U.S. patent application number 10/105521 was filed with the patent office on 2003-10-02 for gradient barrier layer for copper back-end-of-line technology.
Invention is credited to Hung, Cheng-Yu, Liou, Fu-Tai, Yew, Tri-Rung.
Application Number | 20030186087 10/105521 |
Document ID | / |
Family ID | 28452433 |
Filed Date | 2003-10-02 |
United States Patent
Application |
20030186087 |
Kind Code |
A1 |
Liou, Fu-Tai ; et
al. |
October 2, 2003 |
Gradient barrier layer for copper back-end-of-line technology
Abstract
The present invention is directed to a structure of a gradient
barrier layer. The gradient barrier with a composite structure of
metal/metal salt of different composition/metal such as
Ta/Ta.sub.xN.sub.1-x/TaN/Ta.s- ub.xN.sub.1-x/Ta
(tantalum/tantalum.sub.x nitride.sub.1-x/tantalum
nitride/tantalum.sub.x nitride.sub.1-x/tantalum) is proposed to
replace the conventional barrier for copper metallization. The
gradient barrier can be formed in a chemical vapor deposition (CVD)
process or a multi-target physical vapor deposition (PVD) process.
For CVD process, using the characteristics of well-controlled
reaction gas injection, the ratio of tantalum (Ta) and nitrogen (N)
can be modulated gradually to form the gradient barrier. For the
multi-target PVD process, the gradient barrier is formed by
depositing multi-layers of different composition Ta.sub.xN.sub.1-x
films. After subsequent thermal cycle processes such as metal
alloy, the inter-layer diffusion occurs and a more smooth
distribution of Ta and N is achieved for the gradient barrier. The
advantages of forming the gradient barrier include a
well-controlled process, a strong adhesion between via and landing
metal, more uniform step coverage, and less brittle to reduce
crack.
Inventors: |
Liou, Fu-Tai; (Hsin-Chu
City, TW) ; Hung, Cheng-Yu; (Taipei City, TW)
; Yew, Tri-Rung; (Hsin-Chu, TW) |
Correspondence
Address: |
Dickinson Wright PLLC
1901 L Street NW,
Suite 800
Washington
DC
20036
US
|
Family ID: |
28452433 |
Appl. No.: |
10/105521 |
Filed: |
March 26, 2002 |
Current U.S.
Class: |
428/698 ;
257/E21.169; 257/E21.17; 427/255.28; 427/419.1; 427/419.7; 428/332;
428/472 |
Current CPC
Class: |
H01L 2924/00 20130101;
C23C 14/027 20130101; H01L 21/2855 20130101; H01L 21/76864
20130101; Y10T 428/26 20150115; H01L 2924/0002 20130101; H01L
21/76846 20130101; H01L 2924/0002 20130101; C23C 14/0641 20130101;
H01L 21/28556 20130101; C23C 16/029 20130101; C23C 16/34 20130101;
H01L 23/53238 20130101; H01L 21/76805 20130101 |
Class at
Publication: |
428/698 ;
428/472; 428/332; 427/255.28; 427/419.1; 427/419.7 |
International
Class: |
B32B 015/04; C23C
016/00; B05D 001/36 |
Claims
What is claimed is:
1. A gradient barrier structure comprising: a first metal layer; a
plurality of layers of a metal salt with different composition; and
a second metal layer.
2. The structure according to claim 1, wherein said first metal
layer is selected from the group comprising of tantalum, titanium,
and tungsten layer.
3. The structure according to claim 1, wherein said metal salt
comprises elements of tantalum (Ta) and nitrogen (N).
4. The structure according to claim 3, wherein said plurality of
layers of said metal salt with different composition comprises a
plurality of Ta.sub.xN.sub.1-x layers, wherein said x varies in the
range between about 0.5 and 1.
5. The structure according to claim 3, wherein said plurality of
layers of said metal salt with different composition comprises a
plurality of Ta.sub.x1N.sub.1-x1 layers, a TaN (tantalum nitride)
layer, and a plurality of Ta.sub.x2N.sub.1-x2 layer, wherein said
x1 is decreasing from about 1 to 0.5, and said x2 is increasing
from about 0.5 to 1.
