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Auger electron spectroscopy of deposited silane layers
NOTES Auger Electron Spectroscopy of Deposited Silane Layers of 20 A, and appears identical to that in Fig. 1. That is, while AES is sampling the nitrogen in the silane coating, it is sampling the silicon in the substrate surface layer beneath it. This is characteristic of "punch through" also known as "knock on" (2), a term used to indicate electron penetration of a loose nondense structure, to a depth greater than the first few monolayers. It is found predominantly for silicon and its cause is unknown. Here, it indicates a loose, possibly incomplete silane coating, which is, however, substantially thicker than one monolayer, as seen from the 60-A nitrogen-containing layer. This is in agreement with previous work (8-13), which also found thicknesses substantially greater than one monolayer. Such "punch through" is not found for a silane layer deposited from y-aminopropyltriethoxysilane
We wish to note a new application of Auger Electron Spectroscopy (AES) (1, 2). This note presents preliminary results, intended to indicate the possibilities of using AES in the study of deposited silane layers. AES probes the first few monolayers of a surface (with some exceptions). Its peak positions are characteristic of the atoms which ejected the Auger electrons (3, 4). When used with Reactive Ion Etching (RIE), in which argon ions bombard the surface and sputter off surface atoms, AES may be used to give a depth profile of the layer. This permits the approximate evaluation of the layer thickness and that of the layer structure, as well. The silanes used were all commercially available and were obtained fresh. They were prepared as 1% (w/w) solutions in deionized water, except where noted, and an equimolar amount of n-propyl amine was used to catalyze the deposition of those which were not amine containing (5, 6). Silicon wafers were immersed and coated for 10 sec; after drying overnight at room temperature they were heated at 110°C for 10 min, to drive off residual water. Auger spectra were obtained on a Physical Electronics Industries Model 545A Scanning Auger Microprobe. Initial experiments on sputtered quartz indicated that no beam-induced reduction of SiO2 to Si (7) occurred under the conditions used. Samples were evacuated for at least 75 hr, in order to reach an operating pressure of less than 2 × 10-t° Torr. RIE was carried out at an etch rate of approximately 20 A/min, as determined on SiO2. Since silanes are not expected to sputter at the same rate as SiOz, the sputter depths indicated throughout are only approximate. Figure 1 shows the etch profile of as-received silicon wafers, cleaned with an isopropyl alcohol wash. AES shows a 20-A Si(O) layer, most probably SiO2 produced by surface oxidation, a 20-A nitrogencontaining layer whose source is unknown, and both carbon- and oxygen-containing impurities extending substantially into the bulk. Figure 2 shows a silane layer deposited from 7-glycidoxypropyltrimethoxysilane (Union Carbide A 187). While the nitrogen-containing layer has now grown to 60 A, due to the incorporation of the catalyst, the Si(O) layer still remains at a thickness
100-
Si 80-
>, "~ 60-
E ._~ m 40-
20-
C
o
2'0
4=0
6'0
Depth 1~)
FIG. 1. A thickness profile of an as-received silicon wafer. The absolute magnitudes should not be taken as indicative of relative concentrations; they have been chosen to clarify plotting.
538 0021-9797/78/0673-0538502.00/0 Copyright © 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 67, No, 3, December 1978
NOTES
100-
Si
801
539
6O
80>
_c = 40-
-~
6o-
2040N
0 20.
2fO
40
610
810
Depth 1~) FIG. 4. A thickness profile of a silane layer deposited from 7-aminopropyltriethoxysilane in 50% water/50% methanol (v/v).
-C
20
4'0 Depth (~)
(~0
FIG. 2. A thickness profile of a silane layer deposited from y-glycidoxypropyltr/methoxysilane in water. (Union Carbide All00), as seen in Fig. 3, since the depth of the Si(O) layer is the same as that of the nitrogen-containing layer; this indicates a denser, more complete silane coating.
