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Sputter yields and erosion rates for low energy ion bombardment of multielemental powders
Nuclear Instruments and Methods in Physics Research B 83 (1993) 339-344 North-Holland
Beam Interactions with Materials 8 Atoms
Sputter yields and erosion rates for low energy ion bombardment of multielemental powders J. Goschnick, J. Schuricht, A. Schweiker and H.J. Ache Kernforschungszentrum
Karlsruhe, Institut fiir Radiochemie,
Postfach 3640, 76050 Karlsruhe,
Germany
Received 3 April 1992 and in revised form 28 June 1993
For depth-resolved analysis of particulate material such as pigments, ceramic powders or environmental microparticles with secondary neutral mass spectrometry (SNMS), erosion rates and sputter yields of 16 powders, including salts, metal and nonmetal oxides, have been measured to provide data for the conversion of sputter time into depth values. For that purpose a new gravimetric method was developed, which allows the measurement of depth propagation within particles even for bad electrical conductors. With about 1 mA/cm’ of 400 eV argon ions for the 16 powders an average sputter yield of Y = 1.2 and a mean erosion rate of i = 0.4 rim/s were measured. In spite of the chemical variety of the powders a standard deviation of only 40% was observed, which gives the relative error for a depth calibration in case of an unknown analyte. Comparison between compact and particulate samples showed the powder erosion data to be valid within a factor of 2 even for compact samples with flat surfaces. No
correlation between the heat of atomization and the sputter yield could be recognized.
1. Introduction Plasma-based secondary neutral mass spectrometry (SNMS) was found to be appropriate to measure the elemental composition of particulate material containing oxides, salts or organic compounds [1,2]. In order to use the high performance of this method [3,4] to determine quantitative depth profiles, erosion rates are needed to convert sputter times into depth values. Neither for the special kind of material nor for the applied low primary energies of a few hundred eV sputter yields were -known. Moreover, typical literature data are given for normal incidence, while in the case of powder analysis the multitude of local surface orientations results in a wide distribution of incidence angles. Therefore a special weight loss method was developed to measure erosion rates for powders and applied for a variety of representative compounds. This technique turned out to be of particular advantage for measuring electrically nonconducting substances.
2. Experimental Measurements were performed with the INAinstrument manufactured by the Leybold company (ColCorrespondence to: J. Goschnick, Karlsruhe, Institut fiir Radiochemie, Karlsruhe, Germany. 0168-583X/93/$06.00
Kernforschungszentrum Postfach 3640, 76050
0 1993 - Elsevier
Science
Publishers
ogne, Germany). Principles, design and operating conditions have been described in detail elsewhere [5,6]. Briefly, a metal foil supporting impressed powder grains is mounted behind an orifice in the wall of the chamber containing the rf-excited argon plasma. A negative voltage applied to the metal carrier accelerates argon ions out of the plasma towards the sample. With respect to the carrier foil normal incidence is obtained. On arrival at the carrier surface the energy of the bombarding ions is given by the potential difference between plasma and sample. A geometrically given fraction of the sputtered neutrals enters the plasma, becomes partially ionized and leaves the plasma towards the analysis section. There, an energy filter discriminates thermal particles of the plasma to optimize the subsequent mass analysis by a quadrupole mass spectrometer. The argon pressure of the plasma was set to p = 2 x 10m3 mbar. Plasma conditions were used providing electrons and argon ions with a density of about 3 X lOlo particles/cm3 and an average unidimensional energy E, = 4 eV for the electrons. For the 3.6 mm distance between the grounded aperture exposed to the plasma and the sample a target potential of -360 eV was found to be appropriate to achieve a primary ion current density with lateral uniformity across the sample. The sputtered sample area was 0.38 cm2. Using a profilometer the flatness of the crater bottoms of copper samples was checked for adjustment. A depth resolution of less than 10 nm was measured at the sharp interface of a 107 nm thick B.V. All rights reserved
340
J. Goschnick et al. / Sputter yields of multielemental powders
silicon dioxide layer on Si(100) wafer samples confirming the high lateral uniformity of the bombardment. As the potential inside the plasma was measured to be ca. 40 V, the applied target potential supplied argon ions of an energy of 400 eV. The primary ion current density incident to the samples was determined to be 0.6-1.0 mA/cm* (according to No = 3.8-6.3 x 1015 projectiles/(scm2)) from the weight loss of polycrystalline copper samples for which a sputter yield of 1.8 was taken from the literature [7]. A Faraday cup arrangement would disturb the critical ion optical conditions of the ambient geometry of the sample and was therefore not applied for the primary current measurement. Several salts, some oxides and copper granulate were chosen to be investigated. Salts and oxides are typical components of environmental material, for which a depth-resolved analysis should be developed. Reagent grade powders (particle sizes < 100 pm) were pressed into indium foil covering less than half of the plasma exposed area (7 mm in diameter). Keeping the coverage lower than this limit turned out to be sufficient to avoid interference of the measurements by charging. However, with the same salts in compact form, such as pressed tablets, nearly no sputtering was possible due to severe charging of the electrically insulating material under ion bombardment. From each powder at least five samples with different coverages were prepared. A microbalance with 10 pg resolution was used to measure the mass loss of the samples before and after sputtering. The initial mass was measured after the sample had been in the vacuum for several hours and sputtered for some seconds. This procedure prevented errors in the determination of the sputter mass loss due to pumped off water, mechanical falloff of particles during sample transfer and loss of bound particles being charged up and electrically forced to leave the carrier when the target potential is switched on. The samples were exposed to the bombardment some 1000 s to give mass losses in the range of 1 mg. During sputtering the atomic SNMS-signals of the powder constituents and the supporting indium foil were recorded. The temperature of the samples was kept below 0°C.
rial, the mean mass of sputtered atoms @ (g/mol) and the fraction of the total primary flux hia (projectiles/s) the powder particles or the uncovered area of the indium foil are exposed to. Let 0 be the fraction of the primary flux incident to the particles, the total mass loss rate is given as ti = Y&&(1
- 0) + Y&M,.
