Patent Document

BACKGROUND 
       [0001]    Technical Field 
         [0002]    The present disclosure regards a vertical conduction integrated electronic device that is protected against the so-called “latch-up” phenomenon; further, the present disclosure regards the corresponding manufacturing process. 
         [0003]    Description of the Related Art 
         [0004]    As is known, electronic devices are today available, such as for example MOSFETs or insulated-gate bipolar transistors (IGBTs), which are able to conduct high currents and withstand high voltages. These devices, however, may be subjected to the so-called latch-up phenomenon. 
         [0005]    For instance, as shown in  FIG. 1  with reference to an IGBT  1 , this transistor has a parasitic circuit, which includes a first parasitic transistor  2  and a second parasitic transistor  3 , which are, respectively, of a PNP and an NPN type. In addition, the collector of the first parasitic transistor  2  is connected to the base of the second parasitic transistor  3 , the collector of which is connected to the base of the first parasitic transistor  2 , whereas the emitters of the first and second parasitic transistors  2 ,  3  are connected to the drain terminal and to the source terminal, respectively, of the IGBT  1 . This being said, in latch-up conditions, the first and second parasitic transistors  2 ,  3  form a closed path flowing in which is a current that is self-sustaining, irrespective of the value of the voltage that controls the IGBT  1 . Likewise, in the case of a power MOSFET (not shown), in latch-up conditions, within the corresponding body region, and thus between the source and drain, a current is found to flow also in the case where the gate terminal is set at a zero voltage, which entails, in practice, the impossibility of switching off the MOSFET. 
       BRIEF SUMMARY 
       [0006]    One embodiment of the present disclosure is an integrated electronic device that solves at least in part the drawbacks of the known art. 
         [0007]    According to the present disclosure, a vertical conduction integrated electronic device includes a semiconductor body, a trench that extends through part of the semiconductor body and delimits a portion of the semiconductor body, and a gate region of conductive material which extends within the trench. The portion of the semiconductor body forms a first conduction region having a first type of conductivity and a body region having a second type of conductivity, which is arranged on top of the first conduction region. The device also includes an insulation region of dielectric material, which extends within the trench and is arranged between the gate region and the body region; and a second conduction region arranged on top of the body region. The second conduction region is made of an undoped conductor. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    For a better understanding of the present disclosure preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
           [0009]      FIG. 1  shows a circuit diagram of an IGBT and of corresponding parasitic transistors; 
           [0010]      FIGS. 2, 21, 23, and 24  are schematic cross sections (not in scale) of embodiments of the present electronic device; 
           [0011]      FIGS. 3-18  are schematic cross sections (not in scale) of the embodiment shown in  FIG. 2 , during successive steps of a manufacturing process; 
           [0012]      FIGS. 19-20  are schematic cross sections (not in scale) of the embodiment shown in  FIG. 21 , during successive steps of a manufacturing process; and 
           [0013]      FIG. 22  is a schematic cross section (not in scale) of the embodiment shown in  FIG. 23 , during a step of a manufacturing process. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 2  shows a transistor  10 , which is a trench MOSFET, with vertical current flow. 
         [0015]    In detail, the transistor  10  comprises a semiconductor body  12  made, for example, of silicon and comprises a substrate  14 , of an N++ type and an epitaxial layer  16  of an N type extending over the substrate. Further, the semiconductor body  12  comprises a region  18  of a P type, which will be referred to in what follows as the top semiconductor region  18 . The top semiconductor region  18  extends over the epitaxial layer  16 , with which it is in direct contact. 
         [0016]    Present on the top semiconductor region  18 , and in direct contact with the latter, is a source region  20 , of undoped conductive material, such as for example a metal material. 
         [0017]    As previously mentioned, the transistor  10  comprises a trench  22 , which in top plan view has an annular shape. In particular, the trench  22  extends through a bottom portion of the source region  20 , as well as through the top semiconductor region  18  and a top portion of the epitaxial layer  16 . Consequently, the trench  22  does not extend within the substrate  14 ; further, the trench  22  surrounds an active region  24 . 
         [0018]    Present within the trench  22  is a gate region  30 , which in top plan view thus has an annular shape. The gate region  30  is made of conductive material, such as for example polysilicon. 
         [0019]    Further present within the trench  22  is an insulation region  32 , which is made of dielectric material and surrounds all sides of the gate region  30 . In particular, the insulation region  32  includes a first insulation subregion  36 , which overlies the gate region  30  and is made, for example, of deposited silicon oxide (TEOS), and a second insulation subregion  38 , which surrounds at the lateral sides and underneath the gate region  30  and is made, for example, of silicon oxide. 
