Laterally transmitting thin film electroluminescent device

A multi-purpose light source comprising a laterally transmitting TFEL device (1,2a,2b,4) with a means to de-couple the intense lateral transmission via reflecting microstructures (11) is formed onto substrates in which, in the preferred embodiment, there is pre-fabricated addressing/drive circuitry (36 through 42). Such light sources are applicable to displays, particularly small area high resolution displays, and to linear imaging arrays, such as in electrophotographic printers.

THE RELATED FIELD OF INVENTION 
The present invention relates to a laterally transmitting thin film 
electroluminescent device, for use as a multi-purpose light source for 
display and electrographic printing applications. 
BACKGROUND OF THE INVENTION 
Thin film electroluminescent (TFEL) devices are utilised primarily to 
provide the light source for flat panel emissive displays, see, for 
example reference (1) and references therein. A typical TFEL display 
structure is shown in schematic cross-section in FIG. 1, where a light 
emitting phosphor thin film is sandwiched between dielectric cladding 
layers to form a capacitative structure. Electrodes are disposed on the 
outer surfaces of the cladding dielectrics, such that the application of a 
high voltage ac signal between these electrodes results in the emission of 
light form the phosphor thin film, due to the high electric field strength 
in the phosphor film being greater than a threshold value. For typical 
display applications, the thin films are deposited onto a transparent 
substrate, wish one of the electrode layers also being transparent, such 
at light generated within the phosphor layer may be viewed directly. This 
is termed surface emission. 
A significant disadvantage associated with conventional surface emission is 
the fact that up to 90% of the light generated by thin film phosphors is 
tripped within the phosphor layer by virtue of internal reflection, and is 
transmitted laterally, as shown in FIG. 2. This occurs because phosphor 
thin films generally have a refractive index that is higher than that of 
the materials used to clad them. This effect was first reported by D H 
Smith in 1982, J Lum 23 (1983) 209 when he observed that the light 
intensity emitted at the edge of a TFEL thin film display was much greater 
than that emitted directly though the surface. Recognising this as a loss 
mechanism, several groups have attempted to improve the luminous surface 
emission by reducing the internal reflection, so as to allow more light to 
be emitted directly through the thin film surface. Described, for example 
in U.S. Pat. No. 5,131,065 (Boeing) is a method of improving the surface 
emission from TFEL panels by index matching the optical properties of the 
phosphor and dielectric layers, such that internal reflection is 
minimised. Alternatively, U.S. Pat. No. 5,072,152 (Planar Systems), and 
U.S. Pat. No. 4,774,435 (GTE Labs) both teach the use of rough non-planar 
interfaces between the phosphor thin film and the cladding layers. The 
Planar Method is to initially deposit a dielectric layer onto a planar 
substrate, where the dielectric is deposited by electron-beam evaporation 
in order to generate a rough surface contour. This "convoluted" surface 
contour is then replicated by the thin films deposited on top, such that 
the internal light reflection is reduced and more light output is provided 
at the front of the panel. The GTE method is to produce the same effect--a 
rough, non-planar surface at the phosphor/dielectric interface, but in 
this case it is achieved by depositing the thin films onto a substrate 
that has a rough non-planar surface. In each case, the technique used is 
intended to overcome the inherent internal reflection by increasing the 
probability that the angle of incidence for a light ray arriving at the 
phosphor/dielectric interface will be less that the critical angle and 
will thus be transmitted. 
The methods described above, if successfully implemented, will undoubtedly 
improve the direct surface emission through the various thin films. 
However, in doing so they effectively eradicate a property of TFEL devices 
that may in fact be highly beneficial; i.e. the confinement of up to 90% 
of the generated light to within the geometrical limits of the phosphor 
thin film. Since such phosphor thin films are in general of the order of 1 
micron thick (10.sup.-6 m), the result is the concentration of the light 
energy to a microscopic area. This consequently increases the intensity 
relative to that of conventional surface emission, where approximately 10% 
of the light energy is spread over an area of typically 100 micron.sup.2. 
The gain in emitted light intensity (luminance) that may be attained via 
the utilisation of laterally transmitted light was demonstrated by D H 
Smith in 1982, J Lum 23 (1983) 209 when he observed that the luminous 
efficiency of a TFEL display could be increased by including the light 
that was unintentionally emitted from the edge of the thin film stack. 
