Tomonori Mukai

New radical control method for high-performance dielectric etching with nonperfluorocompound gas chemistries in ultrahigh-frequency plasma

A new method for controlling radical generation in the etching of silicon dioxide is described. In an ultrahigh-frequency (UHF) plasma, the mean electron energy is about 2–3 eV and there are a small number of high-energy electrons. The plasma mainly dissociates C–I (2.4 eV), C=C (2.8 eV), C–C (4.3 eV), and C–Br (3.0 eV) bonds in the CF3I, C2F4, CF2Br2, C2F6, and C4F8 plasmas, and it mainly generates CF3 and CF2 radicals because the bond energies of these bonds are lower than the bond energies of C–F (5.6 eV in CF4) bonds. We found that the densities of these radicals were inversely proportional to the bond dissociation energy in these gases. That is, we found that C–I and C=C bonds are ideal for selective radical generation in the UHF plasma. The ratio of each radical density can be precisely controlled by changing the ratio of the mixture of these gases. As a result, etching selectivity and etching rate are improved considerably. From an environmental viewpoint, CF3I and C2F4 are also good alternatives t...

Differences in radical generation due to chemical bonding of gas molecules in a high-density fluorocarbon plasma: Effects of the C=C bond in fluorocarbon gases

We investigated the differences in radical generation due to chemical bonding of fluorocarbon gas molecules in the plasma. We found that dissociation of the C=C bond is five times easier than that of the C–C bond in a fluorocarbon gas plasma. As a result, a C2F4 plasma could generate a higher density of CF2 radicals than a C4F8 plasma. Additionally, the same dissociation processes occurred in the C3F6 and C5F8 plasma, which both have the C=C bond and C–C bond in their molecules. In the C3F6 plasma, the density of generated CF2 radicals was 3.5 times higher than that for CF or CF3 radicals, whose radical densities were the same. The C5F8 gas plasma mainly produced CF2 and CF radicals, and the CF radical density was higher in comparison to other fluorocarbon gas plasmas.

High-performance silicon dioxide etching for less than 0.1-μm-high-aspect contact holes

We describe a method for controlling radicals in high-performance SiO2 etching of contact holes smaller than 0.1 μm using nonperfluorocompound gases (CF3I and C2F4) in an ultrahigh-frequency (UHF) plasma. Because this method allows the independent control of polymerization and etching through the selective generation of CF2 and CF3 radicals, both high etching selectivity and a high etching rate were achieved without microloading and etching stop, even for a 0.05 μm contact hole. Using this new gas chemistry, we achieved an optimum balance between polymerization and etching in 0.05-μm-diameter contact holes by controlling the flow ratio of C2F4/CF3I gas

Magnetic Tunnel Junction (MTJ) Patterning for Magnetic Random Access Memory (MRAM) Process Applications

We have developed a top free type magnetic tunnel junction (MTJ) patterning technique that involves tunnel barrier etching with enough time margins for chemical assisted ion etching (CAIE). The approximately 1 nm thick tunnel barrier enables stopping the etching process in a shorter time margin. We have found that no aluminum-oxide barrier shorting by re-deposition occurred in chlorine based CAIE, when the etching depth into anti-ferromagnetic IrMn layer was less than 5 nm. The magnetoresistance (MR) ratio of the whole MTJ patterns reached 35% on a 6-inch wafer. The time margin of the etching was 120 s, which is long enough for an magnetic random access memory (MRAM) process.

High-performance and damage-free magnetic film etching using pulse-time-modulated Cl/sub 2/ plasma

Magnetic films have been used to fabricate magnetic random access memory (MRAM) devices. A key issue for producing magnetic devices is the problems associated with magnetic film etching, in particular a low etching rate and sidewall deposition caused by non-volatile etching products. We recently found that plasma radiations degraded magnetic properties. To overcome these problems, we investigated the effects of a few tens of micro-second pulse-time-modulated (TM) plasma on magnetic film etching. In this study, we found that injecting negative ions from TM plasma produced no sidewall deposition and accomplished damage-free magnetic film etching.

