Jason T. Wright

Photolithography with polymethyl methacrylate (PMMA)

Polymethyl methacrylate (PMMA) is widely used as an electron beam resist but is not used as a photoresist because of its insensitivity to electromagnetic radiation with wavelengths longer than about 300 nm. In this paper we describe a technique for performing conventional photolithography with high molecular weight PMMA at the widely used 365 nm i-line wavelength. The technique involves photosensitizing PMMA with Irgacure 651—a commercially available photo-initiator that can cause PMMA strands to cross-link. Optimum amount of Irgacure can produce a negative tone photoresist with adequate photosensitivity and plasma etch resistance. We describe this technique in detail with complete processing conditions and discuss the effects of varying Irgacure 651 concentration in PMMA as well as changes in UV exposure dose. We also show example structures patterned with commonly available materials and equipment. Finally, we show that it is possible to carry out gradient lithography with this approach, in order to produce structures in relief in photosensitive PMMA.

Electronic cigarette vapor alters the lateral structure but not tensiometric properties of calf lung surfactant

BackgroundDespite their growing popularity, the potential respiratory toxicity of electronic cigarettes (e-cigarettes) remains largely unknown. One potential aspect of e-cigarette toxicity is the effect of e-cigarette vapor on lung surfactant function. Lung surfactant is a mixture of lipids and proteins that lines the alveolar region. The surfactant layer reduces the surface tension of the alveolar fluid, thereby playing a crucial role in lung stability. Due to their small size, particulates in e-cigarette vapor can penetrate the deep lungs and come into contact with the lung surfactant. The current study sought to examine the potential adverse effects of e-cigarette vapor and conventional cigarette smoke on lung surfactant interfacial properties.MethodsInfasurf®, a clinically used and commercially available calf lung surfactant extract, was used as lung surfactant model. Infasurf® films were spread on top of an aqueous subphase in a Langmuir trough with smoke particulates from conventional cigarettes or vapor from different flavors of e-cigarettes dispersed in the subphase. Surfactant interfacial properties were measured in real-time upon surface compression while surfactant lateral structure after exposure to smoke or vapor was examined using atomic force microscopy (AFM).ResultsE-cigarette vapor regardless of the dose and flavoring of the e-liquid did not affect surfactant interfacial properties. In contrast, smoke from conventional cigarettes had a drastic, dose-dependent effect on Infasurf® interfacial properties reducing the maximum surface pressure from 65.1 ± 0.2 mN/m to 46.1 ± 1.3 mN/m at the highest dose. Cigarette smoke and e-cigarette vapor both altered surfactant microstructure resulting in an increase in the area of lipid multilayers. Studies with individual smoke components revealed that tar was the smoke component most disruptive to surfactant function.ConclusionsWhile both e-cigarette vapor and conventional cigarette smoke affect surfactant lateral structure, only cigarette smoke disrupts surfactant interfacial properties. The surfactant inhibitory compound in conventional cigarettes is tar, which is a product of burning and is thus absent in e-cigarette vapor.

Development-less deep ultraviolet positive tone photolithography with polymethyl methacrylate

The authors describe a new lithography technique that relies on spatially thinning down a polymethyl methacrylate (PMMA) film through ultraviolet (UV) radiation exposure. Patterns on chrome-on-quartz mask plates or shadow masks can be transferred to an underlying PMMA film as UV light at 254 nm is projected through the mask. This work made use of cheap and easily available low pressure hot filament mercury discharge tubes as the UV radiation source. UV irradiation causes chain scission in PMMA followed by the removal of chain fragments. The process is synergistically aided by heating the PMMA-covered sample. This process thins down the PMMA wherever it receives UV irradiation, creating a topographic pattern in the polymer film. With sufficient irradiation dose, PMMA can be completely removed, all the way down to the substrate. The UV-induced decomposition of PMMA is shown to be aided by a secondary exposure from photoelectrically generated electrons ejected from the substrate. Subsequently, both additive ...

