U.S. patent number RE38,319 [Application Number 09/961,341] was granted by the patent office on 2003-11-18 for dual-node capacitor coupled mosfet for improving esd performance.
This patent grant is currently assigned to Winbond Electronics Corporation. Invention is credited to Shin-Tron Lin, Shyh-Chyi Wong.
United States Patent |
RE38,319 |
Lin , et al. |
November 18, 2003 |
Dual-node capacitor coupled MOSFET for improving ESD
performance
Abstract
A dual-node capacitor coupling technique is used to lower the
trigger voltage and to improve the uniform turn-on of a
multi-finger MOSFET transistor. Preferably, each MOSFET is an NMOS
device. Specifically, each NMOS device includes a capacitor that is
connected between the gate of the NMOS device and the pad terminal.
A first resistor is connected between the gate and the p-well,
while a second resistor is connected between the p-well and the
grounded source. For a positive ESD pulse to VSS, the p-well is
pulled up to approximately 0.7 V during the initial ESD event, such
that the source junction is forward biased and that the trigger
voltage of the NMOS device is lowered. At the same time the gate
voltage is coupled within the range of approximately 1 to 2 V to
promote the uniform turn on of the gate fingers of the NMOS devices
during the initial ESD event.
Inventors: |
Lin; Shin-Tron (Taipei,
TW), Wong; Shyh-Chyi (Hsinchu, TW) |
Assignee: |
Winbond Electronics Corporation
(TW)
|
Family
ID: |
21757411 |
Appl.
No.: |
09/961,341 |
Filed: |
September 25, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
012928 |
Jan 24, 1998 |
05959488 |
Sep 28, 1999 |
|
|
Current U.S.
Class: |
327/313; 327/318;
327/566 |
Current CPC
Class: |
H01L
27/0251 (20130101); H01L 2924/0002 (20130101); H01L
2924/0002 (20130101); H01L 2924/00 (20130101) |
Current International
Class: |
H01L
27/02 (20060101); H03K 005/08 () |
Field of
Search: |
;327/309,310,313,318,319,321,328,333,530,534,564,565,566 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Zweizig; Jeffrey
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner, L.L.P.
Claims
What is claimed is:
1. An ESD protection circuit for lowering the trigger voltage and
improving the turn-on consistency of a MOSFET device, comprising: a
MOSFET device having a source, a drain, a gate and a bulk; a
capacitor connected the gate of said MOSFET and a pad terminal; a
first resistor connected between the gate and the bulk of said
MOSFET; and a second resistor connected between the bulk and a
power bus.
2. The circuit of claim 1, wherein said source is connected to said
power bus.
3. The circuit of claim 1, wherein said MOSFET is a PMOSFET.
4. The circuit of claim 3, wherein said power bus is VDD.
5. The circuit of claim 1, further comprising multiple MOSFET
devices connected in parallel, wherein all gates of said multiple
MOSFET devices are electrically connected together.
6. The circuit of claim 5, wherein at least two drains of said
multiple MOSFET devices are electrically connected together.
7. The circuit of claim 5, wherein at least two MOSFET devices
share a common drain diffusion region.
8. The circuit of claim 5, wherein at least two sources of said
multiple MOSFET devices are electrically connected together.
9. The circuit of claim 5, wherein at least two MOSFET devices
share a common source diffusion region.
10. The circuit of claim 5, wherein said multiple MOSFET devices
form a multi-finger structure.
11. The circuit of claim 1, wherein said drain is connected between
said pad and a buffer.
12. The circuit of claim 11, wherein said bulk is a p-well, said
p-well having a voltage potential of approximately 0.7 V, and said
gate has a voltage potential approximately in the range of 1.0 to
2.0 V during as ESD event.
13. The circuit of claim 1, wherein said MOSFET is an NMOSFET.
14. The circuit of claim 13, wherein said power bus is a GND.
15. The circuit of claim 1, further comprising a third resistor
connected between said pad terminal and a buffer.
16. The circuit of claim 15, further comprising a second MOSFET
coupled between said buffer and said power bus.
17. An ESD protection circuit for lowering the trigger voltage and
improving the turn-on consistency of a MOSFET device, comprising: a
MOSFET device having a source, a drain, a gate and a bulk; a
capacitor connected between the gate of said MOSFET and a first
power bus; a first resistor connected between the gate and the bulk
of said MOSFET; and second resistor connected between the bulk and
a second power bus.
18. The circuit of claim 17, wherein said first power bus is a VDD
bus and said second power bus is a VSS bus.
19. The circuit of claim 17, wherein said first power bus is a VDD
bus and said second power bus is GND.
