AN-248 Electrostatic Discharge Prevention-Input Protection Circuits and Handling Guide for CMOS Devices
AN-248
Fairchild Semiconductor
Application Note
October 1987
Revised March 2003
Electrostatic Discharge Prevention-Input Protection
Circuits and Handling Guide for CMOS Devices
Introduction
During the past few years, there have been significant
increase in the usage of low-power CMOS devices in sys-
tem designs. This has resulted in more stringent attention
to handling techniques of these devices, due to their static
sensitivity, than ever before.
All CMOS devices, which are composed of complementary
pairs of N- and P-channel MOSFETs, are susceptible to
damage by the discharge of electrostatic energy between
any two pins. This sensitivity to static charge is due to the
fact that gate input capacitance (5 pF typical) in parallel
with an extremely high input resistance (10
12
Ω
typical)
lends itself to a high input impedance and hence readily
builds up the electrostatic charges, unless proper precau-
tionary measures are taken. This voltage build-up on the
gate can easily break down the thin (1000Å) gate oxide
insulator beneath the gate metal. Local defects such as
pinholes or lattice defects of gate oxide can substantially
reduce the dielectric strength from a breakdown field of
8–10
×
10
6
V/cm to 3–4
×
10
6
V/cm. This then becomes the
limiting factor on how much voltage can be applied safely
to the gates of CMOS devices.
When a higher voltage, resulting from a static discharge, is
applied to the device, permanent damage like a short to
substrate, V
DD
pin, V
SS
pin, or output can occur. Now static
electricity is always present in any manufacturing environ-
ment. It is generated whenever two different materials are
rubbed together. A person walking across a production
floor can generate a charge of thousands of volts. A person
working at a bench, sliding around on a stool or rubbing his
arms on the work bench can develop a high static potential.
Table 1 shows the results of work done by Speakman
1
on
various static potentials developed in a common environ-
ment. The ambient relative humidity, of course, has a great
effect on the amount of static charge developed, as mois-
ture tends to provide a leakage path to ground and helps
reduce the static charge accumulation.
TABLE 1. Various Voltages Generated in 15%–30%
Relative Humidity (after Speakman
1
)
Condition
Most
Common
Reading
(Volts)
12,000
4,000
500
3,500
500
Highest
Reading
(Volts)
39,000
13,000
3,000
12,000
3,000
Standard Input Protection
Networks
In order to protect the gate oxide against moderate levels
of electrostatic discharge, protective networks are provided
on all Fairchild CMOS devices, as described below.
Figure 1 shows the standard protection circuit used on all
A, B, and 74C series CMOS devices. The series resistance
of 200
Ω
using a P
+
diffusion helps limit the current when
the input is subjected to a high-voltage zap. Associated
with this resistance is a distributed diode network to V
DD
which protects against positive transients. An additional
diode to V
SS
helps to shunt negative surges by forward
conduction. Development work is currently being done at
Fairchild on various other input protection schemes.
Diode Breakdown
D
1
=
25V
D
2
=
60V
D
3
=
100V
*These are intrinsic diodes
FIGURE 1. Standard Input Protection Network
Person walking across carpet
Person walking across vinyl
floor
Person working at bench
16-lead DIPs in plastic box
16-lead DIPs in plastic shipping
tube
© 2003 Fairchild Semiconductor Corporation
AN006029
www.fairchildsemi.com
AN-248
Other Protective Networks
Figure 2 shows the modified protective network for
CD4049/4050 buffer. The input diode to V
DD
is deleted
here so that level shifting can be achieved where inputs are
higher than V
DD
.
Mode
1
2
3
4
TABLE 2. Modes of High-Voltage Test
+
Terminal
Input
V
DD
Input
Associated Output
−
Terminal
V
SS
Input
Associated Output
Input
Pre- and post-zap performance is monitored on the input
leakage parameter at V
DD
=
18V. It has been found that all
Fairchild’s CMOS devices of CD4000 and 74C families can
withstand 400V zap testing with above mentioned condi-
tions and still be under the pre- and post-zap input leakage
conditions of
±
10 nA.
Handling Guide for CMOS Devices
From Table 1, it is apparent that extremely high static volt-
ages generated in a manufacturing environment can
destroy even the optimally protected devices by reaching
their threshold failure energy levels. For preventing such
catastrophes, simple precautions taken could save thou-
sands of dollars for both the manufacturer and the user.
In handling unmounted chips, care should be taken to
avoid differences in voltage potential between pins. Con-
ductive carriers such as conductive foams or conductive
rails should be used in transporting devices. The following
simple precautions should also be observed.
