Abstract:
A trench DMOS transistor cell is provided, which is formed on a substrate of a first conductivity type. A body region, which has a second conductivity type, is located on the substrate. At least one trench extends through the body region and the substrate. An insulating layer lines the trench. The insulating layer includes first and second portions that contact one another at an interface. The first portion of the insulating layer has a layer thickness greater than the second portion. The interface is located at a depth above a lower boundary of the body region. A conductive electrode is formed in the trench so that it overlies the insulating layer. A source region of the first conductivity type is formed in the body region adjacent to the trench.

Description:
This application is a divisional of co-pending U.S. patent application Ser No. 09/531,138, filed Mar. 17, 2000, entitled “Trench DMOS Transistor Having A Double Gate Structure ”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to MOSFET transistors and more generally to DMOS transistors having a trench structure. 
     BACKGROUND OF THE INVENTION 
     DMOS (Double diffused MOS) transistors are a type of MOSFET (Metal On Semiconductor Field Effect Transistor) that use two sequential diffusion steps aligned to the same edge to form the transistor regions. DMOS transistors are typically employed as power transistors to provide high voltage, high current devices for power integrated circuit applications. DMOS transistors provide higher current per unit area when low forward voltage drops are required. 
     A typical discrete DMOS circuit includes two or more individual DMOS transistor cells which are fabricated in parallel. The individual DMOS transistor cells share a common drain contact (the substrate), while their sources are all shorted together with metal and their gates are shorted together by polysilicon. Thus, even though the discrete DMOS circuit is constructed from a matrix of smaller transistors, it behaves as if it were a single large transistor. For a discrete DMOS circuit it is desirable to maximize the conductivity per unit area when the transistor matrix is turned on by the gate. While the individual DMOS transistor cells are typically rectangular in shape, they can in general have an open or closed cell geometry. 
     One particular type of DMOS transistor is a so-called trench DMOS transistor in which the channel is formed vertically and the gate is formed in a trench extending between the source and drain. The trench, which is lined with a thin oxide layer and filled with polysilicon, allows less constricted current flow and thereby provides lower values of specific on-resistance. Examples of trench DMOS transistors are disclosed in U.S. Pat. Nos. 5,072,266, 5,541,425, and 5,866,931. 
     One example is the low voltage prior art trenched DMOS transistor shown in the cross-sectional view of FIG.  1 . As shown in FIG. 1, trenched DMOS transistor  10  includes heavily doped substrate  11 , upon which is formed an epitaxial layer  12 , which is more lightly doped than substrate  11 . Metallic layer  13  is formed on the bottom of substrate  11 , allowing an electrical contact  14  to be made to substrate  11 . As is known to those of ordinary skill in the art, DMOS transistors also include source regions  16   a ,  16   b ,  16   c , and  16   d , and body regions  15   a  and  15   b . Epitaxial region  12  serves as the drain. Substrate  11  is relatively highly doped with N-type dopants, epitaxial layer  12  is relatively lightly doped with N type dopants, source regions  16   a ,  16   b ,  16   c , and  16   d  are relatively highly doped with N type dopants, and body regions  15   a  and  15   b  are relatively highly doped with P type dopants. A doped polycrystalline silicon gate electrode  18  is formed within a trench, and is electrically insulated from other regions by gate dielectric layer  17  formed on the bottom and sides of the trench containing gate electrode  18 . The trench extends into the heavily doped substrate  11  to reduce any resistance caused by the flow of carriers through the lightly doped epitaxial layer  12 , but this structure also limits the drain-to-source breakdown voltage of the transistor. A drain electrode  14  is connected to the back surface of the substrate  11 , a source electrode  22  is connected to the source regions  16  and the body regions  15 , and a gate electrode  19  is connected to the polysilicon  18  that fills the trench. 
     In the DMOS transistor shown in FIG. 1 there is a trade-off between the device&#39;s on-resistance and its drain-to-source breakdown voltage. As the depth of the trench increases, the on-resistance decreases because an accumulation layer forms along the side-wall of the trench. However, the drain-to-source breakdown voltage decreases with increasing trench depth. This latter trend occurs because the depletion layer extending along the trench upon application of a reverse bias voltage cannot spread as the distance between the substrate and the bottom of the trench decreases. As a result, the electric field is concentrated at the bottom corner of the trench and thus breakdown occurs at this point. While the electric field can be reduced by increasing the thickness of the gate oxide layer lining the trench, this adversely effects the on-resistance of the device. 
     Y. Baba et al., in Proc. of ISPSD &amp; IC, p300, 1992, discloses a trench DMOS transistor having a relatively low on-resistance and a high drain-to-source breakdown voltage. A transistor with such characteristics is accomplished by providing a double gate oxide structure that has a thicker gate oxide layer at the bottom of the trench and a thinner gate oxide layer along the side-walls of the upper portion of the trench. This arrangement provides a more optimal trade-off between the device&#39;s on-resistance and its drain-to-source breakdown voltage. Specifically, while the trench is sufficiently deep so that the on-resistance of the device is adequately low, the thickness of the gate oxide region is increased where it can most effectively reduce the electric field at the bottom of the trench; however, the remainder of the gate oxide layer has a reduced thickness so that the on-resistance is minimally impacted. 
