U.S. patent application number 10/457955 was filed with the patent office on 2004-01-22 for low on-resistance trench lateral misfet with better switching characteristics and method for manufacturing same.
Invention is credited to Fujishima, Naoto, Salama, C. Andre T..
Application Number | 20040014263 10/457955 |
Document ID | / |
Family ID | 26918863 |
Filed Date | 2004-01-22 |
United States Patent
Application |
20040014263 |
Kind Code |
A1 |
Fujishima, Naoto ; et
al. |
January 22, 2004 |
Low on-resistance trench lateral MISFET with better switching
characteristics and method for manufacturing same
Abstract
A high-voltage and low on-resistance semiconductor device
incorporates a trench structure that provides improved switching
characteristics. In a preferred embodiment, a Trench Lateral Power
MISFET is provided having a gate, channel and drift regions that
are built on the side-walls of the trench. The process used to form
the MISFET involves a self-aligned trench bottom contact hole to
contact a source provided at the bottom of the trench to achieve
minimum pitch and very low on-resistance. An example of a MISFET
with 80 V breakdown voltage having a cell pitch of 3.4 microns is
disclosed in which an on-resistance of 0.7 m.OMEGA.-cm.sup.2 is
realized. The switching characteristics of the MISFET are twice as
good as that of prior MISFET device structures.
Inventors: |
Fujishima, Naoto; (Toronto,
CA) ; Salama, C. Andre T.; (Toronto, CA) |
Correspondence
Address: |
Marc A. Rossi
ROSSI & ASSOCIATES
P.O. Box 826
Ashburn
VA
20146-0826
US
|
Family ID: |
26918863 |
Appl. No.: |
10/457955 |
Filed: |
June 10, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10457955 |
Jun 10, 2003 |
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10007081 |
Nov 13, 2001 |
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10007081 |
Nov 13, 2001 |
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09224605 |
Dec 31, 1998 |
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6316807 |
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09224605 |
Dec 31, 1998 |
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08985762 |
Dec 5, 1997 |
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Current U.S.
Class: |
438/193 ;
257/E29.118; 257/E29.121; 257/E29.258; 257/E29.262;
257/E29.267 |
Current CPC
Class: |
H01L 29/41766 20130101;
H01L 29/41741 20130101; H01L 29/7827 20130101; H01L 29/7809
20130101; H01L 29/7813 20130101; H01L 29/7834 20130101 |
Class at
Publication: |
438/193 |
International
Class: |
H01L 021/337 |
Claims
What is claimed is:
1. A semiconductor device comprising: a substrate of a first
conductivity type including a trench formed therein that extends
from a top surface of the substrate to a defined depth into the
substrate; a dielectric material formed on sidewalls of the trench,
wherein a thickness of the dielectric material at the bottom of the
trench is smaller than a thickness of the dielectric material at
the top of the trench, and wherein a contact hole extends through
the dielectric material at the bottom of the trench to the
substrate; a region of a second conductivity type formed in the
substrate beneath the contact hole; and an electrical
interconnection material formed in the trench that extends from the
top of the trench through the contact hole to contact the region of
second conductivity type.
2. A semiconductor device as claimed in claim 1, wherein a drain
region of the second conductivity type is formed at the surface of
the substrate adjacent to the trench.
3. A semiconductor device as claimed in claim 1, further comprising
a first conductivity type diffusion region that extends from the
upper portions of the sidewalls.
4. A semiconductor device as claimed in claim 3, further comprising
a second conductivity type extended drain region formed in said
first conductivity type diffusion region.
5. A semiconductor device as claimed in claim 1, further comprising
a gate located in said trench and separated from the sidewall of
the trench and the electrical interconnection material by a
dielectric material.
6. A semiconductor device as claimed in claim 5, further comprising
a first conductivity type base at the lower portion and bottom of
the trench.
7. A semiconductor device as claimed in claim 6, further comprising
a second conductive type source in said base at the lower portion
and bottom of the trench.
8. A semiconductor device as claimed in claim 2, further comprising
a metal drain electrode formed on said source region, a metal
electrode formed on said electrical interconnection material and a
metal electrode extended from said gate.
