Low work function, stable compound clusters and generation process

Low work function, stable compound clusters are generated by co-evaporation of a solid semiconductor (i.e., Si) and alkali metal (i.e., Cs) elements in an oxygen environment. The compound clusters are easily patterned during deposition on substrate surfaces using a conventional photo-resist technique. The cluster size distribution is narrow, with a peak range of angstroms to nanometers depending on the oxygen pressure and the Si source temperature. Tests have shown that compound clusters when deposited on a carbon substrate contain the desired low work function property and are stable up to 600.degree. C. Using the patterned cluster containing plate as a cathode baseplate and a faceplate covered with phosphor as an anode, one can apply a positive bias to the faceplate to easily extract electrons and obtain illumination.

BACKGROUND OF THE INVENTION 
The present invention relates to electron emission, particularly to 
patterned electron emitting devices, and more particularly to low work 
function, stable compound cluster generation and the patterned deposition 
thereof on surfaces whereby extraction of electrons and obtaining 
illumination is easily accomplished. 
Visual displays of various types are being widely utilized. A considerable 
fraction of the total energy consumption in the United States and other 
technically advanced countries is spent in powering visual displays, such 
as televisions and computer monitors. Flat panel displays are increasing 
in popularity since they are light and small, and are becoming an integral 
part of visual displays, particularly since they are less bulky than 
conventional cathode ray tubes (CRTs). Liquid crystal displays are 
commercially available and are mainly used for portable computers. The 
disadvantages of the liquid crystal display include high energy 
consumption, high cost of production, low intensity, and difficulties 
associated with large diameter display production. Among a few 
alternatives to this technology is the field emission display. This type 
of display relies on the emission of electrons from an array usually made 
up of sharp tips. The ease of electron emission, and therefore the amount 
of energy consumed by the display, depends on the work function of the 
emitter material and the sharpness of the tips. The work function of a 
material is defined as the minimum energy needed to remove an electron 
from a material to a point at an infinite distance away outside the 
material's surface. The production of sharp tips and reliable low work 
function materials has not been achieved successfully and substantial 
effort has been directed to this problem. A low work function is 
considered to be &lt;3.5 electron-volts (eV). Recently, a process and 
apparatus have been developed which enables the deposition of low work 
function material on the field-emission tips and which is described and 
claimed in co-pending U.S. application Ser. No. 09/080,109, filed May 18, 
1998, entitled "Low Work Function Surface Layers Produced by Laser 
Ablation Using Short-Wavelength Photons," assigned to the same assignee. 
The present invention provides an alternative to the sharp tip 
electron-emission devices. The invention involves the production of 
compound clusters with small radius having low work function, which 
enables efficient electron-emission and illumination with low energy 
consumption and ease of fabrication. The low work function, stable 
silicon-based compound cluster generation, and their patterned deposition 
on surfaces enable their use, for example, as cathode material for field 
emission flat panel displays, electron guns, and cold cathode electron 
guns. The low work function compound clusters are produced by 
co-evaporation of solid semiconductor (i.e., Si) and alkali metal (i.e., 
Cs) elements in an oxygen environment, and can be easily patterned during 
cluster film deposition. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide low work function 
compound clusters for field emission devices. 
A further object of the invention is to provide a process for producing low 
work function compound clusters. 
A further object of the invention is to produce compound clusters having 
low work function by co-evaporation of solid semiconductor and alkali 
metal elements in an oxygen environment. 
Another object of the invention is to provide low work function, stable 
compound cluster generation, and their patterned deposition on surfaces. 
Another object of the invention is to provide low work function clusters 
formed by co-evaporation of silicon and an alkali metal in an oxygen 
buffer gas. 
Another object of the invention is to provide a low work function, stable 
silicon-based compound clusters utilizing a process wherein evaporated Si 
atoms coalesce and react with oxygen and an alkali metal to produce 
compound clusters. 
Another object of the invention is to provide a process for producing 
compound clusters, such as Si/Cs/O, on a patterned deposition surface. 
Another object of the invention is to provide patterned compound clusters 
deposited on a substrate having the capability to function as cathode 
material for field emission devices, such as flat panel displays, electron 
guns, and cold cathode electron guns. 
Another object of the invention is to provide silicon-based compound 
clusters which are stable up to about 600.degree. C., have a low work 
function property, and which are easily patterned during cluster film 
deposition. 
