Radiation Effects on Solid Surfaces

1 Physical Sputtering: A Discussion of Experiment and Theory

Downloaded by NORTHERN ILLINOIS UNIV on August 19, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0158.ch001

HAROLD F. WINTERS IBM Research Laboratory, San Jose, Calif. 95193 The theory of sputtering of amorphous and polycrystalline materials has been further developed. Experimental results for the energy, mass, and angular dependence of sputtering yields and recent experiments on the sputtering of chemi sorbed gas and sputtering by molecular ions illustrate sev eral effects that are important in determining sputtering yields and, in particular, the relationship of these effects to Sigmund theory. There are several mechanisms which might lead to discrepancies between theory and experiment. Τ"* he ejection of material from solid surfaces under bombardment by energetic ions (or neutrals) is known as sputtering. The sputtering yield, S, is defined as the number of target atoms ejected per incident ion. Review articles on physical sputtering have been published by Giintherschulze and Meyer ( I ), Wehner (2), Behrish ( 3 ) , Kaminsky ( 4 ) , Carter and Colligon (5), M c D o n a l d (6), and Tsong and Barber (7). McCracken (8) has published a paper recently on the interaction of ions with solid surfaces i n which there is a long section on sputtering. In addition, Sigmund has published a series of review articles on the collision theory of displacement damage (9) which includes a long chapter on sputtering (JO). A l l of these articles, and i n particular, various articles by Sigmund and co-workers, have been of material benefit i n preparing this paper. This is all the more true because the author, while very interested in the interaction of low energy ions ( < 1000 e V ) with solid surfaces, is not a specialist in the field of physical sputtering. It is hoped that this chapter w i l l benefit from his slightly detached point of view rather than suffering from a lack of intimacy with the subject. The thrust of this paper is to outline concisely the current theo retical approaches to the sputtering of amorphous and polycrystalline targets and then to interpret some of the important experiments on the basis of the derived results. N o attempt at comprehensive coverage w i l l 1 Kaminsky; Radiation Effects on Solid Surfaces Advances in Chemistry; American Chemical Society: Washington, DC, 1976.


2

R A D I A T I O N

Figure I . Schematic of possible collision processes which occur under ion bombardment. (1) Sur face atom receives energy and after several collisions is reflected away from the target; (2) the incoming ion creates a primary recoil which in turn produces a collision cascade that penetrates the surface; (3) collision cascade which does not penetrate the sur face; (4) reflected ion creates a cascade which penetrates the sur face; (5) reflected ion gives en ergy to a surface atom which is sputtered; (6) ion reflected into the vacuum with kinetic energy; and (7) atom with momentum component directed away from the surface returns because of attractive forces.

E F F E C T S

O N

SOLID

SURFACES

ο ,on

be made because of the number of review articles already i n the literature. A l l modern approaches to the theory of physical sputtering are based on the binary collision model which assumes that energy is transferred from the impinging ion to the target atoms by a sequence of binary collisions; i.e., the ion only interacts with one target atom at a time. This process is illustrated schematically i n Figure 1. The first collision does not, i n general, lead directly to sputtering since the hit target atom has a momentum component i n the direction away from the surface. There fore, sputtering is a multiple collision process involving a cascade of moving target atoms. Whereas the concept of a collision cascade govern ing the sputtering process is a common feature i n a l l recent sputtering theories (11,12,13,14), there are some differences i n the processes that various authors consider important and consequently i n the approxima tions made to solve the problem (see Réf. I I ) . A yield calculation consists of a number of steps: (1) T o determine the differential cross section, da, for the transfer of energy between Τ and Γ + dT from the ion to the target atom and from one target atom to another (this step primarily involves the ap proximation of interaction potentials). (2) T o determine the amount of energy deposited near the surface. (3) T o convert this energy into the density of low energy recoil atoms.

Kaminsky; Radiation Effects on Solid Surfaces Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

1.

W I N T E R S

3

Physical Sputtering

(4) T o detennine the number of recoil atoms w h i c h reach the very surface. (5) T o select those atoms w h i c h are able to overcome surface b i n d ing forces and thus be emitted into the gas phase.


