A111Q3 Dfi755b

NATL ia«#^ltpECIAL PUBLICATION 400-39

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NATIONAL BUREAU OF STANDARDS

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AUG 1 19//

Semiconductor Measurement Technology:

\oo

Some Aspects of Dose Measurement for Accurate ^jd^^nL-:

Ion Implantation

Douglas M. Jamba

Hughes Research Laboratories

MaHbu, CaUfornia 90265

Jointly supported by: The Defense Advanced Research Projects Agency

and The National Bureau of Standards

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

'6. NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

k

Issued July 1977
Library of Congress Cataloging in Publication Data

Jamba, Douglas M. Some aspects of dose measurement for accurate ion implantation.

(Semiconductor measurement technology) (National Bureau of Standards special publication ; 400-39)

Supt. of Docs. no. €13.10:400-39

1. Radiation dosimetry. 2. Ion implantation. I. United States. National Bureau of Standards. II. Title. III. Series. IV. Series: United

States. National Bureau of Standards. Special publication ; 400-39. QC100.U57 no. 400-39 [QC795.32.R3] 602Ms [621.38I5'2'028]

77-24724

National Bureau of Standards Special Publication 400-39

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-39, 44 ' pages (July 1977) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $1.60 Stock No. 003-003-01808-2
CONTENTS

PAGE

1. Introduction and Summary 1

2. Ion Beam Scanning 4

2.1. Methods 4

2.2. Electrostatic Scanning 6

a. Efficiency . 7

b. Unifoirmity 7

c. Ion Beam Control 8

2.3. Target Area 10

3. Secondary Suppression 11

3.1. Electrode Design 11

3.2. Measurement of Currents 13

3.3. Recommended Circuit 13

3.4. Negative Secondary Emission 15

3.5. Positive Secondary Emission 18

3.6. Effect of Current Density 19

4. Current Measurement 22

4.1. Current Integrators 22

4.2. ' Measurement of dc Current 24

4.3. Off-Scale Current 25

4.4. Measurement of Pulsed Current 26

4.5. Effect of Pulse Width 27

5. Other Factors Affecting Dose Measurement 31

5.1. Neutrals , . . . 31

5.2. ac Interference - 33

iii
PAGE

5.3. Insulator Leakage . 34

5.4. Dose Calculation 35

References ......... 36

LIST OF FIGURES

Figure 1. CMOS p-channel threshold shift versus implanted boron

ion dose 2

Figure 2. Schematic of a 150-kV ion implantation system .... 4

Figure 3. Typical target configuration ..... 5

Figure 4. Theoretical effects of beam scanning ^. 6

Figure 5. Target current density as a function of scanning

voltage for 84-keV boron ions 8

Figure 6. Schematic drawing showing the effect of a drifting

ion beam at the scanner 9

Figure 7. Secondary suppression geometries 12

Figure 8. Implantation target current measuring circuitry ... 14

Figure 9. Implantation target currents versus suppressor

voltage using the circuitry and geometry of figure

8(b) with 3-cm^ target beam area 16

Figure 10. Suppressor current versus suppressor voltage for N"*" ion energies from 10 to 300 keV. Circuitry and geometry

are shown in figure 8(b) with target beam area

9 (5-cm area for 10- and 20-keV data). Inset shows

secondary coefficient versus ion energy. Total ion

current was 1.3 yA 17

Figure 11. Collector current versus suppressor voltage for N"^ ion energies from 50 to 300 keV. Total ion current

was 1.3 yA .... 18

Figure 12. Positive secondary ion current as a function of N"^ ion energy. Circuitry and geometry are shown in fig- ure 8(b) with S-cm^ target beam area (5-cm^ area for

10- and 20-keV data). Total ion current was 1.3 yA • 20

Figure 13. Positive secondary ion emission as a function of

primary 50-keV argon ion beam current density .... 22

iv
PAGE

Figure 14, Comparison of integrator dc current readings

(Integrator A used as a reference) 24

Figure 15. Off-scale current reading errors 25

Figure 15. Pulsed current comparison of Integrator B to reference 27

Figure 17. Pulsed current comparison of Integrator C to reference 28

Figure 18. Pulsed current comparison of Integrator D to reference 29

Figure 19. Pulsed current comparison of Integrator E to reference 30

Figure 20. Pulsed current comparison of Integrator F to reference 31

Figure 21. Pulsed current comparison of Integrator B to reference using 2500 pps 32

Figure 22. Pulsed current comparison of Integrator C to reference using 2500 pps 32

Figure 23. Pulsed current comparison of Integrator D to reference using 2500 pps 33

Figure 24. Pulsed current comparison of Integrator E to reference using 2500 pps 34

Figure 25. Pulsed current comparison of Integrator F to reference using 2500 pps 35

LIST OF TABLES

Table 1. Secondary Positive Ion Emission Measured for Some Ions on Stainless Steel. (Surface Conditions Not Controlled) 21

Table 2. Secondary Positive Ion Emission Measured for Different

Targets. (Surface Conditions Not Controlled) 21

Table 3. Comparison of Some Specifications of Selected Current

Integrators 23

V

PREFACE

This study was carried out at the Hughes Research Laboratories as a part

of the Semiconductor Technology Program in the Electronic Technology

Division at the National Bureau of Standards. The Semiconductor Technology Program serves to focus NBS efforts to enhance the performance,

interchangeability , and reliability of discrete semiconductor devices

and integrated circuits through improvements in measurement technology

for use in specifying materials and devices in national and international

commerce and for use by industry in controlling device fabrication processes. The work was supported by the Defense Advanced Research Projects

Agency, ARPA Order 2397, Program Code 6D10, through the National Bureau

of Standards Semiconductor Technology Program, Contract 5-35891.

The contract was monitored by Richard L. Raybold as the Contracting Officer's Technical Representative. Drs . W. M. Bullis, K. F. Galloway,

and D. R. Myers provided technical review of this report for the National

Bureau of Standards

The author wishes to acknowledge the valuable assistance of Dr. Robert

G. Wilson of the Hughes Research Laboratories in the preparation of this

report.

vii

SEMICONDUCTOR MEASUREMENT TECHNOLOGY: Some Aspects of Dose Measurement

For Accurate Ion Implantation

by

Douglas M. Jamba

Abstract : An investigation of various phases of ion implantation dose measurement was carried out, covering in detail

ion beam scanning, secondary particle suppression, and current

measurement instruments. Problems are discussed and preferred

techniques, electrode structures, and measurement circuitry are presented. Five current integrators were tested and are compared, especially in regard to pulsed current measurement.

