A blog from Intermolecular
NVM Memories: A Look at the Advantages of ALD Chalcogenides
Chalcogenide materials produce fast, low power non-volatile memories (NVMs).
In this webinar, Valerio Adinolfi, Sr. Scientist here at Intermolecular, discusses ALD chalcogenides for non-volatile memories and the advantages of ALD over other deposition techniques.
NVM memory arrays comprise OTS switches and PCM memories. OTSs are two-terminal devices whose conductivity abruptly increases by orders of magnitude, from a high resistive state, above a threshold voltage. OTS are used as selective elements for Phase Change Memories (PCMs), whose morphology can be switched between an amorphous and a crystalline state upon application of an appropriate electrical stimulus. The information stored in a PCM is therefore non-volatile.
The state-of-the-art architecture used for memory arrays is the 3D Xpoint; here a memory and a selector are stacked one above the other in a planar fashion using PVD deposition. However, to increase the chip density and the memory storage capacity more sophisticated 3D structures must be used.
The image above shows a vertical 3D Xpoint structure where the active films are conformally deposited around cylindrical contacts. Such a level of conformality can be provided only by ALD.
Many challenges are associated with the ALD deposition of chalcogenide films such as composition control and conformality. ALD films must be conformal and present negligible surface roughness to produce quality electronic interfaces with metal contacts, isolation layers, etc.
By alternating layers of different unit films in a super-cycle fashion, we achieve control over the film composition, as schematized in the image below. Our ALD chamber hosts 300 mm wafers and is equipped with an in-situ ellipsometer allowing real-time characterization of the film growth. Real-time ellipsometry greatly enhances the understanding of the growth mechanisms and reduces the time required to develop a deposition process.
We deposited chalcogenides on silicon trench test-structures and image the films using our in-house SEM to test conformality. We then used our internal test vehicle to produce OTS and PCM devices. IMI internal test-vehicles feature a tungsten plug connecting the film to a line resistor. Thanks to our e-test capabilities, we carried out a full electrical characterization on a large number of devices to perform statistical and failure analysis.
Here at IMI, we have a large number of tools for advanced material characterization and film integration into device test-structures. To develop material systems and devices at a faster pace, we use our custom combinatorial-ALD chambers that allow the growth of four different film compositions in four different quadrants of a 300 mm wafer in one deposition run.