The Formalism of Sputtering Theory Classical Mechanics. Two-particle, collisions are often adequately described using the ideas of classical dynamics, and a l l modern sputter ing theories make this assumption. The quantity, da(T), which is needed by sputtering theorists, can be formally obtained from the following w e l l known equations ( 5 , 1 5 ) . T = r

' -

m

s i n - ,

* ~

2 P

I

T

m

=

Γ,

F

M

i

+

U (R) B

L

B 0

{

d

R

R

M

2

)

E

2

P H " R]

(1)

( 2 )

2

>

M

da (θ) _

E

-2πΡάΡ

(3)

where (see Figure 2) Μ χ and M are the masses of the colliding atoms, Ε the initial energy, Τ the energy transfer (the energy of the recoiling atom), θ the center of mass scattering angle, Ρ the impact parameter, R the internuclear distance, R the distance of closest approach, and U(R) the interatomic potential. R is the root of the equation (15): 2

0

0

T. V" Figure 2. Scattering of two particles viewed from the laboratory system, φ is the laboratory scattering angle.


R A D I A T I O N E F F E C T S O N SOLED

SURFACES

1,0 He—Νι |ψ=45°.$=9σ) 0,8

*+ "ΓΤ+J " %ô~° υ υ g

0,6

Χ χ χ

φ +

•

Ne—Ag(i|> = 30°.d = 60°)

° χ • κ

Κ

*

Ne-Ni (ψ=38°.0 = 60°)

*

w

Η - 0 (ιΜ5°.θ=90°) β

Ne—Ni (ψ =£5°.$=90°)


0.2

Δ

·»

**

500

"Ne—S (ψ=45°.θ=90°Γ

1000 Energy Ε

1500

2000

Figure 3. Fractional energy loss (E — T J / E υ*, primary energy at constant φ for ion scattering experiments. Data from Ref. 17.

da(T) is formally obtained by inverting Equation 2 to obtain Ρ as a function of θ. Ρ and dP are t i e n substituted into Equation 3 yielding άσ(θ) which is subsequently changed to da(T) using Equation 1. The input quantity needed to use this procedure is the interaction potential, U(R).

The binary collision approximation is crucial to sputtering theorists, and hence we make a few comments at this time. The best experimental evidence supporting the model is from investigations conducted at many laboratories involving the scattering of ion beams from surfaces and the subsequent measurement of the angle and energy of the reflected ions (16). F o r binary collisions, the quantity (E — T)/E (see Figure 2) is independent of Ε for constant θ (see Equation 1). (Constant θ implies constant φ.) Figure 3 shows data from Heiland and Taglauer (17) which demonstrate that the binary approximation is valid to quite low energies. This is consistent w i t h computer-simulated results of Karpuzov and Yurasova (18) who investigated the reflection of 50-500 eV argon ions from a copper crystal and concluded that ion reflection is adequately described by the binary collision model. Interatomic Potentials. It is not absolutely necessary to have accu rate interatomic potentials to calculate reasonably good sputtering yields because the many collisions involved tend to obscure the details of the interaction. This, together with the fact that accurate potentials are only known for a few sysems makes the Thomas-Fermi approach quite attrac tive. The Thomas-Fermi statistical model assumes that V ( r ) varies slowly enough within an electron wavelength that many electrons can be localized within a volume over which the potential changes by a fraction of itself. The electrons can then be treated by statistical mechan-


1.

5

Physical Sputtering

W I N T E R S

ics and obey F e r m i - D i r a c statistics. I n this approximation, the potential in the atom is given b y : F ( r ) = ^ Q ;

(4)

a = 0.885a Z- '* 0

1

where Ζ is the atomic number and a the Bohr radius, 0.529 A . F o r a derivation of Equation 4, see Ref. 19. φ (r/a) is the T h o m a s - F e r m i screening function shown i n Figure 4. It is convenient to describe the interatomic potential, U(R), w i t h the same functional form as Equation 4. This has been accomplished b y Bohr (21) who estimated the interaction energy between two atoms b y the formula: TT/r>\

ΖχΖζβ

R

2

- _ e x p -

T

where R is the internuclear distance and exp( — R/a) the screening function. Subsequent authors usually represented their interaction poten tials i n the same form but with modified screening functions. Firsov (22) showed that, within the limits of accuracy of the T h o m a s - F e r m i statisti^

