embodiment, the filling material  22   c  is removed prior to the application of the gate material  70   c  using an additional lithographic method only in a central region of the fins  56 . At the ends of the fins  56 , by contrast, the filling material  22  remains as a support. The gate material  70  and  72  is then deposited simultaneously, so that only one planarization step is required for planarizing gate material. 
     The gates are subsequently patterned, e.g. using a hard mask in accordance with the first exemplary embodiment or by means of a photolithographic step for the production of minimum feature sizes. 
     As illustrated by dashed lines in  FIG. 2C , in another exemplary embodiment, a thin oxidation protective layer  80  is deposited after the production of the gate electrode layer  62 . Anisotropic etching is subsequently effected, so that the oxidation protective layer  80  remains only on the sidewalls of the fins  56  which are remote from the cutout  18   d . The anisotropic etching effects etching as far as the semiconductor substrate  10   c . By way of example, RIE is suitable for the anisotropic etching. 
     As is furthermore illustrated by dashed lines in  FIG. 2C , a thermal oxidation of the semiconductor substrate  10   c  is then carried out, oxide regions  82  forming at the uncovered regions of the semiconductor substrate  10   c  and at the base of the fins  56 , said oxide regions electrically insulating the fins  56  from the semiconductor substrate  10   c.    
     The underoxidation of the fins  56  is facilitated if the distance D already amounts to several nanometers prior to the oxidation. In addition or as an alternative, during the anisotropic etching of the oxidation protective layer, too, it is possible to etch further into the semiconductor substrate  10   c , for example by more than 5 nanometers. Furthermore as an alternative or in addition, the underoxidation is promoted if the semiconductor substrate  10   c  is isotropically etched prior to the oxidation, a cutout in each case being produced below the fin  56 , but the fins  56  not being completely separated from the semiconductor substrate  10   c . The mechanical support for the fins  56  is formed by the filling material  22   c  during the underoxidation and the isotropic undercutting of the fins that is possibly carried out, said filling material either not yet having been removed from the cutout  18   d  or having been removed only in a partial region of the cutout  18   d.    
     The further method implementation corresponds to the method implementation explained with reference to  FIG. 2D , that is to say application of the filling material  70  and  72  according to one of the methods explained. 
       FIGS. 3A and 3B  show further production stages for producing a double-fin field effect transistor  100 .  FIG. 3A  shows the transistor  100  after the patterning of the gate stack as described above. The gate electrodes are embodied in the form of a narrow strip  72   a  above the fins  56 . Situated at the ends of the strip are e.g. square contact areas for the connection of the gate. The gate length is given by the width of the strip and is e.g. 20 nanometers given a width of the fins  56  of in each case 6 nanometers. 
     As is illustrated in  FIG. 3B , what is subsequently carried out involves a thermal oxidation or an oxide deposition e.g. by means of a CVD method (chemical vapor deposition) with subsequent anisotropic etching-back, oxide spacers  102  being formed at uncovered areas of the fins  56  and at uncovered sides of the gate material  72   a . The oxide spacers  102  subsequently insulate the gate material from the source connection material and/or from the drain connection material, inter alia. Instead of the oxide spacers, it is also possible to use spacers made from a different material, e.g. nitride spacers, in particular silicon nitride spacers. 
     The next step involves effecting an optional implantation for source extensions and/or drain extensions with a comparatively low dopant concentration. The implantation is carried out e.g. obliquely toward or counter to the direction of the normal to the active surface of the semiconductor substrate  10   c . After a further oxidation and an etching-back step for producing further spacers, the connection regions for the source zone and/or for the drain zone are implanted, a higher dopant concentration than before being implanted. The second implantation is also carried out e.g. obliquely or counter to the direction of the normal. 
     Drain and source contact holes  104  and  106  are subsequently produced. A double-fin field effect transistor  100  arises which has closely adjacent fins and a sublithographic fin width. By way of example, the following steps are carried out during the production of the source connection regions  104  or  106 :
         Self-aligning silicide production in accordance with the salicide technique by application of the metal for the silicide formation, selective siliciding and removal of the non-silicided metal. In this case, a silicide formation is prevented on the planar substrate basic area e.g. by the oxide produced during the underoxidation or by an additionally applied oxide layer which is applied instead of the underoxidation layer.   Planarization e.g. by application of a dielectric, e.g. an oxide, followed by e.g. CMP planarization.   Production of contact holes  104 ,  106  to the connection regions.       

     The method in accordance with  FIGS. 1A to 1D  enables deeper structures or structures with higher sidewalls to be etched more easily in comparison with the method in accordance with  FIGS. 2A to 2D . 
     With the same method steps, it is also possible to produce single-fin field effect transistors if the fins are removed by means of the trimming mask in the relevant regions.