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NBSIR 81-2398

Investigation of the Two-Dimensional

Shape of Ion-Implanted Regions

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Devices and Circuits Division

Washington, DC 20234

and

Sandia National Laboratories

(U.S. Department of Energy Facility)

Albuquerque, NM 87185

December 1 981

100

.U56

81-2398

1981

c. 2

Naval Ocean Systems Center

San Diego, CA 92152

NBSIR 81-2398

INVESTIGATION OF THE TWO-DIMENSIONAL

SHAPE OF ION-IMPLANTED REGIONS

P. Roitman, J. Albers, J. R. Ehrstein

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Devices and Circuits Division

Washington, DC 20234

and D. R. Myers

Sandia National Laboratories

(U S. Department of Energy Facility)

Albuquerque, NM 87185

December 1981

Prepared for Naval Ocean Systems Center

San Diego, CA 92152

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
s«ai*aiuta to

?a«4tlEU #:
Table of Contents

Page

Abstract 1

1 . Introduction 2

2. Ion Channeling During Implantation 4

3. Review of Implantation Modeling .. 6

A) Fundamentals of Ion Penetration Theory 6

i) Introduction 6

ii) Interaction Mechanisms 6

B) Ion Range Theories in One Dimension for Amorphous Targets . . 8

i) Introduction 8

ii ) Analytic Codes 12

iii) Monte-Carlo Codes 14

iv) Comparison to Experiments 14

C) Ion Range Theories for Point Sources 18

i) Introduction 18

ii) Analytic Theories ..... 18

iii) Monte-Carlo Codes 19

iv) Comparison of Predictions 19

D) Scattering Under a Rectangular Mask Edge 21

i) Introduction 21 ii) Comparison of Theories 21 E) Ion Channeling 25

i) Introduction 25

ii) The Continuum Model of Lindhard 25

iii) The Effect of Amorphous Surface Layers 28

iv) "Lateral Channeling" 31

F) Electrical Activation of Arsenic- Implanted Silicon 31

4. Experimental 33

A) Sample Fabrication 33

B) Spreading Resistance 37

C) Etch Technique 41

i) Contrast Mechanism 41

ii) Etch E)ata 41 iii) Interpretation and Problems with Etch Data 45

D) Induced Current Techniques ..... 50

i) Physics of Electron-Beam- Induced Currents 50

ii) EBIC Data 51 iii) Interpretation of EBIC Data 51

5. Conclusions 55

References 59

iii
List of Figures

Page

3o1 Plots of the f((})) function as given as eq (3.10) for several

representative potentials parameterized by the values of X, m, and q as given in table 3.1.

(a) Results for the Thomas-Fermi (Lindhard), the Chu-Murley

(Wilson), the Moliere, the Lenz-Jensen, and the Bohr potentials 10

(b) The function S^(e) for figure 3.1a, along with values of

electronic stepping power common to typical implantations ... 11

3.2 Predictions of the Winterbon amorphous target code with several

choices of the potential for the case of a 150-keV implant of

arsenic into silicon at a dose of 5 x 10^^ cm“^ 15

3.3 Comparison of the TRIM (Monte-Carlo) code and the Winterbon

amorphous target code for 150-keV implant of arsenic into silicon at a dose of 1 x 10^^ cm"^ 16

3.4 Comparison of profiles for 150-keV arsenic implanted into silicon 17

3.5 Predicted ion density distributions resulting from a point

source of 75-keV arsenic ions incident on amorphous silicon at

0-deg incidence 20

3.6

Predicted ion density distributions resulting from 75-keV

arsenic implantation into amorphous silicon partially covered

by a rectangular mask.

(a) The density contour after Furukawa using a product of a com- plementary error function vertically and a Gaussian horizontally 22

(b) The density contour after Ryssell using a product of a Pearson distribution vertically and a Gaussian horizontally . . 23

(c) The density contour calculated by the TRIM code using the

Moliere potential 24

3.7 The Lindhard string model for the interaction of an incident

ion traveling along a channel 26

3.8 Variation in probability of ion channeling versus tilt angle,

and step function approximation 30

4.1 Schematic diagram of the resist structure used to mask the implants 34

4.2 SEM micrograph of the photoresist structure used to mask the ' implants 35

! I 5

IV
Page

4.3 SEM micrograph of the photoresist structure used to mask the

implants 36

4.4 Schematic diagram of an etch step at the corner of the grating

pattern 38

4.5 SEM micrograph of an etch step 39

4.6 Comparison of a typical spreading resistance profile with the

predictions of the SUPREM model using joined half-Gaussians

and with the amorphous target code 42

4.7 Schematic diagram of the electron-beam-sample interaction near

an etch step 43

4.8 Secondary electron picture of a cleaved and etched sample

tilted 0.5 deg to normal (at 75 keV and 0-deg tilt) 44

4.9 Secondary electron picture of a cleaved and etched sample

tilted 0.5 deg to normal (at 150 keV and 7-deg tilt) 46

4.10 Secondary electron pictures of cleaved and etched samples

tilted 0.5 deg to normal (at 75 keV and 7-deg tilt).

(a) Annealed at 1000°C for 30 min 47

(b) Annealed at 1 000°C for 60 min 48

4.11 Secondary electron picture of a cleaved and etched sample 0.5

deg to normal 49

4.12 Composite secondary electron and EBIC micrograph 52

4.13 Higher magnification of figure 4.12 53

4.14 Deflection modulation EBIC picture 54

5.1 Comparison of junction depths determined by junction etch and by

the as-implanted joined half-Gaussian model used in SUPREM for a sample implanted at 75 keV and annealed at 600°C 56

5.2 Comparison of junction depths determined by junction etch and by

several theoretical models for a sample implanted at 150 keV and

annealed at 57

5.3 Comparison of junction depths determined by junction etch and by

two models for a 5 x 10^^ cm“^ 150-keV arsenic-implanted sample

annealed at 1 000°C for 30 min 58

V
List of Tables

Page

3.1 Parameterization of the Cross Section in Terms of the Potential 9

3.2 Dechanneling of Arsenic Implants into (100) Silicon by Amorphous

Surface Layers for 0-Deg Incident Beam 29

4.1 Comparison of Junction Depth Locations Obtained from Spreading

Resistance and Stain Techniques 40
Preface

The work described in this report was sponsored by the Naval Ocean Systems

Center (Contract #MIPR N66001 79MP00052) and the National Bureau of Standards.

A special acknowledgment is given to Dr. Isaac Lagnado of the Naval Ocean

Systems Center for his insight and understanding of the need for the present

investigation as well as his continued support and encouragement during the course of the work.

The Sandia National Laboratories (Contract DE-AC04-76-DP00789 ) provided

computer facilities and time for the Monte-Carlo calculations of the twodimensional distributions. Dr. D. R. Myers was with the National Bureau of

Standards when this project was initiated.

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INVESTIGATION OF THE TWO-DIMENSIONAL SHAPE OF ION-IMPLANTED REGIONS

Peter Roitman, John Albers, James R. Ehrstein

Semiconductor Devices and Circuits Division

National Bureau of Standards

Washington, D.C. 20234

and

David R. Myers

Sandia National Laboratories

(U.S. Department of Energy Facility)

Albuquerque, NM 87185

Abstract

The two-dimensional shape of arsenic ion-implanted regions in

single-crystal silicon was investigated both experimentally and

theoretically. Experimentally, two techniques were shown to have

the necessary submicron resolution; a junction etch process and an SEM-induced current collection method. A comparison of junction

depths determined by the etch technique, the EBIC technique, and

spreading resistance with the depths calculated using several

amorphous target codes was made. For the case of low temperature

(600°C) anneals, the etch technique agrees very well with the

junction depths predicted by the amorphous target code due to Winterbon. The lateral junction locations obtained from the etch

technique are in good agreement with the predictions of a twodimensional Monte-Carlo code (TRIM) which indicates that arsenic

does not show any significant lateral scattering under mask edges.

For the case of high temperature (1000°C) anneals, the etch and

EBIC techniques agree with each other, but show consistently deeper

junction locations than does the spreading resistance technique.

Comparison with arsenic-diffusion models shows that concentrationdependent effects are important. Presently available processing

models do not appear to adequately forecast junction depths.

Key words: arsenic ion implantation; electron-beam-induced

current; etching; integrated circuit processing; ion channeling;

ion implantation; lateral profiles; scanning electron microscopy;

silicon technology; spreading resistance; two-dimensional profiles.

1
1 INTRODUCTION

Ion implantation is the preferred means of dopant introduction for semiconductor integrated circuit fabrication due to the reproducibility, lateral

uniformity, dopant level control, and lateral registration which can be

achieved. These features give ion implantation a significant advantage over diffusion technology in the fabrication of LSI devices; however, as device

dimensions in integrated circuits are reduced for very high speed, very large

scale integrated circuits, device performance will become increasingly sensitive to the details of the depth distribution of dopants, and secondary processes characterized by the two-dimensional nature of implanted (and annealed) regions will become limiting factors in device performance. Despite

the present recognition that dopant control in two dimensions is of critical

importance, the processes that control the ultimate shape of the doped regions especially during implantation are not adequately understood.

To illustrate the increasing importance of two-dimensional dopant profile

control, it is useful to examine the application of ion implantation to the

fabrication of silicon enhancement-mode metal -oxide -semiconductor fieldeffect transistors (MOSFETs ) for use in integrated circuits. In the late

1960s, typical variations in substrate background doping were 20 percent,

leading to unacceptable variations in threshold voltage across an integrated

circuit [1]. Implantation was then applied to control the doping in the

channel [2]; and it was found that for shallow enough implants at low enough

doses, the implanted dopants were always depleted for zero-bias conditions.

