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D.c. dielectric breakdown in phosphosilicate glass films prepared by low temperature chemical vapour deposition
Thin Solid Films, 186 (1990)L25-L28
L25
LeRer
D.c. dielectric breakdown in phosphosilicate glass films prepared by low temperature chemical vapour deposition DANIEL SERGHI AND CRISTIAN PAVELESCU R&D Center for Electronic Components (CCSIT-CE), Str. Erou lancu Nicolae, Nr. 32 B, R-72996, Bucharest (Romania)
(ReceivedNovember 16, 1989;acceptedDecember7, 1989) The effect of phosphorous concentration on some dielectric characteristics (i.e. breakdown voltage and dielectric constant) of phosphosilicate glass (PSG) films chemically vapour deposited (CVD) was previously reported 1. The aim of this work is to study the influence of phosphorus concentration, annealing temperature and medium on the d.c. dielectric breakdown of CVD PSG films deposited in the S i H 4 - P H 3 - O 2 - N 2 system at atmospheric pressure and low temperature. PSG films in the 150-200 nm thickness range (ellipsometrically measured) were deposited in a nozzle type reactor at 410 °C 2 on n-type silicon wafers of resistivity 4-6 D cm. The O2:SiH 4 ratio was kept at 20 (Fsm , = 38 cm 3 min-1, where F is the flow t~ate) and the PH 3 flow rate was varied between 0 and 5.5 cm 3 m i n - 1, in a total flow (N 2 balanced) of 9.81 min-1. Films within the 0-8 wt.~oP concentration were deposited, as determined from wet chemical etch rate data 3. Annealings were performed in dry N2, for 0.5 h, at temperatures in the range 600-1000 °C, in 93~o H 2 0 relative humidity, for 150 h, at 40 °C and in steam, for 0.5 h, at 600 °C. The d.c. destructive dielectric breakdown was investigated on metal-semiconductor capacitors of area 0.84mm 2, with counterelectrodes deliniated by a photolithographic process on 1 ~tm thick electron gun evaporated aluminium films (unsintered). Measurements were done with silicon in accumulation as shown previously ~, the destructive breakdown event being recorded at a current flow through the capacitor of 10 ~tA. Hystograms for 300 samples on wafers are presented (Figs. 1, 2 and 3) in terms of percentage breakdown events vs. breakdown field, with phosphorus concentration, annealing temperature and medium as parameters. The incidence of events due to gross structural defects (e.g. pinholes) recorded at fields in the range 0-1 MV cm-1 remains insensitive to any change, and the peak corresponding to processrelated defects (usually between 1-5 M V c m - 1 ) 5 is absent (excepting a sample annealed in dry N 2 at 1000 °C). These facts prove a good process consistency. The peak corresponding to the intrinsic breakdown strength s increases markedly vs. phosphorus concentration for as-deposited PSG films, the average breakdown field increasing from 8 MV c m - 1 for 0 wt.~o P to 10 MV c m - 1 for 4 wt.~ P and even stronger to 14 MV cm-1 for 8 wt.~o P (Figs. l(a), l(b) and l(c) 0040-6090/90/$3.50
© ElsevierSequoia/Printedin The Netherlands
L26
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Fig. 1. The effect of phosphorus concentration (wt.~, P) on the hystogram of breakdown events (~o) vs. breakdown field (MV cm-1) of PSG films: (a) 0 wt.°/o P; (b) 4 wt.°,~ P; (c) 8 wt./o°/p,. (deposition conditions: temperature, 410'C; O2:SiH,,=20; FsiH4=38 cm3min-~: F p H , = 0 5.5 cmamin 1; F,otal = 9.81min 1).
