Wall profile of thick photoresist generated via contact printing

Yao, Cheng Te; Lin, C.-Y.; Wei, Der-Hsin; Loechel, B.; Gruetzner, G.

doi:10.1109/84.749398

Cited by 44 publications

(38 citation statements)

References 12 publications

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“…1b used in backside exposures leads to sidewall deformation of the developed pattern due to diffraction of light passing through the substrate. Recent studies have investigated sidewall exposures of thick positive and negative resists for microstructures resulting from a small gap varying from a fraction of a lm to about 100 lm between the mask and the photoresist normally present in contact or proximity printing (Cheng et al 1999;Chuang et al 2002). Diffraction arising from the wave nature of light can be described by contributions resulting from Fresnel and Fraunhofer components with their relative importance being dependent on the location of the surface relative to the mask.…”

Section: Introductionmentioning

confidence: 99%

Microlens array fabrication by backside exposure using Fraunhofer diffraction

et al. 2008

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In UV-lithography, a gap between photoresist and UV-mask results in diffraction. Fresnel or near-field diffraction in thick positive and negative resists for microstructures resulting from a small gap in contact or proximity printing has been previously investigated. In this work, Fraunhofer or far-field diffraction is utilized to form microlens arrays. Backside-exposure of SU-8 resist through Pyrex 7740 transparent glass substrate is conducted. The exposure intensity profile on the interface between Pyrex 7740 glass wafer and negative SU-8 resist is modeled taking into account Fraunhofer diffraction for a circular aperture opening. The effects of varying applied UV-doses and aperture diameters on the formation of microlens arrays are described. The simulated surface profile shows a good agreement with the experimentally observed surface profiles of the microstructures. The paper demonstrates the ease with which a microlens array can be fabricated by backside exposure technique using Fraunhofer diffraction.

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Section: Introductionmentioning

confidence: 99%

Microlens array fabrication by backside exposure using Fraunhofer diffraction

et al. 2008

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“…To calculate the average energy of the photons absorbed on the sidewall, we applied the approximation of the Fresnel diffraction I F (u) as a function of the Fresnel number u ¼ x ffiffiffiffiffiffiffiffiffi ffi 2=kr p near the sidewall u $ 0 [5]:…”

Section: Calculationsmentioning

confidence: 99%

“…1 is mainly formed by the competition of two effects, i.e. attenuation and diffraction [1,5]. The first effect to be discussed is the wavelength-dependent attenuation in the vertical direction [4].…”

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confidence: 99%

Surface roughness control by energy shift in deep X-ray lithography

Yao

Chen

Chieng

et al. 2003

Microsystem Technologies

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Previous studies show that the surface roughness of the sidewall generated by deep X-ray lithography (DXL) is a function of the photon energy of X-rays. Present study demonstrates and reveals the ideas of controlling surface roughness by tuning irradiation photon energy on the sidewall using the developing temperature. The ideas are resulted from two observations (1) X-rays of higher energy induce photoelectrons of higher energy in the resist and the corresponding scattering distance is nearly a square function of electron energy [1], and (2) high-energy X-rays are expected to induce more surface roughness and this effect has been observed by different laboratory [2]. IntroductionThe synchrotron radiation is a white light source containing radiation ranging from IR to hard X-rays. The 1.5 GeV synchrotron Taiwan Light Source (TLS) with a characteristic wavelength of 0.58 nm provides main radiation of the soft X-rays. The micromachining beamline at the TLS selects the X-rays from 0.2 to 0.4 nm for the DXL and the ultra-deep X-ray lithography (UDXL) [3]. As X-rays penetrate the resist, the spectra are moving from low-energy to high-energy region due to the faster absorption of low-energy X-rays [4]. The developed sidewall of constant dosage as shown in Fig. 1 is mainly formed by the competition of two effects, i.e. attenuation and diffraction [1,5]. The first effect to be discussed is the wavelength-dependent attenuation in the vertical direction [4]. The intensity decays exponentially with coefficient nearly proportional to the cubic power of the wavelength. Therefore, low-energy X-rays decay faster than high-energy X-rays. The average energy of X-rays increases as the X-rays penetrate in depth. The second effect of the Fresnel diffraction deals mainly in the horizontal direction. As illustrated in Fig. 1, the X-ray mask is placed on top of photoresist with a gap g. Absorbers on the mask block the X-ray irradiation at the opaque region. However, the X-rays have a gray exposure at the opaque edge due to the diffraction. Higher-energy X-rays are less diffractive. The gray exposure is again wavelength-dependent. The diffracted intensity spectrum is nearly a square function of the wavelength. Present study utilizes these two contradictory phenomena to control the surface roughness of the sidewall by changing the average photon-energy. ExperimentThe experiments are designed to understand the relation between the surface roughness and the induced photoelectron energy using the spectrum-shift along the penetration path. Figure 2 illustrates the schematic drawing of the experimental setup. Two freestanding thin PMMA sheets [3] of 200 microns in thickness are irradiated at our 'micromachining beamline' with an optic cubic prism (beam splitter) as mask. After the irradiation, the sample was immediately placed in the developing bath of the G-G developer [3] for 2 h, and therefore developed from both sides. The development should stop at the surface of threshold dosage determined by the developing temperature of G-G developer....

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“…Prior research (Kupka et al 2000;Cheng et al 1999) has found that diffraction at the edge of the dark field of the mask strongly affects the pattern size and sidewall profile in SU-8 contact UV lithography. Because of the high sensitivity of SU-8, even a little ''stray'' diffracted UV energy dose beneath the dark fields can cause gelation and cross-linking leading to enlarged SU-8 dimensions compared to the mask.…”

Section: Introductionmentioning

confidence: 98%

Reduction of diffraction effect for fabrication of very high aspect ratio microchannels in SU-8 over large area by soft cushion technology

et al. 2005

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Edge bead due to spin coating has been found to cause an air gap as large as 53 lm for a 99-lm thick (measured at the wafer center) SU-8 coating over a 4-in. wafer. This caused poor mask width replication and non-uniformity of SU-8 pattern width. We have devised a process using a soft cushion technology to improve mask dimension replication and large-area pattern uniformity. A soft cushion was placed beneath the substrate to produce convex bending of the wafer in order to improve the contact between the mask and photoresist top surface during UV exposure. Dramatic improvements in pattern uniformity, from over 30% variation in SU-8 width across the wafer to less than 10%, and mask width replication, from 54% deviation from mask width to 20%, have been demonstrated. Numerically calculated increases in SU-8 width from the wafer center to the edge bead using the Fresnel diffraction equation match well with observed values. Further, employment of this technique with a narrow (10 lm) dark-field mask enabled the fabrication over the entire surface of 4-in. diameter wafers of dense SU-8 gratings separated by microchannels with aspect ratio over 18.

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Wall profile of thick photoresist generated via contact printing

Cited by 44 publications

References 12 publications

Microlens array fabrication by backside exposure using Fraunhofer diffraction

Microlens array fabrication by backside exposure using Fraunhofer diffraction

Surface roughness control by energy shift in deep X-ray lithography

Reduction of diffraction effect for fabrication of very high aspect ratio microchannels in SU-8 over large area by soft cushion technology

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