Abstract:
In one embodiment, a transistor fabricated on a semiconductor die includes a first section of transistor segments disposed in a first area of the semiconductor die, and a second section of transistor segments disposed in a second area of the semiconductor die adjacent the first area. Each of the transistor segments in the first and second sections includes a pillar of a semiconductor material that extends in a vertical direction. First and second dielectric regions are disposed on opposite sides of the pillar. First and second field plates are respectively disposed in the first and second dielectric regions. Outer field plates of transistor segments adjoining first and second sections are either separated or partially merged. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to semiconductor device structures and processes for fabricating high-voltage transistors. 
       BACKGROUND 
       [0002]    High-voltage, field-effect transistors (HVFETs) are well known in the semiconductor arts. Many HVFETs employ a device structure that includes an extended drain region that supports or blocks the applied high-voltage (e.g., several hundred volts) when the device is in the “off” state. In a conventional vertical HVFET structure, a mesa or pillar of semiconductor material forms the extended drain or drift region for current flow in the on-state. A trench gate structure is formed near the top of the substrate, adjacent the sidewall regions of the mesa where a body region is disposed above the extended drain region. Application of an appropriate voltage potential to the gate causes a conductive channel to be formed along the vertical sidewall portion of the body region such that current may flow vertically through the semiconductor material, i.e., from a top surface of the substrate where the source region is disposed, down to the bottom of the substrate where the drain region is located. 
         [0003]    In a traditional layout, a vertical HVFET consists of long continuous silicon pillar structure that extends across the semiconductor die, with the pillar structure being repeated in a direction perpendicular to the pillar length. One problem that arises with this layout, however, is that it tends to produce large warping of the silicon wafer during high temperature processing steps. In many processes, the warping is permanent and large enough to prevent the wafer from tool handling during subsequent processing steps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The present disclosure will be understood more fully from the detailed description that follows and from the accompanying drawings, which however, should not be taken to limit the invention to the specific embodiments shown, but are for explanation and understanding only. 
           [0005]      FIG. 1  illustrates an example cross-sectional side view of a vertical HVFET structure. 
           [0006]      FIG. 2A  illustrates an example layout of the vertical HVFET structure shown in  FIG. 1 . 
           [0007]      FIG. 2B  is an expanded view of one portion of the example layout shown in  FIG. 2A . 
           [0008]      FIG. 3A  illustrates another example layout of the vertical HVFET structure shown in  FIG. 1 . 
           [0009]      FIG. 3B  is an expanded view of one portion of the example layout shown in  FIG. 3A . 
           [0010]      FIG. 4A  illustrates yet another example layout of the vertical HVFET structure shown in  FIG. 1 . 
           [0011]      FIG. 4B  is an expanded view of one portion of the example layout shown in  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    In the following description specific details are set forth, such as material types, dimensions, structural features, processing steps, etc., in order to provide a thorough understanding of the present invention. However, persons having ordinary skill in the relevant arts will appreciate that these specific details may not be needed to practice the present invention. It should also be understood that the elements in the figures are representational, and are not drawn to scale in the interest of clarity. 
         [0013]      FIG. 1  illustrates an example cross-sectional side view of a vertical HVFET  10  having a structure that includes an extended drain region  12  of N-type silicon formed on an N+ doped silicon substrate  11 . Substrate  11  is heavily doped to minimize its resistance to current flowing through to the drain electrode, which is located on the bottom of the substrate in the completed device. In one embodiment, extended drain region  12  is part of an epitaxial layer that extends from substrate  11  to a top surface of the silicon wafer. A P-type body region  13  and N+ doped source regions  14   a  &amp;  14   b  laterally separated by a P-type region  16 , are formed near a top surface of the epitaxial layer. As can be seen, P-type body region  13  is disposed above and vertically separates extended drain region  12  from N+ source regions  14   a  &amp;  14   b  and P-type region  16 . 