(2)
Index P denotes the powder and index C stands for the carrier, respectively. Obviously the prefactors of 0 and (1 - 0) are the mass loss rates lit: of the powder only and ri$, the empty carrier foil, respectively. Thus, the total mass loss is a linear function of 0: ti = ti”p + (tie, - $)o.
(3)
0 is necessarily coverage dependent. The more particles are deposited on the carrier surface, the higher is the part of the projectile flux impinging onto particle surfaces. Indeed, as long as the primary current density is laterally constant, 0 is identical with the relative coverage of the powder on the carrier surface. But if the particles charge up, some of the positive projectiles originally travelling towards a particle, may be diverted by the positive charge of the powder grains. Consequently the fraction of projectiles impinging onto particles becomes lower than the relative coverage. However, even in this case eq. (3) stays valid. Only for severe charging the change of the sputtering yield due to lowered primary energy and an alteration of the distribution of incidence angles of the projectiles have to be considered. The linear relationship of eq. (3) forms the basis of a simple way to determine the mass loss rate tit of the powder, replacing 0 by the corresponding atomic SNMS-signal of the carrier. This can be done because the atomic SNMS-signal of the carrier foil (here indium) is proportional to (1 - O), the fraction of the primary flux bombarding the metal foil. Using the basic equation for the intensity of atomic SNMS-signals [1,3], Z(C) = Y&D(C),
(4)
where Z(C) is the intensity of the atomic signal D(C) the detection factor for the carrier element, (3) can be rewritten as
and eq.
3. Mass loss method for sputter erosion measurements of supported powders The total mass loss rate ti of a sample is given by the sum of the individual mass loss rates of the powder particles ti, and the carrier foil tic: ti =ti,+tic.
(1)
The individual mass loss rates are determined by the sputter yield Y (in atoms/projectile) of the mate-
with Z,,(C) denoting the atomic carrier signal for the empty foil. Accordingly, the evaluation of data from samples with a variety of particle coverages should yield a straight line when the measured total mass loss rates are plotted versus the corresponding SNMS-signals of the carrier. Extrapolation of this line to Z(C) = 0, according to a totally covered carrier, should give an intercept with the &axis identical to the mass loss rate
J. Goschnick et al. / Sputter yields of multielemental
ti”p of the powder itself. The method uses the carrier signal only as indication for complete particle coverage. Neither the sputter yield of the empty carrier nor the absolute coverage of the samples are relevant for the mass loss rate determination of the powder. The sputter yields can be obtained from these mass loss rates and the primary flux No according to
where NA denotes Avogadro’s number. The powder sputter yield should be different to the one obtained for a flat surface under normal incidence bombardment of the same material. This is because the powder particle is necessarily sputtered under a variety of incidence angles and compared to a flat sample with the size of the particle’s basal plane, it has a higher surface area exposed to the projectile flux. Defined as depth propagation i perpendicular to the bombarded surface the erosion rate is calculated from the mass loss rate, the density p and the surface area of the powder. Assuming the particles to behave like half-spheres, the powder surface has 2 times the area the particles are covering. At complete coverage of the carrier, for which ti! is valid, the particles occupy the total bombarded area A,. Accordingly, the erosion rate can be calculated by the equation
(7) This is only an average depth propagation into the particle bulk. Due to the incidence angle dependence of the sputter yield, the erosion cannot be strictly uniform across the particle. The more the bombardment direction deviates from normal incidence the higher the sputter yield will be until at an incidence angle (Y of about 70” the maximum yield is reached. For even more oblique incidence a steep decrease of the yield occurs [8]. However, the total angle dependence of the erosion rate is given by the product of bombarding current density and yield. While the first is proportional to cos(cu), according to Zalm [8] for incidence angles lower than for maximum erosion, the yield depends on incidence angles approximately as Y= Y&oP((Y),
(8)
whereby Y,, denotes the value for normal incidence ((u = P>. Accordingly, the depth propagation should show only a weak dependence proportional to l/ cos”.7(~) resulting in a nearly surface-parallel erosion for (Y< 70”. Because only a minor part of the total surface is exposed to glancing bombardment with considerably lower erosion, eq. (7) should be a good approximation to the depth propagation at least in these areas of the particle surface which dominate the mea-
341
powders
sured SNMS-intensity. As the acceptance of the spectrometer is directed normal to the plane of the carrier foil, the erosion flux of accordingly oriented surface areas is preferably detected. These are the areas of the grains which are bombarded under near-normal incidence. Additionally, the incidence angle dependence and the higher area of a rough surface should result in a sputter yield for the powder being moderately higher than a flat sample of the same material. Integration of eq. (8) over a half-sphere up to 70” and neglecting the small fraction of atoms coming off at higher angles results in about 1.8 times more sputtered atoms compared to a flat sample with a surface area equal to the basal plane of the spherical particle. Thus, the sputter yield of the powder should be slightly higher than the sputter yield for a flat sample at normal incidence. However, as can be seen from eqs. (6) and (7), the depth propagation rate for the powder should be about the same as for an even surface.
4. Results and discussions Fig. 1 shows for example the relevant SNMS-signals during sputtering of K,CrO,. Indium foil was used as carrier material in all cases. Between 50 and 200 s were usually enough to obtain stationary signals from the powder particles, indicating a sputter equilibrium with constant surface concentrations and a stable topography. As for all other powders a number (at least 5, in this case 10) of samples with different K,CrO, coverages were sputtered. Fig. 2 presents the mass loss data plotted against Z(In), the SNMS intensity of the indium foil. According to eq. (7) a linear relationship is obtained between the total mass loss rates and Z(In). Therefore for all powders a linear fit was used to determine
the
intercept
at Z(In) = 0. The
mass
loss
lE6 i ____________________--_________^_____________ In(115) .,_-‘iii
a lE5
s
: K (39)
;,._._ __- -.-.--.-.-.-
.s E 2 .c
Cr (9) 1 E4 i- _.....-..-.- ....__.._..__.__._ _.__.._.._..__..__.._ -
lE3’
0
’ 500
’
loo0
’
1500
0 (16)
’ 2ooo
2500
3000
sputter time [s] Fig. 1. Time profile of the atomic SNMS-signals during sputtering of K,CrO, on indium foil with 0.9 mA/cm’ of 400 eV argon ions.
