Phase stepping digital interference microscopy (PSM) is a measuring technique that can be used for the three-dimensional analysis of small surface defects in many microelectronic applications on bulk materials and epitaxial layers. If the resulting intensities for the interference after regular phase stepping are I1, I2,...,Ip, where p equals 3,4,5,... according to the method, then the relief h(x,y) is given as a well known function of the Ii. We have used fuzzy logic to improve the algorithms in order to secure the final result and also to afford a possibility for a subsample resolution: first we define into the I-space N fuzzy classes in a non linear way; then the regularity needed in phase stepping leads us to define theoretical p-uples of fuzzy classes. Finally we correct the p-uples of measured values (I1, I2,..., Ip) according to the membershipness to the theoretical p-uples of fuzzy classes. These algorithms lead to important image improvements, offering better accuracy and a partial solution to the problem of image noise.