Billah, M M

Islam, S M A

Zaman, MA

IACS Institutional Repository

Indian J. Phys. «8A (1), 77 -  83 (1994)U P A-aBimBnwitomljwwMiNuclear reaction analysis as a technit|ue of measurement of some oxide layer thicknessesS M A Islam, M A Zlaman and M M BillahDepartment of Physics, Jahangimagar University, Savar, BangladeshandLGEarwakerDepartment of Physics, University of Birmingham, U K Received 21 October 1992, accepted i 9 July 1993Abstract : Nuclclaj reaction aiialyvSis (NRA) has been carried out through (d,p) reaction for evaluating oxide layer thicknesses with 1 M%V deuterons. Oxides of silicon and tantalum of layer thicknesses up to 13CXX) A were investigated using integrated charge and beam current nespectively 30 |iiC and 100 nA. The technique is found to be excellent for relative measurements of layer thicknesses A calibration curve for future measurements of thickness of thin films, of silicon and tantalum oxides is provided. The results so obtained are then compared with those of Rutherford backscaltcring spectrometry (RBS) technique.Keywords : Measurements of oxide layer ihickncvss, appheation of nuclear reaction analysis, surface barrier detector, (d.p) reactionPACS Nos. ; 24.50,+g. 29.40.-n, 25.40.-h1. IntroductionOver more than a decade  ^charged partit l^e nuclear reactions and scattering methods have ix;comc increasingly popular in depth profiling of different nuclides in various materials* It is an established fact that the composition and thickness of oxides and nitride films on bulk elemental surface can be evaluated using NRA technique [1-6]. The technique has been developed for near surface analysis during the past few decades [1,7-14]. Experimentally, a light ion beam taken from a particle accelerator is incident on the surface of a target resulting in nuclear reactions of different types depending on the nature and energy of the ions and the nature of the target. Identification and depth profiling of the elements making up the target can he made. Using NRA t^hnique concentration profiles over a few microns of the surface with resolution of the order of 1000 A can be attained where depth resolution attainable is betterthan 200 A [3,6], ^   ^ '^©1994rIACSh8A (^l)U‘Side by side, the appJication of RBS for near surface analysis has been developed during the past three decades. It is mainly applied in the semiconductor field and has been extensively applied in investigating silicon and its compounds. The method has taken a good place besides other modern surface analytical techniques such as secondary ion mass spectrometry (SIMS), auger electron spectrometry (AES) and X-ray fluorescence (XRF).In this work, (d,p) reaction has been used in evaluating the layer thickness of some oxide samples. The reaction is favoured because of its general applicability and also since it provides a useful rnethexi of analysis of reactions involving all the major isotopes of elements having Z = 4 to Z = 17 which have positive Q values of the order of few MeV [15] thus making multiclemcntal analysis possible. The results obtained from the (d,p) reaction analysis are then compared with those obtained through RBS technique on the same samples.7H S M A Islam, M A Zaman, M M Billah and L G Earwaker2. PrinciplesIf we consider a thin^and single element target on which a beam of ions with energy sui to overcome the Coulomb barrier is allowed to incident (Figure la) then some of theseblemsF11iL i_ h_Proton EnergyTorget tNot to scale)SubloverIncident deuteron Beam ( E^)Ic)\ \ __ Foil usedin ?)«*?»«V^^urfoce barrier detector >Thin meleneM window to minimise X^ roy absorption Twelve position target cassetteRcmoveable beam collimator and electron screen assem blySi surface barrier holder and collimofor(b) ( d )Figure 1. (a) Mechanism of a nuclear reaction, (b) The yield/energy spectrum of emergent protons at the detector, (c) Po.sition of the dclecior with respect to the incident deuteron beam, (d) Schematic diagram of general purpose nuclear analy.si.s target chamber.undergo nuclear reactions with the target atoms. The energy of the light particles emitted from the reaction can easily be evaluated from the standard kinematic considerations (161.Nuclear reaction analysis as a technique of measurement etc 79The yicld/cnergy spectrum of emergent protons that results when a large number of deutcrons are incident on a thin target is an shown in Figure lb. Any particular incident jon- target nucleus interaction can give rise to a scries