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High-resolution electron microscopy of structural features at the SiSiO2 interface
Volume 5, number 3
MATERIALS LETTERS
February 1987
HIGH-RESOLUTION ELECTRON MICROSCOPY OF STRUCTURAL FEATURES AT THE Si/SiOz INTERFACE A.H. CARIM and R. SINCLAIR Departmentof~ater~als Science and Engineering, Stanford Un~~ersi fy~Stanford, CA 943~~-1698, USA Received 25 November 1986
High-resolution transmission electron microscopy (HREM) allows observation of interface morphology at close to the atomic level. Careful examination of the images yields a wealth of info~ation about the structure studied. Cross-sectional micro~aphs through a silicon/silicon dioxide/~lysilicon structure are presented here, showing interface roughness at the substrate/oxide boundary. The various image features in each region are elucidated. HREM provides a precise method for measuring tilm thickness and determining the structural uniformity of the oxide and its interfaces. The temptation to regard electron micrographs as two-dimensional slices through the material should, however, be resisted; we briefly discuss the superposition effects that may result from finite sample thickness.
One of the most valuable tools at the disposal of the materials scientist for examining the structure of solids is the transmission electron microscope (TEM). Use of this instrument at high resolution allows the direct observation of atomic arrangements in the material. Much info~ation is implicit in each micrograph, although caution should be exercised in deriving the actual morphological features from the recorded image. In this report, we present considerations on high-resolution cross-sectional micro~aphs of a somewhat rough Si/SiOz interface. The image features in the substrate, the oxide, and a capping polycrystalline silicon layer are discussed. We also point out that the sample does have finite thickness in the beam directiotrand cannot always be regarded as a simple two-dimensional slice through the material. Some consequences of the sample thickness on image interpretation and on the conclusions drawn from high-resolution electron microscopy (HREM ) images are mentioned. The material in question consisted of 4-6 fJ cm ptype < 100) Si that had been oxidized at 900°C and subsequently capped with polyc~stalline silicon grown by chemical vapor deposition (CVD) at 620” C, as described earlier [ 11. Cross-sectional TEM samples were produced by standard techniques [ 21 and viewed in a Philips EM430ST electron microscope at an accelerating voltage of 200 kV. Axial illu94
mination was employed, and the objective aperture was removed in order to allow the transmitted beam and all diffracted beams to contribute to the image. The instrument has a point-to-point resolution of under 0.25 nm when operated in this mode. Fig. 1 shows the S~SiO~/polysilicon thin-fdrn structure at high resolution. Overall brightness in the various regions of the micrograph depends to some extent on minor variations in the sample thickness over the area imaged. The inset in fig. 1 shows the detailed image ch~acte~stics in the substrate silicon. It is convenient to think of the prominent, elongated white spots as representing the lattice points in the diamond cubic crystal structure of silicon. These points are illustrated schematically in fig. 2. The diagram also indicates the lattice-point and lattice-plane spacings associated with the image that allow precise determination of the scale of features within each micrograph. The actual observed image (fig. 1 inset) contains additional weaker spots interposed between those that represent the crystal lattice. These features arise from interference between higher-order reflections at a characteristic specimen thickness and objective lens defocus value, and are not representative of the crystal structure [ 31. They are rarely observed in images taken with lower-resolution instruments. Silicon dioxide, when imaged at high resolution,
0167-577x/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
February 1987
MATERIALS LETTERS
Volume 5. number 3
Fig. 1.High-resolution micrograph of cross section through WSiOJpoly-Si structure. Inset shows detail of image features in the substt -ate silicon region.
has an appearance that is typical of amorphous materials. Crystalline inclusions, pinholes, precipitates, and similar defects in the SO2 layer should be discernible down to the nm level in such micro-
graphs; no such inhomogeneities have been detected in our studies. Interfacial roughness at the substrate/oxide boundary is, however, evident in fig. 1. This arises as a consequence of the thermal oxidation con-
%I,=
33 nm
0.384
nm
Fig. 2. (110) projection of the lattice points in the diamond cubic structure, with several prominent lattice-point and lattice-plane separations for silicon noted.