         [0020]    In detail, the top semiconductor region  18  forms a body region  40 , which is arranged in the active region  24  (and is thus surrounded by the trench  22 ), and a peripheral semiconductor region  19 , which is arranged on the outside of the trench  22 . The body region  24  and the peripheral semiconductor region  19  are thus separated from one another on account of interposition of the trench  22 . Further, extending underneath the body region  40  is a portion of the epitaxial layer  16 . 
         [0021]    In greater detail, the semiconductor body  12  is delimited at the top and at the bottom, respectively, by a top surface S a  and a bottom surface S b , which are formed, respectively, by the top semiconductor region  18  and by the substrate  14 . 
         [0022]    Yet in greater detail,  FIG. 2  shows a first top portion  39   a  of the second insulation subregion  38 , which is arranged laterally with respect to the gate region  30 , contacts the body region  40  and to a first approximation is oriented perpendicular to the top surface S a . The first top portion  39   a  coats the inner lateral wall of the trench  22  and is delimited laterally by a first lateral surface S c1  and a second lateral surface S c2 , which contact, respectively, i) the body region  40  and the source region  20 , and ii) the gate region  30 . Further,  FIG. 2  also shows a second top portion  39   b  of the second insulation subregion  38 , which surrounds, at a distance, the aforementioned first top portion  39   a  of the second insulation subregion  38  and is delimited laterally by a third lateral surface S 3  and a fourth lateral surface S c4 , which contact, respectively, i) the peripheral semiconductor region  19  and the source region  20 , and ii) the gate region  30 . In practice, the second top portion  39   b  coats the outer lateral wall of the trench  22 . Further, the second and fourth lateral surfaces S c2 , S c4  face the gate region  30 , whereas the first and third lateral surfaces S c1 , S 3  face the body region  40  and the peripheral semiconductor region  19 , respectively. 
         [0023]    This being said, assuming a reference system oriented perpendicular to the aforementioned surfaces S a  and S b  and directed from the bottom surface S b  towards the top surface S a , the top surface S a  extends to a height lower than the height of the portion of gate region  30  arranged in contact with the second lateral surface S c2 . In other words, if we denote by h 30  the maximum height of the portion of the gate region  30  in contact with the second lateral surface S c2 , the body region  40 , and in particular the portion of the body region  40  in contact with the first lateral surface S c1 , extends up to a corresponding maximum height, which is lower than the height h 30 . Equivalently, the portion of source region  20  that contacts the body region  40  and the first lateral surface S c1  extends at the bottom up to a height lower than the height h 30 . In this connection,  FIG. 2  shows, purely by way of example, an embodiment in which the gate region  30  has a non-uniform height. In particular, the height of the gate region  30  decreases starting from the peripheral portions closest to the top semiconductor region  18  towards a central portion of the gate region  30 . In other words, in cross-sectional view the gate region  30  exhibits a cusp-shaped profile, with the cusp facing downwards, this cusp being arranged, in top plan view, approximately at the middle of the gate region  30 . However possible are embodiments in which the gate region  30  has, for example, a maximum height that is substantially uniform in a direction parallel to the top surface S a . 
         [0024]    In practice, a lateral overlap is created between the gate region  30  and the source region  20 . In use, the epitaxial layer  16  forms the drain of the transistor  10 , whereas the first top portion  39   a  of the second insulation subregion  38  functions as gate oxide. Consequently, when the gate region  30  is biased at a voltage higher than the threshold voltage of the transistor  10 , in the portion of the body region  40  arranged in contact with the first lateral surface S c1  the (vertical) conduction channel of the transistor  10  is formed. The lateral overlap between the gate region  30  and the source region  20  guarantees that the source is electrically coupled to the channel. 
         [0025]    For practical purposes, since the source region  20  is made of an undoped conductive material, in the transistor  10  no parasitic transistor of an NPN type is present, and consequently latch-up may not occur. 
         [0026]    The transistor  10  may be obtained with the manufacturing process described in what follows. 
         [0027]    Initially, as shown in  FIG. 3 , the semiconductor body  12  is provided, which comprises the substrate  14 , the epitaxial layer  16 , and a region  18 ′ that is to form the top semiconductor region  18 , which will be referred to in what follows as the preliminary top semiconductor region  18 ′. Formed on the preliminary top semiconductor region  18 ′ is a layer  44  of dielectric material (for example, silicon oxide or TEOS), which will be referred to in what follows as the temporary layer  44 . For instance, the temporary layer  44  is formed by thermal oxidation or by chemical deposition. 
