US20070040214A1 - Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and Miller charge - Google Patents
Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and Miller charge Download PDFInfo
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- US20070040214A1 US20070040214A1 US11/502,594 US50259406A US2007040214A1 US 20070040214 A1 US20070040214 A1 US 20070040214A1 US 50259406 A US50259406 A US 50259406A US 2007040214 A1 US2007040214 A1 US 2007040214A1
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- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- H01L29/76—Unipolar devices, e.g. field effect transistors
- H01L29/772—Field effect transistors
- H01L29/78—Field effect transistors with field effect produced by an insulated gate
- H01L29/7801—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
- H01L29/7802—Vertical DMOS transistors, i.e. VDMOS transistors
- H01L29/7813—Vertical DMOS transistors, i.e. VDMOS transistors with trench gate electrode, e.g. UMOS transistors
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/40—Electrodes ; Multistep manufacturing processes therefor
- H01L29/402—Field plates
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/40—Electrodes ; Multistep manufacturing processes therefor
- H01L29/402—Field plates
- H01L29/407—Recessed field plates, e.g. trench field plates, buried field plates
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- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/66007—Multistep manufacturing processes
- H01L29/66075—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials
- H01L29/66227—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
- H01L29/66409—Unipolar field-effect transistors
- H01L29/66477—Unipolar field-effect transistors with an insulated gate, i.e. MISFET
- H01L29/66674—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
- H01L29/66712—Vertical DMOS transistors, i.e. VDMOS transistors
- H01L29/66734—Vertical DMOS transistors, i.e. VDMOS transistors with a step of recessing the gate electrode, e.g. to form a trench gate electrode
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- H01L29/70—Bipolar devices
- H01L29/72—Transistor-type devices, i.e. able to continuously respond to applied control signals
- H01L29/739—Transistor-type devices, i.e. able to continuously respond to applied control signals controlled by field-effect, e.g. bipolar static induction transistors [BSIT]
- H01L29/7393—Insulated gate bipolar mode transistors, i.e. IGBT; IGT; COMFET
- H01L29/7395—Vertical transistors, e.g. vertical IGBT
- H01L29/7396—Vertical transistors, e.g. vertical IGBT with a non planar surface, e.g. with a non planar gate or with a trench or recess or pillar in the surface of the emitter, base or collector region for improving current density or short circuiting the emitter and base regions
- H01L29/7397—Vertical transistors, e.g. vertical IGBT with a non planar surface, e.g. with a non planar gate or with a trench or recess or pillar in the surface of the emitter, base or collector region for improving current density or short circuiting the emitter and base regions and a gate structure lying on a slanted or vertical surface or formed in a groove, e.g. trench gate IGBT
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- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- H01L29/70—Bipolar devices
- H01L29/74—Thyristor-type devices, e.g. having four-zone regenerative action
- H01L29/749—Thyristor-type devices, e.g. having four-zone regenerative action with turn-on by field effect
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- H01L29/40—Electrodes ; Multistep manufacturing processes therefor
- H01L29/41—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions
- H01L29/417—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions carrying the current to be rectified, amplified or switched
- H01L29/41725—Source or drain electrodes for field effect devices
- H01L29/41741—Source or drain electrodes for field effect devices for vertical or pseudo-vertical devices
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- H01L29/40—Electrodes ; Multistep manufacturing processes therefor
- H01L29/41—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions
- H01L29/423—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions not carrying the current to be rectified, amplified or switched
- H01L29/42312—Gate electrodes for field effect devices
- H01L29/42316—Gate electrodes for field effect devices for field-effect transistors
- H01L29/4232—Gate electrodes for field effect devices for field-effect transistors with insulated gate
- H01L29/42356—Disposition, e.g. buried gate electrode
- H01L29/4236—Disposition, e.g. buried gate electrode within a trench, e.g. trench gate electrode, groove gate electrode
Definitions
- the present invention is directed to semiconductor devices and, more particularly, to a trench MOSFET with reduced Miller capacitance having improved switching speed characteristics.
- the power MOSFETS which are used as the upper and the lower switches in a DC-DC converter must have both low switching power losses and low conduction power losses. The switching losses can be reduced by lowering on-resistance. Unfortunately, lowering the on-resistance raises the Miller capacitance.
