High-k cobalt–titanium oxide dielectrics formed by oxidation of sputtered Co/Ti or Ti/Co
films
Tung Ming Pan, Tan Fu Lei, and Tien Sheng Chao
Citation: Applied Physics Letters 78, 1439 (2001); doi: 10.1063/1.1352044
View online: http://dx.doi.org/10.1063/1.1352044
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/78/10?ver=pdfcov Published by the AIP Publishing
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High-
k
cobalt–titanium oxide dielectrics formed by oxidation of sputtered
Co
Õ
Ti or Ti
Õ
Co films
Tung Ming Pan and Tan Fu Lei
Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China
Tien Sheng Chaoa)
National Nano Device Laboratories, and Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China
共Received 14 August 2000; accepted for publication 4 January 2001兲
High-k cobalt–titanium oxide共CoTiO3) film was formed by directly oxidizing sputtered Co/Ti or Ti/Co films. Al/CoTiO3/Si3N4/Si capacitor structures were fabricated and measured. Excellent electrical properties with an effective dielectric constant 共i.e., k value兲 as high as 40 have been achieved for a CoTiO3gate dielectric with a buffer layer. The metal–oxide thus appears to be a very promising high-k gate dielectric for future ultralarge scale integrated devices. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1352044兴
As conventional SiO2gate dielectric scales down to less than 20 Å, a high leakage current is inevitable due to the occurrence of direct tunneling. To solve this problem, high-k 共⬎3.9 of oxide兲 dielectric materials that allow a physically thicker film for the required equivalent oxide thickness (EOT) are proposed to replace the conventional SiO2.1Thus, gate dielectric materials having high dielectric constant, low interface state density and good thermal stability appear to be promising for future gate dielectric application. Recently, Si3N4 (k⫽7), Al2O3 (k⫽9), Ta2O5 (k⫽25), and TiO2 (k ⫽40) gate dielectric films have been widely studied.2–5 However, these high-k films still exhibit undesirable high leakage current. The formation of an interfacial silicon oxide layer during the metal–oxide deposition process is a serious issue in high-k gate dielectric development. An interfacial SiO2 layer with a thickness over 20 Å was obtained when Ta2O5 was deposited directly on silicon. This interfacial ox-ide layer seriously limits the scalability of high-k dielectrics and causes poor interface quality.6Besides, thermal stability of the high-k dielectric material is another major concern. Severe degradation of the dielectric quality has been shown to occur after Ta2O5 is subjected to processing temperature above 800 °C.7 In this letter, we reported a cobalt–titanium oxide (CoTiO3) film as an alternative gate dielectric. This CoTiO3film is formed by direct oxidation of Co/Ti or Ti/Co films. From our results, the dielectric constant can reach as high as 40, while depicting excellent electrical properties.
Samples were fabricated on p-type共100兲-oriented Si wa-fers with resistivity of 14–21 ⍀ cm. All wafers were first cleaned by a standard Radio Corporation of America clean. To avoid reaction between metal and silicon during the sput-tering process and later high-temperature oxidation step, a 10 Å Si3N4 film was first grown by NH3 nitridation of the Si substrate in low-pressure chemical vapor deposition system at 800 °C for 1 h. Afterwards, samples were immediately deposited in sequence first with a 50 Å Ti and then a 50 Å Co共Co/Ti兲, or first a 50 Å Co and then 50 Å Ti 共Ti/Co兲 film
from independent targets by using a physical vapor deposi-tion method. Direct thermal oxidadeposi-tion was carried out at 700 or 800 °C in diluted O2 (N2/O2⫽2/1) gas for 5 min and annealed in N2 ambient for 5 min to form CoTiO3 films. A 5000 Å Al film was deposited on the wafer by a thermal coater to serve as the gate electrode. The gate of the metal– oxide–semiconductor 共MOS兲 capacitor was defined by li-thography, and then the Al was etched by a wet etching solution. Finally, a 5000 Å Al film was also deposited on the back side of the wafers after stripping the oxide on the back side. X-ray diffraction 共XRD兲 was used to identify the com-position and the phase of these new metal–oxide films. The gate dielectrics of MOS capacitors with an area of 2.5 ⫻10⫺5 cm2 were measured. The E
OT 共17.9–21.2 Å兲 of CoTiO3 with a Si3N4 buffered layer structure was obtained by high frequency capacitance–voltage (C – V) of 0.1 MHz at an operating range of ⫺2–2 V in a strong accumulation
a兲Electronic mail: [email protected] FIG. 1. XRD spectra of CoTiOtemperatures. 3 films with various stack structures and
APPLIED PHYSICS LETTERS VOLUME 78, NUMBER 10 5 MARCH 2001
1439
0003-6951/2001/78(10)/1439/3/$18.00 © 2001 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
region without considering quantum mechanical effects. The physical thickness 共⬃200 Å兲 was doubly checked by trans-mission electron microscopy to obtain the k value. The k value共36.8–43.6兲 is calculated by timing 3.9 to the physical thickness and dividing it by the EOT. The electrical proper-ties and reliability characteristics of the metal oxide were measured by using an Hewlett–Packard 4156 semiconductor parameter analyzer.
