AVS2011 Session TF1+EM-WeA: Nonvolatile Memory

Phase-Change Random Access Memories (PCRAM) are very promising candidates for next generation of non-volatile memories. Those devices store information using the high electrical contrast between the amorphous and crystalline phases of chalcogenide alloys such as Ge₂Sb₂Te₅ (also called GST) which are mainly deposited by physical vapor deposition (PVD) [1]. However, two important drawbacks have to be overcome: a too short archival life for GST at high operating temperatures and a too high power consumption. The binary compound GeTe seems to be a promising candidate for high temperature applications since it has an estimated archival life up to ten years at 110°C. A pronounced increase in thermal stability of N- and C-doped GeTe was also demonstrated [2].

Concerning the power consumption, it was shown that a way to reduce the high operating currents is to confine the material. In this way, several groups try to develop a new deposition process to achieve the gap filling of the confined structure. Atomic Layer Deposition (ALD) of GST has shown very good step coverage but the deposition rate remains very low. Therefore, we proposed to study the deposition of GeTe in a shower-head type 200mm plasma-enhanced pulsed liquid injection CVD reactor (AltaCVD200). Ge and Te liquid precursors are introduced into the deposition chamber as vapours through a pulsed injection system and an evaporating furnace. A capacitively-coupled RF plasma is applied between the shower head and the substrate heater to decompose the precursors. Chamber walls are maintained at 70°C to avoid condensation of precursors vapours. The deposition chamber is mounted on a cluster tool which allows quasi in situ analysis of the deposited films by angle-resolved XPS while the plasma is analyzed by optical emission spectroscopy (OES).

In this paper, we investigate the optical emission spectra of high pressure (2-20 Torr) Ge- and Te-containing plasmas. Reference UV-visible emission spectra (200-800 nm) are recorded for various conditions of RF power, pressure, and flow rate. The role of electrons and H atoms in the decomposition of Ge and Te precursors is discussed by correlating the OES of the plasma with the chemico-physical properties of the deposited material. It is shown that H atoms induce a competitive effect between deposition (due to the decomposition of the precursors) and etching of Ge and Te atoms at the growing film surface. Role of C contamination in the phase change properties is also discussed and phase transitions of plasma deposited GeTe-C materials are compared with sputtered GeTe-C.

[1] G. W. Burr et al, J. Vac. Soc. Technol. B 2010, 28 (223)

The integration of various functionality to (Bi)CMOS circuits is in the focus of the “More than Moore” approach. Here, we demonstrate the incorporation of nonvolatile memories (NVM) into the Back end of line (BEOL) of Bi(CMOS) circuits. The added functionalities open new technological possibilities for high value microelectronics systems.

Embedded nonvolatile memories (NVM) with high-density, high-speed, and low-power are attractive for a growing number of applications. One promising candidate for next-generation nonvolatile memories is based on the electrically switchable resistance change between a high (OFF-state) and a low (ON-state) resistive state of a metal-insulator-metal (MIM) structure. This approach is often termed resistance random access memory (RRAM) technologies. Due to the cost effectivity and BEOL compatibility with (Bi)CMOS technologies, this approach is highly attractive. By combining the MIM devices (R) with selection transistors (T), the 1T-1R cells offer good scalability, long retention time, and rapid read/write times. In this letter, the reliable bipolar resistive switching of TiN/HfO₂/Ti/TiN devices embedded into 1T-1R cells is demonstrated.

The current-voltage (I-V) characteristics of the TiN/HfO₂/Ti/TiN diodes and 1T-1R cells were studied by DC voltage sweep measurements. By applying a positive voltage at the top electrode, the resistance is reduced for V > V_Set. The device can be switched back into the OFF-state when a negative voltage is applied beyond V_Reset. In case of the 1T-1R cell, the forming and set processes can be controlled by changing the gate voltage (V_G) of the select transistor.

Multiferroic materials exhibit two or more forms of ferroic order such as (anti)ferroelectricity, (anti)ferromagnetism, ferroelasticity, or ferrotoroidicity. Materials containing both ferroelectricity and ferromagnetism will exhibit some amount of magnetoelectric coupling which is a desirable aspect for the future of non-volatile memory, as these materials could potentially be used for devices that will be written magnetically and read electronically or vice versa, as well as the prospect of four-state memory devices. Materials which exhibit magnetoelectric coupling have been well studied, however, the synthesis methods may not easily translate into large scale integration.

One possible route for synthesis on a commercial scale, atomic layer deposition (ALD) is a thin-film processing technique which involves alternatively flowing non-self reacting precursor vapors or gases onto a substrate. As a result of the self limiting reaction, the precursors only form a single monolayer per cycle. The sequential and self-limiting nature of the deposition is used to deposit thin films with good compositional control, high conformity, high uniformity, and excellent thickness control.

To create multiferroic crystal structures, a 1:1 stoichiometric ratio between cations is desired with low contamination by organic ligands in order to form the crystal phases that permit multiferroicity. Therefore, in this work, multiferroic YMnO₃ and BiFeO₃ on various substrates are synthesized by radical enhanced atomic layer deposition (RE-ALD) using Y(tmhd)₃ (tmhd = 2,2,6,6-tetramethylheptane-3,5 dione), Mn(tmhd)₃, Fe(tmhd)₃, and Bi(tmhd)₃ as metal precursors and oxygen radicals as the oxidizer. By varying the cycle sequences, controlled composition is demonstrated and verified through XPS. Growth rates are shown on a thickness per cycle basis as a function of deposition temperature, precursor pulse times, and substrate. The crystal structure as well as atomic environment are examined by XRD and extended x-ray absorption fine structure spectroscopy (EXAFS) respectively and are accompanied by TEM micrographs. Finally, magnetic measurements made by a super conducting quantum interference device (SQUID) magnetometer, zero-field cooled and field cooled (ZFC-FC) M vs. T and M vs. H, are shown on 1:1 YMnO₃ stoichiometric films showing a Néel temperature T_N = ~45 K and a coercive magnetic field H_C = 130 Oe for Si(111) and H_C = 300 Oe for YSZ(111).