20. The circuit of claim 17, wherein said drain of said MOSFET is
connected to said first power bus and said source of said MOSFET is
connected to said second power bus.
21. The circuit of claim 17, wherein said second resistor is in
series with said first resistor.
22. An ESD protection structure, comprising: a MOSFET device
comprising a source, a drain, a gate and a body; a capacitor
coupled between said gate and said drain; a first resistor coupled
between said gate and said body; and a second resistor coupled
between said body and said source.
23. The circuit of claim 22, wherein said source is connected to a
power bus.
24. The circuit of claim 23, wherein said power bus is a GND.
25. The circuit of claim 22, wherein said MOSFET is a PMOSFET.
26. The circuit of claim 22, further comprising multiple MOSFET
devices connected in parallel, wherein all gates of said multiple
MOSFET devices are electrically connected together.
27. The circuit of claim 26, wherein at least two drains of said
multiple MOSFET devices are electrically connected together.
28. The circuit of claim 26, wherein at least two MOSFET device
share a common drain diffusion region.
29. The circuit of claim 26, wherein at least two sources of said
multiple MOSFET devices are electrically connected together.
30. The circuit of claim 26, wherein at least two MOSFET device
share a common source diffusion region.
31. The circuit of claim 26, wherein said multiple MOSFET devices
form a multi-finger structure.
32. The circuit of claim 22, wherein said drain is connected
between a pad and a buffer.
33. The circuit of claim 22, wherein said MOSFET is an NMOSFET.
34. The circuit of claim 22, further comprising a third resistor
connected between said drain and a buffer..Iadd.
35. An ESD protection circuit for an integrated circuit,
comprising: a MOS transistor having a source node, a drain node, a
gate node and a base node, wherein said gate node is coupled to a
first node through a capacitor and is coupled to a second node
through a first resistor, wherein said base node is coupled to the
first node through said capacitor and is coupled to the second node
through said first resistor, and wherein said first node is subject
to electrostatic-discharge stress during an ESD
event..Iaddend..Iadd.
36. The circuit of claim 35, wherein said first node is coupled to
said drain node..Iaddend..Iadd.
37. The circuit of claim 35, wherein said first node is coupled to
a signal pad..Iaddend..Iadd.
38. The circuit of claim 35, wherein said first node is coupled to
a first power bus..Iaddend..Iadd.
39. The circuit of claim 35, wherein said second node is coupled to
said source node..Iaddend..Iadd.
40. The circuit of claim 35, wherein said second node is coupled to
a second power bus..Iaddend..Iadd.
41. The circuit of claim 35, wherein said MOS transistor is a NMOS
transistor, said base node being a p-well, and said second node
being coupled to VSS..Iaddend..Iadd.
42. The circuit of claim 35, wherein said MOS transistor is a PMOS
transistor, said base note being an n-well, and said second node
being coupled to VDD..Iaddend..Iadd.
43. The circuit of claim 35, wherein said MOS transistor is a
multi-gate-finger MOS transistor..Iaddend..Iadd.
44. The circuit of claim 35 further comprising an electrically
conducting element coupled between said gate node and said base
node..Iaddend..Iadd.
45. The circuit of claim 44, wherein said electrically conducting
element is a second resistor..Iaddend..Iadd.
46. An ESD protection circuit, comprising: a MOS transistor having
a source node, a drain node, a gate node, and a base node, wherein
said gate node is coupled to a first node through a first impedance
and is coupled to a second node through a second impedance, wherein
said base node is coupled to the first node through said first
impedance and is coupled to the second node through said second
impedance, and wherein said first node is subject to
electrostatic-discharge stress during an ESD
event..Iaddend..Iadd.
47. The circuit of claim 46, wherein said first impedance comprises
a capacitive element..Iaddend..Iadd.
48. The circuit of claim 46, wherein said first node is coupled to
said drain node..Iaddend..Iadd.
49. The circuit of claim 46, wherein said first node is coupled to
a single pad..Iaddend..Iadd.
50. The circuit of claim 46, wherein said first node is coupled to
a first power bus..Iaddend..Iadd.
51. The circuit of claim 46, wherein said second node is coupled to
said source node..Iaddend..Iadd.
52. The circuit of claim 46, wherein said second node is coupled a
second power bus..Iaddend..Iadd.
53. The circuit of claim 46, wherein said MOS transistor is a NMOS
transistor, said base region being a p-well, and said second node
being coupled to ground..Iaddend..Iadd.
54. The circuit of claim 46, wherein said MOS transistor is a PMOS
transistor, said base node being an n-well, and said second node
being coupled to VDD..Iaddend..Iadd.
