Research On LED Solid State Film Thickness Online Measurement System

It is a big problem that measuring film thickness online for each film manufacturers because the film thickness is an important factor to guarantee the quality of the film.In this paper, compared with traditional thickness measurement, proposed a new way to measure film thickness which is combined with infrared spectroscopy. The system use reference method with a fixed optical path. Using two modulated infrared pulses exposure one position of the film at the same time. The infrared is received by the infrared sensor and then through the pre-amplifier and signal processing circuit we can get a voltage representing the film thickness. After ADC we could figure out the film thickness. The characteristic of the system is using reference method can eliminate the environment, temperature and other common mode interference. And using high-speed modulation of infrared light can eliminate the phase error. It still can improve the accuracy and reliability of the system.Thirdly, using LED light source we don't need use modulation disk, so there is no mechanical movement in the system which can increase the reliability of the system. This paper designed the film thickness measurement system based on above method. The system is divided into two parts: measurement and data process. The measurement part includes light source, light modulation circuit, infrared sensor, pre-amplifier and filter. Data acquisition and processing mainly use 16bit A/D converter AD976 and DSP TMS320F2812. This paper mainly focuses on design and principle of hardware circuit. According to the characteristics of LED and infrared sensor, we designed the LED driver and the regulator. For the LED's power is small, it needs pre-amplifier and filter. On the base of hardware design, this paper also describes the data acquisition software system. At last, the paper analyzed some elements which have influence on the precision, such as environment, material, LED driver, regulator, signal processing circuit, and puts forward the improvement of the method.