6O ==
~ o
40.
tr =._I
Si
2O-
si(o) --N 0
40
20
8'0
Depth (~) FXG. 3. A thickness profile of a silane layer deposited from y-aminopropyltriethoxysilane in water.
The layer deposited from 7-aminopropyltriethoxysilane in 50% water/50% methanol (v/v), is seen in Fig. 4. Note that "punch through" occurs, as evidenced by the 20-• Si(O) layer and the 80-A nitrogen-containing layers. This indicates that the solvent may influence the deposition density. It is evident from the figures that all of the species have thickness-dependent concentrations. In Fig. 2, the presence of nitrogen throughout the thickness indicates that the catalyst is incorporated into the layer and is chemically bound, since it is not lost at the low pressures used. The decrease in nitrogen concentration with thickness in Figs. 3 and 4 indicates that the amine groups are preferentially concentrated toward the free surface, as is necessary for adhesion promotion through chemical reaction of the amine group with an appropriate coating resin. In the case of the dense silane layer deposited from 7-aminopropyltriethoxysilane, in Fig. 3, the carbon and nitrogen profiles seem to be in an almost mirror image relationship: The nitrogen concentration is highest at the outer edge and as one approaches the interface, while the carbon concentration is lowest in these regions. Since the nitrogen is that on the terminal amino group and the carbon that on an adjacent methylene group, the profiles appear to indicate that the outer layers of the silane differ from the b u l k , in that the amino groups are oriented toward the outer surface. How they differ is, at present, unknown although suggestions have been made as to possible structures (11 - 13). It is anticipated that work presently in progress will make clearer the unanswered questions on silane coating density, structural variations, and the influence Journal of Colloid and Interface Science, Vol.67, No. 3, December1978
540
NOTES
of the solvent. Another important point to resolve is why the oxygen- and carbon-containing signals of Figs. 2 and 4, which have not been reported to demonstrate "punch through," are almost identical to those found for the impurities contained in the uncoated substrate, as seen in Fig. 1. In summary, AES has great potential in delineating silane layer thickness and orientation. Using this method, we have thus far identified two types of thickness profiles: type I, exemplified by Fig. 3, in which the Si(O) peak decays in a manner similar to the decay of the O, C, and N peaks, indicates a dense silane network. Type II, exemplified by Figs. 2 and 4, in which "punch-through" causes the exhibition of the underlying SiO2 layer, indicates a loose silane network. REFERENCES 1. Chang, C. C., Surface Sci. 25, 53 (1971). 2. Chang, C. C., in "Characterization of Solid Surfaces" (P. F. Kane and G. B. Larrabee, Eds.), Chap. 20. Plenum Press, New York, 1974. 3. Palmberg, P. W., Riach, G. E., Weber, R. E., and MacDonald, N. C., "Handbook of Auger Electron Spectroscopy." Physical Electronics Industries, Edina, MN, 1972. 4. Coughlan, W. A., and Clausing, R. E., Atom. Data 5, 317 (1973).
Journal of Colloidand Interface Science. Vol.67. No. 3, December1978
5. Kaas, R. L., and Kardos, J. L., Polym. Eng. Sci. 11, 11 (1971). 6. Gent, A. N., and Hsu, E. C., Macromolecules 7, 933 (1974). 7. Thomas, S., J. Appl. Phys. 45, 161 (1974). 8. Koenig, J. L., and Shih, P. T. K., J. Colloid Interface Sci. 36, 247 (1971). 9. Bascom, W. D., Macromolecules 5, 792 (1972). 10. Shih, P. T. K., and Koenig, J. L., Mat. Sci. Eng. 20, 137, 145 (1975). 11. Boerio, F. J., and Grievenkamp, J, E., 32nd ANTEC, Reinf. Plast./Comp. Inst., SPI. 1977, Section 4-A. 12. Ishida, H., and Koenig, J. L., 32nd ANTEC, Reinf. Plast./Comp. Inst., SPI, 1977, Section 4-B. 13. Kritchevsky, G., and Uhlmann, D. R., Coatings Plast. Preprints 37(2), 446 (1977). J. F. CAIN* E. SACHERt'1 *Failure Analysis and Environmental Test and #Materials and Engineering Analysis IBM Corporation P.O. Box 6 Endicott, New York 13760 Received March 23. 1978; accepted July 27, 1978
i Author to whom correspondence should be sent.
