Medium energy ion scattering study of Ni on ultrathin films of SiO2 on Si(111)

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Medium energy ion scattering study of Ni on ultrathin films of SiO, on Si( 111) J.B. Zhou, T. Gustafsson Department of Physics and Astronomy & Laboratory for Surface Modification, P.O. Box 849, Rutgers - The State University of New Jersey, Piscataway, NJ 08855-0849, USA

R.F. Lin and E. Garfunkel Department of Chemistry & Laboratory for Surface Modification, P.O. Box 939, Rutgers - The State University of New Jersey, Piscataway, NJ 08855-0939, USA

Received 26 May 1992; accepted for publication 8 October 1992

The structure of thin Ni films (- 7 ;i> deposited at room temperature on ultrathin SiO, layers on Si(ll1) has been studied using medium energy ion scattering. Our results indicate that the Ni films deposited at room temperature are not uniform. The data have been fitted with two_model approximations. In one, the overlayer structures are characteriied by spherical caps with a radius of 39 and height of 22 A, while in the second they are characterized by a thickness distribution function with an average thickness of 14 and deviation of 7 A. Above 750 K, Ni atoms start to diffuse through the SiO, layer and subsequently into the near-surface region of the crystalline Si substrate. After annealing to 1075 K, some of the Ni has diffused into the substrate to a depth of at least 800 A. In this final state, the SiO, is completely desorbed and nickel silicides are formed.

1. Introduction The study of metal adsorption on silicon dioxide is of importance for a number of technological applications. Firstly, studies of metal-oxidesemiconductor CMOS) structures are of relevance to many aspects of semiconductor technology including the design of MOS devices, metallization, and multilayered structures [ll. Secondly, oxides are widely used as high surface area supports for dispersed metal catalyst systems [2], and studies of metal-oxide bonding and thermal stability will lead to a better understanding of oxide-supported catalyst systems. Past RBS studies of transition metal thin films (for thicknesses 2 1000 & on SiO, substrates have indicated that metals of the first transition period to the right of V, i.e. Cr, Mn, Fe, Co, Ni, and Cu, as well as Pd and Pt do not react with

SiO,, while most metals of Group IVB and VB (Ti, Zr, Hf, V and Nb) do react with SiO, to form metal silicides and metal oxides [3]. Because of a decreased ability to react and an increased rate of diffusion, the tendency of a transition metal to coalesce into islands on SiO, should increase as one goes to the right in the periodic table. The Ni/SiO,/Si system is an appropriate model system as the behavior of the Ni/Si interface has been well characterized by various groups [4-81. Different structural and spectroscopic techniques including RBS, SEM, TEM, AES, XPS, UPS, HREELS, LEED and AFM have been employed for the study of Ni/SiO,/Si interface structures [3,9-111. In our previous work, we came to the tentative conclusion that Ni islanding occurs at lower temperatures (100-850 IQ, and that Ni diffusion through the SiOz film occurs at higher temperatures (700-1050 K). We report in this

0039~6028/93/$06.00 0 1993 - Elsevier Science Publishers B.V. Ah rights reserved

68

J.B. Zhou et al. / Ni on ultrathin jilms of SO, on Si(III)

paper further, more quantitative studies of thin Ni films deposited on thin SiO, layers using medium-energy ion-scattering with channeling and blocking (MEIS) [12]. The main objective of this study is to characterize the morphology of adsorbed Ni films and the temperature-dependent diffusion of Ni through the SiO, layers into the Si substrates.

2. Experimental 2.1. Sample preparation Si(ll1) wafers (Virginia Semiconductor, Pdoped, 20-40 R * cm) were cut to 19 x 6 X 0.4 mm3 to fit into the sample holder. They were chemically etched and oxidized prior to insertion into the vacuum chamber [13]. They were mounted on the sample holder with a pair of Ta clamps in order to allow resistive heating. In situ cleaning was performed by annealing to _ 1150 K for 10 min. Under such preparation conditions, the Si(ll1) surfaces exhibit sharp (7 X 7) LEED patterns. Auger and ion scattering indicated that the surfaces prepared under such conditions had no detectable contaminants. SiO, was subsequently grown by heating the Si substrate to N 1090 K for 30 min in a dry 0, environment with a pressure of N 2.0 X low4 Torr. Under our growth conditions, the SiO, thickness was N 10 A as measured by ion scattering. The growth of SiO, thin films on Si in a UHV system has been discussed in detail in ref. [14]. It is generally believed that both the quality and growth rate depends on the 0, pressure and the Si substrate temperature, and the gas and surface impurity concentration. Thin Ni overlayers were deposited onto the SiO, films by sublimation. The Ni source consists of a Ni wire (99.994% pure) spot welded to a Ta foil; resistive heating of the tantalum foil leads to Ni evaporation. The coverage of Ni was measured directly by MEIS with an accuracy of + 2%. The rates of deposition were between 4.0 X lOi and 8.0 x 1014 atoms cmp2 s-l. The samples that were used for annealing studies were annealed for 10 min. The MEIS spectra were taken after