55. The circuit of claim 46, wherein said MOS transistor is a
multi-gate-finger MOS transistor..Iaddend..Iadd.
56. The circuit of claim 46, further comprising an electrically
conducting element coupled between said gate node and said base
node..Iaddend..Iadd.
57. The circuit of claim 56, wherein said electrically conducting
element is a resistor..Iaddend.
Description
FIELD OF THE INVENTION
The present invention relates to a primary ESD protection circuit,
and more particularly, the present invention relates to a primary
ESD protection circuit having a dual-node capacitor coupled
multi-finger NMOSFET structure.
BACKGROUND OF THE INVENTION
Electrostatic discharges (ESDs) from human handling of a
metal-oxide semiconductor (MOS) IC chip, or from other causes,
permanently damage the IC chip. Often the thin-oxide layer that
isolates the gate electrode from the substrate of a MOS field
effect transistor is irreparably ruptured by a voltage spike
applied across it. A voltage spike or ESD is often applied to the
gate, because the gate electrode is connected to an external
terminal or pin of the IC chip. The external terminals are formed
on an input or output pad. To prevent such damage from excessive
electrostatic discharges, a protective device is often connected
between the pad and the internal circuits.
As CMOS technology is scaled down into submicron regime, the
processes and the structures, such as a thinner gate oxide, shorter
channel length, shallower source/drain junction, LDD (Lightly-Doped
Drain) structure, and silicided diffusion, greatly degrade the ESD
robustness of submicron CMOS ICs. Submicron CMOS devices, such as
short channel thin-oxide MOS devices, are extremely susceptible to
ESD damage. Therefore, ESD protection has become one of the most
important elements with respect to the reliability of submicron
CMOS ICs.
An NMOSFET is a very effective ESD protection device. Specifically,
NMOS devices, either with the gate grounded (GGNMOS) or with the
gate coupled to the positive ESD transient voltage (GCNMOS), have
been commonly used as primary ESD protection elements for
integrated circuits.
GGNMOS or GCNMOS can be used as the primary ESD protection element
for ESD protection of an input pin. The input pad is connected to
the drain of the NMOS, whose gate is either grounded or coupled to
the drain and VSS by a capacitor and a resistor. The drain of the
NMOS transistor is then connected to a series resistor of the order
of 200 ohms, and then a secondary ESD protection element (say, a
smaller GGNMOS) before connected to the first input gate.
In one application, an NMOS is used as the pull down transistor of
a CMOS buffer to drive an output voltage for an external device. In
this type of application, the gate of the NMOS is connected to an
output driving signal.
In another common NMOS application, the gate is electrically
connected to ground, and the NMOS is used as an ESD protection
device for an input pin or a power bus during an ESD event.
The ESD protective action of an NMOS is based on the device's
snap-back mechanism, which enables the NMOS to conduct a high level
of ESD current between its drain and source. This occurs when a
strong electric field across the depletion region in the drain
substrate junction becomes high enough to begin avalanche
breakdown, which in turn causes impact ionization, resulting in the
generation of both minority and majority carriers. The minority
carriers flow toward the drain contact, and the majority carrier
flow toward the substrate/p-well contact, causing a local potential
build up across the current path in the p-well substrate.
When the local substrate potential is 0.6 V high than an adjacent
n+ source potential, the source junction becomes forward biased.
The forward biased source junction then injects minority carriers
(electrons) into the p-well, and these carriers eventually reach
the drain junction to further enhance the impact ionization effect
(see "ESD in Silicon Integrated Circuits", by A. Amerasekera and C.
Duvvury Chap. 3, Sec. 1., John Wiley & Sons, 1995). Eventually,
the NMOS reaches a low impedance (snap-back) state, which enables
it to conduct a large amount of ESD current.
To enhance the ESD protection capabilities of a MOSFET device, it
is desirable to have a rapid turn on with a high degree of
uniformity throughout the device. A known technique for
accomplishing this objective utilizes a multi-gate-finger
configuration to increase the gate effectiveness. However, in a
typical multi-gate-finger NMOS structure, as shown in FIGS. 1 and
2, not all the poly gate fingers may turn on during an ESD event.
That is, when the first few gate fingers reach their snap-back low
impedance mode, the drain terminal to source terminal voltage is
reduced to a value, called the snap-back voltage, which is less
than the trigger voltage of the NMOS device. This has the effect of
preventing the remaining gate fingers from being turned on. As a
result, only a partial number of the gate fingers are available to
absorb the ESD energy. Therefore, the ESD protection provided by
the NMOS is significantly reduced.
