Download Deep trench Polysilicon

A 0.25µ
µ m CMOS based 70V smart power technology with deep trench
for high-voltage isolation
V. Parthasarathy, R. Zhu, V. Khemka, T. Roggenbauer, A. Bose, P. Hui, P. Rodriquez,
J. Nivison, D. Collins, Z. Wu, I. Puchades and M. Butner
Semiconductor Products Sector, Motorola Inc.
2100 East Elliot Road-EL709, Tempe, AZ85284
Tel: (480) 413-3878 Fax: (480) 413-3970
Email: [email protected]
Abstract
We present a 0.25µm CMOS based smart power
platform on a P++ substrate with a deep trench high-voltage
isolation as a low-cost alternative to SOI in realizing
significant analog shrink, reduction of substrate parasitics and
70V high-side capability without affecting analog matching
and process complexity.
designed to minimize the impact on the shallow trench
isolation module which is used for logic isolation. Hence,
maximum re-use of shallow trench process steps becomes
critical.
Introduction
Consumer and automotive applications such as
printer head drivers, motor control, anti-lock brake systems
(ABS) and electronic vehicular stability systems are driving
the integration of high-voltage (40-70V) power and analog
components onto advanced logic platforms [1] to enable
inexpensive on-board DSP and micro-processor cores. Highvoltage Rdson and energy limited power MOSFETs with highside capability impose minimum epi thickness requirements
on the process architecture. However, a larger epi thickness
diminishes the effectiveness of a P++ substrate in minimizing
system parasitics such as substrate PNP bipolars, lateral NPN
bipolars resulting from electron injection into the substrate
and latchup structures. Although SOI BCD platforms [2,3,4]
are very effective in addressing these concerns, the design
complexity, higher wafer cost and lower heat removal
capability limit the use of these technologies to niche
applications. In this paper, we present a 0.25µm 70V smart
power platform with a deep trench for high-voltage isolation
combined with a bulk-silicon P++ substrate that is effective
in reducing these parasitics to an acceptable level, while
allowing for significantly higher packing densities.
Deep trench process architecture
The process flow for this technology is based on a
Pepi on P++ substrate starting material. A shallow trench
isolation is employed for the CMOS logic isolation. P-body
and N-body implants, a split gate oxide module and highvoltage well modules, which are used for medium and high
voltage devices, are added to the conventional CMOS
processing which includes logic low-voltage wells. An SEM
of the processed deep trench is shown in Fig.1. A highquality thermal oxide liner is grown along the side-walls of
the deep trench and the remaining oxide is deposited at low
temperature. A polysilicon stress-relief layer [5] is deposited
so that it fills the deep trench and is recessed below the
silicon surface. The deep trench process integration is
Polysilicon
Deep
trench
Fig.1 SEM cross-section of fabricated deep trench showing
polysilicon fill and oxide side-wall lining
No deep trench
With deep trench
-10
10
-11
10
10-12
10-13
10-14
-15
10
-16
10
0
10
20
30 40 50 60
V
(V)
70
80
NWELL
Fig.2 Comparison of nwell to substrate breakdown and offstate leakage with and without deep trench abutting nwell.
Leakage current levels of nwell to P-type substrate
are very low with deep trench abutting junction and are no
different from junction without deep trench (Fig.2),
indicating that there is no additional leakage due to deep
trench presence. The deep trench oxide thickness is sufficient
to sustain a breakdown voltage from nwell to substrate of
74V. This breakdown voltage has been found to be stable
even with repeated stress, which is made possible by good
interface between the deep trench silicon sidewall and the
high quality thermal oxide liner.
The deep trench in this technology is used to
significantly increase the analog packing density by bringing
devices adjacent to each other across the deep trench, thereby
enabling shrinks ranging from 50% for medium-voltage
analog to >80% for high-voltage analog components
compared to the 0.35µm SMOS7 technology [6]. In SMOS7,
high-energy implant chains replaced the deep diffused
implants, which were present in older diffusion based
technologies [7]. Any further shrink in analog component
size from here onwards becomes difficult to accomplish due
to voltage requirements and significant reductions may
become possible only with the deep trench isolation.
