Electrostatic analysis of backscattered heavy ions for semiconductor surface investigation

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Electrostatic analysis of backscattered heavy ions for

semiconductor surface investigation

M. Hage-Ali, P. Siffert

To cite this version:

Electrostatic

_analysis

of

backscattered

_heavy

ions

for

semiconductor surface

_{investigation}

M.

_Hage-Ali

and P. Siffert

Centre de Recherches _{Nucléaires, Groupe} de Physique et _Applications des Semiconducteurs (PHASE),

67037 _{Strasbourg Cedex,} France.

(Reçu le 22 octobre 1980, révisé le 15 janvier 1981, accepté le 19 _{janvier 1981)}

Résumé. 2014

Les

possibilités d’analyse de surface par rétrodiffusion de _{particules chargées} sont _limitées, à l’heure

actuelle, par la résolution limitée des détecteurs à semiconducteurs et par la dégradation rapide de leurs per-formances pour des

projectiles

lourds. Dans cet _article, nous décrivons les possibilités offertes par un _analyseur

électrostatique, capable

de détecter des ions lourds

_{(7Li+, 12C+)}

_pour

_l’analyse

_{des surfaces de semiconducteurs}

composés. Un intérêt

_particulier

est _porté aux _problèmes de résolution en masse et de résolution en _profondeur.

Abstract. 2014 The

capabilities of Rutherford

_{backscattering}

in surface

_analysis

are limited _{by the energy}

reso-lution of the solid state detectors and their _{rapid degradation} for heavier

_projectiles.

_Here, we

_investigate

the

possibilities of an electrostatic

_analyser

_(ESA)

_detecting

_heavy

_projectiles

_{(7Li+, 12C+)}

backscattered from various

compound semiconductor _{surfaces, essentially} with _respect to mass and _depth resolution.

Classification

Physics Abstracts

29.30h - 29.70e - 61.80Mk

1. Introduction. - Surface

analysis

by

means of

ion beams in the keV-MeV _{energy range} becomes a very

powerful

method for

investigating

solids.

_Among

the various

_{techniques employed,}

Rutherford

back-scattering (RBS)

of

1H+

or

4He+

ions in the 1-5 MeV range, the detector

being

a silicon

_Schottky

_barrier,

offers _many

_{advantages :}

non

_{destructability,}

abso-lute concentration and location of

_impurities,

etc.

Many

publications

on the

_subject

are available,

espe-cially

concerning

semi-conductor surfaces

_[1-3].

The

light projectiles,

mentioned above have been chosen

essentially

for the

_{easy beam production}

in the Van de Graaff accelerator and the

_good

detection capa-bilities of silicon

_Schottky

diodes for these

_particles.

In

_principle,

_however,

heavier ions should offer several

_{advantages [4], particularly}

better mass and

depth

resolution and

_high

cross section.

_Therefore,

several _groups have tried to

_employ

either

12C+

[5-8],

14N+

_[9-11],

160+

_[12]

or even ’Li+

_[13-15]

beams.

_But,

several

_problems

arise when solid state

detectors are used with

_heavy

ions

_{[16, 17] :}

- the detector

degrades quickly

due to the radia-tion

_damage

induced in the silicon and the

_Schottky

barrier

_(Fig.

_{1) ;}

- the

energy resolution is much poorer for

heavy

ions than for _protons

_(Fig.

2)

due to _{fluctuations in}

energy loss

by

nuclear

_collisions,

which

_give

no

elec-tron-hole

_{pairs ;}

REVUE DE PHYSIQUE APPLIQUÉE. 2013 T. 16, N" 4, AVRIL 19si

Fig. 1. - Resolution

degradation of _implanted silicon detectors

versus various _heavy ion doses _[29].

- the

amplitude

of the

_signal

is smaller and

depends

on the

_type

of

_particle,

because of window effect and nuclear collisions

_(Fig.

_3).

To

_improve

the resolution of

_RBS,

some authors

suggested

the use of

_magnetic

_{analysers [18-19],}

which are rather _{cumbersome systems} or electrosta-tic

_analysers

_(ESA),

which are

_usually

limited to use

with low _energy ions

₍

500

_{keV) [7,}

_21,

_22].

Here,

we consider the

_{possibilities}

_{of ESA for}

light heavy projectiles

of

_energies

_up to 1 MeV and also the

_{advantages attending}

the use of the heavier

ions when the drawbacks due to other detection

systems are eliminated.

12

166

Fig. 2. - Evolution of silicon detector

energy resolution with ions

species as a function of energy.

Fig. 3. - Efiect of nuclear collisions _on pulse amplitude delivered

by a silicon detector versus _energy for various heavy ions.

2. Ion

_scattering.

- 2.1 KINEMATICS. - Under

the Rutherford elastic

_scattering

collision

_assumption,

a

_projectile

of _energy

_Eo

and mass

_Ml

scattered at a

lab

_angle

0 from an atom

_M2

at rest

_(p

=

Mi/M2)

looses

_part

of its initial _energy, such that its new

value is :

where the factor K is

_expressed

_{by :}

with

The evolution of K as a function of

_1/03BC

and 0 is

shown in

_figure

4.

