1. Field of the Invention
This invention relates to ferroelectric field effect transistors, and more particularly to ferroelectric field effect transistors containing a thin film of crystallographically oriented ferroelectric material.
2. Statement of the Problem
It has been known since at least the 1950""s that if a practical ferroelectric memory could be made, it would provide a fast, dense, non-volatile memory that could be operated at relatively low voltages. See Orlando Auciello et al., xe2x80x9cThe Physics of Ferroelectric Memoriesxe2x80x9d, Physics Today, July 1998, pp. 22-27. The principal type of ferroelectric memory being explored today is the non-volatile ferroelectric random access memory or NVFRAM. Ibid. A disadvantage of the NVFRAM is that, in the process of reading it, the information it holds is destroyed and, therefore, the read function must be followed by a rewrite function.
It has been postulated for at least 40 years, however, that it may be possible to design a nonvolatile, nondestructive read-out (xe2x80x9cNDROxe2x80x9d) memory in which the memory element is a single ferroelectric field effect transistor (xe2x80x9cFETxe2x80x9d), thereby reducing some of the complexity of conventional ferroelectric memory architecture and function. See U.S. Pat. No. 2,791,758 issued to D. H. Looney on May 7, 1957; Shu-Yau Wu, xe2x80x9cA New Ferroelectric Memory Device, Metal-Ferroelectric-Semiconductor Transistorxe2x80x9d, in IEEE Transactions On Electron Devices, pp. 499-504, August 1974; S. Y. Wu, xe2x80x9cMemory Retention and Switching Behavior Of Metal-Ferroelectric-Semiconductor Transistorsxe2x80x9d, in Ferroelectrics, Vol. 11, pp. 379-383, 1976; and J. F. Scott, C. A. Paz de Araujo, and L. D. McMillan, xe2x80x9cIntegrated Ferroelectricsxe2x80x9d, in Condensed Matter News, Vol.1, No.3, pp. 15-20,1992. Because the ferroelectric memory effect measured in the devices of Wu was only a temporary, single state effect rather than a long lived two state effect, it has been generally believed that this effect was a carrier injection effect rather than an effect due to ferroelectric switching. See U.S. Pat. No. 5146,299 issued to Donald R. Lampe, Samaar Sinharoy, Shu Y. Wu, et al. on Sep. 8, 1992, col. 1, line 63-col. 2, line 5. Thus, up to the time of the present invention, the ferroelectric FET memory has been only a theoretical concept, and no actual memory devices were built.
The primary problem in making ferroelectric FETs is one of insufficient voltage for switching the polarization in the ferroelectric material. This problem can be understood by considering an exemplary ferroelectric FET structure of a so-called metal-ferroelectric-semiconductor FET (xe2x80x9cMFS-FETxe2x80x9d), in which a ferroelectric oxide is formed on the semiconductor substrate, and the metal gate electrode is located on the ferroelectric oxide. When such a structure is formed, oxide from the ferroelectric interacts with the semiconductor substrate, typically silicon, and forms a thin semiconductor oxide layer between the ferroelectric material and the semiconductor, typically, silicon dioxide. The ferroelectric thin film and the semiconductor oxide layer may be viewed as two capacitors in series. The dielectric constant of the ferroelectric thin film (usually 400-1000) is much higher than the dielectric constant of typical semiconductor oxides, which is usually about 3-5. As a result, most of the voltage drop occurs across the low dielectric constant semiconductor oxide, and a high operational voltage is required to switch the polarization of the ferroelectric thin film, typically, about 15 volts. This can lead to electrical breakdown of the ferroelectric material, semiconductor oxide, and other materials in the circuit. Further, a high operational voltage in excess of 3-5 volts renders the device incompatible with conventional integrated circuit art.
Another problem encountered is that a high leakage current in the ferroelectric thin films of the prior art makes the materials poorly suited for use in a nonvolatile memory since the information-storing polarization charge slowly dissipates.
One attempt to solve the above problem has been the growth of single ferroelectric crystals directly on a semiconductor substrate, such as sapphire. See U.S. Pat. No. 5,146,299 issued to Donald R. Lampe et al. referenced above. However, single crystal films have coercive fields that are too high to make a useful low-voltage memory. See Auciello et al., supra, p. 26, FIG. 4.
Thus, the high dielectric constant and high leakage current of ferroelectrics present significant obstacles to the use of ferroelectric FETs as integrated circuit memories.
The invention solves the problems described above by providing a novel ferroelectric FET device containing a thin film of polycrystalline, crystallographically oriented ferroelectric material, as well as a method of making such devices.
A device in accordance with the invention preferably is a ferroelectric FET containing a polycrystalline thin film of ferroelectric layered superlattice material having c-axis orientation; for example, c-axis oriented strontium bismuth tantalate having a stoichiometric formula SrBi2Ta2O8. Nevertheless, the oriented ferroelectric material can be formed of other metal oxides; for example, oriented polycrystalline ABO3-type perovskites. Oriented ABO3-type metal oxide perovskites include, but are not limited to titanates (e.g., BaTiO3, SrTiO3, PbTiO3, PbZrTiO3) and niobates (e.g., KNbO3). The oriented polycrystalline ferroelectric material may also be a non-oxide metal compound, such as a metal fluoride, or a crystallographically oriented nonmetallic organic compound.
The polycrystalline crystallographically oriented ferroelectric materials have a significantly lower dielectric constant and lower leakage current while retaining high polarizability and low coercive voltage. Thus, the ferroelectric FETs made with the polycrystalline crystallographically oriented material have a relatively low switching voltage while retaining the advantageous non-destructive read out properties of ferroelectric FETs.
In accordance with the invention, the thin film of polycrystalline crystallographically oriented ferroelectric material may be applied using any number of techniques for applying thin films in integrated circuits. Preferably, metal organic precursors suitable for metal organic decomposition (xe2x80x9cMODxe2x80x9d) techniques of thin film deposition are used. MOD methods enable convenient and accurate control of precursor concentrations. Preferably, a liquid source chemical deposition method is used such as misted deposition or chemical vapor deposition (xe2x80x9cCVDxe2x80x9d), or a spin-on or dipping method may be used instead.
In preferred embodiments of the invention, the write bias is applied between the substrate and the FET gate. Preferably, the ferroelectric FET memory cell is read by sensing the source/drain current when a voltage difference is placed across the source and drain.
An effective method for fabricating a ferroelectric FET memory in accordance with the invention is the use of UV radiation during treatment of the ferroelectric thin film to enhance c-axis orientation.