the sample cooled down. The beam dose was kept below 3 x 1015 ions cmv2 to ensure that no significant ion-beam-induced damage occurred. 2.2. Ion scattering on thin films and inte$aces As the principles of using MEIS to study the growth mode and morphology of thin films based on peak shape analysis have been discussed in detail by van der Veen [12] and van Loenen et al. [15], only relevant aspects will be discussed here. MEIS is a quantitative surface structural and compositional probe which measures the absolute area1 atomic density of a thin film. Ions scattered off the 0, Si and Ni atoms on a surface will yield three distinct peaks in the energy spectra (mass specificity). For a fixed scattering angle, H+ ions scattered off Ni lose the least energy, and ions scattered off 0 lose the most energy. In addition, ions also lose energy along their in- and out-going paths due to inelastic collisions with the electrons in the solid. The energy scale in the spectrum is therefore also a depth scale, permitting one to determine a depth profile. Compared to conventional RBS, MEIS has an advantage in that the depth resolution is higher due to the use of a high-resolution electrostatic ion analyzer. When an incident ion beam is aligned with a low-index crystallographic direction, all atoms in the crystal lie in rows parallel to the ion beam. Scattering by the first atom in each of these rows results in a shadow cone that reduces the probability of atoms further along the row being hit by the ion beam. Only a few monolayers of the surface atoms will dominate the ion-scattering process - resulting in a surface peak in an energy spectrum. The ion scattering yield from the bulk is greatly reduced due to this “channeling” effect [12,16,17]. When the ion analyzer is placed along a crystallographic direction, the background yield is further reduced due to “blocking” [12,18-201. Thermal vibrations, surface reconstructions or surface and bulk defects make shadowing and blocking less effective. Since the 0 peak is at a lower energy than the Si peak, the 0 peak is on a background of the Si bulk yield even in channeling. In a channeling and blocking configuration, this Si bulk yield drops by more than an order of

J.B. 2%~ et al. / Ni on ultrathin f&s of SiO, on Si(ll1)

BadcpcangedEnergy

Fig. 1. Possible structures of Ni films on SiO,/Si, and their corresponding schematic MEIS spectra. (a) A thin SiOz film on a Si surface. (b) A uniform Ni film on the SiO, surface. 63 Cluster of Ni atoms on the SiO, surface, assuming all clusters of equal height. (d) Clusters of Ni atoms with variable heights on the SiO, surface.

magnitude, enabling quantitative analyses of the 0 and Si spectra from the SiO, film and the SiO,/Si interface. Fig. 1 shows three possible structures of the Ni film and the corresponding ion-scattering peak shapes of 0, Si and Ni. Fig. l(a) shows schematically a thin fihn of SiO, on a Si(lll) substrate. The corresponding spectrum consists of an 0 peak and a Si peak. Their peak widths are indicative of the thickness of the SiOz film and broadened by the finite energy resolution of the analyzer. If Ni grows as a uniform polycrystalline film [fig. l(b)], both the 0 and Si peaks should be shifted rigidly to lower energies due to the energy loss of the ions as they travel through the Ni overlayer. The size of the shift should reflect the thickness of the Ni film, and the Ni peak should be essentially symmetric. If Ni forms clusters [fig. l(c)] and part of the SiO, surface is uncovered, both the 0 and Si peaks should consist of two parts, one from the uncovered SiO, located at

69

the same energies as in fig. l(a), and another from the Ni covered SiO, at lower energies as in fig. l(b), resulting in broadened peaks. If the Ni film has a wide distribution in thickness, the Ni peak becomes asymmetric [fig. l(d)], and the degree of asymmetry reflects the width of the thickness distribution and fractional coverage. The sticking coefficient of Ni on SiO, at 300 K was measured and found to be equal to unity even at low coverage. This is quite different from the case of Cu adsorption on SiO,, where the sticking coefficient is significantly less than 1 as long as bare SiO, patches remain on the surface. This behavior is attributed to the weak Cu-SiO, bond [21j. 98.0 keV Hf ions were used as projectiles. The UHV scattering chamber is equipped with a high-resolution toroidal electrostatic ion energy analyzer, LEED, double pass CMA and a mass spectrometer.