6. The structure according to claim 5, wherein said first metal
layer is a tantalum layer, and said tantalum layer and said
Ta.sub.x1N.sub.1-x1 layers has a total thickness between 10 and 100
angstroms.
7. The structure according to claim 5, wherein said tantalum
nitride layer has a thickness between 100 and 200 angstroms.
8. The structure according to claim 5, wherein said second metal
layer is a tantalum layer, and said Ta.sub.x2N.sub.1-x2 layers and
said tantalum layer has a total thickness between 100 and 200
angstroms.
9. The structure according to claim 1, wherein said second metal
layer is selected from the group comprising of tantalum, titanium,
and tungsten layer.
10. A method for forming a gradient barrier on a substrate, said
method comprising: forming a first metal layer on said substrate;
forming a plurality of layers of a metal salt with different
composition on said first metal layer; and forming a second metal
layer on said plurality of layers of said metal salt with different
composition.
11. The method according to claim 10, wherein said step of forming
said first metal layer comprises by introducing a first reaction
gas to form said first metal layer in a chemical vapor deposition
process.
12. The method according to claim 11, wherein said step of forming
said plurality of layers of said metal salt with different
composition comprises introducing a second reaction gas of varying
flow rates to react with said first reaction gas to form said
plurality of layers of said metal salt with different
composition.
13. The method according to claim 12, wherein said step of forming
said second metal layer comprising a step of stopping introducing
said second reaction gas to form said second metal by use of said
first reaction gas.
14. The method according to claim 10, wherein said first metal
layer is a tantalum layer.
15. The method according to claim 14, wherein said step of forming
said plurality of layers of said metal salt with different
composition comprises forming a plurality of Ta.sub.xN.sub.1-x
(tantalum nitride) layers, wherein said x varies in a range between
about 0.5 to 1.
16. The method according to claim 15, wherein said step of forming
said plurality of layers of said metal salt with different
composition comprises: forming a Ta.sub.x1N.sub.1-x1 (tantalum
nitride) layer, wherein x1 is less than 1 and greater than 0.5;
forming a TaN (tantalum nitride) layer; and forming a
Ta.sub.x2N.sub.1-x2 (tantalum nitride) layer, wherein x2 is greater
than 0.5 and less than 1.
17. The method according to claim 10, further comprising a step of
performing a thermal cycle process.
18. A method for forming a gradient barrier on a substrate, said
method comprising: forming a first tantalum layer on said
substrate; forming a plurality of Ta.sub.x1N.sub.1-x1 (tantalum
nitride) layers, wherein said x1 gradually decreases from about 1
to 0.5; forming a TaN (tantalum nitride) layer on said plurality of
said Ta.sub.x1N.sub.1-x1 (tantalum nitride) layers; forming a
plurality of Ta.sub.x2N.sub.1-x2 (tantalum nitride) layers, wherein
said x2 gradually increases from about 0.5 to 1 ; forming a second
tantalum layer on said plurality of said Ta.sub.x2N.sub.1-x2
(tantalum nitride) layers.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to a method for
forming a gradient barrier layer, and more particularly to a method
for forming a gradient barrier layer with a composite structure of
Ta /Ta.sub.xN.sub.1-x/TaN/Ta.sub.xN.sub.1-x/Ta
(tantalum/tantalum.sub.x nitride.sub.1-x/tantalum
nitride/tantalum.sub.x nitride.sub.1-x/tantalum) for VLSI copper
back end of the line (BOEL) technology.
[0003] 2. Description of the Prior Art
[0004] As feature sizes shrink, copper metallization has been
proposed to answer the need of high performance and reliable
interconnect for high-density integrated circuits since copper has
improved stress and electromigration properties and reduced
resistivity over the aluminum. However, copper readily diffuses
through many materials, including both metals and dielectrics,
potentially affecting dielectric constants of insulating material.
For example, copper diffusion into the inter-meal dielectric (IMD)
such as silicon oxide results in current leakage between adjacent
lines and degradation of inter-level dielectric (ILD) breakdown
field. Therefore, difficulties with forming copper interconnects
have lead to the development of barrier layers that hinder the
diffusion of copper into the vulnerable regions.
