- Email: [email protected]
Secondary photon emission studies of ion bombarded beryllium
Nuclear Instruments and Methods 170 (1980) 577-583 © North-Holland Publishing Company
577
SECONDARY PHOTON EMISSION STUDIES OF ION BOMBARDED BERYLLIUM *
R.B. WRIGHT and D.M. GRUEN Chemistry Division, Argonne National Laboratory, Argonne, 1L 60439, U.S.A.
The secondary photon emission which arises during ion bombardment of Be metal foil by 200-3000 eV Kr÷, Ar÷, Ne+, OF and N~ was studied. By measuring the secondary photon emission yield of four Be(1) and one Be(II) emission lines as a function of incident ion energy information pertinent to the excitation process was obtained. Using a model based on a velocity dependent excitation process, the random collision cascade theory of sputtering; and making allowance for non-radiative de-excitation of the excited sputtered atoms and/or ions, it was possible to account for the observed energy dependent yields. The results of this model indicate that the secondary photon emission yield, yex of a given emission line, i, can be expressed as: Y~/x(om) ~ J(gA/cm 2) • S(no./ion)-. exp[-(A/a)i/Om] , where J is the incident ion current density, S the sputtering yield; Omis the velocity corresponding to the maximum energy transferred between the incident ion of energy El, mass M1 and the target atom of mass M2, i.e. Om = (8M1EI/(MI + M2)2) 1/2 ;(A/a) i is the non-radiative de-excitation parameter for state i. Values for the (A/a) i parameters were found to be ~(1-3) x 107 cm/s for the Be(I) and Be(II) states; a decrease to -~(5-7) × 106 cm/s upon O2÷ bombardment was observed for the Be(1) states, the value for the Be(lI) state did not change.
1. Introduction During energetic particle irradiation o f solid targets the target surface is eroded as evidenced b y the emission o f the constituents o f the target surface and near surface region. This phenomena is generally collectively referred to as sputtering, although it may in turn be described as comprising physical sputtering and chemical sputtering or chemical erosion. We will be concerned exclusively with the emitted particles arising from the physcical sputtering process. Among those particles which are emitted some are electronically excited atoms, ions, molecules and molecular ions; the latter two groups o f particles generally occur to a lesser degree than the first two. As the excited states have a finite lifetime after leaving the target surface they may radiate their excitation energy giving rise to secondary photon emission, the subject o f this paper. Despite extensive work in this field [1,2] there does not yet exist a detailed understanding of the origin o f the electronic excitation o f the sputtered particles [3].
* Work performed under the auspices of the U.S. Department of Energy.
In order to arrive at a better understanding o f the excitation and de-excitation mechanisms at surfaces undergoing ion b o m b a r d m e n t the dependence o f the secondary photon emission yield o f Be metal bombarded by various ions has been measured as a function o f the incident ion energy.
2. Experimental procedure In fig. 1 is shown a diagram o f the experimental system used in this study. A mass analyzed, 5 - 1 5 keV Ar ÷ beam was produced by an ion accelerator with a duoplasmatron ion source. The 200 e V - 3 keV Kr ÷, Ar ÷, Ne ÷, He ÷, O~, N~, D~ and ~ beams were produced by a variable energy sputter-ion gun. The operation o f this gun required that the target chamber be backfilled to 5 × 10 -s Torr (N:-equivalent) with the desired gas (all gases were research grade with > 99.999% purity). In our case this was not a static backfill, instead we chose to partially throttle the main gate valve leading to the pumping system and then increase the target chamber pressure with the desired gas until a suitable stable pressure was achieved. The ion beam produced b y the ion gun was not mass analyzed. When using 02, N2, D: and H2 gas XIII. ELECTRON AND PHOTON EMISSION
578
R.B. Wright, D.M. Gruen /Ion bombarded beryllium
Rotary motion UHV feedthrough -
OO 1 ~ OO , ~
+90 volt bias battery
UHV high voltage insulated flange
,--0-3 keY sputter ion gun control unit
Target holder
6 mm dia beam aperture
Current integrator and picoammeter
i 1Oo O0 0 O0 o
/ Mass analyzed ion beam from duo-plasmatron ion source
/i
valve J u.v ,,,e----r-
UHV gas leak valve connected to gas manifold
tt
UHV LiF window
(5-18 keV)
=,
~
Supracil-1 quartz lens
~
I II--I o
McPherson Model 218 ~
/
single monochromator
[ 2400groove/ram1 | 3o00 A blaze
~ IL;~Ogroovelmm°r'
/
I
I ~ . ~ L ~ * ~ _ ~ , ~, ~. . . / '~-
/~/ /~
Preamplifier (X10)
I ~
~X,'~l~'X~
I//~ ;
~ I
PM tube cooled
Photomultiplier tube
0
I--]
V-----1 0 I o OlO
0 0
(1) (2)
0 0
(3)
0 0
(4)
S~Or~cil-]
quartz lens
(l) High voltage power supply for PM tube (2) Amplifier/ discriminator (3) Photon counter (4) D/A converter (5) Strip chart recorder
\ 5000A blaze \ grating
(5)
Fig.1.Diagram of the experimental apparatus used in this study. (Refer to text for a detailed description.)
the dominant ion was M~ ,~96%, with -~,4% being the M ~ ion. We have not corrected our data for this difference. All currents were measured with a +90 V bias applied to the target. When we discuss data pertaining to when O~, N~, D~ or ~ was the incident ion it will be treated as due to )14" ions o f twice the measured ion current and half the energy of the a c t u a l ~ species. The ion beam was incident normal to the plane of the target for all the measurements herein presented. The target was a piece o f polycrystalline Be metal foil, 0.25 mm thick of 99.88% purity supplied by Kawecki Berylco Industries, Inc. Prior to insertion into the vacuum chamber the unpolished Be foil was solvent degreased and etched slightly in acid. After insertion, the chamber was evacuated, baked, and the target was extensively sputtered cleaned with a rastered (1 cm X 1 cm) 3 keV, 20/aA beam o f Kr ÷.
