The electrical and reliability characteristics of ferroelectric capacitors fabricated using sol-gel derived 50/50 lead-zirconate-titanate (PZT) thin films have been examined for ULSI DRAM (dynamic random access memory) applications. Various electrode materials, film thicknesses (200 nm to 600 nm) and capacitor areas were used. A large stored-energy density (Q(c)) of 15 muC/cm(2) (at 125 kV/cm) was measured using different methods. The results indicate that PZT thin films exhibit material properties which might satisfy the requirements of ULSI DRAMs.
Modified lead titanate ferroelectdc thin films are currently utilized in conjunction with silicon CMOS technology for non-volatile memory applications. The electrical measurement of ferroelectric films has been common practice for many years but the parameters necessary for optimizing their use as memory have not been routinely recorded. Remanent polarization, spontaneous polarization and their change through fatigue and ageing are still the dominant parameters but the pulsing conditions of interest are driven by the circuit requirements. The operation of a memory circuit will be discussed along with implications for testing. Memory is written by applying a pulse to the capacitor. It is read by applying a pulse and sensing whether or not the polarization switched. Read and write pulses will apply different voltages to the ferroelectric capacitor in the circuit because a sense capacitor will be in series with it during the read. A new test, pulsed hysteresis, which is based on circuit operation will be presented. Parameter space of interest will also be discussed.
Invited s-D-5 domain greatly exceeds the available thermal energy lnd the domain is stable. This mechanism leads directly to the advantages these materials have when used as a memory element. We divide the advantages into three categories; circuit performance and design issues, reliability issues qnd manufacturing issues. Ferroelectric circuits are fast, dense, and can operate at low voltages. The speed of the switching has been calculated2 to be < 0.5 ns. Our measurements which are limited by our current circuits show it to be faster than 3 ns. The memory cell can be scaled down in size as it is similar to a DRAM cell consisting of one transistor and one capacitor which can be stacked above the transistor. This structural similarity should allow the density of ferroelectric circuits to evolve rapidly until it catches leading edge processing. Switching at less than 2 volts has been demonstrated on 1000A films3. This wilt allow operation in the planned 3.3 or 2.0 volt supply voltage circuits without special high voltage transistors. Reliability defines the limits of performance of the ferroelectric as with any technology. It has been demonstrated that a ferroelectric film can be switched '1.e12 times and have only a 20To reduction in remanent polarization (Figure 1). Soft errors from alpha partictes are a major factor limiting the reduction of the size of the storage capacitor in
This paper investigates the issues in the scaling of thin film PZT (Lead-Zirconate-Titanate) capacitors for DRAM (Dynamic Random Access Memories) applications. The test structures used were MIM (metal-insulator-metal) capacitors with platinum electrodes and PZT deposited using a sol-gel process. Charge storage density (Q'c), leakage current density (JL), unipolar switching time to 10% decay (ts), time dependent dielectric breakdown (TDDB) and electrical fatigue have been analyzed. Unipolar switching time has been modeled as an RC time constant, where C is electric-field dependent. Q'c at a given electric field appears to remain constant over the range of film thicknesses and electrode areas studied. Leakage current density and time-to-breakdown (tBD) for a given electric field degrade with decreasing film thickness. Unipolar stressing causes considerably less fatigue than bipolar stressing, and after 2 × 1011 cycles, a 400nm film still exhibits sufficient Q'c for DRAM operation.
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