However, when two nwell regions are placed adjacent with a
deep trench separation, punchthrough under the deep trench
becomes a concern. This can be avoided in P- substrates with
punchthrough retardation implants [8] or simply by
increasing the depth of the trench significantly. In our case,
the problem is eliminated by optimized epi thickness and
trench depth in conjunction with the P++ substrate, which
acts as a very effective punch-through barrier. This is evident
in Fig.3, where breakdown between an nwell1 and substrate
is reached before punchthrough to an adjacent nwell2.
In smart power applications, the harsh operating
conditions of the power MOSFETs in the IC create ample
situations for chip-level parasitics to impact circuit operation.
Certain circuit topologies [10,11] cause transient electron
injection from a negatively biased drain of a high-voltage
high-side capable power MOSFET. While high-side
requirements drive the need for thick epitaxial regions to
prevent punch-through from source to substrate, this
diminishes the effectiveness of the P++ substrate in lifetime
killing of electrons. Here, the presence of a deep trench
confines the injected to the P++ substrate where they
recombine in a very short distance. Through appropriate
design, the substrate injection collection ratio (Ic/Ie) can be
reduced by more than 6 orders of magnitude from ~2X10-3
(no trench) to ~1.5X10-9 (with trench), as seen in Fig.4.
10-1
-3
10
-5
10
Ie (A)
Ic: No deep trench
10-7
Ic: With deep trench
10-9
Inwell1
Inwell2
10-7
10
-8
10
-9
10-11
10-13
-10
10
0
-0.5
-1
-1.5
Ve (V)
Fig.4 Comparison of collector current collection in parasitic
substrate NPN transistor with and without deep trench
10-11
10-12
10-13
10
-14
0
10
10-15
Emitter to Collector Space=110µm
P- sub
0
10
20
30
V
40 50
(V)
60
70
NWELL1
Fig.3 Off-state leakage currents of adjacent nwell regions
across deep trench showing absence of punch-through
10
-2
10µm P-/P++ (No DT)
5µm P-/P++ (No DT) [1]
10-4
10-6
Parasitics suppression with deep trench
Deep trench based processes have been integrated
previously in CMOS architectures to suppress inter-well
parasitics and CMOS thyristor latchup [9]. In mixed-signal
signal applications, an immediate benefit of the deep trench is
to reduce capacitances to substrate. In the present case,
measured capacitance of a 500µm2 nwell to substrate junction
is reduced from ~1.38X10-13 F to ~3.2X10-14 F, a reduction of
4X by the presence of a deep trench surrounding the nwell.
0.25µm SMOS P-/P++
Nominal DT
10-8
10-10
0
50
100
150
200
Collector Distance (µm)
Fig.5 Substrate injection suppression ratio (Ic/Ie) as a
function of distance to collector for different substrate types
with and without deep trench
Fig.5 shows the effectiveness of the deep trench in
conjunction with the P++ substrate in suppressing minority
carrier injection. For example if a few amps of injected
current need to be reduced to few µAs of current, a 60µm
region would suffice with the nominal deep trench. The
impact of the deep trench on substrate injection suppression
is more dramatic as the trench depth is increased. By
comparison, even 5µm of P-epi on P++ substrate would
require large moat areas around the power devices for such
conditions. While a reduction in moat size can be
accomplished by various passive and active guard ring
schemes [11], these schemes consume area and can cause
potentially unforeseen problems such as latchup or reduction
in effectiveness due to nearby ground ties. With 5µm P-epi
on P++ substrate, high-side capability is very difficult to
realize. In the present technology, the deep trench enables the
very low substrate injection ratios of 2X10-6 to be attained in
distances of 50µm with high-side capability of 74V (Fig.6).
Technology
Platform
SMOS5 [7]
SMOS7 [6]
BCD6 [1]
0.25µm SMOS
Substrate
Type
P- substrate (thick epi)
P- substrate (thin epi)
5µm P-/P++ (no DT)
P-/P++ with Deep Tr.
