LithoTalk

[guest1]

Dear all ...

I use a positive photoresist (spectral sensitivity between 310 and 420nm) and I need to have some 1 micron features resolved. The mask aligner I use cannot resolve these features. The mask aligner has a broadband (i, g and h line) spectrum. Could this be a problem due to the use of a broadband spectrum? I tried using an i-line sensitive mask aligner alternatively and found that I can resolve the 1 micron features. Can someone tell me the reason behind this?. Do I need i-line filters to get the features resolved?

[Neil]

The resolution or achievable critical dimension is proportional to the wavelength of UV used. It also depends on the absoption bands within the photoactive compound within the photoresist. Due to the appearance of more possible absorption transitions involving larger energy differences, the absorption tends to increase with shorter wavelength, or larger photon energy. If photons are used with energy above the ionisation potential of the photoresist, electrons can be released which further exposes the photoresist. I think as a rule of thumb i-line UV can define structures to around 0.35um.

[Guest]

Hello,

in principle, small wavelength allow a higher resolution. However, as Neil already mentioned, some things have to be considered:

1) The resist has to be compatible with the small wavelengths applied. A standard positive tone resist should NOT be exposed with < 320 nm light, since the penetration depth will become rather small. Thus, the required exposure dose for through-exposure becomes rather high, which results in scattering which again deteriorates the attained resolution.

2) A modern resist such as the AZ 701 MiR or AZ ECI 3000 allows a resolution of less than 500 nm via i-line (365 nm) exposure if all process parameters are optimized. If even 1 micron resolution cannot be attained, smaller wavelengths or another resist will not help ... please find enclosed some trouble-shooting regarding high resolution requirements deim our recent litho-booklet:

Suitable Photoresist? Positive resists such as AZ® 1505, AZ® 1512 HS, or AZ® 6612 allow structure sizes below 1 µm, AZ® 701 MiR and the AZ® ECI 3000 series even below 0.5 µm. If high-resolution negative tone resists are required, thin films of the AZ® nLOF 2000 series allow sub-µm features for e. g. lift-off application.

Gap between Photomask and Resist Surface? Particles, bubbles in the resist film, or an resist edge bead may cause a gap between mask and resist which degrades the theoretical resolution by diffraction and light scattering.

Thermal Decomposition of the Photo Active Compound? If not exposed, the photosensitive compound of photoresists is a solubility inhibitor in alkaline solutions, thus decreasing the dark erosion in developers. If the softbake temperature and -time are higher than recommended, this inhibitor is partially decomposed. Since the development rate decreases at the same time, the contrast of the resist and hereby its resolution performance deteriorates.

Compatible Developers? Certain developers such as AZ® Developer or AZ® 826 MIF have higher dark erosion rates for photoresists than other developers, making them less suited for high-resolution demands.

Exposure Dose Too High? In this case, light scattering in the resist film also exposes also ‘dark’ parts of the resist, which hereby become soluble in developers and prevents the realization of very small structures.

Maybe this helps a little bit?