[0005] Referring to FIG. 1, a copper metallization implemented in
an integrated circuit technology is illustrated. A barrier layer
includes tens nanometer of TaN (tantalum nitride) 108 and Ta
(tantalum) 110 sandwiched in between copper layer 112 of the dual
damascene structure and an inter-metal dielectric (IMD) 106 such as
silicon oxide layer and electrically contacted a copper structure
102 within a substrate 100. In general, the inter-metal dielectric
layer 106 is formed on a silicon nitride layer 104 which overlies
on the substrate 100 and serves as a passive layer. It is noted
that TaN has been proposed as a good copper diffusion barrier and
the adhesion of TaN to insulators is adequate, while Ta adheres
poorly to oxide-like dielectric but acts better for copper seed
formation. Thus, the Ta layer is typically formed on the TaN layer
to enhance the adhesion of copper to TaN. In the conventional
copper interconnect technology, when oxide-like materials act as
the inter-metal dielectric, the adhesion of TaN layer 108 of the
barrier layer to copper 102 isn't an issue. Therefore, the
conventional barrier layer used in copper back end of the line
(BOEL) technology is mainly for preventing copper out-diffusion
from the structure 102 and 112 as depicted Arrows.
[0006] However, in the new low-k inter-metal dielectric (IMD)
material systems, due to the larger thermal expansion coefficients
of low-k materials 210 and the poor adhesion of TaN 108 and Cu
landing pad 102, the interface 212 of TaN and Cu (108/102) becomes
weak and very easy to separate, as shown in FIG. 2. Moreover, the
TaN is more brittle and easy to crack. These cause the
interconnection open issue and even serious fails in reliability
tests such as thermal cycle test (TCT) and stress migration (SM).
Therefore, approaches to the adhesion problem induced in the low-k
dielectric material systems are prosperously progressing, and the
argon (Ar) pre-clean technique is one of many.
[0007] The Ar-preclean process has been implemented to removed TaN
at via bottom to make Ta film directly contact with Cu surface to
increase the adhesion strength. However, due to the TaN layer at
the via bottom is extremely thin, the Ar-preclean process margin is
very difficult to control. Many side effects, such as
micro-trenches 310, materials re-deposition on via sidewall 320,
barrier thinning in trench bottom 330, are created and induce more
reliability issue as depicted in FIG. 3. Micro-trenches 310 are
created due to unevenly over etched. Original via bottom material
even including copper residue is re-deposited on via sidewall or
diffuses into the low-k dielectric that causes the increase in
possibility of electrically discontinuity and changes the
characteristic of the low-k dielectric. When the via bottom portion
of the TaN layer 108 is removed, a partial of TaN layer 108 at
trench bottom is also removed resulting in barrier thinning problem
or, even worse, no TaN layer reserved, as respectively indicated by
reference numbers 330 and 340.
[0008] In view of the prior art described, it is a desire to
provide a barrier layer with a low diffusion coefficient for metal
conductive layers, excellent adhesion and more tensile properties,
and more uniform step coverage characteristic.
SUMMARY OF THE INVENTION
[0009] The present invention is directed to a method for forming a
gradient barrier layer. The gradient barrier with a composite
structure of metal/metal salt of different composition/metal such
as Ta/Ta.sub.xN.sub.1-x/TaN/Ta.sub.xN.sub.1-x/Ta
(tantalum/tantalum.sub.x nitride.sub.1-x/tantalum
nitride/tantalum.sub.x nitride.sub.1-x/tantalum) is proposed to
replace the conventional barrier for copper metallization. The
gradient barrier can be formed in a chemical vapor deposition (CVD)
process or a multi-target physical vapor deposition (PVD) process.
For CVD process, using the characteristics of well-controlled
reaction gas injection, the ratio of tantalum (Ta) and nitrogen (N)
can be modulated gradually to form the gradient barrier. For the
multi-target PVD process, the gradient barrier is formed by
depositing multi-layers of different composition Ta.sub.xN.sub.1-x
films. After subsequent thermal cycle processes such as metal
alloy, the inter-layer diffusion occurs and a more smooth
distribution of Ta and N is achieved for the gradient barrier. The
advantages of forming the gradient barrier include a
well-controlled process, a strong adhesion between via and landing
metal, more uniform step coverage, and less brittle to reduce
crack.
[0010] It is another object of this invention that a method for
forming a gradient barrier, which provides a good nucleation
surface for supporting deposition of copper in an overlying copper
layer, is provided.
