We present here the design of a broadband antenna array for photonic beam-steering at W-band. The array unit-cell consists of two stacked microstrip patches that are cavity backed, and fed by aperturecoupling using an hourglass slot. The unit-cell shows a simulated impedance bandwidth of 37% for SWR<2 from 77 GHz to 112 GHz, thus covering most of the W band. The feeding network is designed using a low-loss Substrate Integrated Waveguide (SIW) and respects nearly the same bandwidth as the unit cell for a total SWR<2. Therefore, the structure is well-suited for fabrication by PCB technology. The final array is composed of 4 sub-arrays of 2x8 unit cells and exhibits a broadside directivity of 26 dBi. Each sub-array is expected to be fed by a Uni-Traveling-Carrier (UTC) photodiode, which enables power combining to overcome the low emitted power at mm-waves and also the use of optical phase shifters in the antenna architecture to achieve a +/-5 degrees beam steering with reasonable grating lobes.