Abstract:
A method and structure for a composite trench fill for silicon electronic devices. On a planar silicon substrate having a first deposited layer of oxide and a second deposited layer of polysilicon, a trench is etched. Deposition and etch processes using a combination of oxide and polysilicon are used to fabricate a composite trench fill. The trench bottom and a lower portion of the walls are covered with oxide. The remaining portion of the trench volume is filled with polysilicon. The method may be used for junction field effect transistors (JFETs) and metal oxide semiconductor field effect transistors (MOSFETs).

Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention relate to the field of silicon electronic devices. In particular, embodiments of the present invention relate to a trench fill method and structure. 
     BACKGROUND ART 
     Junction field effect transistors (JFETs) are majority carrier devices that conduct current through a channel that is controlled by the application of a voltage to a p-n junction. JFETs may be constructed as p-channel or n-channel and may be operated as enhancement mode devices or depletion mode devices. 
     The most common JFET type is the depletion mode type. The depletion mode device is a normally “on” device that is turned off by reverse biasing the p-n junction so that pinch-off occurs in the conduction channel. P-channel depletion mode devices are turned off by the application of a positive voltage between the gate and source (positive V gs ), whereas n-channel depletion mode devices are turned off by the application of a negative voltage between the gate and source (negative V gs ). Since the junction of a depletion mode JFET is reverse biased in normal operation, the input voltage V gs  can be relatively high. However, the supply voltage between the drain and source (V ds ) is usually relatively low when the device is switched on. 
     Enhancement mode, or normally “off” JFETs are characterized by a channel that is sufficiently narrow such that a depletion region at zero applied voltage extends across the entire-width of the channel. Application of a forward bias reduces the width of the depletion region in the channel, thereby creating a conduction path in the channel. P-channel enhancement mode JFETs are turned on by the application of a negative V gs , and n-channel enhancement mode JFETs are turned on by the application of a positive V gs . The input voltage of an enhancement mode JFET is limited by the forward breakdown voltage of the p-n junction. 
     Historically, high voltage applications for transistors have relied chiefly on bipolar junction transistors (BJTs), insulated gate bipolar transistors (IGBTs), and metal oxide semiconductor field effect transistors (MOSFETs). IGBTs and MOSFETs have the disadvantage of being susceptible to gate damage due to static discharge, and BJTs are susceptible to thermal runaway. JFETs do not have these disadvantages. JFETs share the transconductance/temperature behavior of MOSFETs, but they do not rely on an insulated gate. 
     In the fabrication of silicon electronic devices such as JFETs and MOSFETs, trenches may be etched that require a subsequent fill with a dielectric material. Silicon dioxide is commonly used for trench fill, but there are some disadvantages. As smaller critical dimensions are adopted for the fabrication of silicon electronic devices, thinner layers and finer registration are required, and devices are less tolerant of defects. 
     With typical oxide deposition processes, voids occasionally form in the region of a filled trench. These voids may form as a result of trench geometry or the characteristics of the oxide deposition process. 
     Another problem with conventional oxide fills is the difficulty in avoiding the unwanted removal of oxide during subsequent etchback. Unwanted material removal may be the result of over-etching in the vertical dimension or may result from poor lateral registration during an etch step. Shallow trenches are particularly-susceptible, since a given amount of over-etching will constitute a greater fraction of the fill. The problem of over-etching of an oxide trench fill may be compounded by a subsequent metal deposition step, resulting in a substitution of metal for a portion of the dielectric oxide fill. 
     Thus, a need exists for a trench fill method that is capable of reducing the formation of voids in the vicinity of the trench. There is also a need for a trench fill method that protects against oxide over-etching, particularly for shallow trenches. 
     SUMMARY OF INVENTION 
     A method and structure for a composite trench fill for silicon electronic devices is disclosed. On a planar silicon substrate having a first deposited layer of oxide and a second deposited layer of polysilicon, a trench is etched. Deposition and etch processes using a combination of oxide and polysilicon are used to fabricate a composite trench fill. The trench bottom and a lower portion of the walls are covered with oxide. The remaining portion of the trench volume is filled with polysilicon. The method may be used for junction field effect transistors (JFETs) and metal oxide semiconductor field effect transistors (MOSFETs). 
     In an embodiment of the present invention, a planar silicon substrate is prepared by forming a first oxide layer on the surface of the substrate and the subsequent formation of a first polysilicon layer on the first oxide layer. A trench is then etched in the prepared substrate. A second oxide layer is then formed on the substrate and trench surfaces. A second polysilicon layer is deposited over the second oxide layer and etched back so that a polysilicon fill that is approximately level with the surface of the substrate remains within the trench. The second oxide layer is then etched back to produce a gap between the polysilicon fill and the substrate. Finally, a third polysilicon layer is deposited and etched back to fill the gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a general schematic for an n-channel depletion mode junction field effect transistor (JFET). 
         FIG. 1B  shows a general schematic for an n-channel enhancement mode junction field effect transistor (JFET). 
