J. R. Childress

Patterning of Cu, Co, Fe, and Ag for magnetic nanostructures

Wet and dry etching of thin metallic multilayer structures is necessary for the development of sensitive magnetic field sensors and memory devices based on spin–valve giant magnetoresistance elements. While it is well established that Cu, Co, and Fe are soluble in HNO3 and H3PO4 at room temperature, little effort has been made to investigate selective wet and dry etch chemistries. For example, we find Ag is not etched in H2SO4, HCl, or H3PO4 under conditions where etch rates for the other metals are in the range of 2000–60 000 A/min. Electron cyclotron resonance (ECR) SF6/Ar plasmas provide etch selectivities of ⩾5:1 for Ag over Cu, Co, and Fe, while lower selectivities are obtained with CH4/H2/Ar. Cl2-based plasma chemistries leave significant metal–chlorine surface residues, which can be removed in situ by low ion energy H2 or Ar plasma treatments that eliminate corrosion problems. Cu etch rates in excess of 3000 A/min at 25 °C can be obtained in ECR Cl2/Ar discharges because the high ion flux prevents ...

Development of electron cyclotron resonance and inductively coupled plasma high density plasma etching for patterning of NiFe and NiFeCo

Two different kinds of high density plasma reactors are found to be effective for dry etching of Ni0.8Fe0.2 and Ni0.8Fe0.13Co0.07. Using a Cl2/Ar plasma chemistry, electron cyclotron resonance and inductively coupled plasma system produce a factor of 2 higher etch rates than for pure Ar sputtering under the same conditions. The etch rates are a strong function of ion flux, ion energy, and plasma gas composition, all of which may be interpreted in terms of balancing formation of chloride etch products with efficient ion-assisted desorption of these products. Typical peak etch selectivities of ∼5 and ∼4, respectively, were obtained for NiFe over SiO2 and SiNx masks. Post-etch corrosion was also studied, and found to be strongly dependent on the conditioning of the reactor walls.

Copper Dry Etching with Cl2 / Ar Plasma Chemistry

Etch rates ≥5000 A min 1 were obtained for Cu in electron cyclotron resonance Cl 2 /Ar discharges at a sample temperature of 200°C for ion-neutral ratios ≥ 0.02. The rates are a strong function of ion-neutral ratio, ion flux, and ion energy through the need to have CuCl x desorption rate faster than the CuCl x , generation rate in order to avoid formation of a chlorinated selvedge layer. Postetch, in situ H 2 plasma cleaning removes most of the chlorine residues and allows creation of clean, anistropic Cu features.

Patterning of NiFe and NiFeCo in CO/NH3 high density plasmas

The CO/NH3 plasma chemistry operated under conventional reactive ion etching conditions does not etch NiFe or NiFeCo. However, under high density plasma conditions, etch rates up to ∼500 A min−1 are obtained for both materials provided optimized ratios of CO:NH3 and values of ion flux and ion energy are employed. The etch mechanism still has a strong physical component and appears to depend on having sufficient CO to form carbonyl etch products, and to avoid formation of a carbide-like surface layer. Under nonoptimized conditions, the latter can lead to net deposition rather than etching.

Fabrication of spin-current field-effect transistor structures

It has been theoretically proposed that a narrow gap semiconductor field-effect transistor with ferromagnetic contacts for injection of spin-polarized electrons may be used for current modulation. This raises the possibility of electronic devices based on transport of spin-up and spin-down electrons biased and controlled by internal magnetic fields. As an initial step in this direction, we have fabricated GaAs/AlGaAs and InGaAs/AlInAs high electron mobility transistor structures, where the normal AuGeNi source and drain contacts are replaced by sputter-deposited Fe contacts. Mesa fabrication to allow the Fe to contact the two-dimensional electron gas formed at the interface between the doped AlGaAs or AlInAs and the lower bandgap (GaAs or InGaAs) undoped channel was achieved by either wet chemical etching or angled Ar+ ion milling. TiPtAu (1 μm gate width) was employed in both materials systems as a gate contact. The Fe is found to form a rectifying contact to the GaAs channel devices, preventing current ...

