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Description  |
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FIELD OF INVENTION
This invention relates to displays and more particularly to exceptionally
large flexible displays utilizing plastic film with embedded active
display elements.
BACKGROUND OF THE INVENTION
In an effort to provide large, wall size displays, many projection
techniques have been utilized in which video information is projected on
to a screen. Such systems are bulky and expensive, and do not lend
themselves easily to outdoor applications such as billboards, signage or
where large flat screens are required. Moreover, these techniques are not
well adopted to wide screen TV or theatrical applications even though
there have been many attempts to do so.
There are presently efforts under way to use large scale integrated circuit
technology to extend the solid state display out to much larger
dimensions. This is done primarily through the use of active semiconductor
elements placed in an X/Y addressable array. However, these active matrix
TV techniques are not well adapted to large screen displays over more than
tens of inches due to registration problems, poor yields and the
requirement for the maintenance of strict manufacturing tolerances. As a
result, as photolithography is applied at larger scales, fundamental
problems arise which drastically impact the yield and reliability of the
resulting devices. As the size and complexity of the circuits increase,
this multiplicative effect can drive the yield and reliability of the
circuits to unacceptably low numbers.
A second problem with applying photolithography to large screen displays is
an incompatibility of scale. Producing active components requires
registration of multiple masks to dimensions on the order of a micron.
Unfortunately, for a 19 inch TV, a 19 inch substrate will deform due to
thermal expansion so that it is impossible to properly register the masks.
A technically intensive solution to this problem has been devised by MRS
Technology of Burlington, Mass. in which the required TV aperture is
divided into sub-apertures which are imaged separately. As the imaging
system steps from one section to another it must sense the location of the
previous mask layers and properly register the next mask. Together the
problems with component yield and incompatibility of scale have delayed
the realization of large active-matrix flat panel displays. Thus prior art
large screen displays have suffered from the problems of component yield,
correlation of component failures and mask registration.
As illustrated in U.S. Pat. No. 4,136,436, and by way of further
background, Texas Instruments, Inc. of Dallas, Tex., has provided
concentric spherical semiconductor devices embedded in a rigid epoxy used
as photovoltaic sources with the use of an electrolyte. While this
technique provides pn junctions in spherical form distributed across a
rigid place, it will be appreciated that this technology does not result
in a display and is non-flexible, precluding use in a large size
application requiring an easily erectable flexible display.
SUMMARY OF THE INVENTION
In order to solve the problems of component yield, localized component
failures and mask registration over large areas, individual active
semiconductor devices, such as diodes, are fabricated in bulk and
incorporated into a thin flexible transparent plastic film which is
subsequently metallized with a transparent array of electrodes. This
produces a flexible display of plastic wrap type material which can be
rolled up and unrolled for the particular application, with flexibility
being a key factor.
In one embodiment, the devices are micron-size particles distributed in the
film as a powder when the plastic sheet is made, such that large numbers
of active devices are addressed by a single pair of crossed X/Y buses
which are part of the row-column addressing structure. Because of the
large number of active devices at a row-column junction, the display is
immune to individual active device failure.
In a preferred embodiment, the active components extend through the film to
permit metallization on both surfaces of the film to provide
interconnects. Moreover, by fabricating the individual devices first,
their yield is no longer tied to registration problems which depend on the
scale of the final circuit. Also, since the devices are formed in bulk,
their positions in the final circuit become randomized when distributed in
the plastic film, thereby removing any spatial correlations in the device
failures.
More specifically, in one embodiment, spherical diode elements are formed
as a powder having diameters on the order of 5 microns-2 mm. These formed
elements are distributed across the film as it is being formed, with an
electric field used to orient the diodes as the plastic film is formed.
Thereafter, appropriate metallization is applied.
Alternatively, silicon spheres can be embedded in the film, without diode
junctions being formed. Thereafter the spheres can be activated to produce
visible light by an etching process which forms microchannels in the
spheres. When a voltage is applied across the etched spheres, they emit
light without the necessity of initially forming a pn junction.
