Backscatter communication, an emerging ultra-low-power wireless communication paradigm, has been studied as a promising solution for wireless communications in some Internet-of-Things (IoT) applications. The required ubiquitous data transmission in the IoT world is demanding information security. Different from mathematical encryption at higher protocol layers, the directional modulation (DM) technology can provide physical-layer wireless security, which does not require digital and computation resources that are otherwise scarce in low-power IoT devices. In this paper, a synthesis-free Van-Atta backscatter array is constructed and studied. The authors show that a Van-Atta array can be made to project orthogonal interference upon the backscattered signals in the radio frequency domain. The design details of the Van-Atta DM array are elaborated, and its efficacy is validated via bit error rate simulations.