U.S. patent application number 10/861651 was filed with the patent office on 2004-11-11 for method for fabricating an integrated circuit with a transistor electrode.
Invention is credited to Li, Li-Chun, Lo, Wen-Wei, Pittikoun, Saysamone, Wu, Chung-Cheng, Wu, Huoy-Jong.
Application Number | 20040224452 10/861651 |
Document ID | / |
Family ID | 25412719 |
Filed Date | 2004-11-11 |
United States Patent
Application |
20040224452 |
Kind Code |
A1 |
Li, Li-Chun ; et
al. |
November 11, 2004 |
Method for fabricating an integrated circuit with a transistor
electrode
Abstract
Dynamic Random Access Memory (DRAM) cells are formed in a P well
formed in a biased deep N well (DNW). PMOS transistors are formed
in N wells. The NMOS channels stop implant mask is modified not to
be a reverse of the N well mask in order to block the channels stop
implant from an N+ contact region used for DNW biasing. In DRAMs
and other integrated circuits, a minimal spacing requirement
between a well of an integrated circuit on the one hand and
adjacent circuitry on the other hand is eliminated by laying out
the adjacent circuitry so that the well is located adjacent to a
transistor having an electrode connected to the same voltage as the
voltage that biases the well. For example, in DRAMs, the minimal
spacing requirement between the DNW and the read/write circuitry is
eliminated by locating the DNW next to a transistor precharging the
bit lines before memory accesses. One electrode of the transistor
is connected to a precharge voltage. This electrode overlaps the
DNW which is biased to the same precharge voltage. This electrode
provides the DNW N+ contact region.
Inventors: |
Li, Li-Chun; (Los Gatos,
CA) ; Wu, Huoy-Jong; (Taichung City, TW) ; Wu,
Chung-Cheng; (Tou-Cheng City, TW) ; Pittikoun,
Saysamone; (Hsinchu City, TW) ; Lo, Wen-Wei;
(Pan-Chiao City, TW) |
Correspondence
Address: |
MACPHERSON KWOK CHEN & HEID LLP
1762 TECHNOLOGY DRIVE, SUITE 226
SAN JOSE
CA
95110
US
|
Family ID: |
25412719 |
Appl. No.: |
10/861651 |
Filed: |
June 3, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10861651 |
Jun 3, 2004 |
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10136498 |
Apr 30, 2002 |
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6777280 |
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10136498 |
Apr 30, 2002 |
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09053557 |
Apr 1, 1998 |
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6406953 |
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09053557 |
Apr 1, 1998 |
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08900560 |
Jul 25, 1997 |
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6133597 |
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Current U.S.
Class: |
438/199 ;
257/E21.544; 257/E21.552; 257/E21.66; 257/E27.097 |
Current CPC
Class: |
H01L 27/10897 20130101;
H01L 27/10894 20130101; H01L 21/76202 20130101; H01L 21/761
20130101 |
Class at
Publication: |
438/199 |
International
Class: |
H01L 021/8238 |
Claims
1-10. (canceled).
11. A method for fabricating an integrated circuit, the method
comprising: introducing a dopant of a first conductivity type into
a semiconductor substrate to form a first well; and forming a
transistor T1 having first and second electrodes of the first
conductivity type, the first electrode having at least a portion
inside the first well, wherein the first electrode is connected to
a voltage for: (1) biasing the first well, and (2) coupling the
voltage to the second electrode when the transistor is on.
12. The method of claim 11 further comprising: introducing a dopant
of a second conductivity type into the first well to form a second
well inside the first well; and forming one or more transistors T2
having electrodes of the first conductivity type in the second
well.
13-20. (canceled).
21. The method of claim 11 wherein the voltage is between about
1.6V and about 2.5V.
22. The method of claim 11 wherein the dopant of a first
conductivity type comprises phosphorous.
23. The method of claim 11 wherein transistor T1 further comprises
a gate, wherein the gate receives an equalization signal.
24. The method of claim 12 wherein the dopant of a second
conductivity type comprises boron.
25. The method of claim 12 wherein at least one of the electrodes
of the first conductivity type in the second well is connected to a
capacitor.
26. The method of claim 12 wherein at least one of the electrodes
of the first conductivity type in the second well is connected to a
bit line, wherein the bit line is further connected to the second
electrode of transistor T1.