This concept was then further developed by workers from Westinghouse Corp 
and Edge Emission Incorporated into methods of producing high intensity 
light sources for image bar arrays intended to work in electrophotographic 
printers. Described in US and European patents: U.S. Pat. No. 4,535,341, 
U.S. Pat. No. 4,685,448 and EP 0 398 591 A2 are devices where the light is 
emitted in a direction parallel to the substrate, directly from an exposed 
edge of the phosphor film. This is similar, in principle, to the structure 
of an edge emitting LED, or diode laser, where spatially confined light is 
channelled to an emitting facet, which is typically formed by cleaving the 
substrate as described by J. Wilson, and J. F. B. Hawkes; in 
OptoElectronics An Introduction, Prentice Hall International, (1983) 211. 
However, as detailed in U.S. Pat. No. 4,535,341, the TFEL edge emitting 
linear arrays are formed by defining an exposed emitting facet in the TFEL 
stack directly at the edge of the substrate. In such a configuration, the 
proximity of both the emitting facet, and the electrodes to the substrate 
edge is problematic due to the high voltages required to operate TFEL 
devices and due to the sensitivity of the EL materials to the effects of 
exposure to moisture and other contaminants. Attempts to overcome these 
problems are consequently concerned with protection of the emitting edge, 
and the isolation of the electrodes across which the high voltage drive 
signal is applied. For example, in U.S. Pat. No. 4,734,723, an optical 
printer head is disclosed that is formed from a plurality of edge emitting 
TFEL devices positioned along one edge of the substrate, with waveguide 
strips to transmit the light from the EL device to the other end of the 
substrate, similar to the operation of an edge emitting LED. While 
protecting the TFEL edge emitter, this presents additional optical 
attenuation, and may thus reduce the benefits of edge emission in regard 
to the intensity of light produced. Alternatively, as disclosed in EP 0 
398 591 A2, a practical edge emitter light source requires an integral 
housing, or packaging assembly to protect the device. Such an assembly is 
described, which consists of a series of spacers and packaging members 
arranged around the TFEL substrate and sealed to prevent contamination. 
Again, the extra optical interfaces that result from such packaging may be 
detrimental to the emitted light intensity, and the packaging process 
itself, requiring precision mechanical alignment, is costly and difficult. 
SUMMARY OF THE INVENTION 
Therefore, as a significant improvement over the prior art described above, 
one aspect of the present invention makes use of the high intensity light 
emission that is possible when utilising the laterally transmitted, 
internally reflected light of a TFEL device, but in a manner that does not 
require the formation of emitting facets at the edge of the substrate, 
thus eradicating the problems associated with such a configuration. 
According to a first aspect of the present invention there is provided a 
thin film electroluminescent (TFEL) device including an active phosphor 
layer sandwiched between dielectric layers with electrodes formed an the 
outer surfaces of the dielectric layers which provide the means to apply 
an AC drive signal to generate light within the phosphor thin-film via the 
process of electroluminescence. The inherent internal reflection of the 
thin-film structure is used to provide lateral transmission of the 
generated light. The present invention relates to the product of 
high-intensity illuminous emission via the effective decoupling of this 
laterally transmitted light away from the substrate plane. This is 
achieved, via the inclusion of reflecting micro-structures (micro-mirrors) 
within, or adjacent to the laterally transmitting TFEL device. Such 
reflecting micro-structures thus provide the decoupling means and may be 
fabricated in or onto the substrate, such that following the deposition 
and fabrication of the TFEL device(s), light transmitted laterally within 
the TFEL device(s) will encounter a decoupling means (reflecting 
micro-structures) and will be decoupled from the substrate plane. Examples 
of how such a decoupling means may be included on a substrate with TFEL 
devices are shown in FIGS. 3-8. 
In the preferred embodiment of the invention, photolithographic techniques 
are used to define emitting regions in the TFEL devices with de-coupling 
means in the form of reflecting micro-structures formed onto or into the 
substrate so as to effectively de-couple the intense laterally transmitted 
light in a direction away from the substrate plane. 
In order to further enhance the optical de-coupling, cladding/outcoupling 
layers are deposited in/over the emitting regions, and these additional 
layers also serve to protect the device. Fabrication of the TFEL device, 
reflecting micro-structure, and cladding/outcoupling layer may be achieved 
using standard microelectronic device processing techniques, resulting in 
a protected and hermetically sealed high intensity light source. 
