Mechanisms of Area Selective- Atomic Layer Deposition and Their Impact on Feature Sizes

As devices have continued to scale down in size, fabrication of smaller features using novel methods has become a necessity. Strict requirements of location and alignment are often necessary and difficult to achieve with prior techniques, which has led to studies of area selective deposition. Area selective deposition, such as area selective-atomic layer deposition (AS-ALD), can be achieved by manipulating deposition preferences for different materials. A material is deposited selectively at certain locations based on the interaction of the precursors with different surfaces, so etching is not usually necessary and sometimes lithography is not necessary. AS-ALD can take advantage of the preferential deposition of the ALD precursors onto different types of materials like oxides or metals, etc. In fact, these techniques can sometimes be the only option for patterning different materials if the geometry of the surface or substrate cannot be patterned with lithography. Also, some SAM or polymer masks that have very low deposition can be patterned onto a surface so that AS-ALD can be used to pattern a single material (since the mask is later removed). This technique is especially useful for patterning materials or substrates that may be sensitive to etching. However, the selectivity between two different materials under certain deposition parameters limits the use of some materials and ALD precursors. Thus, a deeper understanding on the mechanism of area selective deposition is necessary to understand limitations on feature sizes.<br/>This study describes the mechanisms for area selective- atomic layer deposition of common oxide materials (e.g., TiO₂ and HfO₂) on PMMA and Si and the effects of these deposition mechanisms on feature sizes. PMMA was patterned onto Si using electron beam lithography with feature sizes ranging from 5 µm down to 60 nm. ALD of TiO₂ and HfO₂ were studied to understand the selectivity of the deposition on Si vs PMMA and how that selectivity affected feature size dimensions and film thickness. The mechanisms of deposition were studied by characterizing the extent of ALD deposition before and after PMMA removal. TiO₂ was not detected on the surface of the PMMA, but after PMMA removal there was a very small amount of deposition in the location that had been coated by PMMA. It was determined that the ALD precursors are likely able to diffuse into the PMMA and still deposit at the interface between the Si and PMMA, even if it is only a trace amount. Thus, ALD of TiO₂ is highly selective for Si in comparison to PMMA; however, the effects of the PMMA side walls inhibit deposition so that the dimensions of the TiO₂ feature is smaller than the PMMA pattern. This side wall inhibition significantly affects possible feature sizes using TiO₂ and PMMA patterns. In contrast, HfO₂ is less selective than TiO₂ and demonstrates a mechanism combining selective deposition and lift-off. This lower selectivity limits possible HfO₂ thicknesses before there is blanket coverage, but it also exhibits less side wall inhibition. Significantly smaller feature sizes were obtained with HfO₂ compared to TiO₂ in these ALD conditions. Though the ALD parameters could be optimized further, these results suggest that the deposition mechanism itself, whether it is a truly area selective deposition or a combination of area selectivity and liftoff will always affect possible feature sizes.