3. Experimental results and discussion In figs. 2, 3 and 4 are shown the measured secondary photon photon emission yield for the 234.86 nm Be (I) the 332.11 nm Be (I), and the 313.07 nm Be (II) emission lines [4] respectively as a function of incident ion energy for the various incident ions used in this study. The data shown in each figure are relative values as indicated; the abscissa scales as given in each figure are also the relative, as measured, values. However, since the sensitivity o f the spectroscopic detection system was not determined one should not interpret these ordinate values as given in figs. 2, 3 and 4 as being relative to one another. Obviously from figs. 2, 3 and 4 the secondary photon emission is not a linear function of the incident ion energy, nor is it proportional to the physical sputtering yield which is a function of the incident ion energy, mass
579
R.B. Wright, D.M. Gruen ~Ion bombarded beryllium I0,000
IOOO BERYLLIUM 5
BERYLLIUM
o
Be(I)- 3521 I,~
Be(T)- 23486 A
IOOO I00
E
5
5
E
2 I00
2
ct:
~.
5
3 t.u
2
~-
I0
.d iII I 0
•
• Ar + • Ne +
•
5-
21
5
•
Kr +
• Ar+ (125~.amp/cm21
Kr +
(125Fomp/cm2}
He+
o O"
[] ND ÷+ (245FomP/crn2) o H*
:f"
021.
10 20 30 INCIDENT10N ENERGY(xl03eV)
0
•
Ne +
•
He +
O 0÷ a N+
z~ D+ (245/~amp/cm2)o H+ ,
I 1.0
I
I
2.0
I 50
INCIDENT ION ENERGY(xlO3eV)
Fig. 2. Relative secondary photon emission yield values for the 234.86 nm Be(I) emission line as a function of the incident ion energy for the various incident ions used in this study.
Fig. 3. Relative secondary photon emission yield values for the 332.11 nm Be(l) emission line as a function of the incident ion energy for the various incident ions used in this study.
and atomic number. The theoretical energy dependence of the sputtering yields o f Be for the various incident ions are shown in fig. 5 as calculated using the semi-empirical theory o f Smith [5]. The formula proposed by Smith is given in the figure as well as the values o f the variables used in the calculations. This model as well as that o f Sigmund [6,7] would predict that the relative sputtering yield is greater the higher the mass of the incident ion. The incident ion energy dependence o f the secondary photon emission yields for the Be emission lines shown in figs. 2, 3 and 4 as well as that of the two other Be (I) lines studied display considerable variation with the predicted sputtering yield dependence. There is even a considerable variation in the incident ion dependence amongst the various Be (I) and Be (II) lines studied. However, with O~ incident on the target the greatest secondary pho-
ton emission yield was observed for all of the Be emission lines studied. The oxygen enhancement of secondary photon emission yields [1,2] as well as its enhancement of secondary ion yields [8,9] is well known but the detailed understanding o f its physical (or chemical) origin is still lacking [3]. The observed enhancement due to N~ is of interest as N2 does not appear to chemisorb on Be metal [10]. A beryllium nitride may possibly be formed as a result o f ion implantation, perhaps leading to the observed enhancement. The present data do not allow one to attribute any special significance to the apparent enhancement due to He +, D~ or H~ bombardment as with these incident ions the Be surface may have a higher surface oxygen coverage than with the other incident ions [111. It is presently believed that an excited sputtered XIlI. ELECTRON AND PHOTON EMISSION
R.B. Wright, D.M. Gruen lion bombarded beryllium
580
,02[ se000Be0
. . . . . . .
,
BERYLLIUM Be(~)-31307A
I00 F:
5
~2
I
w
5
_V 2 ,,-4, i.o
• Kr*
n,."
3
4
5
6
? 8 9 Ej(×lO3eV)
]0
II
12
13
f4
15
Fig. 5. Calculated sputtering yields of Be metal as a function of the incident ion energy for the various incident ions used in this study. The semi-empkical model of Smith [51 was used. His expression for the sputtering yield is shown in the figure as well as the values of the parameters used to calculate the curves.
I0 m ),-
2
• Ar* 12¢
• Ne+
5
oFamp/cm2
• He +
2~ Ol
°0Di(N245/~amp/cm2)t3 ÷ A o
001
0
H÷
I0 20 3.0 INCIDEN]" ION ENER6Y(xlO3eV)
Fig. 4. Relative secondary photon emission yield values for
the 313.07 nm Be(II) emission line as a function of the incident ion energy for the various incident ions used in this study.
