1. A method of fast neural network training for library-based critical dimension (CD) metrology, the method comprising: providing a training target for a first neural network; training the first neural network, the training comprising starting with a first number of neurons and iteratively increasing the number of neurons until the training target is reached using a second, larger, number of neurons; generating a second neural network based on the training and the second number of neurons; and providing a spectral library based on the second neural network.