U.S. patent application number 09/031251 was filed with the patent office on 2001-08-16 for dual layer etch stop barrier.
Invention is credited to LAM, CHUNG HON, LEE, ERIC SEUNG, WHITE, FRANCIS ROGER.
Application Number | 20010013638 09/031251 |
Document ID | / |
Family ID | 21858411 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010013638 |
Kind Code |
A1 |
LAM, CHUNG HON ; et
al. |
August 16, 2001 |
DUAL LAYER ETCH STOP BARRIER
Abstract
A method for reactive ion etching of SiO.sub.2 and an etch stop
barrier for use in such an etching is provided. A silicon nitride
(Si.sub.xN.sub.y) barrier having a Si.sub.x to N.sub.y ratio (x:y)
of less than about 0.8 and preferably the stoichiometric amount of
0.75 provides excellent resilience to positive mobile ion
contamination, but poor etch selectivity. However, a silicon
nitride barrier having a ratio of Si.sub.x to N.sub.x (x:y) of 1.0
or greater has excellent etch selectivity with respect to SiO.sub.2
but a poor barrier to positive mobile ion contamination. A barrier
of silicon nitride is formed on a doped silicon substrate which
barrier has two sections. One section has a greater etch
selectivity with respect to silicon dioxide than the second section
and the second section has a greater resistance to transmission of
positive mobile ions than the first section. One section adjacent
the silicon substrate has a silicon to nitrogen ratio of less than
about 0.8. The second section, formed on top of the first section
is formed with the ratio of the silicon to nitrogen of greater than
about 0.8. Preferably the two sections together are from about 50
to about 100 nanometers thick.
Inventors: |
LAM, CHUNG HON; (WILLISTON,
VT) ; LEE, ERIC SEUNG; (ESSEX JUNCTION, VT) ;
WHITE, FRANCIS ROGER; (ESSEX JUNCTION, VT) |
Correspondence
Address: |
DRIGGS, LUCAS BRUBAKER & HOGG CO. L.P.A.
DEPT. IBU
8522 EAST AVENUE
MENTOR
OH
44060
US
|
Family ID: |
21858411 |
Appl. No.: |
09/031251 |
Filed: |
February 26, 1998 |
Current U.S.
Class: |
257/640 ;
257/E21.252; 257/E21.293 |
Current CPC
Class: |
H01L 21/02271 20130101;
H01L 21/3185 20130101; H01L 21/022 20130101; H01L 21/31116
20130101; Y10S 438/97 20130101; H01L 21/76832 20130101; H01L
21/0217 20130101; H01L 21/76834 20130101; H01L 21/76802
20130101 |
Class at
Publication: |
257/640 |
International
Class: |
H01L 023/58 |
Claims
We claim:
1. A semi conductor structure comprising; a semi conductor
substrate, a barrier of silicon nitride disposed on said substrate,
said barrier of silicon nitride having a first layer and a second
layer, the silicon nitride in said second layer having a Si:N ratio
greater than the Si:N ratio of the silicon nitride in said first
layer.
2. The invention as defined in claim 1 whereas the Si:N ratio in
said second layer is at least about 0.8 and the ratio of Si:N in
said first layer is less than about 0.8.
3. The invention as defined in claim 2 wherein said first layer has
a ratio of Si:N of about 0.75.
4. The invention as defined in claim 2 wherein said second layer
has a ratio of Si:N of at least about 1.0.
5. The invention as defined in claim 4 wherein said first layer has
a ratio of Si:N of about 0.75.
6. The invention as defined in claim 2 wherein said barrier is
between 50 and 100 nanometers thick.
7. The invention as defined in claim 6 wherein each of said first
and second layers is below about 25 and about 50 nanometers
thick.
8. The invention as defined in claim 1 wherein Si:N ratio in the
SiN barrier progressively changes from said substrate through said
second layer.
9. The invention as defined in claim 1 wherein a layer of silicon
dioxide overlies said barrier of silicon nitride, and wherein at
least one opening extends through said silicon dioxide layer and
said silicon nitride barrier and having a conductor disposed
therein.
