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Metrology of semiconductor device structures by cross-sectional AFM
Materials Science and Engineering B80 (2001) 138– 141 www.elsevier.com/locate/mseb
Metrology of semiconductor device structures by cross-sectional AFM C. Jenkins a,*, D.I. Westwood a, M. Elliott a, J.E. Macdonald a, C. Meaton b, S. Bland b a b
Department of Physics and Astronomy, Cardiff Uni6ersity, PO Box 913, Cardiff, CF2 3YB, Wales, UK Epitaxial Products International, Pascal Close, Cypress Dri6e, St. Mellons, Cardiff, CF3 0EG, Wales, UK
Abstract Atomic force microscopy (AFM) in air has been used to study various III– V semiconductor heterostructures. Topography of the (110) cleaved cross-sections has been examined where oxidation processes modify the surface and allow the structures to be investigated. It is shown that surface height differences of as little as 0.4 A, are sufficient to distinguish between layers, and that quantum wells of as little as 7 nm width are detectable. © 2001 Elsevier Science B.V. All rights reserved. Keywords: AFM, Metrology; Semiconductors
1. Introduction The precise measurement of all the individual layer thicknesses in complex epitaxially grown device structures has always been difficult [1]. Optical or X-ray diffraction techniques only give reliable values for the simplest structures, and more direct methods such as transmission electron microscopy are very time consuming. However, with the advent of atomic force microscopes, which can operate in air whilst maintaining atomic resolution, there now exists the possibility of quick, simple, accurate and precise thickness measurements on even the most complex structures. This approach involves the cleaving of an epitaxial device structure to reveal its cross-section, followed by the measurement of its topography to determine the layer dimensions. Contrast between layers results from the different oxidation rates for different material compositions [2,3]. The oxidation mechanism of III – V semiconductors has been widely discussed [2] and will not be dealt with here. Important questions to be addressed are, how accurately can layer thicknesses be determined, the minimum step height measurable, and what time-scale is involved in obtaining good images. We have examined materials grown on GaAs and InP (001) substrates, i.e. AlGaAs, AlInGaP and InGaP, * Corresponding author. E-mail address: [email protected] (C. Jenkins).
and considered the effects of oxide thickness, varied by the exposure time to air and compositional dependent oxidation rates, on surface morphology and consequently the ability to determine the true position of interfaces. Below we discuss the performance of AFM during this study, and report a number of different issues that arose from the study. 2. Experimental considerations Heteroepitaxial semiconductor structures have been grown using metal organic vapour phase epitaxy (MOVPE) by Epitaxial Products International, St. Mellons, Cardiff, UK. For cross-sectional studies, the samples were cleaved in air at room temperature, and allowed to oxidise naturally. Normally, the time taken between cleaving and measurement is 15 min. It is possible to thermally oxidise or etch the samples, but that will not be considered here. The quality of the cleave is an important factor in the ability to extract accurate height profiles from the oxidised heterostructure. Indeed, steps as small as 0.3 nm on the surface can make some samples unsuitable for such investigation. Studies were carried out using a Digital Instruments NanoScope™ IIIa MultiMode™ SPM operating under TappingMode™ AFM.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5107(00)00634-6
C. Jenkins et al. / Materials Science and Engineering B80 (2001) 138–141
139
3. Technique capabilities The first type of structure we investigated was a vertical cavity surface emitting laser (VCSEL). The superlattice of the Bragg reflectors made this an ideal structure to study. Added to this were the quantum wells in the active region — an ideal test of the techniques’ ability to detect very thin layers. Fig. 1 shows a close up of the active region of one such VCSEL structure, produced 1 h after cleaving. In this case, the Bragg mirrors are made up of alternating layers of Al0.8Ga0.2As/Al0.15Ga0.85As. The 80% Al layers have undergone the most rapid oxidation, the oxide layer being 3.5 nm high with respect to the GaAs substrate. These layers therefore appear as the brightest bands in the image. The active region consists of three GaAs quantum wells, sandwiched between layers of Al0.3Ga0.7As. Each well was specified as being 66 A, wide, but was measured as 71 A, . The observed contrast is produced by an oxide height difference of 2 A, . Of the investigated semiconductor compounds, AlGaAs was found to oxidise the most quickly, the rate increasing with Al concentration. Phosphide based materials were found to oxidise the least, with combinations of Al and P oxidising the most slowly.
4. Oxidation rate differences between two GaAs layers In addition to distinguishing between different materials, steps have occasionally been observed between two GaAs layers. This is illustrated in Fig. 2. Both layers are silicon doped, with the layer on the left-hand side (the substrate) having a dopant concentration of 1018 cm − 3 and the layer on the right (a buffer layer) having one of 1019 cm − 3.
