Background Art Display devices are conventionally classified into two basic categories including active and passive displays. Active displays which are light generating devices include such technologies as Cathode Ray Tubes (CRT), Light Emitting Diodes (LED) and Plasma Display Panels (PDP). While passive displays are light modulating devices where the light source is either ambient or light from a separate source and includes such technologies as Liquid Crystal Displays (LCD), Electrochromic Displays (ECD) and Electrophoretic Displays (EPID).

Another classification for displays relates to the physical size or geometry of the device. Flat Panel displays are generally more compact and energy efficient, and utilize practically all of the above mentioned technologies except for CRT technology.

Attempts to flatten the conventional CRT have been unsuccessful since the devices produced have had either poor picture quality or excessive manufacturing costs.

A very successful type of Flat Panel

Display is the LCD device. The LCD includes a plurality of pixels arranged in a matrix configuration utilized to either transmit or block light. Whether light is transmitted or blocked, depends on the alignment of the liquid crystal molecules which is controlled by an electrical current. The early LCDs utilized a"passive matrix" scheme in order to address the individual pixels when producing images. This scheme consists of applying a voltage to a single row and then adjusting the column voltages to produce a large combined voltage across the selected pixels in that row. This addressing scheme enabled the early LCDs to be efficient and low cost. However, due to a cross talk condition, the Passive Matrix LCDs cannot provide both good contrast and resolution.

In order overcome the cross talk problem, the"active matrix"scheme was developed for LCD devices. This scheme utilizes an array of transistors in order to address the individual pixels. Each pixel receives a voltage from its column line only when its own transistor is switched on. This enables Active Matrix LCDs to provide good resolution as well as good contrast. However, these devices have some drawbacks. First of all, these type of displays draw more power than a display utilizing the"passive matrix'scheme. These types of displays are also more expensive and complicated to produce. Another drawback is that these type of displays tend to have lower yields due to the difficulty of fabricating the transistor arrays which are included to perform the"active matrix" addressing.

Another type of flat panel display is the ECD device. The ECD device generally includes a cell including at least two electrodes where at least one consists of electrochromic material, an electrolyte and at times an insulator. Applying a voltage across the electrodes causes ions present in the electrolyte to be absorbed by one of the electrodes thereby producing a change of color or transmissive property in the electrode. The change in color or transmissive property is the affect that enables these types of displays to produce images. Such a display is disclosed in U. S. Patent 3,995,943 to Jasinski, Issued December 7,1976, entitled ALL SOLID ELECTROCHROMIC DISPLAY. This patent discloses a display which utilizes either Tungsten Oxide (W03) or Vanadium Oxide (V205) as a display electrode.

Vanadium Oxide (VO2) is material that has been utilized in various electrical and optical applications. These applications have included being utilized as a medium for holographic optical recording, a temperature stabilizer and controller, an electronic switch and, for screening and modulating microwave radiation. VO2 exhibits a phase transition property which is accompanied by a significant change in optical properties. One of the optical properties which is significantly changed is the index of refraction, which would enable V02to optically modulate light. The phase transition in Vomis capable of being thermally induced by utilizing heater elements disposed under films of this material.

It is therefore, an object of the present invention to provide an improved Flat Panel Display

by utilizing the phase transition property of VO2in order to optically modulate light.

Disclosure of the Invention A display device is disclosed including a plurality of pixels arranged in a predetermined configuration. Each pixel includes a mirror element disposed over a flat surface. A light modulating material disposed over the mirror element for selectively modulating a predetermined wave length of light received from an external source by transitioning between a first and a second state.

The light modulating material in the first state causes destructive interference in the predetermined wave length of light and in the second state causes constructive interference in the predetermined wave length of light.

Additionally features are also disclosed which includes a heating element, a first insulating layer, a second insulating layer, a protective layer and a p-n junction. The heating element is disposed beneath the mirror element and is coupled to the p-n junction. The first insulating layer is disposed beneath the heating element, while the second insulating layer is disposed between the heating and mirror element. The protective coating is further disposed over the light modulating material.

