Abstract:
A metal oxide semiconductor field effect transistor (MOSFET) power device with multi gates connection includes a first-conductive type substrate, a first-conductive type epitaxial layer arranged on the first-conductive type substrate, a plurality of device trenches defined on an upper face of the first-conductive type epitaxial layer. Each of the device trenches has, from bottom of the trench to top of the trench, a bottom gate, a split gate and a trench gate. A bottom insulating layer is formed between the bottom gate and the bottom of the trench, an intermediate insulating layer is formed between the bottom gate and the split gate, an upper insulating layer is formed between the split gate and the trench gate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    The present invention relates to a metal oxide semiconductor field effect transistor (MOSFET) power device, especially to a MOSFET power device with multi gates connection. 
         [0003]    Description of Prior Art 
         [0004]    Metal oxide semiconductor field effect transistor (MOSFET) power device is a field effect transistor with extensive applications in analog and digital circuits and is a main stream device for power device in power electronic usage. The MOSFET power device has low power dissipation due to very low conduction resistance and high input impedance. In comparison with power bipolar transistor, the MOSFET power device further has the advantage of high switching speed for its single carrier nature (no minority carrier). For now, MOSFET power devices are popular for high frequency and low voltage applications. 
         [0005]    To further increase device density and reduce on resistance for device, MOSFET power devices with trench gate structure are important issues. However, the gate-drain charge (Qgd) increases as the device density increases; therefore and the charging-discharging speed of gate is affected. Even though split gate structure is developed to reduce gate-drain area and reduce gate-drain capacitance. The gate-drain capacitance of the MOSFET power devices still needs improvements. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to provide a metal oxide semiconductor field effect transistor (MOSFET) power device with reduced capacitance. 
         [0007]    Accordingly, the present invention provides a metal oxide semiconductor field effect transistor (MOSFET) power device with multi gates connection, comprising: a first-conductive type substrate; a first-conductive type epitaxial layer arranged on the first-conductive type substrate; and a plurality of device trenches defined on an upper face of the first-conductive type epitaxial layer, each of the device trenches having, from bottom of the trench to top of the trench, a bottom gate, a split gate and a trench gate, wherein a bottom insulating layer is formed between the bottom gate and the first-conductive type epitaxial layer, an intermediate insulating layer is formed between the bottom gate and the split gate, and an upper insulating layer is formed between the split gate and the trench gate. 
         [0008]    Accordingly, the present invention provides a method for manufacturing metal oxide semiconductor field effect transistor (MOSFET) power device with multi gates connection, comprising: providing a first-conductive type substrate and a first-conductive type epitaxial layer arranged on the first-conductive type substrate; defining a plurality of device trenches defined on an upper face of the first-conductive type epitaxial layer, each of the device trenches having, from bottom of the trench to top of the trench, a bottom gate, a split gate and a trench gate, wherein a bottom insulating layer is formed between the bottom gate and the first-conductive type epitaxial layer, an intermediate insulating layer is formed between the bottom gate and the split gate, and an upper insulating layer is formed between the split gate and the trench gate. 
         [0009]    The gate-source area of the MOSFET power device with multi gates connection according to the present invention can be reduced because the bottom gate is electrically isolated with other elements. The capacitance and the resistance of the MOSFET power device can be reduced to enhance operation bandwidth. 
     
    
     
       BRIEF DESCRIPTION OF DRAWING 
         [0010]    One or more embodiments of the present disclosure are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements. These drawings are not necessarily drawn to scale. 