Furthermore, since the invention does not require the formation of an 
array of emitting facets at the edge of a substrate, the geometrical 
configuration of light sources is defined photolithographically. Those 
skilled in the art of microelectronic device fabrication will consequently 
recognise that the invention permits the flexibility to define a 
multi-purpose high intensity, high resolution light source satisfying the 
requirements for both display and printing applications. For example, the 
combination of TFEL devices and reflecting micro-structures may be 
arranged so as to form the matrix of emitting regions necessary for a 
video graphics display, or in fixed shape; for fixed legend displays, or 
in linear arrays for printing applications. Considering the latter 
example, it is a further benefit of the invention that a linear array of 
light sources so formed may consist of reflecting micro-structures with 
two mutually inclined reflecting surfaces so as to combine the laterally 
transmitted light from two opposing TFEL emitting facets into one light 
source. This clearly enhances the light energy emitted relative to that 
obtained from one light source. 
A further development of the invention is based on the fact that the use of 
laterally transmitted light combined with reflecting microstructure(s) and 
cladding/outcoupling layer(s) does not require a transparent substrate. 
Indeed, the use of microelectronic device fabrication processes implies 
that the preferred substrate would be one which is compatible with such 
fabrication, e.g. a silicson wafer. As such, it will be obvious to those 
skilled in the art, that the present invention facilitates the combination 
of the high intensity, high resolution, multi-purpose light source with 
integrated drive and/or addressing circuitry that is pre-fabricated into 
the, for example, silicon substrate. This further permits the use of high 
frequency drive signals which will enhance the emitted light intensity. 
The benefits of using such active matrix addressing for increasing the 
luminance of conventional surface emitting TFEL devices is well known. 
Vanterleteren et al in the Conference Record of the 1991 International 
Display, 134 Conference, for example, demonstrated an active matrix 
circuit where surface emitting TFEL devices are addressed via high voltage 
(200 volt) thin film transistors (TFT's), and light is emitted through a 
transparent electrode. However, as described in U.S. Pat. No. 5,436,279, 
such a TFT based circuit precludes the production of a high resolution 
display due to the channel dimensions of a TFT, and the need for high 
voltage capacitors to be included in the circuit. Proposed in U.S. Pat No. 
5,463,279, therefore is an alternative configuration for an active matrix 
surface emitting TFEL device, where circuit elements are disposed on the 
substrate, such that the dimensions are reduced, and the gating device 
operates in its breakdown region to avoid the use of a high voltage 
capacitor. Again, in this active matrix device, the light is emitted 
through a transparent upper electrode. Hence, while these configurations 
both provide a means to fabricate active matrix EL pixels, they both rely 
upon the surface emission of light through transparent electrodes. Much 
light energy is thus wasted due to the internal reflection effect, and the 
realisation of both high resolution and high intensity is thus precluded. 
This is illustrated by the published luminance values obtained from 
devices fabricated according to U.S. Pat. No. 5,463,279, where displays 
intended primarily for head mounted applications are presented having 
luminances of the order of 100 fL as reported by C. King; proceedings of 
the EL 96 Berlin (1996) 375. Unfortunately, this level of luminance is 
only acceptable for a limited range of applications, where background 
lighting is dim, or where the display is shrouded in a closed ocular 
enclosure. A universally applicable display technology would have 
sufficient luminance to be used in an open-ocular system, (i.e. where the 
display is superimposed on the users normal field of view) and would 
provide the necessary contrast against the background illumination. This 
requires luminances of the order of 3000-5000 fL, which is clearly not 
produced by the conventional surface emission TFEL de ices. The present 
invention therefore describes a device structure that by combining the 
reflection of laterally transmitted light with the benefit of integrated 
drive circuitry, is a major improvement over the prior art. Furthermore, 
since the light from active matrix EL devices described previously is 
emitted only through the transparent electrode, religions of the pixel 
which are not generating light, such as active circuit elements, 
contribute to wasted area. This is contention of U.S. Pat. No. 5,463,279 
against Vanterleteren et al, since the circuit described by the latter 
requires a high voltage Capacitor which takes up valuable space. A 
significant improvement over both of these designs is therefore 
facilitated by the present invention, since the reflecting microstructures 
are passive regions, and may thus be fabricated over active circuit 
elements with no detrimental effect. In this way, valuable pixel area is 
efficiently utilised. 