27. The method of claim 12 wherein the second well contains
electrodes of a dynamic random access memory (DRAM) cell
transistor.
28. The method of claim 12 further comprising: introducing a dopant
of the first conductivity type into the semiconductor substrate to
form a third well of the first conductivity type, wherein the third
well is for holding second-conductivity-type electrodes of one or
more transistors, and wherein the third well does not overlap with
the first well.
29. The method of claim 11, wherein the second electrode is outside
of the first well.
30. The method of claim 11, wherein the transistor T1 has a channel
region of the second conductivity type.
31. The method of claim 11, wherein the first well is of the first
conductivity type.
32. The method of claim 12, wherein the second well is of the
second conductivity type.
33. A method for fabricating an integrated circuit, the method
comprising: introducing a dopant of a first conductivity type into
a semiconductor substrate to form a first well of a first
conductivity type; and forming a transistor T1 having first and
second electrodes of the first conductivity type, the first
electrode having at least a portion inside the first well and a
portion outside the first well, wherein the first electrode is
connected to a voltage for: (1) biasing the first well, and (2)
coupling the voltage to the second electrode when the transistor is
on.
34. The method of claim 33 wherein the portion of the first
electrode outside the first well is in a region of a second
conductivity type.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to semiconductor circuits, and
more particularly to wells used in semiconductor circuits.
[0002] Some semiconductor circuits use wells biased at a
predetermined voltage to obtain needed functionality or performance
characteristics. For example, biased wells can be used to isolate
transistors from each other. Thus, in some dynamic random access
memories (DRAMs), NMOS transistors of DRAM cells are formed in a
P-well formed in a biased deep N well (DNW) that isolates the P
well from the P doped substrate. The P well itself is biased at a
lower voltage than the substrate. Hence, the body regions of DRAM
cell transistors in the P well are biased at a lower voltage than
the body regions of NMOS transistors of read/write circuitry (for
example, of sense amplifiers) that are fabricated in the substrate.
The lower bias voltage in the P well reduces the leakage current
through the DRAM cell transistors. The leakage current through
these transistors is of concern because it could discharge the
cells. At the same time, the lower bias voltage is not suitable for
read/write NMOS transistors because the lower bias voltage would
make these transistors slower. (Of note, the leakage current is not
as big a concern for the read/write transistors as for the DRAM
cell transistors.) The biased DNW isolates the DRAM cell
transistors from the read/write transistors.
[0003] In order to improve the electrical contact between a voltage
source biasing the DNW and the DNW itself, the DNW is provided with
a low-resistance, heavily-doped N+ contact region located at the
substrate surface. The N+ contact region is formed in a separate N
well which itself is formed in the DNW. The reason for the separate
N well is as follows.
[0004] One of the DRAM fabrications steps is a channel stop
implant. The channel stop implant is a P-type implant performed
into the NMOS transistor active areas and into field isolation
regions. The purpose of the channel stop implant is to increase the
punch-through voltages of NMOS transistors and the punch-through
and threshold voltages of parasitic field transistors. The channel
stop implant is blocked from N wells in which PMOS transistors are
formed. To simplify mask generation, the mask for the channel stop
implant is made to be a reverse of the mask used for the N-type
implant that creates the N wells. Thus, the channel stop implant is
implanted precisely into those areas which are blocked from the
N-well implant.
[0005] Besides the N wells containing the PMOS transistors, the
channel stop implant is also blocked from the N+ contact region
used to bias the DNW. This is done to prevent the channel stop
P-type dopant from impeding electrical contact between the N+
contact region and the DNW. In order to enable the channel stop
implant mask to be the reverse of the N well mask and still to
block the channel stop implant from the N+contact region, the N+
contact region is formed in the separate N well which is formed
with the same N well mask as used for the N wells containing the
PMOS transistors.
[0006] It is desirable to reduce spacings associated with wells in
the integrated circuit. Of note, a minimal spacing is typically
required between a well and transistors outside the well. For
example, in DRAMs a minimal spacing is required between the DNW and
read/write circuitry transistors. It is desirable to reduce such
spacings.
SUMMARY OF THE INVENTION
[0007] According to the present invention, integrated circuit
spacing requirements are reduced. In some embodiments, spacing
requirements between wells and transistors outside the wells are
eliminated. Therefore, the integrated circuit size can be
reduced.