9. A method of manufacturing a MISFET comprising the steps of: a)
forming a trench in a substrate of first conductive type; b)
forming in it a first region of the first conductivity type and a
second region of the second conductivity type into the substrate
through portions of the trench; c) depositing an oxide layer on
portions of sidewalls of trench, wherein said oxide layer extends
from the top of the trench; d) forming an extended trench with
retaining said oxide layer on the upper portion of said trench
sidewall; e) forming a gate oxide layer on the portion of the
sidewalls of said extended trench; f) forming a gate layer on the
gate oxide layer; selectively etching the gate layer, and the gate
oxide layer so that the surface of the substrate is exposed in
regions adjacent to the trench and residual films of the gate layer
and the thick oxide are left on the sidewalls of the trench; g)
forming a base of the first conductivity type and a source of the
second conductivity type at the bottom of the trench; h) forming an
oxide layer inside the trench and on the surface of the substrate
over the drain by a method where oxide growth rate is slower inside
the trench than at the surface of the substrate, wherein the
thickness of the oxide layer within the trench is less than the
thickness of the oxide layer on the surface of the substrate; i)
etching the oxide layer at the bottom of the trench to form a
contact hole that extends to the substrate while maintaining a
thickness of the oxide layer on the sidewalls of the trench and
surface of the substrate using a directional etching method; and j)
forming an electrical interconnection material in the trench that
extends through the contact hole.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application, and claims
priority from, U.S. patent application Ser. No. 09/224,605 filed on
Dec. 31, 1998, which in turn is a continuation-in-part application,
and claims priority from, U.S. patent application Ser. No.
08/985,762 filed on Dec. 5, 1997.
FIELD OF THE INVENTION
[0002] The invention relates in general to lateral semiconductor
devices including a trench structure, and a method of manufacturing
such devices. More specifically, the invention relates to MISFETs
with a high breakdown voltage and a low on-resistance, which can be
incorporated in integrated circuits, power supplies, motors and
other devices.
BACKGROUND OF THE INVENTION
[0003] An example of one type of conventional high voltage lateral
MISFETs with low on-resistance characteristics is shown in FIG. 1.
A high resistive n.sup.- extended drain 10 is formed in a p.sup.-
substrate 8 between a p base region 12 and an n.sup.+ drain region
14 to reduce an electric field between a source region 16 and the
drain region 14. A gate oxide layer 18 under a gate electrode 20 is
thicker at the drain side in order to reduce electric field in the
n.sup.- extended drain 10. Generally, lateral MISFETs consist of
the following four regions shown in FIG. 1: (1) a source region
with a distance of I.sub.1, (2) a channel region with a distance of
I.sub.2, (3) an extended drain region with a distance of I.sub.3,
and (4) a drain region with a distance of I.sub.4. The pitch of the
device is the sum of I.sub.1+I.sub.2+I.sub.3+I.sub.4 and determines
the packing density of the device and its specific on-resistance.
The smaller the pitch, the higher the packing density and the lower
the on-resistance per unit area. Present state of the art MISFETs
with a breakdown voltage of 80 V require I.sub.3 to be 3 .mu.m to
reduce the electric field near the drain and prevent premature
breakdown. The remaining parameters (I.sub.1, I.sub.2, and I.sub.4)
do not influence the breakdown voltage significantly and are
required to be 1.5 .mu.m, 2 .mu.m, and 1.5 .mu.m respectively for
I.sub.1, I.sub.2 and I.sub.4 (for a 1 .mu.m design rule). Thus, the
distance or length of the n.sup.- extended drain 10 is the largest
among all of the regions and must be increased as the breakdown
voltage of the MISFET increases. As a result, the packing density
of the MISFET is sacrificed and on-resistance increases. MISFETS
with the above-described structure have already been described.
See, for example, T. Efland, et al., "Self-Aligned RESURF To LOCOS
Region LDMOS Characterization shows Excellent Rsp vs BV
Performance" Proceedings ISPSD'96, pp. 147-150, 1996, the contents
of which are incorporated herein by reference.
[0004] Results of on-state simulations performed for the structure
shown in FIG. 1 with a substrate doping level of 7.times.10.sup.14
cm.sup.-3, an n.sup.- extended drain surface doping concentration
of 7.times.10.sup.17 cm.sup.-3, and a junction depth of 1.4 .mu.m
are illustrated in FIG. 2. For such simulations, the specific
on-resistance of the device is estimated to be 1.6
m.OMEGA.-cm.sup.2 for a breakdown voltage of 80 V.
[0005] To overcome the packing density limitation discussed above,
MISFETs using trench structures have been proposed by N. Fujishima,
et al. in U.S. patent application Ser. No. 08/547,910. As
illustrated in FIG. 3, a channel 24 and an n.sup.- extended drain
26 are located vertically at a side-wall of a trench formed in a
substrate 28. Since the trench MISFET has the n.sup.- extended
drain 26 between a source region 31 and a drain region 32, and a
thick gate oxide 34 between a gate electrode 36 and the drain
region 32, it is possible to optimize the structure to get almost
the same current handling capability in the unit cell as the
conventional MISFET without reducing the breakdown voltage. The
pitch in this case is determined by the sum of I.sub.1, I.sub.6,
and I.sub.5, which typically have values of 1.5 .mu.m, 2.0 .mu.m
and 0.5 .mu.m respectively (for minimum 1 .mu.m design rules)
resulting in half the pitch of the structure in FIG. 1. Therefore,
packing density per unit area of the MISFET can be increased and a
reduction in on-resistance per unit area achieved.
