PROCESS TO MODIFY WORK FUNCTIONS USING ION IMPLANTATION

Abstract: The work function of electron emitters can be modified by forming a modifying layer at the surface using low energy ion implantation, in a controlled environment, placing chosen elements below the surface of electron emitters as Cs implanted in Si(100) at four different doses illustrates. Sometimes implanted species are deep enough that they do not react with the atmosphere during subsequent low-temperature processing. Then, species implanted in the emitting surfaces are segregated using elevated temperature treatment of the emitters in vacuum and/or reactive gases. The implanted ions modify the work function at the surface, via thin layers of the implanted species on top of the emitter surfaces, or compounds or alloy layers at the surface of the emitters. Depending on the implanted species, the initial emitter material, and the environment, these layers can either increase or decease the work function of the emitter.

Description: PROCESS TO MODIFY WORK FUNCTIONS USING ION IMPLANTATION The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. BACKGROUND OF THE INVENTION The present invention relates to electron emitters, particularly to modifying the work function of an electron emitter, and more particularly to a process using ion implantation to reduce or increase the work function of electron emitting surfaces. In certain fields of use it has been determined that modifying the work function of an electron emitter surface (e.g., semiconductor, metal, alloy or compound) is beneficial. For example, in field emission video displays, reducing the work function of the emitters reduces the grid operating voltage thereof, thereby permitting construction of the displays with lower power requirements, higher image resolution, and increased brightness. Furthermore, in field emission video displays and other devices, such as electron gun tubes, increasing the work function of some surfaces functions to suppress undesirable emission of electrons. A need exists for a process to reduce the work function of field emitters and to increase the work function of other surfaces. It is well known that the emission of electrons by a material can be modified by adding dopant materials to that of the electron emitter. Ion implantation is commonly used to dope materials in a uniform manner laterally. Additionally, ion implantation can be used to control the composition as a function of depth via multiple implants using different energies and an appropriate fluence (ions/cm^) at each energy. G.S. Tompa, W.E. Carr, and M. Seidl, "Cesium Coverage on Molybdenum due to Cesium Ion Bombardment," Applied Physics Letters & (16), 1048-50 (1986) describes implantation of 10-500 eV Cs+ ions into Mo to doses of 4xl0 12 - 2xl0 16 Cs + /cm 2 . For Mo the minimum work function of about 1.6 eV occurred for 100 eV Cs + and increased to a plateau for 500 eV Cs + . The minimum work function was obtained for doses above lxlO 1 ^ Cs + /cm 2 . The work did not show the formation of Cs-O-metal compound layers nor a process involving diffusion and /or segregation of Cs to the surface. J.P. Girardeau-Montaut, C. Girardeau-Montaut, M. Afif, A. Perez, and S.D. Moustaizis, "Enhancement of Photoelectric Emission Sensitivity of Tungsten by Potassium Ion Implantation," Applied Physics Letters ££ (15), 1886-8 (1995) describes implantation of 30 keV K+ into W to a dose of 3x10 6 K+/cm 2 . The work did not show the formation of K-O-metal compound layers nor a process involving diffusion and/or segregation of Cs to the surface. A.E. Souzis, H. Huang, W.E. Carr, and M. Seidl, "Catalytic Oxidation of Silicon by Cesium Ion Bombardment," J. Appl. Phys. 6_9_ (1), 452-8 (1991) describes growth of oxide layers on Si by bombarding surfaces with 20-2000 eV Cs + in θ2 pressure of IO -9 to IO -6 Torr. After heating to 600°C, Cs is removed, leaving a clean Siθ2 film. Using 20 eV Cs + , lμA/cm 2 , 1 hr (i.e., 2.25xl0 16 Cs + /cm 2 ) in 5xl0 -9 Torr of θ2 gave surface with work function of <. 1.3 eV. The work was not directed at producing a work function modifying layer; the Cs ions were used to enhance the growth rate of the oxide layer and were then removed. The work did not show a process involving diffusion and /or segregation of the Cs to the surface nor the formation of compound layers by implantation into an oxide layer or by treatment in a reactive gas after implantation. SUMMARY OF THE INVENTION It is an object of the present invention to modify the work function of electron emitters. A further object of the invention is to provide a process for modifying the work function of electron emitters. Another object of the invention is to provide a process to modify emitter work functions by ion implantation. A further object of the invention is to provide a process involving ion implantation of selected species to either increase or reduce the work function of electron emitters. Another object of the invention is to provide a process for modifying the work function of an electron emitter by low-energy implantation of chosen elements below the surface of the emitter, and segregation of the implanted species to the emitting surface using thermal treatment in a controlled environment. A further object of the invention is to provide a process for modifying electron emitter work function by producing thin layers of an implanted species on top of the emitter surface, or forming a compound or alloy layer at the surface of the emitter. Another object of the