⎇ⓥ⺖㗴ฬ㧦MIS ᭴ㅧ⚛ሶߩ᡼኿✢ᾖ኿ലᨐߦ㑐ߔࠆ⎇ⓥ⎇ⓥઍ⴫⠪㧦ᄢ⷏ ৻ഞ (ᣣᧄᄢቇℂᎿቇㇱ㔚ሶᖱႎᎿቇ⑼)⎇ⓥᓥ੐⠪㧦㜞ᯅ ⧐ᶈ (ᣣᧄᄢቇℂᎿቇㇱ㔚ሶᖱႎᎿቇ⑼)‫⎇ޣ‬ⓥ⋡⊛‫ޤ‬ ቝቮⓨ㑆ߪ᡼኿✢ⅣႺߢ޽ࠅ㧘ቝቮ㘧⠍૕ߦ៞タߔࠆඨዉ૕⚛ሶߦߪ㜞޿⠴᡼኿✢ᕈ߇ⷐ᳞ߐࠇ
ࠆ㧚․ߦඨዉ૕㓸Ⓧ࿁〝ߩၮᧄ᭴ㅧߢ޽ࠆ MIS ᭴ㅧߩ᡼኿✢ᾖ኿ലᨐߩᛠីߪ㧘⠴᡼኿✢ᕈ⚛ሶ㐿⊒ߩ਄ߢ㕖Ᏹߦ㊀ⷐߢ޽ࠆ㧚ߎࠇ߹ߢߦ㧘MIS ᭴ㅧߩ⛘✼⤑ߣߒߡ㉄ൻ⤑࡮⓸ൻ⤑ߩᄙጀ᭴ㅧࠍ᦭
ߔࠆ MNOS (Metal-Nitride-Oxide-Semiconductor) ᭴ㅧߪ⠴᡼኿✢ᕈߦఝࠇߡ޿ࠆߣ޿߁ႎ๔߇޽ࠆ
߇㧘ߘߩ⹦⚦ߥࡔࠞ࠾࠭ࡓ߿ฦ⛘✼⤑ෘߩᦨㆡൻ╬ߦߟ޿ߡߪ㧘߶ߣࠎߤ⼏⺰ߐࠇߡ޿ߥ޿㧚ߘߎߢᚒ‫ߪޘ‬㧘᡼኿✢ᾖ኿ߦࠃࠆ MNOS ⛘✼⤑ਛߢߩ㔚⩄⊒↢߅ࠃ߮࿕ቯ㔚⩄㧔᝝₪㔚⩄㧕ߩᄌൻߦᵈ⋡ߒߡ⎇ⓥࠍⴕߞߡ߈ߚ㧚᡼኿✢ᾖ኿ߦ⿠࿃ߒߚ⛘✼⤑ਛ㔚⩄ߩᄌൻߪ㧘࠻࡜ࡦࠫࠬ࠲ߩߒ߈޿
୯㔚࿶ᄌേߩਥⷐ࿃ߢ޽ࠅ㧘㔚⩄᝝₪ࡔࠞ࠾࠭ࡓߩቯ㊂⊛ߥ⸃᣿ߪ࠺ࡃࠗࠬߩା㗬ᕈ⏕଻ߩ਄ߢ㕖Ᏹߦ㊀ⷐߣߥࠆ㧚ᧄ⎇ⓥߩ⋡⊛ߪ㧘MNOS ᭴ㅧࠍ↪޿ߚ MIS ᭴ㅧߩ᡼኿✢ᾖ኿ലᨐߩ⹦⚦ߥᬌ⸛㧘߅ࠃ߮⠴᡼኿✢ᒝൻ᭴ㅧߣߒߡߩᦨㆡൻߢ޽ࠆ㧚‫⎇ޣ‬ⓥ᭎ⷐ‫ޤ‬ ㉄ൻ⤑ෘ㧘⓸ൻ⤑ෘߩฦ‫ ࠆߥ⇣ޘ‬MNOS ࠳ࠗࠝ࡯࠼ࠍ૞⵾ߒ㧘᡼኿✢ᾖ኿ታ㛎㧔60Co-ǫ✢㧘1[MradSi]㧕ࠍⴕ޿㧘ᾖ኿೨ᓟߦ߅޿ߡ㔚᳇⊛․ᕈ᷹ቯࠍⴕߞߚ㧚᷹ቯ⚿ᨐࠃࠅ㧘ᾖ኿ߦࠃࠆࡒ࠶࠼ࠡࡖ࠶
ࡊ㔚࿶㧔⛘✼⤑ਛ㔚⩄ኒᐲࠍ෻ᤋߔࠆ㔚࿶㧕ߩᄌൻ㊂ǍVmg ࠍ⹏ଔߒߚ⚿ᨐ㧘ࠥ࡯࠻ߦᱜߩ㔚࿶ࠍශടߒߥ߇ࠄᾖ኿ࠍⴕߞߚ႐วߦߪ㧘ᾖ኿ߦࠃࠅ Vng ߇ᱜᣇะߦᄌൻߔࠆ⤑ෘ᧦ઙ߇ሽ࿷ߔࠆߎߣ
߇ࠊ߆ߞߚ㧚ߎࠇߪ㧘MOS ᭴ㅧߢߪ⏕⹺ߢ߈ߥ޿⃻⽎ߢ޽ࠅ㧘ᾖ኿ߦࠃࠅ⛘✼⤑ਛߦ㔚ሶ߇᝝₪ߐࠇߚߎߣࠍᗧ๧ߔࠆ㧚৻ᣇ㧘ᾖ኿ߦࠃࠆࠪ࡝ࠦࡦг㉄ൻ⤑⇇㕙Ḱ૏ኒᐲߩჇടǍDit ߦߟ޿ߡ߽⹏
ଔࠍⴕߞߚ㧚ǍDit ߣǍVmg ߣߩ㑐ଥߦߟ޿ߡ⹏ଔߒߚ⚿ᨐ㧘ǍDit ߪǍVmg㧔⛘ኻ୯㧕ߣ౒ߦჇട
ߔࠆߎߣࠍ⏕⹺ߒߚ㧚ߚߛߒ㧘ǍDit ߩฦ⛘✼⤑ෘ߿ᾖ኿ਛߩࠥ࡯࠻ශട㔚࿶ߦኻߔࠆ᣿⏕ߥଐሽᕈߪ⷗ࠄࠇߥ߆ߞߚ㧚ߎࠇࠄߩ⚿ᨐߪ㧘ᾖ኿ߦࠃࠆ Vmg ߩᄌേ߇ዊߐߥ MNOS ࠻࡜ࡦࠫࠬ࠲ߪ㧘ᾖ኿ߦࠃࠆߒ߈޿୯㔚࿶ᄌേ߅ࠃ߮⋧੕ࠦࡦ࠳ࠢ࠲ࡦࠬߩᷫዋࠍᛥ࿶ߢ߈ࠆߎߣࠍ␜ໂߒߡ߅ࠅ㧘
MNOS ᭴ㅧߩ⠴᡼኿✢ᕈ⚛ሶߣߒߡߩน⢻ᕈࠍ␜ߔ߽ߩߢ޽ࠆ㧚 ߎߩࠃ߁ߥ MNOS ᭴ㅧ․᦭ߩ᡼኿✢ᾖ኿ߦࠃࠆ㔚⩄᝝₪⃻⽎ߦߟ޿ߡࡕ࠺࡞ൻࠍⴕ߁ߚ߼㧘ᾖ኿೨ߦ߅ߌࠆ⛘✼⤑ਛߩ㔚⩄ಽᏓߦߟ޿ߡᬌ⸛ࠍⴕߞߚ㧚ห৻ߩၮ᧼਄ߢ႐ᚲ⊛ߦ⤑ෘࠍᄌൻߒ㧘㔚᳇⊛․ᕈߩ⤑ෘଐሽᕈࠍ⹦⚦ߦ⹏ଔߢ߈ࠆ‫ޟ‬௑ᢳࠛ࠶࠴ࡦࠣᴺ‫ࠍޠ‬ឭ᩺ߒ㧘᡼኿✢ᾖ኿೨ߦ߅ߌ
ࠆ MNOS ᭴ㅧ⛘✼⤑ਛߩ࿕ቯ㔚⩄ಽᏓ⹏ଔࠍⴕߞߚ㧚ߘߩ⚿ᨐ㧘ࠪ࡝ࠦࡦ-㉄ൻ⤑⇇㕙㧘߅ࠃ߮㉄ൻ⤑г⓸ൻ⤑⇇㕙ㄭறߦᱜ㔚⩄߇ዪ࿷ߒ㧘߹ߚ㉄ൻ⤑г⓸ൻ⤑⇇㕙ߩ㔚⩄㊂ߪ㉄ൻ⤑ෘჇᄢߣ౒ߦᷫ
ዋߔࠆߎߣ߇ࠊ߆ߞߚ㧚ߎߩ⚿ᨐࠍ↪޿ߡ㧘ᾖ኿ਛߩ㔚⩄⊒↢࡮᝝₪⃻⽎ߦߟ޿ߡᬌ⸛ࠍⴕߞߚ㧚ߘߩ⚿ᨐ㧘ࠪ࡝ࠦࡦг㉄ൻ⤑⇇㕙ㄭறߩ㉄ൻ⤑ਛߦߪᱜሹ࠻࡜࠶ࡊ߇㧘߹ߚ㉄ൻ⤑г⓸ൻ⤑⇇㕙ㄭறߩ⓸ൻ⤑ਛߦߪᱜሹ߅ࠃ߮㔚ሶ࠻࡜࠶ࡊ߇ዪ࿷ߒ㧘ฦ⛘✼⤑߅ࠃ߮ඨዉ૕ⓨਲጀਛߢᾖ኿ߦࠃࠅ⊒↢ߒߚࠠࡖ࡝ࠕߩ৻ㇱ߇ߘࠇࠄߩ࠻࡜࠶ࡊߦ᝝₪ߔࠆߣ઒ቯߒߚࡕ࠺࡞ࠍ↪޿ࠆߎߣߦࠃࠅ㧘ǍVmg ߩฦ⛘✼⤑ෘ߅ࠃ߮ᾖ኿ਛߩࠥ࡯࠻ශട㔚࿶ଐሽᕈࠍ♖ᐲ⦟ߊ⴫⃻ߢ߈ࠆߎߣ߇᣿ࠄ߆ߣߥߞߚ㧚 ߹ߚ㧘ឭ᩺ߒߚࡕ࠺࡞ࠍ↪޿ߚࠪࡒࡘ࡟࡯࡚ࠪࡦߦࠃࠅ㧘⠴᡼኿✢ᕈ⚛ሶߣߒߡߩᦨㆡ⛘✼⤑ෘߦኻߔࠆ⹏ଔࠍⴕߞߚ㧚ߘߩ⚿ᨐ㧘ᾖ኿ਛᱜߩࠥ࡯࠻㔚࿶߇ශടߐࠇߚ႐ว㧘ǍVmg=0 ߣߥࠆࠃ߁ߥᦨㆡ⤑ෘ߇ሽ࿷ߔࠆߎߣ㧘߹ߚ⛘✼⤑ߩ㉄ൻ⤑឵▚⤑ෘ߇ 100nm એਅߩ႐วߦߪ㧘ᦨㆡ⤑ෘߪోๆ෼✢㊂߅ࠃ߮ශട㔚࿶ߦᄢ߈ߊଐሽߒߥ޿ߎߣࠍ␜ߒ㧘MNOS ᭴ㅧߩ⠴᡼኿✢ᕈ⚛ሶߣߒߡߩ
น⢻ᕈࠍ᣿ࠄ߆ߦߒߚ㧚 ߹ߚ㧘࠻࡜ࡦࠫࠬ࠲᭴ㅧࠍ↪޿ߚᾖ኿ታ㛎ࠍ⋡ᜰߒ㧘p ࠴ࡖࡀ࡞ဳ MNOS 㔚⇇ലᨐ࠻࡜ࡦࠫࠬ࠲
㧔p-MNOSFET㧕ߩ૞⵾ࡊࡠ࠮ࠬߦߟ޿ߡ⹏ଔࠍⴕߞߚ㧚ߘߩ⚿ᨐ㧘⦟ᅢߥඨዉ૕г⛘✼⤑⇇㕙ߩᒻᚑߦߪ㧘ਇ⚐‛㧔ࡏࡠࡦ㧕᜛ᢔಣℂᓟߩඨዉ૕⴫㕙ᵞᵺࠍᔅⷐߣߔࠆߎߣ㧘߹ߚ㧘CVD ᴺߦࠃࠅ૞⵾ߒߚࠪ࡝ࠦࡦ⓸ൻ⤑ߩᓸ⚦ߥࡄ࠲࡯ࡦ࠾ࡦࠣߦߪ㧘⓸ൻ⤑ౣ⴫㕙ߩਇ⚐‛ጀߩ㒰෰߇᦭ലߢ޽
ࠆߎߣ ߥߤࠍ⷗಴ߒߚ㧚ߎࠇࠄߩᬌ⸛⚿ᨐࠍၮߦ㧘ࡏࡠࡦ᜛ᢔጀࠍ᦭ߔࠆ p-MNOSFET㧔ࠥ࡯࠻
㐳㧦10ޯ100Ǵ㨙㧕ࠍ⹜૞ߒ㧘㔚᳇⊛․ᕈ᷹ቯࠍⴕߞߚ⚿ᨐ㧘࠺ࡃࠗࠬࠪࡒࡘ࡟࡯࡚ࠪࡦߩ⚿ᨐߣ߶߷ห╬ߥ ⦟ᅢߥ․ᕈ ࠍᓧࠄࠇࠆ ߎߣࠍ⏕⹺ ߒߚ㧚ߎࠇ ࠄߩᚑᨐࠍ ߽ߣߦ㧘⤑ ෘߩ⇣ߥ ࠆ
p-MNOSFET ߦኻߒᾖ኿ታ㛎ࠍⴕ޿㧘ᾖ኿೨ᓟߦ߅ߌࠆ㔚᳇⊛․ᕈ᷹ቯࠍⴕߞߚ㧚ߘߩ⚿ᨐ㧘᭴▽ߐࠇߚࡕ࠺࡞⸘▚ࠃࠅᓧࠄࠇߚᦨㆡ⤑ෘࠍ᦭ߔࠆ MNOSFET ࠍ↪޿ߚ႐ว㧘ᾖ኿ߦࠃࠆߒ߈޿୯㔚࿶ߩᄌൻ߇ᛥ࿶น⢻ߢ޽ࠆߎߣ߇ታ⸽ߐࠇ㧘⠴᡼኿✢ᕈ࠺ࡃࠗࠬߣߒߡߩ MNOS ᭴ㅧ⚛ሶߩน⢻ᕈࠍ᣿ࠄ߆ߦߒߚ㧚‫ޤ߼ߣ߹ޣ‬ ㉄ൻ⤑࡮⓸ൻ⤑ߩᄙጀ᭴ㅧࠍ᦭ߔࠆ MIS ᭴ㅧߢ޽ࠆ MNOS ᭴ㅧߩ᡼኿✢ᾖ኿ലᨐߦߟ޿ߡᬌ⸛
ࠍⴕߞߚ㧚ߘߩ⚿ᨐ㧘ᾖ኿ߦࠃࠅ⛘✼⤑ਛߦ㔚ሶ߇᝝₪ߐࠇࠆࠃ߁ߥ⤑ෘ᧦ઙ߇ሽ࿷ߔࠆߎߣࠍታ
㛎ߦࠃࠅ᣿ࠄ߆ߦߒߚ㧚߹ߚ௑ᢳࠛ࠶࠴ࡦࠣᴺߩ㐿⊒ߦࠃࠅ㧘⛘✼⤑ਛߩ㔚⩄ߪฦ⇇㕙ઃㄭߦዪ࿷
