Hitoshi Nagata

Photolithography system using annular illumination

Photolithography is a leading technique in LSI device fabrication. The LSI pattern size has approached the exposure wavelength such as the g or i-line of a Hg lamp. This fact indicates that the shorter wavelength or some novel technique will be needed in order to cope with finer patterns. It is known that annular illumination can improve the depth of focus and resolution. We applied the annular illumination method to the step and repeat exposure system. Experiments and simulations using annular illumination were carried out and subhalf-micron patterns were produced. The process latitudes of the annular illumination method are evaluated.

Etching yields of SiO2 by low energy CFx+ and F+ ions

Etching yields of SiO2 by mass‐separated F+, CF+, CF2+, CF3+, and Ar+ ions have been measured at low ion energies ranging from 80 to 350 eV. CF3+ and CF2+ ions have higher etching yields than CF+ and F+ ions. At low ion energies, SiO2 cannot be etched and some film deposition is observed on the SiO2 surface. For example, in the case of CF+ ions, SiO2 can be etched at ion energies above 200 eV. This film deposition is caused by reactions of CFx+ ions at the SiO2 surface, and neutrals coming from the ion source also have some effect on this deposition.

Wetting Behavior of Liquid on Geometrical Rough Surface Formed by Photolithography

The effect of roughness on the wettability is studied. The rough surface is constructed with photoresist micropatterns which are formed by photolithography. The aspect ratio is defined as the length ratio between the pattern space and its height. Contact angles of three kinds of liquids, pure water, diiodomethane and ethylene glycol, increase and saturate with the increase of the aspect ratio. The wetting behavior of pure water of which higher surface energy is extremely complex.

Photolithography System Using Modified Illumination

Since the LSI pattern size has approached the exposure wavelength, various methods have been developed to overcome limitations in photolithography. This fact indicates that a shorter wavelength or some novel techniques will be needed in order to cope with finer patterns. The modified illumination method was proposed in order to improve the depth of focus and resolution limit. We applied the modified illumination method to the step-and-repeat exposure system. Experiments using modified illumination were carried out and sub-half-micron patterns were produced. The process latitudes of the modified illumination method are evaluated.

Adhesion of Photoresist Pattern Baked at 80 to 325°C in Tetramethyl-ammonium-hydroxide Aqueous Solution

Adhesion characteristics of a photoresist thin film baked onto a spin-on SiO2 layer at 80 to 325°C are studied in a tetramethyl-ammonium-hydroxide (TMAH) developer. The maximum value of adhesion strength is observed in the temperature range of 150 to 250°C. It is found that the surface energy model cannot explain all of the experimental results. The following two models are introduced as new adhesion factors: (i) decrease of surface energy of the TMAH developer due to dissolution of a photoresist into it above 250°C and (ii) decrease of mechanical strength of a photoresist pattern by swelling under 150°C.

Characterization of surface energetic behavior by atomic force microscopy

Inorganic solid surfaces which have various surface free energies, 30~65 dyn/cm, were probed using a Si3N4 cantilever tip. It was found that the attractive force (~10-7 N), which is generated when a cantilever tip moves away from a solid surface after making contact, is more sensitive to the behavior of surface free energy than that (~10-9 N) which is generated when the tip moves toward the surface. The relationship between the elastic potential energy of the cantilever and the adhesion energy of the tip-surface system is also discussed.

Characteristics of Adhesion between Photoresist and Inorganic Substrate

The adhesion of positive photoresist film to some inorganic substrates used as LSI substrate material in dry and wet conditions has been investigated based on surface chemistry. The method of surface energy treatment is used to predict the adhesion energy. The contact angle method is used to measure the surface energy of each substrate. The correlation between the value of two components of surface energy, the polar and the dispersion parts, and adhesion strength is discussed. It is important to control the polar part of surface energy of a substrate in order to achieve high adhesion strength.

Photolithographic system using modified illumination

A modified annular illumination method known as PHOENEX (Photolithography enhanced by modified exposure) is discussed. Compared with annular illumination, the modified illumination effectively cuts off the background element, which does not contribute to the imaging. In this study, the process latitudes of the modified illumination are evaluated using experiments and simulations. The DOF limit is doubled compared to the conventional method. The optical contrast is improved by the shifter-shade-type phase shift mask. The resolution limit is also improved by the modified illumination.<<ETX>>

Blister Formation at Photoresist-Substrate Interface

We report the formation mechanism of a blister at the photoresist-substrate interface during exposure to ultraviolet (UV) light. It is found that the blister formation strongly depends on the following factors: (i) photoactive-compound (PAC) concentration of the photoresist film and (ii) adhesion energy between the photoresist film and the substrate. Formation parameter U, which represents the difference between the adhesion energy and the strain energy of the photoresist film, can be utilized for predicting the blister formation.

Adhesion between Photoresist and Inorganic Substrate

Adhesion strength of a photoresist during pattern development decreases as penetration of an alkaline aqueous solution into the photoresist film increases. The penetration is accelerated by increasing the quantity of residual solvent in the photoresist film. Resistance measurements confirmed the penetration. The tensile stress of photoresist films increases as the penetration increases. The creation of inner stress also causes adhesion failure during HF etching. These phenomena cannot be explained by a balance model of surface energy. Therefore, we conclude that the increase in tensile stress causes adhesion loss during both pattern development and HF etching.