We wish to note a new application of Auger Electron Spectroscopy (AES) (1, 2). This note presents preliminary results, intended to indicate the possibilities of using AES in the study of deposited silane layers. AES probes the first few monolayers of a surface (with some exceptions). Its peak positions are characteristic of the atoms which ejected the Auger electrons (3, 4). When used with Reactive Ion Etching (RIE), in which argon ions bombard the surface and sputter off surface atoms, AES may be used to give a depth profile of the layer. This permits the approximate evaluation of the layer thickness and that of the layer structure, as well. The silanes used were all commercially available and were obtained fresh. They were prepared as 1% (w/w) solutions in deionized water, except where noted, and an equimolar amount of n-propyl amine was used to catalyze the deposition of those which were not amine containing (5, 6). Silicon wafers were immersed and coated for 10 sec; after drying overnight at room temperature they were heated at 110°C for 10 min, to drive off residual water. Auger spectra were obtained on a Physical Electronics Industries Model 545A Scanning Auger Microprobe. Initial experiments on sputtered quartz indicated that no beam-induced reduction of SiO2 to Si (7) occurred under the conditions used. Samples were evacuated for at least 75 hr, in order to reach an operating pressure of less than 2 × 10-t° Torr. RIE was carried out at an etch rate of approximately 20 A/min, as determined on SiO2. Since silanes are not expected to sputter at the same rate as SiOz, the sputter depths indicated throughout are only approximate. Figure 1 shows the etch profile of as-received silicon wafers, cleaned with an isopropyl alcohol wash. AES shows a 20-A Si(O) layer, most probably SiO2 produced by surface oxidation, a 20-A nitrogencontaining layer whose source is unknown, and both carbon- and oxygen-containing impurities extending substantially into the bulk. Figure 2 shows a silane layer deposited from 7-glycidoxypropyltrimethoxysilane (Union Carbide A 187). While the nitrogen-containing layer has now grown to 60 A, due to the incorporation of the catalyst, the Si(O) layer still remains at a thickness
100-
Si 80-
>, "~ 60-
E ._~ m 40-
20-
C
o
2'0
4=0
6'0
Depth 1~)
FIG. 1. A thickness profile of an as-received silicon wafer. The absolute magnitudes should not be taken as indicative of relative concentrations; they have been chosen to clarify plotting.
538 0021-9797/78/0673-0538502.00/0 Copyright © 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 67, No, 3, December 1978
NOTES
100-
Si
801
539
6O
80>
_c = 40-
-~
6o-
2040N
0 20.
2fO
40
610
810
Depth 1~) FIG. 4. A thickness profile of a silane layer deposited from 7-aminopropyltriethoxysilane in 50% water/50% methanol (v/v).
-C
20
4'0 Depth (~)
(~0
FIG. 2. A thickness profile of a silane layer deposited from y-glycidoxypropyltr/methoxysilane in water. (Union Carbide All00), as seen in Fig. 3, since the depth of the Si(O) layer is the same as that of the nitrogen-containing layer; this indicates a denser, more complete silane coating.
6O ==
~ o
40.
tr =._I
Si
2O-
si(o) --N 0
40
20
8'0
Depth (~) FXG. 3. A thickness profile of a silane layer deposited from y-aminopropyltriethoxysilane in water.
