A DEEPER LOOK Photolithography

Metarhodopsin II is then recycled back into rhodopsin by a multistep sequence involving cleavage to all-fcraus-retinal and cis-trans isomerization back to 11-ds-retinaI. 

Fifty years ago, someone interested in owning a computer would have paid approximately 150,000 for 16 megabytes of random-access memory that would have occupied a volume the size of a small desk. Today, anyone can buy 60 times as much computer memory for 20 and fit the small chip into their shirt pocket. The difference between then and now is due to improvements in photolithography, the process by which integrated-circuit chips are made. 

Manufacturing the ultrathin circuitry on this computer chip depends on the organic chemicai reactions of speciai poiymers. 

The polymer resist currently used in chip manufacturing is based on the two-component diazoquinone-novolac system. Novolac resin is a soft, relatively low-molecular-weight polymer made from methylphenol and formaldehyde, while the diazoquinone is a bicyclic (two-ring) molecule containing a diazo group (=N = N) adjacent to a ketone carbonyl (C=0). The diazoquinone-novolac mix is relatively insoluble when fresh, but on exposure to ultraviolet light and water vapor, the diazoquinone component undergoes reaction to yield N2 and a carboxylic acid, which can be washed away with dilute base. Novolac-diazoquinone 

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