[0006] However, for the device of FIG. 3, two additional masks are
needed to define the silicon trench and the drain contact holes.
The resulting process also requires strict alignment tolerance
among these three masks. In addition, two deep directional etching
steps are needed to define the gate and make the drain contact hole
inside the initial silicon trench.
[0007] In view of the above, it is an object of the present
invention to provide a lateral MISFET incorporating a high packing
density trench structure and offering high breakdown voltage with
low on-resistance and a method of manufacturing the lateral
MISFET.
SUMMARY OF THE INVENTION
[0008] The present invention provides a semiconductor device
incorporating a trench structure that combines high breakdown
voltage with low on-resistance characteristics and a method for
manufacturing the same. In a first embodiment, a Trench Lateral
Power MISFET (T-LPM) is provided having a gate and channel regions
that are built on the side-wall of the trench. The process used to
form the T-LPM uses self-aligned trench bottom contact holes to
contact a drain at the bottom of the trench to achieve minimum
pitch and very low on-resistance. An example of a T-LPM with 80 V
breakdown voltage having a cell pitch of four microns is disclosed
in which an on-resistance of 0.8 m.OMEGA.-cm.sup.2 is realized.
[0009] More specifically, a semiconductor device is provided that
includes a substrate of a first conductivity type having a trench
formed therein that extends from a top surface of the substrate to
a defined depth into the substrate. A dielectric material is formed
on sidewalls of the trench, wherein a thickness of the dielectric
material at the bottom of the trench is greater than a thickness of
the dielectric material at the top of the trench. A contact hole
extends through the dielectric material at the bottom of the trench
to the substrate. A region of a second conductivity type is formed
in the substrate beneath the contact hole, and an electrical
interconnection material is formed in the trench that extends from
the top of the trench through the contact hole to contact the
region of second conductivity type.
[0010] In the first embodiment a MISFET is provided in which a base
region of the first conductivity type is formed near a surface
region of the substrate adjacent to the trench, and a source region
is formed at the surface of the substrate above the base region. A
first conductivity type diffusion region that extends from portions
of the lower side walls and bottom of the trench, and a second
conductivity type extended drain region is formed in said first
conductivity type diffusion region. The region of second
conductivity type formed under said contact hole is located in the
extended drain region. A gate is located in the trench and is
separated from the side walls of the trench and the electrical
interconnection material by the dielectric material.
[0011] Process steps for forming the first embodiment include:
[0012] a) forming a trench in a substrate of first conductivity
type;
[0013] b) growing a pad oxide in the trench;
[0014] c) depositing a nitride layer and etching the nitride layer
to leave residual nitride layers that extend from the top of the
trench and along the side walls of the trench;
[0015] d) extending the depth of the trench into the substrate;
[0016] e) depositing a thick oxide layer on the top of the
substrate, the portions of the sidewall of the trench not covered
by the residual nitride layer and the bottom of the trench;
[0017] f) removing the residual nitride layer and pad oxide and
forming a gate oxide layer on the portions of the side walls that
were previously covered by the residual nitride layer;
[0018] g) forming a gate layer on the gate oxide layer;
[0019] h) forming an oxide layer over the gate layer;
[0020] i) selectively etching the oxide layer formed over the gate
layer, the thick oxide layer and the gate layer so that the surface
of the substrate is exposed in regions adjacent to the trench and
residual films of the gate layer and the thick oxide are left at
the side-walls of the trench;
[0021] j) forming an oxide layer inside the trench and on the
surface of the substrate by a method where oxide growth rate is
slower inside the trench than at the surface of the substrate,
wherein the thickness of the oxide layer within the trench is less
than the thickness of the oxide layer on the surface of the
substrate;
[0022] k) etching the oxide layer at the bottom of the trench to
form a contact hole that extends to the substrate while maintaining
a thickness of the oxide layer on the side walls of the trench and
a thickness of the oxide layer on the surface of the substrate
using a directional etching method; and
[0023] l) forming an electrical interconnection material in the
trench that extends through the contact hole.