3. Morphology of the Ni film after deposition Fig. 2 shows MEIS spectra in the region of the 0 and Si peaks before and after Ni deposition.

Fig. 2. MEIS spectra of 0 and Si from (a) SiO,/Si and (b,> Ni/SiO,/Si taken with a 98.0 keV II+ beam. The _ 10 A thick SiO, film was formed by annealing a Si(ll1) substrate at 1025 K in an O2 pressure of * 2.0~ 10W4 Torr for 30 min. The Ni coverage was 6.7 x 10” atoms/cm-2 (3.6 close-packed layers for a uniform Ni film). Ions were incident and detected atong the [OOi] and [IlO] directions, respectively (90” scattering angle). Curve (a) uses the scale of the left axis and curve (b) uses the scale of the right axis.

70

J.B. Zhou et al. / Ni on ultrathin films of SO,

The area of the peaks (background-subtracted) give the total number of atoms visible to the ion beam. The 0 spectra consist of ions scattered off 0 atoms from within the amorphous SiO, film. The Si spectra consist of two parts: one from ions scattered off the Si atoms from within the amorphous SiO, film and the other from a few monolayers of the crystalline Si substrate. When comparing results before and after room temperature Ni deposition, two important features can be observed. First, both the 0 and Si peaks [fig. 2(b)] are broadened and lowered upon the deposition of Ni films. Second, after Ni deposition [fig. 2(b)], the high energy side of both the 0 and Si peaks start at the same energies as before deposition [fig. 2(a)]. Therefore [cf. fig. l(b)], we can directly eliminate the uniform Ni layer structure based on the observed features in the 0 and Si spectra. The morphology of the Ni film is further examined by modeling the Ni and Si peak shapes. For an Ni overlayer structure of arbitrary shape, the Ni spectra should be given by:

-d(E)] dx dy dz,

(I)

where H(E) is the intensity at energy E, F the detector function which can be approximated by a Gaussian having a FWHM of 405 eV for 100 keV protons [22], C, a constant determined by the total coverage of Ni, p(x, y, z) the path length of ions scattered at a point (x, y, z> and d(E) the path length of ions carrying energy E: c _c d(E)

=

!l!!-r

dE/dx

’

(2)

where E, is the energy of ions scattered off surface atoms and d E/dx the energy loss function. Films of different structures give different sets of p(x, y, z) which in turn give different peak shapes. The integral is carried over the volume u of the overlayer. Eq. (1) relies on an input of the overlayer structure. One must start with a model overlayer structure and then calculate the spectra which can be compared to the experimental data. For a film with true irregularity in its structure, u ex-

e

10

h -7 6

on Si(lll)

--urdform Ni film - --clusters same height -clusters variable height

g

[ 6 2 y 4 2 z $2 0 88

89

90

91 92 Energy WV)

93

94

Fig. 3. MEIS spectra of Ni from Ni/SiO, /Si (0) taken with a 98 keV H+ ion beam at a 120” scattering angle and calculated spectra with uniform Ni films (. . . .), Ni clusters with identical thickness (- - -_), and clusters with varying thicknesses ). The incident and exit directions were not aligned (with any Si crystallographic axes.

tends to the entire volume of the film. We have calculated the spectra in two model approximations. One is a slowly-varying thickness model (SVT) in which the thickness variations between the incident and exit point on the surface of the film is small. Under this approximation, eq. (1) can be simplified using a thickness distribution function of the film: H(E)

= C,j-amP( t, ai) dt/‘-dF(u) -d

du,

(3)

where P(t, a,) is the thickness distribution function, t the thickness and ai some parameters associated with the distribution function (see eq. (4)). However, eq. (3), similar to the one used in ref. [15], does not specify the lateral size or shape of the overlayer structure. The validity of the SVT model depends also on the incident and exit angle of the ion beam. It is strictly valid for 180” scattering at normal incidence. In fig. 3, the Ni spectra are shown together with simulations based on the SVT model. The Ni spectrum in this graph was taken at random incidence and random exit angles (i.e., not aligned with any Si crystallographic axes). The data clearly show an asymmetric peak, reflecting the spread of the thickness in the Ni film.

