Atomic Layer Deposition of Germanium Telluride Thin Films Using Intermediate Precursor Formation Method for Phase-change Memory Application

Abstract

Current information technology industry requires high speed, high density, low power consumption memory devices. However, the present semiconductor industry which is represented by dynamic random access memory (DRAM) and NAND flash, has reached the limit of scaling, thus, researches on next-generation memory has been continued. Phase-change random access memory (PRAM), which records data through the resistivity difference between amorphous and crystalline phase of phase-change materials (PCM) is one of the strongest candidate for next-generation non-volatile memory. The most widely studied materials for PCM are GeTe-Sb2Te3 pseudobinary materials for its fast phase transition, superior retention property, and low power consumption. Meanwhile, the early stage of researches on PCRAM has mushroom structure that forms a small electrode contact at planar PCM for its operation. However, the mushroom structured PCRAM requires improvement due to very low thermal efficiency and cross-talk issue between adjacent cells. Therefore, new structure was proposed that fills PCM into a small contact hole, called confined structure. The confined structure has become new standard of PCRAM by much higher thermal efficiency, improved cross-talk problem, and even strong resistivity on etch damage during device fabrication process. In order to fabricate a PCRAM device with confined structure, deposition process with excellent step coverage properties became important. By the requirement, atomic layer deposition (ALD) of PCM became necessary. Previous researches on atomic layer deposition of ternary GeSbTe materials left many challenging tasks. One of the most important issue at previous researches on GeSbTe ALD was the composition of the material, which lies on GeTe2-Sb2Te3 tie line rather than desired GeTe-Sb2Te3 tie line. The problem was originated from characteristics of the Ge-precursor used in the process, wherein +4 oxidation state to form GeTe2 by reaction of Te precursor with -2 oxidation state. Because the most stable oxidation state of Ge element is +4, there have been many difficulties in the development of Ge(II) precursors and the deposition process using the precursor such as polymerization of the precursor molecule by its chemical instability. In this work, novel processes for atomic layer deposition of GeTe films were suggested. In common for both processes, a newly suggested methods are used in which the form of the precursor and the molecules actually taking place in the deposition are changed. The first process was developed using Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Te as the Ge- and Te-precursors, wherein the Ge atom has +2 oxidation state. The Ge-precursor was introduced to chamber with methanol vapor to form intermediate precursor, Ge(OMe)2, which is more reactive, but has no long term stability as precursor, by gas phase reaction. The Te-precursor was also introduced with methanol vapor to form H2Te. The intermediate precursors described above played the role as precursors in the deposition to form GeTe films. Mechanism of chemical reactions in deposition process was studied. Combined process with previously settled Sb2Te3 deposition process using Sb(OC2H5)3 and ((CH3)3Si)2Te was also attempted. The second process using HGeCl3 and ((CH3)3Si)2Te as the Ge- and Te-precursors was also suggested. The Ge-precursor wherein the Ge atom is +4 oxidation state at original form, cleaves into HCl and GeCl2 by hydrogen chloride elimination reaction. The cleaved molecule take place in the reaction as Ge(II) precursor, GeCl2 to form stoichiometric GeTe through reaction with the Te precursor. This process was also combined with Sb2Te3 deposition process to obtain GeTe-Sb2Te3 pseubobinary films. Mechanism study on the deposition processes of binary GeTe and ternary GeSbTe films was performed.