neutral atom may undergo non-radiative de-excitation processes with the solid surface [ 3 , 1 2 - 1 4 ] . Such processes as resonant ionization and the two-electron Auger de-excitation process are in principle permitted. For a sputtered excited ion the resonant neutralization, Auger neutralization and the Auger de-excitation processes are permitted. These processes have been described in detail by Hagstrum [13,14] and by Hagstrum and Becker [15]. The probability that an excited atom or ion in state i will not undergo one o f the non-radiative de-excitation processes has been expressed in the form [ 3 , 1 2 - 1 4 ] N t~iur(o,O)=exp(_~(A/a)l/ocosO} . (1) \ j z
The total survival probability, ~ u r , for the N possible processes for state i, involves A, the transition rate relevant to the de-excitation process, and a, which is related to the effective distance over which the de-excitation is likely to occur. In eq. (1), u is the velocity of the escaping particle, 0 is the emission angle defined with respect to the surface normal. Note that v cos 0 is the escaping particle's perpendicular velocity and that the longer the particle spends close to the target surface, the greater the probability for it to undergo non-radiative de-excitation. Eq. (1) indicates that even relatively small changes in the value of (A/a) can make considerable differences in the survival probability for an atom moving with a velocity u and with a given emission angle 0. Wright and Gruen [3] have assumed that the excited sputtered particle in state i originates from those atoms sputtered by a collision cascade process, and that these particles have an excitation probability, as yet unspecified, which may be dependent on the sputtered neutral particle velocity and angle of emission, PeX(v, 0). The excited particles would then have a flux density velocity distribution [number. (unit area)-l . (unit time) -1 • (unit velocity) -I ] given by [31
~ x (o, 0) sin 0 do dO d4~ = ~°(o, O)l~iX(v,O) sin 0 do dO d4~,
(2)
R.B. Wright, D.M. Gruen ~Ion bombarded beryllium where ~b is the azimuthal angle of emission. Using the model of Thompson [16] and Sigmund [6,7] for the flux density energy distribution, 'I)°(E,®), of the sputtered neutral atoms resulting from a collision cascade process [since ~(v, O) = ¢b(E, O)(dE/dv)= v4~(E, 0)] then [3]
'l,°(v, O) C(n) =
u3
cos 0
(3)
(0 5 + v~). + ~ ,
where 1 ~< n ~< 2 (generally n is assumed to be n = 2, but recent measurements [17,18] of neutral sputtered atom flux density energy distributions have indicated 1.5 ~
///m 1 ~Tr
0
0
v3 cos 0 sin 0 do dO d¢/(v 2 + v~)n+ 1
0
(4) J is the incident ion beam current density in/~A/cm 2 ; S is the neutral atom sputtering yield (number sputtered per incident ion) for the collision cascade process; Vm is the velocity corresponding to the maximum energy which can be transferred to a target atom of mass M2 by an incident ion of mass Mt and energy Ea as given by: 1
Om F 4MIM2
EI(2/M2)I~
-- L ( ~ ~-~)~ = [7.72
1
X
1012M1E1/(M1 +M2)2]~ (cm/s) ,
(5)
with M~ and 3'/2 given in g/mol, and El in eV. vb is the velocity of a target atom which corresponds to a surface binding energy, Eb, usually approximated by the target sublimation energy, i.e. 1
Vb = [1.92 X lOl2Eb/M2]~ (cm/s),
(6)
with E b in eV. The yield of excited sputtered atoms can be then expressed using eqs. (1)-(6) as Y~iX(v,0) sin 0 do dO de = ~I'°(v, 0) p~iX(v, 0) ~Ur(v, 0) sin 0 dv dO dq~. (7) Wright and Gruen [3] have also shown that if: W/X(v, 0) = constant; WiX(v, 0) ~ v m cos m 0 ; or Wix(v, 0)
581
o: exp(_Bi/o cos 0), then the total secondary photon emission yield [determined by integrating eq. (7) over do dO dO], when A ~ vb and Vm >>Vb is approximately given by:
Y~ix ~,IS exp(-A~/vm) .
(8)
In eq. (8) if/ffix(v, 0) = exp(-Bi/v cos 0) then A~ = ~ ( A / a ) / + Bi; if/~ix(v, 0) is a constant or a power of the velocity thenA} = ZN(A/a)/. Therefore, eq. (8) can be used to determine the value of A~ since Y~ix is experimentally determined as a function of the incident ion energy, and S and Vm are also functions of the incident ion energy. In table 1 are shown the values of A~ calculated using the experimental secondary photon emission yield values for the four Be(I) lines and the one Be(II) line by the relation given in eq. (8). The correlation coefficients for all of the A~ values were >0.98. The range of the incident ion energy used in the calculations was 400 eV to 3 keV. We also used the model of Smith [5] to calculate the energy dependence of the collision cascade sputtering yield (refer to fig. 5) in eq. (8). Obviously the model used to calculate the energy dependence of the sputtering yield will effect the values of A[ given in table 1. As there have not been extensive measurements of the incident ion energy dependence of the sputtering yield for Be metal for the various incident ions used in this study we simply do not know if this model accurately reflects the true values. However, Smith's model reproduces the experimental data on other targets as a function of the incident ion energy for various incident ions quite well. The values for A~ listed in table 1 for the inert gas incident ions generally decrease in the order He÷> Ne÷> At÷> Kr ÷ for the five Be emission lines studied. The A} values for the Be(II) emission line are definitely greater than those for the Be(I) emission lines. This is not unexpected because an excited beryllium ion has more channels available for nonradiative de-excitation than does an excited beryllium atom. The values of A~ for the four Be(I) emission ÷ lines investigated when 02 was used as the incident ion are especially interesting in that they are approximately half the values of A~ as determined with the other incident ions. However, the A} value for the Be(II) emission line when O~ is used changes very little as compared to the other incident ions. It may XIII. ELECTRON AND PHOTON EMISSION
582
R.B. Wright, D.M. Gruen /Ion bombarded beryllium
Table 1 Values of A[ as experimentally determined using the secondary photon emission yield values as a function of the incident ion and the incident ion energy a for the various Be(I) and Be(II) energy levels studied in this work (A I in cm/s). Incident ion He+ Ne+ Ar+ Kr + H+2 D2 N~ O~ b Ar+c
Be(1) 234.86 nm 1.97 X 107 1.88 X 1 0 7 1.75 X 107 1.34 X 107 Average: (1.74 X 107) 1.34 X 107 1.91 X 107 1.42 × 107 6.36 X 106 4.12 × 107
Be(I) 249.47 nm
Be(1) 265.07 nm
Be(1) 332.11 nm
2.32 X 107
3.86 X 107
--
--
1.48 × 107 2.04 X 107 1.44 X 107 8.40 X 106 (1.45 X 107) 1.31 X 107 1.67 X 107 2.12 X 107 7.42 X 106 4.19 X 107
2.51 (2.42 1.74 2.33 3.40 1.50 -
X 107 X 107) X 107 X 107 X 107 × 107
2.30 (3.08 2.10 3.01 2.64 1.48 -
X 107 × 107) X 107 X 107 X 107 X 107
Be(lI) 313.07 nm 3.76 3.40 2.46 2.43 (3.01 2.49 3.40 3.18 2.64 8.64
X 107 X 10 7
X 107 X 107 X 107) X 107 X 107 X 107 × 107 X 107
a Energy range used to determine theA~ values was 400 eV-3 keV except for case (c). b The sputtering yield for BeO as a function of incident ion energy was used to calculate A~. c The duoplasmatron ion source was used to provide the mass analysed Ar+ beam; energy range was 5-15 keV.
