Lon Implantation in Semiconductors. Silicon and Germanium

285 Pages·1970·7.37 MB·English

by  James Mayer (Auth.)

I on I m p l a n t a t i on in S e m i c o n d u c t o rs
SILICON AND GERMANIUM
James W. Mayer Lennart Eriksson
CALIFORNIA INSTITUTE OF TECHNOLOGY RESEARCH INSTITUTE FOR PHYSICS
PASADENA, CALIFORNIA STOCKHOLM, SWEDEN
and John A. Davies
CHALK RIVER NUCLEAR LABORATORIES
CHALK RIVER, CANADA
A C A D E M IC P R E SS
A SUBSIDIARY OF HARCOURT BRACE JOVANOVICH, PUBLISHERS
N ew Y o rk L o n d on T o r o n to S y d n ey S an F r a n c i s co
COPYRIGHT © 1970, BY ACADEMIC PRESS, INC.
ALL RIGHTS RESERVED
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Many of the discussions that led to the writing of this book
(and many of our experimental results)
occurred at odd hours, weekends, and holidays.
This book is dedicated, therefore,
to our wives—Betty, Sylvia, and Flo—
for their support, patience, and understanding.
Preface
Interest in ion implantation as a method to introduce atoms into the sur
face layer of a solid has been growing steadily for at least a decade. Until a
few years ago this interest was confined to a limited number of research
laboratories where implantation was manifesting itself as a useful and versatile
tool in many areas of atomic, nuclear, and solid-state physics. The marked
upsurge of interest in ion implantation since 1966 can be attributed largely to
its emergence as a potentially useful technique to produce electronic com
ponents. Indeed, most of the current studies are being made in silicon.
This monograph reviews the recent developments in ion implantation in
silicon and germanium and emphasizes the basic aspects of these studies; we
feel that experimental work on other semiconductor materials has not been
carried through extensively enough to permit a comprehensive picture to
emerge, and we have therefore limited our coverage of the subject to the two
most studied elements. We have tried to cover in some detail each of the major
basic aspects of experimental study: dopant distribution, radiation damage,
dopant location, and electrical characteristics.
This is not an historical survey; it is a guide to the more recent develop
ments in the field. Because of this approach, we may not always have given
credit to those pioneers in the field of ion-implantation doping who stimulated
many of the later investigations. In this regard, the work of M. Bredov,
J. O. McCaldin, D. Medved, W. King, and R. Ohl and their colleagues should
be noted.
It is our hope that this monograph will serve as a useful summary of the
efforts to date in the field of ion implantation of semiconductors. It is aimed
%
at both the specialists in the implantation field and those in other fields who
wish to acquaint themselves with the problems and merits of ion implantation.
April 1970
xi
Acknowledgments
The writing of this book has grown out of a joint experimental program between the
three authors, extending back to 1965 and involving five different research institutes. We
are very deeply indebted to our many colleagues at these institutes for their encouragement,
assistance, and criticism and for their stimulating discussions with us during our own in­
vestigations. In particular, we wish to mention F. Brown, L. Cheng, J. R. Parsons,
D. Marsden, I. Mitchell, and J. L. Whitton at Chalk River Nuclear Laboratories (Canada);
J. U. Andersen, E. B0gh, K. O. Nielsen, H. Schiott, and J. Lindhard at the University of
Aarhus (Denmark). I. Bergstrom, K. Bjorkqvist, B. Domeij, G. Fladda, N. G. E. Johansson,
and D. Sigurd at the Research Institute for Physics, Stockholm (Sweden); R. Baron,
R. W. Bower, R. R. Hart, O. J. Marsh, and G. A. Shifrin at Hughes Research Laboratories
(California); and S. T. Picraux and J. E. Westmoreland at the California Institute of
Technology. In writing this book we acknowledge the major contributions of O. J. Marsh
who collaborated in writing Chapter 5, R. W. Bower who wrote Chapter 6, and H. Schiott
who furnished many of the range and straggling data in Chapter 2. The comments of
P. Sigmund and F. Eisen on Chapter 3 were very valuable. A special acknowledgment is
given to Mrs. N. Kosowicz for her secretarial assistance.
The U.S.A.F. laboratories at Cambridge, Massachusetts and at Wright-Patterson Air
Force base, Ohio, are acknowledged for their valuable support in stimulating many of
these ion-implantation studies.
