Research On Some Key Technologies Of Laser Lithography On Curved-surface Substrate

A novel system of laser lithography including polar-coordinate and Cartesian-coordinate configurations is constructed in this project. The system is different from previous conventional system of laser lithography. Because it is based on a creative architecture of constant posture. The whole optical route of the system consists of two separated parts, the subsystem of blue-ray lithography, which is applied to accomplish exposure, and the closed-loop subsystem of red-ray feedback detection, which is directed at focusing and positioning of focal spot on curved-surface substrate. The two subsystems are built on an one-dimensional translational guide rail with high precision. Optical routes of laser lithography and feedback detection keep moving together during exposing, which differs from the fixed model of detection route in conventional laser-lithography system. The novel laser lithographic system of constant posture can overcome some limitations and defects of conventional laser-lithography system, such as incorrect positioning, terrible shift between focus exposing spot and detection spot, and so on. Furthermore, personal computer is used to control exposing time and exposure digitally, which can overcome the desynchrony and non-realtime of acousto-optic modulation. The lithography system is applied to carry out research and development of laser lithography on curved-surface substrate and relevant techniques.Planar techniques of laser lithography have been mature over the past 30 years. However, curved-surface lithography is just at the starting stage. Compared with planar micro-optical element (PMOE), including diffractive optical element(PDOE) and planar micro-optical-mechanical-electronic system (PMOMES), curved-surface diffractive optical element (CDOE) and curved-surface micro-optical-mechanical-electronic system (CMOMES) have more design freedoms, better optical performances and better micro-structural mechanical characteristics. Nevertheless, the fabrication of curved-surface micro-optical element (CMOE) is more difficult. Based on this constant-posture lithography system, the topic of this project is focused on the micro-fabrication of CMOE and related techniques. The dissertation is concentrated on some key problems in laser lithography on curved-surface substrate and some primary applications of constant-posture lithography. These analytical problems and primary applications include the following aspects: technique of photoresist spin-coating on curved-surface substrate, especially on convex-surface substrate, full-field measurement of photoresist-layer thickness on curved-surface substrate by means of spin coating, real-time centering and focusing technologies during lithography on curved-surface substrate, exposure model in curved-surface laser lithography and fabrication of CMOE and CMOMES, etc. The brief description of detail work in this dissertation is shown as below:Chapter 1.This chapter is the prolegomenon of this dissertation, which introduce the research progress and application prospect of some conventional micro-optical fabrication technologies. Additionally, the research background of laser lithography on curved-surface substrate is analyzed and discussed. Some common fabricating technologies of MOEs and MOMES are introduced. These technologies include projection exposure, laser lithography, diamond turning, injection moulding of optical plastics, electroforming and LIGA (Lithographie, Galanoformung, Abformung). Correspondingly the comparisons of their advantages and disadvantages are made. It is concluded that even if laser lithography is one kind of mature and old technology, its comparative advantages still show great research value and applicable potential. Laser lithography is especially suitable for the fabrication of photomask machining and CMOE. Current situation and application limitations of planar technique development of conventional laser lithography are reviewed and summarized. In the meanwhile, better optical performances and micro-structural properties of CMOE over PMOE are introduced simply. Based on the above discussion and analysis, the necessity of application and technical feasibility to fabricate CMOE is provided in the end of this chapter.Chapter 2. A constant-posture system of laser lithography on curved-surface substrate is designed and constructed. The total composition and functional configurations in all subsystems of this system are introduced and illustrated. Based on the designal principles and optical-mechanical configuration, feasibilities and advantages of this constant-posture system to laser lithography on curved-surface substrate are discussed. The focusing process of this system is alayzed theoretically, which is based on the power detection of PSD (Position Sensitive Detector, PSD). In the last part of chapter 2, the procedures of microfabricating MOEs are introduced briefly.Chapter 3 analyzes the disadvantages of worsen distribution of photoresist-layer thickness and photoresist-layer uniformity due to the gravity component of photoresist infinitesimal when spin-coating on curved-surface substrate by conventional photoresist