Synthesis and characterization of porous polymeric low dielectric constant films

Journal of Electronic Materials, Apr 2001 by Xu, Yuhuan, Zheng, D W, Tsai, Yipin, Tu, K N, Et al

This paper reports the synthesis and dielectric properties of a porous poly(arylether) material with an ultra-low dielectric constant for interlayer dielectric applications in microelectronics. The porous polymer films were successfully fabricated by a method of organic phase separation and evaporation. A dielectric constant kappa of 1.8 was achieved for a porous film with an estimated porosity of 40% and average pore size of 3 nm. Electrical and mechanical properties as well as coefficient of thermal expansion for both dense and porous polymer films were measured.

Key words: Ultra-low dielectric constant, poly(arylene) ethers, porous polymer film

INTRODUCTION

Continuing improvement of the very large scale integration of microelectronic circuits has reached the point where multi-layer metallization and the interlayer dielectric have become the limiting factors in process integration and device performance. The major improvements in performance of microelectronics come from device size miniaturization, which leads to faster device speeds, higher device packing density, and more functions on a chip. As the devices scale to smaller feature sizes, the transistor capacitance and resistance are reduced, yet the line-to-line capacitance and resistance of the metal lines increase. As a result, the resistance-capacitance (RC) delay caused by interconnect tends to limit the chip performance. A new generation of low dielectric constant materials is required to achieve the advantages in high speed, low power dissipation, and low cross talk noise. According to the Semiconductor Industry Association (SIA) roadmap,1 by the year 2001 the minimum feature size will be 0.15 gm, which requires insulation with a dielectric material having a dielectric constant kappa = 2.3. An ultra low dielectric material (kappa

MEASUREMENT OF MECHANICAL PROPERTIES

We measured the mechanical properties of dense and porous PAE films, by three different methods: membrane bulge test,20,21 nanoindentation,22 and single-substrate bending beam method.23,24 The elastic modulus and initial stress measured from these three methods are in good agreement.25 For the dense film samples, a Young's modulus of 3.9 GPa was obtained independently from both the bulge test and nanoindentation methods. The initial stress of 35 MPa and 16 MPa for the dense and porous film samples, respectively, was also obtained from both the bulge test and single-substrate bending beam methods. Hardness of 0.32 GPa and 0.29 GPa for the dense and porous 1 (mu)m-thick films, respectively, were obtained from nanoindentation. CTE of 26.8-32.6 ppm/ deg C and 56.1-72.5 ppm/ deg C for dense and porous PAE films, respectively, were obtained by the single-substrate bending beam method. Table II summarizes the data obtained by the above three techniques.

The substrate effect was observed in the measurements by nanoindentation. Figure 4 shows the dependence of elastic modulus, E, and hardness, H, on the thickness of porous PAE film samples.

We attempted to use different values of Poisson's ratio from 0.40 to 0.43 for calculating the elastic modulus from the measured biaxial modulus. We found that when gamma = 0.42 was used, we obtained the best match between the initial stress measured by bulge test and that calculated from the stress-versustemperature curve by the single-substrate bending beam method. Also, the elastic modulus calculated from the bulge test is in good agreement with that measured by nanoindentation.

CONCLUSION AND DISCUSSION

In conclusion, a porous polymer PAE film was successfully fabricated by a simple method of organic materials phase separation and evaporation. The dielectric constant decreased from 2.7 to 1.8 with an estimated porosity of 40%. Adding more solution-B, the dielectric constant of the film became lower. The high resolution transmission electron microscopy (HRTEM) observation showed numerous nanopores with an average size of 3 nm distributed uniformly inside the film. In addition, the dielectric dissipation of the porous films is lower than that of the dense film because of the very low dissipation of the gas phase in the film. Since polymer PAE has a very small amount of water absorption, both dense and porous films have good aging behavior (time-independence) for dielectric properties. The mechanical properties and CTE for both dense and porous PAE low-kappa films have been measured by different methods: membrane bulge test, nanoindentation, and single-substrate bending beam method. The results measured by these methods are in good agreement. The substrate effect was observed in the measurements by nanoindentation.

ACKNOWLEDGEMENT

This work was supported by UC-MICRO and UCSMART programs.

(Received December 2, 2000; accepted January 9, 2001)

REFERENCES

1. The National Technology Roadmap for Semiconductors (San Jose, CA: Semiconductor Industry Association, 1997).

2. J.P. Sullivan, D.R. Denison, J.C. Barbour, P.P. Newcomer, CA. Apblett, C.H. Seager, and A.G. Baca, Mater. Res. Soc. Symp. Proc. 443, 149 (1997).