Figures 2.3, 2.9a, 2.10, 2.15, 2.17, 2.19, 2.20, 2.23, 2.24, 2.26, 2.27, 2.30, 2.32, 3.14,
3.19, 3.21, 3.27, 3.31, 3.32, 4.9, 4.12, 4.13, 4.17, and 5.18 are reproduced by permission of
the National Research Council of Canada from the Canadian Journal of Physics.
Figure 2.4 is reproduced by permission of the National Research Council of Canada from
the Canadian Journal of Chemistry.
Figure 3.34 is reproduced by permission from C. Jech and R. Kelly, J. Phys. Chem.
Solids 30, 465 (1969). New York, Pergamon Press.
Figure 5.7 is reproduced from A. H. Clark and Κ. E. Manchester, Hall measurements of
ion-implanted layers in silicon, Trans. TMS ΛΙΜΕ 242, 1173-1180 (1968). New York,
American Institute of Mining, Metallurgical,and Petroleum Engineers.
xiii
1
General Features of Ion Implantation
Ion implantation is the introduction of atoms into the surface layer of a
solid substrate by bombardment of the solid with ions in the keV to MeV
energy range. The solid-state aspects are particularly broad because of the
range of physical properties that are sensitive to the presence of a trace
amount of foreign atoms. Mechanical, electrical, optical, magnetic, and
superconducting properties are all affected and indeed may even be dominated
by the presence of such foreign atoms. Use of implantation techniques affords
the possibility of introducing a wide range of atomic species, thus making it
possible to obtain impurity concentrations and distributions of particular
interest; in many cases, these distributions would not be otherwise attainable.
Recent interest in ion implantation has focused on the study of dopant
behavior in implanted semiconductors and has been stimulated by the possi
bilities of fabricating novel device structures in this way. We will therefore
direct our attention to those factors which affect the electrical characteristics
of implanted layers in silicon and germanium—such factors as range distribu
tions of dopant species, lattice disorder, and location of dopant species on
substitutional and interstitial sites in the lattice.
The application of semiconductors in electronic circuitry has been based
upon control of the thermal diffusion of dopant elements into semiconduct
ing crystals, normally silicon. These dopants occupy silicon lattice sites and
determine the electrical properties of the device. Their concentration is
determined by the equilibrium solubility at the process temperature (900-
1100° C), and the distribution in depth is given by the diffusion constant and
process time.
Ion implantation provides an alternative method of introducing dopant
atoms into the lattice. In this case, a beam of dopant ions accelerated through
1
2 / GENERAL FEATURES OF ION IMPLANTATION
a potential of typically 10-100 kV is allowed to impinge on the semiconductor
surface. The implantation system shown in Fig. 1.1 illustrates the basic
elements required in this technique. Using different types of available ion
sources, a wide variety of beams may be produced with sufficient intensity for
implantation purposes: 1014-1015 ions/cm2 (less than a "monolayer") is a
representative ion dose. Note that a mass-separating magnet is almost
mandatory to eliminate unwanted species that often dominate the extracted
beam. Beyond this, however, the basic instrumentation can be quite simple.
An important aspect of the application of implantation to semiconductor
technology, in contrast to diffusion processes, is that the number of implanted
ions is controlled by the external system, rather than by the physical properties
of the substrate. For example, dopants can be implanted at temperatures at
which normal diffusion is completely negligible. Also, the dopant concentra
tion is not limited by ordinary solubility considerations, and so a much wider
variety of dopant elements may be used. Thus, one potential application of
ion implantation is that it might allow the investigation of the properties of
species which cannot be introduced into semiconductors by conventional
means.
ION
Fig. 1.1. Schematic drawing of an ion-implantation system. A mass-separating magnet
is used to select the ion species of interest. Beam-sweeping facilities are provided for large-
area uniform implantations.
LI RANGE DISTRIBUTIONS 3
The major factors governing the successful exploitation of ion implanta­
tion are the range distribution of the implanted atoms, the amount and nature
of the lattice disorder that is created, the location of the implanted atoms with­
in the unit cell of the crystal, and (ultimately) the electrical characteristics
that result from the implantation and subsequent annealing treatment. We
will consider all of these factors briefly in the present chapter in order to
obtain an overall picture of the problems involved. Subsequent chapters will
then treat each one in detail.