Importantly, the final product is a continuous, flexible, transparent film
which is easily fabricated through the use of continuous film forming
technology.
Note that reliability is provided by interconnecting many discrete devices
in parallel to provide redundancy. This is made possible through the use
of micron-size spheres and buses which activate large numbers of spheres.
Considering the application of this technology to TV screens, a film of
LED's or other luminescent devices is created and row and column lines are
provided on either side. Where the lines happen to cross the devices, they
are interconnected and a "pixel" is formed. Clearly this process requires
no registration since misalignment of either set of lines will only shift
the effective pixel location.
In one embodiment, there are fundamentally four steps which are used to
produce flexible extended films with embedded light-emitting elements.
First, devices are fabricated in a bulk process. Secondly, these devices
are incorporated in a plastic material and formed into a film. Thirdly,
the active devices are aligned in the film or are such that they emit
light in a given direction with respect to the film. Fourth, both sides of
the film are metalized to provide a row-column structure.
Note that Chemical Vapor Deposition (CVD) which is commonly used in the
fabrication of integrated circuits may be used in device formation,
specifically amorphous light emitting diodes or LED's. Note also that
spherical devices can be formed by fabricating thin SiC:H layers which are
removed from the substrate and ground to form spherical devices with
diameters on the order of 5-10 microns.
As mentioned above, when it is inconvenient to orient the active devices in
the film during film formation, suitable active elements can take the form
of porous silicon light-emitting diodes in the form of spheres embedded in
the flexible film and etched with hydroflouric acid to produce the
required porous material. It has been found that these microporous spheres
have the same photoluminescent behavior as the original wafer.
Other active elements include the manufacture of electro-luminescent
phosphors for thin film displays or diodes produced by the annealing of
semi-molten p and n spheres.
There are currently several common processes for commercially producing
thin plastic films, namely, casting, stretching and blowing. The process
of casting involves a process in which the plastic is extruded through
narrow die-lips and is cooled either by passing over a chilled roller or
through a water bath. The film can be made thinner and the polymers more
oriented by stretching or "drawing" the material. The process in which the
cast film is stretched in two dimensions below its melting point includes
inlet rolls, exit rolls and grippers to stretch the film as it moves
between rolls. This process produces films, primarily of polyethylene
terephthalate, PET, and polypropylene, PP, with thicknesses ranging from
20 microns to 2 mm.
Alternatively the films can be formed by film blowing; either melt
film-blowing or biaxial-draw film blowing. Film blowing is used to produce
thin sheets of plastic more rapidly and economically than the casting
process, and has been in widespread use in producing plastic wrap and
garbage bags. As will be appreciated, film blowing can create films with
thicknesses of 6 microns.
In one embodiment, the melt film-blowing process is modified for producing
extended films as follows. The molten plastic, including the active
devices, is extruded through an annular die creating a cylindrical film.
This film is drawn upwards, or downwards, through a cylindrical capacitor
which is added to the process to orient the active components. Air is
injected about the film for cooling and inside the film to "blow" or
expand the film radially. At a point, the film cools below the melting
point and it is then constrained in one dimension by guide rolls, after
which it is wound on a take-up reel. Note that the initial extrusion may
first be cooled below the melting point and then heated just above the
melting point prior to drawing and blowing.
It will be appreciated that as an alternative process, another blown film
technique can be used which involves fixing a cast film over a fixture and
blowing it into a "bubble".
As to diode junction orientation, although there are several ways to orient
components in a flexible film, in one embodiment, an electric field is
used to orient an active component by taking advantage of the dipole
moment of the depletion region. When a cylindrical capacitor is used in
the film-blowing process, as the film is drawn between the walls of the
capacitor, the electric field acts to rotate all the individual active
devices into alignment because of the built-in electric field across the
depletion region of each device. The process is designed so that the field
is strong enough, the plastic viscosity low enough, and the take-up rate
slow enough to insure that the devices are properly oriented as they exit
the capacitor.