[0011] It is a further object of this invention that a method for
forming a gradient barrier layer with excellent barrier properties
to prevent copper out-diffusion is provided.
[0012] It is another further object of this invention that a method
for in-situ forming a gradient barrier with a composite structure
of Ta/Ta.sub.xN.sub.1-x/TaN/Ta.sub.xN.sub.1-x/Ta
(tantalum/tantalum.sub.x nitride.sub.1-x /tantalum
nitride/tantalum.sub.x nitride.sub.1-x/tantalum- ) is provided.
[0013] In accordance with the present invention, in one embodiment,
a gradient barrier structure comprises a first metal layer, a
plurality of layers of a metal salt with different composition, and
a second metal layer. The first and the second metal layer can be
selected from the group comprising of tantalum, titanium, and
tungsten layer. The first and the second metal layer are tantalum
layers, and the plurality of layers of the metal salt includes a
plurality of Ta.sub.xN.sub.1-x layers, wherein x varies in the
range between about 0.5 and 1. The Ta.sub.xN.sub.1-x layers
comprises a plurality of Ta.sub.x1N.sub.1-x1 layers, a TaN
(tantalum nitride) layer, and a plurality of Ta.sub.x2N.sub.1-x2
layer, wherein the x1 is decreasing from about 1 to 0.5, and the x2
is increasing from about 0.5 to 1. The total thickness of the first
tantalum layer and the Ta.sub.x1N.sub.1-x1 layers is between 10 and
100 angstroms. The TaN layer has a thickness between 100 and 200
angstroms. The total thickness of the second tantalum layer and the
Ta.sub.x2N.sub.1-x2 layers is between 100 and 200 angstroms.
[0014] In accordance with the present invention, in another
embodiment, a method for forming a gradient barrier on a substrate
is also provided. The method comprises steps of forming a first
metal layer on the substrate, forming a plurality of layers of a
metal salt with different composition on the first metal layer, and
forming a second metal layer on the plurality of layers of the
metal salt with different composition. By introducing a first
reaction gas in a chemical vapor deposition process, the first
metal layer is formed. Then, by introducing a second reaction gas
of varying flow rates to react with the first reaction gas, the
plurality of layers of the metal salt with different composition is
formed. Next, by stopping the introduction of the second reaction
gas, the second metal is formed. The method further comprises a
step of performing a thermal cycle process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
becomes better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0016] FIG. 1 is a schematic cross-section view of the conventional
barrier for preventing copper diffusion;
[0017] FIG. 2 is a schematic cross-sectional view of the adhesion
problem of the conventional barrier at via bottom and the landing
metal in prior art;
[0018] FIG. 3 is a schematic cross-sectional view of showing side
effects occurred in the formation of a barrier by use of the
Ar-preclean technique in the prior art;
[0019] FIG. 4A and 4B are a schematic cross-sectional view of
forming the gradient barrier and the structure composition of the
gradient barrier in one embodiment in accordance with the present
invention; and
[0020] FIG. 5A and 5B are a schematic cross-sectional view of
forming the gradient barrier and a structure composition of the
gradient barrier in another embodiment in accordance with the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] Some sample embodiments of the invention will now be
described in greater detail. Nevertheless, it should be noted that
the present invention can be practiced in a wide range of other
embodiments besides those explicitly described, and the scope of
the present invention is expressly not limited except as specified
in the accompanying claims.
[0022] Referring to FIG. 4A, in one embodiment, a gradient barrier
for integrated circuit metallization processes is disclosed. The
key aspect of the present invention is that the gradient barrier
with a composite structure of metal/metal salt of different
composition/metal such as
Ta/Ta.sub.xN.sub.1-x/TaN/Ta.sub.xN.sub.1-x/Ta (tantalum/
tantalum.sub.x nitride.sub.1-x/tantalum nitride/tantalum.sub.x
nitride.sub.1-x/tantalum) is proposed to replace the conventional
barrier for copper metallization. A substrate 400 with a conductive
structure 410 is shown. The substrate 400 can be any substrate at
any semiconductor processing stage such as substrate with copper
landing pad 410 in the copper metallization. An optional passive
layer 430 such as silicon nitride layer is formed on the substrate
400 with the copper landing pad 410 to maintain copper's electrical
conductivity characteristics. The gradient barrier 450 of a dual
damascene structure is formed in a dielectric layer 440 such as
low-k dielectric or oxide-like dielectric. A barrier layer 420 used
to prevent copper out-diffusion from the copper landing pad 410
into the dielectric layer 440 can be also formed in accordance with
the present invention. A metal layer 460 is formed on the gradient
barrier 450 to accomplish the dual damascene interconnect. It is
noted that in this configuration the gradient barrier is
effectively sandwiched between the metal layer 460 and the
dielectric layer 440 and electrically connected the copper landing
pad 410 to prevent out-diffusion of the metallic material.
