Scanning Hall Probe Microscope and Imaging of Vortex Penetration into Nb

We report on the construction of a scanning Hall probe microscope with 100 nm lateral resolution and a large scan range, which exceeds 40 $\mu$m at 4.2 K. The microscope is based on the beetle design and operates between room temperature and 1.5 K. The DSP-based control electronics achieves a high (100 kHz) sampling rate and a low noise. The system is capable of simultaneous tunneling and Hall signal acquisition. The Hall sensor for measuring local magnetic fields is fabricated on a GaAs heterostructure through standard EBL and wet etching. It has an active area of 300 nm x 300 nm and a sensistivity of ~ 0.2 $\Omega$/Gauss. We will present (1) a description of the microscope, and (2) progress on imaging the penetration of vortices and the growth of vortex dendritic patterns into thin Nb films \footnote{ Altshuler E. \textit{et al.}, Rev. Mod. Phys.\textbf{76}, 471 (April 2004)} and grid arrays \footnote{Hallen H. D. \textit{et al.}, Solid State Commumications \textbf{99} (9), 651-654 (SEP 1996)}.