U.S. patent application number 12/021292 was filed with the patent office on 2008-06-12 for process for manufacturing a semiconductor device, a semiconductor device and a high-frequency circuit.
Invention is credited to Christoph Bromberger.
Application Number | 20080135850 12/021292 |
Document ID | / |
Family ID | 36062199 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080135850 |
Kind Code |
A1 |
Bromberger; Christoph |
June 12, 2008 |
PROCESS FOR MANUFACTURING A SEMICONDUCTOR DEVICE, A SEMICONDUCTOR
DEVICE AND A HIGH-FREQUENCY CIRCUIT
Abstract
A process for manufacturing a semiconductor device, provides
that a silicide layer is formed, an amorphous semiconductor layer
is applied both to the silicide layer and to an open
monocrystalline semiconductor region, adjacent to the silicide
layer, and during a subsequent temperature treatment, the amorphous
semiconductor layer is crystallized proceeding from the open,
monocrystalline semiconductor region, acting as a crystallization
nucleus, so that the silicide layer is covered at least partially
by a crystallized, monocrystalline semiconductor layer.
Inventors: |
Bromberger; Christoph;
(Heilbronn, DE) |
Correspondence
Address: |
Muncy, Geissler, Olds & Lowe, PLLC
P.O. BOX 1364
FAIRFAX
VA
22038-1364
US
|
Family ID: |
36062199 |
Appl. No.: |
12/021292 |
Filed: |
January 28, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11242792 |
Oct 5, 2005 |
7348221 |
|
|
12021292 |
|
|
|
|
Current U.S.
Class: |
257/66 ; 257/75;
257/E21.09; 257/E21.133; 257/E21.165; 257/E21.375; 257/E29.003;
257/E29.043; 438/486 |
Current CPC
Class: |
H01L 29/66272 20130101;
H01L 21/02667 20130101; H01L 21/02598 20130101; H01L 21/2022
20130101; H01L 21/28518 20130101 |
Class at
Publication: |
257/66 ; 257/75;
438/486; 257/E29.003; 257/E29.043; 257/E21.09 |
International
Class: |
H01L 29/10 20060101
H01L029/10; H01L 29/04 20060101 H01L029/04; H01L 21/20 20060101
H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 5, 2004 |
DE |
102004048332.9-33 |
Claims
1. A semiconductor device comprising: a monocrystalline
semiconductor layer; and at least one suicide layer, the silicide
layer being at least partially covered by the monocrystalline
semiconductor layer, which is crystallized from an amorphous
semiconductor material applied to the silicide layer proceeding
from a monocrystalline semiconductor region as a crystallization
nucleus.
2. The semiconductor device according to claim 1, wherein the
monocrystalline semiconductor region forms a substantially flat
surface with the silicide layer, at least in a region of
crystallization.
3. The semiconductor device according to claim 1, wherein a
dielectric at least partially covers the silicide layer, and is
adjacent to the crystallized monocrystalline semiconductor
layer.
4. The semiconductor device according to claim 1, further
comprising a trench structure that is provided in an area along
which the silicide layer is applied to the monocrystalline
semiconductor region.
5. The semiconductor device according to claim 4, wherein the
trench structure is at least partially filled with the
dielectric.
6. The semiconductor device according to claim 5, wherein the
dielectric is planarized.
7. The semiconductor device according to claim 1, wherein the
monocrystalline semiconductor layer is doped by dopants, which
diffuse out of the silicide layer, which functions as a dopant
source.
8. The semiconductor device according to claim 1, wherein the
semiconductor device is formed by the steps comprising: forming the
silicide layer; applying the amorphous semiconductor layer to the
silicide layer and to the monocrystalline semiconductor region that
is adjacent to the silicide layer; and crystallizing, during a
subsequent temperature treatment, the amorphous semiconductor layer
proceeding from the open monocrystalline semiconductor region,
which functions as a crystallization nucleus, so that the silicide
layer is at least partially covered by a crystallized,
monocrystalline semiconductor layer.
9. A high-frequency circuit having a semiconductor device with at
least one silicide layer, which connects electrically a buried,
doped semiconductor layer of an active high-frequency component,
whereby the silicide layer is covered at least partially with a
monocrystalline semiconductor layer crystallized from an amorphous
semiconductor material.
