Long-term continuous wireless monitoring of neural signals enables early detection of neurological disorders without restricting the patient's mobility. Existing wireless neural recording systems cannot effectively address the technical challenges associated with long-term implantation due to their large size, high power consumption, and high failure rate. In this paper, we present a miniaturized fully-passive wireless neural recorder thin enough to be implanted under the skull for long-term continuous neural recordings. Based on RF backscattering telemetry, the fully-passive sensor features a diameter of 6 mm, and has a near-zero power consumption. An artifact reduction method based on optical signal chopping was developed to improve the wireless signal integrity at low frequencies. Benchtop characterization verified the neural recorder's capability to accurately measure electrical signals as low as 300 μVpp at a distance of 45 mm. In addition, in vivo verification was conducted by implanting the recorder onto rodent skull surface. Seizure activity was successfully detected using two deep brain electrodes inserted through burr holes. These results demonstrate the potential of our proposed wireless neural recorder for long-term and continuous neural recording applications.