342
J. Goschnick et al. / Sputter yields of multielemental powders
,I-_ al
E 8
....-_,-
,
_,_...~~l~-
_o
:: 9
0.5
t
t
0’0
’
*
0.5
s
s
1
.
’ 1.5
’
’
2
*
’
2.5
intensity of In [I 06 cpsl Fig. 2. Total mass loss rates of 10 samples of K,CrO, on indium foil versus the SNMS-signal intensity of the indium carrier. Bombardment being done with 0.9 mA/cm* of 400 eV argon ions.
of K,CrO, was found to be tit = 6.1 X lop8 g/s. Additionally two separate K,CrO, samples and an empty indium foil were sputtered until stationary conditions had been established. The measured indium intensities were used to calculate 0, the fraction of the bombarding current incident on the powder. According to eq. (4) and using Z&n) for the intensity of the empty carrier 0 = [Z&n) - Z(In)]/ZO(In). Values of 0 = 0.023 and 0 = 0.080 were obtained. Subsequently micrographs of the two powder samples were taken rate
Table 1 Sputter yields and erosion rates of powdered salts and are given for a primary current density of 1 mA/cm*. atomization are given too. The latter were calculated enthalpies for the solid elements and the dissociation taken from Sigmund ill].
with scanning electron microscopy (SEMI. From these images the coverage of the powder was determined with a computer-aided grain analysis system by numerical integration of the particle covered areas. Coverages of 0.023 and 0.074 were obtained. The close agreement with the above primary current fractions indicates the absence of electrical charging induced by the ion bombardment. From the linear relation between the measured mass loss rates and the corresponding indium intensities the absence of charging can be concluded also for higher coverages. The low coverages in this investigation were necessary to obtain reliable coverage data from the SEM-images of isolated single particles on the carrier foil. The emission of secondary electrons from the carrier metal is assumed to inhibit the chargeup of the particles. Table 1 contains the resulting sputter yields and erosion rates calculated with eqs. (6) and (7) from the measured mass loss rates of the powders Ijli. The given error margins are derived from the standard deviation of the intersect determination by the linear extrapolation. The average relative error is below +30%. However, in some cases the scatter of data caused considerably higher errors. Maybe because of morphological reasons, grains of these powders were not held tight enough by the metal foil, so that some of them eventually fall off after some sputtering. Moreover, indium forms sputter-induced cones typical for smooth metals [9], which indeed had been found by electron microscopy for several samples. Probably be-
oxides for bombardment with argon ions of Ea = 400 eV. The erosion rates The mass dependent factors a, energy transfer factors T, and the heats of from standard enthalpies of formation of the compounds, the sublimation enthalpies for the gaseous elements given in ref. [14]. The values for a are
Compound
Heat of atomization [kcal/mol]
Transfer factor
a-factor
Yield [atoms/ projectile]
Erosion rate
Error margin
h/s1
[%I
NaNO,
86 86 82 112 99 99 96 152 81 149 147 102 128 115 1.59 180
0.84 0.89 0.93 0.85 0.89 0.95 0.98 0.95 0.95 0.89 0.89 0.97 0.98 0.99 0.78 1.0
0.19 0.20 0.26 0.19 0.20 0.21 0.30 0.21 0.31 0.20 0.20 0.21 0.22 0.22 0.45 0.24
1.0 1.0 0.58 1.7 1.8 1.35 1.17 1.93 1.53 1.3 0.54 0.67 0.96 0.95 0.7 1.5
0.38 0.47 0.24 0.60 0.66 0.48 0.48 0.62 0.54 0.50 0.15 0.30 0.28 0.30 0.46 0.54
58 40 36 50 29 20 8 23 12 19 24 14 11 5 50 21
f=O, PMNO,), Na,CO, NazSO, K2SO.4
PbSO, TiO, (rutile) Cu (powder) SiO, A12o3
K,CrO, Cr203 Fe203
PbO cue
J. Goschnick et al. / Sputter
cause of the developing sputter cones, the indium signal of the carrier always passed through a slight m~mum. This topographical change could be assumed to be influenced by the redeposition of sputtered material from the powder [9] resulting in slightly variable traces of the indium SNMS-signal. Additionally, the injury of the metal foil surface by the impressed particles could contribute to some variation of ItIn). Finally, to avoid interference by charging in some cases the coverages had to be kept quite low, which introduced additional errors due to a long range extrapolation. The sputter yields as well as the erosion rates of all 16 powders do not show much variation. The mean values are Y = 1.2 and i = 0.4 rim/s (for 1 mA/s primary current density) with a standard deviation of ca. 40% only. Therefore, the conversion of time into values of depth based simply on the average depth propagation rate seems to be a suitable approximation even for the depth-resolved analysis of material composed of a mixture of oxides and salts. The measured sputter yields for copper Y(G) = 1.5 and quartz powder Y&O,) = 1.3 do not differ significantly from values for flat surfaces Y_(G) = 1.8 [7] and Y_(SiO,) = 0.9. The latter value was obtained from the time interval of 156 s (i, = 1 mA/cm’) necessary to sputter through a 107 nm thick SiO, layer on a silicon wafer. Thus, the expected enhancement for sputtering of powders could only be measured for SiO,. Probably the rough topography of the metal particle surfaces consists of deep valleys with steep sides and causes a certain amount of the sputtered mass to be redeposited, which results in a compensation of the erosion enhancement caused by nonnormal incidence and increased surface area [lo]. On the other hand the accordance of the sputter yields derived from the mass loss of powders with those for flat surfaces confirms the reliability of the method. Thus, the sputter yields determined by the powder method seem to provide a suitable approximation for normal incidence values with an inaccuracy less than a factor of 2. Sigmund’s model [ll] is still being accepted 19,121to give a generally correct description of the sputtering erosion for elastic collision dominated conditions. The influence of the bombarded material on the yield is given by