Key Words: Accurate ion beam current measurement; current

integrators; ion beam scanning; ion implantation; ion

implantation dose measurement and control; secondary particle

suppression.

1. INTRODUCTION AND SUMMARY

After Itss than ten years of development, ion implantation has reached

a stage where it is routinely used by most semiconductor processing

facilities. The growth of the technique has been so rapid that often

little time has been available for the production and manufacturing personnel to become familiar with the sophisticated details developed in

the research laboratories.

Measurement of the ion dose is one of the most important steps in a re- producible and well-characterized ion implantation process. In most

applications of the semiconductor industry, the required dose is determined empirically based on the testing of the completed devices. For

any particular system and target combination, reproducing a particular

ion current and implantation time will usually produce the same ion dose,

and the absolute value of that dose will not be a major concern. However,

if more than one system and target material are involved or if it becomes

desirable to transfer operations to another facility or the implant technology to another device processing line, there is a need for absolute

dose calibration.

The accuracy of calibration required usually depends on how variations

in dose affect the operation of devices. The use of ion implantation

for threshold voltage control in CMOS devices may be cited as an example

of a critical application [ll . Figure 1 taken from Ref . 1 shows a curve

of the threshold shift obtained for boron doses implanted into the

p-channel of a CMOS device. An implantation must be controlled to within 3 X 10-'-^ /cm2 in order to control the threshold shift to within 0.1 V

of any desired value

1
0 12 3 4 5

p-CHANNEL BORON ION DOSE, lO'Vcm^

Figure 1. CMOS p-channel threshold shift versus

implanted boron ion dose.

As another example, an accurately controlled dose has been shown to be

required in the fabrication of ion implanted bipolar transistors [2]

.

The current gain of these all-implanted transistors can be set at desired

values by controlling the doping density in the range of less than ±3%

in both the emitter and the base regions of the devices.

The subject of ion implantation dose measurement was discussed briefly

by Wilson and Brewer [3] who showed methods for electrostatic suppression

2
of secondary charged particles at the target. Dearnaley [4] , in reviewing the problem, emphasized the use of magnetic field suppression of

secondary electrons to obtain accurate dose measurement. However,

neither of these references treats the subject with enough detail to

enable the operator of an ion implantation facility to be sure making accurate measurements.

This report presents the results of a detailed study of methods for

making accurate current measurements and points out places where errors

are likely to occur. It is hoped that the transfer of this information

to the semiconductor device and integrated circuit industry will result

in a more efficient and useful application of ion implantation techniques.

Three main aspects are covered in this investigation. The first deals

with uniformly distributing the ions over the target and accurately

calculating the implanted area and dose. Since the dose is a direct

function of the target area, it is essential that the area covered by

the beam be determined carefully.

The second section discusses the techniques for proper suppression of

secondary charged particles. There are significant quantities of positive secondary ions as well as secondary electrons emitted from the

target, and both cannot be suppressed at the same time. Biasing the

suppressor electrode negatively prevents secondary electrons from leaving

the target but collects all the positively charged particles emitted as

a result of the primary beam hitting the target. Secondary positive ions can have energies in the range of several thousands of volts [5] resulting in the release of secondary electrons from the suppressor electrode

which are collected by the target. These effects reduce the collected

target current to less than the beam current. A grounded bias circuit

can result in errors of 20% or more between the true and measured beam

current in some cases when secondary positive ions are neglected. There

is no assurance that these effects will be reproducible because secondary emission changes with system pressure, adsorbed gases, and surface

conditions [3,5,6]. The recommendations offered in this report include

measurement circuitry designed to determine the true incident ion beam

current in the presence of all secondary particles.

The third section compares five different current integrators emphasizing the need to check carefully the specifications and the operating

characteristics of each unit. Factors such as the capability of measuring scanned beams common to ion implantation, integrator scaling, and

current indicator off-scale operation can be responsible for errors even when using sophisticated integrators if the operator is not aware of

proper handling procedures. Parts of this report dealing with the measurement of high current levels are of particular relevance to the production industry where implant times must be kept to a minimum. Another

section discusses problems arising from the effects of neutral atoms,

ac interference, and insulator leakage.

3
2. ION BEAM SCANNING

2.1. Methods

In order to implant 2- or 3-in. diamfeter silicon wafers, typically either

a focused ion beam is electrostatically scanned or the wafers are mechanically scanned. The ions must be distributed uniformly over the target

surface and current measurements must confirm that a uniform current distribution exists before implantation proceeds. This section deals with

the important aspect of setting up the ion beam at the target in preparation for an implantation.

A typical ion implantation system is shown in figure 2. All of the data

reported were obtained in our laboratory in two implantation systems

,

one of which is shown in the figure. However, the conclusions and rec- ommendations resulting from the study are general and can be applied to

most other systems

.

MASS

SEPARATION

(MAGNET)

STEERING

ION BEAM

RASTERING

BEAM

APERTURE

— u ^j~a SLIT

POR

MASS

SEPARATION

DEFINITION

1/ DRIFT SPACE '^^^

MULTIPLE

TARGETS

ACCELERATOR

VALVE

DRIFT SPACE

PROFILER

VALVE

> I t B II I I B I ION SOURCE

—a DSOURCE/ACCELERATOR

VACUUM SYSTEM

ION PUMP

TARGET

VACUUM

SYSTEM

Figure 2. Schematic of a 150-kV ion implantation system.

4
One target configuration with a broad ion beam impinging on the target

collector is shown in figure 3. Some target chambers employ a wafer

holder that rotates in steps as this one does, exposing wafers one at a time to the ion beam. Some systems have wafer holders that continuously

rotate the wafers past the ion beam (mechanical scanning) . In both cases

it should be rioted that the ions impinge perpendicularly to the wafer

surface only at the center of the wafer. In the case of electrostatic

scanning, the impingement angle depends on the distance between the scan- ner and the target. In the case of some mechanical scanning the wafers

are tipped as the target drum rotates past the ion beam causing the impingement angle to depend on the diameter of the drum.

Figure 3. Typical target configuration.

The impingement angle does not have an effect on the measurement of the

ion dose and will not be discussed further here. The effects of angular

variation of the impinging ion beam on the implanted ion distributions in

silicon wafers is dealt with in another report. [7]

5
2.2. Electrostatic Scanning

In order to obtain a uniform beam over a large area without continuous

mechanical motion, the beam must be electrostatically scanned in both

the vertical and horizontal directions, creating a raster similar to a television display. This type of scanning produces a square pattern over

the defining aperture (usually circular), as shown in figure 4(a).