         [0028]    Next, as shown in  FIG. 4 , a photolithographic process and a subsequent anisotropic etch are carried out in order to remove selectively a portion of the temporary layer  44  for forming a window  46  of an annular shape in the temporary layer  44 . 
         [0029]    Next, as shown in  FIG. 5 , the window  46  is used in a subsequent etch, which enables selective removal of a portion of the preliminary top semiconductor region  18 ′ and an underlying portion of the epitaxial layer  16 , to form the trench  22 . This operation entails separation, within the preliminary top semiconductor region  18 ′, of a region  40 ′, which is to form the body region  40 , and a region  19 ′, which is to form the peripheral semiconductor region  19 , which will be referred to in what follows as the preliminary body region  40 ′ and the preliminary peripheral semiconductor region  19 ′, respectively. 
         [0030]    Next, as shown in  FIG. 6 , the remaining portion of the temporary layer  44  is removed. 
         [0031]    Next, as shown in  FIG. 7 , formed in a per se known manner is a layer  50  of dielectric material, which will be referred to in what follows as the thin dielectric layer  50 . For instance, the thin dielectric layer  50  is made of silicon oxide and is obtained by thermal oxidation, or else is made of TEOS oxide, formed by deposition. Further, the thin dielectric layer  50  has a thickness of, for example, to 50 nm. 
         [0032]    In greater detail, the thin dielectric layer  50  extends on the preliminary top semiconductor region  18 ′, as well as within the trench  22 , for coating the bottom and the lateral walls of the latter. 
         [0033]    Next, as shown in  FIG. 8 , formed on the thin dielectric layer  50  is a further dielectric layer  52 , which will be referred to in what follows as the thick dielectric layer  52 . 
         [0034]    The thick dielectric layer  52  is made, for example, of silicon nitride (Si 3 N 4 ) and has a thickness, for example, comprised between 70 nm and 100 nm. The presence of the thin dielectric layer  50  enables reduction of the mechanical stresses induced in the semiconductor body  12  during the subsequent steps of the manufacturing process. 
         [0035]    Next, as shown in  FIG. 9 , selective removal is carried out (for example, by an anisotropic chemical etch) of portions of the thin dielectric layer  50  and of the thick dielectric layer  52  arranged on the preliminary body region  40 ′ and the preliminary peripheral semiconductor region  19 ′, thus outside the trench  22 , as well as portions of the thin dielectric layer  50  and of the thick dielectric layer  52  that coat the bottom of the trench  22 . In this connection, in what follows referred to, respectively, as the first lateral wall P 1  and second lateral wall P 2  of the trench  22  are the inner lateral wall and the outer lateral wall of the trench  22 , as well as the bottom wall P 3  of the trench  22 . Following upon the operations described previously, the bottom wall P 3  of the trench  22  is exposed, whereas the first and second lateral walls P 1 , P 2  of the trench  22  are coated by a first coating layer  56  and a second coating layer  58 , respectively, which are formed by residual portions of the thin dielectric layer  50 ; in turn, the first and second coating layers  56 ,  58  are coated, respectively, by a first spacer  60  and a second spacer  62 , which are formed by residual portions of the thick dielectric layer  52 . 
         [0036]    Next, as shown in  FIG. 10 , a process of thermal oxidation is carried out, which entails oxidation of the exposed portions of semiconductor material, not coated either by the first spacer  60  or by the second spacer  62 . This operation entails formation, on the preliminary body region  40 ′, of a corresponding dielectric region, which will be referred to in what follows as the central dielectric region  66 . Further, this operation entails formation, on the peripheral semiconductor region  19 , of a corresponding dielectric region  68 , which will be referred to in what follows as the peripheral dielectric region  68 ; for example, the central dielectric region  66  and the peripheral dielectric region  68  have a thickness comprised between 0.2 μm and 0.3 μm. In addition, this operation of oxidation entails formation, by the central dielectric region  66  and the peripheral dielectric region  68 , of corresponding projections that extend towards the trench  22 , as well as entailing curving in the direction of the trench  22  of the top portions and bottom portions of the first and second spacers  60 ,  62 . In particular, the projections of the central dielectric region  66  and of the peripheral dielectric region  68  project towards the inside of the trench  22  with respect to the preliminary body region  40 ′. Further, said operation of oxidation entails formation, on the bottom of the trench  22 , of a further dielectric region  70 , which will be referred to in what follows as the bottom dielectric region  70 . 
