Role of Ce³⁺Ions in Achieving High Silicon Nitride Polish Rates

Abstract

We show that Ce 3+ ions when used as an additive to ceria dispersions enhance plasma-enhanced chemical-vapor-deposited silicon nitride polish or removal rates (RRs). Ceria slurries (0.1 wt% and 140 nm avg. size) containing 2.3 mM Ce(NO 3 ) 3 and no other additive gave nitride RRs of ∼300 nm/min at 4 psi and ∼350 nm/min at 5 psi, both at pH4. The nitride RRs measured in the presence of Ce(NO 3 ) 3 , Ce(CH 3 COO) 3 and KNO 3 suggest that the rate enhancement is solely due to the presence of Ce 3+ ions. We discuss the underlying mechanism causing high silicon nitride RRs in the presence of Ce 3+ ions based on XPS analysis of pre-and post-polished silicon nitride and oxynitride film surfaces, streaming potential data and high oxynitride RRs obtained using the same ceria particle-based slurries as above. It is suggested that the Ce 3+ ions convert the upper layers of the nitride film into an oxynitride that is polished by the abrasives at a high rate. In this process, the conversion of the nitride film seems to be the rate controlling step with the oxidation and polishing occurring in a repetitive manner. © The Author Semiconductor device scaling led to the shrinking of device structures and also created new challenges in the front end of line integration schemes that required several additional new chemical mechanical planarization (CMP) steps. One such new integration scheme in the replacement metal gate (RMG) process is the self-aligned contact (SAC) module where a silicon nitride CMP step is required as shown in Figure 1 (Redrawn from Ref. 1). Since the composition of this silicon nitride film and those used in our experiments is nonstoichiometric, we will refer to them by the generic symbol SiN. In this SAC module, one of the challenges is the misalignment of the contact metal and source/drain, leading to device failure. In order to provide a wider process margin in contact alignment, a SiN cap is deposited on top of the metal gate.1 In the process of depositing this SiN cap, there will always be some SiN overburden, as shown in Fig. 1, which needs to be removed using a highly selective nitride slurry that has a very low oxide removal rate to minimize loss of the underneath oxide. This removal rate selectivity requirement in this SiN cap CMP process is the reverse of that needed in the more conventional shallow trench isolation (STI) CMP and is often referred to as reverse STI selectivity. 2-8Several authors 2-8 investigated reverse STI selectivity CMP processes using ceria and silica based dispersions with various additives. Recently Bae et al.2 achieved highly selective Si 3 N 4 to SiO 2 removal rate (RR) ratio of ∼ 95.0 at pH 1.5 using modified silica abrasives. They modified the surface charge of the silica abrasives toward more negative values (−50 mV) since creating higher negative charge on silica abrasives enables stronger attractive forces between them Si 3 N 4 films and a stronger repulsive force toward silica films in a low pH environment, which can lead to high nitride to oxide removal rate selectiv...

Photo-oxidative degradation of polyacids derived ceria nanoparticle modulation for chemical mechanical polishing

“…The trivalent state of cerium promotes Ce-O-Si bonding with an enhancement of the interaction between ceria particles and silicon dioxide wafer. Since the formation of Ce-O-Si bonding in an oxide CMP using ceria abrasives is directly related to the material removal rate, high concentrations of Ce 3+ ions can provide high CMP performance 18 , 21 , 57 . Figure 4 shows CMP performance including the removal rate and finished surface roughness of the oxide film using photo-reduced ceria nanoparticles according to UV irradiation time.…”

A review on chemical and mechanical phenomena at the wafer interface during chemical mechanical planarization

“…The details of the mechanism of Si 3 N 4 removal rate suppression will be discussed in detail. Later, Alety et al showed that the removal rate of Si 3 N 4 film in the presence of 0.1 wt% ceria particles + 2.3 mM Ce(NO 3 ) 3 at pH 4 was ∼300 nm/min, whereas it is ∼10 nm/min with only ceria particle [55]. X-ray photoelectron spectroscopy of the polished Si 3 N 4 films is similar to that of an as-received silicon oxynitride film which led them to suggest that, in the presence of Ce 3+ , the surface of Si 3 N 4 film can be converted to silicon oxynitride which was then polished at a high rate [55].…”

Photo-Assisted Chemical Mechanical Polishing of Si Wafer Using Phosphorescent Particles as a Luminescent Agent

“…34 The Ce 3+ -rich CeO 2 can reduce the formed oxide to eliminate oxygen atoms from SiO 2 , resulting in an improved MRR in pH 9 with a smooth polished surface. 35 Next, the dependence of the material removal characteristics on the polishing rotation rate was examined to obtain a further improved MRR. For conventional polishing, the MRR is proportional to the relative velocity between the material and the polishing tool, which is known as Preston's law of polishing.…”