Accelerator Surface Phase Associated with Superconformal Cu Electrodeposition

Abstract

Superconformal film growth is a key process in state-of-the-art Cu metallization of electronic devices. Superfilling of recessed surface features results from the competition between electrolyte additives that accelerate or inhibit Cu electroplating. In situ scanning tunneling microscopy is used to image the accelerating bis-͑3-sodiumsulfopropyl disulfide͒ ͑SPS͒-Cl − surfactant phase that is responsible for disrupting and preventing the formation of the inhibiting poly͑ethylene glycol͒-Cl − layer. Various aspects of competitive and coadsorption of Cl − and SPS on Cu͑100͒ were examined for industrially relevant additive concentrations. At potentials associated with superfilling, a saturated, c͑2 ϫ 2͒ Cl − ordered adlayer forms on the surface. When as-received SPS is added, individual SPS and ͑3-mercaptopropyl͒sulfonate ͑MPS͒ molecules are imaged as a mobile two-dimensional gas diffusing on the Cl − adlattice. The SPS-Cl − surfactant accounts for many aspects of the additive function previously observed and stipulated by the curvature enhanced accelerator model of superconformal film growth. SPS-derived species of differing mobility and tunneling contrast appear with exposure time. The lattice gas species are sensitive to the imaging conditions with tip-molecule interactions particularly evident at higher tunneling currents. At negative potentials, the c͑2 ϫ 2͒ Cl − adlayer is disrupted by an order-disorder transition, followed by desorption at more negative potentials. This allows direct access of SPS to the Cu metal whereupon irreversible sulfide formation occurs.In the past decade, Cu electrodeposition has become the dominant technology for fabricating interconnects for microelectronic devices over length scales ranging from those of damascene processing of submicrometer logic and memory circuitry to through silicon vias ͑TSVs͒ for chip stacking and circuit board metallization. 1 Central to the success of this technology is its ability to generate voidand seam-free bottom-up filling of trenches and vias by additive derived superconformal growth. 2 The desired bottom-up superconformal growth mode derives from a competition between rateaccelerating and rate-inhibiting two-dimensional ͑2D͒ phases for the available surface area within filling features. Area change is a key aspect that accompanies deposition on any nonplanar interface. 3,4 Feature filling behavior is well described by a mass balance construct known as the curvature enhanced adsorbate coverage ͑CEAC͒ model. 3,4 In this model, the coverage of the more strongly bound accelerator surface phase increases during area reduction accompanying deposition within recessed features, thereby giving rise to the bottom-up superfilling dynamic. For a prototypical acid CuSO 4 electrolyte, the suppressing or inhibiting phase involves coadsorption of chloride with polyether, whereas the accelerating phase derives from the combination of chloride with either a sulfonate-terminated disulfide or thiol. The most widely discussed additive system to date is chloride ͑Cl − ͒, ...

Superconformal Electrodeposition for 3‐Dimensional Interconnects

“…Values scatter between 10 -13 cm 2 s -1 and 10 -16 cm 2 s -1 for the various observed species. [15] This is similar in magnitude to the transport parameters recently measured in video rate STM studies of individual S adatoms diffusing within the c(22) Cl -adlayer on Cu(100). [16] [15]…”

“…A variety of other species are also evident, ranging from larger clusters to individual species whose dimensions approach that of a single unit cell of the Cl -adlayer, the latter probably corresponding to the SPS thiol derivative MPS. [15] [17] tour indicates that the diffusion coefficient of the surfactant species must be 10 -13 cm 2 s -1 or less, congruent with the STM studies. [15] The plurality of SPS-derived species, and the nature and evolution of binding states, deserves further consideration.…”

“…[5] same time ensures that SPS behaves as a surfactant during metal deposition. [5,15] In-situ STM studies of SPS and Cl -co-adsorption at the same potential used in our feature filling work, but in the absence of Cu deposition, reveal individual SPS-derived species tethered to the four-fold symmetric c(22) Cl -adlayer of Cu(100), as shown in Figure 5a-c. [15] A schematic representation of the disulfide SPS lattice gas on the Cl-saturated surface is shown in Figure 5d. [15] Sequential images shown in Figure 6 reveal movement of the individual SPS-derived species.…”

“…[5,15] In-situ STM studies of SPS and Cl -co-adsorption at the same potential used in our feature filling work, but in the absence of Cu deposition, reveal individual SPS-derived species tethered to the four-fold symmetric c(22) Cl -adlayer of Cu(100), as shown in Figure 5a-c. [15] A schematic representation of the disulfide SPS lattice gas on the Cl-saturated surface is shown in Figure 5d. [15] Sequential images shown in Figure 6 reveal movement of the individual SPS-derived species. The weak binding of the lattice gas species leads to a notable sensitivity to the imaging conditions, as indicated by the sudden increase in the observed coverage revealed by stepping the tunneling current from 0.731 nA (Figure 6a) to 1.738 nA (between 11 s and 22 s) (Figure 6b).…”

“…(E Cu = -0.2 V, E tip = -0.1 V). [15] Figure 7. The Cl -adlayer serves as an internal standard to determine the dimensions and coverage of the SPS derived surface species (E = -0.2 V, E tip = -0.1 V, i tunnel = 2.051 nA).…”