Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-06-28
2004-11-02
Tsai, H. Jey (Department: 2812)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S040000, C257S298000, C257S410000
Reexamination Certificate
active
06812509
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an organic semiconductor device, more particularly to an organic field effect transistor memory cell having an organic semiconductor material forming the field effect transistor and a ferroelectric thin film polymer as gate dielectric, and methods of fabricating such a device.
BACKGROUND OF THE INVENTION
Semiconductor memories are configured as either read-only memories (ROM) such as EPROM (Electrically Programmable ROM), EEPROM (Electrically Erasable ROM), flash ROM or as volatile, random access memories (RAM) such as SRAM (Static RAM) and DRAM (Dynamic RAM). The processing required to produce these memory types are complicated and the necessary facilities are expensive due to the high temperature processing required. Ferroelectric ceramic random access memories and field effect transistor memories can be configured to be both read-write and nonvolatile, but again the processing conditions require processing at temperatures in excess of about 600° C. Furthermore, these silicon-based or ferroelectric ceramic-based memories are expensive as the inorganic raw materials used are expensive when compared to many organic materials in addition to the high costs involved in the processing.
Ferroelectric materials possess the unique properties of a spontaneous polarization which can be re-oriented with an applied field, and that the polarization state can be retained even after the removal of electric field. Hence ferroelectric materials can contain two data states (“+” and “−” polarization states) which are very stable over a variety of environmental conditions. These properties allow ferroelectric materials to be one of the best materials for production of digital computer memories. Research activities on ferroelectric-based computer memories commenced in the 1950s, just following the appearance of computers. However, because these early researches focused on using bulk ferroelectric materials which required very high applied voltages to be used to re-orient the polarization, the research activities were discontinued and no commercial products developed.
In the 1980s, with the advances of ferroelectric thin film deposition technology and integration of ferroelectric thin films with silicon microelectronics, practical ferroelectric memories were developed and commercial products were introduced in the market. These advances allowed for the manufacture of ferroelectric thin film based memories which use a standard 5V or 3V voltage to re-orient the polarization or that is, to read and write data. These ferroelectric random access memories (FRAM) combine the advantages of read-on memories (ROM) and volatile random access memories. FRAM have the same advantages of DRAM and SRAM in that they are easy to write, but are superior to DRAM and SRAM due to their nonvolatility. That is, FRAM store the data even in the absence of power. FRAM also have the same advantages of EPROM, EEPROM and Flash ROM in that they are easy to read, but are superior to EPROM and EEPROM as the write speed of FRAM is much faster than that of EPROM, EEPROM and Flash ROM as well as having a higher number of allowed write cycles. However, FRAM does have one drawback. This major drawback is the destructive readout. In order to determine if the polarization of the ferroelectric thin film cell is positive (e.g., representing a “0”) or negative (e.g., representing a “1”), a positive (or a negative) pulse is applied to the cell. The induced charge will be significantly different between positively and negatively polarized ferroelectric cells. However, if the original state of the ferroelectric cell was a negative polarization state, it will change to positive polarization state after reading via a positive pulse being applied to the cell. Likewise, if the original state of the ferroelectric cell was a positive polarization state, it will change to negative polarization state after reading via a negative pulse being applied to the cell. This destructive readout requires that each read access be accompanied by a pre-charge operation to restore the memory state.
In order to solve the destructive readout problem, ferroelectric thin film-based field effect transistors (FETs) have been proposed as the next-generation ferroelectric memories. The ferroelectric FETs use a ferroelectric thin film as a gate dielectric. The ferroelectric thin film is deposited on a silicon substrate, either with or without a thin dielectric layer such as silicon dioxide (SiO
2
) or silicon nitride (Si
3
N
4
) between the silicon substrate and the ferroelectric thin film. When a gate voltage is applied, the polarization of the ferroelectric thin film can be either positive or negative and the polarization state can be retained after the removal of gate voltage. This positive or negative polarization can affect the source-drain current or the source-drain resistance. As the source-drain current or resistance can be controlled by the polarization state of the ferroelectric thin film, a single ferroelectric FET can be used as a memory cell. It can be seen that the ferroelectric FET memory cells have all the advantages of FRAM, such as nonvolatility, easy to read and write, lower power consumption, plus the additional advantage of a nondestructive readout. Furthermore, FRAMs utilizing ferroelectric thin films have a larger remnant polarization (usually larger than 10 &mgr;C/cm
2
), while a remnant polarization of at the order of one-tenth &mgr;C/cm
2
can effectively change the source-drain current in ferroelectric FET memories.
It should be noted that all current ferroelectric FET memory cells use ferroelectric ceramic thin films such as lead zirconate titanate (PZT) or strontium bismuth tantalate (SrBi2Ta2O9 or SBT) with Si-based semiconductors. Therefore, both to deposit the ferroelectric film and to make the FET requires high temperature processes with temperatures in excess of about 600° C. More recently, a research group in France has demonstrated (G. Velu, C. Legrand, O. Tharaud, A. Chapoton, D. Remiens, and G. Horowitz, Appl. Phys. Lett, 79, 659, 2001) the memory effect of a ferroelectric FET using PZT thin film as gate dielectric and &agr;6T (sexithiophene) organic thin film transistor. The deposition of &agr;6T organic thin film can be done at 100° C., but the preparation of PZT film needs a post annealing treatment at 625° C.
In contrast to ferroelectric ceramic thin films, ferroelectric polymer thin films, such as in the family of poly(vinyidiene-trifluoroethylene) (P(VDF-TrFE)) copolymers can be easily deposited on silicon or other substrates using solution spin coating, casting, evaporation or Langmuir-Blodgett (LB) growth method, with the growth temperature lower than 200° C. The remnant polarization of these polymer thin films can be higher than 40 mC/m
2
, or 4 &mgr;C/cm
2
, which is large enough to change the source-drain current and suitable for use in a ferroelectric memory device. Thus organic, nonvolatile, nondestructive readout ferroelectric memory cells can be developed by combining ferroelectric polymer thin film technology and organic thin film transistor technology.
SUMMARY OF THE INVENTION
There is provided a FET memory cell that comprises a substrate which could be made from a wide variety of materials such as silicon, metal, glass, or plastic, a polymer ferroelectric thin film gate dielectric such as P(VDF-TrFE) copolymer thin film, an organic thin film semiconductor such as a pentacene film, and gate, source, and drain electrodes which could be constructed using a variety of conducting materials such as a thin metal film, conducting oxide, or conducting polymer. The memory cell may also contain a dielectric polymer layer between the ferroelectric polymer thin film and the organic semiconductor thin film and a floating gate electrode.
There are many candidate ferroelectric polymer materials that can be used in the above memory structures, including but not limited to poly(vinylidene fluoride) (PVDF), poly(vinyidiene-trifluoroethylene) (P(VDF-TrFE)) copolymers, o
McBain Nola Mae
Palo Alto Research Center Inc.
Tsai H. Jey
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