- Email: [email protected]
Effect of surface topography on scanning RBS microbeam measurements
\
Vacuum/volume 50/number 3–4/pages 503 to 506/1998 © 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042–207X/98 $19.00+.00
Pergamon
PII : S0042–207X(98)00090–6
Effect of surface topography on scanning RBS microbeam measurements A Simon,a* F Pa´szti,b I Uzonyi,a A Manuabab and A´ Z Kiss,a aInstitute of Nuclear Research of the Hungarian Academy of Sciences, H-4001 Debrecen, P.O. Box 51, Hungary ; bKFKI-Research Institute for Particle and Nuclear Physics, H-1525 Budapest, P.O. Box 49, Hungary received for publication 20 January 1998
Scanning Rutherford Backscattering Spectrometry (RBS) microprobe in the standard large angle backscattering geometry is not sensitive for the surface topography of samples consisting of homogeneous material. Whereas moving the detector into grazing exit angle the surface irregularities will be enhanced since surface elevations absorb the backscattered ions casting shadows on RBS mapping images ; slopes and edges facing the detector emit ions with decreased energy loss increasing in this way the yield for those positions. In such a way the scanning RBS microprobe will be capable of studying surface topography. Examples for test Si samples will be presented and observed effects are discussed. The method can also be used for the determination of the depth of ditches. © 1998 Elsevier Science Ltd. All rights reserved
0[ Introduction 0
1
Nuclear microprobes are able to scan ½ 0 mm dimension sur! faces with proton\ helium ion or other beams focused to a few microns and successfully have used in measuring 1 dimensional "1D\ or lateral# elemental distributions\ e[g[ applying proton induced X!ray emission "PIXE#[ The combination of PIXE with traditional Rutherford!backscattering "RBS# technique can pro! vide 2D distributions for the investigated sample1 where the depth as the third dimension is characterised by the energy loss during the incoming and outgoing paths of the scattered ions[ Here the ions enter the sample perpendicular to the surface and due to the nearly 079> scattering angle\ the incident and backscattered ions follow almost the same trajectory but in opposite directions[ Since the surface of the samples generally are not ~at "they have elevations\ depressions\ craters\ etc[# and this arrangement is not sensitive for such details\ it gives the 2D structure with a peculiar distortion\ namely although the depth is still determined from the point where the incident ions _rst reach the sample the true surface topography is completely ignored[ The e}ect of surface topography on RBS measurements is frequently investigated from the beginning of the technique\ but just from the point of view that how these e}ects might distort the obtained results and how can they be avoided[2\3 It is chal! lenging to improve the RBS microprobe in such a way that it should provide together the distorted 2D distribution with the details of the surface topology as well[ As we will demonstrate in this paper\ the task can be ful_lled without applying additional
Corresponding author[
methods or detectors simply by moving the detector into a graz! ing exit angle[ In this case the energy loss of ions along the incoming path is almost negligible in comparison with that along the outgoing path\ which is predominantly determined by the surface topography[ Furthermore\ by using two detectors put on the left and right side of the incident beam\ one may directly separate the e}ects of surface topography from those of other origin "e[g[ that are related to changes in material composition#[ While there are some trials to evaluate the surface shape from the distortions of the energy spectra in conventional RBS\4Ð00 to our knowledge this is the _rst attempt to intentionally enhance these e}ects in improving the performance of the RBS mic! roprobe technique[ 1[ Experimental The nuclear microprobe facility in ATOMKI\ Debrecen01 was applied for the measurements with use of 3He¦ beam[ The sample chamber described in detail in Ref[ ð00Ł was complemented with two movable surface barrier detectors "ORTEC# of 04 keV nom! inal resolution[ The detectors were collimated down to 7 msr by 1 mm wide slits at a distance of 34 mm and their position "the scattering angle# was adjustable with a precision of ½ 9[4> in the 099>Ð069> range without breaking the vacuum[ For the present test measurements secondary electron images were also taken simultaneously using a Philips type single channel electron mul! tiplier[ The layout of the sample chamber is shown in Fig[ 0[ Test samples of special structures consisting of only compact Si were prepared02 in the technological laboratory of KFKI! Research Institute of Materials Science\ Budapest in the fol! 503
