Method of forming a zener diode

Details Method of forming a zener diode Method of forming a zener diode

2001-06-08

2003-11-11

Tsai, Jey (Department: 2812)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

Having insulated gate

C438S983000, C438S328000

Reexamination Certificate

active

06645802

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to over-voltage protection circuits, and in particular to electrostatic-discharge protection circuits.
BACKGROUND
An electrostatic discharge, or ESD, is a transient discharge of static charge. A familiar example of an ESD is the spark that can occur between a person and a grounded object after the person walks across a carpet. The person acquires a static charge from the carpet; contact with the grounded object allows the static charge to discharge.
The energy associated with an ESD event can easily damage sensitive integrated circuit (IC) components. Protection circuits that can handle the high energies of ESD events are therefore integrated with sensitive IC components so that the protection circuitry can dissipate ESD energy. Typically, a voltage clamp limits the voltage on a selected external IC pin to a level that will not damage ESD-sensitive components. For a discussion of voltage clamps for ESD protection, see Ajith Amerasekera and Charvaka Duvvury,
ESD in Silicon Intearated Circuits
, pp. 30-52 (1995), and U.S. patent application Ser. No. 09/150,503, entitled “Electrostatic Discharge Protection Circuit,” by Shahin Toutounchi and Sheau-Suey Li, filed Sep. 9, 1998. Both of these documents are incorporated herein by reference.
FIG. 1A
is a schematic diagram of a conventional silicon-controlled rectifier (SCR)
100
. SCRs are used extensively to protect ESD-sensitive components. SCR
100
is a two-terminal voltage clamp having an anode
102
and a cathode
104
. SCR
100
responds to ESD events on anode
102
by sinking current to cathode
104
, primarily via a pair of current paths: a PNP transistor
106
and a resistor
108
define the first current path; an NPN transistor
110
and a resistor
112
define the second. SCR
100
also includes a zener diode
114
connected between the bases of transistors
106
and
110
. Zener diode
114
exhibits a reverse-bias breakdown voltage that is low relative to standard diodes. As described below, zener diode
114
acts as a trigger element to help turn on transistors
106
and
110
in response to ESD events on anode
102
.
Anode
102
remains in some active voltage range relative to cathode
104
during normal circuit operation. In a typical logic circuit, for example, cathode
104
might be grounded (i.e., held at zero volts) and anode
102
might transition between zero and five volts or zero and 2.5 volts. Such differences in potential between anode
102
and cathode
104
are insufficient to turn on zener diode
114
, so very little current passes through resistors
108
and
112
. As a result, the voltages dropped across resistors
108
and
112
are normally insufficient to turn on respective transistors
110
and
106
.
An ESD on anode
102
can raise the voltage between anode
102
and cathode
104
well above normal operating levels. Significant increases will exceed the break-down voltage of zener diode
114
, causing zener diode
114
to conduct. The resulting voltages developed across resistors
108
and
112
will then turn on respective transistors
110
and
106
, thereby sinking ESD current from anode
102
to cathode
104
.
FIG. 1B
is a graph of an illustrative I-V curve
116
for SCR
100
(FIG.
1
A): the x-axis represents the voltage difference between anode
102
and cathode
104
(i.e., V
A
-V
c
) and the y-axis represents the current I
scr
through SCR
100
between anode
102
and cathode
104
.
In the absence of an ESD (or some other over-voltage event), the anode voltage V
A
on anode
102
remains below the so-called “trigger” voltage V
T
required to turn on SCR
100
. The current through SCR
100
therefore remains very low. When an ESD raises the anode voltage V
A
above trigger voltage V
T
, the anode voltage V
A
will “snap back” to a holding voltage V
H
. Once triggered, SCR
100
sinks current from anode
102
to cathode
104
until most of the energy of the ESD event is dissipated. The trigger voltage V
T
should be selected to ensure that SCR
100
triggers fast enough to avoid damaging any associated ESD-sensitive components (not shown).
Integrated circuits are becoming more complex as device engineers are able to pack more devices on each chip. These improvements are primarily due to advances in semiconductor processing technologies that afford the use of ever smaller circuit features. As features become smaller, reducing junction capacitance becomes increasingly critical to speed performance. One method of reducing junction capacitance involves the use of lower doping levels when forming substrates and well diffusions. Unfortunately, reducing doping levels complicates the task of providing adequate ESD protection.
ESD protection circuits typically include triggering mechanisms that depend upon the breakdown voltage of a selected junction. In general, the breakdown voltage of a given junction is inversely related to doping level. That is, lower doping levels provide higher breakdown voltages. The low well and substrate doping levels preferred for circuits with very small features can increase the breakdown voltage of ESD trigger mechanisms to unacceptably high levels. In modern 0.18-micron processes, the breakdown voltage of trigger mechanisms can approach the breakdown voltage of gate oxides. Consequently, an ESD-protection circuit can fail to trigger in response to an ESD event in time to avoid irreversibly damaging a neighboring gate oxide.
FIG. 1C
is a cross-sectional diagram of an example of SCR
100
that addresses the problem of providing an adequate trigger mechanism for circuits with very small feature sizes. SCR
100
is formed on a p-type silicon substrate
118
using a conventional CMOS process. SCR
100
includes a number of diffusion regions, some of which are isolated from others by isolation regions
120
. Isolation regions
120
are typically silicon dioxide formed using a conventional isoplanar isolation scheme. The diffusion regions include p+ regions
122
,
124
, and
126
, n+ regions
128
,
130
, and
132
, and an n− region
136
. Of these, p+ diffusion
126
is formed within an n-well
134
. A layer of silicide is divided into areas
138
that conventionally establish low-impedance electrical contact to the diffusion regions.
The various components of
FIG. 1A
are instantiated in substrate
118
as shown. For example, zener diode
114
is formed laterally between diffusion regions
124
and
130
. A silicide block
140
prevents the zener junction formed between n− diffusion
136
and p+ diffusion
124
from shorting. Silicide block
140
is typically silicon dioxide. The break-down voltage of zener diode
114
, and therefore the trigger voltage V
T
of SCR
100
, depends primarily on the doping concentration of n− diffusion
136
.
Instantiating zener diode
114
laterally, as depicted in
FIG. 1C
, allows process engineers a degree of flexibility in establishing the breakdown voltage of zener diode
114
. The breakdown voltage of zener diode
114
can be adjusted by selecting an appropriate dopant dose for n− diffusion
136
. Silicide block
140
, typically silicon dioxide, then prevents zener diode
114
from shorting upon the formation of silicide layer
138
. Unfortunately, the silicide blocking process is expensive and time consuming. Further, residual oxides from the formation of silicide block
140
can contaminate the subsequently formed silicide layers
138
, and consequently increase their resistance. Finally, providing a sufficiently low breakdown voltage for zener diode
114
can be difficult for very dense ICs due to the use of reduced doping levels. There is therefore a need for an improved ESD protection circuit that works well in circuits with very small features and that does not require a silicide blocking process.
SUMMARY
The present invention is directed to a cost-effective ESD protection circuit that is easily integrated with circuits having very small features. One ESD protection circuit in accordance with the invention includes a bipolar t

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