Water-soluble film forming composition

A water-soluble composition comprising a water-soluble N-vinylpyrrolidone copolymer and a fluorinated organic acid in a weight ratio of from 20:80 to 70:30 is useful in forming a water-soluble overlying film on a chemically amplified resist layer. The overlying film functions as both an anti-reflective film and a protective film during resist pattern formation by photolithography.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a water-soluble film forming composition and more 
particularly, a water-soluble composition for forming a film on a 
chemically amplified resist layer which lends itself to micro-processing 
technology in that it has high sensitivity to high energy radiation such 
as deep ultraviolet lights, electron beams and X-rays and is developable 
with alkaline aqueous solution to form a pattern. 
2. Prior Art 
To cope with the increased integration and speed of LSIs, chemically 
amplified positive working resist compositions using acid catalysts were 
developed as disclosed in U.S. Pat. No. 4,491,628 and 5,310,619, Japanese 
Patent Publication (JP-B) No. 27660/1990, and Japanese Patent Application 
Kokai (JP-A) No. 27829/1988. Because of high sensitivity, resolution and 
dry etching resistance, they are promising resist materials especially 
suited for deep ultraviolet lithography. 
The deep-UV lithography, however, has the problem that processing of a 
resist layer to an accurate pattern size is difficult since a resist image 
is reduced in dimensional accuracy by the influence of standing waves 
associated with the use of monochromatic light and if a substrate has 
steps, by the influence of optical interference due to variations in 
thickness of the resist layer at the steps and the influence of halation 
at the steps. 
Also chemically amplified resists suffer from the problem known as 
post-exposure delay (PED) that when deep-UV, electron beam or X-ray 
lithography is carried out, line patterns would have a T-top 
configuration, that is, patterns become thick at the top if the 
leave-to-stand or delay time from exposure to post-exposure baking (PEB) 
is extended. This problem, which arises probably because the resist 
surface is reduced in solubility, becomes a serious drawback on practical 
application. This not only makes difficult dimensional control in the 
lithographic process, but also adversely affects dimensional control in 
the processing of substrates using dry etching. In this regard, reference 
is made to W. Hinsberg et al., J. Photopolym. Sci. Technol., 6 (4), 
535-546 (1993), T. Kumada et al., J. Photopolym. Sci. Technol., 6 (4), 
571-574 (1993), and Hatanaka et al., Preprint of 1994 Spring Meeting of 
the Applied Physical Society, page 567, 29p, MB-2. 
It is understood that the PED problem of chemically amplified resists is 
dependent on the environment, that is, basic compounds in the air largely 
participate in the PED problem. In the case of positive resists, light 
exposure generates acids at the resist surface which react with basic 
compounds in the air and are thereby deactivated. As the delay time until 
PEB is extended, more amounts of acids are deactivated and accordingly, 
decomposition of acid unstable groups are more unlikely to occur. As a 
consequence, an insolubilized layer is formed at the resist surface, 
resulting in a T-top configured pattern. 
Several patterning techniques were proposed as having solved the 
above-mentioned problems associated with steps, for example, a multi-layer 
resist technique as disclosed in JP-A 10775/1976, an ARC technique 
(anti-reflective coating beneath resist) as disclosed in JP-A 93448/1984, 
and an ARCOR technique (anti-reflective coating on resist) as disclosed in 
JP-A 62520/1987, 62521/1987, 188598/1993, 118630/1994, and 148896/1994. 
The multi-layer technique involves forming two or three layers of resist 
and transferring a pattern to form a resist pattern serving as a mask. 
Undesirably the multi-layer technique requires a number of steps and is 
low in production efficiency. Light reflection from an intermediate layer 
can cause a lowering of dimensional accuracy. The ARC technique is by 
etching an anti-reflective film formed beneath the resist layer. The 
dimensional accuracy is substantially lost by etching and the extra 
etching step lowers production efficiency. 
In contrast, the ARCOR technique which involves forming a transparent 
anti-reflective film on a resist layer and peeling the film after exposure 
is able to form a fine resist pattern to high dimensional and alignment 
accuracy in a convenient way. By using low index of refraction materials, 
for example, perfluoroalkyl compounds (e.g., perfluoroalkyl polyethers and 
perfluoroalkylamines) as the anti-reflective film, the ARCOR technique of 
JP-A 62520/1987 minimizes reflection light at the resist 
layer/anti-reflective film interface, thereby reducing the variation of 
the pattern size of a resist image to one third as compared with a resist 
layer used alone. 