Fig. 1. The triple quantum well structure of a VCSEL (the three narrow dark bands running just to the left of the centre of the image). The wells are embedded in an active region, either side of which are the Bragg mirror stacks (the thick alternating light/dark layers). Each well is 66 A, wide, and the observed contrast is produced by an oxide height difference of 2 A, .
Fig. 2. Contrast produced between two differently doped GaAs layers. The layer to the right has the highest dopant level.
The image was taken 2 h after the cleave, and produced our smallest measured step height of 0.4 A, . This is comparable with the smallest possible height measurement capable with AFM. At this stage, the cause of the observed contrast is unclear. A number of factors could be responsible. The different growth conditions for the substrate and overlayer may lead to differences in defects and impurities or strain.
5. Oxide layer spreading The form of the growth of oxide layers has an important effect on any layer thickness measurements made. For example, apparent interface positions change with an increasing oxide layer. The surface transformation is best illustrated by the measurement of a superlattice structure. Fig. 3 shows such a structure 1 h after it has been cleaved. To the left
Fig. 3. Oxide layer induced contrast for an Al0.45Ga0.55As/AlAs superlattice structure after 1 h of oxidation.
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C. Jenkins et al. / Materials Science and Engineering B80 (2001) 138–141
Fig. 5. An AlGaAs based superlattice with cleave damage running throughout unhindered by the layer interfaces. Fig. 4. The same sample as in Fig. 3 after a day of oxidation. Note that the oxide layers have grown and merged together.
is the GaAs substrate, and to the right is the superlattice composed of layers of Al0.45Ga0.55As/AlAs, each being 20 nm wide. In this case the oxide on the AlAs has grown to a height of 10 nm. Fig. 4 shows the same sample a day latter. As well as increasing in height to 16 nm, the oxide on the AlAs layers has spread and merged together. As can be seen, oxide layers formed on the AlAs grow laterally as well as vertically, smearing out the original interface until they eventually merge together. The implications for the accurate determination of layer interfaces are immediately obvious. Although this was found to be the extreme case (as high concentration Al layers are involved), extensive oxidation of samples is to be avoided when layer thicknesses are being assessed. It is therefore advantageous to image a sample as soon after cleaving as possible. It must be noted, however, that in this case the determination of the superlattice repeat distance is still possible. Oxide layer growth can have other deleterious effects. For example, although very thin quantum wells are detectable soon after cleaving, a day later, surrounding material has obliterated the wells completely. Finally, oxide growth appears to not always be uniform. One can note from Fig. 3 and Fig. 4 the large variations in oxide growth along the AlAs layers. This effect is observed in AlGaAs layers down to about 80% Al, and has been noted in other work [3]. As of yet, no reason has been assigned to this.
a superlattice structure composed of Al0.8Ga0.2As/ Al0.15Ga0.85As layers. In this case, the cleave damage readily propagates through the layers unhindered. The effect is somewhat different in Fig. 6. Here we have an Al0.7Ga0.3As/AlInGaP interface. One can easily see the cleave damage on the Al0.7Ga0.3As layer to the right. However, no cleave damage appears in the AlInGaP layers to the immediate left. Throughout the study, 2° off Al based materials were routinely cleaved and imaged. 10° off phosphide based structures on the other hand, proved to be more problematical. The wafers required thinning in order to give a sufficient cleave quality.
7. Discussion AFM has demonstrated itself to be a powerful tool in III –V semiconductor device structure metrology. The results of this study indicate that oxide height differences of as little as 0.4 A, are sufficient to distinguish between layers. Although an effective contrast mechanism, the nature of oxide growth (i.e. its tendency to
6. Cleave damage As previously mentioned, an important factor in the ability to obtain layer dimensions is the quality of the cleave, especially if the material is slow to oxidise. The situation is complicated by the observation that cleave damage can relate to the device structure. Fig. 5 shows
Fig. 6. An AlGaAs layer (right) with cleave damage that clearly does not propagate through to the phosphide layers to its immediate left.
C. Jenkins et al. / Materials Science and Engineering B80 (2001) 138–141
spread in both the lateral and vertical directions), places an upper limit on the extent of oxidation before the uncertainty in interface position becomes intolerable. In general, the interfaces can be determined to within 10–50 nm, depending on the extent of the oxidation. For superlattice structures, the determination of repeat distances is only limited by the accuracy of the microscope itself (3%).
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141
References [1] A. Rudra, L. Sagalowicz, K. Leifer, J. Behrend, C.A. Berseth, O. Dehaese, J.F. Carlin, et al., J. Crystal Growth 188 (1–4) (1998) 300– 306. [2] F. Reinhardt, B. Dwir, E. Kapon, Appl. Phys. Lett. 68 (22) (1996) 3168– 3170. [3] F. Reinhardt, B. Dwir, G. Biasiol, E. Kapon, Mater. Sci. Eng. B Solid State Mater. Adv. Technol. 37 (1 – 3) (1996) 83–88.
