# Recent Advances in Flexible Resistive Random Access Memory
## 4 Fabrication Process of Flexible RRAM  
The typical steps in making flexible RRAM are shown in Figure 13:  
As a typical film electronic device, the fabrication of flexible RRAM mainly relies on various film deposition methods. Generally speaking, the fabrication process of flexible RRAM is divided into dry methods (DMs) and wet methods (WMs) [136]. DMs are mainly vacuum evaporation, including chemical vapor deposition (CVD) [137; 138], physical vapor deposition (PVD) [22; 139] and molecular beam epitaxy (MBE) [140; 141]. These methods are less affected by impurities because they are operated in a vacuum environment. In the process of industrialization, the flexible film can be improved by changing the gas flow rate, gas pressure, ambient temperature and operating voltage. However, these methods have high costs, long synthesis times and difficulty in mass production; hence the emergence of WMs. WMs include coatings (spin coatings [142], spraying [143] and casting [144]), printing (screen printing [145], inkjet printing [146; 15] and nano imprinting [147]) and immersion (dipping method [142], Langmuir-Blodgett (LB) method [148] and self-assembly [88]). WMs are inexpensive and easy to manufacture in large areas, but these methods often introduce impurities, resulting in less uniform films and less flexibility. On the other hand, they are difficult to graph, making them difficult for operations that require high precision and resolution. Table 4 below summarizes the advantages and disadvantages of each method: \begin{table}
\begin{tabular}{c c c} \hline \hline
**Methods** & **Advantages** & **Disadvantages** \\ \hline CVD [137,138] & Pure film and good density and uniformity & High temperature of reaction, difficulty in mass production and toxic byproducts \\ \hline PVD [22,139] & Low temperature of reaction and high degree of process automation & Poor uniformity and difficulty in mass production \\ \hline MBE [140,141] & Pure film, grows at low temperatures and has atomic interfaces & High cost and difficulty in mass production \\ \hline spin coatings [142] & Uniform film thickness & Mass solution loss \\ \hline spraying [143] & No solution loss and quick film forming & Difficulties in process optimization of solvent mixing and solution adjustment \\ \hline casting [144] & High degree of crystallinity & Poor uniformity and long molding cycle \\ \hline screen printing [145] & Simple manufacturing process and wide range of application & The large thickness is not conducive to miniaturization \\ \hline inkjet printing [146,15] & Automatic control, small material loss, rapid production of multilayer structure and no selectivity for substrates & High requirements for ink conditions and droplets coalesce easily \\ \hline nano imprinting [147] & Composition of tens of nanometers can be achieved under normal temperature and pressure & Immature technology and expensive equipment \\ \hline dipping [142] & Large area of film forming & Large thickness relatively and inability to perform fine manipulation \\ \hline LB [148] & Single-layer controlled production and nanoscale accuracy & Corresponding materials are limited and poor mechanical properties \\ \hline self-assembly [88] & Highly oriented monolayer and quick film forming & High requirements of process and strict conditions of preparation \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Advantages and disadvantages of various film deposition methods. Figure 13: Typical steps in making flexible RRAM. In recent years, some new fabrication methods have been developed, such as atomic layer deposition (ALD) [9; 17; 149; 150], pulsed laser deposition (PLD) [151; 152], layer-by-layer (LBL) [108; 108], the sol-gel process (SGP) [21; 85; 92] and drop-on-demand (DOD) [154]. ALD can be used for the deposition of metal, dielectric, polymers, etc.