WO2009060406A1 - A trench-gate semiconductor device and method of manufacturing the same - Google Patents

A trench-gate semiconductor device and method of manufacturing the same Download PDF

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Publication number
WO2009060406A1
WO2009060406A1 PCT/IB2008/054642 IB2008054642W WO2009060406A1 WO 2009060406 A1 WO2009060406 A1 WO 2009060406A1 IB 2008054642 W IB2008054642 W IB 2008054642W WO 2009060406 A1 WO2009060406 A1 WO 2009060406A1
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region
trench
semiconductor device
gate semiconductor
conductivity type
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PCT/IB2008/054642
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French (fr)
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Steven T. Peake
Philip Rutter
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Nxp B.V.
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Publication of WO2009060406A1 publication Critical patent/WO2009060406A1/en

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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
    • H01L29/76—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
    • 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
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H01L29/10—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions with semiconductor regions connected to an electrode not carrying current to be rectified, amplified or switched and such electrode being part of a semiconductor device which comprises three or more electrodes
    • H01L29/1095—Body region, i.e. base region, of DMOS transistors or IGBTs
    • 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
    • H01L29/66007—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
    • 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
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H01L29/0603—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by particular constructional design considerations, e.g. for preventing surface leakage, for controlling electric field concentration or for internal isolations regions
    • H01L29/0607—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by particular constructional design considerations, e.g. for preventing surface leakage, for controlling electric field concentration or for internal isolations regions for preventing surface leakage or controlling electric field concentration
    • H01L29/0611—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by particular constructional design considerations, e.g. for preventing surface leakage, for controlling electric field concentration or for internal isolations regions for preventing surface leakage or controlling electric field concentration for increasing or controlling the breakdown voltage of reverse biased devices
    • H01L29/0615—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by particular constructional design considerations, e.g. for preventing surface leakage, for controlling electric field concentration or for internal isolations regions for preventing surface leakage or controlling electric field concentration for increasing or controlling the breakdown voltage of reverse biased devices by the doping profile or the shape or the arrangement of the PN junction, or with supplementary regions, e.g. junction termination extension [JTE]

Definitions

  • This invention relates to semiconductor devices and methods of manufacturing semiconductor devices, and in particular to trench-gate semiconductor devices and methods of manufacturing the same.
  • Trench-gate semiconductor devices are well-known, in which a semiconductor body includes a source region towards a first major surface of the device, and a drain region deeper in the device.
  • the source and drain region are doped n-type. Lying between the source and drain is a body region, which is doped p-type.
  • a channel In order to turn on the device, so that it conducts an electrical current between the source and drain, a channel must be created through the p-type body region.
  • a gate electrode is provided, the gate material being located close to the body region in trench in the device.
  • the trench is typically lined with oxide, to electrically isolate the gate from the body.
  • Providing a positive bias to the gate produces an electrical field which locally deletes the p-type body, and inverts it to become locally conducting n-type.
  • a low resistance Rdson
  • this is effected by providing a short vertical channel through the body, which is relatively high doped.
  • a further desirable attribute of such devices is to withstand a high reverse bias between the source and drain, without the device breaking down or passing any significant reverse current.
  • One way of effecting this is to provide a low doped "drift" region, as part of the drain structure.
  • the low doped drift region forms a n-/p+ junction with the body region, which can hold off a large reverse bias.
  • the n- region contributes a higher resistance in the on-state, than would an equivalent higher doped n+ region.
  • the n- drift layer is typically provided as a high quality epitaxial layer, and its thickness is limited such that it is fully depleted at the desired maximum reverse bias condition.
  • the remainder of the drain structure is provided by a more highly doped n+ substrate, (for devices with a rear-side drain contact), or more highly-doped laterally spaced drain contact right (for devices with a top surface drain contact).
  • US Patent number 5,864,159 to Takahashi discloses a trench-gate semiconductor device, which includes a p- layer between the p-body and the n drain region, adjacent the trench.
  • the dopant is leached, during manufacture, from the regions immediately adjacent the trench, to provide a continuation of the n-drain region.
  • the p- layer is introduced to modify the field distribution adjacent the side and tip of the trench.