- the most efficient way is to reduce the device cell pitch and increase the total channel width. Both of these result in an increase of the drain-gate overlay area. As the consequence, the device's Miller capacitance, or Miller charge increases.
- a power device having features of the present invention comprises a first substrate layer that is highly doped with a dopant of a first conductivity type, forming a drain. Over this first layer is a second layer that is lightly doped with the same conductivity dopant as the first layer. Above this second layer is a third layer, doped with a second conductivity dopant that is opposite in polarity to the first conductivity type. A fourth layer highly doped with the first conductivity dopant, is on the opposite surface of the semiconductor substrate. A trench extends from this fourth layer, into the second layer. This trench divides the fourth layer into a plurality of source regions. The trench also has sidewalls adjacent to the third and fourth layers for controlling a channel layer. Finally, this trench also has upper and lower conductive layers that are separated by a dielectric layer.
- the upper conductive layer in the trench forms a gate electrode for controlling current through a channel adjacent the sidewall of the trench.
- the polysilicon gate layer, the polysilicon shield layer and the interlevel dielectric layer are suitably sized so that the bottom of the polysilicon gate layer is proximate the curvature of the well region. This will minimize the overlap between the gate and drain, and therefore minimize the gate-to-drain capacitance.
- the device originally described can have a source metal layer over the device and in electrical contact with the fourth layer, so that it is in contact with the source regions.
- This source metal also is in contact with the third layer doped with the second conductivity type.
- This metal layer will also be in contact with the lower conductive layer of the trench at peripheral locations around the cells of the device.
- the lower conductive layer, or shield layer will be at the same electrostatic potential as the source. Now the capacitance at the bottom of the trench is no longer gate-to-drain it is now gate-to-source, because the shield is tied to the source.
- FIG. 1 is a cross-section of a DMOS cell structure.
- FIG. 1A is a table showing comparative results.
- FIG. 2 is a first graph showing gate voltage as a function of gate charge for new and conventional 0.2 micron cell pitch devices.
- FIG. 3 is a second graph similar to FIG. 2 for 0.3 micron cell pitch devices.
- FIG. 4 is an n-type doped epi-layer grown on the N+ substrate.
- FIG. 5 is pad oxidation followed by the Boron implant and annealing.
- FIG. 6 is LTO oxide deposition, followed by the LTO pattern definition by using photo-mask/etch steps.
- FIG. 7 is silicon etch to form the trench structure.
- FIG. 8 is LTO removal followed by Sacrificial Oxidation and Gate oxidation.
- FIG. 9 is polysilicon fill.
- FIG. 10 is polysilicon recess etch forming the polysilicon shield layer.
- FIG. 11 is IDL (SiN) deposition
- FIG. 12 is IDL (SiN) recess etch.
- FIG. 13 is polysilicon refill.
- FIG. 14 is polysilicon recess etch forming the polysilicon gate.
- FIG. 15 is IDL (BPSG) deposition.
- FIG. 16 is IDL (BPSG) etch back and planarization.
- FIG. 17 is boron/phosphorous implant and drive to form the P+ body/N+ source.
- FIG. 18 is BOE dip, followed by Ti/TiN barrier layer formation and Metal deposition (SiAl).
- FIG. 19 is a section taken through the center of the trench of FIG. 18 .
- the new device structure is shown in FIG. 1 .
- This new device 10 includes a buried polysilicon shield layer 38 between the polysilicon gate 34 and the drain terminal 12 .
- the polysilicon shield layer 38 contacts the source metal 20 at periphery of the die, and has the same electrostatic potential as the Source Metal 20 . See FIG. 19 .
- the gate and drain displacement currents flow through the source 18 and the polysilicon shield 38 during the switching transient, and the feedback between the drain terminal and the gate terminal during the gate turn on/off transients is greatly suppressed.
- a much lower Miller Charge can be obtained with minimum sacrifice of the on-resistance.
- the contradiction between the on-resistance and Miller Charge can be depressed significantly.
- the electrical characteristics of both the new device and the conventional trench-gated power MOSFET were evaluated by using the computer simulations. The results are summarized in FIG. 1A .
- the new device 10 disclosed in this invention has much less Miller capacitance, Cgd, with a small increase of the on-resistance when compared to the conventional device without a shield.
