Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state
Reexamination Certificate
2002-04-19
2004-06-15
Norton, Nadine G. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
C117S003000, C117S088000, C117S089000, C117S092000
Reexamination Certificate
active
06749686
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a crystal growth method of an oxide and a multi-layered structure of oxides particularly suitable for application to oxide electronics developed on silicon substrates.
2. Description of the Related Art
Silicon oxide (SiO
2
) films made by thermal oxidation of silicon (Si) have been exclusively used as gate insulating films of MOS-FET (metal-oxide-semiconductor FET) because of their high electric insulating ability, low interface state density, easiness to process, thermal stability, and other advantages. These SiO
2
films made by thermal oxidation for use as gate insulating films, however, have low specific dielectric constants (&egr;
r
~3.8) and must be formed very thin on Si substrates. Along with the progress toward thinner gate insulating films, short channels and other requirements to meet the demand for integration, various problems arose such as dielectric break-down of gate insulating films and pinch-off of channels caused by influences from source-drain voltages (short channel effect), and gate insulating films will soon come to the limit in terms of their materials. Under the circumstances, the need for new gate insulating films having high dielectric constants is being advocated as a technical subject of MOS-FET of the sub 0.1 micron generation, in addition to a further progress of lithographic technologies, needless to say (for example, (1) MTL VLSI Seminar (Massachusetts Institute of Technology)). If EL gate insulating film can be made by using a material of a high dielectric constant, it will need not be so thin. Therefore, gate leakage will be prevented, and short-channel effects will be prevented as well.
On the other hand, researches on ferroelectric non-volatile memories (FeRAM) have come to be active (for example, (2) Appl. Phys. Lett., 48(1986)1439, (3) IEDM Tech. Dig., (1987)850, (4) IEEE J. Solid State Circuits, 23(1988)1171, (5) 1988 IEEE Int. Solid-State Circuits Conf. (ISSCC88), (6) Digest of Technical Papers, THAM 10.6 (1988)130, and (7) Oyo Butsuri, 62(1993)1212). Among these ferroelectric non-volatile memories, what is considered to be closest to practical use is a ferroelectric non-volatile memory of a quasi-DRAM structure (using two-transistors and two-capacitors type memory cells, or one-transistor and one-capacitor type memory cells). This structure is advantageous in making it easier to prevent interference with the Si process because CMOS process and ferroelectric capacitor process can be separated by using an inter-layer insulating film. However, the structure of this ferroelectric non-volatile memory does not meet the use of a scaling law of a Si device. Therefore, as microminiaturization progresses, it is necessary to employ a more complex structure or use a material with a larger value of residual polarization in order to ensure a certain amount of charge storage in the capacitor. On the other hand, there are many research institutes tackling with the study of ferroelectric non-volatile memories using MFS (metal-ferroelectrics-semiconductor)-FET memory cells, including MFMIS (metal-ferroelectrics-metal-insulator-semiconductor)-FET memory cells, FCG (ferroelectric capacitor gate) memory cells, and so forth), which constitute two major subjects together with those of a quasi-DRAM structure mentioned above. The latter type ferroelectric non-volatile memories match with the scaling low, and merely need a very small value of residual polarization (about ~0.1 &mgr;C/cm
2
). Additionally, since they need only one transistor for storage and hence contribute to a decrease of the cell size, they are advantageous for high integration. Furthermore, since they are of a nondestructive readout type, they are more advantageous also against fatigue, which might be an essential problem of ferroelectric materials, than destructive readout type memory cells with two transistors and two capacitors, or one transistor and one capacitor, and are also available for high-speed operations. Because these excellent properties are expected, MFS-FET ferroelectric non-volatile memories are now recognized as ultimate memories ((8) Appl. Surf. Sci. 113/114(1997)656).
What is preventing practical use of these MFS-FET ferroelectric non-volatile memories is problems with their manufacturing process. It is extremely difficult to grow ferroelectric materials directly on Si substrates. Therefore, growth of buffer layers of insulating materials on Si substrates is recognized as one of most important technologies. In case of a MFIS (metal-ferroelectrics-insulator-semiconductor)-FET which is one of MFS-FET ferroelectric non-volatile memories, gate voltage is distributed to an insulating layer as well, and this invites the drawback that the write voltage is high. To prevent it, the insulating layer must be one with a high dielectric constant. On the other hand, material properties required as a ferroelectric material used here are a low dielectric constant, appropriate value of residual polarization (typically around ~0.1 &mgr;C/cm
2
, although depending upon the device design), and most seriously, good squareness ratio. Additionally, for the purpose of realizing a better interface, one of important conditions is that these materials can be grown at low temperatures. Thus, choice and development of materials is required from the standpoint different from that of the one-transistor and one-capacitor type. There are a lot of research reports on MFIS-FET ferroelectric non-volatile memories. However, because of insufficient interface properties, there is almost no reports about practically usable ones including the requirement for retention (charge retaining property).
On the other hand, it is greatly significant to introduce oxide materials other than SiO
2
into the semiconductor industry. High-temperature superconductive materials discovered in 1986 ((9) Z. Phys. B., 64, 189-193(1986)), needless to say, and oxide materials especially having perovskite or related structures have very important physical properties for semiconductor devices, such as ferroelectricity, high dielectric constant, superconductivity, colossal magnetoresistance, and so forth ((10) Mater. Sci. Eng., B41(1996)166, and (11) J. Ceram. Soc. Japan, Int. Ed., 103(1995)1088). For example, among ferroelectric materials of capacitors for ferroelectric non-volatile memories mentioned above, zirconium titanate (PZT) having a large value of spontaneous polarization and a low process temperature (for example, (12) J. Appl. Phys. 70, 382-388(1991)) and bismuth strontium tantalate (Bi
2
SrTa
2
O
9
((13) Nature, 374(1995)627, (14) Appl. Phys. Lett., 66(1995)221, (15) Mater. Sci. Eng., B32(1995)75, (16) Mater. Sci. Eng., B32(1995)83, (17) Appl. Phys. Lett., 67(1995)572, (18) J. Appl. Phys., 78(1995)5073, (19) Appl. Phys. Lett., 68(1996)566, (20) Appl. Phys. Lett., 68(1996)690, and (21) International Laid-Open Publication WO93/12542) are the twin greatest materials. Furthermore, including the discovery of colossal magnetoresistance materials (CMR materials) in the group of Mn oxides, which are variable in resistivity over some digits under application of a magnetic field ((22) Phys. Rev. Lett. 74(1995)5108), great interest has come to be attracted to how high potential capacities these oxide materials have ((23) Mater. Sci. Eng., B41(1996)166, and (24) J. Ceram. Soc. Japan, Int. Ed., 103(1995)1088), and technologies for making thin oxide films have been developed remarkably in these ten years or so.
If oxide materials having these very high functional physical properties can be developed on Si which is the basis of the semiconductor industry, these materials will get a high marketability. However, because of difficulties between these functional oxide materials and Si, such as mutual thermal diffusion and differences in thermal expansion coefficient, it is usually difficult to directly grow these functional oxide materials on Si.
As discussed above, almost all of these functional oxide materials have structure based on a perovskite structure. Man
Ami Takaaki
Ishida Yuichi
Machida Akio
Nagasawa Naomi
Suzuki Masayuki
Anderson Matthew
Norton Nadine G.
Sonenschein, Nath & Rosenthal LLP
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