[0008] More particularly, in some embodiments, the separate N wells
containing the N+ contact regions in the DNWs are eliminated. This
is made possible by modifying the channel stop mask not to be a
reverse of the N well mask.
[0009] Further, spacing requirements between wells and transistors
outside the wells are eliminated as follows. When transistors
outside the well (e.g., a DNW) are laid out, the transistor placed
adjacent to the well is a transistor that can be used to bias the
well. This transistor couples a predetermined voltage from one of
its electrodes to the other. For example, in a DRAM, this
transistor can be a precharge transistor that couples a
predetermined voltage to a bit line to precharge the bit line
before a memory access (e.g., a memory read operation). The
predetermined voltage is also suitable to bias the well. The
transistor electrode that receives the predetermined voltage is at
least partially inside the well, biasing the well to the
predetermined voltage. Therefore, the minimal spacing requirement
between the well and the transistor is eliminated.
[0010] In some DRAM embodiments, the channel stop implant mask
blocks at least a portion of an area in which the DNW overlaps the
precharge transistor drain region. Hence, the channel stop P dopant
is prevented from impeding the electrical contact between the DNW
and the drain region.
[0011] In some embodiments, P and N conductivity types are
reversed.
[0012] Other features of the invention are described below. The
invention is defined by the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a block diagram of a DRAM according to the present
invention.
[0014] FIG. 2 shows a cross-section of a portion of the DRAM of
FIG. 1.
[0015] FIGS. 3A, 3B are a circuit diagram of a portion of the DRAM
of FIG. 1.
[0016] FIGS. 4A, 4B are a top layout view of the portion of the
DRAM of FIG. 1.
[0017] FIGS. 5 and 6 are cross-section illustrations of the DRAM of
FIG. 1 in the process of fabrication.
[0018] FIG. 7 is a cross-sectional view showing drawn dimensions in
some embodiments of FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0019] FIG. 1 shows an integrated DRAM 110. Memory arrays 120.1,
120.2 in DRAM 110 alternate with read/write (R/W) circuitry blocks
130.1, 130.2, 130.3 along the horizontal direction in FIG. 1 (the
memory column direction). Each memory array 120 is surrounded on
the left and right by R/W circuitry blocks 130. Each memory array
120 is an array of memory cells. Each cell includes an NMOS
transistor 210 (FIG. 2). Source regions 210S and drain regions 210D
of transistors 210 of each memory array are formed in one or more P
wells 216. Each P well 216 is formed in a corresponding deep N well
(DNW) 220.
[0020] Drain 210D of each transistor 210 is connected to a bit line
BL (or a complimentary bit line {overscore (BL)}, not shown in FIG.
2). Source 210S is connected to a memory capacitor 226 which is
also connected to a reference voltage VREF. P well 216 is biased to
a negative voltage, -1.0V in some embodiments in which VCC=3.3V, to
reduce leakage through transistors 210. Deep N well 220 is biased
to a positive voltage HVCC (half VCC) to isolate the P well 216
from P substrate 230. VCC is 3.3V or 5.0V in some embodiments.
[0021] In an adjacent read/write circuitry block 130 on the right
of memory array 120 in FIG. 2, the transistor adjacent to DNW 220
is a bit line precharge transistor 236. Drain 236D of transistor
236 is connected to the same voltage source HVCC that biases the
deep N well 220. Source 236S is connected to a bit line BL (or
{overscore (BL)}). Gate 236G receives an equalization signal EQ.
Signal EQ is asserted high before a memory access operation to
precharge the bit lines to HVCC.
[0022] A portion of drain 236D is inside DNW 220 so that the DNW
overlaps the drain 236D. Therefore, no minimal spacing is required
between the R/W circuitry block 130 and the deep N well.
[0023] Similarly, in R/W block 130 on the left of the memory array
(not shown in FIG. 2), a bit line precharge transistor has a drain
region overlapping DNW 220. The drain region is connected to HVCC.
Therefore, no minimal spacing is required.
[0024] In some embodiments, the drains of the bit line precharge
transistors 236 are the only N+ contact regions in the DNW that
connect the DNW to HVCC.
