- Email: [email protected]
Thin Si oxide films for MIS tunnel emitter by hollow cathode enhanced plasma oxidation
ELSEVIER
Thin Solid Films 281-282 (1996) 412-414
Thin Si oxide films for MIS tunnel emitter by hollow cathode enhanced plasma oxidation K. Usami *, I. Takahashi, E. Miyake, M. Moriya, X.Y. Cai, T. Kobayashi, T. Goto The faculty of Electrical Communications, The University of Electra-Communicutions, /-5-/ Chofugao!m, Chofu-Shi, Tokyo, 182, Japan
Abstract . ADC plasma oxidation system with a hollow cathode which consists of a pair ofparallel Siplates was developed. Using this system, thin Si oxide films of less than 40 nm thickness were grown on n-type Si( 100) substrates, for the application to the tunnel devices. The film quality and the oxide stoichiometry were estimated by XPS measurements. Onthe oxide films, the MIS (Metal-Insulator-Semiconductor) diode type tunnel emitters were fabricated. The electrical properties of the diodes, such as I-V characteristics and electron emission into the vacuum were measured. For a typical sample, anelectron emission current density of 800 pA/mm 2 into thevacuum was obtained. Keywords: Electron emission; Plasma processing and deposition; Silicon oxide; Tunneling
-_._--------------------------------------1. Intrcduetion
The tunnel devices onSisubstrates such as thin gate MOS transistors [1J, MIS solar cells [2], tunnel emitters [3], etc. are expected tohave very promising characteristics asdevices for the near future in semiconductor applications. Butthese devices require a high quality ultrathin and uniform insulator layer with a thickness in the range from 1 nm to 10 nm for the quantum mechanical tunnelling current through it. Butit is difficult to obtain such a ultrathin and uniform Si oxide layer reproducibly and controllably using conventional thermal oxidation processes in spite of its excellent film quality [4]. In addition, a temperature rise above 900 °Cduring the oxidation process causes serious thermal damage onthepreviously fabricated part of thedevice. The growth ofSioxide film ata lower process temperature has been approached byusing ion assisted oxidation, such as plasma oxidation or ion beam oxidation. In this experiment, we have developed a hollow cathode enhanced plasma oxidation system and tried to grow thin Sioxide films for tunnel emitters on Si substrates. The properties of these Si oxide films were estimated by ellipsometry for film thickness and by XPS (X-ray Photoemission Spectroscopy) for oxide stoichiometry and depth profile respectively. In order to investigate oxide quality as thetunnel barrier, we fabricated MIS diode type tunnel emitters bydepositing thin metal counter electrodes on the oxide
* Corresponding author. 0040·6090/96/$15.00 © 1996 Elsevier Science SA All rights reserved PllS0040·6090(96)08689-0
films. The electrical properties of MIS diodes, such as I-V characteristics and electron emission into the vacuum were also measured.
2. Experimental
2.J. Development of the hollow cathode enhanced plasma oxidation system In the hollow discharge plasma system with a cylindrical or a pair of parallel plate cathodes, we can obtain a higher degree of ionization than that of the conventional glowdischarge system with a plane cathode, Hence a large enough and a stable discharge current can be obtained at a lower anode voltage and lower discharge gas pressure. As the negative oxygen ions in the plasma chamber are accelerated by the anode voltage and strike the substrate, the lower anode voltage oxidation such as hollow discharge oxidation is expected to cause less bombardment damage of the oxide film during oxidation. In this experiment, plasma discharge was enhanced by the hollow cathode which consists of a pair of rectangular Si plates (12 rom X 40mm X 0.4mm). Inorder toprevent metal contamination by cathode sputtering, Si plate was used as a cathode material. Theplasma chamber in which the hollow cathode was placed, was a 30mm diameter and 45mm height fused silica tube. It was set up in the 40 em diameter glass bell-jar and evacuated to less than 10- 5 Pa. by a 600litres/
K. Usaml et a/./Thill Solid Films 28/-282 (/996) 4/2-414
min. and 6 inch oildiffusion pump. The 5-nine purity oxygen gas was directly introduced into the plasma chamber through a variable leakvalve. Thegas pressure atthe plasma chamber was kept in the range 1.~3-2.66 Pa. throughout the experiment. The schematic diagram ofthe hollow cathode enhanced plasma oxidation system is shown inFig. I. Thedischarge characteristics of the oxidation system with a normal plane cathode and with a hollow cathode areshown inFig. 2. Forthe normal plane cathode system, the discharge current Id was less than 1 rnA atthe anode voltage of Vd == 500 V, butfor the hollow cathode system /d was enhanced to 26 mA at the same anode voltage. In both cases, oxygen gas pressure was kept constant at the pressure of 1.33 Pa. therefore the enhancement of the oxidation rate of the Si surface was also expected.
