High resolution depth profiling of nitrogen in A1N layers

Nuclear Instruments and Methods in Physics Research B66 (1992) 262-266 North-Holland

NuclearInstrnnnents & Methods in PhysicsResearch Section 8

High resolution depth profiling of nitrogen in AIN layers G. Terwagne, S . Lucas and F. Bodart Laboratoire d Analyses par Réactions Nucléaires (LARN) and Institute for Studies in Interfaces Sciences (ISIS), Facultés Universitaires Notre-Dame de la Paix, 22, rue Muzet, B-5000 Namur, Belgium

Nitrogen implantation into aluminum offers many industrial applications. It has already been observed that the shape and the thickness of the layer formed depend strongly on the implantation doses for a given energy. The aim of this work is to investigate the atomic movements of nitrogen during implantation which leads to the formation of the aluminum nitride layer . Resonant nuclear reaction analysis (RNRA) allows for the profiling of both stable nitrogen isotopes ('SN and 14N) by means of the ' 5N(p,ay)' Z C reaction at 429 keV and the ' 4N(a,y)'8F reaction at 1531 keV. Therefore, 100 keV' 4Nz and 100 keV 's'NZ were alternatively implanted into aluminum, and the behavior of each isotope was followed using the appropriate nuclear ruction. The results indic..te that some of the nitrogen atoms already present in the matrix are displaced and moved out the sample by the incident nitrogen atoms during the implantation . Consequently, the distribution of the original isotope is broadened due to the collisional processes occurring when nitrogen doses increase .

1 . Introduction Over the last decase, ion implantation has become a powerful tool for the modification of surface properties of materials . It is used extensively in tribology and in the particular case of nitrogen implantation into aluminum or iron, several interesting tribological modifications have been shown including hardness increase and friction reduction. In parallel with the studies of mechanical properties performed by other teams [1-3], we started, at the LARN, to study the dependence of implantation parameters (dose, temperature, flux, etc .) on the nitrogen depth profiles using resonant nuclear reactions. The use of nuclear techniques has several advantages over more conventional ones such as SIMS, Auger, etc . : it is non destructive, highly quantitative and very fast. These advantages have been successfully demonstrated in the discovery of a nitrogen surface peak [4,5] . To get nitrogen depth profiles, two nuclear reactions are used: '4N(a,y)18F, "N(p,aY)'2C . One particular feature of these nuclear reactions is the separate isotope measurements that can be done on nitrogen 'SN and 14 N. This is very useful for studying the atomic movements of nitrogen implanted into material . I4N (or 'IN) can be initially implanted, followed by implantation of 'IN (or 14N) in the same matrix . Using the above nuclear reactions, it is possible to follow the redistribution of the r4N (or 'IN) due to ballastic process.

The aim of this paper is to present our investigation of nitrogen atomic movements in aluminum when it is implanted to high doses . The first part concerns the description of the characteristics of the nuclear reactions on '4N and 'SN and the particular procedure to extract the actual depth profile . In the second part, we discuss the results of experiments carried out with I successive implantations of "N and ' N into aluminum. This experimental procedure has been used to study the formation of a buried aluminum nitride layer formed by ion implantation. 2. Nitrogen depth profile technique

r4N and 'SN distributions can be measured selectively using the two resonant nuclear reactions: r4 N (a,y)' 8 F and ' sN(p,ay)' Z C. The capture reaction '4N(a,y)raF presents a narrow resonance at Ea = 1531 keV. The natural width of this resonance rR is 0 .6 keV [6] . The emitted -y-rays have an energy between 3 .5 and 5 .3 MeV. The next resonance occurs at 1618 keV leaving a free region of bombarding energy of about 87 keV for profiling. The 'IN distribution is measured using the wellknown 'SN(p,ay)' Z C resonant nuclear reaction which occurs at EP = 429 keV. This very intense resonance is isolated and has a natural width of 120 eV [7]. Among the emitted particles, we detect the 4.43 MeV -y-rays of 12C. The nearest resonance appears at 898 keV, which

