Photomask Materials Research Articles

Assessing stability of metal tellurides as alternative photomask materials for extreme ultraviolet lithography

Tellurium (Te) is one of the elements with highest extinction coefficient κ at the 13.5 nm extreme-ultraviolet (EUV) wavelength. It is being considered as an alternative absorber material for binary photomasks in EUV lithography. The absorber material is required to remain chemically stable during EUV exposure, at elevated temperatures up to 150 °C, during mask cleaning, and in the low hydrogen pressure environment that is present in the EUV scanner. However, Te is known to react with oxygen and hydrogen, forming less EUV absorbing TeO2 and more volatile H2Te, respectively. Since the melting temperature of Te is only 449.5 °C at normal pressure, alloying Te with a more stable metal might result in a high κ material that will remain thermally and chemically stable over a wider range of operating conditions. In this paper, the authors report on the stability assessment of metal telluride (M-Te) alloys for the EUV absorber material. They combined Te with high κ metals, noble metals, and etchable metals. High κ and noble M-Te materials are both thermally more stable than etchable M-Te, but they cannot be patterned easily for use in an EUV photomask. High κ M-Te exhibits polycrystal morphology at room temperature compared to quasiamorphous noble M-Te though both can crystallize at a higher temperature. Hydrogen stability and cleaning solution stability of M-Te materials are improved considerably compared to Te, but their higher surface reactivity cannot be completely mitigated without the addition of an inert capping layer. Furthermore, etchable M-Te alloys are easily oxidized during deposition, resulting in lower electron density and hence lower κ. Nevertheless, M-Te alloys may be a way to stabilize Te for usage as the EUV absorber material.

Immersion Lithography: Photomask and Wafer-Level Materials

Optical immersion lithography utilizes liquids with refractive indices >1 (the index of air) below the last lens element to enhance numerical aperture and resolution, enabling sub-40-nm feature patterning. This shift from conventional dry optical lithography introduces numerous challenges requiring innovations in materials at all imaging stack levels. In this article, we highlight the recent materials advances in photomasks, immersion fluids, topcoats, and photoresists. Some of the challenges encountered include the fluids' and photomask materials' UV durability, the high-index liquids' compatibility with topcoats and photoresists, and overall immersion imaging and defectivity performance. In addition, we include a section on novel materials and methods for double-patterning lithography—a technique that may further extend immersion technology by effectively doubling a less dense pattern's line density.

Mask-induced polarization effects at high numerical aperture

Degradation in image contrast becomes a concern at higher numerical apertures (NAs) due to mask-induced polarization effects. We study how different photomask materials (binary and attenuated phase shift), feature sizes and shapes, pitch values, duty ratios (line to space), and wavelengths effect the polarization of transmitted radiation. Rigorous coupled-wave analysis (RCWA) is used to simulate the polarization of radiation by the photomask. The results show that higher NA leads to greater polarization effects in all cases. Off-axis illumination increases polarization in one of the first orders, decreasing it in the other. Nonvertical sidewall angles and rounded corners can also impact polarization, but the wavelength of incident radiation has no effect on polarization effects at the same NA values. In general, materials with higher refractive indices and lower extinction coefficients tend to pass more of the TM polarization state, whereas materials with lower refractive indices and a relatively wider range of extinction coefficients pass more TE polarized radiation. These properties can provide new design considerations for the development of next-generation masking materials.

New Aqueous-base Soluble Resin and Its Applicationto Negative and Positive Tone EB Resists for Mask-making

Acid-catalyzed condensation resins of naphthol compounds (1-Naphthol/1,6- Dihydroxynaphthalene (1,6-DHN)) with 2,6-Bis(hydroxymethyl)-p-cresol (BHMC) were investigated for photo-mask materials shading that has a 248-nm wavelength. The new naphthol resins were aqueous-base soluble and have sufficiently large optical density (Absorbance>11/?m) at the wavelength. The new resins have alternative structures of naphthol units bonded p-cresol by a methylene bridge. Evaluated trial naphthol-resin-based negative and positive EB resists showed sub-micrometer resolution (0.5?m lines and spaces) and high absorbance, 9/μm and 12/μm, respectively, at a wavelength of 248 nm.