2.2. Plasma oxidation andfabrication ofMIS diode type tunnel emitter Thegrowth of thin Si oxide film and the MIS diode type tunnel emitter fabrication process were as follows. First of all, 6 mm X 6 mm X 0.4 mm n-type Si( 100) substrate with a resistivity of ]-2 ohm-ern was cleaned by using standard chemical solvents. The thin oxide film was grown on the substrate by exposing the hollow cathode enhanced oxygen plasma as mentioned above. The substrate temperature during oxidation was varied from 80°Cto400 °C, which Hollow Dischargc Chamber
Oxygen Gas
413
Table 1 Typical hollow discharge oxidation conditions Discharge gas
Pure oxygen (5.N)
Gas pressure Discharge voltage Discharge current Oxidation temperature Oxidation time
1.33-2.66 Pa 300-500 V 0-30 rnA 80-400·C 5-120 min
was calibrated by an Almel-Clomel thermocouple attached to the substrate. Typical oxidation conditions are listed in Table I. The film thickness measurements were made with a PLASMOS ellipsometer. The oxide stoichiometries were also measured by XPS using a SHIMAZU spectrometer. A thin Au counter electrode (about 10 nm thick) was deposited on the oxide film by a conventional vacuum evaporation system, and the MIS diode type tunnel emitter was fabricated as shown inFig. 3.The I-V characteristics and the electron emission into the vacuum were measured inthe highvacuum chamber evacuated to less than 10- 6 Pa. by using a turbo-molecular pump system, where the electron collector in the chamber was a lOX 10 mm area Ta plate and it was separated by about 5 mm from the counter electrode of the emitter. The collector applied voltage was 55 V, and the collector current or the emission current was directly measured by a high sensitivity electrometer.
3. Results and discussion
•
Diffusion Pump
Fig. I. Schematic diagram ofthe hollow cathode enhanced plasma oxidation system.
P02=2.66PL
.,
RoDoll' Cathode
I/
Fig. 4 shows the relations between oxidation time and Si oxide film thickness measured by ellipsometer asa parameter of oxidation temperature. The oxide thickness in the range 5 nm-37 nm can be reproducibly controlled with oxidation time. The thickness ofthe oxide film almost linearly depends on the oxidation time but slightly saturates with increasing oxidation time. On the other hand, the oxidation rate also depends on the deposition temperature and it increases with increasing substrate temperature. Fig. 5 shows the depth profiles of XPS measurements of the Si2pcore level peaks for oxide films prepared atoxidation temperatures of 80°C and 400 "C respectively, where the film thickness of the each sample is about 10 nm. Near the
/
\; .'
..- NOl'III&1 Calhodc
I
Au SiO SiOx n-Si AI
Discharge Vollage [V]
Fig. 2.Discharge characteristics ofa plasma oxidation system with a normal cathode and with a hollow cathode.
Fig. 3. Schematic diagram of MIS diode type tunnel emitter.
K. Usami et al. /71/in Solid Films 281-282 (1996) 412-414
414
10r-------~t----,1000
~
o
Au/Si02fSi • Diode Currenl
200 'C
Ts=400 ' C - 0 '\.
,
80 'C
//
/
o
o Emission Current
•
Di!eharac Vollqe=500V D1acharlc Current=lOmA Subllra\e Temperature Ts
2
o
OIidaUon Time ( min ] Fig. 4, Relations between oxidation time and oxide thickness measured by elipsometer asa parameter of substrate temperature.
•••• ••••••
•
dl
nO
{) 10 Diode Voltage [ V]
Fig. 6. The I-V characteristics and electron emission into thevacuum of a MIS diode type tunnel emitter.
4. Conclusions . 0 0 0
~ ~
,..,
~....
>
~ 10( ~
I!P
is
~
5101
103
• Ts=400 'C
0
• _~
I
~ ~
:9
'»:
Dlsclwllc Vollalle=500V .... Dllchargc CW1'ClIt=lOmA
I
51
0
.. •
80 'C
0
.0
o
•
•.....,,.!~~e.o 1 I 5101 Sl
0
ElebiDg Time [ sec ]
Fig. 5. Depth profiles of XPS measurements of the Si 2pcore level peaks for oxide film atthesubstrate temperatures of80°C and 400°C respectively.