be that upon oxidation one o f the non-radiative de-excitation processes for the Be(I) levels is prevented, or at least its probability diminished, whereas those de-excitation processes allowed for the Be(II) level do not change. The present method o f analysis cannot exclude the possibility that the initial excitation probability is given by W/x(v, O ) ~ e x p ( - B i / o cos O) as previously discussed. In this case it would be impossible at the present stage o f understanding to separate the excitation mechanism from the de-excitation mechanism or mechanisms. Previous workers have found a similar functional dependence of the secondary photon emission yields on the incident ion energy as given by eq. (8). Heiland et al. [19] employed a different interpretation o f the origin o f the excited sputtered particles choosing to neglect collision cascade sputtering, and only consider the "direct knock-off" and the "reflected-ion" contributions [20]. They did not report any A~ values as such, although they did interpret the exponential fit as arising due to the occurrence of a survival probability in the secondary photon emission yield expression. The fact still remained that the secondary photon emission yield, normalized to certain components o f the total sputtering yield depends exponentially on the velocity o f the excited atom (or ion). Similarly, Bhattacharya et al. [21]
used the relation given in eq. (8) with S being the "direct knock-off" [20] contribution only. They obtained values o f 1.7 X 107 to 2.6 X 107 cm/s forA~ by studying various excited states o f sputtered Si(I), Si(II) and Si(III) produced using 5 0 - 5 0 0 keV Ar ÷ incident ions. Van der Weg and Bierman [22], and White and Totk [23] have reported values o f approximately 2 X 1 0 6 cm/s for A~ for sputtered Cu(I) (324.75 nm emission line). These authors assumed that the binary collision between the impinging ion and atoms in the first layer o f the target as being the liberating and exciting event for the emission o f the detected excited particle. A similar approach was taken by Hippler et al. [24] who used line shape measurements of the Doppler shifted secondary photon emission lines to determine A~ values for selected excited states of A1, Cu and Zn. They observed a value ofA~ = 5 X l 0 s cm/s for the 307.95 nm line of Zn(I) which has the excited level below the Fermi level of Zn; but for the 481.05 nm Z n ( I ) l i n e which has its excited level lying above the Fermi level a value of A~ = 5 X 106 cm/s. Presumably, the possibility that the latter level can undergo the resonance ionization de-excitation process in addition to the Auger de-excitation process accounts for the increase in the effective A~ value. This observation supports our findings in that the total probability for the nonradiative de-excitation of a sputtered excited atom or
R.B. Wright, D.M. Gruen ~1on bombarded beryllium ion is the p r o d u c t o f the probabilities [i.e. the sum o f the (A/a)i values] for all o f the allowed de-excitation channels for a given excited state.
References [1] C.B. Kerkdijk, J. Kistemaker and F.W. Saris, in Physics of Ionized Gases, 1976, ed. B. Navinsek. (J. Stefan Institute, Univ. of Ljubljana, 1976) p. 357. [2] C.W. White, E.W. Thomas, W.F. van der Weg and N.H. Tolk, in Inelastic ion-surface collisions, eds. N.H. Tolk, J.C. Tully, W. Heiland and C.W. White (Academic Press, New York, 1977) p. 201. [3] R.B. Wright and D.M. Gruen, J. Chem. Phys. (to be published, 1979). [4] W.L. Weise, M.W. Smith and B.M. Glennon, Atomic transition Probabilities, Vol. I (Nat. Bureau of Standards, Washington, D.C., 1966). [5] D.L. Smith, J. Nucl. Mater. 75 (1978) 20. [6] P. Sigmund, Phys. Rev. 184 (1969) 383. [7] P. Sigmund, Rev. Roum. Phys. 17 (1972) 767, 897, 1005. [8] K. Wittmazck, in ref. 2, p. 153. [9] G. Blaise, Surface Sci. 60 (1976) 65.
583
[10] J.T. Hurd and R.O. Adams, J. Vac. Sci. Technol. 6 (1968) 229. [11] R.B. Wright, Ming-Biann Liu and D.M. Gruen, J. Nucl. Mater. 76, 77 (1978) 205. [12] W.F. van der Weg and P.K. Rol, Nucl. Instr. and Meth. 38 (1965) 274. [13] H.D. Hagstrum, Phys. Rev. 96 (1954) 336. [14] H.D. Hagstrum, in ref. 2, p. 1. [15] H.D. Hagstrum and G.E. Becker, Phys. Rev. B. 8 (1973) 107. [16] M.W. Thompson, Phil. Mag. 18 (1968) 377. [ 17 ] P. Hucks, Thesis, Cologne (1978). [18] P. Hucks, G. StScklin, E. Vietzke and K. Vogelbruch, J. Nucl. Mater. 76, 77 (1978) 136. [19] W. Heiland, J. Krauss, S. Leung and N.H. Tolk, Surface Sci. 67 (1977) 437. [20] H.F. Winters and P. Sigmund, J. Appl. Phys. 45 (1974) 4760. [21] R.S. Bhattacharya, D. Hasselkamp and K.H. Schartner, J. Phys. D.: Appl. Phys. 12 (1979) L55. [22] W.F. van der Weg and D.J. Bierman, Physica 44 (1969) 206. [23] C.W. White and N.H. Tolk, Phys. Rev. Lett. 26 (1971) 486. [24] R. Hippler, W. Kr~iger, A. Scharmann and K.H. Schartner, Nucl. Instr. and Meth. 132 (1976) 439.