10. The invention as defined in claim 1 wherein said first layer of
silicon nitride overlies said substrate and said second layer of
silicon nitride overlies said first layer.
11. The invention as defined in claim 10 wherein a layer of silicon
dioxide overlies said second layer of silicon nitride, and wherein
there is at least one opening in said silicon dioxide layer and
said silicon nitride, and a conductive material is disposed in said
at least one opening.
12. A method of reactive ion etching SiO.sub.2 comprising the steps
of; providing a substrate, forming a barrier of silicon nitride on
said substrate having a first layer and a second layer, said second
layer having a greater ratio of Si:N than said first layer, forming
a layer of SiO.sub.2 on said barrier, and reactive ion etching at
least one opening in said SiO.sub.2 using said silicon nitride
barrier as an etch stop.
13. The invention as defined in claim 12 further comprising the
steps of controlling the Si:N ratio in said second layer to at
least about 0.8 and the ratio of Si:N in said first layer to less
than about 0.8.
14. The invention as defined in claim 13 wherein said first layer
has a ratio of Si:N of about 0.75.
15. The invention as defined in claim 13 wherein said second layer
has a ratio of Si:N of at least about 1.0.
16. The invention as defined in claim 15 wherein said first layer
has a ratio of Si:N of about 0.75.
17. The invention as defined in claim 13 wherein said barrier is
between 50 and 100 nanometers thick.
18. The invention as defined in claim 17 wherein each of said first
and second layers is between about 25 and about 50 nanometers
thick.
19. The invention as defined in claim 12 wherein Si:N ratio in the
SiN barrier progressively increases from said substrate through
said second layer.
20. The invention as defined in claim 12 wherein said first layer
of silicon nitride is deposited on said substrate and said second
layer of silicon nitride is deposited on said first layer a silicon
nitride.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to the manufacture of
integrated circuit (I/C) chips and particularly to the fabrication
or processing of a silicon substrate to form the circuitry for the
I/C chip. During one stage of manufacture of I/C chips, a silicon
dioxide layer is applied over a silicon substrate. The silicon
dioxide must be etched at various places to provide openings to the
substrate for electrical connections. One conventional technique of
etching is by means of reactive ion etching (RIE). With reactive
ion etching it is conventional to provide an etch stop barrier
between the silicon substrate and the silicon dioxide layer formed
thereon. One conventional etch stop barrier is silicon nitride
(Si.sub.xN.sub.y). These silicon nitride barriers are
conventionally deposited by low pressure chemical vapor deposition
(LPCVD) utilizing conventional equipment. In one embodiment
mixtures of silane (SiH.sub.4) and ammonia (NH.sub.3) are utilized
as an ambient to provide the necessary silicon and nitrogen
moieties for the formation of the silicon nitride.
[0002] However, it has been found in the past that there were
variations from process to process of forming the Si.sub.xN.sub.y
barrier in the effectiveness of the nitride barrier in its
selectivity with respect to SiO.sub.2 when reactive ion etching the
SiO.sub.2. When etching SiO.sub.2 it is desirable to have as much
selectivity as possible of the etch stop with respect to the
SiO.sub.2 so as to allow a minimum thickness of the etch stop to be
applied. It was also found that there were variations in the
resulting barrier in the effectiveness of the silicon nitride to
prevent passing of positive mobile ions (PMI) which may occur
during subsequent processing due primarily to contaminants
introduced into the SiO.sub.2 layer. Positive mobile ion
contamination (PMIC) such as in a gate oxide of CMOS devices must
be reduced to a minimum. Thus a requirement of the silicon nitride
barrier is that it act to effectively block positive mobile ions
from penetrating into the substrate during subsequent processing
steps.
[0003] Therefore it is desirable to provide a silicon nitride
barrier that is both highly selective to etching of SiO.sub.2 and
also effective to block the passage of positive mobile ions in
subsequent processing steps.
