Subharmonic contrast imaging provides improved ultrasound imaging quality by taking advantage of increased contrast to tissue ratio. This article presents a novl approach to enhance the subharmonic emission from microbubbles by using a chip excitation technique. The chirp excitation pressure threshold and the sound pessure dependence of subharmonic are theoretically evaluated based on the Churc's model. Measurements validate that the optimized bandwidth and time duration of the chirp excitation can improve subharmonic amplitude and reduce side-lobe level. Both theoretical and experimental results demonstrate that the chirp excitation can improve the amplitude up to 22dB over the pulse excitation at the subharmonic frequency when comparing the coded excitation with traditional pulse excitation.