XIII. ELECTRON AND PHOTON EMISSION
577
SECONDARY PHOTON EMISSION STUDIES OF ION BOMBARDED BERYLLIUM *
R.B. WRIGHT and D.M. GRUEN Chemistry Division, Argonne National Laboratory, Argonne, 1L 60439, U.S.A.
The secondary photon emission which arises during ion bombardment of Be metal foil by 200-3000 eV Kr÷, Ar÷, Ne+, OF and N~ was studied. By measuring the secondary photon emission yield of four Be(1) and one Be(II) emission lines as a function of incident ion energy information pertinent to the excitation process was obtained. Using a model based on a velocity dependent excitation process, the random collision cascade theory of sputtering; and making allowance for non-radiative de-excitation of the excited sputtered atoms and/or ions, it was possible to account for the observed energy dependent yields. The results of this model indicate that the secondary photon emission yield, yex of a given emission line, i, can be expressed as: Y~/x(om) ~ J(gA/cm 2) • S(no./ion)-. exp[-(A/a)i/Om] , where J is the incident ion current density, S the sputtering yield; Omis the velocity corresponding to the maximum energy transferred between the incident ion of energy El, mass M1 and the target atom of mass M2, i.e. Om = (8M1EI/(MI + M2)2) 1/2 ;(A/a) i is the non-radiative de-excitation parameter for state i. Values for the (A/a) i parameters were found to be ~(1-3) x 107 cm/s for the Be(I) and Be(II) states; a decrease to -~(5-7) × 106 cm/s upon O2÷ bombardment was observed for the Be(1) states, the value for the Be(lI) state did not change.
1. Introduction During energetic particle irradiation o f solid targets the target surface is eroded as evidenced b y the emission o f the constituents o f the target surface and near surface region. This phenomena is generally collectively referred to as sputtering, although it may in turn be described as comprising physical sputtering and chemical sputtering or chemical erosion. We will be concerned exclusively with the emitted particles arising from the physcical sputtering process. Among those particles which are emitted some are electronically excited atoms, ions, molecules and molecular ions; the latter two groups o f particles generally occur to a lesser degree than the first two. As the excited states have a finite lifetime after leaving the target surface they may radiate their excitation energy giving rise to secondary photon emission, the subject o f this paper. Despite extensive work in this field [1,2] there does not yet exist a detailed understanding of the origin o f the electronic excitation o f the sputtered particles [3].
* Work performed under the auspices of the U.S. Department of Energy.
In order to arrive at a better understanding o f the excitation and de-excitation mechanisms at surfaces undergoing ion b o m b a r d m e n t the dependence o f the secondary photon emission yield o f Be metal bombarded by various ions has been measured as a function o f the incident ion energy.
2. Experimental procedure In fig. 1 is shown a diagram o f the experimental system used in this study. A mass analyzed, 5 - 1 5 keV Ar ÷ beam was produced by an ion accelerator with a duoplasmatron ion source. The 200 e V - 3 keV Kr ÷, Ar ÷, Ne ÷, He ÷, O~, N~, D~ and ~ beams were produced by a variable energy sputter-ion gun. The operation o f this gun required that the target chamber be backfilled to 5 × 10 -s Torr (N:-equivalent) with the desired gas (all gases were research grade with > 99.999% purity). In our case this was not a static backfill, instead we chose to partially throttle the main gate valve leading to the pumping system and then increase the target chamber pressure with the desired gas until a suitable stable pressure was achieved. The ion beam produced b y the ion gun was not mass analyzed. When using 02, N2, D: and H2 gas XIII. ELECTRON AND PHOTON EMISSION
578
R.B. Wright, D.M. Gruen /Ion bombarded beryllium
Rotary motion UHV feedthrough -
OO 1 ~ OO , ~
+90 volt bias battery
UHV high voltage insulated flange
,--0-3 keY sputter ion gun control unit
Target holder
6 mm dia beam aperture
Current integrator and picoammeter
i 1Oo O0 0 O0 o
/ Mass analyzed ion beam from duo-plasmatron ion source
/i
valve J u.v ,,,e----r-
UHV gas leak valve connected to gas manifold
tt
UHV LiF window
(5-18 keV)
=,
~
Supracil-1 quartz lens
~
I II--I o
McPherson Model 218 ~
/
single monochromator
[ 2400groove/ram1 | 3o00 A blaze
~ IL;~Ogroovelmm°r'
/
I
I ~ . ~ L ~ * ~ _ ~ , ~, ~. . . / '~-
/~/ /~
Preamplifier (X10)
I ~
~X,'~l~'X~
I//~ ;
~ I
PM tube cooled
Photomultiplier tube
0
I--]
V-----1 0 I o OlO
0 0
(1) (2)
0 0
(3)
0 0
(4)
S~Or~cil-]
quartz lens
(l) High voltage power supply for PM tube (2) Amplifier/ discriminator (3) Photon counter (4) D/A converter (5) Strip chart recorder
\ 5000A blaze \ grating
(5)
Fig.1.Diagram of the experimental apparatus used in this study. (Refer to text for a detailed description.)
the dominant ion was M~ ,~96%, with -~,4% being the M ~ ion. We have not corrected our data for this difference. All currents were measured with a +90 V bias applied to the target. When we discuss data pertaining to when O~, N~, D~ or ~ was the incident ion it will be treated as due to )14" ions o f twice the measured ion current and half the energy of the a c t u a l ~ species. The ion beam was incident normal to the plane of the target for all the measurements herein presented. The target was a piece o f polycrystalline Be metal foil, 0.25 mm thick of 99.88% purity supplied by Kawecki Berylco Industries, Inc. Prior to insertion into the vacuum chamber the unpolished Be foil was solvent degreased and etched slightly in acid. After insertion, the chamber was evacuated, baked, and the target was extensively sputtered cleaned with a rastered (1 cm X 1 cm) 3 keV, 20/aA beam o f Kr ÷.