(a)

k d ! I

'

I \< D ^

0 20 40 60 80 100 120 140 160 180 200

OVER-SCAN,

%

Figure 4. Theoretical effects of beam scanning.

The scan frequency in one direction must be significantly higher than in

the other direction so that the traces of the ion beam overlap and cover

the surface of the wafer uniformly. To accomplish this the line trace

separation must be smaller than the beam diameter. Ion beam diameters

can be as small as 1 mm, which then requires approximately 75 traces over

6
a 3- in. diameter wafer. It is, therefore, recommended that the ratio of

scanning frequencies be 100:1. Synchronizing of the scanning voltage

signals causes the beam to retrace one particular pattern and is to be

avoided. Both free-running scan voltage sweeps and high scan-frequency

ratios enhance the achievement of uniformly implanted wafers.

a. Efficiency

If the ion beam shape is irregular, it is important to scan the beam

sufficiently past the edges of the defining aperture to maintain uniformity. The more the beam is overscanned, the more of the beam that is lost.

Therefore, it is important to have some guidelines to follow in setting

up the scan voltage amplitudes. Figure 4(b) is a plot of the efficiency

of utilization of the ion beam as a function of how far the beam is scan- ned past a circular defining aperture. For a small beam (beam diameter

<< defining aperture diameter) scanned to the edges of the aperture

(overscan = 0) the maximiim efficiency is 80%. This ideal beam shape is

difficult to achieve and should not be expected in ordinary systems. A

typical condition found to be satisfactory is to scan the beam such that

the average current in each of the horizontal and vertical directions

is reduced to one-half of the unscanned current, resulting in an average

current one-fourth that of the unscanned beam current. This condition

occurs when the beam deflection distance D shown in figure 4(a) is equal

to 1.8 times the target diameter, or 80% overscanning

b. Uniformity

A verification of beam uniformity at the target can be obtained by com- paring the current measurements of the target collector and the Faraday

cup as shown in figure 3. The current densities shown in figure 5 were measured using both the target collector and the Faraday cup collector

as a function of the beam scanner voltage. With no scanning voltage

applied, the steady state ion beam is centered on the target collector

at the Faraday cup and shows a relatively high reading depending on how

well the beam can be focused. The total beam current is the sum of the

cup and target current values. When the scanner voltage is applied, the

beam is deflected back and forth across the target producing a series of

pulses collected by the cup current meter. With increasing scanner voltage these pulses get smaller in width and the cup current decreases. The

target collector current increases in proportion to the decrease in cup

current until the scanner voltage is high enough to deflect the beam past

the aperture defining the target collector area. As the voltage in-

^creases beyond this point, the target current becomes a series of pulses

and the fraction of the beam that is collected by the target collector

decreases in proportion to the scanner voltage. When the beam is swept

far enough to completely cover the defining aperture and average out any

nonuniformities in the beam shape, the Faraday cup and target collector

current densities become equal and a satisfactory beam uniformity is obtained over the entire target area. The appropriate scanning voltage for

any particular system depends on the scanning electrode geometry and

dimensions, the distance from the scanner to the target, the target area,

and the beam accelerating voltage.

7
0 200 400 600 800 1000

SCANNER VOLTAGE,

PEAK-TO-PEAK TRIANGULAR WAVE , V

Figure 5. Target current density as a function of scanning

voltage for 84-keV boron ions.

It is recommended that implantation systems have methods built in for

doiible checking measurements in case of operator error or equipment failure such as a loss of bias voltage. The verification of proper current

density as measured by both the target collector and the Faraday cup is an example of such a doiible check.

c. Ion Beam Control

The placement of current collector plates or Faraday cups near the edges

of the defining aperture as shown in figure 6 provides another way to set

up the scanning voltages and to monitor the beam position. The scanning

voltage can be increased until a current is obtained in both collectors

8
BEAM DRIFT

LIMITING APERTURE

E ^ IMPLANTED AREA H

Figure 6. Schematic drawing showing the effect of a drifting ion beam at the scanner.

9
indicating that the beam is completely filling the defining aperture.

Four collectors are necessary to monitor both scanning directions; and

when properly uniform conditions are obtained, the current in all four

collectors will be equal. Any one or all of these collectors can be

used to monitor the target current density and therefore can be used as

the input to an integrator for measurement of the ion dose. This technique is useful when it is impossible or impractical to measure the cur- rent on the target itself (for example, when the target cannot be isolated from ground or the samples have insulating surfaces)

.

Another important use of these peripheral collectors is to provide a means for feedback stabilization control of the ion beam position. When

this technique is used the currents collected by an opposite pair of

collectors are fed into a differential amplifier that is able to detect

shifts of the beam toward one side or the other. This signal is processed

to supply a correcting voltage applied to a beam steering electrode which

returns the beam to the center of the target.

2.3. Target Area

In order to calculate the current density, the exact target area the ion

beam strikes must be known. A straightforward method for this purpose

is to implant a piece of sensitive (e.g. , zinc oxide coated) paper placed

in the target position until the paper shows the outline of the beam area. This technique is useful for checking beam location. However, it is

difficult to measure the area accurately because a sharp edge is not always obtained as is illustrated in figure 6. When the defining aperture

is placed away from the target and when the ion beam diameter is smaller

than the limiting aperture at the scanner, there can be a shift of the

implanted area caused by a drifting of the ion beam position at the scanner. Drifting of the ion beam is common in the horizontal direction

and is caused by variations in the beam energy or in the electromagnetic

separator current. Such shifts, while causing a diffuse edge, do not

significantly affect the dose measurement because the area of the implanted region remains essentially the same.

A better method of calculating the area of the beam at the target is to use the dimensions of the system components. The implanted area is

TT (r|^£ij,/£^) 2 , for the recommended circular aperture where the quantities

are defined in figure 6. The use of this value is recommended for calculating the current density at the target because, with proper scanning of

the beam past the edges of the defining aperture, its use will minimize

errors in dose determination.

It was mentioned that continuous mechanical motion of the target can be

employed to provide uniform ion beam exposure over the target. One way

to accomplish this is to have a circular rotation coupled with a radial

oscillation to provide complete coverage of wafers while the ion beam

stays in a fixed position. Another method is to have a circular mechanical rotation combined with a one-dimensional electrostatically scanned

ion beam. In both of these cases, the ion beam current must be measured

10
on the moving target and the current density must be calculated from the

total area of the rotating surface exposed to the beam.

