Abstract:
A split gate field effect transistor device. The device includes a split gate structure having a trench, a gate electrode and a source electrode. A first poly layer is disposed within the trench and is connected to the gate electrode. A second poly layer connected to the source electrode, wherein the first poly layer and the second poly layer are independent.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/253,455, filed Oct. 20, 2009, Attorney Docket No. VISH-8792.PRO “STRUCTURES OF AND METHODS OF FABRICATING SPLIT GATE MIS DEVICES”, by Terrill, et al., which is incorporated herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to trench MOS transistors. 
       BACKGROUND OF THE INVENTION 
       [0003]    Power MOSFETs (metal-oxide-semiconductor field-effect transistors) comprise one of the most useful field effect transistors implemented in both analog and digital circuit applications as energy saving switches. 
         [0004]    In general, a trench-based power MOSFET is built using a vertical structure as opposed to a planar structure. The vertical structure enables the transistor to sustain both high blocking voltage and high current. 
         [0005]    The conventional trench MOS transistors have realized much higher cell density than the planar MOS transistors. But the denser pitches and the trench structures have increased gate-drain overlap capacitances and gate-drain charges. At high density the resistance of these structures is mainly limited by the epitaxial resistance for a given breakdown voltage. The so called split gate structure was proposed to overcome several drawbacks of the conventional trench structure performance. In this structure a shielded poly, which is connected to the source, is placed under the gate poly inside the trench. 
         [0006]    Split gate structures have been known to have better switching, breakdown voltage, and lower on-resistance characteristics. But due to its complexity the split gate structure is more difficult to manufacture. Also at high density it is necessary to bury the split gate structure under a top isolation oxide so that a space saving self-aligned contact technique can be utilized. Under these conditions difficulties in forming a isolation oxide, gate poly, inter poly oxide and shield poly inside a trench are very challenging. 
       SUMMARY OF THE INVENTION 
       [0007]    Embodiments of the present invention implement high density power field effect transistor that avoids the channel mobility problems caused by gate oxide scattering, that exhibits lower forward voltage (Vf) rated at high current; and that shows shorter channel length for faster switching. This invention can apply to DC-DC conversion as a synchronized rectifier transistor. 
         [0008]    In one embodiment, the present invention is implemented as a split gate field effect transistor device. The device includes a split gate structure having a trench, a gate electrode and a source electrode, a first poly layer a disposed within the trench and connected to the source electrode. A second poly layer is disposed within the trench and connected to the gate electrode, wherein the first poly layer and the second poly layer are independent. 
         [0009]    In one embodiment, the device further includes a gate contact connecting the second poly layer to the gate electrode, and a source contact connecting the first poly layer to the source electrode. Both contacts are made within the trench region. 
         [0010]    In one embodiment, the device further includes an active region body and a source contact, wherein the active region body and the source contact are disposed at a same surface plane. 
         [0011]    In one embodiment, the same surface plane is established via a CMP compatible process. 
         [0012]    In one embodiment, a layout method is used to enable a CMP compatible process to connect the first poly layer to the source electrode and the second poly layer to the gate electrode the same surface plane with active region source contacts. 
         [0013]    In one embodiment, the present invention is implemented as a CMP compatible split gate field effect transistor device. The device includes a split gate structure having a trench, a gate electrode and a source electrode. The device further includes a first poly layer disposed within the trench and connected to the source electrode, a second poly layer disposed within the trench and connected to the gate electrode, wherein the first poly layer and the second poly layer are independent. The device further includes a metal layer disposed over the split gate structure. 
         [0014]    In one embodiment, the device further includes a gate contact connecting the second poly layer to the gate electrode, and a source contact connecting the first poly layer to the source electrode. Both contacts are made within the trench region. 
         [0015]    In one embodiment, the device further includes an active region body and a source contact, wherein the active region body and the source contact are disposed at a same surface plane. 
         [0016]    In one embodiment, the same surface plane is established via a CMP compatible process. 
         [0017]    In one embodiment, a layout method is used to enable a CMP compatible process to connect the first poly layer to the source electrode and the second poly layer to the gate electrode the same surface plane with active region source contacts. 
         [0018]    In one embodiment, the present invention is implemented as a planar split gate field effect transistor device, comprising: 
         [0019]    The device includes a split gate structure having a trench, a gate electrode and a source electrode. The device further includes a first poly layer disposed within the trench and connected to the source electrode, a second poly layer disposed within the trench and connected to the gate electrode, wherein the first poly layer and the second poly layer are independent. The device further includes a metal layer disposed over the split gate structure, and wherein the first poly layer and the second poly layer are coplanar. 
         [0020]    In one embodiment, the device further includes a gate contact connecting the second poly layer to the gate electrode, and a source contact connecting the first poly layer to the source electrode. Both contacts are made within the trench region. 
         [0021]    In one embodiment, the device further includes an active region body and a source contact, wherein the active region body and the source contact are disposed at a same surface plane. 
         [0022]    In one embodiment, the same surface plane is established via a CMP compatible process. 
         [0023]    In one embodiment, a layout method is used to enable a CMP compatible process to connect the first poly layer to the source electrode and the second poly layer to the gate electrode in the same surface plane with active region source contacts. 
         [0024]    The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
           [0026]      FIG. 1  shows a top-down view of a split gate structure in accordance with one embodiment of the present invention. 