     One limitation of the trench DMOS transistor shown in the previously mentioned reference is that it can be difficult to produce the double gate oxide structure, particularly at high transistor cell densities when the width of the trench becomes narrow. Another limitation of the device shown in FIG. 1 is that at high switching speeds its switching losses are relatively large because of its gate charge, which leads to increased capacitance. 
     Accordingly, it would be desirable to provide a trench DMOS transistor having a double gate oxide structure that is relatively simple to manufacture, particularly at high trench cell densities when the trench is narrow and which has a reduced gate charge to reduce switching losses. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a trench DMOS transistor cell is formed on a substrate of a first conductivity type. A body region, which has a second conductivity type, is located on the substrate. At least one trench extends through the body region and the substrate. An insulating layer lines the trench. The insulating layer includes first and second portions that contact one another at an interface. The first portion of the insulating layer has a layer thickness greater than the second portion. The interface is located at a depth above a lower boundary of the body region. A conductive electrode is formed in the trench so that it overlies the insulating layer. A source region of the first conductivity type is formed in the body region adjacent to the trench. 
     In accordance with one aspect of the invention, the interface is located at a depth between an upper and lower boundary of the body region. 
     In accordance with another aspect of the invention, the conductive electrode is formed from polysilicon. Alternatively, the conductive electrode may be formed in whole or in part from silicide. 
     In accordance with yet another aspect of the invention, the insulating layer is an oxide layer. 
     In accordance with another aspect of the invention, a trench DMOS transistor structure is provided, which includes a plurality of individual trench DMOS transistor cells formed on a substrate of a first conductivity type. Each of the individual trench DMOS transistor cells include a body region, which has a second conductivity type, located on the substrate. At least one trench extends through the body region and the substrate. An insulating layer lines the trench. The insulating layer includes first and second portions that contact one another at an interface. The first portion of the insulating layer has a layer thickness greater than the second portion. The interface is located at a depth above a lower boundary of the body region. A conductive electrode is formed in the trench so that it overlies the insulating layer. A source region of the first conductivity type is formed in the body region adjacent to the trench. 
     In accordance with another aspect of the invention, at least one of the individual trench DMOS transistor cells has a closed cell geometry. Alternatively, at least one of the individual trench DMOS transistor cells has an open cell geometry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional view of a conventional DMOS transistor. 
     FIG. 2 shows a cross-sectional view of another conventional DMOS transistor that employs a double gate structure. 
     FIG. 3 shows a cross-sectional view of one embodiment of the DMOS transistor constructed in accordance with the present invention. 
     FIG. 4 is a simulation showing the on-resistance for the DMOS transistor shown in FIG. 3 when the reverse bias applied between the gate and source is 10 V and 4.5 V. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 shows a conventional DMOS transistor having a double oxide gate structure such as disclosed in the previously cited reference to Y. Baba et al. The trench DMOS transistor  110  includes heavily doped substrate  111  upon which is formed an epitaxial layer  112 , which is more lightly doped than substrate  111 . Metallic layer  113  is formed on the bottom of substrate  111 , allowing an electrical contact  114  to be made to substrate  111 . Metallic layer  21  is similarly formed allowing an electrical contact  22  to be made to body regions  15  and source regions  16 . The DMOS transistor also includes source regions  116   a ,  116   b ,  116   c , and  116   d , and body regions  115   a  and  115   b . Epitaxial region  112  serves as the drain. In the example shown in FIG. 2, substrate  111  is relatively highly doped with N-type dopants, epitaxial layer  112  is relatively lightly doped with N type dopants, source regions  116   a ,  116   b ,  116   c , and  116   d  are relatively highly doped with N type dopants, and body regions  115   a  and  115   b  are relatively highly doped with P type dopants. A doped polycrystalline silicon gate electrode  118  is formed within a trench, and is electrically insulated from other regions by gate dielectric layer  117  formed on the bottom and sides of the trench containing gate electrode  118 . The trench extends into the heavily doped substrate  111  to reduce any resistance caused by the flow of carriers through the lightly doped epitaxial layer  112 . However, as previously mentioned, this structure also limits the drain-to-source breakdown voltage of the transistor. This problem is alleviated in FIG. 2 by increasing the thickness of the gate oxide layer in the bottom portion of the trench to define thick oxide layer  125  and decreasing its thickness in the upper portion of the trench to define thin oxide layer  127 . As shown, the interface  129  between the thick gate oxide layer  125  and the thin gate oxide layer  127  is located in epitaxial region  112 . As a result of this structure, the electric field at the bottom of the trench is reduced, thus increasing the drain-to-source breakdown voltage, while the on-resistance of the device remains low because the thick gate oxide layer  125  does not extend throughout the entire trench. Finally, the device is completed in a conventional manner by connecting a drain electrode  114  to the back surface of the substrate  111 , connecting a source electrode  122  to the source regions  116  and the body regions  115 , through a conventional metallization layer  121 , and connecting a gate electrode  119  to the polysilicon  118  that fills the trench. 