[0024] In the preferred method of forming a MISFET device, a first
region of the first conductivity type and a second region of a
second conductivity type are formed in the substrate through the
portions of the sidewall of the trench not covered by the residual
nitride layer after step (d) and before step (e). A contact region
of second conductivity type in the second region of second
conductivity type is formed through the contact hole after step (k)
and before step (i). A base of the first conductive type and a
source of the second conductive type are formed in the substrate in
the exposed regions adjacent to the trench after step (i) and
before step (j). The second region of second conductivity type
comprises an extended drain region.
[0025] In a second embodiment of the invention, a semiconductor
device is provided comprising a substrate of a first conductivity
type including a trench formed therein that extends from a top
surface of the substrate to a defined depth into the substrate. A
dielectric material formed on sidewalls of the trench, wherein a
thickness of the dielectric material at the bottom of the trench is
smaller than a thickness of the dielectric material at the top of
the trench. A contact hole extends through the dielectric material
at the bottom of the trench to the substrate. A region of a second
conductivity type formed in the substrate beneath the contact hole;
and an electrical interconnection material formed in the trench
that extends from the top of the trench through the contact hole to
contact the region of second conductivity type.
[0026] In the second embodiment a MISFET is provided in which a
drain region of the second conductivity type is formed at the
surface of the substrate adjacent to the trench. A first
conductivity type diffusion region that extends from the upper
portions of the sidewalls, and a second conductivity type extended
drain region formed in said first conductivity type diffusion
region. A gate located in said trench and separated from the
sidewall of the trench and the electrical interconnection material
by a dielectric material. A first conductivity type base at the
lower portion and bottom of the trench, and a second conductive
type source in said base at the lower portion and bottom of the
trench. A metal drain electrode is formed on the source region, a
metal electrode is formed on the electrical interconnection
material and a metal electrode is extended from the gate.
[0027] Process steps for forming the second embodiment include:
[0028] a) forming a trench in a substrate of first conductive
type;
[0029] b) forming in it a first region of the first conductivity
type and a second region of the second conductivity type into the
substrate through portions of the trench;
[0030] c) depositing an oxide layer on portions of sidewalls of
trench, wherein said oxide layer extends from the top of the
trench;
[0031] d) forming an extended trench with retaining said oxide
layer on the upper portion of said trench sidewall;
[0032] e) forming a gate oxide layer on the portion of the
sidewalls of said extended trench;
[0033] f) forming a gate layer on the gate oxide layer;
[0034] selectively etching the gate layer, and the gate oxide layer
so that the surface of the substrate is exposed in regions adjacent
to the trench and residual films of the gate layer and the thick
oxide are left on the sidewalls of the trench;
[0035] g) forming a base of the first conductivity type and a
source of the second conductivity type at the bottom of the
trench;
[0036] h) forming an oxide layer inside the trench and on the
surface of the substrate over the drain by a method where oxide
growth rate is slower inside the trench than at the surface of the
substrate, wherein the thickness of the oxide layer within the
trench is less than the thickness of the oxide layer on the surface
of the substrate;
[0037] i) etching the oxide layer at the bottom of the trench to
form a contact hole that extends to the substrate while maintaining
a thickness of the oxide layer on the sidewalls of the trench and
surface of the substrate using a directional etching method;
and
[0038] j) forming an electrical interconnection material in the
trench that extends through the contact hole.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The invention will be described in greater detail with
reference to certain preferred embodiments thereof and the
accompanying drawings, wherein:
[0040] FIG. 1 is a cross-sectional view of a conventional lateral
power MISFET;
[0041] FIG. 2 shows on-state simulation for the conventional
lateral power MISFET of the type illustrated in FIG. 1;
[0042] FIG. 3 is a cross-sectional view of a trench lateral power
MISFET;
[0043] FIG. 4 is a top view of a trench lateral power MISFET in
accordance with a first embodiment of the present invention;
[0044] FIG. 5 is a cross-sectional view of the power MISFET
illustrated in FIG. 4 taken along the line A-B;
[0045] FIG. 6 is a cross-sectional view of the power MISFET
illustrated in FIG. 4 taken along the line C-D;
[0046] FIG. 7 illustrates an oxide deposition and etching step in
the process used to form the power MISFET of FIG. 4;
[0047] FIG. 8 illustrates a silicon trench etching step in the
process used to form the power MISFET illustrated in FIG. 4;
[0048] FIG. 9 illustrates pad oxidation and nitride deposition in
the process used to form the power MISFET of FIG. 4;
[0049] FIG. 10 illustrates nitride etching in the process used to
form the power MISFET of FIG. 4;
[0050] FIG. 11 illustrates silicon trench etching a p-body/n-drain
formation in the process used to form the power MISFET of FIG.
