Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
1999-04-28
2002-06-25
Ghyka, Alexander G. (Department: 2812)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S239000, C438S933000
Reexamination Certificate
active
06410343
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates generally to the fabrication of ferroelectric memory devices and, more particularly, with a method to maximize the ferroelectric properties of Lead Germanium Oxide (PGO) thin films for ferroelectric memory applications, by orienting crystallization of the PGO film along the c-axis.
Ferroelectric thin films for use in electro-optics, pyroelectric, frequency agile electronics, and non-volatile memories have drawn much attention in recent years due to their bi-stable nature. Most of the studies on Ferroelectric Random Access Memories (FRAMs) have concentrated on the memory structure with one transistor and one capacitor. The capacitor is made by a thin ferroelectric film sandwiched between two conductive electrodes. The circuit configuration and read/write sequence of this type memory are similar to that of DRAMs except no data refreshing is necessary in FRAMs. The fatigue problem observed in ferroelectric capacitor, therefore, becomes one of the major obstacles that limit the realization of these memories on a commercial scale. Lead germanate (Pb
5
Ge
3
O
11
) thin films exhibit excellent fatigue properties, so these PGO thin films are very attractive material for FRAM device applications.
The non-perovskite uniaxial ferroelectric Pb
5
Ge
3
O
11
, with polar direction parallel to the c-axis, belongs to the trigonal space group P
3
at room temperature. This material transforms to the hexagonal, space group P
6
(=P
3
/m) paraelectric phase above the Curie temperature (Tc=178° C.). Since this uniaxial ferroelectric Pb
5
Ge
3
O
11
possesses only 180° domains, there are no ferroelastic effects that tend to reorient domains through 90° in order to relax the polarization. Interesting feature of this material are the small dielectric constant and small remanent polarization, which are also suitable for ferroelectric non-volatile memory devices, especially for one transistor memory applications. Pb
5
Ge
3
O
11
also has some potential for thermal detector applications because of its pyroelectric and dielectric characteristics.
Another area of research in ferroelectric non-volatile memory is the deposition ferroelectric thin film directly onto the gate area of FET, to form a ferroelectric-gate controlled FET. Ferroelectric-gate controlled devices such as metal-ferroelectric-silicon (MFS) FET have been studied as early as 1950s. Various modified MFSFET structures have been proposed. For example, Metal-Ferroelectric-Insulator-Silicon (MFIS) FET, Metal-Ferroelectric-Metal-Silicon (MFMS) FET, and Metal-Ferroelectric-Metal-Oxide-Silicon (MFMOS) FET. In response to the requirements of one transistor memory applications, ferroelectric materials should have low dielectric constant and small remanent polarization. Therefore, ferroelectric Pb5Ge
3
O
11
thin films, which have a smaller remanent polarization of 4 &mgr;C/cm
2
and dielectric constant of about 50 in their bulk materials, have been sought.
The thin films of lead germanate were made in the past by thermal evaporation and flash evaporation (A. Mansingh and S. B. Krupanidhi, J. Appl. Phys. 51, 5408, 1980), dc reactive sputtering (H. Schmitt, H. E. Mueser, and R. Karthein, Ferroelectrics 56, 141, 1984), laser ablation (S. B. Krupanidhi, D. Roy. N. Maffei and C. J. Peng, Proceedings of 3
rd
fInternational Symp. on Integrated Ferroelectrics, 100, 1991), and sol-gel technique (J. J. Lee and S. K. Dey, Appl. Phys. Lett. 60, 2487, 1992).
Previously, single crystal Pb
5
Ge
3
O
11
have been reported, with spontaneous polarization and coercive field of 4 &mgr;C/cm
2
and 14 kV/cm, respectively, in the direction along the c-axis. These c-axis oriented Pb
5
Ge
3
O
11
thin films exhibit poor ferroelectric properties: lower polarization (2-3 &mgr;C/cm
2
, higher coercive field (55-135 kV/cm ), and their hysteresis loops were not saturated and square. In order to switch the PGO ferroelectric domains, very high operation voltages are required, which precludes their use in the memory devices.