Film crystallization and degradation during a back-end thermal process is a major concern for high-dielectric-constant metal–oxide materials. Figure 1 shows the resultant XRD spectra. From the results, Co/Ti and Ti/Co samples oxidized at either 700 or 800 °C are found to react with oxygen and form CoTiO3 films as shown in Fig. 1. The sample oxidized at 800 °C has a stronger spectrum than that of the sample oxidized at 700 °C. No noticeable CoTiO3
crystal peak is observed for oxidation at 700 °C, suggesting insufficient time for crystallization, as shown in Fig. 1. Fig-ure 2共a兲 shows current density–voltage (J – V) curves for different gate dielectric films. With a thinner EOTof 1.8 nm, the CoTiO3/Si3N4/Si sample shows a lower leakage at low field and a higher breakdown voltage than those of Ta2O5 (EOT⫽2.0 nm, in Ref. 4兲, TiO2(EOT⫽2.2 nm, in Ref. 5兲, or oxynitride (EOT⫽2.5 nm兲. Figure 2共b兲 shows J–V curves of CoTiO3 capacitors. Co/Ti and Ti/Co capacitors oxidized at 700 and 800 °C were compared. It is found that the Co/Ti capacitor oxidized at 800 °C demonstrates the highest break-down voltage among all samples. In addition, the metal ox-ide oxidized at 800 °C exhibits a lower leakage current than that oxidized at 700 °C, implying that samples oxidized at 800 °C may exhibit CoTiO3 crystallization.
The formation of an interfacial silicon oxide layer during sputtering and thermal processing makes it very difficult to
FIG. 2. J–V characteristics of共a兲 different gate dielectric material films with various equivalent oxide thicknesses and 共b兲 CoTiO3 films with various stack structures and temperatures.
FIG. 3. High-frequency C–V curves of Co/Ti and Ti/Co films with 700 and 800 °C oxidation.
FIG. 4. SILC for Co/Ti capacitors with 800 °C oxidation. 1440 Appl. Phys. Lett., Vol. 78, No. 10, 5 March 2001 Pan, Lei, and Chao
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realize a high-k value. An effective method to solve this problem is to use a high quality silicon nitride buffer layer.8 Figure 3 shows the high frequency (C – V) curves for Co/Ti and Ti/Co capacitors oxidized at 700 and 800 °C. It is found that the Co/Ti stack capacitor exhibits a higher capacitance 共k value兲 than the Ti/Co stack capacitors. Figure 4 shows the result after constant voltage stress at⫺4.5 V for Co/Ti stack capacitors. No significant stress induced leakage current 共SILC兲 was observed for these four samples even after 104 s stressing.
In summary, we have demonstrated a high-k CoTiO3 which is formed by direct oxidation of the sputtered Co/Ti and Ti/Co film. The CoTiO3/Si3N4/Si stack by sputtering Co/Ti film oxidation shows higher k value and better electri-cal properties, such as low gate leakage current at low volt-age operation, and high reliability after stressing. This high-k material with CoTiO3 thus appears to be very promising for future ultralarge scale integrated devices.
The authors would like to express their appreciation to all the staff of the Semiconductor Research Center, National Chiao Tung University for their technique help. Financial support was from National Science Council Taiwan, Repub-lic of China under Contract No. NSC89-2215-E009-306.
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Appl. Phys. Lett., Vol. 78, No. 10, 5 March 2001 Pan, Lei, and Chao
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