Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2002-11-21
2004-11-30
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
active
06825054
ABSTRACT:
REFERENCE TO A MICROFICHE APPENDIX:
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to configurations and fabrication methodologies for light-emitting devices which are comprised of multiple ceramic layers constructed on a supporting substrate and which use electroluminescent phosphors as a light source.
2. Description of Related Art
The fabrication and commercial application of electroluminescent lamps (EL) is a well established art spanning more than five (5) decades. Typically, EL devices use doped zinc sulfide phosphors dispersed in a dielectric material and placed between conductive electrode surfaces. The application of a suitable AC voltage creates an electric field in the dielectric material exciting the phosphors into luminescence. A transparent electrode is used adjacent to the phosphor material permitting the generated light to escape, forming a lamp.
The prior art includes multiple examples of both plastic and ceramic configurations. Ceramic devices received intense development attention over approximately a 10 year period from 1960 to 1970. However, due to the complexity of the ceramic EL devices, there was little success in developing a viable configuration and production process that was competitive. Virtually all successful commercial applications over the intervening several decades have been based on plastic materials and associated processing systems.
U.S. Pat. No. 4,482,580 by Emmett et al attempted to develop and commercialize a variant of device concepts first defined by Buck in U.S. Pat. No. 3,073,982 and Westerveld in U.S. Pat. Nos. 3,201,632 and 3,200,279. The manufacturing yields of the Emmett design proved too low, and the power dissipation levels too high, to successfully compete with plastic EL devices. The Emmett design has other significant performance difficulties as will become evident in comparison with this invention.
Most recently, Winsor (U.S. Pat. No. 6,091,192) sought to improve the Emmett design by adding two (2) new layers (an insulation layer plus a base electrode layer) with the expectation of reduced dissipation levels and improved manufacturing control. The Winsor approach is a variant of device concepts first defined by Diemer (U.S. Pat. No. 3,275,870) and Rulon (U.S. Pat. No. 3,103,607). Although some performance improvement relative to Emmett would be expected, the production costs would be significantly increased due to the additional layers. Further, Winsor fails to address other performance difficulties as will become evident in comparison with this invention.
In short, the substantive prior art related to this invention dates largely to the 1960s. This body of work, now in the public domain, is for the most part conceptual, with validation limited to small area devices and few specific performance requirements. In fact, this prior art (and subsequent noted improvements to the same) have mostly served to confirm that useful devices are possible but have failed to define integrated material systems and processes which could realize this potential in a commercially viable product. There are significant omissions in the set of required device attributes considered, and inadequate attention to the complex interactions between the material compositions in the various device layers. The special challenges of large area devices (e.g. >1 sq-ft) both in terms of production cost and performance were not considered in the prior art.
It is the object of this invention to describe and demonstrate an integrated set of materials and processes which achieve a dramatic improvement in both ceramic EL performance and cost relative to the prior art. This new fabrication methodology and associated materials system are applicable to large areas and enables devices which are superior to plastic EL in many important applications, including those with severe environmental exposure requirements.
A conceptual layout for a typical prior-art ceramic EL device is illustrated in
FIG. 1. A
metal substrate
1
provides structural support while also serving as a base electrode. An insulating layer
3
is constructed on the substrate providing break-down isolation for an overlying ceramic matrix encasing the EL phosphor
5
. A transparent top electrode
7
completes the electrical circuit permitting an intense AC field to be established across the ceramic stack. A transparent ceramic cover layer
9
is used to protect the top electrode
7
and the phosphor layer
5
from the ambient environment. Given a nominal 10:1 ratio between the dielectric constant of the insulation layer relative to the phosphor layer, most of the applied voltage will appear across the ceramic matrix containing the phosphor. Yet in the event of short-circuit through the phosphor layer, the insulating layer will limit the current and help minimize degradation in device performance. This model is deceptively simplistic because the ceramic stack is a blended mechanical, chemical, electrical, and optical system formed through a sequence of molten states, each with a unique time-temperature profile providing the opportunity for diffusion of various constituents between layers and contributing complex residual mechanical stresses upon cool-down to ambient temperature.
In terms of design priorities, a fundamental requirement is that the stack must bond together mechanically with minimal distortions and fracturing due to residual stresses arising from mismatched coefficients of thermal expansion which are compounded by temperature gradients during processing. The cool-down time-temperature profile is often as important as the peak temperatures reached. The metal-to-ceramic bond lines at the substrate are particularly troublesome because the coefficients of thermal expansion cannot be exactly matched and the constituents in the ceramic mix which contribute to a strong bond (generally metal oxides) have an adverse effect on the electrical properties of the insulation (dielectric) layer. Further, given device areas of several square feet and the inevitable temperature gradients induced by the high temperature oven system, there will be micro-cracking penetrating multiple levels. In practical terms, the device design must accommodate a significant level of statistically certain imperfection while minimizing the adverse performance effects. Therefore optimum device performance is not a simple summation of optimum components. The complexity of the ceramic EL system, and the difficulty in achieving viable commercialization in prior art devices, arises in no small part from these non-linear interactions, especially including a tolerance for some number of localized faults in large area devices.
An exemplary prior art method, as taught by Buck, et al. (U.S. Pat. Nos. 3,073,982 and 3,275,870) is illustrated in FIG.
2
. Buck uses low carbon enameling steel as the substrate
11
providing a reasonable thermal expansion coefficient match with an overlying ceramic layer. The mechanical bond to the substrate is achieved through an overlying semiconductive ceramic layer
12
formed by a materials mix containing titanium oxide which becomes semiconducting upon diffusion of iron from the steel substrate. The conductivity of this semiconducting ceramic is sufficiently high as to substantially reduce the power dissipation in this layer. The dielectric layer
13
is formed as a matrix of finely ground barium titanate combined with a glass which also contains titanium oxide. This layer serves to slow the diffusion of iron toward the vulnerable phosphor layer while providing improved break-down protection. The EL phosphor is encased in a glass matrix
14
with an overlying transparent conductive top electrode
15
. A protective top cover is provided by bonding an organic adhesive layer
16
and a top electrode
15
wherein the rolled glass provides an improved environmental durability in comparison with enameling processes using glass frits. Buck reports an overall performance level of 7 mw/sq-in of power dissipation at 120 volts, 60 Hz, with an illumination
Begley Richard
Brown Wayne Douglas
Edwards Doreen D.
Norton Michael
Osborne Cathy
Mulpuri Savitri
Waters Robert R.
Waters Law Office
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