         FIG. 2A  shows a cross-section of a planar silicon substrate prepared for a trench fill in accordance with an embodiment of the present claimed invention. 
         FIG. 2B  shows the substrate of  FIG. 2A  with a second oxide coating in accordance with an embodiment of the present claimed invention. 
         FIG. 2C  shows the substrate of  FIG. 2B  with first polysilicon fills in accordance with an embodiment of the present claimed invention. 
         FIG. 2D  shows the substrate of  FIG. 2C  with the second oxide coating etched back to produce a gap between the substrate and the first polysilicon fill in accordance with an embodiment of the present claimed invention. 
         FIG. 2E  shows the substrate of  FIG. 2D  with second polysilicon fills in accordance with an embodiment of the present claimed invention. 
         FIG. 2F  shows the substrate of  FIG. 2E  with the oxide layer removed in accordance with an embodiment of the present claimed invention. 
         FIG. 3  shows a cross-section of a JFET structure with a composite trench fill in accordance with an embodiment of the present claimed invention. 
         FIG. 4  shows a flow chart for a method embodiment of the present claimed invention. 
         FIG. 5  shows a flow chart for a method embodiment of the present claimed invention using discrete polysilicon depositions. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the present invention, a method and structure for a composite trench, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances well known methods involving deposition and etch, photolithography, etc., and well known structures such as ohmic contacts and barrier metallization, etc., have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     U.S. Pat. No. 6,251,716 entitled “JFET Structure and Manufacture Method for Low On-Resistance and Low Voltage Application,” issued Jun. 26, 2001, and assigned to the assignee of the present invention, is hereby incorporated herein by reference. 
       FIG. 1A  shows a general schematic for an n-channel depletion mode JFET with V gs =V ds =0. The JFET has two opposed gate regions  10 , a drain  11  and source  12 . The drain  11  and source  12  are located in the n-doped region of the device and the gates  10  are p-doped. Two p-n junctions are present in the device, each having an associated depletion region  13 . A conductive channel region  14  is shown between the two depletion regions  13  associated with the p-n junctions. In operation, the voltage variable width of the depletion regions  13  is used to control the effective cross-sectional area the of conductive channel region  14 . The application of a voltage V gs  between the gates  10  and source  12  will cause the conductive channel region to vary in width, thereby controlling the resistance between the drain  11  and the source  12 . A reverse bias, (e.g., a negative V gs ), will cause the depletion regions to expand, and at a sufficiently negative value cause the conductive channel to “pinch off,” thereby turning off the device. 
     The width of the depletion regions  13  and the conductive channel region  14  are determined by the width of the n-doped region and the dopant levels in the n-doped and p-doped regions. If the device shown in  FIG. 1A  were constructed with a narrow n-doped region, such that the two depletion regions merged into a single continuous depletion region and the conductive channel region  14  had zero width, the result would be the device shown in FIG.  1 B. 
       FIG. 1B  shows a general schematic of an n-channel enhancement mode JFET with V gs =V ds =0. The enhancement mode device is normally “off” since the conductive channel width is zero due to the extent of the two depletion regions  13 B. The application of a sufficient forward bias (e.g. positive V gs ) to the device of  FIG. 1B  will cause the depletion regions  13 B to contract, thereby opening a conductive channel. 
     Although the depletion mode and enhancement mode devices shown schematically in FIG.  1 A and  FIG. 1B  are n-channel devices, depletion mode and enhancement mode devices could be constructed with a reversed doping scheme to provide p-channel devices. 
       FIG. 2A  shows a cross-section of a planar silicon substrate  200  prepared for a composite trench fill in accordance with an embodiment of the present claimed invention. The substrate  200  comprises an oxide layer  201  (e.g. thermally grown or deposited SiO 2 ), and a first layer of deposited polysilicon  202 . The oxide layer  201  and polysilicon layer  202  are preferably deposited prior to etching of the trenches  203 . The thickness of the oxide coating  201  is preferably 100 angstroms to 5000 angstroms, and the thickness of the polysilicon layer  202  is preferably 1000 angstroms to 3000 angstroms. 
       FIG. 2B  shows the substrate of  FIG. 2A  with a second oxide coating  204  in accordance with an embodiment of the present claimed invention. The second oxide coating may be thermally grown, or may be deposited by CVD or other deposition process. The second oxide coating  204  is preferably between 100 and 3000 angstroms in thickness. 
       FIG. 2C  shows the substrate of  FIG. 2B  with first polysilicon fills  205  in accordance with an embodiment of the present claimed invention. The polysilicon fills  205  are preferably made by depositing a second coating of polysilicon and subsequently etching back the deposited polysilicon so that the surface of the polysilicon remaining in the trenches is nominally at the same level as the surface of the substrate  200 . 
