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Water sorbability of low-k dielectrics measured by thermal desorption spectroscopy
Surface Science 566–568 (2004) 566–570 www.elsevier.com/locate/susc
Water sorbability of low-k dielectrics measured by thermal desorption spectroscopy Hiroshi Yanazawa *, Takuya Fukuda, Yoko Uchida, Ichiro Katou Association of Super-Advanced Electronic Technologies (ASET), Time 24 BLD. 10F. 2-45 Aomi, Koutouku, 135-8073 Tokyo, Japan Available online 19 June 2004
Abstract With the aim of collecting information for a reliability database, a method of quantitatively determining the amount of water that adsorbs on dielectric films was established that employs thermal desorption spectroscopy (TDS). The issues of measurement repeatability and the stability of the background level were examined, and a noble dehalogenation treatment was developed to improve these factors. In this study, the amount of water that adsorbs on SiLK resin ( Trademark of The Dow Chemical Company) and on porous silica film was measured and compared with values for p-TEOS and wet thermal SiO2 films. Samples stored in air in a clean room, stored in air with a relative humidity of 100%, and subjected to pressure-cook treatment were investigated by TDS. Reflecting the strong affinity to water, the amount of water that adsorbed on porous silica and p-TEOS films after pressure-cook treatments were 49 · 1016 and 30 · 1016 molecules/cm2 , respectively. In contrast, the amount for SiLK resin was only 8 · 1016 molecules/cm2 , which is very close to 6 · 1016 molecules/cm2 for thermally grown SiO2 . These differences in water sorbability are discussed based on the chemical and physical properties of the materials. 2004 Elsevier B.V. All rights reserved. Keywords: Dielectric phenomena; Silicon oxides; Porous solids; Water; Physical adsorption; Thermal desorption spectroscopy
1. Introduction Over the past several years, a great deal of effort has gone into the development of new ultralow-k dielectric materials. Porous silica and organic polymers seem to be promising candidates. However, several reliability issues must be resolved before they can be used confidently in mass production. The problem of water adsorption is particularly troubling as it can lead to device degradation and process incompatibility. In this study, *
Corresponding author. Tel.: +81-3-5531-0091; fax: +81-35531-0093. E-mail address: [email protected] (H. Yanazawa).
the amount of water that adsorbs on SiLK resin and on porous silica film was measured and compared to values for thermally grown SiO2 and p-TEOS films. Special care was taken in TDS measurements, because it is rather difficult to quantitatively determine the amount of water in thin films. 2. Experimental 2.1. Dielectric materials Commercially available SiLK resin was spincoated on a Si wafer covered with SiO2 , and cured at 450 C for 5 min on a hot plate to form a thin
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.118
H. Yanazawa et al. / Surface Science 566–568 (2004) 566–570
2.2. Thermal desorption spectroscopy A WA1000S system (Denshi-Kagaku, Japan) was used for the TDS measurements, and the measurement chamber was evacuated with a turbomolecular pump down to 5 · 10 8 Pa. The samples were cut into 8-mm-square specimens and weighed on a microbalance (Mettler Toledo AB204S, Switzerland). Then, the surface area was calculated from the known density and thickness of the Si substrate. Samples were (1) stored in air in a clean room; (2) stored in air with relative humidity of 100%; and (3), for comparison, subjected to pressure-cook treatment. The pressure-cook treatment is employed as a standard procedure for device degradation studies, in which samples are exposed to 85% humidity at 120 C for 2 h. Spectra were taken at temperatures from 100 to 500 C for SiLK film and porous silica, and from 100 to 900 C for thermally grown SiO2 and p-TEOS, considering the thermal stability of each material. The amount of water desorbed in the temperature range from 150 to 450 C was calculated using installed software after subtraction of the background level for the day of the measurement. A noble method for dehalogenation was developed in this study. This treatment [1] involves placing a specimen in an analysis chamber along with special dehalogenizing agent, heating it slowly from 100 to 520 C in 60 min, and cooling it down to room temperature. 3. Results and discussion 3.1. Stability of background level and repeatability of TDS measurements Fig. 1 shows the day by day TDS background spectra for 35 days within a nine-month period.
16 Amount of desorbed gas (1015 molecules)
film 400 nm thick. Porous silica film was synthesised by the surfactant template method (Courtesy of Dr. Susa of Hitachi Chemical), spin-coated on a Si wafer, and baked at 400 C for 6 min. The thickness of the resulting film was 450 nm. Wet thermal SiO2 and plasma CVD TEOS films were formed by conventional methods, and had thicknesses of 400 and 630 nm, respectively.