0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V . All rights reserved

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G. Terwagne et al. / High resolution depth profiling ofNin AIN layers

provides a large region of bombarding energy . In AIN matrix, we can profile an overall depth of a few Rm under the surface . To obtain an excitation curve at high resolution, we use an automatic energy scan system installed on our 2.5 MeV Van de Graaff accelerator [8]. The incident particle energy is varied by small increments from the resonance energy to a higher value correlated to the depth of the implanted layer. In our system, the energy change is performed by a transverse electric field applied along the beam line and each step corresponds to a correction of the accelerator voltage of 150 eV for the proton beam at 429 keV, and of 1.5 keV for the (x-particle at 1531 keV. For both nuclear reactions, -y-rays are detected in a 4 x 4 inches Nal well detector. A full description of the system can be found in ref. [9] . The depth resolution achieved for each nuclear reaction depends on the natural width of the resonance (TR), the energy spread of the beam (TB) and the straggling effect (Ts) due to unequal energy loss of identical incident particles. The latter can be estimated by a gaussian distribution at a depth greater than 400 nm for pure aluminum, and asymmetric distributions just below the surface which can be described by the Vavilov functions [10,11] . The overall effect of these three phenomena is to limit the depth resolution as observed in fig. 1. To compute the curves, we have chosen an energy spread of 400 and 1500 eV for the incident beam of our machine for proton and a-particles, respectively . It is clearly shown in fig. 1 that the depth resolution for the 14 N(a'_y) 1s F reaction is twice as good as for the other reaction. As the measured excitation curve is the convolution of the actual depth profile, a function including the energy distribution of the beam (TB +l's) and the

Ec c

0 ôN rr

Fig. 1 . Calculated depth resolution of the two nuclear reac tions on 14 N and 15 N prior to the deconvolution for an aluminum and an aluminum nitride sample .

width of the nuclear reaction (rR), a deconvolution procedure is necessary to obtain this actual concentration distribution. This deconvolution can be done with the help of a deconvolution program which uses a procedure developed by Deconninck et al . [12] . In addition, the depth profile is quantified by comparison with a TiN standard with a statistical accuracy on the concentration around 5% . The final accuracy on the depth profile expressed in at ./cm2 is better than 1.5%. In the situation where we have to quantify the total amount of a species present in the matrix, we do not necessarily need to deconvolute the excitation curves. This amount can be roughly estimated by the following relation : N=

A,.

S

1

* S, .( E )

n AE F, N~, 

,=r where N is the number of atoms (in 10 17 at ./cm'), fs,, is the atomic fraction of the measured isotope in the standard, Sst(E*) is the standard stopping power in eV/(10 15 at ./cm') at E*, AE is the energy change at each step of the profile, E"_I N, is the total number of detected -y-rays in the profile (n channels), and E* is the arithmetic mean energy over the whole energy scan. One disadvantage of the 14 N(a,Y)18 F reaction is the low cross section for the a capture reaction . To obtain an accurate profile over 2 x 101s at ./cm2, high beam currents (=500 nA) and long exposure times (=2 h) are required. In this case, the accumulation of a-particles (=5 x 10 1 He +/cm2) into the sample can lead to the formation of blisters and radiation damages and as has been previously observed in the study of nitrogen implantation into iron [9]. However, on the opposite side, the 15 N(p,ay)12 C reaction has a high cross section which allows the measurement of a 15N distribution on 200 nm in less than 10 min with a proton beam of 150 nA . Therefore, the influence of 1° N implantation will be preferentially studied by the measurement of the additional amount of 15N. s,.

3. Study of the atomic movements of nitrogen implanted into aluminum In previous work, we have shown that nitrogen implantation into aluminum leads to the formation of a nitride layer which is broadened when the implantation dose is very high [13,14]. It is possible to understand the mechanisms involved with the help of the isotopic selectivity of nuclear resonant reactions on 'IN and 15N. To do this, we studied the migration of isotopic nitrogen atoms freshly implanted in a layer contaminated with nitrogen of the other isotope. 111. CONTRIBUTED PAPERS

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G. Terwagne et al. / High resolution depth profiling ofN in AIN layers 10 .--rr-,Trt-

3.1. Sample preparation Thick aluminum polycrystalline discs (15 mm diameter, 2 mm thick, 99 .5% purity) were mechanically polished down to a grain size of 0 .5 pm, and implanted with molecular 14N2' or ' S Nz at 100 keV. We have shown previously that the depth distributions of the 14N or 15N are the same within the experimental errors [91. For the rest of the paper, we shall define a sample preimplanted with A and postimplanted with B as a sample which is first bombarded with A ions, and afterwards implanted with B ions. In order to understand the evolution of the depth distributions during the implantation of nitrogen into aluminum, we have prepared two sets of specimens. The first one consists of a pre-implantation of '4 N to doses from 3.6 X 10' 7 to 11 .4 X 10'7 14 N + /cm 2 followed by postbombardment of 15 N at a dose of 1 .1 X 1017 15 N'/cm 2. The second one consists of one implantation of 'IN at a dose of 3 X 10" ions/cmZ , followed by postbombardment with 14N to doses from 1 .1 X 10' 7 to 11 X 10' 7 ions/cmZ by steps of 1 .1 X 10" ions/cm 2. During all the implantation procedures, the temperature was monitored and maintained at 30°C and the residual vacuum was better than 2 X 10 -6 Torr . The current density was about 10 p,A/cm 2. 3.2. Depth distribution of the freshly implanted nitrogen Fig . 2 shows the 15N depth profile for the aluminum specimens preimplanted with different 14N doses (between 3 .6 X 10' 7 and 11 .4 X 1017 14N/CM2), and one