[0025] As seen in FIG. 1, DRAM 110 includes four boundaries between
memory arrays 120 and R/W circuitry blocks 130. Hence, four minimal
spacings are eliminated. The number of spacings eliminated
increases with the number of memory arrays. Some 4 Mb DRAMs include
16 memory arrays, and hence 32 boundaries between memory arrays and
read/write circuitry blocks. Significant size reduction is
therefore achieved.
[0026] In FIG. 1, memory columns and their respective bit lines BL,
{overscore (BL)}, extend horizontally. Memory rows and their
respective word lines WL extend vertically. Each memory array 120.i
is subdivided into a number of identical memory blocks M-i.j. (A
circuit diagram of block M-1.2 is shown in FIG. 3A.) Each memory
block M-i.j is a single column of memory array 120.i. Each word
line WL of memory array 120.i runs through all blocks M-i.j of the
array. Only one pair of bit lines BL, {overscore (BL)} runs through
any given memory block M-i.j.
[0027] The bit line pair BL, {overscore (BL)} of block M-i.j is
connected to a read/write block RW-k.j (FIGS. 1-3) in an adjacent
R/W circuitry block 130.k (where k=i or k=i+1) in a staggered
fashion. Thus, the bit lines of memory block M-1.1 are connected to
read/write block (RW block) RW-1.1 on the left of memory array
120.1. The bit lines of memory block M-1.2 are connected to RW
block RW-2.2 on the right of memory array 120.1, and so on. Block
RW-2.2 is also connected to memory block M-2.2. Additional details
of this staggered architecture are described in U.S. patent
application "DRAM With Staggered Shared Bit Line Sense Amplifier
Architecture", attorney docket number M-3880 US, filed by Li-Chun
Li et al. on Dec. 3, 1996 and incorporated herein by reference.
[0028] In memory block M-1.2 (FIG. 3A), and hence in every memory
block M-i.j, a memory cell is located at the intersection of bit
line BL with every other word line WL, and at the intersection of
bit line {overscore (BL)} with each of the remaining word lines.
The gate of each memory cell transistor 210 is connected to a
respective word line WL, and the drain is connected to a respective
bit line BL or {overscore (BL)}.
[0029] All memory cell transistors 210 of a single memory array 120
are formed in the same P well 216 located in the same deep N well
220 (FIG. 2).
[0030] As shown in FIGS. 3A, 3B, RW block RW-1.2 includes: (1)
precharge block 310 (FIG. 3A) for precharging the bit line segments
running through memory block M-1.2; (2) sensing block 314 (FIG. 3B)
including a sense amplifier 320; and (3) precharge block 326 for
precharging the bit line segments of memory block M-2.2. All the RW
blocks RW-i.j are identical to each other, except that the leftmost
blocks RW-1.j omit the precharge block 310, and the rightmost
blocks RW-3.j omit precharge block 326.
[0031] In precharge block 310, precharge transistor 236 (FIGS. 2,
3A) has its drain connected to a metal-1 line HVCC-M1 running
vertically (in the plan view of FIGS. 1, 3A) through all the RW
blocks in the R/W circuitry 130.2 (i.e. through blocks RW-2.2,
RW-2.4). This metal-1 line HVCC-M1 carries the constant voltage
HVCC. The source of transistor 236 is connected to bit line BL. The
gate is connected to a metal-1 line EQ-M1.
[0032] This line carries equalization signal EQ. Metal line EQ-M1
runs vertically through all the RW blocks of R/W circuitry
130.2.
[0033] Metal line EQ-M1 is connected also to the gate of NMOS
equalization transistor 330 interconnecting the bit lines BL,
{overscore (BL)}.
[0034] NMOS transistors 334, 340 connect bit line segments BL,
{overscore (BL)} of memory block M-1.2 to respective bit line
segments BL, {overscore (BL)} of sensing block 314 (FIG. 3B).
Similar NMOS transistors 344, 350 connect the bit line segments of
sensing block 314 to respective bit line segments of memory block
M-2.2. The gates of transistors 334, 340 receive signal SS1
distributed on a vertical metal-1 line SS1-M1. The gates of
transistors 344, 350 receive signal SS2 distributed on a vertical
metal-1 line SS2-M1. Lines SS1-M1, SS2-M1 run through all the RW
blocks of R/W circuitry 130.2. When signal SS1 is asserted high,
sense amplifier 320 amplifies the signals from memory block M-1.2.