Metrology of semiconductor device structures by cross-sectional AFM C. Jenkins a,*, D.I. Westwood a, M. Elliott a, J.E. Macdonald a, C. Meaton b, S. Bland b a b
Department of Physics and Astronomy, Cardiff Uni6ersity, PO Box 913, Cardiff, CF2 3YB, Wales, UK Epitaxial Products International, Pascal Close, Cypress Dri6e, St. Mellons, Cardiff, CF3 0EG, Wales, UK
Abstract Atomic force microscopy (AFM) in air has been used to study various III– V semiconductor heterostructures. Topography of the (110) cleaved cross-sections has been examined where oxidation processes modify the surface and allow the structures to be investigated. It is shown that surface height differences of as little as 0.4 A, are sufficient to distinguish between layers, and that quantum wells of as little as 7 nm width are detectable. © 2001 Elsevier Science B.V. All rights reserved. Keywords: AFM, Metrology; Semiconductors
1. Introduction The precise measurement of all the individual layer thicknesses in complex epitaxially grown device structures has always been difficult [1]. Optical or X-ray diffraction techniques only give reliable values for the simplest structures, and more direct methods such as transmission electron microscopy are very time consuming. However, with the advent of atomic force microscopes, which can operate in air whilst maintaining atomic resolution, there now exists the possibility of quick, simple, accurate and precise thickness measurements on even the most complex structures. This approach involves the cleaving of an epitaxial device structure to reveal its cross-section, followed by the measurement of its topography to determine the layer dimensions. Contrast between layers results from the different oxidation rates for different material compositions [2,3]. The oxidation mechanism of III – V semiconductors has been widely discussed [2] and will not be dealt with here. Important questions to be addressed are, how accurately can layer thicknesses be determined, the minimum step height measurable, and what time-scale is involved in obtaining good images. We have examined materials grown on GaAs and InP (001) substrates, i.e. AlGaAs, AlInGaP and InGaP, * Corresponding author. E-mail address: [email protected] (C. Jenkins).
and considered the effects of oxide thickness, varied by the exposure time to air and compositional dependent oxidation rates, on surface morphology and consequently the ability to determine the true position of interfaces. Below we discuss the performance of AFM during this study, and report a number of different issues that arose from the study. 2. Experimental considerations Heteroepitaxial semiconductor structures have been grown using metal organic vapour phase epitaxy (MOVPE) by Epitaxial Products International, St. Mellons, Cardiff, UK. For cross-sectional studies, the samples were cleaved in air at room temperature, and allowed to oxidise naturally. Normally, the time taken between cleaving and measurement is 15 min. It is possible to thermally oxidise or etch the samples, but that will not be considered here. The quality of the cleave is an important factor in the ability to extract accurate height profiles from the oxidised heterostructure. Indeed, steps as small as 0.3 nm on the surface can make some samples unsuitable for such investigation. Studies were carried out using a Digital Instruments NanoScope™ IIIa MultiMode™ SPM operating under TappingMode™ AFM.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5107(00)00634-6
C. Jenkins et al. / Materials Science and Engineering B80 (2001) 138–141
139
3. Technique capabilities The first type of structure we investigated was a vertical cavity surface emitting laser (VCSEL). The superlattice of the Bragg reflectors made this an ideal structure to study. Added to this were the quantum wells in the active region — an ideal test of the techniques’ ability to detect very thin layers. Fig. 1 shows a close up of the active region of one such VCSEL structure, produced 1 h after cleaving. In this case, the Bragg mirrors are made up of alternating layers of Al0.8Ga0.2As/Al0.15Ga0.85As. The 80% Al layers have undergone the most rapid oxidation, the oxide layer being 3.5 nm high with respect to the GaAs substrate. These layers therefore appear as the brightest bands in the image. The active region consists of three GaAs quantum wells, sandwiched between layers of Al0.3Ga0.7As. Each well was specified as being 66 A, wide, but was measured as 71 A, . The observed contrast is produced by an oxide height difference of 2 A, . Of the investigated semiconductor compounds, AlGaAs was found to oxidise the most quickly, the rate increasing with Al concentration. Phosphide based materials were found to oxidise the least, with combinations of Al and P oxidising the most slowly.
4. Oxidation rate differences between two GaAs layers In addition to distinguishing between different materials, steps have occasionally been observed between two GaAs layers. This is illustrated in Fig. 2. Both layers are silicon doped, with the layer on the left-hand side (the substrate) having a dopant concentration of 1018 cm − 3 and the layer on the right (a buffer layer) having one of 1019 cm − 3.
Fig. 1. The triple quantum well structure of a VCSEL (the three narrow dark bands running just to the left of the centre of the image). The wells are embedded in an active region, either side of which are the Bragg mirror stacks (the thick alternating light/dark layers). Each well is 66 A, wide, and the observed contrast is produced by an oxide height difference of 2 A, .
