US Patent 7223703 Method of forming patterns

In forming a mask pattern on a circuit board, a mask pattern of N-layer structure is formed in a region where the mechanical strength of the circuit board needs to be increased. N photosensitive layers are first stacked on a substrate so that they becomes lower in sensitivity from the first photosensitive layer toward the Nth photosensitive layer. In the first photosensitive layer (bottom layer), a first pattern is formed and has the same shape as a predetermined pattern to be formed on the circuit board. In the Kth photosensitive layer (N≧K≧2), a Kth pattern is formed so that the Kth pattern is smaller than a (K−1)st pattern formed in the (K−1)st photosensitive layer and arranged inside the (K−1)st pattern.