yields of multielemental
343
powders
Because the atomic SNMS signals dominate the spectra (> 90% of the total intensity) for all analysed samples and their ratios were found to be consistent to corresponding ratios of atomic ionization probabilities [l], most of the eroded material leaves the samples as sputtered atoms. Substantial release of thermal species would give a reduced atomic intensity due to the energy discrimination prior to mass analysis. Accordingly, thermal processes which are not considered in the coliision-type sputter model such as simple evaporation of the bombarded material is excluded to contribute considerably. Therefore in a first attempt relation (9) was used to systematize the yield data. Although structural disruption of the surface and con~ntrational changes due to the bombardment are not considered within the average heat of atomization (AH,), this type of energy is assumed to be at least a rough estimate of the binding forces between the atoms. In accordance to the usual procedure [8,12] and as other more appropriate data are not available, U, is therefore replaced by the average heat of atomization (AH,). The latter was calculated from standard enthalpies of formation for the compounds added by sublimation enthalpies for the solid elements and dissociation enthalpies for the gaseous elements (all taken from ref. 1131). Transfer factors were determined from the average atomic masses of the compounds and values for the factor a were taken from ref. [ll] for low bombarding energy condition. Fig. 3 shows the sputter yields of the measured powders as a function of the UT/AH, ratios. Although the heats of atomization differ by a factor of 2 a simple increasing trend for the yield with lowered average atomic binding forces is not perceptible. No substantial
‘;i;
2.5
I
, .
l
Na2COs
Cu.
C”p K2 SO, 302
l
9 Cr203=
l
PbSO1,
KN03* l NaN03 l
Fe203
PbO
.
l
Al;03
Y-a(M,,M,)T(N,,M,)/U,,
N4S04
TiOz .
K2C*4
(9)
with U, as binding energy of the atoms to the surface, with T being the collision energy transfer factor given by T = 4M,M,/(M, + M2j2 (M, denotes the mass of the projectile ion while MZ represents the mass of the sample atoms), and a being a correction factor which is dependent on the mass ratio of projectile and target atoms as well as on the primary energy range.
Pb;NO&
i 0.005
a+T /AH,
0.01
0.015
[Mol/kcal]
Fig. 3. Compound sputter yields (in atoms/projectile) shown versus the ratio of UT (Sigmund’s correction factor a was taken from ref. [Ill, the energy transfer factor T see text) and the average heats of atomization. The data are given in table 1 together with the experimental error margins.
344
J. Goschnick et al. / Sputter yields of multielemental powders
difference was found when only the reciprocal heat of atomization was used as abscissa. Therefore AH, is the most essential entity for the distribution of the data. Either the heat of atomization is not a suitable measure for the atomic binding forces or additional parameters are of considerable influence.
5. Conclusions
The presented method allows an easy determination of the depth propagation during sputter erosion of particulate material even if high electrical resistance prevents crater measurements of such samples in compact form. Normalized to 1 mA/cm* for 400 eV argon ions the erosion rates of 15 salts and oxides were found to be spread about an average of 0.4 rim/s within a limited range of +40% variation (standard deviation) only. Thus, for sputter-based depth-resolved analysis of particles composed of these materials (such as pigments, ceramic powders or environmental microparticles) the conversion of time into values of depth seems to be feasible with acceptable accuracy using one average depth propagation regardless of the instantaneous composition during sputtering. Studies of erosion rates for organic material are under way and will be published soon. Furthermore, the sputter yields of macroscopic flat surfaces and powder particles were found to differ not much. Thus, the erosion rates and sputter yields derived from powder measurements are approximately within a factor of 2 also valid for compact samples. The determined sputter yields between 0.5
and 1.7 show no simple heat of atomization.
dependence
on the average
References t11 M. Fichtner, J. Goschnick, U.C. Schmidt, A. Schweiker and H.J. Ache, J. Vat. Sci. Technol. A 10 (1992) 362. PI J.W.G. Bentz, H.-P. Ewinger, J. Goschnick, G. Kannen and H.J. Ache, Proc. 7th Working Conf. on Applied Surface Analysis, Jiilich, Germany, 1992, Fres. J. Anal. Chem. 346 (1993) 123. [31 H. Oechsner, in: Thin Film And Depth Profile Analysis, chap. 4, ed. H. Oechsner, Topics in Current Physics 34 (Springer, Berlin, 1984). [41 E. Stumpe, H. Oechsner and H. Schoof, Appl. Phys. 20 (1979) 55, [51 R. Jede, H. Peters, G. Diinnebier, 0. Ganschow, U. Kaiser and K. Seifert, J. Vat. Sci. Technol. A 6 (1988) 2271. 161A. Wucher, J. Vat. Sci. Technol. A 6 (1988) 2293. [71 H.H. Andersen and H.L. Bay, in: Sputtering by Particle Bombardment I, ed. R. Behrisch, Topics in Applied Physics 47 (Springer, Berlin, 1981) pp. 145-218. 181P.C. Zalm, Surf. Interf. Anal. 11 (1988) 1. [91 W.K. Leung, Y. Hiroka, R.W. Corm, D.M. Goebel, B. Labombard and R. Nygren, J. Vat. Sci. Technol. A 7 (1988) 21. I101 P. Sigmund, J. Mater. Sci. 8 (1973) 1545. [ill P. Sigmund, in: Sputtering by Particle Bombardment I, ed. R. Behrisch, Topics in Applied Physics 47 (Springer, Berlin, 1981) pp. 9-71e. WI R. Kelly, Nucl. Instr. and Meth. B 18 (1987) 388. [131 Handbook of Chemistry and Physics, 68th ed. (CRC Press, Boca Raton, FL, 1987).