The layer deposited from 7-aminopropyltriethoxysilane in 50% water/50% methanol (v/v), is seen in Fig. 4. Note that "punch through" occurs, as evidenced by the 20-• Si(O) layer and the 80-A nitrogen-containing layers. This indicates that the solvent may influence the deposition density. It is evident from the figures that all of the species have thickness-dependent concentrations. In Fig. 2, the presence of nitrogen throughout the thickness indicates that the catalyst is incorporated into the layer and is chemically bound, since it is not lost at the low pressures used. The decrease in nitrogen concentration with thickness in Figs. 3 and 4 indicates that the amine groups are preferentially concentrated toward the free surface, as is necessary for adhesion promotion through chemical reaction of the amine group with an appropriate coating resin. In the case of the dense silane layer deposited from 7-aminopropyltriethoxysilane, in Fig. 3, the carbon and nitrogen profiles seem to be in an almost mirror image relationship: The nitrogen concentration is highest at the outer edge and as one approaches the interface, while the carbon concentration is lowest in these regions. Since the nitrogen is that on the terminal amino group and the carbon that on an adjacent methylene group, the profiles appear to indicate that the outer layers of the silane differ from the b u l k , in that the amino groups are oriented toward the outer surface. How they differ is, at present, unknown although suggestions have been made as to possible structures (11 - 13). It is anticipated that work presently in progress will make clearer the unanswered questions on silane coating density, structural variations, and the influence Journal of Colloid and Interface Science, Vol.67, No. 3, December1978
540
NOTES
of the solvent. Another important point to resolve is why the oxygen- and carbon-containing signals of Figs. 2 and 4, which have not been reported to demonstrate "punch through," are almost identical to those found for the impurities contained in the uncoated substrate, as seen in Fig. 1. In summary, AES has great potential in delineating silane layer thickness and orientation. Using this method, we have thus far identified two types of thickness profiles: type I, exemplified by Fig. 3, in which the Si(O) peak decays in a manner similar to the decay of the O, C, and N peaks, indicates a dense silane network. Type II, exemplified by Figs. 2 and 4, in which "punch-through" causes the exhibition of the underlying SiO2 layer, indicates a loose silane network. REFERENCES 1. Chang, C. C., Surface Sci. 25, 53 (1971). 2. Chang, C. C., in "Characterization of Solid Surfaces" (P. F. Kane and G. B. Larrabee, Eds.), Chap. 20. Plenum Press, New York, 1974. 3. Palmberg, P. W., Riach, G. E., Weber, R. E., and MacDonald, N. C., "Handbook of Auger Electron Spectroscopy." Physical Electronics Industries, Edina, MN, 1972. 4. Coughlan, W. A., and Clausing, R. E., Atom. Data 5, 317 (1973).
Journal of Colloidand Interface Science. Vol.67. No. 3, December1978
5. Kaas, R. L., and Kardos, J. L., Polym. Eng. Sci. 11, 11 (1971). 6. Gent, A. N., and Hsu, E. C., Macromolecules 7, 933 (1974). 7. Thomas, S., J. Appl. Phys. 45, 161 (1974). 8. Koenig, J. L., and Shih, P. T. K., J. Colloid Interface Sci. 36, 247 (1971). 9. Bascom, W. D., Macromolecules 5, 792 (1972). 10. Shih, P. T. K., and Koenig, J. L., Mat. Sci. Eng. 20, 137, 145 (1975). 11. Boerio, F. J., and Grievenkamp, J, E., 32nd ANTEC, Reinf. Plast./Comp. Inst., SPI. 1977, Section 4-A. 12. Ishida, H., and Koenig, J. L., 32nd ANTEC, Reinf. Plast./Comp. Inst., SPI, 1977, Section 4-B. 13. Kritchevsky, G., and Uhlmann, D. R., Coatings Plast. Preprints 37(2), 446 (1977). J. F. CAIN* E. SACHERt'1 *Failure Analysis and Environmental Test and #Materials and Engineering Analysis IBM Corporation P.O. Box 6 Endicott, New York 13760 Received March 23. 1978; accepted July 27, 1978
i Author to whom correspondence should be sent.