A Simon et al : Effects on RBS microbeam measurements
Figure 0[ Layout of the sample chamber equipped with two movable surface barrier detectors[
lowing way ] p!type Si wafers were masked before doping with phosphorus to convert the _rst 6 mm layer into n!type Si[ After! wards they were subjected to anodisation in ethanolÐwaterÐHF solution\ where the _rst 049 mm or 69mm "depending on the sample# of the p!type areas transformed into porous Si[ The n! type layer and the area under it remained compact[ Around the borders of p!type regions the formed porous Si extended several mm over the borders leaving behind an n!type compact sheet at the surface[ The applied test samples were obtained after etching away the porous Si in KOH solution[ RBS mappings were taken either by 1[9 MeV or 1[4 MeV 3He¦ beam of 799 pA beam current and 2×2 mm1 beam size at normal incidence and detected simultaneously by the two detectors men! tioned above set left and right of the incident beam both at 009> scattering angle[ The data acquisition system was running under WINDOWS 84 and the signals were collected in listing mode enabling us to produce di}erent kinds of mapping images\ line! and selected!area scans o}!line[ 2[ Results and discussion The RBS mapping images of the _rst sample taken by the two detectors simultaneously are shown in Fig[ 1[ The brightness of the image represents the count number of the scattered ions collected at that position "the brighter the image\ the higher the count number#[ This sample has two rectangular ditches perpendicular to each other "in a cross!shape# containing a bridge in the middle and the horizontal one is opened to the left into a larger cavity and is closed on the opposite side[ The vertical one turns to the right at the bottom of the picture[ Considering the horizontal ditch\ its di}erent endings "opened and closed# causes a di}erence in the two RBS images[ There is no shadow "dark! ness# in the left image horizontally along the bridge while a de_nite shadow appears in the right one[ At the bottom of the vertical ditch an opposite e}ect is seen[ Considering only the right side detector the depth of the hori! zontal ditch can be determined in the following way ] the majority of the incident ions could not scatter back to the right because their outgoing trajectory is intercepted by a compact Si wall at the end of the ditch "right side of the picture#[ However those ions which hit the sample at a distance farther away from the wall than a minimum distance\ d\ could reach the detector\ since they can ~y over the wall[ The minimum distance is measurable 504
Figure 1[ RBS mapping images of a silicon sample taken by the left "a# and the right "b# surface barrier detectors presenting di}erent bright and dark areas[ 1[9 MeV 3He¦\ scan area 559×559 mm1[
on the picture\ and by simple trigonometry the depth\ h of the cavity can be calculated as h dtga where a is the angle between the detector direction and sample surface[ The surface topography was studied on another more complex sample\ as well[ Its mapping image taken by the left detector together with the cross!sectional view of the sample is shown in Fig[ 2[ The mapping image\ contrary to the real structure of the sample\ is non!symmetric[ On the right side two bright stripes appear\ because there are also particles coming from the bottom
A Simon et al : Effects on RBS microbeam measurements
Figure 3[ RBS spectra for the second silicon sample[ Data collected from the full area "a#\ and from the b and d region only "b#[ 1[4 MeV 3He¦\ backscattering angle left 009>\ scan size 499×499 mm1[
Figure 2[ RBS mapping image "a# and cross!sectional view "b# of the second Si sample[ 1[4 MeV 3He¦\ backscattering angle left 009>\ scan size 499×499 mm1[