       FIG. 2D  shows the substrate of  FIG. 2C  with the second oxide coating  204  etched back to produce a gap between the substrate  200  and the first polysilicon fill  205  in accordance with an embodiment of the present claimed invention. As shown, the second oxide coating is etched back a distance D below the surface of the substrate  200  and the surface of the first polysilicon fill  205 . 
       FIG. 2E  shows the substrate of  FIG. 2D  with second polysilicon fills  206  in accordance with an embodiment of the present claimed invention. The polysilicon fills  206  are preferably made by depositing a layer of polysilicon in the range of 500 to 5000 angstroms and subsequently etching back the deposited polysilicon (polysilicon  202  is also removed) so that the gap between the substrate  200  and the first polysilicon fill  205  is filled, producing a continuous polysilicon bridge between the first polysilicon fill  205  and the substrate  200 . The combination of the first polysilicon fill  205  and the second polysilicon fill  206  provide an etch stop that protects the underlying oxide  204 . 
       FIG. 2F  shows the substrate of  FIG. 2E  with the oxide layer  201  removed, and the surface ready for further processing (e.g., top layer metallization). 
     In silicon electronic devices, oxide filled trenches are typically used for electrical isolation, and the substitution of a portion of the conventional trench oxide with metallization through process alignment error may lead to increased leakage current or a short circuit. 
       FIG. 3  shows a cross-section of a JFET structure with a composite trench fill in accordance with an embodiment of the present claimed invention. A planar n-type silicon substrate  300  is shown with n +  contact surfaces on top and bottom for source and drain  311 , respectively. Source metallization  305  collects the current that flows through the channel region  314  that is bounded by the gate regions  310 . 
     A composite trench fill comprising an oxide fill  304 , a first polysilicon fill  305 , and a second polysilicon fill  306  is shown. The composite trench fill provides a continuous polysilicon surface that supports an oxide layer  301 . As can be seen in  FIG. 3 , the etch stop capability of the composite trench fill allows the source contact metallization to be designed for maximum contact area without concern for inadvertent oxide etch, since there is not exposed oxide in the trench fill. For shallow trenches, accurate planarization is more readily achieved when working with a homogeneous surface, as opposed to a heterogeneous polysilicon/oxide surface. In addition, the homogeneous polysilicon surface may be etched (e.g., wet HF or plasma) prior to metallization without damaging exposed oxide dielectric. 
       FIG. 4  shows a flow chart  400  for a method embodiment of the present claimed invention. This fabrication method applies to a trench in a silicon substrate such as that shown in FIG.  2 A. In step  405 , an oxide coating is formed on the trench wall and bottom, as shown in FIG.  2 B. In step  410 , the oxide coating is etched back to expose the upper portion of the trench wall. This may be done by low angle ion milling of the substrate under rotation, or other process. In step  415 , polysilicon is deposited to completely fill the trench. This typically will involve an overfill. In step  420  the excess polysilicon is removed to level the polysilicon fill with surface of the substrate. This may be done by etching, chemical mechanical polishing (CMP) or other process. 
       FIG. 5  shows a flow chart for a method embodiment of the present claimed invention using discrete polysilicon depositions. The method shown  FIG. 5  is more involved than the method shown in  FIG. 4 ; however, the more involved method provides better dimension control. 
     In step  505 , a first oxide coating is formed on a silicon substrate. The first oxide coating may be thermally grown, or it may be deposited by chemical vapor deposition (CVD) or other deposition process. 
     In step  510 , a first polysilicon coating is formed on top of the first oxide coating. This coating is preferably formed using CVD. 
     In step  515 , a trench is etched into the silicon substrate through the first oxide coating and the first polysilicon coating. The result of this step is shown in FIG.  2 A. 
     In step  520 , a second oxide coating is formed on the surface of the substrate and on the wall and bottom of the trench, as shown in FIG.  2 B. 
     In step  525 , a second polysilicon coating is formed on the first oxide coating, filling the remainder of the trench. This step is preferably done by CVD and may involve a degree of overfill. 
     In step  530 , the excess polysilicon from the second polysilicon coating is removed to create a first polysilicon fill that is nominally level with the surface of the substrate as shown in FIG.  2 C. This step is preferably done with a reactive ion etch. 
     In step  535 , a portion of the first oxide coating is etched back below the surface of the substrate to create a gap between the silicon substrate and the first polysilicon fill. This etch effectively creates a “moat” around the first polysilicon fill with a depth D as shown in FIG.  2 D. Depth D is preferably less than 5000 angstroms. 
     In step  540 , a third polysilicon coating is formed to fill the gap (moat) between the silicon substrate and the first polysilicon fill. This step is preferably done by CVD and may involve a degree of overfill. 
     In step  545 , the excess polysilicon from the third polysilicon coating is removed to create a second polysilicon that is nominally level with the surface of the substrate as shown in FIG.  2 C. This step is preferably done with a reactive ion etch as shown by the structure of  FIG. 2E ; however, mechanical polishing or CMP may be used to fully planarize the surface. 
     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 to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications 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.