Abstract:
A vertical diffused metal oxide semiconductor (DMOS) field-effect transistors (FET), has a cell structure with a substrate; an epitaxial layer or well of the first conductivity type on the substrate; first and second base regions of the second conductivity type arranged within the epitaxial layer or well and spaced apart by a predefined distance; first and second source regions of a first conductivity type arranged within the first and second base region, respectively; a gate structure insulated from the epitaxial layer or well by an insulation layer and arranged above the region between the first and second base regions and covering at least partly the first and second base region, wherein the gate structure comprises first and second gates being spaced apart wherein each gate covers a respective portion of the base region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/415,449 filed on Nov. 19, 2010, entitled “FORMING A LOW CAPACITANCE FIELD EFFECT TRANSISTOR BY REMOVAL OF A PORTION OF THE CONTROL GATE”, which is incorporated herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This application concerns a vertical DMOS-Field Effect Transistor (FET). 
       BACKGROUND 
       [0003]    Power metal oxide semiconductor field-effect transistors (MOSFET) are generally used to handle high power levels in comparison to lateral transistors in integrated circuits.  FIG. 5  shows a typical MOSFET which uses a vertical diffused MOSFET structure, also called double-diffused MOSFET structure (DMOS or VDMOS). 
         [0004]    As shown for example, in  FIG. 5 , on an N+ substrate  415  there is a N-epitaxial layer formed whose thickness and doping generally determines the voltage rating of the device. From the top into the epitaxial layer  410  there are formed N+ doped left and right source regions  430  surrounded by P-doped region  420  which forms the P-base surrounded by its out diffusion area  425 . A source contact  460  generally contacts both regions  430  and  420  on the surface of the die and is generally formed by a metal layer that connects both left and right source region. An insulating layer  450 , typically silicon dioxide or any other suitable material, insulates a polysilicon gate  440  which covers a part of the P-base region  420  and out diffusion area  425 . The gate  440  is connected to a gate contact  470  which is usually formed by another metal layer. The bottom side of this vertical transistor has another metal layer  405  forming the drain contact  480 . In summary,  FIG. 5  shows a typical elementary cell of a MOSFET that can be very small and comprises a common drain, a common gate and two source regions and two channels. Other similar cells may be used in a vertical power MOS-FET. A plurality of such cells may generally be connected in parallel to form a power MOSFET. 
         [0005]    In the On-state, a channel is formed within the area of regions  420  and  425  covered by the gate reaching from the surface into the regions  420  and  425 , respectively. Thus, current can flow as indicated by the horizontal arrow. The cell structure must provide for a sufficient width d of gate  440  to allow for this current to turn into a vertical current flowing to the drain side as indicated by the vertical arrows. 
         [0006]    Such structures have a relatively high gate to Drain capacitance due to the necessary width of the gate which is undesirable, in particular, in high frequency switching applications such as switched mode power supplies. 
       SUMMARY 
       [0007]    According to an embodiment, a vertical diffused metal oxide semiconductor (DMOS) field-effect transistors (FET), may have a cell structure comprising a substrate; an epitaxial layer or well of the first conductivity type on said substrate; first and second base regions of the second conductivity type arranged within said epitaxial layer or well and spaced apart by a predefined distance; first and second source regions of a first conductivity type arranged within said first and second base region, respectively; and a gate structure insulated from said epitaxial layer or well by an insulation layer and arranged above the region between the first and second base regions and covering at least partly said first and second base region, wherein the gate structure comprises first and second gates being spaced apart wherein each gate covers a respective portion of said base region. 
         [0008]    According to a further embodiment, the base region may further comprise first and second diffusion areas of said second conductivity type surrounding said first and second base regions, respectively. According to a further embodiment, the vertical DMOS-FET may further comprise a source metal layer connecting said first and second source region and said first and second base region. According to a further embodiment, the vertical DMOS-FET may further comprise a gate metal layer connecting said first and second gate. According to a further embodiment, the first and second gate can be formed by a gate layer that connects the first and second gate. According to a further embodiment, the first and second gate can be connected outside the cell structure. According to a further embodiment, the first and second gate can be connected by wire bonding. According to a further embodiment, the vertical DMOS-FET may further comprise a drain metal layer on the backside of the substrate. According to a further embodiment, the cell structure or a plurality of cell structures can be formed in an integrated circuit device. According to a further embodiment, the integrated circuit device may provide for control functions for a switched mode power supply. According to a further embodiment, the first conductivity type can be P-type and the second conductivity type can be N-type. According to a further embodiment, the first conductivity type can be N-type and the second conductivity type can be P-type. According to a further embodiment, the substrate can be of the first or second conductivity type. According to a further embodiment, if said substrate is of the second conductivity type, the drain is connected through a top surface. 
