s in order to
enable electrical contact to be made to underlying
metallic layers (e.g., tungsten).
Preferably, etching should be
carried out just to the point at
which the dielectric material is
completely removed from the
surface of the metallic regions.
Prior to that point, incomplete
etching leads to
poor contact to the underlying
metallic layer.
Subsequent to that point,
undesirable undercutting of the
dielectric layer occurs.
The present use of calibration curves
of etch rate as a function of temperature to
achieve the desired end point does not always
lead to consistent results.
.br
.ul
Summary of the Invention
	In accordance with the
invention, a method is disclosed for monitoring
the progress of chemical etching,
which is used
to open up contact
windows in a
dielectric layer.
.ta 10
The method, which is electrochemical
in nature, requires both an electrode under
the dielectric layer to which electrical
connection may be easily made and
sufficient current to be easily
measured.
To determine when etching
is complete, a
metallic region
beneath the dielectric layer
serves as one monitoring
electrode.
Prior to the completion
of chemical etching,
this electrode is biased at a instant potential
relative to a second electrode,
which is also immersed in the
chemical etchant.
A measurement is made of
current, which is
proportional
to the area of metal exposed through
the dielectric as the chemical
etching proceeds.
The chemical etching is considered
complete when the
current rises to and
attains a substantially constant value.
	A preferred embodiment
is directed to the
processing of integrated
circuit devices on
semiconductor slices,
in which windows or apertures must be
etched through a dielectric layer of
silicon nitride to an underlying
patterned tungsten layer in order to
prepare the devices for connection
to external circuitry.
In the practice of the invention,
a separate tungsten test region may be formed
along the perimeter of the slice at
the same time that the patterned tungsten layer
is formed.
The test region may be
of known dimensions, and the
dielectric layer above it may be
completely etched. 
Alternatively, apertures or windows
of known area in the
dielectric layer
may be etched.
	Where the substrate is
a semiconductor having
a doping level sufficiently high to
support conduction
and where portions of the
patterned metallic layer make electrical
contact with portions of the
semiconductor,
then the metallic layer itself may serve
as the test region.
In such a case, monitoring may
be done by contacting
the backside of the
semiconductor slice.
.br
.ul
Brief Description of the Drawing
	The Figure is a section view, partly schematic,
of apparatus used in the
practice of the invention.
.br
.ul
Detailed Description of the Invention
.ta 10
	The Drawing is discussed
in terms of the preferred
embodiment, which is
the monitoring of
chemical etching of
portions of a
protective dielectric
layer overlying
integrated circuit devices.
The chemical etching
procedure is performed
just prior to a
final metallization step
for interconnecting
the devices to
external circuitry.
	As is shown
in the Figure, a
substrate|13,
consisting of a
semiconductor material such as
silicon and the like, has already been
processed by methods well-known in the art
of fabricating integrated circuit devices.
A protective film|14
of silicon dioxide,
for example,
formed by well-known methods,
covers the surface of the substrate.
Apertures 14a in the protective film
enable regions of a metallic layer|16, 
consisting of
tungsten, for example, to
make electrical contact to selected portions
of substrate|13.
The apertures may be produced by
well-known photoresist and
etching techniques.
An insulating film|17,
consisting of
silicon nitride, for example,
portions of which are to be etched,
is shown deposited on
both metallic layer|16
and protective film|14.
As described in Vol.|114,
.ul
Journal of Electrochemical Society,
pp.|869-872 (1967),
a second protective film|18
is shown deposited on the
silicon nitride film|17.
The second film,
which may be
any mask material
including silicon dioxide,
is shown with
windows|18a already having been etched
using well-known photoresist techniques.
It is desired to etch the portions of the
silicon nitride layer|17 thus exposed by
the windows|18a in layer|18.
As shown in the
Figure, this is accomplished
by partly immersing the
substrate in a
chemical etchant|12,
contained in container|11.