Brief Description of the Drawings The above objects, further features and advantages of the present invention are described in detail below in conjunction with the drawings, of which:

FIGURE 1 is a graph plotting the conductivity (l/f2) of V02as a function of temperature; FIGURE 2 is a graph plotting the index of refraction (n) of VO2as a function of the wavelength (X) of light; FIGURE 3 is a diagram of the optical resonator according to the present invention; FIGURE 4 is a graph plotting the percent change in the reflective coefficient (R) of the optical resonator at the critical Temperature (Tc) as a function of wavelength; FIGURE 5 is a graph plotting the change in the critical temperature (Tc) of Vouas a function of the percentage of Niobium (Nb) dopant; FIGURE 6 is a diagram of the architecture of the Phase Transition Display (PTD) according to the present invention; FIGURE 7 is a diagram illustrating the addressing scheme utilized by the Phase Transition Display (PTD) according to the present invention; FIGURE 8 is a side view of an individual pixel included in the Phase Transition Display (PTD) according to the present invention; FIGURES 9A & 9B are graphs illustrating the operation of the pixels included in the Phase Transition Display (PTD) according to the present invention; and FIGURE 10 is a side view of another embodiment of an individual pixel included in the Phase Transition Display (PTD) according to the present invention.

Best Mode for Carrying Out the Invention The present invention is directed to a Phase Transition Display (PTD) which is capable of being implemented in a flat panel configuration. The phase transition display utilizes the thermally induced phase transition property of Vanadium Oxide (VO2) films included in the pixels of the display in order to optically modulate light for producing images. The use of VO2-based pixels has a number of advantages. One advantage is that enables the use of a Silicon substrate, which makes the device processing compatible with Silicon IC technology.

Another advantage is that it enables the use of a "passive matrix"addressing scheme, which implies high manufacturing yield and low production costs.

The phase transition property of VO2 relates to this material transitioning between an insulator and metal state. In the insulator state VO2has a relatively lower conductivity and index of refraction, while in the metal state VO2has a higher conductivity and index of refraction. This change in the index of refraction is what enables the V02 films to optically modulate light. The transition from the insulator to the metal state is achieved by heating the VO2above its critical temperature (Tc) which is approximately 68°C, while the transition to the insulator state occurs when the VO2is cooled to a temperature below its Tc.

Referring to FIGURE 1, there is shown a graph plotting the conductivity (1/n) of Vouas a function of temperature, which has been taken from an article entitled PRESSURE DEPENDENCE OF PROPERTIES OF

VO2, by?????,??????, P. 1035. As can be seen, the conductivity change due to the insulator-to-metal phase transition of V02iS considerable. The observed conductivity change at the Of 68C which corresponds to a value of 3 on the x coordinate of this graph exceeds four orders of magnitude. The hysteretic loop which is not shown is approximately 0.5°C in the single crystal and 1-2°C in the films for VO2with good stoichiometry.

Referring to FIGURE 2, a graph plotting the index of refraction (n) of VO2as a function of the wavelength (X) of light. This graph illustrates the spectral dependance of n for the two phases of VO2.

Curves 1 & 2 represent a film of VO2having a thickness of 1850 Angstroms, where Curve 1 is for the metal state and Curve 2 is for the insulator state.

While Curves 3 & 4 represent a film of VO2having a thickness of 600 Angstroms, where Curve 3 is for the metal state and Curve 4 is for the insulator state.

As can be seen, there is a relatively large change in the index of refraction for these films of VO2 in the visible spectral range. The following table summarizes the change in the index of refraction (An) that occurs due to the phase transition of VO2for the three important wavelengths (X) in the visible spectrum of blue, green and red: (um) An 0.44 (blue) 0.28 0.50 (green) 0.32 0.62 (red) 0.54 The large observed An in the above table is an important feature of the present invention since it enables the above three wavelengths to be

optically modulated.

Referring to FIGURE 3, there is shown a diagram of an optical resonator consisting of a film of V02 12 deposited on an layer of Aluminum (Al) 14 serving as a mirror. The optical resonator 10 demonstrates the basic operation of an individual VO2-based pixel according to the present invention.