- Cgd Miller capacitance
- the new device achieves about 84% reduction of gate-to-drain capacitance (Cgd) with only 18% increase of the on-resistance per unit area, RSP, when compared to the conventional device.
- the figure of merit, RSP*Cgd is defined as the product of on-resistance and Miller capacitance.
- Results show about 80% improvement when comparing the new device to the conventional device.
- the gate voltage vs. gate charge characteristics of both devices are given in FIGS. 2 and 3 , respectively for 2.0 ⁇ m cell pitch and 3.0 ⁇ m cell pitch. These figures demonstrate that the new device disclosed in this invention has dramatically reduced the Miller charge, which is represented by the width of plateau portion in the curves of the gate voltage vs. gate charge. Additionally, because of the reduction in gate-to-drain capacitance, the device can be made very dense without sacrificing switching speed.
- FIG. 1 is a cross-sectional view of a single cell of a multi-cellular trench gate device 10 of the present invention.
- the trench gate device 10 may be used in connection with power devices such as DMOS devices as well as other power devices, including insulated gate bipolar transistors (IGBTs) and MOS gated thyristors.
- IGBTs insulated gate bipolar transistors
- MOS gated thyristors MOS gated thyristors.
- the cellular pattern shown in FIG. 1 is repeated by having multiple cells, all of which are divided by the trench gate.
- the cellular structure of a typical MOSFET 10 includes a substrate 12 that has a highly doped drain or N+ region. Over the substrate 12 there is a more lightly doped epitaxial layer 14 of the same doping polarity. Above the epitaxial layer 14 is a well region 16 formed of opposite or P-type doping. Covering the P-wells 16 is an upper source layer 18 that is heavily N-type doped.
- the trench structure 30 includes a sidewall oxide 32 or other suitable insulating material that covers the sidewalls of the trench 30 . The bottom of the trench 30 is filled with a polysilicon shield 38 . An interlevel dielectric such as silicon nitride 36 covers the shield 38 .
- the gate 34 is formed by another layer of highly doped polysilicon.
- a second interlevel dielectric 33 typically borophosphosilicate glass (BPSG) covers the gate 34 . In operation, current flows vertically between the source 18 and the drain 12 through a channel in the well 16 when a suitable voltage is applied to the gate 34 .
- the device 10 is formed by the series of process steps shown in FIGS. 4-17 .
- a suitable substrate 8 is highly doped to form an N+ region 12 .
- an epitaxial layer 14 is grown on the other surface of the substrate 8 .
- the epitaxial layer 14 is either grown as a lightly doped N ⁇ layer or is then suitably doped as N ⁇ after it is grown.
- the epitaxial layer 14 is shown larger than the substrate, the difference in size is strictly for purposes of explaining the invention. Those skilled in the art understand that the substrate is normally significantly greater in thickness than is the epitaxial layer.
- FIG. 5 shows boron ions 40 implanted into the upper surface of the epitaxial layer 14 to form a latent P-well 16 in FIG. 6 .
- FIG. 6 also shows the surface of the epitaxial layer covered with a low-temperature oxide 42 that is patterned to expose future trench regions. Trenches are etched as shown in FIG. 7 .
- the trenches 30 may be etched by any suitable process, including a wet etch or a dry plasm etch. Such etching is conventional and is known to those skilled in the art.
- the entire wafer is subjected to an oxidation process in order to grow a relatively thin gate oxide layer 32 over the entire wafer, including the sidewalls of the trench 30 .
- the low-temperature oxide mask Prior to growing the gate oxide layer, the low-temperature oxide mask is stripped. The oxidation step is performed at a temperature high enough to drive in the boron implants and form well region 16 .
- FIG. 9 the entire wafer is covered with a polysilicon layer 38 that fills the trench 30 .
- the polysilicon layer 38 is etched away until it leaves a residual polysilicon shield 38 in the bottom of the trenches 30 , as shown in FIG. 10 .
- FIG. 11 an interlevel dielectric layer 36 is formed from silicon nitride or other suitable insulator over the entire wafer, including in the trenches 30 .
- the interlevel dielectric layer 36 is suitably etched, as shown in FIG. 12 , to leave a layer within the trenches and covering the polysilicon shield layer 38 .