3. SECONDARY SUPPRESSION

When the primary ions penetrate the target surface, secondary particles

are emitted. These particles, many of which are charged, cannot be

neglected because they can introduce losses or additions to the ion dose.

Secondary electrons and negatively charged ions leaving the target cause

the target current to increase. Positively charged particles leaving the

target and secondary electrons from other sources hitting the target

cause a decrease in target current. This section presents techniques of

electrode and circuit design for accurate target current measurement in

the presence of emitted secondary electrons and ions.

3.1. Electrode Design

The most common method used to prevent the loss of charged secondary

particles is to place a suppression electrode in front of the target and

to apply a voltage to repel the charges and -keep them on the target surface. The suppression electrode can be either a cylinder or a flat plate

containing a hole. A typical target with a cylindrical suppressor electrode and defining aperture is shown in figure 7 (a) . The ion beam must

be prevented from touching the suppressor by the use of a collimator

in front of the suppressor. If the beam were allowed to hit the suppressor, with the suppressor at a negative voltage, the secondary electrons

genera.ted would be collected by the target causing the measured beam cur- rent to be lower than the true value. The cylindrical suppression geometry is superior to the flat plate because it is more efficient for supplying a potential barrier alcJng the beam axis and because it concentrates

a stronger electric field near the outer edges of the implanted area and assures that no secondary electrons escape to the walls of the vacuum

chamber to affect current measurements.

The suppression geometries shown in these figures and used in this study

resulted from analytical (electrolytic tank) and experimental studies [3]

of electrode geometries to determine suitable potential contours for

secondary suppression across the beam aperture, especially on the beam

axis. The function of the suppressor electrode is not only to establish

a potential field to keep secondary electrons from leaving the target, but

also to keep other electrons from being collected by the target. For

example, any secondary electrons generated from the ion beam striking the vacuum chamber walls upstream from the target or the defining aperture

must be repelled by the supression voltage. This accounts for the long

length between the defining aperture and the target. A general rule of

thumb is to allow a space between each electrode equal to the maximum

diameter of the aperture. Thus, it is recommended that the distance

between the defining aperture and the target be two aperture diameters.

In some instances it is desirable to change the size of the defining

aperture to implant smaller wafers. For example, if heavy ion doses are

11
SUPPRESSOR

ELECTRODE

ION

BEAM

<

DEFINING

APERTURE

(a) PERMANENTLY INSTALLED SUPPRESSOR ELECTRODE

VARIABLE

APERTURE

INSERT

W////A )

11}

(b) REMOVABLE INSERT FOR SMALLER AREA IMPLANTS

Figure 7. Secondary suppression geometries.

12
desired, reducing the size of the implanted area results in more efficient

use of the total ion beam available and reduces the time required for implantation. One technique used at this laboratory is to insert a smaller

diameter electrode unit as shown in figure 7(b). Other types of variable

apertures have been used where the defining apertures are holes in a ro- tating disc controlled from outside the vacuum system.

3.2. Measurement of Currents

We began the investigation of secondary suppression electrodes by recording the currents collected by each electrode of the system as shown in

figure 8(a). The voltage (Vg) on the suppressor electrode was continuously varied from positive values through zero to negative values of

several hundreds of volts. When is positive, the secondary electrons

and negative ions are attracted to the suppressor and the suppressor

current (Ig) has a high negative value. In this case, for an ideal suppressor, the secondary positive ions are repelled, back to the target.

The secondary current depends mainly on the ion beam energy and can be

anywhere from one to more than ten times the primary ion beam current.

In this case the current (I^) collected by the target is the sum of the

incoming positive ion current plus the secondary negative particle cur- rent leaving. The true ion current is the algebraic sum of Ig and IcIf there were no suppressor electrode, the secondaries would be lost to

the chamber walls and 1q would be in error from the true beam current by

the magnitude of the secondary current.

When Vg is negative, the current Ig has a positive value as a result of

collecting the secondary positive ions. In this case, for an ideal

suppressor, the secondary negative particles are repelled back to the

target. The number of the positive ions also depends on the beam energy

but is usually only a small fraction of the number of primary ions.

This number typically has a maximum at a beam energy less than 50 keV.

Positive ion emission occurs in conjunction with the emission of neutral

atoms during sputtering. In this case, I^ is the difference between the

incoming ions and the secondary ions leaving. The true ion current is

again the sum of I^^, and Ig.

3.3. Recommended Circuit

It is obvious that both negative and positive secondary particles cannot

be suppressed at the same time. Therefore, methods must be employed to assure collection of all the currents and to present the true collected

ion charges to the integrator. The method we have used to accomplish

this is shown in figure 8(b). With the suppressor operating at either

positive or negative voltage, the current Ig is cycled in a looped circuit floating on the total current I^j^. The collected current I^ indicates both the secondary and true currents but Irp records the true incident ion beam current.

13
SUPPRESSOR

(a) INDEPENDENTLY GROUNDED SUPPRESSION CIRCUIT

(b) FLOATING LOOPED SUPPRESSION CIRCUIT

Figure 8. Implantation target current measuring circuitry.
We recorded data for the circuit of figure 8(b) in our 300-kV implantation system using N and N2 ion beams in the energy range from 50 to

300 keV. Curves typical of these data are shown in figure 9 for 50-keV

With the suppressor bias looped to the target, the total current

was approximately 1.3 yA throughout the measurements. The total current,

Irp is the sum of the suppressor (1^) and collector (I^) currents for the

high voltage biased conditions. When the bias voltage is near zero, a discrepancy appears which represents the secondaries lost to the sur- rounding environment (the target chamber walls) either directly back

along the beam axis or out of the space between the suppressor electrode

and the target. These curves indicate that for these conditions with

typical negative Vg operation, any voltage larger than -30 V is sufficient

to suppress the secondary electrons.

3.4. Negative Secondary Emission

A measure of the negative secondary emission (primarily electrons, but including negative ions) can be obtained by dividing the suppressor current

obtained with the suppressor at high positive values by the total current

(the incident ion current) , 5.85 for this case of 50-keV Nt ions (see fig- ure 9) .

A set of data was taken with an N"*" ion beam for a series of energies from

10 to 300 keV to show the variation in secondary emission with ion energy.

The resulting increase in suppressor current with ion energy is shown in

figure 10. The inset in the figure plots the changes in the calculated « negative secondary coefficient as a function of the ion beam energy. The

coefficient increases sharply at the lower ion energies. Above 100 keV

it continues to increase, but more slowly. A value of 10 is reached at

about 300 keV.