ߒߡ޿ࠆߎߣ߇ࠊ߆ߞߚ㧚ߎࠇࠄߩ⚿ᨐࠍ߽ߣߦ㧘ᾖ኿ਛߩ㔚⩄⊒↢࡮᝝₪ࡕ࠺࡞ࠍ᭴▽ߒߚ㧚ߘߩ⚿ᨐ㧘ᧄࡕ࠺࡞⸘▚ߦࠃࠅታ㛎⚿ᨐࠍቯ㊂⊛ߦ⺑᣿ߢ߈ࠆߎߣ߇ࠊ߆ߞߚ㧚ᦝߦߎߩࡕ࠺࡞ࠍ↪
޿ߚࠪࡒࡘ࡟࡯࡚ࠪࡦߦࠃࠅ⠴᡼኿✢ᕈ⚛ሶߣߒߡߩᦨㆡ⛘✼⤑ࠍ⹏ଔߒߚ㧚߹ߚ㧘࠻࡜ࡦࠫࠬ࠲
᭴ㅧ⚛ሶࠍ⹜૞ߒ㧘ᾖ኿ታ㛎ࠍⴕ߁ߎߣߦࠃࠅ㧘⠴᡼኿✢ᕈ࠺ࡃࠗࠬߣߒߡߩ MNOS ᭴ㅧ⚛ሶߩน⢻ᕈࠍ᣿ࠄ߆ߦߒߚ㧚‫⎇ޣ‬ⓥᬺ❣╬‫ޤ‬࡮⊒⴫⺰ᢥ(1) ᄢ⷏৻ഞ㧘᧻↰⚐ᄦ (2002) ඨዉ૕⚛ሶߦኻߔࠆ᡼኿✢ᾖ኿ലᨐ ޯᦨㄭߩ⎇ⓥ௑ะޯ㧘㔚ሶᖱႎㅢାቇળ⹹㧘Vol.85, No.9, pp.662-669
࡮⪺ᦠ‫⺑⸃ޔ‬(1) ᄢ⷏৻ഞ㧘⮮↰ታ ౒⪺ (2002) ⛗ߣ߈ߢࠊ߆ࠆ ඨዉ૕࠺ࡃࠗࠬ㧘ࠝ࡯ࡓ␠(2) 㜞ᯅ⧐ᶈ (2003) ⴡᤊ៞タ↪ඨዉ૕࠺ࡃࠗࠬߦ㑐ߔࠆ⎇ⓥ, ࠹ࠢࡁ࠾ࡘ࡯ࠬߜ߫, Vol.29,
pp.10-11࡮ቇળ⊒⴫(1) Y. Takahashi and K. Ohnishi (2000) Total Dose Effect of Metal-Nitride-Oxide-Semiconductor Structure,
Proc. of the International Workshop on Radiation Effects on Semiconductor Devices for Space
Application(2) K. Ohnishi and Y. Takahashi (2002) Total Dose Effect on MIS structures, The International Workshop
on Radiation Effects on Semiconductor Devices for Space Application(3) ᄢ⷏৻ഞ (2003) SOI MOS ⚛ሶߩ⠴᡼኿✢ᕈ㧘╙ 3 ࿁ඨዉ૕ߩ᡼኿✢ᾖ኿ലᨐ⎇ⓥળ(4) ⦼↰೑ᒾ㧘㒙ㇱື↵㧘㜞ᯅ⧐ᶈ㧘ᄢ⷏৻ഞ (2003) ඨዉ૕⸥ᙘ⚛ሶߩࡑ࠶ࡇࡦࠣᣇᴺߩᬌ⸛㧘╙ 3 ࿁ඨዉ૕ߩ᡼኿✢ᾖ኿ലᨐ⎇ⓥળ
(5) T. Hirao, T. Shibata, J. S. Laird, S. Onoda, Y. Takahashi, K. Ohnishi, and T. Kamiya (2003)
Experimental Study of Single-Event Transient Current in SOI Devices, Radiation and its effects oncomponents and system(6) ⦼↰೑ᒾ㧘ᐔየᢅ㓶㧘J.S.Laird㧘ዊ㊁↰ᔋ㧘␹⼱ን⵨㧘᧛ἑ␭ఽ㧘㜞ᯅ⧐ᶈ㧘ᄢ⷏৻ഞ (2004)
SOI ⚛ሶߩࠪࡦࠣ࡞ࠗࡌࡦ࠻⠴ᕈߦ㑐ߔࠆ⎇ⓥ ޯMOS ࠠࡖࡄࠪ࠲ߦ߅ߌࠆࠪࡦࠣ࡞ࠗࡌࡦ
࠻ㆊᷰ㔚ᵹߩ᷹ቯޯ㧘╙ 4 ࿁ඨዉ૕ߩ᡼኿✢ᾖ኿ലᨐ⎇ⓥળ
(7) ᄢ⷏৻ഞ (2004) ඨዉ૕ࠪࡦࠣ࡞ࠗࡌࡦ࠻ലᨐ⎇ⓥߩ⃻⁁㧘ᔕ↪‛ℂቇ㑐ଥㅪว⻠Ṷળ(8) 㜞ᯅ⧐ᶈ㧘⦼↰೑ᒾ㧘᧛ἑ⵨ఽ㧘ᄢ⷏৻ഞ㧘ᐔየᢅ㓶㧘␹⼱ን⵨ (2004) MOS ᭴ㅧߦ߅ߌࠆ㊀ࠗࠝࡦᾖ኿⺃⿠㔚ᵹ㧘ᔕ↪‛ℂቇ㑐ଥㅪว⻠Ṷળ(9) K. Ohnishi (2004) Progress of the Radiation Effects Research on Semiconductor Devices in Japan, The
International Workshop on Radiation Effects on Semiconductor Devices for Space Application(10) T. Hirao, J.S. Laird, S. Onoda, T. Shibata, T. Wakasa, T. Yamakawa, H. Abe, Y. Takahashi, K. Ohnishi,
H. Itoh (2004) Charge collected in Si MOS Capacitors and SOI Devices P+N Diodes due to Heavy Ion
Radiation, The International Workshop on Radiation Effects on Semiconductor Devices for Space
Application(11) Y. Takahashi, T. Shibata, Y. Murase, K. Ohnishi, T. Hirao, T. Kamiya (2004) Heavy-Ion induced Current
in MOS Structure, The International Workshop on Radiation Effects on Semiconductor Devices forSpace Application(12) T. Abe, K. Ohnishi, Y. Takahashi, T. Hirao (2004) Consideration to Reliability of Laser Testing for
Evaluating SEU Tolerance, The International Workshop on Radiation Effects on Semiconductor Devices
for Space Application