  • the p- layer of Takahashi lies at a shallower depth than the trench depth. Since, in reverse bias, the p- side of the body diode (that is, the diode between the drain and body, which in this case is the diode between the n- drift layer and the p- layer) will be depleted to a greater distance than would be the case for an equivalent p+ layer. Thus, to provide effective reverse protection, this layer needs to be thicker than need be the case for an equivalent more heavily doped layer, and thus the trench is deeper than would otherwise be the case. This has deleterious effects on the device properties, in particular the gate-source charge (Qgs) is greater than would otherwise be the case for a shallower trench.
  • Qgs gate-source charge
  • a trench-gate semiconductor device comprising a semiconductor body having a first major surface and a trench extending into the semiconductor body from the first major surface to a first depth, the semiconductor body comprising a first region of a first conductivity type adjacent a sidewall of the trench at the first major surface, a second region of the first conductivity type adjacent the trench at a position distant from the first region, and a channel-accommodating region, of a second conductivity type opposite to the first conductivity type, adjacent the sidewall of the trench between the first region and the second region, wherein the semiconductor body further comprises a third region of the second conductivity type, spaced apart from the trench and extending from the channel-accommodating region to within the second region and extending to a depth into the semiconductor body which is as least as great as the first depth and having a doping concentration of the second conductivity type that is lower than that of the channel-accommodating region.
  • the third region extends deeper into the semiconductor body than does the trench. This is particularly effective for minimising the required trench depth.
  • the third region may extend deeper into the semiconductor body than does the trench by between 0.1 and 0.6 ⁇ m.
  • the doping concentration of the second region is greater than the doping concentration of the third region, such that in use when the trench-gate semiconductor device is reverse biased, the depletion region extends further into the third region than into the second region.
  • Such relative doping concentrations allows for enhanced reverse breakdown characteristics, moreover, since a reduced thickness of the second - drift - region is depleted; the thickness of the epitaxial layer can thus be optimised.
  • the doping concentration in the second region may be the same as the doping concentration in the third region, such that in use when the trench-gate semiconductor device is reverse biased, the depletion region extends the same distance into the third region and the second region. This may allow for an optimal use of the available thickness of epitaxial material, and similar advantages to that described above.
  • the third region has a doping concentration of the second conductivity type between 10 15 and 10 17 atoms/cm 3 .
  • This doping range is particularly convenient to ensure an appropriate depth of material is depleted either side of the junction.
  • the optimum doping concentration in any particular device will depend on the doping concentration of the second region.
  • the device may further comprise a substrate region of the first conductivity type, farther from the first major surface than the second region and have a doping concentration of the first conductivity type which is greater than that of the second region.
  • the invention also encompasses devices having an insulating substrate, such as SOS (semiconductor on sapphire) or SOI (semiconductor on insulator) devices.
  • a method of manufacturing a trench-gate semiconductor device as described above including the steps of masking part of the surface of the device, and implanting a p-type dopant to a depth which exceeds the depth of the channel- accommodating region, to over-dope part of the second region and change its conductivity from n-type to p-type and form a p-type layer which has a lower doping concentration than the second region.
  • Fig. 1 shows a partial cross-section through a part of trench-gate semiconductor device according to an aspect of the present invention
  • Fig. 2 shows a schematic comparison between a prior art body diode of a trench-gate semiconductor device (at Fig 2a), and a non-abrupt body diode according to the embodiment shown in Fig. 1 ;
  • Fig. 3 shows the electric field distribution for a prior art device (at Fig. 3a), and a device according to the embodiment shown in Fig. 1 ;
  • FIG. 4 shows schematics of a prior art device (at Fig. 4a), and devices according to the embodiment shown in Fig. 1 a, having p- regions of various widths (at Fig. 4b, 4c and 4d); and
  • Fig. 5 shows the simulation results of the reverse breakdown voltages for the devices of Fig. 4.
  • FIG. 1 shows a partial cross-section through a part of trench-gate semiconductor device according to an embodiment of the present invention.
  • a trench-gate semiconductor device or Trench-FET has a source region 1 adjacent its first major surface 11 , and a substrate 2 which forms the drain of the device. Between the source and drain regions there are body regions 3, and drift regions 6. Extending downwards into the device from the first major surface, is a trench 4. The walls of the trench are lined with oxide 5 or other insulating material (such as silicon nitride), which serves to isolate the gate 8 which lies within trench 4 from the semiconducting layers. Wells, or moats 9 are formed through the source region, to allow contact to be made to the source and body regions.
  • oxide 5 or other insulating material such as silicon nitride
  • the source region 3 and drain region 2 are doped n, or n+.
  • the epitaxial drift region 6 also has n-type conductivity, but has a lower n- doping than the source or drain regions.