RBS

is,

therefore,

useful for mass

_analysis

with a

rather

_complicated

conversion scale

_{K, except}

for

two

angles

0 = 90 and I80°, where :

Fig. 4. - Evolution of kinematic factor K _versus laboratory angle 03B8

and mass ratio _{1 /u.}

In

_figure

4 we have also

_plotted

the K values for

03BC

1 even

_though

this condition is

_generally

not

used in

_RBS,

but

_{which, nevertheless,}

constitues a

domain of interest in some situations as will be consi-dered later. It is

_interesting

to notice at this

_point

that for relative

light

elements, the factor K

is rather uniform for all

_(ju)

_values,

_leading

to a rather _poor

mass resolution. For

_heavy

_projectiles,

K

_changes

more

_drastically,

especially

for

particular (0)

and

(03BC)

values. It

_is,

therefore

_interesting

to evaluate the

mass resolution.

2.2 MASS RESOLUTION IN RBS. - Let us establish

the

_{relationship existing}

between the _energy resolu-tion AE and the mass resolution

AM2-By

differentiating equation (1)

we can write :

From

_equation

_(2),

we have :

and

and

Fig. 5. - Evolution of the resolution in

mass _{M 2/ Õ’M 2} as a function of _angle 0 mass _{ratio p} for _0394E/E = 10-3, with extension to _{M1 > M2.}

Figure

5

_gives

the evolution of mass resolution

versus 0 and Il for a relative resolution in _energy

AE/Eo

= 10-3. It should be

pointed

out that the maximum in

_{mass resolution}

increases from 380

to 1 000 when _ji increases from 0.26 to 1 and 0 varies

from 180 to 90° :

_{for ji a}

1 and 0° 0 90,,,

M2/0394M2

reaches _very

_high

_values,

_going

to oo for

0 = _arc sin

1/03BC.

As mentioned

above,

these conditions

are not used in RBS

_today.

The

_{backscattering}

cross

_section,

_{expressed by :}

its variation is shown on

_figure

_{6. It appears,} in

par-ticular,

that the

_angles

between 0 and

_90°;

_together

with a value

_{of p}

close to

_unity

are still better than an

angle

of 180°.

A

_{specific example, perhaps,}

shows the

improve-ment better : from

_figure

7 it _appears that for

_M2

= 70

(gallium),

0

= 1500,

AE/Eo

=

40/00’

M2/0394M2

increa-ses from 43 with’

4He+

_(IAM2

_{= 1.6)}

to 98

for

a

12C+

beam

_(0394M2

=

0.7)

and

0394M2

reaches even 6 for

4He+

ion and a

conventional

surface barrier

detector

_(AE/Eo

=

1.7 %).

Further conclusion can be drawn from

_equation

(8) :

. .

- the

impinging

beam should have as

_high

as

possible

energy

Eo,

Fig. 6. - Evolution of the relative

backscattering cross-section versus 03B8 and y.

- the resolution AE should be the best.

It

should

be mentioned that a few hundred eV beam energy

dispersion

are attainable

_{today [23]}

with Van de

Graaff accelerators.

_Ideally,

a detection _system should

be able to measure

_this,

which,

of _course, is not

the case with solid state

_detectors,

as indicated above.

3.

_Depth

résolution in RBS. - For an energy

reso-lution

_AE,

the

_depth

resolution

_AD/D

can be

168

in which

_S,

_{and S2}

are the _energy loss of the

projectile

in the

_impinging

and backscattered direction. To

simplify,

we can choose a mean _energy loss S such

that :

and

Fig. 7. - Evolution of _mass resolution

M2/0394M2 as a function of ratio y and energy resolution AE/Eo for an _{angle 03B8} = 150°. Notice

the _progress when going from Schottky silicon detector _(resolution

2 %) to the electrostatic system (resolution 4

_%0)’

Fig. 8. -

Principle of backscattering analysis.

As we did

previously

for the mass

resolution,

we

report on

_figure

9 the évolution of

_depth

resolution

Fig. 9. - Evolution of

depth resolution versus 0 and 03BC.

versus 03B8 and _11. When

_going

from

_light

to

_heavy

projectiles,

the kinematics

_depth

resolution

_degrades

at least a factor of two at 0 =

180°,

for Il going

from

10 to 03BC

= 1 but in

reality, this

value must be

_multiplied

by

the

_{stopping power S and}

divided

_by

the _energy resolution. The

_optimum

_depth

resolution

is,

there-fore,

achieved when the

following

conditions are

fullfiled :

- a

good

energy resolution

AE;

-

as

large

a thickness D as

_possible.

This can be

artificially

obtained

_{by using glancing angle}

_{geometry ;}

- use of

particules

and _energy

_Eo

_{corresponding}

to the maximum

_stopping

_{power S.} This maximum

starts around 100 keV

for 1H+

and increases with the mass of the

_projectile

_{(Fig. 10)}

_being

around 3 MeV

for 12C+.

It

is, therefore,

_advantageous

to use an ESA

operating

at

_high

_energy.