[0023] The gradient barrier 450 is a composite structure of
metal/metal salt of different composition/metal formed
sequentially. The first formed metal layer, such as tantalum,
titanium, and tungsten layer, is any metal layer with good adhesion
to the underlying conductive structure (copper landing pad for
example) and more tensile to prevent cracking in subsequent thermal
cycle processes when high thermal expansion coefficient dielectric
material such as low-k material serves as the inter-metal
dielectric layer. The metal salt layers of different composition
can be any metal salt layers with great barrier properties to
prevent materials in the overlying conductive layer diffusion into
the dielectric. The later formed metal can provides a good
nucleation surface for supporting deposition of metallic material
in the overlying conductive layer. For the great adhesion to
copper, the good step coverage, the excellent copper seed formation
properties, and a well-controlled process, tantalum (Ta) is
proposed to serve as the first and the later formed metal, while
different composition tantalum nitride layers (Ta.sub.xN.sub.1-x
films) sandwiched in between act as the metal salt layers to
prevent copper diffusion.
[0024] Referring to 4B, the composition of the gradient barrier 450
with Ta/Ta.sub.x1N.sub.1-x1/TaN/Ta.sub.x2N.sub.1-x2/Ta (tantalum/
tantalum.sub.x1 nitride.sub.1-x1/ tantalum nitride/tantalum.sub.x2
nitride.sub.1-x2/tantalum) structure is shown. The total thickness
of the Ta and Ta.sub.x1N.sub.1-x1 is in the range between about 10
and 100 angstroms depicted as Region 1 in FIG. 4B, and it is noted
that the metal ingredient (Ta) of Ta.sub.x1N.sub.1-x1 is decreased
as the thickness increases till reaching the ratio of Ta to N
(nitrogen) is 1, that is x1 is a descent number from about 1 to 0.5
. According to Region 2, the thickness of the TaN is between about
100 and 200 angstroms as depicted in FIG. 4B. The total thickness
of the Ta.sub.x2N.sub.1-x2 and Ta is in the range between about 100
and 200 angstroms depicted as Region 3, and it is noted that the
metal ingredient (Ta) of Ta.sub.x2N.sub.1-x2 is increased as the
thickness increases from the ratio of Ta to N being 1 to almost
100% Ta being reached, that is x2 is an ascent number from about
0.5 to 1.
[0025] In accordance with the present invention, a method for
forming the gradient barrier 450 is also disclosed. The gradient
barrier can be formed in a chemical vapor deposition (CVD) process
or a multi-target physical vapor deposition (PVD) process. For CVD
process, in another embodiment, using the characteristics of
well-controlled reaction gas injection, the ratio of tantalum (Ta)
and nitrogen (N) can be modulated gradually to form the gradient
barrier in-situ. Referring to FIG. 4A again, the method comprises a
step of providing a substrate 400 having a conductive structure 410
therein and an inter-layer dielectric layer 440 thereon. An
optional passive layer 430 such as silicon nitride layer is formed
underlying the inter-layer dielectric layer 440 to maintain the
conductive layer's 410 electrical characteristics. Then, a dual
damascene topography including trenches and vias is formed by use
of a conventional dual damascene process flow such as self-aligned,
via-first, or trench-first.