  • the body region is oppositely doped, that is it is doped p-type. In use, when a forward bias is applied to the gate in the trench, a field is set up which locally depletes the p- body, adjacent the trench, of holes, and allows type inversion and thus a current to flow between the source 3 and drift 6 regions.
  • n- drift region 6 It is particularly convenient to use a conventional epitaxial n- drift region 6, since this provides a uniformly doped layer (or graded layer where appropriate), without the complexity or processing difficulties associated with multiple implants or high temperature anneals, to produce a suitable drift region by other means.
  • the device of Fig. 1 is the same as prior art Trench-FETs.
  • a further layer 13 doped lightly p-type, is included, which extends from the p-body, into the n- drift region.
  • This body- extension layer is doped more lightly that the body layer proper 3, and extends farther into the device than does the trench 4.
  • the lightly doped layer is spaced apart from the trench, and so does not directly affect the length of the channel, which is determined by the thickness of the p body region adjacent the trench.
  • the device is manufactured using conventional techniques.
  • the drift region 6 is conveniently high quality, low doped epitaxial material; the p+ doped body region is formed by ion implantation and activation anneal; the source regions are formed by ion implantation to over-dope the body region; the trenches are formed by an-isotropic etching (typically carried out by a dry process); the trench is lined by oxide 5, typically by low temperature growth of LOCOS oxide material; polysilicon material is deposited to form the gate 8 within the trench 4, and the wells or moats 9 are typically formed by planarising the surface (in practise, the surface may not be planar, and the entire region between the trenches are over-doped into n+; planarisation then has the effect of forming virtual moats by removing all of the over-doped material remote from the trenches, and provided access for contacting the body along with the source).
  • the lightly doped p- layer is formed by high energy implantation of a low dose of p- dopant such as Boron.
  • the doping is carried out through the same oxide mask which is used for defining the source contact (which source contact electrically shorts the source and the parts of the p-body region which were exposed to the surface at the moats or wells); thus the lightly doped layer is auto-aligned away from the sidewalls of the trench.
  • the lightly doped p- layer 13 is remote from sidewalls of the trench.
  • the layer thereby does not contribute to the length of the channel, and thus does not contribute to the on-resistance Rdson.
  • the channel in the on-state would have to extend not only across the p-body, but also across the lightly doped layer. This would inevitably lead to an increase in the channel resistance, and thus to an undesirable increase in Rdson.
  • the lightly doped layer is remote from the trench, it can also extend deeper into the device than the trench. Were thus not the case, the lightly doped layer would surround the base of the trench.
  • the lightly doped layer extends deeper than the trench, it does not consume any of the volume required by the body region. Further, since the lightly doped layer forms a non-abrupt junction with the drift region 6, incorporation of this layer into the volume conventionally occupied by the epitaxial drift region 6 does not result in a need for thicker epitaxial layer, since during use in reverse bias the drift region is depleted to a shallower distance than would the case for a conventional body diode. This will be considered in more detail with reference to figures 2 and 3.
  • the lightly doped layer extends farther into the device than does the trench, the blocking capability is enhanced: that is, the presence of the deep non-abrupt junction acts to shield, to some extent, the trench (and particularly the bottom corners of the trench), from the extremes of voltage seen at the drain, in reverse bias.
  • the depth of the trench is 1.0 ⁇ m, and the lightly doped layer extends to a depth of 1.5 ⁇ m. That is, the lightly doped layer extends 0.5 ⁇ m deeper than the trench. Furthermore, to provide for a wide process window for the trench depth, for instance between 0.9 and 1.3 ⁇ m, the depth of the lightly doped layer must at least equal or exceed the depth of the trench - that is, it must be greater than, or equal to 1.3 ⁇ m in this case. Application of a process tolerance to this depth results in a target depth of the lightly doped layer, which is at least, for example, 1.4 ⁇ m in this case.
  • the trench may be spaced apart from the trench by typically 0.2 ⁇ m, 0.4 ⁇ m or 0.6 ⁇ m (and thus have a nominal with of approximately 1.1 , 0.7 or 0.4 ⁇ m respectively).
  • Figure 2 shows the depletion region for a conventional body diode (at
  • Fig. 2a compared with a non-abrupt body diode according to the embodiment shown in Fig. 1 (at Fig 2b).
  • Fig. 2 shows an ideal 1 dimensional structure; the corresponding simulated field distribution and depletion regions in a device are shown in Fig. 3.