The PGO film of the present invention was developed meet the requirements of ferroelectric memory devices. The present invention concerns a pure c-axis oriented PGO thin films have a smaller Pr value, smaller dielectric constant and largest Ec value. Such a film is useful in making one transistor (
1
T) memory cells. In co-pending patent application Ser. No. 09/301,435, entitled “Multi-Phase Lead Germanate Film and Deposition Method”, invented by Tingkai Li et al., filed on Apr. 28, 1999, attorney docket No. SLA400, a second phase of Pb
3
GeO
5
is added to the Pb
5
Ge
3
O
11
, increasing grain sizes without an increase in c-axis orientation. The resultant film had increased Pr values and dielectric constants, and decreased Ec values. Such a film is useful in microelectromechanical systems (MEMS), high speed multichip module (MCM), DRAM, and FeRAM applications.
In co-pending patent application Ser. No. 09/302,272, entitled “Epitaxially Grown Lead Germanate Film and Deposition Method”, invented by Tingkai Li et al., filed on Apr. 28, 1999, which issued on Feb. 20, 2001 as U.S. Pat. No. 6,190,925 B1, attorney docket No. SLA402, an appropriate content of the second phase Pb
3
GeO
5
is added to Pb
5
Ge
3
O
11
, forming large grain sizes with extremely high c-axis orientation and completely epitaxial c-axis ferroelectric lead germanate film. As a result, high Pr and Ec values, as well as lower dielectric constant, is obtained. Such a film is useful in
1
T, one transistor/one capacitor (
1
T
1
C) FeRAM memory devices.
In co-pending patent application Ser. No. 09/301,434, entitled “Ferroelastic Lead Germanate Film and Deposition Method”, invented by Tingkai Li et al., filed on Apr. 28, 1999, attorney docket No. SLA403, a CVD Pb
3
GeO
5
film is formed having improved ferroelastic properties useful in making MEM and MCM devices. The above-mentioned co-pending patent applications are incorporated herein by reference.
It would be advantageous if a single phase PGO film could be developed with ferroelectric properties sufficient for use in the non-volatile memories.
It would be advantageous if the ferroelectric properties of a single phase polycrystalline PGO film could be enhanced by crystallographic alignment. Further, it would be advantageous if the crystalline PGO film could be aligned primarily along the c-axis.
It would be advantageous if a PGO film could be formed having a small, homogeneous, grain size for use in high density non-volatile ferroelectric memories.
Accordingly, in a lead germanium oxide (PGO) film, a method has been provided for forming a polycrystalline PGO film, having a c-axis orientation, on a IC film in a reactor chamber. The method comprising the steps of:
a) mixing [Pb(thd)
2
] and [Ge(ETO)
4
] to form a PGO mixture having a molar ratio in the range of approximately 5:3;
b) dissolving the mixture of Step a) with a solvent of tetrahydrofuran, isopropanol, and tetraglyme in a molar ratio of approximately 8:2:1, respectively, to form a precursor solution having a concentration of approximately 0.1 to 0.3 moles of PGO mixture per liter of solvent;
c) using a precursor vaporizer, heating the precursor solution to a temperature in the range of approximately 130 to 180 degrees C, creating a precursor gas;
c
1
) mixing the precursor gas in the chamber with an argon gas shroud flow in the range of approximately 1000 to 6000 standard cubic centimeters per minute (sccm), preheated to a temperature in the range of approximately 130 to 180 degrees C;
c
2
); introducing an oxygen flow to the chamber in the range of approximately 500 to 3000 sccm;
d) heating the wafer to a temperature in the range of approximately 450 to 500 degrees C, to decompose the precursor gas formed in Step c) on the wafer;
e) forming a PGO film, including a first phase of Pb
5
Ge
3
O
11
with a small, homogeneous, crystal grain size; and
f) forming a c-axis crystallographic orientation of approximately 70%, or more, in the Pb
5
Ge
3
O
11
phase of the PGO film, and a grain size
Hsu Sheng Teng
Li Tingkai
Ono Yoshi
Zhang Fengyan
Ghyka Alexander G.
Krieger Scott C.
Rabdau Matthew D.
Ripma David C.
Sharp Laboratories of America Inc.
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