Page 130 - Semiconductor Manufacturing Handbook

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ION IMPLANTATION AND RAPID THERMAL PROCESSING

ION IMPLANTATION AND RAPID THERMAL PROCESSING 10.3

(germanium tetrafluoride) and SiF (silicon tetrafluoride) are used for preamorphization of the crys-
4
tal lattice prior to ultralow energy implantation for shallow junction formation. Nitrogen and carbon
are occasionally used for material property modification in some applications as well.

10.2 COMPONENTS OF AN ION IMPLANTATION SYSTEM

10.2.1 Ion Source
The productivity demands of commercially viable ion implantation tools are perhaps most evident
in the development of ion source technologies for this industry. Beam current requirements that can
be as high as tens of milliamperes have led to the adoption of hot cathode ion sources almost exclu-
sively for mainstream production needs. The earliest versions of these ion sources were derived
2
1
directly from the Freeman and Bernas style sources that had been developed for use in isotope sep-
aration. The basic operating principle of these sources relies on thermionic emission from a filament
to support a dc plasma discharge between a cathode and anode separated by a potential difference of
the order of 100 V. The electrons in the discharge are confined by an externally applied magnetic
−3
field of the order of 100 G. Plasma ion densities in these sources can be as high as 10 12 cm .
Relatively simple triode extraction systems are typically used to extract and shape ion beams from
these sources and inject them into beamlines of various designs for delivery to the wafers being
processed. Beginning in the mid-nineties and driven largely by the desire to increase the operating
lifetime of the ion source, a migration from a hot filament that was immersed in the plasma discharge
to an indirectly heated cathode (IHC) where the filament was shielded from the discharge, took
3
place. Today, the majority of new implant systems make use of this IHC technology and generally
realize source operating lifetimes that are improved by factors of two to five. 4
10.2.2 Beamline Architectures

There are elements of implanter beamlines that are generally common among the three major types
of tools. All beamlines begin with an ion source and extraction optics, which are responsible for
injecting an appropriately shaped beam of ions into the subsequent elements of the beamline.
Virtually all implanter beamlines also require a mass analysis device, which is almost universally a
dipole electromagnet that provides momentum dispersion and transverse focusing of the ion beam. 5
The force applied to an ion passing through this dipole field is described by
F = qv × B (10.1)
where q = ion charge
v = ion velocity
B = magnetic field strength

The direction of this force is perpendicular to both the velocity of the ion and the magnetic field.
In a uniform magnetic field, the ions follow circular trajectories and can be described by balancing
the Lorentz force above with the centrifugal force as

qvB = mv 2 (10.2)
R
where m is the ion mass and R is the radius of the circular trajectory. Typically, the velocity of the
ion can be determined from the electrostatic potential V, through which it was accelerated when leav-
ing the ion source and being injected into the rest of the beamline as
qV = 1 mv 2 (10.3)
2

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