The Silicon Precursor Toolbox for Low-temperature Deposition
The drive toward making electronics faster, denser, and cheaper continues unabated.
The drive toward making electronics faster, denser, and cheaper continues unabated. Shrinking device dimensions and changes in structure place additional demands on the materials used in all steps of semiconductor processing, including depositing silicon nitride (SixNy, or SiN) and silicon oxide (SiO2) films. With horizontal dimensions of transistors already near their lower limit, the path forward for Moore’s Law requires building upward. Increasing use of FinFET transistor structures and 3D NAND memory devices is driving the move from planar coatings on horizontal surfaces to conformal coatings on vertical and topologically complex surfaces. Aspect ratios are growing to the point where conformal coating performance is becoming a potential roadblock.
Silicon nitride and oxide films serve two primary types of functions in semiconductor device fabrication. Some are used for patterning, and others are used for electrical insulation. Within these broad categories, each application comes with a slightly different set of challenges. In this white paper, we explain the role of precursors in depositing highquality silicon-containing films under a wide range of challenging conditions.
SILICON-CONTAINING FILMS USED FOR PATTERNING
SiN and SiO2 function as disposable films for patterning layers in semiconductor devices. Self-aligned double and quadruple patterning, which is becoming increasingly common, is an effective method of creating extremely fine lines. This approach uses films deposited on the side-walls of patterned structures to circumvent the dimensional limitations of photolithography, fabricating narrower lines than individually patterned layers can achieve using 193 nm photolithography. The deposited side-wall layer is used as a mask to pattern underlying layers during subsequent etch steps. When the sacrificial layer is etched away, whether by a dry or wet etch process, the underlying layer has received the pattern at twice the spatial density of the structure before deposition of the patterning film. With quadruple patterning, line widths are one-quarter of the width that would be possible without implementing multiple patterning steps.
There are two primary issues with these side-wall deposited films used for patterning. The first is etch selectivity and the second is step coverage, sometimes called conformality. Etch selectivity occurs when the film used for patterning resists etching in a specific environment while the layer below is receiving the desired pattern by etching more quickly. If the side-wall film is lacking resistance to this etch condition, then it will need to be deposited over a higher aspect ratio structure so that it is effectively thicker during the etch step. However, uniform deposition is more difficult over the higher aspect ratio structure. When the patterning function is complete, the layer needs to be easy to remove.
The composition and the density of the deposited film controls the etch selectivity. Excess hydrogen in the form of terminal Si-H, O-H, or N-H bonds tends to decrease the density of the film, making it susceptible to attack by both dry and wet etchant species. Carbon can be added to oxide or nitride films to increase their resistance to fluorine or HF attack, but terminal CH3 groups can be vulnerable to dry etchants that contain active oxygen. Silicon rich nitrides have been used for enhanced resistance to HF wet etchants.
The second challenge is achieving appropriate step coverage during film deposition as aspect ratios increase. Ideally, all surfaces of each trench should be coated uniformly, but if the process and precursor are not properly matched and optimized, the film will preferentially coat the bottom and top surfaces before completely coating the sidewalls. To achieve a precise pattern transfer, the film’s etch rate should be the same on the top and the sidewalls. Proper control of film deposition can create films with uniform step coverage and sufficient thickness, but as aspect ratios and the number of layers increase, it becomes more difficult to achieve a high-quality conformal coating.