Other objects and advantages of the present invention will become apparent 
from the following description and accompanying drawings. Basically, the 
invention involves low work function, stable compound cluster generation, 
and their patterned deposition on surfaces. Compound clusters having low 
work function can be produced by the apparatus and process of the present 
invention, which involves co-evaporation of solid semiconductor and alkali 
metal elements in an oxygen environment. The cluster size distribution is 
narrow, with a peak range of angstroms to nanometers, depending on oxygen 
pressure and the semiconductor source temperature. Compound clusters 
composed of Si/Cs/O deposited on a carbon substrate have been shown to 
have a low work function property by ultraviolet photoelectron 
spectroscopy (UPS), are stable up to 600.degree. C. as measured by x-ray 
photoelectron spectroscopy (XPS), and for certain size range and chemical 
composition, negative electron affinity (NEA) was detected even after 
annealing to 600.degree. C. Also, by an easily patterned technique of 
compound cluster film deposition, it has been demonstrated that the 
compound clusters can easily be deposited in a desired pattern. Using a 
patterned plate containing the compound clusters as a cathode baseplate 
and a faceplate covered with phosphor as an anode, application of a 
positive bias to the faceplate easily extracts electrons and obtains 
illumination. Thus, the patterned compound clusters of this invention may 
be effectively utilized, for example, as cathode material for field 
emission flat panel displays, electron guns, and cold cathode electron 
guns. The production of compound clusters with small radius having low 
work function meet the above-referenced challenges regarding visual 
displays, wherein such have low energy consumption, low cost of 
production, high intensity, and reduce the prior difficulties associated 
with large diameter display production, thereby advancing the state of the 
visual display technology.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention involves low work function, stable compound cluster 
generation, and patterned deposition thereof on surfaces. Compound 
clusters having low work function have been produced by co-evaporation of 
solid semiconductor and alkali metal elements in an oxygen environment. 
The cluster size distribution is narrow, with a peak range of angstroms to 
nanometers (10.ANG. to 10 nm) depending on oxygen pressure (10.sup.-4 Torr 
to 10.sup.-1 Torr) and solid semiconductor source temperature. While the 
experimental verification tests utilized silicon (Si) as the solid 
semiconductor, other materials, such as Ge, GaAs, SiC and other 
semiconductors, could be used; and while the alkali metal used in the 
verification tests was cesium (Cs), other alkali metals, such as K, Li, 
Rb, Fr, and Na may be used. Compound clusters composed of Si/Cs/O 
deposited on a graphite substrate in accordance with the present invention 
were tested for low work function properties' stability, negative electron 
affinity, and patterning capability. The low work function property of the 
clusters was determined by ultraviolet photoelectron spectroscopy (UPS). 
The clusters are stable up to 600.degree. C., as measured by x-ray 
photoelectron spectroscopy (XPS). For a certain combination of cluster 
size range (1 nm to 6 nm) and chemical composition (i.e., Cs concentration 
&gt;10%) negative electron affinity (NEA) was detected even after annealing 
to 600.degree. C. by UPS and electron emission microscopy (FEEM). Testing 
established that a patterned compound cluster film can be easily obtained 
by using a conventional photo-resist technique. Thus, the patterned, low 
work function, and stable compound clusters of this invention may be 
utilized effectively in applications such as cathode material for field 
emission flat panel displays, electron guns, and cold cathode electron 
guns. The compound clusters with small radius having low work function, 
enable low energy consumption, low cost of production, high intensity, and 
reduce difficulties associated with large diameter display production, and 
thus provide a significant advance in the field of visual displays. 
The low work function compound clusters of the present invention are formed 
by co-evaporation, for example, of silicon (Si) and an alkali metal, such 
as cesium (Cs), in an oxygen buffer gas. The Si source may be a heated 
boat positioned in line-of-sight of the substrate. The Si source may also 
be an excimer laser. An alkali metal dispenser directed at the Si source 
provides alkali metal atoms. The dispenser may be a holder connected to a 
power supply. The evaporated Si atoms coalesce and react with the oxygen 
and the alkali metal atoms to produce compound clusters composed of 
Si/Cs/O. The oxygen background gas is maintained at a fixed pressure in 
the range of 10.sup.-4 Torr to 10.sup.-1 Torr. The thus-produced compound 
clusters can be transported toward a target or substrate by convection for 
the purpose of film deposition. The film that is formed on the target or 
substrate, therefore, consists of an ensemble of compound clusters. A 
patterned film of compound clusters can be readily obtained by using on 
the target or substrate a conventional photo-resist technique where the 
clusters form only on openings or bias in the photo-resist, as known in 
the art. 
