TECHNOLOGY
Electronics Industry Opens Frontiers for Photoresist Chemistry New lithographic methods are fitting smaller features on silicon chips; processes using electron beam, x-rays, and deep-UV hold promise Steven C. Stinson, C&EN New York
Chemists today are facing growing demands for supplying new com pounds to the electronics industry. Perhaps the greatest opportunities lie in inventing resists for photo lithographic techniques of ever higher resolution. As makers of integrated circuits work to fit increasing numbers of components onto silicon chips, the sizes of features on a chip keep get ting smaller. Indeed, some observ ers say that resolution of such min ute features almost exceeds capabili ties of present photolithographic methods. The new lithographic processes that integrated-circuit makers are looking at include those based on deep-ultraviolet (less than 300 nm), electron beam, and x-ray irradiation. Until now, the near-UV region has been satisfactory, such as at the 436-, 404-, and 366-nm lines of mercury sources. But the present concern is that feature sizes are approaching those of the wavelengths used to resolve them. For example, since 1970, the industry has advanced from IK (1024-byte) random access memory (RAM) chips, with their 8- to 10-μπ\ features, to the threshold of the 256K RAM age, whose chips hold 262,144 bytes of memory and need resolu tions of 1 μιη or less. Among chip makers, resolution means ±10%, and 1 μιη is 1000 nm, which is close to the "length" of a photon.
phy at wavelengths from 350 to 450 nm still can meet demands for reso lution for the near future. Here also, however, the need is for new photo resist types, such as multilayer sys tems or resists that can be devel oped by plasma etching rather than by dissolving in liquids. Whichever school of thought is correct, the re sult will be more opportunities for the creation of new materials. One group that has reported prog ress this year in resists for the midUV range is that of C. Grant Willson, manager of lithography at the IBM research laboratory in San Jose, Calif. IBM scientists Robert D. Miller, Dennis R. McKean, Terry Tompkins, and Nick Clecak worked with physi-
Use of deep-UV of wavelengths from 230 to 270 nm could let indus try begin escaping from this limit, though equipment to use such wave lengths would need quartz optics and very "bright" sources. Then there is the mid-UV range around 310 to 340 nm, where borosilicate glass optics are still usable. Also, x-rays have wavelengths of 10 down to 0.1 nm, and electrons have effec tive wavelengths of 0.2 down to 0.01 nm. So one challenge for chem ists is to synthesize resins with sensitivities lying in these regions and with response speeds that al low high production rates. But there are scientists who say that present near-UV photolithogra
Photosensitive groups attach to polyamic acids CH3 CH 3 N
T ~0~
CHr
W~
NC
C
C
\J^ \=y^
N-CH2CH2CH2-Si-0-Si-CH2CH2CH2+
H0 2 C
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i
n
0=C=N-CH2CH2-02C - C = C H 2 CH a
2 N - C h L2C H2. - C L C - C ^ Η ^CH,
Of
. that are cross-linked, insoiubilized in exposed areas . . . /
C H
3
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I C H 3 X3H CHLCH \ CO,
. . . and form polyimide-siloxanes on baking CH3 ,
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Il
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X
September 26, 1983 C&EN 23
Technology
Photoresists, light-sensitive resins used for imaging, may be positive or negative For the electronics industry, resists are resins that can be applied to a substrate in a pattern of exposed and covered areas to leave some areas open for later etching or plating while protecting others. For many types of printed circuit boards, whose lines and spaces are 40 mm or more wide, screening is sufficient to give needed resolution. But integrated circuit chips need the finer resolution afforded by photolithography, which uses lightsensitive resists for imaging.
to ultraviolet light. UV irradiation photolyzes bis(azides) to bis(nitrenes), which form aziridine groups with dou ble bonds of neighboring chains. De veloping exposed negative photoresists in a solvent dissolves away resin from unexposed areas. Positive resists are formulations like phenolic resins mixed with relatively large amounts of light-sensitive solubili ty inhibitors, such as diazonaphthoquinones. On exposure to light, the solu bility inhibitors rearrange to acidic com pounds that can be leached out by bases. Thus, exposed areas of posi tive resists are the ones dissolved away on development. Alternatively, posi tive resists can be resins that are depolymerized by irradiation.
Such photoresists may be either positive or negative. Negative photo resists include resins such as cyclized polybutadiene or polyisoprene, mixed with a bis(azide), which become crosslinked and insolubilized by exposure
cal chemistry professor Josef Michl and research associate John Downing at the University of Utah to opti mize absorptions of 2-diazo-l,2-naphthoquinones at both the 313- and 334-nm mercury lines. Michl and Downing used semiempirical calculations to predict sub stituent effects on UV absorption.
These led the IBM group to synthe size a mixed 4- and 5-sulfonate diester of a long-chain primary diol, which had a broad absorption over the desired range. Preliminary for mulation of the mixed diester as photosensitizer in a phenolic-resin novolac has led to imaging at 1.25-μπι resolution.
Chemical amplification intensifies images Photochemical reaction generates an a c i d . . UV,256nm