The V02 f ilM 12 deposited on the Al mirror 14 represents an optical resonator having a reflective coefficient (R) which is dependant on the phase transition state of the V0212. For certain wavelengths satisfying the resonant conditions, a change of in the index of refraction alters the optical interference pattern causing a strong modulation of the optical reflection. Depending on thickness (d) of the VO2film 12 and the wavelength of the reflected light, two waves reflected from the top of the V0212 and Al mirror 14 creates either a constructive interference pattern or a destructive one depending on the phase transition state of the VO2film 12. The constructive interference causes the intensities of the two beams to be combined providing the maximum amount of reflection or the largest R value, which is satisfied by the following equation: 2d = mX, m=1, 2,3... (1) The destructive interference causes the two beams to be out of phase and thus cancel each other out, which provides the minimum amount of reflection or R value, where this condition is satisfied by the following equation: 2d = (2m-1) X/2, m=1, 2,3... (2) It should be further noted that varying the

thickness (d) of the VO2film 12 changes the wavelength corresponding to the resonant conditions of the optical resonator 10. Thus, providing a basis for color display operation, which will be described in detail later.

Referring to FIGURE 4, a graph plotting the percent change in the reflective coefficient (R) for the previously described optical resonator as a function of wavelength (X) is shown. As can be readily observed, the value of R which depends on the X for both the metal state (T : T,) and insulator state (T (Te). At the green wave length of light (X=0. 5um), the contrast ratio is approximately 15 which is desirable for display devices.

Referring to FIGURE 5, there is shown a graph plotting the change in the critical temperature (Tc) of Vouas a function of the percentage of Niobium (Nb) dopant. This graph illustrates that is possible to change the Tcof VO2by doping it with a small amount of Nb. As can be seen, the addition of 0.2% Nb reduces the T, down to 45°C. Reducing the Tcof VO2 is desirable since it can be utilized for reducing the power required to operating the VO2-based pixels of the present invention.

Referring to FIGURE 6, there is shown a diagram of the architecture of the Phase Transition Display (PTD) according to the present invention.

The architecture 16 consists of a plurality of individual VO2-based pixels 18 arranged in a conventional two dimensional matrix array which is adaptable to be fabricated on a Silicon substrate (not shown). Each pixel 18 is interconnected by a row and column line 22,24 similar to other flat panel

displays. Coupled between each pixel 18 and column line 22 is a diode or p-n junction 20 which are also fabricated on the Silicon substrate. The p-n junctions 20 are utilized to prevent current spread and possible cross-talk between the pixel elements 18. Leakage current in such an architecture 16 is likely since there are four loops of parallel connection through three neighboring pixels. The p-n junctions 20 being placed as shown blocks any leakage current by being placed twice in each loop.

The architecture 16 of the present invention is desirable because it enables it to be driven by utilizing a"passive matrix"addressing scheme or circuit. As described in the prior art section and shown in FIGURE 7, this scheme consists of data being received by the columns in parallel while a particular pixel is selected by a sequential row pulse. The architecture 16 of the present invention can utilize a row pulse as narrow as 1-10 us. Utilizing such a narrow pulse shortens both the turn on and decay times, which increases the capability to drive a larger number of pixels. Thus, providing video frequencies for display operation.

The use of the"passive matrix"scheme is also desirable because it does not require the use of transistors as in active matrix LCD devices. This significantly affects the yield and manufacturing costs, since fabricating p-n junctions on Si is standard and has very high yields.

Referring to FIGURE 8, a side view of an individual pixel included in the Phase Transition Display (PTD) according to the present invention is shown. The individual pixel 18 consists of a first

insulating film 24 which is preferably a film of Silicon Dioxide (SiO2) grown on a Silicon Substrate 22. The first insulating film 24 is utilized to control heat dissipation. The dissipation time is variable in broad limits, from seconds to fractions of a ms, by varying the thickness of this film 24.

The thickness of the first insulating film 24 is preferably adjusted to provide a heat dissipation time of 40 ms, which is most suitable for display operation. Calculations show that without a Si02 film, Silicon substrates having thicknesses in the range of 0.3-1 mm absorb heat in a time period (td) according to the following formula: td Z (1) 2/DT (3) where DT is the heat diffusion coefficient of Silicon. With DT = 0.8 cm 2/s a td of 3 ms is obtained which is too short for efficient display operation.

Deposited and disposed over the first insulating film 24 is a heater element 26 which is preferably a film of Nickel Chromium (NiCr) having a thickness of 20 nm. The heater element 26 is utilized to provide heat to the pixel 18 in order to induce the phase transition in a VO2 film 32 located above. While the heater element 26 is disclosed as thin layer of Nickel Chromium, other materials including Silicon can be utilized as well, as such other materials that function to provide heat according to the resistance of the layer. Power is applied to the heater element 26 through a pair of contacts 36,38 which are preferably films of gold.