invention is to provide a process for forming a low work function layer through ion implantation in a reactive gas environment to form a layer consisting of elements of the implant, the substrate and the gas. Other objects and advantages of the present invention will become apparent from the following description. Basically, the invention is directed to modifying the work function of electron emitters by ion implantation. The work function of the surface is modified, either increased or decreased, by creating a work function modifying layer of implanted ions at the surface, by implanting the ions and by binding the ions to the surface. The ions are implanted below the surface, at a depth determined mainly by the ion energy. For ions of lower energy, which are deposited very near the surface, the ions may be bound at the surface, e.g. by exposure to an oxidizing or other reactive atmosphere, or by compounding or alloying the elements, to form the work function modifying layer from the elements of the substrate, implant, and/or gas. For ions of higher energy, which are deposited at a depth too great to immediately form the work function modifying layer, the electron emitter with implanted ions may be, if needed, subjected to heat treatment to segregate the ions, i.e. cause ion diffusion toward the surface, where the ions may be similarly bound, compounded or alloyed at the surface to form the work function modifying layer. The above-described need is satisfied by the present invention which utilizes ion implantation, whereby the work function of electron emitters can be modified, either decreased or increased, depending on the implanted species, the initial emitter material, and the controlled environment (vacuum, reactive gases, atmospheric air) in which the ion implantation and post-implant heat treatment are carried out. The process of this invention results in either thin layers of the implanted species on top of the emitter surfaces, or the formation of compounds or alloy layers at the surface of the emitters. This enables enhancement or suppression of electron emission from surfaces, such as in vacuum micro-electronics. For example, implantation of alkali elements (alkali metals) into a silicon or molybdenum surface with exposure to oxygen-bearing gases forms a compound surface layer resulting in reduction of the work function of the silicon and molybdenum surfaces. A first embodiment of this invention involves: 1) low- energy ion implantation of selected elements below the surface of electron emitters, 2) segregation of the implanted species to the emitting surfaces using thermal treatment in a controlled environment (e.g., vacuum, or reactive gases including atmospheric air), and 3) formation of a work function modifying layer, made of either: a) thin layers of the implanted species on top of the emitter surfaces, or b) compounds or alloy layers at the surfaces of the emitters. Depending on the implanted species, the initial emitter material, and the controlled environment, these layers could either increase or decrease the work function of the emitter. A second embodiment is similar to the first except that it essentially omits step 2) segregation of the implanted species to the surface. This embodiment can be used when the ion implantation is sufficiently near the surface to permit formation of the work function modifying layer. The work function modifying layer can be formed by several processes, including 1) direct implantation of the ions into a surface oxide or other compound or alloy on the material, 2) implantation of the ions in the presence of a partial pressure of a reactive gas (e.g. oxidizing gas), and 3) modification, including oxidation, of the surface after implantation. For example, reducing the work function in silicon and molybdenum surfaces by forming a compound or alloy layer can be accomplished using implanted alkali elements (alkali metals), i.e., Li, Na, K, Rb and Cs, and exposure to oxygen-bearing gases. The processes of this invention enable enhancement or suppression of electron emission from surfaces in vacuum micro¬ electronics, and as pointed out above, as an example, permit operation of field emission video displays at greatly reduced grid operating voltages. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Figs. 1A-C illustrate processing steps for changing work function by ion implantation followed by segregation of the implanted ions. Figs. 2A,B illustrate alternate processing steps for changing work function by low energy ion implantation without segregation of the implanted ions. Fig. 3 shows the work function of silicon as a function of implanted 35 keV cesium ion fluence. Fig. 4 shows both cesium concentration at the surface and work function as a function of annealing temperature for one fluence of Fig. 3. DETAILED DESCRIPTION QF THE INVENTION The invention is directed to a process to modify the work function of electron emitters, and enables enhancement or suppression of electron emission from surfaces. According to the invention, a work function modifying layer is formed at the electron emitting surface by ion implantation and immobilization of the implanted atoms at the surface. The immobilized atoms decrease or increase the work function, depending on the atoms. The atoms are immobilized by binding the atoms to the surface or by forming compounds or alloys at the surface. The immobilized layer of atoms leads to a stable, changed work function of the surface. The invention includes various related