The present invention therefore provides significant improvements over the 
prior art in that by combining reflecting micro-structures with laterally 
emitting TFEL devices, and using micro-electronic fabrication techniques 
to form, in combination, cladding/outcoupling layers and/or integrated 
drive/addressing circuitry, a general purpose high intensity, high 
resolution light source is provided. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As previously described, up to 90% of the light generated by a TFEL 
phosphor thin film can be trapped within it due to internal reflection, as 
illustrated by FIG. 2. Furthermore, since the thickness of a phosphor thin 
film is of the order of 1 micrometre, the result is that laterally 
transmitted (internally reflected) light can provide a high intensity 
light source, provided that 
(i) the light is effectively de-coupled from the phosphor film, and 
(ii) the dimensions of the de-coupling region are maintained to be of the 
order of the phosphor film thickness (.apprxeq.microns). 
One technique, that is useful for the evaluation of lateral emission, is to 
form an edgy, or emitting facet in the phosphor film by etching or 
cleaving perpendicular to the substrate plane, thus providing edge 
emission. Such a technique, however, is difficult to apply to practical 
applications, particularly displays, where it is necessary to provide 
surface emission. Consequently, we have developed the technique that is 
the premise of this invention, namely the reflection of laterally 
transmitted light via micro-mirrors (reflecting micro-structures) 
fabricated onto the substrate. These reflecting micro-structures provide 
the means to de-couple the laterally transmitted light whilst maintaining 
the benefit of high intensity that results from internal reflection within 
the phosphor film. Examples are illustrated in FIGS. 3-5. 
In summary, the invention Provides a TFEL device structure with a light 
emitting phosphor thin film disposed between dielectric thin films having 
a lower refractive index. Electrodes formed on the outer surfaces of the 
dielectric thin films provide the means to apply an ac drive signal to 
generate light within the phosphor film via the process of 
electroluminescence. A light emitting region is formed which consists of a 
reflecting microstructure configured so as to maximize the de-coupling of 
laterally transmitted light from the phosphor thin film in a direction 
away from the substrate. The light emitting region is protected with a 
cladding/outcoupling layer(s) which may be formed from a material having 
an index of refraction that is greater than or equal to the phosphor layer 
so as to increase the outcoupling efficiency. The TFEL device so described 
is fabricated onto a substrate in, or on which there is pre-fabricated the 
necessary electronic circuitry to provide individually addressable light 
emitting regions. The whole device thus forms an opto-electronically 
integrated high intensity multi-purpose light source. 
According to the present invention there is provided a TFEL device 
structure with a light emitting phosphor thin film disposed between 
dielectric thin films having a lower refractive index. Electrodes formed 
on the outer surfaces of the dielectric thin films provide the means to 
apply an ac drive signal to generate light within the phosphor film via 
the process of electroluminescence. The device includes, or is fabricated 
adjacent to, reflecting micro-structures configured so as to maximize the 
amount of laterally transmitted light which may be de-coupled from the 
phosphor light generating film. 
The reflecting micro-structures may take the form of micro-mirrors, 
micro-grooves, or tapered emitting facets in the phosphor or cladding 
materials, or any combination of these. 
Photolithographic fabrication processes are preferably used to define the 
reflecting mi-crostructures into, or onto the substrate. They may take the 
form of dielectric structures with, or without an additional reflective 
coating, and may be shaped to provide the maximum reflection of light in 
the desired direction. Reflective coatings, such as metallic, or 
multi-layer dielectric mirrors may be added to the reflecting surface of 
the reflecting microstructure, but if such a reflective coating is 
electrically conducting it shall be isolated from the electrodes of the 
device, and/or addressing circuit. In such a way the reflecting 
microstructure is defined as a passive optical element within, or adjacent 
to the active light emitter (TFEL) device, also termed the light 
generating region. 
TFEL device layers are then deposited via physical vapor, or chemical 
vapour deposition methods onto a substrate containing the reflecting 
micro-structures. Photolithographic techniques are used to define the 
surface electrodes and expose emitting regions corresponding to each 
reflecting micro-structure. The TFEL layers may be etched to expose an 
emitting facet corresponding to each reflecting micro-structure. 