« o —ι ι ι ι 1 1 1 I ^ \ ^^>\S=2

VI I \ \S=3

S=3/2^V

II

1 1 1 1 1

1

1

1

IIIIL

J

1 1 1 11 1

\

I

I

\\

-

ιι ι ι


0

\

V 0.1

1

J

1

l i t

\

l-Ll

6 8 1.0

1

1

R/a

1 _l_ ι

ι ι ι ι

6 8 10

1

6 8

100

Figure 4. Thomas-Fermi screening function, 0(R/a,), (see Equation 4) for neutral atoms ( ) and power approximations ( ) from Equation 7. Values of «^fR/aJ are from Ref. 20. Constants used in Equation 7 are: k = 0.591, k = 0.833, k = 2.75. See Equation 7 for definition of s. t 5

2

s


6

RADIATION E F F E C T S O N SOLID SURFACES

cal model, the interaction between atoms at a distance less than 10~ cm could be described by the potential: 8

where φ ( Κ / α ) is the T h o m a s - F e r m i screening function shown i n F i g ure 4 and Downloaded by NORTHERN ILLINOIS UNIV on August 19, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0158.ch001

Ρ

a = 0.8853 α ( Ζ ι F

0

1/2

+

Z Y m

m

2

L i n d h a r d et a l . (23) preferred a screening radius: a — 0.8853 a [ Z i L

0

2 / 3

+ Z

2

2 / 3

]"

1 / 2

for the same functional form of Equation 5. The two screening radii are numerically equal within the accuracy of the Thomas-Fermi approach. In subsequent sections we w i l l use a and refer to it as a. Equation 5 is often used to describe the interaction between the incoming ion and the target atoms. The interaction between two target atoms generally occurs at low energy where the Thomas-Fermi potential overestimates the interaction. Under this situation a B o r n - M a y e r poten tial is more appropriate ( I I ) , i.e.: h

U(R)

=Ae-™

(6)

where A and b are constants. Typical values for A and b have been tabulated by Abrahamson (24). A n especially useful approximation for the Thomas-Fermi potential has been developed by Lindhard (23) and co-workers where the screen ing function is assumed to have the form:

where k and s are constants. 17(R) then becomes: 8

-h?>*gLl

U{B)

(8)

Figure 4 shows that Equation 7 reasonably approximates the screening function over limited energy ranges. The inverse power approximation made i n Equation 7 is quite attractive since it allows Equation 2 to be integrated i n closed form for several values of s ( 2 5 ) .


1.

W I N T E R S

7

Physical Sputtering

The substitution of Equation 8 into Equation 2 followed by ap proximations (see Ref. 23) and integration leads to: da(T)

= CE^T'

m = s

- dT;

1

m

(9)


where

and λ™=ι = 0.5, A i = 0.327, X m i — 1.309. m = 1 corresponds to Rutherford scattering, and Sigmund (11 ) has shown that m — 0 approxi mates scattering from a B o r n - M a y e r potential. F o r m = 0, C = ( 1/2)πλοα where λ = 24 and α = 0.219 A . F o r m — 0 α is assumed to be independent of z. Sigmund also suggests that for medium mass ions and atoms over most of the k e V range, m — 1/2 is a fair approximation while i n the lower k e V and upper eV region, m~ 1/3 should be adequate. Equation 9 is an extremely useful description for the differential cross section and has been used extensively i n a variety of applications. W i t h the use of Equation 9, the nuclear stopping power becomes: m =

/ 2

e

/ 3

0

2

0

This is one of the basic input quantities needed i n Sigmund s sputtering theory. Basic Equations—Sigmund's Theory (11). There have been three important theories i n recent years on the sputtering of amorphous and polycrystalline solids. They are attributed to Sigmund ( I I ) , Thompson (12), and Brandt and Laubert (13). The predictions about various aspects of sputtering often agree. Sigmund's theory, however, is the most general and latest and therefore is outlined briefly. Suppose an atom starts its motion with an arbitrary velocity vector ν at t = 0 i n a plane χ — 0 i n an infinite medium (Figure 5 ) . The basic quantity of interest is the function G(x,v >v,t)d v dx which is the average number of atoms moving at time t i n a layer (x,dx) with a velocity (v ,d v ). The number of atoms with a velocity (v ,d v ) penetrating the plane χ i n the time interval dt is given by: 3

