Abstract:
An integrated electronic device includes a semiconductive film above a buried insulating layer that is situated above a supporting substrate. An active zone is delimited within the semiconductive film. A MOS transistor supported within the active zone includes a gate region situated above the active zone. The gate region includes a rectilinear part situated between source and drain regions. The gate region further includes a forked part extending from the rectilinear part. A raised semiconductive region situated above the active zone is positioned at least partly between portions of the forked part. A substrate contact for the transistor is electrically coupled to the raised semiconductive region.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority from French Application for Patent No. 1652717 filed Mar. 30, 2016, the disclosure of which is incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    Embodiments relate to integrated circuits and more particularly MOS transistors with hybrid operation produced on substrates of silicon on insulator type, commonly referred to by those skilled in the art by the acronym “SOI”, in particular a substrate of the fully depleted silicon on insulator type, known to those skilled in the art by the acronym “FDSOI”. 
       BACKGROUND 
       [0003]    MOS transistors with hybrid operation are known, which are of interest notably for electrostatic discharge (ESD) protection applications. A person skilled in the art will for example be able to refer to U.S. Pat. No. 9,019,666 (incorporated by reference) which describes this type of transistor. 
         [0004]    These transistors are produced on bulk substrates. Now, electrical simulations have shown (see, for example, Galy, et al., “BIMOS transistor in thin silicon film and new solutions for ESD protection in FDSOI UTBB CMOS technology”, EUROSOI-ULIS 2015, 26-28 Jan. 2015, Bologna, Italy (incorporated by reference)), that there would be advantages from an electrical point of view in producing these transistors with hybrid operation on a substrate of FDSOI type for an ESD protection application. 
         [0005]    However, the very small thickness of the semiconductive film (typically of the order of 7 nm) does not make it possible to directly produce a contact on an FDSOI substrate for this type of transistor. 
         [0006]    U.S. application patent Ser. No. 15/041,593 filed Feb. 11, 2016 (corresponding to French Application for Patent No. 1556515), incorporated by reference, describes means that make it possible to produce a substrate contact by the use of additional junction-free transistor(s) as connection element(s). Although satisfactory, this solution can however, in some cases, generate spurious effects and offers an integration density which can prove limited in certain applications. 
       SUMMARY 
       [0007]    Thus, according to one implementation and embodiment, it is proposed to provide a substrate contact for a transistor produced in a substrate of SOI type, in particular of FDSOI type, resulting in reduced spurious effects, notably because of a more compact geometry. 
         [0008]    According to one aspect, there is proposed a method for producing at least one substrate contact for an MOS transistor produced in and on an active zone of a substrate of silicon on insulator type, comprising:
       formation on top of the active zone of a gate region of the transistor having a rectilinear part situated between the source and drain regions of the transistor and extended by at least one first forked part,   formation of at least one first raised semiconductive region above the active zone and at least partly within said first forked part, and formation of said at least one substrate contact electrically coupled to, for example on, said at least one first raised semiconductive region.       
 
         [0011]    The substrate contact is electrically coupled to the first raised semiconductive region in as much as it can for example be directly formed on the raised semiconductive region, or possibly on the gate region if the forked part of the gate region is in electrical contact with the raised semiconductive region. 
         [0012]    In other words, the production of a junction-free transistor is dispensed with by producing a contact electrically coupled to the raised silicon region. The forked part of the gate notably serves as mask which makes it possible to simplify the delimiting and the production of the raised silicon region. 
         [0013]    The distance between the contact and the substrate is reduced, which makes it possible on the one hand to reduce the spurious capacitive effects between these elements and, on the other hand, to reduce the substrate access resistance. 
         [0014]    According to one implementation, the formation of said gate region further comprises a formation of a second forked part extending said rectilinear part opposite the first forked part, the method further comprising: formation of a second raised semiconductive region above the active zone and at least partly within said second forked part, and formation of a second substrate contact for the first transistor electrically coupled to, for example on, said second raised semiconductive region. 
         [0015]    The formation of each raised semiconductive region can comprise an epitaxy of a semiconductive material. 
