- Email: [email protected]
Fabrication of MIS-Schottky diodes with thermal nitroxide film
Volume 2, number 2
October 1983
MATERIALS LETTERS
FABRICATION OF MIS-SCHOTTKY DIODES WITH THERMAL NITROXIDE FILM *
Q.H. HUA *, D.K. YANG + and E.S. YANG Department of Electrical Engineering, Columbia University,New York, NY 10027, USA Received 26 July 1983
We report a thermal nitridation process which is used to produce nitroxide films ranging between 15 and 50 A in thickness. Fabricated MIS-Schottky barrier diodes with various turnon voltages show an improvement in breakdown voltage.
In the development of ultra-small electronic devices, it has been found that thin gate oxide less than 200 A often deteriorates after silicide-gate, ion-beam or plasma-assisted processing. Yet, because of scaling, future generations of megabit RAMSwill require a film thickness of 100 A or less [ 1,2]. The integrity of such thin insulator films is most important in achieving highly reliable device structures. Recently, thermal nitridation of thin oxide films has been found to improve MOS characteristics [3]. Incorporation of nitrogen in an oxide produces a dense, uniform and pinhole-free film known as oxynitride or nitroxide. We have used this technique to fabricate MIS rectifiers whose forward turn-on voltage can be controlled by varying the thickness of the oxynitride film. This letter reports the results of the process when a closed tube moving furnace is used. I- V characteristics of MIS-Schottky diodes fabricated by this process are also presented. Thermal nitridation of a silicon wafer is achieved by reaction with ammonia in the system illustrated in fig. 1. The substrates are (111) n/n+ polished Si. The surface cleaning procedure consists of: (1) degreasing in organic solvents in a ultra-sonic bath; (2) dipping in H2S04 at 80°C for 5 min to remove heavy ions; (3) immersion in HN03 at 80” for 5 min to form a thin * Supported by JSEP contract No. DAAG29-82-K-0080 by NSF grants 81-16163 and 82-17677. * Permanent address: Tianjian Electronic Materials Research Institute, Tianjian, China. $ Permanent address: Dunster House, Harvard University, Cambridge, MA 02138, USA.
i!Y I
I
a
d
e
Fig. 1. Schematic diagram of experiment. (a) Gas cylinders, (b) quartz tube, (c) rolling furnace, (d) pump and (e) gas trap.
native oxide; and (4) etching in buffered HF for 30 s to remove the oxide. After these steps the silicon surface should be hydrophobic. In order to obtain a thin nitroxide film on Si, we dip the wafer in HNO, at 80°C for 5 min to form a controlled SiO, about 1S-20 A. Then we load the wafer into the quartz reaction tube and purge the tube with pure N2 for 30 min after sealing and evacuating the tube to 0.1 Torr. Subsequently, NH3 is introduced into the quartz tube and the preheated furnace is moved to encompass the wafers. The insulator-film will become a nitroxide whose thickness depends on the growth time and temperature. In our experience, the thickness of the film depends slightly on the growth time, but strongly depends on the temperature. In some cases, in order to avoid using high temperature, we can grow thicker nitroxide on Si by preheating the wafer in an oxygen environment for several minutes, purging the oxygen with N2, and heating the wafer in NH,. After the desired film thick147
01
10
I 20
30
GROWTH
TIME
40
50
60 V (VOLTS)
(mms)
Fig. 2. Dependence of nitroxide film thickness on growth time.
ness is achieved, the furnace is removed, and the wafers are allowed to cool down to room temperature. The tube is then purged with N2 before u~oa~ng the wafers. ~u~nurn is evaporated on the back side of the wafer to form an ohmic contact, and aluminum dots are deposited on the front surface to produce MIS-Schottky diodes. The film thickness is measured by ellipsometry, and its dependence on growth time is shown in fig. 2. The refractive index is found to be between 19 and 2.2. At 920°C, the film thickness remains essentially constant beyond a heating period of 40 min. Therefore, a higher temperature (96O’C) is used to obtain a thicker film. Our experience shows that the thickness can be controlled easily by faring the temperature whereas the growth time is relatively unimportant after the initial growth. This indicates that the mechanism is likely to be limited by the diffusion of the string species through the insulator. In fig. 3, the forward current-voltage characteristics are shown for diodes fabricated on the same substrate with different insulator film thickness, The turn-on voltage increases with the film thickness as expected from known theory [4]. The dashed line represents a diode with Si02 insulator which is used as a reference. It is ~terest~g to note that the behavior of these MIS-Schottkry diodes is not sensitive to the band-gap value of different dielectrics and that the turn-on voltage follows the insulator thickness. Similar results have been noted by Card and Rhoderick in their work [4]. On the other hand, the reverse-biased 148
October 1983
MATERIALS LETTERS
Volume 2, number 2
Fig. 3. Forward current-voltage Schottky diodes.