  • the depletion region extends further into the n- drift region, than into the p+ body region.
  • the depletion region lies predominantly in the lightly doped layer.
  • Fig. 3 shows the region of a device bounded by outline 9 of Fig. 1 In each case the metallurgical boundary between the p and n regions are shown at lines 27. It is clear from the figures, that in the embodiment shown, the depletion region extends less distance into the device, compared with the conventional arrangement. Further, the field is more evenly and more widely distributed, as the equipotentials are more dispersed, compared with the conventional arrangements.
  • Fig. 4 shows schematics of a prior art device (at Fig.
  • Fig. 4a and example devices according to the illustrative embodiment (at Fig. 4b, 4c and 4d).
  • the example devices have p- regions of various widths. That is, the conventional structure of Fig. 4a is the same as that shown in Fig. 3a, and the structures shown in Fig 4b, 4c, and 4d are of devices having lightly doped p- layers which are increasingly spaced apart from the side of the trench.
  • Fig. 4 The simulated reverse breakdown performance of the structures shown in Fig. 4 are given in Fig 5.
  • Curve A shows the reverse breakdown voltage of the conventional structure (of Fig 4a)
  • curves B, C and D show the reverse breakdown voltage for the structures of Fig 4b, 4c and 4d respectively (that is, with the lightly doped layer spaced apart from the trench by approximately 0.2 ⁇ m, 0.4 ⁇ m and 0.6 ⁇ m respectively).
  • the variation of breakdown voltage with depth of the trench is plotted.
  • all the devices of this embodiment show an improved or increased reverse bias breakdown for all trench depths, in comparison with a conventional device.
  • the breakdown voltages are highest for the structure which approaches closest to the trench.
  • the implementation of the embodiment described above benefits from the processing simplicity of a auto- aligned masked implantation to form the lightly doped layer, it is equally possibly to tailor the width of the layer to suit particular applications, by using one or more dedicated masks with the implantation step or steps.
  • the implanted lightly doped region is spaced apart from the trench by a sufficient distance so as not to noticeably or even measurably increase the spreading resistance, in the on-state, of carriers leaving from or arriving at the channel.
  • the depletion in a reverse biased device according to this embodiment is predominantly in the lightly doped layer.
  • the n- (epitaxial) drift layer which can thus be thinner, relative to an equivalently rated conventional device.
  • This can be carried out be appropriate control of the p- lightly doped layer doping and thickness, and the n- epitaxial layer; however, the requirements are not as stringent as those required in conventional, vertical charge-balanced full RESURF (reduced surface field) structures.
  • references to doping concentration used herein normally relate to the free carrier concentrations in each region; however, where context requires they refer to the absolute n or p type levels of each region.

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Abstract

A trench-gate semiconductor device is disclosed which comprises a lightly- doped p- region (13) between the p-body (3), and the n- drift region (6). The lightly doped region, which extends to a depth which is at least as great as that of the gate thench (4), serves to form a non-abrupt junction with the n- drift which enhances the field distribution in reverse bias, and improves reverse bias hold-off performance.

Description

DESCRIPTION
A TRENCH-GATE SEMICONDUCTOR DEVICE AND METHOD OF MANUFACTURING THE SAME
Field of the Invention
This invention relates to semiconductor devices and methods of manufacturing semiconductor devices, and in particular to trench-gate semiconductor devices and methods of manufacturing the same.
Background of the Invention
Trench-gate semiconductor devices are well-known, in which a semiconductor body includes a source region towards a first major surface of the device, and a drain region deeper in the device. Typically the source and drain region are doped n-type. Lying between the source and drain is a body region, which is doped p-type. In order to turn on the device, so that it conducts an electrical current between the source and drain, a channel must be created through the p-type body region. To create the channel, a gate electrode is provided, the gate material being located close to the body region in trench in the device. The trench is typically lined with oxide, to electrically isolate the gate from the body. Providing a positive bias to the gate produces an electrical field which locally deletes the p-type body, and inverts it to become locally conducting n-type. When conducting, a low resistance (Rdson) is required through the device. Conventionally, this is effected by providing a short vertical channel through the body, which is relatively high doped.
