Hydrogen Silsesquioxane-Based, Ultralow-k Dielectric Films

The physical and electrical characteristics of silsesquioxane-based, ultralow-permittivity (ultralow-k) dielectric films have been investigated in collaborative work by IMEC (Leuven, Belgium) and Dow Corning Corp. (Seneffe, Belgium and Midland, Mich.). It was shown that the exposure of these films to plasma environments resulted in a change of porosity. The low-k dielectric film with a pore size ~4 nm has been successfully integrated into 200 nm single-damascene structures.

As device dimensions fall toward the 100 nm region, it becomes necessary to employ materials with a k value of <2.0 to reduce parasitic capacitances in interconnect schemes. Ultralow-k dielectrics can be prepared from hydrogen silsesquioxane (HSQ) resin by adding a solvent with a high boiling point to the resin formulation, followed by wet ammonia treatment and thermal cure. Important film characteristics, such as the refractive index and SiH content, depend on the ammonia treatment time. The remaining high boiling point solvent is removed during thermal treatment, and a stronger network structure is thus formed. Higher temperatures and longer cure times produce films with lower SiH bond density, higher modulus and higher refractive index because of the greater amount of cross-linking in the film.

Cross-sectional SEM image of 250 nm trenches patterned in a Dow Corning silsesquioxane-based ultralow-k dielectric material. (Source: Dow Corning Corp., Belgium)

The stability of the cured films was investigated by Fourier transform infrared (FTIR) spectroscopy. No change in the FTIR spectra was found during storage for one month in a cleanroom environment, even in those samples with the lowest SiH content (~40%), which would be most susceptible to moisture. Thus, the films had very good stability and resistance to moisture uptake. The low stress value of 20 ±5 MPa was maintained after a cycle of one hour at 425°C.

Plasma processing with the commonly used fluorine, oxygen and hydrogen chemistries can modify the material porosity and affect the permittivity. The workers used wafer-level ellipsometric porosimetry (EP) to measure the pore size distribution of the films both before and after various plasma treatments. A very uniform pore size distribution of ~3.5 nm was found. The optical porosity of the freshly prepared film was ~50%, indicating that all of the pores were interconnected. Films exposed to oxygen and hydrogen plasmas showed a decrease in the optical and absorption porosity and a shrinkage of 5-10%. Wafers treated in a C₂F₆/Ar plasma showed an optical porosity equal to the freshly prepared film, but a decreased absorption porosity. The group suggests that this is due to fluorocarbon polymers being formed in the plasma "filling/closing" part of the open pores, resulting in less absorption.

The permittivity of the films was found from measurements of capacitor structures at 1 MHz. Capping layers of PECVD oxide from silane (k=4.2) and SiC from trimethylsilane (k=4.4) were used. An initial k value of 1.85 was found for both cases, but capacitors with the oxide cap degraded to a k value of 2.18 after three weeks.

Trenches were etched in a two-step etch process. An Ar/CF/CHF₃/O₂ step opened the 100 nm SiC hard mask with limited etching of the low-k material. The second step opened the trenches. The Figure shows etching ceased successfully on the 50 nm SiC stop layer.