For those implants, the threshold voltage was found to scale linearly with

implanted dose [1,2], and for those devices, implanted ions acted as a variation in channel doping level. Prior to the introduction of implantation,

lateral diffusion under the gate during source and drain formation led to

increased parasitic capacitances that reduced device operating speed [1]. In

addition to allowing controlled variations in doping level, the introduction

of implantation also allowed for the formation of self-aligned devices [3]

that greatly reduced device parasitics, due to the reduced penetration of

implanted dopants under a mask edge [4] compared to the extent of lateral

diffusion typical in diffused devices. As device dimensions are reduced to

produce faster and denser circuits, linear scaling requires that junction

depths and lateral dimensions be decreased and dopant levels increased [5].

However, as the dose of the threshold adjustment implant is increased, all

the implanted dopants are no longer totally depleted, and the threshold voltage no longer scales linearly with ion dose [6]. Of even greater importance

is the fact that linear scaling is no longer sufficient to ensure proper

device operation [7], as secondary effects such as short-channel effects [8],

drain-induced barrier lowering [9], punch through [10], substrate reversebias sensitivity [7], subthreshold conduction [11], snap-back [12], and hot

carrier effects [13] become the limiting factors in device performance. All

of these effects are determined by the details of the electric field distribution determined by the two-dimensional interactions among the doped regions

and thus demand control over not only the depth distribution of the implanted

ions but their lateral spreading as well.

The importance of two-dimensional dopant profile control is not limited to

silicon enchancement-mode MOSFETs. Dopant profile effects in two dimensions

also control device operation in depletion -mode transistors in silicon or in

2
gallium arsenide [14]. Finally, in silicon bipolar technologies the net

dopant profile at the junction between the emitter and the active base controls transistor turn-on and turn-off characteristics by determining emitterbase capacitance [15] and such other parameters as base width, forward gain,

and base spreading resistance [16]. Thus, control over dopant profiles re- sulting from implantation is of great importance for future device technologies.

3
2 ION CHANNELING DURING IMPLANTATION

It has been long recognized that ion distributions resulting from implantation into initially crystalline targets are affected by ion channeling [17],

in which ions are steered down axial lattice channels by correlated collisions with the ordered arrangement of lattice atoms. While this phenomenon

is qualitatively understood on the basis of an analytic theory of Lindhard

[18], the major advantage of this theory is that it scales implant ion mass,

energy, and alignment to substrate crystallographic orientation in terms of a calculated critical angle for channeling. This angle varies from 2 to 7 deg

for common industrial implantation [19], with the larger critical angles

being found for implantations of heavier ions at low energies, as will be

used to produce shallow doped regions for future devices. What is not generally understood, however, is that the critical angle so calculated corresponds only to the limits of applicability of the assumptions made in the

theory and not to the actual limits of angular alignment over which substantial channeling can occur [20]. Studies of ion channeling during implantation [19,21] indicate that channeling characteristics persist until ion beam

alignment to the crystallographic orientations exceeds twice the critical

angle. It is important to note that the angle between the ion beam and the

nearest low index crystallographic direction required to minimize unintentional channeling can easily exceed the 7-deg tilt angle commonly used for

industrial implantation and that a substantial amount of ion channeling can

occur at 7-deg tilt angles for implantation of heavy ions at low energy. As an example, 53.5 percent of 75-keV arsenic ions implanted into (111) silicon

at a 7-deg tilt angle penetrate deeper than the LSS Gaussian distribution

[19,21], with even greater amounts of ion channeling occurring for arsenic

implantations at lower energies for nominal "random equivalent" implantation.

In considering implantation for device structures, several important factors

must be considered. Junctions between doped regions occur at impurity densities typically four or more orders of magnitude lower than the peak dopant

densities in the doped regions; thus, the thirty or more times greater penetration of channeled heavy ions [19,21] can lead to junction locations re- moved from that predicted on the basis of range theories for amorphous targets before implantation-induced damage reduces the possibility of ion channeling. In addition, since ion trajectories in crystalline substrates are determined by the conditions under which the ion enters the crystalline region [18-20], the presence of an amorphous covering layer is not always sufficient to eliminate unintentional channeling in underlying crystalline substrates [22]. The presence of an amorphous surface layer has been shown to

reduce the extent of ion penetration for directly aligned implants [23]. It

has also been found, however, that for certain implant conditions (ion species, implant energy, substrate orientation, and amorphous surface layer

thickness) the angular divergence introduced into the ion beam by random

collisions with atoms in the amorphous surface layer can compensate for the

effects of substrate tilt angle and actually increase the extent of unintentional channeling by orienting the ions back toward the major (low index)

crystallographic directions [24].

4
From the above discussion, several important facts should be noted:

(1) Unintentional channeling strongly affects the ultimate depth distribution

of dopants resulting from implantation.

( 2) The extent of unintentional channeling is commonly underestimated for

typical device implantations.

(3) Neither a 7-deg tilt angle nor a surface layer of silicon dioxide is

sufficient to prevent unintentional channeling during implantation.

(4) No adequate theoretical treatment of the effects of unintentional channeling on the two-dimensional penetration of implanted dopants exists.

Because of the increasing importance of lateral control of implanted dopant

profiles, an empirical study of the effects of unintentional channeling on

the two-dimensional profile of implanted regions has been performed to determine data for device designers, as well as to generate a data base on the

systematics of lateral penetration during implantation so that physical understanding can be developed. This study is especially timely as these factors control the depths of the 0.2-ym deep junctions required by scaling laws

for 1 . 0-ym lateral surface dimension geometries [25].

5
3. REVIEW OF IMPLANTATION MODELING

A) Fundamentals of Ion Penetration Theory

i) Introduction

When an energetic ion enters a solid, it undergoes collisions with the lattice core ions and electrons. These collisions are responsible for slowing

down the incident ion and eventually bringing it to rest at some vector distance from the original point of entry. In addition to the stopping of the

incident ion, several other phenomena are possible. If the target is crystalline, the incident ion may destroy the regular long-range order of the

crystal by displacing the lattice ions from their equilibrium positions.

This phenomenon, referred to as radiation damage, usually proceeds from isolated damage regions at low incident ion fluences to overlapping damage regions, i.e., complete amorphization, at high incident ion fluences. The

onset of amorphization depends upon the ratio of the masses of the incident

ion and the target ion. Generally, lower fluences are needed for amorphization when the ratio is larger than unity, and vice versa.

As the effects of lattice damage (and the subsequent incomplete activation of

the implanted dopant atoms) must be eliminated by either thermal or laser

annealing, the principal concern of ion implantation modeling is the prediction of the spatial distribution of the implanted dopant atoms and their

subsequent redistribution and activation by the annealing process.

This section of the report will be concerned with the development of a physical understanding of the lateral spreading process and the application of

predictive models.

To develop this necessary physical understanding, the basic mechanisms by

which ions come to rest in a solid will be briefly reviewed. It will be seen

that there are a number of different models for the ion-target interaction,

and these will be evaluated on the basis of their predictions of the depth

distribution of impurities resulting from implantation into bare silicon

surfaces. Both analytic and statistical (Monte-Carlo) methods will be examined. The approach will then be generalized to the case of two-dimensional

distributions, where it will be seen that the Monte-Carlo methods offer significant advantages. The effects of crystalline order on ion trajectories

will then be discussed, where it will be shown that ion channeling affects

the vertical, but not the lateral, dimensions. Finally, the modification of

the implanted profiles by diffusion during annealing will be briefly discussed.

ii) Interaction Mechanisms

As indicated above, the interaction between the incident energetic ion and

the target atoms is central to the prediction and understanding of ion ranges

in solids. Ions lose energy in a solid by two distinct mechanisms: through

collisions with the screened nuclei of the target atoms (nuclear stopping),

and also through interaction with the electrons surrounding the nuclei of the

target atoms (electronic stopping). In early formulations of ion range theories, the two processes were viewed as independent processes; but more recent

6
formulations allow for correlation of the two processes. As excellent re- views of the subject exist (e.g., [26]), only the salient features will be

summarized here to provide a background for the range calculations and to

interpret the experimental results.

Nuclear stopping arises from the interaction between the incident ion and the core ions in the solid. This process is viewed as being a seguence of twobody nuclear encounters; i.e., the incident ion scatters off one core ion at

a time. Further, for a given ion-ion potential, the dynamics of the collision process is formulated in terms of classical two-body scattering theory.

Also, because of the nature of the ion-atom scattering, it is this portion of

the interaction between the incident ion and the solid which is primarily

responsible for directional effects, i.e., the change of direction of motion

of the incident ion from that of the incident beam. These nuclear encounters

are responsible not only for the scattering of ions under a mask edge for

implantation into an amorphous target, but also for the correlated collisions

that produce ion channeling in a crystalline target.

Ions moving through a solid lose energy to electrons as well. Starting from

two different physical models, either Firsov [27] or Lindhard and Winther

[28] have derived that electronic stopping should be directly proportional to

the ion's velocity, similar to a viscous drag force. Also similar to a viscous drag force, electronic stopping is usually viewed as having no effect on

the particle's direction.

The relative importance of nuclear and electronic stopping in determining ion

penetration depths is influenced both by the ion's incident energy and by

crystal lattice effects. For ions that follow a trajectory in which the

substrate atoms appear to be randomly arranged, the lower the ion's incident

energy, the more important nuclear stopping will become in determining the

ultimate ion range. Ions that channel, however, do not approach the target

atoms closely enough to lose appreciable energy in nuclear collisions, and

thus lose most of their energy by electronic stopping. For heavy ions at low

energies (such as those studied here), the excess penetration of the ions

that channel over those that follow a random trajectory is extremely important in establishing vertical junction depth.

