David J. Duquette

Chemical processes in the chemical mechanical polishing of copper

The mechanisms by which removal and planarization occur during the chemical mechanical polishing (CMP) of copper, used for pattern delineation in a multilevel metallization scheme, are investigated in this paper. We propose that removal occurs as mechanical abrasion of the surface followed by chemical dissolution of the abraded species. Planarization is achieved by the use of a rigid polishing pad that provides mechanical abrasion only to the high areas on the copper surface and by the formation of a surface layer on the copper during polishing to prevent dissolution of copper in the low areas. Fundamentals of electrochemistry are used to explain and predict both the dissolution of copper and the formation of a surface layer in the CMP slurry. Examples of polishing slurries are presented to demonstrate our hypotheses, including a complexing agent (ammonia) plus oxidizer (ferricyanide ion or nitrate ion) slurry and an oxidizing acid (nitric acid) plus corrosion inhibitor (benzotriazole) slurry. Finally, the mechanisms used to explain the CMP of copper are used to explain anomalous behavior during the CMP of titanium, in which the presence of copper ions in the polish slurry accelerates the polish rate of titanium. Titanium is used as a diffusion barrier and adhesion promoter for copper.

The role of hydrogen in corrosion fatigue of high purity Al-Zn-Mg exposed to water vapor

Corrosion fatigue tests were performed on samples of a high purity Al-Zn-Mg alloy in humid nitrogen gas after pre-exposure to either vacuum or humid air. The results of these tests were compared to the results of fatigue tests performed in dry nitrogen, used as an inert reference environment, after the same pre-exposure treatments. The pre-exposure times were calculated by assuming that bulk diffusion of hydrogen was the rate limiting process in either hydrogen adsorption or desorption. Water vapor in the testing environment resulted in reduced fatigue lives; however, pre-exposure to humid air was just as detrimental as water vapor in the test environment. The pre-exposure embrittlement effect of humid air was found to be completely reversible when the samples were stored in a vacuum long enough to remove hydrogen, assuming a bulk diffusion coefficient of 1 x 10-13 m2/sec. These results confirm the hypothesis that the reduced fatigue lives of Al-Zn-Mg alloys in water vapor is due to hydrogen embrittlement.

Chemical-mechanical polishing of copper with oxide and polymer interlevel dielectrics

Chemical-mechanical polishing (CMP) of copper with oxide interlevel dielectrics has been demonstrated as a viable patterning approach for copper interconnect structures. This paper summarizes our understanding of the mechanisms involved in copper CMP and presents our results with both oxide interlevel dielectrics (ILDs) and low dielectric constant polymer ILDs. Our two-step model of copper CMP consists of mechanical abrasion of the copper surface followed by removal of the abraded material from the vicinity of the surface and has been developed after extensive electrochemical and CMP experiments with alternative slurries. Although the softer and less process tolerant polymers result in Damascene patterning difficulties compared with oxide ILDs, our results with both benzocyclobutene and parylene indicate that manufacturable processes for on-chip interconnect structures can be established with additional fundamental understanding and further development.

Electrochemical Potential Measurements during the Chemical‐Mechanical Polishing of Copper Thin Films

A description is given of the mixed electrochemical potential measured in situ during the chemical-mechanical polishing of copper. Potential measurements are indicative of the dissolution rate of copper and of the equilibrium form of the polished copper by-products. These measurements are used to explain the polish performance in several ammonia-based slurries. Specifically, the polish rate is shown to correlate with the potential and the change in potential during polishing. In addition, complexing of copper ions with dissolved ammonia is discussed and shown to be an effective method for increasing the solubility of copper ions in the slurry and thereby increasing the polish rate.

The effect of order on hydrogen embrittlement of Ni3Fe

Abstract The susceptibility of a cold pressed and sintered Ni 3 Fe alloy to hydrogen embrittlement has been investigated by means of room temperature tensile tests in air and with hydrogen charging in both fully ordered and disordered conditions. Pre-charging with hydrogen and simultaneous charging with hydrogen resulted in a fracture morphology which ranged from a mixed transgranular/intergranular mode to a totally intergranular depending on the type of exposure to hydrogen and the degree of ordering. Associated reductions in the tensile elongations as well as in the ultimate tensile strengths were also observed. Hydrogen diffusion along the grain boundaries led to intergranular fracture concentrated near the surfaces of the alloy when cathodic pre-charging conditions were used, with a deeper penetration due to higher hydrogen diffusivity in the case of the disordered alloy. Susceptibility to hydrogen embrittlement has been found to be largely promoted by ordering under simultaneous charging conditions with a resulting totally intergranular fracture. The results suggest that enhanced diffusitivity of hydrogen to grain boundaries, associated with dislocation transport, occurs in combination with planar slip in the ordered alloy.