10. The high-frequency circuit according to claim 9, wherein a
dielectric, which fills the trench structure, is placed between a
base connection and the silicide layer.
11. The high-frequency circuit according to claim 9, wherein the
semiconductor device is formed by the steps comprising: forming the
silicide layer; applying the amorphous semiconductor layer to the
silicide layer and to the monocrystalline semiconductor region that
is adjacent to the silicide layer; and crystallizing, during a
subsequent temperature treatment, the amorphous semiconductor layer
proceeding from the open monocrystalline semiconductor region,
which functions as a crystallization nucleus, so that the silicide
layer is at least partially covered by a crystallized,
monocrystalline semiconductor layer.
Description
[0001] This application is a divisional of U.S. application Ser.
No. 11/242,792, which was filed on Oct. 5, 2005, and which claims
priority under 35 U.S.C. .sctn. 119(a) to German Patent Application
No. DE 102004048332.9, which was filed in Germany on Oct. 5, 2004,
and which is herein incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a process for manufacturing
a semiconductor device, a semiconductor device, and a
high-frequency circuit.
[0004] 2. Description of the Background Art
[0005] Various manufacturing variants for semiconductor components
by partial overgrowth of oxide layers with monocrystalline silicon
by solid phase epitaxy are described in the Journal of the
Electrochemical Society, 138 (1991), No. 12, pp. 3771-3777; Journal
of Crystal Growth 98 (1989), pp. 519-530; Applied Physics Letters,
49(7), 1996, pp. 397-399; Applied Physics Letters, 60(1), 1992, pp.
80-81; Applied Physics Letters, 52(20), 1988, pp. 1681-1683;
Applied Physics Letters, 43(11), 1983, pp. 1028-1030; Applied
Physics Letters, 52(21), 1988, pp. 1788-1790; Applied Physics
Letters, 56(6), 1990, pp. 560-562; Applied Physics Letters, 48(12),
1986, pp. 773-775; Applied Physics Letters, 53(26), 1988, pp.
2626-2628; Applied Physics Letters, 49(20), 1986, pp. 1363-1365;
Journal of Applied Physics, 64(6), 1988, pp. 3018-3023; Japanese
Journal of Applied Physics, 35, 1996, pp. 1605-1610; and the
Japanese Journal of Applied Physics, 31, 1992, pp. 1695-1701. Here,
an oxide layer is first applied to a silicon wafer. Seed windows
where the monocrystalline lattice of the wafer is exposed are
opened in the oxide layer. An amorphous silicon layer is then
applied and crystallized outwardly from the seed openings.
[0006] It is known from U.S. Pat. No. 5,534,716 to crystallize a
film layer of silicon in a predetermined direction. In this regard,
selective metal atoms are added that have a catalytic action for
the crystallization of amorphous silicon. This is used for
manufacturing a TFT (Thin Film Transistor) with a high critical
frequency or a TFT with a low leak current on the same substrate.
It is specified that transition metal impurities lower the
nucleation temperature on the silicon below the growth temperature
for nuclei.
SUMMARY OF THE INVENTION
[0007] It is therefore an object of the present invention to
provide a process for manufacturing a semiconductor device, a
semiconductor device and a high-frequency circuit, monocrystalline
semiconductor material being produced over a silicide layer to
improve the electrical properties as much as possible.
[0008] A feature of the invention is a manufacturing process step
in which during a temperature treatment, an amorphous semiconductor
layer is crystallized proceeding from an open, monocrystalline
semiconductor region, acting as a crystallization nucleus, so that
a silicide layer is covered at least partially by a crystallized,
monocrystalline semiconductor layer. To this end, the silicide
layer is formed beforehand and preferably the amorphous
semiconductor layer is applied afterwards both to the silicide
layer and to the open, monocrystalline semiconductor region
adjacent to the silicide layer.