TOTAL DOSE EFFECTS ON MIS STRUCUTRES
Kazunori Ohnishi and Yoshihiro Takahashi
College of Science and Technology, Nihon University
7-24-1 Narashinodai, Funabashi-shi, Chiba 274-8501, Japan
Abstract

Characteristics and role of hydrogen and nitrogen related trap centers effects on radiation induced positive
charges and Si-SiO2 interface states generation were discussed from experimental results of MOS capacitors
annealed in H2, N2 and ammonia. The charge distributions in insulators of MOS and MNOS were also discussed
by using the slanted etching method. Experimental results make clear that the nitrogen related trap center isamphoteric. Most optimum thickness of the oxide and the nitride films of radiation hardened MNOS structure isdiscussed too.

1.

Introduction
Total dose effects on MOS structures have been previously discussed for long time. When a MOS FET and IC
are irradiated to ionized radiation, positive charges are
induced into the oxide and generate the Si-SiO2 interface
states density increases. The characteristics of devices arechanged by this total dose ionizing effects such as thethreshold voltage shift, degradation of mutual conductance,increased leakage currents and so on. (1) In this paper, we
will summarize our works concerned to total dose effects
that have been carrying out in our laboratory since 1985.
The basic mechanisms underlying the oxide charge
trapping and interface states generation are explained asfollows. Electron-hole pairs are generated in the oxide byionizing irradiation such as X and gamma rays.(2-3) Someof the pairs recombine there; the rest of them are separated due to the field in the oxide. Electrons are transported to the metal and/or silicon rapidly and holes moveslowly through the oxide to the Si-SiO2 interface becauseof the large mobility difference between the electron andthe hole. (4) Positive charge buildup is produced by trappedholes in the oxide traps close to the Si-SiO2 interface states
and this is discussed in connection with the Eí center.(5)The radiation interface states are believed to be generated
from broken bonds of SiH or SiOH at Si-SiO2 interface
by irradiation.(6) Holes transported from the oxide to theinterface interact with bonds and break them.(3,7) This interface trap coincides with the Pb center.(5) The relation

between hydrogen related trap center and radiation effects
on MOS structure is continuing controversial problems.(811)

There are many works concerned to radiation hardening gate oxide and gate insulators.(3,7,12-14)
We also have been studied about radiation-hardened oxide and insulator films on the silicon, and estimated thecharge distribution through the insulator films.(15)In this paper we will explain three things as follows;
1) the annealing atmospheres dependency on the radiation effects of the oxide, 2) ammonia annealing effects on
the irradiated oxide and trapped charge distribution throughthe oxide, 3) radiation effects on the MNOS structures
and to optimum thickness of insulator films for radiation
hardening.
2.

The annealing atmosphere dependency on theradiation effects on the oxide
2.1 Experiments
MOS capacitors used in this experiments were fabricated on n-type <100> oriented, 3-6 1-cm Si substrates
and oxidized in dry oxygen or diluted oxygen with Ar or
nitrogen at a temperatures 1000°C or 1100°C, and all theoxidized silicon films were annealed in nitrogen at 950°C.
Oxide thickness was grown to 37~100nm and Al gate of
1mm diameter were evaporated and then annealed at
400°C for 10 minutes, 30 minutes and 90minutes in either
nitrogen or hydrogen. (This process is called Post Metal-

ized Anneal, PMA.)
60

Co-gamma ray irradiation tests were carried out up
to the total dose of 500 krad(Si) at the dose rate of 1.0×105
rad/h and 1Mrad(Si) at the dose rate of 3~4.13×105 rad/h.
2.2 Experimental Results
Figure 1 shows the MOS C-V characteristics measured
at frequencies of 10 kHz, 100 kHz and 1 MHz before and5 hours after irradiation. This figure shows the frequency
difference between before and after irradiation at 1
Mrad(Si), but it does not show at 500 krad(Si). The shift

between before and after irradiation respectively. Figure
4 shows the corelation between 6Not versus 6Dit for various process conditions. Figure 5 shows the ratio between
6Dit and 6Not for the cases PMA in N2 and in H2. In Fig.
4, the gradient of the slope of 6Dit against 6Not shows
0.75, however it changes with oxide thickness for PMA
in N2 and in H2 as shown in Fig. 5.(18)
2.3 Discussions
The oxide charge of MOS structure is often estimatedfrom the flat bandvoltage shift. However at the flatband
condition, Si-SiO2 interface states above midgap are considered to be an acceptor-like states and states below
midgap is donor-like states.(19-23) Therefore, the flatband
voltage shifts by the trapped positive charge in the oxide
and the negative charged interface states distributed from

Fig. 1: C-V characteristics of MOS structure before and5 hours after irradiation.

Vfb and Vmg [V]of C-V curves is more negative as the frequency is low
and is larger for PMA in nitrogen than in hydrogen.
Figure 2 shows midgap voltages (Vmg) and flatband
voltages (Vfb) estimated from C-V characteristics. Vmg and
Vfb were measured at 5 hours and 70 hours after 1Mrad(Si) and compared with before irradiation, which do notchange with measurement frequency. Midgap voltages(Vmg) were almost the same at 5 hours and 70 hours after
irradiation regardless of measurement frequency. Contrary,flatband voltages depended on measured frequencies.
Though it does not shown in Fig. 2, Vmg and Vfb did notdepend on the frequency in the case of 500 krad(Si), theshift of Vmg and Vfb are around 1V. Figures 3(a) and (b)show the calculated Si-SiO2 interface states densities (Dit)of MOS capacitors PMA in H2 and in N2. Practically, 6Dit
and 6Not decreased with annealing time for PMA in N2
and increased in H2 for longer annealing time.(16-17)
We defined the 6Dit and the 6Not as the difference of
the Si-SiO2 interface states and trapped charge densities

Co-60 Gamma dose
1 Mrad(Si)
-6

Vmg

5 hr
70 hr

-4

N2
Vfb

H2
5 hr

-2
70 hr
Vmg

0

Pre Rad
Vfb
10k

100k
1M
Frequency [Hz]Fig. 2: Vmg and Vfb of MOS structure before and after
irradiation.
midgap to flatband energy levels. If the interface statestrap can exchange charges with silicon surface in response
to measurement signal, this frequency response makes
large the semiconductor capacitance virtually by adding
this fast surface states capacitance. C-V curves in Fig. 1
and Vfb change in Fig. 2 includes this frequency dependence of fast surface states. When the measurement frequency is high as 1MHz, the interface trap cannot responseto the frequency, captured charges are kept trapped in thestates and it contribute only Vfb shift. The interface trap
response to the frequency arround 10kHz to 100kHz has
relatively small time constant and coincide with bordertrap.(24)The positive charges trapped in the oxide are usuallyestimated by midgap voltage shift 6Vmg and this seemstrue from our experiments. We also showed the fact thatthe increased interface states density after irradiation(6Dit) is correlated strongly with positive charge buildup
due to irradiation as shown in Fig. 4 and it depends on theannealing atmosphere as shown in Fig. 5. 6Not and 6Dit
of MOS capacitor PMA in H2 was larger than PMA in N2.
From these results, we conclude that the introduced hy(a) PMA
in H2

drogen during oxidation process break the weak bonding
Si-O, and bond with it to make Si-H or Si-OH in the oxide. Hydrogen also bonds with oxygen vacancy in the oxideand making same defects. The hydrogen combines withsilicon dangling bond at the Si-SiO2 interface and making
Si-H. These hydrogen related defect bonds can be precursor of radiation introduce positive charge and generate theinterface states. When the MOS structure is radiated byionizing radiation, Si-H and Si-OH can be break easily by(b) PMA
in N2

irradiation and capture holes (E’ center) and generate the
Si-SiO2 interface states (Pb center).
3.

6Dit [1×1011 cm-2]

Fig. 3: Si-SiO2 interface states densities (Dit) of MOS
capacitors PMA in H2 (a) and in N2 (b).
10

Pure O2
N2 Dilution
N2 Dilution

5

a<ray: 1Mrad(Si)Dox = 50nm
05106Not [1×1011 cm-2]

Fig. 4: Relation between 6Dit and 6Not.

We also tried to estimate the charge distribution throughthe oxide by using the slanted etching method that we proposed formally, and it can be estimated by measure the

1.0
6Dit / 6Not

PMA in H2
PMA in N2
0.5

a<ray: 1Mrad(Si)050Ammonia annealing effects on the irradiatedoxide and trapped charge distribution through theoxide
3.1 Experiments
In this experiment, we used the n-type <100> oriented,
0.8-8 1-cm Si wafers and oxidized in dry oxygen at atemperatures 1000°C and then annealed in N2 for 30minutes at same temperature. Ammonia annealing was carried out for 0.5, 1, 3 hours at 950°C and 1000°C, and thenanneal in N2 for 30minutes. Oxide thickness was grown
to 20~100nm and Al gate of 1mm diameter were evaporated and then PMA in N2 at 400°C for 30 minutes. 60Cogamma ray irradiation tests were carried out up to the total dose of 1Mrad(Si) at the dose rate of 5×105 rad/h.

100

Oxide thickness [nm]Fig. 5: 6Dit/6Not for the cases PMA in N2 and in H2.

midgap voltages of the MOS structures with different
thickness of oxide on the same substrate which were prepared by the slanted etching method.(15)
3.2 Experimental results
Figure 6 shows the Vfb characteristics of MOS capacitors annealed in ammonia at 950°C and 1000°C, and shows
the Vfb depends on annealing time and annealing temperature. Figures 7 and 8 show the 6Vmg and 6Dit of ammonia
annealed MOS capacitors after irradiation.(25) Figure 9

Figure 10 shows the midgap voltages of the MOS capacitors prepared by slanted etching method. MOS capacitors used in this experiments were oxidized at 1045°C
in dry O2 with standard fabrication process in our laboratory, and the annealed in ammonia for 0,30 60 nd 180
minutes.
The second derivative of Vmg to the thickness manifests the density of trapped charge at the point x as following equation.

near Si-SiO2 surface and almost coincide with mounted
nitride density distribution. From these experiments, it iscleared that the characteristic of nitride related trap center is amphoteric. Figure 12 shows mid-gap voltage distribution after irradiation and Fig. 13 shows the trappedcharge distribution.
1016

6Dit [m-2]shows the composition of the oxide annealed in NH3 at
950 °C analyzed by X-ray Photoelectron Spectroscopy(XPS). Nitrogen density mounted up near the Si-SiO2 interface and the surface of the oxide.

Oxide thickness=50nm

1015

1014
2

d Vmg ( x)dl ( x) =(¡ 0 ¡ ox E ox ) = <¡ 0 ¡ ox
dx
dx 2

(1)

When charges are distributed in the form of Gaussian, theprofile of the charge estimated from Fig. 10 would beshown in Fig. 11. This figure shows that the ammonia annealing causes charge trap centers at around 10 to 20nm

1013

0

1

2
3
NH3 annealing time [h]Fig. 8: 6Dit of ammonia annealed MOS capacitors after
irradiation.

Vfb [V]

Oxide thickness=50nm
2NH3 annealing temp.
=1000°C

0
-2

950°C

1

2
3
NH3 annealing time [h]6Vmg [V]Fig. 6: Vfb characteristics of MOS capacitors annealed inammonia at 950°C and 1000°C.
2
Oxide thickness=50nm
0-2Fig. 9: Composition of the oxide annealed in NH3 at 950°C analyzed by XPS.
mid-gap voltage [V]

-4

-0.5
180min

-1.0

60min

Ammonia
Annealling time
0min

-1.5

-4

30min
initial oxide layer
thickness: 50(nm)

-2.0

0

2
3
NH3 annealing time [h]Fig. 7: 6Vmg of ammonia annealed MOS capacitors after

irradiation.

1

0
10
20
30
40
50
remaining oxide layer thickness [nm]Fig. 10: Midgap voltages of the MOS capacitors preparedby slanted etching method.

3.3 Conclusions
Ammonia anneal after oxidation is effect to reduce thegeneration of Si-SiO2 interface states by irradiation. Ammonia annealing does not effective to decrease the charge
trap center affects to mid-gap voltage until oxide thickness is 500nm. This trap center distribution is almost samewith nitrogen distribution near Si-SiO2 interface, and thetrap is amphoteric.
4.

Radiation effects on the MNOS structures and to18- 3

number of trapped charge[x10 c m ]

1.0
0,30,
60,180

30,60

0.5
0.0

0

-0.5
180

-1.0

6 0

Ammonia Annealing
time [min]

-1.5
0
10
20
30
40
distance from Si-SiO2 interface [nm]mid-gap voltage [V]Fig. 11: Estimated charge profile in oxide layer annealedin NH3 before irradiation.
-0.5
0,180

-1.0
-1.5
Ammonia
annealing time

-2.0

0min
30min
60min
180min

-2.5

30,60

-3.0
0
10
20
30
40
50
remaining oxide layer thickness [nm]trapped charge density [cm -3 ]Fig. 12: Midgap voltages distribution of MOS structure
after irradiation.
2.0[x1018 ]0, 30, 60, 180

1.5
1.0

Ammonia annealing
time [min]
30,60

0.5
0.0

optimum thickness of insulator films for radiation
hardening.

