Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
1999-04-12
2001-05-29
Berman, Jack (Department: 2878)
Radiant energy
Ion generation
Field ionization type
C250S492210, C315S111810
Reexamination Certificate
active
06239440
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods for fabricating materials and structures having mechanical, thermal and electrical properties suitable for use in a wide variety of applications.
Materials fabricated from powder have in the past been fabricated using a sintering process but do not have a sufficient density to provide a product having sufficient mechanical strength or thermal stability.
For example, boron has a wide variety of uses, but it is a difficult material to form in a desired geometry and is also difficult to machine. Arsenic, phosphorus, antimony and boron are all used as dopants in the fabrication of semiconductor devices. These materials are selectively ionized and implanted using an ion implantation system. These systems have an ion source that is used to generate a beam of ionized particles which are directed onto a target such as a semiconductor wafer. These systems are complex and expensive to fabricate, operate and maintain. A particular problem in the use of these ion implanters is the level of impurities generated during use which increases maintenance, increases defect density in the materials produced and reduces production yield in the manufacture of devices.
The housing for the ion source in an implanter is often referred to as an arc chamber. Arc chambers have usually been made of graphite, molybdenum or tungsten. These materials contribute to the contamination of the beam, and consequently, they contaminate the final product.
In one type of arc chamber electrons are emitted by a cathode, usually by thermionic emission, and accelerated to an anode. Some of these electrons have collisions with gas atoms or molecules and ionize them. Secondary electrons from these collisions can be accelerated toward the anode to energies depending on the potential distribution and the starting point of the electron. Ions can be extracted through the anode, perpendicular to it, or through the cathode area depending upon the type of source.
To increase the ionization efficiency of the electrons in electron bombardment ion sources, several modifications have been introduced in existing systems. An additional small magnetic field confines electrons inside the anode and lets them spiral along the magnetic field lines, multiplying on their way to the anode and increasing the ionization efficiency of the ion source. By using a cylindrical anode and a reflector electrode, the electron path is further enlarged. Many mass separator ion sources are this type, such as the Nier, Bernas, Nielsen, Freeman, Cusp and other sources.
The Bernas ion source, for example, has a rectangular or cylindrical arc chamber positioned in an external magnetic field. The source can contain a single-turn helical filament (cathode) at one side of the arc chamber and a reflector at the other end. Electrons from the cathode are confined inside the anode cylinder by the magnetic field and can oscillate between the filament and the reflector resulting in a high ionization efficiency. Ions are extracted perpendicular to the anode axis through a slit of about 2 mm width and about 40 mm length. However, the dimensions can vary, depending on the specific design.
A continuing need exists for improvements in the field of materials fabrication to provide structures having desired mechanical, thermal and electrical properties. In particular, there is a need for improvements in ion implantation systems used for the fabrication of semiconductor devices.
SUMMARY OF THE INVENTION
The present invention relates to devices and methods of fabricating components for use in ion implantation systems. More particularly, the invention relates to the fabrication of boron arc chambers and other boron components for ion implantation systems. With the use of boron components in ion implantation systems a number of advantages are realized, including a reduction in contaminants due to the use of boron instead of other materials such as graphite, molybdenum or tungsten; the enhancement of beam current that can be accommodated due to the lower level of contaminants; the lighter weight of these components and the ability to retrofit them onto existing systems as well as their use in new systems, and the ability to use these components with the electrical system (e.g. as electrodes) and as a source of boron particles for ionization.
The refractory metals are problematic for the ion source because they are heavy, difficult to fabricate, and highly reactive with boron trifluoride, a gas used in many systems to provide a source of boron for ionization.
Tungsten, for example, is the current preferred material for the Bernas ion source but is far from ideal. It is one of the heaviest of all engineering materials having a density of 19.3 gm/cc, is difficult and expensive to machine, and reacts with boron trifluoride to form another gas, tungsten hexafluoride. The chemical reaction between fluorine and tungsten not only erodes the interior of the arc chamber but also acts as a material transport mechanism for depositing tungsten metal at other regions of the chamber. This effect shortens the chamber lifetime and considerably alters its interior geometry. Additionally, tungsten hexafluoride formation acts to pump unwanted tungsten ions into the boron beam current, some of which invariably ends up in the target, which is typically a single crystal silicon wafer or silicon-on-insulator (SOI) structure used for the manufacture of integrated circuits.
Boron is very light, having a density of 2.46 gm/cc (about 13% the weight of tungsten) and therefore, is less demanding on mounting fixtures and is easier to handle. It is also very hard and strong, even at the elevated operating temperatures of an arc chamber. It is more durable than graphite and tungsten, which is prone to creep (permanent displacement under an applied load). A boron arc chamber enhances the source beam current by reaction with free fluorine ions in applications involving the use of boron trifluoride as a source for boron ions.
Solid boron has not been utilized in semiconductor processing systems because it is not a conventional engineering material. There are currently no known manufacturers of dense boron products, mainly because specialized materials techniques are required to form this type of boron.
Structures made from boron for use in the fabrication of implanter components can be made using several distinct processes. A preferred embodiment of a method for making such boron structures includes providing a mold or die having the desired shape for the part to be fabricated, positioning a boron material such as an amorphous boron powder into the mold, treating the boron powder under selected conditions of temperature and pressure to crystallize the powder into a more crystalline state to form a solid unitary boron structure, removing the structure from the mold and machining the structure as necessary. In many applications it is desirable to produce a structure having a polycrystalline lattice with an average crystal size in the range of 1 to 10 microns. In some applications it is desirable to form a structure having a crystal size in excess of ten microns, including single crystal material. In some applications with lower tolerance requirements there may remain a large population of crystals with diameters of less than 1 micron, typically in the range of 0.5-1 micron.
It is also preferred that the density of the material produced be at least 50% of its maximum (theoretical) density, and preferably in the range of 80-100% in order to increase the mechanical strength and resistance to erosion. A preferred embodiment employs boron having a high purity level having an atomic percentage of elemental boron of at least 95%, and preferably of at least 99.99% or greater.
The fabrication process can be pressure sintering methods such as uniaxial hot pressing and hot isostatic pressing, or a casting method, a single crystal growth method, by deposition from the vapor or liquid phase, or by spray forming. The specific technique employed for a given workpar
Berman Jack
Hamilton Brook Smith & Reynolds P.C.
Thermoceramix, L.L.C.
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