0

ô cd

U o0 U z n

0

500

1000

1500

2000

Depth (1015 at./cmZ) Fig. 2. Nitrogen depth profiles of 15N (1 .1 X 10'"5 N+/cmZ) implanted into different aluminum matrices which have been pre-implanted to different doses of 14N . The labels on the curves are the .4N dose expressed in 1017 at ./cm2.

a

rr

r

6 4

c

0. . 0

.~ . 5

.~ . 10

15

Total dose (1017at ./cm Z) Fig . 3 . Evolution of the projected range (R P), standard deviation (ARp), and retained dose of an 1 .1 x 10'7 ' 5N/cmZ implantation into different aluminum samples containing '4 N . The units of the vertical scale see given in 10 17 at ./c.2. The total dose is the sum of .4N and '5 N. single implantation of 15N (1 .1 X 10' 7 15 N/cm 2 ). The shape of all depth distributions is nearly gaussian and we can calculate for those profiles : the projected range, the standard deviation of the distribution and the retained dose of 'IN (using eq. (1)). îhe results are presented in fig . 3 . The retained dose versus the total implanted dose is constant (fig. 3c), meaning that all nitrogen post-implanted remains in the matrix whatever the dose of nitrogen already present. The projected range Rp (fig. 3a) and the width AR P (fig . 3b) of the distributions are also constant when the total dose is increased. Moreover, the mean projected range RP (8 .3 X 10" at./cm 2 ) and the width of the distribution ARP (2 .8 X 10" at./cm2 ) are close to the data calculated with the TRIDYN code [151 for an aluminum nitride matrix implanted with 50 keV atomic 15N ions, i .e. RP = 8.4 X 10' 7 at./CM2 and AR P = 2.7 X 10' 7 at./cm 2 . To prove the reliability of the profiling technique of 14N and ' I N, we have measured successively the depth profile of 14N and ' 5 N on an aluminum sample preimplanted with 8.9 X 10' 7 14 N/cm 2 and postbombarded with 1 .1 X 10' 7 15 N/cm2 (fig . 4). The measurements were performed after the implantation of both isotopes . We can observe that the 14N concentration (triangles) does not exceed 40% and remains flat over the implanted layer, while the 'IN distribution (crosses) in fig . 4 is nearly Gaussian as discussed above. Previous experiments have shown that the local nitrogen concentration of 50% is always reached when

G. Terwagne et al. /Nigh resolution depth profiling of N in AINlayers

26 5

ions/C .2 by steps of 1.1 X 10 17 14 N/cm 2 and the depth profile of 15N was measured at each step . 14N At low doses of (lower than 3.3 X 10 17 ions/cm2), the depth profile of 15N is broadened and 14N for higher doses, the depth distribution of initially implanted nitrogen is split into two components . The first component "migrates" to the surface of the sam-

ple, whereas the second component penetrates deeper and deeper into the bulk. We can observe also in fig. 5 that the deeper component is larger than the one at the surface .

4. Discussion and conclusion Fig. 4. Nitrogen depth profiles obtained using the two nuclear reactions, 14N(a,i')18F and 15 N(p,aÏ) 12 C. (+) and (o) sample pre-implanted with 14N and post-implanted with 'IN; ( ) algebraic sum of the 14N and 15 N distributions; (o) nitrogen depth profile of a sample implanted with the same dose of 15N than for the full circle profile .

