Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2002-04-30
2003-12-02
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298110, C445S036000, C445S038000
Reexamination Certificate
active
06656331
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to video display devices of the self-emitting type such as cathode ray tubes (CRTs) and is particularly directed to a method and apparatus for applying an antistatic/antireflective surface coating to the display screen of a CRT.
BACKGROUND OF THE INVENTION
In a typical CRT, approximately 4-8% of the light incident on the CRT's glass display screen is reflected. This reflected light not only degrades the resolution and contrast of the video image presented on the display screen, but also may cause eye fatigue and in some cases eye damage to the viewer. In addition, the high voltage, e.g., on the order of 25 KeV, typically generated in the CRT results in the buildup of electrostatic charge on the display screen. Dust tends to collect on the outer surface of the display screen because of the presence of this electrostatic charge, resulting in degradation of the video image. In addition, this electrostatic charge buildup may cause an electrical shock to the viewer, particularly in low relative humidity conditions.
In addressing the aforementioned problems, the outer surface of the CRT's display screen is typically provided with a coating having antistatic, antiglare and antireflective characteristics. One approach to applying an antistatic/antireflective coating to the display screen's outer surface employs a “wet” process known as spin or spray coating. Shown in
FIG. 1
is a partial sectional view of a sealed glass envelope
10
of a CRT including a glass display screen
12
having disposed thereon a composite antistatic/antireflective coating
14
such as applied by one of the aforementioned wet coating processes. In FIG.
1
and the remaining figures shown and discussed below, common identifying numbers are used to identify the same element appearing in more than one figure. The composite antistatic/antireflective coating
14
applied to the outer surface of the glass display screen
12
is in the form of a single layer comprised of staggered molecules
16
dispersed within an insulating SiO
2
layer
18
. The inter-molecular spacing is relatively large and there is diffusion between the antistatic and antireflective layers so as to form a single conductive coating having a surface resistance on the order of 10
4
-10
5
ohm/cm
2
. The composite antistatic/antireflective coating
14
is grounded by electrically coupling the antistatic/antireflective coating to the CRT's grounded conductive implosion protection, or tension, band
20
by means of a conductive Al tape layer
22
. A plastic film of a conductive Al foil
24
is then placed over the conductive Al tape layer
22
to provide physical protection and electrical insulation for the conductive Al tape layer. Because the composite antistatic/antireflective coating
14
is a good electrical conductor, electrostatic charge on the outer surface of the glass display screen
12
is effectively directed to neutral ground via the conductive Al tape layer
22
and the implosion protection band
20
.
An antistatic/antireflective coating may also be applied to the glass display screen's outer surface by a “dry” deposition process such as by sputtering. A sputtered antistatic/antireflective coating differs in several characteristics from an antistatic/antireflective coating deposited by spray or spin coating. For example, as shown in
FIG. 2
, an antistatic/antireflective coating
32
deposited by sputtering is comprised of an inner antistatic layer
26
and an outer antireflective layer
28
. The reason for this difference is that in the sputter-deposited coating, the individual atoms are arranged in a staggered array, rather than the molecules as in the spray or spin-deposited coating, producing a far more compact structure. The inner antistatic layer
26
deposited directly on the outer surface of the glass display screen
22
is highly conductive, while the outer antireflective layer
28
is an insulator. Each of the layers in the sputter-deposited antistatic/antireflective coating
32
is clearly distinguishable from the other layer in the coating and the two layers have fundamentally different characteristics. Indium-doped tin oxide (ITO) is a typical composition for the inner conductive antistatic layer
26
, while insulating SiO
2
is a typical composition for the outer antireflective layer
28
. A layer of Ti may also be disposed between the inner antistatic layer
26
and the outer antireflective layer
28
. Attempting to use the grounding approach of
FIG. 1
in the sputter-deposited, layered antistatic/antireflective coating
32
of
FIG. 2
does not provide effective grounding for the CRT's glass display screen
12
. For example, placing a conductive adhesive layer
22
such as of Al tape in contact with and between the coating's outer antireflective layer
28
and the CRT's implosion protection band
20
does not provide an effective electrical conducting path for grounding electrostatic charge buildup on the display screen because the outer antireflective layer is not a good electrical conductor. Similarly, positioning a plastic film
24
such as of conductive Al foil
24
on the outer surface of the conductive adhesive layer
22
also does not provide an effective path to neutral ground for an electrostatic charge on the outer surface of the CRT's glass display screen
12
.
Referring to
FIG. 3
, there are shown another arrangement for directing an electrostatic charge on the display screen
12
of a sealed glass envelope
10
of a CRT to neutral ground. In the arrangement shown in
FIG. 3
, the layered antistatic/antireflective coating
32
is also formed by sputtering and is comprised of an inner antistatic layer
26
and an outer antireflective layer
28
. A gap, or opening, has been formed in the outer antireflective layer
28
adjacent its peripheral edge and extends down to the conductive inner antistatic layer
26
. A conductive element
42
is then inserted or formed in the opening in the outer antireflective layer
28
and is positioned in contact with the conductive inner antistatic layer
26
. The conductive element
42
is typically comprised of a conductive metal and is formed by conventional means such as ultrasonic spot welding as in the “Sunbonder” technique practiced by Asahi Glass Company of Japan. A conductive Al tape layer
44
is then positioned on the conductive element
42
and extends to the CRT's implosion protection band
20
. The conductive element
42
reduces the electrical resistance between the inner antistatic layer
26
and the conductive Al tape layer
44
to provide an effective electrical conductive path to neutral ground via the implosion protection band
20
. A plastic film of conductive Al foil
46
is then applied over the conductive Al layer
44
as in the previously described other prior art approaches.
The conductive aluminum tape strip used in the prior art grounding arrangements described above and shown in
FIGS. 1-3
is relatively expensive and thus increases the manufacturing cost of the CRT. In addition, the conductive aluminum tape strip is typically applied by hand by a worker which further increases CRT manufacturing costs. Finally, while incorporating a conductive element within the antistatic/antireflective coating by means of ultrasonic spot welding provides a good conductive path from the inner antistatic layer to neutral ground, this approach also increases the complexity of CRT manufacture resulting in a corresponding increase in manufacturing costs.
The present invention addresses the aforementioned limitations of the prior art by providing for the electrical grounding of an inner electrically conductive antistatic layer disposed on the outer surface of a CRT's glass display screen where an outer non-conductive antireflective layer is disposed over the inner antistatic layer. The inner antistatic layer is first deposited on the entire outer surface of the display screen. A portion of the deposited antistatic layer is then masked prior to deposition of
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