Field emission displays with reduced light leakage

Abstract
Semiconductor devices may be made by forming a silicided layer on a silicon material such as that used to form the extractor of a field emission display. The silicided layer may be self-aligned with the emitter of a field emission display. It the silicided layer is treated at a temperature above 1000° C. by exposure to a nitrogen source, the silicide is resistant to subsequent chemical attack such as that involved in a buffered oxide etching process.
Description




BACKGROUND OF THE INVENTION




This invention relates broadly to semiconductor devices, and to methods for making such devices and particularly to display devices such as field emission displays.




Because of their exceedingly small size, it is often very difficult to align one feature in a semiconductor device with another. The problem arises from the fact that once one feature is in place on a semiconductor layer it is very hard to subsequently align another feature with that feature. For example, it may be difficult to align an etching mask with the first feature because there is no physical way to guide the mask into alignment with the first feature.




One way of overcoming these problems is to self-align two features to one another. For example, one self-alignment technique is to use the gate electrode as a mask for the subsequent source/drain ion implantation. Because the gate electrode acts as a mask for the implantation, the source and drain regions become aligned to the gate electrode.




Self-alignment may be advantageous since it may allow devices to be made smaller because some misalignment tolerances need not be included in the design. These misalignment tolerances may require the device to be larger to compensate for the misalignment between features.




Moreover, in many applications the misalignment influences the operating effectiveness of the device. Thus, it is highly desirable to coordinate features such that subsequent features are aligned with previous ones.




Another example of a self-alignment technique applied to semiconductor devices arises in the field of field emission displays. Field emission displays use electron emission from an emitter to illuminate a screen which displays a corresponding image for the user. These devices can be used in a variety of electronic displays such as laptop computer displays.




In field emission displays it is desirable to align the extractor or grid to the emitter. The extractor, situated between the emitter and the screen, is charged such that it can extract electrons from the emitter and accelerate them toward the screen.




One technique for self-aligning the extractor to the emitter is to use a chemical mechanical polishing process. After the emitters are formed they may be covered with a generally conformal layer of oxide followed by a generally conformal layer of a conductive material such as silicon. Because of the conical shape of the emitter, the portion of the dielectric and conductive layers over the emitter, forms a hump or hillock on the semiconductor surface. By using a chemical mechanical polishing process the hillock can be removed down to the oxide layer, leaving in effect, an opening in the conductive layer which is self-aligned to the emitter. The conductive layer opening is self-aligned to the emitter because it was the shape of the emitter itself, in cooperation with the chemical mechanical polishing process, which defined the opening in the conductive layer. The conductive layer ultimately becomes the extractor or grid associated with the emitter after a portion of the oxide layer between the extractor and the emitter has been removed.




An emitter, formed over a junction in a semiconductor layer, can be controlled to emit electrons through its tip. These electrons pass from the emitter tip through the opening in the extractor and are accelerated by the extractor potential towards a screen. When the electrons hit the screen they cause luminescence which the user perceives as an image.




Sometimes field emission displays suffer from a leakage problem which causes lighter regions to appear on the screen. A leakage problem may arise from light that enters the junction under the emitter in one of several ways. Light can enter this region by being reflected back from the screen towards the underlying semiconductor layer. Also light from a variety of sources outside the display may enter the junction through the opening in the extractor and by actually passing through the extractor itself.




As a result of photoelectric affects, electrons may be created in the junction which are emitted through the emitter, operating the emitter even when the emitter is effectively turned off. Similarly, leakage from the junction to the underlying substrate may adversely affect the operation of the display.




The active matrix driving scheme of field emission displays requires integration of silicon devices and tips on a single silicon substrate. Such a scheme can be easily implemented by using MOS devices as shown in FIG.


1


. Although this is a very efficient way of manufacturing small area field emission displays, it also suffers from serious drawbacks.




The main problem with the scheme illustrated in

FIG. 1

is the fact that photons from the phosphor anode can easily pass through the extraction grid and consequently generate electron hole pairs in the MOS devices. Generated electrons will be attracted by the higher positive field in the tip area and cause a bright background. This problem is even more severe for color displays where three colors, red, blue, and green are used as the main ingredients of phosphor anodes. If the background light is not extremely dim, it can turn the wrong color on, cause cross talking among different colors, and distort the image quality of a display.