of the cavity without being shadowed[ These bright stripes indi! cate high intensity regions[ Their origin can be understood with the help of the cross!sectional view in Fig[ 2[ Beam trajectories in the cross sectional view and features in the mapping image indicated by a\ b\ c and d correspond to each other[ Ions following trajectory a those hitting the bottom of the ditch on its left side will be absorbed by the top cap casting a shadow on the RBS mapping image[ The shadow would appear also in the case of c\ but in this case ions scattered from the membrane produce almost the same intensity as that observed on the unetched parts of the sample "standard intensity#[ Between a and b the scattered beam from the bottom can reach the detector without any further decrease in the energy\ hence the intensity here is standard again[ From right of the beam line b a new region starts with an increased intensity[ Here from the membrane we get the standard intensity\ and ions scattered from the bottom of the ditch also contribute[ Because the incoming 1[4 MeV particles crossing the membrane of 6 mm thickness backscattered with a
much smaller energy E\ than the incident ions and according to the Rutherford formula the cross section of the elastic scattering increases as "0:E#1\ the contribution of these ions will be large[ "In our other measurements of 1[9 MeV incoming energy the ions could not reach the bottom\ they stopped in the membrane\ and the extra yield did not appear[# The situation is similar in case d[ The extra yield from the bottom can also be seen in the spectra in Fig[ 3[ Spectrum "a# was taken from the whole region "full scan# while the other one "b# was collected from the selected bright area only[ As it is seen in the region of 099Ð199 channels\ due to the ions backscattered from the bottom under the cap\ the detected counts increased[ From the energy value of these extra counts it was possible to determine the thickness of the membrane[ The disappearance of the extra intensity between b and d is caused by the shadow cast by the self!supporting mem! brane in the middle of the map[ From the width of the shadow in region "a# we could determine also the depth of the ditch as in the previous sample[ Both the depth of the ditch and the thickness of the membrane were in reasonable agreement with the real values[ 3[ Conclusion Our results clearly show the capability of the RBS microprobe equipped with movable detectors at grazing angle in determining 505
A Simon et al : Effects on RBS microbeam measurements
the surface topography[ Investigation on the e}ect of surface topography in RBS tomographic images "position!energy maps# as well as on the application of the method for multielemental sample are in progress[03\04[ Further study\ including simulation program\ is needed for the full exploitation of the method[ Acknowledgements The support from the Hungarian National Science Research Foundation "OTKA# under Research Contracts Nos[ T908036 and A979 is gratefully acknowledged[ References 0[ Breese\ M[ B[ H[\ Grime\ G[ W[ and Watt\ F[\ Annu[ Rev[ Nucl[ Part[ Sci[\ 0881\ 31\ 0[ 1[ Doyle\ B[ L[\ Nucl[ Instr[ and Meth[\ 0875\ B04\ 543[
506
2[ Hobbs\ C[ P[\ McMillan\ J[ W[ and Palmer\ D[ W[\ Nucl[ Instr[ and Meth[\ 0877\ B29\ 231[ 3[ Malmqvist\ K[ G[\ Nucl[ Instr[ and Meth[\ 0884\ B093\ 027[ 4[ Edge\ R[ D[\ IEEE Trans[ on Nucl[ Sci[\ 0872\ NS29\ 0574[ 5[ Schmid\ K[ and Ryssel\ H[\ Nucl[ Instr[ and Meth[\ 0863\ 008\ 176[ 6[ Edge\ R[ D[ and Bill\ U[\ Nucl[ Instr[ and Meth[\ 0879\ 057\ 046[ 7[ Knudson\ A[ R[\ Nucl[ Instr[ and Meth[\ 0879\ 057\ 052[ 8[ Bird\ R[\ Duerden\ P[\ Cohen\ D[ D[\ Smith\ G[ B[ and Hillery\ P[\ Nucl[ Instr[ and Meth[\ 0872\ 107\ 42[ 09[ Takai\ M[\ Nucl[ Instr[ and Meth[\ 0884\ B093\ 490[ 00[ Churms\ C[ L[ and Pretorius\ R[\ Nucl[ Instr[ and Meth[\ 0883\ B74\ 588[ 01[ Rajta\ I[\ Borbely!Kiss\ I[\ Morik\ Gy[\ Bartha\ L[\ Koltay\ E[ and ł [ Z[\ Nucl[ Instr[ and Meth[\ 0885\ B098:009\ 037[ Kiss\ A ł dam\ M[\ Szabo\ I[\ Barsony\ I[\ 02[ Ducso\ Cs[\ Vazsonyi\ E ł[\ A Gardeniers\ J[ G[ E[ and van den Berg\ A[\ Sensors and Actuators A\ 0886\ 59\ 124[ ł [ Z[\ Nucl[ 03[ Simon\ A[\ Paszti\ F[\ Uzonyi\ I[\ Manuaba\ A[ and Kiss\ A Instr[ and Meth[\ 0887\ B025Ð027\ 249[ 04[ Simon\ A[\ Paszti\ F[\ Uzonyi\ I[\ Manuaba\ A[\ Rajta\ I[ and Kiss\ ł [ Z[\ Nucl[ Instr[ and Meth[\ 0887\ B025Ð027\ 233[ A