157 nm: Deepest deep-ultraviolet yet

Lithography at 157 nm is rapidly emerging as the industry-preferred technology for the post-193 nm era. Its target application is for the 100 to 70 nm generations, and it is therefore widely viewed as a “bridge” technology before the next-generation lithographies are ready for insertion into manufacturing. Its attractiveness stems from the overlap in many areas with current practice and shared infrastructure developed for longer wavelengths. This article will review the present status of 157 nm lithography, emphasizing the technological challenges in the various subsystems: lasers, optical materials and coatings, photomask materials, photoresists, and projection tool development. Viewed as a whole, recent developments in these diverse areas are cause for cautious optimism that indeed 157 nm lithography will be ready in time, without encountering unforeseen obstacles.

Effects of fluorine dimer excimer laser radiation on the optical transmission and defect formation of various types of synthetic SiO2 glasses

Changes in the optical absorption and the formation of point defects in three types of synthetic SiO2 glasses, wet (OH content; 120 ppm), dry (OH content <1 ppm), and fluorine (F)-doped (∼1 mol %) SiO2 glasses, by irradiation with fluorine dimer (F2) excimer laser light pulses (∼8 mJ/cm2/pulse×3.6×105 pulses) were examined by various spectroscopic methods. Intense optical absorptions were induced in the wet and dry silicas in the range of 4–8 eV, whereas the intensity of absorptions induced in the F-doped silica was smaller by an order of magnitude than that in the F-free glasses. The optical transmission at the wavelength of 157 nm after the irradiation was F-doped silica≫wet silica>dry silica. The dominant electron spin resonance-active defect in the irradiated specimens was the nonbridging oxygen-hole center (NBOHC) for the wet silica, or the E′ center in the dry silica. The concentration of NBOHCs or E′ centers in the F-doped silica was lower by an order of magnitude than that in the wet or dry silica. The present results suggest the possibility of using F-doped silica glasses as photomask materials for F2 laser lithography.

High-performance VLSI photomask with a molybdenum silicide film

A molybdenum silicide (MoSi) film deposited on a quartz substrate offers major advantages as a high-performance photomask material for VLSI fabrication. There is no missing pattern due to exfoliation after ultrasonic cleaning with a frequency of 28 kHz and 300 W of power, and after being scrubbed over fifteen cycles with a high-pressure water jet. Reflectivity and optical density of the MoSi film are not affected by acidic chemicals. Dry etching with CF 4 + O 2 gas plasma can be performed at a rate of 70 nm/min, which is about five times as fast as the etch rate for chromium (Cr) masks. Moreover fabrication of submicron patterns using poly(buten-1 sulfone) (PBS) resist, a positive electron beam resist with very poor resistance to dry etching, has been realized. In addition a photomask with a MoSi film is easier to repair by focused ion beam than one with a Crfilm because of the higher sputter yields for a MoSi film.

Optical Lithography at 248 nm

The krypton fluoride excimer laser has provided a means of exploiting the deep‐ultraviolet for optical lithography. The use of this new light source requires the development of new photoresists and opens up a new set of concerns for the lithographer. Durability of lenses, coatings, and photomask materials are potential problems because of the very high peak power emitted by the pulsed laser. Optical interference effects, caused by the coherence of the laser light, could also cause trouble. Finally, the laser itself has high requirements for reliability and safety.

Photomask and reticle materials review : Ron Iscoff. Semiconductor int., 82 (March 1986)

Photomask and reticle materials review : Ron Iscoff. Semiconductor int., 82 (March 1986)

High performance very large scale integrated photomask with a silicide film

A molybdenum silicide (Mo-silicide) film deposited on a quartz glass substrate offers major advantages as a high performance photomask material for very large scale integrated (VLSI) fabrication. There is no pattern missing due to exfoliation after ultrasonic cleaning with frequency of 28 kHz and 300 W of power, and after being scrubbed over 10 cycles with a high pressure water jet. Reflectivity and optical density of the Mo-silicide film are not affected by acidic chemicals. Moreover, dry etching can be done at a rate of 50 nm/min; more than five times as fast as etching of a chromium (Cr) mask.