3. Experimental results and discussion In figs. 2, 3 and 4 are shown the measured secondary photon photon emission yield for the 234.86 nm Be (I) the 332.11 nm Be (I), and the 313.07 nm Be (II) emission lines [4] respectively as a function of incident ion energy for the various incident ions used in this study. The data shown in each figure are relative values as indicated; the abscissa scales as given in each figure are also the relative, as measured, values. However, since the sensitivity o f the spectroscopic detection system was not determined one should not interpret these ordinate values as given in figs. 2, 3 and 4 as being relative to one another. Obviously from figs. 2, 3 and 4 the secondary photon emission is not a linear function of the incident ion energy, nor is it proportional to the physical sputtering yield which is a function of the incident ion energy, mass
579
R.B. Wright, D.M. Gruen ~Ion bombarded beryllium I0,000
IOOO BERYLLIUM 5
BERYLLIUM
o
Be(I)- 3521 I,~
Be(T)- 23486 A
IOOO I00
E
5
5
E
2 I00
2
ct:
~.
5
3 t.u
2
~-
I0
.d iII I 0
•
• Ar + • Ne +
•
5-
21
5
•
Kr +
• Ar+ (125~.amp/cm21
Kr +
(125Fomp/cm2}
He+
o O"
[] ND ÷+ (245FomP/crn2) o H*
:f"
021.
10 20 30 INCIDENT10N ENERGY(xl03eV)
0
•
Ne +
•
He +
O 0÷ a N+
z~ D+ (245/~amp/cm2)o H+ ,
I 1.0
I
I
2.0
I 50
INCIDENT ION ENERGY(xlO3eV)
Fig. 2. Relative secondary photon emission yield values for the 234.86 nm Be(I) emission line as a function of the incident ion energy for the various incident ions used in this study.
Fig. 3. Relative secondary photon emission yield values for the 332.11 nm Be(l) emission line as a function of the incident ion energy for the various incident ions used in this study.
and atomic number. The theoretical energy dependence of the sputtering yields o f Be for the various incident ions are shown in fig. 5 as calculated using the semi-empirical theory o f Smith [5]. The formula proposed by Smith is given in the figure as well as the values o f the variables used in the calculations. This model as well as that o f Sigmund [6,7] would predict that the relative sputtering yield is greater the higher the mass of the incident ion. The incident ion energy dependence o f the secondary photon emission yields for the Be emission lines shown in figs. 2, 3 and 4 as well as that of the two other Be (I) lines studied display considerable variation with the predicted sputtering yield dependence. There is even a considerable variation in the incident ion dependence amongst the various Be (I) and Be (II) lines studied. However, with O~ incident on the target the greatest secondary pho-
ton emission yield was observed for all of the Be emission lines studied. The oxygen enhancement of secondary photon emission yields [1,2] as well as its enhancement of secondary ion yields [8,9] is well known but the detailed understanding o f its physical (or chemical) origin is still lacking [3]. The observed enhancement due to N~ is of interest as N2 does not appear to chemisorb on Be metal [10]. A beryllium nitride may possibly be formed as a result o f ion implantation, perhaps leading to the observed enhancement. The present data do not allow one to attribute any special significance to the apparent enhancement due to He +, D~ or H~ bombardment as with these incident ions the Be surface may have a higher surface oxygen coverage than with the other incident ions [111. It is presently believed that an excited sputtered XIlI. ELECTRON AND PHOTON EMISSION
R.B. Wright, D.M. Gruen lion bombarded beryllium
580
,02[ se000Be0
. . . . . . .
,
BERYLLIUM Be(~)-31307A
I00 F:
5
~2
I
w
5
_V 2 ,,-4, i.o
• Kr*
n,."
3
4
5
6
? 8 9 Ej(×lO3eV)
]0
II
12
13
f4
15
Fig. 5. Calculated sputtering yields of Be metal as a function of the incident ion energy for the various incident ions used in this study. The semi-empkical model of Smith [51 was used. His expression for the sputtering yield is shown in the figure as well as the values of the parameters used to calculate the curves.
I0 m ),-
2
• Ar* 12¢
• Ne+
5
oFamp/cm2
• He +
2~ Ol
°0Di(N245/~amp/cm2)t3 ÷ A o
001
0
H÷
I0 20 3.0 INCIDEN]" ION ENER6Y(xlO3eV)
Fig. 4. Relative secondary photon emission yield values for
the 313.07 nm Be(II) emission line as a function of the incident ion energy for the various incident ions used in this study.