of peaks due to the different possible excited states of the daughter nucleus (Figure lb). A calibration curve between emitted particle energy and peak channel number is sketched to identify unknown peaks. The usual way to reveal the depth profiles from the spectrum is to create a depth scale from stopping power tables or from the parameterized slopping powers including the effect of any detector foil.3 Experimentalllic ’^’0(d,p) reaction has been used to evaluate the oxide layer thicknesses of the samples cited in the Table I. The experiment was carried out using the 3 MV Dynamilron acceleratorTable 1. List of the samplesSI.no.Name of the sourceComposition Quoted layer thickness (A)Inieciatcd urea under ’^ 0(d,pi^)*^0 peakC’akulated layer thickne.s.s fioin RBS1 Plc.ssey Lid Si()2 IfXK) 499 ± 2« 1040 ±602 PIc.s.sey Ltd Si02 I3.S04 .57.-S0±8I 135(X)± 1803 GEC Si02 647 323 ± 22 640 ±604 GEC Si02 1030 514 ±29 1040 ±60S GEC S1O2 2590 1294 ±37 26(X)± 120h GEC Si02 41K)7 20.S0±49 4020± 1807 Chalk River Ta20.s 707 353 ±22 710±40K Chalk River T3205 1741 870 ±31 1760 ±409 Chalk River Ta2C)5 5435 27I6±5! .5470 ±80\\] ihe samples are tnadc of oxiHi* i.'iv^ i* rat Bmmngham Radiation Center, University of Birmingham. The geometries investigated to compare the depth resolution attainable using the *^0(d,p)*^0 reaction is similar to that described previously by Simpson et al [17]. Only the angle 135° has been considered in this work (Figure Ic).Among the samples used in the work two oxide samples known to be Si02 were supplied by Plessey Ltd and four oxide s^amples all known to be the same composition were provided by GEC Research Laboratories (Hirst Research Centre). Three other oxide samples known to be Ta205 were obtained from Chalk River, Canada. All the samples possess their quoted oxide layer thicknesses as mentioned in the fourth column ol Table I. The Ta^Os films were developed on tantalum and Si02 on silicon. 1 MeV deuleron beam was allowed to uiipinge on the samples mounted on the target holder (cassette) at 30° ajiart from each other carefully in such a way that the incoming beam strikes these sides of the samples normally (Figure Id). A surface barrier detector (S 199) with depletion layer 700 p.m was placed at them SMA tslamiMA Zaman, MMBUlah andLG EarwakersaidangJe e.g. 135^  ^to the incident beam direction. Beam current used was 100 nA and iniegraled charge was 30 A foil of thickness 25 was used to stoptbe alpha particles which also come out of this reaction. The beam collimation and detector bias used were respectively I mm and 2(X) volts. The main amplifier gains were as follows :Course gain = 200 Fine gain * 0.9 Shaping time = 0.25 jisecThe beam tube and thc target chamber were evacuated by vacuum pumps to I O’* Torr. The targets (samples) were slriken by the incident beam of deuterons one after another normal to the plane of the target surface. Signals from the detector were fed by short cables to an Ortec 125 pre-amplifier and than to an Oriec 472A main amplifier by a long cable at coupling station. The resulting pulses were passed in turn to a Hewlet Packard (HP) 5416B ADCt and 2100 HP Computer Multichannel Analyzer for the spectrum storage and display. ]Thc spectrum obtained was stored on the disk and then copied to the magnetic tape for analysis!For (d,p) reaction, generally, an absorber foil thick enough to stop elastically scattered deuterons from the target but thin enough to allow the expected reaction product (protons) to pass through is placed over the detector. This reduces the pile up effect and hence increases the sensitivity of the technique.4 . Results and discussionFigures 2 and 3 show two of the yiclcl/cnergy spectra obtained in this experimeat. The highest energy group e.g. the '^O(d,po)’^ 0 arising from the '^0(d,p) reaction has been consideredFigure 2. Energy spectrum of the (d.p) reaction for I MeV deutcrort on S1O2 (GEC) sample hoving quoted layer thickness 2S90 A.only. Other groups were contaminated by Si and Ta groups and ht^nct were avoided. Integrated area of the (d.pj peak of each spectrum has been calculated. Figure 4a is a plotshowing the vartoion of tht integrated areas with resjpect to the quoted oxide layer thickness of the samples. In the work the cross section was supposed not to very appreciably over theNuclear reaciion analysis as a technique ofn^asuremenr etc 81Emitted particle energy (MeV)Figure 3. Energy spectrum of the (d,p>reaction for I MeV deuteron on Ta205  (Chalk