SUMMARY OF THE INVENTION
[0004] According to the present invention, a method for reactive
ion etching of SiO.sub.2 with an etch stop barrier for use in such
an etching is provided. It has been found that a silicon nitride
(Si.sub.xN.sub.y) barrier having a Si.sub.x to N.sub.y ratio (x:y)
of less than about 0.8 and preferably the stoichiometric amount of
0.75 provides excellent resilience to positive mobile ion
contamination, but poor etch selectivity. However, a silicon
nitride barrier having a ratio of Si.sub.x to N.sub.x (x:y) of 1.0
or greater has excellent etch selectivity with respect to SiO.sub.2
but a poor barrier to positive mobile ion contamination. The
technique of the present invention includes providing a substrate
which conventionally is a doped silicon substrate, and forming a
barrier of silicon nitride on the substrate which barrier has two
sections or layers. One section has a greater etch selectivity with
respect to silicon dioxide than the second section and the second
section has a greater resistance to transmission of positive mobile
ions than the first section. Preferably the two sections are formed
by forming one section, referred to as the lower section adjacent
to silicon substrate with a silicon to nitrogen ratio of less than
about 0.8 and preferably about 0.75 which is the stoichiometric
ratio of silicon to nitrogen. The second section, or upper section
is preferably formed with the ratio of the silicon to nitrogen of
greater than about 0.8 and preferably at least about 1.0.
Preferably the two sections together are from about 50 to about 100
nanometers thick and in the preferred embodiment, each section is
about 25 to 50 nanometers thick.
DESCRIPTION OF THE DRAWING
[0005] FIG. 1 is a graph of the etch rate of silicon nitride
(Si.sub.xN.sub.y) in Ar:CHF.sub.3 CF.sub.4 at various silicon to
nitrogen ratios (x:y) of the silicon nitride;
[0006] FIG. 2 is a bar graph showing the V.sub.t shift of a
substrate after reactive ion etching using silicon nitride
(Si.sub.xN.sub.y) barriers of various ratios of silicon to nitrogen
(x:y);
[0007] FIG. 3 is a graph similar to FIG. 2 graphing the positive
ion density in the substrate as a function of the ratio of the
silicon to nitrogen (x:y) in silicon nitride; and
[0008] FIGS. 4A through 4G show the steps of the method of the
present invention somewhat diagrammatically.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0009] The use of silicon nitride as an etch stop barrier is well
known in the art especially for stopping the etch during reactive
ion etching (RIE) of silicon dioxide disposed over a silicon or
doped silicon substrate in the manufacture of integrated circuit
chips. Reactive ion etching is used in chip manufacturing to form
openings through the silicon dioxide so as to provide access to the
substrate. Typically the opening will be filled with metal such as
tungsten or other metal as is well known. In etching the silicon
dioxide an etch stop layer is used so as to allow the etching to
stop or essentially terminate once the etching has penetrated
through the silicon dioxide layer. Expressed another way, when the
etching has pierced the silicon dioxide layer it is desired that
the etching not continue to any significant extent. The barrier
layer of etch stop material is to ensure that the etch stops
substantially uniformly at all the various locations being etched
through the silicon dioxide. Thus, one of the principal
requirements of the etch stop material is that it have a relatively
high selectivity of etching with respect to the material which is
intended to be etched i.e. silicon dioxide. Expressed another way,
once the silicon dioxide has been etched it is desirable that there
be very little etching taking place after that.
[0010] FIG. 1 shows the etching rate of Si.sub.xN.sub.y in
nanometers per minute using an AME 5000 tool with Ar:CHF.sub.3
atmosphere at various ratios of silicon to nitrogen in a silicon
nitride (Si.sub.xN.sub.y) barrier. As can be seen, when the ratio
of silicon to nitrogen is 0.75 (which is the stoichiometric ratio)
the etch rate is between 140 and 160 nanometers per minute, but as
the ratio of silicon to nitrogen increases, this etch rate
decreases dramatically to a point where when the ratio of Si to N
is about 1.0 the etch rate has dropped down to about 20 nanometers
per minute. With a ratio greater than 1.0 no improvement in the
etch rate resistance is achieved. Thus, based on this particular
characterization, in order to get the lowest etch rate of silicon
nitride and thus the highest etch selectivity, it is desirable to
have a ratio of silicon to nitrogen of at least about 1.0.