Light sensitivity is basically originated from the presence of the pn junction right under the tip, where generated electron hole pairs can be separated and find their paths to the tip area and substrate, respectively. This disclosure describes a new technique which can significantly reduce the light leakage problem.




SUMMARY OF THE INVENTION




In accordance with one aspect of the present invention, a process for forming a field emission display involves forming an emitter. A conductive layer is then formed over the emitter with an opening in the conductive layer being self-aligned to the emitter. A second layer is formed over the conductive layer with an opening in the second layer that is also self-aligned to the emitter.




In accordance with another aspect of the present invention, a process of forming a field emission display involves forming a grid with an opening self-aligned to the emitter. A light blocking layer is self-aligned on the grid to the opening in the grid.




In accordance with still another aspect of the present invention, a process for forming a field emission display with an emitter for emitting electrons which impact a screen, involves forming a silicon layer with an opening that is self-aligned to the emitter. A metal layer is deposited on the silicon layer and a silicide is formed where metal contacts the silicon layer. The metal not in contact with the silicon layer is then removed to self-align the silicide layer to the emitter.




In accordance with yet another aspect of the present invention, a field emission display includes an emitter and an overlaying extractor. A light blocking layer self-aligned to the emitter is formed on the extractor.




In accordance with still another aspect of the present invention, a method for forming a field emission display having an emitter that emits electrons which impact a screen, includes the step of forming a silicided grid with an opening over the emitter. The grid is treated to resist damage from an ensuing oxide etch.




In accordance with but another aspect of the present invention, a process of forming a field emission display includes the step of forming a grid with an opening self-aligned to an emitter. A light blocking layer is self-aligned on the grid to the opening in the grid. The light blocking layer is treated to resist damage from an ensuing oxide etch.




In accordance with another aspect of the present invention, a semiconductor device is made by the process that includes the step of forming a silicide. The silicide is exposed to a source of nitrogen at a temperature over 1000° C.




In accordance with still another aspect of the present invention, a computer system comprises a computer and a field emission display. The display includes a screen, an emitter and an extractor arranged such that the emitter can emit electrons through the extractor to impact the screen. The extractor is treated to prevent light from passing through the extractor.











BRIEF DESCRIPTION OF THE DRAWING





FIG. 1

is a schematic depiction of a conventional field emission display emitter;





FIG. 2

is an enlarged, schematic cross-sectional depiction of a conventional field emission display of the type shown in

FIG. 1

;





FIGS. 3



a


-


3




f


are an enlarged, cross-sectional view showing the process steps utilized in forming one of the emitters in a field emission display;





FIG. 4

is a perspective view of the field emission display in use in a computer system;





FIG. 5

is a graph of penetration depth of photons versus wavelength for silicon; and





FIG. 6

is a graph of penetration depth of photons versus wavelength for TiSi


x


N


y


.











DESCRIPTION OF THE PREFERRED EMBODIMENTS




Referring to the drawing wherein like reference characters are used for like parts throughout the several views, a field emission display


10


includes an emitter


12


, an extractor or grid


14


, and a screen


16


, as shown in FIG.


1


. The emitter


12


emits a stream of electrons past the positively charged extractor


14


so that they impact the screen


16


. Since the screen


16


is electroluminescent, the impacting electrons cause an image to appear to the user on the opposite side of the screen


16


.




The emitter


12


, which may be formed on a semiconductor layer, is situated on the drain electrode


18


of a transistor


20


which includes a grounded source


22


and a gate


24


. Control over the emitter


12


can be maintained by controlling the potential on the gate in accordance with conventional field effect transistor technology.




Referring to

FIG. 2

, the display


10


may be implemented in the semiconductor layer


26


which advantageously may be formed of p-type silicon. A doped region


28


formed in the semiconductor layer


26


may be of a conductivity type opposite to the conductivity type of the semiconductor layer


26


. Thus, the doped region


28


, which acts as the drain of the transistor


20


, may be formed by modifying the p-type semiconductor material


26


using conventional techniques to form an n-type doped region


28


.




A plurality of emitters


12


, situated directly on top of the semiconductor layer


26


, are located in openings


30


in a surrounding oxide layer


32


. Over the oxide layer


32


and extending slightly into the openings


30


is the grid or extractor


14


having openings


34


. Spaced above the extractor


14


is the screen


16


. Exemplary patents on field emission displays include U.S. Pat. Nos. 5,585,301, 5,525,865, 5,410,218, 5,151,061, 5,186,670 and 5,210,472, hereby expressly incorporated by reference herein.