We have used the isotopic selectivity of resonant 14N 15N nuclear reactions on and to follow the atomic movements which take place when aluminum is bombarded with nitrogen . An extensive description of the mechanism taking

part in the formation of the buried nitride layer has already been reported elsewhere [16]. The results presented in fig. 5 suggest that when an incident nitrogen the incident dose is greater than 3 X 10 17 ions/cm2 14N [13]. The nitrogen distribution observed for can be explained by a collisional process. The 14N atoms are 15N displaced by the incoming I atoms which leads to a decrease of the maximum ' N concentration and a 14N broadening of the depth profile. The total amount of nitrogen present in this sample is the arithmetic sum 14N of the and 15N distribution (full line in fig . 4) . We 1-IN can compare this calculated depth profile with a distribution measured on a different sample, implanted 15N 15 with only at a dose equal to 10 18 N/cm 2, the sum of both implantations with 14N and 15N in the same

ion collides with nitrogen atoms bounded with aluminum, some of them move due to collisional processes. When they are free to move, they diffuse in the Al and AIN phases according to their diffusion coefficients. Therefore, at a depth where the number of collisional events is maximum, we expect a lower concentration for the preimplanted nitrogen distribution as observed in fig . 5. We conclude that the mechanisms

taking place during broadening of the nitrogen distribution are purely ballistic and are followed by a diffu-

conditions (filled circles in fig. 4). Both depth profiles are the same within the experimental errors (10%). 3.3. Evolution of the depth distribution of nitrogen firstly implanted

C

By inverting the sequence of isotopic nitrogen implantations (preimplantation with 15N followed by 14N postbombardment with in the same conditions), we have measured the evolution of the depth profile of 14N 'IN when more and more is implanted in the specimen and therefore the evolution of the depth 15N distribution of which was firstly implanted. The results of such experiments are presented in fig. 5. The 15N aluminum sample has been implanted first with at a dose of 3 X 10 17 ions/cm' . The depth profile, shown

c 0

ô

in the bottom of fig. 5, is nearly Gaussian and corresponds to the results obtained previously [13]. After-

wards, the same sample was postbombarded with different doses of 14 N between 1.1 X 10 1 and 11 X 10 17

ô

Û ô

U

0

500 1000 1500 2000 2500 Depth (1015 at ./cm 2)

Fig. 5. Evolution of a pre-implantation of 3.3 X 10 17 15 N/cm2 into aluminum . The sample was post-implanted step by step with 14 N ions. The labels on the curves are the 14N doses expressed in 10 17 ions/cm 2. III . CONTRIBUTED PAPERS

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G. Terwagne et al. / High resolution depth profiling of N in AIN layers

sion of the delocalized atoms in either the AI or AIN phase. Acknowledgements The present research was supported by the Interuniversity Research Center (PAD program of the Belgian Ministry of Science Policy . Special thanks are due to Y. Morciaux and J . Nackers for their technical support. References [Il G . Dearnaley, Mater . Sei . Eng. 69 (1985) 139. [2] B. Doyle, D!M. Follstaedt and S .T. Picraux, Nuci. Instr. and Meth . B7/8 (1985) 166. [31 B. Rauschenback, Nucl. Instr . and Meth. B15 (1986) 756. [4] N . Moncoffre, G. Marest, S . Hiadsi and J. Tousset, Nucl. Instr. and Meth . B15 (1986) 620. [5] G . Terwagne, M. Piette, P. Bertrand and F. Bodart, Mater. Sci . Eng . B2 (1989) 195 .

[6] C.R. Gossett, Nucl. Instr. and Meth. BIO/Bll (1985) 722. [7] B. Maurel and G. Amsel, Nucl . Instr. and Meth. 218 (1983) 159. [81 G. Amsel, F. D'Artemaere and F . Girard, Nucl. Instr . and Meth. 205 (1983) 5 . [9] G. Terwagne, M. Piette and F . Bodart, Nucl . Instr . and Meth. 1319/20 (1987) 145. [10] G. Deconninck, Introduction to Radioanalytical Physics, Akad6miai Kiado, Budapest (Elsevier, Amsterdam, 1978). [111 P.V. Vavilov, J . Exp. Theor. Phys . 5 (1957) 749. [121 G. Deconninck and B. Van Oystaeycn, Nucl . Instr. and Meth . 218 (1983) 165. [131 S. Lucas, G. Terwagne and F. Bodart, Nucl . Instr. and Meth . B50 (1990) 401. [14] S. Lucas, G. Terwagne and F. Bodart, Nucl . Instr. and Meth. B59/60 (1991) 925 . [151 W . MbIler and E . Eckstein, Nucl . Instr. and Meth. B2 (1984) 814. [161 G. Terwagne and S . Lucas, Proc. SM 2 1B Conf, Washington, 1991, accepted for publication in Surf . Coat . Technol.