It is obvious from these curves that the suppressor is repelling the sec- ondary electrons effectively for negative voltage exceeding -30 V. However, xt is possible for some secondaries to be lost from the target

current reading under such limited bias voltage conditions. For example,

secondaries from the target with enough energy can pass directly through

the center of the suppressor back toward the ion beam source and cause

the total current reading to be larger than the true beam current. In

such a case the suppressor current may be constant above -30 V because no more electrons are being collected. However, examination of the collector current may show that the target is still losing some electrons.

This effect is shown more clearly in figure 11 which is an expanded scale

plot of the collector current as a function of suppressor bias voltage

for the same set of N"*" ion beam measurements of figure 10. Here we can indeed see that the current does not level off until the bias voltage is

around 100 V for the 300-keV N"^ ion beam.

It should be emphasized here that these data were taken with what we consider to be a good suppression configuration (that of figures 7 and

8(b)). Other systems should be checked to verify that complete suppression is being achieved. This can be done most easily by increasing the

suppressor voltage until there is no change in the target current.

15
-10 -

-100 -200 -300

SUPPRESSOR VOLTAGE, V

-400

Figure 9, Implantation target currents versus suppressor voltage

using the cxrcuitry and geometry of figure 8(b) with 3-cm target

beam area.

16
2

ION ENERGY, keV

120 80 40 0 -40 -80 -120 -160

SUPPRESSOR VOLTAGE , V

Figure 10. Suppressor current versus suppressor voltage for N"*" ion energies from 10 to 300 keV. Circuitry and geometry are shown in figure 8(b) with 3-cm^ target beam area (5-cm^ area for

10- and 20-keV data). Inset shows secondary coefficient versus

ion energy. Total ion current was 1.3 yA.

There are two other methods used to suppress secondary electrons. The

first is to operate the target at a positive voltage, thus preventing the

secondary electrons from leaving the target. The second is to utilize

a magnetic field at the target surface to return the secondary electrons

to the surface. Magnetic field suppression has the advantage of not re- quiring any elements of the system (either target or suppressor electrodes) to operate with a bias voltage across insulating supports which

might lead to leakage current losses. However, neither of these methods

is effective in preventing positively charged particles from escaping, and

they are not recommended for this reason.

17
0 -20 -40 -60 -80 -100 -120

SUPPRESSOR VOLTAGE, V

Figure 11. Collector current versus suppressor voltage for N

ion energies from 50 to 300 keV. Total ion current was 1.3 yA.

3.5. Positive Secondary Emission

The secondary positive ion emission coefficient can be obtained by

dividing the positive secondary ion current measured in the suppressor

circuit under negative bias by the true total incident ion current. In

the specific case of figure 9, the value is 0.12 yA/l . 3 yA = 0.09.

18
Operation in the grounded suppressor mode under these conditions and

interpreting I^^ as the beam current would result in an error in implantation dose 9% too large.

The variation of positive secondary emission current for both monatomic

and diatomic ions as a function of ion beam energy is shown in figure 12.

These curves show an emission maximum at 50 keV for N"*" and 100 keV for or a value of 50 keV per incident atom. The nitrogen secondary emission coefficients calculated from these curves are seen to be essentially

coincident because the values of positive ion current are twice as large

at twice the energy for N2 as for N"*", leading to identical coefficients

for one atom at a given energy.

The magnitude of positive secondary ion emission changes with other ions

and targets as well as with beam energy. Some additional data are included in tables 1 and 2 to provide a rough idea of the variations that

might be expected for different ion beams and different target surfaces.

Even for the same surfaces, the secondary emission can change with vacuum

conditions, adsorbed gases, method of surface preparation, type of cleaning process used, orientation of crystal lattice, etc. No attempts were

made to control or reproduce these conditions exactly from run to run. Therefore, the data should only be taken as a relative comparison with

no absolute significance given to the percentages. General trends are obvious, showing that higher atomic mass elements produce higher secondary emission at the same energy, and that higher secondary emission is

obtained with semiconductor samples compared with aluminum foil targets.

It was previously shown in connection with the discussion of figure 12

that the beam energy is also a factor in secondary emission.

It is important to be aware that the number of secondary positive ions

may be in the range of 15 to 20% of the true ion beam current. This

magnitude of emission is capable of causing significant errors in dose

measurement when using an independently grounded suppressor circuit. In

a system using the floating loop circuit shown in figure 8(b) , the positive secondary ions are collected by the suppressor electrode and the

current circulates only through the floating loop such that the integrator reading (Irp) is not affected.

3.6. Effect of Current Density

Our investigation also revealed a variation in secondary emission as a result of the ion beam current density. Measurements were taken at

eight different levels of current from 1.5 x 10~^ to 5 x 10~^ A/cm^

Figure 13 shows the percentage of the true ion beam current being emitted

as secondary positive ions plotted as a function of the current density

of a 50-keV argon ion beam. The reduction in emission at higher current

density may be attributed to a surface change caused by the higher target

temperature or to an increase in positive beam potential, caused by the

higher ion space charge, which suppresses the secondary positive ions more effectively.

19
Figure 12. Positive secondary ion current as a function of

N"^ ion energy. Circuitry and geometry are shown in figure

8(b) with 3-cin^ target beam area (5-cm^ area for 10- and 20-

keV data). Total ion current was 1.3 yA.

20
Table 1. Secondary Positive Ion Emission Measured For Some Ions

On Stainless Steel. (Surface Conditions Not Controlled)

Primary

Ion

Energy

,

keV

Total Current,

yA

Percent Secondary

Ions

Be 100 1.05 2.6

B 20 1.5 3.0

N 50 1.3 5.2

50 1.3 9.2

Ar 50 2.5 15.6

Ga 40 5.0 17.7

As 50 1.3 18.9

Table 2. Secondary Positive Ion Emission Measured For Different

Targets. (Surface Conditions Not Controlled)

Primary

Ion

Energy,

keV Target

Percent Secondary

Ions

Be 100 aluminum foil 2.6

Be 50 gallium arsenide 7.4

Ar 50 aluminum foil 8.4

Ar 50 silicon 15.6

21
12

g 10

C 2

I I I I I

1

I I I II M I I

I

I I II

10-9

50 keV Ar IONS

10

-8

11 I I II MM I I I I I 10~'

CURRENT DENSITY Acm

10- 10-5

-2

Figure 13. Positive secondary ion emission as a function of primary

50-keV argon ion beam current density.

