Patent Publication Number: US-7224027-B2

Title: High voltage power MOSFET having a voltage sustaining region that includes doped columns formed by trench etching and diffusion from regions of oppositely doped polysilicon

Description:
RELATED APPLICATIONS 
   This application is a divisional and claims the benefit of priority of U.S. patent application Ser. No. 10/414,949, filed Apr. 16, 2003, now U.S Pat. No. 6,794,251 and entitled “High Voltage Power MOSFET Having A Voltage Sustaining Region That Includes Doped Columns Formed By Trench Etching And Diffusion From Regions of Oppositely Doped Polysilicon,” which is a divisional of U.S. patent application Ser. No. 10/039,241, filed Dec. 31, 2001, now U.S. Pat. No. 6,576,516. The disclosure of each of these prior applications is incorporated by reference herein in their entirety. 
   This application is also related to U.S. patent application Ser. No. 09/970,972 entitled “Method for Fabricating a Power Semiconductor Device Having a Floating Island Voltage Sustaining Layer,” filed in the United States Patent and Trademark Office on Oct. 4, 2001, now U.S. Pat. No. 6,465,304, which is incorporated by reference herein in its entirety. 
   This application is also related to U.S. patent application Ser. No. 10/039,068 entitled “Method For Fabricating A High Voltage Power MOSFET Having A Voltage Sustaining Region That Includes Doped Columns Formed By Rapid Diffusion,” filed in the United States Patent and Trademark Office on Dec. 31, 2001, now U.S. Pat. No. 6,566,201, incorporated herein by reference in their entirety. 
   This application is also related to U.S. patent application Ser. No. 10/038,845 entitled “High Voltage MOSFET Having A Voltage Sustaining Region That Includes Doped Columns Formed By Trench Etching And Ion Implantation,” filed in the United States Patent and Trademark Office on Dec. 31, 2001, now U.S. Pat. No. 6,656,797, incorporated herein by reference in their entirety. 
   This application is also related to U.S. patent application Ser. No. 10/039,284 entitled “High Voltage Power MOSFET Having A Voltage Sustaining Region That Includes Doped Columns Formed By Trench Etching Using An Etchant Gas That Is Also a Doping Source,” filed in the United States Patent and Trademark Office on Dec. 31, 2001, now U.S. Pat. No. 6,750,104, incorporated herein by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to semiconductor devices, and more particularly to power MOSFET devices. 
   BACKGROUND OF THE INVENTION 
   Power MOSFET devices are employed in applications such as automobile electrical systems, power supplies, and power management applications. Such devices should sustain high voltage in the off-state while having a low voltage drop and high current flow in the on-state. 
     FIG. 1  illustrates a typical structure for an N-channel power MOSFET. An N-epitaxial silicon layer  1  formed over an N+ silicon substrate  2  contains p-body regions  5   a  and  6   a , and N+ source regions  7  and  8  for two MOSFET cells in the device. P-body regions  5  and  6  may also include deep p-body regions  5   b  and  6   b . A source-body electrode  12  extends across certain surface portions of epitaxial layer  1  to contact the source and body regions. The N-type drain for both cells is formed by the portion of N-epitaxial layer  1  extending to the upper semiconductor surface in  FIG. 1 . A drain electrode is provided at the bottom of N+ substrate  2 . An insulated gate electrode  18  typically of polysilicon lies primarily over the body and portions of the drain of the device, separated from the body and drain by a thin layer of dielectric, often silicon dioxide. A channel is formed between the source and drain at the surface of the body region when the appropriate positive voltage is applied to the gate with respect to the source and body electrode. 
   The on-resistance of the conventional MOSFET shown in  FIG. 1  is determined largely by the drift zone resistance in epitaxial layer  1 . The drift zone resistance is in turn determined by the doping and the layer thickness of epitaxial layer  1 . However, to increase the breakdown voltage of the device, the doping concentration of epitaxial layer  1  must be reduced while the layer thickness is increased. Curve  20  in FIG.  2  shows the on-resistance per unit area as a function of the breakdown voltage for a conventional MOSFET. Unfortunately, as curve  20  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 paper No. 26.2 in the Proceedings of the IEDM, 1998, p. 683. This MOSFET is similar to the conventional MOSFET shown in  FIG. 2  except that it includes p-type doped regions  40  and  42  which extend from beneath the body regions  5  and  6  into the drift region of the device. The p-type doped regions  40  and  42  define columns in the drift region that are separated by n-type doped columns, which are defined by the portions of the epitaxial layer  1  adjacent the p-doped regions  40  and  42 . The alternating columns of opposite doping type cause the reverse voltage to be built up not only in the vertical direction, as in a conventional MOSFET, but in the horizontal direction as well. As a result, this device can achieve the same reverse voltage as in the conventional device with a reduced layer thickness of epitaxial layer  1  and with increased doping concentration in the drift zone. Curve  25  in  FIG. 2  shows the on-resistance per unit area as a function of the breakdown voltage of the MOSFET shown in  FIG. 3 . Clearly, at higher operating voltages, the on-resistance of this device is substantially reduced relative to the device shown in  FIG. 1 , essentially increasing linearly with the breakdown voltage. 
