Metal treatment – Process of modifying or maintaining internal physical... – With ion implantation
Reexamination Certificate
2000-07-20
2003-07-08
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With ion implantation
C148S525000
Reexamination Certificate
active
06589364
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to formation of silicon-germanium alloys and, in particular, to the formation of silicon-germanium alloys utilizing energy from an applied laser.
2. Description of the Related Art
Silicon-germanium alloys are finding increased use in semiconductor manufacturing due to the emergence of band gap engineering to control conductance. Silicon-germanium alloys are particularly useful for forming the base material of high speed heterojunction bipolar transistor (HBT) devices.
FIGS. 1A-1F
show a conventional process flow for forming an HBT.
FIG. 1A
shows the starting point for the process, wherein single crystal silicon workpiece
100
containing dopant of a first conductivity type is exposed to an ambient containing a mixture of silane (SiH
4
)
102
and germine (GeH
4
)
104
gases.
FIG. 1B
shows the resulting deposition of silicon-germanium alloy
106
over the surface of single crystal silicon workpiece
100
.
FIG. 1C
shows the ion-implantation of dopant
108
of a second conductivity type opposite the first conductivity type into silicon-germanium alloy
106
.
FIG. 1D
shows the subsequent formation of polysilicon layer
110
over the doped silicon-germanium alloy
106
. Polysilicon layer
110
is then highly doped by ion-implantation of dopant
112
of the first conductivity type, as shown by FIG.
1
E.
FIG. 1F
shows completion of fabrication of HBT structure
114
, wherein polysilicon layer
110
and silicon-germanium alloy layer
106
are selectively removed to reveal polysilicon emitter
116
overlying and separated from single crystal silicon collector
118
by silicon-germanium base
120
.
While the above
FIGS. 1C and 1E
depict introduction of conductivity-altering dopant by ion-implantation, it is also well known in the art to introduce dopant by chemical vapor deposition followed by thermal drive-in.
While satisfactory for some applications, the conventional process for forming the HBT suffers from a number of disadvantages. In particular, formation of the silicon-germanium alloy of the base by co-deposition of silicon and germanium-containing gases, as shown in
FIGS. 1A-1B
, produces an alloy having uneven concentrations of silicon and germanium. In addition, the conventional co-deposition technique produces a silicon-germanium alloy of relatively low crystal quality. Both the uneven Si—Ge concentration and the poor crystal structure of the conventionally-formed alloy degrade the operational characteristics of the HBT.
Therefore, there is a need in the art for a process for forming a high quality silicon-germanium alloy having uniform silicon and germanium concentrations.
SUMMARY OF THE INVENTION
The present invention relates to a process for forming a high quality crystalline doped silicon-germanium alloy utilizing, laser annealing. An amorphous or polycrystalline doped germanium film is first formed over epitaxial silicon. Application of radiation from a laser beam to the amorphous/polycrystalline doped germanium layer melts both the germanium layer and a portion of the underlying epitaxial silicon. The high temperature of melting promotes diffusion of both germanium and dopant into the underlying silicon. Removal of the laser beam causes the silicon-germanium-dopant melt to cool and recrystallize in high quality crystalline form. Diffusion of germanium and dopant during the melting step ensures uniform incorporation of these materials into the silicon-germanium lattice.
A first embodiment of a process for forming a silicon-germanium alloy in accordance with the present invention comprises the steps of forming a doped amorphous/polycrystalline germanium layer over a single crystal silicon workpiece. A laser beam is applied to melt the doped polycrystalline/amorphous germanium layer and single crystal silicon and cause dopant from the doped germanium layer to diffuse into the silicon. Removing the laser beam causes melted germanium and silicon to solidify to form a silicon-germanium alloy incorporating dopant from the amorphous/polycrystalline germanium layer.
REFERENCES:
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patent: 4415383 (1983-11-01), Naem et al.
patent: 4849371 (1989-07-01), Hansen et al.
patent: 5114876 (1992-05-01), Weiner
patent: 5225371 (1993-07-01), Sexton et al.
patent: 5889292 (1999-03-01), Sameshima et al.
Erin C. Jones et al., “Modeling of Leakage Mechanisms in Sub-50 nm p+−n Junctions”, J. Vac. Sci. Technol. B14(1), Jan./Feb. '96, pp. 236-241.
Katsuyoshi Washio, “SiGe HBTs and ICs for Optical-Fiber Communication Systems”, Solid-State Electronics 43(1999) pp. 1619-1625.
K.-Josef Kramer et al., “Resistless, Area-Selective Ultrashallow P+ IN Junction Fabrication Using Projection Gas Immersion Laser Doping”, Appl. Phys. Lett. 68(17), Apr. 22, 1996, pp. 2320-2322.
Essaian Stepan
Naem Abdalla A.
National Semiconductor Corporation
Stallman & Pollock LLP
Wyszomierski George
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