Thus far, nothing has been said about the specific form of the two-body potential which is responsible for the incident-ion core-ion scattering process. Historically, the work of Lindhard et dl. [29] made use of a screened

Coulomb potential. They have shown for several forms of this potential that

the energy and angular dependence of the cross section for ion scattering

from a target atom may be expressed as

where the function f((j>) depends upon the screening length, a; while the

variable (]> is an energy transfer variable defined by

(3.1 )

(|)2 = e2t/(ye) , (3.2)

where T is the energy transferred in the collision process, y® is the

maximum energy transfer, and e is a dimensionless energy

7
-1

e = M2E

+ M2 a4ire (3.3)

where M

M

12 ^2

e

ion mass target atom mass atomic numbers of the ion and target atoms, respectively, and

electronic charge in MKS units.

For purposes of analysis and calculation, the function f (<j)) may be expressed

in the form

which leads to a nuclear stopping power

S ((})) = [2<t)(2-2 )] n ' m

'

( 1 -u ) ^ du . q

(3.4)

(3.5)

This parameterization has been used by Winterbon to generate a code to

predict ion ranges in amorphous targets [30], and in that work the parameters

were also calculated for a number of potentials other than the Thomas-Fermi

form. A number of experimental studies (e.g. , [31]) have demonstrated that

the Thomas-Fermi potential overestimates nuclear stopping at low energies (e

from eg (3.3) < 0. 1 ) . Wilson, Haggmark, and Biersack (WHB) [32] have calculated nuclear stopping, power, range, and straggling in the low-energy region

for 14 diatomic interactions and have generated an average potential as a "universal" approximation. This work also demonstrated that, of the previously existing expressions for the interatomic potential, the Moliere potential was closest to the WHB average potential. The WHB average potential has

been parameterized by Chu et at, [31] in a form appropriate for the Winterbon

code and has been found to yield results in excellent agreement with experiment. Table 3.1 presents the choices of the parameters X, m, and q which

correspond to several choices of potential. Also, for purposes of comparison, the function Sj^(e) is plotted in figure 3.1. From this plot, it can

be seen that the various choices of the parameters represent various degrees

of potential range and strength, and thus each potential would predict different ion ranges and lateral spreading for a given particular implant. Also

shown are representative particular implant values of electronic stopping

power.

B) Ion Range Theories in One Dimension for Amorphous Targets

i) Introduction

Collisions with substrate atoms necessarily produce three-dimensional scattering of the implanted ions; however, implantation into large unmasked surface areas produces an overlap of large numbers of ion trajectories, thus

resulting in a laterally uniform ion density distribution. When the distribution of implanted ions varies only with depth into the solid, the distribution can be more easily determined than a two-dimensional distribution, since

most experimental techniques sample an area large compared to the lateral

8
TABLE 3.1 Parameterization of the Cross Section in Terms of the Potential

POTENTIAL X m q

Thomas-Fermi (Lindhard) 1.309 0.33333 0.666666

Chu-Murley (WHB) 2.542 0.25 0.50

Moliere 3.071 0.2157 0.5303

Lenz-Jensen 2.917 0.1909 0.5116

Bohr 2.37 0.1032 0.5696

Kalbitzer (Wilson) 2.54 0.26 0.475

The cross section as given by eq (3.1) is related

means of the appropriate choice of the parameters

to the potential listed by

X, m, and q as listed.

9
f|0)

r T 10

10

10

0

(a) Results for the Thomas-Fermi (Lindhard) (A), the Chu-Murley (WHB) (B),

the Moliere (C), the Lenz-Jensen (D), and the Bohr (E) potentials.

Figure 3.1 Plots of the fCcfi) function as given as eg (3.10) for several

representative potentials parameterized by the values of X, m, and q as given

in table 3.1.

10

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•H (—1 0 P

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01

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01 « o to 01 p > p

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11
spreading of an ion beam. These one -dimensional distributions thus provide

an important check of computational and analytic procedures and provide a basis for the two-dimensional studies to be described later.

Sanders [33] has shown that the spatial distribution in one dimension of ions

implanted into an amorphous target follows the linear Boltzmann equation of

the form

where F(X,V) is the distribution function of the implanted ions which depends upon the vector position, X, and upon the particle velocity, V; N is

the number density of target atoms; da is the scattering cross section which

depends upon the potential between the incident ion and the core-ions of the

target; and is the electronic stopping power. Central to the use of

this equation is the assumption that the collisions between the incident ion

and the lattice core-ions are binary and that the collision is of effectively

zero-time duration. The justification of the use of the linear Boltzmann

equation and a derivation have been given by Hynes and Deutch [34] following

the previous work of Albers and Deutch [35].

The linear Boltzmann equation describing the distribution of implanted ions

is an intergo-dif f erential equation which is difficult to solve in general.

Instead of solving for the distribution function directly, the theories presented by Lindhard, Scharff, and Schi^tt [29] and by Winterbon, Sigmund,

and Sanders [36] rely upon the solution of a set of coupled equations for the

moments of the distribution. These methods will be referred to as analytic

solutions, since they attempt to solve the Boltzmann equation for the form of

the range distribution directly. An alternative method (as exemplified by

the TRIM code [37]) relies on Monte-Carlo methods [38] to construct the ion

distribution by solving the equations of motion for a single ion and then

repeating the calculation for a statistically significant number of ions with

randomly chosen initial collisions.

ii) Analytic Codes

In this study, the amorphous target range code generated by Winterbon [39] was used because of the following advantages:

1. The code is available from its author on request.

2. The code allows the option of correlating nuclear and electronic

stopping.

3. The code allows the use of different ion-atom potentials.

4. The code solves the moment equations directly rather than iterati vein general, the moments of the distribution (in one dimension) are given by

the equation for a point or plane-sources and a subsequent expansion of the

angular dependence of the equation in Legendre polynomials. The first condition for the plane source reduces eq (3.1) to

V 9F(X, V)

v| 3X (3.5)

ly for both x- and y-directions

.

12
while the Legendre polynomial expansion as given by

F(cos Q) = I (2A + 1) , a leads to a further reduction as

(3.8)

9F 9F

-i - (A + 1)

^ = (2JI + 1) S + (2£ + 1) n/ fa {f (X,E) - I 3X 8X e 3E e

- (cos (|)') F^(x,E') - e P^(cos (|)") F^(S,E")} . (3.9)

Introducing the moment equation, which in the present case, takes the form

•: />F^(X) dX , (3.10)

leads to the equation

+ (z+i) fj;',)

2£+1

3F

S = e 3E f

Z + N I da{F^^(E) - Pj^(cos' )Fj^^(E'

)

(3.11)

- ^ P. (cos 0") f/(E")} .

This equation represents a recursion relation, in an integro-dif ferential

equation form, for the Legendre expansion coefficients for the moments of the

distribution.

Without going into the details of the analysis and the accompanying code, the

version of the Winterbon program used here solves the system of equations for

the moments by means of a spline routine.

Once the moments have been calculated from the code, construction of the

distribution from the moments is required. Unfortunately, this construction

is not necessarily unique. The specific technique used most frequently in

the construction of the distribution from a limited number of moments (four

in this case) is due to Pearson (e.g., [40]). The input information in this

scheme for the construction of the distribution are the mean (first moment

relative to the surface), the standard deviation (the second moment relative

to the surface), the skewness (related to the third central moment), and the

kurtosis (related to the fourth central moment). The mean and standard deviation are given by eg (3.6) for the cases of n = 1 and n = 2, respectively.

The skewness and the kurtosis are defined relative to the mean and the standard deviation by

skewness = j ? . ? , (3.12)

(standard deviation)^

and

, ,

. <(X-<X>)^>

kurtosis = (standard deviation)^(3.13)

13
respectively. Depending upon the values of the skewness and the kurtosis,

several forms of the resulting distribution may arise. These are usually

referred to by the appropriate Pearson type.

It should be noted that this scheme, the Pearson scheme, is not the only

scheme for the construction of the distribution from the moments. There are

a number of other schemes which might be used. However, in this report,

attention is primarily focused upon the Pearson systems.

The results of the Winterbon code are sensitive not only to the choice of the

potential, but also to the inclusion of correlated electronic scattering

(through an impact -parameter dependent electronic coupling). Figure 3.2

represents the results of the calculation of arsenic implanted into silicon

at 150 keV at a fluence of 5 x 10^^ cm“^ for several choices of the potential. It can be seen that the potentials predict approximately the same modal depths but significantly different distributions, including peak densities and junction depths.

iii) Monte-Carlo Code

An alternative method which has been used in the calculation of ion ranges

makes use of the Mojite-Carlo technique [38]. The version of this method

employed in this study is due to Biersack and Haggmark, the TRIM (Transport

of Ions in Matter) code [37]. The central point of the Monte-Carlo technique

which TRDi uses is not to solve an equation for the distribution for the

implanted ions but rather to consider the details of the motion of individual

ions as they travel into the target. For a given form of the scattering

potential, the motion of individual incident ions is calculated until the

energy falls below a predetermined limit at which point the ion is considered

to have come to rest. The histories of a large number of incident ions (usually at least 1,000) are used to construct histograms of the number of ions

which have come to rest in given depth intervals from the surface. These

histograms represent the Monte-Carlo predictions of the range distributions.