95
Volume 5, number 3
MATERIALS LETTERS
ditions and primarily reflects the presence of unoxidized silicon protrusions remaining at the interface [ 41. The qualitative effects of sample thickness on the imaging of these asperities will be discussed shortly. The capping layer of polycrystalline silicon is apparent above the thin oxide layer. The individual grains in the polysilicon film have various orientations, leading to different diffraction conditions for each crystallite and corresponding differences in transmitted electron current density and the resulting image intensity. Orientation contrast is very strong when only the transmitted beam is included in the objective aperture, but is less prominent in the present images where the objective aperture is removed entirely. Some grains may be oriented such that there is sufficient contrast between the polysilicon and the adjoining oxide to delineate the boundary between them. It is difficult, however, to precisely locate that interface by such contrast alone on a scale of tenths of nanometers. Instead, the lattice fringes that occur in a number of polysilicon grains provide a more accurate indication of the location of the interface. Single sets of lattice fringes are often discernible whenever any particular type of closelyspaced lattice planes (especially the { 111) planes, with a spacing of d, , ,= 0.314 nm) are almost parallel to the incident electron beam. Since alignment of the polysilicon grain along a specific low-index zone axis is not required, these conditions are considerably less restrictive than those necessary for observing the projection of lattice points as desired in the single-crystal silicon substrate. Polysilicon fringes are often visible in regions where cross-fringes appear in the substrate, thus allowing clear definition of both the substrate/oxide and oxidelpolysilicon interfaces and an accurate measurement of the local oxide thickness. Lattice imaging has demonstrated that the interface between the thin oxide and the capping polysilicon layer is normally very flat, even in cases where the corresponding substrate/oxide boundary contains substantial asperities. We can also verify that the overlying crystalline material is, indeed, polysilicon by measuring the width of the lattice fringes and confirming that they correspond to planar spacings in silicon, using the known substrate lattice spacings as a magnification reference. Ellipsometric measurements made on the same 96
February 1987
Fig. 3. High-resolution image illustrating a large protrusion at the substrate/oxide interface.
wafers before the polysilicon deposition indicated an oxide thickness of 8.4 nm. HREM shows that the actual oxide thickness ranges from 2.7 to 5.6 nm, with much of the variation arising from extreme interfacial roughness at the substrate/oxide boundary (see, e.g., fig. 3) which is not reproduced at the oxidelpolysilicon interface. The local differences in oxide thickness indicate that the reaction rate varies over the surface in the initial stages of oxidation. Further information on this process of non-uniform oxidation and on the discrepancies between ellipsometric and HREM determinations of oxide thickness has been presented elsewhere [ 41. A number of HREM studies have been conducted that examine the morphology of the interface between single-crystal silicon and its thermally grown oxide. Early results indicated that interface was essentially flat, with occasional atomic steps of only one plane in height [ 51. Recent work, however, has shown that certain processing procedures, including the growth of thin oxides at relatively low temperatures [ 1,4,6,7] and oxidation in chlorinated ambients [ 8,9], can lead to substantial Si/SiOZ roughness. The characterization of such roughness by cross-sectional microscopy is complicated by the effects of sample thickness in the beam direction. In fig. 4, we illustrate several situations that may arise when a non-uniform surface or interface is viewed in cross section. The two factors that are explicitly considered here are the density of features (in this case, Si protrusions) at the interface and the sample thickness. Fig. 4a schematically depicts an interface with a
MATERIALS LETTERS
Volume 5. number 3
viewing
direction
viewing
direction
February 1987
Fig. 4. Schematic representations of surface or interface roughness. (a) “Low” density of protrusions. Thin samples (2,) may contain partial features or none at all; protrusions in thicker samples (z2) may be obscured by the oxide layer in the viewing derection. (b) “High” density of protrusions. Features may superpose, as shown in (c), a projection of (b) along the viewing direction. Dotted lines indicate protrusion outlines which are obscured by overlapping features in the image.