         [0016]    According to one implementation, the formation of at least one substrate contact is performed on the corresponding raised semiconductive region. 
         [0017]    As a variant, at least one raised semiconductive region is in contact with at least one forked part of the gate region and the formation of at least one substrate contact is performed on said gate region. 
         [0018]    According to another aspect, an integrated electronic device is proposed that comprises a semiconductive film, for example fully depleted, above a buried insulating layer, which is itself situated above a supporting substrate, an active zone produced within the semiconductive film, at least one first MOS transistor produced in and on the active zone and comprising a gate region produced above the active zone and having a rectilinear part situated between the source and drain regions and extended by at least one first forked part, at least one first raised semiconductive region situated above the active zone and at least partly within said first forked part, and at least one first substrate contact for the first transistor electrically coupled to, for example on, said first raised semiconductive region. 
         [0019]    According to one embodiment of this aspect, the gate region comprises a second forked part extending said rectilinear part opposite the first forked part, the device further comprising a second raised semiconductive region situated above the active zone and at least partly within said second forked part, and a second substrate contact for the first transistor electrically coupled to, for example on, said second raised semiconductive region. 
         [0020]    Each forked part can comprise an extension extending at right angles on either side of the rectilinear part out of the source and drain regions, a first branch connected to said extension and extending in the extension of the source region and a second branch connected to said extension and extending in the extension of the drain region, and each raised semiconductive region can extend at least partly between the corresponding first branch and second branch. 
         [0021]    According to one embodiment, the device can comprise, within the supporting substrate, a semiconductive well situated under said active zone, and a well contact intended to bias said well. 
         [0022]    According to one embodiment, the device can comprise a number of MOS transistors of which the rectilinear gate parts are parallel and mutually electrically connected via their forked part, so that all of the extensions of the transistors form a single line of gate material at right angles to each rectilinear gate part and from which extend said corresponding branches. 
         [0023]    Two neighboring transistors can have their source regions or their drain regions in common. 
         [0024]    According to one embodiment, at least one substrate contact is situated on the corresponding raised semiconductive region. 
         [0025]    As a variant, at least one raised semiconductive region is in contact with at least one forked part of the gate region and at least one substrate contact is situated on said gate region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Other advantages and features of the invention will become apparent on studying the detailed description of non-limiting embodiments, and the attached drawings in which: 
           [0027]      FIGS. 1 to 7  illustrate embodiments of a substrate contact for a transistor. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 1  illustrates a plan view of an integrated device DIS according to one embodiment, for which  FIGS. 2 and 3  are cross-sectional views along the lines II-II and of  FIG. 1 . 
         [0029]    The device DIS comprises a substrate of fully depleted silicon on insulator (FDSOI) type (FDSOI being an acronym well known to those skilled in the art), which comprises a semiconductive film  1  situated above a buried insulating layer  2  (“BOX”, Buried Oxide), which is itself situated above a supporting substrate comprising a semiconductive well  3 . 
         [0030]    The well is here of P type and comprises an upper zone  30  (in contact with the BOX) of P+ type which forms a rear buried gate making it possible to bias the channel of a transistor TR via the rear face. In this respect, the device DIS further comprises a back gate contact BG making it possible to bias the well  3 . 
         [0031]    An insulating region  4 , of shallow trench insulation (STI) type delimits an active zone ZA in the semiconductive film  1 . 
         [0032]    The semiconductive film  1  comprises a fully depleted semiconductive material which in practice is an intrinsic material, for example intrinsic silicon of P type, that is to say very weakly doped (10 15  atoms·cm −3 ). 
         [0033]    An MOS transistor TR, for example an NMOS transistor, is produced in and on the active zone ZA. 
         [0034]    This transistor TR comprises source S and drain D semiconductive regions, doped of N+ type, an insulated gate region G and a channel region  8  adapted to be formed under the gate. 
         [0035]    The insulated gate region G comprises a rectilinear part G 1  produced above the channel region  8 , and a forked part G 2  having an extension G 20  extending at right angles on either side of the gate line G 1 . 