However, since the perfluoroalkyl compounds are less soluble in organic 
solvents, they must be diluted with such diluents as Freon solvents in 
order to control the thickness of a coating film. Freon solvents are also 
used in peeling off anti-reflective films of perfluoroalkyl compounds. The 
use of Freon is now undesirable from the standpoint of environmental 
protection. An increased number of steps is another problem. 
Using water-soluble polysaccharides as the anti-reflective film material, 
the technique of JP-A 62521/1987 eliminates intermixing at the 
resist/anti-reflective film interface and permits removal of the 
anti-reflective film to be done at the same time as the development step, 
offering a simple process. However, since the polysaccharides are not so 
low in refractive index as the perfluoroalkyl compounds, the variation of 
the pattern size is suppressed to only two thirds as compared with a 
resist layer used alone. 
JP-A 188598/1993 discloses an anti-reflective film forming composition of a 
two component system comprising a film-forming polymeric binder which is 
soluble or dispersible in water or aqueous alkaline solution and a low 
refractive index fluorocarbon compound which is soluble or dispersible in 
water or aqueous alkaline solution. It is effective as an anti-reflective 
film material for conventional resists using diazonaphthoquinone 
compounds. Since all the fluorocarbon compounds used therein have an 
ammonium ion which can deactivate acid on the chemically amplified resist 
surface, the above-mentioned PED problem cannot be overcome. 
JP-A 118630/1994 discloses an anti-reflective film forming composition 
comprising at least 90% by weight of a water-soluble film-forming 
composition and up to 10% by weight of a proton-generating material, based 
on the solids in the composition. Like JP-A 62521/1987, this composition 
also lacks the function as an anti-reflective film material for chemically 
amplified resists because the refractive index is not low. If 
water-soluble inorganic and organic acids which are exemplified as the 
proton-generating material are added in amounts of more than 10% by weight 
based on the solids, the pattern profile is deteriorated by the excess 
supply of acid. Ammonium weak acid salts which are also exemplified as the 
proton-generating material can deactivate acid on the chemically amplified 
resist surface, also failing to overcome the above-mentioned PED problem. 
Finally JP-A 148896/1994 discloses an anti-reflective film forming 
composition comprising a polyvinyl pyrrolidone homopolymer and a 
fluorinated organic acid ammonium salt. It is used only with conventional 
resists. Like JP-A 188598/1993, this does not solve the PED problem of 
chemically amplified resists. Since this composition contains a polyvinyl 
pyrrolidone homopolymer and is thus free of a hydrophobic unit, a film of 
this composition can be formed on a resist film with difficulty and has a 
high refractive index. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a water-soluble 
film-forming composition which can effectively form an overlying film on a 
chemically amplified resist film, the overlying film having the function 
of both an anti-reflective film and a protective film and enabling 
formation of a fine resist pattern at high dimensional accuracy and 
alignment accuracy in an easy, efficient, reproducible manner. 
The inventors have found that when a water-soluble copolymer of 
N-vinylpyrrolidone with another vinyl monomer (often referred to as an 
N-vinylpyrrolidone copolymer, hereinafter) is blended with a fluorinated 
organic acid which is substantially insoluble in water, but well 
compatible with the N-vinylpyrrolidone copolymer together with water and 
the resulting aqueous mixture is used as an anti-reflective film material 
to form an overlying film on a chemically amplified resist layer, this 
overlying film is an effective anti-reflective and protective film on the 
chemically amplified resist layer which can reduce reflected light at the 
resist layer surface without a loss of incident light, substantially 
suppress the variation of a pattern size due to multiple interference at 
the resist layer as compared with the use of a resist layer alone, and 
overcome the PED problem. 
More particularly, the fluorinated organic acid is a low refractive index 
compound substantially insoluble in water. An aqueous mixture of this 
compound and an N-vinylpyrrolidone copolymer well compatible therewith as 
major components is used to form an overlying layer on a chemically 
amplified resist layer. The overlying layer constitutes an anti-reflective 
layer having a refractive index of up to 1.55 at wavelength 248 nm. This 
anti-reflective layer is effective for substantially reducing optical 
reflectance and hence, improving the dimensional accuracy of a resist 
image, suppressing the variation of a pattern size due to optical multiple 
interference to one half or less as compared with a resist layer used 
alone. Formation of a film from the aqueous mixture is easy. No 
intermixing occurs at the resist layer/anti-reflective film interface. 