         [0037]    Once again with reference to  FIG. 10 , here the central dielectric region  66 , the peripheral dielectric region  68 , the bottom dielectric region  70 , and the first and second coating layers  56 ,  58  are shown in a distinct way, for reasons of clarity, even though they may be made of a same material and may thus form a single dielectric region, made, for example, of oxide. 
         [0038]    Next, as shown in  FIG. 11 , an isotropic etch is made to remove the first and second spacers  60 ,  62 . 
         [0039]    Then, as shown in  FIG. 12 , a further etch is made (for example, an isotropic chemical etch in a liquid or nebulized environment) to remove the first and second coating layers  56 ,  58 , which may have previously undergone contamination. Albeit not shown, this operation entails a slight reduction of the thickness of the central dielectric region  66 , of the peripheral dielectric region  68 , and of the bottom dielectric region  70 . 
         [0040]    Next, as shown in  FIG. 13 , a new oxidation process is carried out. In this way, on the first and second lateral walls P 1 , P 2  of the trench  22  a first oxide layer  72  and a second oxide layer  74  are formed, respectively, which will be referred to in what follows as the first and second oxide layers  72 ,  74 . The first and second oxide layers  72 ,  74  contact the bottom dielectric region  70  for forming the second insulation subregion  38 . 
         [0041]    In greater detail, albeit not shown, the oxidation process described with reference to  FIG. 13  entails also a slight increase in the thickness of the central dielectric region  66 , of the peripheral dielectric region  68 , and of the bottom dielectric region  70 . Further, even though in  FIG. 13  the first and second oxide layers  72 ,  74  are shown as distinct with respect to the central dielectric region  66  and the peripheral dielectric region  68 , they may be made of the same material of which the latter are made. 
         [0042]    Once again with reference to  FIG. 13 , this shows how, thanks to the prior use of the first and second spacers  60 ,  62 , it is possible to coat the bottom wall P 3  of the trench  22  with an insulating region (in the case in point, the bottom dielectric region  70 ) having a thickness greater than the thickness of the first and second oxide layers  72 ,  74 . In this way, insulation of the gate region  30  towards the drain region is improved, without this entailing an increase of the threshold voltage of the transistor  10 . 
         [0043]    Next, as shown in  FIG. 14 , a conductive region  78 , made, for example, of polysilicon is formed. For instance, the conductive region  78  may be formed by successive deposition of layers. 
         [0044]    In detail, the conductive region  78  overlies the central dielectric region  66  and the peripheral dielectric region  68 . In addition, the conductive region  78  fills the trench  22  completely. In this connection, without this implying any loss of generality, the trench  22  has a depth that is, for example, twice the respective width. 
         [0045]    Next, as shown in  FIG. 15 , an anisotropic etch is made, in order to reduce the thickness of the conductive region  78  so that the residual portion of conductive region  78  forms the gate region  30 . In other words, following upon this etch, just a portion of conductive region  78  remains, which occupies the trench  22  starting from the bottom up to a height lower than the maximum height of the semiconductor body  12 . For instance, the residual portion of the conductive region  78  has a maximum height 0.4 μm lower than the maximum height of the semiconductor body  12 . 
         [0046]    In greater detail, and without any loss of generality, etching of the conductive region  78  may be carried out by a homogeneous “etch back”, in which case the gate region  30  assumes the aforementioned cusp shape. 
         [0047]    Next, as shown in  FIG. 16 , dielectric material (for example, silicon oxide) is deposited for forming a top dielectric region  80 , which is arranged on top of the central dielectric region  66  and of the peripheral dielectric region  68 . Further, the top dielectric region  80  extends within a top portion of the trench  22  until it contacts the gate region  30 . 
         [0048]    Next, as shown in  FIG. 17 , a new anisotropic etch is made in order to remove a top portion of the top dielectric region  80 , the central dielectric region  66 , and the peripheral dielectric region  68  for exposing the preliminary body region  40 ′ and the preliminary peripheral semiconductor region  19 ′. In addition, this etch entails removal of a portion of the top dielectric region  80  arranged inside the trench  22 . In this way, the residual portion of top dielectric region  80  forms the first insulation subregion  36 , the maximum height of which is, for example, 0.2 μm lower than the maximum height of the preliminary top semiconductor region  18 ′. 