4;
[0051] FIG. 12 illustrates formation of a thick oxide layer in the
process used to form the power MISFET of FIG. 4;
[0052] FIGS. 13(a), 13(b) illustrate gate oxidation and polysilicon
gate deposition in the process used to form the power MISFET along
lines A-B and C-D of FIG. 4, respectively;
[0053] FIGS. 14(a), 14(b) illustrate polysilicon gate definition
and formation of p-base and n+ and p+ regions in the process used
to form the power MISFET along lines A-B and C-D of FIG. 4,
respectively;
[0054] FIGS. 15(a), 15(b) illustrate oxide deposition in the
process used to form the power MISFET along lines A-B and C-D of
FIG. 4, respectively;
[0055] FIGS. 16(a), 16(b) illustrate oxide etching and n+ region
formation at the bottom of the trench in the process used to form
the power MISFET along the lines A-B and C-D of FIG. 4,
respectively;
[0056] FIGS. 17(a), 17(b) illustrate polysilicon drain definition
in the process used to form the power MISFET along the lines A-B
and C-D of FIG. 4, respectively;
[0057] FIGS. 18(a), 18(b) illustrate contact opening and
metallization in the process used to form the power MISFET along
the lines A-B and C-D of FIG. 4, respectively;
[0058] FIG. 19 illustrates on-state simulation for the trench
lateral power MISFET illustrated in FIG. 4;
[0059] FIG. 20 is an SEM micrograph after oxide deposition;
[0060] FIG. 21 illustrates the ratio of oxide thickness between
surface and bottom as a function of trench width;
[0061] FIG. 22 is an SEM micrograph after RIE etching of oxide;
[0062] FIG. 23 illustrates the size of the contact hole and oxide
thickness at side-wall as a function of trench width;
[0063] FIG. 24 SEM micrograph of a top view of a lateral trench
power MISFET in accordance with the invention;
[0064] FIG. 25 is a graph illustrating trade-offs between specific
on-resistance and breakdown voltage;
[0065] FIG. 26 is a top view of a trench lateral MISFET according
to a second embodiment of the invention;
[0066] FIG. 27 is a cross-sectional view of the trench lateral
MISFET illustrated in FIG. 26;
[0067] FIG. 28 is a cross-sectional view of the trench lateral
MISFET with device dimensions;
[0068] FIG. 29 is a cross-sectional view of silicon etching and p
body/n.sup.- drain formation;
[0069] FIG. 30 is a cross-sectional view of P body/n.sup.- drain
drive and growth of thick oxide;
[0070] FIG. 31 is a cross-sectional view of oxide and second
silicon trench etching;
[0071] FIG. 32 is a cross-sectional view of gate oxidation,
polysilicon deposition;
[0072] FIG. 33 is a cross-sectional view of mask oxide patterning,
RIE of polysilicon and p base/n.sup.+ source formation;
[0073] FIGS. 34(a), 34(b) illustrate deposition of oxide by CVD
along the lines A-B and C-D of FIG. 26, respectively;
[0074] FIGS. 35(a), 35(b) illustrate the source contact hole
opening along the lines A-B and CD of FIG. 26, respectively;
[0075] FIGS. 36(a), 36(b) illustrate source polysilicon deposition,
leveling, contact opening and metallization along the lines A-B and
C-D of FIG. 26, respectively;
[0076] FIG. 37 is a graph of the trade-off between specific
on-resistance and breakdown voltage; and
[0077] FIG. 38 is a table illustrating switching times.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0078] A top view and cross-sectional view of a first embodiment of
the Trench Lateral MISFET are shown in FIGS. 4, 5, and 6. In order
to realize a wider channel and increase the current handling
capability of the device, a source electrode 50 and a drain
electrode 52 having an interdigitated form are provided as shown in
FIG. 4. Cross-sectional views of an active area (taken along line
A-B in FIG. 4) and a gate area 55 coupled to a gate electrode 54
(taken along line C-D in FIG. 4) are respectively shown in FIGS. 5
and 6. The MISFET has a channel region 56 along the sidewall of a
trench formed in a p.sup.- substrate 62, a source region 60 located
at the top of the trench, and a drain region 58 located at the
bottom of the trench. The trench extends from a top surface of the
substrate 62 to a defined depth I.sub.1, and an n.sup.- extended
drain 66 and p body 68 are formed around a lower portion of the
trench by implantation through a window defined by the sidewalls
and bottom of the trench as will be described in greater detail.
