Nobuyuki Negishi

Investigation of Bowing Reduction in SiO2 Etching Taking into Account Radical Sticking in a Hole

The bowing mechanism in high-aspect-ratio contact hole (HARC) etching was investigated by taking into account reactive sticking on the sidewall of the hole. Sticking coefficients of radicals on the sidewall have been estimated by comparing the observed deposition profile with the calculated one. It was found that the coefficients of C rich radicals and CFx radicals were 0.5 and 0.004, respectively, and that F radical reaction probability to the fluorocarbon polymer is 0.07. These coefficient values were deduced that the excessive flux of O and F onto the sidewall of a hole causes bowing during HARC etching. It was also indicated that the bowing can be suppressed by reducing of the flux of oxygen. These findings were confirmed by the results of experiments using an ultra-high frequency-electron cyclotron resonance (UHF-ECR) plasma.

Effect of oxygen addition to an argon plasma on etching selectivity of poly(methyl methacrylate) to polystyrene

Abstract. The effect of oxygen addition to an argon plasma on the etching selectivity of poly(methyl methacrylate) (PMMA) to polystyrene (PS) (hereafter “PMMA/PS etching selectivity”) was investigated. The PMMA/PS etching selectivity was evaluated by using inductively coupled plasmas composed of argon and oxygen. The etching selectivity in the case of argon plasma was estimated to be 3.9, which is higher than that of oxygen plasma, which is 1.7. The time dependence of etching depth shows that the etching rate of PMMA is reduced to less than one half of its initial value after the etching depth exceeds 15 nm. X-ray photoelectron spectroscopy of the PMMA surface revealed that the reduction of etching rate is caused by a depletion of oxygen concentration by argon-ion bombardment. To compensate the oxygen-concentration depletion, 1% oxygen was added to the argon plasma. As a result, the reduction of PMMA etching rate was suppressed, and constant etching rate was obtained even when etching depth exceeded 50 nm. The mixed argon-oxygen plasma was used to fabricate a PS mask pattern with a full pitch in the range of 25.5 to 77 nm.

Effects of Mask and Necking Deformation on Bowing and Twisting in High-Aspect-Ratio Contact Hole Etching

The effects of mask characteristics on high-aspect-ratio contact hole (HARC) etching profiles were investigated. The evaluation of etching profiles produced with different taper angle masks confirmed that the bowing amount and mask selectivity worsened with decreasing mask taper angle. The relationship between mask taper angle and distribution of scattered ion flux on the sidewall of a tapered mask was calculated. The scattered ion flux was heavily concentrated in the upper part of the sidewall in the case of a tapered mask, and this was considered to be the main cause of the bowing formation. Direct observation of an etched sidewall by atomic force microscopy (AFM) revealed that the roughness of the necking was strongly related to the roughness of the bottom part of the etched sidewall. To evaluate the dependence of twisting on nonuniform necking, the incident ion flux in a circular hole was calculated. As a result, in the case of nonaxisymmetric necking, an imbalance of ion flux in the bottom of the hole appeared and broke the etching symmetry in the bottom part of the hole, causing twisting. In addition, the probability of twisting was found to increase with increasing necking growth rate irrespective of mask electrification. Therefore, mask deformation and nonuniform necking in the upper part of the sidewall during HARC etching are considered the main factors causing bottom degradation. Accordingly, a vertical and nondeformed mask is very important for a smaller critical dimension (CD) and HARC etching.

Line-edge roughness increase due to wiggling enhanced by initial pattern waviness

To clarify whether pattern waviness due to line-edge-roughness enhances wiggling, distortion of straight and wavy patterns was numerically analyzed by the three-dimensional (3D) elastic finite element method. Wiggling occurs only in wavy patterns but not in straight patterns at a stress or aspect ratio much lower than their buckling thresholds. More severe wiggling occurs when the wavelength of initial waviness approaches a value that is 3.3 times the pattern height. These phenomena were experimentally confirmed in the etching of amorphous carbon with a SiON mask. We consider that precise etching without wiggling is achieved by the elimination of the original line-edge roughness and the reduction in mechanical stress in an underlying film to which the pattern is transferred.

Characteristics of selective PMMA etching for forming a PS mask

The characteristics of poly(methyl methacrylate) (PMMA) etching of self-assembled poly(styrene-block-methyl methacrylate) (PS-b-PMMA) thin film for forming a polystyrene (PS) mask were investigated. In this investigation, first, the etching selectivity of PMMA to PS under argon- and oxygen-plasma processes was evaluated. Higher selectivity was obtained in the case of argon plasma (3.9) compared to that of oxygen plasma (1.7). Second, to investigate the argon process in detail, the time dependence of etching depth was evaluated. It was found that PMMA etching rate decreases by more than half after etching to a depth of around 15 nm. To investigate the mechanism of this decrease in PMMA etching rate, the surface composition of PMMA was measured by X-ray photoelectron spectroscopy (XPS). The XPS result revealed that the reduction of etching rate is caused by a depletion of oxygen by argon ions, and the depleted oxygen attaches to the PMMA film in air exposure. In accordance with these results, to compensate the decrease in oxygen concentration, oxygen was added to the argon plasma at a composition of 1%. As a result of this oxygen addition, constant PMMA etching rate was confirmed, even beyond etching depth of 50 nm. It is thus concluded from these results that a PS lamellar mask pattern with a pitch from 25.5 to 70 nm could be successfully formed by using selective PMMA etching.