When signal SS2 is asserted high, sense amplifier 320 amplifies
signals from memory block M-2.2. At most one of signals SS1, SS2 is
high at any given time.
[0035] In sensing block 314, bit line BL is connected to a
source/drain region of NMOS pass transistor 354. The other
source/drain region of pass transistor 354 is connected to data bit
output line {overscore (DB)}. Bit line {overscore (BL)} is
connected to a source/drain region of pass transistor 360 whose
other source/drain region is connected to complimentary data bit
output line DB. The gates of transistors 354, 360 receive a column
select signal YS. Data lines {overscore (DB)}, DB are metal-1 lines
running vertically through all the RW blocks of R/W circuitry
130.2.
[0036] PMOS transistors 364, 370 and NMOS transistors 374, 380 form
two cross-coupled latches which form sense amplifier 320. Bit line
BL is connected to the gates of transistors 364, 374, and the
drains of transistors 370, 380. Bit line {overscore (BL)} is
connected to the gates of transistors 370, 380, and the drains of
transistors 364, 374. The sources of PMOS transistors 364, 370 are
connected to a vertical metal-2 line SLP. The sources of NMOS
transistors 374, 380 are connected to a vertical metal-2 line SLN.
Lines SLP, SLN run through all the RW blocks of R/W circuitry
130.2. During amplification, line SLP is connected to a positive
voltage, and line SLN is connected to ground. During precharge,
both lines SLP, SLN are connected to the same precharge voltage
HVCC. The sense amplifier operation and timing are described in
U.S. patent application "Charging a Sense Amplifier", Ser. No.
08/760,121, filed Dec. 3, 1996 by L. Liu et al. and incorporated
herein by reference.
[0037] Precharge block 326 is similar to block 310. In particular,
block 326 includes NMOS equalization transistor 381 connected
between the bit lines BL, {overscore (BL)} and NMOS precharge
transistor 383 connected to bit line BL. The drain of transistor
383 is connected to a vertical metal-1 line receiving the voltage
HVCC and running through all the RW blocks of circuitry 130.2. The
gates of transistors 381, 383 receive equalization signals EQ
provided on a vertical metal-1 line running through all the RW
blocks of circuitry 130.2. The drain of transistor 383 overlaps
with the deep N well (not shown) of memory block M-2.2.
[0038] FIGS. 4A, 4B are a layout view showing masks used to
manufacture the blocks 310, 314. DRAM 110 includes four polysilicon
layers and two metal layers over the polysilicon layers. Bit lines
BL, {overscore (BL)} are formed from the fourth polysilicon layer
("poly 4"). The bit line boundaries are shown by dashed lines.
[0039] Stippled areas are mask openings through which N+ or P+
implants are performed into substrate 230. Stippled region 210S is
the N+ source region of transistor 210 of the rightmost memory cell
of memory block M-1.2. See also FIG. 2. In capacitors 226 (FIG. 2),
the capacitor plates connected to memory cell transistors 210 are
formed from the second polysilicon layer (not shown). The capacitor
plate connected to voltage VREF is formed from the third
polysilicon layer (not shown). This plate is shared by a number of
memory blocks in a memory array. This poly-3 plate is interrupted
between some memory blocks M-i.j of the array to allow metal-1 word
lines WL to contact poly-1 word lines WL (each word line WL
includes a metal-1 line running over a poly-1 line).
[0040] Stippled region 410 (FIG. 4A) includes N+ drain 236D of
precharge transistor 236.
[0041] Metal-1 line HVCC-M1 contacts drain region 236D in contact
region 418. (In region 418, line HVCC-M1 contacts a doped poly-4
region. The poly-4 region contacts the drain region.)
[0042] Metal-1 line EQ-M1 contacts a poly-1 line EQ-P1 in region
422. Poly-1 line EQ-P1 provides the gates for transistors 236, 330
(FIG. 3A).
[0043] Poly-1 line SS1-P1 provides the gates of transistors 334,
340. Line SS1-P1 contacts metal line SS1-M1 in region 426. Bit
lines BL, {overscore (BL)} contact the source/drain regions of
transistors 334, 340 in contact regions 430.
