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Hydrophobic Films Surface Preparation And Its Impact On Wet Cleaning


 
 Keywords: hydrophobic films, drying marks, SiCN, amorphous carbon, chemical oxide growth
 Introduction
 Integrated circuits manufacturing requires use of hydrophobic materials, as low-k films. Maintaining their integrity during wet cleaning is challenging. Indeed, the aqueous liquid film break-up on these surfaces during the cleaning process can result in undesired drying marks defects [1] which can cause open-circuits and be yield killers by blocking proper patterning. Use of dry-cleaning methods instead of wet chemistries can be a solution to avoid drying marks formation. However, while relevant in particle removal they cannot remove chemisorbed defects. Therefore, the use of aqueous solutions cannot be always avoided to clean hydrophobic films. In this case, inducing surface modification of the hydrophobic surface before the aqueous-cleaning step can be a solution. Since drying is carried out after the surface becomes hydrophilic, the occurrence of drying marks can be prevented.
 In this work we first study how chemical treatments can modify hydrophobic surfaces, so they get a hydrophilic nature. Based on that, we evaluate the interest that can have such step before aqueous wafer cleaning techniques.
 Experimental and characterizations
 Silicon carbon nitrogen (SiCN) and amorphous carbon (a-C) films are deposited by plasma enhanced chemical vapor deposition method on 300mm p-type (011) silicon substrate. The a-C composition is of 65% carbon,35% hydrogen while the SiCN composition is of: 42% Hydrogen, 20% carbon, 14% nitrogen, 24% silicon. They have been obtained with elastic recoil detection analysis, nuclear reaction analysis and Rutherford backscattering spectroscopy. All wet chemical surface preparations have been carried out on spin dry single wafer platforms from SCREEN. Two wet chemistries have been evaluated: Standard Clean 1 (mix of ammonia, hydrogen peroxide and water with a respective volume ratio of 1/2/80, fresh chemistry at 65°C) and ozonated water (20ppm). A treatment with an excimer ultraviolet lamp with a 172nm wavelength has been also done.
 The reactions between these films and treatments are characterized by AR-XPS and XRR.
 Artificial contamination of SiCN wafers for particle removal tests has been done by spin-drying using isopropanol containing monodisperse SiO2 particles of 80nm.
 Results and discussion
 The surface preparations of a-C and SiCN films impact the water contact angles, as reported in Figure 1.
 A-C surface presents a contact angle of 5° with water, after UV or ozonated water treatments, while stays unaltered with the SC1. This difference between the ozonated water and SC1 dispense can be related to the oxidizing power difference as ozonated water features higher redox potential than ammonia. Proposed mechanism of amorphous carbon oxidation by ozone is the breaking of C=C bonds and the formation of carboxyl —COOH groups [2].
 On the other hand, SiCN contact angle is reduced by SC1, ozonated water and UV (respectively 45, 30 and 5°). AR-XPS characterizations have been used to identify the bonds modification during these treatments. The results lead to the conclusion of an oxidation mechanism for ozonated water, with an oxide layer thickness of less than 5nm (XRR) (Table 1). SiCN bonds oxidation are thermodynamically favored, even though kinetics of these reactions has not been studied (Table 2).
 The development of a hydrophilic behavior on these surfaces suggests that we avoid watermarks formations. A comparison of SC1 spray cleaning on modified and as deposited SiCN wafers has been carried, to determine the impact of surface modification on particle removal efficiency.
 Nevertheless, in the specific case of low-k films two potential impacts of these surface modifications also need to be investigated. Indeed, electrical properties of low-k films are key parameters, that is why the consequences of surface modification on them need to be evaluated. And delamination of low-k interfaces is already a failure risk. It would be enhanced by an adhesion strength reduction due to surface energy change. Contact angle measurements and the Fowkes model have been used to deduce the influence of these treatments on surface energy.
 Conclusion
 The cleaning of hydrophobic films is a key challenge for yield benefits. These films are prone to drying marks formation during aqueous cleaning steps. In this work, we study how surface preparation can break non-polar bonds, leading to the development of a hydrophilic surface. Several treatments (UV, SC1 and ozonated water) and materials (a-C, SiCN) have been studied. A comparison of particle removal efficiency with SC1 spray method between as deposited and modified films have been carried out, to quantify the impact of these modifications on wet surface cleaning.
 
 References
 
 [1] Han, J. and al, Solid State Phenomena, 134, p295–298, 2007
 [2] Mawhinney, D. and al. Carbon, 39(8), p1167–1173, 2001
 
 
 
 
 
 Figure 1