         [0009]    According to another embodiments, a method for manufacturing a cell structure of a vertical diffused metal oxide semiconductor (DMOS) field-effect transistors (FET), may comprise: forming a cell structure comprising first and second source regions of a first conductivity type for a vertical DMOS-FET in an epitaxial layer or well of a second conductivity type arranged on a substrate, wherein the first and second source regions are spaced apart by a predefined distance; forming an insulated gate layer on top of said epitaxial layer or well; patterning the gate layer to form first and second gates being spaced apart from each other. 
         [0010]    According to a further embodiment of the method, the step of patterning can be performed in a single step. According to a further embodiment of the method, the step of patterning the gate layer may provide for a bridging area of the gate layer connecting the first and second gates. According to a further embodiment of the method, the bridging area can be located outside the cell structure. According to a further embodiment of the method, the method may further comprise connecting the first and second gates by a metal layer. According to a further embodiment of the method, the method may further comprise connecting the first and second gates by wire bonding. According to a further embodiment, the substrate can be of the first or second conductivity type. According to a further embodiment, if said substrate is of the second conductivity type, the drain is connected through a top surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows an embodiment of an improved vertical DMOS-FET. 
           [0012]      FIG. 2A-2F  shows several exemplary process steps for manufacturing a device as shown in  FIG. 1 . 
           [0013]      FIG. 3  shows an exemplary partial top view of the device as shown in  FIG. 1 ; and 
           [0014]      FIGS. 4A and 4B  show applications of the improved vertical DMOS-FET in single integrated chip. 
           [0015]      FIG. 5  shows a conventional vertical DMOS-FET. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows a cross-sectional view of a vertical DMOS-FET according to various embodiments. Again, an N+ substrate  115  is provided on top of which an N-epitaxial layer  110  is formed. Alternatively, a N-well  110  can be formed on top of the substrate  115 . The substrate can be either of N-type or of P-type as will be explained in more detail below. In the example shown in  FIG. 1 , layer  115  is an N+− substrate and from the top into the epitaxial layer  110  there are formed N+ doped left and right source regions  130  each surrounded by a P-doped region  120  which forms the P-base. Each P-base  120  is surrounded by an associated out diffusion area  125 . Similar as for the transistor shown in  FIG. 4 , a source contact  160  generally contacts both regions  130  and  120  on the surface of the die and is generally formed by a metal layer that connects both left and right source region. Contrary to the conventional vertical DMOS-FET, an insulating layer  150  insulates separate left and right gates  140  and  145  each covering a part of the respective left and right P-base region  120  and associated out diffusion area  125 . The gates are interconnected, for example by means of a metal or contact layer  170  or outside the gate effective area as will be explained in more detail below. Thus, according to various embodiments, the cell proposed structure does not only create two source regions  120 ,  125 ,  130  and two channels but also two gates  140  and  145 . The gates can be formed by polysilicon, amorphous silicon or any other suitable conductive materials The bottom side of this vertical transistor has again another metal layer  105  forming the drain contact  180 . 
         [0017]    As mentioned above, according to various embodiments, the gates  140  and  145  do substantially not overlap such that two distinct gates are formed. Thus, the combined gate area for gates  140  and  145  when seen from atop is smaller than that of a conventional vertical transistor. Hence, the resulting individual gate-source and gate-drain capacitances are effectively are in sum smaller than the respective gate capacitances of a conventional vertical DMOS-FET as for example shown in  FIG. 4 . The various embodiments thus effectively take out the middle portion of the gate  440  of a conventional DMOS-FET thereby splitting the gate into two distinct gates  140  and  145 . This can be done as much of the gate material is unnecessary for channel control. Thus, by removing the middle portion, the effective gate capacitance of this cell can be lowered without affecting the performance of the device. Depending on the manufacturing process, the split gate can be created by patterning of the gate layer in a single step. Hence, no additional masking steps are required. The middle section of gate  440  that is to be taken out may be very small, however, available lithography techniques will be capable of resolving the spaces involved and thus allow to create such a structure. 
         [0018]    Alternatively, as mentioned above different types of substrate  110  can be used. For example, the substrate  110  can be a N+, a N++, or an N substrate, or can even be a P-type substrate. Thus, layer  110  can be an epitaxial layer or just a diffused N-type well. In case the substrate is N-doped, and a N-type well  110  is formed, the same structure as mentioned above with respect to the N-epitaxial layer will be formed. In case the substrate is P-doped, while the remaining structure and conductivity types remain as mentioned above, the substrate could not be used as the drain anymore. In this case, the drain would be connected through the top surface instead of the substrate layer. However, the device would still be considered to be a vertical transistor because current would generally flow vertically as indicated in  FIG. 5  but would then also move laterally through the N-well and be collected on the top side. 