[0044] Poly-1 line YS-P1 provides the gates of pass transistors
354, 360. Poly-1 line YS-P1 contacts a metal-1 region in contact
region 434. The metal-1 region contacts a metal-2 region which
provides Y-select signal YS. Stippled region 440 includes source
and drain regions of transistor 360. (Region 440 is a mask opening
through which the dopant is implanted. This implant is also masked
by poly-1 line YS-P1, causing the source and drain regions to be
spaced from each other.) A source/drain region of transistor 360
contacts the metal-1 line DB in region 444.
[0045] Similarly, stippled region 450 includes the source and drain
regions of transistor 354 of RW block RW-2.2 and of transistor 354
of the next RW block RW-2.4. Poly-1 line YS2-P1 provides the gates
of the pass transistors of RW block RW-2.4. The common source/drain
region of transistors 354 of the two RW blocks contacts the data
line {overscore (DB )} in contact region 454.
[0046] Bit lines BL, {overscore (BL)} contact the source/drain
regions of transistors 360, 354 in contact regions 455.
[0047] In FIG. 4B, poly-1 line 364-P1 extending essentially
directly below the bit line {overscore (BL)} provides the gate of
transistor 364. Poly-1 line 370-P1 extending essentially directly
below the bit line BL provides the gate of transistor 370. Stippled
region 460 includes the P+ sources and drains of the two PMOS
transistors. Line 370-P1 contacts bit line {overscore (BL)} in
contact region 464. Poly-1 line 364-P1 contacts bit line BL and the
drain of transistor 370 in contact region 468. Bit line {overscore
(BL)} contacts the drain of transistor 364 in contact region 472.
Metal-2 line SLP (not shown in FIG. 4B) contacts the common source
of transistors 364, 370 in contact region 476.
[0048] Poly-1 line 374-P1 provides the gate of transistor 374.
Poly-1 line 380-P1 provides the gate of transistor 380. The two
poly-1 lines extend between the bit lines essentially in parallel
with the bit lines. Poly-1 line 380-P1 contacts bit line {overscore
(BL)} in contact region 480. Poly-1 line 374-P1 contacts bit line
BL in contact region 482.
[0049] Stippled region 484 includes the sources and drains of
transistors 374, 380. Bit line BL contacts the drain of transistor
380 in contact regions 486. Bit line {overscore (BL)} contacts the
drain of transistor 374 in contact regions 490. Metal-2 line SLN
(not shown) contacts the common source of transistors 374, 380 in
contact region 494.
[0050] Bit lines BL, {overscore (BL)} contact the source/drain
regions of transistors 344, 350 (FIG. 3B) in contact regions 430
(FIG. 4B). Transistors 344, 350 are not shown in FIG. 4B.
[0051] FIG. 5 shows the beginning stages of fabrication of DRAM
110. Wafer 230 doped with boron has a doping concentration of
3.times.10.sup.15 cm.sup.-3. Initial silicon dioxide layer 510 is
grown by thermal oxidation to a thickness of 300 to 1000 nm. Oxide
510 is patterned by standard photolithographic techniques to expose
a region 520 into which dopants will be implanted for P well 216
and DNW 220.
[0052] A protective silicon dioxide layer 530 is grown by thermal
oxidation to a thickness of 30 to 300 nm (100 nm in some
embodiments). Phosphorous is implanted into region 520 at the
energy 180 keV to create DNW 220. The ion dose is 1 to 9 times
10.sup.13 atoms/cm.sup.2 (1.5.times.10.sup.13 atoms/cm.sup.2 in
some embodiments.)
[0053] Phosphorous is driven in by heating the wafer in nitrogen
atmosphere at a temperature of 1150.degree. C. for 510 to 1000
minutes (950 minutes in some embodiments). The deep N well diffuses
laterally and downward as shown in FIG. 6.
[0054] A blanket etch removes protective oxide 530 and a small
portion of oxide 510. Protective silicon dioxide 610 is grown
thermally to a thickness of 30 to 300 nm (100 nm in some
embodiments) by wet oxidation performed at 950.degree. C. for 10 to
60 minutes. Boron is implanted at an energy of 30 to 180 keV (60
keV in some embodiments to form P well 216. The ion dose is
1.times.10.sup.13 to 9.times.10.sup.13cm.sup.-2 (2.times.10 .sup.13
cm.sup.-2 in some embodiments).