567
H2 H2O CO/N2
14 12
Sample
CO2
10 8 6
Background
4 2 0 0
20 Experimental run
40
60
Fig. 1. Variation in TDS background level for different days.
Some scatter of the data was observed, especially in the beginning, but there is no clear correlation between operation schedule and background level. For comparison, the amount of gas desorbed from a sample, in this case SiLK, is shown on the right side of the figure. The values are more than 10 times the background level. For the quantitative analysis of TDS spectra, the background on the day of the measurement was subtracted from the measurement results. Dehalogenation is a very effective way to keep the background level stable when the chamber is contaminated. Fig. 2 shows an example. When the chamber was contaminated, the fluorine signal reached as high as 4 · 10 10 A; but after the dehalogenation treatment, it dropped to 1 · 10 12 A, which is an improvement of more than two orders of magnitude. The repeatability of the TDS measurements was tested using p-TEOS samples. Fig. 3 shows the results of measurements on three separate days of (1) five samples stored in air in a clean room and (2) eight samples stored in air with a relative humidity of 100%. That is, the first two samples of (1) and the first three samples of (2) were measured on one day; and the next sample of (1) and the next two samples of (2) were measured on another day; and the last two samples of (1) and the last three samples of (2) were measured on a third day. The amount of water desorbed from the (1) samples is
568
H. Yanazawa et al. / Surface Science 566–568 (2004) 566–570
Fig. 2. Effect of dehalogenation treatment.
Air, 100% humidity
25
Air
20
however, a scatter of several percent must be taken into account in the water measurements. Fig. 4 compares the water sorbability of SiLK film, porous silica, p-TEOS and wet thermal SiO2 . The two columns for each material are for the amount of water that desorbed from (1) samples stored in air in a clean room and (3) samples
15 50
10
Air
5 0
1
2
3 4 5 6 Experimental run
7
8
Fig. 3. Measurement repeatability.
about half that from the (2) samples. Since the data show no marked differences between experimental runs, it can be concluded that the storage conditions are not an important factor in measurement repeatability. And at the same time,
Amount of desorbed H2O (1016 molecules/cm2 )
Amount of desorbed H2O (10 16 molecules/cm2)
30
pressure-cook 40
30
20
10
0
SiLK
Porous silica
P-TEOS
Wet thermal SiO2
Material
Fig. 4. Comparison of water sorbability of dielectric films.
H. Yanazawa et al. / Surface Science 566–568 (2004) 566–570
569
Table 1 Water sorbability of dielectric films Sample
Pressure cook
450 400 360 1100
1.1 19 6.5 0.8
7.2 48 28 5.6
Trademark of The Dow Chemical Company.
12
60
2
subjected to pressure-cook treatment. As shown in Table 1, after pressure-cook treatments, porous silica adsorbed six times more water than SiLK film, and p-TEOS adsorbed 4 times more. These differences are attributed to differences in the affinity of the materials to water; that is, SiLK resin is hydrophobic and SiO2 is very hydrophilic. Thermally grown SiO2 is inert to water because its affinity is lessened during high-temperature oxidation and is not restored by the pressure-cook treatment. To discover more about the affinity of these materials to water, the following experiments were carried out to examine the relationship between the amount of water adsorbed and the temperature of the heat treatment. The first TDS spectra of pTEOS were taken at temperatures up to a specific final temperature, namely, 500, 600, 700, 800 or 900 C. Next, the sample was transferred to a storage box containing air with a relative humidity of 100% and kept for 2 weeks. Then, the second TDS spectrum was taken. The amount of desorbed H2 O obtained from the second TDS spectrum was plotted against the final temperature of the first TDS measurement, or in other words, the heat treatment temperature. As shown by the solid squares in Fig. 5, the affinity of the surface of pTEOS to water decreases with temperature, which suggests the formation and stabilization of siloxane bridges on the surface. When the p-TEOS data is extrapolated to 1100 C, as indicated by the broken line, we find that the data point for wet thermal SiO2 lies on that line. This shows the typical surface characteristics of SiO2 . Solid circles in Fig. 5 show data for porous silica. In this case, the amount of desorbed water is 50 · 1016 molecules/cm2 , regardless of the temperature. This amount is over five
16
a
SiLK resin Porous silica TEOS Wet-oxidized SiO2
Atmosphere
Amount of desorbed H2O (10 molecules/cm )
1 2 3 4
a
H2 O desorbed (1016 molecules/cm2 )
Temperature processed (C)
Porous Silica 10
50
8
40
6
p-TEOS
30
4
20
SiLK
Wet Thermal SiO2 10
2
0 200
0 400
600
800
1000
1200
Temperature(°C) Fig. 5. Effect of heat treatment on water sorbability.
times greater than that for p-TEOS. This is probably due to the porous structure; namely, even
Fig. 6. Size distribution of pores in dielectric films.
