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Comparative study of RBS, SIMS and VASE for characterisation of high electron mobility transistors
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 482±486
www.elsevier.nl/locate/nimb
Comparative study of RBS, SIMS and VASE for characterisation of high electron mobility transistors L. Persson a
a,*
, N.G. L ovestam b, U. S odervall b, U. W atjen
a
EC-JRC, Institute for Reference Materials and Measurements (IRMM), Retieseweg, B-2440 Geel, Belgium b Chalmers University of Technology, S- 412 96 G oteborg, Sweden
Abstract The investigated devices consist of InGaAs/InAlAs multilayer ®lms grown by molecular beam epitaxy on InP. Results from two samples with the same epitaxial layer composition but with dierent layer thicknesses are presented. Of the three techniques investigated, Rutherford backscattering spectrometry (RBS), variable angle spectroscopic ellipsometry (VASE) and secondary ion mass spectrometry (SIMS), VASE appears to be the choice for routine work. Both RBS and SIMS are shown to be valuable complementary techniques. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 82.80.Ms; 82.80.Yc Keywords: RBS; SIMS; Ellipsometry; HEMT; III±V; Characterisation
1. Introduction The high electron mobility transistor (HEMT) is based on III±V semiconductor materials. It is the fastest three-terminal device known today and thus well suited for millimeterwave (high frequency) applications. A HEMT is fabricated on an epitaxial layer structure, in this study by means of molecular beam epitaxy (MBE). At the interface between layers of materials with dierent bandgaps a triangular quantum well is formed. If the high band-gap material is n-doped, a two-dimen-
*
Corresponding author. Tel.: +32-14-571-385; fax: +32-14571-376. E-mail address: [email protected] (L. Persson).
sional electron gas is con®ned in the quantum well giving the device excellent high-speed characteristics. The perfection in the fabrication procedure is, however, of critical importance for the ®nal electrical performance and deviations in layer thicknesses, bad interface quality, contaminations, etc may be fatal. The objective of this study was to compare various techniques for layer thickness measurements on HEMT devices. Considering that the selected technique will be used for analysis of a large number of in-house grown MBE materials, important issues in addition to pure measurement performance were reasonable high analysis throughput and measurement precision. Three techniques were studied: Rutherford backscattering spectrometry (RBS), secondary ion mass
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 9 5 - 7
L. Persson et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 482±486
spectrometry (SIMS) and variable angle spectroscopic ellipsometry (VASE).
2. Materials and methods 2.1. Samples Commercially available epitaxial wafers from quantum epitaxial design (QED) were analysed. Two single-crystal InP wafers, S1 and S2, with equal epitaxial layer composition but with dierent layer thickness pro®les as shown in Table 1 were used. From both wafers two samples were cut out for analysis, one for the RBS and VASE analyses (non-destructive) and one for the SIMS analysis (destructive). This type of sample grows an oxide layer when handled in air that might complicate the analyses. 2.2. RBS The RBS measurements were performed using He ions of 2.0 and 2.8 MeV energies at backscattering angles between 120° and 165°. The beam current was typically 20±25 nA and was measured with a transmission Faraday cup [1]. The energy resolution was around 20 keV FWHM. Due to the low stopping power of the material this corresponds to a depth resolution of around 25 nm in
483
the best case. In order to minimize the risk of accidental channelling the beam was incident on the samples at 7° with respect to the sample normal. The vacuum was typically better than 2 10ÿ4 Pa during the measurements. A versatile target chamber facilitates other analytical methods such as particle induced X-ray emission (PIXE) and/or nuclear reaction analysis (NRA) for contamination detection simultaneously with RBS [2]. In this study PIXE was utilised to detect possible contamination heavier than Al in the samples. 2.3. SIMS The SIMS measurements were done using a Cameca IMS-3F instrument. Low energy primary ions, 1.7 keV, of O 2 were used and raster scanned over an area of 500 500 lm2 to improve crater bottom ¯atness, while only ions from the central 100 lm were detected for analysis. The primary probe diameter was about 100 lm and using 100 nA gave an appropriate sputtering rate of 0.02 nm/ s. Secondary ions of masses 27 Al, 28 Si, 31 P, 71 Ga, 75 As and 113 In were registered during the analysis. Results from sample S1 are shown in Fig. 1. To determine an absolute depth scale of SIMS measurements, the crater depth must be measured independently after analysis. For a multilayer structure it is more complicated as dierent layers
Table 1 Nominal layer thicknesses together with thickness obtained from the dierent methodsa
a
Nominal thickness
RBS
SIMS
VASEb
VASEc
S1 InGaAs InAlAs InGaAs InAlAs
5 28 25 500
(5) 30 (7) 26 (6) 510 (51)
4 29 22 490
(3) (5) (5) (10)
5.8 (0.6) 27.9 (0.8) 32.1 (0.6) 484 (3)
5.8 (0.7) 29.2 (1.2) 28.5 (0.9) 488 (3)
S2 InGaAs InAlAs InGaAs InAlAs
5 24 30 500
(5) 22 (5) 31 (7) 490 (49)
4 27 27 485
(3) (3) (3) (10)
4.6 (0.1) 24.8 (0.2) 29.0 (0.4) 480 (5)
All thickness are in nm and the substrate is InP. Assuming nominal elemental fractions. c Using RBS results of the elemental fractions. b
484
L. Persson et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 482±486
and perpendicular (Rs ) to the plane of incidence according to [4] q
Fig. 1. A plot of secondary ions registered during the SIMS analysis, in this case for sample S1. Vertical arrows indicate the interfaces.