           [0027]      FIG. 2A  shows a first cross sectional view of cut line A-A′ from  FIG. 1  in accordance with one embodiment of the present invention. 
           [0028]      FIG. 2B  shows a second cross sectional view of cut line B-B′ from  FIG. 1  in accordance with one embodiment of the present invention. 
           [0029]      FIG. 2C  shows a third cross sectional view of cut line C-C′ from  FIG. 1  in accordance with one embodiment of the present invention. 
           [0030]      FIG. 3A  shows a first exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. 
           [0031]      FIG. 3B  shows a second exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. 
           [0032]      FIG. 3C  shows a third exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. This figure shows the process profile after first poly CMP. 
           [0033]      FIG. 3D  shows a fourth exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. 
           [0034]      FIG. 3E  shows a fifth exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. 
           [0035]      FIG. 4A  shows a sixth exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. This figure shows the process profile after second poly CMP. 
           [0036]      FIG. 4B  shows a seventh exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. 
           [0037]      FIG. 4C  shows an eighth exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. This figure shows the process profile after isolation oxide CMP. 
           [0038]      FIG. 4D  shows a ninth exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. 
           [0039]      FIG. 4E  shows a tenth exemplary sequential process step implementing split gate structures using surface planarization by CMP in accordance with a one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0040]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of embodiments of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the embodiments of the present invention. 
         [0041]    Embodiments of the present invention function with trench MOS transistors having much higher cell density than conventional planar MOS transistors. Embodiments of the present invention utilize a split gate structure that overcome certain drawbacks of the conventional trench structure performance. Embodiments of the present invention employ a split gate structure having a shielded poly, which is connected to the source, is placed under the gate poly inside the trench. This feature provides better switching, breakdown voltage, and lower on-resistance characteristics. 
         [0042]    Embodiments of the present invention advantageously utilize chemical mechanical polishing (CMP) to facilitate the fabrication of the complex split gate structure. The split gate structure is buried under a top isolation oxide so that a space saving self-aligned contact technique can be utilized. The use of chemical mechanical polishing facilitates the forming a isolation oxide, gate poly and shield poly inside a trench. 
         [0043]      FIG. 1  shows a top-down view  100  of a split gate structure in accordance with one embodiment of the present invention. As depicted in  FIG. 1 , the view  100  shows an active region source contact  104  and a poly1 contact  103  connected the source electrode and a poly2 contact  102  connected to the gate electrode at the same surface plane. A legend  101  indicates the nature of the shaded areas of the view  100 . 
         [0044]      FIG. 1  embodiment comprises a chip design with oxide CMP and poly CMP compatible process and layout specifically tailored to fabricate a split gate power MOSFET. In the  FIG. 1  embodiment, both the gate poly and source poly are picked up inside the trench. In one embodiment, device cell contact is realized by a self-aligned contact method. This process is highly scalable due to its inherit CMP compatibility. 
         [0045]    As described above, embodiments of the present invention utilize chemical mechanical polishing to facilitate fabrication of the split gate structure. The use of CMP allows for the planarization of each film inside the trench. This aspect results in better structure control and improved process margin. In order for CMP to be used, both the process and device layout need to be optimized to generate a planner structure. An extra benefit of such a planner structure is the improvement in photo lithography depth of focus. The above described features of embodiments of the present invention enable an improved ability to scale the features of the process to a smaller dimension. 
         [0046]      FIGS. 2A-C  shows delineates the cross sectional views of the cut-lines of A-A′, B-B′, and C-C′ of  FIG. 1 . Specifically,  FIG. 2A  shows the cross sectional view  201  of cut line A-A′.  FIG. 2B  shows the cross sectional view  202  of cut line B-B′.  FIG. 2C  shows the cross sectional view  203  of cut line C-C′. 
         [0047]    As depicted in the  FIGS. 2A-C , the views  201 - 203  show the structures of the fabricated trench gated MIS device shown in  FIG. 1 . As described above, the fabricated trench gated MIS device incorporates split gate structures that have two independent poly layers inside the trench connected to the gate electrode and the source electrode, respectively. 
         [0048]    In accordance with embodiments of the present invention, the fabricated trench gated MIS device implements the split gate structures having gate contact connecting the second poly layer and source contact connecting the first poly layer, and active region body and source contact at the same surface plane through CMP compatible processes. 
         [0049]    It should be noted that in addition to the illustrated  FIG. 1  method, there exist a number of different layout methods to connect poly1 layer to the source electrode effectively at the same surface plane with the other gate poly2 contacts and the active region source contacts that are within the scope of embodiments of the present invention. 
         [0050]      FIGS. 3A-3E  shows exemplary sequential process steps implementing split gate structures using surface planarization by CMP. Specifically,  FIG. 3C  shows the vertical structures after the application of poly1 and planarization via CMP. 
         [0051]      FIGS. 4A-4E  shows further exemplary sequential process steps implementing split gate structures using surface planarization by CMP. Specifically,  FIG. 4A  shows the vertical structures after the application of poly2 and planarization via CMP. Additionally,  FIG. 4C  shows the vertical structures after the application of oxide and planarization via CMP. In this manner, embodiments of the present invention implement a chip design with oxide CMP and poly CMP compatible process and layout specifically tailored for CMP to fabricate a split gate power MOSFET. 
         [0052]    The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order best to explain the principles of the invention and its practical application, thereby to enable others skilled in the art best to utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.