Cladding/outcoupling layers are then deposited to protect the emitting 
region. These layers may be composed of dielectric materials such as 
SiO.sub.2, Si.sub.3 N.sub.4, BaTiO.sub.3, Y.sub.2 O.sub.3, or graded index 
combinations thereof to provide an optimum transmission of the particular 
wavelength of light generated by the phosphor layer, or they may be 
composed of materials with a refractive index.gtoreq.that of the phosphor 
thin film in order to maximize the optical outcoupling, e.g. ZnSe, TiO. 
Electrical addressing of t he TFEL structure so described is facilitated by 
electrically connecting the bottom electrode of the device to the drain, 
or extended drain of a MOSFET, and/or onto the electrode of a shunt 
capacitor, grounding the source of the MOSFET and providing a high voltage 
ac supply line to the top electrode of the TFEL device, such that when the 
MOSFET is turned on, emission from the TFEL device is initiated. Said 
emission being reflected by the reflecting micro-structures to produce a 
high intensity light source that may be turned on or off by the 
application of a small voltage signal to the gate of the MOSFET. 
Individually addressable light sources may thus be configured to form a 
linear array of high intensity light sources, a matrix of light sources, 
or a group of fixed legend light sources. The geometrical arrangement of 
the reflecting microstructures may further be configured so as to provide 
the maximum light intensity by optimizing the spacing between them, i.e. 
adjusting the ratio of active light generation area to 
reflecting/de-coupling area. 
The configuration of devices forming an array of high intensity light 
sources is of particular importance for application electrophotographic 
printing Market factors dictate that the resolution of such printers must 
be greater than 300 dots per inch (dpi). Apart form laser/polygon exposure 
systems, low cost solutions include arrays of discrete surface emitting 
LEDs, but ultimately the resolution of such an array is limited by the 
necessity to form a wire bond to each device. The edge emitting TFEL 
device structures proposed by Westinghouse and Edge Emission Inc., as 
discussed in the prior art section above, provide another alternative. 
However, due to the problems associated with using an emitting facet at 
the edge of a substrate such technology has yet to form a commercially 
viable product. The present invention, therefore, provides an improved 
light source for this application. Accordingly, there is provided an array 
of TFEL devices formed in combination with reflecting micro-structures as 
described above, where each individually addressable device forms the 
light source corresponding to a printed picture element. A further 
possible embodiment of such an array is one in which selection of a 
particular device addresses a pair of TFEL structures where a reflecting 
microstructure located between this pair of light generation regions 
results in the reflection of light away from the substrate to form a 
single light source. This increases the active light generating area that 
is contributing to the light source, and so increases the intensity to 
facilitate the exposure of a light sensitive medium. Furthermore, by 
combining laterally transmitting TFEL devices with reflecting 
micro-structures, cladding/outcoupling layer(s), and integrated addressing 
drive circuitry, it is possible to increase the switching frequency of the 
devices and thus increase the achievable printing speed. 
It will be obvious to those skilled in the art that the invention thus 
described may be applied to any TFEL materials where lateral transmission 
of light is facilitated by internal reflection. Consequently, the 
invention applies to the full range of thin film phosphors that may be 
employed in TFEL devices, and can therefore produce high intensity light 
sources with an emission wavelength that is designed for the application 
under consideration. For example, ZnS:Mn may be used to produce broad-band 
yellow light for multi-colour display, and/or printing applications,; 
SrS:Ce for white light emission, and ZnS:TmF.sub.3 could be used to 
produce blue light which is useful for displays, and infra-red emission 
that is suitable for printing. Finally, it is possible to produce devices, 
arrays of devices, or matrices of devices, where multiple deposition 
steps, or ion implantation is used to selectively dope different regions 
of the phosphor thin film. In this way a multi-colour, or full-colour 
light source is defined.

Referring to the drawings, FIG. 1 depicts a typical conventional TFEL 
structure comprising an active phosphor layer 1 sandwiched between 
dielectric layers 2a and 2b (which may or may not be the same material) 
with a transparent electrode 3 on one outer surface and an electrode 4 on 
the opposite outer surface. The device is foxed onto a transparent 
substrate 5 such that when an ac voltage of sufficient value to initiate 
electroluminescence is applied across the electrodes 6 light 7 is 
generated, with surface emission viewed (8) through the substrate 5. 