The screen


16


is designed to shine light upwardly away from the display


10


to be perceived by the user. However, light from screen


16


also is reflected downwardly, together with light from the external surroundings. This downwardly directed light may impact the doped region


28


in one of two ways. It may pass through the openings


34


in the extractor


14


or it may pass through the extractor


14


and the dielectric layer


32


to impact the doped region


28


. Light impacting the doped region


28


creates a photoelectric effect wherein mobile electrons may either pass from the doped region


28


into the substrate


26


, as indicated by the arrow “c”, or they may be drawn into the emitter


12


and pass upwardly towards the screen


16


, as indicated by the arrows “a” and “b”. Either of these effects is generally undesirable.




Ideally, the field emission display


10


is controlled by the conducting state of the transistor


20


as controlled by its gate bias. As a result of leakage created by stray light sources, the field emission display may not operate in accordance with its design parameters. This may include the possibility that display light may appear on the display


16


when the transistor


20


is in the “off” condition.




Conventionally the extractor


14


is made of silicon. Silicon passes some light which may adversely affect the operation of the display


11


. The silicon forming the extractor may be made more opaque to light by coating it with appropriate materials to prevent light passage through the extractor


14


.




The predominant source of leakage is due to light that passes through the extraction grid


14


, as opposed to the tip or emitter


12


. For example, for 100 μm


2


of cavity, tips


12


may occupy 2-3 μm


2


of the area and are as light sensitive as the material forming the grid. In other words, if grid


14


is replaced with silicide materials, light sensitivity can be improved by a factor of


43


.




Certain silicided silicon materials are sufficiently opaque to prevent light entering the field emission display from passing through the extractor


14


and reaching the semiconductor junction


28


. The inventors of the present invention have appreciated that approximately 500 angstroms of titanium silicide (TiSi


x


) or titanium silicide nitride (TiSi


x


N


y


) are sufficiently opaque to light to prevent undesirable photon induced leakage. Those skilled in the art will appreciate other silicided materials which are also sufficiently opaque to light to prevent adverse leakage including silicides of tungsten, cobalt, niobium, and molybdenum.




Light sensitivity can be reduced if the extraction grid is formed from materials with a large extinction coefficient. Since a=4πK/λ (where a is the absorption coefficient, K is the extinction coefficient, and λ is the wavelength), a high K results in a high absorption coefficient and less light penetration (penetration depth is inversely proportional to the absorption coefficient). Photons penetrate silicon thickness of approximately 10,000 Angstroms at wavelengths of 400 to 800 nanometers (nm.) as shown in FIG.


5


. In comparison, photons penetrate through only about 120 to 180 Angstroms of titanium silicide nitride (TiSi


x


N


y


) at wavelengths of 200 to 800 nms. as shown in FIG.


6


.




A process for forming an improved device of the type shown in

FIG. 2

, with reduced light leakage, many utilize chemical mechanical polishing techniques disclosed for example in U.S. Pat. No. 5,229,331 to Doan et al. hereby expressly incorporated by reference herein. The aforementioned patent describes the technique for forming the emitters


12


, shown on a smaller scale which depicts a single emitter in

FIG. 3



a,


on the substrate


26


. After the conical emitter


12


has been formed, the substrate and the emitter


12


may have a dielectric layer


40


formed over them by any conventional technique. For example, the dielectric layer could be formed of oxide which is grown or deposited. On top of the dielectric layer


40


may be deposited a silicon layer


42


. The silicon layer


42


would conventionally be doped to make it conductive.




The structure shown in

FIG. 3



b


is then subjected to chemical mechanical polishing in accordance with the aforementioned patent to create the structure shown in FIG.


3




c.


A hillock or hump


43


formed by the imposition of the emitter


12


is polished away to form a flat surface having an extractor


14


with an opening


34


. As discussed above, the opening


34


is self-aligned to the emitter


12


.




Next, a metal layer


45


is deposited over the entire structure, as shown in

FIG. 3



d.


Advantageously the metal is titanium since titanium has good opaqueness to light energy. For example, 500 angstroms of titanium may be sputter deposited.