Beam Interactions with Materials 8 Atoms
Sputter yields and erosion rates for low energy ion bombardment of multielemental powders J. Goschnick, J. Schuricht, A. Schweiker and H.J. Ache Kernforschungszentrum
Karlsruhe, Institut fiir Radiochemie,
Postfach 3640, 76050 Karlsruhe,
Germany
Received 3 April 1992 and in revised form 28 June 1993
For depth-resolved analysis of particulate material such as pigments, ceramic powders or environmental microparticles with secondary neutral mass spectrometry (SNMS), erosion rates and sputter yields of 16 powders, including salts, metal and nonmetal oxides, have been measured to provide data for the conversion of sputter time into depth values. For that purpose a new gravimetric method was developed, which allows the measurement of depth propagation within particles even for bad electrical conductors. With about 1 mA/cm’ of 400 eV argon ions for the 16 powders an average sputter yield of Y = 1.2 and a mean erosion rate of i = 0.4 rim/s were measured. In spite of the chemical variety of the powders a standard deviation of only 40% was observed, which gives the relative error for a depth calibration in case of an unknown analyte. Comparison between compact and particulate samples showed the powder erosion data to be valid within a factor of 2 even for compact samples with flat surfaces. No
correlation between the heat of atomization and the sputter yield could be recognized.
1. Introduction Plasma-based secondary neutral mass spectrometry (SNMS) was found to be appropriate to measure the elemental composition of particulate material containing oxides, salts or organic compounds [1,2]. In order to use the high performance of this method [3,4] to determine quantitative depth profiles, erosion rates are needed to convert sputter times into depth values. Neither for the special kind of material nor for the applied low primary energies of a few hundred eV sputter yields were -known. Moreover, typical literature data are given for normal incidence, while in the case of powder analysis the multitude of local surface orientations results in a wide distribution of incidence angles. Therefore a special weight loss method was developed to measure erosion rates for powders and applied for a variety of representative compounds. This technique turned out to be of particular advantage for measuring electrically nonconducting substances.
2. Experimental Measurements were performed with the INAinstrument manufactured by the Leybold company (ColCorrespondence to: J. Goschnick, Karlsruhe, Institut fiir Radiochemie, Karlsruhe, Germany. 0168-583X/93/$06.00
Kernforschungszentrum Postfach 3640, 76050
0 1993 - Elsevier
Science
Publishers
ogne, Germany). Principles, design and operating conditions have been described in detail elsewhere [5,6]. Briefly, a metal foil supporting impressed powder grains is mounted behind an orifice in the wall of the chamber containing the rf-excited argon plasma. A negative voltage applied to the metal carrier accelerates argon ions out of the plasma towards the sample. With respect to the carrier foil normal incidence is obtained. On arrival at the carrier surface the energy of the bombarding ions is given by the potential difference between plasma and sample. A geometrically given fraction of the sputtered neutrals enters the plasma, becomes partially ionized and leaves the plasma towards the analysis section. There, an energy filter discriminates thermal particles of the plasma to optimize the subsequent mass analysis by a quadrupole mass spectrometer. The argon pressure of the plasma was set to p = 2 x 10m3 mbar. Plasma conditions were used providing electrons and argon ions with a density of about 3 X lOlo particles/cm3 and an average unidimensional energy E, = 4 eV for the electrons. For the 3.6 mm distance between the grounded aperture exposed to the plasma and the sample a target potential of -360 eV was found to be appropriate to achieve a primary ion current density with lateral uniformity across the sample. The sputtered sample area was 0.38 cm2. Using a profilometer the flatness of the crater bottoms of copper samples was checked for adjustment. A depth resolution of less than 10 nm was measured at the sharp interface of a 107 nm thick B.V. All rights reserved
340
J. Goschnick et al. / Sputter yields of multielemental powders
silicon dioxide layer on Si(100) wafer samples confirming the high lateral uniformity of the bombardment. As the potential inside the plasma was measured to be ca. 40 V, the applied target potential supplied argon ions of an energy of 400 eV. The primary ion current density incident to the samples was determined to be 0.6-1.0 mA/cm* (according to No = 3.8-6.3 x 1015 projectiles/(scm2)) from the weight loss of polycrystalline copper samples for which a sputter yield of 1.8 was taken from the literature [7]. A Faraday cup arrangement would disturb the critical ion optical conditions of the ambient geometry of the sample and was therefore not applied for the primary current measurement. Several salts, some oxides and copper granulate were chosen to be investigated. Salts and oxides are typical components of environmental material, for which a depth-resolved analysis should be developed. Reagent grade powders (particle sizes < 100 pm) were pressed into indium foil covering less than half of the plasma exposed area (7 mm in diameter). Keeping the coverage lower than this limit turned out to be sufficient to avoid interference of the measurements by charging. However, with the same salts in compact form, such as pressed tablets, nearly no sputtering was possible due to severe charging of the electrically insulating material under ion bombardment. From each powder at least five samples with different coverages were prepared. A microbalance with 10 pg resolution was used to measure the mass loss of the samples before and after sputtering. The initial mass was measured after the sample had been in the vacuum for several hours and sputtered for some seconds. This procedure prevented errors in the determination of the sputter mass loss due to pumped off water, mechanical falloff of particles during sample transfer and loss of bound particles being charged up and electrically forced to leave the carrier when the target potential is switched on. The samples were exposed to the bombardment some 1000 s to give mass losses in the range of 1 mg. During sputtering the atomic SNMS-signals of the powder constituents and the supporting indium foil were recorded. The temperature of the samples was kept below 0°C.
rial, the mean mass of sputtered atoms @ (g/mol) and the fraction of the total primary flux hia (projectiles/s) the powder particles or the uncovered area of the indium foil are exposed to. Let 0 be the fraction of the primary flux incident to the particles, the total mass loss rate is given as ti = Y&&(1
- 0) + Y&M,.
