The Degradation of Characteristics in DRAM by ILD HDP Process Condition
Authors: KIM JI Hoon , Choi Byoung Deog 
Published: December 2020

As the size of DRAM devices are increasingly scaling down as time goes by, materials in the back end of line process are having a significant impact on DRAM device characteristics. [1]

To ensure DRAM characteristics and reliability, it is necessary to identify the impact of subsequent processes. In this experiment, we studied the DRAM device electrical characteristics according to HDP CVD oxide process temperature which is used as ILD between metal lines in the back end of line.

For the experiment, DRAM device wafer with a design rule of sub 20nm was prepared, and the p process temperature was divided into three way. (Ref, Ref -5%, Ref +5%)

First of all, the concentration of the atomic bonding according to the process temperature was identified using FT-IR (Fourier Transform – Infra Red Spectroscopy).

Through FT-IR, the presence of atomic bonding in the material can be identified and quantified.

Next the electrical characteristics of the DRAM device according to the process temperature were evaluated with the corresponding sample.

We measured capacitors leakage current and NBTI, which are important properties in DRAM devices.

Fig 1 illustrates the tendency of hydrogen content to decrease as the temperature of HDP deposition increases.

It seems that hydrogen out-gassing decreases as the HDP process temperature decreases, so that hydrogen in the film material diffuses in a subsequent thermal process.[2]

Fig 2 shows a tendency for leakage current to increase as process temperature decreases. That means the leakage current increased as the hydrogen concentration increased.

In addition, the EDS Test can prove the failure cell by capacitor leakage, and the result is that the same tendency was found in Fig 3.

This is explained by the increase in trap density by hydrogen in Poole–Frenkel Emission among dielectric conduct mechanism. [3]

The change in threshold voltage can be seen in Fig 4 according to the change in process temperature.

Obviously, it is confirmed that hydrogen increase due to process temperature decrease has a significant effect on threshold voltage shift.

As the hydrogen concentration increases, the Si-H bonds of interface are increased, then the Si-H bonds are broken after stress, contributing to NBTI degradation. [4]

To summarize this experiment, changes in hydrogen concentration were observed with the temperature change of the HDP CVD process used as ILD.

It has been confirmed that the increase in hydrogen concentration has greatly affected the increase in leakage current of capacitors and the deterioration of PMOSFET NBTI.

ILD process temperature conditions have been identified as factors that have a clear effect on the characteristics of DRAM.

We can propose the process conditions in the future, taking into account the characteristics.

References

[1] Khamankar, R., Kim, J., Jiang, B., Sudhama, C., Maniar, P., Moazzami, R., ... & Lee, J. (1994, December). Impact of post processing damage on the performance of high dielectric constant PLZT thin film capacitors for ULSI DRAM applications. In Proceedings of 1994 IEEE International Electron Devices Meeting (pp. 337-340). IEEE.

[2] Li, Q., Goosen, J. F. L., Van Beek, J. T. M., Van Keulen, F., & Zhang, G. Q. (2009, August). Out gassing of materials used for thin film vacuum packages. In 2009 International Conference on Electronic Packaging Technology & High Density Packaging (pp. 802-806). IEEE.

[3] Knebel, S., Schroeder, U., Zhou, D., Mikolajick, T., & Krautheim, G. (2012). " Conduction Mechanisms and Breakdown Characteristics of Al2O3-Doped ZrO2 High-k Dielectrics for Three-Dimensional Stacked Metal–Insulator–Metal Capacitors " IEEE Transactions on Device and Materials Reliability, 14(1), 154-160.

[4] Huard, V., Denais, M., Perrier, F., Revil, N., Parthasarathy, C., Bravaix, A., & Vincent, E. (2005). A thorough investigation of MOSFETs NBTI degradation. Microelectronics Reliability, 45(1), 83-98.

Figure 1