There are various types of electronic devices that comprise thin-film magnetic components, such as magnetic-random-access-memories and magnetic recording heads. The thin-film magnetic components in these devices are often processed by standard photolithography and etching techniques during fabrication. For electronic devices having magnetic thin-film components and stacks of other thin-film components of different natures or chemical properties, it becomes difficult to efficiently and successfully processing the thin-films using standard lithography and etching techniques. It becomes even more difficult to use standard photolithography and etching techniques to process the thin-films when the thin-films are to be defined into features with characteristic dimensions matching today's technology nodes, such as 130 nm or less.
Taking magnetic-random-access memories (MRAM or MRAM cell) as an example, MRAMs are a new non-volatile memory technology and have been drown great attention in both scientific research laboratories and industries. Their advantageous properties over existing memory technologies for storing digital signals have proved MRAMs to become a promising mainstream memory technology in the recent future.
A MRAM cell uses a magnetic tunnel junction (MTJ) as a storage element; and the MTJ comprises of two magnetic layers separated by a thin (such as 1 nm) insulating layer. One of the two magnetic layers, which is referred to as a reference layer, is characterized by fixed magnetization. The other magnetic layer, which is referred to as a storage layer, is characterized by variable magnetization orientation.
The two magnetic layers of the MTJ are often based on 3d metals (such as Fe, Co, Ni) and 3d metal alloys. The insulating layer laminated between the two magnetic layers in the MTJ often comprises of alumina (Al2O3) or magnesium oxide (MgO) although many other oxide/nitride materials could in principle be used. In one example, one of the two magnetic layers of the MTJ is made of a synthetic antiferromagnet that involves ultra thin layers of Co-alloys and ruthenium (Ru). One or both of the magnetic layers in the MTJ can be coupled with an anti-ferromagnetic layer, which can be a Mn alloy, such as FeMn, PtMn, and IrMn.
A MRAM cell may comprise additional functional elements, such as, buffer layers to promote adhesion and texture, capping layers prevent corrosion or materials inter-diffusion, electrical contact layers, thermal barriers, and spin polarizing layers. Because of different desired functions of different elements, a MRAM cell may comprise of various materials, some of which can be uncommon materials such as NiFe, CuN, NiFeCr, Pt, GeTeSb, and BiTe, as well as usual semiconductor materials such as Ti, TiN, TiW, TiWN, W, Ta, Cu, and CoSiN.
The combination of many materials with very different chemical natures makes it difficult if not impossible to etch using existing semiconductor processing techniques, especially when it is to be patterned into small individual elements (“cells”) at features sizes matching today's technology nodes (130, 90, 65 nm going down to 45, 32, 22 nm). In addition, it is always desired to preserve the chemical/crystalline nature of the tunnel barrier/storage and reference layers interfaces so as to achieve desired electrical characteristics of the MRAM cell. In particular it is always desired to avoid disturbing the magnetic properties of the reference and storage layers. It is also desired to avoid variation of the critical dimension of the MRAM cell. It is also desired to avoid damages of the tunnel barrier layer by means of atomic diffusion of metallic species and/or modification of oxygen content. It is further desired to avoid electrical shorting of the tunnel barrier layer by metallic sidewalls re-depositions during fabrications.
Amongst the existing etch techniques that are commonly used in the semiconductor and thin film industry, wet etch is unsuitable for processing MRAM features, especially those features with critical dimensions. Ion beam etching (IBE) is unsuitable either for processing MRAM features due to the following reasons. An IBE etch is often driven by high energy ions that sputter off the target material. Heavy sidewall re-deposition occurs as the sputter species being non-volatile by nature. Although etching at a grazing incidence may reduce sidewall re-deposition, such grazing incidence is primarily practical for isolated devices (such as recording heads) but not for dense MRAM cells, such as an array of MRAM cells, which is necessary for practical memory applications. Moreover, IBE etch may result in etched sidewalls being slopped due to non-isotropic etch, which in turn, causes severe critical dimension gain at the tunnel junctions of MRAM cells.
Reactive ion etching, which is capable of achieving features of critical dimensions in MRAM cells and clean vertical sidewalls, is however difficult to implement due to the multiple and often chemically incompatible elements in MTJs of MRAM cells. This arises from the fact that some of MTJs are comprised of highly non volatile elements (such as Pt and Co). Some of MTJs are comprised of highly volatile elements (such as Ge and Te); while some of MTJs are comprised of elements that are highly sensitive to corrosion (such as elements Fe and Ni). Some MTJs are comprised of elements prone to solid state diffusion (such as elements Mn, Cu, and Sb). It is therefore very difficult to find an appropriate combination of chemistry and etching parameters (such as temperature and power) to achieve a proper etch.
MTJs of MRAM cells patterning is currently performed either by means of IBE in the data storage market, where structures are isolated by nature and grazing incidences can be used, or by means of RIE in MRAM applications, wherein large densities are required. It is believed however that as the feature size is decreased and the complexity of MTJ stacks are increased, existing etching techniques will become more and more difficult to implement in processing MRAM cells.
In view of the foregoing, it is desired for a method of processing thin-film components in electronic devices.