           [0011]      FIGS. 1 to 9  show the sectional views for illustrating the manufacture process for the metal oxide semiconductor field effect transistor (MOSFET) power device with multi gates connection of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    As shown in  FIG. 1 , a substrate structure is provided, which includes a heavily-doped N type silicon substrate  101  (N+ silicon substrate) and a lightly-doped doped N type silicon epitaxial layer  102  (N− silicon epitaxial layer). In the shown embodiment, the lightly-doped doped N type silicon epitaxial layer  102  is drawn to be thicker than the heavily-doped N type silicon substrate  101 . However, in practical device, the lightly-doped doped N type silicon epitaxial layer  102  can be thinner than the heavily-doped N type silicon substrate  101  and the scope of the present invention is not limited by the shown embodiment. A plurality of photoresist patterns (not shown) are formed by using photolithography process and the photoresist patterns are used as etching masks to define a plurality of device trenches  200  and at least one termination trench  300  on the lightly-doped doped N type silicon epitaxial layer  102 . The device trenches  200  on the left side of the dashed line in  FIG. 1  are corresponding to the device region of the MOSFET power device and the termination trench  300  on the right side of the dashed line in  FIG. 1  are corresponding to the termination region of the MOSFET power device. After the formation of the trenches  200 ,  300 , an optional sacrificial oxidation can be performed, namely by forming a thin oxide layer and then performing an oxide etching step, the damaged surface of the trenches  200 ,  300  can be removed to make the sidewall of trenches  200 ,  300  become smooth. As also shown in  FIG. 1 , a thermal oxidation process is performed for the lightly-doped doped N type silicon epitaxial layer  102  formed with the trenches  200 ,  300  to form an oxide layer  30 , which is arranged on inner wall of the trenches  200 ,  300  and the exposed surface of the lightly-doped doped N type silicon epitaxial layer  102 . The thickness of the oxide layer  30  is, for example, 3000-6000 angstrom (Å). Moreover, the oxide layer  30  can also be formed by deposition instead of thermal oxidation. 
         [0013]    As shown in  FIG. 2 , a polysilicon layer  20 A is formed atop the oxide layer  30  to fill the trenches  200 ,  300  and cover the lightly-doped doped N type silicon epitaxial layer  102 . The thickness of the polysilicon layer  20 A, counted from an upper face of the oxide layer  30  on the lightly-doped doped N type silicon epitaxial layer  102 , is for example 1.5-2.5 um. 
         [0014]    As shown in  FIG. 3 , after forming the polysilicon layer  20 A, an etching back process (such as a dry etching process) is performed to remove part of the polysilicon layer  20 A until no polysilicon layer  20 A is present in termination trench  300  and part of polysilicon layer  20 A is present in device trenches  200 . As also shown in  FIG. 3 , after the etching back process, part of polysilicon layer  20 A remains in device trench  200 , which will be used as a bottom gate  20  in the MOSFET power device of the present invention. Moreover, the part of the oxide layer  30  between the bottom gate  20  and the N type silicon epitaxial layer  102  is a bottom insulating layer  32 . 
         [0015]    As shown in  FIG. 4 , an oxidation process, such as Tetraethyl Orthosilicate (LPTEOS) process or CVD process, is further conducted to form a deposited oxide layer  22 A, which is formed atop the bottom gate  20  and fills the trenches  200 ,  300  as well as is formed atop the oxide layer  30  on the N type silicon epitaxial layer  102 . The thickness of the deposited oxide layer  22 A, counted from an upper face of the oxide layer  30  on the lightly-doped doped N type silicon epitaxial layer  102 , is for example 1000-3000 angstrom. Moreover, as shown in  FIG. 5 , a CMP process is conducted to remove the part of the deposited oxide layer  22 A and the part of the oxide layer  30  on the upper face of the N type silicon epitaxial layer  102  such that the followed etching step for the oxide layer can be better controlled. 
         [0016]    As shown in  FIG. 6 , a dry etching step is then performed to remove the part of the deposited oxide layer  22 A in the trenches  200 ,  300  until a layer of oxide remains atop the bottom gate  20 , which functions as an intermediate insulating layer  34  between the bottom gate  20  and a split gate (not shown) to be formed. 
         [0017]    As shown in  FIG. 7 , steps similar to those shown in  FIGS. 2-6  are performed. Namely, a polysilicon layer with thickness of 2-3 um is grown and etched back until the polysilicon layer only remains in the device trenches  200 . As also shown in  FIG. 7 , a polysilicon layer remains atop the intermediate insulating layer  34  in the device trench  200 , which will function as split gate  22 . Afterward, an oxidation process, such as Tetraethyl Orthosilicate (LPTEOS) process or CVD process, is further conducted to form a deposited oxide layer (not labeled). Moreover, a CMP process is conducted to remove the part of the deposited oxide layer on the upper face of the N type silicon epitaxial layer  102 , and a dry etching step is performed to remove the part of the deposited oxide layer in the trenches  200 ,  300  until a layer of oxide remains atop the split gate  22 , which functions as an upper insulating layer  36  between the split gate  22  and a trench gate (not shown) to be formed. 
