## 1 Introduction  
Nowadays, micro-structures have acted as an element in various systems play an important role in measurement [1, 2], optical fiber communication [3, 4], display [5, 6] and other fields [7, 8]. In order to ensure the performance of the micro-structure, the characterization of surface topography of micro-structure is crucial. Digital holography microscopy (DHM) provides a high-efficiency and accuracy as a measuring tool for measurement of micro-structures which can achieve lateral resolution down to a few hundred nanometers and an axial resolution of less than \(\lambda/150\), simultaneously [9, 10]. Nevertheless, the conventional DHM system is not suitable for the measurement of micro-structures with large gradient topography, which can generate the object wave of large slope angle. When the largest slope angle of the object wave exceeds the range of angles that the microscopic objective (MO) of DHM system can receive, the corresponding large gradient regions cannot be measured effectively since the object wave is limited by the small numerical aperture (NA) of MO of the DHM system. To solve the problem, some researchers introduced different illumination modes to the DHM system to achieve surface shapes of objects with large gradients. Generally, there are two typical solutions. The first solution is to use an illumination mode of multidirectional plane wave. [PERSON] et al. [11] was inspired by the concept of super-resolution, in that the illumination mode was realized by introducing the sinusoidal grating, and three illumination beams are generated by the diffraction order (\(-1\) 0 +1) of grating. Therefore, the method has the problem of aliasing which needs to carry out the additional diffraction calculations for separation of beams. Moreover, the period and orientation of grating is fixed so that the grating provided only stationary direction and angle of illumination, so the measurement of multiple objects can only be implemented by replacing the corresponding grating, which is inefficient. This method, based on the super-resolution concept, was then developed by [PERSON] et al. [12]. They present a more flexible approach which replaces the sinusoidal grating with a blazed diffraction grating produced by the phase spatial light modulator (SLM) that can generate a grating of multiple periods and arbitrary direction, therefore, the introduction of SLM makes the method more practical and robust. However, the resolution of SLM, which cannot generate a blazed grating fringe smaller than pixel size, is finite, so the inclination angle of the illumination beam generated by the blazed grating from SLM is limited, and it cannot implement oblique illumination at an arbitrary angle of tilt. In this case, the measurable gradient of this method is restricted. Another solution which utilize spherical wave illumination mode, which was proposed by [PERSON] et al. [13], and this measuring system use the MO to generate the point source illumination, which can effectively decrease the NA of the object wave and extend the capabilities of transmission systems for characterization of large gradient objects. However, the application range of this method is only limited for focusing on micro-structures, such as: Spherical/aspherical surfaces. In addition, all of the above solutions based on different illumination modes have a common disadvantage, while the large steepness surface shape is measured in reflection configurations through the above solutions, the illumination wave of the large slope angle has the risk of being partially blocked by MO so that this form of illumination cannot be effectively implemented. In our work, we introduce multi-angle stitching DHM to solve the problem of large gradient micro-structure measurement. In system configuration, we only need to replace the ordinary sample carrier platform with a four degrees of freedom stage in the conventional DHM system.