As mentioned above, another method to achieve electroluminescence is
through the utilization of spheres of silicon first embedded into the
flexible plastic sheet and then microetched with hydroflouric acid to
produce active devices which, upon application of an electric current emit
radiation in the visible portion of the electromagnetic spectrum. It has
been found that light is emitted under such circumstances without the need
for doping. The advantage to this process is that the active devices need
not be manufactured in bulk and oriented during film manufacture. Rather,
the spheres may be added to the material used to make the plastic film
prior to film formation, with the top of the spheres being exposed after
film formation to the microetchant imparted to the spheres from a top
surface of an already formed film. Thus active element orientation need
not involve the application of orienting electric fields.
In summary, what is provided is a flexible transparent film with active
elements imbedded in a dense random distribution in the film. The
individual active devices are activated via a matrix of buses which may be
transparent and which are laid across the elements, with the elements
extending from one surface of the flexible sheet to the other. Note that
the top and bottom sides of the elements are connected from the top and
bottom of the sheet via the use of appropriate metallization, such that
the elements lie between the crossed buses to produce light.
It will be appreciated that the flexible display has application as
electroluminescent tape for automotive pin striping and sheet material for
signs. The displays may be provided as a flexible glue-on sheet in the
automotive field for dash and windshield applications. This same glue-on
sheet can be utilized in the home product market for clock displays and
appliance displays. The glue-on sheet can be utilized in advertising for
printed signs and billboards and in the novelty field for static curved
displays on cups and glasses or other tableware.
Finally, the subject flexible sheets may be utilized as video displays, for
work stations, in home HDTV, for theaters and for outdoor size billboards.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects will be better understood taken in conjunction with
the Detailed Description in conjunction with the Drawings of which:
FIG. 1 is a diagrammatic representation of the utilization of a flexible
plastic sheet with embedded light emitting elements to produce a large
screen display in such a manner that the display material can be rolled up
and distributed from a roll;
FIG. 2 is a diagrammatic illustration of the utilization of the subject
display across the face of a building, illustrating the scale to which
such displays may be utilized;
FIG. 3 is a diagrammatic representation of a kiosk on which the subject
display is affixed, illustrating the flexibility of the display and its
use around cylindrical or other mounting substrates;
FIG. 4 is a top and diagrammatic view of a portion of the subject display
illustrating a portion of film carrying micron-sized elements sandwiched
between crossed electrodes such that those elements at the junction of the
crossed electrodes are activated with the application of appropriate
voltage;
FIG. 5 is a diagrammatic illustration of the subject display illustrating
the imbedding active elements within a flexible sheet, with the active
elements being exposed at the top and bottom surfaces of the sheet which
are adapted to be contacted via overlying electrodes in the form of buses;
FIGS. 6A and 6B are, respectively, isometric and top views of a portion of
the subject display, illustrating the sandwiching of multiple micron sized
active elements in a plastic sheet surrounded top and bottom by
appropriate electrode buses;
FIG. 7 is a schematic diagram illustrating the addressing of diodes within
the matrix utilizing scan and data lines;
FIG. 8 is a sectional view of the subject display in which the diodes of
FIG. 7 are addressed via overlapping and staggered electrodes on opposite
sides of the flexible sheet;
FIG. 9 is a flow chart illustrating one embodiment of the fabrication of
active elements in a film in which spherical pn junctions are first formed
with the resulting diodes subsequently being mixed into plastic and
oriented during film forming;
FIG. 10 is a diagrammatic illustration of one embodiment of film forming in
which diode spheres are mixed with the granular material utilized to make
the plastic film, with the subsequent mixing and heating resulting in an
extruded film, and with the spheres being oriented via an electrostatic
field during the cooling process in which the final film is formed;