may have dierent sputtering rates. In order to correct for dierent sputtering rates in this SIMS analysis, several analyses were carried out to different depths. Except for the ®rst GaAs cap layer, each pro®le was stopped manually when the intensity of either mass 27 Al or 71 Ga decreased to half its maximum value at the interface of interest. At least one SIMS pro®le was done to each interface and the corresponding crater depth was measured by a surface pro®lometer, an Alfastep with a resolution of typically 2 nm. However, an accurate measurement of a sputtered crater also demands a very ¯at surface over the distance of the crater (typically 500±1000 lm). We observed that S1 had a signi®cantly rougher and more curved surface than S2. This rendered a worse accuracy in the determination for each crater of S1.
Rp tan w eiD : Rs
The phase information (iD) makes SE very sensitive to small changes in thickness (a few tens of a nm) and optical constants. During an analysis W and D are calculated for the assumed model and compared with experimental data. A regression analysis is then performed to minimise the dierences between calculated and experimental data. It is important to make sure that the model obtained is physically reasonable. It is sometimes possible to obtain a better overall ®t with an unphysical model. SE is a very precise method and it can provide very accurate data on dielectric functions, depthpro®les and composition of alloys or composites. SE works best when the sample under study has a smooth surface and the wavelength of the incoming light is about the same as the ®lm thickness. Both ®lm thickness and optical constants can be determined simultaneously if there is a wavelength regime for which the ®lm is transparent. SE becomes more powerful when combined with variable angle of incidence as is done in VASE [5]. The user can then optimise the sensitivity of the measurement to the parameters of interest. The instrument used in this study was a J.A. Woollam Co., Inc. VASEâ . The measurements were carried out in the wavelength region 260±950 nm in steps of 10 nm, for angles of coincidence between 65±75° in steps of 2.5°. An extensive library of optical constants that originates from several research projects accompanies the instrument and was used for the analyses. 3. Results
2.4. VASE
3.1. RBS
In spectroscopic ellipsometry (SE) [3] the change in polarization state of usually re¯ected light is studied. The measured values can be expressed as W and D and are related to the re¯ection coecients for light linearly polarized parallel (Rp )
The RBS spectra were analysed using RUMP [6]. In order to convert the thicknesses from atoms/ cm2 to dimensional thicknesses an interpolation was performed between the density values for InAs (5.66 g/cm3 ), GaAs (5.32 g/cm3 ) and AlAs (3.79 g/
L. Persson et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 482±486
485
Table 2 Nominal elemental concentrations for S1 together with concentrations obtained from RBS S1
Nominal fraction In
In
Ga
InGaAs InAlAs InGaAs InAlAs
0.53 0.52 0.60 0.52
0.53 0.45 0.47 0.52
0.47
cm3 ) [7]. The thickness of the top layer in both samples was chosen to correspond to 5 nm, as in this study RBS does not have the depth resolution needed to determine thicknesses this small. When trying to determine the layer thicknesses for the layers approximately 20 nm thick a careful examination of the Ga signal was necessary. As expected due to the poor depth resolution no signi®cant deviations from nominal thicknesses were detected and the quoted uncertainties in Table 1 were estimated from stopping uncertainties (10%) and sensitivity to change of ®lm thicknesses during RBS spectrum evaluation (up to 20%). RBS was also used to check for deviations from the nominal layer compositions. In Table 2 the nominal elemental composition values are compared with the values obtained from RBS. When growing S1 a lattice mismatch was attempted for the second InGaAs layer and the aim was 60 mole% In. RBS indicates, however, that the result was instead a drop with only 47 mole% In, see Table 2 and Fig. 2. Also the ®rst InAlAs layer appears to have a de®ciency of In, approximately 45 mole% instead of 52. No discrepancies were seen for S2. No contaminants were found using He-induced X-ray emission. 3.2. SIMS The thickness of each layer was determined from the position when the intensity of either mass 27 Al or 71 Ga decreased to half its maximum value as described previously. The major uncertainties arise from deciding when to stop at each interface, accuracy in Alfastep measurements, dierences in sputter rate and sputter yield, and surface roughness. Considering this the uncertainties are esti-
0.53
Al 0.55 0.48
As 1 1 1 1
Fig. 2. RBS data for sample S1 taken at 120° at 2.0 MeV together with ®ts for nominal structure (dotted line) and suggested model (dashed line). The improved ®t of the new model compared to the nominal model is especially pronounced in the region between channels 750 and 800.