0

0

3

0

0

0

3

0

G (xpp,t ) cPv 1 Vox I dt 0

wher Vos is the χ component of v . surface at χ — 0 is then: 0

The backward sputtering yield for a


δ


S— Γ\v ,\ o^'^ ~ lJi G(x,v ,v,t) G

x

1}

r

NÎ da[G(x,v ,v,t) J v.v"

0

-G(x,v ,v't)

0

=

-G(x,Vo,v",t)]

0

(13)

Equation 13 applies to an ion of the same species as the target, but similar arguments lead to an analogous equation for an arbitrary ion. F o r simplicity, electronic stopping has also been neglected but could easily be included as was done i n the original treatment by Sigmund. The function: F(x,v ,v) 0

=

j

G(x,v ,v,t)dt 0

is the total number of atoms which penetrate the plane χ w i t h a velocity (v ,dvo ) during the development of the collision cascade. F(x,v ,v) satisfies an equation that follows from integration of Equation 13 over t, i.e.: 0

3

0

1 -S(x)S(v-v )

d - -^F(x,v ,v)

0

ΝJ

da[F(x,v ,v) 0

—

0

v

-FfavoF)

-F(z,v ,V")] 0

(14)

The function H(x,v) represents the backward sputtering yield of a target atom for the case of a source at χ = 0 and the sputtered surface i n the plane χ where: H(x,v) = J

d v \vo \F{x,v v) z

Q

x

0

The integrations over v obey the conditions: 0

Vox ^

Λ


10

RADIATION E F F E C T S O N SOLID SUBFACES

E =

±Mv *>U( )

0

0

Vo

where ϋ(η ) is a surface binding energy. Multiplying both sides of Equation 14 by v and integrating over v and also changing the velocity variables to energy variables yields: 0


0x

0

= NJda[H(x,E, ) v

- Η(χβ', ') η

-

ff(ï^V)]

Here, θ(£) is the Heaviside step function; θ(ξ) = 0 for £ < 0 and θ(ξ) 1 for £ > 1. The backward sputtering yield is: S(E„)

=H(x

= 0,E, ) v

(15) =»

(16)

The fundamental problem i n determining yield is to solve Equation 15 for H. Sigmund solves the problem by using a standard technique involv ing expansion i n Legendre polynomials. The final result is given by:

where F(x,E^) is now defined as the amount of energy deposited i n a layer (x,dx) by an ion of energy Ε starting at χ = 0 and by a l l recoil atoms. See Ref. 11 for the mathematical details used i n obtaining Equation 17 from Equation 15. F o r power scattering, it can be shown that: F(0fi„)-aNs (E) m

(18)

where s (E) is the nuclear stopping power and α is a dimensionless quan tity depending on the relative masses and angle of incidence, a is shown as a function of M / M i i n Figure 6 for perpendicular incidence. Direct energy dependence in a drops out for power scattering. Using Equations 17 and 18, the sputtering yield becomes: a

2

Although Equation 19 has been derived for the case where the ion and target are of the same substance, it is also valid for an arbitrary ion (see Réf. I I ) .


1.

W I N T E R S

Physical Sputtering

11


1.0

0.5

2 0.01

10

0.1

Physical Review

Figure 6. Factor a in sputtering yield formula (Equation 21) calcu lated for power scattering and aver aged between m = 1/3 and m = 1/2 (H)

F o r low energies (m — 0; Ε £ 1 k e V ) , the yield becomes: (20) where the value for s ( E ) was obtained from Equation 10. F o r keV ener gies and heavy-to-medium mass ions, the expression for the nuclear stop ping power calculated by Lindhard (23), assuming a T h o m a s - F e r m i interaction, is used, i.e.: n

where s (e) is the reduced stopping power plotted i n Figure 7. T h e sputtering yield from Equation 19 is therefore: n

S(E„)

= 3.56 a

[

V

/

3

+\

M]V%

[M +M ] A

1

2

^

(

2

1

)

where U is i n eV. The relationship between the ion energy Ε and reduced energy € is given b y : 0