The first contact 36 which is disposed over the substrate 22 is coupled to the heater element 26

through a p-n junction which also fabricated on the substrate 22. The second contact 38 is disposed over the first insulating film 24 and is directly coupled to the heating element 24.

Disposed over the heater element 26 is a second insulating film 28 which is preferably a film of Aluminum Oxide (Al203). The second insulating film 28 is utilized to isolate a mirror element 30 located above, from the heating element 26. Disposed over the second insulating film 28 is the mirror element 30 which is preferably a film of Aluminum.

Again while Aluminum is described other highly reflective materials can be utilized as well including Chromium, Nickel and so on. Disposed over the mirror element 30 is the film of VO2 32. As previously described, the VO2 32 along with the mirror element 30 forms an optical resonator, which is utilized to optically modulate light according to the phase transition state of the VO2 32.

The VO2film 32 along with mirror element 30 determines the reflective coefficient (R) of each pixel 18, which depends on the phase transition state of the VO2film 32. The V0232 along with the mirror creates either a constructive interference pattern or a destructive one depending on the phase transition state of the VO2film 32 and the wavelength of light being modulated. The constructive interference pattern provides the maximum value of R for each pixel 18, while the destructive interference provides the minimum value of R.

Grown and disposed over the VO2 32 is a protective layer 34 having an anti-reflective property which is preferably a film of Vanadium Oxide

(V205). The protective film 34 represents a stable and transparent insulator in the temperature range of interest. Both the VO2 32 and protective film 34 are applied by being sputtered in the same chamber and grown sequentially under different oxygen pressure.

The above described pixel 16 structure is preferably fabricated by utilizing modern Silicon IC technology. This technology enables the pixel size to be reduced to 10-20 um. Even at such a small size, the heat transfer from one pixel to another is negligible. This is because the heat is first absorbed by the silicon wafer which functions as a heat sink. Also, the distance from the heater element 26 to the top active film 32 is only 100-200 nm, which is more than a magnitude less than the distance between neighboring pixels. Under these conditions potential temperature induced cross talk is greatly reduced.

Referring to FIGURES 9A & 9B, there are shown graphs illustrating the operation of the pixels included in the phase transition display according to the present invention. During operation, a short electrical pulse ranging from 1-10 us is applied to the heater element of a particular pixel, which phase transitions the VO2film to the metal state. This transition causes the brightness and color of the pixel to be changed, for example from a bright green to a dark green. The pulse is assumed to be powerful enough to raise the temperature of the pixel well above the Tcas shown in FIGURE 9B.

At the end of the pulse, temperature decreases with time. If its is desired to maintain the pixel in the metallic state, the next pulse

should arrive at a time when the temperature of the pixel is still above the Te as demonstrated by Curve 1 of FIGURE 9B. With a display having a frame frequency of 60 Hz, the next pulse arrives at a time period (tf) of 16.7 ms.

Curve 2 of FIGURE 9B represents the situation where the initial temperature of the pixel is significantly lower. This can cause the temperature of a pixel to fall below the Tcwithin a frame period and thus cause the pixel return to its original brightness and color which for example is a dark to bright green. The time spent in the dark state is controlled by the pulse amplitude or width as shown in FIGURE 9A and thus implies a simple method of providing grey levels. Another method of providing grey levels includes modulating the pulses in the frame cycle. For example skipping a pulse within the frame period causes the pixel temperature to fall below the Tc in order to produce grey levels.

Utilizing a combination of both methods provides a sufficiently high number of grey levels.

An approximation of the power required to perform the pixel modulation is as follows. The power (Ql) to drive a single pixel having an area of 20 x 20 um and a thickness of 100 nm to a change in temperature (AT) of 60°C above normal is calculated using the following equation: Q1 = CmAT (4) where C is the heat capacity of the VO2 film and m is the film mass. For a C of 25 J/ (mole k), the power Q1= 3.6 x 10-9 Joules. For one million pixels turned on, the power obtained for a 60 Hz pulse repetition is Ql/s = 0.2W of power per inch squared of the

display area. Another component of the energy required originates from the latent heat, which is associated with the first order phase transition.