respectively). This behaviour was previously interpreted in terms of defect density dropping in SiO2 films with increase in phosphorus content 6. Also, correlations can be made between the stress reduction due to phosphorus incorporation ~ and the evolution of the P = O absorption band v s . phosphorus concentration which affects the density and refractive index of PSG films near 8 wt.~o P concentration 7. Annealing in dry N z of phosphorus-doped PSG films clearly reduces the dielectric strength when the annealing temperature increases from 600 to 1000 °C. Thus, the average breakdown field of 8wt.~o P doped films decreases from 14 MV cm ~ for as-deposited film, to 10 MV c m - 1 for films annealed at 600 and 800°C (Figs. 2(a), 2(b) and 2(c)), and the hystogram approaches a narrower distribution at 800 °C. We infer from here not only the effect of stress (evoluting from tensile to zero to compressive when the annealing temperature increases 1) or the possible effect of O H removal 4, but also an effect of superior vitrification of the asdeposited, phosphorus-doped PSG films during annealing in dry N~. However, 8 wt.~o P doped PSG films annealed at 1000 °C in dry N 2 shows a strong decrease in average breakdown field at 5 MV c m - 1, and a strong increase in the number of breakdown events at low breakdown fields ( 0 - 2 M V c m -1) (Fig. 2(d)), this behaviour being interpreted in terms of the flow phenomenon which appears at temperatures above 900 °C 8, and determines an important structural change of asdeposited films, which push the film to a more random (amorphous) state, with a large number of defects. PSG films with 8 wt.~ P content present a decrease in average breakdown strength from 14 MV cm-~ for as-deposited films, to 11 MV c m - ~ for films annealed in 93~o HzO relative humidity, for 150 h, at 40 °C, and to 8 MV cm- ~ for films annealed in steam, for 0.5 h, at 600 °C (Figs. 3(a), 3(b) and 3(c) respectively).
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Fig. 2. B r e a k d o w n events (%) vs. b r e a k d o w n field ( M V c m - t ) with annealing temperature T in dry N z for 0.5 h as a parameter, for 8 wt.% P doped PSG films (deposition conditions as for Fig. 1): (a) as deposited; (b) T = 600 °C; (c) T = 800 °C; (d) T = 1000 °C. Fig. 3. Breakdown events (%) vs. breakdown field (MV c m - ~) with annealing ambient and temperature as parameters, for 8 wt.% P doped PSG films (deposition conditions as for Fig. 1): (a) as deposited; (b) annealed in 90% H zO relative humidity at 40 °C for 150 h; (c) annealed in steam at 600 °C for 0.5 h; (d) annealed in dry N 2 at 600 °C for 0.5 h.
These variations may be related to the removal of phosphorus atoms from 8 wt.% P doped as-deposited PSG films, during annealing in water containing ambients, this behaviour being noticed previously for as-deposited PSG films with a phosphorus concentration above 6 wt.% P when subjected to a moisture test in saturated water vapour for 4 h at 120 °C and a vapour pressure of 1.96 atmospheres 9. However for 8 wt~ P doped PSG films annealed in steam at 600 °C, the decrease in average breakdown strength (Fig. 3(c)) may be explained by a cumulative effect between phosphorus removal and the structural change suffered by the films during thermal annealing, the second effect being evinced by the decrease in average breakdown field from 14 MVcm -1 for as deposited films to 11 MVcm -1 for films annealed in dry N 2 at 600 °C (Fig. 3(a) and 3(d) respectively). In conclusion, the increase in phosphorus concentration in 0-8 wt.~o P range in as-deposited PSG films, also increases the intrinsic breakdown field, from 8 to 14 MV cm-1, this fact being explained in terms of defect density dropping in SiO2 film with an increase in phosphorus content. The average breakdown field of 8 wt.~o P doped PSG films decreases from 14 to 5 MV cm- 1 when annealing temperature, in dry N 2 , increases from 600 to 1000 °C, this behaviour being interpreted by the fact that as-deposited PSG films which have
L28
LETTERS
an intrinsic degree of order, are p u s h e d to a m o r e r a n d o m ( a m o r p h o u s ) state by annealing, with a higher n u m b e r of defects. T h e decrease in dielectric strength of 8 wt.~o P d o p e d P S G films after a n n e a l i n g in water c o n t a i n i n g ambients, is explained b y the p h o s p h o r u s r e m o v a l from the films which has a deleterious effect on the b r e a k d o w n characteristics of P S G films. 1 2 3 4 5 6 7 8 9
Y. Shioya and M. Maeda, J. Electrochem. Soc., 133 (1986) 109. W. Kern, J. Vac. Sci. Technol., 14 (1977) 1082. W. Kern, RCA Rev., 37 (1976) 78. C. Cobianu and C. Pavelescu, Thin Solid Films, 143 (1986) 109. N.J. Chou and J. M. Elridge, J. Electrochem. Soc., 117 (1970) 1287. C.M. Osburn and D. W. Ormond, J. Electrochem. Soc., 119 (1972) 597. C. Pavelescu, C. Cobianu, N. Vlahovici and C. Ghita, Thin Solid Films, 161 (1988) L71. R.A. Levy, Solid State Technol., 29 (1986) 123. N, Nagasima, H. Suzuki, K. Tanaka and S. Nishida, J. Electrochem. Sot., 121 (1974) 434.