12

RADIATION

E F F E C T S

O N

SOLID

SURFACES

+

Ion Energy - A r / C u (keV) 0 0.71

1—I

1 INI

I I I

Τ—I

I

10 I I MM

100 I I I MM

1—I

1—I

I I I

1000 III


Thomas Fermi

r~

0.1

2

Potential

h

J—I

ιο

I I I I III

I

I

I I I MM

ίο- 1

ί ο

I

I

I I I MM

I

L 10

1

Reduced Energy, e

Figure 7. Nuclear stopping power, s«, as a function of ε for bottom scale and of Ε for Ar*-Cu top scale. Horizontal line is for an R~ potential. Data from Ref. 23. 2

~[Z Z eHM +M )]

€

1

2

1

2

Equations 20 and 21 along with Figures 6 and 7 are used as a framework to interpret most of the experimental data presented i n this paper. Comparison of Experiment with Theory To interpret experimentation results i n terms of Equations 20 and 21, we are interested i n the sputtering yield expressed as a function of (1) ion energy; (2) the angle of incidence; (3) the masses of the incident ion and target material; (4) the surface binding forces, i.e., t7 , and (5) energy E of the sputtered atoms. There have been numerous sputtering yield measurements on amor phous and polycrystalline targets during the past 20 years. Unfortunately many of the measurements are difficult to interpret because of uncon trolled experimental conditions. F o r example, the presence of chemisorbed gas often reduces the sputtering rate (26). This phenomenon is 0

0


1.

W I N T E R S

Physical Sputtering

13

well known to people involved i n the growth of thin films (27). A n example of the influence of an adsorbed layer is seen i n Figure 8 where the variation i n sputtering yield w i t h ion dose is shown for 600-eV N ions on gold. The initial rapid increase was attributed by Colligon et al. (28) to the removal of an adsorbed layer. It is, therefore, quite clear that atomically clean surfaces should be used for accurate yield measurements. This requires good vacuum conditions. It is becoming clear that many of the reported yields are dose dependent. Almén and Bruce (29) published some examples of 45-keV V ion irradiation of copper and tantalum targets where they found a marked decrease i n yield as a function of dose. F o r 45-keV C a ions on the same target, they found a weight gain. In the same article, the authors reported measurements of the Z i variation of the 45-keV sputtering yield on copper, silver, and tantalum targets. In the Z variation they found peculiar oscillations following the chemical properties of the incident ions and ascribed these variations to a change i n target material caused by the accumulation of projectile atoms. This assumption has recently been shown to be correct (30, 31) since the periodicity disappears for smaller doses. A dose dependence is expected under situations where a change of chemical composition occurs because the development of the collision cascade must depend to some extent on the masses of the target atoms. 2

+


+

+

x

Figure 8.

Variation of sputtering yield with ion dose for 600~eV N + ions on gold (28)


2

14


Accumulation of significant amounts of projectile material near the sur face is expected under many situations. Therefore, a definite need for yield measurements made under low dose conditions exists. Figure 9 shows the concentration of nitrogen near the surface of a polycrystalline tungsten sample as a function of the number of bombard ing N ions. The saturation coverages are 8 Χ 10 and 9 Χ 1 0 atoms/ c m at 300 and 450 eV, respectively. Based on this information, one would expect the sputtering yield for an initially clean surface to change until the surface had been bombarded with 4 Χ 1 0 N V c m and then to remain relatively constant. Noble gases are attracted to surfaces b y weak van der Waals forces, and therefore when they reach the surface they are almost immediately desorbed into the vacuum. Consequently, ion bambardment effectively releases noble gas atoms trapped near the surface (33), and therefore the saturation concentration does not become nearly so large as for other projectiles (34) (compare Figures 9 and 10). W e think that yield measurements involving noble gas ions are i n general more reliable than those involving other ions, but even for noble gases, there appears to be a dose dependence (35). Variation of Sputtering Yield with Ion Energy. F o r scattering using an inverse power potential (Equation 8 ) , the quantity a (see E q u a tion 21) is independent of energy and is only a function of the angle of incidence and relative masses. The energy dependence of the sputtering yield is therefore determined b y the energy dependence of the stopping power. Brandt and Laubert (13) make similar predictions, and their 2