1.1 Range Distributions
One of the most important considerations, obviously, in any description
of implantation processes is the depth (range) distribution of the implanted
ions. In recent years, a large amount of experimental and theoretical work
has been devoted to the task of understanding the energy-loss processes that
govern the range distribution, and it is now possible to predict fairly accurately
most of the factors involved. For example, a typical range distribution in an
amorphous substrate is approximately Gaussian in shape, and may therefore
be characterized by a mean range and a straggling about this mean value, as
depicted in Fig. 1.2. As discussed in Chapter 2, both these quantities depend
in a complex but predictable fashion on many variables. It is evident from
Depth Depth
Fig. 1.2. The depth distribution of implanted atoms in an amorphous target for the
case in which the ion mass is less than or greater than the mass of the substrate atoms. To a
first approximation the mean depth R depends on ion mass M and incident energy E,
p x
whereas the relative width AR/R of the distribution depends primarily on the ratio between
P P
ion mass and that of the substrate atoms, Μ.
2
4 1 GENERAL FEATURES OF ION IMPLANTATION
Fig. 1.2 that implanted distributions contrast strongly with the monotonically
decreasing profiles that are typical of diffusion processes. Furthermore, by
varying the energy continuously during the implantation, one may achieve
(in principle) almost any type of dopant profile. Typical values of the mean
range for 100-keV ions are ~0.1 micron, whereas diffusion doping usually
produces a mean depth of 1-10 microns.
Numerous experiments have shown that in monocrystalline substrates,
the range distribution depends strongly on the orientation of the crystal with
respect to the implantation direction, i.e., on the "channeling effect." If an
ion enters almost parallel to a major axis or plane, then a correlated series of
collisions may steer it gently through the lattice, thus reducing its rate of
energy loss and increasing its penetration depth. This may result in profiles
of the type indicated in Fig. 1.3. In most implantations, only a small fraction
of the implanted ions manage to stay channeled throughout their path, and
the shape of such a distribution is sensitive to many factors that are difficult
to control.
1.2 Lattice Disorder
Other problems inherent in the use of implantation techniques arise from
the lattice-disorder (Chapter 3) and radiation-damage effects produced by the
incident ion. As an implanted ion slows down and comes to rest, it makes
Fig. 1.3. The depth distribution of implanted atoms in a single crystal under conditions
such that the beam is aligned with a major crystallographic axis. The shaded portion shows
the distribution of perfectly "channeled" ions which penetrate nearly to the maximum
channeling range R . The distribution of atoms is sensitive to many factors, such as beam
max
alignment, lattice vibrations, and surface disorder. The dashed curves indicate the type of
distributions that might be obtained under typical implantation conditions in silicon and
germanium.
1.2 LATTICE DISORDER 5
many violent collisions with lattice atoms, displacing them from their lattice
sites. These displaced atoms can in turn displace others, and the net result is
the production of a highly disordered region around the path of the ion, as
shown schematically in Fig. 1.4 for the case of a heavy implanted atom at
typically 10-100 keV. At sufficiently high doses, these individual disordered
regions may overlap, and a noncrystalline or amorphous layer is formed.
The isolated disordered regions and the amorphous layer have widely
different anneal behavior. In the case of germanium and silicon, the isolated
disordered regions anneal at moderate temperatures of approximately 200°
and 300° C, respectively. The amorphous layers also anneal in a characteristic
fashion, but at appreciably higher temperatures, i.e., at approximately 600° C
in silicon and 400° C in germanium. It should be noted that even though both
types of disorder can be annealed at temperatures well below those where
diffusion of the dopant species occurs, there are still defects present. These
defects are most evident in the growth of dislocations that occur in silicon
at anneal temperatures above 600° C.
If the implantation is performed at a temperature greater than approxi-
Lattice Disorder
Fig. 1.4. A schematic representation of the disorder produced in room-temperature
implantations of heavy ions at energies of 10-100 keV. At low doses, the highly disordered
regions around the tracks of the ions are spatially separated from each other. The volume of
the disordered region is determined primarily by the stopping point of the ion and the range
of the displaced lattice atoms (dashed arrows). At high doses, the disordered regions can
overlap to form an amorphous layer.