S

increases from 5

_eV/Å

for

1H+

to 30

_eV/A

for

4He+

and 120

_eV/Â

for

12C+,

in

_Al,

all

_{projectiles having}

_{1 MeV energy.} This value is even

higher, by

a factor 2-3, for

_heavy

targets like Au or

_{Ni ;}

- reduction

in the energy

straggling

[20].

The energy fluctuation of the machine must be as small as

_possible.

However, after a

_penetration

of about 1 000

_Á,

the beam

_straggling

becomes

_equivalent

Fig. 10. -

Stopping power of various ions in aluminium, arrows

and brackets _give maximum stopping power for some heavier

targets (Be, Ti, Ag, Au, Ni) expressed in _eV/A.

sophisticated

ESA is

_really

of full use in

investi-gating

close surface

layers.

4. The electrostatic

_{analyser (ESA).}

- The

prin-ciple

and

_practical

realization of our

_ESA,

which

operates up to 1 MeV for all

_particles

have been

published

elsewhere

_[20].

For

_theory

see references

[24-26].

Here we restrict ourself to

_figures

11 and 12

showing

the schematic of the _system.

5. Results. - As

previously

indicated,

an ESA

system

using heavy

ions in the MeV _range is best suited for near surface

_analysis,

at

_depths

in the order of about 1 000

A.

The

_depth

resolution

_capabilities

are illustrated

_by

the

_figures

13a and b for thin

_gold

layers deposited

on silicon and

_{analysed, respectively}

by

a conventional surface barrier detector and ESA. It should be noticed on

figure

13a that the

_depth

Fig. 11. - Schematic

drawing of our electrostatic _analyser.

resolution

_degrades

_quickly

with

_penetration

of the

projectile

in the

_film,

due to the beam

_straggling.

The

_high

mass resolution allows the determination

of the surface

_{stoichiometry}

of

_compound

semi-conductors :

_depending

on the mass resolution and the nature of the

_projectile,

the theoretical _spectra,

taking

into account the various

_isotopes

of GaAs and CdTe are shown on

_figures

14 and 15. The expe-rimental _spectra

_depend

also on

_{crystal quality}

and real

_{stoichiometry;}

some results are

_reported

on

figures

16 to 19 for cleaved

surfaces,

first

_analysed

with

_{light projectiles}

and then with ’Li+ and

12C+

beams. For

_GaAs,

the mass resolution

_improves

from 1.7 for

4He+

to 1.4 for ’Li+ and even 0.8 for

12C+.

In the case

_{of CdTe,}

the values

improve

from

5 to 2.0 for the same

_projectiles.

In the case of a

Fig. 12. - Electronic

170

Fig. 13. - (a) ESA analysis of _a

gold layer 270 A thick, showing

the surface and depth resolution. (b) Comparison of ESA and

Schottky barrier detector _performance in _analysing a diffused gold layer into silicon.

M

Fig. 14. - Calculated surface

spectrum of AsGa as seen _by

back-scattering under channelling conditions for various mass

reso-lutions.

conventional

_{Schottky barrier,}

_having

a resolution

of 16

keV,

the mass resolution would

become

17.

When

_channelling

conditions are

_requested,

the use of

_heavy

ions constitutes a further

_advantage,

Fig. 15. - Calculated surface

spectrum of CdTe as seen _by

back-scattering under channelling conditions for various mass

reso-lutions.

Figs. 16-17. -

Experimental spectra obtained on GaAs observed

by backscattering with 1 MeV 12C+, 4He+, ’Li+ ions.

since the critical

_angle

becomes

_larger,

as shown on

figure

20 for

CdTe,

investigated

by

means of a 7Li +

beam. The

_expected

theoretical values

_{[27, 28]}

are

also

_reported.

However, it should be

_noticed,

as

already

indicated,

that for some

crystals

the

aligne-ment is still difficult when

_compound

semiconductors

Fur-Mg. 18. -

Backscattering spectrum obtained with 12C+ ions

impinging under random incidence on GaAs.

Fig. 19. - Surface

analysis of CdTe under _channelling conditions

as seen with 1 MeV 12C+ and 4He+ projectiles.

thermore,

care

must

also be

_given

to the eventual radiation

_damage

created on the material

_by

the

analysing heavy projectiles :

in some of our

experi-Fig.

20. -

Angular scanning of _{an 110 )} axis of CdTe as seen by 1 MeV 7Li + ions.

ments, the

_target

was moved

_during

the beam

ana-lysis,

which needs _very

_good

mechanical

_alignement

of the whole

_goniometer

_set-up.

6. Conclusion. -

The combination of ESA and

heavy projectiles

open new

_{possibilities}

of RBS in

surface

_analysis.

In

_fact,

both

_depth

and mass

reso-lution can be enhanced. Besides the

_{stoichiometry}

measurements, we have shown that this

_procedure

is also of

_importance

for

_heavy

ions diffusion

experi-ments as well as _range of

_particles

in

_light

_targets

[14, 21].

The main drawback of this

_technique

is related to the _very

_{long counting}

_rate, of several hours

per

spectrum,

for low cross section elements. In this

case, care must be devoted to the

_{damage resulting}

from

the

_analysing

beam itself.

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