1. First Presentation; Project Defence
Multifunctional PDMS tips fabricated by polymer casting of KOH etched pyramidal pits
First Presentation; Project Defence
Kees Schilder
Welcome today, here at my project defence of my master thesis. The title of the thesis is Multifunctional PDMS tips fabricated by polymer casting of KOH etched pyramidal pits. This thesis is not very theoretical, it is pretty practical, so don’t expect any mathematical derivations.

2. Introduction Polymer tip array Increase of throughput
Challenge is inclination of tip array
Advantage of PDMS
Compliance
Stiffness
The goal is to fabricate a polymer tip array, where the tip apex is as small as possible. With such an array it is possible to increase the throughput of micro-printing techniques. Challenging in such micro-printing is the unavoidable inclination of the tip array. This results in an inhomogeneous force distribution over the array.
When we have a stiff tip, than the tips on one side of the array will touch the surface and the tips on the other side of the array will not touch the array. But if we have a compliant tip, which can deform under the action of a force, than the contact area between the tip and the surface is not the same at all locations.
A solution to this is the use of a multilayer tip, which consists of a stiff tip apex and a compliant base, such that the base can deform.
Now the contact area is always the same and every tip touches the surface.
With PDMS as polymer it is possible to play with certain parameters – like bake temperature, pressure or the time in the oven – so that we can create different characteristics.

3. Applications Proteome analysis Cleaning Direct write lithography
C. Wingren et al., Progress in miniaturization of protein arrays – a step closer to high-density nanoarrays, Drug Discovery Today (2007)
F. Huo et al., Polymer Pen Lithography, Science (2008)

4. Fabrication Process - Pits

5. Fabrication Process - Pits & Holes – 1.2

6. Fabrication Process - Pits & Holes – 2.3

7. Mask Design Simple design Various configurations
It is a very simple design which only consist of a lot squares.
There are 21 dies on 1 wafer. Most of the dies are divided into 9 squares and each square contains an array of squares with constantly different values for the width of the hole and the offset between the holes. This is to see whether the offset has influences on the quality of the PDMS tips and to check from which width of the squares a good PDMS tip is created.

8. Timeline - Milestones
What has to be done
What has been done

9. Expected Challenges
Producing of (very) small tips due to the viscosity of PDMS
PDMS to SiO2 bonding
The making of small tips is challenging because of the small structures which has to be copied by the PDMS from the silicon master. The making of the silicon master is not a problem, because the shape is determined by the KOH etch, but PDMS is probably a problem because of its viscous characteristics. Due to this viscosity the shape of the master could not be reproduced at very small dimensions.
I heard that the bonding of PDMS to silicon or silicon nitride could be challenging, but I read in a couple of papers that the writers succeed in the bonding process. So I have to wait to see whether this becomes a problem, not a problem, a challenge.

10. Questions & Discussion