mated to be about 4 nm for layers 1, 2 and 3 close to the surface, while for the buer layer it is about 10 nm. In that case the SIMS results give good agreement with nominal thickness for the buer layer and the next two layers, while it is dicult to give good enough accuracy for the cap layer. 3.3. VASE There are too many unknowns (elemental fractions, interlayer mixing, contaminations, thicknesses) for it to be possible to ®t for all unknowns. This would lead to large correlation coecients between dierent variables. In a ®rst attempt the nominal values for elemental fractions
486
L. Persson et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 482±486
were used. The samples were assumed to be perfect except for a possible oxide layer on top, which has been included in the top InGaAs thickness in the results. For S1 the optical constants were obtained from an extrapolation of the values in the library as such a large deviation from 53 mol% In in InGaAs is not included. The extrapolation introduces an extra uncertainty and the resulting ®t is not perfect. The result appears, however, to be fairly good with the exception being the second InGaAs layer which seems to be 32 nm thick instead of 25 nm, see Table 1. The uncertainties (repeatability only) are estimated from measurements on dierent spots and the extrapolation has not been taken into account. For S2 a good ®t is obtained and the agreement with nominal thicknesses is very good. A second ®t was performed for S1 using the elemental compositions obtained from RBS and the result was a better ®t to the optical data. A somewhat better agreement with nominal thickness values for layers #2 and #3 is achieved but there is still a discrepancy between nominal values and the result obtained from VASE. An attempt was also made to study the interface quality but including interface layers did not signi®cantly improve the ®t of the model. 4. Discussion The complexity of the investigated structures makes it very dicult for one technique to meet all the requirements. The technique that appears to give best agreement with nominal values is SIMS. However, it is destructive and quite time-consuming considering that each layer has to be investigated individually. RBS suers from poor depth resolution and the uncertainty of layer densities when converting from number of atoms per unit area to dimensional thickness. RBS has
the advantage that one obtains the elemental compositions of the layers in the sample without having to rely on any nominal values. VASE is a very precise technique and it is a relatively quick method to apply. The simplicity of the instrumentation and user handling as compared to the other techniques must be considered to be advantageous when doing pure routine epitaxial layer thickness measurements, for example as production control, once the elemental composition of the layers is reliably known. If only one technique is to be chosen VASE would be the choice taking all aspects into consideration. However, it would be strongly recommended to complement with RBS and/or SIMS whenever there is any doubt with regards to sample composition, if the obtained ®t is not satisfactory or if any contaminants are suspected.
Acknowledgements The VdG operators at IRMM are gratefully acknowledged for providing the ion beam. Dr. Thomas Wagner, L.O.T.-Oriel, GmbH is also acknowledged for valuable discussions concerning VASE measurements.
