A method of determining process or structure parameters for semiconductor structures, the method comprising: acquiring a plurality of optical signals from one or more targets located in a plurality of fields on a semiconductor wafer, wherein the fields are associated with different process parameters for fabricating the one or more targets, wherein the acquired optical signals contain information regarding a parameter of interest for a top structure and information regarding one or more underlayer parameters for one or more underlayers formed below such top structure; generating a feature extraction model to extract a plurality of feature signals from such acquired optical signals so that the feature signals contain information for the POI and exclude information for the underlayer parameters, wherein generating the feature extraction model includes performing a dataset reduction technique on the acquired optical signals to produce transformed optical signals data and generating the feature extraction model to extract the feature signals from the transformed optical signals data; and determining a POI value for each top structure of each field based on the feature signals extracted by the feature extraction model, characterized in that determining the POI is accomplished by training a parameter model to determine the POI value for each top structure based on the feature signals, and in the method further comprising: acquiring a plurality of optical signals from a plurality of unknown structures on one or more subsequent wafers; and using the feature extraction model and the parameter model to determine a plurality of POI values for the unknown structures. acquiring a plurality of optical signals from one or more targets located in a plurality of fields on a semiconductor wafer, wherein the fields are associated with different process parameters for fabricating the one or more targets, wherein the acquired optical signals contain information regarding a parameter of interest for a top structure and information regarding one or more underlayer parameters for one or more underlayers formed below such top structure; generating a feature extraction model to extract a plurality of feature signals from such acquired optical signals so that the feature signals contain information for the POI and exclude information for the underlayer parameters, wherein generating the feature extraction model includes performing a dataset reduction technique on the acquired optical signals to produce transformed optical signals data and generating the feature extraction model to extract the feature signals from the transformed optical signals data; and determining a POI value for each top structure of each field based on the feature signals extracted by the feature extraction model, characterized in that determining the POI is accomplished by training a parameter model to determine the POI value for each top structure based on the feature signals, and in the method further comprising: acquiring a plurality of optical signals from a plurality of unknown structures on one or more subsequent wafers; and using the feature extraction model and the parameter model to determine a plurality of POI values for the unknown structures. acquiring a plurality of optical signals from a plurality of unknown structures on one or more subsequent wafers; and using the feature extraction model and the parameter model to determine a plurality of POI values for the unknown structures.