Hariklia Deligianni

Damascene Copper electroplating for chip interconnections

Damascene Cu electroplating for on-chip metallization, which we conceived and developed in the early 1990s, has been central to IBMs Cu chip interconnection technology. We review here the challenges of filling trenches and vias with Cu without creating a void or seam, and the discovery that electrodeposition can be engineered to give filling performance significantly better than that achievable with conformal step coverage. This attribute of superconformal deposition, which we call superfilling, and its relation to plating additives are discussed, and we present a numerical model that represents the shape-change behavior of this system.

In situ surface pH measurement during electrolysis using a rotating pH electrode

An in situ technique has been developed for measuring the surface pH adjacent to a solid electrode/liquid interface during electrolysis. Measurements of the surface pH can be used to obtain insights regarding the electrodeposition of various transition metals and to obtain a better understanding of associated in situ surface chemistry effects. Many transition metals and alloys deposit with simultaneous hydrogen evolution and, as a result, are accompanied by a pH rise near the cathode, thereby affecting the reactivity of the nearby metal-ion species. Measurements of the surface pH of a solution containing simple salts during hydrogen evolution from a cathode were performed. The surface pH of a cathode during Ni and NiFe electrodeposition was also measured. The experiments demonstrated that, in the absence of buffers or metal ions, the surface pH rises many pH units above the bulk value. During Ni and NiFe electrodeposition, however, the surface pH of solutions consisting of simple salts and starting from a bulk pH level of 2 does not increase more than 3 pH units from the bulk value. In the case of Ni and NiFe electrodeposition, surface buffering occurs because of the hydrolysis of the metal-ion species present. Additionally, it is found that during the anomalous codeposition of NiFe, the surface pH is much lower than that required by the Dahms-Croll hypothesis.

Effect of Additives on Shape Evolution during Electrodeposition II. Parameter Estimation from Roughness Evolution Experiments

Numerical analysis procedures are used to identify the most sensitive parameters and estimate their values based on experimental measurements and a hypothesized mechanism for the effect of additives on copper electrodeposition. The electrolysis system consists of an acid copper sulfate bath containing accelerator, suppressor, and leveler additives in the range of concentrations typically used for deposition of on-chip wiring interconnections. Experimental measurements, obtained with an impinging jet apparatus, included surface roughness evolution and potential-time curves under constant current deposition in a set of 36 electrolyte compositions, selected by a D-optimal design procedure. The hypothesized additive mechanism consisted of a set of 15 surface reactions and 3 homogeneous chemical reactions. A mathematical model was composed that couples kinetic Monte Carlo simulation (for roughness evolution) with a finite volume method (for computing surface concentrations and potential). After identifying the most sensitive parameters and estimating their values, the surface concentrations of key species associated with the additive mechanism were simulated. The methods reported here are reusable and may be applied to other applications beyond the copper electrodeposition used here for its initial development.

Electrochemical Fabrication of Mechanically Robust PbSn C4 Interconnections

Electrochemical fabrication of PbSn C4s (controlled collapse chip connection) offers significant cost, reliability, and environmental advantages over the currently employed evaporation technology. A continuous seed layer is required for through-mask electrodeposition of the solder alloy. This layer becomes the ball limiting metallurgy (BLM) for the solder pad after etching. The seed layer metallurgy and the BLM etching are crucial to obtaining mechanically robust C4s. In the present study, the issues related to the selection of seed layer metallurgy, uniformity of plating and etching, and mechanical integrity of C4s have been investigated. The results demonstrate the feasibility of electrochemically fabricating highly reliable PbSn (97/3) C4 structures with a high degree of dimensional uniformity on a variety of wafer sizes ranging up to 200 mm.

Simultaneous fabrication of RF MEMS switches and resonators using copper-based CMOS interconnect manufacturing methods

This paper describes the successful concurrent fabrication of micro-electro-mechanical (MEM) electrostatic switches and resonators on the same wafer. Base processes from copper interconnect technology were used to fabricate devices allowing for easy introduction of MEMS technology into CMOS IC manufacturing. Both switches and resonators were electrically tested in a controlled ambient to determine performance and characteristics.