FIG. 11 is a diagrammatic illustration of device orientation during film
formation in which the active elements are rotated via the application of
an electrostatic field, with the final position such that opposite pn
portions of the spheres are exposed at the top and bottom surfaces of the
finally formed film;
FIG. 12 is a diagram illustrating forces induced on an active device by an
applied field;
FIG. 13 is a graph illustrating the viscosity of polystyrene as a function
of sheer rate at 180.degree. C.;
FIGS. 14A and B are diagrams illustrating two approximations to the induced
torque on a sphere rotating in a fluid involving a sphere inscribed in a
cylinder and many cylinders used to approximate a sphere;
FIG. 15 is a graph of the effect of an applied electric field on junction
orientation;
FIG. 16 is a graph of the effect on an applied electric field on junction
orientation with junction asymmetry;
FIGS. 17A-17F illustrate various methods of sphere formation, respectively,
gas atomization, chemical sphere formation by accretion to form micro
balloons, mask-etch-dicing, fluidized bed sphere accretion, rotating
electrode ablation and laser ablation;
FIG. 18 illustrates one embodiment of a blow-forming operation for the
formation of film;
FIG. 19 is a flow chart illustrating the formation of electroluminescent
porous silicon spheres with anodic etchant to create the porous silicon
structure;
FIGS. 20A-20E are diagrammatic and sectional views illustrating the
fabrication of the porous silicon elements after the silicon spheres are
first imbedded in the flexible sheet; and
FIG. 21 is a diagrammatic illustration of an emersion scanning process for
the formation of porous silicon spheres.
DETAILED DESCRIPTION
Referring now to FIG. 1, a flexible display 10 includes a flexible sheet 12
provided in a roll 14 in which micron sized active elements are imbedded
in the sheet, with the sheet having an array of electrodes metalized top
and bottom to be able to actuate the active elements for providing an
image 16 across the surface of the sheet. It is the purpose of the subject
display to provide anything from a small size display to a billboard-size
display through the utilization of the flexible material which, in one
embodiment, resembles transparent plastic wrap. As will be discussed,
embedded in the plastic wrap there is a randomly distributed powder-size
formation of active elements, with literally dozens of elements activated
to emit light at a crossed junction of activating electrodes or buses.
Rather than having each element addressed by a single bus, in the Subject
Invention, multitudes of active elements are addressed at a single bus
crossover such that relatively large displays may be fabricated without
regard to active device yield, or in fact the exact location of the active
devices relative to the overlying and underlying bus matrix.
Referring to FIG. 2, such a display may be building-size as illustrated by
building 18 such that the sheet 12 may be draped down over the side wall
20 of the building. Heretofore, such displays have been fabricated with
individual light emitting devices, with each activated by its own set of
electrodes. The size of the display is of course determined by the size of
the light emitting elements or lightbulbs. Note that the size of the light
emitting area is determined solely by the width of the electrodes which
activate the elements, with the elements sandwiched between the crossed
electrodes being that which makes up the pixel of the image to be
displayed.
Referring to FIG. 3, a kiosk 22 is illustrated with display 10 wrapped
around the surface of the cylindrical portion of the kiosk. It will be
appreciated that due to the flexibility of the display, the display may be
utilized for non-flat applications.
Referring now to FIG. 4, it will be appreciated that sheet 12 is
illustrated with a random distribution of active elements 30, with those
elements that are activated being in the overlap between electrode 32 and
electrode 34. This region of electroluminescence or light emitting is
illustrated via lines 36.
Referring to FIG. 5, 6A and 6B film 12 is shown illustrated with spherical
active elements 30 being exposed at top portions 40 to be contacted via
the under side 42 of electrodes 32. Top side 44 of electrodes 34 contact
the portions of the active elements 30 exposed at the bottom surface of
film 12 to provide row-column activation.
Ideally, the mean diameter of the spheres is on the order of 5 to 10
microns to provide maximum resolution for a display or more importantly to
provide more dense packing of the individual active elements. However, it
is useful sometimes to have the mean diameter be as much as two
millimeters.