4.1 Experiment
Figure 14 is the energy band diagram and charge generation and trapping model of MNOS structures by ionized irradiation. MNOS capacitors used in this experiments
were fabricated on n-type <100> substrate. Oxide wasgrown at 1040°C in dry O2. Nitride layer was depositedon the oxide by LPCVD at 700°C with flow ratio of NH3
to SiH2Cl2 is 3:1. Al electrode formed by vacuum deposition. MNOS capacitors were irradiated to 60Co-gamma ray
up to 1Mrad(Si) at dose rate with 5×105rad(Si)/h.
4.2 Experimental Results
Figure 15 shows the midgap voltage Vmg of MNOS
capacitors before and after irradiation. Vmg characteristic
after irradiation varied the gradient of the slope at the Si3N4and SiO2 interface. (26) This shows that charges are trappedat SiO2-Si3N4 interface due to irradiation. Figure 16 shows
the 6Vmg of MNOS capacitor with oxide thickness is 70
and 120nm for various nitride thickness. The gate voltages during irradiation were +6V and –6V. This figuresuggests the negative charges trapping in the insulators
when the gate bias is positive. In MOS capacitors, generally positive charges are trapped in the oxide by irradiation. However, nitride related trap center is able to capture the positive and negative charges. From this point ofview, we simulate the radiation induced charge trapping
using electron and hole trapping model at the Si-SiO2 interface and SiO2-Si3N4 interface and also calculate theoptimum thickness of nitride and oxide films to radiation
tolerant. Figure 17 shows the simulated 6Vmg of the MNOS
capacitor irradiated at 1Mrad(Si) for the combination ofoxide layer thickness and nitride layer thickness when thegate voltage is 5V.
4.3 Conclusions
By using the slant etching method, the charge depth
profiles were estimated before and after irradiation.
Trapped charges were located mainly at the Si-SiO2 andSiO 2-Si 3N4 interfaces, and density at SiO2- Si 3N4 waschanged by the oxide thickness.

-0.5
0
10
20
30
40
50
distance from Si-SiO2 interface [nm]Fig. 13: Estimated charge profile in oxide layer annealedin NH3 after irradiation.

5.

Conclusion Remarks
From this work, we infer that the hydrogen in the ox-

ide affects importantly to the radiation induced positive
charge and Si-SiO2 interface states generation due to breakhole and electron
traps (Nhn, Nen )

-

hole
traps
(Nho )-

Gate

NitrideOxide+
generated charge(sweep direction
depends on field)

pensate the charges captured in the insulators. MNOS
structure is a candidate of the radiation-hardened insulator for IC by using the nitride related trap site. We also+
Si

proposed the slanted etching method to determine thetrapped charge distribution and it can be applied to the+

mid-gap voltage [V]Fig. 14: Charge generation and trapping model of MNOS
structure by ionized irradiation.
0complex insulator films such as MNOS structure.
References(1)-1
beforeirradiation-2

after irradiation

Si N

-3

3

S i O2

4

-4
-5
0

50
100
150
insulator thickness [nm](converted into oxide thickness)

Fig. 15: Vmg of MNOS capacitors before and after irradiation.

(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)

5
6Vmg [V]the hydrogen related bonding site. Nitrogen related bonding site has an amphoteric trap, so that this trap captures
the positive and the negative charges. This allows to com-

Oxide thickness(dox) =
0

(12)

70nm

Vg = +6 V-5-10

Vg = -6 V

120nm
60Co-a ray, 1Mrad(Si)

(16)
(17)

-15
0

(13)
(14)
(15)

50
100
150
Nitride layer thickness (dni) [nm](18)Fig. 16: 6Vmg of MNOS structures due to irradiation.
(19)
(20)
(21)

1 Mrad(Si), Vg = +5 V

Nitride layer thickness (dni ) [nm]

100

+1.3
6Vmg
[V] =

80

+3.3
+3

(22)
(23)
(24)

60
+0.5

+1.8
40

+2
+0.5

20

(25)

+1
0

-5.1

-0.6
0

0

20

-1 -2 -3
-4

40

60

-6
80

(26)
100

Oxide layer thickness (dox ) [nm]Fig. 17: Simulated 6Vmg of the MNOS structure.

T.P.Ma and P.V.Dressendorfer, Ionizing Radiation Effects in MOS
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K.K.Hung and Y.C.Cheng, J.Appl.Phys. 59 (1986) 816.
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21st International Symposium for Testing and Failure Analysis
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TOTAL DOSE EFFECT OF
METAL-NITRIDE-OXIDE-SEMICONDUCTOR STRUCTURE
Yoshihiro Takahashi and Kazunori Ohnishi
Nihon University, 7-24-1 Narashinodai, Funabashi-shi, Chiba, 274-8501 Japan
Abstract
The radiation-induced trapping charge in the insulation layer and generation of interface
state of Metal-Nitride-Oxide-Semiconductor (MNOS) structure have been investigated. The
irradiation tests were carried out using Co-60 gamma ray source up to 1 Mrad(Si) with the gate
voltage of +6 or −6 V. The charge trapping mechanism under irradiation is studied by the radiationinduced mid-gap voltage shift. The simple trap model was proposed to calculate the voltage shift,and it was confirmed that the calculated results can be fitted well to the experimental results. The
strong correlation was observed between the mid-gap voltage shift and the generation of interface
state. From these results, the optimum oxide and nitride thicknesses for radiation hardened device
are evaluated.
1.

Introduction
When a MOS structure is irradiated by gamma rays, the positive charge build-up at Si-SiO2
interface and the interface state creation occur [1]. A Metal-Nitride-Oxide-Si (MNOS) structure
which has stacked insulation layers composed silicon nitride and silicon dioxide has been reported
to reduce these irradiation effects [2]-[5]. It has been also reported that the flat-band voltage shift
due to irradiation depends on both the applied gate voltage during irradiation and the insulation
layer thickness.
In this study, the radiation-induced trapping charge in the insulation layer and generation ofinterface state of MNOS structure have been investigated. The charge trapping during irradiation is
discussed by analyzing the radiation-induced mid-gap voltage shift (ΔVmg). The charging in theinterface state is neutral at the mid-gap condition. That indicates the ΔVmg is not affected by theinterface state generation and is very useful to investigate the radiation-induced charge trapping in
the insulation layer. A simple charge trap model is proposed to assess the charge trapping mechanism,and the ΔVmg is calculated using the model. By fitting the calculated results to the experimental
results, the charge generation rate and the charge trap densities in the insulators and these capture
cross sections are evaluated. The optimum oxide and nitride thicknesses for minimizing ΔVmg areevaluated by the simulation, and the possibility of MNOS structure for radiation hardened device isdiscussed.
2.

Experiments
The MNOS capacitors were fabricated on n-type Si <100> substrates. After conventional
cleaning processes, thermal oxidation was performed at 1040˚C in dry O2 ambient. Following theoxidation, the samples were annealed in N2 ambient at 1040˚C for 60 min. Nitride layer was depositedon the oxide by LPCVD at 700˚C with the gas flow ratio of NH3 to SiH2Cl2 is 3 : 1. Capacitors wereExperimental Results
Figure 1 shows the ΔVmg of MOS structures
as a function of the oxide thickness (dox). Vmg shifts
toward negative due to irradiation. The negative
shift indicates the radiation-induced holes aretrapped in the oxide layer. Figure 2 shows the ΔVmg
of MNOS structures in which dox = 70 and 120 nm
as a function of the nitride thickness (dni). The
negative shift is observed regardless of the insulator
thickness when Vg = −6 V, and the positive shift isobserved when Vg = +6 V. In the case when Vg is
positive, ΔVmg changes toward positive with dni, andis saturated or decreased when dni becomes thick. It
is considered that these phenomena are caused by
the radiation induced trapped charges in nitride
layer.
Figure 3 shows the radiation-induced increaseof the density of interface state (ΔDit) in MOS andMNOS structures as a function of the magnitude of
ΔVmg. In this study, Dit is determined as averagedensity from mid-gap to fermi energy and isevaluated by the diference of flat-band voltage andVmg. It is found that ΔDit increases with ΔVmg, andthe strong correlation can be observed between ΔDit
and ΔVmg. However the definite relations with theinsulation layer thicknesses and with the gate
voltage are not observed.
4.
4.1

Discussions
Charge trapping model
We discuss the charge trapping due to

ΔVmg [V]V g = -6 V

-5V g = +6 V

-10
60 Co-γ ray, 1Mrad(Si)

-15

0

50
100
150
Oxide layer thickness (dox) [nm]Figure 1:

Radiation-induced V mg shift of MOS
structures. Points show the experimental
results. Lines show the calculated results.

ΔVmg [V]3.

0

5Oxide layer thickness(dox) =0

V g = +6 V

70nm

-5
-10
-15

120nm
60 Co-γ ray, 1Mrad(Si)V g = -6 V

0

50
100
150
Nitride layer thickness (dni) [nm]Figure 2:

Radiation-induced V mg shift of MNOS
structures in which the oxide thicknesses
are 70 and 120 nm. Points show theexperimental results. Lines show thecalculated results.

ΔDit [x 1012 cm-2eV-1]

formed by vacuum deposition of aluminumelectrodes (1mm of diameter). The obtained
capacitors were irradiated to Co-60 gamma rays up
to 1 Mrad(Si) at room temperature. The dose rate
was 500 krad(Si)/h. The gate voltage (Vg) of +6 or−6 V was applied during irradiation. High-frequency(1 MHz) C-V characteristics were measured before
and after irradiation to evaluate the mid-gap voltage(Vmg) and the density of interface state (Dit).

4

MOS V g = +6V

MOS V g = −6V

MNOS V g = +6V
MNOS V g = −6V

3
2
1
0

0

Figure 3:

5

10ΔVmg [V]15

Radiation-induced Dit shift in MOS andMNOS structures as a function of themagnitude of Vmg shift.

hole and electron
hole
irradiation by constructing a simple charge trap
traps (Nhn, Nen )traps
model as shown in fig. 4. In this model, we assumed(Nho )
that the hole traps are located in the oxide near Sioxide (S-O) interface, and the hole and electron traps
Gate
are located in the nitride near oxide-nitride (O-N)
NitrideOxide+
Siinterface. Nho and σho are defined as the local density
and the capture cross section of the hole traps in+oxide, and Nhn and σhn (Nen and σen) are those ofgenerated charge(sweep direction+
hole (or electron) traps in nitride. When the MNOS
depends on field)
structure is irradiated, the electron-hole pairs are
generated throughout the insulation layers, and a Figure 4: Charge generation and trapping model of
MNOS structure under irradiation.
fraction that escaped initial recombination yields
free electrons and holes. These free carriers are
swept by electric field, and some of them are captured by the charge traps.
When the oxide and the nitride layers are irradiated by gamma rays of unit dose (ΔD), theelectron-hole pairs (Δρox (ni)) are generated throughout the films as follows [6]:Δρox (ni) = Kox (ni) fyox (ni) ΔD dox (ni)(1)

where Kox (ni) [C/cm3/rad(SiO2)] is the generation constant in the oxide (nitride) layer, fy is thefractional charge yield that depends on the electric field. Benedetto et al. have evaluated theseparameters as Kox = 1.30 × 10-6 [C/cm3/rad(SiO2)] and fy = (0.27 /(Eox + 0.084) + 1) -1 in case of Co60 gamma ray irradiation, where Eox [V/cm] is the oxide field [7]. When Vg is positive, the generated
holes in oxide drift toward the hole traps at S-O interface, and some of them are captured. The
amount of captured hole (Δnho) can be expressed as:

Δnho = Fho σho [ Nho - nho ] ,(2)where Fho is the hole fluence at the traps, nho is the trapped hole density before irradiation of ΔD, andthe capture cross section depends on the electric field as Eox−0.5 [8], [9]. When Vg is negative, thegenerated electrons drift toward the silicon. In this model, we assumed the charges are generated insilicon depletion layer by impact ionization due to injected electrons from oxide [10]. In that case,
Fho can be expressed as γ · Δρox where γ is the quantum yield. The amount of captured hole andelectron by the traps at O-N interface may be calculated by the same manner as the S-O interface.
The calculation of ΔVmg is carried out by repeating the evaluation of radiation-induced trappedcharge due to small amount of dose (ΔD = 2 krad(Si)) because the electric field in each insulator
should be changed during irradiation.
In order to calculate ΔVmg, the trapped charge density before irradiation have to be fed. We
have ever reported the depth profile of fixed charge in the insulation layers [11], [12]. The profile
was evaluated by slanted etching method, and it was found that the charges were located mainly ateach interface, and the densities were obtained as:
Qso = 1.6 × 10 -8 [C/cm2] ,(3)

Qon = 6.2 × 10 -8 - 5.2 × 10 -3 dox [C/cm2] ,(4)

where Qso and Qon are the charge densities at S-O and O-N interfaces, respectively. Using theseresults, the charge density and the electric field before irradiation were defined.
Calculated results of ΔVmg
The ΔVmg of MOS structures are calculated by varying Nho, σho and γ. The lines in Fig. 1 show
the fitted results, and it is found that the calcurated results can be fit to the experimental results byusing following parameters: Nho = 5.0 × 10 18 cm-3, σho = 2.0 × 10 -14 cm2 (@ 1MV/cm), and γ = 0.25.
Next the ΔVmg of MNOS structures are discussed. The charge trap density in the nitride layer
and its profile have been investigated [13]-[15]. From these reports, we assumed that the electron
and hole trap centers are distributed rectangularly within 10 nm from O-N interface in nitride layer
as Nhn = 1.2 × 10 20 cm-3 and Nen = 2.0 × 10 19 cm-3. In that case, the unknown parameters forcalculation are σhn, σen and the charge generation coefficients in nitride (Kni and fyni). However Kni
and fyni are very difficult to evaluate independently. So we assume the electric field dependence offyni is same as fyox, and α is defined as:
4.2

α = (Kni fyni) / (Kox fyox) .

(5)

Photocurrent density [nA/cm2]

As the results of calculation, the calculated ΔVmg can be fitted successfully to the experimental data
by using following parameters: σhn = 1.5 × 10 -16 cm2 (@ 1MV/cm), σen = 1.5 × 10 -14 cm2 (@ 1MV/cm), and α = 2 ~ 5 [%]. The lines in Fig. 2 show the fitted results.
From these fitting procedures, the small value of α was obtained. That indicates the amountof radiation-induced generated carriers in the nitride layer is much smaller than that in the oxide
layer. In order to confirm the result physically, the photocurrents of MOS and MNS (Metal-NitrideSi) structures are measured. The measurements were carried out using SEM system with 35 keVelectron beam. The samples were set in a vacuum
150
condition (1.5 × 10 -6 Torr) to eliminate air ionization
35 keV electron-beameffects. Figure 5 shows the photocurrent densitiy
Insulator thickness: 100 nm
as a function of the applied electric field during100
measurement. The insulator thickness of each
MOS structuresample is 100 nm. The current of MNS structure is50
about 10 ~ 20 % of the current of MOS structure
under same electric field. From these results, it was
MNS structure
confirmed that the charge generation rate due to
0
0
1
2
3
4
irradiation in nitride is less than that in oxide. It is
Electric field in insulator [MV/cm]
considered that the small generation rate in nitride
is caused by the high initial recombination rate. Figure 5: 35 keV electron-beam photocurrent
However the detail discussions, for example thedensities of MOS and MNS structures as afunction of electric field.
radiation source or the thickness dependence, arethe subject for a future study.
From above mentioned investigations, the charge trapping mechanism in MNOS structure
under irradiation was clarified. When Vg is positive, the amount of radiation-induced trapped electronsat O-N interface is larger than that of holes at S-O interface because Nen >> Nho, and is also largerthan that of holes at O-N interface because α is small. The effect of trapped electrons at O-Ninterface on ΔVmg becomes large with nitride thickness, so the ΔVmg changes from negative to positive
with the thickness. And when the amount of electron increases, the oxide electric field is reduced.
This reduction causes the decrease of the charge generation rate in oxide because the generated
electron-hole pairs recombine easily under the weak field, and the amount of trapped electrons issaturated. When dni is thick, the oxide field can be suppressed by the small amount of trappedelectrons because the initial field is weak, and the positive ΔVmg is reduced. Therefore, ΔVmg issaturated when nitride becomes thick. On the other hand when Vg is negative, the amount of trappedholes at O-N interface is larger than that of electrons. Hence the ΔVmg is negative, and the shift
increases with the increment of each insulator thickness.
From these results, it is confirmed that the proposed model is very useful to estimate theradiation-induced trapped charge in insulators of MNOS structure.
The optimum insulator thickness for radiation hardened structure
It is considered that the ΔVmg = 0 can be obtained using MNOS structure with specific oxide
and nitride thicknesses when Vg is positive, as shown in Fig. 2. These thicknesses are considered tobe the optimum thicknesses in terms of radiation-induced trapped charge. On the other hand, it was
confirmed that the small ΔDit is observed when ΔVmg is small, as shown in Fig. 3. That indicates theinsulation layer thicknesses of MNOS structure tominimize ΔVmg is also optimum thicknesses for100
minimizing the radiation-induced interface state
generation. In order to obtain these thicknesses,
1 Mrad(Si)80
Vg = + 2.5 V
ΔVmg of MNOS structures with varying dox and dni
1 Mrad(Si)are calculated. Figure 6 shows the obtained
Vg = + 5 V
60
optimum thicknesses with different radiation
conditions when Vg > 0. In this figure, the equivalent
ΔVmg = 040oxide thickness is the same on each dashed line. It
0.5 Mrad(Si)is found that the optimum thicknesses do not depend
Vg = + 5 V
20much on the total dose and on the gate voltage
during irradiation when the equivalent oxide0
thickness is less than about 100 nm. It was also
20
40
60
80
100
0
Oxide
layer
thickness
(dox
)
[nm]
confirmed that ΔVmg of MNOS structure is almost
same as that of MOS structure which has same
Figure 6: The optimum insulator thicknesses of
equivalent oxide thickness when Vg < 0. From these
MNOS structures with different radiation
results, it is expected that the radiation hardened
conditions when Vg > 0. The equivalentdevice can be achieved using MNOS structure inoxide thickness is the same on each dashedline.
terms of total dose effects.
Nitride layer thickness (dni ) [nm]

4.3

5.

Conclusions
The charge trapping in the insulation layers in MNOS structure under irradiaiton has been
investigated by analyzing ΔVmg. The direction and the magnitude of ΔVmg depended on the insulation