fairly low density of protrusions. A thin sample sec-
tion is indicated by the shaded region labelled zI. The image from this specimen would not show any of the existing asperities. The extent of the prot~sions would only be evident if one were fortunate enough to intersect one or more at their maximum height, which should occasionally be the case for a random distribution. On the other hand, let us consider a thicker sample (of width z2). In this case, numerous entire prot~sions would be included in the sample cross section examined. These features may or may
not be apparent in the high-resolution image depending on their thickness in the viewing direction relative to the total sample thickness. For SiOt on Si, the irregular a~angement of the atoms in the amorphous oxide on both sides of the protrusion in the beam direction will obscure the projection of the crystalline structure. As a result, the protrusion may appear smaller or may not appear at all in the image. A larger density of protrusions is sketched in fig. 4b. Here we can see that a sufficiently thin region of the specimen may give an accurate representation of 97
Volume 5. number 3
MATERIALS LETTERS
interface roughness. In a thicker sample, however, the protrusions may superpose as shown in the profile in fig. 4c. The net result of all of these thickness and superposition effects is that the apparent interface roughness observed in the image is less than the actual interface roughness in the sample. This occurs when the imaging conditions are not optimal, i.e. when the sample is too thick or when the density of protrusions is small and the sample is very thin. Such effects are not considered in some recent studies and may account for some of the discrepancies in the literature. The maximum protrusion height observed is thus the most appropriate quantity to use in characterizing the extent of surface or interface roughening. If one attempts instead to determine an average height for the protrusions, features that appear smaller due to projection effects will be averaged in along with features that actually are physically smaller. An alternative approach is to determine a rootmean-square roughness (d) and correlation length (L) from the Fourier transform of the image [ 8, lo]. While this analysis might be useful for modelling the effect of roughness on electrical properties such as carrier mobility, it may provide a misleading impression of the physical situation. The actual interface is not an undulating boundary with some mean value of roughness; rather, our studies indicate that much of the interface is flat with occasional Si protrusions whose size and impact are not well-represented by A. Maximum observed protrusion height is a better indicator of the local inhomogeneities that may result under a given set of oxidation conditions. With respect to dielectric breakdown, the oxide is weakest at the largest protrusions due to reduced oxide thickness and electric field enhancement in the silicon. Furthermore, Fourier analysis is not always valuable for electrical comparisons; in at least one study, there was a large discrepancy between the A and L values deduced from electron micrographs and those that were indicated by the electrical data [ lo]. Several difficulties are encountered in the Fourier transform approach: reduction of the apparent roughness must again be considered; the accuracy of sample thickness determination is poor for the extremely thin samples necessary for HREM; and
98
February 1987
specimen thickness also varies considerably over the field of view necessary for a representative Fourier transform. These problems are circumvented by examining a considerable length of the interface at varying sample thickness and characterizing the roughness by the size of the largest protrusions observed. In each of our TEM specimens, a total of a micron or more of material spaced at irregular intervals along the interface was examined and photographed at high resolution. The protrusions of the greatest base-to-tip height were then readily located and measured. Details of that work are given in the references cited earlier [ 1,4]. The authors appreciate the support and involvement of Philips Research Laboratories Sunnyvale I Signetics Corporation in this research, and particularly wish to thank Charles Vorst and W.T. Stacy for their contributions.
References [ 1] A.H. Carim and A. Bhattacharyya, Appl. Phys. Letters 46 (1985) 872. [2] J.C. Bravman and R. Sinclair, J. Electron Microsc. Technique 1 (1984) 53. [ 31 J.C.H. Spence, Experimental high-resolution electron microscopy (Clarendon Press, Oxford, 198 1) p. 323. [4] A.H. Carim and R. Sinclair, J. Electrochem. Sot., to be published; in: Semiconductor silicon 1986, eds. H.R. Huff, T. Abe and B. Kolbesen (The Electrochemical Society, Pennington, 1986) p. 458. [ 5 ] O.L. Krivanek, T.T. Sheng, D.C. Tsui and A. Kamgar, in: The physics of SiOZ and its interfaces, ed. S.T. Pantelides (Pergamon, New York, 1978) p. 356. [6] O.L. Krivanek and J.H. Mazur, Appl. Phys. Letters 37 (1980) 392. [ 71 N.M. Ravindra, D. Fathy, J. Narayan, J.K. Srivastava and E.A. Irene, Mat. Letters 4 (1986) 337. [8] 2. Liliental, O.L. Krivanek, SM. Goodnick and C.W. Wilmsen, Mat. Res. Sot. Symp. Proc. 37 (1985) 193. [ 91 C. Claeys, J. Vanhellemont, G. Declerck, J. van Landuyt, R. van Overstraeten and S. Amelinckx, in: VLSI science and technologyll985, eds. W.M. Bullis and S. Broydo (The Electrochemical Society, Pennington, 1985) p. 329. [lo] S.M. Goodnick, R.G. Gann, J.R. Sites, D.K. Ferry, C.W. Wilmsen, D. Fathy and O.L. Krivanek, J. Vacuum Sci. Technol. Bl (1983) 803.