Referring now to FIG. 7, the schematic diagram shows diodes 50 addressed
via data lines D1, D2 and D3 and via scan lines S1, S2 and S3. It will be
appreciated that the above data and scan lines are conventional for the
activation of individual diodes.
As illustrated in FIG. 8, multiple diodes are activated at the intersection
of crossed electrodes. Here, the sandwich structure shows active elements
30 addressed by electrodes 32 and 34 corresponding to the above-mentioned
data lines and scan lines.
Films of light-emitting diodes can also be produced and used to produce
full color flat panel displays. Note three separate films of red, green
and blue LED's can be bonded to a transparent or reflective panel to
produce a display. Electroluminescent displays can also be fabricated by
using electroluminescent material in place of LEDs as the spheres in the
film. Referring now to FIG. 9, a flow chart is illustrated for the
fabrication of active devices in the form of diodes. The spherical p/n
junctions are formed as illustrated at 70 by either forming p/n films and
dicing or grinding the film, or by forming silicon spheres, utilizing tape
casting, or utilizing doping. As illustrated at 72, the surfaces of the
spheres are passivated after which they are mixed as illustrated at 74 in
the raw plastic and oriented such that the junctions are all in one
direction while forming the film. Then passivation is removed on the
surfaces of the spheres to provide contact locations for the overlying
electrodes as illustrated at 76. Thereafter metallization films are formed
over both sides of the sheet, with selective etching as illustrated at 80
and 82 forming data lines on one side of the film and scan lines on the
other side.
More particularly, for the diode-matrix implementation, a single diode film
is metallized with data lines on one side and scan lines on the other. The
scan lines are metallized on the back of the liquid crystal and the two
bonded. In one embodiment, addition of a color mask, driving circuitry and
back lighting produces a full color display. It is important to note that
transparent metallizations such as indium-tin-oxides (ITO) are currently
used to form data and scan lines for such applications.
Thus, as to metallization and passivation of the plastic film,
Indium-tin-oxide (ITO) may be used in the fabrication of displays where it
is essential that light pass through the metallization layer or layers.
For applying metal to plastics as indicated by the article by E. Miller,
Plastics Products Design Handbook, Marcel Dekker Inc., New York, 1983, pp.
211-221 is instructive. Conductive polymers are also a candidate for
interconnecting the active components as described in the article by P.
Stipp, "Scientists Say They Increaased Electricity That Plastic can
Conduct by a Factor of 10", The Wall Street Journal, Thurs. Mar. 19, 1992.
Referring now to FIG. 10, in order to provide oriented active devices in a
film, in one embodiment, active devices are first manufactured as
illustrated at 90, where they are placed in a hopper 92 connected to a
mixer or oven-type configuration 94 which mixes these active devices with
granulated plastic material 96 in hopper 98, such that when mixed and
extruded at die lip 100 individual active devices 102 exit the die lip in
non-oriented form. As the film cools it moves away from die lip 100 and an
electric field is applied via electrodes 104 and 106 to provide an
electrostatic field via the application of a voltage from voltage source
108. The die lip 100 may be linear or circular in cross section. As will
be described hereinafter, the subjecting of the active devices to an
electrostatic field orients the devices such that they are all oriented in
one direction with the junctions being generally along the center line of
the film. After cooling, the film is extracted via a roll 109 and provided
in roll form 110 as illustrated.
Referring now to FIG. 11, as can be seen, positively charged electrode 104
is located and spaced adjacent to negatively charged electrode 106, with
film 112 initially carrying unoriented devices 102 that have a built in
dipole moment derived from prior doping. These devices are rotated as
illustrated by arrow 116 due to the electrostatic field while the film is
still fluid as it cools. What is now presented is an explanation of the
action of electrostatic field on the individual active elements indicating
the mechanism by which the elements can be rotated.
The subject flexible thin film active device structures permit many flat
panel designs to be inexpensively produced using flexible extended films
to form active circuits for liquid crystal displays (LCDs), full color
light emitting diode displays (LED) and electroluminescent displays. Note
that active matrix displays can be designed in a diode-matrix
configuration.