surface of the Si oxide film, the relatively sharp and symmetrical photoemission peak associated with Si dioxide is observed at 104 eV (which isslightly larger than the standard Si dioxide value because of electrical charge above the film surface) but thelow energy peak associated with theSi suboxide peak is notobserved. Hence thehigh quality insulating layer is obtained under the film surface. A noticeable differenceis not observed between the oxidation temperatures of 80°C and400°C. Incases of conventional thermal oxidation, a typical thickness of Si-Si dioxide interfacial transition layer is approximately 1 nm [5]. Butin this case it was about 5 nm, which is considerably larger than that of thermal oxidation film. 'The electron emission of the tunnel emitter which was fabricated ontheplasma oxidized Si oxide films grown at the substrate temperature of 80°C was measured, where thedischarge voltage is 500V andoxide thickness is 15 nm respectively. A typical result is shown in Fig. 6 and the I-V characteristic of thesame sample isalso plotted. The electron emission current rises at Vd = 12.7 V and the maximum value is 800pA/mm2•
Thin Sioxide films onSisubstrates grown byusing hollow cathode enhanced plasma oxidation were investigated as a tunnel barrier insulator. On the oxide film, MIS diode type tunnel emitter was fabricated and electrical properties of the diode were measured. From these experiments, we conclude thefollowing results, ( 1) Oxygen plasma was enhanced by a Si plate hollow cathode without metal contamination. (2) Thin Si oxide films in the thickness range 5 nm-37 nm were grown on n-type Si( 100) substrates controllably and reproducibly by controlling the plasma discharge conditions or the oxidation time. (3) A maximum electron emission current density of 800 pA/mm2 was obtained for typical MIS diode type tunnel emitter fabricated on 15 nm thick Si oxide film grown at a lower temperature (80°C) than that of conventional thermal oxidation.
Acknowledgements The authors acknowledge Prof. S.Ono andProf. K. Yokoo of Tohoku University for useful discussions of this study.
References [1 JK.M. Chu and D.L. Puifrey, IEEE Trans. Electron Devices, 35 (1988) 188. [2] D.L. Puifrey, IEEE Trans. Electron Devices, 25 (1978) 1308. [3] Y. Yokoo, H. Tanaka, S. Sato, 1. Murota and S. Ono, 1., Vac. Sci. Technol., B 2 (1993) 429. [4] J.M.E.Harper, M. Heibum, l.L.Speidel! and J.1. Cuomo. J. Appl. Phys.• 52 (1981) 4118. [5] T. Hattori and T. Nishina, Surf. Sci.• 86 (]979) 555.
Thin Solid Films 281-282 (1996) 412-414
Thin Si oxide films for MIS tunnel emitter by hollow cathode enhanced plasma oxidation K. Usami *, I. Takahashi, E. Miyake, M. Moriya, X.Y. Cai, T. Kobayashi, T. Goto The faculty of Electrical Communications, The University of Electra-Communicutions, /-5-/ Chofugao!m, Chofu-Shi, Tokyo, 182, Japan
Abstract . ADC plasma oxidation system with a hollow cathode which consists of a pair ofparallel Siplates was developed. Using this system, thin Si oxide films of less than 40 nm thickness were grown on n-type Si( 100) substrates, for the application to the tunnel devices. The film quality and the oxide stoichiometry were estimated by XPS measurements. Onthe oxide films, the MIS (Metal-Insulator-Semiconductor) diode type tunnel emitters were fabricated. The electrical properties of the diodes, such as I-V characteristics and electron emission into the vacuum were measured. For a typical sample, anelectron emission current density of 800 pA/mm 2 into thevacuum was obtained. Keywords: Electron emission; Plasma processing and deposition; Silicon oxide; Tunneling
-_._--------------------------------------1. Intrcduetion
The tunnel devices onSisubstrates such as thin gate MOS transistors [1J, MIS solar cells [2], tunnel emitters [3], etc. are expected tohave very promising characteristics asdevices for the near future in semiconductor applications. Butthese devices require a high quality ultrathin and uniform insulator layer with a thickness in the range from 1 nm to 10 nm for the quantum mechanical tunnelling current through it. Butit is difficult to obtain such a ultrathin and uniform Si oxide layer reproducibly and controllably using conventional thermal oxidation processes in spite of its excellent film quality [4]. In addition, a temperature rise above 900 °Cduring the oxidation process causes serious thermal damage onthepreviously fabricated part of thedevice. The growth ofSioxide film ata lower process temperature has been approached byusing ion assisted oxidation, such as plasma oxidation or ion beam oxidation. In this experiment, we have developed a hollow cathode enhanced plasma oxidation system and tried to grow thin Sioxide films for tunnel emitters on Si substrates. The properties of these Si oxide films were estimated by ellipsometry for film thickness and by XPS (X-ray Photoemission Spectroscopy) for oxide stoichiometry and depth profile respectively. In order to investigate oxide quality as thetunnel barrier, we fabricated MIS diode type tunnel emitters bydepositing thin metal counter electrodes on the oxide
* Corresponding author. 0040·6090/96/$15.00 © 1996 Elsevier Science SA All rights reserved PllS0040·6090(96)08689-0
films. The electrical properties of MIS diodes, such as I-V characteristics and electron emission into the vacuum were also measured.