characteristic of MIS-
breakdown voltage shown in fig. 4 indicates that the nitroxide films are better. More important is the fact that the devices with the nitroxide fti do not seem to degrade after months whereas diodes with SiO, insulator degrade in days. It appears that the MISSchottky structure may also be useful in evaluating the stability of insulator films less than 50 A in thickness [5]. In conclusion, we have fabricated aluminumnitroxide-silicon Schottky diodes and found that their electrical characteristics are similar to US-Schott~
60-
0%
10
20
30 THICKNESS
40
50
60
(8,
Fig. 4. Reverse breakdown voltage as a function of nitroxide film thickness.
Volume 2, number 2
MATERIALS LETTERS
junctions with SiO, insulator. However, the better breakdown and degradation characteristics indicates that the integrity of nitroxide films is significantly better than pure oxide films.
October 1983
[2] H.H. Chao, R.H. Dennard, M.Y. Tsai, M.R. Wordeman and A. Cramer, ISSCC Digest Tech. Papers (1981) p. 152. [3] T. Ito, T. Nakamura and H. Ishikawa, IEEE Trans. Electron Devices ED-29 (1982) 498. [4] H.C. Card and E.IL Rhoderick, J. Phys. D4 (1971) 1589. [5] L.C. Olsen, D.L. Barton and W. Miller, J. Appl. Phys. 51 (1980) 6393.
References [l] J.D. Meindl, KH. Ratnakumar, L. Gerzbert and K.C. Saraswat, ISSCC Digest Tech. Papers (1981) p. 36.
149
October 1983
MATERIALS LETTERS
FABRICATION OF MIS-SCHOTTKY DIODES WITH THERMAL NITROXIDE FILM *
Q.H. HUA *, D.K. YANG + and E.S. YANG Department of Electrical Engineering, Columbia University,New York, NY 10027, USA Received 26 July 1983
We report a thermal nitridation process which is used to produce nitroxide films ranging between 15 and 50 A in thickness. Fabricated MIS-Schottky barrier diodes with various turnon voltages show an improvement in breakdown voltage.
In the development of ultra-small electronic devices, it has been found that thin gate oxide less than 200 A often deteriorates after silicide-gate, ion-beam or plasma-assisted processing. Yet, because of scaling, future generations of megabit RAMSwill require a film thickness of 100 A or less [ 1,2]. The integrity of such thin insulator films is most important in achieving highly reliable device structures. Recently, thermal nitridation of thin oxide films has been found to improve MOS characteristics [3]. Incorporation of nitrogen in an oxide produces a dense, uniform and pinhole-free film known as oxynitride or nitroxide. We have used this technique to fabricate MIS rectifiers whose forward turn-on voltage can be controlled by varying the thickness of the oxynitride film. This letter reports the results of the process when a closed tube moving furnace is used. I- V characteristics of MIS-Schottky diodes fabricated by this process are also presented. Thermal nitridation of a silicon wafer is achieved by reaction with ammonia in the system illustrated in fig. 1. The substrates are (111) n/n+ polished Si. The surface cleaning procedure consists of: (1) degreasing in organic solvents in a ultra-sonic bath; (2) dipping in H2S04 at 80°C for 5 min to remove heavy ions; (3) immersion in HN03 at 80” for 5 min to form a thin * Supported by JSEP contract No. DAAG29-82-K-0080 by NSF grants 81-16163 and 82-17677. * Permanent address: Tianjian Electronic Materials Research Institute, Tianjian, China. $ Permanent address: Dunster House, Harvard University, Cambridge, MA 02138, USA.