- FIG. 12 the interlevel dielectric layer 36 is suitably etched, as shown in FIG. 12 , to leave a layer within the trenches and covering the polysilicon shield layer 38 .
- the polysilicon gate includes highly doped polysilicon so as to provide a conductive gate material. Other materials are possible, but polysilicon is the preferred material.
- the polysilicon gate layer and the polysilicon shield layer 38 and the interlevel dielectric layer 36 are suitably sized so that the bottom of the polysilicon gate layer 34 is proximate the curvature of the well region 16 . This will minimize the overlap between the gate and drain, and therefore minimize the gate-to-drain capacitance.
- the polysilicon gate layer 34 is etched in order to provide room in the trench for an interlevel dielectric layer.
- a suitable IDL layer 33 as shown in FIG. 15 , comprising BPSG is deposited over the entire wafer 10 and into the remainder of the trenches.
- the BPSG layer 33 is planarized, as shown in FIG. 16 , to approximately the same level as the top of the epitaxial layer 14 .
- the entire surface of the device is subjected to a suitable N+ source implant such as phosphorous or arsenic.
- the source implant is suitably driven into an appropriate depth in order to form the source regions 18 as shown in FIG. 17 .
- the wafer is then covered with a layer of metal 20 that forms the source metal which is then suitably patterned and finished to create the device.
- a section 19 - 19 ′ is taken through the center of the trench of FIG. 18 .
- a portion of the resulting profile at the edge of the wafer is shown in FIG. 19 .
- the polysilicon shield 38 is in electrical and mechanical contact with the source 20 at the die periphery. As a result, the polysilicon shield layer 38 is electrically shorted to the source metal 20 . This source metal is also in contact with the third layer. (not shown)
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Abstract
The cellular structure of the power device includes a substrate that has a highly doped drain region. Over the substrate there is a more lightly doped epitaxial layer of the same doping. Above the epitaxial layer is a well region formed of an opposite type doping. Covering the wells is an upper source layer of the first conductivity type that is heavily doped. The trench structure includes a sidewall oxide or other suitable insulating material that covers the sidewalls of the trench. The bottom of the trench is filled with a doped polysilicon shield. An interlevel dielectric such as silicon nitride covers the shield. The gate region is formed by another layer of doped polysilicon. A second interlevel dielectric, typically borophosphosilicate glass (BPSG) covers the gate. In operation, current flows vertically between the source and the drain through a channel in the well when a suitable voltage is applied to the gate.
Description
- This application is a continuation of U.S. patent application Ser. No. 11/178,215 filed Jul. 8, 20005 which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/274,760, filed Mar. 9, 2001 (expired), U.S. patent application Ser. No. 10/092,692 filed Mar. 7, 2002, now U.S. Pat. No. 6,683,346 and U.S. patent application Ser. No. 10/678,444 filed Oct. 1, 2003, now U.S. Pat. No. 6,929,988.
- The present invention is directed to semiconductor devices and, more particularly, to a trench MOSFET with reduced Miller capacitance having improved switching speed characteristics.
- The semiconductor industry is witnessing an increasing demand for low-output-voltage DC-DC converters with very fast transient response and higher power efficiency for high frequency power conversion applications. When the operation frequency reaches 1 MHz or even higher, the power losses of a synchronous buck DC-DC converter will be dominated by the switching losses. Switching losses in a power MOSFET occur during charging/discharging the drain-gate feedback capacitance. The corresponding gate charge is called Miller Charge. Thus, the reduction of Miller capacitance is one of most important focus to improve DC-DC converter efficiency.
- Also, as the cell density and speed of a microprocessor increases, more current is needed to power the microprocessor. This means that the DC-DC converter is required to provide a higher output current. The increase of the output current raises the conduction loss of not only the lower switches but also the upper switches in synchronous DC-DC converter. Therefore, in order to power an advanced microprocessor, the power MOSFETS, which are used as the upper and the lower switches in a DC-DC converter must have both low switching power losses and low conduction power losses. The switching losses can be reduced by lowering on-resistance. Unfortunately, lowering the on-resistance raises the Miller capacitance. For example, in order to reduce the on-resistance of a power MOSFET, the most efficient way is to reduce the device cell pitch and increase the total channel width. Both of these result in an increase of the drain-gate overlay area. As the consequence, the device's Miller capacitance, or Miller charge increases.
