Abstract:
A method is provided for forming a power semiconductor device. The method begins by providing a substrate of a second conductivity type and then forming a voltage sustaining region on the substrate. The voltage sustaining region is formed by depositing an epitaxial layer of a first conductivity type on the substrate and forming at least one terraced trench in the epitaxial layer. The terraced trench has a plurality of portions that differ in width to define at least one annular ledge therebetween. A barrier material is deposited along the walls of the trench. A dopant of a second conductivity type is implanted through the barrier material lining the annular ledge and said trench bottom and into adjacent portions of the epitaxial layer. The dopant is diffused to form at least one annular doped region in the epitaxial layer and at least one other region located below the annular doped region. A filler material is deposited in the terraced trench to substantially fill the trench, thus completing the voltage sustaining region. At least one region of the second conductivity type is formed over the voltage sustaining region to define a junction therebetween.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/950,156, filed Dec. 4, 2007, which is a continuation in part of U.S. patent application Ser. No. 10/712,810, filed Nov. 13, 2003, entitled “Method for Fabricating a Power Semiconductor Device Having a Voltage Sustaining Layer with a Terraced Trench Facilitating Formation of Floating Islands”, now U.S. Pat. No. 7,304,347, which is a divisional of U.S. patent application Ser. No. 09/970,758, filed Oct. 4, 2001, entitled “Method for Fabricating a Power Semiconductor Device Having a Voltage Sustaining Layer with a Terraced Trench Facilitating Formation of Floating Islands”, now U.S. Pat. No. 6,649,477, which is related to U.S. patent application Ser. No. 09/970,972, filed Oct. 4, 2001, entitled “Method for Fabricating a Power Semiconductor Device Having a Floating Island Voltage Sustaining Layer”, now U.S. Pat. No. 6,465,304. Each of the above applications is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor power devices, and more particularly to a semiconductor power device such as a MOSFET and other power devices that use floating islands of oppositely doped material to form the voltage sustaining layer. 
     BACKGROUND OF THE INVENTION 
     Semiconductor power devices such as vertical DMOS, V-groove DMOS, and trench DMOS MOSFETs, IGBTs as well as diodes and bipolar transistors are employed in applications such as automobile electrical systems, power supplies, motor drives, and other power control applications. Such devices are required to sustain high voltage in the off-state while having low on-resistance or a low voltage drop with high current density in the on-state. 
       FIG. 1  illustrates a typical structure for an N-channel power MOSFET. An N-epitaxial silicon layer  101  formed over an N+ doped silicon substrate  102  contains p-body regions  105   a  and  106   a , and N+ source regions  107  and  108  for two MOSFET cells in the device. P-body regions  105  and  106  may also include deep p-body regions  105   b  and  106   b . A source-body electrode  112  extends across certain surface portions of epitaxial layer  101  to contact the source and body regions. The N-type drain for both cells is formed by the portion of N-type epitaxial layer  101  extending to the upper semiconductor surface in  FIG. 1 . A drain electrode is provided at the bottom of N+ doped substrate  102 . An insulated gate electrode  118  comprising insulating and conducting layers, e.g., oxide and polysilicon layers, lies over the body where the channel will be formed and over drain portions of the epitaxial layer. 
     The on-resistance of the conventional MOSFET shown in  FIG. 1  is determined largely by the drift zone resistance in epitaxial layer  101 . Epitaxial layer  101  is also sometimes referred to as a voltage sustaining layer since the reverse voltage applied between the N+ doped substrate and the P+ doped deep body regions is sustained by epitaxial layer  101 . The drift zone resistance is in turn determined by the doping concentration and the thickness of epitaxial layer  101 . However, to increase the breakdown voltage of the device, the doping concentration of epitaxial layer  101  must be reduced while the layer thickness is increased. The curve in  FIG. 2  shows the on-resistance per unit area as a function of the breakdown voltage for a conventional MOSFET. Unfortunately, as the curve shows, the on-resistance of the device increases rapidly as its breakdown voltage increases. This rapid increase in resistance presents a problem when the MOSFET is to be operated at higher voltages, particularly at voltages greater than a few hundred volts. 
       FIG. 3  shows a MOSFET that is designed to operate at higher voltages with a reduced on-resistance. This MOSFET is disclosed in Cezac et al.,  Proceedings of the ISPSD , May 2000, pp. 69-72, and Chen et al.,  IEEE Transactions on Electron Devices , Vol. 47, No. 6, June 2000, pp. 1280-1285, which are hereby incorporated by reference in their entirety. This MOSFET is similar to the conventional MOSFET shown in  FIG. 1  except that it includes a series of vertically separated P− doped layers  310   1 ,  310   2 ,  310   3 , . . .  310   n  (so-called “floating islands”), which are located in the drift region of the voltage sustaining layer  301 . The floating islands  310   1 ,  310   2 ,  310   3 , . . .  310   n  produce an electric field that is lower than for a structure with no floating islands. The lower electric field allows a higher dopant concentration to be used in the epitaxial layer that in part, forms the voltage sustaining layer  301 . The floating islands produce a saw-shaped electric field profile, the integral of which leads to a sustained voltage obtained with a higher dopant concentration than the concentration used in conventional devices. This higher dopant concentration, in turn, produces a device having an on-resistance that is lower than that of a device without one or more layers of floating islands. 