4. CURRENT MEASUREMENT

Measurement of the ion beam current density and integration of the beam

current for dose measurement are important parts of the implantation

process and must be done before and during implantation. The section

of this report on beam scanning has suggested ways to set up proper

conditions before the implant. Assuming proper suppression and collection of secondary ions and electrons, the remaining possibility for error

is the current collecting instrument or integrator itself.

4.1. Current Integrators

We studied the characteristics of five models of current integrators from

three manufacturers. These instruments will be referred to as integrators

A through E. Additional data, referred to as F, were taken using integrator E equipped with an input circuit designed to increase its frequency response. The manufacturers' specifications of these instruments

are listed in table 3. Many of the specifications for these integrators

do not describe how the units respond to pulsed current conditions, es- pecially pulse rates. The absolute accuracy of any of these integrators

is not considered a major issue as far as this work is, concerned. What

is important here is the reaction of the integrators to the test conditions and the different results that would be obtained using each integrator measuring the same test signal.

22
+J -P -H -H

13 0 0 M JJ

+J +J 3 -H

3 C C (0 -H

M V4 M C

23
One of the integrators (A) was chosen to be a basis of comparison because of its demonstrated satisfactory performance during many years of use in our laboratory. Signals from either an ion beam or a current

source were measured by each of the units separately and each reading

was compared with the reading obtained by integrator A. These ratios

are plotted in the subsequent graphs.

4.2. Measurement of dc Current

The integrators were first compared measuring steady dc currents typical

of unscanned beams over the range from 5 x 10"^ to 5 x 10"^ A. The units were not especially calibrated for this comparison but were removed from

their regular operations and tested as they would normally be used. In

general, they were all found to compare very favorably, as seen in figure

14. Integrator E was within 1% of the reference. Integrators B and C

varied less than 1% over the range but were off-set in absolute value to

0.99 and 0.98 respectively. Integrator D varied as much as 2% low at low

current levels. Integrator F was the unit equipped with the input circuit to improve the frequency response when measuring pulsed currents;

the use of such a circuit is undesirable for dc current measurements

above 10"^ A.

1.04

1.02

0.92

TTTT] I I I lllll| I I I I llll| 1 I I TTT

INTEGRATOR

E

10-7

I I I m ill I i\ mi ll LLL

10"^ 10"^ 10~^

REFERENCE CURRENT, A

Figure 14. Comparison ojf integrator dc current readings.

(Integrator A used as a reference)

.

24
4.3. Off-Scale Current

Two of the integrators were compared in regard to their accuracy when the

indicator of the current went past full scale. These tests were conducted

using steady dc currents monitored by another reference current meter.

Even though the current meters on both integrators indicated full scale,

charge counting continued and was recorded as a function of time to determine integrator accuracy. Integrator A was found to give a charge

collection count equivalent to the full scale reading no matter how high

the current was increased beyond this level. The result, as shown in

figure 15, was a sharp increase in error when attempting to measure cur- rents above full scale. Integrator D was found to continue counting the

collected charge with a much slower increase in error. Even when the

current was increased to 1.2 times full scale, the integrator still re- corded this collected charge with an error of less than 1%.

60 80 100 120 140 160 180

RATIO OF CURRENT TO FULL SCALE ,%

Figure 15. Off-scale current reading errors.

25
This potential cause of error must be considered if the current being

monitored is drifting or jumping to levels exceeding full scale. It is

recommended that a high enough integrator scale setting be chosen so that

the indicated current is less than mid scale with no chance of exceeding

full scale.

4.4. Measurement of Pulsed Current

Referring back to figure 4, as the beam passes over the center of the

target in the typical 80% overscan case, the integrator measures a square wave type pulse. For this case the width of each pulse at a scan

rate of 2000 pulses per second (pps) is about 250 ys. The pulse width

gradually decreases with distance from the center, becoming zero during

the maximum levels of the slower sweep voltage. The integrator must be

capable of measuring this wide variety of pulse widths.

In order to investigate the accuracy of the current measurements under

these pseudo-pulsed conditions, two sources of current were used: 1)

actual ion beam currents obtained from an implantation system and 2) simulated beam current pulses obtained from a pulse generator. The currents

in both instances were measured and compared using the five different

integrators as well as other current measuring instruments. The readings were taken from dc conditions to 10,000 pps. The ion beam scanner unit

was variable over a range from 20 to 2000 pps; the remaining frequencies

were covered with the pulse generator

.

Lower pulse rates (from 2.5 to about 30 pps) usually produced a fluctuation in the indicator meter needle making it impossible to record the

current indicated by the indicator. For example, at 2.5 pps and with a 50% duty cycle, the pulse was on for 0.4 s and off for 0.4 s. For these

low pulse rates the integrator count was measured as a function of timed

intervals. At the higher pulse rates it was found that the indicated

current accurately matched the integrated charge divided by the time

interval and could be used for the comparison of the test integrator

with the reference unit.

—8

Average current readings were compared over the range from 5 x 10 to

5 X 10"^ A, maintaining a 50% duty cycle. The results are plotted in

figures 16 through 20, each figure presenting the data for one integrator with the parameter being the five current levels that were measured.

Integrator C (figure 17) was found to record nearly the same currents as the reference. The average values were around 0.98 of the reference,

being very close to the dc current comparison. The variation in pulsed

current readings around the average was less than 1.5% showing that this

integrator was reproducing the reference very closely over the complete

range of pulse rates tested. The maximum current scale on this unit was

10~ A. Therefore, the highest average current tested was 5 x 10~^ A.

Integrators B and D (figure 16 and 18) were found to have significant

variations (in the range of 10%) but at different ends of the pulse rate

"26
spectrum. Both integrators were satisfactory (±1.5%) over the central

range of 50 to 1000 pulses per second for all current levels tested.

Integrators E and F (figures 19 and 20) were found to be unsatisfactory

for use in measuring pulsed current. Use between 100 and 10,000 pps

should be possible if limited to low current levels (less than 5 x 10"^ A

for E and less than 5 x 10~^ A for F) . The curves for 5 x

10"'^ A in f ig-

— A — 5 ure 19 and 5 x 10 and 5 x 10 A m figure 20 are not shown because

they were mostly outside the range of the graphs.

10° 10^ 10^ 10? 10"*

PULSE RATE .s""*

Figure 16. Pulsed current comparison of Integrator B to reference.