neutral atom may undergo non-radiative de-excitation processes with the solid surface [ 3 , 1 2 - 1 4 ] . Such processes as resonant ionization and the two-electron Auger de-excitation process are in principle permitted. For a sputtered excited ion the resonant neutralization, Auger neutralization and the Auger de-excitation processes are permitted. These processes have been described in detail by Hagstrum [13,14] and by Hagstrum and Becker [15]. The probability that an excited atom or ion in state i will not undergo one o f the non-radiative de-excitation processes has been expressed in the form [ 3 , 1 2 - 1 4 ] N t~iur(o,O)=exp(_~(A/a)l/ocosO} . (1) \ j z
The total survival probability, ~ u r , for the N possible processes for state i, involves A, the transition rate relevant to the de-excitation process, and a, which is related to the effective distance over which the de-excitation is likely to occur. In eq. (1), u is the velocity of the escaping particle, 0 is the emission angle defined with respect to the surface normal. Note that v cos 0 is the escaping particle's perpendicular velocity and that the longer the particle spends close to the target surface, the greater the probability for it to undergo non-radiative de-excitation. Eq. (1) indicates that even relatively small changes in the value of (A/a) can make considerable differences in the survival probability for an atom moving with a velocity u and with a given emission angle 0. Wright and Gruen [3] have assumed that the excited sputtered particle in state i originates from those atoms sputtered by a collision cascade process, and that these particles have an excitation probability, as yet unspecified, which may be dependent on the sputtered neutral particle velocity and angle of emission, PeX(v, 0). The excited particles would then have a flux density velocity distribution [number. (unit area)-l . (unit time) -1 • (unit velocity) -I ] given by [31
~ x (o, 0) sin 0 do dO d4~ = ~°(o, O)l~iX(v,O) sin 0 do dO d4~,
(2)
R.B. Wright, D.M. Gruen ~Ion bombarded beryllium where ~b is the azimuthal angle of emission. Using the model of Thompson [16] and Sigmund [6,7] for the flux density energy distribution, 'I)°(E,®), of the sputtered neutral atoms resulting from a collision cascade process [since ~(v, O) = ¢b(E, O)(dE/dv)= v4~(E, 0)] then [3]
'l,°(v, O) C(n) =
u3
cos 0
(3)
(0 5 + v~). + ~ ,
where 1 ~< n ~< 2 (generally n is assumed to be n = 2, but recent measurements [17,18] of neutral sputtered atom flux density energy distributions have indicated 1.5 ~
///m 1 ~Tr
0
0
v3 cos 0 sin 0 do dO d¢/(v 2 + v~)n+ 1
0
(4) J is the incident ion beam current density in/~A/cm 2 ; S is the neutral atom sputtering yield (number sputtered per incident ion) for the collision cascade process; Vm is the velocity corresponding to the maximum energy which can be transferred to a target atom of mass M2 by an incident ion of mass Mt and energy Ea as given by: 1
Om F 4MIM2
EI(2/M2)I~
-- L ( ~ ~-~)~ = [7.72
1
X
1012M1E1/(M1 +M2)2]~ (cm/s) ,
(5)
with M~ and 3'/2 given in g/mol, and El in eV. vb is the velocity of a target atom which corresponds to a surface binding energy, Eb, usually approximated by the target sublimation energy, i.e. 1
Vb = [1.92 X lOl2Eb/M2]~ (cm/s),
(6)
with E b in eV. The yield of excited sputtered atoms can be then expressed using eqs. (1)-(6) as Y~iX(v,0) sin 0 do dO de = ~I'°(v, 0) p~iX(v, 0) ~Ur(v, 0) sin 0 dv dO dq~. (7) Wright and Gruen [3] have also shown that if: W/X(v, 0) = constant; WiX(v, 0) ~ v m cos m 0 ; or Wix(v, 0)
581
o: exp(_Bi/o cos 0), then the total secondary photon emission yield [determined by integrating eq. (7) over do dO dO], when A ~ vb and Vm >>Vb is approximately given by:
Y~ix ~,IS exp(-A~/vm) .
(8)
In eq. (8) if/ffix(v, 0) = exp(-Bi/v cos 0) then A~ = ~ ( A / a ) / + Bi; if/~ix(v, 0) is a constant or a power of the velocity thenA} = ZN(A/a)/. Therefore, eq. (8) can be used to determine the value of A~ since Y~ix is experimentally determined as a function of the incident ion energy, and S and Vm are also functions of the incident ion energy. In table 1 are shown the values of A~ calculated using the experimental secondary photon emission yield values for the four Be(I) lines and the one Be(II) line by the relation given in eq. (8). The correlation coefficients for all of the A~ values were >0.98. The range of the incident ion energy used in the calculations was 400 eV to 3 keV. We also used the model of Smith [5] to calculate the energy dependence of the collision cascade sputtering yield (refer to fig. 5) in eq. (8). Obviously the model used to calculate the energy dependence of the sputtering yield will effect the values of A[ given in table 1. As there have not been extensive measurements of the incident ion energy dependence of the sputtering yield for Be metal for the various incident ions used in this study we simply do not know if this model accurately reflects the true values. However, Smith's model reproduces the experimental data on other targets as a function of the incident ion energy for various incident ions quite well. The values for A~ listed in table 1 for the inert gas incident ions generally decrease in the order He÷> Ne÷> At÷> Kr ÷ for the five Be emission lines studied. The A} values for the Be(II) emission line are definitely greater than those for the Be(I) emission lines. This is not unexpected because an excited beryllium ion has more channels available for nonradiative de-excitation than does an excited beryllium atom. The values of A~ for the four Be(I) emission ÷ lines investigated when 02 was used as the incident ion are especially interesting in that they are approximately half the values of A~ as determined with the other incident ions. However, the A} value for the Be(II) emission line when O~ is used changes very little as compared to the other incident ions. It may XIII. ELECTRON AND PHOTON EMISSION
582
R.B. Wright, D.M. Gruen /Ion bombarded beryllium
Table 1 Values of A[ as experimentally determined using the secondary photon emission yield values as a function of the incident ion and the incident ion energy a for the various Be(I) and Be(II) energy levels studied in this work (A I in cm/s). Incident ion He+ Ne+ Ar+ Kr + H+2 D2 N~ O~ b Ar+c
Be(1) 234.86 nm 1.97 X 107 1.88 X 1 0 7 1.75 X 107 1.34 X 107 Average: (1.74 X 107) 1.34 X 107 1.91 X 107 1.42 × 107 6.36 X 106 4.12 × 107
Be(I) 249.47 nm
Be(1) 265.07 nm
Be(1) 332.11 nm
2.32 X 107
3.86 X 107
--
--
1.48 × 107 2.04 X 107 1.44 X 107 8.40 X 106 (1.45 X 107) 1.31 X 107 1.67 X 107 2.12 X 107 7.42 X 106 4.19 X 107
2.51 (2.42 1.74 2.33 3.40 1.50 -
X 107 X 107) X 107 X 107 X 107 × 107
2.30 (3.08 2.10 3.01 2.64 1.48 -
X 107 × 107) X 107 X 107 X 107 X 107
Be(lI) 313.07 nm 3.76 3.40 2.46 2.43 (3.01 2.49 3.40 3.18 2.64 8.64
X 107 X 10 7
X 107 X 107 X 107) X 107 X 107 X 107 × 107 X 107
a Energy range used to determine theA~ values was 400 eV-3 keV except for case (c). b The sputtering yield for BeO as a function of incident ion energy was used to calculate A~. c The duoplasmatron ion source was used to provide the mass analysed Ar+ beam; energy range was 5-15 keV.
