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Positive photoresist chemistry

Positive photoresist materials originally developed for the printing industry have found use in the semiconductor industry. The commonly used novolac resins (phenol-formaldehyde copolymer) and (photosensitive) diazoquinone both were products of the printing industry.

The novolak resin is a copolymer of a phenol and formaldehyde ( [link] ). Novolak resins are soluble in common organic solvents (including ethyl cellosolve acetate and diglyme) and aqueous base solutions. Commercial resists usually contain meta-cresol resins formed by the acid-catalyzed condensation of meta-cresol and formaldehyde.

Structure of a novolak resin.

The positive photoresist sensitizers are substituted diazonaphthoquinones. The choice of substituents affects the solubility and the absorption characteristics of the sensitizers. Common substituents are aryl sulfonates. The diazoquinones are formed by a reaction of diazonaphthoquinone sulfonyl chloride with an alcohol to form sulfonate ester; the sensitizers are then incorporated into the resist via a carrier or bonded to the resin. The sensitizer acts as a dissolution inhibitor for the novalac resin and is base-insoluble. The positive photoresist is formulated from a novolac resin, a diazonaphthoquinone sensitizer, and additives dissolved in a 20 - 40 wt% organic solvent. In a typical resist, up to 40 wt% of the resist may be the sensitizer.

The photochemical reaction of quinonediazide is illustrated in [link] . Upon absorption of a photon, the quinonediazide decomposes through Wolff rearrangement, specifically a Sus reaction, and produces gaseous nitrogen as a by-product. In the presence of water, the decomposition product forms an indene carboxylic acid, which is base-soluble. However, the formation of acid may not be the reason for increased solubility; the release of nitrogen gas produces a porous structure through which the developer may readily diffuse, resulting in increased solubility.

Image reversal

By introducing an additive to the novolac resins with diazonaphthaquiones sensitizers, the resultant photoresist may be used to form a negative image. A small amount of a basic additive such as monazoline, imidazole, and triethylamine is mixed into a positive novolac resist. Upon exposure to light, the diazonaphthaquiones sensitizer forms an indene carboxylic acid. During the subsequent baking process, the base catalyzes a thermal decarboxylation, resulting in a substituted indene that is insoluble in aqueous base. Then, the resist is flood exposed destroying the dissolution inhibitors remaining in the previously unexposed regions of the resist. The development of the photoresist in aqueous base results in a negative image of the mask.

Comparison of positive and negative photoresists

Into the 1970s, negative photoresist processes dominated. The poor adhesion and the high cost of positive photoresists prevented its widespread use at the time. As device dimensions grew smaller, the advantages of positive photoresists, better resolution and pinhole protection, suited the changing demands of the semiconductor industry and in the 1980s the positive photoresists came into prominence. A comparison of negative and positive photoresists is given in [link] .

A comparison of negative and positive photoresists.

The better resolution of positive resists over negative resists may be attributed to the swelling and image distortion of negative resists during development; this prevents the formation of sharp vertical walls of negative resist. Disadvantages of positive photoresists include a higher cost and lower sensitivity.

Positive photoresists have become the industry choice over negative photoresists. Negative photoresists have much poorer resolution and the positive photoresists exhibit better etch resistance and better thermal stability. As optical masking processes are still preferred in the semiconductor industry, efforts to improve the processes are ongoing. Currently, researchers are studying various forms of chemical amplification to increase the photon absorption of photoresists.

Bibliography

  • W.M. Alvino, Plastics For Electronics , McGraw-Hill, Inc, New York (1995).
  • R. W. Blevins, R. C. Daly, and S. R. Turner, in Encyclopedia of Polymer Science and Engineering , Ed. J. I. Krocehwitz, Wiley, New York (1985).
  • M. J. Bowden, in Materials for Microlithography: Radiation-Sensitive Polymers , Ed. L. F. Thompson, C. G. Willson, and J. M. J. Frechet, American Chemical Society Symposium Series No. 266, Washington, D.C. (1984).
  • S. J. Moss and A. Ledwith, The Chemistry of the Semiconductor Industry , Blackie&Son Limited, Glasgow (1987).
  • E. Reichmanis, F. M. Houlihan, O. Nalamasu, and T. X. Neenan, in Polymers for Microelectronics , Ed. L. F. Thompson, C. G. Willson, and S. Tagawa, American Chemical Society Symposium Series, No. 537, Washington, D.C. (1994).
  • P. van Zant, Microchip Fabrication , 2nd ed., McGraw-Hill Publishing Company, New York (1990).
  • C. Grant Willson, in Introduction to Microlithography , 2nd ed., Ed. L. F. Thompson, C. G. Willson, M. J. Bowden, American Chemical Society, Washington, D.C. (1983).

Questions & Answers

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Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
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There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
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da
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Bhagvanji
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Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
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revolt
da
Application of nanotechnology in medicine
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RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
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Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
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is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
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LITNING Reply
What is meant by 'nano scale'?
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LITNING
scanning tunneling microscope
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Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
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what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
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what king of growth are you checking .?
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What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
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Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
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what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
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Source:  OpenStax, Chemistry of electronic materials. OpenStax CNX. Aug 09, 2011 Download for free at http://cnx.org/content/col10719/1.9
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