Typically,
hot (160|degrees C to 180|degrees C) phosphoric
acid (85|percent H938P0948 in water) is used 
to chemically etch silicon nitride,
as is well-known in the art.
Following the desired etching
of the silicon nitride
layer, a
patterned metallic layer
is formed on the
surface of layer|18
in order to
enable electrical
connection from the exposed
tungsten regions|16 to be made to external
circuitry.
	In order to determine when
etching is complete
through the insulating film|17
to the underlying
metallic layer|16, and in accordance
with the invention, a method of
monitoring is now described.
A three-electrode
system in conjunction with a commercially
available potentiostat is employed to
monitor the progress of etching.
Such potentiostats may
be used to control potentials in
three-electrode arrangements consisting
of a working electrode, a counter
electrode, and a reference electrode,
all immersed in the electrolyte.
The potential of the working electrode,
here a continuous metallic surface,
is constantly regulated with respect to the reference
electrode.
The potentiostat adjusts the applied
potential differences between the
working electrode and the counter
electrode to maintain the potential of the working
electrode at a constant value
with respect to the reference electrode.
	In order to measure the area
exposed on the working electrode, it is
necessary that the potential at that
electrode be held constant.
If the potential is not held constant,
then the end point of chemical etching is
not readily determinable, and undercutting
of the dielectric may occur as chemical
etching proceeds beyond the end point.
	Accordingly, it is not desirable
to use two electrode systems, since variable
current passes through these electrodes
during the chemical etching of the
dielectric film.
Consequently, the potential of the working
electrode varies, and only the potential
difference between the two electrodes
remains constant.
On the other hand, a three electrode
arrangement combined with a potentiostat,
enables the potential at the working
electrode to be maintained at a constant
value, as discussed above, and hence is
preferred.
	To employ the monitoring technique,
contact is made to a previously bared portion of the
metallic layer|16 by electrode|21,
which is connected to one input of a
potentiostat 20.
However, it will be appreciated that the
individual integrated circuit
devices are each of exceedingly small surface
area.
In order to facilitate making electrical
connection to the metallic layer|16
and in order to ensure that there is
sufficient area of the metallic layer|16
to be exposed to the etchant solution
such that the current resulting from the
applied potential is easily measured,
one of two techniques are conveniently employed.
In the first technique,
electrical connection is made to
a continuous
metallic test region|16a, which has
previously been fabricated simultaneously with
the patterned metallic regions
comprising layer|16.
In the second technique,
contact is made to the backside of
the substrate in cases
where the resistivity
of the substrate is
sufficiently low.
For silicon, a maximum resistivity
of about 10839|ohm-centimeters
is permitted.
In either event, immersion in
solution is made to the extent
that electrode|21 does not make contact
with the solution.
	The test region|16a
may conveniently be a
ring
around the circumference of the slice.
Such a design,
which is incorporated
in the mask used to
define the metallic 
layer|16,
will ordinarily not
interfere with the 
desired contact patterns on the mask.
For example, the
test region may be
fabricated by masking
a ring around the rim
of the slices with photoresist during the
tungsten patterning step.
Thus, following the patterning step,
the
normal conductor
pattern is generated on the
active device region of
each slice, but the
outer ring is continuous
tungsten.
Alternative designs may also be
envisioned which will 
accomplish the same purpose.
	In order to expose at least
portions of the test region to the
electrolyte, a series of windows
of known dimension, shown as 18b
in the Figure,
are formed over such portions
simultaneously when windows|18a are formed
in the second dielectric film|18.
Alternatively, the
test region may be
of known dimensions, and
the portion of the dielectric film|17
above the test region completely
etched.
	A second input of the potentiostat|20 is
connected to a reference electrode 22.
Examples of reference electrodes
include the calomel electrode (mercurous
chloride in contact with mercury, both
immersed in an aqueous potassium
chloride solution of known concentration)
and the silver-silver chloride
electrode (silver chloride in contact
with silver, both immersed in hydrochloric
acid).