+

15

15

2


16

τ

1

1

1

1

2

2

1

1

Γ

2

Number of Incident Ions (Ions/cm ) Journal of Applied Physics

Figure 9. Number of nitrogen atoms trapped near a tungsten surface vs. number of incident N * ions (32) $



Canadian Journal of Physics

Figure 10. The number of argon atoms trapped near a tungsten surface as a function of the number of incident argon ions of various energies (34) calculated results agree reasonably with those based on the Sigmund model. Yields for many ion-target combinations have been calculated using Equations 20 and 21, and the agreement is remarkably good considering that there are no adjustable parameters i n the theory. Figure 11 shows data for inert gas ion bombardment of copper where agreement is very good over the entire energy range. Table I shows calculated and measured yields for bombardment of various metals w i t h 45-keV noble-gas ions. The agreement, in general, is somewhat worse than for copper. However, judging from the work of Andersen and Bay (35), we would expect the experimental yields of Almén and Bruce (29) to be significantly higher (possibly by a factor of two or more) if they had been taken under low dose conditions. This would make the agreement between experiment and theory much better. Wehner (36) has amassed a large amount of data i n the low energy range (0-600 e V ) which, i n our opinion, is quite reliable and relatively free of dose effects. These data are not corrected for secondary electron emission. However, for K r and X e , where Sigmunds theory is most +

+



30

I

I I I

I

m—1

I

• Wehner et al. • Guseva ο Alme'n et al. 20 ~" A Dupp et al.

MM—ι—ΓΤΊΠ— A

25 ~

A

A

— A

A

+

Xe - Cu

S 15


10 5

—

0 .—rr-ïl Ί 10"

ι

Il

1

1

l 1JJ

1

l_l_U 10

10^

10'

ι ι ι τ ι - πT T — ι — Γ Τ η — ι — r m — • Wehner et al. 25 ~ • Guseva Kr -> Cu ο Alme'n et al. 20 - A Dupp et al. • Key well

J

30

+

S 15

—

/ /

10 5

-

J η

0

< i l Il

ι

1 10

10 - 1 30 25 20 S 15

-

1

TTTT—I • • ο A • • I

Ar

1 10'

1 11 1

1—ΓΤΠ—I—ΓΤΤΤ

ΓΤΤΙ

Wehner et al. Guseva Alme'n et al. Dupp et al. Yonts et al. Weijsenfeld Southern et al.

I I Il

+

Cu

10 5 0


10

J

1.

30

TTTT

25

• ο A • •

20 S 15


Physical Sputtering

W I N T E R S

1

Wehner et al. Alme'n et al. Dupp et al. Weijsenfeld Roi et al.

17

ΓΓΠ—I

ΓΤΠ—I

ΓΤΤΓ

+

Ne -> Cu

E (keV) Physical Review

Figure 11. (left and above) Sputtering yields for Cu calculated from Equation 21 ( ) and Equation 20 ( ), compared with experimental results of Refs. 11, 29, 36-43 (11) applicable, the correction would, i n general, change the yield b y less than 1 0 % . Calculated yields using Equation 20 agree excellently with Wehners data for many target materials, but for others the measured values are smaller than the calculated ones by a factor of two. Whereas the sputtering yield is generally proportional to the nuclear stopping power as predicted b y Sigmunds theory, there are systematic variations for large and small mass ratio ( Λί /Λίι ) as well as i n the energy dependence. These discrepancies are discussed i n later sections. Variation of Sputtering Yield with Mass of the Incident Ion. Dose effects make it inconvenient i n some instances to use absolute yield data when comparing experiment with theory. Therefore, Andersen and Bay have presented their data as normalized yields, i.e., the ratio of the sput tering yield for an ion of atomic number Z to the self sputtering yield or to the argon sputtering yield. The normalization tends to eliminate or at least minimize dose effects. Data for Si, C u , A g , and A u from Andersen and Bay (35) are shown in Figure 12. Absolute yields from Almén and Bruce (29) along with calculated values from Equation 21 are shown i n Table I. Both sets of data and also theory indicate that the yield increases with Ζχ. The quanti tative agreement between theory and experiment (Figure 12) is good for amorphous silicon but gets progressively worse as the mass increases. The sputtering yield for heavy ions on heavy targets shows a more pro nounced maximum i n the energy dependence than does the stopping power (see Figure 11—Xe on C u ) . Such a discrepancy between theory 2

x

+





1.