When a MOSFET gate finger is triggered during an ESD event, the
entire finger turns on. This is due to the cascading effect of the
previously described impact ionization and snap-back process along
the entire gate finger. Moreover, experimental data indicates that
a long-gate-finger structure (e.g. 100 um.times.2), as shown in
FIG. 4, has better ESD performance than a short-gate-finger
structure (e.g. 20 um.times.10), of the type shown in FIG. 1, where
both structures have the same total gate width of 200 um. That is,
during an ESD event, the long-finger NMOS structure will have
either one or two gate fingers (50% to 100% of total gate width)
turned on, while the short-finger NMOS may only have a few fingers
(out of 10) turned on, with each finger being only 10% of the total
gate width, thus reducing the short-finger MOSFET's ability to
absorb ESD current as compared to the long finger configuration.
For manufacturing purposes, however, layout area is typically at a
premium, and a conventional long-finger structure may not fit in
the designated layout area. Therefore, both multi-gate-finger
(short) and long-gate-finger (long) types of structures are used,
depending on physical and electrical priorities.
A commonly used multi-gate-finger structure is shown in FIG. 4,
where the poly-gate fingers are connected by a poly-gate bus,
rather than the metal bus of FIG. 1.
One prior art technique for improving the uniform turn on of such a
multi-gage-finger NMOS structure uses a gate coupled technique, as
shown in FIG. 5, and as described in "ESD in Silicon Integrated
Circuits", A. Amerasekera and C. Duvvury, Chap. 4, Sec. 2., John
Wiley & Sons, 1995. In this configuration, the drain is
connected to either VDD or the buffer output line, and the gate is
coupled to the drain via a capacitor C, and is also connected to
ground via a resistor R. The coupling capacitor C and the RC time
constant of the circuit cause the gate potential to rise to 1 to 2
V during the first 5 to 10 ns of an ESD event. The positive gate
voltage reduces the triggering threshold of the NMOS, thereby
enabling a more uniform turn-on of the gate fingers. However,
improving the uniformity of the turn-on state of each finger is
desirable.
SUMMARY OF THE INVENTION
The present invention overcomes the deficiencies of the prior art.
Specifically, a dual-node capacitor coupling technique is used to
lower the trigger voltage and to improve the uniform turn-on of a
multi-finger MOSFET transistor.
Preferably, each MOSFET is an NMOS device, although it will be
understood by those skilled in the art that PMOS devices may be
used as desired. Each NMOS device includes a capacitor that is
connected between the gate of the NMOS device and the pad terminal.
A first resistor is connected between the gate and the p-well,
while a second resistor is connected between the p-well and the
grounded source. For a positive ESD pulse to VSS, the p-well is
pulled up to approximately 0.7 V during the initial ESD event, such
that the source junction is forward biased and that the trigger
voltage of the NMOS device is lowered. At the same time, the gate
voltage is coupled within the range of approximately 1 to 2 V to
promote the uniform turn on of the gate fingers of the NMOS devices
during the initial ESD event.
In another embodiment the input signal line from the pad terminal
is replaced by a VDD line from a VDD source to protect against a
bus-to-bus ESD event, from VDD to VSS.
BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description, giving by way of example and
not intended to limit the present invention solely thereto, will
best be understood in conjunction with the accompanying drawings in
which like reference numbers depict like elements:
FIG. 1 shows a conventional multi-gate-finger NMOS structure;
FIG. 2 is a schematic representation of the structure of FIG.
1;
FIG. 3 shows a conventional long-gate finger structure;
FIG. 4 depicts another conventional multi-gate-finger NMOS
structure;
FIG. 5 is a schematic representation of a gate-coupled MOSFET;
FIG. 6 shows a dual-node capacitor in accordance with a first
embodiment of the present invention;
FIG. 7 shows the inherent capacitor properties of the dual-node
capacitor of FIG. 7 of the present invention; and
FIG. 8 shows a portion of a dual-node capacitor in accordance with
a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention improves over conventional gate-coupling and
well-coupling techniques in that the embodiments provide a lower
trigger voltage for each MOSFET in the multi-finger MOSFET device,
while improving the turn-on consistency for each MOSFET. Note that,
although the present invention is particularly well suited to a
multi-finger MOSFET ESD protection device, the present invention
may also be employed in a single MOSFET device while providing
stellar results.
FIG. 6 shows an ESD protection device 60 having a dual-node
capacitor in accordance with a first embodiment of the present
invention. Protection device 60 is positioned between an IC pad
terminal 10 and a buffer 20 that connects to an internal circuit
(not shown) to be protected from an ESD event.