Removal of the anti-reflective film can be done at the same time as the 
development step, raising no problem from a process aspect. The overall 
process is simple and evokes no environmental problem. Since no basic 
materials such as ammonium salts are contained, the PED problem can be 
solved. Therefore, the overlying film is an effective protective film on 
the chemically amplified resist layer. 
Briefly stated, the present invention provides a water-soluble composition 
for forming an overlying film on a chemically amplified resist layer, 
comprising a water-soluble copolymer of N-vinylpyrrolidone with another 
vinyl monomer and a fluorinated organic acid in a weight ratio of from 
20:80 to 70:30.

BEST MODE FOR CARRYING OUT THE INVENTION 
The water-soluble film forming composition of the invention contains an 
N-vinylpyrrolidone copolymer which is a water-soluble polymer. Examples of 
the N-vinylpyrrolidone copolymer include N-vinylpyrrolidone/vinyl acetate 
copolymers, N-vinylpyrrolidone/vinyl alcohol copolymers, 
N-vinylpyrrolidone/acrylic acid copolymers, N-vinylpyrrolidone/methyl 
acrylate copolymers, N-vinylpyrrolidone/methacrylic acid copolymers, 
N-vinylpyrrolidone/methyl methacrylate copolymers, 
N-vinylpyrrolidone/maleic acid copolymers, N-vinylpyrrolidone/dimethyl 
maleate copolymers, N-vinylpyrrolidone/maleic anhydride copolymers, 
N-vinylpyrrolidone/itaconic acid copolymers, N-vinyl-pyrrolidone/methyl 
itaconate copolymers, and N-vinylpyrrolidone/itaconic anhydride 
copolymers, with the N-vinylpyrrolidone/vinyl acetate copolymers being 
preferred. The proportion of N-vinylpyrrolidone and another vinyl monomer 
polymerizable therewith is not critical although it preferably ranges from 
30:70 to 90:10 in a molar ratio. These water-soluble polymers may be used 
alone or in admixture of two or more. 
The water-soluble film forming composition of the invention also contains a 
fluorinated organic acid which is substantially insoluble in water, but 
well compatible with the water-soluble polymer or N-vinylpyrrolidone 
copolymer. The fluorinated organic acid includes acids of the following 
formulae (1) to (6). 
EQU F(CF.sub.2).sub.n COOH (1) 
EQU H(CF.sub.2).sub.n COOH (2) 
EQU F(CF.sub.2 CF.sub.2 O).sub.m CF.sub.2 COOH (3) 
##STR1## 
EQU F(CF.sub.2).sub.n SO.sub.3 H (5) 
EQU H(CF.sub.2).sub.n SO.sub.3 H (6) 
In the formulae, letter n is an integer of 4 to 15, preferably 6 to 10 and 
m is an integer of 1 to 10, preferably 2 to 4. The fluorinated organic 
acid should not form an ammonium salt. 
Typically the water-soluble film forming composition of the invention is an 
aqueous mixture containing 20 to 70%, preferably 30 to 60% by weight of 
the N-vinylpyrrolidone copolymer or water-soluble polymer and 30 to 80%, 
preferably 40 to 70% by weight of the fluorinated organic acid which is 
substantially insoluble in water, but well compatible with the 
water-soluble polymer, percent being based on the solids in the aqueous 
mixture. The aqueous mixture can be spin coated to form a water-soluble 
film. An N-vinylpyrrolidone copolymer content of less than 20% by weight 
raises problems with respect to compatibility and film formability whereas 
a mixture containing more than 70% by weight of the N-vinylpyrrolidone 
copolymer has a refractive index of more than 1.55 at wavelength 248 nm 
and becomes less effective for anti-reflection. In the water-soluble film 
forming composition, the sum of the N-vinylpyrrolidone copolymer and the 
fluorinated organic acid, that is, the amount of solids is preferably 1 to 
30%, especially 2 to 15% by weight of the entire composition in order that 
the resulting water-soluble layer have a thickness of 300 to 3,000 .ANG. 