layer thickness and the applied gate voltage during irradiation. ΔVmg was calculated by a proposed
simple charge trap model. The calculated results could be fitted to the experimental results byvarying the densities and the capture cross sections of the charge trap centers, and these parameters
were obtained. It is considered that these parameters may be changed by the fabrication processes.
We considered the studies on the process dependencies of these parameters are helpful to verify thegenerality of our model. From these results, the charge trapping mechanism under irradiation wasclarified.
The strong correlation was observed between ΔVmg and ΔDit. That indicates the insulator
thicknesses to minimize ΔVmg are the optimum thicknesses for radiation hardened device in termsof not only radiation-induced trapped charge but also interface state generation. The thicknesses
were evaluated by the model calculations when Vg is positive. It was found that the optimum
thicknesses do not depend much on the total dose and on Vg when the equivalent oxide thickness isless than about 100 nm.
From these results, it was found that the charge trap model is very useful to understand thecharge trapping mechanism in the insulators of MNOS structure under irradiation, and the MNOS
structure can be applied to the radiation hardened device.
Acknowledgements
The authors are very grateful to Masahito Yoshikawa of JAERI for fruitful discussions andtechnical help. We would like to thank T. Fujimaki and S. Miyazaki for device fabrication andirradiation experiments.
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[3] C. W. Perkins, K. G. Aubuchon, and H. C. Dill, IEEE Trans. Nucl. Sci. NS-15, 176 (1968).
[4] P. F. Schmidt, M. J. Rand, J. P. Mitchell, and J. D. Ashner, IEEE Trans. Nucl. Sci. NS-16,
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[5] J. R. Cricchi and D. F. Barbe, Appl. Phys. Lett. 19, 49 (1971).
[6] H.E. Boesch, Jr., F.B. McLean, J.M. Benedetto, and J.M. McGarrity, IEEE Trans. Nucl. Sci.
NS-33, 1191 (1986).
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[8] C.M. Dozier and D.B. Brown, IEEE Trans. Nucl. Sci. NS-27, 1694 (1980).
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⎇ⓥ⺖㗴ฬ㧦శബ⿠ࡊࡠ࠮ࠬߦࠃࠆࠪ࡝ࠦࡦ♽⛘✼⤑ߩૐ᷷ᚑ㐳⎇ⓥઍ⴫⠪㧦ᄢ⷏ ৻ഞ (ᣣᧄᄢቇℂᎿቇㇱ㔚ሶᖱႎᎿቇ⑼)⎇ⓥᓥ੐⠪㧦㜞ᯅ ⧐ᶈ (ᣣᧄᄢቇℂᎿቇㇱ㔚ሶᖱႎᎿቇ⑼)‫⎇ޣ‬ⓥ⋡⊛‫ޤ‬ ㄭᐕߩඨዉ૕㓸Ⓧ࿁〝ߩᓸ⚦ൻߦ઻޿㧘ࠪ࡝ࠦࡦ㉄ൻ⤑ߩ⭯⤑ൻߪ㒢⇇ߦㄭߠ޿ߡ߅ࠅ㧘Ყ⺃㔚₸߇㉄ൻ⤑ࠃࠅᄢ߈ߥ㧘ࠪ࡝ࠦࡦ⓸ൻ⤑ߩࠥ࡯࠻⛘✼⤑߳ߩᔕ↪ߦߟ޿ߡᵈ⋡ߐࠇߡ޿ࠆ㧚৻ᣇ㧘
p+ࡐ࡝ࠪ࡝ࠦࡦࠥ࡯࠻ࠍ᦭ߔࠆ MOSFET ߦ߅޿ߡ㧘ࡐ࡝ࠪ࡝ࠦࡦ૞⵾ᤨߩࠥ࡯࠻㉄ൻ⤑ߢߩࡏࡠࡦ
ߟ߈ᛮߌ⃻⽎ߦࠃࠆၮ᧼ਇ⚐‛Ớᐲᄌൻߦ⿠࿃ߒߚߒ߈޿୯㔚࿶ᄌേߪ㧘࠺ࡃࠗࠬߩା㗬ᕈ⏕଻ߩ਄ߢᄢ߈ߥ໧㗴ߣߥߞߡ޿ࠆ㧚⓸ൻ⤑߅ࠃ߮⓸ൻ㉄ൻ⤑ߪ㧘㉄ൻ⤑ߦᲧߴࡏࡠࡦߟ߈ᛮߌ⃻⽎ࠍᛥ೙ߢ߈ࠆ⛘✼⤑ߣߒߡႎ๔ߐࠇߡ޿ࠆ߇㧘ߘߩ૞⵾ㆊ⒟ߢߪ㜞᷷ᾲಣℂߥߤࠍᔅⷐߣߔࠆ߽ߩ߇ᄙ
޿㧚ᦝߥࠆ㓸Ⓧᐲะ਄㧘ၮ᧼ߩᄢญᓘൻࠍ⠨ᘦߔࠆߣ㧘ࡊࡠ࠮᷷ࠬᐲߩૐ᷷ൻ߇ᔅⷐߢ޽ࠆ㧚ߘߎߢ㧘శബ⿠෻ᔕࠍ↪޿ߚࠪ࡝ࠦࡦ⓸ൻ⤑ߩૐ᷷ᚑ㐳ߩ⏕┙ࠍ⋡⊛ߦᬌ⸛ࠍⴕߞߚ㧚 ৻ᣇ㧘㔚ሶ࠺ࡃࠗࠬ╬ߩ૞⵾ߦ߅޿ߡਇนᰳߥ⛘✼⤑ߢ޽ࠆࠪ࡝ࠦࡦ㉄ൻ⤑ߩ⵾⤑ߦ߅޿ߡ߽㧘ห᭽ߦૐ᷷ൻ߇ⷐ᳞ߐࠇߡ޿ࠆ㧚ᧄ⎇ⓥߢߪ⚐᳓ࠍ↪޿ߚ㓁ᭂ㉄ൻᴺߦ⌕⋡ߒ㧘㓁ᭂ㉄ൻ⤑ߩ․ᕈ
⹏ଔ߅ࠃ߮㉄ൻࡔࠞ࠾࠭ࡓߩ⸃᣿ࠍ⋡⊛ߦ㧘శᾖ኿ࠍ฽߼ߚࡊࡠ࠮ࠬ᧦ઙߦኻߔࠆฦ⒳⤑․ᕈߦߟ޿ߡᬌ⸛ࠍⴕߞߚ㧚‫⎇ޣ‬ⓥ᭎ⷐ‫ޤ‬ ߎࠇ߹ߢߦ㧘ࠕࡦࡕ࠾ࠕ߅ࠃ߮ࠫࠢࡠ࡞ࠪ࡜ࡦࠍ᧚ᢱࠟࠬߣߒߚశ CVD ᴺߦߟ޿ߡᬌ⸛ࠍⴕߞ
ߡ߈ߚ㧚ߘߩ⚿ᨐ㧘ᵄ㐳 254㧘185nm ߦࡇ࡯ࠢࠍ᦭ߔࠆ⚡ᄖశࠍ↪ࠆߎߣߦࠃࠅၮ᧼᷷ᐲ 300͠⒟ᐲߢᲧセ⊛⦟⾰ߥࠪ࡝ࠦࡦ⓸ൻ⤑߇ၸⓍน⢻ߢ޽ࠆߎߣࠍ␜ߒߚ㧚ᧄ⎇ⓥߢߪ㧘⚡ᄖ✢ᾖ኿ࠕࡦࡕ
࠾ࠕࠟࠬߦࠃࠆࠪ࡝ࠦࡦၮ᧼ߩ⋥ធ⓸ൻߦߟ޿ߡᬌ⸛ࠍⴕߞߚ㧚ᷫ࿶ࠕࡦࡕ࠾ࠕࠟࠬ㔓࿐᳇ਛ
㧔300Pa㧕ߩࠪ࡝ࠦࡦၮ᧼ߦ⚡ᄖశ㧔ᵄ㐳㧦254㧘185nm㧕ࠍᾖ኿ߒߚ⹜ᢱ⴫㕙ࠍ⚵ᚑಽᨆߒߚ⚿ᨐ㧘ၮ᧼᷷ᐲ 200͠એਅߩૐ᷷ߢ⋥ធ⓸ൻ߇ㅴⴕน⢻ߢ޽ࠆߎߣࠍ⏕⹺ߒߚ㧚߹ߚ㧘ࠛ࡝ࡊ࠰ࡔ࡯࠲ࠍ
↪޿ߡ⤑ෘ߅ࠃ߮ዮ᛬₸ߩ᷹ቯࠍⴕߞߚ⚿ᨐ㧘⓸ൻᤨ㑆ࠍჇടߐߖߡ߽⤑ෘߪ⚂ 2nm ⒟ᐲߢ㘻๺ߔࠆ௑ะ߇޽ࠆߎߣ㧘૞⵾⤑ߩዮ᛬₸ߪ 2.2ޯ2.3 ⒟ᐲߢ޽ࠆߎߣ߇ࠊ߆ߞߚ㧚߹ߚ㧘ᧄᚻᴺߦࠃࠅ૞⵾ߒߚ⓸ൻ⤑ࠍ↪޿ߡࠕ࡞ࡒࠥ࡯࠻ MIS ᭴ㅧࠍ૞⵾ߒ㧘㔚᳇⊛᷹ቯࠍⴕߞߚߣߎࠈ㧘MIS ᭴ㅧ․᦭ߩኈ㊂г㔚࿶․ᕈ߇ᓧࠄࠇࠆߎߣ߇ࠊ߆ߞߚ߽ߩߩ㧘ૐ㔚⇇ᤨߦ߅ߌࠆ࡝࡯ࠢ㔚ᵹ߇ LPCVD ᴺߥߤ
ߦࠃࠅ૞⵾ߒߚ⓸ൻ⤑ߦᲧߴߡ㕖Ᏹߦᄢ߈ߊߥࠆߎߣࠍ⏕⹺ߒߚ㧚৻ᣇ㧘⓸ൻᓟߩၮ᧼ࠍੇ῎㉄ൻ
ᴺߦࠃࠅ㉄ൻߔࠆߎߣߦࠃࠅ㧘⓸⚛߇ᢙ㧑⒟ᐲᷙ౉ߒߚ⓸ൻ㉄ൻ⤑ߩ૞⵾߇น⢻ߢ޽ࠆߎߣࠍ⏕⹺ߒ㧘ߘߩ㉄ൻㅦᐲߪ⓸ൻಣℂࠍᣉߐߥ޿ၮ᧼ߦᲧߴߡ㕖Ᏹߦㆃߊߥࠆߎߣ߇ࠊ߆ߞߚ㧚 ৻ᣇ㧘ߎࠇ߹ߢߦ⚐᳓ࠍ↪޿ߚ㓁ᭂ㉄ൻᴺߦࠃࠅ㧘ࠪ࡝ࠦࡦ਄ߦቶ᷷ߦߡ㉄ൻ߇ㅴⴕߔࠆߎߣࠍ
␜ߒߚ㧚ᧄ⎇ⓥߢߪ㧘㉄ൻᓟߩᾲಣℂ᷷ᐲߦኻߔࠆ⤑․ᕈᄌൻߦߟ޿ߡ⹏ଔߒߚ㧚ߘߩ⚿ᨐ㧘ᾲಣ೨ߩ㉄ൻ⤑ߢߪૐ㔚⇇ශടᤨߦᄢ߈ߥ࡝࡯ࠢ㔚ᵹ߇᷹ⷰߐࠇߚ߽ߩߩ㧘800͠એ਄ߩᾲಣℂߦࠃࠅᾲ㉄ൻ⤑ห╬ߩ⦟ᅢߥ⛘✼․ᕈࠍ␜ߔߎߣ߇᣿ࠄ߆ߣߥߞߚ㧚߹ߚ㧘⿒ᄖๆ෼ಽᨆ㧔FT-IR㧕ࠍⴕߞߚ⚿ᨐ㧘ᾲಣℂ᷷ᐲߩ਄᣹ߦ઻޿㧘᳓⚛⣕㔌ߦࠃࠅ Si-OH ⚿วߩࡇ࡯ࠢ߇ૐਅߔࠆߎߣࠍ⏕⹺ߒߚ㧚߹ߚ㧘ၮ᧼⴫㕙ߩࠠࡖ࡝ࠕኒᐲ߇ᚑ㐳ㅦᐲߦਈ߃ࠆᓇ㗀ࠍ⹏ଔߔࠆߚ߼㧘⥄↱㔚ሶ࡟࡯ࠩࠍᾖ
኿ߒߥ߇ࠄ㓁ᭂ㉄ൻࠍⴕߞߚ㧚ߘߩ⚿ᨐ㧘శᾖ኿ߦࠃࠆ⵾⤑ਛߩ࿁〝㔚ᵹ߅ࠃ߮ᚑ㐳ㅦᐲߩჇᄢࠍ

⏕⹺ߒߚ㧚
‫ޤ߼ߣ߹ޣ‬ ⚡ᄖ✢ߩࠛࡀ࡞ࠡ࡯ࠍ↪޿ࠆߎߣߦࠃࠅ㧘ࠕࡦࡕ࠾ࠕࠟࠬࠍ↪޿ߡࠪ࡝ࠦࡦࠍ 200͠એਅߩૐ᷷
ߢ⋥ធ⓸ൻߢ߈ࠆߎߣࠍ᣿ࠄ߆ߦߒߚ㧚߹ߚ㧘ߘߩᚑ㐳ㅦᐲߪ㕖Ᏹߦૐߊ㧘ᭂ⭯⤑ߩ⤑ෘࠦࡦ࠻ࡠ
࡯࡞߇น⢻ߢ޽ࠆߎߣ߇ࠊ߆ߞߚ㧚੹ᓟ㧘⥄↱㔚ሶ࡟࡯ࠩࠍ↪޿ߚᵄ㐳ߩᦨㆡൻߥߤߦࠃࠅ㧘⤑⾰ߩᡷༀࠍⴕߞߡⴕ߈ߚ޿ߣ⠨߃ࠆ㧚 ߹ߚ㧘⚐᳓ࠍ↪޿ߚ㓁ᭂ㉄ൻ⤑ߦኻߒ㧘⵾⤑ᓟᾲಣℂࠍᣉߔߎߣߦࠃࠅ⛘✼․ᕈ߇ᡷༀߔࠆߎߣ
ࠍ᣿ࠄ߆ߦߒߚ㧚ߎࠇߪ㧘ᾲಣℂߦࠃࠆ᳓⚛⣕㔌ߦࠃࠆ߽ߩߣ⠨߃ࠄࠇࠆ㧚߹ߚ㧘⵾⤑ਛߩశᾖ኿
ߦߟ޿ߡᬌ⸛ࠍⴕߞߚ⚿ᨐ㧘శᾖ኿ߦࠃࠅᚑ㐳ㅦᐲ߇Ⴧᄢߔࠆߎߣࠍ⏕⹺ߒߚ㧚੹ᓟߎࠇࠄߩේ࿃
ߦߟ޿ߡ⹦⚦ߦᬌ⸛ࠍⴕߞߡⴕ߈ߚ޿ߣ⠨߃ࠆ㧚‫⎇ޣ‬ⓥᬺ❣╬‫ޤ‬࡮⊒⴫⺰ᢥ(1) K. Ohnishi, A. ito, Y. Takahashi and S. Miyazaki: “Growth and Characterization of Anodic Oxidized
Films in Pure Water”, Jpn. J. Appl. Phys., Vol.41, No.3A, pp.1235-1240, 2002.3.
࡮ቇળ⊒⴫(1) K. Ohnishi and A. Ito (2000) Characterization of Anodic Oxide Films in Pure Water, Proc. of the 3rdInternational Symposium on Advanced Science and Technology of Silicon Materials(2) દ⮮㐓㧘ᄢ⷏৻ഞ㧘㜞ᯅ⧐ᶈ (2000) ⚐᳓ਛߢߩ㓁ᭂ㉄ൻᴺߦࠃࠅ૞⵾ߒߚ SiO2 ⤑ߦ㑐ߔࠆ⎇ⓥ㧘ᔕ↪‛ℂቇળ㑐ଥㅪว⻠Ṷળ(3) Ꮉୖ⑲㓶㧘㜞ᯅ⧐ᶈ㧘ᄢ⷏৻ഞ (2000) శ CVD ᴺߦࠃࠅ૞⵾ߒߚࠪ࡝ࠦࡦ⓸ൻ⤑ߩᾲಣℂലᨐ㧘ᔕ↪‛ℂቇળ㑐ଥㅪว⻠Ṷળ(4) ችፒବഥ㧘ᄢ⷏৻ഞ㧘㜞ᯅ⧐ᶈ (2001) 㓁ᭂ㉄ൻᴺߦࠃࠅ૞⵾ߒߚ㉄ൻ⤑ߩᾲಣℂലᨐ㧘ᔕ↪
‛ℂቇળ㑐ଥㅪว⻠Ṷળ(5) ᫪↰ፏ㧘⥓ୖ৻ᢅ㧘ᄢ⷏৻ഞ㧘㜞ᯅ⧐ᶈ (2002) ⚡ᄖ✢ബ⿠ࠕࡦࡕ࠾ࠕࠟࠬࠍ↪޿ߚ㉄⓸ൻ⤑ߩ૞⵾࡮⹏ଔ㧘ᔕ↪‛ℂቇળ㑐ଥㅪว⻠Ṷળ(6) ᷰㄝ┥ᄥ㧘ᩊ↰ᔀ਽㧘ᣂ၂ਭ㧘⮮↰᣿⦟㧘㜞ᯅ⧐ᶈ㧘ᄢ⷏৻ഞ (2004) 㓁ᭂ㉄ൻᴺߢ૞⵾ߒߚ
ࠪ࡝ࠦࡦ㉄ൻ⤑ߩ․ᕈ㧘ᔕ↪‛ℂቇ㑐ଥㅪว⻠Ṷળ(7) ᩊ↰ᔀ਽㧘ᷰㄝ┥ᄥ㧘ᣂ၂ਭ㧘⮮↰᣿⦟㧘㜞ᯅ⧐ᶈ㧘ᄢ⷏৻ഞ (2004) 㓁ᭂ㉄ൻᴺߦࠃࠆࠪ࡝
ࠦࡦ㉄ൻ⤑ߩ૞⵾ߣ⹏ଔ㧘ᔕ↪‛ℂቇ㑐ଥㅪว⻠Ṷળ

Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 1235–1240
Part 1, No. 3A, March 2002
#2002 The Japan Society of Applied Physics

Growth and Characterization of Anodic Oxidized Films in Pure Water
Kazunori O HNISHI* , Akira I TO{ , Yoshihiro TAKAHASHI and Syunsuke M IYAZAKI
Department of Electronic Engineering, Nihon University, 7-24-1, Narashinodai, Funabashi, Chiba 274-8501, Japan(Received March 14, 2001; accepted for publication December 12, 2001)

An anodic oxidized ﬁlm has been deposited in pure water at room temperature. Film thickness increases linearly as a functionof total charge during oxidation. High ﬁlm deposition rate and low surface roughness are obtained by alternately changing thepolarity of the applied voltage. The ﬁlm growth mechanism is discussed and the model of anodic oxidation in pure water is
proposed. The HF etching rate and the oxide thickness are reduced by an increase in annealing temperature. Though theelectrical characteristics of the anodic oxide ﬁlm are inferior to those of the ﬁlm obtained by thermal oxidation, they can be
improved by thermal annealing at a temperature of 400 C. [DOI: 10.1143/JJAP.41.1235]
KEYWORDS: silicon dioxide, anodic oxidation, low-temperature process, thermal annealing

1.

Introduction

With the progressive shrinking of LSI device size, device
and circuit is complexity increasing. The fabrication process
is also becoming increasingly diﬃcult. Moreover, there is agreat demand to reduce energy consumption on the grounds
of preventing global warming and environmental pollution.
An oxidation process in VLSI fabrication requires a hightemperature of around 1000 C and thermal stress damages
the silicon wafer inconspicuously. From this viewpoint, lowtemperature processes have been researched. Many lowtemperature processes have been proposed to fabricate
insulation ﬁlms on silicon such as plasma deposition,
chemical vapor deposition (CVD), photo-CVD,1) jet vapor
deposition (JVD),2) anodic oxidation and so on.
We considered the possibility of applying the electricaleﬀect to the fabrication process of semiconductor devices at
low temperatures. The ﬁrst thing we noted was theoccurrence of electrically enhanced oxidation and anodic
oxidation on the Si substrate. The eﬀect of the electric ﬁeld
on oxidation has been studied previously.3) Anodic oxidation
has been experimentally studied previously as an oxide
formation process. Usually, anodic oxidation was carried out
with electrolytes such as N methylacetamide (NMA)–KNO3
or tetrahydrofurfutyl alcohol (THF)–KNO3 .4–6) An anodic
oxidized silicon ﬁlm in pure water was also reported byDubrovskii et al.7)
Early works on anodic oxidation reported that the etchingrate was greater than that of thermal oxides and was changed
by impurity solutions and the growth rate was a function of
applied voltage in constant current mode.5,6) There is acontroversy regarding the oxidation mechanism. Dubrovskii
et al. proposed the silicon diﬀusion model in which siliconions diﬀused to the oxide–electrode boundary and formed
the oxide,7) whereas Kraitchman and Oroshnik argued thatthe oxygen migrates close to the oxide–silicon interface andoxide growth occurs at the interface.5)
Although the electrical property is improved by hightemperature annealing in He, the oxide ﬁlms were
substantially porous and the electrical property was poor.
For these reasons, the use of anodic oxidation wasabandoned in integrated circuit fabrication processes.
We intend to study the application of the electrical eﬀect

in the fabrication of integrated circuits at low temperatures.
This will contribute to suppressing the energy consumption
in VLSI fabrication systems. Here, we report on theexperimental method and the characteristics of anodic
oxidized silicon dioxide ﬁlms grown in pure water at room
temperature as a possible oxidation process at a lowtemperature. We discuss the electric characteristics andgrowth mechanism. We clarify that the surface roughness ofthe oxide is improved by alternately changing the polarity of
the applied voltage and that the oxide thickness is notincreased by the applied voltage, but increases with the total
charge ﬂowing between electrodes during oxidation. The
electrical characteristic is improved by annealing over400 C after oxidation.
2.

Experimental

Samples used in our experiments were fabricated asshown in Fig. 1. Wafers used in this experiments were ntype and p-type silicon wafers of h100i plane and theircorresponding resistivities were 3–5 cm and 0.8–
1.2 cm. The wafers were cleaned by ths standard RCA
cleaning method. Anodic oxidation was carried out in purewater with resistivity of 16 Mcm at 20 C. After oxidation,
wafers were annealed in nitrogen atmosphere at various
appropriate temperatures. A reference sample was preparedby thermal oxidation at 1045 C in dry oxygen and annealed
at the same temperature in nitrogen. Film thickness was*

E-mail: ohnishi@ecs.cst.nihon-u.ac.jp
Present address: Sanken Electric Co., Ltd., 3-6-3 Kitano, Niiza, Saitama
352-0003, Japan.

RCA cleaning of silicon substrate ( 1.1 cm x 1.1 cm)
Anodic oxidation
Thickness measurement
by ellipsometer

Thermal oxidation(1045 C, O 2: 500 cc/min)

Thermal annealing (30 min, N2: 500 cc/min)Thickness measurement by ellipsometer
Removal of oxide on Si backside by HF
Fabrication of Al gate MOS diode(Diameter of gate electrode: 0.3mm )

{

1235

Fig. 1. Sample preparation process ﬂow.

1236

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 3A

K. OHNISHI et al.

3 cm

Output ofController

I

Pure water(16 M cm)

Output ofVoltage
SourceAmmeter

Pt or Si

(Keithley 2000 Multi-meter)

Teflon water bath

PC

Si

0 [V]

400 ms(Data store)+

Voltage controller

DC voltage source

Fig. 3. Time chart of alternately changed polarity of voltage (AC).

(Alternative voltage generator)

Fig. 2. Anodic oxidation experimental system.

measured by the ellipsometry method. The ﬁlm composition
was analyzed by X-ray photoelectron spectroscopy. Electrical properties were measured using MOS structures withaluminum electrodes. Anodic oxidation was carried out inpure water with the application of a constant voltage acrossthe silicon wafer anode and the other electrode of platinum
or silicon, as shown in Fig. 2. The distance between theelectrodes was 3 cm, and anodic oxidation was carried out
with a constant DC voltage or by alternately changing thepolarity of the voltage (AC). We used the period of 60 s andthe duty ratio of 0.5 for the AC voltage source. Figure 3 is atime chart of applied voltage when the polarity changes
alternately. The current ﬂow during anodic oxidation wasmonitored and integrated to calculate the total charge duringoxidation. Silicon substrates were cut to a 1.1 cm square size,
and the anodic oxidation window was 1 cm square.
3.

600

(a)

400
300

8

200

6

100
0

500
Thickness [ ]

500

300

6

4

2

0

0

y [mm]

0

600

500

500

(c)

400
300
8

200

6
4
8

4
x [mm]

2

0

0

8

2
6

4

y [mm]

20y [mm]

0

(d)

400
300

8

200

6

100
0

2
6

6
4

x [mm]

600

100

8

200

2

8

x [mm]

0

(b)

400

100

4

Thickness [ ]Thickness [ ]Experimental Results

We measured distributions of the thickness of both
samples fabricated with DC and AC voltages. Figure 4
shows proﬁles of oxide ﬁlms in this experiment. Figures 4(a)600

Thickness [ ]400 msPolarity ofElectrode

4
8

2
6

4
x [mm]

200y [mm]

Fig. 4. Proﬁle of oxide thickness. (a) on n-Si with 300 V DC (b) on p-Si with 300 V DC (c) on n-Si with 300 V AC (d) on p-Si with
300 V AC.

Oxide Thickness [ ]800
AC 200400
500
DC 200
500

600

400
200

K. OHNISHI et al.

Etched Oxide thickness [ ]Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 3A

1237

400
Anodic Oxidation Time [min]

300

25
20
15

200

5
100

10

Thermal Oxide

0

0

0

0.1
0.2
0.3
0.4
0.5
0.6
Total Charges during Oxidation [C]Fig. 5. Film thickness vs total charges during anodic oxidation on p-type(100) 3–5 cm Si at 20 C.

and 4(b) show thickness distributions of the oxide ﬁlm on
the n-type and p-type silicon substrates when the voltage
was 300 V DC. Figures 4(c) and 4(d) show distributions of
the oxide thickness when the applied voltage changed
polarity as shown in Fig. 3. The oxide on the n-type
substrate was thinner than that on the p-type substrate andrandomly distributed, but it was improved upon oxidation
with an applied AC voltage as shown in Fig. 4(c). Figure 5
shows the oxide thickness on the p-type silicon substrate
against the total charge ﬂowing in the system duringoxidation for diﬀerent applied voltages. This ﬁgure showsthat the oxide ﬁlm thickness linearly increased with total
charges during oxidation and the oxide obtained with AC
voltage is thicker than that obtained with DC voltage. The
oxide thickness as a function of total charges ﬂowing duringoxidation with AC voltage is twice that with DC voltage.
The composition of the anodic oxidized ﬁlm in our
experiment was analyzed by X-ray photoelectron spectroscopy; it was stoichiometric SiO2 , the same as the thermaloxide formed at 1045 C.
We estimated the etching rate of thermal oxide and anodic
oxide ﬁlms in order to compare the quality. The thermaloxide ﬁlm of 30 nm thickness was grown on a p-type silicon
substrate at a temperature of 1045 C and then it was
reoxidized by anodic oxidation with 300 V AC for 5–25 min.
These samples were etched by a solution of HFð48%Þ :H2 O ¼ 1 : 50 at room temperature. Figure 6 shows theetched oxide thickness against etching time for various
anodic oxidation times. The region of high-etching rate at an
early stage is predominantly of anodic oxide and the regionof low-etching rate a later stage is predominantly thermaloxide being slightly aﬀected by anodic oxidation. This result
reveals that the density of anodic oxide is lower than that ofthermal oxide as reported previously. The etching rate of the and 8–9 A/s for the reoxidized
thermal oxide ﬁlm is 1 A/s,ﬁlm by anodic oxidation. It is conﬁrmed that the etching rateof the oxide ﬁlm fabricated by anodic oxidation (300 V AC)on the silicon substrate is 25 A/s,and the rate of the ﬁlm
grown with DC voltage is higher than that with AC voltage.
The refractive index of the anodic oxide ﬁlm was around however it ranged from 1.3
1.45 for ﬁlms thicker than 200 A,
 The dielectric constants
to 1.4 for ﬁlms thinner than 100 A.

