Polyimide is one of the most popular substrate materials for the microfabrication of flexible electronics,
while polydimethylsiloxane (PDMS) is the most widely used stretchable substrate/encapsulant material.
These two polymers are essential in fabricating devices for microfluidics, bioelectronics, and the internet of
things; bonding these materials together is a crucial challenge.
Reported bonding techniques can be divided by direct bonding and indirect bonding.
Thermal bonding , laser bonding were the commonly seen direct bonding methods. They
might also need oxygen plasma or UV ozone treatment for surface modification. Direct
bonding requires high temperature or voltage. Indirect bonding, which required an intermediate layer to
bond the two incompatible surfaces, could circumvent the problems. These adhesives could be based on
chemical effects like polymerization (e.g.
acrylics), polycondensation (e.g. silicones) or polyaddition (e.g. epoxies) and could often be cured at
elevated temperatures or by UV light. While adhesive bonding enabled high strength bonding at much
lower temperature, this technique also confronted several challenges. Direct laminating a thin layer of
flexible plastic substrate on a rigid substrate could easily introduce unwanted bubbles, and such uneven
surface might cause troubles for sealing.
Adhesion improvement methods reported in the literature by the use of techniques like sputter etch
and/or making use of several intermediate layers like SiC and SiO2 for a good adhesion. However, these
approaches require extensive and expensive fabrication methods, which may not always be easily accessible
Another mechanisms reported in the literature to enhance the bonding properties of PDMS to PI is by
exposing it to a short oxygen plasma which makes the surface active, and thus more chemically reactive to
the polyimide surface .However, this only holds for cases where pre-cured PDMS is bonded to cured PI.
Whereas in most process flows the PDMS is spin coated/casted as the last step of the process. In these
cases there are very limited solutions to improve the adhesion.
By sputtering the vanadium oxide thin film on glass substrates followed by post-annealing in argon. This
method has been proven to be highly effective to improve the bonding strength of PI and PDMS
Polybutadiene is a type of rubber that has low permeability to gases and moisture, as well as
chemicals, and is thus being exploited for its good barrier and chemical properties. The structure of butyl
rubber (Figure 2a) has several double bonds. The double bonds get oxidized in ambient environment to
form –OH groups that react with the PI layer to make polar bonds. The interaction between PI and BR is
completely chemical, and hence always consistent irrespective of the crosslinked or non-cross linked state
of the BR. BR-PDMS is diffusive bonding, which describes the mechanical locking between materials at
molecular level . According to this bonding regime, the adhesion is a result of interdigitation between the
free chains of two polymers. This type of bonding is therefore heavily dependent on the freedom of the
polymer chains to interlock with each other
Strong permanent adhesion between thin-film polyimide (BPDA-PPD) and silicone rubber (MED-1000) was
studied through deposition of a chemically-transitive intermediate adhesion promoting layer.
Plasma-enhanced chemical vapor deposition (PECVD) of SiC and SiO2 was used to grow a thin 50 nm layer
directly on a 5 µm thin polyimide substrate. The deposition at low pressures permitted the fabrication of an
adaptive covalent bond transition from sp2- hybridized carbon (in polyimide) towards the sp3 bonding in
SiC, continuing to SiO2 which provides a good bonding partner for one-component poly-dimethyl siloxane
(PDMS).
Silicon wafers coated with a 5μm thick layer of polyimide were treated with different surface modification
techniques such as chemical adhesion promoters, oxygen plasma and an Ar+ sputter etch. It was found that
commercially available chemical adhesion promoters and oxygen plasma treatment resulted in a very poor
PI/PDMS adhesion, whereas the Ar+ sputter etch resulted in an adhesion so strong that the PDMS could not
be delaminated from the PI surface without the failure of the material.
Click chemistry at room temperature employed to irreversibly bond polyimide and PDMS through
thiol-epoxy bonds using two different methods. In the first method, the surfaces of the PDMS and
polyimide substrates was functionalized with mercaptosilanes and epoxysilanes, respectively, for the
formation of a thiol-epoxy bond in the click reaction. In the second method,one or both surfaces
functionalized with mercaptosilane and introduce an epoxy adhesive layer between the two surfaces. the
novel processing route employing click chemistry can be utilized in various cases of stretchable and flexible
device fabrication.