Effect of Copper Ions in the Slurry on the Chemical‐Mechanical Polish Rate of Titanium

Titanium is being investigated as the adhesion promoter and diffusion barrier between silicon dioxide and copper in a copper metallization scheme. Chemical-mechanical polishing (CMP) is being used to define the inlaid copper interconnections. An investigation into the CMP of titanium has revealed an interaction between the presence of copper ions in the polish slurry and the polish rate of titanium. The polish rate of titanium increases dramatically when copper ions are present in the slurry from the previous copper polish step. In this paper, we present and discuss the results of these investigations

A study of electroless nickel coatings containing low phosphorus

Abstract The structure and composition of as-deposited and heat-treated electroless nickel coatings, plated from an alkaline hypophosphite bath, were studied. The microstructure of the as-deposited coatings consists of a microcrystalline nickel matrix with 2%–3% phosphorus in supersaturated solid solution. The phosphorus is uniformly distrubuted throughout the coating. The microstructural changes which occur with increasing heat treatment temperature eventually transform the coating into a larger grain size nickel matrix with Ni3P precipitates. After heating to 400°C many small coherent precipitates form in the matrix, resulting in significant dispersion hardening. At 600°C incoherent Ni3P precipitates develop in the nickel matrix, together with a precipitous drop in hardness. The plating of the electroless nickel coating is found to begin with nickel nucleating on small islands of the palladium catalyst material. Once the nickel nucleates it spreads laterally over the substrate, quickly forming a continuous coating with a {111} preferred orientation.

Hydrogen embrittlement of amorphous alloys based on iron and nickel

Abstract Hydrogen embrittlement of amorphous alloys based on iron and nickel was examined in constant extension rate tests during cathodic polarization. Tests were carried out at a current density of 50 A·m −2 in 0.1 M H 2 SO 4 with and without addition of NaAsO 2 and NaCl. Hydrogen permeation and polarization curves were measured. Differences in the degree of hydrogen embrittlement were ascribed mainly to differences in the entry of hydrogen into the alloys. It was suggested that a low degree of embrittlement resulted largely from a hindrance of hydrogen entry. For alloys containing phosphorus, this might be associated with phosphorus-containing oxyanions and salts. For the alloy with higher silicon content it may be associated with the formation of oxides of silicon.

The effect of surface dissolution on fatigue deformation and crack nucleation in copper and copper 8% aluminum single crystals

Abstract The effect of surface dissolution on fatigue deformation and crack nucleation in copper and copper-aluminum single crystals has been studied. It is shown that surface dissolution during cyclic deformation results in an increase in fatigue life, delay in crack initiation, an increase in slip offset height and density, and blunting of slip band cracks. Transmission electron microscopy studies of near surface areas of copper specimens shows that dissolution disrupts the normal cyclic deformation generated dislocation substructure in these regions suggesting surface softening. A model of interaction between emerging strained metal associated with dislocations and the dissolution process is suggested to explain observed results.

Effect of Chemical Composition on Adhesion of Directly Electrodeposited Copper Film on TiN

Copper citrate (Cu-Cit) baths are effective mediums for seedless electrochemical deposition (ECD) of copper with strong adhesion property on TiN. The chemical equilibrium diagrams of chemical fraction vs pH were used to explain the cause of good adhesion of copper deposits on TiN. The kind and the amount of Cu-Cit complexes in a bath were determined according to pH and the concentration ratio of copper and citrate, [Cu 2+ ]/[Cit -3 ]. It was found that CuCir among various Cu-Cit complexes participated in adherent copper reduction on TiN. When more Cu 2 (Cit)OH than CuCir was contained in a bath, the reduction of CuCir was hindered and the adhesion property of copper on TiN became degraded. Resultant ECD copper films on TiN, which were deposited in baths of [Cu 2+ ]/[Cit 3- ] = 1 in the existence of CuCit - passed the ASTM D 3359 adhesion test showing strong adhesion.