Authors would like to raise a discussion about image intensity for surface exposure, off course, including optical
lithography. As a springboard for the discussion, a novel definition of image "intensity", which expresses local
irradiance associating with optical image, is proposed. An experimental result, which strongly supports the proposed
"intensity", is also obtained.
To describe exposure dose, energy input for unit area with unit of J/m2, is applied as a measure of this amount. A
phrase of "dose-to-clear" is frequently used to show sensitivity of a resist film. In contrast, conventional image intensity
of optical image is defined as a value, which is proportional to volume energy density associating with image. The value
is described with unit of J/m3. In some papers, it is mentioned that number of photochemical reactions in resist film is
proportional to the volume energy density of electromagnetic filed, that is, conventional image intensity. It seems
unclear what physical value is proper measure of surface exposure.
We considered that, in optical lithography, energy flux is proper value to indicate degree of resist exposure from
experience and some former reports. Then, a novel image "intensity", which expresses local irradiance associating with
optical image, is proposed. The proposed image "intensity" is proportional to surface normal component of Poynting
vector.
A novel process of OPC-free on-grid fine random hole pattern formation is developed. Any random hole pattern with
~120nm diameter on 240 nm base grid can be printed by KrF exposure. In this technique, double resist patterning
scheme is adopted. Dense hole pattern is delineated with first resist process. Quadrupole illumination is applied with
embedded attenuating phase shift mask (EA-PSM) in imaging on this step. As is well known, fine dense hole pattern
is formed with very large process latitude. After development of the first resist, hardening of the resist film by Ar ion
implantation is carried out so as not to mix with second resist at second coating. This hardening process is very robust
such that rework in second resist process can be performed with stripping the resist by a solvent. Then, second resist
patterning is carried out. In the second exposure, cross-pole illumination is applied with high transmission EA-PSM.
By this imaging, very fine dark spot image is generated. Resultantly, fine random pillar patterns, which plug an
underlying hole, are formed in the second resist film. Because function of the pillar is plugging a hole, no precise CD
control is required. Moreover, pattern connection between adjacent pillars does not cause any problem. Hence, no
OPC is needed in the pillar formation, regardless of printed size variation of the pillars. Undesired holes in the dense
holes are plugged by the pillars. As a result of the double resist patterning, on-grid random hole pattern is successfully
delineated. Due to the robustness of each patterning process, very high process latitude is achieved. Off course, this
technique can be carried out under any wavelength on regard of imaging. In other aspect, this technique utilizes only
positive-tone resist. Hence, this technique can be applied with leading-edge ArF immersion lithography. As a
conclusion, this technique is a promising candidate of hole pattern formation in 32nm era and beyond.
A novel RET, "Super Diffraction Lithography" (SDL), which enable 70 nm any pitch line by single exposure in KrF wavelength, has been studied in order to apply for an actual device pattern formation. In a previous work, the concept of SDL has been described with optical image calculations for 1-dimensional patterns and very superior performance has been proved. In this work, imaging characteristics and printing performance of typical 2-dimension patterns are investigated by optical image calculations and printing experiments to realize an application of SDL technique to fabrication of actual device patterns. As a result, very good performance is achieved for the typical 2-dimentional patterns such as line-end, tee-branch. Moreover, good performance is obtained for general SRAM patterns and standard cell of 65 nm node logic device with a little relaxation of design rule. In conclusion, by the application of SDL, 65 nm SoC patterns with a little relaxed design can be formed by single exposure process in KrF wavelength with a simple Atten-PSM. Then, huge cost reduction can be expected by application of SDL.
A simple and high sensitive focus monitoring has been developed utilizing an aperture in Cr film formed on backside of photomask. A special mask for focus monitoring is developed such that two mark patterns on the front side of the mask are irradiated by different illuminations. The different illuminations for the two marks are generated from usually used illumination with modulation by an aperture on the backside of the mask. In this work, two complementally halves of usually used illumination are effectively generated. Because illumination for each mark pattern on front side of the mask is strongly asymmetric in incident angle such that illumination beam impinged from only one side of the space, imaging of the large size mark pattern is carried out obliquely on the wafer. As a result, image is laterally shifted with focus. The direction of lateral image shift is opposite to that of another mark which is irradiated with illumination beams from opposite side of the space. Thus, the relative displacement between the two mark images may become a measure of focus. Because this focus monitor works under purely geometrical optics, focus monitoring of multiple steppers, which are working under different wavelength, can be performed with the same one photomask. In experiments, the two mark patterns, which are inner and outer box patterns, are printed with overlaying each other by double exposure with stepping of wafer stage. Then, mutual displacement of mark patterns is measured by a commercially available overlay measurement tool whose resolution is a few nm. Very high focus sensitivity (Δx/Δz) of ~0.9 is observed for NA=0.68 optics with strong annular illumination. Because of the high focus sensitivity and high resolution of overlay measurement, focus monitoring with very high resolution of a few nm can be achieved.
To get fine patterns of ArF photoresist without pattern collapse, we studied the relation between optical property of pattern image and adhesion of photoresist pattern. In concern of the type of photo mask, we found that using attenuated phase shift mask could make experimental small resolution limit beyond the estimation by simulation. About ARC substrate structure (inorganic ARL/ oxide/ polySi), it was important not only to optimize the reflectivity, but also to optimize the phase of reflectance. Photoresist was easy to collapse when the phase of reflectance at the interface between photoresist and inorganic ARL is near the 0 degree, although the reflectivity was set below 1%. In order to change the phase of reflectance, the film thickness of oxide was varied. In the observation of photoresist profile, bottom profile was changed similar to simulation. In the case of organic bottom ARC, we could not observe the effect of the phase.
We have investigated the influence of a spherical aberration on the printing characteristics with modified illumination. At first, we have developed a simple method for measuring the aberration function with an alternating phase shift mask (PSM), and have measured that in the projection optics of a commercially available KrF stepper. Then the anomalous phenomena observed in the printing with modified illumination are examined with the simulated aerial images with the measured spherical aberration. As a result, we found good coincidence between the simulated images and the anomalies. In conclusion, the origin of the anomalies is ascribed to the spherical aberration in the projection optics.
The etching rates of resist base polymers with several molecular weights were measured against fluorocarbon or chlorine plasma. The rate showed a minimum value at the weight-average molecular weight of approximately 10,000, and increased to a saturated value for higher molecular weights. For poly(p-hydroxystyrene) (PHS) protected with tert- butoxycarbonyl (BOC) or acetal (ACT) group, the rate became larger with increasing the protection ratio and the rate of BOC-protected PHS was lower than that of ACT-protected PHS. It was also found that the rate was significantly influenced by the prebake and increased as the temperature became higher. The etching rate of the resin with gradual cooling after the prebake was slower than that with quick cooling. These results may indicate that the dry etching resistance is independent of the density of resin film and influenced by the stability of arrangements of polymer molecules. The reduction of etching rate by the deep UV cure method has been tried. The rate decreased with the deep UV cure under N2 gas flow, while increased in the presence of O2 such as dry air. The films after deep UV cure under N2 and a dry air was scarcely dissolved in any organic solvents and this implies that the polymers are crosslinking. The absorbance band of carbonyl appeared in FT-IR spectra for the case under dry air may suggest that oxygen atoms in the polymer structure influence the dry etching resistance. We have tried deep UV cure under N2 gas flow for KrF chemically amplified PHS based resists and obtained the same level of etching rates as novolak resin. For ArF resists, the combination of the incorporation of acryl unit into the base polymer and deep UV cure under N2 gas flow may be an effective method for improving the dray etching resistance.
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