Data Storage Systems Center

Magnetic tunnel junctions (MTJ) are of great interest to the data storage community for potential use in magnetic random access memory (MRAM) or as bits for bit patterned media at small enough sizes. We have recently demonstrated using conductive atomic force microscopy (C-AFM) to induce spin torque switching of MTJs as small as 100 nm by 200 nm on an MTJ thin film with RA product of 5.4 Ω-µm2 provided by Everspin Technologies. The stack consists of Si / 200nm SiO2 / 50nm Ta / 20nm PtMn / 2nm CoFe / 0.8nm Ru / 3nm CoFe / 1nm MgO / 2.5nm CoFeB / 8 nm Ta / 10 nm Pt. Features were patterned using electron beam lithography and Ar ion milling. The features were measured by stopping an AFM probe coated with 200 nm of Pt on top of the feature. After correcting for the AFM probe resistance, MR is found to be ~47% with ΔR = 90 Ω on a 200 nm x 100 nm pillar.

The fabrication of magnetic based structures, including MRAM and patterned media, typically requires that etching be performed on one or more magnetic films. Traditionally, ion milling has been used for this process. However, the purely physical nature of the milling process produces several drawbacks, which can include redeposition of etched material, shadowing of the ion beam, poor etch selectivity, cross-wafer non-uniformity, and etch induced damage. Given these drawbacks, researchers have naturally searched for alternative etching methods. One such method with a great deal of potential is known as reactive ion etching (RIE). Already adopted by the semiconductor industry, RIE combines both physical and chemical etching processes to uniformly control the etch profile across the wafer. The inclusion of the chemical etch component helps to increase selectivity, decrease redeposition and shadowing, and to minimize etch damage from physical bombardment by forming volatile compounds that are easily removed from the sample. We have chosen to pursue methanol (CH3OH) as the reactive etching gas due to its ability to etch various magnetic and non-magnetic films down to nanoscale dimensions. In addition, methanol can have very good selectivity over a variety of mask materials, including Ta, TaNx, Ti, SiNx, and various multilayer combinations of these materials. Through the use of a parallel plate RIE system, we have successfully used methanol to etch both isolated and dense discrete tracks into commercially available perpendicular recording media with groove widths as narrow as 20nm, and we have been able to fabricate nanoscale pillars and rings in materials, such as NiFe. More recently, we have obtained an inductively coupled plasma (ICP) RIE system to be used for methanol etching. In the ICP configuration ion bombardment and plasma density can be controlled separately, unlike the parallel plate setup where they work in synergy. Therefore, it is possible to produce faster etching rates, better selectivity, and even less ion induced etching damage. All of these factors will aide in the etching of even smaller features with higher aspect ratios than those obtained in the parallel plate setup.

The future of bit-patterned media implies smaller gaps between bits: how closely can we pack neighbouring features together? Our approach focuses on subtractive patterning processes, and we use nanoparticle arrays as ideal test systems for small-gap etch processes. Their regular ultra-small gaps (2 nm), fast self-assembly, and long-range order come at a price: a high lateral instability during etching. Their surfactant coatings block etchants and must be removed, but we observe that this leads to aggregation, destroying their order. Here we present a novel technique for selectively preserving that order, by using electron beam irradiation to chemically alter (or “cure”) the surfactant, making it more durable. Surfactant is removed with an O2 plasma, and Fourier transforms are used to gauge the degree of aggregation as a function of e-beam dose. We find the aggregation to decrease monotonically with increasing dose, within the range of doses we studied. After removing the surfactant, we use a CF4-based RIE process to transfer the pattern from the nanoparticles into an underlying Si substrate. Uncured regions exhibit pronounced disordering, and outlines of individual particles are not visible. The feature outlines in cured regions become increasingly visible as the dose is increased, and the order in the array is correspondingly better preserved. Compared to our earlier method involving through-wafer etching, e-beam stabilization requires dramatically fewer processing steps, and can be used to pattern a significantly wider variety of materials. We hope this technique will allow us to refine tomorrow’s small-gap etches, today.