- Due to gate to drain capacitance's significant impact on device switching speed, a series of improvements for minimizing it's impact have been proposed. These improvements include tailoring of source-drain ion implant angles and gate spacers, in order to obtain sufficient gate overlap of source-drains for maintaining low channel resistance, while still minimizing the associated capacitance values. One such effort to minimize Miller capacitance is a process step that locally increases the gate oxide thickness in the region of gate to drain overlap. However, that process is difficult to control because you need to maintain overlap while growing the thick oxide in the bottom of the trench and etching back. Therefore, what is needed is a method that will achieve low switching power losses and low conductivity power losses.
- The present invention is directed towards a power device that has low switching power losses and low conductivity power losses. A power device having features of the present invention comprises a first substrate layer that is highly doped with a dopant of a first conductivity type, forming a drain. Over this first layer is a second layer that is lightly doped with the same conductivity dopant as the first layer. Above this second layer is a third layer, doped with a second conductivity dopant that is opposite in polarity to the first conductivity type. A fourth layer highly doped with the first conductivity dopant, is on the opposite surface of the semiconductor substrate. A trench extends from this fourth layer, into the second layer. This trench divides the fourth layer into a plurality of source regions. The trench also has sidewalls adjacent to the third and fourth layers for controlling a channel layer. Finally, this trench also has upper and lower conductive layers that are separated by a dielectric layer.
- According to another aspect of the invention, the upper conductive layer in the trench forms a gate electrode for controlling current through a channel adjacent the sidewall of the trench. The polysilicon gate layer, the polysilicon shield layer and the interlevel dielectric layer are suitably sized so that the bottom of the polysilicon gate layer is proximate the curvature of the well region. This will minimize the overlap between the gate and drain, and therefore minimize the gate-to-drain capacitance.
- According to still another aspect of the invention, the device originally described can have a source metal layer over the device and in electrical contact with the fourth layer, so that it is in contact with the source regions. This source metal also is in contact with the third layer doped with the second conductivity type. This metal layer will also be in contact with the lower conductive layer of the trench at peripheral locations around the cells of the device. The lower conductive layer, or shield layer, will be at the same electrostatic potential as the source. Now the capacitance at the bottom of the trench is no longer gate-to-drain it is now gate-to-source, because the shield is tied to the source.
-
FIG. 1 is a cross-section of a DMOS cell structure. -
FIG. 1A is a table showing comparative results. -
FIG. 2 is a first graph showing gate voltage as a function of gate charge for new and conventional 0.2 micron cell pitch devices. -
FIG. 3 is a second graph similar toFIG. 2 for 0.3 micron cell pitch devices. -
FIG. 4 is an n-type doped epi-layer grown on the N+ substrate. -
FIG. 5 is pad oxidation followed by the Boron implant and annealing. -
FIG. 6 is LTO oxide deposition, followed by the LTO pattern definition by using photo-mask/etch steps. -
FIG. 7 is silicon etch to form the trench structure. -
FIG. 8 . is LTO removal followed by Sacrificial Oxidation and Gate oxidation. -
FIG. 9 is polysilicon fill. -
FIG. 10 is polysilicon recess etch forming the polysilicon shield layer. -
FIG. 11 is IDL (SiN) deposition; -
FIG. 12 is IDL (SiN) recess etch. -
FIG. 13 is polysilicon refill. -
FIG. 14 is polysilicon recess etch forming the polysilicon gate. -
FIG. 15 is IDL (BPSG) deposition. -
FIG. 16 is IDL (BPSG) etch back and planarization. -
FIG. 17 is boron/phosphorous implant and drive to form the P+ body/N+ source. -
FIG. 18 is BOE dip, followed by Ti/TiN barrier layer formation and Metal deposition (SiAl). -
FIG. 19 is a section taken through the center of the trench ofFIG. 18 . - The new device structure is shown in