19

Physical Sputtering

W I N T E R S

- ι — Γ

2.00

™T

" 1— ι — Γ

1

1

_


Silver

1.00

< t < CO

0.50

io-3l

?

ιο

ί ο

ι o-

JL

10" 10~

2

10"

10°

10

_L

1

10

J_ 2

10

3

10

4

10°

E (eV) Q

Radiation Effects

Figure 16. Energy distribution of sputtered gold atoms under irradiation by 20-keV Ar for several different target temperatures (46) +


1.

25

Physical Sputtering

W I N T E R S

4—Figure 1). They evaporated a thin film of copper onto substrates of Be, V , N b , and T a and subsequently measured the yield of copper as a function of substrate material. The various materials produced different intensities of backscattered ions, and therefore the contributions to the sputtering yield of incident and backscattered ions could be separated. Their results indicate that possibly 5 0 % of the sputtering events result from backscattering.


0.5

0.4

cυ ΰ

I CO

0.3

0.2

0.1

0.0 0

5 Energy

10 (keV) Radiation Effects

Figure 17. Theoretical calculations of the reflec tion coefficients as a function of energy for *He* incident on Nb. ( ), with surface correction; ( ), without surface correction (64). Calculations by B0ttiger and Winterbon (64) and others (65, 66) have shown that a large fraction of the incident ions are reflected away from the surface (see process 6—Figure 1). Their calculations for the reflection coefficient of H e are shown i n Figure 17. These results indicate that a significant fraction of the initial kinetic energy is not deposited i n the target material. Furthermore, the results depend sensi tively on a surface correction. This is because i n the theoretical model, an ion can pass through the hypothetical surface several times while i n an actual experiment, it can pass back through the surface only once. 4

+



26


Ion Energy (eV) Journal off Applied Physics

Figure 18. Sputtering yield for nitrogen chemisorbed on tungsten as a function of ion energy, λ is the secondary electron coefficient (67). A theory of sputtering for light ions on heavy substrates should consider the effects of (1) electronic stopping, (2) large angle scattering, and (3) a surface correction. W h e n processes 1 and 2 are included (66), a (see Equation 21), contrary to the case for heavy-keV ions, depends sensitively on ion energy, i.e., the sputtering yield is no longer directly proportional to the nuclear stopping power. Weismann (66) suggests that the rather large values of a at low and intermediate c-energies i n d i cate a considerable contribution to the yield from backscattered ions. Comparing experiment with theory shows the measured values to be about one half the calculated ones. The general shape of the yield curves, however, agree well with Weismann's predictions. The Sputtering Yield for Chemisorbed Gas. Winters and Sigmund (67) have measured the sputtering yield for nitrogen chemisorbed on tungsten for ion energies up to 600 eV (see Figure 18). W h e n this data is contrasted with yields for elemental materials (36), some interesting comparisons can be made. Elemental materials with large sublimation energies generally have small sputtering yields and relatively large appar ent threshold energies. However, nitrogen chemisorbed on tungsten has a large desortpion energy, ~ 6 . 7 eV/atom, a very large sputtering yield (see Table I I I ) , and also a low apparent threshold energy. This is just the opposite of what one would intuitively expect. Moreover, the yield



1.

