**4.6 Summary of unresolved problems for nano-accuracy surface measurement**

There are still many problems that need to be solved in the development of the NSP.


### **5. Calibration of nano-accuracy**

As described above, now the required measurement accuracy is nano-radian and nanometer. If the instrument cannot be calibrated precisely, nano-accuracy metrology achievement is meaningless. Plane mirror measurement by use of traditional phase shift interferometer is difficult to reach /50-/100 accuracy because of the reference surface accuracy limitations. The absolute flatness test of the plane mirror by use of three-flat test method can reach /100 or better, but it is very difficult for large mirror calibration (for example for 500mm to 1000mm dimension). Pencil beam profilers can solve this problem, but it is only in 1-D. When the tested slope range increases, the angle related systematic error of the measurement instrument increases significantly. Though the pencil beam profilers have potential power to reach nano-radian and nanometer accuracy or less in principle, they still need to be calibrated. Calibration equipment should have the accuracy of three times better than the instrument to be calibrated. This is a great challenge to optical metrology.