(=0.03 to 0.3 .mu.m). A concentration of less than 1% by weight would lead 
to formation of a film of less than 300 .ANG. thick which is less 
effective for anti-reflection and protection purposes whereas a 
concentration of more than 30% by weight would lead to formation of a film 
of more than 3,000 .ANG. thick which would undesirably increase the load 
required in the peeling step. 
The water-soluble film forming composition of the invention may be obtained 
by mixing the above-mentioned two components together with water and forms 
a film which not only functions as an anti-reflective film or protective 
film on chemically amplified resists, but can also be used as an 
anti-reflective film for conventional resists using diazonaphthoquinone 
compounds. 
Any desired technique well known for chemically amplified resists may be 
used in forming a resist pattern using the water-soluble film material 
according to the present invention. For example, a typical lithographic 
process for positive resist material is shown in FIG. 1. First a resist 
layer 2 is formed on a substrate 1 such as a silicon wafer by a suitable 
technique such as spin coating as shown in FIG. 1(a). The water-soluble 
film material according to the invention is applied onto the resist layer 
2 as by spin coating, forming a water-soluble layer 3 as shown in FIG. 
1(b). The water-soluble layer 3 is exposed to a desired pattern of 
ultraviolet radiation or excimer laser light 4 having a wavelength of 190 
to 500 nm by a demagnification projection technique. That is, regions A of 
the water-soluble layer 3 and photoresist layer 2 are illuminated as shown 
in FIG. 1(c). After post exposure baking (PEB), the water-soluble layer 3 
is removed with water. The photoresist layer 2 is developed with a 
conventional developer, obtaining a resist pattern 5 as shown in FIG. 
1(d). It is noted that removal of the water-soluble layer 3 and 
development can be done concurrently using an alkaline developer. 
The water-soluble layer 3 preferably has a thickness of 300 to 3,000 .ANG., 
especially a thickness of 400 to 440 .ANG. or greater by a factor of 3 or 
5 when exposure is made with light of 248 nm. 
Since a positive resist is used as the photoresist layer 2 in the 
embodiment of FIG. 1, regions B are left as a resist pattern. The 
chemically amplified resist used herein may be either positive or negative 
type insofar as it has a contrast threshold value of a desired level 
relative to light of a given wavelength. 
Referring to FIGS. 2 and 3, it is described how an anti-reflective film 
made of the water-soluble film forming composition according to the 
invention reduces optical scattering. FIG. 2 shows a prior art structure 
in which only a resist layer 2 is formed on a substrate 1. Incident light 
I.sub.0 reaching the resist layer 2 undergoes substantial reflection 
I.sub.r1 at the air/resist layer interface with a substantial portion of 
the incident light quantity being lost. The light entering the resist 
layer 2 undergoes reflection I.sub.r2 at the resist layer/substrate 
interface whereupon the reflected light I.sub.r2 goes out of the resist 
layer 2 as emergent light while it is reflected I.sub.r3 again at the 
resist layer/air interface. This process is repeated in the resist layer. 
That is, optical multiple interference occurs in the resist layer 2. 
FIG. 3 shows a structure in which an anti-reflective film 3 according to 
the invention is formed on a resist layer 2 on a substrate 1. The 
provision of the anti-reflective film 3 is effective for reducing 
reflection I.sub.r4 of incident light I.sub.0 at the air/anti-reflective 
film interface, reflection I.sub.r5 at the anti-reflective film/resist 
layer interface, reflection I.sub.r6 of the reflected light at the resist 
layer/anti-reflective film interface, and reflection I.sub.r7 of the 
reflected light at the anti-reflective film/air interface. Since I.sub.r4 
and I.sub.r5 are reduced, the loss of incident light quantity is reduced. 
Since I.sub.r6 and I.sub.r7 are reduced, optical multiple interference in 
the resist layer 2 is suppressed. 
