Reason and Role of Atomic Layer Deposition for PCRAM and ReRAM Films
Authors: Karim Zia , Song Kay , Yang Lin , Lu Brian 
Published: February 2020

With a 13.4% CAGR, the demand for diverse memory applications in mobile phones, digital cameras, MP3 players, video game consoles and automobiles are exploding rapidly. To keep up with the diverse applications of memory devices, a number of next generation memory options are being studied, such as STTRAM, PCRAM and ReRAM. Most promising solutions for future non-volatile memories replacing conventional NAND are Phase Change Memory (PCRAM) and Resistive Memory (ReRAM).

The growth of complex thin films through atomic layer deposition (ALD) is finding a broad range of applications for the production of next-generation memory devices. Conformal coating or gap-fill of high-quality thin films helps to mitigate multiple technology challenges encountered in new material development for high aspect ratio device structures. Similar step-coverage and composition controls are needed for fabricating next-generation non-volatile memory devices including phase-change memory (PCRAM) and resistive memory (ReRAM).

This presentation will introduce a multi-wafers ALD system, QXP-8300 equipped with liquid injection vaporizers integrated with close-coupled showerhead in a small volume confined process space. This paper will also discuss recent new developments in equipment design to meet the technology needs as well as practical manufacturing targets (such as throughput and cost of ownership) in order to provide a production-worthy ALD solution. Applications in new memories such as PCRAM materials (such asGST), and O-vacancy controlled ReRAM materials will be reviewed. The formation of such engineered materials often requires chemical precursors that are low vapor-pressure liquids or even solids, posing a significant engineering challenge for production equipment design. With the help of thermal and flow modelling, designs are optimized for precursor evaporation, vapour delivery, and reactor geometry in order to maximize step-coverage and minimize ALD cycle time. Innovative platform architecture further improves system productivity, reduces defects, and enhances overall system cost of ownership. Results on both unit process and electrical characteristics will be presented for multiple ALD films generated from such a high-volume production system.

Among the various candidate PCRAM alloys, GexSbyTez (GST) is the most promising because of its good scalability, low cost, and compatibility with conventional CMOS process. Conventional PCRAM had been researched and developed using physical vapour deposition. As devices scale down to 20 nm and beyond, confined and conformal GST film deposition into hole/trench structures is required, using ALD processes. However, to ensure good gapfill in a sub-20nm structures by ALD process, choice of precursors and precursors’ delivery challenges are far greater and require unique precursor delivery methodology to ensure good void-free gapfill while maintaining higher productivity. This paper highlights the progress made in ALD equipment development offering solutions for critical problems facing sub 20nm ALD PCRAM technology nodes. Using vaporizers suitable for Ge, Sb and Te precursors, ALD GST films were deposited in a Multi-wafer ALD system characterized by Small volume reactor and Closely coupled showerhead process. In this presentation, results are presented showing progress in GST film deposition into sub 20nm structures. ALD GST films were deposited under process conditions which achieved void-free gap-fill. By controlling injection dose and other parameters, such as temperature, pressure, etc., GST composition can be varied from Sb rich to Te rich in the films. Electrical characterization showing reset current <150uA due to confined cell enabled by ALD will be presented in this paper.

Similarly, binary films of HfO2, HfO2/TiO2 and doped HfO2 deposited by ALD process showed lower forming voltage and high resistance ratio for ReRAM application. ALD can be used not only to tailor the composition of the films, it can also be utilized to conformally deposite multi-layers binary films for 3D ReRAM cross-point structure.