The version of the TRIM code used here employs the Moliere potential previously described with uncorrelated nuclear and electronic stopping. The results of this code for the case of 150-keV arsenic implanted into silicon are presented in figure 3.3, where they are contrasted with the results of the

Winterbon code run for identical conditions. As can be seen from the figure,

the two techniques yield very similar results for the same potential, thereby

providing confidence in the numerical methods used for each technique.

iv) Comparison to Experiments

In figure 3.4, experimental results obtained by secondary ion mass spectrometry (SIMS) for a 150-keV arsenic implant at a fluence of 5 x 10^^ cm~^ into a bare silicon surface are compared to the predictions of both the analytic and

the Monte-Carlo codes.

In this figure, the results of the Monte-Carlo code have been left as histograms to help distinguish the various curves. As seen in this figure, both

the analytic and the Monte -Carlo methods produce good agreement on the modal

depth and peak density, as would be expected from figure 3.1. Both calcula14
Figure 3. 2 Predictions of the Winterbon amorphous target code with several

choices of the potential for the case of a 150-keV implant of arsenic into

silicon at a dose of 5 x 10^^ cm“^. The potentials used were: Thomas-Fermi

(Lindhard) (A), Chu-Murley (Wilson) (B), Moliere (C), Lenz-Jensen (D), and

the Kalbitzer (Wilson) (E) forms. It can clearly be seen that the potentials

predict approximately the same peak location but rather different junction

depths

.

15
(eUID/suiojb)

AIISN30

0IN3SHV

Figure 3.3 Comparison of the TRIM (Monte-Carlo) code (A) and the Winterbon

amorphous target code (B) for 150-keV implant of arsenic into silicon at a dose of 1 X 10^^ cm“^. Both calculations make use of the Moliere potential.

16
DEPTH I fx m)

Figure 3.4 Comparison of profiles for 150-keV arsenic implanted into silicon. Curve A: TRIM code (Moliere); curve B: analytic code (WHB average

potential); curve C: atomic profile as determined by secondary ion mass spectroscopy.

17
tions tend to decrease more rapidly with distance away from the surface than

do the SIMS. A "tail" in the distribution has been seen by other techniques

and has been largely attributed to ion channeling during the initial stages

of the implant (before a continuous amorphous layer had been formed) [17].

The topic of ion channeling will be discussed in the next section.

Nevertheless, from figure 3.4 it is seen that the amorphous target codes

provide an excellent estimate of ion ranges for the majority of ions

implanted into the crystal under "random equivalent" (minimum channeling)

conditions.

From the point of view of the prediction of one-dimensional ion ranges in

amorphous targets, each of the above approaches has its strong and weak

points. The Winterbon code is relatively fast, but relies upon a seemingly

inconsistent method for the construction of the distribution from the calculated moments. Nevertheless, the fact that a finite number of parameters can be found to describe a continuous distribution allows rapid estimation of

trends and thus a useful scaling for prediction of implant distributions.

The Monte-Carlo code, on the other hand, follows the details of the motion of

the incident ion and constructs the histogram from the histories of the incident particles. In the region of the peak, where there are a significant

number of the incident particles which have come to rest, the effects of

noise are minimized. However, in the tail of the distribution where the

concentration is approaching the substrate dopant level (forming a junction

if the substrate is of the opposite conductivity type), there is usually an insufficient number of incident particles which have come to rest so as to be

able to predict such important parameters as junction depths. Basically, the

noise level of the data in that region, being inversely proportional to the

square root of the number of particles in the interval, is very large.

Hence, both techniques have their limitations for calculating one -dimensional

distributions. Nonetheless, the agreement between the two predictions for

depth distribution of dopants as presented in the figure is very good.

C) Ion Range Theories for Point Sources

i ) Introduction

As has been discussed in the introduction, two-dimensional effects dominate

the operating characteristics of fine-geometry semiconductor devices. For

this reason, the prediction of the implanted ion density in two dimensions

and its evolution during thermal annealing are of critical importance. Fortunately, Monte-Carlo techniques are particularly well suited to the calculation of ion range distributions in two dimensions, while analytic theories

suffer from difficulties in reconstructing multidimensional distributions

from their moments. As all of the physics of the ion penetration process are identical for one-dimensional and for multidimensional range calculations,

the only differences in the codes are the extraction of additional parameters.

ii) Analytic Theories

The question of lateral spreading of the ion beam was initially addressed by

Lindhard et ai, [29], who developed an integro-differential equation for the

lateral spreading of an ion beam along with the integro-differential equation

18
for the depth distribution of implanted ions. Winterbon et at. [36] were

able to solve these equations for a power law potential to obtain estimated

equal density contours to 1 0 percent of peak density by summing a total of 30

moments in the incident and transverse dimensions. Since such a calculation

would require major revisions of the Winterbon range code for each ion-energy

combination chosen, this method was not adopted in the present study. The

other approaches to constructing two-dimensional distributions from analytic

solutions require the use of simplifying assumptions.

Furukawa et at. [41] assumed a Gaussian distribution in three dimensions as

the shape of the ion distribution from a point source. This particular mathematical form of a Gaussian has particular advantages in constructing the

distributions that result from scattering of a distribution under a vertical

mask edge. The difficulty with applying the Gaussian approximation is that

range distributions, even in one dimension, are rarely Gaussian - especially

for common implants in silicon such as boron [42] or arsenic (e.g., [43]).

iii) Monte-Carlo Codes

While difficulties arise in estimating lateral contours from analytic codes,

the application of Monte-Carlo codes to lateral penetration problems is quite

straightforward, since the code must follow the three-dimensional path of the

ion to calculate the total energy loss along its trajectory. Thus, the program need only note the resting point in three dimensions instead of just the

vertical component of the rest position.

As before, the major difficulty in applying Monte-Carlo techniques is that a significant number of histories must be followed to obtain reasonable statistical accuracy. However, since the ion rest position is being noted in two

dimensions, more histories (typically at least 10,000) are required. In two

dimensions, the Monte-Carlo techniques share a common disadvantage with the

analytic techniques in that there is no convenient (but accurate) finiteparameter description of two-dimensional density contours. A Monte-Carlo

approach to the spreading of boron implanted into silicon was obtained by

Shimizu et at. [44], who found good agreement not only with the depth profile

of dopants but also with junction staining experiments as well. They also

found that in addition to not being Gaussian in the vertical direction,

neither was the lateral spreading well described by a Gaussian.

In the present study a slightly modified version of the TRIM code [37] was used to model the lateral spreading process. The TRIM code has incorporated

techniques that reduce computer usage by an order of magnitude for the same number of histories without sacrificing numerical accuracy, thus making the

calculation of the increased number of trajectories needed to calculate twodimensional distributions feasible. The code was modified to quantize the

lateral spreading in cylindrical coordinates (for point source distribution)

or in rectangular coordinates (for calculation of scattering under a mask

edge )

.

iv) Comparison of Predictions

Shown in figure 3.5 are the equal density contours in cylindrical coordinates

for 75-keV arsenic implanted into amorphous silicon at 0-deg tilt. The pre19
q:

IuHc

(am

(0 >

0)I in

r'

M-tO

<VU

i-t 3o

to

+Jc

•r4aeo

4-1

t7> Cs

to

QJ

>-i •

0)

touc

0) Pa uc

•H -iH P

+J CP

to <U TJ

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20
dictions of the Furukawa theory, the Winterbon code using a Pearson distribution vertically multiplied by a Gaussian distribution laterally (following

[45]), and the TRIM code are shown. From this figure it can be seen that the

lateral spreading, as well as the spreading in the vertical direction, as calculated by the TRIM code, varies drastically from that which would be

expected on the basis of a Gaussian distribution, with the maximum lateral

penetration occurring at the projected range (~50 nm) for the assumption of

uniform lateral scattering, while the TRIM code predicts maximum lateral

penetration at roughly three times the projected range (~150 nm).

D) Scattering Under a Rectangular Mask Edge

i) Introduction

When the ion density distribution in a solid F(x,y,z) resulting from a point

ion source is known (as was solved in the last section), the density distribution G(x,y,z) resulting from implantation into an unmasked surface A(y,z)

follows from superposition:

G(x,y,z) = J

F(x,y-C, z-n)dCdn . (3.14)

A

Thus, as was previously mentioned, the accuracy in calculating the ion

scattering under a mask edge is directly related to the accuracy for which

the point source ion distribution can be obtained.

ii) Comparison of Theories

The advantage in assuming a Gaussian lateral distribution for a point source

is that the solution for implantation into a rectangular mask edge can be

expressed in closed form as a complementary error function [41]. The alternative approach, adopted by Ryssel et at. [45] is to multiply the vertical

distribution by a Gaussian (for reasons that are not readily apparent). When

Monte-Carlo simulation techniques are applied to a grid with depth resolution

element A, eg (3.14) becomes

G(x ,y,z) = ^ ^ F(x,y-nA, z-i•mA) , (3.15)

n,m

where the indices n and m are chosen to fill the area A.

The results of these calculations are shown in figure 3.6 for 75-keV arsenic

implanted into silicon at 0-deg incidence. From this figure, it is seen that

the distribution as predicted by the Monte-Carlo codes (without the use of

simplifying or ad hoc assumptions) penetrates laterally approximately half

the vertical penetration and deviates markedly from the analytic solutions,

especially in the depth of the maximum lateral penetration.