Alloying of a Less Noble Metal in Electrodeposited Cu Through Underpotential Deposition

Underpotential deposition of Pb or Sn on Cu can be used to produce electroplated Cu-Pb and Cu-Sn alloys, with small amounts of alloyed Pb and Sn, from acid solutions that do not contain complexants. Such alloys are of interest as possible on-chip wiring for very large scale integration ; the content of alloying agent must be kept small in order to maintain a low resistivity. The primary requirement for the formation of the alloys is that the deposition process occur in the range of underpotential deposition (UPD) ; this requirement can be met in solutions of methane sulfonic acid (MSA). Both Pb(II) and Sn(II) depolarize Cu deposition from MSA ; the Sn(II) is the weaker depolarizer. Thus, in the absence of other additions to the Cu(II)/MSA solution, electrodeposition proceeds in the UPD range in the presence of Pb(II), producing a Cu-Pb alloy, but positive to the UPD range in the presence of Sn(II), producing pure Cu. The deposition potential can be manipulated by addition agents in solution. A direct relationship exists between the potential and the amount of incorporated Pb or Sn for all solution compositions and plating conditions, provided that the solution does not contain strongly adsorbing species that interfere with the UPD process. The minor component has been shown to be incorporated in the metallic state, even though the deposition potential is positive to its reversible potential.

Single-Crystalline Germanium Thin Films by Electrodeposition and Solid-Phase Epitaxy

Single-crystal germanium films on silicon were prepared by electrodeposition and solid-phase epitaxy. The germanium films were amorphous as-deposited and crystallized into single crystals or polycrystals, depending on the cleanliness of the Si substrates. The low deposition temperature, the ease of thickness control, and the inherit advantage of spatial selectivity of the electrodeposition process make this method a promising way to selectively grow high-quality germanium for device applications.

An alternative low resistance MOL technology with electroplated rhodium as contact plugs for 32nm CMOS and beyond

This paper addresses a critical CMOS challenge of increasing parasitic resistance by introducing electroplated rhodium (Rh) as an alternative middle-of-line (MOL) metallurgy to replace the conventional CVD tungsten (W) processes for lower contact resistance and better extendibility to 32 nm technology and beyond. Electroplating of Rh is shown to have similar to Cu superconformal filling capability, allowing us to successfully fill high aspect ratio vias (40 nm times 240 nm). Plating of 300 mm wafers with 60 nm times 290 nm vias was demonstrated using CVD or ALD ruthenium (Ru) as the seed layer. An annealing process was developed to obtain a thin Rh film resistivity of 6.5 muOmega-cm, which is 1.5 to 3X lower than the resistivity of CVD W films. Since Rh is stable in Si environment, when compared to a fast diffusing Cu, a very thin Ti/Ru layer can be implemented. Therefore we propose to use PVD Ti/ALD Ru/electroplated Rh as the alternative MOL metallurgy. With this simple liner/seed/fill stack, the overall MOL resistance is calculated to be 2x lower than the overall MOL resistance of the conventional W stacks, and slightly lower than Cu fill stacks. In addition, the ability to use a thinner liner layer than that used for Cu-base fill process, provides a greater potential for extendibility of Rh fill into future CMOS MOL generations.

Effect of benzotriazole on the anisotropic electrolytic etching of copper

Electrolytic etching of copper foil at the base of cavities formed by patterned photoresist was investigated in 0.5 M sulfuric acid solutions which either contained 40 mM benzotriazole (BTA) or were free of BTA. It was found that undercutting (metal dissolution beneath the photoresist) was minimized by the action of surface films in both solutions. It was also found that the nature of the surface films and the mechanism by which they enhanced etch anisotropy differed. In additive-free solutions, anisotropic etching was observed under conditions of applied potential and flow for which mass transfer was suppressed in the interior corner regions of cavities. Such operating conditions in additive-free solutions displayed characteristic current transients. In BTA-containing solutions, the etch profiles were highly dependent on applied potential. In contrast to the additive-free solutions, the flow conditions in BTA-containing solutions had little effect on the current transients or on the degree of undercutting within the region of applied potential in which anisotropic etching was achievable.

Electrodeposition of Cu on Ta-Based Layers I. Electrodeposition on Ta

The nucleation and growth of copper directly electroplated on Ta is studied in this paper using electrochemical methods. It is shown that little Cu deposition on Ta occurs from an acid sulfate copper plating electrolyte, while a deposited nuclei density of ∼ 10 9 cm -2 is possible using a basic citrate electrolyte. Experimental results indicate that when plating directly onto Ta, besides a high current efficiency, the nucleation characteristics of an electrolyte are important. Using a basic citrate chemistry, deposition currents were found to be higher on Cu than on Ta, suggesting that when both are present, deposition occurs preferentially on Cu. Because the relative deposition rates are a function of applied potential, deposition on Ta instead of Cu may be encouraged by the judicious control of this deposition parameter.