Optimization and temperature mapping of an ultra-high thermal stability environmental enclosure

Precision metrology, lithography and machining systems will soon require sub-nanometer tolerances in order to meet the evolving needs of industry. This, in turn, requires thermal control of large environmental enclosures with sub-millidegree single-point stability and control of temperature gradients to several millidegrees. In order to optimize the systemʹs thermal controls, it is essential to measure the open-loop transfer function. We report a technique that obtains the open-loop transfer function by utilizing a dynamic signal analyzer to perform a closed-loop frequency response measurement of the thermal system. Based on the transfer function, we designed a PI-lead compensation controller and achieved one-sigma air temperature stability of less than 1 m°C at a single point over 2 h. In order to rapidly map temperature gradients over large regions inside the 7 m3-volume enclosure, we have developed a measurement scheme that involves mechanically scanning a network of thermistors. Accurate cross calibration of the thermistors and a study of self-heating effects on temperature measurement in moving air have also been performed, which assures the relative accuracy of the thermistors is less than 1 m°C. Comparing temperature gradient maps taken before and after control improvements shows improved temperature stability over the mapped volumes.