Pure gentle chemical etching with no attack onto the etching sample by ions; ions and electrons combine before they reach the substrates, therefore high (e.g. Si, SiN, SiO2) resp. infinite (e.g. Au, Cu, Ni, NiFe, NiP) selectivities are achievable.
Because of the low thermal load and the missing ion impact no densifying of the resist takes place. Therefore the strip rate doesn't slow down during strip time; it is independent on resist thickness even for resists thicker than 1 mm.
Inside the microwave plasma more than 95% dissociation takes place and whereas the ions and electrons recombine inside the plasma chamber of the microwave source the neutral long-living oxygen (O*) radicals stream down to the substrates and react with all organic materials which built up volatile etch by-products like CO2; these are pumped away together with the residual gases. A small percentage of fluorine radicals (F*) helps to crack the resist bonds and to increase the etch rate at low temperatures.
The stripping process is a pure isotropic etch process without any directionality (Fig 3). That means that even parts behind sloped walls or resist out of caverns can be removed.
The resist strip rate is accelerated along the metal side walls as it can be seen in Fig 5 with partial stripped resist. That means that the metal structures are cleared before the structures between, and that small structures with high aspect ratio have a higher strip rate than other structures.
Contrary to wet chemical removal of organic materials the removal by oxygen radicals is quite less dependent on the kind of organic material and pre-treatment. As an example the rates for SU-8, KMPR and PMMA are nearly the same. Also the difference in etch rates between resist with 90°C and 200°C post bake are only in the range of 10%.
End point detection
For each of up to nine different samples the strip progress can be tracked and the endpoint of each of these samples can be detected in this way (see Fig 4).
- High aspect ratio structures with vertical sidewalls
- Photo-definable, ultra thick structures
- Outstanding thermal and chemical stability, ideal for electroplating
- Excellent electrical and physical properties for dielectrics
- Highly cross-linked photo-epoxy system
- Chemical and thermal stability
- Excellent dielectric properties
- Film thickness range 500nm >100µm
- High contrast and aspect ratios
- High transparency
- Low temperature processing