Reactive and anisotropic etching of magnetic tunnel junction films using pulse-time-modulated plasma

Reactive and anisotropic etching of magnetic tunnel junction (MTJ) stacked films has been achieved using pulse-time-modulated (TM) plasma. While corrosion and delamination of MTJs are observed in continuous wave discharge plasma, a chlorine pulse-time-modulated plasma achieved a high MTJ etching rate without corrosion or delamination. The authors think that the negative ions enhance the chemical reactions on the surface of magnetic films. The magnetic characteristics are also significantly improved by using TM plasma because of reduced residues and improved tapered profiles. Accordingly, TM plasma etching is a promising candidate for high-rate and damage-free MTJ etching for magnetoresistive random access memory devices.

A 16-Mb Toggle MRAM With Burst Modes

This paper describes a recently developed 16-Mb toggle magnetic random access memory (MRAM). It has 100-MHz burst modes that are compatible with a pseudo-SRAM even though the toggle cell requires reading and comparing sequences in write modes. To accelerate operating clock frequency, we propose a distributed-driver wide-swing current-mirror scheme, an interleaved and pipelined memory-array group activation scheme, and a noise-insulation switch scheme. These circuit schemes compensate the toggle cell timing overhead in write modes and maintain write-current precision that is essential for the wide operational margin of MRAMs. Because toggle cells are very resistant to write disturbance errors, we designed the 16-Mb MRAM to include a toggle MRAM cell. The MRAM was fabricated with 0.13-mum CMOS and 0.24-mum MRAM processes with five metal layers.

Effects of low-molecular-weight radicals for reduction of microloading in high-aspect contact-hole etching

Abstract SiO 2 etching is done by using fluorocarbon gases to deposit a fluoropolymer on the underlying silicon. This deposit enhances the etching selectivity of SiO 2 over silicon or silicon nitride. CF 2 radicals are used as the main gas precursor for polymer deposition. In a conventional gas plasma, however, the CF 2 radicals and other radicals (high-molecular-weight radicals: C x F y ) lead to polymerization. This condition causes microloading and etching-stop in high-aspect contact-hole patterning due to the sidewall polymerization during SiO 2 etching processes. Conversely, by using new fluorocarbon gas chemistries (C 2 F 4 /CF 3 I), we achieved selective radical generation of CF 2 and eliminated high-molecular-weight radicals. Under this condition, microloading-free and etching-stop-free high-aspect-ratio contact-hole patterning of SiO 2 was accomplished. Thus, the higher molecular weight radicals play an important role in the sidewall polymerization in contact holes because these radicals have a higher sticking coefficient than CF 2 radicals. Selective generation of CF 2 radicals and suppression of C x F y radicals are thus necessary to eliminate microloading and etching-stop in the formation of high-aspect-ratio contact holes.

Ion-Beam-Etched Profile Control of MTJ Cells for Improving the Switching Characteristics of High-Density MRAM

The effect of the reduction of the sidewall redeposition layer of magnetic materials is investigated for submicron-patterned magnetic random access memory (MRAM) cells. The sidewall redeposition layer is made at the first etch step of a magnetic tunnel junction (MTJ) with ion beam etching (IBE) in the case that the sidewall angle of a hard mask is steep. By controlling the etched profile at the time of the first IBE step, formation of the redeposition layer is prevented. Functional test results of 1-Kb MRAM arrays show that the sidewall redeposition layer enlarges fluctuation of switching current, and reduces the write operation region. The effect of the sidewall redeposition on the switching characteristics of MRAM arrays is explained qualitatively by micromagnetic simulation solving the Landau-Lifshitz-Gilbert (LLG) equation

Characterization of process-induced charging damage in scaled-down devices and reliability improvement using time-modulated plasma

The charging damage from metal etching and dielectric etching was studied using metal–oxide–semiconductor devices with an oxide thickness of 1.9–6.0 nm, and the impact of the charging on reliability of scaled-down devices, as well as damage monitoring methods appropriate for each plasma process and oxide thickness, were investigated. For metal etching, in which the electron shading effect is a major cause of charging, hot carrier degradation dominated oxide degradation for oxides of 3.5–6.0 nm. For thinner oxides (<3.0 nm), however, a gate leakage failure dominated, but the failure rate decreased with gate oxide thinning below 3.0 nm and became negligibly small below 2.2 nm. For dielectric etching, the gate leakage current was an effective damage monitor. To detect the latent damage accurately, use of a high oxide electric field of 5–9 MV/cm was effective. Like the metal etching damage, the failure rate was lower for a thinner oxide of <3.0 nm. The hot carrier degradation was less sensitive to the dielect...