Device 60 includes a primary protection circuit that includes
MOSFET 40, capacitor 12, and first and second resistors 14, 16,
respectively. Illustratively, MOSFET 40 is an NMOS transistor but
as previously discussed, may be a PMOS transistor as well. In
addition, although only one NMOS transistor is shown, it is to be
understood that multiple transistors may be connected in parallel,
with their poly gates connected together to a capacitor and first
and second resistors, to form a dual-node capacitor coupled
multi-finger MOSFET device. Accordingly, the protection device of
FIG. 6 preferably has a multi-finger structure.
Referring back to FIG. 6, capacitor 12 is connected between the
gate of NMOS device 40 and pad 10. First resistor 14 is connected
between the gate and the p-well (or bulk), while second resistor 16
is connected between the p-well and the grounded source (to VSS) of
the NMOS. In addition, a resistor 18 is located between pad 10 and
buffer 20. Further, an optional secondary ESD protection circuit,
comprising a smaller NMOSFET 50, is included as well.
Note that if MOSFET device 40 was a PMOSFET (so that its bulk is
formed of an n-well region), its drain would be connected between
pad 10 and buffer 20, and its source would be connected to power
source VDD.
The basic operation of FIG. 6 will now be discussed. In particular,
the unique structure of a single capacitor and two serially
connected resistors simultaneously pulls the p-well voltage
potential, of each NMOS in the multi-finger structure, to
approximately 0.7 V. In addition, the gate voltage potential is
pulled within the approximate range of 1 to 2 V during the first 5
to 10 ns of the ESD event. As an example, the resistor ratio
R.sub.1 R/R.sub.2 is in the range of 0.2 to 5, and preferably in
the range of 0.5 to 2.
As discussed, a 1 to 2 V gate potential assists in the uniform
turn-on of NMOS fingers during an ESD event. An approximate 0.7 V
p-well potential, during the initial ESD event, causes the source
junction to become forward-biased such that minority carriers are
injected which, when flowing towards the drain, help to reduce the
trigger voltage of the snap-back mechanism so as to improve the ESD
robustness of the NMOS transistor.
FIG. 7 shows the inherent parasitic capacitor properties of the
dual-node capacitor of FIG. 6 of the present invention.
Specifically, device 60 is now shown having parasitic capacitor 32
(ground-to-drain), 34 (ground-to-bulk), 36 (source-to-bulk), 39
(gate-to-source), as well as diode 38.
The operation of FIGS. 6 will now be discussed with greater detail
with reference to FIG. 7. During the first 5 to 10 ns of an ESD
event, the voltage potential of the p-well (Vb) is pulled up and
pinned at the forward-bias voltage (to approximately 0.7 V) of the
p-well to n-source junction diode. The gate potential (Vg) is
pulled up to within the range of approximately 1 to 2 V due to the
capacitor coupling 12, while the pad terminal 10 is around 1.5 to
20 V. The coupling ratio is determined by the following
equation:
where C.sub.C is the coupling capacitance between the drain and the
gate, C.sub.GD is the gate-to-drain overlap capacitance, C.sub.GS
is the gate-to-source overlap capacitance and C.sub.GB is the
gate-oxide capacitance. The initial R-C relaxation time constant
when the source junction is pinned at forward biasing may be
expressed by the following equation:
which is preferably designed to be within the range of
approximately 10 to 30 ns. As an example, the coupling capacitor
used in the order of 0.5 to 3 PF, and R1 (resistor 14) and R2
(resistor 16) are in the order to 4 to 20 Kohms.
FIG. 8 illustrates and alternative embodiment of the present
invention. Specifically, the inventive primary ESD protection
circuit 80 is employed to protect against a bus-to-bus ESD event,
from VDD to VSS. Accordingly, the input signal line from pad
terminal 10 of FIG. 6 is replaced by a VDD line from VDD source 30,
as shown in FIG. 8.
.Iadd.Furthermore, the pad terminal 10 in FIG. 6 or the first power
bus (VDD) in FIG. 8 can be deemed as a first stress node, and the
second power bus (VSS) can be deemed as a second stress node. Since
the gate of the MOSFET device 40 and the p-well (base) are coupled
to the first stress at least by a first common element (e.g., the
capacitor 12) and also coupled to the second stress node at least
by a second common element (e.g., the resistor 16), during an ESD
event, either the first and second stress node may see a high ESD
voltage, and the first and second stress nodes can bypass an ESD
current when the ESD protection circuit is triggered to conduct a
larger ESD transient current..Iaddend.
Finally, the above-discussion is intended to be merely illustrative
of the invention. Numerous alternative embodiments may be devised
by those having ordinary skill in the art without departing from
the spirit and scope of the following claims.
* * * * *