Page 131 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH10.qxd 04/04/2005 19:46 Page 10.4

ION IMPLANTATION AND RAPID THERMAL PROCESSING

10.4 WAFER PROCESSING

Combining Eqs. (10.1), (10.2), and (10.3) gives

BR = 2 mV (10.4)
q

as a fundamental equation describing mass analysis (actually a misnomer, since it is really momen-
tum analysis) in every beamline ion implanter. What follows the ion source, extraction, and mass
analysis is more clearly differentiated among the three major tool segments.

10.2.3 High-Current Beamlines

The primary objective of high-current implanters is to deliver multimilliampere beams at specific
energies, typically below 200 keV, and more common recently, below 80 keV. The lower energy
requirements of high-current implanters grow ever lower, with production needs now reaching the
sub-keV level, with active process development down to energies below 0.2 keV. Various estimates
of whether a practical production lower limit for minimum energy exists (based on concerns of
implanted dose retention due to an equilibrium between surface deposition and sputtering, for exam-
6
ple) have placed this limit in the vicinity of 0.2 to 0.5 keV. All of these constraints drive the design
of high-current beamlines in a particular direction in which they tend to be relatively short and have
large cross sections. Each of these attributes is favorable for delivering the highest possible usable
beam current to the wafer.
The primary challenges to delivering high beam currents at lower energies center around the
effects of space charge forces on these beams. Given that the ions in an ion beam experience a repul-
sive force exerted by all neighboring ions, there is a tendency for the beam to expand in size as it
propagates through the beamline. This beam size expansion typically gets worse as the beam current
or ion mass is increased, or as the energy of the beam is decreased (as a result of a lower energy beam
moving more slowly, thereby allowing more time for the expansion forces to act on the beam
between points A and B). A typical parameter for understanding the scaling of space charge forces
acting on an ion beam is known as the beam perveance and is usually written as 7

P = m I (10.5)
i
/
E 32
where m = ion mass
E = energy of the beam
I = net nonneutralized ion current in the beam
This net nonneutralized ion current can be thought of as only the fraction of the ion beam popula-
tion which is in excess of any electron population that may also be present in the beam and the sur-
rounding beam plasma. In regions where the beam plasma is excluded (such as regions of high
electric field), the net nonneutralized beam current is equal to the total beam current. In field-free
regions where there is a beam plasma, the net nonneutralized beam current can be as little as 1 percent
of the beam current.
Beam size expansion due to space charge is a problem primarily due to the loss of ion current
(and hence productivity) whenever the beam passes through an aperture in the beamline that is smaller
than the beam.
Most common high-current beamlines are as simple from an optics standpoint as having only one
ion source, an analyzer magnet, and a resolving aperture, and allow the beam to travel through the
entire beamline without any externally imposed electrostatic fields present. This mode of operation
is referred to as drift mode since the ions are given their final energy via the ion source and extrac-
tion optics alone, and are left to “drift” through the remainder of the beamline at that energy. It is
advantageous to operate high-current tools in drift mode, since the presence of any electrostatic fields
in the beamline creates regions of very high space charge, by virtue of the fact that any electrons that

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