[0011] However, in subsequent processing during chip manufacture
there can be generated positive mobile ions (PMI), in particular
Na.sup.+ and K.sup.+, principally from contamination in the
SiO.sub.2 layer. If these positive ions diffuse even in small
amounts into the silicon substrate they can cause significant
degradation of the substrate material in some structures. Thus, it
is desirable and often even necessary that these ions be
essentially excluded from penetrating the barrier and diffusing
into the substrate. FIGS. 2 and 3 show the amount of diffusion of
positive mobile ions especially sodium (Na.sup.+) as measured by Vt
Shift (mV) shown in FIG. 2 and ion density in 10.sup.10
Ions/cm.sup.2 shown in FIG. 3 in substrates with Si.sub.xN.sub.y
nitride barriers having various ratios of Si to N in the silicon
nitride. At a Si to N ratio of 1.05 there is a very high number of
mobile ions passing through the silicon nitride barrier, and even
at a ratio of 1.0 there is an appreciable amount of these ions
penetrating; indeed even at a ratio of silicon to nitrogen of 0.8
there is a significant amount of PMIC (positive mobile ion
contamination) . It is not until the ratio of silicon to nitrogen
is 0.75 (i.e. the stoichiometric ratio) that the PMIC is
essentially eliminated.
[0012] Thus, if one were to design the barrier to maximize
resistance to positive mobile ion penetration one would use a ratio
of silicon to nitrogen of 0.75. However, as shown above, this would
provide very poor etch selectivity. On the other hand, if one were
to design for the best etch selectivity, one would design a nitride
barrier having a ratio of silicon to nitrogen of 1.0 or greater;
but this would provide poor resistance to positive mobile ion
penetration.
[0013] According to the present invention, a barrier is provided
which will achieve both high resistance to positive mobile ion
penetration and very good etch selectivity with respect to
SiO.sub.2. This is accomplished by providing a barrier having two
separate sections or layers. A first layer of silicon nitride is
tailored to have excellent resistance to positive mobile ion
penetration and thus has a ratio of silicon to nitrogen of less
than about 0.8 and preferable about 0.75. A second layer of silicon
nitride is provided which has a silicon to nitrogen ratio of
greater than about 0.8 preferably about 1.05. This will provide
excellent etch selectivity. By having a dual layer barrier as
described, the barrier will provide not only good etch selectivity
but resistance to positive mobile ion contamination.
[0014] Referring now to FIGS. 4A through 4G, various steps of the
present invention are depicted in very diagrammatic fashion. As
seen in FIG. 4A a silicon substrate 10 is provided which has a gate
device 12 separated from the substrate 10 by means of a gate oxide
layer 13. The substrate has a region 14 of opposite polarity (shown
as N.sup.+) on top of which is a silicided layer 15, which
silicided layer 15 also overlies the gate 12.
[0015] A first layer of silicon nitride (Si.sub.xN.sub.y) 16 is
deposited over the substrate 10 and the gate device 12. The first
layer of silicon nitride 16 in the preferred embodiment is formed
in an AME 5000 tool sold by Applied Materials, Inc. with an
atmosphere of SiH.sub.4 and NH.sub.3 to form a silicon nitride
having a ratio of silicon to nitrogen of about 0.75. The ratio of
silicon to nitrogen is controlled by controlling the ratio of
SiH.sub.4 to NH.sub.3 in a well known manner. Preferably this first
layer 16 is from about 25 to about 50 nanometers thick.