The MISFET exhibits a low on-resistance and a high breakdown
voltage when the impurity profile between the p body 68 and n.sup.-
extended drain 66 are optimized. When a positive potential higher
than the threshold average is applied to the gate electrode 54,
which is coupled to a polysilicon gate layer 82, an inversion layer
is created along the sidewall of the trench in a p base region 64
located under the source region 60, the p.sup.- substrate 62 and
the p body 68. The current in the drain is collected through drain
electrode 52, which is coupled to the drain region 58 at the bottom
of the trench by an electrical interconnection material such as a
polysilicon plug 61.
[0079] In this structure, the channel region 56 and the extended
drain 66 need to be long enough to achieve the required breakdown
voltage. For an 80 V MISFET, the channel length I.sub.7 needs to be
2 .mu.m and the length of the extended drain I.sub.8 needs to be 3
.mu.m. However, since the structure is vertical, these dimensions
do not affect the device pitch, which is determined by half the
contact opening at the drain I.sub.5, the lateral distance I.sub.6
between the edge of the drain region 58 and the edge of the source
region, and the length I.sub.1 of the source region. For a 1 .mu.m
minimum design rule, I.sub.5=0.5 .mu.m, I.sub.6=2 .mu.m, and
I.sub.1=1.5 .mu.m resulting in a device pitch of 4 .mu.m, which is
half the value of the pitch in the conventional MISFET of FIG.
1.
[0080] The (100) silicon plane is used to implement the device by
orienting the main sidewall plane in the trench 45 degrees away
from the <110> axis of the (100) orientation wafer, which
results in very high electron mobility in the channel. In addition,
the current in the extended drain 66 flows mainly in the bulk,
instead of at the surface, thus avoiding mobility degradation due
to damage associated with trench formation.
[0081] Process flow of the Trench Lateral MISFET is will now be
described with reference to FIGS. 7-17. First, an oxide film 70 is
deposited on the p-type substrate 62 and then selectively etched
after photolithography (first mask) as shown in FIG. 7. The silicon
substrate 62 is then etched by RIE to form a trench 71, as
illustrated in FIG. 8, and a pad oxide 73 is grown in the trench.
Following the growth of the pad oxide 73, a silicon nitride layer
72 is deposited on the pad oxide film 73 (FIG. 9) and etched by RIE
to leave residual portions of the nitride layer 72 on side-walls of
the trench 71 as shown in FIG. 10. Thereafter, the silicon
substrate 62 is etched by RIE once again to extend the depth of the
trench 71 past the residual nitride layer 72. Tilted
ion-implantation of boron is then performed at the side-walls of
the trench 71 and the boron is driven into the substrate 62 to
create the p body 68. Next, tilted ion-implantation of phosphorus
is performed and annealed to create the n.sup.- extended drain 66
as shown in FIG. 11.
[0082] Wet oxidation is used to grow a thick oxide layer 74 at the
surface of the substrate 62 and at the lower portions of the
sidewalls, which extend beyond the residual nitride layer 72, and
bottom of the trench 71 as shown in FIG. 12. The residual nitride
layer 72 and the pad oxide 73 are then removed. A gate oxide layer
80 and gate polysilicon layer 82 are then deposited, and followed
by the deposition of a further oxide layer 84. The oxide layer 84
at the top is selectively etched using a second mask to define the
actual gate region as shown in FIGS. 13(a), 13(b). Next, the
polysilicon layer 82 and the thick oxide 74 are etched with RIE and
residual portions of polysilicon layer 82 and the thick oxide 74
are left at the side-walls. Thereafter, p base ion-implantation is
performed selectively using a third mask and annealed to form the p
base 64. This step is followed by the formation by ion implantation
of an n+ region 88 and p+ region 90 of the source region 60 using
fourth and fifth masks, respectively, as shown in FIGS.
14(a),14(b). An oxide layer 92 is then deposition by LPCVD as shown
in FIGS. 15(a), 15(b). Because the reactants do not migrate rapidly
along the surface at the temperature used for LPCVD, the thickness
t.sub.2 of the oxide inside the trench is thinner than the
thickness of the oxide t.sub.1 at the surface of the substrate
(t.sub.2<t.sub.1). RIE is then used to create a contact hole at
the bottom of the trench as shown in FIG. 16(a), namely, since RIE
has strong directional etching properties, the oxide film 92 at the
bottom of the trench 71 is completely removed and silicon substrate
62 is exposed. On the other hand, the oxide 92 at the side-walls
and at the top surface is retained and is thick enough to provide
good electrical isolation between the gate and the drain. An n+
drain region 58 is formed at the bottom of the trench by
ion-implantation shown in FIG. 16(a). Thereafter, the polysilicon
plug 61 is deposited to fill the trench and patterned using a sixth
mask as shown in FIGS. 17(a), 17(b). Finally contact windows are
opened, using a seventh mask, at the surface of the substrate, and
metal is deposited for the source electrode 50, drain electrode 52
and gate electrode 54 as illustrated in FIG. 18 using an eighth
mask.