FIG. 1 . Thisnew device 10 includes a buriedpolysilicon shield layer 38 between thepolysilicon gate 34 and thedrain terminal 12. Thepolysilicon shield layer 38 contacts thesource metal 20 at periphery of the die, and has the same electrostatic potential as theSource Metal 20. SeeFIG. 19 . As the consequence, the gate and drain displacement currents flow through thesource 18 and thepolysilicon shield 38 during the switching transient, and the feedback between the drain terminal and the gate terminal during the gate turn on/off transients is greatly suppressed. Making use of this new device structure, a much lower Miller Charge can be obtained with minimum sacrifice of the on-resistance. Furthermore, the contradiction between the on-resistance and Miller Charge can be depressed significantly. The electrical characteristics of both the new device and the conventional trench-gated power MOSFET were evaluated by using the computer simulations. The results are summarized inFIG. 1A . Thenew device 10 disclosed in this invention has much less Miller capacitance, Cgd, with a small increase of the on-resistance when compared to the conventional device without a shield. For example, for the device with cell pitch 2.0 μm, the new device achieves about 84% reduction of gate-to-drain capacitance (Cgd) with only 18% increase of the on-resistance per unit area, RSP, when compared to the conventional device. The figure of merit, RSP*Cgd, is defined as the product of on-resistance and Miller capacitance. Results show about 80% improvement when comparing the new device to the conventional device. The gate voltage vs. gate charge characteristics of both devices are given inFIGS. 2 and 3 , respectively for 2.0 μm cell pitch and 3.0 μm cell pitch. These figures demonstrate that the new device disclosed in this invention has dramatically reduced the Miller charge, which is represented by the width of plateau portion in the curves of the gate voltage vs. gate charge. Additionally, because of the reduction in gate-to-drain capacitance, the device can be made very dense without sacrificing switching speed. -
FIG. 1 is a cross-sectional view of a single cell of a multi-cellulartrench gate device 10 of the present invention. Those skilled in the art will understand that thetrench gate device 10 may be used in connection with power devices such as DMOS devices as well as other power devices, including insulated gate bipolar transistors (IGBTs) and MOS gated thyristors. In such devices the cellular pattern shown inFIG. 1 is repeated by having multiple cells, all of which are divided by the trench gate. - The cellular structure of a
typical MOSFET 10 includes asubstrate 12 that has a highly doped drain or N+ region. Over thesubstrate 12 there is a more lightly dopedepitaxial layer 14 of the same doping polarity. Above theepitaxial layer 14 is awell region 16 formed of opposite or P-type doping. Covering the P-wells 16 is anupper source layer 18 that is heavily N-type doped. Thetrench structure 30 includes asidewall oxide 32 or other suitable insulating material that covers the sidewalls of thetrench 30. The bottom of thetrench 30 is filled with apolysilicon shield 38. An interlevel dielectric such assilicon nitride 36 covers theshield 38. Thegate 34 is formed by another layer of highly doped polysilicon. Asecond interlevel dielectric 33, typically borophosphosilicate glass (BPSG) covers thegate 34. In operation, current flows vertically between thesource 18 and thedrain 12 through a channel in the well 16 when a suitable voltage is applied to thegate 34. - The
device 10 is formed by the series of process steps shown inFIGS. 4-17 . Initially, asuitable substrate 8 is highly doped to form anN+ region 12. Then, anepitaxial layer 14 is grown on the other surface of thesubstrate 8. Theepitaxial layer 14 is either grown as a lightly doped N− layer or is then suitably doped as N− after it is grown. Please note that although theepitaxial layer 14 is shown larger than the substrate, the difference in size is strictly for purposes of explaining the invention. Those skilled in the art understand that the substrate is normally significantly greater in thickness than is the epitaxial layer. -
FIG. 5 showsboron ions 40 implanted into the upper surface of theepitaxial layer 14 to form a latent P-well 16 inFIG. 6 .FIG. 6 also shows the surface of the epitaxial layer covered with a low-temperature oxide 42 that is patterned to expose future trench regions. Trenches are etched as shown inFIG. 7 . Turning toFIG. 8 , thetrenches 30 may be etched by any suitable process, including a wet etch or a dry plasm etch. Such etching is conventional and is known to those skilled in the art. After completion of the etching, the entire wafer is subjected to an oxidation process in order to grow a relatively thingate oxide layer 32 over the entire wafer, including the sidewalls of thetrench 30. Prior to growing the gate oxide layer, the low-temperature oxide mask is stripped. The oxidation step is performed at a temperature high enough to drive in the boron implants and form wellregion 16. - In a next step, shown in
