LITHOGRAPHY ERNET India（光刻ERNET印度）.pdfVIP

LITHOGRAPHY Lithos – stone + graphein – to write Process invented in 1796 by Aloys senfelder , used inked limestone (used chemically treated stones affinity to water/oil affected by chemical treatment ) for printing images on paper . Lithography in MEMS • Photolithography is the most widely used • Widely used in IC industry for pattern transfer, for multilayered IC’s, accurate registration and exposing a series of successive patterns. • This is a 2 D process. • What is the science involved? o Miniaturizing feature sizes. o Improving resolution. Pattern transfer by photolithography: Figure shows basics of photolithography and pattern transfer. Example uses an oxidized Si wafer and a negative photoresist system . Process steps 1. Oxidized wafer is coated with negative photo resist layer. 2. Exposure with a mask on glass plate, using UV light. 3. Wafer is rinsed in developing solution, to remove unexposed areas of photo resist. This results in bare and photo resist coated areas of oxide Note: Photo resist pattern now is the negative image of the pattern on the photo mask. 4. Wafer is placed in an etchant (eg HF or NH4F) , this attacks oxide, and not the photo resist. ( Wet anisotropic etching) 5. After etching is completed, remaining photo resist is stripped off with a Strong acid ( H SO ), 2 4 this attacks only the photo resist , not Si or SiO2 Some Definitions of Lithography Critical dimension : Absolute size of a minimum feature in an IC (line width/ spacing / contact dimension) Overall resolution of a process describes consistent ability to point a minimum size image, a critical dimension under conditions of reasonable manufacturing variation