   The improved operating characteristics of the device shown in  FIG. 3  are based on charge compensation in the drift region of the transistor. That is, the doping in the drift region is substantially increased, e.g., by an order of magnitude or more, and the additional charge is counterbalanced by the addition of columns of opposite doping type. The blocking voltage of the transistor thus remains unaltered. The charge compensating columns do not contribute to the current conduction when the device is in its on state. These desirable properties of the transistor depend critically on the degree of charge compensation that is achieved between adjacent columns of opposite doping type. Unfortunately, nonuniformities in the dopant gradient of the columns can be difficult to avoid as a result of limitations in the control of process parameters during their fabrication. For example, diffusion across the interface between the columns and the substrate and the interface between the columns and the p-body region will give rise to changes in the dopant concentration of the portions of the columns near those interfaces. 
   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 this structure is expensive to manufacture. Another technique for fabricating these devices is shown in copending U.S. application Ser. No. 09/970,972, in which a trench is successively etched to different depths. A dopant material is implanted and diffused through the bottom of the trench after each etching step to form a series of doped regions (so-called “floating islands”) that collectively function like the p-type doped regions  40  and  42  seen in  FIG. 3 . However, the on-resistance of a device that uses the floating island technique is not as low as an identical device that uses continuous columns. 
   Accordingly, it would be desirable to provide a method of fabricating the MOSFET structure shown in  FIG. 3  that requires a minimum number of epitaxial deposition steps so that it can be produced less expensively while also allowing sufficient control of process parameters so that a high degree of charge compensation can be achieved in adjacent columns of opposite doping type in the drift region of the device. 
   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 first or 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 trench in the epitaxial layer. A first layer of polysilicon having a second dopant of the second conductivity type is deposited in the trench. The second dopant is diffused to form a doped epitaxial region adjacent to the trench and in the epitaxial layer. A second layer of polysilicon having a first dopant of the first conductivity type is subsequently deposited in the trench. The first and second dopants respectively located in the second and first layers of polysilicon are interdiffused to achieve electrical compensation in the first and second layers of polysilicon. Finally, 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 a diode. 
   In accordance with another aspect of the invention, a power semiconductor device is provided. The device includes a substrate of a first or second conductivity type and a voltage sustaining region disposed on the substrate. The voltage sustaining region includes an epitaxial layer having a first conductivity type and at least one trench located in the epitaxial layer. At least one doped column having a dopant of a second conductivity type is located in the epitaxial layer, adjacent a sidewall of the trench. A first layer of polysilicon located in the trench and a second layer of polysilicon is located over the first layer of polysilicon. The column is formed by diffusion of the second dopant from the first layer of the polysilicon into the epitaxial layer. At least one region of the second conductivity is disposed over the voltage sustaining region to define a junction therebetween. 

   
     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 columns of p-type dopant 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 . 
       FIGS. 4(   a )- 4 ( d ) 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 
   In accordance with the present invention, a method of forming the p-type columns in the voltage sustaining layer of a semiconductor power device may be generally described as follows. First, one or more trenches are etched in the n-type doped epitaxial layer that is to form the voltage sustaining region of the device. Each trench is centered where a doped column is to be located. A first layer of p-type doped polysilicon is deposited in the trench. The p-type dopant in the polysilicon is diffused into the n-type doped epitaxial layer that surrounds the trench. Next, a second layer of n-type doped polysilicon is deposited to fill the trench. The dopants from the oppositely doped polysilicon layers interdiffuse, imparting electrical compensation to each other. However, the p-type doped region formed in the epitaxial layer does not undergo significant charge compensation because the diffusion rate of the n-type dopant is greater in polysilicon than in the single crystal silicon that forms the epitaxial layer. The resulting nearly electrically neutral column of polysilicon exhibits a high resistivity and does not contribute to device performance in any significant manner, while the p-type doped single crystal silicon region forms a continuous doped column with smooth sides similar to the one depicted in  FIG. 3 . In some embodiments of the invention the p-type dopant that is employed is boron and the n-type dopant that is employed is phosphorus, arsenic, or a combination of both. 
   A power semiconductor device similar to the one shown in  FIG. 3  may be fabricated in accordance with the following exemplary steps, which are illustrated in  FIGS. 4(   a )- 4 ( d ). 