FIG. 9 , the entire wafer is covered with apolysilicon layer 38 that fills thetrench 30. Thepolysilicon layer 38 is etched away until it leaves aresidual polysilicon shield 38 in the bottom of thetrenches 30, as shown inFIG. 10 . Turning now toFIG. 11 , an interleveldielectric layer 36 is formed from silicon nitride or other suitable insulator over the entire wafer, including in thetrenches 30. The interleveldielectric layer 36 is suitably etched, as shown inFIG. 12 , to leave a layer within the trenches and covering thepolysilicon shield layer 38. Next,FIG. 13 shows apolysilicon gate layer 34 deposited over the entire wafer and into thetrenches 30. The polysilicon gate includes highly doped polysilicon so as to provide a conductive gate material. Other materials are possible, but polysilicon is the preferred material. The polysilicon gate layer and thepolysilicon shield layer 38 and the interleveldielectric layer 36 are suitably sized so that the bottom of thepolysilicon gate layer 34 is proximate the curvature of thewell region 16. This will minimize the overlap between the gate and drain, and therefore minimize the gate-to-drain capacitance. - Turning to
FIG. 14 , thepolysilicon gate layer 34 is etched in order to provide room in the trench for an interlevel dielectric layer. Asuitable IDL layer 33, as shown inFIG. 15 , comprising BPSG is deposited over theentire wafer 10 and into the remainder of the trenches. TheBPSG layer 33 is planarized, as shown inFIG. 16 , to approximately the same level as the top of theepitaxial layer 14. The entire surface of the device is subjected to a suitable N+ source implant such as phosphorous or arsenic. The source implant is suitably driven into an appropriate depth in order to form thesource regions 18 as shown inFIG. 17 . The wafer is then covered with a layer ofmetal 20 that forms the source metal which is then suitably patterned and finished to create the device. A section 19-19′ is taken through the center of the trench ofFIG. 18 . A portion of the resulting profile at the edge of the wafer is shown inFIG. 19 . Thepolysilicon shield 38 is in electrical and mechanical contact with thesource 20 at the die periphery. As a result, thepolysilicon shield layer 38 is electrically shorted to thesource metal 20. This source metal is also in contact with the third layer. (not shown) - The remaining process steps are standard backend process steps for power semiconductor devices, including top surface tape, wafer grind, removal of tape and backside metalization, etc. While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the present invention using the general principles disclosed herein. Furthermore, this application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
Claims (17)
1. A semiconductor device having improved and reduced Miller capacitance in a repeated cellular structure, wherein the cells of the device comprise:
a substrate having one surface with a first layer highly doped with a first conductivity dopant and forming a drain;
a second layer over the first layer and lightly doped with a first conductivity dopant;
a third layer over the second layer and doped with a second conductivity dopant opposite in polarity to the first conductivity component, and forming a PN junction with the second layer;
a fourth layer on the opposite surface of the semiconductor substrate and highly doped with a first conductivity dopant;
a trench structure extending from the fourth layer into the substrate and dividing the fourth layer into a plurality of source regions, said trench having spaced apart sidewalls and a floor with an insulating layer on the sidewalls and floor, upper and lower conductive layers separated by a dielectric layer; and
a source metal layer over the device and in electrical contact with the fourth layer to contact the source regions and in electrical contact with the lower conductive layer of the trenches at peripheral locations around the cells of the device.
2. The semiconductor device of claim 1 wherein the length of the upper conductive layer is greater than the length of the lower conductive layer.
3. The semiconductor device of claim 1 wherein the width of the trench is less than the depth of the trench.
4. The semiconductor device of claim 1 wherein top of the upper conductive layer in the trench is proximate the bottom of the fourth highly doped layer.
5. The semiconductor device of claim 1 wherein a floor portion of the trench is disposed below the lower level of the third layer and said floor portion is rounded to reduce the concentration of electromagnetic field at the bottom of the trench.
6. The semiconductor device of claim 1 wherein the conductive layers in the trench comprise doped polysilicon.
7. The semiconductor device of claim 1 wherein the dielectric layer separating the two conductive layers is silicon nitride.
