ABSTRACT
A measurement system with three degrees of freedom (3 DOF) that compensates for errors caused by incident beam drift is proposed. The system's measurement model (i.e. its mathematical foundation) is analyzed, and a measurement module (i.e. the designed orientation measurement unit) is developed and adopted to measure simultaneously straightness errors and the incident beam direction; thus, the errors due to incident beam drift can be compensated. The experimental results show that the proposed system has a deviation of 1 µm in the range of 200 mm for distance measurements, and a deviation of 1.3 µm in the range of 2 mm for straightness error measurements.
ABSTRACT
The phase modulation technique is adopted to reduce the coherent noise that arises from spurious interference. By choosing an appropriate driving signal, the method can reduce the coherent function of coherent noise to a great degree while keeping the coherent function of a coherent signal nearly unchanged. Simulation results show that for the grating interferometer, the phase error caused by coherent noise is reduced by 81.53% on average. For the Twyman interferometer, the fringe quality and contrast deteriorated by coherent noise are significantly improved. Furthermore, an experiment is set up in the phase-modulated Twyman interferometer to verify the feasibility of the principle. It is concluded that the method is effective to reduce the coherent noise in interference systems.
ABSTRACT
This paper presents the design and realization of a three degrees of freedom (DOFs) displacement measurement system composed of Hall sensors, which is built for the XYθz displacement measurement of the short stroke stage of the reticle stage of lithography. The measurement system consists of three pairs of permanent magnets mounted on the same plane on the short stroke stage along the Y, Y, X directions, and three single axis Hall sensors correspondingly mounted on the frame of the reticle stage. The emphasis is placed on the decoupling and magnetic field fitting of the three DOFs measurement system. The model of the measurement system is illustrated, and the XY positions and θZ rotation of the short stroke stage can be obtained by decoupling the sensor outputs. A magnetic field fitting by an elliptic function-based compensation method is proposed. The practical field intensity of a permanent magnet at a certain plane height can be substituted for the output voltage of a Hall sensors, which can be expressed by the elliptic function through experimental data as the crucial issue to calculate the three DOFs displacement. Experimental results of the Hall sensor displacement measurement system are presented to validate the proposed three DOFs measurement system.
ABSTRACT
A balanced plane-mirror heterodyne interferometer with a polarizing beam splitter used to recombine the reference and measurement beams is proposed to reduce periodic nonlinearity and to eliminate thermal error. Experimental results indicated that the periodic error due to ghost reflection was kept within ±36 pm, and the interferometer proposed was immune from thermal error.
ABSTRACT
A fiber-coupled displacement measuring interferometer capable of determining of the posture of a reflective surface of a measuring mirror is proposed. The newly constructed instrument combines fiber-coupled displacement and angular measurement technologies. The proposed interferometer has advantages of both the fiber-coupled and the spatially beam-separated interferometer. A portable dual-position sensitive detector (PSD)-based unit within this proposed interferometer measures the parallelism of the two source beams to guide the fiber-coupling adjustment. The portable dual PSD-based unit measures not only the pitch and yaw of the retro-reflector but also measures the posture of the reflective surface. The experimental results of displacement calibration show that the deviations between the proposed interferometer and a reference one, Agilent 5530, at two different common beam directions are both less than ±35 nm, thus verifying the effectiveness of the beam parallelism measurement. The experimental results of angular calibration show that deviations of pitch and yaw with the auto-collimator (as a reference) are less than ±2 arc sec, thus proving the proposed interferometer's effectiveness for determination of the posture of a reflective surface.