71

LB. Zhou et al. / Ni on ultrathin films of SO, on Si(lll)

The simulated peak for a uniform Ni film (dotted line) is much higher and narrower than the experimental data (the peak areas are conserved). This is consistent with what we concluded from the 0 and Si spectra. Islanding of the Ni atoms (for a given total coverage) will inevitably lead to the formation of some islands having thicknesses greater than that of a uniform Ni film and, therefore, to a broadened Ni peak. If we allow the Ni to form islands and assume all islands have the same height [a pillbox model as in fig. l(c)], then the fit (dashed line) is much closer to the experimental data. However, there are still significant disagreements between the two, both at the peak and in the low energy tail of the spectra. The calculation fails to reproduce the asymmetry in the experimental spectra. As mentioned above, the asymmetry implies a spread in island thicknesses. When we ailow the islands to have variable thicknesses [as in (fig. l(d)] the simulated spectra (solid line) fit the data much better. For the thickness distribution function, we use a simple, modified Gaussian function #I: P( X, a,) = G(x,

X, a), x 2 0,

(4) where G is a true Gaussian function and i, cr are the mean and standard deviation of the Gaussian function. The use of such a distribution function is purely empirical Our simulation with this function indeed yields a good fit to the data. The total Ni coverage is f3= 6.7 x lOi atoms cme2. This coverage corresponds to a nominal thickness of 7.3 A (for a uniform film). Our simulation shows an average film thickness of 14.0, and the variation of the thickness is characterized by a ‘“standard deviation” u, of 7.1 A. The fraction F of the SiO, surface covered with Ni islands, can be calculated by the following relation [ 151: F = 0/n% = 0.52,

(5) where 8 is the total coverage of Ni and n is the atomic density of Ni (n = 9.14 x 10” atoms cmw3, as in bulk Ni).

#’ Pfx, ai) is not a true Gaussian function since we onty allow positive values of x.

0 88

89

91

92

Fig. 4. MEIS spectra of (a) Ni and (b) Si from Ni/SiO, /Si and calculated spectra (solid lines) based on spherical caps of Ni islands with radius 38.8 and height 22.0 A. The Ni spectrum was taken at a 120” scattering angle and the Si spectrum was taken at a 90” scattering angle with a 98.0 keV H+ beam.

The SVT model, although involving approximations in the formula itself, is simple, and often can yield conclusions about the roughness of the film directly. Nevertheless, the SVT model does not yield any ~fo~ation about the lateral dimensions of the clusters. In the second model calculation, we assume Ni forms islands of spherical-cap shape and we calculate the spectra and optimize the size and contact angle of the island (see below). The spherical-cap model comes from continuum theories which minimize the surface free energy of the substrate and the overlayer. We also note that STM data for larger Ni islands appear to support spherical-cap islands 1231. In fig. 4, simulations of Ni, Si and 0 based on the spherical-cap model are shown together with the spectra. The calculation is optimized to obtain an average island size and contact angle (see fig. 4 inset) using eq. (1) directly. The simulations (solid lines) shown are for spherical caps with r = 38.8, and h = 22.0 A (19 = 59”). The fit is reasonably good except in the low energy region of the spectra. We believe that this is mainly due to some variations of size or shape of the islands since our calculations only account for an average

LB. Zhou et al. / Ni on ultrathin films of SiO, on Si(llI)

12

size and shape. The advantage of this model is that eq. (l), which involves no approximations, is solvable under the model. It has an additional advantage in that the island density (as well as the coverage) can be derived from the model:

DLL

I1V

8

&r(

h3 + 3hr2) ’

(6)

where 8 is the total coverage of Ni, and v is the volume of the island. For f3= 6.7 X lOi atoms cmm2, r = 38.8 and h = 22.0 A, this gives an island density of 1.27 x 1012 cmP2. The fraction of the SiO, surface covered by Ni can also be derived from the model: F = Drr2

= 0.60.