8. The semiconductor device of claim 1 wherein the upper conductive layer in the trench forms a gate electrode for controlling current through a channel adjacent the sidewall of the trench.
9. The semiconductor device of claim 1 wherein the second layer comprises a layer of dopant that curves upwardly toward the surface of the substrate proximate the sidewalls of the trench and the bottom of the upper layer of the trench conductor is at about the same level as the PN junction proximate the sidewall of the trench.
10. The semiconductor device of claim 1 wherein the top of the lower conductive layer is below the bottom of the third layer of second conductivity.
11. The semiconductor device of claim 1 wherein said upper and lower conducting layers have approximately the same width in their respective regions adjacent the dielectric layer separating them.
12. The semiconductor device of claim 11 wherein the source metal is in contact with the third layer of the second conductivity type.
13. The semiconductor device of claim 1 further comprising a second interlevel dielectric layer above said upper conductive layer.
14. The semiconductor device of claim 13 wherein said second interlevel dielectric layer is borophosphosilicate glass.
15. The semiconductor device of claim 1 wherein the sidewalls and floor of said trench include an oxide layer.
16. The semiconductor device of claim 1 wherein the first dopant is p-type and the second dopant is n-type.
17. The semiconductor device of claim 1 wherein the first dopant is n-type and the second dopant is p-type.
Priority Applications (3)
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US11/502,594 US20070040214A1 (en) | 2001-03-09 | 2006-08-10 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and Miller charge |
US11/930,686 US20080211014A1 (en) | 2001-03-09 | 2007-10-31 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
US11/930,673 US20080142909A1 (en) | 2001-03-09 | 2007-10-31 | Ultra dense trench-gated power device with reduced drain source feedback capacitance and miller charge |
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US10/092,692 US6683346B2 (en) | 2001-03-09 | 2002-03-07 | Ultra dense trench-gated power-device with the reduced drain-source feedback capacitance and Miller charge |
US10/678,444 US6929988B2 (en) | 2001-03-09 | 2003-10-01 | Method of making an ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
US11/178,215 US7098500B2 (en) | 2001-03-09 | 2005-07-08 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
US11/502,594 US20070040214A1 (en) | 2001-03-09 | 2006-08-10 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and Miller charge |
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US11/930,686 Continuation US20080211014A1 (en) | 2001-03-09 | 2007-10-31 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
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US10/678,444 Expired - Fee Related US6929988B2 (en) | 2001-03-09 | 2003-10-01 | Method of making an ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
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US11/502,594 Abandoned US20070040214A1 (en) | 2001-03-09 | 2006-08-10 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and Miller charge |
US11/930,673 Abandoned US20080142909A1 (en) | 2001-03-09 | 2007-10-31 | Ultra dense trench-gated power device with reduced drain source feedback capacitance and miller charge |
US11/930,686 Abandoned US20080211014A1 (en) | 2001-03-09 | 2007-10-31 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
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US10/678,444 Expired - Fee Related US6929988B2 (en) | 2001-03-09 | 2003-10-01 | Method of making an ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
US11/178,215 Expired - Fee Related US7098500B2 (en) | 2001-03-09 | 2005-07-08 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
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US11/930,686 Abandoned US20080211014A1 (en) | 2001-03-09 | 2007-10-31 | Ultra dense trench-gated power device with the reduced drain-source feedback capacitance and miller charge |
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JP (1) | JP2005505912A (en) |
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US8357971B2 (en) | 2007-10-29 | 2013-01-22 | Nxp B.V. | Trench gate MOSFET and method of manufacturing the same |
Also Published As
Publication number | Publication date |
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DE10296457T5 (en) | 2004-04-29 |
US20060006460A1 (en) | 2006-01-12 |
US20040061171A1 (en) | 2004-04-01 |
TW543146B (en) | 2003-07-21 |
US20080211014A1 (en) | 2008-09-04 |
JP2005505912A (en) | 2005-02-24 |
US20080142909A1 (en) | 2008-06-19 |
US6929988B2 (en) | 2005-08-16 |
US20020125529A1 (en) | 2002-09-12 |
US6683346B2 (en) | 2004-01-27 |
WO2002078092A1 (en) | 2002-10-03 |
US7098500B2 (en) | 2006-08-29 |
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