According to the principle of reflection prevention, provided that the 
resist has an index of refraction n to illuminating light and the 
illuminating light has a wavelength .lambda., the reflectivity (amplitude 
ratio) of an anti-reflective film is reduced as the index of refraction n' 
of the anti-reflective film approaches to .sqroot.n and the thickness 
thereof approaches to an odd multiple of .lambda./4n. In one example, a 
phenolic resin material having an index of refraction of about 1.78 is 
used as the chemically amplified resist material, a water-soluble film 
according to the invention has an index of refraction of less than 1.50 at 
248 nm, and the light used is KrF excimer laser light having a wavelength 
of 248 nm. Then the optimum thickness of the anti-reflective film is 400 
to 440 .ANG. or greater by a factor of 3 or 5. Under these conditions, the 
use of the anti-reflective film according to the invention is effective 
for reducing the reflected light and suppressing the optical multiple 
interference. 
EXAMPLE 
Examples of the invention are given below by way of illustration and not by 
way of limitation. 
EXAMPLE 1 
A positive working chemical amplification type resist material had the 
following composition. 
______________________________________ 
Component pbw 
______________________________________ 
Polyhydroxystyrene having some hydroxyl groups pro- 
75 
tected with t-butoxycarbonyl groups 
Triphenyl trifluoromethanesulfonate sulfonium 
5 
2,2'-bis(4-tert-butoxycarbonyloxyphenyl)propane 
20 
1-ethoxy-2-propanol 450-550 
______________________________________ 
A water-soluble film forming composition consisted of the following 
components. A film of this composition had a refractive index of 1.51 at 
wavelength 248 nm. 
______________________________________ 
Component pbw 
______________________________________ 
Luviskol VA-64 (BASF Japan K.K., copolymer of 6/4 N- 
2.0 
vinylpyrrolidone/vinyl acetate) 
C-5800 (Daikin Fine Chemical Research K.K., 9H-hexade- 
1.3 
cafluorononanoic acid which is a fluorinated organic acid of 
general formula (2) wherein n = 8 
Ultra-pure water 96.7 
______________________________________ 
A resist pattern was formed according to the lithographic process shown in 
FIG. 1. First, the positive working chemical amplification type resist 
material was spin coated on a substrate 1 and pre-baked at 100.degree. C. 
for 120 seconds to form a resist layer 2 of 0.6 to 1.0 .mu.m thick (see 
FIG. 1(a)). The water-soluble film forming composition was spin coated on 
the resist layer 2 to form a water-soluble layer 3 of 410 .ANG. thick (see 
FIG. 1(b)). Using an excimer laser stepper (Nikon K. K., NSR-2005EX8A, 
NA=0.5), selected regions are exposed to light (see FIG. 1(c)). The 
water-soluble layer 3 was removed with ultra-pure water. Post-exposure 
baking (PEB) was done at 75.degree. C. for 120 seconds and development was 
done with an aqueous solution of 2.38% tetramethylammonium hydroxide, 
obtaining a positive pattern (see FIG. 1(d)). 
The resulting 0.35-.mu.m line-and-space resist pattern was examined for 
dimensional variation. The variation was about .+-.0.050 .mu.m or more 
with lithography using a resist layer alone. The lithography using a 
resist layer and a water-soluble layer as mentioned above could reduce the 
variation to about .+-.0.020 .mu.m. When the leave-to-stand or delay time 
from exposure to PEB was 1 hour, the dimensional variation was about 
.+-.0.020 .mu.m. 
Equivalent results were obtained where removal of the water-soluble layer 3 
with ultra-pure water was not effected prior to PEB, but after PEB, and 
where removal of the water-soluble layer 3 with ultra-pure water was not 
effected before or after PEB, but concurrently with the development with 
an aqueous solution of 2.38% tetramethylammonium hydroxide. 
EXAMPLE 2 
A water-soluble film forming composition consisted of the following 
components. A film of this composition had a refractive index of 1.45 at 
wavelength 248 nm. 
______________________________________ 
Component pbw 
______________________________________ 
Luviskol VA-64 (BASF Japan K.K., copolymer of 6/4 N- 
1.3 
vinylpyrrolidone/vinyl acetate) 
C-1800 (Daikin Fine Chemical Research K.K., perfluoro- 
1.9 
nonanoic acid which is a fluorinated organic acid of general 
formula (1) wherein n = 8 
Ultra-pure water 96.8 
______________________________________ 
A resist pattern was formed according to the lithographic process shown in 
FIG. 1. First, the positive working chemical amplification type resist 
material was spin coated on a substrate 1 and pre-baked at 100.degree. C. 
