Kazunori Tsujimoto

Low‐temperature reactive ion etching and microwave plasma etching of silicon

A new low‐temperature reactive ion etching and microwave plasma etching method is described. Highly anisotropic silicon etching with extremely small width shifts has been performed with high selectivities of 30 for organic resist films. High etch rates of 500 and 1000 nm/min by reactive ion etching and microwave plasma etching, respectively, were achieved with a SF6 gas plasma at low wafer temperatures from −130 to −100 °C. It is concluded that lower temperatures during plasma treatment yield lower side etching and increase the dry etch resistance of organic masks.

Low‐temperature dry etching

Low‐temperature electron‐cyclotron‐resonance microwave plasma etching and reactive ion etching are described for ULSI device fabrication. Highly selective anisotropic etching at a high rate, which implies dry etching without tradeoffs, is performed without changing the discharge parameters. This etching is only achieved at reduced wafer temperatures. The etching mechanism and the model are discussed based on the etching yield results obtained by the mass‐selected reactive ion beam etching experiments. The new etching system and the etching properties obtained for the low‐temperature etching are reviewed comparing those obtained in the conventional reactive ion etching and electron‐cyclotron‐resonance microwave plasma etching.

Deposition in Dry-Etching Gas Plasmas

Polymer deposition on Si and SiO2 surfaces has been investigated in CH2F2, CHF3, CF4, and CHClF2 gas plasmas, using a microwave plasma etching system. The dependence of the deposition rate on gas pressure, RF bias power, and substrate temperature was measured at a temperature between -120°C and 150°C. The deposition rate increased with decreasing temperature in CH2F2, CHF3, and CHClF2 plasmas. The deposition of polymers occured only below -60°C in the CF4 plasma. The obtained dependence of the deposition rate on gas pressure was examined in terms of the volume of adsorbed particles. X-ray photoelectron spectroscopy measurement showed that the number of bondings between C and F atoms in deposited polymers increases with decreasing temperature and RF power, and increasing gas pressure.

Low-Damage Damascene Patterning Using Porous Inorganic Low-Dielectric-Constant Materials

The degradation of porous low-dielectric-constant (low-k) materials, such as porous methyl silsesquioxane (MSQ) and porous chemical-vapor-deposited SiOCH films, by damascene etch and ash processes has been investigated. The influence of etch damage becomes relatively stronger as ash damage improves. The etch damage to porous low-k dielectric is found particularly at the sidewall of the isolated lines. By investigating the influence of etch conditions on the damage, it is found that O2 and Ar addition causes large sidewall damage. O2-added etch condition, as well as the O2 ash process, produces oxygen radicals, which extract CH3 groups from porous low-k films. On the other hand, Ar plasma does not efficiently extract CH3 groups differently from O2 plasma, but it changes the bonding states of CH3 groups and causes H2O adsorption. This change in film characteristics increases dielectric constant. Finally, we successfully achieved low-damage dual damascene patterning using a porous SiOCH material whose dielectric constant is 2.2.

Low-Temperature Microwave Plasma Etching of Crystalline Silicon

Low-temperature microwave plasma etching of crystalline silicons is described. Vertical and lateral etch rates of Si and the selectivities of Si to photoresist are measured as a function of water temperature within a range of -150 to +30° for SF6, and (3) the selectivities become high at low temperatures (e.g., >40 at -90°C and 2.3 Pa). This etching enables highly anisotropic Si etching at a high etch rate and high selectivity with fluoride gases. Less-polymerizing-type gases can provide high etch rates. An etching model of the ion-bombarded surfaces in discussed. The model implies that separate control of the side wall reaction and the horizontal surface reaction is archived by the low-temperature etching.

Mechanism of Radiation Damage in SiO2/Si Induced by vuv Photons

The generation yield (Rf) of effective positive charges at an SiO2/Si interface, induced by a vacuum ultraviolet (vuv) photon, is measured as a function of the substrate temperature by a low-temperature microwave plasma etching apparatus. The Rf decreases monotonously with decreasing temperatures from 300 K down to 120 K. The model is verified by analyzing the experimental results according to the effective positive charge generation model previously proposed. From this analysis, it is proposed that an ultra low-damage plasma process becomes possible by eliminating the holes generated by the vuv photons from the SiO2 film keeping the specimens at low temperatures.

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...

Novel short‐gas‐residence‐time electron cyclotron resonance plasma etching

Novel short‐gas‐residence‐time electron cyclotron resonance (ECR) plasma etching is described. Using a newly equipped high‐pumping‐rate etching system (5000 l/s), we obtained a high etch rate and high anisotropic etching of silicon at low pressure and high gas flow rate. The residence time obtained was 30 ms. The silicon etch rate with Cl2 dramatically increased up to 1 μm/min as the gas flow rate increased to 90 sccm at 0.5 mTorr. It was proven by plasma emission measurement that the reaction products were minimized by the present method. These results indicate that the short‐gas residence time produces a small amount of reaction products and a large amount of etching species.

Lower plasma‐induced damage in SiO2/Si at lower temperatures

We found that the radiation damage induced in a SiO2/Si system during plasma processing depends strongly on the specimen temperature. The surfaces of the SiO2 have been exposed to a microwave plasma at different temperatures and the resultant damage has been evaluated by capacitance‐voltage (C‐V) measurements. The flatband voltage shift (ΔVFB) for the specimen exposed to the plasma at 126 K has been found to be only 1/3 of that at 300 K. In case of vacuum ultraviolet photon irradiation through a thin Al film, the ΔVFB for the irradiation at 126 K has been only 1/5 of that at 300 K. It is believed that this lower plasma‐induced damage at the lower temperature is due to the small mobility of hole in SiO2 at lower temperatures. Plasma etching at low temperature has the advantage of low damage generation in the SiO2/Si structures.