FIG. 1 schematically illustrates the low work function, compound cluster 
generation and deposition, and apparatus for carrying out the process. As 
shown schematically, the process is carried out in a deposition chamber 
that is originally evacuated to 10.sup.-8 Torr. The apparatus, as shown in 
FIG. 1, comprises a silicon source including an Si evaporator 10 
comprising a coil 11 and a power supply 12, and a silicon boat 13; a Cs 
dispenser 14 directed at the Si source and consisting of an alkali metal 
holder 15 and a power supply 16; an oxygen supply 17 and oxygen control 
means 18; a mechanical shutter 19 having an actuator or driver 20; and 
target, substrate, or sample holder 21 containing a patterned sample 22, 
and having a drive mechanism 23, whereby the target can at least be 
rotated, if desired. Note that Si boat 13 is positioned in line-of-sight 
of the sample 22. 
In operation, the Si boat 13 is heated to a temperature in the range of 
1500.degree. C. to 2000.degree. C. by evaporator 10 to produce Si atoms, 
by evaporation, Cs dispenser 14 is activated to produce Cs atoms at a rate 
of 10.sup.15 -10.sup.17 atoms/sec. over a 5 mm long opening slit on the 
dispenser indicated by arrow 24, and oxygen supply 17 is activated to 
produce an oxygen flow indicated by arrows 25 at a rate of flow or 
background gas of oxygen resulting in 10.sup.-4 Torr to 10.sup.-1 Torr of 
oxygen prior to turning on the Si or the Cs sources; whereby Si atoms, at 
a vapor pressure in the range of 10.sup.-3 -10.degree. Torr, coalesce and 
react with the oxygen and the Cs atoms to produce Si/Cs/O compound 
clusters indicated by arrows 26, which are transported by convection 
toward the patterned target, substrate, or sample 22, and pass through the 
mechanical shutter 19, which can be moved in a linear direction as 
indicated by arrow 27, or in a rotational motion, by driver 19, whereby 
the compound clusters 26 are deposited of the patterned surfaces 28 of 
sample or substrate 22. Thus a patterned substrate containing Si/Cs/O 
compound clusters is produced within deposition chamber 29, and which can 
be utilized, as described above or as in FIG. 2, described hereinafter. 
After deposition, the photo-resist utilized to pattern the surfaces 28 can 
be washed away. 
As shown in FIG. 2, the patterned substrate, produced as described above, 
may be used as a cathode baseplate 30 composed of a substrate 31 and 
patterned Si/Cs/O compound cluster films 32, and a faceplate 33 composed 
of a substrate 34 covered with a phosphor layer 35 is used as an anode. By 
connecting the baseplate 30 and faceplate 33 to a power supply 36, and 
applying a positive bias to the faceplate 33, electrons can be easily 
extracted from the compound cluster films 32 and illumination on the 
phosphor layer 35 obtained. The patterned compound cluster films 32 
usually have clusters in the size range of 1 nm to 6 nm, depending on the 
Si evaporation temperature and/or the oxygen pressure. 
If the Si source temperature was increased to 2000.degree. C. and the 
oxygen pressure increased to 10.sup.-1 Torr, the size distribution of the 
compound clusters would increase to about .gtoreq.10 nm. 
While the invention was experimentally verified by producing Si/Cs/O 
compound clusters, other clusters composed of semiconductor/alkali 
metal/oxygen may be produced. It is understood that the processing will 
vary depending on the materials involved in producing different clusters. 
It has thus been shown that the present invention provides low work 
function, stable compound clusters that can be patterned during deposition 
on surfaces. The low work function, stable compound clusters are produced 
by co-evaporation of solid semiconductor and alkali metal elements, such 
as Si and Cs, in an oxygen environment. The cluster size distribution is 
dependent on oxygen pressure and Si source temperature, and the size 
distribution is narrow, with a peak range of angstroms to nanometers. 
Compound clusters of Si/Cs/O have been shown to be stable up to 
600.degree. C. with a work function in the range of .ltoreq.3.5 eV or even 
NEA (the condition in which the vacuum level is below the conduction band 
in semiconductor and insulator). Patterning during deposition of the 
compound clusters can be easily carried using known techniques. 
While particular embodiments of the compound clusters, deposition 
apparatus, operation sequence, materials, temperatures, parameters, etc., 
have been described or illustrated to exemplify and teach the principles 
of the invention, such are not intended to be limiting. Modifications and 
changes may become apparent to those skilled in the art, and it is 
intended that the invention be limited only by the scope of the appended 
claims.