2. Experimental
2.J. Development of the hollow cathode enhanced plasma oxidation system In the hollow discharge plasma system with a cylindrical or a pair of parallel plate cathodes, we can obtain a higher degree of ionization than that of the conventional glowdischarge system with a plane cathode, Hence a large enough and a stable discharge current can be obtained at a lower anode voltage and lower discharge gas pressure. As the negative oxygen ions in the plasma chamber are accelerated by the anode voltage and strike the substrate, the lower anode voltage oxidation such as hollow discharge oxidation is expected to cause less bombardment damage of the oxide film during oxidation. In this experiment, plasma discharge was enhanced by the hollow cathode which consists of a pair of rectangular Si plates (12 rom X 40mm X 0.4mm). Inorder toprevent metal contamination by cathode sputtering, Si plate was used as a cathode material. Theplasma chamber in which the hollow cathode was placed, was a 30mm diameter and 45mm height fused silica tube. It was set up in the 40 em diameter glass bell-jar and evacuated to less than 10- 5 Pa. by a 600litres/
K. Usaml et a/./Thill Solid Films 28/-282 (/996) 4/2-414
min. and 6 inch oildiffusion pump. The 5-nine purity oxygen gas was directly introduced into the plasma chamber through a variable leakvalve. Thegas pressure atthe plasma chamber was kept in the range 1.~3-2.66 Pa. throughout the experiment. The schematic diagram ofthe hollow cathode enhanced plasma oxidation system is shown inFig. I. Thedischarge characteristics of the oxidation system with a normal plane cathode and with a hollow cathode areshown inFig. 2. Forthe normal plane cathode system, the discharge current Id was less than 1 rnA atthe anode voltage of Vd == 500 V, butfor the hollow cathode system /d was enhanced to 26 mA at the same anode voltage. In both cases, oxygen gas pressure was kept constant at the pressure of 1.33 Pa. therefore the enhancement of the oxidation rate of the Si surface was also expected.
2.2. Plasma oxidation andfabrication ofMIS diode type tunnel emitter Thegrowth of thin Si oxide film and the MIS diode type tunnel emitter fabrication process were as follows. First of all, 6 mm X 6 mm X 0.4 mm n-type Si( 100) substrate with a resistivity of ]-2 ohm-ern was cleaned by using standard chemical solvents. The thin oxide film was grown on the substrate by exposing the hollow cathode enhanced oxygen plasma as mentioned above. The substrate temperature during oxidation was varied from 80°Cto400 °C, which Hollow Dischargc Chamber
Oxygen Gas
413
Table 1 Typical hollow discharge oxidation conditions Discharge gas
Pure oxygen (5.N)
Gas pressure Discharge voltage Discharge current Oxidation temperature Oxidation time
1.33-2.66 Pa 300-500 V 0-30 rnA 80-400·C 5-120 min
was calibrated by an Almel-Clomel thermocouple attached to the substrate. Typical oxidation conditions are listed in Table I. The film thickness measurements were made with a PLASMOS ellipsometer. The oxide stoichiometries were also measured by XPS using a SHIMAZU spectrometer. A thin Au counter electrode (about 10 nm thick) was deposited on the oxide film by a conventional vacuum evaporation system, and the MIS diode type tunnel emitter was fabricated as shown inFig. 3.The I-V characteristics and the electron emission into the vacuum were measured inthe highvacuum chamber evacuated to less than 10- 6 Pa. by using a turbo-molecular pump system, where the electron collector in the chamber was a lOX 10 mm area Ta plate and it was separated by about 5 mm from the counter electrode of the emitter. The collector applied voltage was 55 V, and the collector current or the emission current was directly measured by a high sensitivity electrometer.
3. Results and discussion
•
Diffusion Pump
Fig. I. Schematic diagram ofthe hollow cathode enhanced plasma oxidation system.