     The structure shown in  FIG. 3  can be fabricated with a process sequence that includes multiple epitaxial deposition steps, each followed by the introduction of the appropriate dopant. Unfortunately, epitaxial deposition steps are expensive to perform and thus a structure that uses multiple epitaxial deposition steps is expensive to manufacture. 
     Accordingly, it would be desirable to provide a method of fabricating a power semiconductor device such as the MOSFET structure shown in  FIG. 3 , which method requires a minimum number of epitaxial deposition steps so that the device can be produced less expensively. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method is provided for forming a power semiconductor device. The method begins by providing a substrate of a second conductivity type and then forming a voltage sustaining region on the substrate. The voltage sustaining region is formed by depositing an epitaxial layer of a first conductivity type on the substrate and forming at least one terraced trench in the epitaxial layer. The terraced trench has a plurality of portions that differ in width to define at least one annular ledge therebetween. A barrier material is deposited along the walls of the trench. A dopant of a second conductivity type is implanted through the barrier material lining the annular ledge and said trench bottom and into adjacent portions of the epitaxial layer. The dopant is diffused to form at least one annular doped region in the epitaxial layer. One other region located below the annular doped region may also be formed. A filler material is deposited in the terraced trench to substantially fill the trench, thus completing the voltage sustaining region. At least one region of the second conductivity type is formed over the voltage sustaining region to define a junction therebetween. 
     The power semiconductor device formed by the inventive method may be selected from the group consisting of a vertical DMOS, V-groove DMOS, and a trench DMOS MOSFET, an IGBT, a bipolar transistor, and diodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of a conventional power MOSFET structure. 
         FIG. 2  shows the on-resistance per unit area as a function of the breakdown voltage for a conventional power MOSFET. 
         FIG. 3  shows a MOSFET structure that includes a voltage sustaining region with floating islands located below the body region, which is designed to operate with a lower on-resistance per unit area at the same voltage than the structure depicted in  FIG. 1 . 
         FIG. 4  shows a MOSFET structure that includes a voltage sustaining region with floating islands both below and between the body regions. 
         FIGS. 5(   a )- 5 ( g ) show a sequence of exemplary process steps that may be employed to fabricate a voltage sustaining region constructed in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  shows a power semiconductor device having floating islands of the type disclosed in co-pending U.S. application Ser. No. 09/970,972 (now U.S. Pat. No. 6,465,304). In this device the trenches are assumed to be circular and therefore the floating islands are depicted as donut-shaped. Of course, the trenches may have other shapes such squares, rectangles, hexagons, or the like, which in turn will determine the shape of the floating islands. An N-type epitaxial silicon layer  401  formed over an N+ silicon substrate  402  contains P-body regions  405 , and N+ source regions  407  for two MOSFET cells in the device. As shown, P-body regions  405   a  may also include deep P-body regions  405   b . A source-body electrode  412  extends across certain surface portions of epitaxial layer  401  to contact the source and body regions. The N-type drain for both cells is formed by the portion of N-epitaxial layer  401  extending to the upper semiconductor surface. A drain electrode is provided at the bottom of N+ substrate  402 . An insulated gate electrode  418  comprising oxide and polysilicon layers lies over the channel and drain portions of the body. A series of floating islands  410  are located in the voltage sustaining region of the device defined by epitaxial silicon layer  401 . The floating islands are arranged in an array when viewed from the top of the device. For instance, in  FIG. 4 , in the “y” direction, floating islands are denoted by reference numerals  410   11 ,  410   12 ,  410   13 , . . .  410   1m  and in the “z” direction floating islands are denoted by reference numerals  410   11 ,  410   21 ,  410   31 , . . .  410   m1 . While the column of floating islands  410  located below the gate  418  may or may not be employed, they are preferably employed when required for the device geometry and the resistivity of epitaxial layer  401 . 
     In the device of  FIG. 4 , each horizontal row of floating islands, such as row  410   11 ,  410   12 ,  410   13 , . . .  410   1m , is formed in a separate implantation step. While this fabrication technique advantageously reduces the required number of epitaxial deposition steps in comparison to the known fabrication technique discussed in connection with  FIG. 3 , it would nevertheless be desirable to further simply the fabrication process by reducing the number of implantation steps that are required. 