1. Soldering-iron tips, metal parts of fixtures and tools,
and handling facilities should be grounded.
2. Devices should not be inserted into or removed from
circuits with the power on because transient voltages
may cause permanent damage.
3. Table tops should be covered with grounded conduc-
tive tops. Also test areas should have conductive floor
mats.
Diode Breakdown
D
1
=
25V
D
2
=
60V
D
3
=
100V
*These are intrinsic diodes
FIGURE 2. Protective Network for CD4049/50
and MM74C901/2
Figure 3 shows a transmission gate with the intrinsic diode
protection. No additional series resistors are used so the
on resistance of the transmission gate is not affected.
All CMOS circuits from Fairchild’s CD4000 Series and 74C
Series meet MIL-STD-38510 zap test requirements of
400V from a 100 pF charging capacitor and 1.5 k
Ω
series
resistance. This human body simulated model of 100 pF
capacitance in series with 1.5 k
Ω
series resistance was
proposed by Lenzlinger
2
and has been widely accepted by
the industry. The set-up used to perform the zap test is
shown in Figure 4.
FIGURE 4. Equivalent RC Network to Simulate Human
Body Static Discharge (after Lenzlinger
2
)
Diode Breakdown
D
1
=
25V
D
2
=
60V
*These are intrinsic diodes
FIGURE 3. Transmission Gate with Intrinsic Diodes to
Protect Against Static Discharge
V
ZAP
is applied to DUT in the following modes by charging
the 100 pF capacitor to V
ZAP
with the switch S
1
in position 1
and then switching to position 2, thus discharging the
charge through 1.5 k
Ω
series resistance into the device
under test. Table 2 shows the various modes used for test-
ing.
Above all, there should be static awareness amongst all
personnel involved who handle CMOS devices or the sub-
assembly boards. Automated feed mechanisms for testing
of devices, for example, must be insulated from the device
under test at the point where devices are connected to the
test set. This is necessary as the transport path of devices
can generate very high levels of static electricity due to
continuous sliding of devices. Proper grounding of equip-
ment or presence of ionized-air blowers can eliminate all
these problems.
At Fairchild all CMOS devices are handled using all the
precautions described above. The devices are also trans-
ported in anti-static rails or conductive foams. Anti-static,
by definition
3
means a container which resists generation
of triboelectric charge (frictionally generated) as the device
is inserted into, removed from, or allowed to slide around in
it. It must be emphasized here that packaging problems will
2
www.fairchildsemi.com
AN-248 Electrostatic Discharge Prevention-Input Protection Circuits and Handling Guide for CMOS Devices
Handling Guide for CMOS Devices
not be solved merely by using anti-static rails or containers
as they do not necessarily shield devices from external
static fields, such as those generated by a charged person.
Commercially available static shielding bags, such as 3M
company’s low resistivity (
≤
10
4
Ω
/sq.) metallic coated poly-
ester bags, will help prevent damages due to external stray
fields. These bags work on the well-known Faraday cage
principle. Other commercially available materials are
Legge company’s conductive wrist straps, conductive floor
coating, and various other grounding straps which help
prevent against the electrostatic damage by providing con-
ductive paths for the generated charge and equipotential
surfaces.
It can be concluded that electrostatic discharge prevention
is achievable with simple awareness and careful handling
of CMOS devices. This will mean wide and useful applica-
tions of CMOS in system designs.
(Continued)
Footnotes
1. T.S. Speakman, “A Model for the Failure of Bipolar Sili-
con Integrated Circuits Subjected to ESD,” 12th Annual
Proc. of Reliability Physics, 1974.
2. M. Lenzlinger, “Gate Protection of MIS Devices,” IEEE
Transac. on Electron Devices, ED-18, No. 4, April
1971.
3. J.R. Huntsman, D.M. Yenni, G. Mueller, “Fundamental
Requirements for Static Protective Containers.” Pre-
sented at 1980 Nepcon/West Conference, Application
Note—3M Static Control Systems.
Fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and
Fairchild reserves the right at any time without notice to change said circuitry and specifications.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD
SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the
body, or (b) support or sustain life, and (c) whose failure
to perform when properly used in accordance with
instructions for use provided in the labeling, can be rea-
sonably expected to result in a significant injury to the
user.
3
2. A critical component in any component of a life support
device or system whose failure to perform can be rea-
sonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
www.fairchildsemi.com
www.fairchildsemi.com
京公网安备 11010802033920号
EEWORLD