         [0014]    In one embodiment, the doping concentration of the portion of epitaxial layer which comprises extended drain region  12  is linearly graded to produce an extended drain region that exhibits a substantially uniform electric-field distribution. Linear grading may stop at some point below the top surface of the epitaxial layer  12 . 
         [0015]    Extended drain region  12 , body region  13 , source regions  14   a  &amp;  14   b  and P-type region  16  collectively comprise a mesa or pillar  17  (both terms are used synonymously in the present application) of silicon material in the example vertical transistor of  FIG. 1 . Vertical trenches formed on opposite sides of pillar  17  are filled with a layer of dielectric material (e.g., oxide) that makes up dielectric region  15 . The height and width of pillar  17 , as well as the spacing between adjacent vertical trenches may be determined by the breakdown voltage requirements of the device. In various embodiments, mesa  17  has a vertical height (thickness) in a range of about 30 μm to 120 μm thick. For example, a HVFET formed on a die approximately 1 mm×1 mm in size may have a pillar  17  with a vertical thickness of about 60 μm. By way of further example, a transistor structure formed on a die of about 2 mm-4 mm on each side may have a pillar structure of approximately 30 μm thick. In certain embodiments, the lateral width of pillar  17  is as narrow as can be reliably manufactured (e.g., about 0.4 μm to 0.8 μm wide) in order to achieve a very high breakdown voltage (e.g., 600-800V). 
         [0016]    In another embodiment, instead of arranging P-type region  16  between N+ source regions  14   a  &amp;  14   b  across the lateral width of pillar  17  (as shown in  FIG. 1 ), N+ source regions and P-type regions may be alternately formed at the top of pillar  17  across the lateral length of pillar  17 . In other words, a given cross-sectional view such as that shown in  FIG. 1  would have either an N+ source region  14 , or a P-type region  16 , that extends across the full lateral width of pillar  17 , depending upon where the cross-section is taken. In such an embodiment, each N+ source region  14  is adjoined on both sides (along the lateral length of the pillar) by P-type regions  16 . Similarly, each P-type region  16  is adjoined on both sides (along the lateral length of the pillar) by N+ source regions  14 . 
         [0017]    Dielectric regions  15   a  &amp;  15   b  may comprise silicon dioxide, silicon nitride, or other suitable dielectric materials. Dielectric regions  15  may be formed using a variety of well-known methods, including thermal growth and chemical vapor deposition. Disposed within each of the dielectric layers  15 , and fully insulated from substrate  11  and pillar  17 , is a field plate  19 . The conductive material used to from field plates  19  may comprise a heavily doped polysilicon, a metal (or metal alloys), a silicide, or other suitable materials. In the completed device structure, field plates  19   a  &amp;  19   b  normally function as capacitive plates that may be used to deplete the extended drain region of charge when the HVFET is in the off state (i.e., when the drain is raised to a high voltage potential). In one embodiment, the lateral thickness of oxide region  15  that separates each field plate  19  from the sidewall of pillar  17  is approximately 4 μm. 
         [0018]    The trench gate structure of vertical HVFET transistor  80  comprises gate members  18   a  &amp;  18   b , each respectively disposed in oxide regions  15   a  &amp;  15   b  on opposite sides of pillar  17  between field plates  19   a  &amp;  19   b  and body region  13 . A high-quality, thin (e.g., ˜500 Å) gate oxide layer separates gate members  18  from the sidewalls of pillar  17  adjacent body region  13 . Gate members  18  may comprise polysilicon, or some other suitable material. In one embodiment, each gate member  18  has a lateral width of approximately 1.5 μm and a depth of about 3.5 μm. 
         [0019]    Practitioners in the art will appreciate that N+ source regions  14  and P-type body region  13  near the top of pillar  17  may each be formed using ordinary deposition, diffusion, and/or implantation processing techniques. After formation of the N+ source region  38 , HVFET  10  may be completed by forming source, drain, gate, and field plate electrodes that electrically connect to the respective regions/materials of the device using conventional fabrication methods (not shown in the figures for clarity reasons). 