4.5. Effect of Pulse Width

An operating property of integrators that must receive separate attention can be referred to as scaling. Scaling means switching scales on

the integrator to record the charge at the most convenient current indicating level. In some instances errors can be introduced when the ratio

27
of the pulse peak height to the average current reading is high. For

example, when a high current beam is scanned excessively to reduce the

average current to a low value for a light dose implant, the pulses become very narrow. This type of test should not be confused with the offscale tests where the current indicator exceeded full scale. In this

case, even though the individual pulse heights may exceed full scale, the

averaged current reading remains on scale.

A set of data was taken with all the integrators recording the current

readings as the width of the pulse from the pulse generator was reduced.

A pulse repetition rate of 2500 pps was used in this test. The integrators were set to read the average current at the most convenient scale

setting. This means that in most cases the peak of the pulse was higher

than the full scale setting of the current meter. For example, a current

pulse with a peak value of 100 yA could be scanned to 25 yA average and

1.25

1.20 -

1.15 -

1.10

O Z

I- UJ < cc CC LU (J ULu m

I- QC

O I- 1.05

O Ei <

cc g 1.00

0.95 -

0.90

10^ 10^ 10^ 10^

PULSE RATE,s-1

Figure 17. Pulsed current comparison of Integrator C to reference.

28
.4

Figure 18. Pulsed current comparison of Integrator D to reference.

-4

measured on the 10 A full scale setting (reading 1/4 of f.s.). Higher

scanning voltage might reduce the same. pulse to an average value of 5 yA,

allowing a measurement to be made on the 10~^ A f.s. setting (reading 1/2

of f.s.). The object of these tests was to compare the readings obtained

at such different scale settings. The data obtained were compared and

normalized to integrator A. The specifications of the manufacturer state

that this integrator can correctly measure pulses having peak amplitudes

as high as 20 mA on any scale as long as the average value of the indicator current is on the scale being used. The same kind of errors result

with pulsed currents as with dc currents (see figure 15) if the average

value is allowed to exceed the full scale of the indicator meter. Curves

for these tests are shown in figures 21 through 25. The parameters are marked showing the peak steady state values of the current before generating the pulses. For example, the lowest level tested was a steady

dc current of 1 x 10"^ a. At a 50% pulse duty cycle the current represented the average value of a square wave pulse with the current indicating approximately 5 x 10~8'A. The pulse width was continually reduced

— 8 by one-half for each step, resulting in current readings of 2.5 x 10 ,

29
1.25 X 10 and 6.25 x 10 A. The integrator scale setting for the

lowest reading was typically 10~^ A full scale, two orders of magnitude

lower than the original pulse height.

10° 10^ 102 10^ io4

PULSE RATE,s~''

Figure 19. Pulsed current comparison of Integrator E to reference.

All of the integrators were able to give correct readings within a few

percent for such low level currents. Even the next two curves (10~^ and

— 5 —4

10 A) were satisfactory. However, 10 A and higher were not measured

correctly by three of the units, giving errors greater than 10%. This means that steady beam current levels between 10 and 100 yA might be

causing measurement errors when scanned across the target. The best way

to check for such discrepancies is to compare readings on different

scales. If at all reasonable, always record implantation doses using

the higher full scale settings.

These errors are usually caused by frequency response limitations of the

input amplifiers of the integrators, some of which are not able to handle

the high current, high frequency components of the pulses. Integrators

specified to be used for the measurement of pulses will most likely cause

30
no problem. It is recommended that a second integrator or other electrometers be available for periodically double checking accuracy or for

replacement use during calibration.

CC O

o

<

QCCD

I-E o

o t

1.20

1.15 -

1.10

1.05

O LU

ccg 1.00

0.95

0.90

I MM

10^

5x 10

JLLL I I I I M ill I I Mi ll I I III!

10^ 10^

PULSE RATE .s

10-^ 10'

-1

Figure 20. Pulsed current comparison of Integrator F to reference.

5. OTHER FACTORS AFFECTING DOSE iXEASUREMENT

There can be other conditions that interfere with the accurate measurement of ion dose. A few of these will be discussed briefly in this

section.

5.1. Neutrals

Energetic neutral atoms or molecules can originate in the ion source or anywhere along the ion beam path as a result of charge-exchange collisions.

If these neutrals are allowed to penetrate the wafers at the target, they

will contribute to the doping density in the wafer but will not be measured by the integrator, thus leading to errors in the implanted dose.

Since they are not affected by electrostatic scanning or magnetic fields

they can also be present at the target in a nonuniform pattern leading

to nonreproducible devices.

31
1.06

oc o 1.04

O z

\- HI

QC uJ '-^^

(J ulIj hi 1.00 z o

O ^ 0.98

O LU

"=3

!^ IT 0.96

0.94

0.92

0.0625 0.125 0.25

PULSE DUTY CYCLE

1

1 1

PULSE PEAK CURRENT

^^>^07iX^ 10-5 - 10"^

1 1 I 0.5 1.0

Figure 21. Pulsed current comparison of Integrator B to reference

using 2500 pps.

1.06

1 04 DC CJ O Z

h- UJ <ac 1.02

QC UJ O u.

UU UJ l-QC 1.00

0.98

O LU 0.96

0.94

0.92

1 1

1

PULSE PEAK CURRENT^- ^

10-5 Ax

10-6

10-^

1 1 1 0.0625 0.125 0.25

PULSE DUTY CYCLE

0.5 1.0

Figure 22. Pulsed current comparison of Integrator C to reference

using 2500 pps.

32
Figure 23. Pulsed current comparison of Integrator D to reference

using 2500 pps.

Neutrals from the ion source directed along the beam line do not reach

the target if the ion beam is bent in a magnetic field for mass separation. If a combination electrostatic and magnetic separator (EXB) is

used where the desired ion component is passed straight through, the use

of a separate beam deflector should be employed to bend the beam slightly

displacing the target from the path of the neutrals. In addition to deflecting the beam, charge-exchange in the target end of the system can

be minimized by improving the vacuum pumping to keep the pressure as low

as possible.

5.2. ac Interference

Another problem arises when ac operated equipment is used for any function in the target. Some examples are target heaters, bias power supplies,

or thermocouple meters. These units can be isolated from the system

ground with isolation transformers, but there is usually some leakage

current depending on the quality of the transformer and the operating

level of the ac voltage. If the leakage is low enough on the particular

33
PULSE DUTY CYCLE

Figure 24. Pulsed current comparison of Integrator E to reference

using 2500 pps.

current scale being used, the effect can be neglected. If not, some current integrators are equipped with zero off-set adjustments that allow

the operator to eliminate the effect of 'such leakage. When making implantations, especially at low dose, in the presence of ac operated

equipment, care should be taken to check the leakage current on lower

scale settings of the integrator before proceeding with the implantation.