[0055] Then oxide layers 510, 610 are removed. A protective 100 nm
layer of silicon dioxide ("third protective oxide", not shown) is
grown by wet oxidation performed at 950.degree. C. for 10 to 60
minutes. Photoresist (not shown) is deposited and patterned to
expose N well regions 620 (FIG. 2) in which the PMOS transistor 370
and other PMOS transistors will be-formed. Phosphorous is implanted
at an energy of 30 to 180 keV to form the N wells. The ion dose is
1.times.10.sup.13 to 9.times.10.sup.13cm.sup- .-2
(1.2.times.10.sup.13 cm.sup.-2 in some embodiments). Then a well
drive-in step is performed at a temperature of 1150.degree. C. for
200 to 800 minutes (250 minutes in some embodiments). The resulting
depth of DNWs 220 is about 5 .mu.m. The depth of P wells 216 is 2
.mu.m. The depth of N wells 620 is 3 .mu.m. The distance dw between
the right edge of P well 216 and the right edge of respective DNW
220 is 2 .mu.m.
[0056] The third protective oxide is removed. 470 nm thick field
oxide regions 630 are grown between transistor active areas by
LOCOS oxidation performed at 1000.degree. C. for 90 minutes using
methods known in the art.
[0057] A 30 nm sacrificial layer of silicone dioxide (not shown) is
grown by wet oxidation performed at 850.degree. C. for 40 minutes.
Blanket ion implantation of BF2 is performed through this
sacrificial oxide at an energy of 70 keV to adjust transistor
threshold voltages. The ion dose is 3.2.times.10.sup.12
cm.sup.-2.
[0058] Then a deep P type implant (channel stop implant) is
performed into NMOS transistor active regions to enhance the P type
dopant concentration under the NMOS transistors and the field oxide
regions. The resulting P- channel stop regions are shown at 640.
Regions 640 increase the punch-through voltages of NMOS transistors
and parasitic transistors formed under field oxide 630. Regions 640
also increase the parasitic transistor threshold voltages. Regions
640 are formed by implanting boron at the energy 120 keV. The ion
dose is 8.times.10.sup.12 atoms/cm.sup.2. The implant mask protects
N wells 620 during this implant. The implant mask also protects
regions CN of equalization transistor drains 236D. Each region CN
overlaps an area in which the respective drain 236D meets the
respective DNW 220. Protecting the regions CN from the channel stop
implant serves to improve the electrical contact between drains
236D and DNWs 220.
[0059] In some embodiments, the channel stop implant mask is the
reverse of the N well 620 mask except that the channel stop implant
mask also covers regions CN.
[0060] Each region CN is spaced from the respective gate 236G. This
spacing allows a portion 640.1 of channel stop region 640 to extend
under the drain 236D, thus increasing the punch-through voltage of
transistor 236. Each region CN is also spaced from the edge of the
respective drain 236D where the drain meets field oxide 630 (the
left edge of drain 236D in FIG. 2). This spacing allows a portion
640.2 of channel stop region 640 to extend from under the field
oxide to a region under the drain 236D. This helps to improve the
punch-through voltage and the threshold voltage of the field
transistor formed at the location of oxide 630.
[0061] In some embodiments, after formation of the implant mask
used to form channel stop regions 640, but before the boron
implantation forming the channel stop regions, another boron
implant is performed through the sacrificial oxide at an energy of
30 keV to adjust the threshold voltages of NMOS transistors. The
ion dose of this implant is 2.times.10.sup.12 cm.sup.-2.
[0062] After the channel stop implant, the sacrificial oxide is
removed.
[0063] Gate oxide (not shown) is grown to a thickness of 5 to 18 nm
(8 nm in some embodiments) by oxidizing the structure at 700 to
1000.degree. C. (850.degree. C. in some embodiments) for 10 to 60
minutes. Polysilicon or polycide gates of transistors 210, 236,
370, and other transistors, are formed by known techniques.
[0064] Phosphorous is ion-implanted at 25 keV to form LDD regions
of NMOS transistors. The ion dose is 2.times.10.sup.13 cm.sup.-2.
Then pocket ion implantation of boron is performed at an angle of
25.degree. and an energy of 60 keV into regions underlying NMOS
sources and drains, to increase the NMOS punch-through voltages.
