A Research For Improve CD Uniformity By Lens Adjustment

With the development of lithography, microelectronics and integrated circuit technology has developed rapidly during the past fifty years. As one of the most important factor in lithography technology, CD getting more smaller, so the within shot Critical Dimension Uniformity(CDU) performance requirements has become more strictly. The Total Focus Deviation(TFD) performance have strong relationship with the within shot CD uniformity, which will also impact the minimum CD of product. To improve the within shot CD uniformity and yield, we need improve the TFD performance for stepper lens at first.Firstly, this paper will try to study the function of the scanner tool lens TFD, and will try to find the better way to improve TFD performance after research the relation ship between TFD and depth of focus(DOF). Because the TFD can impact the efficient scope of lens depth of focus(DOF), which is the basic condition to get the minimum CD. But in fact, we can only control it within a reasonable range due to the influence of various factors. The CD uniformity cannot be guaranteed if the focal plane curvature accuracy out of control limit, then wafer will drift from the focal plane(Defocus) during exposure, which will induce low yield rate of the product. There are many factors can impact the TFD range, including the uniformity of the light source, lens transmittance, refractive rate change and etc, and different factors will cause different impact on the TFD range during exposure.Secondly, this paper will try to study the basic theory of lithography process, understand the main factors which impact the TFD of photo scanner/stepper machine. After compare the traditional method and the method by adjusting the optical projection lens to improve CD uniformity, we can summary the advantages and disadvantages of different methods and to discuss the necessity and feasibility of the method by adjusting the optical projection lens to improve CD uniformity in this paper. There are some solutions to improve CD uniformity: First is improving expose light uniformity within one Field Size/Shot by adjust the lamp position. For worse case, we need replace the optical filter but the cost will be more expensive. The second is to clean optical lens or replace the lens component. The third one is improving the lens TFD and distortion through adjusting tilt and height between lens component, calibration expose light angle etc. It is also the solution we will perform in this paper. Finally, after collect test data from experiment product, the CD uniformity improve about 36.4% reach the target. With the CD uniformity improvement, we can also reduce production cost, and increase the process capacity.