for 120 seconds to form a resist layer 2 of 0.6 to 1.0 .mu.m thick (see 
FIG. 1(a)). The water-soluble film forming composition was spin coated on 
the resist layer 2 to form a water-soluble layer 3 of 420 .ANG. thick (see 
FIG. 1(b)). Using an excimer laser stepper (Nikon K. K., NSR-2005EX8A, 
NA=0.5), selected regions are exposed to light (see FIG. 1(c)). The 
water-soluble layer 3 was removed with ultra-pure water. Post-exposure 
baking (PEB) was done at 75.degree. C. for 120 seconds and development was 
done with an aqueous solution of 2.38% tetramethylammonium hydroxide, 
obtaining a positive pattern (see FIG. 1(d)). 
The resulting 0.35-.mu.m line-and-space resist pattern was examined for 
dimensional variation. The variation was about .+-.0.050 .mu.m or more 
with lithography using a resist layer alone. The lithography using a 
resist layer and a water-soluble layer as mentioned above could reduce the 
variation to about .+-.0.015 .mu.m. When the leave-to-stand or delay time 
from exposure to PEB was 1 hour, the dimensional variation was about 
.+-.0.015 .mu.m. 
Equivalent results were obtained where removal of the water-soluble layer 3 
with ultra-pure water was not effected prior to PEB, but after PEB, and 
where removal of the water-soluble layer 3 with ultra-pure water was not 
effected before or after PEB, but concurrently with the development with 
an aqueous solution of 2.38% tetramethylammonium hydroxide. 
EXAMPLE 3 
A water-soluble film forming composition consisted of the following 
components. A film of this composition had a refractive index of 1.46 at 
wavelength 248 nm. 
______________________________________ 
Component pbw 
______________________________________ 
Copolymer of 6/4 N-vinylpyrrolidone/vinyl alcohol 
1.6 
C-1800 (Daikin Fine Chemical Research K.K., perfluoro- 
1.6 
nonanoic acid which is a fluorinated organic acid of general 
formula (1) wherein n = 8 
Ultra-pure water 96.8 
______________________________________ 
The resist pattern was formed by the same procedure as in Example 2 except 
that the water-soluble film forming composition mentioned above was used, 
obtaining equivalent results. 
EXAMPLE 4 
A water-soluble film forming composition consisted of the following 
components. A film of this composition had a refractive index of 1.49 at 
wavelength 248 nm. 
______________________________________ 
Component pbw 
______________________________________ 
Copolymer of 7/3 N-vinylpyrrolidone/acrylic acid 
1.6 
C-5800 (Daikin Fine Chemical Research K.K., 9H-hexade- 
1.6 
cafluorononanoic acid which is a fluorinated organic acid of 
general formula (2) wherein n = 8 
Ultra-pure water 96.8 
______________________________________ 
The resist pattern was formed by the same procedure as in Example 1 except 
that the water-soluble film forming composition mentioned above was used, 
obtaining equivalent results. 
EXAMPLE 5 
A water-soluble film forming composition consisted of the following 
components. A film of this composition had a refractive index of 1.50 at 
wavelength 248 nm. 
______________________________________ 
Component pbw 
______________________________________ 
Copolymer of 7/3 N-vinylpyrrolidone/methyl acrylate 
1.6 
C-5800 (Daikin Fine Chemical Research K.K., 9H-hexade- 
1.6 
cafluorononanoic acid which is a fluorinated organic acid of 
general formula (2) wherein n = 8 
Ultra-pure water 96.8 
______________________________________ 
The resist pattern was formed by the same procedure as in Example 1 except 
that the water-soluble film forming composition mentioned above was used, 
obtaining equivalent results. 
There has been described a water-soluble film forming composition which can 
effectively form an overlying film on a chemically amplified resist film. 
The overlying film has the function of both an anti-reflective film and a 
protective film and enables formation of a fine resist pattern at high 
dimensional accuracy and alignment accuracy in an easy, efficient, 
reproducible manner. Therefore, in the pattern forming step of 
photolithography for the manufacture of semiconductor integrated circuits, 
high precision micro-processing can be done even on a substrate having an 
irregular surface using the composition of the invention. 
Japanese Patent Application No. 252849/1994 is incorporated herein by 
reference. 
Although some preferred embodiments have been described, many modifications 
and variations may be made thereto in the light of the above teachings. It 
is therefore to be understood that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically 
described.