L25
LeRer
D.c. dielectric breakdown in phosphosilicate glass films prepared by low temperature chemical vapour deposition DANIEL SERGHI AND CRISTIAN PAVELESCU R&D Center for Electronic Components (CCSIT-CE), Str. Erou lancu Nicolae, Nr. 32 B, R-72996, Bucharest (Romania)
(ReceivedNovember 16, 1989;acceptedDecember7, 1989) The effect of phosphorous concentration on some dielectric characteristics (i.e. breakdown voltage and dielectric constant) of phosphosilicate glass (PSG) films chemically vapour deposited (CVD) was previously reported 1. The aim of this work is to study the influence of phosphorus concentration, annealing temperature and medium on the d.c. dielectric breakdown of CVD PSG films deposited in the S i H 4 - P H 3 - O 2 - N 2 system at atmospheric pressure and low temperature. PSG films in the 150-200 nm thickness range (ellipsometrically measured) were deposited in a nozzle type reactor at 410 °C 2 on n-type silicon wafers of resistivity 4-6 D cm. The O2:SiH 4 ratio was kept at 20 (Fsm , = 38 cm 3 min-1, where F is the flow t~ate) and the PH 3 flow rate was varied between 0 and 5.5 cm 3 m i n - 1, in a total flow (N 2 balanced) of 9.81 min-1. Films within the 0-8 wt.~oP concentration were deposited, as determined from wet chemical etch rate data 3. Annealings were performed in dry N2, for 0.5 h, at temperatures in the range 600-1000 °C, in 93~o H 2 0 relative humidity, for 150 h, at 40 °C and in steam, for 0.5 h, at 600 °C. The d.c. destructive dielectric breakdown was investigated on metal-semiconductor capacitors of area 0.84mm 2, with counterelectrodes deliniated by a photolithographic process on 1 ~tm thick electron gun evaporated aluminium films (unsintered). Measurements were done with silicon in accumulation as shown previously ~, the destructive breakdown event being recorded at a current flow through the capacitor of 10 ~tA. Hystograms for 300 samples on wafers are presented (Figs. 1, 2 and 3) in terms of percentage breakdown events vs. breakdown field, with phosphorus concentration, annealing temperature and medium as parameters. The incidence of events due to gross structural defects (e.g. pinholes) recorded at fields in the range 0-1 MV cm-1 remains insensitive to any change, and the peak corresponding to processrelated defects (usually between 1-5 M V c m - 1 ) 5 is absent (excepting a sample annealed in dry N 2 at 1000 °C). These facts prove a good process consistency. The peak corresponding to the intrinsic breakdown strength s increases markedly vs. phosphorus concentration for as-deposited PSG films, the average breakdown field increasing from 8 MV c m - 1 for 0 wt.~o P to 10 MV c m - 1 for 4 wt.~ P and even stronger to 14 MV cm-1 for 8 wt.~o P (Figs. l(a), l(b) and l(c) 0040-6090/90/$3.50
© ElsevierSequoia/Printedin The Netherlands
L26
~"
LETTERS
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5 BREAKDOWN
10 15 FIELD {MV/cm)
Fig. 1. The effect of phosphorus concentration (wt.~, P) on the hystogram of breakdown events (~o) vs. breakdown field (MV cm-1) of PSG films: (a) 0 wt.°/o P; (b) 4 wt.°,~ P; (c) 8 wt./o°/p,. (deposition conditions: temperature, 410'C; O2:SiH,,=20; FsiH4=38 cm3min-~: F p H , = 0 5.5 cmamin 1; F,otal = 9.81min 1).