[0083] Process and device simulation was performed for the Trench
Lateral MISFET utilizing the same doping levels and junction depths
for the simulation illustrated in FIG. 2. A result for the on-state
characteristics of the device is shown in FIG. 19. From these
simulations, the specific on-resistance of 0.8 m.OMEGA.-cm.sup.2 at
a breakdown voltage of 80V.
[0084] An experimental verification of the feasibility of the
trench bottom contact implementation is illustrated in the SEM
micrographs of FIGS. 20 and 21. FIG. 20 shows the trench after the
deposition of the thick oxide. In this case, the deposited oxide
thickness t.sub.1 at the surface is 2.5 .mu.m, while the oxide
thickness t.sub.2 at the bottom of the trench is 1.2 .mu.m, and the
trench bottom width 2.times.I.sub.5 is 1.3 .mu.m. The ratio of
oxide thickness between the surface and the trench bottom as a
function of the bottom trench width is illustrated in FIG. 21. FIG.
22 shows the defined bottom contact. The contact hole realized by
RIE has a width t.sub.4 of 0.7 .mu.m in a 3.5 .mu.m deep trench
with a side-wall oxide thickness t.sub.5 of 0.3 .mu.m. Size of the
contact holes and the oxide thicknesses at the side-wall are shown
as a function of trench width in FIG. 23.
[0085] The invention makes it possible to increase packing density
of the MISFET while keeping current handling capability in the unit
area high resulting in significant reduction of specific
on-resistance for the MISFET. FIG. 24 is a SEM micrograph of a top
view of a lateral trench MISFET in accordance with the invention.
The trench lateral MISFET exhibits one of the lowest specific
on-resistance values ever reported as illustrated in FIG. 25. See,
for example, T. Efland, et al., "An Optimized RESURF LDMOS Power
Device Module Compatible with Advanced Logic Processes" IEDM Tech
Dig., pp. 237-240, 1992, the contents of which are incorporated
herein by reference.
[0086] In second embodiment of the invention, the structure of the
device is modified to improve switching performance. The objective
of this work is to implement a device structure with a trench
bottom contact hole for the source which offers a smaller Miller
capacitance in order to improve switching performance of the
device. In order to realize a smaller Miller capacitance, a source
is formed at the bottom of the trench. A top view and a
cross-sectional view of the improved trench lateral MISFET are
respectively shown in FIGS. 26 and 27.
[0087] The major difference between the second embodiment shown in
FIG. 27 and the first embodiment, shown in FIG. 15, is that a
source 202 is formed at the bottom of a trench 204 and an extended
drain 206 is located at the upper portion of the trench sidewall.
One of the advantages of this structure is that Cgd (Miller
capacitance) would be reduced to less than half of that of the
first embodiment because the plugged polysilicon 208 at the bottom
of the trench 204 contacts the source 202 instead of the drain 206.
Thus Cgd is generated only between the gate 210 and n.sup.- drain
206.
[0088] The process to manufacture the second embodiment will
utilize a self-aligned method to form the gate electrodes and the
trench bottom contact holes to the source to achieve minimum pitch
and very low on-resistance. The source contact holes will be filled
with polysilicon to access the bottom source from the surface and
to level the surface for metal formation.
[0089] The MISFET shows a driving current of 55 mA per mm of
channel width and a specific on-resistance of 0.8 m.OMEGA.-cm.sup.2
for an 80V device when the design uses a 1 .mu.m minimum feature
size. When a 0.6 .mu.m feature size were allowed, then the specific
on-resistance of the MISFET would become 0.7 m.OMEGA.-cm.sup.2 for
an 80V device.
[0090] As shown in FIG. 27, the MISFET is formed along the
sidewalls of the trenches. The drain contact 212 is located at the
surface. The channel, n.sup.- drain 206, gate oxide 214, thick
oxide 216, and gate electrode 210 are formed along the sidewalls.
The n.sup.+ source 202 at the bottom of the trench 204 is connected
and brought to the surface through a polysilicon plug 208. When a
positive bias, higher than the threshold voltage, is applied to the
gate 210, an inversion layer is created and an electron current
flows from the source electrode 218 through the polysilicon plug
208 to the n.sup.+ source 202 at the bottom of the trench 204, and
is collected by the drain 220 at the surface. In order to decrease
the electric field under the gate 210, a thick oxide 216 is used at
the top of the sidewall. The (100) sidewall plane, which has been
shown to have the lowest interface-trap density and the highest
surface electron mobility, is used in the implementation of the
device by orienting the main sidewall plane 45.degree. away from
the <110> axis of the (100) orientation wafer. In addition,
the current in the n.sup.- drain 206 flows mainly in the bulk
instead of at the surface, thus avoiding mobility degradation due
to damage associated with trench formation.