W I N T E R S

Physical Sputtering

27

for helium reaches a maximum at ~ 1 0 0 eV and then remains constant to 600 eV. Neon shows a tendency in the same direction. B y comparison, elemental yields never reach a maximum i n this energy range. Whereas the sputtering of elemental materials is dominated by the development of a collision cascade (process 2 — F i g u r e 1), the sputtering of a light gas chemisorbed on a target of large atomic weight appears to be dominated by direct collisions between the incoming and/or reflected ion and the adsorbed species (67) (processes 1 and 5—Figure 1). This difference i n behavior occurs because energy is not effectively trans ferred between the nitrogen and tungsten (i.e., 4ΜχΜ /(Μι + M ) ^ 0.25). F o r example, a sputtering event involving transfer of energy from an H e ion to a tungsten atom and then to a nitrogen atom could not occur for ion energies less than several hundred eV. Calculations based on direct collisions between the incoming ion and the adsorbed gas, including sputtering events resulting from devel opment of a collision cascade, agree with experiment within approxi mately a factor of two. The difference in the shape of the yield curves between H e , Ne, and the heavier noble gases results from the fact that a Thomas-Fermi interaction is appropriate for He,Ne in this energy range (see Equation 5) while a B o r n - M a y e r potential (see Equation 6) is more appropriate for the heavier gases. The large yields and low thresholds occur because the energy initially given to the nitrogen is not easily transferred to the tungsten, and therefore the nitrogen atoms are reflected away from the surface with a large fraction of their initial kinetic energy. Winters and Sigmund predict that the cascade mechanism would begin to dominate at higher energies because of the increasing yield of sputtered tungsten and also because of the decreasing cross section for ion-nitrogen collisions. Under this situation the adsorbed nitrogen should have an exceptionally low yield because of ineffective energy transfer 2

2

2

+

Table III.

He Ne Ar Kr Xe

Ratio of Sputtering Yields

a

S (nitrogen on tungsten)

S (nitrogen on tungsten)

S (silver)

S (tungsten)

100 eV

500 eV

2.9 1.6 0.77 1.2 1.1

0.62 0.39 0.31 0.41 0.51

100 eV > 100 11.5 8.0 >6 >6

500 eV > 10 2.4 1.6 1.4 1.7

Data from Ref. 67. The nitrogen yields have been extrapolated to a coverage of 1.2 Χ 10 atoms/cm (~ 1 monolayer) for comparison purposes. The tungsten and silver yields are from Ref. 36. Silver was chosen because it has one of the largest elemental sputtering yields. β

15

2


28


from the moving tungsten to the adsorbed gas. As of now there is no experimental evidence to verify this prediction. Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

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44. Andersen, H. H., Bay, H. L., "Ion Surface Interaction, Sputtering, and Re latedPhenomena,"p. 63, Gorden and Breach, 1973. 45. Sigmund, P., unpublished data. 46. Chapman, G. E., Farmery, B. W., Thompson, M. W., Wilson, I. H., Radiat. Eff. (1972) 13, 121. 47. Thompson, M. W., Nelson, R. S., Philos. Mag. (1962) 7, 2015. 48. Nelson, R. S., Philos. Mag. (1965) 11, 291. 49. Honig, R. E., RCA Rev. (1962) 23, 567. 50. Dupp, G., Scharman, Α., Ζ. Physik (1966) 194, 448. 51. Cheney, Κ. B., Pitkin, E. T., J. Appl. Phys. (1965) 36, 3542. 52. Colombie, Ν., University of Toulouse, 1964, unpublished data. 53. Rol, P. K., Fluit, J. M., Kistemaker, J., Physica (1960) 26, 1009. 54. Oechsner, Η., Z. Physik (1973) 261, 37. 55. Bach, H., J. Noncryst. Solids (1970) 3, 1. 56. Molchanov, V. Α., Tel'Kovskii, V. G., Dokl. Akad. Nauk SSSR (1961) 136, 801 [English Trans.: Sov. Phys.-Dokl. (1961) 6, 137]. 57. Winters, H. F., Phys. Rev. (1974) 10, 55. 58. Sigmund, P., unpublished data. 59. Robinson, M. T., Philos. Mag. (1965) 12, 145. 60. Stuart, R. V., Wehner, G. K., Anderson, G. S., J. Appl. Phys. (1969) 40, 803. 61. Oechsner, H., Z. Phys. (1970) 238, 433. 62. Politiek, J., Kistemaker, J., Radiat. Eff. (1969) 2, 129. 63. Weissmann, R., Behrisch, R., Radiat. Eff. (1973) 19, 69. 64. Bøttiger, J., Winterbon, K. B., Radiat. Eff. (1973) 20, 65. 65. Robinson, J. E., Radiat. Eff. (1974) 23, 29. 66. Weissmann, R., Sigmund, P., Radiat. Eff. (1973) 19, 7. 67. Winters, H. F., Sigmund, P., J. Appl. Phys. (1974) 45, 4760. RECEIVED January 5, 1976.


Radiation Effects on Solid Surfaces

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