(Invited) Supercritical Drying: A Sustainable Solution to Pattern Collapse of High-Aspect-Ratio and Low-Mechanical-Strength Device Structures

Collapse of nanostructures with high aspect ratio (STI, 3D NAND, capacitor, FinFET, etc.) and/or low mechanical strength (ultra low-k trench, EUV resist pattern, etc.) during drying step in wet process has become a critical issue in semiconductor industry.  Even current workable drying practices are facing steeply rising challenges from rapid device scaling advancements.  Imbalanced capillary pressure induced at the liquid-gas interface contributes to line distortion during conventional drying process.  Commencement of pattern collapse or stiction depends not only on structure aspect ratio, but also on material strength.  Figure 1 depicts non-stiction limit of conventional liquid drying with respect to feature critical dimensions and device material strength.  Regular 2x STI trench structures start leaning at aspect ratios 17 or higher, while pattern collapse can be observed on ultra low-k features (k ~2.55) with aspect ratio as low as 5 due to porous nature of the material. 

The objective of this work, following evaluation of multiple advanced drying technique in previous publication (H.-W. Chen et al., ECS Trans. 2013 58(6): 205-211), is to develop a sustainable non-stiction drying solution incorporating supercritical fluids.  Supercritical fluids exhibit both liquid-like (transport/ dissolution) and gas-like (low viscosity/ negligible surface tension) properties, leading to effective extraction of wet clean chemicals from nanostructures with extremely low capillary force for stiction-free drying.  As an example, the principle of supercritical drying is illustrated in Figure 2 with Carbon Dioxide, which has been preferably employed for commercial and industrial practices due to its high stability, insignificant toxicity, and low environmental impact.  The process started with liquefied CO2 to mix and displace chemicals from the normal wet process.  Subsequently, both temperature and pressure are elevated to establish supercritical state for CO2 (30.8°C, 73 bar) in the system.  Isothermal depressurization then follows to transition the CO2 from supercritical state back to gaseous phase for drying without surface tension.       

A comparison of our approach with other drying schemes showed that the integrated supercritical drying method is superior in non-collapse performance to all conventional (e.g. spinning, solvent-assisted, etc.) and advanced (self-assembled monolayer formation, sublimation, etc.) practices.  Figure 3 presents the study results of various drying technique containing sublimation drying (~90% stiction), SAM (self-assembled monolayer) for surface tension modification (~60% stiction), solvent-assisted Marangoni drying (~40% stiction), and supercritical CO2 drying (no stiction).

Leaning-free performance have been consistently demonstrated with our supercritical drying sequence on 2x NAND STI (trench aspect ratio ~20) and low-k trench (aspect ratio ~6.0) structures, where pattern collapses were observed under conventional solvent-assisted drying or even with advanced SAM approach. Figure 4 exhibits non-stiction results of complete integration of standard aqueous chemical clean (HF-SC1-SC2), DI water rinse, and supercritical drying.  We also demonstrated de-stiction as shown in Figure 4(b), where light leaning of lines prior to the process flow was totally released.  Collapse-free drying on 2x NAND STI structures was further verified on 300mm wafers with 36-spot inspection (Figure 5).  Extendibility to ultra low-k (k~ 2.55) features was successfully achieved as displayed in Figure 6, where supercritical drying clearly presented stiction-free performance over simple N2 blow dry. 

Particle adder performance was also evaluated in this work.  We found that cleanliness of process chemicals and hardware, suppression of flow turbulence, and control of depressurization sequence are critical to particle reduction.  Figure 7 sketches particle adder data of the integrated supercritical drying process.  With background particles subtracted, in average < 50 adders @0.09μm was attained from a mini-marathon fulfilled.  Metallic contamination analysis with TXRF also demonstrated all metal components interested were below detection limit (Table 1). 

The significance of supercritical drying lies in its unrivalled extendibility to future technology nodes due to the negligible surface tension.  This unique drying scheme not only sustains continuous device scaling, but also enables respective material innovation, e.g., ultra ultra low-k (k ≤ 2.2) features, EUV resist patterning, DSAL (directed self-assembly lithography) wet development, etc.  Supercritical drying has lately attracted great attention due to the eager demand for pattern leaning suppression in the semiconductor industry.  As a result, this emerging technology is expected to play a prominent role as the ultimate solution to device structure collapse problem in the near future.