         [0018]    As shown in  FIG. 8 , a polysilicon layer with thickness of 2-3 um is grown and etched back until the polysilicon layer only remains in the device trenches  200 . In  FIG. 8 , a remained polysilicon layer functioning as trench gate  24  is placed atop the upper insulating layer  36 . Afterward, an etching back step for oxide is performed. 
         [0019]    As shown in  FIG. 9 , after forming the trench gate  24 , ion implantation and driving-in processes are performed to form P body area  40  and N type source regions  42 , which are near the upper face of the N type silicon epitaxial layer  102  and outside the device trench  200 . Afterward, interlayer dielectric (ILD) layer  44  is formed atop the resulting structure and then photolithography process is performed on the ILD layer to define source trench  400 . Contact metal layer  46  is then formed atop the source trench  400 , and the contact metal layer  46  can be Ti or TiN layer such that silicide can be formed between a later-formed metal electrode and the underlying silicon layer to reduce electrical resistance. After forming the metal contact layer  46 , a metal electrode layer  48  and a passivation layer (not shown) are respectively formed. 
         [0020]    With reference again to  FIG. 9 , this figure also shows a sectional view of the MOSFET power device with multi gates connection of the present invention. The MOSFET power device comprises an N type substrate structure  100  (including a heavily-doped N type silicon substrate  101  and a lightly-doped doped N type silicon epitaxial layer  102 ), a plurality of device trenches  200  in the device region, and at least one termination trench  300  in the termination region. Moreover, the MOSFET power device further comprises, in each device trench  200  and from the bottom to the top of the trench, a bottom gate  20 , a split gate  22  and a trench gate  24 , where a bottom insulating layer  32  is placed between the bottom gate  20  and the lightly-doped doped N type silicon epitaxial layer  102 , an intermediate insulating layer  34  is placed between the bottom gate  20  and the split gate  22 , and an upper insulating layer  36  is placed between the split gate  22  and the trench gate  24 . Moreover, the MOSFET power device further comprises a P body area  40  and N type source regions  42 , which are near the upper face of the N type silicon epitaxial layer  102  and outside the device trench  200 . The N type source regions  42  are placed within the P body area  40 . Moreover, the MOSFET power device further comprises gate oxide layer  38  between the trench gate  24  in the device trench  200  and the N type source region  42  outside the device trench  200 . Moreover, the MOSFET power device further comprises source trenches  400 , each between the adjacent device trenches  200  and ILD layer  44  beside the source trench  400  and atop the trench gate  24  and the N type source regions  42 . The MOSFET power device further comprises a metal contact layer  46  on inner wall of the source trench  400  and atop the ILD layer  44 , and comprises a metal electrode layer  48  atop the metal contact layer  46  to function as source electrode. 
         [0021]    In the MOSFET power device shown in  FIG. 9 , the trench gate  24  electrically connects with gate electrode (not shown) to obtain operation voltage, and the split gate  22  electrically connects with the N type source regions  42  through buried-in electrode (not shown). Moreover, the bottom gate  20  is electrically isolated with the split gate  22  through the intermediate insulating layer  34  therebetween and is not electrically connected with other elements of the MOSFET power device. By the provision of the bottom gate  20 , the gate-drain area can be further reduced such that the equivalent capacitance and equivalent resistance of the MOSFET power device can be further reduced to enhance the operation bandwidth. 
         [0022]    The person skilled in the art can know other implementations are also feasible for above-mentioned embodiment. For example, the N type substrate structure  100  can be replaced with P type substrate structure, and correspondingly the N type source regions  42  are replaced with P type source regions, and the P body area  40  is replaced with N body area. 
         [0023]    Thus, particular embodiments have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims may be performed in a different order and still achieve desirable results.