570
H. Yanazawa et al. / Surface Science 566–568 (2004) 566–570
though the porous silica surface partially loses its affinity to water, a large amount of water can adsorb in micropores through what is called capillary condensation. This is consistent with the fact that the amount of desorbed H2 O is independent of temperature, reflecting the total volume of micropores. Fig. 6 shows some results for Kr gas adsorption [2], which indicate that, among the dielectrics tested in this study, micropores are present only in the porous silica. The small amount of water that desorbed from SiLK can be explained by the hydrophobic nature of its surface.
4. Conclusion A method of quantitatively measuring the amount of water that adsorbs on thin films was developed that employs thermal desorption spectroscopy. Special care was taken to ensure a stable background level and good measurement repeatability; and success in this regard was due in part to the development of a noble dehalogenation treatment. A comparison of water sorbability revealed that porous silica and p-TEOS have a
strong affinity to water, while SiLK and thermally grown SiO2 have less affinity. The difference between the two groups differs by a factor as great as 5–10. Pressure-cook treatments enhanced water adsorption for every material, pointing to a possible reliability issue in future device development.
Acknowledgements The authors are grateful to Dr. Susa of Hitachi Chemical for supplying the porous silica. They also wish to thank Mr. Yamaguchi of Tokyo Electron, Ltd. for the sample preparation. They also wish to express deep appreciation to Mr. Maejima of Denshi Kagaku, for his help in TDS measurements. This work was performed under the management of ASET in a METI R & D program supported by NEDO.
References [1] Japanese Patent 2003-13732. [2] H. Yanazawa et al., J. Vac. Sci. Technol. B 20 (2002) 1833.
Water sorbability of low-k dielectrics measured by thermal desorption spectroscopy Hiroshi Yanazawa *, Takuya Fukuda, Yoko Uchida, Ichiro Katou Association of Super-Advanced Electronic Technologies (ASET), Time 24 BLD. 10F. 2-45 Aomi, Koutouku, 135-8073 Tokyo, Japan Available online 19 June 2004
Abstract With the aim of collecting information for a reliability database, a method of quantitatively determining the amount of water that adsorbs on dielectric films was established that employs thermal desorption spectroscopy (TDS). The issues of measurement repeatability and the stability of the background level were examined, and a noble dehalogenation treatment was developed to improve these factors. In this study, the amount of water that adsorbs on SiLK resin ( Trademark of The Dow Chemical Company) and on porous silica film was measured and compared with values for p-TEOS and wet thermal SiO2 films. Samples stored in air in a clean room, stored in air with a relative humidity of 100%, and subjected to pressure-cook treatment were investigated by TDS. Reflecting the strong affinity to water, the amount of water that adsorbed on porous silica and p-TEOS films after pressure-cook treatments were 49 · 1016 and 30 · 1016 molecules/cm2 , respectively. In contrast, the amount for SiLK resin was only 8 · 1016 molecules/cm2 , which is very close to 6 · 1016 molecules/cm2 for thermally grown SiO2 . These differences in water sorbability are discussed based on the chemical and physical properties of the materials. 2004 Elsevier B.V. All rights reserved. Keywords: Dielectric phenomena; Silicon oxides; Porous solids; Water; Physical adsorption; Thermal desorption spectroscopy
1. Introduction Over the past several years, a great deal of effort has gone into the development of new ultralow-k dielectric materials. Porous silica and organic polymers seem to be promising candidates. However, several reliability issues must be resolved before they can be used confidently in mass production. The problem of water adsorption is particularly troubling as it can lead to device degradation and process incompatibility. In this study, *
Corresponding author. Tel.: +81-3-5531-0091; fax: +81-35531-0093. E-mail address: [email protected] (H. Yanazawa).
