SHIELDED FIELD EMISSION DISPLAY

This invention was made with government support under Contract

No. DABT-63-93-C-0025 awarded by Advanced Research Projects Agency (ARPA). The government has certain rights in this invention.

Technical Field This invention relates generally to field emission displays, and more particularly, to driving circuit structures in field emission displays.

Background of the Invention

Flat panel displays are widely used in a variety of applications, including computer displays. One type of device suited for such flat panel displays is the field emission display.

Field emission displays typically include a generally planar emitter beneath a display screen. The emitting panel is a substrate having an array of surface discontinuities projecting from an upper surface. In many cases, the surface discontinuities are conical projections, or "emitters" integral to the substrate. Typically, the emitters are grouped into emitter sets where the bases of the emitters in the emitter sets arc commonly connected. A conductive extraction grid is positioned above the emitters and driven with a voltage of about 30V- 120V. The emitter sets are then selectively activated to produce an electric field extending from the extraction grid to the emitters by providing a current path between the bases of the emitters and ground. In response to the electric field, the emitter sets emit electrons.

The display screen is mounted directly above the extraction grid, and it is coated with a transparent conductive material to form an anode biased to about l-2kV. The anode attracts the emitted electrons, causing the electrons to pass through the extraction grid. A cathodoluminescent layer covers the anode and faces the extraction grid to intercept the electrons as they travel toward the l-2kV potential of the anode. The electrons strike the cathodoluminescent layer causing the cathodoluminescent layer to emit light at the impact site. The emitted light then passes through the anode and display screen where it is visible to a viewer.

The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which electrons strike the cathodoluminescent layer, which in turn depends upon the amount of current available to provide electrons to the emitter sets. The brightness of each area can then be controlled by controlling the current flow to the respective emitter set. Thus, by selectively controlling the current flow to the emitter sets, the light from each area of the display can be controlled and an image can be produced. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel." One problem in such field emission displays is consistent control of current to the emitters, especially where driving circuits are integrated into the same substrate as the emitters. Such integrated driving circuits typically include field effect transistors driven by external signals to selectively couple the emitters to ground. The high impedance of such transistors permits little current leakage through the transistors. Consequently, current leaking into or out of substrate may cause charge accumulation that is not dispersed by the integrated driving circuit. Such charge accumulation can have detrimental effects on the operation of the field emission display. For example, where the integrated transistors are intended to be OFF, charge buildup can affect the biasing of the transistors to allow current to bleed through the transistor channel. Such current bleeding through the transistors may provide electrons to the emitters and cause unwanted emission of light.

Summary of the Invention

A field emission display includes an emitter panel formed from a substrate having several emitters on an upper surface of the substrate. An insulative layer surrounds the emitters and supports an extraction grid. The extraction grid is a conductive layer that encircles the emitters and provides a grid voltage to establish an electric field between the extraction grid and the emitters. If the voltage differential between the emitters and the extraction grid is sufficiently high, the resulting electric field causes the emitters to emit electrons.

A transparent plate covered by a transparent conductive coating forms an anode. The anode is positioned above the emitters and a positive voltage on the order of 1-2 kV is applied to the anode to cause it to attract the

emitted electrons. A cathodoluminescent layer covers the transparent conductive anode such that electrons traveling toward the anode strike the cathodoluminescent layer. In response, the cathodoluminescent layer emits light that passes through the transparent plate to be viewed by an observer. Control of current to the emitters is achieved by driving circuitry integrated into the same substrate that carries the emitters. The integrated driving circuitry includes multiple transistors integrated into the substrate and covered by an insulative layer.

A charge shield covers the insulative layer above the integrated driving circuit to provide a protective barrier for the driving circuit. A passivation layer covers the charge shield to protect and insulate the charge shield. The charge shield provides a conductive ground plane between the passivation layer and the insulative layer to bleed charges to ground, thus preventing the charges from affecting the integrated driving circuitry. Additionally, the charge shield forms a conductive plane that terminates electric field within the passivation layer. By terminating the electric fields, the charge shield reduces the effects of transient changes in surface charge that may otherwise couple to the integrated driving circuitry.

Brief Description of the Figures

Figure 1 is a side elevational view in cross-section of a portion of a conventional field emission display with no charge shield.

Figure 2 is a partial schematic, partial diagrammatic view of the portion of the field emission display of Figure 1. Figure 3 is a side elevational view in cross-section of a portion of a field emission display according to the invention, including a charge shield.

Detailed Description of the Invention

As shown in Figure 1, a portion of a conventional field emission display 100 includes a section of an emitting panel 102 beneath a screen 104. The emitting panel 102 is formed on a substrate 106 of single crystal, p-type silicon with a pair of emitters 108 projecting upwardly from the upper surface of the substrate 106. The emitters 108 are known electron emitting structures for field emission displays and are fabricated according to conventional fabrication techniques. One skilled in the art will understand that although only two emitters

108 are shown for clarity of presentation, the number of emitters 108 typically will be much larger than two.

