| The Schwarzschild system, composed by two spherical mirrors, enjoys the excellent optical characteristics of no3rd order spherical aberration, coma, astigmatism and chromatic aberration. It has been widely used in the field of astronomical observations. With the help of micro-optics concept and MEMS technology, the one-century-old Schwarzschild system would be given a new life by miniaturization into micron-scale structure. Hence it is suitable for applications which are critical for the size and weight of the microscope objective, such as in vivo detection, space exploration, data read/write or storage, and various handheld consumer electronics products, etc.This paper designs a micro-Schwarzschild objective fabricated by MEMS technology. The primary mirror and secondary mirror are designed and fabricated with the help of the research on the mirolens with large size and small contact angle, and on the spherical mirocavity fabrication. Different integration schemes are discussed, and then the prototype of micro-Schwarzschild objective is fabricated and tested. The main contents and results are summarized as follows:A micro-Schwarzschild objective design is proposed for MEMS fabrication by the discussion on various two-mirror configurations. Then the geometric parameters are designed by deduction of the aberration and the study of limitations of MEMS technology. The light trace software is used to analyse the optical scheme. The property of excellent aberration performance is confirmed by the ray tracing simulation results. And the tolerace characteristics of the system parameters are analysed.Different microlens fabrication methods are discussed. The method of acetone vapor reflow is chosen for the fabrication of primary mirror with large size and small contact angle. Then the relationship between the process parameters and microlens radius of curvarure and crown height is derived from the mathematical model. Photo resists with different thickness are used to produce microlenses with the range of bottom diameter of80-1000μm and the crown height of about10～120μm.After the study on the thickness of aluminum film and the deposit method, a layer of reflective film is coated on the surface to transform the microlens into micro-mirror. And then the primary mirror is successfully fabricated.Kuiken’s mathematical model on the diffusion limited isotropic etching is introducted for the fabrication of the secondary mirror. The impacts of the agitation method, the etching time, the etching window size and other parameters on the profile of etched cavities are studied. The experimental results are analyzed and compared with Kuiken’s model on the two different time periods. The recommended range of process parameters are then proposed for spherical micro-cavity fabrication. The secondary mirror that fits the design values is successfully fabricated.Two sets of cantilever designs for the integration of the primary mirror are proposed. Cantilever stress distribution is analyzed by the finite element method. The cantilever around the primary mirror is fabricated by the MEMS technology, and the apertures on the secondary mirror and middle gasket are produced by laser positioning drilling. The middle layer alignment bonding is used for the integration of the primary and the secondary mirrors to obtain the prototype of the miro-Schwarzschild objective.A testing miroscope system is built to test the optical performance of the prototype. The spot shapes at different image plane positions are captured by CCD. The working distance and numerical aperture are then deducted by the data, and compared with the simulation results and design values. Light sources with different wavelengths are used to test the system’s chromatic aberration. Test results confirm that the system is not wavelength secletive.Finally the impovement of the process parameters and the directions of future work are pointed out. |
