Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
2001-12-05
2003-09-02
Wells, Nikita (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S492200, C250S492210, C250S398000
Reexamination Certificate
active
06614033
ABSTRACT:
BACKGROUND OF THE INVENTION
Recently, most computers and communications apparatus use large-scale integration (LSI) circuits each having large numbers of transistors and resistors integrated into a single chip with interconnections. Thus, the performance of the entire apparatus depends greatly on the performance of the LSI chip. The performance of the LSI chip can be upgraded by increasing the packing density, that is, scaling down the dimensions of on-chip devices.
Scaling down the dimensions of devices can be achieved by optimizing the ion implantation and subsequent thermal annealing in forming diffusions such as source/drain diffusions. This allows MOS devices with shallow source/drain diffusions of 0.2 &mgr;m or less in depth to be realized.
In order to form such shallow diffusions, it is required to make a low thermal budget so that impurity atoms are distributed shallow upon ion implanting and are not diffused deep in the subsequent thermal process.
On the other hand, in order to form through impurity doping a well in which a device, such as a MOS transistor, is formed and a region (a channel doped layer) in which the channel of the MOS transistor is induced, it is required to control precisely the implant dose.
The production of MOS transistors having channels of opposite conductivity type or MOS transistors having different threshold voltages in the same substrate inevitably requires the use of a resist mask in each of ion implantation processes for wells, channels, or polysilicon gate electrodes.
That is, it is required to coat a layer of resist onto the entire surface, remove portions of the resist that are located above regions where ion implantation should take place to thereby define a resist pattern, and ion-implant impurities into the regions using this resist pattern as a mask.
This approach involves a sequence of steps of resist coating, exposure to light, resist development (resist pattern formation), ion implantation, resist ashing, and wet cleaning using H
2
SO
4
—H
2
O
2
mixture.
The ion implantation (ion irradiation) has been extensively used as a method of forming pn junctions by introducing impurities, such as boron (B), phosphorus (P), arsenic (As), etc., into a semiconductor substrate. This ion implantation method allows impurities to be introduced into target sites with their concentration and depth controlled precisely.
The ion source chambers at the heart of ion implantation apparatus are roughly classified into three: the Burnus type, the Freeman type, and the microwave type that uses a magnetron.
FIGS. 16A and 16B
show, in sectional view, the conventional Burnus type ion source chamber. More specifically,
FIG. 16A
is a sectional view taken parallel to the top of the ion source chamber, and
FIG. 16B
is a sectional view taken parallel to the side of the chamber. On one side of an arc chamber
71
is mounted a tungsten filament
77
by insulating supports
75
and reflectors (spacers). On the opposite side is mounted an electrode
74
by an insulating support
75
so as to be opposed to the filament
77
.
Next, description is given of a method of extracting ions using this apparatus. A gas, such as an Ar gas, is introduced into the arc chamber through a gas inlet
72
and thermal electrons are released from the tungsten filament
77
. The direction of movement of the thermal electrons is changed to the reverse direction to the direction of emission from the filament by the opposed electrode
74
, thereby increasing the probability of collision of the thermal electrons with the Ar gas introduced into the arc chamber to ionize the Ar gas. The resulting ions are taken out of the chamber through an ion outlet
23
provided in a front plate
78
.
FIGS. 17A and 17B
show, in sectional view, the conventional Freeman type ion source chamber. More specifically,
FIG. 17A
is a sectional view taken parallel to the top of the chamber, and
FIG. 17B
is a sectional view taken parallel to the side of the chamber. On the opposed sides of an arc chamber
91
are mounted reflectors
96
by insulating supports
95
. A bar-like tungsten filament
99
is attached to the opposed reflectors
96
.
Next, description is given of a method of taking out ions using this apparatus. A gas, such as an Ar gas, is introduced into the arc chamber through a gas inlet
92
and thermal electrons are released from the tungsten filament
97
. At the same time, a magnetic field parallel to the filament
97
is produced by electromagnets
100
and a rotating magnetic field is produced by a current in the filament electrode. Within the arc chamber
91
the movement of electrons is disturbed by the action of the reflectors
96
, thereby increasing the probability of collision of thermal electrons emitted by the tungsten filament
97
with the Ar gas introduced into the arc chamber. The resulting ions are taken out of the chamber through an ion outlet
93
provided in a front plate
98
.
FIG. 18
shows, in sectional view, of the microwave type ion source chamber. To take out ions using this apparatus, microwaves are generated by a magnetron
111
and then introduced into a discharge box
113
through a waveguide
112
, thereby generating a plasma in the discharge box, which corresponds to the above-described arc chamber. The resulting ions are taken out through an electrode
114
.
In these conventional ion source chambers, ions to be implanted are generally obtained by introducing a gas or vapor produced by sublimating a solid into the arc chamber and ionizing the gas or vapor by the aforementioned plasma. That is, in the conventional ion source chambers, ions are required to be supplied in the form of vapor or gas. However, with a refractory metal such as boron or titan, in order to obtain a vapor pressure of the order of 1E-4TORR necessary for ion implantation, it is required to heat the metal to a very high temperature (for example, 1400° C. or above for titan). In practice, ion implantation is impossible with this method.
Conversely, indium, having a melting point as low as 156° C., melts easily in plasma and hence is very inconvenient to use.
On the other hand, an ion implantation method has been developed which uses gases of chlorides or fluorides of those metals, enabling those low melting point metals to be used. However, this method inevitably causes corrosion of the inner walls of the arc chamber and the thermal electrons emitting filament due to chlorine, fluorine, chloride compounds, or fluoride compounds resulting from chloride gases or fluoride gases.
For indium as well, an attempt was made to use its chloride gas. For example, when vapor obtained by heating InCl
3
to 330° C. is introduced into the conventional ion source chamber shown in
FIGS. 16A and 16B
for the purpose of ion implantation, chlorine ions or radicals dissociated from InCl
3
etch not only the inner walls of the arc chamber that is made mainly of tungsten but even the tungsten filament. As a result, the filament becomes thinned considerably, resulting in an increase in resistance and failure to perform necessary control for arc discharge. In addition, even the outlet electrode is etched, disabling ions from being taken out stably. As a result, a large number of abnormal discharges comes to occur in about five hours, disabling ion implantation.
Thus, so long as chlorine-based compounds are used to ionize the refractory metals and indium, etching reaction due to chlorine ions or chlorine radicals resulting from the ionization inevitably occurs in the inner walls of the arc chamber and the tungsten filament.
Moreover, when a chloride gas, such as indium chlorine, and a fluoride gas, such as boron fluoride or germanium fluoride, are alternately introduced into the same arc chamber and then ionized, fluorine is attracted to the walls at the time when the boron fluoride is introduced and then reacts with chlorine at the time when the chloride gas is introduced to form chlorine fluoride that is a strong oxidizing agent. This accelerates the corrosion of the inner walls of the arch chamber and the thermal el
Murakoshi Atsushi
Okumura Katsuya
Suguro Kyoichi
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Wells Nikita
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