UV Curing Effects of Low-k Materials under Reactive Conditions
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1079-N02-04
UV Curing Effects of Low-k Materials under Reactive Conditions Darren Moore, Carlo Waldfried, Palani Sakthivel, and Orlando Escorcia Axcelis Technologies, 108 Cherry Hill Drive, Beverly, MA, 01915 ABSTRACT This work describes the effect of addition of O2 and NH3 to the N2 ambient used during the UV cure process of low-k materials. The effects of O2 give an acceleration of the cure process, resulting in increased film modulus and shrinkage. The use of NH3 resulted in a retardation of UV cure, explained by the higher absorption cross section of NH3 at the wavelengths used. INTRODUCTION UV curing of low-k films has received particular attention as a post deposition treatment due to its ability to effectively improve film modulus [1] while removing porogen and retaining porosity, all with low thermal budget and little increase in k value [2]. In general increasing cure time will result in increased modulus, eventually at the disadvantage of an increase in film k. This work will describe the use of oxygen addition to the cure process to achieve an accelerated cure process, enabling a targeted film modulus to be achieved with shorter cure time, with little or no impact on k value. EXPERIMENT Three carbon doped SiOCH film types were used for this study. The first was a porous SOG, prepared by a typical spin-on and soft bake process with target k=2.5. The second was a PECVD non-porous film with target k=3.0. The final film was a porous PECVD film formed by the co deposition of a lattive and cyclic organic porogen material. The removal of the porogen achieved the target k=2.5. All films used were of nominal thickness ~ 10kA. The cure process used the Axcelis RapidCure 320FC UV cure tool. The process uses microwave driven electrodless bulbs emitting UV radiation in the 100-400 nm range with the wafer temperature maintained at 400oC with thermal chuck. Standard process conditions used a constant purge of N2 gas at atmospheric pressure. Oxygen additions to the ambient were set at 50 and 100 ppm in N2, while ammonia concentrations tested were 50, 100 and 750 ppm. All tests were processed for 3 mins, and compared to the cure process with N2 only ambient. K values were determined using Al shadow mask techniques. Mechanical properties were determined using an MTS nanoindentor XP. Solid state Si29 MAS NMR was carried out on a Bruker Avance NMR spectrometer (11.7 Tesla, 500.2MHz, 4mm Zr O2 rotor @ 10kHz). DISCUSSION Effect of UV cure with N2 ambient Although cure of dielectric films is well documented, the effects of increasing cure time for the films used in this work will first be summarized. Figure (1) illustrates film shrinkage for cure times of 1, 3, 6 and 13 minutes for all three dielectrics studied. The relative modulus and
hardness increases for the SOG 2.5 and CVD 3.0 films, (Figure (2)) show a ~70% increase for the CVD 3.0 and a 400-500% increase for the porous SOG 2.5 film when cured to 13 minutes. Effects of UV cure on the FTIR properties of films are well known and typically show an increase in the SiO net
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