         [0036]    The forked part G 2  further comprises a first branch G 21  and a second branch G 22  extending from the extension G 20  in the extension of the source S and drain D regions. 
         [0037]    The reference B denotes the substrate of the transistor TR. 
         [0038]    According to a conventional embodiment in substrates of FDSOI type, the drain D and source S regions are produced in a raised fashion by epitaxial rework (i.e., growth), in order to allow for the contacts. 
         [0039]    The semiconductive film further comprises a doped region of P+ type on which has been produced a raised silicon region  5  by epitaxial rework. The biasing of this region makes it possible to bias the substrate  1  of the transistor TR. 
         [0040]    This raised region  5 , here of P type, is produced between the first branch G 21  and the second branch G 22 . 
         [0041]    Here, the first branch G 21  and the second branch G 22  allow for a greater accuracy in the production of the region  5  because they delimit the epitaxied region. 
         [0042]    Zones of metal silicide are, in this example, produced respectively on the gate G, drain D and source S regions, and on the region  5 , and respectively allow for the gate PCG, drain PCD, source PCS and substrate PCB contacts. 
         [0043]    The gate contacts are here produced on the branches G 21  and G 22  of the forked part G 2  of the gate region G, via the metal silicide zone. 
         [0044]    The device DIS therefore comprises the transistor TR situated in and on a substrate of FDSOI type comprising a substrate contact produced simply and accurately by epitaxial rework and siliciding. Thus, the contact is close to the substrate of the transistor which makes it possible to reduce the spurious capacitive effects and the substrate access resistance. 
         [0045]      FIG. 4  illustrates a schematic representation of the device from an electrical point of view. 
         [0046]    Represented therein are the transistor TR, comprising its drain D, source S and gate G regions, the contacts PCS, PCD, PCG and PCB, and the contact of the well BG. 
         [0047]    A capacitor C schematically represents the capacitor formed under the transistor TR by the semiconductive film  1 , the insulating layer  2  and the well  3 . 
         [0048]    The gate contacts situated on each of the branches G 21  and G 22  are represented by one and the same contact PCG. 
         [0049]    Such a device notably makes it possible to obtain a very significant current gain, of the order of 10 5 . 
         [0050]      FIG. 5  illustrates an embodiment in which the device DIS comprises a number of analog MOS transistors TR 1 , TR 2  and TR 3  similar to that described previously and illustrated in  FIGS. 1 to 4 . 
         [0051]    The three transistors (the number three not being limiting) are produced in and on the same silicon film  1 , and their gates are mutually electrically connected via the extension of their respective forked part. 
         [0052]    The device can therefore be considered as having a common gate G, comprising 3 rectilinear gate parts G 10 , G 11  and G 12 , and an extension G 3  at right angles to the three gate lines from which extend a number of branches G 30 , G 31 , G 32  and G 33  in the extension of the source and drain regions of each transistor. 
         [0053]    Here, each transistor is produced in such a way as to have its source region and/or its drain region in common with the neighboring transistor. 
         [0054]    Thus, the transistor TR 1  comprises the first rectilinear part G 10  and the source and drain regions D 1  and S 1 , the transistor TR 2  comprises the second rectilinear part G 11  and the source and drain regions S 1  and D 2 , and the third transistor TR 3  comprises the third rectilinear part G 12  and the source and drain regions D 2  and S 2 . 
         [0055]    Each transistor TR 1 , TR 2  or TR 3  also comprises a raised region  51 ,  52  or  53  in the extension of its gate, between the branches of its forked part, and on which is produced a zone of metal silicide (not represented) allowing for a substrate contact. 
         [0056]    The device also comprises a well contact BG making it possible to bias the well common to each transistor via the rear face. Given that the wells of the transistors TR 1 , TR 2  and TR 3  are common, the contact BG makes it possible to bias each of the transistors via its rear face. 
         [0057]    The production of the substrate contacts on the raised silicon regions makes it possible to obtain a more compact structure. 