the amount of water that adsorbs on SiLK resin and on porous silica film was measured and compared to values for thermally grown SiO2 and p-TEOS films. Special care was taken in TDS measurements, because it is rather difficult to quantitatively determine the amount of water in thin films. 2. Experimental 2.1. Dielectric materials Commercially available SiLK resin was spincoated on a Si wafer covered with SiO2 , and cured at 450 C for 5 min on a hot plate to form a thin
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.118
H. Yanazawa et al. / Surface Science 566–568 (2004) 566–570
2.2. Thermal desorption spectroscopy A WA1000S system (Denshi-Kagaku, Japan) was used for the TDS measurements, and the measurement chamber was evacuated with a turbomolecular pump down to 5 · 10 8 Pa. The samples were cut into 8-mm-square specimens and weighed on a microbalance (Mettler Toledo AB204S, Switzerland). Then, the surface area was calculated from the known density and thickness of the Si substrate. Samples were (1) stored in air in a clean room; (2) stored in air with relative humidity of 100%; and (3), for comparison, subjected to pressure-cook treatment. The pressure-cook treatment is employed as a standard procedure for device degradation studies, in which samples are exposed to 85% humidity at 120 C for 2 h. Spectra were taken at temperatures from 100 to 500 C for SiLK film and porous silica, and from 100 to 900 C for thermally grown SiO2 and p-TEOS, considering the thermal stability of each material. The amount of water desorbed in the temperature range from 150 to 450 C was calculated using installed software after subtraction of the background level for the day of the measurement. A noble method for dehalogenation was developed in this study. This treatment [1] involves placing a specimen in an analysis chamber along with special dehalogenizing agent, heating it slowly from 100 to 520 C in 60 min, and cooling it down to room temperature. 3. Results and discussion 3.1. Stability of background level and repeatability of TDS measurements Fig. 1 shows the day by day TDS background spectra for 35 days within a nine-month period.
16 Amount of desorbed gas (1015 molecules)
film 400 nm thick. Porous silica film was synthesised by the surfactant template method (Courtesy of Dr. Susa of Hitachi Chemical), spin-coated on a Si wafer, and baked at 400 C for 6 min. The thickness of the resulting film was 450 nm. Wet thermal SiO2 and plasma CVD TEOS films were formed by conventional methods, and had thicknesses of 400 and 630 nm, respectively.
567
H2 H2O CO/N2
14 12
Sample
CO2
10 8 6
Background
4 2 0 0
20 Experimental run
40
60
Fig. 1. Variation in TDS background level for different days.
Some scatter of the data was observed, especially in the beginning, but there is no clear correlation between operation schedule and background level. For comparison, the amount of gas desorbed from a sample, in this case SiLK, is shown on the right side of the figure. The values are more than 10 times the background level. For the quantitative analysis of TDS spectra, the background on the day of the measurement was subtracted from the measurement results. Dehalogenation is a very effective way to keep the background level stable when the chamber is contaminated. Fig. 2 shows an example. When the chamber was contaminated, the fluorine signal reached as high as 4 · 10 10 A; but after the dehalogenation treatment, it dropped to 1 · 10 12 A, which is an improvement of more than two orders of magnitude. The repeatability of the TDS measurements was tested using p-TEOS samples. Fig. 3 shows the results of measurements on three separate days of (1) five samples stored in air in a clean room and (2) eight samples stored in air with a relative humidity of 100%. That is, the first two samples of (1) and the first three samples of (2) were measured on one day; and the next sample of (1) and the next two samples of (2) were measured on another day; and the last two samples of (1) and the last three samples of (2) were measured on a third day. The amount of water desorbed from the (1) samples is
568
H. Yanazawa et al. / Surface Science 566–568 (2004) 566–570
Fig. 2. Effect of dehalogenation treatment.
Air, 100% humidity
25
Air
20
however, a scatter of several percent must be taken into account in the water measurements. Fig. 4 compares the water sorbability of SiLK film, porous silica, p-TEOS and wet thermal SiO2 . The two columns for each material are for the amount of water that desorbed from (1) samples stored in air in a clean room and (3) samples
15 50
10
Air
5 0
1
2
3 4 5 6 Experimental run
7
8
Fig. 3. Measurement repeatability.
about half that from the (2) samples. Since the data show no marked differences between experimental runs, it can be concluded that the storage conditions are not an important factor in measurement repeatability. And at the same time,
Amount of desorbed H2O (1016 molecules/cm2 )
Amount of desorbed H2O (10 16 molecules/cm2)
30
pressure-cook 40
30
20
10
0
SiLK
Porous silica
P-TEOS
Wet thermal SiO2
Material
Fig. 4. Comparison of water sorbability of dielectric films.