A further desirable attribute of such devices, is to withstand a high reverse bias between the source and drain, without the device breaking down or passing any significant reverse current. One way of effecting this is to provide a low doped "drift" region, as part of the drain structure. The low doped drift region forms a n-/p+ junction with the body region, which can hold off a large reverse bias. However, the n- region contributes a higher resistance in the on-state, than would an equivalent higher doped n+ region. To mitigate this contribution to Rdson, the n- drift layer is typically provided as a high quality epitaxial layer, and its thickness is limited such that it is fully depleted at the desired maximum reverse bias condition. The remainder of the drain structure is provided by a more highly doped n+ substrate, (for devices with a rear-side drain contact), or more highly-doped laterally spaced drain contact right (for devices with a top surface drain contact).
US Patent number 5,864,159 to Takahashi discloses a trench-gate semiconductor device, which includes a p- layer between the p-body and the n drain region, adjacent the trench. In one embodiment the dopant is leached, during manufacture, from the regions immediately adjacent the trench, to provide a continuation of the n-drain region. The p- layer is introduced to modify the field distribution adjacent the side and tip of the trench.
The p- layer of Takahashi lies at a shallower depth than the trench depth. Since, in reverse bias, the p- side of the body diode (that is, the diode between the drain and body, which in this case is the diode between the n- drift layer and the p- layer) will be depleted to a greater distance than would be the case for an equivalent p+ layer. Thus, to provide effective reverse protection, this layer needs to be thicker than need be the case for an equivalent more heavily doped layer, and thus the trench is deeper than would otherwise be the case. This has deleterious effects on the device properties, in particular the gate-source charge (Qgs) is greater than would otherwise be the case for a shallower trench.
Summary of the invention It is an object of the present invention to provide a trench-gate semiconductor device with improved characteristics, including in particular a non-abrupt body diode characteristic.
According to the present invention there is provided a trench-gate semiconductor device comprising a semiconductor body having a first major surface and a trench extending into the semiconductor body from the first major surface to a first depth, the semiconductor body comprising a first region of a first conductivity type adjacent a sidewall of the trench at the first major surface, a second region of the first conductivity type adjacent the trench at a position distant from the first region, and a channel-accommodating region, of a second conductivity type opposite to the first conductivity type, adjacent the sidewall of the trench between the first region and the second region, wherein the semiconductor body further comprises a third region of the second conductivity type, spaced apart from the trench and extending from the channel-accommodating region to within the second region and extending to a depth into the semiconductor body which is as least as great as the first depth and having a doping concentration of the second conductivity type that is lower than that of the channel-accommodating region.
Preferably the third region extends deeper into the semiconductor body than does the trench. This is particularly effective for minimising the required trench depth.
Advantageously, the third region may extend deeper into the semiconductor body than does the trench by between 0.1 and 0.6 μm. This provides for particularly convenient manufacturing processes and device properties. However, the invention is not restricted to this depth range, and in particular, the depth of implantation may be conveniently only be limited by the capabilities of the available equipment. Preferably, the doping concentration of the second region is greater than the doping concentration of the third region, such that in use when the trench-gate semiconductor device is reverse biased, the depletion region extends further into the third region than into the second region. Such relative doping concentrations allows for enhanced reverse breakdown characteristics, moreover, since a reduced thickness of the second - drift - region is depleted; the thickness of the epitaxial layer can thus be optimised.
Advantageously, the doping concentration in the second region may be the same as the doping concentration in the third region, such that in use when the trench-gate semiconductor device is reverse biased, the depletion region extends the same distance into the third region and the second region. This may allow for an optimal use of the available thickness of epitaxial material, and similar advantages to that described above.
Preferably the third region has a doping concentration of the second conductivity type between 1015 and 1017 atoms/cm3. This doping range is particularly convenient to ensure an appropriate depth of material is depleted either side of the junction. However, the optimum doping concentration in any particular device will depend on the doping concentration of the second region.
Advantageously, the device may further comprise a substrate region of the first conductivity type, farther from the first major surface than the second region and have a doping concentration of the first conductivity type which is greater than that of the second region. However, the invention also encompasses devices having an insulating substrate, such as SOS (semiconductor on sapphire) or SOI (semiconductor on insulator) devices.
According to another aspect of the invention, there is provided a method of manufacturing a trench-gate semiconductor device as described above, including the steps of masking part of the surface of the device, and implanting a p-type dopant to a depth which exceeds the depth of the channel- accommodating region, to over-dope part of the second region and change its conductivity from n-type to p-type and form a p-type layer which has a lower doping concentration than the second region.
These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiment described hereinafter.