(7)

The fraction of coverage derived in this model differs slightly but not unreasonably from that of the SVT model (0.52). Using the above values, we then calculate the Si spectra [fig. 4(b)]. These are in quite reasonable agreement with the experimental data. The two model calculations reflect that the Ni films deposited at room temperature consist of clusters. These clusters can be understood by the results of the model calculations. It should be also stressed here however, that the models we used are not unique. The calculations do not uniquely determine the exact shape of the clusters. They are rather meant to characterize the overall structure and morphology of the film.

4. Diffusion of Ni atoms with annealing The Ni/SiO,/Si structure can be significantly modified by annealing. At temperatures above 750 K, Ni diffusion through the SiO, layer into the Si is observed, and the diffusion increases rapidly as the annealing temperature increases. In fig. 5, the Ni spectra taken after several annealing temperatures are plotted. We distinguish between two different regions, labeled I and II. Region I (II) consists of ions scattered off Ni atoms located mainly on (below) the surface. For T I 700 IS, changes in the spectra are mainly in region I [figs. 5(b) and 5(c)]. The Ni peak is

88

89

90

91 92 Energy (keV)

93

94

Fig. 5. MEIS spectra of Ni (98.0 keV Hf ions, 120” scattering angle) after annealing to temperatures of (a) 300, (b) 625, (c) 690, Cd) 750, (e) 785, (0 825, (g) 925, (h) 975 and (i) 1075 K. Yields in region I (II) correspond to ions scattered from near the surface (below the surface). The increase around 88 keV marks the start of the Si peak.

slightly broadened after annealing (see also fig. 8). This broadening is due to Ni cluster growth upon annealing [lo]. After annealing to T 2 750 K, the Ni yield in region II rises from essentially zero to a finite value. The increase in the yield in region II could be explained by two possible changes in the morphology of the Ni film. One possibility is that Ni forms large, high islands. Ions scattered off Ni atoms near the bottom of the islands would lose more energy (proportional to the depth of scattering) and cause the increase in the yield in region II. The second possibility is that Ni diffuses into the Si substrate. Spectra (d)-(i) all show a non-zero yield in region II extending all the way to the Si edge. For the first possibility (high islands) to account for the yield

73

LB. Zhou et al. / Ni on ultrathin f%ns of SO, on Si(lll)

increase in region II, the islands must haye grown by 750 K to a nominal height of > 100 A. This is estimated by noting that the energy separation between the Si and Ni peaks (120” scattering, E, = 98.0 keV) is 5.2 keV. This corresponds to ,a depth scale in pure Ni (dE/dx = 23.15 eV/A

Random irIddencre

WI) of 5.2 x lo3 d-

dE/dx

G( 6,) IQ - 100 A,

(8)

where cos 6, cos 32

G(%, 82) =

cos 6, + cos 6,

(9)

is a geometrical factor depending on the incident and exit angles 4, (31.4”) and 6, (28.6”). We note that this would require significant island agglomeration, much more than could be accounted for by our XPS results presented elsewhere [lo]. The Ni-Si inter diffusion possibility on the other hand, is more plausible [9-111. In order to determine which process is truly happening, we resort to the ion blocking effect [12]. Ni and Si compounds are known to exist in many thermodynamically stable phases such as Ni,Si,, Ni,Si, Ni,Si,, Nisi and Nisi, [25]. Since we start from a few monolayers of Ni on SiOJSi, it is likely that after diffusing through the SiO, layer, Ni will react with Si to form Nisi,. Further support for the assignment of Nisi, as the high temperature phase comes from our XPS results [lo], SEM data [26], and the non-existence of stable Ni-Si compounds having a higher Si/Ni ratio. The Nisi, structure has a lattice parameter very close to that of Si [7] and is known to grow epitaxially on Si(ll1). In MEIS spectra, blocking along a Si crystallographic direction should be observed for ions scattered off Ni atoms in the Nisi, islands. Fig. 6a shows the Si (li0) scattering plane. Assuming a Ni atom is on or near a Si bulk lattice site (as in a Nisi, island), ions scattered off this Ni atom will be blocked along the [llOl exit direction, and consequently, a blocking dip will be seen in the angular spectra. If such a feature is observed, we can strongly argue for a Ni-Si inter-diffusion process (not surface clustering) since such blocking would only occur for Ni