spinner. Therefore, it is necessary to research on spin coating and photoresist-layer uniformity on curved-surface substrate during curved-surface laser lithography. Thickness distribution model of photoresist layer when spin-coating on convex-surface substrate is proposed theoretically. Based on the distribution model and controllable technical parameters of spin coating, the optimal uniformity model is acquired. Finally some experiments verify the effectiveness of the two models.Chapter 4. In conventional spin-coating and laser lithography on planar substrate, the exact thickness distribution and uniformity of photoresist-layer is unnecessary to be known, but only acquired approximately based on technical experience. As far as curved-surface laser lithography is concerned, measurement of photoresist-layer thickness is important, especially for the fabrication of CMOE requiring fine control in the depth direction of lithographic lines. Common methods to measure thickness of transparent film include profilometer, colorimeters, fringe interferometry, white-light interferometry ellipsometry to measure thickness of transparent films with sharp steps, dynamic capacitance measurement, SEM (Scanning Electronic Microscope), angle lap stain method, stacking fault, infrared reflecting method, and back scattering. Limitations exist in all these methods, step and transparent of film are required for profilometer and SEM, dynamic capacitance measurement stacking fault need both the underside and upside of film to be metallic. Additionally colorimetry, fringe interferometry, infrared reflecting method, back scattering method, and ellipsometry are restricted to plane and material-dependent measurement. Furthermore, some methods of them such as ellipsometry and infrared reflecting method are expensive and time-consuming. The method of indirect interferometry put forward causal positional and repeatability error usually because of the procedures necessary to measure twice before and after film-coating. All these above methods are not suitable for the thickness measurement of curved-surface photoresist layer. A full-field method based on digital wavefront interferometry to measure curved-surface photoresist-layer thickness is presented theoretically. The method makes use of processing double interferometric fringes to acquire phase information of photoresist-layer thickness on curved-surface substrate, so the photoresist-layer thickness is acquired. Another method of measuring the thickness distribution of curved-surface photoresist layer presented theoretically and experimentally in this chapter is the method of multispectral transmittance. By examining the comparison of transmitted optical intensity before and after spin coating, the thickness distribution of photoresist-layer on curved-surface substrate can be acquired.Chapter 5. Based on the constant-posture system of laser lithography and theory of Fresnel diffraction, the optical field distribution on the upper surface of curved-surface substrate and exposing process inside thin photoresist layer are acquired and discussed on curved-surface substrate. A model using point-wise equivalent slant to fit curved surface is built during curved-surface laser lithography. Classic model of Dill exposure is improved to applicable for the laser lithography on curved-surface substrate, so that the correction exposure model and line-width model of curved-surface laser lithography are got theoretically. Theoretical simulations show that the cross-section symmetries of lithographic lines will worsen with the increase of horizontal vector radius of substrate. Subsequent experimental data accord well with the simulation results, which shows the geometrically equivalent model and the correction model of Dill exposure are effective. It is instructive and meaningful for curved-surface laser lithography technically.Chapter 6 primarily analyses and researches the performance evaluation and basic application of constant-posture laser lithography system. Based on the parameters of electronic-mechanical components, the limitations of centering and real-time focusing during lithography, and the geometrically optical relation between the assembly components of the constant-posture laser lithography system, the constraint conditions of these dependant parameters are presented. According to these constraint conditions and the geometrical design of optical path, the minimal curvature radius, the maximal aperture and the maximal sag rise of convex-surface substrate which can be processed in this system are derived theoretically. Based on these arguments and technique experience, the preliminary evaluation of lithographic performance indices are deduced to carry out further research. In the end of chapter 6, a round grating on a convex-surface substrate has been fabricated by means of the constant-posture laser lithography system. Grating constant and etching depth of the convex-surface grating are measured.Chapter 7 is the final chapter of this dissertation. Recapitulative reviews and brief summarization to the full dissertation are presented. The contribution and creative points of this dissertation are concluded. The corresponding future research fields are proposed.