The following is an analysis of active device orientation using an electric
field. The electric field induces several forces on the individual active
components. The first is a torque on the dipole moment which results from
the built in electric field of the depletion region. The second is a
torque on the dipole moment induced by the electric field acting on a
slightly non-spherical dielectric. These forces are opposed by the moment
of inertia of the particle and the viscosity of the plastic. The viscosity
of the plastic depends on its temperature we well as the local strain
placed on the plastic by the drawing process and rotation of the
particles. As the film is drawn to a thickness near the diameter of the
particles, the effects of surface tension may also play an important role.
FIG. 12 illustrates the forces induced on the particle by the electric
field. The dipole moment (p) of the device junction can be estimated as
the product of the surface charge density (), the area of the junction
(A=.pi.r.sup.2), and the effective width of the junction (w). Based on
nominal parameters for a p-n junction the dipole moment of a 10 micron
diameter sphere can be estimated at 10.sup.-20 (coul-m). The torque on
this sphere, given in equation (1), would then be: (10.sup.-20 E sin
.theta.).
##EQU1##
where: p is the dipole moment of the depletion region
E is the applied electric field
.epsilon..sub.o is the free space permittivity (8.85.times.10.sup.-12 F/m)
.epsilon..sub.r is the relative dielectric constant of the sphere (-10)
n is the depolarization coefficient along the axis
.gamma. is a redefinition of n: (.gamma.=1-3n)=0.8.times. (% asymmetry)
V is the volume of the sphere
.theta. is the angle between the dipole moment and electric field
.alpha. is the angle between the dipole moment and major axis of the
ellipsoid as described in FIG. 12.
The torque induced by the electric field on an asymmetrical spheroid is
well known and is expressed in equation (2). I have simplified the
equation given in the reference first by converting from gaussian units to
MKSA units (as in equation (2)) and then by making the substitutions:
(.gamma.=1-3n) and (.epsilon..sub.r =10) as given in equation (3).
Assuming a nominal diameter of 10 microns and 12% asymmetry (.gamma.=0.1),
the torque on the induced dipole moment is: (6.times.10.sup.-27 E.sup.2
sin 2(.theta.-.alpha.)). It is very important that the induced torque is
much less than the torque on the junction so that the junction of the
active device will be vertically aligned rather than the axis of the
ellipsoid (which may be random relative to the junction).
The rate at which the sphere can be rotated by these torques depends on the
rotational inertia of the sphere and the drag of the molten plastic about
the sphere (angular acceleration=torque/rotational inertia). The
rotational inertia of the sphere is given in equation (4).
##EQU2##
Where: I is the rotational inertia
.rho. is the density of the material (.about.5.times.10.sup.3 kg/m.sup.3
for silicon)
R is the radius of the sphere (5 microns in our example case)
With these assumptions the rotational inertia is 2.6.times.10.sup.-23 kg
m.sup.2.
The remaining question is the drag placed on the sphere by the molten
plastic. The choice of: plastic material, extrusion temperature, draw rate
and cooling rate will all effect the viscosity of the material during
particle alignment. The choice of material is not a simple one. Although
linear low-density polyethylene (LLDPE) is commonly used in the production
of thin films and can be blown to thicknesses of 6 microns, its melting
temperature may make it inappropriate for application with active
components with high junction temperatures. Polystyrene is more frequently
used in electronic packaging. For an estimate of the viscous drag on the
spheres we can use the data in FIG. 13 for polystyrene. Plastics are
non-Newtonian fluids meaning that their viscosity is actually dependent on
the rate of sheer (.gamma.). This dependence is illustrated in FIG. 13.
Temperature can plan an important role in the viscosity of a molten
plastic. Equation (5) provides a means of extrapolating viscosity data
over a range of temperatures which exceed the melting point (T.sub.g).
