Dual-Fluid Spray Process for Particle and Fluorocarbon-Polymer Removal in BEOL Applications
Recently the feature size of devices has decreased to <20nm, so the manufacturing process has become more complicated since the number of steps for the pattern technology and its complexity have increased. One of the most critical challenges for BEOL is a post-etch-residue cleaning in the trenches and vias. Not only the chemical step but also rinse step is required to completely remove fluorocarbon-based polymer residue (CF-polymer residue) in the structure.
Conventional dual-fluid spray is widely used to improve the cleaning efficiency; however, it has not been used on advanced fragile structures because the physical force can easily induce pattern damage.
According to a previous paper, smaller droplets at higher velocity are required to reduce pattern damage and to increase particle removal efficiency . In addition, the spray should be able to dissolve the residue.
In this paper, we have demonstrated a dual-fluid spray with chemical additive that dissolves the residue and has less pattern damage as an alternative rinse process.
In order to study the characteristics between de-ionized water (DIW) and its chemical additive (Chem. A), a Dynamic Direct Injection system that can mix them at point-of-use and Nanospray as a dual-fluid spray were used. Particle removal efficiency (PRE), CF-polymer removal efficiency (RRE) and distribution of droplets (size and velocity) were investigated. All tests were carried out with DIW, Chem. A and DIW-Chem. A mixtures in a conventional spin processor.
(1) SiN PRE vs. pattern damage
The number of removed particles was evaluated by an intentional contamination with 45- nm SiN particles on hydrophilic silicon wafer and measured by KLA SP-2. Pattern wafers with aspect ratio 6~7 were prepared as damage test samples. The pattern damage after Nanospray was counted by SEM observation.
(2) Residue removal efficiency (RRE)
CF-polymer (2x2 cm spots at 37-40 nm thick) was intentionally deposited on low-k material as a test vehicle. The polymer removal efficiency was evaluated by ATR-FTIR spectroscopy.
(3) Distribution of droplets (size and velocity)
The size (diameter) and velocity of droplets were measured by a droplet measurement system consisting of a laser, high speed camera and image processor.
Results and Discussion
(1) SiN PRE vs. Pattern damage
Fig. 1 shows result of SiN PRE and pattern damage as a function of several Nanospray conditions. The PRE of 100% Chem. A decreased from 50% to 20% compared to DIW; however, its additive mixtures (20% and 40%) showed almost the same PRE as DIW Nanospray. On the other hand, pattern damage was significantly reduced with increasing Chem. A concentration.
(2) CF polymer-RRE
As shown in Fig. 2, CF-polymer indicates absorbance of functional groups such as C-C, C=O and C-F at the range of 1500 ~ 1900 cm-1. The absorbance of additive mixtures (20% and 40%) or Chem. A Nanospray indicates lower intensity than DIW Nanospray which enables the removal of the CF-polymer by adding Chem. A.
(3) Droplets distributions
Fig. 3 shows distribution of droplet size as a function of Chem. A concentration. Droplets of DIW Nanospray were widely distributed in size range from 2 to 38µm. In contrast, droplets of Chem.A or its additive mixture Nanospray were much narrower distributed with increasing Chem. A concentration, more than 40% Chem. A Nanospray was achieved in size range from 2 to 17µm.By the addition of the Chem.A to the water, droplet size is decreased than DIW by the role of the surfactant of Chem.A. Therefore, the distribution of large droplets with high kinetic energy decreased and resulted in reduced pattern damage.
Dual-fluid spray process for post-etch-residue cleaning was developed. An optimized process could enable simultaneous higher PRE and CF-polymer RRE. The additive made smaller droplet size and higher velocity in addition to excellent CF-polymer dissolved characteristics compared to conventional DIW Nanospray.
 M. Sato et al, ECS Trans., 41(5) 75-82, 2011