The long-term extensibility of bit-patterned media requires etch processes which can transfer patterns with very small gaps between features. Self-assembled nanoparticle arrays exhibit the smallest gap sizes of any densely patterned etch mask, making them ideal for exploring the limits of these processes. We prepared self-assembled monolayers of Fe3O4 nanoparticles with gap sizes as small as 2 nm, and equivalent feature densities greater than 3 Tbit/in2. At such small gap sizes, the lateral stability of the features (nanoparticles) in the mask, during the etch process, was found to be of paramount importance. We developed a novel procedure, involving a structural pinning layer and through-wafer etching, which successfully enabled this lateral stability, allowing me to move forward and develop etch processes compatible with small gaps. Using a CF4-based Reactive Ion Etch (RIE) at low power and low pressure allowed me to pattern an SiO2 layer, which could be used as an intermediate mask for further pattern transfer. By rinsing away the nanoparticle mask using H3PO4, the patterned silica is exposed, and plan view High Resolution Scanning Electron Microscopy (HRSEM) confirmed that we had successfully etched between the 2 nm gaps and transferred the pattern. Future work consists in two main goals. First, we will use cross-sectional Transmission Electron Microscopy (TEM) to precisely characterize the sidewall angle and etch depth in the silica. Subsequently, we will use the patterned silica as an etch mask for further RIE-based pattern transfer into underlying magnetic films.

Magnetic tunnel junctions (MTJ) are of great interest to the data storage community for potential use in magnetic random access memory (MRAM) or as bits for bit patterned media at small enough sizes. We have recently demonstrated using conductive atomic force microscopy (C-AFM) to induce spin torque switching of MTJs as small as 200 nm by 400 nm on an MTJ thin film with RA product of 5 Ω-µm2 provided by Everspin Technologies. The stack consists of Si / 200nm SiO2 / 50nm Ta / 20nm PtMn / 2nm CoFe / 0.8nm Ru / 3nm CoFe / 1nm MgO / 2.5nm CoFeB / 10nm Ta. Features were patterned using focused ion beam (FIB) etching. The features were measured by stopping an AFM probe coated with 200 nm of Pt on top of the feature. After correcting for the AFM probe resistance, MR is found to be ~34% with ΔR = 30 Ω.

The fabrication of magnetic based structures, including MRAM or patterned media, typically requires that etching be performed on one or more magnetic films. Traditionally, ion milling has been used for this process. However, the purely physical nature of the milling process produces several drawbacks, which can include redeposition of etched material, shadowing of the ion beam, poor etch selectivity, cross-wafer non-uniformity, and etch induced damage. Given these drawbacks, researchers have naturally searched for alternative etching methods. One such method with a great deal of potential is known as reactive ion etching (RIE). Already adopted by the semiconductor industry, RIE combines both physical and chemical etching processes to uniformly control the etch profile across the wafer. The inclusion of the chemical etch component helps to increase selectivity, decrease redeposition and shadowing, and to minimize etch damage from physical bombardment by forming volatile compounds that are easily removed from the sample. In order to produce volatile compounds with magnetic materials, many researchers have concentrated on etch chemistries that include gases, such as Cl2 or NH3. However, these highly toxic, corrosive materials require special tooling and post etch treatments to remove corrosive residue that may damage magnetic films. On the other hand, methanol gas (CH3OH) requires no special tooling and produces no corrosive residue. Therefore, we have chosen to pursue methanol etching of magnetic devices and magnetic recording media using a standard parallel plate RIE process. We have demonstrated that various magnetic films, as well as some non-magnetic films can be etched to nanoscale dimensions with very good selectivity over a variety of mask materials, including Ta, TaN, Ti, SiNx, and various multilayer combinations of these materials. Consequently, we have been able to successfully etch both isolated and dense discrete tracks of varying linewidth into commercially available perpendicular recording media, and we have been able to fabricate nanoscale pillars and rings in materials, such as NiFe.