FIG. 2 illustrates the process of lateral transmission of internally 
reflected light. A conventional TFEL device as in FIG. 1 has dielectric 
layers 2 whose refractive index n.sub.2 is less than that n.sub.1 of the 
phosphor layer 1. Light generated via electroluminescence 7 is trapped 
within the phosphor thin film if the angles of incidence with the 
dielectric layers 9 are less than the critical angle for internal 
reflection, resulting in laterally transmitted light 10. 
In FIGS. 3-7 the substrates may be transparent, or opaque. In the preferred 
embodiment, the device forms an opto-electronic integrated circuit, with 
the laterally transmitting TFEL device, reflecting microstructures, 
cladding/outcoupling layers and integrated addressing/drive circuitry. In 
one version of the preferred embodiment of this aspect of the invention, 
the TFEL device is deposited onto, or near to a drive/addressing circuit 
pre-fabricated into or onto the substrate e.g. silicon, SOI, or SOS. Said 
circuit is fabricated such that when activated via low voltage data 
signals, a high voltage drive signal is applied across the TFEL device and 
light emission from the TFEL device is initiated. 
In the Figures depicting possible embodiments of the invention, therefore, 
it is implicit that the substrate shown may contain such pre-fabricated 
circuitry. Furthermore, the active elements of such circuitry may be so 
configured as to be located beneath the reflecting micro-structures, which 
are passive elements, or beneath the TFEL layers, or a combination. 
Furthermore, in such a configuration, each individual light source would 
be associated with its own addressing/drive circuit (termed 
collectively--latching element). In such a configuration the light sources 
may form an array, for printing applications, or a matrix, or fixed 
legends, for display applications. An alterative configuration of this 
aspect of the invention would thus be the integration of addressing/drive 
circuitry along the periphery of the display/printing array regions. With 
this configuration a refresh mode of operation (which may be 
termed--passive matrix) is employed. For both configurations, the 
integration of laterally transmitting TFEL devices with reflecting 
microstructures, cladding/outcoupling layer(s), and latching circuit 
elements, or any sub-combination of these, results in devices to satisfy 
the requirement for multi-purpose high intensity, high resolution light 
sources. 
As illustrated in FIGS. 3a, and 3b, photolithography and fabrication 
processes can define reflecting microstructures consisting of 
micro-mirrors 11 (points, wedges), on a substrate 5 which would preferably 
contain pre-fabricated addressing/drive circuitry (latching elements plus 
logic stages) thus facilitating active, or passive addressing. These 
processes are performed prior to the deposition of the TFEL layers 
comprising phosphor thin film 1, dielectric layers 2a, 2b and electrode 4, 
where the surface electrode layer 4 may be opaque, or transparent. 
Ideally, the reflecting microstructure is reflecting for the wavelength 
emitted by the particular phosphor employed, hence an additional 
reflective coating 14 may be formed over the reflecting microstructure, as 
shown in FIG. 3d where this coating is a metallic, or dielectric mirror. 
If the coating is metallic then it is isolated electrically from the 
substrate, TFEL device, and top electrode. Micro-machining by, for 
example, etching, ion milling, or laser ablation can be used to define an 
emitting facet 12 in the TFEL device in the region of the reflecting 
microstructures. These facets are in close proximity to the reflecting 
microstructures as shown in FIG. 3a, resulting in the reflection of 
laterally transmitted light 13. FIGS. 3a-3c show possible variations of 
this configuration. 
The profile of the reflecting microstructure 11 is such that it enables 
laterally transmitted light to contribute to surface emission. A shaped 
mirror surface (not shown) allows control of the preferred direction of 
the surface emission. Etching of the TFEL emitting facets may be performed 
such that all of the TFEL layers 4, 2a, 1, and 2b are removed (as shown in 
FIG. 3a), or a reduced combination of these, as shown in FIGS. 3b and 3c. 
Where layer 2b is not removed by the etching process (FIG. 3b) this layer 
forms a coating of the reflecting microstructure and may thus also fulfill 
the requirement of the reflective coating 14. 
FIGS. 3e, and 3f illustrate the preferred embodiments of the invention. 
TFEL devices (layers 1,2a,2b,4) are formed onto substrates, 5 with 
addressing/drive circuitry pre-fabricated, and reflecting microstructures 
11, with the light emitting region being coated with an additional layer 
15, to form a protective cladding, and improve the outcoupling of emitted 
light. This layer may thus have refractive index that is greater than or 
equal to that of the phosphor layer 1, such that light outcoupling is 
improved. Additionally, further layers 16 may be deposited to form an 
anti-reflection coating tailored to the wavelength emitted by the 
phosphor. 