H. Yanazawa et al. / Surface Science 566–568 (2004) 566–570
569
Table 1 Water sorbability of dielectric films Sample
Pressure cook
450 400 360 1100
1.1 19 6.5 0.8
7.2 48 28 5.6
Trademark of The Dow Chemical Company.
12
60
2
subjected to pressure-cook treatment. As shown in Table 1, after pressure-cook treatments, porous silica adsorbed six times more water than SiLK film, and p-TEOS adsorbed 4 times more. These differences are attributed to differences in the affinity of the materials to water; that is, SiLK resin is hydrophobic and SiO2 is very hydrophilic. Thermally grown SiO2 is inert to water because its affinity is lessened during high-temperature oxidation and is not restored by the pressure-cook treatment. To discover more about the affinity of these materials to water, the following experiments were carried out to examine the relationship between the amount of water adsorbed and the temperature of the heat treatment. The first TDS spectra of pTEOS were taken at temperatures up to a specific final temperature, namely, 500, 600, 700, 800 or 900 C. Next, the sample was transferred to a storage box containing air with a relative humidity of 100% and kept for 2 weeks. Then, the second TDS spectrum was taken. The amount of desorbed H2 O obtained from the second TDS spectrum was plotted against the final temperature of the first TDS measurement, or in other words, the heat treatment temperature. As shown by the solid squares in Fig. 5, the affinity of the surface of pTEOS to water decreases with temperature, which suggests the formation and stabilization of siloxane bridges on the surface. When the p-TEOS data is extrapolated to 1100 C, as indicated by the broken line, we find that the data point for wet thermal SiO2 lies on that line. This shows the typical surface characteristics of SiO2 . Solid circles in Fig. 5 show data for porous silica. In this case, the amount of desorbed water is 50 · 1016 molecules/cm2 , regardless of the temperature. This amount is over five
16
a
SiLK resin Porous silica TEOS Wet-oxidized SiO2
Atmosphere
Amount of desorbed H2O (10 molecules/cm )
1 2 3 4
a
H2 O desorbed (1016 molecules/cm2 )
Temperature processed (C)
Porous Silica 10
50
8
40
6
p-TEOS
30
4
20
SiLK
Wet Thermal SiO2 10
2
0 200
0 400
600
800
1000
1200
Temperature(°C) Fig. 5. Effect of heat treatment on water sorbability.
times greater than that for p-TEOS. This is probably due to the porous structure; namely, even
Fig. 6. Size distribution of pores in dielectric films.
570
H. Yanazawa et al. / Surface Science 566–568 (2004) 566–570
though the porous silica surface partially loses its affinity to water, a large amount of water can adsorb in micropores through what is called capillary condensation. This is consistent with the fact that the amount of desorbed H2 O is independent of temperature, reflecting the total volume of micropores. Fig. 6 shows some results for Kr gas adsorption [2], which indicate that, among the dielectrics tested in this study, micropores are present only in the porous silica. The small amount of water that desorbed from SiLK can be explained by the hydrophobic nature of its surface.
4. Conclusion A method of quantitatively measuring the amount of water that adsorbs on thin films was developed that employs thermal desorption spectroscopy. Special care was taken to ensure a stable background level and good measurement repeatability; and success in this regard was due in part to the development of a noble dehalogenation treatment. A comparison of water sorbability revealed that porous silica and p-TEOS have a
strong affinity to water, while SiLK and thermally grown SiO2 have less affinity. The difference between the two groups differs by a factor as great as 5–10. Pressure-cook treatments enhanced water adsorption for every material, pointing to a possible reliability issue in future device development.
Acknowledgements The authors are grateful to Dr. Susa of Hitachi Chemical for supplying the porous silica. They also wish to thank Mr. Yamaguchi of Tokyo Electron, Ltd. for the sample preparation. They also wish to express deep appreciation to Mr. Maejima of Denshi Kagaku, for his help in TDS measurements. This work was performed under the management of ASET in a METI R & D program supported by NEDO.
References [1] Japanese Patent 2003-13732. [2] H. Yanazawa et al., J. Vac. Sci. Technol. B 20 (2002) 1833.