MATERIALS LETTERS
February 1987
HIGH-RESOLUTION ELECTRON MICROSCOPY OF STRUCTURAL FEATURES AT THE Si/SiOz INTERFACE A.H. CARIM and R. SINCLAIR Departmentof~ater~als Science and Engineering, Stanford Un~~ersi fy~Stanford, CA 943~~-1698, USA Received 25 November 1986
High-resolution transmission electron microscopy (HREM) allows observation of interface morphology at close to the atomic level. Careful examination of the images yields a wealth of info~ation about the structure studied. Cross-sectional micro~aphs through a silicon/silicon dioxide/~lysilicon structure are presented here, showing interface roughness at the substrate/oxide boundary. The various image features in each region are elucidated. HREM provides a precise method for measuring tilm thickness and determining the structural uniformity of the oxide and its interfaces. The temptation to regard electron micrographs as two-dimensional slices through the material should, however, be resisted; we briefly discuss the superposition effects that may result from finite sample thickness.
One of the most valuable tools at the disposal of the materials scientist for examining the structure of solids is the transmission electron microscope (TEM). Use of this instrument at high resolution allows the direct observation of atomic arrangements in the material. Much info~ation is implicit in each micrograph, although caution should be exercised in deriving the actual morphological features from the recorded image. In this report, we present considerations on high-resolution cross-sectional micro~aphs of a somewhat rough Si/SiOz interface. The image features in the substrate, the oxide, and a capping polycrystalline silicon layer are discussed. We also point out that the sample does have finite thickness in the beam directiotrand cannot always be regarded as a simple two-dimensional slice through the material. Some consequences of the sample thickness on image interpretation and on the conclusions drawn from high-resolution electron microscopy (HREM ) images are mentioned. The material in question consisted of 4-6 fJ cm ptype < 100) Si that had been oxidized at 900°C and subsequently capped with polyc~stalline silicon grown by chemical vapor deposition (CVD) at 620” C, as described earlier [ 11. Cross-sectional TEM samples were produced by standard techniques [ 21 and viewed in a Philips EM430ST electron microscope at an accelerating voltage of 200 kV. Axial illu94
mination was employed, and the objective aperture was removed in order to allow the transmitted beam and all diffracted beams to contribute to the image. The instrument has a point-to-point resolution of under 0.25 nm when operated in this mode. Fig. 1 shows the S~SiO~/polysilicon thin-fdrn structure at high resolution. Overall brightness in the various regions of the micrograph depends to some extent on minor variations in the sample thickness over the area imaged. The inset in fig. 1 shows the detailed image ch~acte~stics in the substrate silicon. It is convenient to think of the prominent, elongated white spots as representing the lattice points in the diamond cubic crystal structure of silicon. These points are illustrated schematically in fig. 2. The diagram also indicates the lattice-point and lattice-plane spacings associated with the image that allow precise determination of the scale of features within each micrograph. The actual observed image (fig. 1 inset) contains additional weaker spots interposed between those that represent the crystal lattice. These features arise from interference between higher-order reflections at a characteristic specimen thickness and objective lens defocus value, and are not representative of the crystal structure [ 31. They are rarely observed in images taken with lower-resolution instruments. Silicon dioxide, when imaged at high resolution,
0167-577x/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
February 1987
MATERIALS LETTERS
Volume 5. number 3
Fig. 1.High-resolution micrograph of cross section through WSiOJpoly-Si structure. Inset shows detail of image features in the substt -ate silicon region.
has an appearance that is typical of amorphous materials. Crystalline inclusions, pinholes, precipitates, and similar defects in the SO2 layer should be discernible down to the nm level in such micro-
graphs; no such inhomogeneities have been detected in our studies. Interfacial roughness at the substrate/oxide boundary is, however, evident in fig. 1. This arises as a consequence of the thermal oxidation con-
%I,=
33 nm
0.384
nm
Fig. 2. (110) projection of the lattice points in the diamond cubic structure, with several prominent lattice-point and lattice-plane separations for silicon noted.
