Lift-off Attempts for a Bone-Invasion Mimetic with Porous Roof using 100 nm NPN

ByGreg Madejski

This post is a continuation of the project found here:  Fabricating 500 nm Channels in Silicon Wafers. The chief limitation of the previous device design was that the long narrow nanochannels were not effectively wetted. To combat this problem, we would like to fabricate the channels in situ on top of NPN. This could be accomplished with either a bottom up approach (lift-off) or a top down strategy (sputter/litho/etch). I chose to try the lift-off process first, as it would not appreciably change the NPN layer.

Lift off Process. The photoresist layer must be thick enough to create a discontinuous boundary in the deposited thin film (often quoted as 3:1 in literature). Having sharp anisotropic sidewalls is also essential to keep the patterning separate.
Top Down process for comparison. Nanopores would be filled by the evaporated sidewall material. Unknown if they could be cleared during the etching process.

 

The photoresist that we have been using with the laserwriter is S1805, which has the necessary resolution to get down to 500 nm wide channels, however, it may not be able to get thick enough to perform good lift off. S1813 resist can be layered on twice as thick, but it is unknown if the laserwriter will work with this resist (It might not have the right dye sensitizer). Both of these resists are positive-tone, meaning that an exposure of the right light energy will cause the resist to dissolve in developer solution (SU8 is a counter example as a negative tone resist, which solidifies with exposure to light energy), so in order to create the channel void, two precisely positioned laser paths must be written with only a 500 nm gap between them. Some work has been done a few years ago to do lift-off processing with SU8 for Josh Miller’s Sr Design project (SU8 lift-off characterization, Feasability of SU8 3010 Negative Resist for Lift-off., Nano-porous SiN Lift-off with SU8 3010 Negative Resist).

5 um spacing exposure on NPN chip (bulk region). The resist (white lines) is well developed at ~600 nm thick).
1 um spacing on NPN chip (bulk region). This resist did not develop as evenly, leaving a residue on the surface of the chip. Lift off processes must be cleared, else they will lift off with the rest of the photoresist pattern.

 

Spacings from top to bottom: 5 um, 1 um, 0.75 um
5 um spacing was roughly produced, though there appear to be more residues than anticipated from the optical images. The bar pattern is created by the beam raster of the laser writer, suggesting more power or a longer development time is necessary.
Smaller channels do not have good anisotropy. The tiered layers have different thicknesses and widths, meaning that it will be difficult to lift off films deposited upon them.
The resist will sag near the edge of the membrane, causing a shift in focus for exposure. This has also been observed when SU8 was used to pattern waveguides on our chips.

 

As S1805 will not coat to a thickness to reliably lift off the deposited films, I will have to switch to S1813 or Su8, which will have different patterning parameters. Creating excellent sharp patterns on the resist is necessary for the lift-off process to work well.

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