Green Manufacturing of Silyl-Phosphate for Use in 3D NAND Flash Memory Fabrication

Abstract

Three-dimensional NAND flash memory featuring dozens of vertically stacked memory cells is the state-of-the-art technology for most storage platforms. To fabricate threedimensional (3D) NAND memory, lateral etching of the Si 3 N 4 layer over SiO 2 is an essential step that is conducted through a wet etching process using a phosphoric acid-based etchant. Silylphosphate or highly selective nitride serves as an etching solution additive to control the SiO 2 layer dissolution rate. However, silylphosphate is prepared with an expensive monomeric silica precursor and at high reaction temperatures and generates environmentally harmful byproduct gases, such as HCl, HF, and CH 3 OH. This study demonstrates that silyl-phosphate can be prepared using low-cost polymeric silica under a mild reaction temperature by changing the characteristic acidity of phosphoric acid. The possibility of tuning the phosphoric acid acidity was first studied by molecular dynamics simulations, and phosphoric acids with stronger acidity were prepared by the evaporation of water from H 3 PO 4 (85%). The concentrated phosphoric acid enabled a fast reaction of polymeric silica and phosphate at a low reaction temperature (80 °C). The obtained silyl-phosphate lowered the SiO 2 layer dissolution rate, thereby yielding a Si 3 N 4 /SiO 2 layer etching ratio of up to 940. The proposed method offers an environmentally friendly production process for special chemicals used in 3D NAND flash memory fabrication. KEYWORDS: 3D NAND, concentrated H 3 PO 4 , fumed silica, Si 3 N 4 layer, SiO 2 layer, wet etching

Nanoscale self-assembly: concepts, applications and challenges

“…For example, the use of complex patterning of nanocrystal films and quantum dots in photonics [483][484][485], with applications such as fabrication of high resolution and flexible displays [486], directional emission using waveguides [15,487], and for improved plasmonic sensing using gold-coated optical fibres [488,489]. In addition, nanoscale patterning is used in a number of applications involving miniaturization of ICs and devices such as FinFETs [490], in 3D integrated circuit applications such as dynamic random access memory (DRAM) and 3D NAND [491][492][493], fabrication of superlattice layers for three dimensional, selfaligned stacked nanoribbon transistors [494], and the use of block copolymers for the development of photonic band gap materials [495] and directed self-assembly (DSA) (discussed further below).…”

“…Figure 125(c) shows step-by-step details for the fabrication of this type of nanosheet transistor. Such 3Dintegration taking advantage of the vertical dimension is also used extensively for modern high-density solid-state memories, including ALD for high-aspect ratio DRAM storage capacitors [513] and 3D NAND Flash ICs and solid-state drives with well over 100 stacked layers requiring intricate growth and etching steps [492,514].…”

Selectivity between SiO2 and SiNx during Thermal Atomic Layer Etching Using Al(CH3)3/HF and Spontaneous Etching Using HF and Effect of HF + NH3 Codosing

“…This challenge motivated the current study of selective etching of silicon dioxide (SiO 2 ) with the retention of silicon nitride (SiN x ), and the reciprocal challenge of selective SiN x etching while retaining SiO 2 . This etch selectivity should be useful for the selective removal of sacrificial SiO 2 or SiN x in composite structures that are employed for fabricating devices such as 3D NAND. , …”

“…This etch selectivity should be useful for the selective removal of sacrificial SiO 2 or SiN x in composite structures that are employed for fabricating devices such as 3D NAND. 6,7 Etching can be accomplished by wet or dry methods. Wet etching in solution can suffer from pattern collapse due to the forces of capillary action.…”

“…Other studies have achieved selectivity between SiO 2 and SiN x using wet or plasma etching approaches. 6,7,12,32,33 The present work examined the selectivity between SiO 2 and SiN x for thermal ALE using TMA and HF as the reactants. This study also focused on the spontaneous etching of SiO 2 versus SiN x using HF alone.…”