[0009] Tests by the applicant have shown that, contrary to the
prevalent bias, it is possible to apply monocrystalline
semiconductor regions to silicide layers according to the process
of the invention without costly processes, such as a process for
growing monocrystalline silicon layers on certain silicon
lattice-adapted silicides such as ErSi.sub.2 or Co--Si.sub.2, by
molecular beam epitaxy; or a process for growing layers, which
includes a matrix continuing the crystal lattice of the substrate
epitactically, in which non-lattice-adapted silicide crystallites
are embedded in a thin layer, the crystallites joining together
into a continuous silicide layer by subsequent annealing.
[0010] In an embodiment of the invention, the silicide layer is
made of titanium as a transition metal and silicon as the
semiconductor material.
[0011] In fact, a crystallization direction, exclusively vertical
relative to the wafer surface, is basically possible; the amorphous
semiconductor layer is also crystallized substantially laterally in
a preferred embodiment of the invention. This can naturally be
combined advantageously with a vertical crystallization
direction.
[0012] A further embodiment of the invention provides that the
crystallization occurs within a temperature range between, for
example, 400.degree. C. and 600.degree. C. In so doing, the
temperature during the process depends on the employed silicide.
The temperature in this embodiment can be selected in such a way
that metal ions of the silicide layer do not significantly
contaminate the adjacent monocrystalline semiconductor layer for
the critical functioning of the semiconductor layer.
[0013] Advantageously, the amorphous silicon layer is applied with
a layer thickness of at least 300 nm, preferably at least one
region of the amorphous semiconductor layer being doped with at
least one dopant, particularly with germanium, phosphorus, boron,
and/or arsenic, to control the crystal growth and the nucleation
rate during the manufacturing process.
[0014] In a further embodiment, the dopant can be introduced in a
preferably 250 nm-thick surface region of the amorphous silicon
layer, in particular by implantation. To again remove this
high-doped region, which interferes with functioning, regions,
doped with the dopant, of the crystallized monocrystalline
semiconductor layer, are selectively etched, particularly wet
chemically, in that the regions doped with the dopant have a higher
etching rate than a less doped region of the crystallized,
monocrystalline semiconductor layer.
[0015] After the etching of the doped regions of the crystallized,
monocrystalline semiconductor layer, the crystallized,
monocrystalline semiconductor layer can be oxidized at the surface.
The oxide can be subsequently removed, preferably by a wet chemical
process.
[0016] A further embodiment includes the selective removal of
non-monocrystalline regions of the semiconductor layer after the
crystallization. Preferably, the monocrystalline semiconductor
layer is subsequently planarized to the level of the surface of the
dielectric.
[0017] A first example embodiment of the invention enables a
selective removal of the non-monocrystalline regions of the
semiconductor layer by a poly-etching. A second possible example
embodiment provides an alternative of a selective removal of the
non-monocrystalline regions of the semiconductor layer by a masked
dry etching.
[0018] In order to produce additional active regions of a
component, after the removal of the non-monocrystalline regions of
the semiconductor layer, the monocrystalline layer can be thickened
by epitaxy, preferably by selective vertical epitaxy.
[0019] Several different semiconductor layers, made of different
semiconductor materials, such as Si or SiGe, may be applied by
epitaxial thickening. Moreover, semiconductor regions with
different conduction types, n-conducting or p-conducting, are
provided to form pn junctions of a bipolar transistor or a
diode.
[0020] Dislocation of the surface between the silicide layer and
the seed opening is also possible. The silicide layer can be formed
in such a way that the monocrystalline semiconductor region forms a
substantially flat surface with the silicide layer at least in the
region of crystallization.
[0021] Another aspect of the invention is a process for
manufacturing a semiconductor device, which can be combined with
the previously disclosed process steps. To this end, at least one
trench structure is introduced in a region of the substrate of a
wafer with monocrystalline semiconductor material. The trench
structure may be introduced, for example, by isotropic or
anisotropic etching and, for example, form a V-shaped trench along
the <111> orientation of the semiconductor lattice of the
substrate.
[0022] Furthermore, a silicide layer, for example, a titanium
silicide layer, can be formed at least in subregions of the trench
structure. The trench structure is filled with a dielectric at
least above the silicide layer. That dielectric can be made of
silicon dioxide.