##STR1##
Where: .eta..sub.o =viscosity
T.sub.i =melting point in .degree.C. (100.degree. C. for Polystyrene)
n.sub.o =reference viscosity
T=Temperature in .degree.C.
For our application, we will take a nominal viscosity of n=10.sup.5
(gm/cm-sec)=10.sup.4 (kg/m-sec) at 180.degree. C. as our reference.
Extrusion temperatures for polystyrene can range from
200.degree.-500.degree. C. Using equation (2) and our assumed nominal
viscosity of 10.sup.4 (kg/m-sec), we can anticipate a range of viscosity
of 10.sup.2 -10.sup.4 (kg/m-sec).
The next step in the analysis is to formulate an expression of the torque
induced on a rotating sphere by the viscosity of the molten plastic. A
solution to this problem can be found through the use of the Navier-Stokes
equations. One closed form solution which can be used to approximate the
above problem is the solution to a rotating cylinder in an infinite
Newtonian fluid.
As shown in FIG. 14(a) illustrates the above-mentioned sphere inscribed in
a cylinder of equal radius and height equaling 2R. The solution is
.tau..sub.v =4.pi..mu.hR.sup.2 .omega., (6)
where:
.tau..sub.r is the induced torque due to viscosity,
.mu. is the viscosity of the fluid,
h is the height of the cylinder,
R is the radius of the cylinder,
.omega. is the angular velocity of the cylinder.
When equation (6) is applied to the cylinder in FIG. 14(a) the result is:
.tau..sub.v .apprxeq.-.epsilon..pi..mu.R.sup.3 .omega.. (7)
The minus sign has been added to indicate that the resulting torque acts in
opposition to the rotation of the sphere. It seems reasonable to assume
that the induced torque on the cylinder would exceed the torque induced on
the inscribed sphere. In order to improve the estimate, the sphere is
assumed to be constructed from many thin cylinders as illustrated in FIG.
14(b). The effective torque is then computed by integrating over the many
small cylinders to arrive at the estimate in equation (8).
##EQU3##
We can now use equations (1), (3), (4) and (8) to estimate the equation of
motion of the sphere in an external electric field. Equation (9) gives the
basic equation of motion and equation (10) makes the substitutions derived
earlier. Note that (.omega.=.theta.).
##EQU4##
Two solutions to this equation will be presented. In the first the induced
torque on the sphere and the rotational inertia are assumed insignificant.
This leads to the simplified equation (12) with solution given in
equations (13-14) and plotted in FIG. 15.
##EQU5##
An exact solution to equation equation (11) can be found by numerical
solution to the differential equation. Table 1 lists the assumed film
parameters and FIG. 16 shows the resulting solution to the differential
equation. There are clearly two different results.
First, when the applied electric field is low (<10.sup.5 V/m), the torque
on the junction will dominate the induced torque. The junction will align
properly but with a conventional plastic viscosity (.about.10.sup.4
kg/m-s) this can take too long (hours). An increase in the electric field
and reduction in the viscosity can yield a solution with acceptable
alignment time (.about.second).
TABLE 1
______________________________________
Parameters Assumed for FIG. 11
.sigma. =
4.5E-4
.UPSILON. =
1.0E-2
.omega. =
3.3E-7
R = 5.0E-6
.mu. =
1.0E-2
.alpha. =
45.degree.
______________________________________
The second result occurs when the electric field is too strong (>10.sup.8
V/m) in which case the induced torque can dominate and the sphere aligns
improperly. In all cases the rotational inertia of the sphere was found to
be insignificant.
From equation (11) we can also see that reducing the radius of the sphere
has two beneficial effects. It reduces the torque from the asymmetry of
the sphere and the torque due to viscosity.
Referring now to FIG. 17A, microspheres can be provided in a process in
which molten silicon 120 is ejected from a nozzle 122 and is subjected to
argon jets 124 so as to provide the microspheres 126.