embodiments for preparing a low (or high) work function electron emitting surface. There are two basic steps in all embodiments, implantation and formation of the work function modifying layer. In some instances the two steps can be performed essentially simultaneously. Various specific treatment steps can be used to form the work function modifying layer, including preparing (e.g., oxidizing) the surface of the material prior to implantation, performing the implantation in a reactive (e.g. oxidizing) environment, or treating (e.g., oxidizing) the surface after implantation. The implantation step can also be carried out under various controlled conditions (e.g. gaseous environment). In some embodiments it is also necessary to diffuse or segregate the implanted element to the surface. In a first step, shown in Fig. IA, low-energy implantation of ions of chosen elements is done with a mean depth of penetration below at least one atom layer of the surface of an electron emitter at temperatures at which the implanted species is not mobile in the host material and at depths such that essentially all the implanted element is inside the surface of the material. As shown, the implanted element concentration profile has a peak at a depth Rp. The implanted species are segregated (diffused) to the emitting surfaces using thermal treatment in a controlled environment and a work function modifying layer is formed. As shown in Fig. IB, the as-implanted material from Fig. IA is heated in a controlled environment of oxidizing or other reactive gases (e.g. chlorine), including atmospheric air, to diffuse and to segregate the implanted element near the surface by forming a compound or alloy layer at the surface. As shown, the work function modifying layer, formed of a compound or alloy of the implanted species, the host material, and the gas environment, has a thickness Xi. Alternatively, as shown in Fig. IC, the as-implanted material from Fig. IA is heated in a vacuum or other environment free of significant reactive gases to cause the implanted element to diffuse and to segregate at the surface. The implanted element forms a thin layer on top of the host material, and /or can also form a compound or alloy layer with the elements present in the host material, including, for example, oxygen present initially in a surface oxide. As shown, the work function modifying layer, formed of a layer of the implanted species, or of a compound or alloy of the implanted species and the host material, has a thickness X 2 . The implantation can also be done at depths such that all the implanted element is essentially inside the surface of the host material, but at temperatures at which the implanted element and /or desirable elements of the gaseous environment are mobile in the host material and can diffuse and segregate during the implantation process itself. Thus the intermediary product of Fig. IA is not formed. If the implantation is done in a reactive environment as described above with Fig. IB, a similar compound or alloy work function modifying layer as shown in Fig. IB is formed. If the implantation is done in a vacuum or nonreactive environment as described above with Fig. IC, a similar thin layer of implanted element or alloy or compound as shown in Fig. IC is formed. The segregated implanted ion layer thus leads to either: a) thin layers of the implanted species on top of the emitter surfaces, or b) the formation of compounds or alloy layers at the surfaces of the emitters. Depending on the implanted species, the initial emitter material, and the controlled environment, these layers will either increase or decrease the work function of the emitter. In further embodiments, a work function modifying layer is formed without the need to carry out the segregation step. Low- energy implantation of ions of chosen elements is done closer to the surface of an electron emitter than in the above embodiments by using lower energy ions, and a work function modifying layer is formed by a variety of specific techniques. As shown in Fig. 2A, a compound (e.g., oxide) layer of thickness X 3 is present on the host material before implantation. The implanted element is all essentially within the initially present surface compound and forms a compound or alloy layer, of thickness X 3 , with the compound and the other elements in the host material. Post-implant heating or annealing may help to homogenize this alloy or compound layer, changing depth profile 10 to depth profile 12, as shown in Fig. 2A. The host material may not have an initial compound (e.g., oxide) layer, and the implantation may be carried out in the presence of a partial pressure of a desirable (e.g., oxidizing) gas. The implanted element is concentrated at the surface, either by low energy implantation or by higher energy implantation followed by removal of the top surface of the host material by sputtering or other suitable process. The implantation process itself may sputter away the top surface and /or enhance the diffusion of the desirable gas elements. An alloy or compound layer with depth profile 14 is thus formed, which can be further heated or annealed to form depth profile 16, as shown in Fig. 2B. In a further alternative to the processes described with Figs. 2A,B, ions can be similarly implanted into a material with no surface compound (e.g. oxide) and without a reactive atmosphere. Post-implant reaction (e.g., oxidation) can be carried out by exposing the surface to reactive (e.g., oxygen) bearing gases at