21
(S

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cr2

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U-

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(C P0 1—1 E 1—1 m

nJO o'

*l^ 4-1 -P 3 p •H

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<U P 3 X

(0 (U o

>1

X CO CO P T3 p

fO

C (0 -3 P ‘H 3 c p; > 4-1 (U CD (0 E H M O

<D e 1 •iH fH p in T5

Q^ (U 3 E (P •iH 0 1 E U CO 0 CO

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(C E 4-1 0

>p £ 31 CO 0 H 3 (0

•rl

g

p 0) 4-1 o ,3 r—C 3 +J 3

T3 CO Eh 0 CO 0) P fO P

CU > •

CO rid <0 "3 3 CO

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-Q (C 3 (C •H 1—1

*Hp 3

IP

4-1 4J 3> S 3 CO 3

IC 0) •H cC 4J T3 4-1 3 0 U P a Sn 3 3 4-1 P b (D •r^ X CO 3 P E-i 3

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4-1 X! P 3

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CO 3 4-1 c (C5 P

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22
23

The

density

contour

after

Ryssell

using

a

product

of

a

Pearson

distribution

vertically

and

a

Gaussian

horizontally.

The

potential

used

was

the

Wilson-Haggmark-Biersack

average

potential.
(c)

The

density

contour

calculated

by

the

TRIM

code

using

the

Moliere

potential
E) Ion Channeling

i ) Introduction

The discussion of ion ranges has thus far been concerned with implantation

into amorphous targets. When implantations are performed into crystalline

targets, there is the possibility that the incident ions will travel much

farther than would be predicted by range theories for amorphous targets.

This excess penetration is caused by the correlated small-angle collisions

between the incident ion and substrate atoms lying along low-index crystallographic directions which prevent the transfer of large amounts of the energy

of the incident ion to substrate atoms (i.e., the trajectories reduce or eliminate nuclear stopping effects). Thus, ions that follow that type of

trajectory encounter a greatly reduced stopping power and thus penetrate

significantly greater distances compared to those ions which follow a trajectory for which the substrate atoms appear randomly arranged. The topic of

ion channeling has been the subject of a number of excellent reviews (e.g.,

[46,47]), and has been understood both on the basis of Monte-Carlo simulations and also as a result of analytic theories. This section employs the

analytic model of Lindhard [18] since this model is of sufficient generality

to provide a means to characterize a wide variety of implantation parameters

and to explain trends in the data. Lindhard 's model begins with the concept

of a string of atoms, which leads to the classification of an ion's trajectory in a single crystal according to its "transverse energy," i.e., the

component of ion's energy in the plane perpendicular to the atomic string.

Classification of the ion's trajectory according to transverse energy in turn

leads to the concept of a critical angle. It is then shown that while the

critical angle describes only the limit of applicability of the continuum

model, this concept provides a useful means of characterizing the implantation conditions. The effect of amorphous surface layers will be discussed

and the effects of ion channeling on the lateral implant profile will then be

discussed.

ii) The Continuum Model of Lindhard

To model the channeling effect, Lindhard approximated the substrate lattice

by a series of strings of atoms parallel to the low index crystalline directions. If V(r) is the potential energy between an isolated atom and the

incident ion, then the potential of an ion due to an infinite string of atoms

at an average spacing d along the x-axis (fig. 3.7) is given by:

Lindhard then argued that since the ions are moving with high velocity, and

since the collision angles are small for channeled trajectories, the channeled ion would interact with many atoms at once. For these ions, details of

the atomic nature of the lattice would become blurred, so that the atomic

distribution would appear uniform. Mathematically, this would imply the

limit of a continuous string

(3.16)

25
26
(3.17)

CO

Since the potential energy now depends only on p (the distance from the

string), the only effective forces on the ion are normal to the row. This

treatment neglects the contribution of electronic stopping which, since it is

purely frictional, provides no directional effects. The model has thus reduced the classification of the ion's trajectory in a three-dimensional crystal to the problem of the motion of the ion in a plane perpendicular to the

atomic string. Neglecting electronic stopping, the total energy of the ion

in this transverse plane (Ej^) is a constant, equal to the sum of the potential energy U(p) plus the transverse kinetic energy Ej^. For an ion

incident to the string at an angle ij; with initial kinetic energy E given by

where is the mass of the ion and v is the incident velocity of the ion,

the transverse kinetic energy is given by

1 1 E^ = — (vsinij;)^ ”

"2 ~ • (3.19)

Thus, the total transverse energy is the sum of the kinetic energy plus the

(repulsive) potential energy of the ion with respect to the string

= U(p) + = U(p) + E\j;2 . (3.20)

According to the continuum model, ion trajectories can be classified ac- cording to their transverse energy, the effects of which on ion range profiles has been discussed in [19]. Of greater concern in the present study

are the conditions under which an ion can be expected not to channel. Ions

with transverse energy greater than some critical value (E,,) approach the

string of atoms closely enough to see the substrate atoms as individual scattering centers. Transverse energy is no longer conserved, and the continuum

model of the atomic string no longer holds. Since the orderly arrangement of

lattice atoms plays no part in influencing the trajectory of this last class

of ions, the substrate atoms are randomly arranged in regard to the ion's

motion. The distribution of this type of ion should therefore be described

by the range theories for amorphous targets.

The continuum model does not predict channeling ions with transverse energy

above the critical value E^. By eg (3,20), the transverse energy is a sum

of a potential energy term and a kinetic energy term. Two concepts are related to that of a critical transverse energy, i.e. , the critical approach

distance and the critical angle. The critical approach distance (Pc) is

the distance from an atomic string within which an ion can approach the

string with zero transverse kinetic enerw (4* = 0) and can still exceed the

critical energy. The critical angle ('['c) i® angle at which an ion can

enter the crystal in a region of zero potential energy (U = 0) and have

transverse kinetic energy egual to the critical energy. Thus,

E = = ufp I

. ( 3. 21 ) c c ^ c' 21
The ability to predict channeling behavior from the continu’om model thus

reduces to the problem of finding the critical approach distance (Pc) as a function of incident ion energy E. Fortunately, a solution to this problem

has been given by Morgan and Van Vliet [20], on the basis of computer modeling. Using their model to calculate the minimum approach distance (Pmin}

and including the thermal vibrations of the lattice, (typically

as modeled in the Debye approximation [47]), the critical approach distance

is given by [46]:

p2 = p2. + u^ , (3.22)

c min rms

The critical angle can then be calculated from eg (3.21) by substituting

p^ into the expression for string potential,

= V^u[p^)/E . (3.23)

Parameters for the implant conditions studied here are summarized in table

3. 2.

It is important to realize that the calculated critical angle corresponds

only to the limits of applicability of the continuum model and not to the

angular extent of channeling. A number of studies (e.g., [22,46]) have demonstrated that channeling characteristics typically continue until the ion

beam is oriented at least twice the classical critical angle from the nearest

low index direction. Nevertheless, the critical angle does provide a convenient scaling parameter to characterize implant conditions and can be used to

calculate the substrate tilt angles for which unintentional channeling can be

minimized [22]. In addition, the continuum model can also be used to estimate the effects of amorphous surface layers on trajectories in underlying

crystals and to estimate the effect of channeling on lateral profiles.

iii) The Effect of Amorphous Surface Layers

Implantations into silicon for the fabrication of electronic devices are often performed through amorphous surface layers resulting from surface oxidation or from ion sputtering [48]. The effect of a surface amorphous layer

is to introduce an angular divergence in the ion beam due to nuclear collisions in the amorphous layer [49]. The result of this angular divergence is

a distribution of angles at which the ions transmitted through the amorphous

surface layer encounter the crystallographic axes of the substrate, and thus

a variation in the probability of ion channeling (i.e., deep penetration) by

the incident ions [23].

This decrease in the probability of ion channeling can be estimated using a procedure developed by Lugujjo and Mayer [50]. This method approximates the

variation in the probability of ion channeling from nearly unity at direct

angular alignment to zero probability at twice the critical angle by a step

function probability at the critical angle; i.e., in this approximation, an

ion has unity probability for channeling when incident at an angle above the

critical angle (fig. 3.8). This assumption is essentially egui valent to

assuming that the probability of ion channeling as a function of angle of

incidence is symmetric about the critical angle when integrated over an angular distribution of finite width. Note that this is not equivalent to claiming an ion implanted directly into a crystalline substrate would have zero

28
TABLE 3.2 - Dechanneling of Arsenic

Amorphous Surface Layers

Implants into (100) Silicon by

for 0-Deg Incident Beam.

Amorphous Energy Pc i" 9c in Thickness for

Layer (keV) substrate

(nm)

substrate

(degrees)

>90% dechannelii

(nm)

Si 75 0.0349 3.93 8.04

Si 150 0.0274 3.72 14.0

Si02 75 0.0349 3.93 7.41

Si02 150 0.0274 3.72 12.9

29
APPROXIMATE

00 4-1 • 0) cn sj 0) (U 3 +J

!Ti IH •H m

30

Variation

in

probability

of

ion

channeling

versus

tilt

angle,

and

step

function

approximation
probability of channeling for alignment at angles greater than the critical

angle. The actual angular spreading particular to each set of implant conditions is estimated from the plural scattering theory of Meyer [49], The

amorphous layer thicknesses required to produce 90-percent dechanneling of

the ion beam for the arsenic implant conditions studied here are shown in

table 3.2, where it is seen that only 14 nm of surface damage or surface

oxide would be required to totally dechannel the incident ion beam.

iv) "Lateral Channeling"

Ion channeling has been shown to strongly affect the depth distribution of

dopants resulting from implantation into initially crystalline targets. A

natural extension is to inquire to what extent implantation will modify the

lateral distribution of dopants. In a ( 1 00) -oriented wafer, both the [010]

and [001] directions run perpendicular to the [100] surface normal and are atomically identical. By employing the physical principles described in

preceding sections, however, it is possible to show that there is negligible

probability of ions channeling parallel to the implanted surface.