Typ. Ic/Ie ratio
High-side
at 50µm (log) Voltage (typ)
-1.5
>100V
-1.5
72V
-2.5
50V
-5.7
74V
Maximum
BVdss (typ)
85V
54V
48V
55V
Fig.6 Comparison of different technology platforms for highside capability, maximum BVdss and substrate injection ratio
20
no deep trench
with deep trench
15
analog nodes and may also have potential benefits for ESD
protection.
Device Performance
Mismatch measurements of a self-aligned low-voltage thingate NMOS matched pair is shown in Fig.8. As expected, the
drain current mismatch percentage is roughly related to the
area of the device. Also, the drain current mismatch
percentage is increased almost 2X in saturation bias
compared to linear bias. This points to the Vth mismatch
being the dominant factor due to the square dependence of
drain current on threshold voltage in the saturation regime.
The presence of deep trench in the vicinity of the matched
pair results in negligible impact on drain current mismatch,
indicating that trench stress has been minimized in this
process. However, a matched pair with the deep trench
surrounding only one device showed discernable systematic
mismatch which suggests that care must be exercised in
layouts of matched pairs.
Linear bias
(Vg=2.5V, Vd=0.1V)
Saturation bias
(Vg=2.5V, Vd=2.5V)
Systematic mismatch Random mismatch Systematic mismatch Random mismatch
η (∆Ιd/Id)
σ (∆Ιd/Id)
η (∆Ιd/Id)
σ (∆Ιd/Id)
Pair type and size
No deep trench
Wg=4.2µ, Lg=2.1µ
0.10%
0.31%
0.17%
0.57%
With deep trench
Wg=4.2µ, Lg=2.1µ
0.20%
0.40%
0.16%
0.66%
No deep trench
Wg=8.4µ, Lg=4.2µ
0.02%
0.15%
0.05%
0.27%
With deep trench
Wg=8.4µ, Lg=4.2µ
0.08%
0.12%
0.12%
0.24%
Fig.8 Comparison of drain current mismatch data with and
without deep trench in vicinity of the matched pair of a selfaligned low-voltage thin gate oxide NMOS
100
10
15
40
45
10
10
80
0.25 um SMARTMOS
SMARTMOS7
TI LBC6
STM BCD6
20 2 5
-0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1 -1.1 -1.2
Vbe (V)
30
30
35
20
25
45
50
25 20
1 05
15
0
15
35
40
55
30
40
45
50
55
40
P-Body
5
10
0
Nwell
50
5
60
40
P45
20
0
10
20
30
40 50 60
BV (V)
70
80
dss
Fig.7 Comparison of parasitic substrate PNP beta with and
without deep trench isolation
Fig.9 NLDMOS performance: (a) Off-state equipotential
lines and (b) BVdss vs. Rdson tradeoff
By surrounding an nwell isolation with a deep trench
in all directions, the parasitic PNP between an isolated pwell
and substrate is also significantly suppressed (Fig.7).
Suppression of this parasitic PNP is particularly important
when the anti-parallel diode of the LDMOS is used in the Hbridge high-side configuration [1]. In addition to these power
LDMOS related situations, the deep trench is also useful in
reducing system level latchup and parasitics at sensitive
The NLDMOS device in this process is high-side
capable with BVdss of 54V, which is obtained by a double
RESURF action of a P- below an N- drift region [12]. The
NLDMOS Rdson of 40mΩ-mm2 is 20% lower than in the
0.35µm SMOS7 process [13]. This improvement has been
made possible by further optimizing RESURF of the N-drift
from the P-body side (Fig.9(a)). The high-side capable RdsonBVdss trade-off in this process (Fig.9(b)) is the best reported
to date for the voltage range under consideration. Due to the
optimum RESURF action, the nwell drift region dose has
been increased so that it can serve as the well for medium
voltage PMOS devices as well as the base an isolated vertical
PNPV. The resulting PNPV has a beta of 30, Vaf of 60V and
BVceo and BVcbo of 22V and 48V respectively (Fig.10).