References [1] F. Paszti, A. Manuaba, C. Hajdu, A.A. Melo, M.F. Da Silva, Nucl. Instr. and Meth. B 47 (1990) 187. [2] U. W atjen, H. Bax, J. R ais anen, Nucl. Instr. and Meth. B 118 (1996) 676. [3] K. Vedam, Thin Solid Films 313/314 (1998) 1. [4] R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized Light, North Holland, Amsterdam, 1977. [5] J.A. Woollam, P.G. Snyder, Mater. Sci. Eng. B 5 (1990) 279. [6] L.R. Doolittle, Nucl. Instr. and Meth. B 15 (1986) 227. [7] A. Turos, private communication.
www.elsevier.nl/locate/nimb
Comparative study of RBS, SIMS and VASE for characterisation of high electron mobility transistors L. Persson a
a,*
, N.G. L ovestam b, U. S odervall b, U. W atjen
a
EC-JRC, Institute for Reference Materials and Measurements (IRMM), Retieseweg, B-2440 Geel, Belgium b Chalmers University of Technology, S- 412 96 G oteborg, Sweden
Abstract The investigated devices consist of InGaAs/InAlAs multilayer ®lms grown by molecular beam epitaxy on InP. Results from two samples with the same epitaxial layer composition but with dierent layer thicknesses are presented. Of the three techniques investigated, Rutherford backscattering spectrometry (RBS), variable angle spectroscopic ellipsometry (VASE) and secondary ion mass spectrometry (SIMS), VASE appears to be the choice for routine work. Both RBS and SIMS are shown to be valuable complementary techniques. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 82.80.Ms; 82.80.Yc Keywords: RBS; SIMS; Ellipsometry; HEMT; III±V; Characterisation
1. Introduction The high electron mobility transistor (HEMT) is based on III±V semiconductor materials. It is the fastest three-terminal device known today and thus well suited for millimeterwave (high frequency) applications. A HEMT is fabricated on an epitaxial layer structure, in this study by means of molecular beam epitaxy (MBE). At the interface between layers of materials with dierent bandgaps a triangular quantum well is formed. If the high band-gap material is n-doped, a two-dimen-
*
Corresponding author. Tel.: +32-14-571-385; fax: +32-14571-376. E-mail address: [email protected] (L. Persson).
sional electron gas is con®ned in the quantum well giving the device excellent high-speed characteristics. The perfection in the fabrication procedure is, however, of critical importance for the ®nal electrical performance and deviations in layer thicknesses, bad interface quality, contaminations, etc may be fatal. The objective of this study was to compare various techniques for layer thickness measurements on HEMT devices. Considering that the selected technique will be used for analysis of a large number of in-house grown MBE materials, important issues in addition to pure measurement performance were reasonable high analysis throughput and measurement precision. Three techniques were studied: Rutherford backscattering spectrometry (RBS), secondary ion mass
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 9 5 - 7
L. Persson et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 482±486
spectrometry (SIMS) and variable angle spectroscopic ellipsometry (VASE).
2. Materials and methods 2.1. Samples Commercially available epitaxial wafers from quantum epitaxial design (QED) were analysed. Two single-crystal InP wafers, S1 and S2, with equal epitaxial layer composition but with dierent layer thickness pro®les as shown in Table 1 were used. From both wafers two samples were cut out for analysis, one for the RBS and VASE analyses (non-destructive) and one for the SIMS analysis (destructive). This type of sample grows an oxide layer when handled in air that might complicate the analyses. 2.2. RBS The RBS measurements were performed using He ions of 2.0 and 2.8 MeV energies at backscattering angles between 120° and 165°. The beam current was typically 20±25 nA and was measured with a transmission Faraday cup [1]. The energy resolution was around 20 keV FWHM. Due to the low stopping power of the material this corresponds to a depth resolution of around 25 nm in
483
the best case. In order to minimize the risk of accidental channelling the beam was incident on the samples at 7° with respect to the sample normal. The vacuum was typically better than 2 10ÿ4 Pa during the measurements. A versatile target chamber facilitates other analytical methods such as particle induced X-ray emission (PIXE) and/or nuclear reaction analysis (NRA) for contamination detection simultaneously with RBS [2]. In this study PIXE was utilised to detect possible contamination heavier than Al in the samples. 2.3. SIMS The SIMS measurements were done using a Cameca IMS-3F instrument. Low energy primary ions, 1.7 keV, of O 2 were used and raster scanned over an area of 500 500 lm2 to improve crater bottom ¯atness, while only ions from the central 100 lm were detected for analysis. The primary probe diameter was about 100 lm and using 100 nA gave an appropriate sputtering rate of 0.02 nm/ s. Secondary ions of masses 27 Al, 28 Si, 31 P, 71 Ga, 75 As and 113 In were registered during the analysis. Results from sample S1 are shown in Fig. 1. To determine an absolute depth scale of SIMS measurements, the crater depth must be measured independently after analysis. For a multilayer structure it is more complicated as dierent layers
Table 1 Nominal layer thicknesses together with thickness obtained from the dierent methodsa
a
Nominal thickness
RBS
SIMS
VASEb
VASEc
S1 InGaAs InAlAs InGaAs InAlAs
5 28 25 500
(5) 30 (7) 26 (6) 510 (51)
4 29 22 490
(3) (5) (5) (10)
5.8 (0.6) 27.9 (0.8) 32.1 (0.6) 484 (3)
5.8 (0.7) 29.2 (1.2) 28.5 (0.9) 488 (3)
S2 InGaAs InAlAs InGaAs InAlAs
5 24 30 500
(5) 22 (5) 31 (7) 490 (49)
4 27 27 485
(3) (3) (3) (10)
4.6 (0.1) 24.8 (0.2) 29.0 (0.4) 480 (5)
All thickness are in nm and the substrate is InP. Assuming nominal elemental fractions. c Using RBS results of the elemental fractions. b
484
L. Persson et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 482±486
and perpendicular (Rs ) to the plane of incidence according to [4] q
Fig. 1. A plot of secondary ions registered during the SIMS analysis, in this case for sample S1. Vertical arrows indicate the interfaces.
