Properties of Photoresist Polymers

IBM Thomas J. Watson Research Center, 1101 Kitchawan Rd, Route 134 / P.O. Box 218,Yorktown Heights, NY 1059857.1 Introduction . 94157.2 Photoresist Materials and Lithographic Patterning Process 94157.3 Optical Properties of Lithographic Polymers and Photoresists 94357.4 Dissolution Properties of Photoresist Polymers 94457.5 Properties of Photoacid Generators 95157.6 Reactive Ion (PLASMA) Etch Resistance of Photoresist Polymers. . . 952References . . 95457.1 INTRODUCTIONThe explosive growth of semiconductor industry has beenfueled by the relentless pursuit for miniaturization of semi-conductor devices. The minimal feature sizes or criticaldimensions (CDs) of semiconductor devices in mass pro-duction have shrunk from 10mm more than 30 years ago toless than 100 nm in 2005. According to the InternationalTechnology Roadmap for Semiconductors, this miniaturiza-tion trend is expected to continue unabated with the produc-tion of sub-25 nm generations of devices later next decade[1]. The miniaturization of semiconductor devices has madeit possible to offer a host of sophisticated devices andequipment, from super computers, personal computers, per-sonal digital assistants, cellular phones to medical devicesand household appliances, with ever increasing performanceat steadily reduced prices per transistor or bit.This miniaturization trend has been made possible byadvances in a critical device patterning process called photo-lithography, including constantly improved photosensitivepolymeric materials called photoresists, advances in opticallenses,andtheuseofshorterwavelengthsoflightforpattern-ing. In 2004, the semiconductor industry quietly ushered inthe Nanoelectronics Age with the mass production of sub-100 nm node devices. The current leading-edge semicon-ductordevicesinmassproduction—thesocalled90 nmnodedevices—have a transistor gate length of less than 50 nm.These leading-edge devices are fabricated using photoresistsbasedonalicyclicpolymersat193 nmwavelength,aswellasNovolak-based mid-ultra violet (MUV) photoresists orpoly(4-hydroxystyrene)-based deep UV (DUV) photoresistsat wavelengths of 365 and 248 nm, respectively.57.2 PHOTORESIST MATERIALS ANDLITHOGRAPHIC PATTERNING PROCESSIn a typical photolithography process, a UV light is pro-jected by a set of sophisticated lenses onto a silicon wafercoated with a thin layer of photoresist through a mask thatdefines a particular circuitry. Exposure to the UV light,coupled with a subsequent baking, induces photochemicalreactions that change the solubility of the exposed regions ofthe photoresist film. Subsequently an appropriate developer,usually an aqueous base solution, is used to selectivelyremove the photoresist either in the exposed regions(positive-tone photoresists) or in the unexposed regions(negative-tone photoresists). The pattern thus defined isthen imprinted on the wafer by etching away the regionsthat are not protected by the photoresist with reactive ion(plasma) etching (RIE). Figures 57.1 and 57.2 depict sche-matic of a typical photolithographic system and a typicaldevice patterning process. Excellent reviews on photoresistmaterials have been published [2–6].Advanced photoresists, such as 193 and 248 nm photo-resists, are based on chemical amplification concept [7,8].These chemically amplified photoresists generally consistof a base polymer, a photo-sensitive compound calledphotoacid generator (PAG), and sometimes a cross-linking