you should use an antenna array composed of planar elements in the same plane.
Many books on antennas (available in this board) give the basics of arrays and of planar (microstrip) antennas.
Regards
Hi abti
Could tell us what you means the"high directivity"of planar antenna.The limit directivity of microstrip array(travelling or standing wave array) is about 30dBi,if you want to design a planar array above 30dBi,the waveguide slots array is the best one.
Regards
caocao