(2)
Index P denotes the powder and index C stands for the carrier, respectively. Obviously the prefactors of 0 and (1 - 0) are the mass loss rates lit: of the powder only and ri$, the empty carrier foil, respectively. Thus, the total mass loss is a linear function of 0: ti = ti”p + (tie, - $)o.
(3)
0 is necessarily coverage dependent. The more particles are deposited on the carrier surface, the higher is the part of the projectile flux impinging onto particle surfaces. Indeed, as long as the primary current density is laterally constant, 0 is identical with the relative coverage of the powder on the carrier surface. But if the particles charge up, some of the positive projectiles originally travelling towards a particle, may be diverted by the positive charge of the powder grains. Consequently the fraction of projectiles impinging onto particles becomes lower than the relative coverage. However, even in this case eq. (3) stays valid. Only for severe charging the change of the sputtering yield due to lowered primary energy and an alteration of the distribution of incidence angles of the projectiles have to be considered. The linear relationship of eq. (3) forms the basis of a simple way to determine the mass loss rate tit of the powder, replacing 0 by the corresponding atomic SNMS-signal of the carrier. This can be done because the atomic SNMS-signal of the carrier foil (here indium) is proportional to (1 - O), the fraction of the primary flux bombarding the metal foil. Using the basic equation for the intensity of atomic SNMS-signals [1,3], Z(C) = Y&D(C),
(4)
where Z(C) is the intensity of the atomic signal D(C) the detection factor for the carrier element, (3) can be rewritten as
and eq.
3. Mass loss method for sputter erosion measurements of supported powders The total mass loss rate ti of a sample is given by the sum of the individual mass loss rates of the powder particles ti, and the carrier foil tic: ti =ti,+tic.
(1)
The individual mass loss rates are determined by the sputter yield Y (in atoms/projectile) of the mate-
with Z,,(C) denoting the atomic carrier signal for the empty foil. Accordingly, the evaluation of data from samples with a variety of particle coverages should yield a straight line when the measured total mass loss rates are plotted versus the corresponding SNMS-signals of the carrier. Extrapolation of this line to Z(C) = 0, according to a totally covered carrier, should give an intercept with the &axis identical to the mass loss rate
J. Goschnick et al. / Sputter yields of multielemental
ti”p of the powder itself. The method uses the carrier signal only as indication for complete particle coverage. Neither the sputter yield of the empty carrier nor the absolute coverage of the samples are relevant for the mass loss rate determination of the powder. The sputter yields can be obtained from these mass loss rates and the primary flux No according to
where NA denotes Avogadro’s number. The powder sputter yield should be different to the one obtained for a flat surface under normal incidence bombardment of the same material. This is because the powder particle is necessarily sputtered under a variety of incidence angles and compared to a flat sample with the size of the particle’s basal plane, it has a higher surface area exposed to the projectile flux. Defined as depth propagation i perpendicular to the bombarded surface the erosion rate is calculated from the mass loss rate, the density p and the surface area of the powder. Assuming the particles to behave like half-spheres, the powder surface has 2 times the area the particles are covering. At complete coverage of the carrier, for which ti! is valid, the particles occupy the total bombarded area A,. Accordingly, the erosion rate can be calculated by the equation
(7) This is only an average depth propagation into the particle bulk. Due to the incidence angle dependence of the sputter yield, the erosion cannot be strictly uniform across the particle. The more the bombardment direction deviates from normal incidence the higher the sputter yield will be until at an incidence angle (Y of about 70” the maximum yield is reached. For even more oblique incidence a steep decrease of the yield occurs [8]. However, the total angle dependence of the erosion rate is given by the product of bombarding current density and yield. While the first is proportional to cos(cu), according to Zalm [8] for incidence angles lower than for maximum erosion, the yield depends on incidence angles approximately as Y= Y&oP((Y),
(8)
whereby Y,, denotes the value for normal incidence ((u = P>. Accordingly, the depth propagation should show only a weak dependence proportional to l/ cos”.7(~) resulting in a nearly surface-parallel erosion for (Y< 70”. Because only a minor part of the total surface is exposed to glancing bombardment with considerably lower erosion, eq. (7) should be a good approximation to the depth propagation at least in these areas of the particle surface which dominate the mea-
341
powders
sured SNMS-intensity. As the acceptance of the spectrometer is directed normal to the plane of the carrier foil, the erosion flux of accordingly oriented surface areas is preferably detected. These are the areas of the grains which are bombarded under near-normal incidence. Additionally, the incidence angle dependence and the higher area of a rough surface should result in a sputter yield for the powder being moderately higher than a flat sample of the same material. Integration of eq. (8) over a half-sphere up to 70” and neglecting the small fraction of atoms coming off at higher angles results in about 1.8 times more sputtered atoms compared to a flat sample with a surface area equal to the basal plane of the spherical particle. Thus, the sputter yield of the powder should be slightly higher than the sputter yield for a flat sample at normal incidence. However, as can be seen from eqs. (6) and (7), the depth propagation rate for the powder should be about the same as for an even surface.
4. Results and discussions Fig. 1 shows for example the relevant SNMS-signals during sputtering of K,CrO,. Indium foil was used as carrier material in all cases. Between 50 and 200 s were usually enough to obtain stationary signals from the powder particles, indicating a sputter equilibrium with constant surface concentrations and a stable topography. As for all other powders a number (at least 5, in this case 10) of samples with different K,CrO, coverages were sputtered. Fig. 2 presents the mass loss data plotted against Z(In), the SNMS intensity of the indium foil. According to eq. (7) a linear relationship is obtained between the total mass loss rates and Z(In). Therefore for all powders a linear fit was used to determine
the
intercept
at Z(In) = 0. The
mass
loss
lE6 i ____________________--_________^_____________ In(115) .,_-‘iii
a lE5
s
: K (39)
;,._._ __- -.-.--.-.-.-
.s E 2 .c
Cr (9) 1 E4 i- _.....-..-.- ....__.._..__.__._ _.__.._.._..__..__.._ -
lE3’
0
’ 500
’
loo0
’
1500
0 (16)
’ 2ooo
2500
3000
sputter time [s] Fig. 1. Time profile of the atomic SNMS-signals during sputtering of K,CrO, on indium foil with 0.9 mA/cm’ of 400 eV argon ions.
