Bottom-up Filling of Damascene Trenches with Cobalt By Electroplating Process

1.      Introduction:

In advanced semiconductor chips, filling of the circuit such as contact vias, trenches and conductive interconnects typically consists of Cu due to its high electrical conductivity. However, Cu can diffuse into Si and SiO₂and form silicides to alter the properties of the circuit. The Cu diffusion can be much severer in small critical dimensions (CD < 10nm) as it can cause an electrical connection between two interconnects, resulting in an electric shot to damage the circuit.

Cobalt with low diffusion coefficient in Si and SiO₂ was considered as a filling material to replace Cu. In this study, Co filling was obtained by electroplating at a current density of 6.25mA/cm² in a CoSO₄electrolyte.

For the first time, Co bottom-up and conformal growth in the damascene trenches with a CD range of 48-130nm can be achieved by electroplating in the Co electrolyte with different additives.

The current efficiency, morphology, resistivity, uniformity, reflectivity and the deposition rate of the Co film on the blanket coupons (with Co 150Å) were also determined in this study.

2.      Experimental:

The plating electrolyte was made of 120g/L CoSO₄.7H₂O, 30g/L H₃BO₃ and 50mg/L Cl^- at pH<4. The effect of different additives such as CUPUR CSFX, CUPUR DTX and CUPUR DTK was examined as a function of various concentration.

The electrochemical method reported by Broekmann et al [1] was used to characterize the additives. The electrochemical experiment was performed in a three-electrode cell with a Co rod as anode, a standard Ag/AgCl reference electrode and a Pt RDE as cathode.

Galvanostatic plating with the current density range of 6-12 mA/cm²was used for the Co deposition.

3.      Results and Discussion:

3.1 Polarisation/depolarisation behaviours of the additives

Figure 1 shows the electrochemical transient curve of the Co electrolyte and additives. The cathodic overpotential of the Co electrolyte was increased by dosing CUPUR DTX, indicating a polarisation occurred during the plating process. The cathodic overpotential of the Co electrolyte was gradually decreased by dosing CUPUR CSFX, indicating a depolarisation occurred during the plating process. We also found the effect of [Co²⁺] and [H₃BO₃] on the polarisation/depolarisation was not obvious, but the depolarisation of CUPUR CSFX was faster by increasing the concentration of [Cl^-].

Figure 1 Electrochemical transient measurement for Co electrolyte, CUPUR DTX and CUPUR CSFX.

3.2 Partial fill results

Different Co filling structure can be obtained by adding different additives. Bottom-up Co filling in the 130nm trenches (with Cu seeds) shown in Fig 2(a) was obtained by plating in the Co electrolyte with CUPUR DTX and CUPUR CSFX.  Superconformal Co filling shown in Fig 2(b) was obtained by plating in the Co electrolyte with CUPUR DTK and CUPUR CSFX.

Figure 2 SEM image of Co partial fill in the 130nm damascene trenches (a) bottom-up filling; (b) superconformal with V-shape filling.

Further study will be focused on the Co filling performance in the trenches with Co seed.

4.      Reference:

1. P. Broekmann, et al, Electrochimica Acta 56 (2011) 4724.