Square lattices of isolated electron disks of diameters 0.2-2 mu m were obtained by laterally patterning n-type modulation-doped Si/SiGe heterojunctions grown by ultra high vacuum chemical vapor deposition. The disk lattices were fabricated by electron-beam lithography and reactive ion etching. The collective excitations in the disks were studied by microwave magnetotransmission experiments. The resonance dispersion as well as the disk plasma frequencies are theoretically analysed assuming a nearly parabolic confining potential.