     In accordance with the present invention, the p-type floating islands are configured as a series of coaxially located annular ledges. A method of forming such floating islands in the voltage sustaining layer of a semiconductor power device may be generally described as follows. First, a terraced trench is formed in the epitaxial layer that is to form the voltage sustaining region of the device. The terraced trench is formed from two or more co-axially located trenches that are etched at different depths in the epitaxial layer The diameter of each individual trench is greater than the diameter of the trenches located at greater depths in the epitaxial layer. Adjacent trenches meet in horizontal planes to define annular ledges, which arise from the differential in the diameter of the adjacent trenches. P-type dopant material is implanted into both the annular ledges and the bottom of the deepest trench in a single implantation step. If desired, the bottom trench may be continued to form a bottom annular ring of dopant. The implanted material is diffused into the portion of the voltage sustaining region located immediately adjacent to and below the ledges and trench bottom. The implanted material thus forms a series of floating islands that are configured as coaxially-located annular rings. Finally, the trenches are filled with a material that does not adversely affect the characteristics of the device. Exemplary materials that may be used for the material filling the trenches include highly resistive polysilicon, a dielectric such as silicon dioxide, or other materials and combinations of materials. 
     The power semiconductor device of the present invention may be fabricated in accordance with the following exemplary steps, which are illustrated in  FIGS. 5(   a )- 5 ( f ). 
     First, the N-type doped epitaxial layer  501  is grown on a conventionally N+ doped substrate  502 . Epitaxial layer  1  is typically 15-50 microns in thickness for a 400-800 V device with a resistivity of 5-40 ohm-cm. Next, a dielectric masking layer is formed by covering the surface of epitaxial layer  501  with a dielectric layer, which is then conventionally exposed and patterned to leave a mask portion that defines the location of the trench  520   1 . The trench  520   1  is dry etched through the mask openings by reactive ion etching to an initial depth that may range from 5-15 microns. In particular, if “x” is the number of equally spaced horizontal rows of floating islands that are desired, the trench  520  should be initially etched to a depth of approximately 1/(x+1) of the thickness of the portion of epitaxial layer  502  that is between the subsequently-formed bottom of the body region and the top of the N+ doped substrate. The sidewalls of each trench may be smoothed, if needed. First, a dry chemical etch may be used to remove a thin layer of oxide (typically about 500-1000 A) from the trench sidewalls to eliminate damage caused by the reactive ion etching process. Next, a sacrificial silicon dioxide layer is grown over the trench  520   1 . The sacrificial layer is removed either by a buffer oxide etch or an HF etch so that the resulting trench sidewalls are as smooth as possible. 
     In  FIG. 5(   b ), a layer of silicon dioxide  524   1  is grown in trench  520   1 . The thickness of the silicon dioxide layer  524   1  will determine the differential in diameter (and hence the radial width of the resulting annular ledge) between trench  520   1  and the trench that is to be subsequently formed. Oxide layer  524   1  is removed from the bottom of the trench  520   1 . 
     In  FIG. 5(   c ), a second trench  520   2  is etched through the exposed bottom of the trench  520   1 . In this embodiment of the invention the thickness of trench  520   2  is the same as the thickness of trench  520   1 . That is, trench  520   2  is etched by an amount approximately equal to 1/(x+1) of the thickness of the portion of epitaxial layer  501  that is located between the bottom of the body region and the N+-doped substrate. Accordingly, the bottom of trench  520   2  is located at a depth of 2/(x+1) below the bottom of the body region. 
     Next, in  FIG. 5(   d ), a third trench  520   3  (most clearly seen in  FIGS. 3(   e ) and  3 ( f )) may be formed by first growing an oxide layer  524   2  on the walls of trench  520   2 . Once again, the thickness of the silicon dioxide layer  524   2  will determine the differential in diameter (and hence the radial width of the resulting annular ledge) between trench  520   2  and trench  520   3 . Oxide layer  524   2  is removed from the bottom of the trench  520   2 . This process can be repeated as many times as necessary to form the desired number of trenches, which in turn dictates the number of annular ledges that are to be formed. For example, in  FIG. 5(   d ), four trenches  520   1 - 520   4  (more clearly seen in  FIG. 3(   e )) are formed. 
     In  FIG. 5(   e ), the various layers of oxide material located on the sidewalls of the trenches  520   1 - 520   4  are removed by etching to define annular ledges  546   1 - 546   3 . Next, an oxide layer  540  of substantially uniform thickness is grown in the trenches  520   1 - 520   4 . The thickness of oxide layer  540  should be sufficient to prevent implanted atoms from penetrating through the sidewalls of the trenches into the adjacent silicon, while allowing the implanted atoms to penetrate through the portion of oxide layer  540  located on the ledges  546   1 - 546   3  and the trench bottom  555 . 