[0091] Brief explanation of device dimensions are described in FIG.
28. The depth of the second trench I.sub.14 is about 2 .mu.m. Total
depth of the trench I.sub.15 is about 5 .mu.m. The length of the
source contact I.sub.11, the distance between the source and drain
I.sub.12, and the length of the drain I.sub.13 are 0.5, 2.0, and
1.5 .mu.m, respectively under 1 .mu.m design rule. Usage of 0.6
.mu.m minimum feature size reduces I.sub.11 and I.sub.13 to 0.3 and
0.9 .mu.m, respectively.
[0092] The process used in the fabrication of the second embodiment
is illustrated in FIGS. 29 to 36. First, n.sup.- diffusion is
performed using the first mask. The obtained junction depth and
surface concentration of the diffusion are about 1 .mu.m and
le17-cm.sup.-3, respectively. Then a shallow trench whose depth is
3 .mu.m is etched in a p-type silicon substrate using the second
mask. Thereafter the p body and n.sup.- drain are formed by using
tilted ion-implantation (FIG. 29). After the p body and n.sup.-
drain are driven, wet oxidation is performed to grow a thick oxide
at the bottom of the trench as well as at the surface. The junction
depth of the n.sup.- drain is about 1 .mu.m. The surface
concentration of the n.sup.- drain is about le17-cm.sup.-3. The
thickness of the grown oxide is about 0.5 .mu.m (FIG. 30). Next,
the oxide and the second silicon trench is etched using RIE. The
additional etching depth I.sub.14 is 2 .mu.m. Since RIE etching is
anisotropic, most of the oxide remains on the sidewall (FIG. 31).
After the 0.1 .mu.m gate oxide is formed, polysilicon with a
thickness of 0.5 .mu.m is deposited (FIG. 32). A further oxide
layer is deposited on the surface. The oxide layer at the top is
selectively etched using the third mask to define the actual gate
area. The polysilicon is etched by RIE using the top oxide layer as
a mask. Then the p base and n+region are formed using 4th and 5th
masks. The junction depths of the p base and n.sup.+ region are 1.0
and 0.2 .mu.m, respectively. The surface concentration of the
n.sup.+ is le20-cm.sup.-3 (FIGS. 33(a) and (b)).
[0093] A critical part of the process is the creation of contact
holes at the bottom of the trench. A 1.0 .mu.m oxide layer is
deposited by CVD in the trench as illustrated in FIGS. 34(a) and
(b). Because the reactants do not migrate rapidly along the surface
at the temperature of about 400.degree. C. used for CVD, the oxide
inside the trench is thinner than that at the surface
(t.sub.2<t.sub.1) [5]. RIE is then used to remove the oxide by
0.5 .mu.m directionally. This creates a contact hole at the bottom
of the trench as shown in FIG. 35(a). Since RIE has strong
directional etching properties, the oxide film at the bottom of the
trench is completely removed and the silicon is exposed. On the
other hand, the oxide at the sidewalls and at the top surface is
retained and is thick enough to provide good electrical isolation
between the gate and the source.
[0094] Following contact hole formation, the contact hole is
plugged with polysilicon then the polysilicon is patterned by the
6th mask. After contact holes are opened by the 7th mask,
metallization is defined using the 8th mask (FIGS. 36(a) and
(b)).
[0095] FIG. 38 lists a comparison of switching figure of merit
between the first embodiment and second embodiment trench lateral
MISFETs. Since the Cgd for the second embodiment MISFET is half of
the first embodiment MISFET, the figure of merit for the second
embodiment MISFET is twice as good as the conventional one.
[0096] Trade off between specific on-resistance and breakdown
voltage is shown in FIG. 37. On-resistance of the proposed trench
lateral MISFET will be reduced by about 50% (using a 0.6 .mu.m
minimum feature size), bringing it close to the silicon limit (0.7
m.OMEGA.-cm.sup.2 for an 80 V device).
[0097] The invention has been described with reference to certain
preferred embodiments thereof. It will be understood, however, that
modifications and variations are possible within the scope of the
appended claims. The method of forming the thick oxide inside the
trench and creating contact holes at the trench bottom is useful
not only for MISFETs, but can be employed to manufacture other
devices including diodes, bipolar transistors, IGBTs and MESFETs
and DRAM cells which require a contact at the bottom of a
trench.
* * * * *