K. B. Jung

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 ...

300°C GaN/AlGaN Heterojunction Bipolar Transistor

A GaN/AlGaN heterojunction bipolar transistor has been fabricated using Cl 2 /Ar dry etching for mesa formation. As the hole concentration increases due to more efficient ionization of the Mg acceptors at elevated temperatures (> 250°C), the device shows improved gain. Future efforts should focus on methods for reducing base resistance, which are briefly summarized.

Relative merits of Cl2 and CO/NH3 plasma chemistries for dry etching of magnetic random access memory device elements

A typical magnetic random access memory stack consists of NiFe/Cu/NiFeCo multilayers, sandwiched by contact and antioxidation layers. For patterning of submicron features without redeposition on the sidewalls, it is desirable to develop plasma etch processes with a significant chlorinated etch component in addition to simple physical sputtering. Under conventional reactive ion etch conditions with Cl2-based plasmas, the magnetic layers do not etch because of the relatively involatile nature of the chlorinated reaction products. However, in high ion density plasmas, such as inductively coupled plasma, etch rates for NiFe and NiFeCo up to ∼700 A min−1 are achievable. The main disadvantage of the process is residual chlorine on the feature sidewalls, which can lead to corrosion. We have explored several options for avoiding this problem, including use of in situ and ex situ cleaning processes after the Cl2-etching, or by use of a noncorrosive plasma chemistry, namely CO/NH3. In the former case, removal of th...

High rate dry etching of Ni0.8Fe0.2 and NiFeCo

A Cl2/Ar plasma chemistry operated under electron cyclotron resonance (ECR) conditions is found to produce etch rates for NiFe and NiFeCo of ⩾3000 A min−1 at ⩽80 °C. The etch rates are proportional to ion density and average ion energy over a fairly wide range of conditions. Under the same conditions, fluorine or methane/hydrogen plasma chemistries produce rates lower than the Ar sputter rate. The high ion current under ECR conditions appears to balance NiClx, FeClx, and CoClx etch product formation with efficient ion-assisted desorption, and prevents formation of the usual chlorinated selvedge layer that requires elevated ion etching conditions. Post Cl2-etch removal of surface residues is performed with an in situ H2 plasma exposure.

Effect of additive noble gases in chlorine-based inductively coupled plasma etching of GaN, InN, and AlN

The effects of additive noble gases, He, Ar and Xe to chlorine-based inductively coupled plasmas (ICPs) for etching of GaN, AlN and InN were studied in terms of etch rate and selectivity. The etch rates were greatly affected by the chlorine concentration, rf chuck power and ICP source power. The highest etch rates for InN were obtained with Cl2/Xe, while the highest rates for AlN and GaN were obtained with Cl2/He. It was confirmed that efficient breaking of the III-nitrogen bond is crucial for higher etch rates. The InN etching was dominated by physical sputtering; the GaN and AlN etch rates were limited by initial breaking of the III-nitrogen bond. Maximum selectivities of ∼80 for InN to GaN and InN to AlN were obtained with the Cl2-based discharges.

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.

GaN/AlGaN HBT fabrication

Abstract Discrete GaN/AlGaN heterojunction bipolar transistors (HBTs) were fabricated on material grown by both metal organic chemical vapor deposition and molecular beam epitaxy. For both types of material, DC current gains of ∼10 were achieved in 90 μm emitter diameter devices measured at 300°C. Some of the key processing steps, such as ohmic contact annealing temperature and mesa fabrication by low damage dry etching, are described, together with secondary ion mass spectrometry measurements of the dopant and background impurity profiles.

GaN PN junction issues and developments

Critical nitride-based p-n junction issues relating to wide bandgap bipolar device performance include minority carrier lifetime, defect related current characteristics and ohmic contact properties. Recent developments in p-GaN deposition processes resulted in GaN p-i-n UV photodetectors with improved deep UV responsivity, visible light rejection and shunt resistance characteristics. From the device data, the electron diffusion length in p-GaN doped at 1·1018 cm−3 was estimated to be 790 A, and the minority carrier lifetime in the p-GaN was estimated to be 24 ps to 0.24 ns. Improved junction electrical characteristics were achieved using MBE deposition on GaN buffers grown by MOCVD. NiAu ohmic contacts were also made to p-GaN with specific contact resistances less than 10−4 Ω·cm2.

Cl2-based inductively coupled plasma etching of CoFeB, CoSm, CoZr and FeMn

Abstract Thin films of CoFeB, CoSm, CoZr and FeMn have been etched in inductively coupled plasma Cl 2 discharges with He, Ar or Xe as the inert gas additive as an additional physical component to the etch process. The etch rates decrease with pressure and go through maxima with rf chuck power and discharge composition. There is a transition from net deposition to etching with increasing source power and rf chuck power, consistent with the need to provide sufficient ion energy and ion/neutral flux ratio to achieve efficient etching of magnetic materials.

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.