Photolithography: Step by step

cellphones computers televisions and any high technology product requires the use of printed circuit boards microcontrollers or integrated circuits how are they made the components of these devices are extremely small if they are too small to be seen by the naked eye how is this possible these products are made using a technique called photolithography photolithography is the process that transfers shapes from a photo mask to the surface of a silicon wafer using light photolithography comes from the Greek words photo meaning light litho meaning stone and graphene meaning writing photolithography is printing with light Alphonse Putin was the man who invented the process that is still used in modern micro manufacturing today he did this in 1855 by combining light with lithography just to give historical context putin invented the process four years before the completion of the iconic London landmark Big Ben not to be confused with Steelers quarterback big Ben Roethlisberger the modern process that is used today requires many steps and several different tools and chemicals the silicon wafer is treated chemically with hydrogen peroxide to remove any contaminations that may be present on the wafer contaminations may come from the removal of the wafer from the pods they're transported in or from the robot arm which the wafer sits on then a layer of silicon is placed on the wafer to act as a masking film hexa methyl diacyl is Ain or HMDs is placed on the wafer to increase adhesion of photoresist to the wafer this reduces the amount of resist needed in the coating process and is referred to as our RC reduce resist consumption the wafer is covered with photoresist by spin coating photoresist is dispensed onto the wafer and the wafer is spun rapidly to produce a uniformly thick layer the spin coating typically runs at 1,200 to 4,800 rpm for 30 to 60 seconds and produces a layer between 0.5 and 2.5 micrometers thick run off from the extra photoresistor removed through a process called edge bead removal to produce a flat layer of resist the photo resist coated wafer is in pre baked to drive off excess photoresist solvent a mask that resembles the chips geometry is held above the resist coated wafer UV light of extreme intensity is directed downward through the mask and onto the wafer this causes a chemical change in the photoresist that is exposed to the light by the mask developer is then used to wash away certain areas of photoresist positive photoresist the most common type becomes soluble and developer when exposed and is washed away with negative photoresist unexposed regions are soluble and developer and they are washed away in the etching process a chemical agent removes the uppermost layer of the substrate in the areas that are not protected by the photoresist this reveals the silicon wafer to the unprotected areas after a photoresist is no longer needed it must be removed from the substrate this usually requires a liquid resist stripper which chemically alters the resist so that it no longer adheres to the substrate silicon dioxide is deposited onto all regions of the wafer to layer any parts of the wafer that may have been altered in the cleaning process silicon dioxide that layers on top of the photoresist is removed in the first layer of the chip geometry is completed the process is repeated to obtain different geometries at as many necessary layers to complete the chips circuit design a chemical agent removes all silicon dioxide from the wafer and silicon is the only element remaining on the wafer how has photolithography improved from 1855 to the modern day the smallest degree of detail or resolution that photo lithography can be measured by is determined by two factors the numerical aperture of the optical system and a light source the numerical aperture can be approximated with the index of refraction and the max half angle of the collimated light as shown in the picture the index of refraction can be larger or smaller in some cases and similarly the max half angle can be altered by a change in focal length or beam diameter shorter wavelengths of light generate a lower resolution the old standard of light source use was a mercury vapor arc lamp with a wavelength of 365 nanometers the new standard light sources are fluoride based lasers with a smaller wavelength Moore's law states that the number of transistors in a space is doubled every two years smaller resolution allows for higher density of transistors within an integrated circuit this allows for higher capabilities of computer processing with smaller components the new standard of critical dimension the maximum distance between gates on the component of the transistors is 14 nanometer some companies that sell modern industrial photolithography equipment are ASML and saccuzzo that's the basics photo lithography size matters

Channel: Jae-Hwang Lee

Views: 640,944

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Id: oBKhN4n-EGI

Length: 5min 25sec (325 seconds)

Published: Tue Nov 24 2015