         [0058]    The inventors have notably observed that, by comparison to a structure that is functionally equivalent but whose substrate contacts are produced by additional transistors such as those described in the U.S. application Ser. No. 15/041,593 (French Appl. No. 1556515), a reduction of the surface area of the imprint of the circuit is obtained that is of the order of 30%. 
         [0059]    That is notably due, in the case where the contacts are made by additional transistors to the need to produce isolation trenches between each transistor used to take the substrate contact in order to reduce the spurious effects. 
         [0060]      FIG. 6  illustrates an embodiment in which the MOS transistor TR comprises a second forked part G 4 , opposite the first forked part G 2 , comprising a second extension G 40  which extends at right angles on either side of the rectilinear gate part G 1 . 
         [0061]    The second forked part G 4  further comprises a third branch G 41  and a fourth branch G 42  extending from the second extension G 40  in the extension of the source and drain regions S and D. 
         [0062]    A second additional raised silicon region  6  has also been produced above the silicon film  1 , between the third branch G 41  and the fourth branch G 42 , by epitaxial rework. The biasing of this region allows for a second substrate contact PCB 2  and therefore makes it possible to bias the substrate B of the transistor TR. 
         [0063]    Thus, the device comprises two forked parts G 2  and G 4  and two substrate contacts PCB and PCB 2  produced symmetrically on either side of the transistor TR. 
         [0064]    The addition of this second substrate contact PCB 2  makes it possible to more effectively bias the substrate B of the transistor TR. 
         [0065]    Furthermore, as illustrated in  FIG. 7 , the device can be considered functionally as an MOS transistor T with four gates, also known to those skilled in the art by the designation “G 4 -FET”, and comprising 6 contacts. 
         [0066]    In this mode of operation, the two contacts PCB and PCB 2  are used as the electrodes of the transistor T. For example, the first contact PCB corresponds to the source and the second contact PCB 2  corresponds to the drain. 
         [0067]    The source S and the drain D of the transistor TR are used as two gates of a P-channel JFET transistor. They can therefore here be biased in order to modulate the current flowing between the source PCB and the drain PCB 2  of the transistor T. 
         [0068]    The gate G and the rear gate of the transistor TR, linked respectively to the contacts PCG and BG, can also be biased in order to modulate the current, and also the resistance value R of the substrate B. These two gates form the other two gates of the four-gate transistor T. 
         [0069]    It should be noted that the embodiments presented here are in no way limiting. 
         [0070]    More specifically in the devices described previously, the forked parts and the raised regions  5 ,  51 ,  52 ,  53 ,  6  are separated by a thin space and mutually electrically insulated for example by insulating spacers (not represented in the interests of clarity of the figures) situated on the flanks of the forked parts. 
         [0071]    This makes it possible to have different contacts for the gate and the substrate. It then becomes possible to have particular embodiments as described in U.S. Pat. No. 9,019,666. 
         [0072]    More specifically, it is possible to connect a first resistive element between the source and the substrate of the MOS transistor and a second resistive element between the gate and the source of the MOS transistor, the gate and the substrate of the transistor not being connected together. 
         [0073]    A combined bipolar and MOS effect is then obtained through the drain-substrate capacitances and through the drain-gate capacitances. That said, this combined effect is not amplified because of the absence of connection between the substrate and the gate of the transistor. 
         [0074]    It would also be possible, in the context of a reversible operation, to leave the substrate and the gate of the MOS transistor floating. The bipolar and MOS effect is then obtained by the capacitive gate-substrate coupling. 
         [0075]    So as to have an amplified effect, it is possible to electrically link the gate and the substrate of the transistor, and also advantageously provide for a resistor to be connected between the gate and the ground, the value of which can be adjusted to raise the value of the trigger threshold of the device, as explained in the abovementioned international patent application. 
         [0076]    In this respect, it would be perfectly possible to envisage having the forked parts and said raised regions in contact, which amounts to electrically connecting the substrate B and the gate G of the transistor TR by having only a single contact situated for example on the gate region. 
         [0077]    Furthermore, in the embodiment illustrated in  FIG. 5 , it would be possible to equip each transistor with a second forked part and a second substrate contact, as illustrated in  FIG. 6 .