There is presently rapid development of non-volatile memory devices in the form of Magnetic Tunnel Junctions. These structures form the memory elements used in MRAM devices. At the core of the device is an ultra-thin dielectric layer, sandwiched between very thin laminates of magnetic metal layers.
The complete device contains many materials not previously encountered within a mainstream semiconductor fabrication line. These may include Co, Fe, Pt, Mn, MgO and Ru among others. Some of the elements form pure metal layers, others are incorporated in alloys. Some of these materials are intrinsically noble, others can be difficult to etch with high selectivity over adjacent and surrounding materials.
Where there are no viable processes for chemical dry etching these materials, they can be removed by physical sputter etching, or more appropriately, ion beam milling. Such physical processes, however, have a tendency to lead to redeposition of material on the side walls of etched features and the mask. This tends to lead to electrical shorting. The difficulties in developing the patterning steps has contributed to the limited rate of progress in realising the smallest features for ultra-high density devices.
To try and address these problems, reactive dry processes involving halogens have been studied, but the properties of the materials etched are deleteriously effected. Non-corrosive chemistries have also been investigated but the low volatility of the etch products can cause problems.
A technique that has become the method of choice for the deposition of ultra thin films for semiconductor fabrication is Atomic Layer Deposition (ALD). This is a process that utilises alternating self-limiting heterogeneous reaction steps. This technique was independently discovered and developed in Finland, under the name Atomic Layer Epitaxy. Essentially the same technique was developed in the Soviet Union under the name Molecular Layering (ML). A recent historical review of these core technologies is available here. Over time the term ALD has become almost universally adopted.
This style of processing technique has also been applied to etching of thin films. The acronym ALE has now become synonymous with Atomic Layer Etching, rather than epitaxy. As the dimensions of features progressively shrink in silicon semiconductor devices (as well as affiliated technologies) the adoption of sub-10 nm nodes highlight the variability in definition of these tiny structures. The process non-uniformities give rise to larger and larger performance variation in the fabricated device. Potentially ALE may provide a powerful way of overcoming some of these difficulties. An additional degree of control may be possible through adopting reactive ion beam etching rather than the more conventional reactive ion etching. Using an immersion method the ion incidence angle is controlled by the formation of the plasma sheath. The ion beam methodology allows independent control of the ion illumination (bombardment) angle.
Ion beam processes are performed at lower pressures than plasma emersion techniques such as reactive ion etching. The lower pressure also means that the chemical concentration of precursors is reduced. This may result is lower etch rates. The films of interest are, however, very thin (typically < 10 nm) and the lower rate becomes less significant.