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micro resist technology GmbH
The most frequently asked questions about our products:
1) Are there exact data available for the high etch resistance of
ma-P 1200 resists?
The resists of the ma-P 1200 series exhibit a good etch resistance.
E.g. ma-P 1200 gives very good results in dry etching (as with CF4 or
high density SF6/O2 plasma).
The etch rates of the resists strongly depend on the etching conditions.
The etching equipment has an influence, the amount of open wafer surface to
be etched, the etch gas composition and all other parameters such as pressure,
temperature or voltage.
ma-P 1200 resists show a good thermal stability. That enables the
thermal stabilisation of the resists for the subsequent plasma etching. E.g.
a hardbake is recommendable at 120°C in an oven (with a temperature ramp
to minimize the pattern rounding). For that also a stronger prebake is necessary.
In case of a 7.5µm thick film of ma-P 1275 that would be a prebake at
110°C for 5 - 10 min on a hotplate.
We cannot deliver any more detailed data. This is nearly impossible
since etching conditions can differ very much from lab to lab.
2) How can I avoid bubbles in the resist film?
Several reasons for the formation of bubbles in the resist are possible.
Big bubbles could arise if the resist was shaken too much, or if
the resist has undergone a bigger temperature change. They should disappear
when the resist can "relax" a while (i.e. when the bottle is kept
under constant climatic conditions in the cleanroom for some time). This applies
mainly for resists of a higher viscosity.
Bubbles occurring during the prebake of thick resist films are caused
by the solvent evaporation. It is important to maintain a certain relaxation
time between spin-coating and prebake to avoid this. Only high viscosity resists
show this tendency.
Smaller bubbles in the exposed area could have several reasons. Adjusting
the respective process condition prevents the bubble formation:
3) Is ma-P 1200 resistant to strong acids?
In general ma-P 1200 positive photoresists withstand strong acids
very well. Tests have shown that 2.5 µm thick ma-P 1225 and
7.5 µm thick ma-P 1275, both processed under standard conditions,
resist concentrated HCl at 45…50 °C for at least 10 minutes
without any problems. No erosion of the resist film is observed.
Concentrated HF is challenging for all photoresists (cf. QUESTION
4).
Also strongly oxidising acids can cause some problems. The resist
stability depends on the temperature and composition of the etchant
in such cases.
4) Can I use ma-P 1200 resists as etch mask for HF glass etching?
HF etching is a bit demanding. HF doesn´t attack the resist. But it
can diffuse under the photoresist and lift it from below causing bad adhesion
of the resist on the substrate. This is why a film thickness as high as possible
should be chosen, and the resist should be hardened (stronger prebake + hardbake).
Nevertheless it depends strongly on the HF concentration and the etch time
how well the photoresist sustains the etching.
In the literature it is mentioned that ma-P 1200 is suitable for
etching with buffered HF [1].
[1] A. Pozzato, S. Dal Zilio, G. Fois, D. Vendramin, G. Mistura, M. Belotti, Y. Chen, M. Natali, Microelectronic Eng. 83 (2006), 884-888, doi:10.1016/j.mee.2006.01.012
5) Can I use positive photoresists for lift-off processes?
For lift-off processes a bi-layer resist system can be applied. E.g.
LOR (a not photosensitive polymer provided by MicroChem Corp. for various
film thicknesses) can be used as bottom layer. In a second step a positive
resist e.g. from the ma-P 1200 series is applied as top layer. During the
aqueous-alkaline development of the exposed areas of the positive resist film
also the LOR film underneath is dissolved. The undercut profile in the bottom
layer is adjusted by varying the development time and the prebake conditions
for the LOR layer.
For some applications you can do the lift-off with a single-layer
resist which does not give an undercut profile. E.g. the use of ma-P 1200
resist without an additional bottom layer would be sufficient - preferably
with a somewhat higher film thickness to give sidewalls that can be reached
by the stripper. The quality of the edges of the deposited metal layer is
slightly worse than in a bi-layer process in this case.
6) Is there a developer that doesn´t corrode Al and Al containing
substrates, respectively?
We recommend to apply “Developer Concentrate” by Rohm & Haas
for developing resists of the ma-P 1200 series on Al and Al containing substrates.
You can purchase the developer from our company. The developer is based on
metasilicate which practically doesn´t corrode Al.