[0026] The gradient barrier 450 with composite structure of
metal/metal salt of different composition/metal is formed on the
substrate 400 with the dual damascene topography by use of CVD
processes. In other words, by controlling the reaction gas
injection technique, a first reaction gas is injected to form a
first metal layer on the inter-layer-dielectric layer 440 covering
sidewalls and bottoms of the trenches and the vias. Next, by
gradually changing a second reaction gas injected, a plurality of
metal salt layers of different composition is formed on the first
metal layer. Then, a second metal layer is formed on the plurality
of metal salt layers. For example, the material for the first and
the second metal layers is tantalum, and the composition of metal
salt is tantalum and nitrogen. By controlling the reaction gas
injection, a first Ta layer is formed on the inter-layer-dielectric
layer 440. Then, by gradually changing the N.sub.2 flow, a
plurality of different composition Ta.sub.xN.sub.1-x films is
formed on the first Ta layer. Additionally, the plurality of
different composition Ta.sub.xN.sub.1-x films can have a similar
composition as shown in FIG. 4B by gradually increasing the N.sub.2
flow (Region 1), when the ratio of Ta to N reaches 1 maintaining
the N.sub.2 flow till achieving a desired thickness (Region 2),
then gradually decreasing the N.sub.2 flow to approaching zero
(Region 3). Then, stopping introducing the N.sub.2 flow, a second
Ta layer is formed on the plurality of different composition
Ta.sub.xN.sub.1-x, films. A conductive layer 460 such as copper is
then formed on the gradient barrier 450 to accomplish the dual
damascene interconnect structure. It is noted that a barrier layer
420 for preventing conductive materials out-diffusion from the
conductive layer 410 into the dielectric layer 440 can be also
formed in accordance with the present invention.
[0027] In a further embodiment, a gradient barrier with composite
structure of metal/metal salt of different composition/metal is
formed by use of PVD processes. The PVD processes include the
sputter-like technique or the ion metal plasma (IMP) technique with
a multi-target feature. Referring to 4C and also FIG. 4A, after the
dual damascene topography is created, the gradient barrier 450 is
formed by depositing multi-layers of different composition metal
salt films. In other words, a first metal layer 450a is formed on
the inter-layer dielectric 440. Then, a plurality of metal salt
layers of different composition, such as 450b, 450c, 450d, 420e,
and 450f, is formed on the first metal layer 450a. Next, a second
metal layer 450g is formed on the plurality of metal salt layers.
Then, a conductive layer 460 such as copper is then formed on the
gradient barrier 450 to accomplish the dual damascene interconnect
structure.
[0028] For example, the material for the first and the second metal
is tantalum, and the composition of metal salt is tantalum and
nitrogen. A first tantalum layer 450a is formed on the inter-layer
dielectric layer 440 by using a first target (100% Ta) with the
sputter deposition technique. Then, a first Ta.sub.xN.sub.1-x layer
450b is formed on the first Ta layer 450a by using a second target
(about 85% Ta). Subsequently, a second and a third
Ta.sub.xN.sub.1-x layers (about 65% and 50% Ta), 450c and 450d, are
formed on the first Ta.sub.xN.sub.1-x layer 450b. Then, a fourth
and a fifth Ta.sub.xN.sub.1-x layers with increasing Ta ingredient
(such as about 65% and 85% Ta), 450e and 450f , is sequentially
formed. Then, a second Ta layer 450g is formed on the fifth
Ta.sub.xN.sub.1-x layer 450f. After subsequent thermal cycle
processes such as metal alloy, the inter-layer diffusion occurs and
a more smooth distribution of Ta and N is achieved for the gradient
barrier 450 indicated as dotted line 470. The gradient barrier 450
is illustrated as FIG. 5B, which presents a similar composition to
FIG. 4B. It is noted that the number of different composition
Ta.sub.xN.sub.1-x layers is not unique and not restricted to the
example.
[0029] In accordance with the present invention, the gradient
barrier with a composite structure of metal/metal salt of different
composition/metal such as
Ta/Ta.sub.xN.sub.1-x/TaN/Ta.sub.xN.sub.1-x/Ta is proposed to
replace the conventional barrier for copper metallization.
Therefore, the advantages of forming the gradient barrier including
a well-controlled process (in-situ formation), a strong adhesion
between via and landing metal, a good nucleation surface for
supporting deposition of copper in an overlying copper layer, more
uniform step coverage, and less brittle to reduce crack are readily
achieved.
[0030] Although specific embodiments have been illustrated and
described, it will be obvious to those skilled in the art that
various modifications may be made without departing from what is
intended to be limited solely by the appended claims.
* * * * *