[0016] Following the deposition of the first layer 16 a second
layer 18 of silicon nitride is deposited over the first layer 16 as
shown in FIG. 4B. Again this is done in the AME 5000 tool in an
atmosphere of SiH.sub.4 and NH.sub.3. The ratio of SiH.sub.4 to
NH.sub.3 in forming this second layer 18 is controlled so as to
form a silicon nitride with silicon to nitrogen ratio of at least
1.0 and preferably 1.05. This layer 18 is also formed to a
thickness of about 25 to about 50 nanometers so that the total
thickness of the first and second layers 16, 18 is from about 50 to
about 100 nanometers. It is not critical whether the layer 16 or 18
is formed on the substrate; however in the preferred embodiment,
the layer 16 is formed on the substrate 10 and the layer 18 is
formed over the layer 16.
[0017] On top of the layer 18 is deposited a layer of silicon
dioxide (TEOS) 20 preferably doped with boron (BSG) or phosphorous
(PSG) or both (BPSG) as shown in FIG. 4C which also is formed in a
conventional manner again using the AME 5000 tool. This layer 20 is
conventionally at least about 0.6 microns thick.
[0018] As shown in FIG. 4D surface 22 of the TEOS 20 is coated with
a photoresist 24, which is photoimaged and developed in a
conventional manner to provide openings one of which is shown at 26
in the photoresist 22. One photoresist that is especially useful is
positive acting resist 5409 sold by Shipley Corp.
[0019] Following the developing of the photoresist layer 24, the
SiO.sub.2 exposed through the opening 26 is anisotropically etched
preferably in a CHF.sub.3:O.sub.2 atmosphere to form opening 28 in
the SiO.sub.2 as shown in FIG. 4E. Because of the layer 18 of
Si.sub.xN.sub.y has a high Si to N ratio it has a very high
selectivity of etch rate as compared to the silicon dioxide 20, the
layer 18 Si.sub.xN.sub.y acts as an excellent etch stop material.
Never-the-less a certain amount of the layer 18 is removed as shown
as 29 in FIG. 4E.
[0020] Following the reactive ion etching, the remaining
photoresist 24 is stripped and the exposed silicon nitride layers
16 and 18 are removed by dry etching in Ar:CHF.sub.3 to provide the
structures shown in FIG. 4F.
[0021] Following the removal of the Si.sub.xN.sub.y layers in
openings 26, a contact barrier such as TiN 30 is formed on the
SiO.sub.2 wall in opening 26 and surface 22 and on the exposed
substrate 10. This is followed by deposition of a metal such as
Tungsten (W) 32, as shown in FIG. 4G.
[0022] That portion of the Si.sub.xN.sub.y layers remaining under
the SiO.sub.x, which have not been exposed and etched, contain the
layer 16 which has excelled resistance to PMIC during subsequent
processing. Thus the two layers 16 and 18 have together provided
high etch selectivity during RIE of the silicon and also reduced or
eliminating PMIC during subsequent processing.
[0023] Of course it should be understood that the ratios of Si to N
in the two layers can be varied as can be the thicknesses of the
two layers. For example if there is more concern for either more
etch selectivity or improved barrier to positive mobile ion
penetration the thickness of each of the layers 16 and 18 as well
as the ratios of Si to N in each layer can be varied. Also, as
noted above the layer 18 with high etch selectivity can be formed
on the substrate, and the layer 16 with good resistance to PMIC can
be formed on the layer 18.
[0024] Also it should be understood that in using a conventional
tool for forming the silicon nitride, it is possible to provide a
barrier which has a gradient throughout; i.e. a structure which at
the surface of the substrate has excellent barrier properties to
positive mobile ion penetration and then gradually increases the
silicon to nitrogen ratio so that the outer surface has high etch
selectivity (or vice versa). This can be accomplished by starting
with a ratio of SiH.sub.4 to NH.sub.3 that will provide a ratio of
0.75 of Si to N in the silicon nitride, and then gradually changing
the concentrations of SiH.sub.4 and NH.sub.3 such that at the end
of the cycle the ratio of Si to N in the silicon nitride is 1.0 or
more.
[0025] Thus, according to the present invention an improved etch
stop barrier is provided which provides both excellent resistance
to positive mobile ion penetration and also very good etched
selectivity in the same barrier by having multiple layers of
material which are tailored to a specific function.
* * * * *