may have dierent sputtering rates. In order to correct for dierent sputtering rates in this SIMS analysis, several analyses were carried out to different depths. Except for the ®rst GaAs cap layer, each pro®le was stopped manually when the intensity of either mass 27 Al or 71 Ga decreased to half its maximum value at the interface of interest. At least one SIMS pro®le was done to each interface and the corresponding crater depth was measured by a surface pro®lometer, an Alfastep with a resolution of typically 2 nm. However, an accurate measurement of a sputtered crater also demands a very ¯at surface over the distance of the crater (typically 500±1000 lm). We observed that S1 had a signi®cantly rougher and more curved surface than S2. This rendered a worse accuracy in the determination for each crater of S1.
Rp tan w eiD : Rs
The phase information (iD) makes SE very sensitive to small changes in thickness (a few tens of a nm) and optical constants. During an analysis W and D are calculated for the assumed model and compared with experimental data. A regression analysis is then performed to minimise the dierences between calculated and experimental data. It is important to make sure that the model obtained is physically reasonable. It is sometimes possible to obtain a better overall ®t with an unphysical model. SE is a very precise method and it can provide very accurate data on dielectric functions, depthpro®les and composition of alloys or composites. SE works best when the sample under study has a smooth surface and the wavelength of the incoming light is about the same as the ®lm thickness. Both ®lm thickness and optical constants can be determined simultaneously if there is a wavelength regime for which the ®lm is transparent. SE becomes more powerful when combined with variable angle of incidence as is done in VASE [5]. The user can then optimise the sensitivity of the measurement to the parameters of interest. The instrument used in this study was a J.A. Woollam Co., Inc. VASEâ . The measurements were carried out in the wavelength region 260±950 nm in steps of 10 nm, for angles of coincidence between 65±75° in steps of 2.5°. An extensive library of optical constants that originates from several research projects accompanies the instrument and was used for the analyses. 3. Results
2.4. VASE
3.1. RBS
In spectroscopic ellipsometry (SE) [3] the change in polarization state of usually re¯ected light is studied. The measured values can be expressed as W and D and are related to the re¯ection coecients for light linearly polarized parallel (Rp )
The RBS spectra were analysed using RUMP [6]. In order to convert the thicknesses from atoms/ cm2 to dimensional thicknesses an interpolation was performed between the density values for InAs (5.66 g/cm3 ), GaAs (5.32 g/cm3 ) and AlAs (3.79 g/
L. Persson et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 482±486
485
Table 2 Nominal elemental concentrations for S1 together with concentrations obtained from RBS S1
Nominal fraction In
In
Ga
InGaAs InAlAs InGaAs InAlAs
0.53 0.52 0.60 0.52
0.53 0.45 0.47 0.52
0.47
cm3 ) [7]. The thickness of the top layer in both samples was chosen to correspond to 5 nm, as in this study RBS does not have the depth resolution needed to determine thicknesses this small. When trying to determine the layer thicknesses for the layers approximately 20 nm thick a careful examination of the Ga signal was necessary. As expected due to the poor depth resolution no signi®cant deviations from nominal thicknesses were detected and the quoted uncertainties in Table 1 were estimated from stopping uncertainties (10%) and sensitivity to change of ®lm thicknesses during RBS spectrum evaluation (up to 20%). RBS was also used to check for deviations from the nominal layer compositions. In Table 2 the nominal elemental composition values are compared with the values obtained from RBS. When growing S1 a lattice mismatch was attempted for the second InGaAs layer and the aim was 60 mole% In. RBS indicates, however, that the result was instead a drop with only 47 mole% In, see Table 2 and Fig. 2. Also the ®rst InAlAs layer appears to have a de®ciency of In, approximately 45 mole% instead of 52. No discrepancies were seen for S2. No contaminants were found using He-induced X-ray emission. 3.2. SIMS The thickness of each layer was determined from the position when the intensity of either mass 27 Al or 71 Ga decreased to half its maximum value as described previously. The major uncertainties arise from deciding when to stop at each interface, accuracy in Alfastep measurements, dierences in sputter rate and sputter yield, and surface roughness. Considering this the uncertainties are esti-
0.53
Al 0.55 0.48
As 1 1 1 1
Fig. 2. RBS data for sample S1 taken at 120° at 2.0 MeV together with ®ts for nominal structure (dotted line) and suggested model (dashed line). The improved ®t of the new model compared to the nominal model is especially pronounced in the region between channels 750 and 800.
