Study of SU‐8 photoresist cross‐linking process by atomic force acoustic microscopy

Abstract

SummaryIn this paper, a method is presented to detect the different phases of epoxy cross‐linking process and the subsurface structures of SU‐8 thin films by atomic force acoustic microscopy (AFAM). The AFAM imaging of SU‐8 thin films was investigated under different exposure and bake conditions. Optimized conditions were obtained for the cross‐linking of SU‐8 thin film at the exposure does of eight laser pulses with the laser fluence 10 mJ cm–2 per pulse and the post exposure bake (PEB) time at 90 s. The subsurface structures of undeveloped SU‐8 thin films were visible in the AFAM images. This method provides an effective and low‐cost way for the determination of different phases of epoxy cross‐linking process in nanostructured compounds, for the non‐destructive testing of subsurface defects, and for the evaluation of the quality of patterned structures.

Subsurface phase imaging of tapping‐mode atomic force microscopy at phase resonance

“…Structural similarity (SSIM) image quality evaluation method was applied to evaluate the image quality of phase subsurface imaging. The expression for calculating SSIM of two images is as follows 32 $$\begin{equation}SSIM\;\left( {x,y} \right) = \frac{{\left( {2{\mu _x}{\mu _y} + {c_1}} \right)\left( {2{\sigma _{xy}} + {c_2}} \right)}}{{\left( {\mu _x^2 + \mu _y^2 + {c_1}} \right)\left( {\sigma _x^2 + \sigma _y^2 + {c_2}} \right)}}\;,\end{equation}$$where $${\mu _x}$$ and $$\;{\mu _y}\;$$ are the average values of images x and y , respectively; $$\sigma _x^2$$ and $$\sigma _y^2$$ are the variances of images x and y , respectively; $${\sigma _{xy}}$$ is the covariance of images x and y ; and c 1 and c 2 are non‐negative constants used to maintain the stability of SSIM.…”