The pocket implant ion dose is 1.2.times.10.sup.13 cm.sup.-2. These
two implants--NMOS LDD and P-type pocket--are performed using the
same photoresist mask ("NMOS LDD mask", not shown) patterned by
standard photolithographic techniques. The mask covers regions LN
one of which is shown in FIG. 2. Each region LN covers the region
CN and extends to the adjacent field oxide region 630 separating
the respective drain 236D from the respective P well 216. By
covering the regions LN, the mask blocks the pocket-implant boron
from areas in which the drain regions 236D meet the respective DNWs
220. Thus, the mask helps to improve the contact between drain
regions 236D and DNWs. At the same, the mask exposes a portion of
the drains 236D adjacent the respective gates 236G and also exposes
the source regions 236S.
[0065] Pocket implants are described in U.S. Pat. No. 5,618,740
entitled "Method of Making CMOS Output Buffer with Enhanced ESD
Resistance", issued Apr. 8, 1997 to T. Huang and incorporated
herein by reference.
[0066] BF2 is implanted to form LDD regions of PMOS transistors
such as transistor 370. A pocket implant of phosphorous is
performed into the PMOS transistor regions to increase their
punch-through voltages. A 100 nm layer of silicone nitride (not
shown) is deposited by LPCVD and etched to form spacers on
transistor gate sidewalls. The silicon nitride deposition
temperature is 780.degree. C., and the deposition time is 40 mins.
A 20 nm layer of silicone dioxide (not shown) is grown at
875.degree. C. on exposed silicone surfaces. Arsenic and BF2 are
implanted in successive ion implantation steps. The arsenic implant
forms heavily doped portions of NMOS source/drain regions. The BF2
implant forms heavily doped portions of PMOS source/drain regions
and also forms one or more P+ contact regions (not shown) in each P
well 216. The P+ contact regions will contact a voltage source that
will bias the P-wells. In some embodiments, the mask used in the
BF2 implant is the reverse tone of the NMOS LDD mask, except that
both the NMOS LDD mask and the BF2 mask block the regions LN.
[0067] Remaining fabrication steps are known in the art.
[0068] The dopant concentration in DNWs 220 is 1.times.10.sup.16
atoms/cm.sup.3. The dopant concentration in drains 236D is
1.times.10.sup.20 atoms/cm.sup.3. The higher dopant concentration
in the drain regions improves the electrical contact between DNWs
220 and doped poly-4 regions (not shown) contacting the drains 236D
and also contacting the metal-1 lines HVCC-M1.
[0069] FIG. 7 illustrates some drawn mask dimensions in the cross
section of FIG. 2. The lateral distance d1 between the right edge
of the rightmost source region 210S in a P well 216 and the right
edge of the respective DNW 220/P well 216 mask opening
(corresponding to the left edge of oxide 510 in FIG. 5) is only
0.45 .mu.m. This spacing is small due to elimination of a separate
N+ contact region in the DNW and of a separate N well containing
the N+ contact region. The distance d2 between the right edge of
the DNW mask opening and the left edge of the mask opening for
drain region 236D is 1.5 .mu.m. The drawn distance d3 between the
left edge of the region 236D and the left edge of the region CN is
0.4 .mu.m. The width d4 of each region CN is 3.1 .mu.m. The
distance d5 between the right edge of the region CN and the left
edge of gate 236G is 1.0 .mu.m.
[0070] The distance d6 between the left edges of region LN and
drain 236D is 1.0 .mu.m. The rights edges of regions LN, CN
coincide.
[0071] In this embodiment, DNW 220 diffuses laterally after
implantation by 75-80% of the DNW depth. Thus, if the DNW depth is
5 .mu.m, the DNW diffuses laterally by 3.75 .mu.m to 4 .mu.m. The
minimal photolithographic line width is 0.5 .mu.m, and the maximum
alignment error is 0.6 .mu.m. The minimal spacing between the DNW
mask and the gate of the nearest transistor is only d2+d3+d4+d5=6
.mu.m.
[0072] The above embodiments illustrate but do not limit the
invention. In particular, the invention is not limited by any
particular dimensions, fabrication techniques, temperatures,
energies, or other process parameters, or by layer compositions or
layout. The invention is not limited to DRAMs or any other
particular circuitry. Other embodiments and variations are within
the scope of the invention as defined by the following claims.
* * * * *