0

after annealing to 690 K

m afterannealingto75OK

I

I

I

I

I

110

115

120

125

130

7

Angle (degree) Fig. 6. The Si (li0) scattering plane (a) and the Ni angular spectra after annealing to 690 and 750 K (b). Ions were incident in a random direction. Ni atoms on (or near, as in NiSi, domains) Si lattice sites will be blocked along the [llOl direction. The spectra were taken with a 98.0 keV H+ ion beam.

atoms inside the crystalline Si substrate, not for Ni clusters on the surface. In fig. 6b, two angular spectra of Ni are shown. No blocking dip is observed after annealing to 690 K. After annealing to 750 K, however, a blocking dip at 113” has developed, corresponding to blocking along the [llO] direction. The blocking effect increases with annealing temperature. This clearly indicates that Ni has diffused into the Si substrate. The decrease in the overall yield after annealing to 750 K is due to Ni diffusion into the substrate. The observed starting temperature of Ni-Si inter-diffusion is somewhat lower than both our previously reported value [lo] and those found by Liehr et al. [ll]. This is probably due to the thinner SiO, film used in this experiment, since thicker oxides require a higher annealing temperature for Ni penetration through the oxide layer [lO,ll].

74

J.B. Zhou et al. / Ni on ultrathin films of SiOl on Si(llI)

Evidence of Ni-Si inter-diffusion can also be seen from the 0 and Si spectra. The 0 and Si spectra after annealing are shown in fig. 7, and their peak widths are plotted in fig. 8. After annealing to 625 K, the peak widths increase very slightly due to further Ni clustering. They start to decrease after annealing to 690 K. We believe this is the temperature at which Ni starts to diffuse into the SiO, /Si interface. After 750 K, the peak widths further decrease due to Ni-Si inter-diffusion (as also seen in fig. 5). At this stage, the 0 spectra have developed into a shape similar to that of pure SiOJSi. However, the Si spectrum at this stage is still different from that of pure SiO,/Si. An annealing temperature of = 925 K is required for the Si spectrum to reach the same width as that of SiO,/Si. These observations suggest that at 750 K, most of the Ni atoms have diffused through the SiO, layer and reacted with the Si to form nickel silicides. As was mentioned earlier, the Si peak consists of

84

86

88 Energy (keV)

90

92

Fig. 7. MEIS spectra of 0 and Si from Ni/SiO,/Si after annealing to 300,625,690, 750,785,825, 925,975 and 1075 K. Ions (98.0 keV HC) were incident and exited along the [OOi] and [llO] directions, respectively.

I

I

I

I

I

I

1000 400 800 600 Annealing temperature CK) Fig. 8. Peak widths (FWHM) of 0, Si and Ni from Ni/SiO, /Si as a function of annealing temperatures. The two points enclosed in the box are taken from pure SiO,/Si. Ions (98.0 keV H+) were incident and exited along the [OOi] and [llO] directions (90” scattering), respectively, for both the 0 and Si spectra and were at a 120” scattering angle not aligned with any Si crystallographic direction for the Ni spectrum.

ions scattered off the Si atoms from both within the amorphous SiO, film and from a few monolayers of the crystalline Si substrate. When most Ni atoms have penetrated the SiO, layer, they have no effect on the 0 spectra. They do, however, alter the spectra of Si since the Si substrate now contains nickel silicide at or near the SiO,/Si interface. After annealing to 925 K, the effect of Ni on both the 0 and Si spectra is negligibly small. We believe that at 925 K and above, large nickel silicide domains, long in their dimension normal to the substrate surface, have formed (see below), leaving most of the SiOJSi structure unaltered. Finally, in order to assess the diffusion scale and thermal stability of the SiO, film, we have plotted the yield (coverage) for 0, Si and Ni versus annealing temperature in fig. 9. The Ni yield is integrated through an energy range of 4.3 keV. Taking the intrinsic width of the detector function into account, the Ni yield integrated in

J.B. Zhou et al. / Ni on ultrathin fiims ofX0, on Si(ll1)

75

this e_nergy range includes all Ni atoms in the top 135 A of the SiO,/Si surface region. After annealing to 1075 K, the total Ni yield of in this layer has dropped to 1.1 x 10" atoms cmw2 from an initial value of 6.7 x 10" atoms crnm2. At this point, the Ni spectra resemble a step function (see fig. 5). Assuming a constant distribution of Ni atoms along the direction normal to the surface, we can estimate the size of Nisi, domains in the normal direction by a simple consideration of conservation of Ni atoms: 6.7

x

IO=

1.1 x lOiS

x 135 A = 820 A.