342
J. Goschnick et al. / Sputter yields of multielemental powders
,I-_ al
E 8
....-_,-
,
_,_...~~l~-
_o
:: 9
0.5
t
t
0’0
’
*
0.5
s
s
1
.
’ 1.5
’
’
2
*
’
2.5
intensity of In [I 06 cpsl Fig. 2. Total mass loss rates of 10 samples of K,CrO, on indium foil versus the SNMS-signal intensity of the indium carrier. Bombardment being done with 0.9 mA/cm* of 400 eV argon ions.
of K,CrO, was found to be tit = 6.1 X lop8 g/s. Additionally two separate K,CrO, samples and an empty indium foil were sputtered until stationary conditions had been established. The measured indium intensities were used to calculate 0, the fraction of the bombarding current incident on the powder. According to eq. (4) and using Z&n) for the intensity of the empty carrier 0 = [Z&n) - Z(In)]/ZO(In). Values of 0 = 0.023 and 0 = 0.080 were obtained. Subsequently micrographs of the two powder samples were taken rate
Table 1 Sputter yields and erosion rates of powdered salts and are given for a primary current density of 1 mA/cm*. atomization are given too. The latter were calculated enthalpies for the solid elements and the dissociation taken from Sigmund ill].
with scanning electron microscopy (SEMI. From these images the coverage of the powder was determined with a computer-aided grain analysis system by numerical integration of the particle covered areas. Coverages of 0.023 and 0.074 were obtained. The close agreement with the above primary current fractions indicates the absence of electrical charging induced by the ion bombardment. From the linear relation between the measured mass loss rates and the corresponding indium intensities the absence of charging can be concluded also for higher coverages. The low coverages in this investigation were necessary to obtain reliable coverage data from the SEM-images of isolated single particles on the carrier foil. The emission of secondary electrons from the carrier metal is assumed to inhibit the chargeup of the particles. Table 1 contains the resulting sputter yields and erosion rates calculated with eqs. (6) and (7) from the measured mass loss rates of the powders Ijli. The given error margins are derived from the standard deviation of the intersect determination by the linear extrapolation. The average relative error is below +30%. However, in some cases the scatter of data caused considerably higher errors. Maybe because of morphological reasons, grains of these powders were not held tight enough by the metal foil, so that some of them eventually fall off after some sputtering. Moreover, indium forms sputter-induced cones typical for smooth metals [9], which indeed had been found by electron microscopy for several samples. Probably be-
oxides for bombardment with argon ions of Ea = 400 eV. The erosion rates The mass dependent factors a, energy transfer factors T, and the heats of from standard enthalpies of formation of the compounds, the sublimation enthalpies for the gaseous elements given in ref. [14]. The values for a are
Compound
Heat of atomization [kcal/mol]
Transfer factor
a-factor
Yield [atoms/ projectile]
Erosion rate
Error margin
h/s1
[%I
NaNO,
86 86 82 112 99 99 96 152 81 149 147 102 128 115 1.59 180
0.84 0.89 0.93 0.85 0.89 0.95 0.98 0.95 0.95 0.89 0.89 0.97 0.98 0.99 0.78 1.0
0.19 0.20 0.26 0.19 0.20 0.21 0.30 0.21 0.31 0.20 0.20 0.21 0.22 0.22 0.45 0.24
1.0 1.0 0.58 1.7 1.8 1.35 1.17 1.93 1.53 1.3 0.54 0.67 0.96 0.95 0.7 1.5
0.38 0.47 0.24 0.60 0.66 0.48 0.48 0.62 0.54 0.50 0.15 0.30 0.28 0.30 0.46 0.54
58 40 36 50 29 20 8 23 12 19 24 14 11 5 50 21
f=O, PMNO,), Na,CO, NazSO, K2SO.4
PbSO, TiO, (rutile) Cu (powder) SiO, A12o3
K,CrO, Cr203 Fe203
PbO cue
J. Goschnick et al. / Sputter
cause of the developing sputter cones, the indium signal of the carrier always passed through a slight m~mum. This topographical change could be assumed to be influenced by the redeposition of sputtered material from the powder [9] resulting in slightly variable traces of the indium SNMS-signal. Additionally, the injury of the metal foil surface by the impressed particles could contribute to some variation of ItIn). Finally, to avoid interference by charging in some cases the coverages had to be kept quite low, which introduced additional errors due to a long range extrapolation. The sputter yields as well as the erosion rates of all 16 powders do not show much variation. The mean values are Y = 1.2 and i = 0.4 rim/s (for 1 mA/s primary current density) with a standard deviation of ca. 40% only. Therefore, the conversion of time into values of depth based simply on the average depth propagation rate seems to be a suitable approximation even for the depth-resolved analysis of material composed of a mixture of oxides and salts. The measured sputter yields for copper Y(G) = 1.5 and quartz powder Y&O,) = 1.3 do not differ significantly from values for flat surfaces Y_(G) = 1.8 [7] and Y_(SiO,) = 0.9. The latter value was obtained from the time interval of 156 s (i, = 1 mA/cm’) necessary to sputter through a 107 nm thick SiO, layer on a silicon wafer. Thus, the expected enhancement for sputtering of powders could only be measured for SiO,. Probably the rough topography of the metal particle surfaces consists of deep valleys with steep sides and causes a certain amount of the sputtered mass to be redeposited, which results in a compensation of the erosion enhancement caused by nonnormal incidence and increased surface area [lo]. On the other hand the accordance of the sputter yields derived from the mass loss of powders with those for flat surfaces confirms the reliability of the method. Thus, the sputter yields determined by the powder method seem to provide a suitable approximation for normal incidence values with an inaccuracy less than a factor of 2. Sigmund’s model [ll] is still being accepted 19,121to give a generally correct description of the sputtering erosion for elastic collision dominated conditions. The influence of the bombarded material on the yield is given by
yields of multielemental
343
powders
Because the atomic SNMS signals dominate the spectra (> 90% of the total intensity) for all analysed samples and their ratios were found to be consistent to corresponding ratios of atomic ionization probabilities [l], most of the eroded material leaves the samples as sputtered atoms. Substantial release of thermal species would give a reduced atomic intensity due to the energy discrimination prior to mass analysis. Accordingly, thermal processes which are not considered in the coliision-type sputter model such as simple evaporation of the bombarded material is excluded to contribute considerably. Therefore in a first attempt relation (9) was used to systematize the yield data. Although structural disruption of the surface and con~ntrational changes due to the bombardment are not considered within the average heat of atomization (AH,), this type of energy is assumed to be at least a rough estimate of the binding forces between the atoms. In accordance to the usual procedure [8,12] and as other more appropriate data are not available, U, is therefore replaced by the average heat of atomization (AH,). The latter was calculated from standard enthalpies of formation for the compounds added by sublimation enthalpies for the solid elements and dissociation enthalpies for the gaseous elements (all taken from ref. 1131). Transfer factors were determined from the average atomic masses of the compounds and values for the factor a were taken from ref. [ll] for low bombarding energy condition. Fig. 3 shows the sputter yields of the measured powders as a function of the UT/AH, ratios. Although the heats of atomization differ by a factor of 2 a simple increasing trend for the yield with lowered average atomic binding forces is not perceptible. No substantial
‘;i;
2.5
I
, .