Brief description of Drawings
An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which
Fig. 1 shows a partial cross-section through a part of trench-gate semiconductor device according to an aspect of the present invention;
Fig. 2 shows a schematic comparison between a prior art body diode of a trench-gate semiconductor device (at Fig 2a), and a non-abrupt body diode according to the embodiment shown in Fig. 1 ;
Fig. 3 shows the electric field distribution for a prior art device (at Fig. 3a), and a device according to the embodiment shown in Fig. 1 ;
Fig. 4 shows schematics of a prior art device (at Fig. 4a), and devices according to the embodiment shown in Fig. 1 a, having p- regions of various widths (at Fig. 4b, 4c and 4d); and
Fig. 5 shows the simulation results of the reverse breakdown voltages for the devices of Fig. 4.
It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments
Detailed description of embodiments Fig. 1 shows a partial cross-section through a part of trench-gate semiconductor device according to an embodiment of the present invention. A trench-gate semiconductor device or Trench-FET has a source region 1 adjacent its first major surface 11 , and a substrate 2 which forms the drain of the device. Between the source and drain regions there are body regions 3, and drift regions 6. Extending downwards into the device from the first major surface, is a trench 4. The walls of the trench are lined with oxide 5 or other insulating material (such as silicon nitride), which serves to isolate the gate 8 which lies within trench 4 from the semiconducting layers. Wells, or moats 9 are formed through the source region, to allow contact to be made to the source and body regions.
In the device shown, the source region 3 and drain region 2 are doped n, or n+. The epitaxial drift region 6 also has n-type conductivity, but has a lower n- doping than the source or drain regions. The body region is oppositely doped, that is it is doped p-type. In use, when a forward bias is applied to the gate in the trench, a field is set up which locally depletes the p- body, adjacent the trench, of holes, and allows type inversion and thus a current to flow between the source 3 and drift 6 regions.
It is particularly convenient to use a conventional epitaxial n- drift region 6, since this provides a uniformly doped layer (or graded layer where appropriate), without the complexity or processing difficulties associated with multiple implants or high temperature anneals, to produce a suitable drift region by other means.
Thus far, the device of Fig. 1 is the same as prior art Trench-FETs. However in this embodiment, a further layer 13, doped lightly p-type, is included, which extends from the p-body, into the n- drift region. This body- extension layer, is doped more lightly that the body layer proper 3, and extends farther into the device than does the trench 4. Further, the lightly doped layer is spaced apart from the trench, and so does not directly affect the length of the channel, which is determined by the thickness of the p body region adjacent the trench.
The device is manufactured using conventional techniques. The drift region 6 is conveniently high quality, low doped epitaxial material; the p+ doped body region is formed by ion implantation and activation anneal; the source regions are formed by ion implantation to over-dope the body region; the trenches are formed by an-isotropic etching (typically carried out by a dry process); the trench is lined by oxide 5, typically by low temperature growth of LOCOS oxide material; polysilicon material is deposited to form the gate 8 within the trench 4, and the wells or moats 9 are typically formed by planarising the surface (in practise, the surface may not be planar, and the entire region between the trenches are over-doped into n+; planarisation then has the effect of forming virtual moats by removing all of the over-doped material remote from the trenches, and provided access for contacting the body along with the source).
The lightly doped p- layer is formed by high energy implantation of a low dose of p- dopant such as Boron. The doping is carried out through the same oxide mask which is used for defining the source contact (which source contact electrically shorts the source and the parts of the p-body region which were exposed to the surface at the moats or wells); thus the lightly doped layer is auto-aligned away from the sidewalls of the trench.
It is particularly useful that the lightly doped p- layer 13 is remote from sidewalls of the trench. The layer thereby does not contribute to the length of the channel, and thus does not contribute to the on-resistance Rdson. Were the layer to extend laterally adjacent the trench, the channel in the on-state would have to extend not only across the p-body, but also across the lightly doped layer. This would inevitably lead to an increase in the channel resistance, and thus to an undesirable increase in Rdson. Moreover, since the lightly doped layer is remote from the trench, it can also extend deeper into the device than the trench. Were thus not the case, the lightly doped layer would surround the base of the trench. It would then generally not be possible to produce, by biasing the gate within the trench, a channel between the source and drain - so the device could not function at all. Although theoretically it might be possible to utilise a very thin, lightly doped layer under the bottom of the trench, which layer could then become fully depleted, the extremely tight tolerance requirement would render such an arrangement impractical.