         [0020]      FIG. 2A  illustrates an example layout of the vertical HVFET structure shown in  FIG. 1 . The top view of  FIG. 2A  shows a single, discrete HVFET comprising an upper transistor section  30   a  and a lower transistor section  30   b  on a semiconductor die  21 . The two sections are separated by a dummy silicon pillar  32 . Each section  30  comprises a plurality of “racetrack” shaped transistor structures or segments, each transistor segment comprises an elongated ring or oval that includes a silicon pillar  17  surrounded on opposite sides by dielectric regions  15   a  &amp;  15   b . Pillar  17 , itself, extends laterally in the x and y directions to form a continuous, elongated, racetrack-shaped ring or oval. Disposed within dielectric regions  15   a  &amp;  15   b  are respective gate members  18   a  &amp;  18   b  and field plates  19   a  &amp;  19   b . Field plate  19   a  comprises a single elongated member that terminates on either end in a rounded fingertip area. Field plate  19   b , on the other hand, comprises an enlarged ring or oval that encircles pillar  17 . Field plates  19   b  of adjacent racetrack structures are shown merged such that they share a common member on a side. By way of reference, the cross-sectional view of  FIG. 1  may be taken through cut lines A-A′ of the example layout of  FIG. 2A . 
         [0021]    It should be understood that in the example of  FIG. 2A , each of the racetrack transistor segments has a width (i.e., pitch) in the y-direction of approximately 13 μm, a length in the x-direction in a range of about 400 μm to 1000 μm, with a pillar height of about 60 μm. In other words, the length to width ratio of the individual racetrack transistor segments comprising sections  30   a  &amp;  30   b  is in a range of about 30 up to 80. In one embodiment, the length of each racetrack shaped segment is at least 20 times greater than its pitch or width. 
         [0022]    Practitioners in the art will appreciate that in the completed device structure, patterned metal layers are used to interconnect each of the silicon pillars  17  of the individual transistor segments. That is, in a practical embodiment, all of the source regions, gate members, and field plates are respectively wired together to corresponding electrodes on the die. In the embodiment shown, the transistor segments in each section  30  are arranged in a side-by-side relationship in the y-direction substantially across a width of die  21 . Similarly, in the x-direction the additive length of the transistor segments of sections  30   a  &amp;  30   b  extend substantially over the length of die  21 . In the example layout of  FIG. 2A  the width of dielectric regions  15  separating the silicon pillars, as well as the width of the field plates, is substantially uniform across semiconductor die  21 . Laying out the transistor segments with uniform widths and separation distances prevents the formation of voids or holes following the processing steps used to conformably deposit the layers that comprise dielectric regions  15  and field plates  19 . 
         [0023]      FIG. 2B  is an expanded view of one portion of the example layout shown in  FIG. 2A . For purposes of clarity, only pillars  17  and dielectric regions  15   b  of each of the transistor segments is represented. Dummy silicon pillar  32  is shown separating the rounded end areas of dielectric regions  15   b  of respective transistor segment sections  30   a  &amp;  30   b . In other words, the deep vertical trenches that are etched in the semiconductor substrate to define pillars  17  also define dummy silicon pillar  32 . In one embodiment, dummy silicon pillar  32  is made to have a width in the x-direction (i.e., that separates the transistor segment sections) that is as small as can be reliably manufactured. 
         [0024]    The purpose of segmenting the single die HVFET into sections separated by dummy silicon pillar  32  is to introduce lengthwise (x-direction) stress-relief in the elongated racetrack shaped transistor segments. Segmenting or breaking the transistor device structures into two or more sections relieves mechanical stress across the length of the die. This stress is induced by the oxide regions flanking the pillars and normally concentrates at the rounded ends of each racetrack segment. Relieving mechanical stress by segmenting the transistor device structures into two or more sections thus prevents undesirable warping of the silicon pillars and damage (e.g., dislocations) to the silicon caused by stress. 