l
Na2COs
Cu.
C”p K2 SO, 302
l
9 Cr203=
l
PbSO1,
KN03* l NaN03 l
Fe203
PbO
.
l
Al;03
Y-a(M,,M,)T(N,,M,)/U,,
N4S04
TiOz .
K2C*4
(9)
with U, as binding energy of the atoms to the surface, with T being the collision energy transfer factor given by T = 4M,M,/(M, + M2j2 (M, denotes the mass of the projectile ion while MZ represents the mass of the sample atoms), and a being a correction factor which is dependent on the mass ratio of projectile and target atoms as well as on the primary energy range.
Pb;NO&
i 0.005
a+T /AH,
0.01
0.015
[Mol/kcal]
Fig. 3. Compound sputter yields (in atoms/projectile) shown versus the ratio of UT (Sigmund’s correction factor a was taken from ref. [Ill, the energy transfer factor T see text) and the average heats of atomization. The data are given in table 1 together with the experimental error margins.
344
J. Goschnick et al. / Sputter yields of multielemental powders
difference was found when only the reciprocal heat of atomization was used as abscissa. Therefore AH, is the most essential entity for the distribution of the data. Either the heat of atomization is not a suitable measure for the atomic binding forces or additional parameters are of considerable influence.
5. Conclusions
The presented method allows an easy determination of the depth propagation during sputter erosion of particulate material even if high electrical resistance prevents crater measurements of such samples in compact form. Normalized to 1 mA/cm* for 400 eV argon ions the erosion rates of 15 salts and oxides were found to be spread about an average of 0.4 rim/s within a limited range of +40% variation (standard deviation) only. Thus, for sputter-based depth-resolved analysis of particles composed of these materials (such as pigments, ceramic powders or environmental microparticles) the conversion of time into values of depth seems to be feasible with acceptable accuracy using one average depth propagation regardless of the instantaneous composition during sputtering. Studies of erosion rates for organic material are under way and will be published soon. Furthermore, the sputter yields of macroscopic flat surfaces and powder particles were found to differ not much. Thus, the erosion rates and sputter yields derived from powder measurements are approximately within a factor of 2 also valid for compact samples. The determined sputter yields between 0.5
and 1.7 show no simple heat of atomization.
dependence
on the average
References t11 M. Fichtner, J. Goschnick, U.C. Schmidt, A. Schweiker and H.J. Ache, J. Vat. Sci. Technol. A 10 (1992) 362. PI J.W.G. Bentz, H.-P. Ewinger, J. Goschnick, G. Kannen and H.J. Ache, Proc. 7th Working Conf. on Applied Surface Analysis, Jiilich, Germany, 1992, Fres. J. Anal. Chem. 346 (1993) 123. [31 H. Oechsner, in: Thin Film And Depth Profile Analysis, chap. 4, ed. H. Oechsner, Topics in Current Physics 34 (Springer, Berlin, 1984). [41 E. Stumpe, H. Oechsner and H. Schoof, Appl. Phys. 20 (1979) 55, [51 R. Jede, H. Peters, G. Diinnebier, 0. Ganschow, U. Kaiser and K. Seifert, J. Vat. Sci. Technol. A 6 (1988) 2271. 161A. Wucher, J. Vat. Sci. Technol. A 6 (1988) 2293. [71 H.H. Andersen and H.L. Bay, in: Sputtering by Particle Bombardment I, ed. R. Behrisch, Topics in Applied Physics 47 (Springer, Berlin, 1981) pp. 145-218. 181P.C. Zalm, Surf. Interf. Anal. 11 (1988) 1. [91 W.K. Leung, Y. Hiroka, R.W. Corm, D.M. Goebel, B. Labombard and R. Nygren, J. Vat. Sci. Technol. A 7 (1988) 21. I101 P. Sigmund, J. Mater. Sci. 8 (1973) 1545. [ill P. Sigmund, in: Sputtering by Particle Bombardment I, ed. R. Behrisch, Topics in Applied Physics 47 (Springer, Berlin, 1981) pp. 9-71e. WI R. Kelly, Nucl. Instr. and Meth. B 18 (1987) 388. [131 Handbook of Chemistry and Physics, 68th ed. (CRC Press, Boca Raton, FL, 1987).