Radiant energy – Luminophor irradiation
Reexamination Certificate
1998-09-16
2001-04-10
Anderson, Bruce C. (Department: 2881)
Radiant energy
Luminophor irradiation
C250S42300F, C315S111810
Reexamination Certificate
active
06215125
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention relates to ion implantation, and in particular to a method for extending the lifetime of a hot cathode discharge ion source which is utilized in an ion implantation apparatus to generate source ions.
2. Background of the Invention
Ge
+
ion implants have been widely used in the semiconductor industry to pre-amorphize silicon wafers in order to prevent channeling effects. The demands for these pre-amorphizing implants are expected to increase greatly in future semiconductor device manufacturing. The most popular ion feed gas for Ge
+
beams is GeF
4
, because of its stable chemical properties and cost effectiveness. However, very short lifetimes, on the order of 12 hours or less, of the hot cathode discharge ion sources have been observed while operating with GeF
4
gas.
The common source failure mode is that some materials deposit on the cathode surfaces of the hot cathode discharge ion source during extended use of the ion implantation apparatus. This deposition reduces the thermionic emission rate of the source ions from the hot cathode surfaces. Consequently, the desired arc currents can not be obtained and the hot cathode discharge sources have to be replaced in order to maintain normal source operation. The short source life greatly reduces the productivity of an ion implanter.
The cause of the short source life in GeF
4
ion implantation is believed to be excessive, free fluorine atoms in the ion source due to the chemical dissociation of GeF
4
molecules. The arc chamber material is etched away by chemical reaction of the fluorine atoms with the material of the arc chamber. Some of the arc chamber material may eventually deposit on the hot cathode resulting in the degradation of electron emissions from the hot cathode discharge source.
Other implantation gases besides GeF
4
are employed in ion implantation and these other gases may cause the same shortening of the lifetime of the hot cathode discharge ion source. The term “hot cathode discharge ion source” is used herein to denote any thermionic emission element which when heated to a temperature of at least 1200° C. emits desired electrons. It is noted that the exact temperature wherein electrons are emitted from such elements is dependent on the material of the element.
A typical prior art ion implantation apparatus, i.e. tool, is illustrated in FIG.
1
. Specifically, the prior art ion implantation apparatus comprises an ion source chamber
10
which generates ions to be implanted into a desired substrate. The generated ions are drawn by drawing electrodes
12
and their mass is analyzed by a separating electromagnet
14
. After mass analysis, the ions are completely separated by slits
16
and the appropriate ions are accelerated by accelerators
18
to a final energy. A beam of ions is converged on the face of a sample or substrate
20
by a quadrupole lens
21
and scanned by scanning electrodes
22
a
and
22
b
. Deflection electrodes
24
,
26
and
28
are designed to deflect the ion beam in order to eliminate uncharged particles caused by collision with residual gas.
The ion source chamber
10
is the heart of the ion implantation tool. Five different kinds of ion source chambers are currently known including: a Freeman-type ion source chamber using thermoelectrodes; a Bernas-type ion source chamber; indirectly heated cathode type ion source; microwave type ion source chamber using magnetrons; and RF sources. It should be understood that the terms “ion source” and “hot cathode discharge ion source” are used interchangeably herein.
In order to better understand the present invention, a brief description of a Freeman-type ion source, a Bernas-type ion source and a microwave type ion source is given herein. The other types of ions sources mentioned hereinabove, i.e. indirectly heated cathode and RF, are not illustrated herein, but are also well known to those skilled in the art.
FIG. 2
is a cross-sectional view of a Freeman-type ion source chamber
10
. Specifically, in this ion source, plasma is generated by emitting thermoelectrons from a bar-shaped filament
30
, an electrical field is generated parallel to filament
30
by an electromagnet
32
, a rotating field is caused by filament current, and electrons are moved in the chamber by a reflector
34
, thereby improving the efficiency in ionization. The ions generated in the chamber pass through slit
36
and are guided in a direction perpendicular to the filament.
FIG. 3
is a cross-sectional view of a Bernas-type ion source chamber
10
containing molybdenum (Mo) as the main ingredient. The ion source chamber
10
includes a tungsten (W) filament
40
and its opposing electrode
44
. The ion source chamber is supplied with the desired gas from gas line
46
and emits thermoelectrons from the filament.
A typical microwave ion source is shown in FIG.
4
. Specifically, in this chamber
10
, plasma is generated in a discharge box
50
using a microwave caused by magnetron
52
. Since this chamber has no filaments, its lifetime is not shortened even by the use of reactive gases. However, metal as well as ions are extracted from the chamber and are attracted to the surfaces of drawing electrodes
54
; therefore, a desired voltage cannot be applied or the metal or ions may reach a sample to contaminate it.
Each of the above described ion sources exhibits the problem mentioned hereinabove. Prior art solutions to the short lifetime problem exhibited by these hot cathode discharge ion sources involve either changing of the hot cathode discharge ion source itself or coating the interior walls of the ion implantation apparatus with a material that is resistant to chemical attack. The latter solution is described, for example, in U.S. Pat. No. 5,656,820 to Murakoshi, et al.
Despite the success of such prior art processes, there exists a need to develop a new and improved method of extending the lifetime of hot cathode discharge ion sources. Such a method is needed since the prior art solutions are either too time consuming or add additional operating costs to the overall process. The prior art solution also yields an unwanted contaminant into the substrate when implanting a BF
2
species (Nb).
SUMMARY OF THE INVENTION
One object of the present invention is to provide a simple, yet cost effective method for extending the lifetime of a hot cathode discharge ion source which is typically employed in the prior art to implant ions into a substrate.
Another object of the present invention is to provide a method which significantly reduces the time required to shut down the ion implantation apparatus to either replace the discharge source or to coat the interior walls of the apparatus thus providing improved productivity to the ion implanter operator.
A still further object of the present invention is to prolong the lifetime of a hot cathode discharge ion source when fluorine-containing gases such as GeF
4
are employed as the implantation, i.e. ion source, gas.
These as well as other objects and advantages can be achieved in the present invention by introducing a nitrogen-containing gas, as a co-bleed gas, into an ion source chamber containing at least an implantation gas and a hot cathode discharge ion source. The method of the present invention is particularly applicable for use in ion implantation apparatuses wherein highly fluorinated gases such as GeF
4
are employed as the implantation gas. The term “highly fluorinated” is used herein to denote a gaseous compound which contains more than a single molecule of fluorine. It has been observed that a 50 to about 120 hour improvement in the lifetime of the hot cathode ion source can be obtained when a nitrogen-containing gas is used in conjunction with GeF
4
source gas. Similar improvements are expected to be observed with other implantation gases.
REFERENCES:
patent: 4881010 (1989-11-01), Jetter
patent: 5306921 (1994-04-01), Tanaka et al.
patent: 5563418 (1996-10-01), Leung
patent: 5640020 (1997-06-01), Murakoshi et al.
patent: 5656820 (1997-08-01), Mu
Chen Jiong
Freer Brian S.
Grant John F.
Jacobs Lawrence T.
Malenfant, Jr. Joseph L.
Anderson Bruce C.
International Business Machines - Corporation
Sabo, Esq. William D.
Scully Scott Murphy & Presser
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