         [0019]      FIG. 2A-2F  show exemplary process steps for manufacturing a device as shown in  FIG. 1 . However, according to the applied technology other steps may be suitable to produce a similar device. As shown in  FIG. 2A , an N-doped epitaxial layer  110  is grown on an N+ substrate  115 . On top of the epitaxial layer  110  an oxide layer  150  is deposited. the oxide layer  150  can be patterned as shown in  FIGS. 2B  and N+-doped source regions  130  and surrounding base regions  120  with associated out diffusion areas  125  can be created with well known diffusion techniques as shown in  FIG. 2C .  FIG. 2D  shows the die with a polysilicon layer  200  which is deposited on top of the die. As mentioned above, amorphous silicon or any other suitable gate material can be deposited as the gate layer  200 . The gate layer  200  can then be patterned using known masking techniques to form gates  140  and  145  as shown in  FIG. 2E .  FIG. 2F  shows the cell structure with an additional metal layer  190  connecting the left and right source regions  130  and associated P-base regions  120 . Furthermore,  FIG. 2F  shows the back metal layer  105  contacting the drain region  115 . 
         [0020]    The step of patterning the gate layer  200  can be performed in one single step. Thus, no additional process step is required. However, according to other embodiments, more than one step may be used. For example, if the gate as shown in  FIG. 4  is used as a mask to form the source regions then splitting the gates into two separate gates may be performed by another step. 
         [0021]      FIG. 3  shows a top view of a cell  300  according to  FIG. 1  wherein only certain areas of the cell are highlighted. As can be seen, the left and right source regions  130  are surrounded by the P-base region  120 . The broken lines indicate the position of the overlaid gates  140  and  145 . Mid section  300  of the gate layer is removed to form individual left gate  145  and right gate  140 . The gate layer  200  may be patterned to completely separate left and right gate by removing the inner section  320  and a metal layer may be used to connect the individual gate portions on the chip. According to other embodiments, well known bonding techniques may be used to connect the gates, for example outside the chip by means of a leadframe. However, the gate layer  200  can also be patterned as shown in  FIG. 3  such that a bridging area  310  is formed outside the cell area. However, according to other embodiments, the bridging area  310  may reach into the cell and cover an insubstantial part of the cell without influencing the gate capacitance significantly. The gate layer  200  may be furthermore patterned to connect a plurality of gates from neighboring cells as indicated by the dotted lines on the left and right and bottom sides of the gate structure shown in  FIG. 3 . 
         [0022]    The cell structure can be a stripe structure as shown in  FIG. 3 . However, according to other embodiments may use square cells, hexagonal shapes or any other suitable cell shape for which the principle of the various embodiments can be applied to. The cell structure or a plurality of cells can be used to form a power DMOS-FET within an integrated circuit or in a discrete transistor device. Such an integrated circuit may provide control circuits for use in a switched mode power supply. Thus, no external power transistors may be necessary. 
         [0023]      FIG. 4A  shows schematically how a microcontroller  660  can be combined with two power transistors  680  and  690  according to various embodiments as shown in  FIGS. 1-3  on a single chip  600 . Alternatively, the microcontroller  660  and the transistors  680 ,  690  may be provided on separate chips within a single housing. Microcontroller  660  may have a plurality of peripheral devices such as controllable drivers, modulators, in particular pulse width modulators, timers etc. and is capable to drive the gates  640  and  650  of transistors  680  and  690  directly or through respective additional drivers. The chip  600  can be configured to make a plurality of functions of the microcontroller available through external connections or pins  670 . The source of first transistor  680  can be connected to external connection or pin  610 . Similarly, external connection  620  provides a connection to the combined drain and source of transistors  680  and  690  and external connection or pin  630  for the drain of the second transistor  630 . Other transistor structures manufactured in accordance with the various embodiments disclosed can be used, such as an H-bridge or multiple single transistors.  FIG. 4B  shows an exemplary plurality of MOSFETs connected to form an H-Bridge  625  that can be coupled with a microcontroller  660  or modulator within a single semiconductor chip  605 . 
         [0024]    Furthermore, the exemplary embodiment shows a N-channel device with appropriate conductivity types of the different regions. A person skilled in the art will appreciate that the embodiments of the present application are not restricted to N-channel devices but can be also applied to P-Channel devices.