Earlier, it was pointed out that ions cannot channel parallel to an atomic

string if they start their trajectories at too high an angle to the string or

if their trajectories begin too near an atomic string. According to table

3,2, the closest an ion can get to the string for these implants in the continuum model and still channel is approximately 3 x 10“^ cm. Collisions that

produce large-angle scattering require impact parameters of approximately

10“^^ cm [51], Thus, collisions that will deflect the incident ions through

an angle large enough to encounter a low index lateral direction also require

the ion to start out with too large a potential energy transverse to the

lateral channel. For this reason, channeling in the lateral direction is a highly unlikely process, and calculations of lateral penetration from the

amorphous target codes should accurately predict the contours for the implants into crystalline substrates as well.

F) Electrical Activation of Arsenic-Implanted Silicon

Previous discussions have described the methods used to predict the resting

points of the implanted ions. However, to fabricate electronic devices,

these implanted impurities must be made electrically active by incorporation

into the silicon lattice. The electrical activation of arsenic implanted

into silicon has been studied for laterally uniform implants, and the understanding thus obtained v/ill be briefly reviewed here.

Room temperature implantation of arsenic at doses above 3 x 10^^ cm“^ leads

to the formation of an amorphous layer at the silicon surface [52], This

layer regrows epitaxially at temperatures below 600°C, with no detectable

redistribution. This regrowth produces excellent incorporation of the implanted arsenic on lattice sites and complete electrical activation of these

impurities up to a solubility limit of approximately 3 x 10^® cm“^ [53], For

( 1 00) -oriented silicon substrates (as are examined in this study), no lattice

disorder has been detected by MeV ion channeling techniques [53], These

studies also determined that incomplete electrical activation of the implant

arsenic (for (100) silicon) occurred only near the peak of the ion distribution, where the implanted ion density exceeded the solubility limit. Thus,

31
the 600°C annealinq temperature examined in this study should provide an electrically active marker of the implanted arsenic distribution for arsenic

densities below the solubility limit. The other annealinq temperatures examined in this study (1000°C), and the fluence level chosen, correspond to

arsenic densities that will redistribute by concentration dependent diffusion

effects [53] - the unders tandinq of which is important to device process

modeling.
4. EXPERIMENTAL

In this section the experimental techniques used in this study are reviewed,

and the results obtained by each are presented. First, the details of the

sample preparation are presented. The spreading resistance technique and

results, the etch technique and results, and the induced current technique

and results are then discussed in turn.

A) Sample Fabrication

Initial sample preparation was performed at Hughes Research Laboratories. A

50-nm oxide was grown on boron-doped silicon wafers. The oxide was then

coated sequentially with 1.2 pm of photoresist, 0.2 ym of aluminum, and 1 pm

of photoresist. The top layer of resist was exposed in a grating pattern

(alternating lines and spaces) of either 6.7-ym or 13-ym period. This layer was developed and etched using conventional wet chemistry. The aluminum was

then plasma etched. The bottom layer of resist and the resist over the aluminum was then reactive ion etched. Finally, the oxide was etched in a conventional acid etch. The resulting mask structure is shown schematically in

figure 4.1. The reactive ion etch was used to create extremely vertical

walls in the resist. The success of this technique can be seen in figures

4.2 and 4.3, which show typical patterned wafers. These vertical walls assure that none of the implanted ions would penetrate through the mask near

the edge of the junction.

The wafers were then implanted using the goniometer end station at Hughes

[19]. All the wafers were (100) oriented, and the grating pattern was also

lined up in the (100) direction. All the implants were 5 x 10^^ arsenic

lons/cm^. The samples were tilted with respect to the ion beam, parallel to

the grating lines, at four angles: 0, 4, 7 and 10 deg. Two implant energies

were used; 75 keV and 150 keV.

At NBS the photoresist and aluminum were stripped off in acetone and the

wafers were cleaned using a standard peroxide solution [54]. The wafers were

then annealed in dry nitrogen. Two annealing temperatures were used: 600°C,

to remove the initial damage and activate the As with minimal redistribution

and 1000°C, to determine the effect of diffusion on ultimate junction shape.

The wafers were then thinned from the back side. This step was found necessary to minimize crack and step formation on the cleaved surface. Finally,

the wafers were cleaved perpendicular to the grating fingers.

Two final steps were employed. For the etch technique, the cleaved edge of

the sample was etched in a solution of two-percent hydrofluoric acid in nitric acid in ambient light for 5 s. This photosensitive etch selectively

attacks n-type, but not p-type, silicon. Care was taken to try to contact

only the cleaved edge with the etch. The samples were then rinsed in water.

For the induced-current technique, a contact was deposited on the top surface. Typically, silver paint was used and contact was therefore made in

parallel to a large number of junction stripes.

Following the photochemical etch, the samples were mounted in the scanning

electron microscope for examination using conventional secondary electron

imaging techniques. It was not necessary to coat the samples. We did find

33
Al

ETCH

MASK

(0.2

^m)

(CHEMICAL

OR

PLASMA

ETCH)

34

Figure

4.1

Schematic

diagram

of

the

resist

structure

used

to

mask

the

implants.

The

top

layer

of

resist,

which

was

used

to

define

the

aluminum,

is

completely

etched

by

the

reactive

ion

etch

and

is

not

shown.

Vertical

and

lateral

dimensions

are

not

to

scale.
35

Figure

4.2

SEM

micrograph

of

the

photoresist

structure

used

to

mask

the

implants.

The

grating

period

is
361

Figure

4.3

SEM

micrograph

of

the

photoresist

structure

used

to

mask

the

implants.

The

grating

period

is
it necessary to short the top n-type regions to ground to prevent charging

effects. The effect of the etch is to create a roughly hemispherical ledge

at the top corner of the cleaved surface. The boundary of the etched volume

on the cleaved surface is determined by the junction, i.e., where the arsenic

concentration from the implant is equal to the boron concentration in the

substrate. The amount of etching along the top surface is determined by the

dynamics of the etching process. Figure 4.4 illustrates the situation schematically and figure 4.5 is a picture of such a ledge at the corner where it

meets the oxide stripe.

B) Spreading Resistance

Beveled spreading resistance measurements were made on unpatterned wafers

implanted with arsenic at 75 keV at a dose of 1 x 10^^ cm“^ . The substrates

were p-type (boron-doped) (100) silicon with resistivities of 0.3 fi»cm and 5 fl*cm (boron concentrations of approximately 6 x 10^^ cm“^ and 2.7 x 10^^

cm“^, respectively). The implanted wafers were annealed for 30 min at 1000°C

in nitrogen and were then beveled at an angle of approximately 4 min of arc. Probes which left a 2-iJm diameter footprint were used, with the loading in

the range from 4 g to 10 g. The step between the data points along the bevel

was between 1 pm and 10 pm. The probe -to -probe spacing was approximately 40

ym.

The beveled spreading resistance data were used to provide information about

the vertical profile as well as the junction depth as obtained from the location of the cusp in the data. In addition, a selective stain was used to

demark the region between the implanted arsenic region (n-type) and the

boron-doped substrate (p-type).

Table 4.1 summarizes the results of the junction location as obtained from

the cusp of the spreading resistance data and as obtained from the stain

technique. There are two observations which can be made. First, the location of the junction as obtained from the spreading resistance cusp is systematically sensitive to the probe load, with the junction appearing deeper

for lighter loads. This is expected on the grounds that heavier loading

causes a strain-induced shorting of the junction. Unfortunately, the loading

is limited by noise in the voltage signal which becomes larger at light

loads. For this reason the load could not be reduced below 4 g. Second, the

stain technique shows the junction to be deeper than that obtained from the

spreading resistance cusp. The reason for this difference is not clear but

may, in part, be due to the probe loading and, in part, to difficulties with

the stain.

As indicated above, the spreading resistance measurements were also used to

obtain the vertical profile of the implanted arsenic. The data were analyzed

according to the technique due to Dickey [55]. Other methods of analysis are possible [56], which have been compared on model data [57]. It is important

to emphasize that the spreading resistance technique provides a profile of

the electrically active dopants. Hence, unlike the SIMS technique which

measures atomic profiles, the spreading resistance technique is sensitive to

the state of activation of the dopant. As a check on the resistivity profile

obtained from the Rgp technique, the sheet resistance was obtained from

integration of the profile and then compared directly with that obtained from
Figure 4.4 Schematic diagram of an etch step at the corner of the grating

pattern.

38
Figure 4.5 SEM micrograph of an etch step. The sample is tilted 30 deg to

normal incidence.

39
TABLE 4.1 - Comparison of Junction Depth Locations Obtained from Spreading Resistance and Stain

Techniques.

