Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2001-11-21
2004-07-20
Berman, Jack (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
Reexamination Certificate
active
06765219
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to the field of ion implantation equipment and, more specifically, to serial ion implantation equipment.
BACKGROUND OF THE INVENTION
In ion implantation, a beam of energetic ions impinges upon a surface of material to imbed or implant those ions into the material. Ion implantation processes are categorized into batch and serial processes. Serial processes are the most common type of ion implantation processes, and are associated with medium dose implantation. Serial processes most often use a plasma ion beam that is subjected to electrostatic deflection processes in both axes normal to the direction of beam propagation. The electrostatic deflection processes are intended to provide a uniform distribution of ions in terms of density and direction of travel, but in practice ion beams vary in angle by as much as 3° relative to the direction of beam propagation. This variance produces undesirable effects in the ion implantation processes, as reported in U.S. Pat. No. 4,726,689 to Pollock.
U.S. Pat. Nos. 5,406,088 and 5,229,615 to Brune et al. describe a parallel beam ion implantation device that was developed in response to increasing commercial use of large wafer diameters. The growth in wafer diameter from 4″ to 6″ and then to 8″ in diameter has generated a need for a serial implantation device capable of producing a beam that strikes the surface of the wafers with a uniform parallel beam while also permitting tilt and rotational control of the wafers.
U.S. Pat. No. 5,350,926 to White et al. describes a high current broad beam ion implanter with emphasis upon systems for beam control to establish uniformity across a large ribbon shaped beam traveling in a single transverse direction. The ion implanter uses a Freeman, Bernas, or microwave source, from which the ion beam is extracted from source plasma through a parallel-sided convex slot. The ion beam passes through a pair of analyzing magnets to render the beam parallel in both axes normal to the direction of beam propagation. U.S. Pat. No. 4,922,106 to Berrian et al. similarly shows an ion beam implantation device having a parallel beam generator together with mechanical and electrical scan controls that facilitate uniform implantation.
Hybrid scanning systems are the type most often used in modem serial processing ion implantation equipment. Processing occurs for one wafer at a time. As shown in
FIG. 1
, which is a midsectional side elevational view, it is common to mechanically scan a wafer
100
in one axis by passing the wafer
100
through a scanned ion beam
104
, i.e., an ion beam
104
that is projected from source
102
. The horizontal ion beam
104
has a transverse axis
106
with respect to the vertical axis
108
of wafer motion. The axis
106
, as shown in
FIG. 1
is an average representation of the beam axis. Portions of the ion beam
104
may be slightly off-axis due to beam shaping field elements, such as are shown in U.S. Pat. No. 5,350,926 to White et al. Generally, the wafer
100
is vertically translated along axis
108
through the horizontally scanned ion beam
104
as a means of distributing the ion beam uniformly over the wafer surface. It is necessary to setup the incoming ion beam
104
prior to implanting the wafer
100
, in order to achieve uniform implantation by this scanning method. These processes occur in a beam implant vacuum chamber
110
. A wafer holder
112
may comprise an arm, a linear conveyor, or any other type of wafer holder. The wafer holder
112
presents a wafer surface
114
that is available for ion implantation through the effects of ion beam
104
.
As shown in
FIG. 2
, which is a midsectional top plan view, setup of the scanned ion beam
104
for uniform implantation is accomplished by sampling with a faraday cup
200
that moves horizontally across the full beam width W in a direction that is normal to the beam axis
106
at the setup plane
202
. The setup plane
202
is ideally located where the wafer implant occurs on surface
114
(see FIG.
1
). The faraday cup
200
is deployed at a plurality of sampling stations, e.g., stations
204
and
206
, to provide a fair representation of the beam uniformity at all positions on setup plane
202
. Ion beam current collected by the faraday cup
200
is measured as a function of faraday cup position. Subsequent adjustments to the ion beam optical elements in source
102
are made by conventional means to even out the beam current, e.g., as taught in U.S. Pat. No. 5,350,926 to White et al. Measurement of beam current and adjustment of the ion optics are repeated according to conventional practices until the beam current is uniform within acceptable limits.
As shown in
FIG. 3
, hybrid implantation systems have process requirements that mandate control of the angle
300
of ion beam incidence with respect to the wafer surface
114
during implantation, for example, as described in U.S. Pat. No. 5,898,179 to Smick et al. This control is usually accomplished by tilting the wafer
100
within the wafer holder
112
. Tilting occurs with respect to the trajectory of ion beam
104
and the mechanical scan axis
108
. This tilting produces an angle
300
of incidence between the incoming ion beam
104
and the wafer surface
114
that is constant everywhere on the wafer. The mechanical translation of wafer
100
continues, as before, in a vertical direction along axis
108
. The incident angle
300
generally ranges from 0° to 45° and is measured in the y-axis plane between the ion beam trajectory along axis
106
and the axis
304
that is normal to the implanted wafer surface
114
. For example, a 0° implant angle occurs when the wafer implant surface
114
is oriented at 90° relative to the ion beam trajectory along axis
106
.
Tilting the wafer
100
with respect to the mechanical scan axis
108
can have a deleterious effect on the uniformity of ion implantation because some regions of the wafer surface
114
are not implanted in the same focal plane as the setup plane
202
. These problems are exacerbated by the current trend of using larger wafers, so that distances between the setup plane
202
and the plane of surface
114
can be significant. Where the wafer
100
is tilted by rotation relative to the mechanical scan axis
108
, one end
306
of the wafer rotates toward the incoming ion beam
104
while the other end
308
rotates away. The middle region
310
of the wafer
100
remains in the setup plane. If, for example, the horizontal tilt axis is located entirely below the wafer
100
, then the entire wafer moves out of the setup plane
202
. Ion beam current uniformity is not specifically known other than in the setup plane
202
where it was actually measured. Therefore, the implant and setup planes should be coplanar.
The ion beam
104
contains positively charged plasma particles, which impinge upon surface
114
to impart a net charge on wafer
100
. The effects of this imparted charge are cancelled, according to conventional practices, by utilizing a flood gun
312
to emit an electron stream
314
. An exemplary ion implantation system including a flood gun for use in neutralizing accumulated plasma charges is the VIISta 80 ion implanter that is produced by Varian Semiconductor Equipment of Glouchester, Mass., as described, for example, in Radonov et al.,
In Situ Charging Potential Monitoring for a High Current Ribbon Beam
(a Varian Trade Publication 2001). The electron stream
314
impinges upon wafer
100
to cancel the net charge. As wafer
100
is tilted in increasing magnitude of angle
300
, surface
114
is increasingly exposed to the electron stream
314
, and there is a corresponding increase in contact from electron stream
314
with associated net charge effects on wafer
100
. Similarly, surface
114
is less exposed to the ion beam
104
by virtue of this tilting with associated net charge effects on wafer
104
. These net charge effects, in combination, produce problematic localized field distortions that vary the uniformity of ion beam
Berrian Donald W.
Pollock John D.
Vanderpot John W.
Berman Jack
Smith, II Johnnie L.
Variah Semiconductor Equipment Associates, Inc.
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