[0023] The silicide layer can be formed by applying a layer of a
transition metal to the device of a semiconductor substrate with an
introduced trench structure and an overlying mask. In this regard,
the transition metal is capable of reacting with the semiconductor
substrate to form a conductive semiconductor-transition metal bond
as the silicide layer. A reaction between the transition metal and
the semiconductor substrate is carried out, the unreacted remainder
of the transition metal being removed afterwards.
[0024] A mask can be applied to the surface regions of the
semiconductor substrate, which is not patterned by the trench
structure. The mask can be formed of a silicon dioxide layer and a
silicon nitride layer, which is thicker than the silicon dioxide
layer, the silicon dioxide layer being applied by thermal oxidation
and the silicon nitride layer being deposited afterwards.
[0025] In order to have a surface as flat as possible available for
the subsequent process steps, the dielectric is planarized down to
the height of the upper edge of the mask in an advantageous
development of the invention. Here, the silicon nitride layer of
the mask advantageously serves as a planarization stop. Preferably,
in so doing, the planarization occurs by means of
chemical-mechanical polishing.
[0026] The mask is removed after the planarization. Therefore,
outside the trench structure, the monocrystalline silicon substrate
and the edge of the silicide layer is open at the surface.
Preferably, after the mask is removed, the amorphous semiconductor
layer is applied, which therefore covers the monocrystalline
silicon substrate and preferably also the dielectric at least
partially.
[0027] The described process steps make possible the manufacture of
semiconductor components with additional process steps, such as the
application of metallizing layers. An essential aspect of the
invention is an application of said manufacturing process for
manufacturing a permeable base transistor, a resonance tunnel
transistor, and/or a resonance tunnel diode, which may be realized
also together on a semiconductor substrate.
[0028] Another aspect of the invention is a semiconductor device
with at least one silicide layer, the silicide layer being covered
at least partially with a monocrystalline semiconductor layer. The
monocrystalline semiconductor layer is crystallized from an
amorphous semiconductor material applied to the silicide layer. In
this regard, the crystallization occurs proceeding from a
monocrystalline semiconductor region as a crystallization
nucleus.
[0029] Preferably, the monocrystalline semiconductor region forms a
substantially flat surface with the silicide layer at least in the
region of crystallization. Above this surface, the monocrystalline
semiconductor layer is applied, which preferably serves as an
active semiconductor region of a component preferably with
additional semiconductor layers.
[0030] Further, a dielectric, which covers the silicide layer at
least partially, can be adjacent to the crystallized,
monocrystalline semiconductor layer. It is preferred, in this
regard, to separate the silicide layer and other active regions of
the same or an adjacent component by the dielectric.
[0031] The silicide layer can be applied to the monocrystalline
semiconductor region at least partially along a trench structure.
It is preferable for the trench structure to be filled at least
partially, preferably completely, with the dielectric. This makes
it possible to distance the silicide layer from the metallization,
contacts, and other active regions of a semiconductor component
placed on the dielectric. The filled dielectric can be planarized,
in order to be able to apply complex metallization.
[0032] The monocrystalline semiconductor layer can be doped by
dopants at the border to the silicide layer, which diffuse out of
the buried silicide layer acting as a dopant source.
[0033] In a further embodiment, a high-frequency circuit is
provided with a semiconductor device having at least one silicide
layer, which connects electrically a buried, doped semiconductor
layer of an active high-frequency component, whereby the silicide
layer can be covered at least partially with a monocrystalline
semiconductor layer crystallized from an amorphous semiconductor
material. A dielectric, which fills a trench structure, can be
placed between a base connection and the silicide layer.
[0034] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus, are
not limitive of the present invention, and wherein:
[0036] FIG. 1a is a schematic drawing, after several first
manufacturing steps, of a manufacturing process according to an
embodiment of the present invention;
[0037] FIG. 1b is a schematic drawing, after several additional
manufacturing steps, of the manufacturing process;
[0038] FIG. 2a is a schematic drawing, after several first
manufacturing steps, of a manufacturing process according to a
second embodiment of the present invention;
[0039] FIG. 2b is a schematic drawing, after several additional
manufacturing steps, of the manufacturing process of the second
embodiment;
[0040] FIG. 2c is a schematic drawing, after several additional
manufacturing steps, of the manufacturing process of the second
embodiment; and
[0041] FIG. 2d is a schematic drawing of a section of a
high-frequency bipolar transistor.