Beneath the emitters 108, a first n+ region 110 is formed in the substrate 106 within an n-region 112. The n regions 110, 112 allow electrical connection to the bases of the emitters 108, as will be described below. An insulative layer 114 of a dielectric material is deposited on the substrate 106.

The insulative layer 1 14 is formed with apertures 113 that surround respective emitters 108. The upper surface of the insulative layer 114 carries a conductive extraction grid 116. The insulative layer 1 14 and extraction grid 1 16 are formed according to known fabrication techniques. A passivation layer 120 covers the extraction grid 1 16 to protect and electrically insulate the extraction grid 1 16.

Within the passivation layer 120 is a conductive line 1 18 that connects the extraction grid 116 to a grid voltage VQ.

As is typical, the screen 104 is above the emitters 108 and the grid 116. The screen 104 includes a glass plate 150 with its inner surface coated with a conductive, transparent material to form an anode 152. A cathodoluminescent layer 154 coats the exposed surface of the anode 152.

In operation, the extraction grid 116 is biased at a grid voltage V^ of about 30- 120V, and the anode 152 is biased at a high voltage V^, such as 1-2 kV. If the emitters 108 are connected to voltage much lower than the grid voltage, such as ground, the voltage difference between the grid 1 16 and the emitters 108 produces a sufficiently intense electric field between the emitters

108 and the extraction grid to cause the emitters 108 to emit electrons according to the Fowler-Nordheim equation. The emitted electrons are attracted by the high anode voltage V^\ and travel toward the anode 152 where they strike the cathodoluminescent layer 154 causing the cathodoluminescent layer 154 to emit light around the impact site. The emitted light passes through the transparent anode 152 and the glass plate 150 where it is visible to an observer.

The intensity of light emitted by the cathodoluminescent layer 154 depends upon the rate at which electrons emitted by the emitters 108 strike the cathodoluminescent layer 154. The rate at which the emitters 108 emit electrons is controlled, in turn, by controlling current flow to the emitters 108. Thus, the intensity of the emitted light can be controlled by controlling the current flow to the emitters 108. Current flow to the emitters 108 is controlled by a driving circuit

109 integrated into the substrate 106. The driving circuit 109 is embedded in the substrate 106 beneath the insulative layer 114 which is, in turn, covered by a

passivation layer 164 to provide additional protection for the driving circuit 109. The passivation layer 164 is a conventionally formed insulative protective layer.

As will be explained below, the n+ region 110 and n- region 112 beneath the emitters 108, in addition to providing a conductive path to the emitters 108, also form a transistor drain and thus are part of the driving circuit 109. To the right of the n- region 112, a portion of a poly layer 123 covering the substrate 106 extends to a second n+ region 124 to form a gate 125 of a field effect transistor 126 positioned over an insulative layer 122. The drain of the transistor 126 is formed by the n-type regions 1 10, 1 12 and is directly connected to the bases of the emitters 108. The source of the transistor 126 is formed by the second n+ region 124, and is connected to conventional current limiting circuitry (not shown), such as a resistor. A gate voltage is applied to the gate 125 through a conductive line 128 embedded in the insulative layer 1 14 and a conductive via 130. The equivalent circuit for the transistor 126 and emitters 108 is presented in Figure 2.

Returning to Figure 1, the conductive line 128 extends from the first via 130 to a second via 132 which connects the conductive line 128 to a third n+ region 134. A second section of the poly layer 123 extends from the third n+ region 134 to a fourth n+ region 138. As with the first portion forming the gate 125, the second section of the poly layer 123 forms the gate 136 of a second field effect transistor 140 positioned over an insulative layer 137. The third n+ region 134 is the source of the second transistor 140 and the fourth n+ region 138 is the drain. A buried isolation region 149 of p+ type material electrically isolates the second n+ region 124 from the third n+ region 134 such that the source of the first transistor 126 is electrically isolated from the source of the second transistor 140. The source of the second transistor 140 is electrically connected through the conductive trace 128 and the vias 130, 132 to the gate 125 of the first transistor 126. The coupling of the transistors 126, 140 is apparent in the circuit diagram of Figure 2. Returning again to Figure 1, an externally supplied row voltage

VR, is applied to the gate 136 of the second transistor 140 through a conductive via 142 and a second embedded line 144. The fourth n+ region 138, which is the drain of the second transistor 140, is connected to an image signal V _j by a third embedded line 146. As can be seen from Figure 2, when the row voltage VR is high, transistor 140 will be ON and will provide the image signal VT to the gate of the first transistor 126. The magnitude of current flowing to the emitters 108 then

corresponds to the amplitude of the image signal Vj applied to the drain of the second transistor 140.

If the row voltage VR is low, the second transistor 140 is OFF, and the initially transferred image signal Vj voltage is retained at the source of the second transistor 140 causing the voltage to be continuously applied to the gate of the first transistor 126. If the gate voltage is sufficiently high, the first transistor 126 will continuously provide current to the emitters 108, causing light to be emitted. If the gate voltage is low, the first transistor 126 will be OFF and no light will be emitted. Thus, when the row voltage VR is high, the intensity of light emission is controlled by the image signal Vp and, when the row voltage VR is low, light is emitted at a level corresponding to the voltage of the image signal V\ immediately before the row voltage VR went low. In typical applications, the image signal Vj is a sample of a video image signal.