95
Volume 5, number 3
MATERIALS LETTERS
ditions and primarily reflects the presence of unoxidized silicon protrusions remaining at the interface [ 41. The qualitative effects of sample thickness on the imaging of these asperities will be discussed shortly. The capping layer of polycrystalline silicon is apparent above the thin oxide layer. The individual grains in the polysilicon film have various orientations, leading to different diffraction conditions for each crystallite and corresponding differences in transmitted electron current density and the resulting image intensity. Orientation contrast is very strong when only the transmitted beam is included in the objective aperture, but is less prominent in the present images where the objective aperture is removed entirely. Some grains may be oriented such that there is sufficient contrast between the polysilicon and the adjoining oxide to delineate the boundary between them. It is difficult, however, to precisely locate that interface by such contrast alone on a scale of tenths of nanometers. Instead, the lattice fringes that occur in a number of polysilicon grains provide a more accurate indication of the location of the interface. Single sets of lattice fringes are often discernible whenever any particular type of closelyspaced lattice planes (especially the { 111) planes, with a spacing of d, , ,= 0.314 nm) are almost parallel to the incident electron beam. Since alignment of the polysilicon grain along a specific low-index zone axis is not required, these conditions are considerably less restrictive than those necessary for observing the projection of lattice points as desired in the single-crystal silicon substrate. Polysilicon fringes are often visible in regions where cross-fringes appear in the substrate, thus allowing clear definition of both the substrate/oxide and oxidelpolysilicon interfaces and an accurate measurement of the local oxide thickness. Lattice imaging has demonstrated that the interface between the thin oxide and the capping polysilicon layer is normally very flat, even in cases where the corresponding substrate/oxide boundary contains substantial asperities. We can also verify that the overlying crystalline material is, indeed, polysilicon by measuring the width of the lattice fringes and confirming that they correspond to planar spacings in silicon, using the known substrate lattice spacings as a magnification reference. Ellipsometric measurements made on the same 96
February 1987
Fig. 3. High-resolution image illustrating a large protrusion at the substrate/oxide interface.
wafers before the polysilicon deposition indicated an oxide thickness of 8.4 nm. HREM shows that the actual oxide thickness ranges from 2.7 to 5.6 nm, with much of the variation arising from extreme interfacial roughness at the substrate/oxide boundary (see, e.g., fig. 3) which is not reproduced at the oxidelpolysilicon interface. The local differences in oxide thickness indicate that the reaction rate varies over the surface in the initial stages of oxidation. Further information on this process of non-uniform oxidation and on the discrepancies between ellipsometric and HREM determinations of oxide thickness has been presented elsewhere [ 41. A number of HREM studies have been conducted that examine the morphology of the interface between single-crystal silicon and its thermally grown oxide. Early results indicated that interface was essentially flat, with occasional atomic steps of only one plane in height [ 51. Recent work, however, has shown that certain processing procedures, including the growth of thin oxides at relatively low temperatures [ 1,4,6,7] and oxidation in chlorinated ambients [ 8,9], can lead to substantial Si/SiOZ roughness. The characterization of such roughness by cross-sectional microscopy is complicated by the effects of sample thickness in the beam direction. In fig. 4, we illustrate several situations that may arise when a non-uniform surface or interface is viewed in cross section. The two factors that are explicitly considered here are the density of features (in this case, Si protrusions) at the interface and the sample thickness. Fig. 4a schematically depicts an interface with a
MATERIALS LETTERS
Volume 5. number 3
viewing
direction
viewing
direction
February 1987
Fig. 4. Schematic representations of surface or interface roughness. (a) “Low” density of protrusions. Thin samples (2,) may contain partial features or none at all; protrusions in thicker samples (z2) may be obscured by the oxide layer in the viewing derection. (b) “High” density of protrusions. Features may superpose, as shown in (c), a projection of (b) along the viewing direction. Dotted lines indicate protrusion outlines which are obscured by overlapping features in the image.