DETAILED DESCRIPTION
[0042] During the crystallization of amorphous substances, two
temperature-dependent variables, the nucleation rate and the
crystal growth rate, influence the quality and properties of the
crystallized layer. If polycrystalline grains have already formed
on an oxide, a lateral solid phase epitaxy, progressing from the
crystallization nuclei, at the grain boundaries stops. With undoped
amorphous Si, in this way, approximately 4-.mu.m oxide can be
overgrown laterally, in a monocrystalline manner, and with a good
crystal quality. It is possible in this regard that the lateral
expansion of the monocrystalline region is about 10 times the layer
thickness. With high phosphorus-, boron-, or germanium-doped
amorphous silicon, in contrast, between 20 .mu.m and 40 .mu.m may
be overgrown. Selective etching of a high-doped layer, moreover,
can also form thinner layers of crystallized amorphous silicon.
[0043] Transition metal impurities, for example, nickel, increase
the crystal formation rate at low temperatures. The doping with
transition metals may be used, for example, for manufacturing
thin-layer transistors. If thereby free metal ions diffuse from a
silicide layer into the overyling amorphous silicon in notable
amounts, overgrowing of silicide layers with lateral solid phase
epitaxy of silicon appears impossible.
[0044] However, it is indicated in the following exemplary
embodiments depicted as drawings that there is a temperature range
for many suicides in which the lateral solid phase epitaxy,
proceeding from a seed opening designated as a crystallization
nucleus, leads to a crystal growth rate, whereas at the same time
the outward diffusion of transition metal ions from the silicide
layer into the overlying silicon layer is so low that the
crystallization rate is not increased to a prohibitively great
extent.
[0045] FIG. 1a shows a schematic drawing after several first
manufacturing steps in the manufacturing process of an initial
embodiment of a semiconductor device. Shown is the state after
application of a silicide layer 2 to a monocrystalline silicon
semiconductor substrate 1. A mask 5, for example, of photoresist,
is applied to the silicide layer 2 and a seed window 3 is etched in
the silicide layer 2.
[0046] FIG. 1b shows the manufacturing state after additional
manufacturing process steps. Here, an amorphous silicon layer is
applied, which is not shown in FIG. 1b and has a thickness of 300
nm. The top 250 nm, with respect to the surface, of the amorphous
silicon layer are doped with high doses of a dopant arsenic by
implanting the arsenic dopants in the amorphous silicon layer.
Other suitable foreign substances are, for example, also boron,
phosphorus, or germanium. These dopants require the subsequent
lateral solid phase epitaxy, whereby FIG. 1b schematically shows
the manufacturing state after a completed solid phase epitaxy.
[0047] The lateral solid phase epitaxy occurs in this case within a
temperature range between 400.degree. C. and 600.degree. C., which
is selected in keeping with a low rate of diffusion of transition
metals from the silicide layer 2 into a crystallized,
monocrystalline semiconductor layer 41, which is, for example, Si
or SiGe. The crystallized, monocrystalline semiconductor layer 41,
obtained by solid phase epitaxy, thereby covers the filled seed
window 3' and also adjacent regions of the silicide layer 2. The
crystallized, monocrystalline semiconductor layer 41, however, ends
at polycrystalline regions, which arise by spontaneous crystal
formation from the amorphous silicon layer at a distance from the
seed window 3.
[0048] In the following steps, not shown in the drawing, the
regions, doped by the foreign substances, of the crystallized,
monocrystallized semiconductor layer 41 are chemically removed,
whereby these foreign substances in sufficiently high concentration
makes sections of the silicon in the crystallized, monocrystalline
semiconductor layer 41 susceptible to chemical removal, and whereby
monocrystalline undoped or low-doped silicon resists etching. In
this exemplary embodiment, a phosphorus doping of 7e18 cm-3 and
etching in HF:HNO3 CH3OOOH=1:3:8 are planned.