This component, Q2, is also estimated to yield a value for Q2/s = 0.2W. Thus, the total power necessary to drive one square inch of the display having one million pixels is: Q/s = Q1 + Q2 (5) which provides a value of 0.4 watts. With a pulse duration of 1 us, a heater resistance of 50 Ohms and a voltage applied of 20 Volts, a reasonable total DC current requirement of 20 milliAmps is obtained. The above calculations are an approximation since it applies only to a situation where all the heat is transferred to the VO2 film. In addition, the power required to drive the electronics for display illumination is not taken into account.

Referring to FIGURE 10, a side view of another embodiment of an individual pixel included in the phase transition display according to the present invention is shown. This embodiment 19 includes many of the same elements which function similarly as described previously in regard to the embodiment of FIGURE 8, as such like numerals implies like elements. Thus, only the differences in the present embodiment of the pixel 19 will be described. These differences include the pixel 19 shown in FIGURE 10 having a heater element 40 and VO2 film 42 which are sub-divided into three sections in order to enable color operation of the Phase Transition Display according to the present invention.

As previously discussed, the optical properties of the pixels according to the present

invention are controlled by the resonant conditions of the two light beams reflected from the VO2 film and mirror element. An appropriate choice of structure parameters enables the fabrication of pixels with the highest reflection contrast ratio at the phase transition for red, green and blue spectral regions. A good contrast ratio is achieved for both green light (f =0.5 um) and red light (X =0.63 um) for a single thickness of the V02 film, where the VO2 film in the metal state is reflective for red light and dark for green light, and vice versa in the insulator state. By thinning the VO2 film, a resonant condition for blue light is achieved. Thus, each pixel according to the present invention includes a VO2 film sub-divided into three sub- sections having two different thicknesses and including separate electrical access in order to provide three different resonant conditions for the red, green and blue spectral regions.

The pixel 19 includes a VO2 film 40, which is sub-divided into three adjacent sections 40A, 40B, 40C. Two of the sections 40A, 40B have the same thickness and are utilized to modulate either green or red light. While the third section 40C has a narrower thickness and is utilized to modulate blue light. The heating element 42 is also sub-divided into three sections 42A, 42B, 42C in order to independently heat each of the VO2 sections 40A, 40B, 40C in order to induce independent phase transitions within the V02 sections 40A, 40B, 40C.

Each of the heating element sections 40A, 40B, 40C is coupled to a respective gold or equivalent type of contact 44,46,48 for transmitting power thereto.

During operation, power is selectively supplied to each of the heating sections 42A, 42B, 42C according to the data supplied to each pixel 19.

This causes heat to be selectively supplied to the associated VO2 sections 40A, 4OB, 40C located above, which selectively transitions each of these sections 40A, 40B, 40C between the insulator and metal states.

These transitions in the VO2 sections 40A, 40B, 40C correspond to a change in the index of refraction, which as previously described causes the appropriate red, green and blue wavelengths of light to be selectively modulated in order to produce color images. Further, the contrast ratio of the pixel 19 is further enhanced by the protective coating 34 having anti-reflective properties disposed over the VO2 film 40 which is preferably a film of V205. This is important since resonant reflective conditions affect the viewing angle. In order to estimate the viewing angle, the wavelength range (AX) is utilized in which the contrast ratio is high enough. A reasonable contrast ratio is achieved with a spectral range of AX z 60 nm, which provides a thickness variation (L) given by the following equation: n = DA (5) For a VO2film thickness L = 100 nm and an index of refraction n = 2.5, a total viewing angle is achieved which ranges from 35-40°C.

The Phase Transition Display (PTD) according to the present invention has a number of advantages over conventional Flat Panel Displays.

The PTD is superior to LCD Displays in many categories except for, perhaps, power consumption.

The advantages include the use of a Passive Matrix architecture fabricated on the Si substrate, which results in low cost and high yield. The speed of the PTD can be varied in the fabrication process enabling video frequencies and also has a high resolution.

The reflective mode of operation of the PTD versus the transmissive mode of LCDs eliminates the problem of device illumination and minimizes the power required. Color operation is achievable in the PAD as a combination of phase transition and optical resonance, wherein additional filters are not required as in LCDs.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the spirit and scope of the present invention. For example, a number of preferred materials and processes have been described for the Phase Transition Display (PTD) according to the present invention, but other equivalent materials and processes such as evaporation and other vapor deposition techniques are also encompassed by the present invention.