Rdson (mΩ-mm2) Vth (V)
6.8
0.8
12
0.8
20
1.4
33
1.4
40
1.4
58
1.4
Device BVdss (V)
NMV1
11.5
NMV2
18
NHV1
23
NHV2
42
NHV3
54
NHV4
75
Device BVdss (V)
PMV1
-10
PMV2
-19
PHV1
-41.5
PHV2
-54
PHV3
-78
Rdson (mΩ-mm2) Vth (V)
25
-1
40
-1.2
95
-1.5
122
-1.5
185
-1.5
Conclusion
Device Bvceo (V) Bvcbo (V) Beta Vaf (V) ft (MHz)
NPNV
22
61
110
100
350
PNPV
22
48
30
60
500
PNPL
28
30
80
>300
Fig.10 Performance summary of NMOS, PMOS and bipolar
device set in the 0.25µm smart power technology with highvoltage deep trench isolation
The p-drift region in the PLDMOS devices is also
optimized through appropriate RESURF action from the Nbody and an n-type region underneath the pwell. In this way,
a maximum BVdss of 78V is obtained. The pwell drift region
is employed as the well for medium voltage (5-10V) NMOS
and for the base of an NPN with a beta of 110, Vaf of 100V
and BVceo and BVcbo of 22V and 61V respectively (Fig.10).
0.25µm SMOS
Area
SMOS7
Analog
1X
0.30X
Power
1X
0.80X
Logic
1X
0.50X
Fig.11 Summary of shrinks in the present 0.25µm smart
power technology with deep trench compared to the 0.35µm
SMOS7 technology.
10600
no deep trench
with deep trench
10400
10000
9800
Voltage coefficient:
-2
No deep trench = 1.32X10 /V
-3
9400
With deep trench = 8.8X10 /V
0
2
4
6
V
well
8
(V)
In conclusion, we have developed and qualified a 0.25µm
CMOS based high-side capable 70V smart power process on
a P++ substrate with a deep trench high-voltage isolation and
logic shallow trench isolation. The NLDMOS power devices
integrated into this platform have the best Rdson-BVdss
reported to date for the 10-70V range. By using a deep trench
combined with a P++ substrate, we have realized significant
analog shrink, reduction of substrate parasitics and 74V highside capability without affecting analog matching and process
complexity.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
10200
9600
A summary of device shrinks obtained in the present 0.25 µm
SMOS technology is shown in Fig.11. Finer lithography
results in a 50% logic shrink while isolating the analog
components with deep trench allows for an average 70%
analog shrink. In addition to reduction of analog device size,
this produces additional performance benefits such as
improvement in the voltage coefficient of an isolated nwell
resistor (Fig.12). The substantially lower shrink of 20% in
power device area is bringing the power device performance
very close to the ideal 1-D silicon limit for lateral power
devices.
10
12
14
Fig.12 Comparison of voltage dependence of resistance of an
isolated nwell resistor with and without deep trench
surrounding the resistor.
A. Moscatelli et. al., pp.323-326, ISPSD, 2000.
A. Ludikhuize et. al., pp.77-80, ISPSD, 2002.
T. Letavic et. al., pp.73-76, ISPSD, 2002.
S. Merchant et. al., pp.185-188, ISPSD, 2002.
W. Yindeepol et. al., pp.151-152, IEEE Int. SOI
Conf., 1998.
V. Parthasarathy et. al., pp.317-320, ISPSD, 2000.
S. Merchant et. al., pp.225-228, ISPSD, 1999.
J. H. Klootwijk et. al., pp.200-204, ICMTS, 2000.
H. Inokawa et. al., pp.218-219, Symp. On VLSI
Technology, 1996.
R. Zhu et. al., pp.184-186, BCTM, 1998.
J. P. Laine et. al., pp.273-276, ISPSD, 2002.
V. Parthasarathy et. al., pp.75-78, IEDM, 2000.
R. Zhu et. al., pp.403-406, ISPSD, 2001.