Since the lightly doped layer extends deeper than the trench, it does not consume any of the volume required by the body region. Further, since the lightly doped layer forms a non-abrupt junction with the drift region 6, incorporation of this layer into the volume conventionally occupied by the epitaxial drift region 6 does not result in a need for thicker epitaxial layer, since during use in reverse bias the drift region is depleted to a shallower distance than would the case for a conventional body diode. This will be considered in more detail with reference to figures 2 and 3.
Furthermore, since in this illustrative embodiment the lightly doped layer extends farther into the device than does the trench, the blocking capability is enhanced: that is, the presence of the deep non-abrupt junction acts to shield, to some extent, the trench (and particularly the bottom corners of the trench), from the extremes of voltage seen at the drain, in reverse bias.
In a typical device, the depth of the trench is 1.0 μm, and the lightly doped layer extends to a depth of 1.5μm. That is, the lightly doped layer extends 0.5 μm deeper than the trench. Furthermore, to provide for a wide process window for the trench depth, for instance between 0.9 and 1.3 μm, the depth of the lightly doped layer must at least equal or exceed the depth of the trench - that is, it must be greater than, or equal to 1.3μm in this case. Application of a process tolerance to this depth results in a target depth of the lightly doped layer, which is at least, for example, 1.4μm in this case. Moreover, for a device with a 2 μm trench pitch, such as a device with trenches of width 0.5μm spaced apart by 1.5μm, the trench may be spaced apart from the trench by typically 0.2μm, 0.4μm or 0.6μm (and thus have a nominal with of approximately 1.1 , 0.7 or 0.4μm respectively).
Figure 2 shows the depletion region for a conventional body diode (at
Fig. 2a), compared with a non-abrupt body diode according to the embodiment shown in Fig. 1 (at Fig 2b). Fig. 2 shows an ideal 1 dimensional structure; the corresponding simulated field distribution and depletion regions in a device are shown in Fig. 3.
As shown in Fig. 2a, in a conventional body diode having a p+ doped body region 3 adjacent the n- drift region 6, in order the achieve charge neutrality, the depletion region extends further into the n- drift region, than into the p+ body region. In contrast, as shown in Fig. 2b for a body diode according to the first embodiment having a lightly doped p- layer 13 between the body 3 and drift 6, which lightly doped layer is doped more lightly than drift region, the depletion region lies predominantly in the lightly doped layer.
Simulated results showing the depletion layer (dotted lines 25 and 26 respectively), and equipotential lines, for the two cases are shown in Fig 3 (Fig. 3a corresponding to the conventional prior art arrangement, and figure 3b including a lightly doped layer). Fig. 3 shows the region of a device bounded by outline 9 of Fig. 1 In each case the metallurgical boundary between the p and n regions are shown at lines 27. It is clear from the figures, that in the embodiment shown, the depletion region extends less distance into the device, compared with the conventional arrangement. Further, the field is more evenly and more widely distributed, as the equipotentials are more dispersed, compared with the conventional arrangements. Fig. 4 shows schematics of a prior art device (at Fig. 4a) and example devices according to the illustrative embodiment (at Fig. 4b, 4c and 4d). The example devices have p- regions of various widths. That is, the conventional structure of Fig. 4a is the same as that shown in Fig. 3a, and the structures shown in Fig 4b, 4c, and 4d are of devices having lightly doped p- layers which are increasingly spaced apart from the side of the trench.
The simulated reverse breakdown performance of the structures shown in Fig. 4 are given in Fig 5. Curve A shows the reverse breakdown voltage of the conventional structure (of Fig 4a), and curves B, C and D show the reverse breakdown voltage for the structures of Fig 4b, 4c and 4d respectively (that is, with the lightly doped layer spaced apart from the trench by approximately 0.2μm, 0.4μm and 0.6μm respectively). In each case, the variation of breakdown voltage with depth of the trench is plotted. As can be seen, all the devices of this embodiment show an improved or increased reverse bias breakdown for all trench depths, in comparison with a conventional device. Moreover, the breakdown voltages are highest for the structure which approaches closest to the trench. Although the implementation of the embodiment described above benefits from the processing simplicity of a auto- aligned masked implantation to form the lightly doped layer, it is equally possibly to tailor the width of the layer to suit particular applications, by using one or more dedicated masks with the implantation step or steps. However, the implanted lightly doped region is spaced apart from the trench by a sufficient distance so as not to noticeably or even measurably increase the spreading resistance, in the on-state, of carriers leaving from or arriving at the channel.
