Tunable, optical microcavities (MC) gain more and more importance for display, laser or other optical applications. The setup of dielectric elastomer actuators (DEA) enables a simple integration of an optical cavity, since reflective electrodes can confine a cavity that is filled with a transparent elastomer. Applying a voltage to the electrodes leads to squeezing of the elastomer and, due to the cavity thickness decrease, the resonator modes of interfering light changes. In this work we present an electrically tunable, optical MC based on ultra-soft poly(dimethylsiloxane) (PDMS). The PDMS gel is coated on a glass substrate with a distributed Bragg reflector, an ITO bottom electrode and a flexible, highly reflective metal electrode and mirror on top. The usage of an ultra-soft PDMS gel, with a storage modulus of about 1kPa, allows to decrease the operating voltage down to a few hundred or even several ten volts. The critical step of fabrication is the metallization of the PDMS gel layer that requires a previous oxidizing surface activation to gain reflective and conductive silver based layers on top. Therefore, the effects of oxygen plasma and UV/ozone treatment on PDMS and the created metal layer were investigated intensively. The performance of the electrically tunable, optical MC is tremendously dependent from an adequate surface activation and structuring of the top electrodes considering the mirror displacement and activation voltage. Here we could show that tunable MCs based on oxygen plasma activated PDMS show a homogenous and high thickness decrease up to 70% at 200V.