Combination photo and electron beam lithography with polymethyl methacrylate (PMMA) resist

We describe techniques for performing photolithography and electron beam lithography in succession on the same resist-covered substrate. Larger openings are defined in the resist film through photolithography whereas smaller openings are defined through conventional electron beam lithography. The two processes are carried out one after the other and without an intermediate wet development step. At the conclusion of the two exposures, the resist film is developed once to reveal both large and small openings. Interestingly, these techniques are applicable to both positive and negative tone lithographies with both optical and electron beam exposure. Polymethyl methacrylate, by itself or mixed with a photocatalytic cross-linking agent, is used for this purpose. We demonstrate that such resists are sensitive to both ultraviolet and electron beam irradiation. All four possible combinations, consisting of optical and electron beam lithographies, carried out in positive and negative tone modes have been described. Demonstration grating structures have been shown and process conditions have been described for all four cases.

Enhancing the dry etch resistance of polymethyl methacrylate patterned with electron beam lithography

Acrylic resists are used for both electron beam lithography and for deep-ultraviolet (UV) lithography at 193 nm wavelength. Polymethyl methacrylate (PMMA) is the most widely used acrylic positive tone electron beam resist. While it offers superb resolution in this role, its dry etch resistance is quite poor. Here, the authors present a new technique for enhancing the dry etch resistance of PMMA. This involves adding Irgacure 651—a photo-cross-linking agent to PMMA. Irgacure-containing PMMA can be spin-coated onto substrates in exactly the same way as pure PMMA. Addition of Irgacure does not impair the chain scissioning properties of PMMA under electron beam irradiation. Electron beam lithography can be carried out with this resist in exactly the same manner as with pure PMMA, although at a higher dose. After electron beam exposure, the exposed sample can be developed in diluted methyl isobutyl ketone solvent, again just as with pure PMMA. A postlithography UV exposure step then cross-links the patterned resist; substantially enhancing its dry etch resistance. This technique enables the fabrication of deeper etched structures than is possible with PMMA alone.Acrylic resists are used for both electron beam lithography and for deep-ultraviolet (UV) lithography at 193 nm wavelength. Polymethyl methacrylate (PMMA) is the most widely used acrylic positive tone electron beam resist. While it offers superb resolution in this role, its dry etch resistance is quite poor. Here, the authors present a new technique for enhancing the dry etch resistance of PMMA. This involves adding Irgacure 651—a photo-cross-linking agent to PMMA. Irgacure-containing PMMA can be spin-coated onto substrates in exactly the same way as pure PMMA. Addition of Irgacure does not impair the chain scissioning properties of PMMA under electron beam irradiation. Electron beam lithography can be carried out with this resist in exactly the same manner as with pure PMMA, although at a higher dose. After electron beam exposure, the exposed sample can be developed in diluted methyl isobutyl ketone solvent, again just as with pure PMMA. A postlithography UV exposure step then cross-links the patterned r...

Thermal oxidation of silicon in a residual oxygen atmosphere—the RESOX process—for self-limiting growth of thin silicon dioxide films

We describe in detail the growth procedures and properties of thermal silicon dioxide grown in a limited and dilute oxygen atmosphere. Thin thermal oxide films have become increasingly important in recent years due to the continuing down-scaling of ultra large scale integration metal oxide silicon field effect transistors. Such films are also of importance for organic transistors where back-gating is needed. The technique described here is novel and allows self-limited formation of high quality thin oxide films on silicon surfaces. This technique is easy to implement in both research laboratory and industrial settings. Growth conditions and their effects on film growth have been described. Properties of the resulting oxide films, relevant for microelectronic device applications, have also been investigated and reported here. Overall, our findings are that thin, high quality, dense silicon dioxide films of thicknesses up to 100 nm can be easily grown in a depleted oxygen environment at temperatures similar to that used for usual silicon dioxide thermal growth in flowing dry oxygen.