Specimen

Substrate

Resistivity

Tangent of

Bevel angle

Probe

step (ym)

Probe

loads ( gf

)

front/rear

Junction

depth from

cusp of

spreading

resistance

data

Junction

depth

from #

steps in

stain

band

Junction depth

from calibrated

magnification

of stain

0.3 0*cm 0.0012 ± 15% 5 ym 7/5 1 38 nm 1 68 nm -

5 ym 7/5 1 44 nm 1 68 nm -

5 ym 7/5 150 nm 1 98 nm -

5 ym 7/4 1 20 nm 1 98 nm -

5 ym 7/5 1 80 nm 1 92 nm -

5 ym 7/4 1 68 nm 1 92 nm -

0.3 J2*cm 0.0043 ± 3% 2.5 10/10 113 nm 161 nm 171 nm

2.5 6/6 1 83 nm 1 94 nm 1 71 nm

2.5 6/6 1 61 nm 1 88 nm 1 79 nm

5 10/10 1 29 nm 1 61 nm 1 79 nm

0.3 0*cm 0.0053 1 ym 6/4 1 60 nm 1 96 nm 235 nm

2.5 ym 6/4 1 46 nm 200 nm 235 nm

1 ym 6/4 1 81 nm 213 nm 235 nm

1 ym 5/4 154 nm 218 nm 235 nm

5 J2*cm 0.00092 1 0 ym 152 nm 1 93 nm 200 nm 0.00092 1 0 ym 1 56 nm 1 93 nm 200 nm 0.00101 1 0 ym 1 62 nm 1 92 nm 1 97 nm

0.001 1

1

5 ym 1 94 nm 1 94 nm 1 97 nm

0.001 1

1

5 ym 1 78 nm 1 89 nm 1 97 nm

40
the four-probe method. In the present case, a comparison of the sheet resistances shows agreement to within 20 percent. On the other hand, the dose

obtained from the profile differs from the value originally used by as much

as 40 to 50 percent. This difference between the integrated carrier density

and the implanted ion dose is probably due to the incomplete activation problem mentioned above. In addition, the use of resistivity-dopant density

relations obtained for phosphorus and applied to arsenic as well as calibration errors may be responsible for this difference. A typical spreading

resistance profile as compared with the predictions of the amorphous target

codes and prediction of arsenic diffusion by SUPREM [58] is presented in

figure 4.6. It is important to emphasize that the amorphous target codes are concerned with the distribution of the implanted ions with no predictions

made about the state of activation of the dopants. The spreading resistance

technique is sensitive to the resistivity variation, and hence the carrier

density variation, of the implanted layer. The reason for the discrepancies

remain to be determined.

C) Etch Technique

i ) Contrast Mechanism

All the secondary electron pictures were taken with a beam energy of 30 keV

and a beam current of 1 0 to 50 pA. These are optimum conditions for high

resolution microscopy in our machine. Under these conditions the diameter of

the electron beam spot at the surface of the sample can be approximately 0.01

ym. Thirty-kiloVO It electrons have a range in silicon of approximately 6.5

ym [59]. Over this range they lose energy in collisions with the electrons

in the sample and scatter laterally. Contours of equal energy deposition in

the silicon therefore have a characteristic "pear" shape. This situation is

suggested schematically in figure 4.7 for a beam incident normal to the

cleaved surface near the edge of an etched step. The secondary electron

spectrum peaks below 50 eV. These low energy electrons have a range of less

than 0.01 ym. When the beam is far from the edge, electrons excited near the

top surface have enough energy to escape the silicon and be collected by the

detector (a conventional phosphor-photomultiplier tube with a +300-V bias).

As the beam is scanned near the edge of the step, electrons excited deeper in

the silicon may escape laterally and be detected. Thus, as the beam moves

over the step, there will be a peak in the secondary signal. When the beam

is moved to the corner of the sample, there will be a large increase in the

secondary signal as secondary electrons excited all along the range of the

beam escape laterally. Two points are worth noting here. First, the geometry of the detector and the sample is in fact as suggested in figure 4.7.

The increase in secondary electron signal at the step is therefore enhanced

because the collection efficiency for electrons emitted laterally is much

larger than for electrons emitted from the top surface. Second, a typical

step height in this work is on the order of 0.2 ym. The 30-keV beam spreads

laterally very little in that distance. Therefore, for this type of sample

the high resolution inherent in the electron optics can be realized.

ii ) Etch Data

Figure 4.8 is a high magnification secondary electron image taken under the

conditions described above. The cleaved surface is the dark area in the

41
ARSENIC

DENSITY

|atoms/cm3|

Figure 4.6 Comparison of a typical spreading resistance profile with the

predictions of the SUPREM model using joined half-Gaussians and with the

amorphous target codes.

42
Incident electron

beam

Region of electron

energy dissipation

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Etched

region

Detector

Figure 4.7 Schematic diagram of the electron-beam-sample interaction near an etch step.

43
Figure 4.8 vSecondary electron picture of a cleaved and etched sample tilted

0.5 deg to normal. This sample was implanted with As'*' to a dose of 5 x 10 ^^/cm^ at 75 keV and 0-deg tilt, then annealed at 600°C for 30 min.

44
lower half of the picture, the 0.05-Mm thick oxide stripe is in the upper

left, and the etched junction area is in the center right. Several points

must be noted here. The cleaved surface is actually at an angle of 89.5 deg

to the incident beam. Dimensions on the cleaved area are given by the calibration bar, to within ~5 percent. The upper half of the picture, the top of

the wafer, is tremendously foreshortened. The edge of the oxide therefore

looks rougher than it really is, and the angle at which the stripes are tilted is exaggerated (the stripe appears to be tilted ~45 deg; actually it

is tilted ~0.5 deg). The symmetry of the particle of dust in the lower left

indicates the system is free of aberrations. Finally, the etched area is

clearly delineated and the edge appears sharp to within ~0.01 ym.

The sample in figure 4.8 was implanted at 75 keV at 0-deg tilt and annealed

at 600°C for 30 min. The junction is very shallow (0.18 ym) and does not

extend under the oxide. The apparent roughness of the depth of the junction

region was repeated over a number of junctions. It may be due either to uneven etching or uneven activation effects at these conditions of low temperature and high concentration.

Figure 4.9 shows a similar picture, at slightly lower magnification, of a sample implanted at 150 keV at 7-deg tilt and annealed at 600°C. The junction is deeper here, 0.27 ym, and again does not extend under the oxide.

Allowing for the increased depth, we note a certain similarity of shape between the lateral profiles. In this case the junction depth is not rough,

further suggesting a possible concentration-dependent problem with the previous sample.

Figure 4.10 contains two pictures, both of samples implanted at 75 keV at 7-

deg tilt. In figure 4.10a, the sample was annealed at 1000°C for 30 min; in

figure 4.10b, at 1 000°C for 60 min. By comparison with figure 4.7, the arsenic has clearly diffused considerably during this treatment. Between figures 4.10a and 4.10b, the depth of the junction does not change drastically,

but there is considerable lateral diffusion under the oxide and the shape of

the lateral profile changes.

iii) Interpretation and Problems with Etch Data

Unfortunately, after a number of other samples were examined, a systematic

problem with the etch was discovered. The difficulty shows clearly in figure

4.11, which is a picture of a sample implanted at 150 keV at 1 0-deg tilt and

annealed at 600°C. Ihe etch has proceeded in two steps. The inner, deeper

region is actually what we would expect for these conditions, while the outer

region is an artifact. Unfortunately, in most cases the distinction is not

really clear.

Two possible explanations are suggested for this behavior. One explanation

is that, due to the high concentrations of arsenic, the small distances, and

the short times involved, the arsenic is not diffusing into the etch but

rather is forming a "puddle" in the etch which then redeposits on the silicon

and causes further etching. The other is simply that, again due to the

scales involved, the etch is simply depleting in the junction region. At

this time we have not determined how to correct this problem. We note in

45
Figure 4.9 Secondary elcc

- o^ jx

0.5 deg to normal. This ra.:r'i-Le ’ 10 at 150 keV and 7 ia:: 7x

tar.i of a cleaved and etched sample tilted

.Hiplanted with As"^ to a dose of 5 x ihea annealed at 600°C for 30 min.

46
(a) Annealed at 1000°C for 30 min.

Fiqiire 4.10 Secondary electron pictures of cleaved and etched samples tilted

0.5 deg to normal. Both samples were implanted with As'*' to a dose of 5 x 10 ^^/cm^ at 75 keV and 7-deg tilt.

47
48
Figure 4.11 Secondary electron picture of a cleaved and etched

deg to normal. This sample was implanted with As"*" to a dose of

at 150 keV and 1 0-deg tilt, then annealed at 600°C for 30 min.

double step.

sample 0.5

5 X 10^^/cm^

Note the

49
closing that submicrometer etching using wet chemistry does not give uniformly good results.

D) Induced Current Techniques

i ) Physics of Electron-Beam-Induced Currents

The SEM has been used to study internal fields and junctions in semiconductor

devices and junctions for some time [39]. While the details of the process

are complex, the underlying principles are well understood. An electron with

a few tens of kilovolts of energy incident on a semiconductor will collide

with the electrons and nuclei in the material and lose energy along a track

of a few microns. In this study 30-keV electrons, which have a range in Si

of 6.5 ym [59], were used. The excited electrons and holes created by the

initial collisions thermalize on a time scale of picoseconds, resulting in a cloud of electrons and holes around the track of the incident beam. In the

absence of any electric fields, the carriers diffuse outward until, after a minority carrier lifetime, they recombine. For the beam currents used in

this study, about 100 pA, plasma effects and high level injection effects do

not occur. Also, for this material the minority carrier lifetime was much

shorter than the time the beam dwelt on any point, so the excited carriers

were in equilibrium with the beam.

The electron -beam-induced current (EBIC) apparatus consisted of a low input

impedence, current-sensitive amplifier connected in series with a bias supply

across a diode. In practice, contact was made to the back of the whole sample (p-side) and to a large number of the n-regions on the top of the sample.

Thus, a large number of diodes were connected in parallel. The resultant

leakage current prohibited the use of any bias. The samples were cleaved and

viewed sideways at 89.5 deg as in the etch technique.

When the incident electron beam is directed at the p-n junction, electrons

and holes are created as described above. However, the built-in field at the

junction causes the carriers to drift apart, electrons to the n-side and

holes to the p-side, before they can recombine to result in a net current in

the external amplifier. If the incident electrons are directed within a diffusion length of the junction, some of the minority carriers will diffuse

to the junction and be swept across it before they recombine. Again, a net

current will be observed.

