PMMA: FAQs

What is the difference between your PMMA A resists and PMMA C resists?
Our PMMA A resists are formulated in anisole, while our PMMA C resists are formulated in chlorobenzene. We introduced the PMMA A resists in response to the environmental concerns expressed by many of our customers with PMMA resists formulated in chlorobenzene solvent.

Will I need to redesign my process if I switch from PMMA resist formulated in chlorobenzene to an equivalent formulated in anisole?
The viscosity of PMMA resists formulated in anisole is slightly lower than those formulated in chlorobenzene. Therefore you may need to use a slightly higher solids PMMA A resist to obtain the same film thickness. Please refer to the spin speed curves located in our PMMA & copolymer technical data sheet to select the appropriate product.

Are there performance differences between PMMA resists formulated in chlorobenzene and those formulated in anisole?
It depends upon your process requirements. In most cases PMMA formulated in anisole will have virtually identical performance characteristics as with PMMA formulated in chlorobenzene. However, if the PMMA resist is placed on top of another resist, such as a DNQ based positive resist, the anisole resist may attack (intermix with) the underlayer more readily than with a chlorobenzene based PMMA resist. For more comparative data between PMMA in anisole and PMMA in chlorobenzene, please refer to the following paper, which can be obtained from our PMMA home page. Characterization of safe solvent PMMA resist variables for electron beam applications. Bruce W. Smith, Todd D. Eakin, Rochester Institute of Technology and Donald W. Johnson, MicroChem Corp., 200 Flanders Road, Westborough, MA 01581.

What is the difference between 50PMMA, 100PMMA, 200PMMA, 495PMMA, 950PMMA and 2.2PMMA?
These are polymethylmethacrylate (PMMA) resists with molecular weights (MW) varying between 50,000 and 2.2 million. 495PMMA and 950PMMA are our standard products. We offer the other MW resists upon request. In general, the higher the molecular weight, the slower it will dissolve in a solvent developer. After exposure (molecular chain scission), the develop contrast between the exposed and unexposed regions of the film becomes higher as MW increases. Thus 950PMMA is typically used in high resolution applications and 495 PMMA in less demanding applications. Proper selection of MW is also critical in bi-layer T-gate processes where contoured sidewall profiles are desired.

How do I remove PMMA and copolymer resists?
PMMA resist films are readily removed in many common positive resist strippers. We have found most N-methyl pyrolidinone (NMP) based strippers to be highly effective, particularly when used at 50-60 C. EBR-PG, our NMP-based stripper is also effective at room temperature and Acryl Strip, our acetic acid based stripper gives extremely clean removal on GaAs substrates.

What is the Index of Refraction (I.R.) of PMMA at the HeNe wavelength?
The I.R. of PMMA is 1.49-1.52 at 632.8nm. Please refer to our PMMA & copolymer technical data sheet for other optical parameters such as cauchy coeffiecients, dispersion curves and n & k values.

What is the glass transition temperature (Tg) of PMMA?
The Tg of 495PMMA and 950PMMA is approximately 95 - 106°C.

Can you recommend any good PMMA reference papers?
Yes we can. They are listed below.

Fabrication limits of nanometer T and G gates: Theory and experiment
B. Maile - Daimler Benz J. Vac. Sci. Tech. B 11(6), Nov/Dec 1993.

Developer Characteristics of Poly-(Methyl Methacrylate) Electron Resist
J. Greeneich - GMRL J. Electrochem. Soc., Vol 122, 970-6, 1975.

Influence of developer and developer conditions on the behavior of high molecular weight e-beam resists
D. Hasko, S. Yasin, A. Mumtaz - U. of Cambridge J. Vac. Sci. Tech. B 18(6), Nov/Dec 2000.

Electron-beam resist technology for GaAs microwave device fabrication
D. Brambley, R. Bennett - GEC GEC Journal of Research, 13,1, 42-53,1996.

Effect of molecular weight on poly (methyl methacrylate) resolution
M. Khoury and D. Ferry - Arizona State J. Vac. Sci. Tech. B 14(1), Jan/Feb 1996.

Effects of molecular properties on nanolithography in PMMA
E. Dobisz, S. Brandow, R. Bass,
J. Mitterender- NRL J. Vac. Sci. Tech. B 18(1), Jan/Feb 2000.


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