i!Y I
I
a
d
e
Fig. 1. Schematic diagram of experiment. (a) Gas cylinders, (b) quartz tube, (c) rolling furnace, (d) pump and (e) gas trap.
native oxide; and (4) etching in buffered HF for 30 s to remove the oxide. After these steps the silicon surface should be hydrophobic. In order to obtain a thin nitroxide film on Si, we dip the wafer in HNO, at 80°C for 5 min to form a controlled SiO, about 1S-20 A. Then we load the wafer into the quartz reaction tube and purge the tube with pure N2 for 30 min after sealing and evacuating the tube to 0.1 Torr. Subsequently, NH3 is introduced into the quartz tube and the preheated furnace is moved to encompass the wafers. The insulator-film will become a nitroxide whose thickness depends on the growth time and temperature. In our experience, the thickness of the film depends slightly on the growth time, but strongly depends on the temperature. In some cases, in order to avoid using high temperature, we can grow thicker nitroxide on Si by preheating the wafer in an oxygen environment for several minutes, purging the oxygen with N2, and heating the wafer in NH,. After the desired film thick147
01
10
I 20
30
GROWTH
TIME
40
50
60 V (VOLTS)
(mms)
Fig. 2. Dependence of nitroxide film thickness on growth time.
ness is achieved, the furnace is removed, and the wafers are allowed to cool down to room temperature. The tube is then purged with N2 before u~oa~ng the wafers. ~u~nurn is evaporated on the back side of the wafer to form an ohmic contact, and aluminum dots are deposited on the front surface to produce MIS-Schottky diodes. The film thickness is measured by ellipsometry, and its dependence on growth time is shown in fig. 2. The refractive index is found to be between 19 and 2.2. At 920°C, the film thickness remains essentially constant beyond a heating period of 40 min. Therefore, a higher temperature (96O’C) is used to obtain a thicker film. Our experience shows that the thickness can be controlled easily by faring the temperature whereas the growth time is relatively unimportant after the initial growth. This indicates that the mechanism is likely to be limited by the diffusion of the string species through the insulator. In fig. 3, the forward current-voltage characteristics are shown for diodes fabricated on the same substrate with different insulator film thickness, The turn-on voltage increases with the film thickness as expected from known theory [4]. The dashed line represents a diode with Si02 insulator which is used as a reference. It is ~terest~g to note that the behavior of these MIS-Schottkry diodes is not sensitive to the band-gap value of different dielectrics and that the turn-on voltage follows the insulator thickness. Similar results have been noted by Card and Rhoderick in their work [4]. On the other hand, the reverse-biased 148
October 1983
MATERIALS LETTERS
Volume 2, number 2
Fig. 3. Forward current-voltage Schottky diodes.
characteristic of MIS-
breakdown voltage shown in fig. 4 indicates that the nitroxide films are better. More important is the fact that the devices with the nitroxide fti do not seem to degrade after months whereas diodes with SiO, insulator degrade in days. It appears that the MISSchottky structure may also be useful in evaluating the stability of insulator films less than 50 A in thickness [5]. In conclusion, we have fabricated aluminumnitroxide-silicon Schottky diodes and found that their electrical characteristics are similar to US-Schott~
60-
0%
10
20
30 THICKNESS
40
50
60
(8,
Fig. 4. Reverse breakdown voltage as a function of nitroxide film thickness.
Volume 2, number 2
MATERIALS LETTERS
junctions with SiO, insulator. However, the better breakdown and degradation characteristics indicates that the integrity of nitroxide films is significantly better than pure oxide films.
October 1983
[2] H.H. Chao, R.H. Dennard, M.Y. Tsai, M.R. Wordeman and A. Cramer, ISSCC Digest Tech. Papers (1981) p. 152. [3] T. Ito, T. Nakamura and H. Ishikawa, IEEE Trans. Electron Devices ED-29 (1982) 498. [4] H.C. Card and E.IL Rhoderick, J. Phys. D4 (1971) 1589. [5] L.C. Olsen, D.L. Barton and W. Miller, J. Appl. Phys. 51 (1980) 6393.
References [l] J.D. Meindl, KH. Ratnakumar, L. Gerzbert and K.C. Saraswat, ISSCC Digest Tech. Papers (1981) p. 36.
149