River) sample having quoted layer thickness 1741 AEnpecled (Quoted) Oxide Layer Thickness ( A )Expected (Quoted) Loyer Thickrtess (A)Figure 4. (a) Plot of expected layer thickness v e m is  integrated oma under ^^0(d.p(^ peak lor I MeV deuteron. (b) Plot of calculated layer thickness versus  expected layer thickness for RBS analysis.range of energy loss, of the incident particles in the layer being investigated. Hence the total reaction yield gives a measure of the number of the atoms present and the energy distribution gives a measum of the dqsth profile.82 S M A Islam, M A Zaman, M M  Billah an d  L G E arw akerThe variation shows a straight line passing through the origin. The peak areas depend on charge integration, cross section for this reaction, geometry, experimental conditions etc. The cross section is not well known for this reaction. Hence we can not get absolute thicknesses using this curve, rather only the relative values of thickness may be obtained.Column 6 of the Table I shows the calculated layer thickness extracted from RBS experiment on the same oxide samples measured in a previous study [18] by us. The corresponding graph showing the viiriation of calculated layer thicknesses with respect to the quoted layer thicknesses is shown in Figure 4b. The graph shows that an absolute measurement of oxide layer thickness is possible using RBS technique because the curve passing through the origin maintains a slope I while showing variation of calculated layer thickness with respect to the quoted layer thicknesses. However for a much thicker layer RBS technique would face difficulty m extraction of depth profiles, where (d,p) reaction analysis method may still be useful. \It seems worthwhile to make a comment here about suitability of NRA as compared t(i other techniques c.g. SIMS, AUGER etc. NRA is a non destructive technique. It has the additional advantage that the analysis lime is usually quite short, of the order of 10 minutes per sample. NRA technique can he used as a very useful alternative to SIMS and AUGER which involve sputtering to provide depth information. NRA is thus a better method than SIMS and AUGER at least in one aspect /.e ., it is free from artifacts inherent in methods involving sputtering.5. Conclusions *The '^0(d,p) reaction has been used lor relative depth measurement on oxide layers. NRA technique using ‘^ '0(d,p)’"0  reaction cannot be employed for absolute depth measurements, where as RBS techniques can be employed for such a purpose. On the other hand NRA technique can be employed for a thicker layer where RBS technique would fail. A few additional advantage of NRA is that it is an non-desiruciivc technique and allows quite a short lime analysis.We have not got reliable data above 13000 A for oxide samples. It is felt that such data and also oxides of materials other than silicon and tantalum would be useful for added check.Refers lu’t'.s|l]  M Bern amf A V Drigo 1982 Nud Jnstrum Meth 201 473[2] G Wcbl^er and L Qiiaglia 1972 y Hadxumul Chem, 12\^\ J A Border'  ^and J M Hams 1978 Nurl Insnum Meth 149 279|4] Y Tamminga, M K C Willcinscn. F H P M Habrakcn and A E T Kuipcr 1982 N iul Instnnn. Mvtii.200 499j.S) E Kotai. G Mc/c>, T Lohner. A Manuaba. F Pas/ii and J Gyulai 1981 A/n7. Melh 180 6 19|6J S T Piciuu.x 1978 /Vwf/ /mtnnn. Meth 149 289(7] 1) W Palmer 1978 5/// S\inf}o\mtn nn Amil\ ws (Oxford)Nuclear reaction analysis as a technique of measunnmt etc 83[8J K Heyl and B Webber 1982 N u cl. in siru m . M eth. 199 387[9) P W Kcaitm, P S Pfceicy. B L Doyle and C J Maggiore 1980 N ucl. Insirum . M eili. 1(8 187[lOJ J A Cookson J982 N ucL Instrum. M e tk  197 255[11] JR Bird 1980 N u cL Instrum . M e tk  168 85[12] E A Schwcikcrt 1981 NucL Instrum. Meth. 195 65[13] I V Mitchell, K M Baifoot and H L, Eschbach 1980 Mud Instrum. Meth. 168 233[14] FShiraishiand YTakami 1982 Nucl. Instrum. Meth. 196 137[15] Ajzcnberg-Sclove 1981 NucL Phys. A360 I[16] J W Mayer and E Rimini (cds) 1977 Ion Beam Handbook Jor Materials Analysis (New York Academic) p 8[17] J C B Simpson. J M C Groves and L G Earwaker 1985 Nucl Instrum. Meth. B9 321[18] S M A Islam, M M Billah, M A Malek and L G Earwaker 1991 7 Banffladesh Acad Sci 15 5

Nuclear reaction Analysis as a Technique of Measurement of some Oxide Layer Thicknesses

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Nuclear reaction Analysis as a Technique of Measurement of some Oxide Layer Thicknesses

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