mated to be about 4 nm for layers 1, 2 and 3 close to the surface, while for the buer layer it is about 10 nm. In that case the SIMS results give good agreement with nominal thickness for the buer layer and the next two layers, while it is dicult to give good enough accuracy for the cap layer. 3.3. VASE There are too many unknowns (elemental fractions, interlayer mixing, contaminations, thicknesses) for it to be possible to ®t for all unknowns. This would lead to large correlation coecients between dierent variables. In a ®rst attempt the nominal values for elemental fractions
486
L. Persson et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 482±486
were used. The samples were assumed to be perfect except for a possible oxide layer on top, which has been included in the top InGaAs thickness in the results. For S1 the optical constants were obtained from an extrapolation of the values in the library as such a large deviation from 53 mol% In in InGaAs is not included. The extrapolation introduces an extra uncertainty and the resulting ®t is not perfect. The result appears, however, to be fairly good with the exception being the second InGaAs layer which seems to be 32 nm thick instead of 25 nm, see Table 1. The uncertainties (repeatability only) are estimated from measurements on dierent spots and the extrapolation has not been taken into account. For S2 a good ®t is obtained and the agreement with nominal thicknesses is very good. A second ®t was performed for S1 using the elemental compositions obtained from RBS and the result was a better ®t to the optical data. A somewhat better agreement with nominal thickness values for layers #2 and #3 is achieved but there is still a discrepancy between nominal values and the result obtained from VASE. An attempt was also made to study the interface quality but including interface layers did not signi®cantly improve the ®t of the model. 4. Discussion The complexity of the investigated structures makes it very dicult for one technique to meet all the requirements. The technique that appears to give best agreement with nominal values is SIMS. However, it is destructive and quite time-consuming considering that each layer has to be investigated individually. RBS suers from poor depth resolution and the uncertainty of layer densities when converting from number of atoms per unit area to dimensional thickness. RBS has
the advantage that one obtains the elemental compositions of the layers in the sample without having to rely on any nominal values. VASE is a very precise technique and it is a relatively quick method to apply. The simplicity of the instrumentation and user handling as compared to the other techniques must be considered to be advantageous when doing pure routine epitaxial layer thickness measurements, for example as production control, once the elemental composition of the layers is reliably known. If only one technique is to be chosen VASE would be the choice taking all aspects into consideration. However, it would be strongly recommended to complement with RBS and/or SIMS whenever there is any doubt with regards to sample composition, if the obtained ®t is not satisfactory or if any contaminants are suspected.
Acknowledgements The VdG operators at IRMM are gratefully acknowledged for providing the ion beam. Dr. Thomas Wagner, L.O.T.-Oriel, GmbH is also acknowledged for valuable discussions concerning VASE measurements.
References [1] F. Paszti, A. Manuaba, C. Hajdu, A.A. Melo, M.F. Da Silva, Nucl. Instr. and Meth. B 47 (1990) 187. [2] U. W atjen, H. Bax, J. R ais anen, Nucl. Instr. and Meth. B 118 (1996) 676. [3] K. Vedam, Thin Solid Films 313/314 (1998) 1. [4] R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized Light, North Holland, Amsterdam, 1977. [5] J.A. Woollam, P.G. Snyder, Mater. Sci. Eng. B 5 (1990) 279. [6] L.R. Doolittle, Nucl. Instr. and Meth. B 15 (1986) 227. [7] A. Turos, private communication.