5.3. Insulator Leakage

There can be leakage currents across insulators that are used to support

the suppressor electrodes or target holders. These leakages are proportional to the voltage across the insulators and the condition of the insulator surfaces. It is important to shield insulators from any sputtered

particles to keep the surfaces clean. Physical placement of the insulators can also be an important factor. For example, if the suppressor

electrode, as seen in figure 8(b), is mounted on the target holder with

insulators, leakage across these insulators will never cause an error in

Irp because the leakage currents will be collected in the same loop as the

secondary particles. Again, a check of the leakage current on a lower

scale setting of the integrator will determine if a problem exists.

34
PULSE DUTY CYCLE

Figure 25. Pulsed current comparison of Integrator F to reference

using 2500 pps.

5.4. Dose Calculation

The ion dose or fluence is the number of ions implanted per unit area of

the target. The number of ions is determined by dividing the collected

charge by the charge per ion (1.602 x lO"-*-^ C times the charged state of

the ion) . Integrators usually display a value for the current being

collected but they are primarily instruments designed to measure the

total charge collected. Therefore, the dose is calculated by using the

following formula:

,2, integrated charge , Dose (lons/cm ) = —

1.602 X 10 X target area (cm )

If an integrator is not available, the dose may be obtained by implanting

for a measured time at a uniform current level. In this case the product

of the current and time is substituted for the integrator charge in equation (1) . The accuracy of this measurement technique depends on maintaining the ion current at a uniform level over the entire time period

of the implantation. Experience at our laboratory has shown that it is

difficult to maintain ion beams at constant levels for an extended length

35
of time because of fluctuatxons m power supplies controlling the beam,

changes in ion source fuel feeding rates, etc. Therefore, this method

of measuring dose is not recommended.

There are occasions when it is appropriate to utilize ion species other

than the normal singly ionized atoms. Multiply ionized single atoms are used to obtain higher ion energies. For example, accelerated doubly

charged atoms gain twice as much energy as the same atoms singly charged

and their use is equivalent to operating at twice the accelerator voltage.

However, it must be remembered that when implanting with multiply ionized

particles, there is only one implanted atom for every two charges (or more depending on the charge state of the atom) and the total charge re- quired to be collected by the integrator must be increased accordingly.

Where lower energies are desired (and not obtainable, perhaps because of

limitations in system operation) diatomic or multiatomic ions can be

used. In this case, there are two or more atoms implanted for each electron charge and the integrator setting must be reduced accordingly.

REFERENCES

1. Coppen, P. J., Aubuchon, K. G. , Bauer, L.-O. , and Moyer, N. E., A

complementary MOS 1.2 volt watch circuit using ion implantation.

Solid-state Eleotronias 1_5, 165-175 (1972).

2. Payne, R. S. , Scavuzzo, R. J., Olson, K. H. , Nacci, J. M. , and

Moline, R. A., Fully ion- implanted bipolar transistors, IEEE Trans, on Eleatvon Devices ED-21 , 273-278 (1974).

3. Wilson, R. G. , and Brewer, G. R. , Ion BeamSj pp. 446-449 (John Wiley

and Sons, New York, NY, 1973).

4. Dearnaley, G. , Freeman, J. H. , Nelson, R. S., and Stephen, J., Ion

Implantation, pp. 403-442 (North-Holland Publishing Co., American

Elsevier Publishing Co., Inc., New York, NY, 1973).

5. Carter, G. , and Colligon, J. S., Ion Bombardment of Solids, Ch. 3 and 4 (American Elsevier Co., Inc., New York, NY, 1968).

6. Kaminsky, M. , Atomic and Ionic Impact Phenomena on Metal Surfaces,

p. 276 (Academic Press, New York, NY, 1965).

7. Wilson, R. G. , Dunlap, H. L. , and Jamba, D. M. , Semiconductor Measurement Technology : Angular Sensitivity of Controlled Implanted

Doping Profiles, NBS Special Publication 400-41 (in preparation)

.

36
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. Special Publication 400-39

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Semtoonduotor Measurement Technologij : Some Aspects of Dose

Measurement for Accurate Ion Implantation

5. Publication Date July 1977

6. Performing Organization Code

7 AIITHDRfSI

Doug]?!.'? M. .Tamba

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Hughes Research Laboratories

Malibu, California 90265

10. Project/Task/Work Unit No.

11. Contract/Grant No. DoC Contract No.

5-35891

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards ARPA

Washington, DC 20234 1400 Wilson Boulevard

Arlington, VA 22209

13. Type of Report & Period Covered Special Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

This work was funded by the Defense Advanced Research Projects Agency under ARPA

Order 2397, Program Code 6D10. Library of Congress Catalog Card Number: 77-24724

16. ABSTRACT (A 200-word or /ess (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) An investigation of various phases of ion implantation dose measurement was carried out, covering in detail ion beam scanning, secondary particle suppression,

and current measurement instruments. Problems are discussed and preferred techniques, electrode structures, and measurement circuitry are presented. Five

current integrators were tested and are compared, especially in regard to pulsed

current measurement

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Accurate ion beam Current measurement; current integrators;

ion beam scanning; ion implaintation; ion implantation dose measurement and

control; secondary particle suppression.

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Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation with interested industries, professional

! organizations, and regulatory bodies. Special Publications—Include proceedings of conferences

, sponsored by NBS, NBS annual reports, and other

' special publications appropriate to this grouping such

as wall charts, pocket cards, and bibliographies.

I Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engi-

] neers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work,

i National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properj ties of materials, compiled from the world's literature

i and critically evaluated. Developed under a world-wide

I program coordinated by NBS. Program under authority

1 of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements available from ACS, 1155 Sixteenth

St. N.W., Wash. D. C. 20056. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental

functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under proce- dures published by the Department of Commerce in Part

10, Title 15, of the Code of Federal Regulations. The

purpose of the standards is to establish nationally rec- ognized requirements for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations.

Consumer Information Series—Practical information,

based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent

of Documents, Government Printing Office, Washington,

D.C. 2()i02. Order following NBS publications—NBSIR's and FIPS

from the National Technical Information Services,

Springfield, Va. 22161. Federal Information Processing Standards Publications

(FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for

outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A

literature survey issued biweekly. Annual subscription: Domestic, 825 00 ; Foreign, 830.00'. Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: 830.00'. Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
U.S. OEPARTMENT OF COMMEIICE

National Buraau of Standards

Washington, D.C. 20234

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