P02=2.66PL
.,
RoDoll' Cathode
I/
Fig. 4 shows the relations between oxidation time and Si oxide film thickness measured by ellipsometer asa parameter of oxidation temperature. The oxide thickness in the range 5 nm-37 nm can be reproducibly controlled with oxidation time. The thickness ofthe oxide film almost linearly depends on the oxidation time but slightly saturates with increasing oxidation time. On the other hand, the oxidation rate also depends on the deposition temperature and it increases with increasing substrate temperature. Fig. 5 shows the depth profiles of XPS measurements of the Si2pcore level peaks for oxide films prepared atoxidation temperatures of 80°C and 400 "C respectively, where the film thickness of the each sample is about 10 nm. Near the
/
\; .'
..- NOl'III&1 Calhodc
I
Au SiO SiOx n-Si AI
Discharge Vollage [V]
Fig. 2.Discharge characteristics ofa plasma oxidation system with a normal cathode and with a hollow cathode.
Fig. 3. Schematic diagram of MIS diode type tunnel emitter.
K. Usami et al. /71/in Solid Films 281-282 (1996) 412-414
414
10r-------~t----,1000
~
o
Au/Si02fSi • Diode Currenl
200 'C
Ts=400 ' C - 0 '\.
,
80 'C
//
/
o
o Emission Current
•
Di!eharac Vollqe=500V D1acharlc Current=lOmA Subllra\e Temperature Ts
2
o
OIidaUon Time ( min ] Fig. 4, Relations between oxidation time and oxide thickness measured by elipsometer asa parameter of substrate temperature.
•••• ••••••
•
dl
nO
{) 10 Diode Voltage [ V]
Fig. 6. The I-V characteristics and electron emission into thevacuum of a MIS diode type tunnel emitter.
4. Conclusions . 0 0 0
~ ~
,..,
~....
>
~ 10( ~
I!P
is
~
5101
103
• Ts=400 'C
0
• _~
I
~ ~
:9
'»:
Dlsclwllc Vollalle=500V .... Dllchargc CW1'ClIt=lOmA
I
51
0
.. •
80 'C
0
.0
o
•
•.....,,.!~~e.o 1 I 5101 Sl
0
ElebiDg Time [ sec ]
Fig. 5. Depth profiles of XPS measurements of the Si 2pcore level peaks for oxide film atthesubstrate temperatures of80°C and 400°C respectively.
surface of the Si oxide film, the relatively sharp and symmetrical photoemission peak associated with Si dioxide is observed at 104 eV (which isslightly larger than the standard Si dioxide value because of electrical charge above the film surface) but thelow energy peak associated with theSi suboxide peak is notobserved. Hence thehigh quality insulating layer is obtained under the film surface. A noticeable differenceis not observed between the oxidation temperatures of 80°C and400°C. Incases of conventional thermal oxidation, a typical thickness of Si-Si dioxide interfacial transition layer is approximately 1 nm [5]. Butin this case it was about 5 nm, which is considerably larger than that of thermal oxidation film. 'The electron emission of the tunnel emitter which was fabricated ontheplasma oxidized Si oxide films grown at the substrate temperature of 80°C was measured, where thedischarge voltage is 500V andoxide thickness is 15 nm respectively. A typical result is shown in Fig. 6 and the I-V characteristic of thesame sample isalso plotted. The electron emission current rises at Vd = 12.7 V and the maximum value is 800pA/mm2•
Thin Sioxide films onSisubstrates grown byusing hollow cathode enhanced plasma oxidation were investigated as a tunnel barrier insulator. On the oxide film, MIS diode type tunnel emitter was fabricated and electrical properties of the diode were measured. From these experiments, we conclude thefollowing results, ( 1) Oxygen plasma was enhanced by a Si plate hollow cathode without metal contamination. (2) Thin Si oxide films in the thickness range 5 nm-37 nm were grown on n-type Si( 100) substrates controllably and reproducibly by controlling the plasma discharge conditions or the oxidation time. (3) A maximum electron emission current density of 800 pA/mm2 was obtained for typical MIS diode type tunnel emitter fabricated on 15 nm thick Si oxide film grown at a lower temperature (80°C) than that of conventional thermal oxidation.
Acknowledgements The authors acknowledge Prof. S.Ono andProf. K. Yokoo of Tohoku University for useful discussions of this study.
References [1 JK.M. Chu and D.L. Puifrey, IEEE Trans. Electron Devices, 35 (1988) 188. [2] D.L. Puifrey, IEEE Trans. Electron Devices, 25 (1978) 1308. [3] Y. Yokoo, H. Tanaka, S. Sato, 1. Murota and S. Ono, 1., Vac. Sci. Technol., B 2 (1993) 429. [4] J.M.E.Harper, M. Heibum, l.L.Speidel! and J.1. Cuomo. J. Appl. Phys.• 52 (1981) 4118. [5] T. Hattori and T. Nishina, Surf. Sci.• 86 (]979) 555.