10

20

30

40

50

60

Etching Time [s]Fig. 6. Etched oxide thickness vs etching time for diﬀerent anodic
oxidation times.

1.0

C / Cmax.

0

Applied Voltage
during Oxidation

0.8
200 [V] (AC)500 [V] (AC)0.6
0.4
400 [V] (AC)0.2

300 [V] (AC)

0.0
-7

-6

-5

-4

-3
-2
-1
Gate Voltage [V]0Fig. 7. C–V curves of MOS structures with anodic oxide fabricated at
diﬀerent voltages.

were determined using the thickness measured by ellipsometry and the capacitance measured under accumulation
condition. The estimated dielectric constant was between 4
and 7 and it was higher than that of the thermal oxide.
The electrical characteristics are analyzed from highfrequency C–V and I–V characteristics. Figure 7 shows theC–V curves for diﬀerent anodic voltages and Fig. 8 shows
the density of Si/SiO2 interface states evaluated by theTerman method. Densities of ﬁxed positive charge in theoxide are evaluated by ﬂat-band voltages, and4:5{4:7  1012 cm2 and 3:2  1012 cm2 are obtained when
the applied voltages during oxidation are 200–400 V and
500 V (AC), respectively.
Anodic oxidized ﬁlms were annealed in nitrogen atmosphere at various temperatures. The etching rate and also
thickness decrease as the temperature increases as shown in
Figs. 9 and 10.
Electrical characteristics are improved by high-temperature annealing. Figure 11 shows the C–V curve of a MOS
structure annealed in nitrogen atmosphere at 400 C for
30 min. The anodic oxidized ﬁlm is fabricated with 300 V
AC and the total charge during oxidation is 0.1 C. The
density of ﬁxed positive charge is 4:3  1011 cm2 . Figure
12 shows the density of Si–SiO2 interface states of the same

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Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 3A

K. OHNISHI et al.

1.0
Applied Voltage
during Oxidation

1013

Capacitance [pF]

Dit [cm-2 eV-1]

1014

200 [V] (AC)1012

400 [V] (AC)300 [V] (AC)

1011
-0.2

0.2

0

0.6
0.4
0.2

500 [V] (AC)

0.4

0.8

0.0
0.6

-5

0.8

-4

-3

-2

0

2

1

Gate Voltage [V]Surface Potential [eV]Fig. 8. Interface states density vs surface potential evaluated from C–V
curves in Fig. 7.

-1

Fig. 11. C–V characteristic of a MOS structure after annealing at 400 C
for 30 min. Anodic oxide is fabricated at 300 V AC, total charge is 0.1 C.

1013

Dit [cm-2 eV-1]

Etching Rate [ /s]

30
25
20
15

1012

1011

10
1010
-0.2

5

0.2

0.4

0.6

Surface Potential [eV]

0
0

200

400

600

800

1200

1000

Annealing Temperature [Fig. 12. Interface states density vs surface potential evaluated from C–V
characteristic of Fig. 11.

]

Fig. 9. Etching rate of anodic oxide after annealing in N2 atmosphere for
30 min at diﬀerent temperature. Anodic oxidized with DC voltage andtotal charge is 1 C.

Oxide Thickness (Normalized
by that before annealing)

0

100

95

90

85
0

200

400
600
800
Annealing Temperature [1000]

Fig. 10. Thickness change at diﬀerent annealing temperature in nitrogen
atmosphere for 30 min. Anodic oxidation condition is the same as inFig. 9.

structure.
4.

Discussions

In this paper, we explain our experimental results ofanodic oxidation on the silicon substrate in pure water atroom temperature. The growth rate of the oxide ﬁlm on the
n-type substrate is smaller than that of the oxide ﬁlm on thep-type substrate and SiO2 grows only under the condition
that the voltage of the silicon wafer is positive. The
thickness of the oxide ﬁlm increases as the total chargeduring oxidation progresses; this result diﬀers from that of
Dubrovskii’s experiment. Oxidation by alternately changing
the voltage polarity makes the surface roughness decrease.
The oxidation rate on the p-type substrate is larger than thaton the n-type substrate and moreover, the surface ﬂatness of
the anodic oxide ﬁlm on the n-type substrate is poorer thanthat on the p-type substrate. When the electrode polarity
changes alternately, the oxide growth rate is twice that of thecase of single polarity and the surface roughness of the oxide
ﬁlm on the n-type substrate is improved.
From these experimental results, we will consider theanodic oxidation mechanism as follows. A water molecule
has a small dipole moment. Electrons of hydrogen atoms
take positions close to the oxygen atom side. A hydrogenJpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 3A
Water
H
H

OH
H

O

O
H

K. OHNISHI et al.

Water

SiliconSilicon
Si

H

H

Si

H
Si

O

H
O
H

H

H

H
Si

O

Si
e

H

O
O

Si
Si
Si

H

Si

OH

Si
O

O

OH

Si

Si

Si

Si

Si

Si
Si

Va

Si

Si

H

Si

Si

Si

O

H
O

Si
Si

H

Si

H

Si

O

Si

1239

Si

Si

e

Si

Si

e

Fig. 13. Proposed model of anodic oxidation in pure water.

Water Hatom is slightly positively charged, þ, and the oxygen of awater molecule is slightly negatively charged, 2 as shownin Fig. 13. When the silicon wafer is biased positive against
a cathode, positive charges exist at the water–Si interface.
Oxygen of a water molecule is directed toward the positively
charged Si–water interface as shown in Fig. 13.8) The watermolecule drifts and/or diﬀuses into the silicon bulk, andhydrogen is dissociated from H2 O. This hydrogen is highly
activated and reacts with Si=Si bonds and/or dangling bonds
forming SiH and also breaks the covalent bond of Si=Si.
The oxygen-hydrogen pair atom combines with Si and forms
SiOH.
When a negative bias is applied to the silicon substrate,
the hydrogen of a water molecule is directed toward the Siwater interface and the water molecule is dissociated tohydrogen and hydroxyl by reacting with electrons from thenegative electrode.5) Hydrogen drifts and/or diﬀuses into thesilicon bulk and reacts with Si=Si forming Si–H. OH drifts
out to the water. Another possible mechanism is that OH and
H in the water are dissociated from a water molecule and H
drifts and/or diﬀuses into the silicon bulk and forms Si–H
and/or is positioned at an interstitial site. The hydrogendiﬀuses deeply into the inner space of the silicon bulk due toa large diﬀusion constant.
When the polarity of the supplied voltage to the silicon
substrate is changed from negative to positive, electrons
move from the Si–metal interface to the water–Siliconinterface and the distributed ions at the interface areneutralized by the electrons. Moreover, activated hydrogenremaining in the oxide approaches the water–Si interface.
The hydrogen distributed in the silicon bulk breaks the
Si=Si bond and forms Si–H, and OH from the water–Siinterface forms the Si–O and SiO2 as shown in Fig. 14. Thus
the oxidation rate against total charges during oxidation incase of AC voltage is roughly twice that with DC voltage asshown in Fig. 5. In the case of the p-substrate, holes
accumulate sharply at the interface as shown in Fig. 15.
Meanwhile, in the case of the n-substrate, donor ions form adepletion layer, holes (minority carrier) are present at theinterface as shown in Fig. 16 and a depletion layer restricts
the growth rate.
The growth rate diﬀerence between SiO2 on the n-type
substrate and on the p-type substrate is considered asfollows. In case of the p-type silicon substrate, positive

H
H H
O

H
H H

OH

H
H
H

Water H
H+
H
H
H

O

H

H
H
H
H
H

Silicon
Si

H H
Si

Si
Si

Si
Si

Si

H
Si

Si

H
Si
H

Si
Si

Si

e

Si

Silicon

OH

H

Si

Si
Si
Si

O
O

Si
Si

Si

O

O
OH

Va

Si
Si

H
Si

Si
Si

Va

Si

e

Fig. 14. Anodic oxidation model when the applied voltage changes fromnegative to positive.

charges near the water–Si interface are holes and are sharply
distributed at the silicon surface. Therefore, OH and H react
easily with silicon covalent bonds and dangling bonds, andform SiOH and SiH, respectively. In the case of an n-typesilicon substrate, positive charges near the water–Si interface
are ionized donors and holes that are minority carriers in an
n-type substrate. There is a small number of holes comparedwith the case of the p-type substrate. Thus, the number of H
and OH reacting with holes is smaller than for the p-type
substrate.
5.

Conclusions
We experimented with anodic oxidation in pure water at

1240

K. OHNISHI et al.

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 3A
H
Water

p-Si

+ Va

O
H
H
HO
OH
H
O

H

E

Si

Energy-band
diagram

Si
Si

Si

accumulation condition

Si

Q
hole
(accumulation)

Si

O
Si

Si

Si

Si

Si

Si

Si

H

Si

Si

Si

H
O

Si

B

Si

H

H

Si

B

water

H

Si

Si

Si

Chargedistribution

H
OH

Si

Si
B

Si

Si

Si

Si

Si
p-Si

Fig. 15. Energy band diagram and charge distribution of n-Si surface.

H
Water

n-Si

+ Va
H

E

O
H
H
HO
OH
H
O

Si
Energy-band
diagram

H
OH

Si

Si
Si

H

Si
Si

water

H

H
H
O H
O

Si
P

Si
Si

Si
Si

Si

depletion condition

Si

Q
hole
(inversion)

Chargedistribution

space
charge

Si

Si
P

Si

Si
Si

Si

Si
Si

Si

Si
Si

Si

Si
P

Si

We also examined the possibility of improving theelectrical characteristic of SiO2 ﬁlms by high-temperatureannealing. The etching rate of the oxide ﬁlm became small,and it was eﬀective in decreasing the density of Si/SiO2
interface states and positive charge density in the oxide.
Our intention in performing this study is to apply theanodic oxide for the following purposes.
1. Low-temperature VLSI processes.
2. Bonded and Etched SOI.
3. Oxide ﬁlm for micro-machining.
It is diﬃcult to apply the anodic ﬁlm to the LSI insulatingﬁlm process directly. However, it will be possible toimprove the quality by controlling the resistivity of purewater, substrate temperature and voltage stability. If theanodic oxide can be applied to an oxidized substrate tofabricate a BE–SOI wafer, an inexpensive SOI substrate canbe obtained. We consider that the most realistic application
at this stage is the use of the anodic oxide for micromachining.
We attempted to apply the anodic oxidation to aid the
suppression of global warming and environmental pollution,and made a tiny step toward the application of the electricaleﬀect to the device fabrication process. We hope this study
will contribute to positive changes in the use of energy.
Acknowledgement

Si

Si
n-Si

Fig. 16. Energy band diagram and charge distribution of p-Si surface.

room temperature as a possible low-temperature process andalso discussed the growth mechanism using a model ofmolecular dipoles of water. We found that alternatelychanging the polarity of the applied voltage operates
eﬀectively to cause anodic oxidation. We also found thatthe composition of the oxide was stoichiometric SiO2 , theoxide thickness increased with charges ﬂowing duringoxidation and the growth rate on p-type silicon was higher
than on n-type silicon. The surface roughness was larger on
n-type silicon than on p-type silicon. When the applied
voltage polarity was changed alternately, surface roughness
became small and the growth rate became twice that in thecase of single polarity.

We would like to thank Professor P. M. Lenahan ofPennsylvania State University for useful suggestions. We
also wish to thank M. Saitou for experimental support andhelp with the ﬁgures. This work was supported partly by
Fund of Research Project of College of Science andTechnology, Nihon University.
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