   First, the N-type doped silicon epitaxial layer  501  is grown on a conventionally N+ doped substrate  502 . Epitaxial layer  501  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 . The trench  520  is dry etched through the mask openings by reactive ion etching to an initial depth that may range, for example, from 10-45 microns. 
   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 . 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. 4(   b ), a layer of p-type doped polysilicon  510  is deposited in the trench  520 . A diffusion step is then performed so that the p-type dopant diffuses from the trench  520  into the surrounding epitaxial layer  501 , thus forming a p-type doped column  512  of single crystal silicon. In general, the trench depth, dopant dose and the magnitude and duration of the diffusion process should be selected to achieve the desired degree of charge compensation. 
   Referring to  FIG. 4(   c ), a layer  516  of n-type doped polysilcon is then deposited to fill the trench. A diffusion step is then performed to cause the n-type dopant in polysilcon layer  516  to interdiffuse with the p-type dopant in the polysilicon layer  510 . The interdiffusion process is allowed to proceed until the p- and n-type dopants electrically compensate one another so that polysilicon layers  510  and  516  are electrically neutral. The charge compensation can be achieved if the amount of n-type and p-type dopants are properly selected, as described in “On the Semi-Insulating Polcrystalline Silicon Resistor,” by M. K. Lee, C. Y. Lu, K. Z. Chang and C. Shih in  Solid State Electronics , Vol. 27, No. 11, pp. 995-1001, 1984, which is hereby incorporated by reference in its entirety. 
   By providing electrical compensation to the polysilicon layers located in trench  520 , a high resistance region is formed within the center of the trench  520 . When a reverse voltage is applied to the completed device, this resistance causes a small leakage current to flow between the two high voltage terminals of the device, assuming that any excess charge in the high resistance polysilicon region has the same conductivity type as the doped column  512  of epitaxial silicon. If, however, the high resistance polysilicon region has a conductivity type opposite to doped column  512 , it will float “electrically” in the doped column  512  unless the critical electric field is exceeded. 
   Finally, as shown in  FIG. 4(   d ), the surface of the structure is planarized by removing the polysilicon from its surface. 
   The aforementioned sequence of processing steps resulting in the structure depicted in  FIG. 4(   d ) provides a voltage sustaining layer with p-type doped columns 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. 3  shows an example of a MOSFET that includes a voltage sustaining layer with doped columns similar to those employed in the present invention, except that in the present invention the doped columns have vertical sidewalls. It should be noted that while  FIG. 4  shows a single trench that is used to form the doped column, the present invention encompasses a voltage sustaining regions having single or multiple trenches to form any number of doped columns. For example, a doped column or columns may be located below the center of the gate or in other locations when appropriate to decrease the on-resistance of the device. 
   Once the voltage sustaining region and the doped column or columns have been formed as shown in  FIG. 4 , a MOSFET similar to that shown in  FIG. 3  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 polysilcon layer is then masked to form the gate regions. The p+ doped deep body regions  5   b  and  6   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 regions  5   a  and  6   a  are 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  7  and  8 . Source regions  7  and  8  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 which an oxide layer is formed on the surface. 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. 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 body region. 
   In some embodiments of the invention it is not necessary to deposit doped layers of polysilicon. Rather, dopant may be added to the first polysilicon layer  510  using gas phase doping. Alternatively, a doped layer of silicon dioxide may be deposited on the polysilicon, which is used as a solid source before it is removed in an etch step. Similarly, dopant may be added to second polysilicon layer  516  from the gas phase or from a doped deposited layer of silicon dioxide, provided the second layer doesn&#39;t fill the trench. If gas phase doping is employed, a dielectric layer or a layer of undoped polysilicon is deposited or grown to fill the trench prior to planarization. In contrast, if doped polysilicon is used as the dopant source, it can be used to fill the trench. It is also possible to deposit only one layer of doped polysilicon (e.g., layer  510 ) and to introduce the dopant that electrically compensates the doped polysilicon using either gas phase doping or by introducing dopant from a solid dopant source as described above. As above, the trench is subsequently filled with a dielectric or undoped polysilicon. 
   The previous description indicates that the n-type dopant in layer  510  does not diffuse into p-type doped column  512 . However, it is in fact possible to use the n-type dopant in layer  516  to compensate some of the p-type dopant in doped column  512 , thus providing a technique for adjusting the charge in doped column  512  to obtain maximum (or optimal) breakdown voltage. It is also possible to initially fill the trench with polysilcon that is undoped, then diffuse the first dopant into the polysilicon and the surrounding portion of the epitaxial layer  501  (to form doped column  512 ) and then diffuse the second dopant into the polysilicon to compensate the first dopant. This approach will produce a doped region that has a dopant concentration gradient that extends from the surface of the wafer to a point beyond the bottom of the trench, which gradient is a function of the trench size, the polysilicon grain size, the trench depth, the amount of dopant introduced, and other variables. 
   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.