Reducing hole-size variation and defect ratio after pattern transfer by using self-assembled polymer with spherical structure

This study presents a method to reduce hole-diameter variation and defect ratio in patterning of a self-assembled block copolymer (BCP) for imprint-mold fabrication. The BCP material used is PMMA-b-poly(methyl acrylate) polyhedral oligomeric silsesquioxane (PMAPOSS) in which PMMA spheres with 18.3-nm-pitch are aligned in the hexagonal close-packed positions in the PMAPOSS matrix. When the self-assembled BCP film was etched in the conventional dry-development process, the hole-diameter variation and the amount of hole defects (defect ratio), defined as “no-opening defects” or “connecting holes,” increased. Variation of PMMA sphere diameter and/or position in the perpendicular direction to the substrate plane was assumed to be the main cause of the increase in hole-diameter variation and defect ratio after BCP development. To optimize the etching conditions for BCP development, a new model representing the relationship between defect ratio and relative standard deviation of PMMA sphere diameter and/or posit...

Decrease in Ozone Density of Atmospheric Surface-Discharge Plasma Source

A surface-discharge plasma source based on dielectric barrier discharge (DBD) has been developed for use in sterilization and cleaning. In these processes, ozone-generation ability is one of the key factors in regard to atmospheric plasma sources. However, it was observed that ozone density decreased during plasma discharge. It is known that an increase in gas temperature decreases the ozone density; thus, the gas temperature in plasma was measured from the rotational temperature of nitrogen molecules. It was confirmed that the gas temperature increases in the case that the ozone density decreases. A dielectric-surface temperature of the plasma source was also measured and its behavior agreed with that of the gas temperature since the thickness of the plasma was small. It is thus confirmed that cooling the dielectric surface of the plasma source is important to increase the ozone density in a surface-discharge plasma source.

Bottom profile degradation mechanism in high aspect ratio feature etching based on pattern transfer observation

The hypothetical mechanism of bottom profile degradation, such as distortion and twisting in high aspect ratio feature etching, was verified based on the pattern transfer observation of etched pattern. The authors mainly focused on trench pattern sample to make the investigation easier, that is, direct observation of the sidewall roughness, using an atomic force microscope, as well as analysis of the depth dependence of pattern deformation in high aspect ratio trench etching. Using Fourier transformation analysis for the trench sidewall roughness, it was found that lower spatial frequency component of the masks sidewall roughness is amplified at the bottom region of the trench and that higher spatial frequency component of over 10 μm−1 disappears. However, the higher spatial frequency component is transferred directly to the upper sidewall of the trench. The observation of the pattern deformation profile, as a function of etch depth, revealed that the ratio of line width roughness to line edge roughness ...

Atomic layer etching of silicon nitride using cyclic process with hydrofluorocarbon chemistry

In this study, atomic layer etching (ALE) of silicon nitride (SiN) using a cyclic process with monofluoromethane chemistry was investigated. Results show that an appropriate desorption time must be chosen at a specific adsorption time to achieve SiN ALE. The results also show that the infinite selectivity of SiN over Si can be achieved using the cycle process. To further understand this behavior, the adsorption and desorption effects were also studied. The results revealed a mechanism to obtain a high Si selectivity and a dominant factor that causes the Si loss. To further understand the ALE capability, we studied and compared the etched profiles and resulting surface roughness obtained by both a conventional process and an ALE process. The results show that the ALE process can achieve a high Si selectivity and a non-detectable level of Si surface damage, compared with a conventional continuous etching process.

Silicon Mold Etching with Hard Mask Stack Using Spherical Structure of Block Copolymer for Bit-Patterned Media with 2.8 Tbit/in.2

We investigated a silicon mold fabrication that uses a hard mask stack by using poly(methyl methacrylate)-block-poly(methacrylate polyhedral oligomeric silsesquioxane) (PMMA-b-PMAPOSS) as the block copolymer (BCP) to assemble nano-patterns for a nano-imprint lithography process during bit-patterned media manufacturing. We developed a dry development process comprised of a single step by taking both the selectivity and anisotropy into consideration, which enables us to create hole patterns by using an array of PMMA spheres embedded in a PMAPOSS matrix. The availability of this process was evaluated from the experimental results that showed that hole patterns at several areal densities were successfully obtained by adjusting the process time under a fixed etching condition. The capability of the pattern transfer to a hard mask from the hole patterns of residual PMAPOSS could be improved by changing the hard mask material from SiO2 to amorphous carbon based on the results from an X-ray photoelectron spectroscopy (XPS) surface analysis. Silicon molds with areal densities of up to 2.8 Tbit/in.2 were successfully fabricated by using an optimized process condition and the hard mask stack.