Parametric Study of NiFe and NiFeCo High Density Plasma Etching Using CO / NH 3

Maximum etch rates of ∼400 A min -1 were obtained for Ni 0.8 Fe 0.2 and Ni 0.8 Fe 0.13 Co 0.07 thin films in CO/NH 3 inductively coupled plasmas (ICP). There is a small chemical contribution to the etch mechanism (i.e., formation of metal carbonyls) as determined by a comparison with Ar and N 2 physical sputtering. The etch rates are a strong function of ion flux, ion energy, pressure, substrate temperature, and discharge composition. The discharge should be NH 3 -rich to achieve the highest etch rates. Several different mask materials were investigated, including photoresist, thermal oxide, and deposited oxide. Photoresist etches very rapidly in CO/NH 3 and use of a hard mask is necessary to achieve pattern transfer. Due to its physically dominated nature, the CO/NH 3 chemistry appears suited to shallow etch depth (≤0.5 μm) applications, but mask erosion leads to sloped feature sidewalls for deeper features.

Comparison of Cl2/He, Cl2/Ar, and Cl2/Xe plasma chemistries for dry etching of NiFe and NiFeCo

Cl 2 /He, Cl 2 /Ar, and Cl 2 /Xe discharges operated under inductively coupled plasma conditions have been compared for patterning of Ni 0.8 Fe 0.2 and Ni 0.8 Fe 0.13 Co 0.07 layers. There is a transition from net deposition to etching with increasing source power, as the relatively involatile chlorinated etch products are removed more efficiently by ion-assisted desorption. This transition occurs at lower ion fluxes for Xe- and Ar-containing discharges than for He due to the more effective momentum transfer. The etch rates with all three mixtures also go through maxima, reflecting the need to balance etch product formation and desorption.

Effect of inert gas additive on Cl2-based inductively coupled plasma etching of NiFe and NiFeCo

NiFe and NiFeCo thin films have been etched in Cl2/He, Cl2/Ar and Cl2/Xe inductively coupled plasmas as a function of pressure, source power, and rf chuck power. The etch rates decrease with increasing pressure, and go through a maximum with both source and chuck power. The results are consistent with a mechanism involving ion-assisted desorption of relatively involatile etch products, and a balance of ion flux, ion energy, and chlorine neutral density is necessary to achieve practical etch rates and smooth surfaces. Under our conditions, Cl2/He provided the best surface morphologies and the least residual chlorine.

Cl2‐Based Inductively Coupled Plasma Etching of NiFe and Related Materials

A parametric study of etch rates, surface quality, and mask materials for inductively coupled plasma (ICP) dry etched NiFe, NiFeCo, TaN, and CrSi in Cl 2 /Ar Cl 2 N 2 , and Cl 2 /H 2 chemistries is reported. The etch rates are a strong function of discharge composition, with maxima at ∼66% Cl 2 in each of the chemistries investigated, as are ion flux, ion energy, and pressure. The etch mechanism appears to be formation of chlorides that are desorbed by ion assistance. If the ion-to-neutral ratio is not optimized, then the etching reverts either to a pure sputtering regime or to net deposition through formation of a thick chlorinated selvedge layer on NiFe and NiFeCo.

Dry and Wet Etch Processes for NiMnSb Heusler Alloy Thin Films

A variety of plasma etching chemistries were examined for patterning NiMnSb Heusler alloy thin films and associated Al 2 O 3 barrier layers. Chemistries based on SF 6 , Cl 2 , and BCl 3 were all found to provide faster etch rates than pure Ar sputtering. In all cases the etch rates were strongly dependent on both the ion flux and ion energy. Selectivities of ≥20 for NiMnSb over Al 2 O 3 were obtained in SF 6 -based discharges, while selectivities ≤5 were typical in Cl 2 , BCl 3 , and CH 4 /H 2 plasma chemistries. Wet etch solutions of HF/H 2 O and HNO 3 /H 2 SO 4 /H 2 O were found to provide reaction-limited etching of NiMnSb that was either nonselective or selective, respectively, to Al 2 O 3 .