appropriate temperatures and pressures. The results, with possible further heating or annealing, are as shown in Fig. 2B. The implanted ion layer thus leads to the formation of compounds or alloy layers at the surfaces of the emitters. Depending on the implanted species, the initial emitter material, and the controlled environment during the step of implantation, these layers will either increase or decrease the work function of the emitter. If the work function of the surface created by the segregation process is not stable in air, then during and after the thermal treatments the emitters must not be exposed to the atmosphere. In the case of video display units, this means that the thermal treatment should occur during vacuum preparation, assembly, and sealing of the display. An example of such a process is the implantation of cesium into silicon or a metal followed by thermal treatment in a vacuum leading to segregation between a fraction of an atomic monolayer and a few atomic monolayers of cesium to the surface of the silicon or metal. It has been found that the work function can be reduced by forming compound or alloy surface layers involving implanted alkali metals and oxygen. The work function of surfaces (e.g., silicon and molybdenum) can be reduced dramatically by using ion implantation of alkali metals (i.e., Li, Na, K, Rb, and Cs) into the surfaces as a step leading to the formation of a compound surface layer. This compound surface layer forms the work function modifying layer. Ion implantation techniques are well known in the art and thus a description thereof is deemed unnecessary. The compound surface layer consisting of the implanted alkali (metal) element, the elements of the surface of the material into which the ions were implanted, and oxygen or other desirable elements can be obtained by several processes, including 1) direct implantation of the alkali element into a surface oxide (e.g., silicon-oxide) on the material, 2) implantation of the alkali element in the presence of a partial pressure of an oxidizing or other desirable gas (e.g., oxygen at 10"5 torr), and 3) oxidation of the surface after implantation. Oxidation of the implanted surface can be obtained by exposure of the surface to oxygen bearing gases at appropriate pressures (e.g., O2 and H2O in atmospheric air) and temperatures (e.g. room temperature). During experimental verification of the process of this invention, it has been shown that implantation of 35-keV Cs+ into monocrystalline silicon (to a dose of about IO Cs/cm 2 ) in a vacuum of about lxl0"6 torr (mostly water vapor) and subsequent exposure to atmospheric air for a time period of a few days to a few weeks and at room temperature, reduced the work function of the silicon from about 4.5 eV to about 2.0 eV. Figure 3 shows the work function of Cs implanted in Si(100) at four different doses. The surface compound or alloy has been shown to consist of cesium, silicon, and oxygen. The work function of this compound surface layer has been shown to be stable during prolonged exposures (e.g., weeks) to atmospheric air and during heating in vacuum to temperatures up to 450°C . Figure 4 shows both Cs atomic percentage at the surface and work function as a function of temperature for Cs implanted into Si(100) with a dose of 9x10*6 Cs/cm 2 with annealing for a half hour at each temperature. In this case, sputtering during implantation resulted in an increased Cs concentration in the surface layer. These stability features are critical for applications to the modification of silicon (and metal) field emitters in micro-electronic devices that rely on field emission of electrons. Field emission flat panel displays are an important class of such devices. By way of example the work function of silicon can be increased by ion implantation of elements such as carbon. It has been shown that the present invention enables enhancement or suppression of electron emission from the surfaces of micro-electronic devices. This is accomplished by ion implantation followed if necessary by segregation in a controlled environment using thermal treatment. A work function modifying layer is formed by binding the implanted atoms, including alloy or compound formation, during or after implantation. In addition to silicon, the process can be carried out by ion implantation in electron emitting metal surfaces such as molybdenum, platinum and nickel. In addition to alkali metals, other elements such as barium and calcium can be implanted. Implantation is carried out at ion energies of about 20 eV - 35 keV, and more particularly 1-35 keV. The lower the energy the closer to the surface that the ions will be implanted. The higher the energy, the deeper the implantation, and the greater the need to perform the segregation step. Ion implant doses will generally fall in the range of about 5x10*4 t 0 about 1x10*? ions/cm 2 . Ion implant temperatures may range from as low as -196°C (liquid N2) to 500°C. Gaseous environments, such as O2 and H2O, during implant may range from about 10"** - 10"* Torr. Post-implant treatment (anneals or oxidation) may be done at temperatures ranging from about 20°C - 500°C. Post- implant ambients during anneals or oxidation can include IO" 9 - 10 " * Torr vacuum, atmospheric air, noble gases at atmospheric pressure, and O2 or H2O in N2 or other nonreactive gas. While specific embodiments of materials and processing parameters have been set forth to exemplify and explain 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.