If the electrode layer 4 is not transparent, then it may be a dark, or 
black material, or may be coated with the same. This facilitates improved 
contrast ratios. Only the intense reflected laterally transmitted light is 
observed/detected. Furthermore, an inherent property of the invention is 
that the use of reflecting microstructures to de-couple laterally 
transmitted light is that light spread between individual pixels, or 
elements is prevented. This means that optical cross-talk is eliminated, 
further enhancing both the efficiency and contrast from these light 
sources. 
Photon emission from radiative centers within the phosphor thin film can 
either contribute to conventional surface emission --a transparent 
electrode is used-, propagate laterally, or be absorbed or scattered into 
an absorbing region. Hence, to maximize the overall luminous efficiency of 
a light source composed of individual light sources as defined by this 
invention, it is necessary to optimize the geometry of the emitting 
regions, i.e. the reflecting microstructures. In particular: 
(i) The area fill-factor of emission within the pixel area should be as 
high as possible. 
(ii) The mean propagation length of laterally transmitted light should be 
optimised as determined by the attenuation coefficient of the particular 
phosphor thin film used. 
(iii) No lateral light should propagate to adjacent pixels 
(iv) The cross-sectional profile of the microstructure(s) should optimize 
the conversion of laterally transmitted light into useful surface 
emission. 
Hence, shown in FIGS. 4-6, by way of example only, are possible geometries 
of microstructures that may be employed for optimizing the light 
out-coupling. The Figures illustrate plan views of typical pixels. 
The area fill factor is dependent upon the spatial frequency and foot print 
size of the emitting apertures 17 as defined by the reflecting 
microstructures and emitting phosphor facets. Various configurations are 
suggested by FIG. 4a-4f, where the spacing period (x and y) would be 
adjusted to optimize the out-coupling efficiency. 
FIG. 5 shows an example of an array of emitting apertures 17, in the form 
of internal reflecting microstructures and emitting facets which form 
rectangular apertures making up a single pixel. Also shown is a perimeter 
reflecting microstructure 18 defining the perimeter of the pixel and 
preventing optical cross-talk. The spacing of the emitting apertures would 
be optimised according to the attenuation properties of the phosphor 
material used. 
FIG. 6 shows yet another possible geometry of reflecting microstructures. 
With this geometry, the extent of lateral propagation can be controlled 
equally in both x and y directions. 
FIG. 7 illustrates yet another possible configuration of the emitting 
aperture 17, where the arms of a three pointed star are formed by the 
reflecting microstructure such that the shape provides multiple angles for 
reflection via its jagged profile. This will improve the light 
out-coupling by increasing the probability of the reflection of laterally 
transmitted light. 
For applications requiring linear arrays of high intensity, high resolution 
light sources, the geometry may be defined as indicated by FIG. 8a, which 
shows a possible etching pattern for such an array. This comprises PECVD 
SiO.sub.2 insulating raft etch 19, polysilicon electrode etch, 20, 
SiO.sub.2 (dielectric reflecting microstructure material) etch 21 for side 
walls, micro-mirrors, and via holes to base electrode (not required if 
addressing/drive circuitry is included into the substrate only top 
electrode connections are required to provide the high voltage drive 
signal), TFEL device etch 22 to form via holes, or TFEL etch 23 to form 
via holes and emitting facets (again, via holes only required when 
addressing circuitry is not included in/on the substrate). FIG. 8b shows a 
cross-section of this possible configuration of light source, comprising 
the substrate 5 which may, or may not contain addressing/drive circuitry 
24, an insulating layer 25, a reflective microstructure 26 which may be 
coated with a reflective coating, the base electrode layer (eg. 
polysilicon) 27 which is connected to the drain or extended drain and or a 
shunt capacitor of the addressing circuit latching element 24 via an 
electrically conducting region 28, TFEL layers 29, and high voltage 
bondpad 30. Alternatively, as also indicated, if no addressing/drive 
circuitry is included in the substrate, then a low voltage bondpad 31 is 
formed to facilitate electrical connection to the base electrode. It is 
thus apparent that the full preferred embodiment of the invention, i.e. 
the integration of the TFEL laterally transmitting device with reflecting 
microstructures, cladding/outcoupling layer(s) and addressing/drive 
circuitry is preferable due to the reduction of fabrication steps, and the 
number of wire bonds that are required, thus improving cost, efficiency, 
and yield. It is clear, however, that any sub combination of the aspects 
of this invention may be employed to provide an improvement over prior art 
structures. 
