Paper AS+EM+MS+TF-TuM12
Synergism of Ellipsometric Porosimetry with Other Complementary Characterization Techniques for Process Control of Ultra Low k Dielectric Films

Tuesday, November 10, 2009, 11:40 am, Room C2

This paper discusses new protocols established for matching material properties of ultra low k films for tool matching and process control required for robust manufacturability. These are based on extensive repeat off-line measurement of 2 different films to determine the precision (3 sigma variation), reproducibility and error bars in the measurements.

At the 45 nm node, porosity was first introduced into the interconnect structures in the form of porous SiCOH (pSiCOH) inter-level dielectric films to reduce the capacitance-resistance delay. pSiCOH films are typically deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) when a precursor containing C, H, Si, O and a hydrocarbon porogen precursor are simultaneously introduced in the processing chamber and RF power is applied. Subsequent exposure to UV irradiation results in the removal of porogen (CHx phase) to create porosity and therefore a lower k value.

Optimization of the UV cure time of the film is critical since overcuring of the film can result in unwanted demethylation and extended network crosslinking of the siloxane backbone which can collapse the pores, altering the porosity, and detrimentally increasing the k value. In-line ellipsometric measurements of refractive index and shrinkage are typically used to monitor the film properties and do not completely reflect the changes in the porosity.

This paper will discuss the methodology for the quantification of the % porosity using Ellipsometry Porosimetry(EP). In addition, measurements of carbon content using XPS, remnant porogen using thermal gravimetric analysis (TGA), and demethylation using FTIR will be presented and correlated to the % porosity. For a 5000A pSiCOH film with 25% porosity, repeatibility studies using off-line EP measurements showed 0.5% variation. Composition from XPS sputter depth profiling resulted in C: 16.0 +/- 4.0 at%; Si: 38.0 +/- 4.0 at%, and O: 46.0 +/- 4.0 at%. The FTIR CH2 peak at 2885 cm-1 normalized to SiOSi peak <0.070 mAU (milliAbsorbance Units) was determined to be an appropriate measure of the porogen removal, along with porogen weight loss in the cured film at 1.0+/-0.5% using TGA. The FTIR methyl silicon CH3 peak at 2974cm-1 at 0.0125 +/- 0.0006 AU (normalized to SiOSi peak) is a sensitive parameter indicative of an overcured condition. Cross linking criteria could not be reliability established, though degree of crosslinking is related to changes in the bulk modulus observed with various cure conditions.

The end result is a more comprehensive and accurate characterization of the ultra low k pSiCOH films, thereby ensuring needed manufacturing controls for chamber matching and tool qualification.