After deposition, the titanium is heated to form a titanium silicide


44


. The formation of the silicide may be done by a rapid thermal process using conventional rapid thermal annealing equipment. For example, a temperature of 600° C. for thirty seconds in a nitrogen atmosphere supplied at three standard liters per minute (SAM) is satisfactory. A silicide is formed in all areas where the metal contacts silicon. As shown in

FIG. 3



d,


this means that the top and sides of the silicon layer


42


have a silicide


44


formed thereon. However, silicide is not formed where there is exposed dielectric layer such as in the opening


34


in the layer


42


. In particular, after the heating step, the metal that was situated on top of the oxide


46


remains in its original unsilicided form.




Thus, the structure shown in

FIG. 3



e


is formed by etching the remaining metal with a suitable etchant. One suitable etchant is an ammonia-peroxide solution, NH


4


OH:H


2


O


2


:H


2


O in a ratio of 1:1:5 for three minutes at 65° C. This etchant does not attack the silicide. It only attacks the metal so that the metal over the oxide


46


(as well as any other unreacted metal) is removed and only silicide remains.




In the next step, the silicide


44


on the surface of the layer


42


is treated to prevent damage from an ensuing oxide etch step. Namely a titanium silicide nitride is formed by heating the silicide to a temperature in excess of 1000° C. Advantageously a temperature of 1050° C. for ninety seconds is utilized using conventional rapid thermal annealing equipment. If the heating step is done in an atmosphere containing nitrogen, such as ammonia (NH


3


), a nitrided silicide is formed. For example, the ammonia may be supplied at ten standard liters per minute (SLM).




At temperatures of 1000° C. or less, the nitride process is not effective in forming a layer which is resistant to ensuing oxide etch steps. While the exact physical and chemical processes involved are not known, it is possible that the higher temperature is more effective in forming more nitrogen reaction sites on the silicide and increasing the amount of reacted nitrogen in the silicide which is effective in preventing chemical attack.




The structure shown in

FIG. 3



e


is then subjected to a buffered oxide etch (BOE). This etch would normally damage silicided material but the preceding treatment in the nitrogen atmosphere at elevated temperature results in the structure which is resistant to the oxide etch. As a result of the oxide etch, the opening


30


is formed in the oxide


32


surrounding the emitter


12


.




The silicidation process is self-aligned to the emitter


12


since the silicon opening


34


was self-aligned by the chemical mechanical processing to the emitter


12


and the silicide


44


is self-aligned to both the silicon opening


34


and the emitter


12


. In this way a light blocking layer which is self-aligned to the emitter can be formed which is effective in blocking light and preventing erroneous device operation, without requiring extensive additional processing steps and/or critical alignments. Moreover, by treating the silicide to prevent damage in subsequent oxide etching steps, the silicide is not substantially removed. Advantageously the extended temperatures significantly reduce the exposure of the silicide


44


to attack during ensuing oxide etch operations.




When an array of emitters


12


are formed and driven in accordance with the aforementioned patent incorporated by reference, images can be caused to appear on the screen


16


which may be perceived by the user. The screen


16


may be utilized, for example, as the display screen in a laptop computer


60


, as shown in FIG.


4


. The operation of the computer system is improved by preventing erroneous operation of the screen in response to stray light effects.




While the present invention has been described with respect to a limited number of preferred embodiments, those skilled in the art will appreciate a number of modifications and variations therefrom and it is intended that the appended claims cover all such modifications and variations that fall within the true spirit and scope of the present invention.



Claims
  • 1. A method for forming a field emission display comprising the steps of:forming an extractor; forming a coating on said extractor to prevent light from passing through said extractor; and treating said coating to make said coating resistant to chemical attack.
  • 2. The method of claim 1 including the steps of forming a silicided extractor and exposing said silicided extractor to a nitrogen source at a temperature of over 1000° C.
  • 3. The method of claim 2 including the step of removing oxide from around said extractor to expose an emitter.
  • 4. The method of claim 3 wherein said removing step includes the step of using a buffered oxide etch that also contacts said coating.
Parent Case Info

This is a divisional of prior application Ser. No. 09/256,882 filed Feb. 24, 1999 now U.S. Pat. No. 6,133,056 which is a divisional of 08/922,871 filed Sep. 3, 1997 now U.S. Pat. No. 5,956,611.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

This invention was made under Government support under Contract No. DABT63-93-C-0025 awarded by Advanced Research Projects Agency (ARPA). The Government has certain rights in this invention.

US Referenced Citations (4)
Number Name Date Kind
4701349 Koyanagi et al. Oct 1987
4772571 Scovell et al. Sep 1988
5103272 Nishiyama Apr 1992
5319279 Watanabe et al. Jun 1994