         [0049]    Next, as shown in  FIG. 18 , a new etch (for example, a chemical etch of silicon in moist, liquid, or nebulized environment) is made in order to reduce the thickness of the preliminary body region  40 ′ and of the preliminary peripheral semiconductor region  19 ′. The residual portions of the preliminary body region  40 ′ and of the preliminary peripheral semiconductor region  19 ′ form, respectively, the body region  40  and the peripheral semiconductor region  19 . 
         [0050]    Next, in a way not shown, the source region  20  is formed for example by deposition of metal material. 
         [0051]    According to a variant of the manufacturing process previously described, following upon execution of the operations described with reference to  FIG. 18 , it is possible to carry out the operations shown in  FIG. 19 . 
         [0052]    In detail, formed, for example by deposition, on the body region  4 , the peripheral semiconductor region  19 , and the first insulation subregion  36  is a further layer  84  of silicon nitride, which will be referred to in what follows as the additional layer  84 . 
         [0053]    Next, as shown in  FIG. 20 , portions of the additional layer  84  that extend over the first insulation subregion  36 , as well as over a central portion of the body region  40 , are selectively removed, for example with an anisotropic etch. The residual portions of the additional layer  84  form a third spacer  86  and a fourth spacer  86 ,  88 . The third spacer  86  coats a top portion of the first lateral surface S c1  of the first top portion  39   a  of the second insulation subregion  38 , until it contacts a peripheral portion of the body region  40 . A central portion of the body region  40  is, instead, in contact with a portion of the source region  20 , which is surrounded by the third spacer  86 . The fourth spacer  88  coats, instead, a top portion of the third lateral surface S c3  of the second top portion  39   b  of the second insulation subregion  38 , until it contacts the peripheral semiconductor region  19 . 
         [0054]    In the case where the operations represented in  FIG. 20  are carried out, the transistor  10  assumes the shape shown in  FIG. 21 . Further, the presence of the third and fourth spacers  86 ,  88  enables improvement of the electrical insulation between the source region  20  and the gate region  30 . 
         [0055]    Optionally, following upon the operations represented in  FIG. 20 , and prior to formation of the source region  20 , it is possible to carry out an ion implantation within the exposed portion of body region  40 , i.e., within the portion of body region  40  not covered by the third spacer  86 . In this way, as shown in  FIG. 22 , an enriched region  90  of a P+ type is formed, which extends within the body region  40 , starting from the top surface S a , without contacting the underlying epitaxial layer  16 . The enriched region  90  is laterally staggered with respect to the third spacer  86 . 
         [0056]    In the case where the operations represented in  FIG. 22  are carried out, the transistor  10  assumes the shape shown in  FIG. 23 . The presence of the enriched region  90  enables improvement of the electrical behavior of the diode formed by the body region  40  and by the underlying portion of epitaxial layer  16 , without affecting the channel of the transistor  10 . 
         [0057]    Further possible are embodiments that are the same as corresponding embodiments described previously, but in which the substrate is of a P+ type, instead of an N+ type. In this case, the transistor, designated by  100 , is of an IGBT type and the epitaxial layer  16  functions as so-called “drift layer”. An example of such embodiments is shown in  FIG. 24 , where the substrate is designated by  99 . In this connection, the substrate  99  functions as collector of the transistor  100 . Albeit not shown, further possible are embodiments in which the substrate is of a P+ type and which include the third and fourth spacers  86 ,  88  as well as possibly the enriched region  90 . 
         [0058]    The electronic device described presents numerous advantages. In particular, thanks to the fact that the source region  20  is made of undoped conductive material, formation of the parasitic transistor of an NPN type is prevented, and thus latch-up thereof is likewise prevented. In particular, in the case of a MOSFET, there is just one diode, formed by the body region  40  and by the underlying portion of the epitaxial layer  16 . Instead, in the case of an IGBT, just the parasitic PNP transistor is present, which in any case does not give rise to latch-up, since it has an h fe  parameter lower than one. 
         [0059]    Finally, it is clear that modifications and variations may be made to the electronic device and to the manufacturing process described and illustrated herein, without thereby departing from the scope of the present disclosure. 
         [0060]    For instance, the types of doping may be reversed with respect to what has been described. 
         [0061]    As regards the trench  22 , it may have, in top plan view, an arbitrary shape, such as for example a circular or elliptical shape. 
         [0062]    Some steps of the manufacturing process may be carried out in a different order with respect to what has been described. In addition, one or more regions of the transistor may be formed in a way different from what has been described. 
         [0063]    The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.

Technology Category: h