It will be apparent from the above, that the depletion in a reverse biased device according to this embodiment is predominantly in the lightly doped layer. Thus less of the depletion is in the n- (epitaxial) drift layer, which can thus be thinner, relative to an equivalently rated conventional device. To maximise performance, it is important that the correct balance must be achieved between the p-body region, the p- lightly doped layer, and the n- drift region. This can be carried out be appropriate control of the p- lightly doped layer doping and thickness, and the n- epitaxial layer; however, the requirements are not as stringent as those required in conventional, vertical charge-balanced full RESURF (reduced surface field) structures.
The person skilled in the art will appreciate that references to doping concentration used herein normally relate to the free carrier concentrations in each region; however, where context requires they refer to the absolute n or p type levels of each region.
From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of trench-gate semiconductor devices, and which may be used instead of, or in addition to, features already described herein.
Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.
The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.
For the sake of completeness it is also stated that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, and reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. A trench-gate semiconductor device comprising a semiconductor body having a first major surface (11 ) and a trench (4) extending into the semiconductor body from the first major surface to a first depth, the semiconductor body comprising a first region (1 ) of a first conductivity type adjacent a sidewall of the trench at the first major surface, a second region (6) of the first conductivity type adjacent the trench at a position distant from the first region, and a channel-accommodating region (3), of a second conductivity type opposite to the first conductivity type, adjacent the sidewall of the trench between the first region and the second region, wherein the semiconductor body further comprises a third region (13) of the second conductivity type, spaced apart from the trench (4) and extending from the channel-accommodating region (3) to within the second region (6) and extending to a depth into the semiconductor body which is as least as great as the first depth and having a doping concentration of the second conductivity type that is lower than that of the channel-accommodating region (3).
2. A trench-gate semiconductor device as claimed in claim 1 , wherein the third region extends deeper into the semiconductor body than does the trench.
3. A trench-gate semiconductor device as claimed in claim 2, wherein the third region extends deeper into the semiconductor body than does the trench by between 0.1 and 0.6 μm.
4. A trench-gate semiconductor device as claimed in any preceding claim, wherein the doping concentration of the second region is greater than the doping concentration of the third region, such that in use when the trench-gate semiconductor device is reverse biased, the depletion region extends further into the third region than into the second region.
5. A trench-gate semiconductor device as claimed in any of claims 1 to 3, wherein the doping concentration in the second region is the same as the doping concentration in the third region, such that in use when the trench-gate semiconductor device is reverse biased, the depletion region extends the same distance into the third region and the second region.
6. A trench-gate semiconductor device as claimed in any preceding claim, wherein the third region has a doping concentration of the second conductivity type between 1014 and 1016 atoms/cm3.
7. A trench-gate semiconductor device as claimed in any preceding claim, farther comprising a substrate region (2) of the first conductivity type, further from the first major surface than the second region and have a doping concentration of the first conductivity type which is greater than that of the second region.
8. A trench-gate semiconductor device as claimed in any preceding claim, wherein the first conductivity type is n- type, and the second conductivity type is p-type.
9. A method of manufacturing a trench-gate semiconductor device according to any preceding claim, including the steps of masking part of the surface of the device, and implanting a p-type dopant to a depth which exceeds the depth of the channel-accommodating region, to over- dope part of the second region (6) and change its conductivity from n- type to p-type and form a p-type layer which has a lower doping concentration than the second region (6).
PCT/IB2008/054642 2007-11-08 2008-11-06 A trench-gate semiconductor device and method of manufacturing the same WO2009060406A1 (en)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH09213939A (en) * 1996-01-30 1997-08-15 Nec Corp Semiconductor device
WO2001099177A2 (en) * 2000-06-16 2001-12-27 General Semiconductor, Inc. Trench mosfet with double-diffused body profile
EP1359624A2 (en) * 2002-04-30 2003-11-05 NEC Electronics Corporation Vertical type MOSFET and manufacturing method thereof

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH09213939A (en) * 1996-01-30 1997-08-15 Nec Corp Semiconductor device
WO2001099177A2 (en) * 2000-06-16 2001-12-27 General Semiconductor, Inc. Trench mosfet with double-diffused body profile
EP1359624A2 (en) * 2002-04-30 2003-11-05 NEC Electronics Corporation Vertical type MOSFET and manufacturing method thereof

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