[0049] FIG. 2a shows a schematic drawing after several first
manufacturing steps in the manufacturing process of a second
embodiment of a semiconductor device for a high-frequency bipolar
transistor.
[0050] In this regard, the manufacturing process comprises the
following process steps. First, a semiconductor substrate 1 is
provided including a first monocrystalline semiconductor material,
in this exemplary embodiment, silicon. A patterned mask is then
applied, which in this exemplary embodiment is a thin silicon
dioxide layer 51 and a thicker silicon nitride layer 52. At trench
structure 4 is patterned by etching at the sites not protected by
the mask 51, 52.
[0051] A silicide layer is then formed by applying a tungsten layer
61 and a cobalt layer 62. This manufacturing state after
application of the tungsten layer 61 and the cobalt layer 62 is
shown in FIG. 2a. The transition metals, tungsten and cobalt, are
capable of reacting with the silicon substrate to form a
semiconductor-transition metal bond.
[0052] After the reaction, unreacted remainders of the transition
metals are removed. The reaction causes a cobalt silicide layer 21
and a tungsten silicide layer 22 adjacent to the cobalt layer, to
form within the trench structure 4. The trench structure 4 is
filled with a dielectric 9, for example, silicon dioxide.
Planarization by chemical-mechanical polishing then occurs down to
the height of the mask layer 52 of the silicon nitride. This
process state is shown in FIG. 2b.
[0053] Next, after removal of the solid mask of oxide 51 and
nitride 52, used for the trench etching, a layer of amorphous
silicon is applied, which is converted by a lateral solid phase
epitaxy, as described for FIGS. 1a and 1b, at least in subregions
into a crystallized, monocrystalline semiconductor layer. This
state is shown in FIG. 2c. In contrast to FIGS. 1a and 1b, the
silicide layers 21 and 22 form a flat surface with the
monocrystalline semiconductor substrate 1 within the seed opening
bounded by the dielectric 9. The region of the crystallized,
monocrystalline semiconductor layer 41, in so doing, extends beyond
the height of the seed opening to regions covered at least
partially by the dielectric 9. The monocrystalline crystal growth
is in turn bounded by a polycrystalline layer 42 forming
spontaneously from the amorphous silicon layer.
[0054] In the next steps, the polycrystalline regions 42 are
removed by appropriate masking and subsequent dry etching. Next,
the monocrystalline silicon layer 41 is planarized by
chemical-mechanical polishing down to the height of the dielectric
9.
[0055] FIG. 2d shows a partial view of the final state of the
manufacturing processes, which in this exemplary embodiment shows a
high-frequency bipolar transistor. A high-doped region 1' is
created in addition in the substrate; it borders the silicide layer
21 and with it enables a low-resistance connection of a collector
semiconductor region 41 of low-doped monocrystalline silicon. An
internal base semiconductor region 81, which is connected via an
external base semiconductor region 81' with the metal connection
181 of the base, is applied to the collector semiconductor region
by vertical solid phase epitaxy.
[0056] A monocrystalline emitter semiconductor region 82, which is
connected to the metal connection 182 for electrical contacting, is
applied in turn, for example, by subsequent solid phase epitaxy via
the internal base semiconductor region 81. For reduced connection
resistance, additional silicide layers can be provided (not shown
in FIG. 2d) between the metal contacts 181 and 182 and the
semiconductor regions 81' and 82. The dielectric 9 separates the
base connection 181 from the silicide layers 21 and 22 for the
low-resistance contacting of the collector semiconductor region 41.
By this means, a low parasitic base collector capacity is achieved
and the high-frequency properties of this bipolar transistor is
significantly improved. Furthermore, the high-frequency properties
of this bipolar transistor are improved by connecting the collector
in a low-resistance manner by means of the silicide layers 21 and
22. The silicide layer is connected via a collector metal contact
141 on the trench structure 9 side opposite the collector
semiconductor region 41.
[0057] The manufacturing process is not limited to the manufacture
of high-frequency bipolar transistors. Thus, for example, tunnel
diodes or permeable base transistors may also be manufactured by
the manufacturing process.
[0058] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are to be included within the scope of the following
claims.
* * * * *