         [0025]    It is appreciated that a tradeoff exists between the stress relief provided by a highly segmented layout and loss of conduction area. More segmentation results in greater stress relief, but at the expense of conduction area. In general, the greater the vertical height of the pillars and the larger the semiconductor die, the greater the number of transistor sections or segments that will be required. In one embodiment, for a 2 mm×2 mm die with 60 μm high pillars, adequate stress relief is provided in a HVFET with an on-resistance of about 1 ohm utilizing a layout comprising four racetrack transistor sections separated by dummy silicon pillars, each having a pitch (y-direction) of about 13 μm and a length (x-direction) of about 450 μm. 
         [0026]    In another embodiment, instead of a dummy pillar of silicon to separate pairs of racetrack transistor segments, each pair being located in a different section, a dummy pillar comprising a different material may be utilized. The material used for the dummy pillar should have a thermal coefficient of expansion close to that of silicon, or sufficiently different from that of the dielectric region so as to relieve the lengthwise stress induced by the dielectric regions flanking the silicon pillars. 
         [0027]      FIG. 3A  illustrates another example layout of the vertical HVFET structure shown in  FIG. 1 .  FIG. 3B  is an expanded view of one portion of the example layout shown in  FIG. 3A , just showing pillars  17 , oxide region  15   b , and an optional dummy silicon pillar  33 . Similar to the embodiment of  FIGS. 2A &amp; 2B ,  FIGS. 3A &amp; 3B  show a single, discrete HVFET comprising an upper transistor section  30   a  and a lower transistor section  30   b  on a semiconductor die  21 . But in the example of  FIGS. 3A &amp; 3B , the deep vertical trenches filled with oxide regions  15   b  and field plates  19   b  of transistor sections  30   a  and  30   b  overlap, or are merged, leaving small, diamond-shaped dummy silicon pillars  33  between the segmented transistor sections. In this embodiment, a single dummy pillar is centrally located between the four rounded ends of adjacent pairs of transistor segments over the two sections. In the example shown, for every N (where N is an integer greater than 1) racetrack segments or structures in a section  30  of the transistor comprising die  21 , there are a total of N−1 dummy pillars  33 . 
         [0028]      FIG. 4A  illustrates yet another example layout of the vertical HVFET structure shown in  FIG. 1 .  FIG. 4B  is an expanded view of one portion of the example layout shown in  FIG. 4A . Pillars  17  and oxide region  15   b  are just shown for clarity reasons in the expanded view of  FIG. 4B . In this example, the transistor segments comprising the HVFET of semiconductor die  21  are alternately shifted by half of the length of each racetrack segment, resulting in racetrack transistor segments that are alternately associated with upper transistor section  40   a  and lower transistor section  40   b . In other words, each of the transistor segments of a row of section  40   a  is separated by a pair of the transistor segments of section  40   b , the pair being arranged in an end-to-end relationship in the x-direction. 
         [0029]    It is appreciated that the alternate shifting of the segments may be any fraction of the segment length. In other words, shifting of the segments is not limited to 50% or half the length. Various embodiments may comprise segments alternately shifted by any percentage or fraction ranging from greater than 0% to less than 100% of the length of the transistor segments. 
         [0030]    In the example of  FIGS. 4A &amp; 4B , the dielectric regions  15   b  of alternating ones of the transistor segments in respective sections  40   a  &amp;  40   b  are merged. In the specific embodiment shown, the rounded ends of the transistor segments associated with different adjacent sections overlap or are merged such that field plates  19   b  of the adjacent sections are merged at the ends (in the x-direction). Also, the extended straight side portions of field plates  19   b  of alternating transistor segments of different sections are merged along a substantial length of each segment. It is appreciated that regions  15   b  and  19   b  may be merged with or without a dummy pillar (or isolated dummy silicon pillars) between the respective sections. 
         [0031]    Although the above embodiments have been described in conjunction with a specific device types, those of ordinary skill in the arts will appreciate that numerous modifications and alterations are well within the scope of the present invention. For instance, although HVFETs have been described, the methods, layouts and structures shown are equally applicable to other structures and device types, including Schottky, diode, IGBT and bipolar structures. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.