Vacuum/volume 50/number 3–4/pages 503 to 506/1998 © 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042–207X/98 $19.00+.00
Pergamon
PII : S0042–207X(98)00090–6
Effect of surface topography on scanning RBS microbeam measurements A Simon,a* F Pa´szti,b I Uzonyi,a A Manuabab and A´ Z Kiss,a aInstitute of Nuclear Research of the Hungarian Academy of Sciences, H-4001 Debrecen, P.O. Box 51, Hungary ; bKFKI-Research Institute for Particle and Nuclear Physics, H-1525 Budapest, P.O. Box 49, Hungary received for publication 20 January 1998
Scanning Rutherford Backscattering Spectrometry (RBS) microprobe in the standard large angle backscattering geometry is not sensitive for the surface topography of samples consisting of homogeneous material. Whereas moving the detector into grazing exit angle the surface irregularities will be enhanced since surface elevations absorb the backscattered ions casting shadows on RBS mapping images ; slopes and edges facing the detector emit ions with decreased energy loss increasing in this way the yield for those positions. In such a way the scanning RBS microprobe will be capable of studying surface topography. Examples for test Si samples will be presented and observed effects are discussed. The method can also be used for the determination of the depth of ditches. © 1998 Elsevier Science Ltd. All rights reserved
0[ Introduction 0
1
Nuclear microprobes are able to scan ½ 0 mm dimension sur! faces with proton\ helium ion or other beams focused to a few microns and successfully have used in measuring 1 dimensional "1D\ or lateral# elemental distributions\ e[g[ applying proton induced X!ray emission "PIXE#[ The combination of PIXE with traditional Rutherford!backscattering "RBS# technique can pro! vide 2D distributions for the investigated sample1 where the depth as the third dimension is characterised by the energy loss during the incoming and outgoing paths of the scattered ions[ Here the ions enter the sample perpendicular to the surface and due to the nearly 079> scattering angle\ the incident and backscattered ions follow almost the same trajectory but in opposite directions[ Since the surface of the samples generally are not ~at "they have elevations\ depressions\ craters\ etc[# and this arrangement is not sensitive for such details\ it gives the 2D structure with a peculiar distortion\ namely although the depth is still determined from the point where the incident ions _rst reach the sample the true surface topography is completely ignored[ The e}ect of surface topography on RBS measurements is frequently investigated from the beginning of the technique\ but just from the point of view that how these e}ects might distort the obtained results and how can they be avoided[2\3 It is chal! lenging to improve the RBS microprobe in such a way that it should provide together the distorted 2D distribution with the details of the surface topology as well[ As we will demonstrate in this paper\ the task can be ful_lled without applying additional
Corresponding author[
methods or detectors simply by moving the detector into a graz! ing exit angle[ In this case the energy loss of ions along the incoming path is almost negligible in comparison with that along the outgoing path\ which is predominantly determined by the surface topography[ Furthermore\ by using two detectors put on the left and right side of the incident beam\ one may directly separate the e}ects of surface topography from those of other origin "e[g[ that are related to changes in material composition#[ While there are some trials to evaluate the surface shape from the distortions of the energy spectra in conventional RBS\4Ð00 to our knowledge this is the _rst attempt to intentionally enhance these e}ects in improving the performance of the RBS mic! roprobe technique[ 1[ Experimental The nuclear microprobe facility in ATOMKI\ Debrecen01 was applied for the measurements with use of 3He¦ beam[ The sample chamber described in detail in Ref[ ð00Ł was complemented with two movable surface barrier detectors "ORTEC# of 04 keV nom! inal resolution[ The detectors were collimated down to 7 msr by 1 mm wide slits at a distance of 34 mm and their position "the scattering angle# was adjustable with a precision of ½ 9[4> in the 099>Ð069> range without breaking the vacuum[ For the present test measurements secondary electron images were also taken simultaneously using a Philips type single channel electron mul! tiplier[ The layout of the sample chamber is shown in Fig[ 0[ Test samples of special structures consisting of only compact Si were prepared02 in the technological laboratory of KFKI! Research Institute of Materials Science\ Budapest in the fol! 503