Fig. 2. Contrast produced between two differently doped GaAs layers. The layer to the right has the highest dopant level.
The image was taken 2 h after the cleave, and produced our smallest measured step height of 0.4 A, . This is comparable with the smallest possible height measurement capable with AFM. At this stage, the cause of the observed contrast is unclear. A number of factors could be responsible. The different growth conditions for the substrate and overlayer may lead to differences in defects and impurities or strain.
5. Oxide layer spreading The form of the growth of oxide layers has an important effect on any layer thickness measurements made. For example, apparent interface positions change with an increasing oxide layer. The surface transformation is best illustrated by the measurement of a superlattice structure. Fig. 3 shows such a structure 1 h after it has been cleaved. To the left
Fig. 3. Oxide layer induced contrast for an Al0.45Ga0.55As/AlAs superlattice structure after 1 h of oxidation.
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C. Jenkins et al. / Materials Science and Engineering B80 (2001) 138–141
Fig. 5. An AlGaAs based superlattice with cleave damage running throughout unhindered by the layer interfaces. Fig. 4. The same sample as in Fig. 3 after a day of oxidation. Note that the oxide layers have grown and merged together.
is the GaAs substrate, and to the right is the superlattice composed of layers of Al0.45Ga0.55As/AlAs, each being 20 nm wide. In this case the oxide on the AlAs has grown to a height of 10 nm. Fig. 4 shows the same sample a day latter. As well as increasing in height to 16 nm, the oxide on the AlAs layers has spread and merged together. As can be seen, oxide layers formed on the AlAs grow laterally as well as vertically, smearing out the original interface until they eventually merge together. The implications for the accurate determination of layer interfaces are immediately obvious. Although this was found to be the extreme case (as high concentration Al layers are involved), extensive oxidation of samples is to be avoided when layer thicknesses are being assessed. It is therefore advantageous to image a sample as soon after cleaving as possible. It must be noted, however, that in this case the determination of the superlattice repeat distance is still possible. Oxide layer growth can have other deleterious effects. For example, although very thin quantum wells are detectable soon after cleaving, a day later, surrounding material has obliterated the wells completely. Finally, oxide growth appears to not always be uniform. One can note from Fig. 3 and Fig. 4 the large variations in oxide growth along the AlAs layers. This effect is observed in AlGaAs layers down to about 80% Al, and has been noted in other work [3]. As of yet, no reason has been assigned to this.
a superlattice structure composed of Al0.8Ga0.2As/ Al0.15Ga0.85As layers. In this case, the cleave damage readily propagates through the layers unhindered. The effect is somewhat different in Fig. 6. Here we have an Al0.7Ga0.3As/AlInGaP interface. One can easily see the cleave damage on the Al0.7Ga0.3As layer to the right. However, no cleave damage appears in the AlInGaP layers to the immediate left. Throughout the study, 2° off Al based materials were routinely cleaved and imaged. 10° off phosphide based structures on the other hand, proved to be more problematical. The wafers required thinning in order to give a sufficient cleave quality.
7. Discussion AFM has demonstrated itself to be a powerful tool in III –V semiconductor device structure metrology. The results of this study indicate that oxide height differences of as little as 0.4 A, are sufficient to distinguish between layers. Although an effective contrast mechanism, the nature of oxide growth (i.e. its tendency to
6. Cleave damage As previously mentioned, an important factor in the ability to obtain layer dimensions is the quality of the cleave, especially if the material is slow to oxidise. The situation is complicated by the observation that cleave damage can relate to the device structure. Fig. 5 shows
Fig. 6. An AlGaAs layer (right) with cleave damage that clearly does not propagate through to the phosphide layers to its immediate left.
C. Jenkins et al. / Materials Science and Engineering B80 (2001) 138–141
spread in both the lateral and vertical directions), places an upper limit on the extent of oxidation before the uncertainty in interface position becomes intolerable. In general, the interfaces can be determined to within 10–50 nm, depending on the extent of the oxidation. For superlattice structures, the determination of repeat distances is only limited by the accuracy of the microscope itself (3%).
.
141
References [1] A. Rudra, L. Sagalowicz, K. Leifer, J. Behrend, C.A. Berseth, O. Dehaese, J.F. Carlin, et al., J. Crystal Growth 188 (1–4) (1998) 300– 306. [2] F. Reinhardt, B. Dwir, E. Kapon, Appl. Phys. Lett. 68 (22) (1996) 3168– 3170. [3] F. Reinhardt, B. Dwir, G. Biasiol, E. Kapon, Mater. Sci. Eng. B Solid State Mater. Adv. Technol. 37 (1 – 3) (1996) 83–88.