     The diameter of trenches  520   1 - 5204  should be selected so that the resulting annular ledges  546   1 - 546   3  and the trench bottom all have the same surface area. In this way, when a dopant is introduced into the ledges and trench bottom, each resulting horizontal plane of floating islands will have the same total charge. 
     Next, in  FIG. 5(   f ), a dopant such as boron is implanted through the portion of oxide layer  540  located on the ledges  546   1 - 546   3  and the trench bottom  555 . The total dose of dopant and the implant energy should be chosen such that the amount of dopant left in the epitaxial layer  501  after the subsequent diffusion step is performed satisfies the breakdown requirements of the resulting device. A high temperature diffusion step is performed to “drive-in” the implanted dopant both vertically and laterally, thus defining the coaxially located floating islands  550   1 - 550   4 . 
     The terraced trench, which is composed of individual trenches  520   1 - 520   4 , is next filled with a material that does not adversely affect the characteristics of the device. Exemplary materials include, but are not limited to, thermally grown silicon dioxide, a deposited dielectric such as silicon dioxide, silicon nitride, or a combination of thermally grown and deposited layers of these or other materials. Finally, the surface of the structure is planarized as shown in  FIG. 5(   f ).  FIG. 5(   g ) shows the structure of  FIG. 5(   f ), but with the bottom trench etched further to form a bottom annular ring of dopant. 
     The aforementioned sequence of processing steps resulting in the structure depicted in  FIGS. 5(   f ) and  5 ( g ) provide a voltage sustaining layer with a series of annular floating islands on which any of a number of different power semiconductor devices can be fabricated. As previously mentioned, such power semiconductor devices include vertical DMOS, V-groove DMOS, and trench DMOS MOSFETs, IGBTs and other MOS-gated devices. For instance,  FIG. 4  shows an example of a MOSFET that may be formed on the voltage sustaining region of  FIG. 5 . It should be noted that while  FIG. 5  shows a single terraced trench, the present invention encompasses a voltage sustaining regions having single or multiple terraced trenches to form any number of columns of annular floating islands. 
     Once the voltage sustaining region and the floating islands have been formed as shown in  FIG. 5 , the MOSFET shown in  FIG. 4  can be completed in the following manner. The gate oxide is grown after an active region mask is formed. Next, a layer of polycrystalline silicon is deposited, doped, and oxidized. The polysilicon layer is then masked to form the gate regions. The p+ doped deep body regions  405   b  are formed using conventional masking, implantation and diffusion steps. For example, the p+-doped deep body regions are boron implanted at 20 to 200 KeV with a dosage from about 1×10 14  to 5×10 15 /cm 2 . The shallow body region  405   a  is formed in a similar fashion. The implant dose for this region will be 1×10 13  to 5×10 14 /cm 2  at an energy of 20 to 100 KeV. 
     Next, a photoresist masking process is used to form a patterned masking layer that defines source regions  407 . Source regions  407  are then formed by an implantation and diffusion process. For example, the source regions may be implanted with arsenic at 20 to 100 KeV to a concentration that is typically in the range of 2×10 15  to 1.2×10 16 /cm 2 . After implantation, the arsenic is diffused to a depth of approximately 0.5 to 2.0 microns. The depth of the body region typically ranges from about 1-3 microns, with the P+ doped deep body region (if present) being slightly deeper. Finally, the masking layer is removed in a conventional manner. The DMOS transistor is completed in a conventional manner by etching the oxide layer to form contact openings on the front surface. A metallization layer is also deposited and masked to define the source-body and gate electrodes. Also, a pad mask is used to define pad contacts. Finally, a drain contact layer is formed on the bottom surface of the substrate. 
     It should be noted that while a specific process sequence for fabricating the power MOSFET is disclosed, other process sequences may be used while remaining within the scope of this invention. For instance, the deep p+ doped body region may be formed before the gate region is defined. It is also possible to form the deep p+ doped body region prior to forming the trenches. In some DMOS structures, the P+ doped deep body region may be shallower than the P-doped body region, or in some cases, there may not even be a P+ doped deep body region. 
     It should also be noted that while in the example present above in connection with  FIG. 5  the substrate  502  and the doped epitaxial layer  501  have the same conductivity type, in other embodiments of the invention a power semiconductor may be formed in which the substrate  502  and the doped epitaxial layer  501  have opposite conductivity types. 
     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, a power semiconductor device in accordance with the present invention may be provided in which the conductivities of the various semiconductor regions are reversed from those described herein. Moreover, while a vertical DMOS transistor has been used to illustrate exemplary steps required to fabricate a device in accordance with the present invention, other DMOS FETs and other power semiconductor devices such as diodes, bipolar transistors, power JFETs, IGBTs, MCTs, and other MOS-gated power devices may also be fabricated following these teachings.