A Simon et al : Effects on RBS microbeam measurements
Figure 0[ Layout of the sample chamber equipped with two movable surface barrier detectors[
lowing way ] p!type Si wafers were masked before doping with phosphorus to convert the _rst 6 mm layer into n!type Si[ After! wards they were subjected to anodisation in ethanolÐwaterÐHF solution\ where the _rst 049 mm or 69mm "depending on the sample# of the p!type areas transformed into porous Si[ The n! type layer and the area under it remained compact[ Around the borders of p!type regions the formed porous Si extended several mm over the borders leaving behind an n!type compact sheet at the surface[ The applied test samples were obtained after etching away the porous Si in KOH solution[ RBS mappings were taken either by 1[9 MeV or 1[4 MeV 3He¦ beam of 799 pA beam current and 2×2 mm1 beam size at normal incidence and detected simultaneously by the two detectors men! tioned above set left and right of the incident beam both at 009> scattering angle[ The data acquisition system was running under WINDOWS 84 and the signals were collected in listing mode enabling us to produce di}erent kinds of mapping images\ line! and selected!area scans o}!line[ 2[ Results and discussion The RBS mapping images of the _rst sample taken by the two detectors simultaneously are shown in Fig[ 1[ The brightness of the image represents the count number of the scattered ions collected at that position "the brighter the image\ the higher the count number#[ This sample has two rectangular ditches perpendicular to each other "in a cross!shape# containing a bridge in the middle and the horizontal one is opened to the left into a larger cavity and is closed on the opposite side[ The vertical one turns to the right at the bottom of the picture[ Considering the horizontal ditch\ its di}erent endings "opened and closed# causes a di}erence in the two RBS images[ There is no shadow "dark! ness# in the left image horizontally along the bridge while a de_nite shadow appears in the right one[ At the bottom of the vertical ditch an opposite e}ect is seen[ Considering only the right side detector the depth of the hori! zontal ditch can be determined in the following way ] the majority of the incident ions could not scatter back to the right because their outgoing trajectory is intercepted by a compact Si wall at the end of the ditch "right side of the picture#[ However those ions which hit the sample at a distance farther away from the wall than a minimum distance\ d\ could reach the detector\ since they can ~y over the wall[ The minimum distance is measurable 504
Figure 1[ RBS mapping images of a silicon sample taken by the left "a# and the right "b# surface barrier detectors presenting di}erent bright and dark areas[ 1[9 MeV 3He¦\ scan area 559×559 mm1[
on the picture\ and by simple trigonometry the depth\ h of the cavity can be calculated as h dtga where a is the angle between the detector direction and sample surface[ The surface topography was studied on another more complex sample\ as well[ Its mapping image taken by the left detector together with the cross!sectional view of the sample is shown in Fig[ 2[ The mapping image\ contrary to the real structure of the sample\ is non!symmetric[ On the right side two bright stripes appear\ because there are also particles coming from the bottom
A Simon et al : Effects on RBS microbeam measurements
Figure 3[ RBS spectra for the second silicon sample[ Data collected from the full area "a#\ and from the b and d region only "b#[ 1[4 MeV 3He¦\ backscattering angle left 009>\ scan size 499×499 mm1[
Figure 2[ RBS mapping image "a# and cross!sectional view "b# of the second Si sample[ 1[4 MeV 3He¦\ backscattering angle left 009>\ scan size 499×499 mm1[
