Effect of pH on CMP of Copper and Tantalum

Abstract

Polishing behavior of copper and tantalum using aqueous slurries containing alumina and silica abrasives with and without H 2 O 2 /glycine has been investigated at different pH values. It was observed that pH-dependent changes in the surface characteristics of the films being polished lead to significant variation in the copper and tantalum removal rates. Some of the surface characteristics that change with the slurry pH include the nature of the passivating layer formed on the metal surface and the hardness of such layers. It is also shown that a favorable copper to tantalum polish rate selectivity can be obtained by adjusting the slurry pH.Copper metallization is achieved by combining the single or dual damascene process with a suitable diffusion barrier layer ͑e.g., tantalum͒ with chemical mechanical planarization ͑CMP͒ to remove excess metal. For an effective application of CMP to sub-0.11 m device fabrication, a complete investigation of the polishing behavior of both copper ͑Cu͒ and tantalum ͑Ta͒ continues to be important. Significant research on Cu CMP using acidic and alkaline slurries with different oxidants and inhibiting agents has been carried out and reported in the literature, 1-20 but much less is accomplished for tantalum CMP. Most of the published polishing mechanisms for Cu and Ta polishing are based on the formation and removal of an oxidized surface layer. [1][2][3][4][5][6][7]20 demonstrated that the electrostatic interactions between the slurry particles and film surface are important during polishing. These electrostatic interactions are strongly influenced by the pH of the slurry. The effect of such interactions during the polishing of oxide and nitride films using silica, ceria, and alumina slurries was investigated by Hegde and Babu. 21 However, the nature and the properties of the oxidized surface layer for Cu vary with changes in the chemistry and the pH of the slurry. Cu forms a variety of oxides and hydroxides depending upon the pH, the potential at the Cu/slurry interface, and the kinetics of the surface reactions, [9][10][11]22 making it difficult to predict the role and impact of electrostatic interactions during copper polishing. Ta is inert due to the formation of a passive layer of tantalum pentoxide (Ta 2 O 5 ) which is stable at all pH values, 23 even though partial dissolution or weakening of such layers in an alkaline pH regime, with and without oxidizers, has also been reported. [24][25][26][27] Various oxidizers, such as ferric nitrate ͓Fe͑NO 3 ) 3 ] 5 and nitric acid (HNO 3 ) 1 in acidic conditions, and ammonium hydroxide (NH 4 OH), 2,3 ammonium nitrate (NH 4 NO 3 ), 2,3 and ammonium chloride (NH 4 Cl) 2 in alkaline conditions, have been used for Cu CMP. Cu CMP in strongly acidic conditions leads to reduced planarization efficiency, while polish selectivity to underlying oxide is poor in alkaline regimes. Therefore, an intermediate pH range ͑pH 4-8͒ is preferred for Cu polishing. Suitable oxidizers in this pH range are hydrogen peroxide (H 2 O 2 ), 4 potassium iodate (KI...

Material Removal Mechanism during Copper Chemical Mechanical Planarization Based on Nano-Scale Material Behavior

“…Other researchers also observed substantial MRRs when copper was polished with DI water containing only abrasive particles. [12][13][14] From those observations, it is apparent that most of the total MRR during copper CMP must be due to the mechanical removal of metallic copper. Removal of the material by the CMP pad material itself was not considered because CMP without abrasive particles yielded negligible MRR.…”

“…This poor correlation is not due to ignoring the effect of direct dissolution of copper, because even the highest dissolution rates seen in the presence of glycine were insignificant compared to the MRR during CMP. 13,16 Note that there is huge difference in the duration of exposure of a given surface element of copper to chemicals during Liao et al's experiments 16 and during CMP. During CMP itself, the probability that a given location on a wafer is contacted by abrasive particles held under a pad asperity during interaction between the asperity and the wafer is defined as the removal efficiency.…”

“…13,18 The slight change of the nanohardness has been attributed to the oxide layer on the surface of copper. 16,18 Poor correlation between the nanohardness measured with nanoindentation experiments and the MRR 13,18 was presumably due to an effect of size on hardness, which was not recognized by the original authors. As discussed above, material properties at very small length scales differ from those at larger length scales.…”

“…The relative magnitudes of the MRRs for the different conditions were determined from Equation 3 by which the MRRs during copper CMP are inversely proportional to the 3/2 power of the hardness at the surface of copper, for constant size of abrasive particles, sliding velocities and down pressures. The predicted rates are graphically compared with the experimental data of Jindal et al 13 in Figure 5. It was assumed that the number of abrasive particles participating in material removal was unaffected by the presence of glycine and the pH of the slurry.…”

Chemical-Mechanical Impact of Nanoparticles and pH Effect of the Slurry on the CMP of the Selective Layer Surfaces

“…In the literature, there are numerous studies on similar topics [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] applied to the copper CMP of integrated circuits. It is worth mentioning that the present study intends to extend to other fields, namely to surfaces coated through selective transfer (the surfaces of the selective layer) in the friction process, considering the slurry pH (important for removal through CMP of the selective layer) [7,[22][23][24][25][26], H 2 O 2 (the most common oxidant) [27][28][29][30][31][32][33], size [21,[28][29][30][31], and concentration [12,21,34] of nanoparticles used in the CMP slurry. Thus, this paper explores the pH effect at a constant H 2 O 2 concentration on the etching and polishing behavior of the selective layer and the influences on size and concentration of nanoparticles during selective layer surface CMP.…”