The pattern of energy deposition of the incident beam affects the interpretation of the EBIC signal in two ways. Most previous work using this technique

has involved more lightly doped, broader junctions in material with relatively longer lifetimes [60] . Under these conditions the lateral resolution was determined by the minority carrier lifetime and the beam scatter could be

ignored. The conditions of this study involved heavily doped, highly damaged

regions. Under these conditions the lateral broadening of the incident beam,

which can be several tenths of a micrometer, must also be considered. The

effects of lateral spread of the incident beam, diffusion length, and

spatially varying dopant and damage concentration all enter the problem with

comparable weight. In the other direction, the long penetration depth of the

incident beam perpendicular to the cleaved surface assures that the EBIC

signal will not be dominated by surface fields or surface recombination. The

50
EBIC signal, therefore, is expected to be very low until the beam is scanned

close to the junction. The current increases, peaks when the beam is at the

junction, then steadily decreases as the beam scans away from the junction.

ii ) EBIC Data

Figure 4.12 shows a composite picture taken with both secondary and EBIC

signals. The sample was implanted with As"*" at a dose of 5 x 10^ ^/cm^ and at

an energy of 75 keV and was then annealed at 1 000°C for 30 min. The sample

was viewed at a 0. 5-deg tilt. The oxide stripes in the top part of the picture were imaged by the secondary electrons, while the bright areas under the

implanted regions were due to the EBIC signal. The EBIC signal is clearly

following the junction shape.

Figure 4.13 is a higher magnification view of the sample shown in figure

4.12. The etch picture for this sample is shown in figure 4.10a. Note the

change of scale. The junction actually occurs almost at the top of the wide

band due to the EBIC signal. This result is to be expected. The region

below the junction is unimplanted, lightly doped, undamaged p-type material.

The diffusion length is long and dominates the current collection process.

The region above the junction is very heavily doped, damaged n-type material.

Lateral spread of the electron beam dominates the current collection, since

the lifetime is very small.

Figure 4.14 is a deflection modulation picture of the EBIC signal for the

sample shown in figure 4.13. In this mode, as the beam is scanned across the

sample from left to right, the amplitude of the signal is added to the ydisplacement of the beam rather than modulating the intensity. The asymmetry

of the signal can be seen clearly: the signal slopes off much more gradually

to the left than to the right. The apparent uniformity of the signal in

figure 4.13 is an artifact of the limited dynamic range of the photographic

films.

iii) Interpretation of EBIC Data

The complexity of the EBIC signal can best be seen in figure 4.14. Several

different slopes are apparent as the beam is scanned across the junction, due

to different processes increasing in importance. Resolution is somewhat

complicated in this context. On fairly general grounds it can be shown that

the junction occurs at the peak of the signal. The location of the peak can

be determined to within 100 nm on the linescan. Careful comparison with the

secondary image gives a junction depth of 0.15 pni (as compared to 0.18 for

the etch).

Further interpretation of the EBIC signal will require comparison with twodimensional model programs, which will in turn require computer data acquisition of the pictures. Such an effort could, however, yield information on the spatial variation of dopants and damage which is unavailable in the etch

data.

51
Figure 4.12 Composite secondary electron and EBIC micrograph. The sample

was implanted with As'*' to a dose of 5 x lO-^-^/cm^ at 75 keV and 7-deg tilt,

then annealed at 1 000°C for 30 min. The oxide shapes are seen in secondary

electron mode, while the bright regions under the implanted states are the

EBIC signal.

52
Figure 4.13 Higher magnification of figure 4.12.

53
Figure 4.14 Deflection modulation EBIC picture. The sample and the are the same as in figure 4.12. The asymmetry of the EBIC signal or unimplanted and implanted sides of the junction can be seen clearly.

geometry

the

'L

54
5. CONCLUSIONS

A number of important conclusions have emerged from this study. In this

section these will be noted, under the caveat that the range of conditions

over which they apply has not been fully determined.

First, junction etch and EBIC techniques have both been demonstrated with

submicrometer resolution. The junction etch technique has an observed edge

resolution of ~1 0 nm; however, control of the etch process may not be that

good. The EBIC technique has an observed resolution of ~1 00 nm; however,

improved data analysis may sharpen this resolution and provide additional

information. The suitability of these techniques on the scale required for

VLSI and VHSIC had not previously been shown.

Second, the etch technique may be compared with existing one-dimensional

theory. Figure 5.1 shows the junction depth determined by etching a sample

annealed at 600°C as com.pared with the junction depth determined by the as- implanted Gaussian profile used in SUPREM. The agreement is within 10 nm and might be even better if there were diffusion on that scale at 600°C.

Figure 5.2 shows the same comparison for the theories discussed in chapter

3. The agreement is again within 1 0 nm and the error is about the same. High dose As"^ implants are nearly ideal cases for these theories, so the

agreement should be taken as proof of the etch technique rather than as proof

of the theories. Figure 5.3 shows a comparison of the junction depth determined by etching and by two one-dimensional diffusion models for a sample

annealed at 1000°C in nitrogen for 30 min. The agreement here is not within

bounds, especially for SUPREM, and the deviation is in the wrong direction.

Diffusion of As in Si at high concentrations is known to be a problem, so

this result should be taken as failure of the models.

Third, the etch technique and the EBIC technique agree to within 100 nm, the

resolution of the peak in the EBIC linescan. This consistency argues well

for the validity of both techniques. Spreading resistance, however, consistently indicates shallower junctions than either the etch, EBIC, or the models. Reasons for this discrepancy have been suggested and are being pursued,

but the validity of submicrometer, high concentration spreading resistance is

in question.

Fourth, the nonlinear diffusion of As in Si is experimentally demonstrated.

Models which accurately predict this behavior, even in one dimension, do not

appear to be available.

Fifth, it has been shown that implanted As"*" does not appreciably scatter

laterally under the edge of a sharp mask. This result runs counter to some suggestions made in the past; however, this result is as expected from the

previous discussion on lateral channeling. The range of validity for this

conclusion must be established, and as more data become available, better

models can be developed.

Sixth, the theories available for one-dimensional ion implantation and diffusion have been examined. The implantation model appears to be in good agreement but the diffusion model appears to be somewhat less accurate. The calculation of two-dimensional implant profiles has been considered and has been

55
DEPTHlMtnl

Figure 5.1 Comparison of junction depths determined by junction etch and by

the as-implanted joined half -Gaussian model used in SUPREM for a sample implanted at 75 keV and annealed at 600°C. The line at 3 x 10^^ indicates the

background, opposite conductivity- type, dopant concentration. The depth at

which the Gaussian intersects this line is the predicted junction depth,

while the arrow indicates the location of the junction obtained by the etch

technique.

{

!

I

i

56
Figure 5.2 Comparison of junction depths determined by junction etch and by

several theoretical models for a sample implanted at 150 keV and annealed at

600°C. The line at 3 x 10^^ indicates the background, opposite conductivitytype, dopant concentration. The depths at which the various curves intersect

this line are the junction depths predicted by the various potential dependent models, while the arrow indicates the location of the junction as obtained by the etch technique. This figure would indicate that the ChuMurley, Moliere, and Kalbitzer forms of the screened potential yield the best

agreement with the observed junction depth.

57
DEPTH (yum)

Figure 5.3 Comparison of junction depths determined by junction etch and by

two models for a 5 x 10^^ cm“^ 150-keV arsenic-implanted sample annealed at

1 000°C for 30 min. The horizontal line represents the background dopant

concentration. The intersections of the curves with this line are the junction depths predicted by the two models for arsenic diffusion. Curve (A) is

the result calculated by SUPREM II, whereas curve (B) is the result calculated by Lowney.
shown to be in qualitative agreement with the experimentally determined twodimensional junction locations.

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63
NBS-114A (REV. 2 -ec) U.S. DEPT. OF COMM. 1 , BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

PUBLICATION OR

REPORT NO.

NBSIR 81-2398

2. Performing Organ. Report No. 3. Publication Date December 1981

Investigation of the Two-Dimsional Shape of Ion-Implanted Regions

5. AUTHOR(S)

P. Roitman, J. Albers, J. R. Ehrstein, and D. R. Myers

6. PERFORMING ORGANIZATION (If joint or other than MBS. see instructions)

national bureau of standards Sandia National Laboratories _

DEPARTMENT OF COMMERCE (U.S. Dept, of Energy Facility) 8 WASHINGTON, D.C. 20234 Albuquerque, NM 87185

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

DE-AC04-76-DP00789

. Type of Report & Period Covered

Final

Naval Ocean Systems Center

San Diego, CA 92152

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200~word or less factual summary of most sign ificant information. If document includes a significant

bi bliography or literature survey, mention it here) The two-dimensional shape of arsenic ion-implanted regions in single-crystal silicon

was investigated both experimentally and theoretically. Experimentally, two techniques were shown to have the necessary submicron resolution: a junction etch process and an SEM-induced current collection method. A comparison of junction depths

determined by the etch technique, the EBIC technique, and spreading resistance with

the depths calculated using several amorphous target codes was made. For the case

of low temperature (600°C) anneals, the etch technique agrees very well with the

junction depths predicted by the amorphous target code due to Winterbon. The lateral junction locations obtained from the etch technique are in good agreement with

the predictions of a two-dimensional Monte-Carlo code (TRIM) which indicates that

arsenic does not show any significant lateral scattering under mask edges. For the case of high temperature (1000°C) anneals, the etch and EBIC techniques agree with

each other, but show consistently deeper junction locations than does the spreading

resistance technique. Comparison with arsenic-diffusion models shows that

concentration-dependent effects are important. Presently available processing

models do not appear to adequately forecast junction depths.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Arsenic ion implantation; electron-beam-induced current; etching; integrated circuit

processing; ion channeling; ion implantation; lateral profiles; scanning electron

microscopy; silicon technology; spreading resistance; two-dimensional profiles.

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