A method of forming 3D micro structures with high aspect ratios includes the steps of: disposing a mask, which has a plurality of through holes having at least two different sizes, on a substrate to expose the substrate through the through holes; forming a negative photoresist layer on the mask and the substrate; providing a light source to illuminate the negative photoresist layer through the substrate and the through holes of the mask so as to form a plurality of exposed portions and an unexposed portion; and removing the unexposed portion and leaving the exposed portions to form a plurality of pillars each having a bottom portion contacting the substrate and a top portion opposite to the bottom portion. A top area of the top portion is slightly smaller than a bottom area of the bottom portion, and the pillars are allowed to have at least two different heights.
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