of the cavity without being shadowed[ These bright stripes indi! cate high intensity regions[ Their origin can be understood with the help of the cross!sectional view in Fig[ 2[ Beam trajectories in the cross sectional view and features in the mapping image indicated by a\ b\ c and d correspond to each other[ Ions following trajectory a those hitting the bottom of the ditch on its left side will be absorbed by the top cap casting a shadow on the RBS mapping image[ The shadow would appear also in the case of c\ but in this case ions scattered from the membrane produce almost the same intensity as that observed on the unetched parts of the sample "standard intensity#[ Between a and b the scattered beam from the bottom can reach the detector without any further decrease in the energy\ hence the intensity here is standard again[ From right of the beam line b a new region starts with an increased intensity[ Here from the membrane we get the standard intensity\ and ions scattered from the bottom of the ditch also contribute[ Because the incoming 1[4 MeV particles crossing the membrane of 6 mm thickness backscattered with a
much smaller energy E\ than the incident ions and according to the Rutherford formula the cross section of the elastic scattering increases as "0:E#1\ the contribution of these ions will be large[ "In our other measurements of 1[9 MeV incoming energy the ions could not reach the bottom\ they stopped in the membrane\ and the extra yield did not appear[# The situation is similar in case d[ The extra yield from the bottom can also be seen in the spectra in Fig[ 3[ Spectrum "a# was taken from the whole region "full scan# while the other one "b# was collected from the selected bright area only[ As it is seen in the region of 099Ð199 channels\ due to the ions backscattered from the bottom under the cap\ the detected counts increased[ From the energy value of these extra counts it was possible to determine the thickness of the membrane[ The disappearance of the extra intensity between b and d is caused by the shadow cast by the self!supporting mem! brane in the middle of the map[ From the width of the shadow in region "a# we could determine also the depth of the ditch as in the previous sample[ Both the depth of the ditch and the thickness of the membrane were in reasonable agreement with the real values[ 3[ Conclusion Our results clearly show the capability of the RBS microprobe equipped with movable detectors at grazing angle in determining 505
A Simon et al : Effects on RBS microbeam measurements
the surface topography[ Investigation on the e}ect of surface topography in RBS tomographic images "position!energy maps# as well as on the application of the method for multielemental sample are in progress[03\04[ Further study\ including simulation program\ is needed for the full exploitation of the method[ Acknowledgements The support from the Hungarian National Science Research Foundation "OTKA# under Research Contracts Nos[ T908036 and A979 is gratefully acknowledged[ References 0[ Breese\ M[ B[ H[\ Grime\ G[ W[ and Watt\ F[\ Annu[ Rev[ Nucl[ Part[ Sci[\ 0881\ 31\ 0[ 1[ Doyle\ B[ L[\ Nucl[ Instr[ and Meth[\ 0875\ B04\ 543[
506
2[ Hobbs\ C[ P[\ McMillan\ J[ W[ and Palmer\ D[ W[\ Nucl[ Instr[ and Meth[\ 0877\ B29\ 231[ 3[ Malmqvist\ K[ G[\ Nucl[ Instr[ and Meth[\ 0884\ B093\ 027[ 4[ Edge\ R[ D[\ IEEE Trans[ on Nucl[ Sci[\ 0872\ NS29\ 0574[ 5[ Schmid\ K[ and Ryssel\ H[\ Nucl[ Instr[ and Meth[\ 0863\ 008\ 176[ 6[ Edge\ R[ D[ and Bill\ U[\ Nucl[ Instr[ and Meth[\ 0879\ 057\ 046[ 7[ Knudson\ A[ R[\ Nucl[ Instr[ and Meth[\ 0879\ 057\ 052[ 8[ Bird\ R[\ Duerden\ P[\ Cohen\ D[ D[\ Smith\ G[ B[ and Hillery\ P[\ Nucl[ Instr[ and Meth[\ 0872\ 107\ 42[ 09[ Takai\ M[\ Nucl[ Instr[ and Meth[\ 0884\ B093\ 490[ 00[ Churms\ C[ L[ and Pretorius\ R[\ Nucl[ Instr[ and Meth[\ 0883\ B74\ 588[ 01[ Rajta\ I[\ Borbely!Kiss\ I[\ Morik\ Gy[\ Bartha\ L[\ Koltay\ E[ and ł [ Z[\ Nucl[ Instr[ and Meth[\ 0885\ B098:009\ 037[ Kiss\ A ł dam\ M[\ Szabo\ I[\ Barsony\ I[\ 02[ Ducso\ Cs[\ Vazsonyi\ E ł[\ A Gardeniers\ J[ G[ E[ and van den Berg\ A[\ Sensors and Actuators A\ 0886\ 59\ 124[ ł [ Z[\ Nucl[ 03[ Simon\ A[\ Paszti\ F[\ Uzonyi\ I[\ Manuaba\ A[ and Kiss\ A Instr[ and Meth[\ 0887\ B025Ð027\ 249[ 04[ Simon\ A[\ Paszti\ F[\ Uzonyi\ I[\ Manuaba\ A[\ Rajta\ I[ and Kiss\ ł [ Z[\ Nucl[ Instr[ and Meth[\ 0887\ B025Ð027\ 233[ A