FIG. 8c depicts an exploded view of an individually addressable light 
source according to the invention that would be suitable for the 
configuration of a linear array, as is required for printing applications. 
The drawing illustrates how the electrical conductivity can be maintained 
around the emitting apertures 17, but with laterally transmitted light 
from the TFEL regions either side of the reflecting microstructures 
contributing to the reflected emission--thus increasing the light energy 
emission and thereby enhancing the luminous efficiency of the light 
sources. Sidewall reflecting microstructures 32 prevent optical cross-talk 
and the electrical path 34 (typically deposited aluminium) weaves in a 
continuous line around the reflecting microstructures 33. Thus only a 
single level of top electrode metallisation is required, for reduced 
processing steps and hence higher process yield. 
Typically, the dimensions of the individual light sources of such a linear 
array for printing applications would be 42.5 micrometres (10.sup.-6 m) 
from sidewall 32 to the next side wall 32, for 600 dots per inch printing 
resolution. Correspondingly smaller dimensions would facilitate higher 
resolutions. 
FIGS. 9(a) and 9(b) depicts examples of the overall electrical geometry of 
the preferred embodiment of the invention--a multi-purpose opto-electronic 
integrated high intensity reflecting laterally transmitting TFEL light 
source. Shown schematically in FIG. 9a is an active matrix configuration, 
and FIG. 9b shows the corresponding passive matrix configuration. For both 
cases the addressing/drive circuitry (or part thereof) is fabricated into 
or onto the substrate prior to the formation of the reflecting 
microstructures and TFEL layers. In active matrix format, each separate 
light source, or light source region 35, is individually controlled via an 
associated latching element 36, such that the application of a low voltage 
data signal via the logic stage(s) 37 permits the selection of the light 
source(s) to be on or off. A linear array of such devices in actively 
addressed configuration would thus correspond to a single row of the 
matrix indicated by FIG. 9a. For passive matrix addressing, as shown in 
FIG. 9b, the addressing/drive circuitry is located on the periphery of the 
light source region(s). The devices are then addressed via a refresh 
operation--row by row. Again a linear array of light sources would thus 
correspond to a single row of such a configuration, or alternatively may 
be composed of groups of several rows to form a multiplexed array. 
FIG. 10 illustrates a possible configuration for the latching element that 
facilitates active addressing. The TFEL device with reflecting 
microstructures 35, is connected to the drain, or extended drain of a 
MOSFET 38 and to the electrode of a shunt capacitor 38. A high voltage ac 
supply 40 is connected to the top electrodes) of the device. The state of 
MOSFET 39 is controlled by the state of the addressing element composed of 
a storage capacitor 41 and a MOSFET 42. Logic signals are applied via the 
interconnects X and Y 43 to control the circuit. As shown in the truth 
table, the selection of appropriate combinations of data signals allows 
the TFEL device to be turned on, turned off, or to remain in its present 
state. When turned on, the channel MOSFET 38 becomes conducting such that 
the drain is connected to the source which is maintained at ground 
potential. The high voltage drive signal 40 is then applied directly 
across the TFEL device such that the light emission is initiated. When 
turned off, closing the channel of MOSFET 38, the high voltage drive 
signal is divided between the capacitative elements of the TFEL device and 
shunt capacitor 39 (which may be an integral part of the circuit, or 
MOSFET 38). The value of shunt capacitor 39 is thus selected so as to 
ensure that the resultant reduced voltage across the TFEL device is below 
the threshold value necessary for electroluminescence, thus turning the 
TFEL device off. 
REFERENCES 
1! Y.A Ono; Electroluminescent Displays, World Scientific, series on 
Information Display (1996) 
2! D.H. Smith; J Lum 23 (1983) 209/ 
3! J. Wilson, and J. F. B. Hawkes; Optoelectronics an Introduction, 
Prentice Hall International, (1983) 211. 
4! J. Vanterfleteren et al; Conference Record of the 1991 International 
Display, 134 Conference. 
5! C King; Proceedings of EL 96, Berlin (1996) 375.