Energy&ew

(10) Such long Nisi, domain structures imply that the diffusion through the SiO, and into the Si is very inhomogeneous. Anisotropic growth of nickel silitides has also been reported for bulk Ni/Si interfaces [25]. The 0 and Si yields are constant up to 925 K. They both start to decrease after annealing to 975 K. This is the onset temperature for the decomposition of SiO,, consistent with our results obtained by XPS [lo]. After annealing to 1075 K, the oxygen yield has completely disappeared indicating a complete desorption of the SiO, layer.

I

I

t

t

400

600

800

8

1000

I

Annealing Temperature 0 Fig. 9. Yield (coverage) of 0, Si and Ni (in the top 135 & as a function of annealing temperature. Both the 0 and Si yields start to decrease at 975 K due to decomposition of SiO,. The yield of Ni decreases due to diffusion into the Si substrate.

Fig. 10. MEIS spectra of Ni from Ni/SiO, /Si @led squares) and Ni/Si (open squares) after annealing to 825 K. The spectra were taken with a 98.0 keV H+ ion beam at a 120” scattering angle not aligned with any Si crystallographic direc tions.

The Si yield has dropped to a value indistinguishable from that of a clean Si(ll1) surface.

5. The influence of SiO, on the Ni/Si

infusion

The behavior of Ni films on SiO, is in clear contrast with that of Ni on pure Si substrates. Ni on Si is known to form nearly perfect epitaxial NiSi,/Si structures [4-61. Tung et al. [63 have shown that single-phase, uniform films of Nisi, on Si with a sharp interface can be grown by UHV deposition followed by low temperature annealing CT = 725 K). To compare the Ni/SiO,/Si and Ni/Si systems, fig. 10 shows two spectra of Ni, one from Ni/SiO,/Si, the other from Ni/Si, both after annealing to 825 K. In contrast to Ni/SiO,/Si, the Ni distribution in the Ni/Si system after annealing is limited to the near surface region (I: 20 A>. The reason why the presence of a thin SiO, film can cause such a big increase in the depth distribution of Ni after annealing is not clear. We believe that the diffusion of Ni through SiO, is mainly through pin-hole defects. If the defect concentration is quite low, then this could result in a Ni depth distribution for the

76

LB. Zhou et al. / Ni on ultrathin ftlms of SiO, on Si(lll)

Ni/SiO,/Si system much larger than what occurs for the Ni/Si system. The presence of a thin SiO, film might also alter the diffusion process through surface stress, field effects or surface and interface energetics.

6. Summary In conclusion, we have studied the morphology of Ni films on SiO,/Si with MEIS for a Ni coverage of N 6.7 x 10” atoms cm-*. We found that the as-deposited Ni forms islands on SiO,/Si at 300 K. This island structure has been characterized with two models representing two extreme possibilities. In one model, the spread in island thickness is characterized by a modified Gaussian distribution function with an average thickness of 14, and a “standard deviation” of 7.1 A. The second model assumes a spherical cap shape for the islands with an0 average size of r = 38.8 and height h = 22.0 A (contact angle 6 = 59”). Upon low temperature annealing CT < 725 K), Ni atoms further cluster on the surface. Above 725 K, Ni starts to diffuse through the SiO, layer and react with Si to form nickel silitides. The extent of the reaction increases with annealing temperature. The silicide formed on this surface is laterally inhomogeneous. After annealing to 1075 K, a reaction with Si has occurred which displaces some of the Ni atoms to at least 800 A below the surface. The observed inhomogeneous diffusion of Ni is in clear contrast to Ni on pure Si where the diffusion is limited to the very near surface region, forming a uniform, planar NiSi,/Si structure. The SiO, layer is stable up to 925 and is completely desorbed on annealing to 1075 K.

Acknowledgements

The authors acknowledge partial support of this work by the National Science Foundation Materials Research Group Program (no. DMR

89-07553). We also thank Mr. Jeff Mayer for help

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