Where the nature of the
solution in which monitoring
is to be performed is
such that the above
reference electrodes
would be chemically attacked,
other suitable reference electrodes 
may be employed.
For example, gold wire is a suitable
reference electrode where the chemical etching
solution is the hot phosphoric
acid described earlier.
Although not a necessary part of
this description, further details of
this aspect may be found in
Vol.|2,
.ul
Electrochemical Technology,
pp.|61-64 (1964).
	Electrical contact with the
electrolyte is provided by 
counter electrode 23, which
is connected to one output side of
potentiostat 20.
Potentiostat circuits have been described
in detail elsewhere, 
and thus do not form a necessary
part of this description.
See, e.g.,
Vol.|35,
.ul
Analytical Chemistry,
pp.|1770-1778 (1963).
	The test region is maintained
at a constant potential relative to
the reference electrode during the
chemical etching, and current is
measured.
Etching is considered to be complete when the current
rises to and attains a substantially
constant value, which is a function
of the
total amount of
surface area
exposed on the
test region.
	In the etching of portions
of a layer of silicon nitride to
expose an underlying layer of
tungsten using hot phosphoric
acid, monitoring in accordance
with the invention is conveniently
done by measuring the reduction
current resulting from the reduction
of hydrogen ion to hydrogen gas
at the exposed tungsten.
Monitoring is performed cathodically
only (i.e., tungsten is the cathode),
since operating anodically would
completely oxidize the tungsten.
The range of potential values
that may be employed is constrained
by the observation that at the
less cathodic potentials, there is
insufficient current to measure
conveniently, while at the more
cathodic potentials, the current
becomes too high to measure
conveniently.
In addition, at the more cathodic
potentials, capious gas evolution
also occurs, which must be avoided,
since it interferes with the monitoring
procedure.
Accordingly, the potential is advantageously
maintained at a voltage in the range
of from -0.55|volts to -0.70|volts
(with respect to the gold reference
electrode; with respect to a
standard hydrogen electrode, the
corresponding range is from -0.21|volts
to -0.36|volts).
No current is observed to flow initially
until the windows 18b are sufficiently
chemically etched through for at least
a portion of the tungsten to be
exposed to the electrolyte|12.
Current then begins to flow, and
when the windows are completely open,
the current reaches a maximum
value and remains constant.
	It is contemplated that the
inventive approach will find application
in other related systems in which
it is desired to chemically etch
portions of a dielectric 
layer covering a metallic layer.
For example, where it is desired to
chemically etch portions of a layer
of P-glass (silicon dioxide containing
from less than 1|percent to about 4|percent
phosphorous) covering a layer of
aluminum, a hydrofluoric acid solution
buffered to a pH of about 7 is used.
A suitable counter electrode is
platinum, and a suitable reference
electrode is the standard hydrogen electrode.
.br
.ul
Example
.ta 10
	Tungsten was deposited 
in the usual manner on integrated
circuit devices previously formed
on silicon slices having a diameter of about
3.2 centimeters,
with the exception that during the tungsten
patterning step, an annular ring of
about 2|millimeters in width was
formed on the surface of the slice
between the periphery of the active
devices and the periphery of the
slice.
In this particular example,
a total window area
of 0.17|square
centimeters
was to be etched
in the deposited
silicon nitride covering the ring.
.ta 10
A platinum
counter electrode and
a gold reference electrode were employed. 
Applying a potential of -0.675|volts 
to the tungsten conductor, no current
was observed to flow until
after about 14|minutes of chemical
etching; the etching was complete
after an additional period of about
3 minutes.
In order to prevent further
chemical
etching, which would result in
deleterious undercutting of the
silicon nitride mask layer, the slice
was removed from the solution.
For the window area of 0.17|square
centimeters, the
current attained a
maximum value
of about 600|microamperes,
equivalent to a current density
of about 3.5|milliamperes per square
centimeter.