The above discussion neglects possible effects on the transistors 126. 140 of electrons emitted from the emitters 108 and effects of the high anode voltage Vy\. Electrons emitted from the emitters 108 can strike the passivation layer 164 and charge the passivation layer negatively. If the electron secondary emission coefficient of the passivation layer 164 is greater than one for the electron impact energy, then the passivation layer can charge positively. The electrons can also cause electron impact ionization of particles either in the gap between the anode 152 and extraction grid 1 16 or at the surface of the anode 152. These ions will then be collected on the passivation layer 120 and 164 and cause it to charge positively. Hence, the electrons may directly or indirectly cause a charge to build up on the passivation layer 164. This charge generates an electric field which can affect the operation of the driving circuit 109.

Additionally, as a consequence of the charge buildup, an electric field E\ is produced within the passivation layer 164 and insulative layer 114. The electric field Ε\ can cause migration of charges, such as free electrons or ion impurities, through the insulative layer 1 14. These charges may cause charge leakage into or out of the substrate 106, especially at regions of the substrate 106 that are uncovered by a metal layer. For example, a portion of the third n+ region at 134 left uncovered by the via 132 is exposed to charge drift through the insulative layer 114.

The electric field E\ is particularly problematic at the third n+ region 134 when the transistor 140 is intended to be OFF. In this condition, the second transistor 140 presents a very high impedance. Consequently, there is no path to bleed current quickly away from the third n+ region 134. If charge leaks

into the n+ region 134, the voltage of the third n+ region 134 may vary, raising the gate voltage of the first transistor 126. In response, the first transistor 126 may no longer be truly OFF, and may allow some current to flow to the emitters 108, thereby causing light to be emitted. Even if charge leakage into the third n+ region 134 does not cause unwanted light emission, the resulting increase in voltage of the n+ region 134 can be detrimental. For example, during extended operation, the voltage may cause aging or breakdown of the integrated components.

An additional effect of surface charge on the passivation layer 164 arises when the surface charge varies dynamically. Transient conditions, such as an increase in electrons due to activation of a local emitter set 108, can cause dynamic changes in the surface charge density. Such variations in surface charge density can cause electric field variations in the passivation layer 164 and insulative layer 1 14 which will affect the integrated driving circuit 109. In response, the voltage at the gate of the transistor 126 may vary, causing variations in current flow to the emitters 108.

The present invention, as embodied in the portion of a field emission display 200 shown in Figure 3, addresses the problem of electric fields in the insulative layer 114 and charge leakage into the substrate 106. Several elements of Figure 3 correspond directly to the elements of the display 100 of Figure 1 and are numbered identically. For example, the screen 104, extraction grid 1 16, emitters 108 and driving circuit 109 are identical to those of the conventional display of Figure 1.

The display 200 of Figure 3 differs from the prior art display 100 of Figure 1 primarily in the use of a charge shield 162 between the passivation layer 164 and the insulative layer 1 14. The charge shield 162 is formed according to conventional integrated circuit fabrication techniques from a conductive material, such as a metallization layer, deposited atop the insulative layer 114. The addition of the charge shield 162 does not change the interconnection of the elements. Therefore, the equivalent circuit of Figure 2 applies equally to the displays of Figures 1 and 3.

The charge shield 162 is connected to ground and thus provides a continuous ground plane beneath the surface charge buildup on the passivation layer 164. The ground plane of the charge shield 162 terminates the electric field E2 caused by charge buildup on the surface of the passivation layer 164. Thus, although the electric field E2 may be high, the electric field E3 in the insulative layer 114 between the substrate 106 and the charge shield 162 is very small.

Because the charge shield 162 blocks the electric field E2, any charge drift within the passivation layer 164 caused by the electric field E2 will be bled to ground. Within the insulative layer 114 beneath the charge shield 162, the charge shift will be minimal due to the low intensity of the electric field E3 between the charge shield 162 and the substrate 106.

In addition to reducing the effects of charge drift, the charge shield 162 also reduces the effects of dynamic changes in the surface charge by terminating the electric field E2. Dynamic variations in surface charge within the passivation layer 164 will affect only the electric field E2, while the electric field E3 in the insulative layer 114 will be substantially unaffected. Consequently, effects of changes in the electric field E2 will be directed to the ground plane of the charge shield 162, not the driving circuit 109.

From the foregoing, it will be appreciated that, although an exemplary embodiment of the invention has been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, various alternative driving circuits for controlling emitter currents may benefit from the effects of the charge shield 162. Moreover, although the preferred embodiment has been described as including the passivation layer 164, in some applications the passivation layer 164 may be eliminated. The exposed conductor will then provide a path for removal of any charge on the surface. Also, although the charge shield 162 has been described as being connected to ground in the preferred embodiment, the charge shield 162 may also be connected to a different voltage. Accordingly, the invention is not limited except as by the appended claims.