be that upon oxidation one o f the non-radiative de-excitation processes for the Be(I) levels is prevented, or at least its probability diminished, whereas those de-excitation processes allowed for the Be(II) level do not change. The present method o f analysis cannot exclude the possibility that the initial excitation probability is given by W/x(v, O ) ~ e x p ( - B i / o cos O) as previously discussed. In this case it would be impossible at the present stage o f understanding to separate the excitation mechanism from the de-excitation mechanism or mechanisms. Previous workers have found a similar functional dependence of the secondary photon emission yields on the incident ion energy as given by eq. (8). Heiland et al. [19] employed a different interpretation o f the origin o f the excited sputtered particles choosing to neglect collision cascade sputtering, and only consider the "direct knock-off" and the "reflected-ion" contributions [20]. They did not report any A~ values as such, although they did interpret the exponential fit as arising due to the occurrence of a survival probability in the secondary photon emission yield expression. The fact still remained that the secondary photon emission yield, normalized to certain components o f the total sputtering yield depends exponentially on the velocity o f the excited atom (or ion). Similarly, Bhattacharya et al. [21]
used the relation given in eq. (8) with S being the "direct knock-off" [20] contribution only. They obtained values o f 1.7 X 107 to 2.6 X 107 cm/s forA~ by studying various excited states o f sputtered Si(I), Si(II) and Si(III) produced using 5 0 - 5 0 0 keV Ar ÷ incident ions. Van der Weg and Bierman [22], and White and Totk [23] have reported values o f approximately 2 X 1 0 6 cm/s for A~ for sputtered Cu(I) (324.75 nm emission line). These authors assumed that the binary collision between the impinging ion and atoms in the first layer o f the target as being the liberating and exciting event for the emission o f the detected excited particle. A similar approach was taken by Hippler et al. [24] who used line shape measurements of the Doppler shifted secondary photon emission lines to determine A~ values for selected excited states of A1, Cu and Zn. They observed a value ofA~ = 5 X l 0 s cm/s for the 307.95 nm line of Zn(I) which has the excited level below the Fermi level of Zn; but for the 481.05 nm Z n ( I ) l i n e which has its excited level lying above the Fermi level a value of A~ = 5 X 106 cm/s. Presumably, the possibility that the latter level can undergo the resonance ionization de-excitation process in addition to the Auger de-excitation process accounts for the increase in the effective A~ value. This observation supports our findings in that the total probability for the nonradiative de-excitation of a sputtered excited atom or
R.B. Wright, D.M. Gruen ~1on bombarded beryllium ion is the p r o d u c t o f the probabilities [i.e. the sum o f the (A/a)i values] for all o f the allowed de-excitation channels for a given excited state.
References [1] C.B. Kerkdijk, J. Kistemaker and F.W. Saris, in Physics of Ionized Gases, 1976, ed. B. Navinsek. (J. Stefan Institute, Univ. of Ljubljana, 1976) p. 357. [2] C.W. White, E.W. Thomas, W.F. van der Weg and N.H. Tolk, in Inelastic ion-surface collisions, eds. N.H. Tolk, J.C. Tully, W. Heiland and C.W. White (Academic Press, New York, 1977) p. 201. [3] R.B. Wright and D.M. Gruen, J. Chem. Phys. (to be published, 1979). [4] W.L. Weise, M.W. Smith and B.M. Glennon, Atomic transition Probabilities, Vol. I (Nat. Bureau of Standards, Washington, D.C., 1966). [5] D.L. Smith, J. Nucl. Mater. 75 (1978) 20. [6] P. Sigmund, Phys. Rev. 184 (1969) 383. [7] P. Sigmund, Rev. Roum. Phys. 17 (1972) 767, 897, 1005. [8] K. Wittmazck, in ref. 2, p. 153. [9] G. Blaise, Surface Sci. 60 (1976) 65.
583
[10] J.T. Hurd and R.O. Adams, J. Vac. Sci. Technol. 6 (1968) 229. [11] R.B. Wright, Ming-Biann Liu and D.M. Gruen, J. Nucl. Mater. 76, 77 (1978) 205. [12] W.F. van der Weg and P.K. Rol, Nucl. Instr. and Meth. 38 (1965) 274. [13] H.D. Hagstrum, Phys. Rev. 96 (1954) 336. [14] H.D. Hagstrum, in ref. 2, p. 1. [15] H.D. Hagstrum and G.E. Becker, Phys. Rev. B. 8 (1973) 107. [16] M.W. Thompson, Phil. Mag. 18 (1968) 377. [ 17 ] P. Hucks, Thesis, Cologne (1978). [18] P. Hucks, G. StScklin, E. Vietzke and K. Vogelbruch, J. Nucl. Mater. 76, 77 (1978) 136. [19] W. Heiland, J. Krauss, S. Leung and N.H. Tolk, Surface Sci. 67 (1977) 437. [20] H.F. Winters and P. Sigmund, J. Appl. Phys. 45 (1974) 4760. [21] R.S. Bhattacharya, D. Hasselkamp and K.H. Schartner, J. Phys. D.: Appl. Phys. 12 (1979) L55. [22] W.F. van der Weg and D.J. Bierman, Physica 44 (1969) 206. [23] C.W. White and N.H. Tolk, Phys. Rev. Lett. 26 (1971) 486. [24] R. Hippler, W. Kr~iger, A. Scharmann and K.H. Schartner, Nucl. Instr. and Meth. 132 (1976) 439.
XIII. ELECTRON AND PHOTON EMISSION