Naoyuki Kofuji

Short‐gas‐residence‐time electron cyclotron resonance plasma etching

This paper describes short‐gas‐residence‐time electron cyclotron resonance plasma etching for high etch rates, reduced contamination, and highly anisotropic etching. The new high‐gas‐flow‐rate (high‐flow) etching system is demonstrated with effective‐pumping rate of 2500 l/s. This method produces very high etch rate while maintaining high anisotropy at very low gas pressure below 1 mTorr. The high etch rate is due to the reduction of reaction products density because of the very short gas‐residence time of 30 ms. This technique also dramatically reduces the contamination of reaction products. Etching of crystalline Si and n+ polycrystalline Si with the high‐flow etching system is demonstrated. For crystalline Si etching with Cl2, a high etch rate up to 1 μm/min is achieved at a high gas flow rate of 90 sccm at 0.5 mTorr.

NEAR-SURFACE INTERACTIONS AND THEIR ETCHING-REACTION MODEL IN METAL PLASMA-ASSISTED ETCHING

Reactive interactions in plasma etching have been investigated. Simple gas-phase transport of etchants and the reaction by-products in the wafer near-surface area are discussed. A new reincidence parameter, determined with a proposed near-surface model, was used to formulate metal etch rates. The experimental results obtained from an electron cyclotron resonance microwave plasma etching system revealed that the measured etching rate agreed well with those obtained by the near-surface model. It was found that reaction by-products repeatedly arrived at the surface depending on the reincidence numbers for the metal etching. The reincidence is the result of the diffusional transport in the vicinity of the wafer and is given by the expression {(one-half of the wafer radius)/(mean-free path)}. The ratio of the by-product flux is expressed by the product of the etching-rate flux times the reincidence number. Then, the resulting ratio of the reaction products in the flux becomes very high when we compare it to th...

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.

Reduction in Microloading by High-Gas-Flow-Rate Electron Cyclotron Resonance Plasma Etching

High-gas-flow-rate electron cyclotron resonance plasma etching was employed to reduce microloading in Si etching with Cl2 at low pressure. Microloading estimated with a conventional etching system increases with decrease in pressure from 5 to 0.5 mTorr. The increase in microloading is attributed to the increase in the ratio of ion flux to reacting neutrals. The ion/neutral ratio was found to be as large as 6.4 at 0.5 mTorr. This large ratio was caused by both the decrease in reacting neutral density and the increase in reaction products. The high gas flow rate with a high effective pumping speed of 2000 l/s reduces the reaction products, increases the reacting neutrals and reduces the ion/neutral ratio to 0.65. As the result, the microloading is reduced. The relative etch rate at an aspect ratio of 7 increases from 0.65 at 136 l/s to 1.00 at 2000 l/s. Thus high-gas-flow-rate-etching solved the problem of large microloading which is not suppressed even with high density plasma and low gas pressure.

Creation of a Degraded Layer on the Surface of Photoresist by Radical Irradiation

A novel mechanism of the inclination of a photoresist mask based on the experimental evaluation of the change in the quality of the irradiated surface layer of a material was proposed. In this proposed mechanism, the irradiation of oxygen and fluorine radicals generates a very thin degraded surface layer with a high tensile stress and causes the inclination of the mask. It was found that a very thin (approximately 5 nm thick) layer with a high tensile stress appeared after the irradiation of radicals on the surface of the photoresist mask. X-ray photoelectron spectroscopy (XPS) confirmed that irradiation of either of these radicals generates a very thin degraded layer on the photoresist surface.

Uniform lateral etching of tungsten in deep trenches utilizing reaction-limited NF3 plasma process

The reaction-limited etching of tungsten (W) with NF3 plasma was performed in an attempt to achieve the uniform lateral etching of W in a deep trench, a capability required by manufacturing processes for three-dimensional NAND flash memory. Reaction-limited etching was found to be possible at high pressures without ion irradiation. An almost constant etching rate that showed no dependence on NF3 pressure was obtained. The effect of varying the wafer temperature was also examined. A higher wafer temperature reduced the threshold pressure for reaction-limited etching and also increased the etching rate in the reaction-limited region. Therefore, the control of the wafer temperature is crucial to controlling the etching amount by this method. We found that the uniform lateral etching of W was possible even in a deep trench where the F radical concentration was low.

Mechanism of wiggling enhancement due to HBr gas addition during amorphous carbon etching

The effect of gas chemistry during etching of an amorphous carbon layer (ACL) on wiggling has been investigated, focusing especially on the changes in residual stress. Although the HBr gas addition reduces critical dimension loss, it enhances the surface stress and therefore increases wiggling. Attenuated total reflectance Fourier transform infrared spectroscopy revealed that the increase in surface stress was caused by hydrogenation of the ACL surface with hydrogen radicals. Three-dimensional (3D) nonlinear finite element method analysis confirmed that the increase in surface stress is large enough to cause the wiggling. These results also suggest that etching with hydrogen compound gases using an ACL mask has high potential to cause the wiggling.

Negative Impact of Etched Si Area on Selectivity and Positive Impact of Photoelectric Current on Etched Profile in Gate Etching with Different Wafer Bias Frequencies

The effect of wafer-bias frequency on the dummy-gate fabrication of fin-shaped field-effect transistor (Fin-FET) was investigated. The clear difference in the selectivity of polycrystalline silicon to SiO2 between 400 kHz and 13.56 MHz decreased when the etched Si area increased. On the other hand, a higher frequency increased such selectivity when Si area decreased. These results can be explained by the effect of by-product deposition. As for the etched profile, the amount of side etching was much larger at 13.56 MHz than at 400 kHz. It was reported that this phenomenon is caused by local charging. It was also suggested that the charging should be suppressed by reducing the ratio of ion saturation current to photoelectric current. Therefore, in this study, we investigated the effect of such current ratio on side etching. The result confirmed that a reduction in current ratio induced by increasing gas pressure decreases the amount of side etching.

Investigation of Mask Inclination Due to Oxygen-Radical Irradiation during Resist Trimming

A novel analytical method for predicting the inclination of a resist mask during its trimming process has been proposed by considering the formation of the surface degraded layer with high compressive stress. It was found that the irradiation of oxygen radicals creates a degraded layer on the resist surface and causes high compressive stress in it. A nonuniform spatial distribution of oxygen radicals, therefore, causes an asymmetrical stress field on the resist-mask surface. Such an asymmetrical stress field distorts the resist mask. The well-known Deal–Groves oxidation model was modified for the trimming process by considering the time-dependent change of the spatial distribution of oxygen radicals on the mask surface. This model successfully explains the observed complicated time-dependent deformation of the resist mask.

Reduction of charging damage of gate oxide by time modulation bias method

The charging damage of gate oxide films caused during plasma etching is reduced by the time modulation (TM) bias method. The radio frequency (RF) bias applied to the substrate is pulse modulated to control the energy and the quantity of accelerated ions. The charging potential of line patterns caused by the electron shading effect decreases during off periods of RF bias. In addition, the emission intensity signal of the optical endpoint monitor indicates suppression of reactive ion etching (RIE) lag. These factors result in the reduction of charging damage.