Claims: THE INVENTION CLAIMED IS 1. A process for modifying the work function of an electron emitter, comprising: implanting ions of an element below at least one atom layer of a surface of an electron emitter material; and forming a work function modifying layer of the implanted element at the surface of the electron emitter material. 2. The process of Claim 1 further comprising: segregating the implanted element to the surface of the material using thermal treatment in a controlled environment. 3. The process of Claim 1 further comprising: removing the top surface of the emitter material following implantation to concentrate the implanted element at the surface. 4. The process of Claim 1 further comprising: sputtering during implantation to remove the top surface of the emitter material and expose the implanted element at the surface. 5. The process of Claim 2, wherein the controlled environment is selected from the group of vacuum, reactive gases, and atmospheric air. 6. The process of Claim 1, wherein the implanted element is selected from alkali metals, barium, calcium, and carbon. 7. The process of Claim 1, wherein the implanted element is selected from the group consisting of Li, Na, K, Rb, and Cs. 8. The process of Claim 1 wherein the emitter material is selected from the group consisting of semiconductors, metals, alloys and compounds. 9. The process of Claim 1, wherein the electron emitter material is selected from the group consisting of silicon, molybdenum, platinum, and nickel. 10. The process of Claim 1, wherein ion implantation is carried out at ion energies of about 0.02-35 keV. 11. The process of Claim 1 wherein the step of forming a work function modifying layer comprises forming a compound or alloy layer. 12. The process of Claim 1 wherein ion implantation is carried out by direct implantation of the element into a surface compound on the material. 13. The process of Claim 12 wherein the element is an alkali metal, the material is silicon, and the surface compound is silicon dioxide. 14. The process of Claim 1 wherein ion implantation is carried out by implantation of the element in the presence of a partial pressure of a reactive gas. 15. The process of Claim 14, wherein the element is an alkali metal, the material is silicon, and the reactive gas is oxygen or water vapor. 16. The process of Claim 1 wherein the step of forming a work function modifying layer comprises exposing the surface of the material to a reactive gas following ion implantation. 17. The process of Claim 16, wherein the surface is oxidized by exposure to at least one of θ2 and H2O in atmospheric air and room temperature. 18. The process of Claim 1 wherein the step of forming a work function modifying layer comprises forming a thin layer of the implanted element on top of the material surface. 19. The process of Claim 1 wherein the step of forming a work function modifying layer comprises binding the implanted element on top of the material surface. 20. A process for enhancement or suppression of electron emission from surfaces of micro-electronic devices, comprising: carrying out low-energy ion implantation of a selected element below the surface of an electron emitting material so that the implanted element does not react with the atmosphere during subsequent handling and processing; if necessary, segregation of the implanted element to the emitting material surface using thermal treatment of the emitting material in a controlled environment; producing thin layers of the implanted element on top of the emitting material surface or forming compounds or alloy layers at the surface of the emitting material. 21. The process of Claim 20, where the controlled environment is selected from vacuum and reactive gases. 22. The process of Claim 20, additionally including selecting the element to be implanted from the group consisting of alkali metals, barium, calcium, and carbon; and selecting the electron emitter material from the group consisting of silicon, molybdenum, platinum, and nickel. 23. The process of Claim 20, wherein ion implantation is carried out at ion energies of about 0.02-35 keV.

Inventors: BALOOCH MEDHI, MUSKET RONALD G, SIEKHAUS WIGBERT J, BALOOCH, MEDHI, MUSKET, RONALD, G., SIEKHAUS, WIGBERT, J.
Applicants: UNIV CALIFORNIA, THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
Application year: 1997
Main classification: B05D5
IPC codes: B05D7, C23C14, H01J1, H01J9, H01L21