fairly low density of protrusions. A thin sample sec-
tion is indicated by the shaded region labelled zI. The image from this specimen would not show any of the existing asperities. The extent of the prot~sions would only be evident if one were fortunate enough to intersect one or more at their maximum height, which should occasionally be the case for a random distribution. On the other hand, let us consider a thicker sample (of width z2). In this case, numerous entire prot~sions would be included in the sample cross section examined. These features may or may
not be apparent in the high-resolution image depending on their thickness in the viewing direction relative to the total sample thickness. For SiOt on Si, the irregular a~angement of the atoms in the amorphous oxide on both sides of the protrusion in the beam direction will obscure the projection of the crystalline structure. As a result, the protrusion may appear smaller or may not appear at all in the image. A larger density of protrusions is sketched in fig. 4b. Here we can see that a sufficiently thin region of the specimen may give an accurate representation of 97
Volume 5. number 3
MATERIALS LETTERS
interface roughness. In a thicker sample, however, the protrusions may superpose as shown in the profile in fig. 4c. The net result of all of these thickness and superposition effects is that the apparent interface roughness observed in the image is less than the actual interface roughness in the sample. This occurs when the imaging conditions are not optimal, i.e. when the sample is too thick or when the density of protrusions is small and the sample is very thin. Such effects are not considered in some recent studies and may account for some of the discrepancies in the literature. The maximum protrusion height observed is thus the most appropriate quantity to use in characterizing the extent of surface or interface roughening. If one attempts instead to determine an average height for the protrusions, features that appear smaller due to projection effects will be averaged in along with features that actually are physically smaller. An alternative approach is to determine a rootmean-square roughness (d) and correlation length (L) from the Fourier transform of the image [ 8, lo]. While this analysis might be useful for modelling the effect of roughness on electrical properties such as carrier mobility, it may provide a misleading impression of the physical situation. The actual interface is not an undulating boundary with some mean value of roughness; rather, our studies indicate that much of the interface is flat with occasional Si protrusions whose size and impact are not well-represented by A. Maximum observed protrusion height is a better indicator of the local inhomogeneities that may result under a given set of oxidation conditions. With respect to dielectric breakdown, the oxide is weakest at the largest protrusions due to reduced oxide thickness and electric field enhancement in the silicon. Furthermore, Fourier analysis is not always valuable for electrical comparisons; in at least one study, there was a large discrepancy between the A and L values deduced from electron micrographs and those that were indicated by the electrical data [ lo]. Several difficulties are encountered in the Fourier transform approach: reduction of the apparent roughness must again be considered; the accuracy of sample thickness determination is poor for the extremely thin samples necessary for HREM; and
98
February 1987
specimen thickness also varies considerably over the field of view necessary for a representative Fourier transform. These problems are circumvented by examining a considerable length of the interface at varying sample thickness and characterizing the roughness by the size of the largest protrusions observed. In each of our TEM specimens, a total of a micron or more of material spaced at irregular intervals along the interface was examined and photographed at high resolution. The protrusions of the greatest base-to-tip height were then readily located and measured. Details of that work are given in the references cited earlier [ 1,4]. The authors appreciate the support and involvement of Philips Research Laboratories Sunnyvale I Signetics Corporation in this research, and particularly wish to thank Charles Vorst and W.T. Stacy for their contributions.
References [ 1] A.H. Carim and A. Bhattacharyya, Appl. Phys. Letters 46 (1985) 872. [2] J.C. Bravman and R. Sinclair, J. Electron Microsc. Technique 1 (1984) 53. [ 31 J.C.H. Spence, Experimental high-resolution electron microscopy (Clarendon Press, Oxford, 198 1) p. 323. [4] A.H. Carim and R. Sinclair, J. Electrochem. Sot., to be published; in: Semiconductor silicon 1986, eds. H.R. Huff, T. Abe and B. Kolbesen (The Electrochemical Society, Pennington, 1986) p. 458. [ 5 ] O.L. Krivanek, T.T. Sheng, D.C. Tsui and A. Kamgar, in: The physics of SiOZ and its interfaces, ed. S.T. Pantelides (Pergamon, New York, 1978) p. 356. [6] O.L. Krivanek and J.H. Mazur, Appl. Phys. Letters 37 (1980) 392. [ 71 N.M. Ravindra, D. Fathy, J. Narayan, J.K. Srivastava and E.A. Irene, Mat. Letters 4 (1986) 337. [8] 2. Liliental, O.L. Krivanek, SM. Goodnick and C.W. Wilmsen, Mat. Res. Sot. Symp. Proc. 37 (1985) 193. [ 91 C. Claeys, J. Vanhellemont, G. Declerck, J. van Landuyt, R. van Overstraeten and S. Amelinckx, in: VLSI science and technologyll985, eds. W.M. Bullis and S. Broydo (The Electrochemical Society, Pennington, 1985) p. 329. [lo] S.M. Goodnick, R.G. Gann, J.R. Sites, D.K. Ferry, C.W. Wilmsen, D. Fathy and O.L. Krivanek, J. Vacuum Sci. Technol. Bl (1983) 803.