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Capacitor Dielectrics

Fig. 1:  Leakage current density vs. capacitance equivalent thickness (CET) for MIM capacitor stacks with different high-k materials characterized at NaMLab.

Continuous research is ongoing to optimize materials with high dielectric constants. After implementation of these dielectrics in a capacitor the correlation between structural and electrical properties can be determined. At NaMLab, the main emphasis is to introduce the material in semiconductor devices like DRAM or backend capacitors.

Fig. 2: Transmission electron microscopy (TEM) image of a ZrO2 based capacitor with interlayer of (a) Al2O3 or (b) SrO2.

For DRAM capacitor applications (see Fig. 1) the main focus was on CaTiO3, ZrO2 and TiO2-based dielectrics. Very promising results for a SrTiO3 based capacitor with a capacitance equivalent thickness (CET) value of 0.2 nm at target leakage current were gained together with RWTH Aachen. Unfortunately, due to a reduced Schottky barrier height of this material a physical thickness of at least 8 nm is required, which would not fit into an 18 nm DRAM structure. Two different new approaches to optimize the current ZrO2 based DRAM capacitor by changing the inter-layer material from Al2O3 to SrO (Fig. 2), and the top electrode from TiN to Pt, were evaluated. A combination of these two approaches leads to a CET value of 0.47 nm. Most important, the physical thickness < 5 nm for the dielectric stack is in accordance with the target specifications. Detailed evaluations of the leakage current characteristics lead to a capacitor model which allows the prediction of the electrical behavior with thickness scaling.

Fig. 3: Time dependent dielectric breakdown behavior for production type crystalline ZrO2 based thin films under DC and AC stress for different stress voltages.

Parallel to the development of new dielectric materials characterization techniques are evaluated. Here, the time dependent dielectric breakdown behavior is investigated for
production type crystalline ZrO2 based thin films under DC and AC stress (Fig. 3). Constant voltage stress measurements over 6 decades in time show that the voltage acceleration of time-tobreakdown follows the conventional exponential law. The effects of AC stress on of time-to-breakdown are studied in detail by changing the experimental parameters including stress voltage, base voltage and frequency. In general, AC stressing gives rise to a gain in lifetime, which may result from less overall charge trapping. This trap dynamic was investigated by dielectric absorption measurements. Overall, the typical DRAM refresh of the capacitor leads to the most critical reliability concern. Variable temperature current vs. voltage measurements revealed a trap assisted leakage mechanism with a mean trap depth of ~0.8 eV which is typically related to oxygen vacancies in these materials. Dielectric relaxation values show a loss of ~5 % for ZrO2 based dielectrics and the lifetime of these capacitors was determined to be longer than 10 years.

In future, further candidate materials will be continuously screened and characterized for new device applications.

 

Contact: Dr. Uwe Schroeder

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Phone: +49.351.21.24.990-00
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