Referring now to FIG. 17B, to provide microspheres microballoons 130 are
produced by in a chamber 132 via a chemical reaction and accretion.
Referring now to FIG. 17C, microspheres 140 can also be formed by doping a
wafer 140, dicing the wafer as illustrated at 142 to provide micron size
particles 144, shown in top view at 146.
Referring now to FIG. 17D, microspheres of silicon can be formed in a
fluidized bed process. Mesh 150 is subjected to a gas stream 152 in an
upward direction as illustrated to provide microspheres 154.
Referring now to FIG. 17E, a boule of silicon 160 may be fragmented or
ablated through the utilization of an electric arc 162 provided by an
electrode 164 in which boule 160 is rotated in the direction of arrow 166.
Finally, referring to FIG. 17F, a disc of silicon 170 may be laser ablated
to knock off particles 172 through the utilization of a laser 174.
Referring now to FIG. 18, one method of forming a thin film is through a
blown film extrusion operation in which granular material in hopper 200 is
fed into an extruder 201 which is a screw-like device. As the material is
conveyed by the screw past a heated portion 202 of the barrel 203 a molten
film forms on the inside diameter of the barrel. As the mixture of molten
and granular material is transported by the screw, energy derived from
sheering of this melt film is responsible for melting the remainder of the
material.
The molten material exits the extruder and enters a set of concentric die
lips 204 and 206. Air is introduced at 208 into the tube to inflate it,
creating a void 212 and stretching the material to the desired thickness.
A second stream of air 213 is directed around the outside of the "bubble"
214 to cool and solidify the melt. It will also be appreciated that as the
walls are formed, the active devices within the melt may be oriented by
appropriately charging walls 204 and 206 of the die lips as illustrated by
the provision of a DC supply 215 connected as illustrated.
Note that once the film has solidified it is folded flat by pinch rollers
216 and rolled up on a high speed winder 218.
Referring now to FIG. 19, electroluminescent porous silicon spheres can be
produced as follows. The first step is to form silicon spheres by any
convenient means as illustrated at 230. Thereafter it is important to
passivate the surface of the spheres as illustrated 232 and to then to mix
the spheres in plastic to form a film as illustrated 234. After the
spheres are mixed in the film and the film has been formed, then the top
surfaces of the spheres are chemically treated to remove passivation as
illustrated 236. It will be appreciated that the spheres are intended to
extend above and below the surfaces of the formed film. As illustrated at
238, the bottom surface of the film is metallized to provide an electrode
for the acceleration of charged etchant.
The next step is to subject the exposed top surfaces of the spheres to a
charged etchant via an anodic etch step to create the porous silicon as
illustrated at 240. In order to create this anodic development, the bottom
layer of the film is metalized as mentioned above.
In one embodiment, hydroflouric acid is utilized in the etching process to
provide the microporous structure at least in the top surface of each
sphere. It will then be appreciated as illustrated at 242 that the front
side of the plastic is metalized with a patterned transparent conductor,
with patterning of the data lines on one side of the film being formed in
a step illustrated at 244, and with the scan lines being formed on the
other side of the film as illustrated at 246.
Referring now to FIG. 20A, in section the completed film 250 is shown with
imbedded silicon spheres 252. As seen in FIG. 20B, a metalized layer 254
is deposited on the back of the film 250, whereas as shown in FIG. 20C a
charged etchant contacts the top surfaces of the spheres.
Referring now to 20D, metallization 260 is patterned as desired.
As can be seen in connection with FIG. 20E, when a voltage is applied
between conductor 260 and conductor 254, which may of course be a
patterned conductor, the microscopic etching results in emitted light 270
from the element or elements sandwiched in between the activated
electrodes or conductors.
Note that several variations on the process for forming etched silicon
sphere are described in E. Bassons, M. Freeman et al. Characterization of
Microporous Silicon Fabricated by Immersion Scanning, Mat. Res. Soc. Symp.
Proc., Vol 256, pp. 23-27, 1992. To date, the best resu | | |