respectively). This behaviour was previously interpreted in terms of defect density dropping in SiO2 films with increase in phosphorus content 6. Also, correlations can be made between the stress reduction due to phosphorus incorporation ~ and the evolution of the P = O absorption band v s . phosphorus concentration which affects the density and refractive index of PSG films near 8 wt.~o P concentration 7. Annealing in dry N z of phosphorus-doped PSG films clearly reduces the dielectric strength when the annealing temperature increases from 600 to 1000 °C. Thus, the average breakdown field of 8wt.~o P doped films decreases from 14 MV cm ~ for as-deposited film, to 10 MV c m - 1 for films annealed at 600 and 800°C (Figs. 2(a), 2(b) and 2(c)), and the hystogram approaches a narrower distribution at 800 °C. We infer from here not only the effect of stress (evoluting from tensile to zero to compressive when the annealing temperature increases 1) or the possible effect of O H removal 4, but also an effect of superior vitrification of the asdeposited, phosphorus-doped PSG films during annealing in dry N~. However, 8 wt.~o P doped PSG films annealed at 1000 °C in dry N 2 shows a strong decrease in average breakdown field at 5 MV c m - 1, and a strong increase in the number of breakdown events at low breakdown fields ( 0 - 2 M V c m -1) (Fig. 2(d)), this behaviour being interpreted in terms of the flow phenomenon which appears at temperatures above 900 °C 8, and determines an important structural change of asdeposited films, which push the film to a more random (amorphous) state, with a large number of defects. PSG films with 8 wt.~ P content present a decrease in average breakdown strength from 14 MV cm-~ for as-deposited films, to 11 MV c m - ~ for films annealed in 93~o HzO relative humidity, for 150 h, at 40 °C, and to 8 MV cm- ~ for films annealed in steam, for 0.5 h, at 600 °C (Figs. 3(a), 3(b) and 3(c) respectively).
LETTERS
L27
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Fig. 2. B r e a k d o w n events (%) vs. b r e a k d o w n field ( M V c m - t ) with annealing temperature T in dry N z for 0.5 h as a parameter, for 8 wt.% P doped PSG films (deposition conditions as for Fig. 1): (a) as deposited; (b) T = 600 °C; (c) T = 800 °C; (d) T = 1000 °C. Fig. 3. Breakdown events (%) vs. breakdown field (MV c m - ~) with annealing ambient and temperature as parameters, for 8 wt.% P doped PSG films (deposition conditions as for Fig. 1): (a) as deposited; (b) annealed in 90% H zO relative humidity at 40 °C for 150 h; (c) annealed in steam at 600 °C for 0.5 h; (d) annealed in dry N 2 at 600 °C for 0.5 h.
These variations may be related to the removal of phosphorus atoms from 8 wt.% P doped as-deposited PSG films, during annealing in water containing ambients, this behaviour being noticed previously for as-deposited PSG films with a phosphorus concentration above 6 wt.% P when subjected to a moisture test in saturated water vapour for 4 h at 120 °C and a vapour pressure of 1.96 atmospheres 9. However for 8 wt~ P doped PSG films annealed in steam at 600 °C, the decrease in average breakdown strength (Fig. 3(c)) may be explained by a cumulative effect between phosphorus removal and the structural change suffered by the films during thermal annealing, the second effect being evinced by the decrease in average breakdown field from 14 MVcm -1 for as deposited films to 11 MVcm -1 for films annealed in dry N 2 at 600 °C (Fig. 3(a) and 3(d) respectively). In conclusion, the increase in phosphorus concentration in 0-8 wt.~o P range in as-deposited PSG films, also increases the intrinsic breakdown field, from 8 to 14 MV cm-1, this fact being explained in terms of defect density dropping in SiO2 film with an increase in phosphorus content. The average breakdown field of 8 wt.~o P doped PSG films decreases from 14 to 5 MV cm- 1 when annealing temperature, in dry N 2 , increases from 600 to 1000 °C, this behaviour being interpreted by the fact that as-deposited PSG films which have
L28
LETTERS
an intrinsic degree of order, are p u s h e d to a m o r e r a n d o m ( a m o r p h o u s ) state by annealing, with a higher n u m b e r of defects. T h e decrease in dielectric strength of 8 wt.~o P d o p e d P S G films after a n n e a l i n g in water c o n t a i n i n g ambients, is explained b y the p h o s p h o r u s r e m o v a l from the films which has a deleterious effect on the b r e a k d o w n characteristics of P S G films. 1 2 3 4 5 6 7 8 9
Y. Shioya and M. Maeda, J. Electrochem. Soc., 133 (1986) 109. W. Kern, J. Vac. Sci. Technol., 14 (1977) 1082. W. Kern, RCA Rev., 37 (1976) 78. C. Cobianu and C. Pavelescu, Thin Solid Films, 143 (1986) 109. N.J. Chou and J. M. Elridge, J. Electrochem. Soc., 117 (1970) 1287. C.M. Osburn and D. W. Ormond, J. Electrochem. Soc., 119 (1972) 597. C. Pavelescu, C. Cobianu, N. Vlahovici and C. Ghita, Thin Solid Films, 161 (1988) L71. R.A. Levy, Solid State Technol., 29 (1986) 123. N, Nagasima, H. Suzuki, K. Tanaka and S. Nishida, J. Electrochem. Sot., 121 (1974) 434.