     The double gate structure shown in FIG. 2 is fabricated by the following process steps. First, the trench is etched after the source regions  116  and body regions  115  have been formed in epitaxial region  112  by diffusion. Next, the thick gate oxide layer  125  is deposited by chemical vapor deposition (CVD) followed by deposition of a first polysilicon layer  130  adjacent to the trench. The thick oxide layer  125  is then etched back to a depth below the body regions to define interface  129 . Finally, the thin oxide layer  127  is deposited followed by deposition of a second polysilicon layer  131 . First and second polysilicon layers  130  and  131  constitute gate electrode  118 . 
     The step of etching back the thick gate oxide layer  125  becomes problematic for narrow and deep trenches. That is, etching is difficult when the trench has a high aspect ratio. This problem arises because a wet etch is employed and it becomes difficult to continuously refresh the etchant in a deep trench. For instance, it is not feasible to form the gate structure shown in FIG. 2 for trenches having a width less than about 0.5 microns. 
     The present inventors have discovered that this fabrication problem can be alleviated by modifying the double gate structure shown in FIG. 2 so that the interface  129  between the thick and thin gate oxide layers is located at a depth above the bottom of the body regions  115   a  and  115   b . FIG. 3 shows an exemplary embodiment of the invention. In FIGS. 2 and 3 like elements are denoted by like reference numerals. More specifically, in the embodiment of the invention shown in FIG. 3 the interface  129  is located at a depth between the top boundary  135  of the body region  115  and a bottom boundary  133  of the body region  115 . In other words, the location of interface  129  in the inventive structure is adjusted so that the thick gate oxide layer  125  does not need to be etched back to an impractical depth when forming the thin oxide layer  127 . In contrast to the structure shown in FIG. 3, the prior art structure shown in FIG. 2 locates the interface  129  at a depth corresponding to epitaxial layer  112  rather than the body regions  115   a  and  115   b.    
     The present invention is easier to fabricate than the prior art structure because the portion of the thick oxide layer  125  that must be etched back to allow formation of the thin oxide layer  127  does not extend as deep within the trench. Accordingly, the problems associated with etching the thick oxide layer that arise when the trench has a high aspect ratio are reduced so that in the present invention the trench can be made correspondingly narrower before etching problems arise. In addition, the inventors have surprisingly found that the inventive structure offers a more optimal trade-off between its on-resistance and drain-to-source breakdown voltage. Most significantly, a primary advantage of the present invention is that because the portion of the total gate oxide layer occupied by the thick oxide layer  125  is increased relative to the prior art structure shown in the FIG. 2, the gate to drain charge of the device and hence its capacitance is reduced without adversely effecting the on-resistance. As previously mentioned, this advantageously reduces switching losses in the device. 
     The inventive DMOS device shown in FIG. 3 may be fabricated in accordance with any conventional processing technique. In particular, the double gate structure may be fabricated in accordance with the process steps set forth above in connection with the FIG. 2 structure and disclosed in the Y. Baba et al. reference. In this reference, when the thin oxide layer  127  is formed, the thick oxide layer  125  is etched back until it is eliminated and then a subsequent oxide layer is deposited to form thin oxide layer  127 . While the present invention may employ this technique, it may also employ an alternative technique in which the thick oxide layer  127  is etched back just enough to form the thin oxide layer  125 . In this way a second oxide deposition step is avoided and both oxide layers  125  and  127  are formed in a single deposition. 
     FIG. 4 is the result of a simulation that was performed showing the on-resistance (normalized to a uniform oxide layer 700 angstroms thick) for the inventive structure when the gate bias applied between the gate and source is 10 V and 4.5 V. In FIG. 4 the abscissa represents the location of the interface  129  in a trench that is 2 microns deep. That is, a depth of zero corresponds to a structure having no thin oxide layer and a depth of 2 microns corresponds to a structure having no thick oxide layer. FIG. 4 shows that there is little benefit from locating the interface at a depth below the body region  115  because below this level the on-resistance does not substantially decrease in comparison to when the interface is located at a depth between the top and boundary and bottom boundaries  135  and  133  of the body region  115 . If the interface is located above the top boundary  135  of the body region, however, the on-resistance significantly increases at low gate-to-source voltages. 
     In an alternative embodiment of the invention, the second polysilicon layer  131  of the gate electrode, which is deposited after the thin gate oxide layer  127 , is formed from silicide rather than polysilicon. Alternatively, the first polysilicon layer  130  or even both polysilicon layers  130  and  131  may be replaced with silicide. Silicide is advantageously employed because of its reduced resistance relative polysilicon and hence it contributes to a reduction in switching losses. This configuration increases the switching speed of the resulting device. 
     Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, the method of the present invention may be used to form a trench DMOS in which the conductivities of the various semiconductor regions are reversed from those described herein.