Introduction to DNQ-Novolac Resists

There are three basic ingredients to all commercial DNQ-novolac photoresists: a phenolic novolac resin, a diazonaphthoquinone (DNQ) type dissolution inhibitor, and an organic casting solvent (see Figure 1).² The novolac resin provides the physical properties required in the photoresist such as good film forming characteristics, etch resistance, and thermal stability. The DNQ makes it possible to image the photoresist by providing a photochemical route to modifying the dissolution rate of the resist in aqueous alkaline developers (see Figures 2). The organic solvent has properties that provide the ability to spin coat the resist to form uniform, glassy thin films.

Novolac resins are soluble in a variety of common organic solvents including cyclohexanone, acetone, ethyl lactate, NMP (1-methyl-2-pyrrolidinone), diglyme (diethyleneglycol dimethyl ether), and PGMEA (propyleneglycol methyl ether acetate). In photoresist applications, the casting solvent must be carefully selected to provide good handling, spinning, and film forming properties. The most commonly used solvent today in DNQ-novolac is PGMEA, due to its relatively benign characteristics with respect to human exposure. Resists are generally formulated with polymer loadings of 15 to 30 percent by weight with respect to the solvent content of the resist solution. The viscosity of the solution can be adjusted by varying the polymer to solvent ratio, thus allowing resists to be formulated for coating a variety of film thicknesses.²

Figure 1 - The two main components of DNQ-novolac resists.

Equation 1: Definition of lithographic contast of a photoresist.

Figure 4 - Comparison of real aerial image produced by projection exposure tools with ideal mask pattern.

Figure 5- (below) Illustration of the importance of a non-linear resist response to exposure dose. A resist that displays a linear rate versus dose behavior will reproduce the sinusoidal behavior of the aerial image, while a non-linear resist response can reproduce the desired binary image from the mask in the final resist profiles.

The second method used to describe the non-linear dissolution behavior of a DNQ-novolac photoresist is to plot the dissolution rate of the resist as a function of the fractional amount of PAC that remains in the resist after exposure (see Figure 6).¹⁸ This type of plot is often referred to as an "R(m) plot" for a photoresist. A non-linear behavior in the R(m) plot for a resist directly translates into high contrast. In order to further understand the function of DNQ-novolac photoresists, a more detailed understanding of their chemical nature is required.

Figure 6- Example of R(M) plots for two DNQ-novolac photoresists showing the difference between high and low contrast responses. A non-linear response results in a high contrast resist.

1. Basic DNQ Novolak Resist Function

1.2. Diazonapthoquinone Dissolution Inhibitors

The most common class of dissolution inhibitors used in non-chemically amplified novolac based resists belong the to the diazonaphthoquinone (DNQ) family of compounds. The basic structure of DNQ is shown in Figure 7, along with the resonance structures that are commonly written for this compound.²

Figure 7- Structure of diazonaphthoquinones, showing both the general formula used to represent these compounds and the possible resonance structures.

The diazonaphthoquinone chromophores used in all commercial resist formulations bear other functional groups.²The most common of these substituted DNQs are derivatives of the 4- and 5-DNQ sulfonyl chlorides shown in Figure 8. The reactive sulfonyl chloride is generally reacted with an alcohol on an organic "ballast group" or "backbone." Figure 9 shows examples of the types of ballast groups and backbones that are used in making DNQ inhibitors. An example of a common multi-functional backbone is 1,2,3-trihydroxybenzophenone. Benzophenones, due to their absorbance in the UV wavelength ranges used for resist exposure, have recently been replaced with other more transparent backbones such as the p-cresol trimer shown in Figure 9. An example of a monofunctional phenol that is used as a ballast group is cumylphenol (see Figure 9). In some cases, the DNQ moieties may even be directly attached to the backbone of the novolac resins. These organic backbones and ballast groups serve a variety of purposes including increasing the inhibition strength of the DNQ and increasing the solubility of the DNQ in resist solutions. More detailed studies of the impact and function of the ballast groups in DNQ-type dissolution inhibitors can be found in the literature.²

Figure 8 - Two precursors to commercial DNQ inhibitors. The chloride groups are generally reacted in a based-catalyzed esterification with an alcohol group on a ballast structure or backbone molecule to make the final inhibitor.

Figure 9 - Examples of mono-functional ballast groups and multi-functional backbones. These compounds are typically alcohols. Multi-functional compounds may be partially or fully esterified, leading to a wide variety of possible inhibitors.

The DNQ family of compounds exhibit strong absorption bands in the ultraviolet wavelength range, from approximately 300 nm to 450 nm, that are utilized in the imaging of DNQ-novolac photoresists (see Figure 10). These absorption bands can be assigned to n-p* (S₀-S₁) and p-p* (S₁-S₂) transitions in the DNQ molecule. Due to the conjugated nature of the DNQ system, the absorption properties of DNQs are strongly affected by the nature and location of substituents.²² Figure 10 shows examples of the absorption spectra for DNQ-4-sulfonate and DNQ-5-sulfonate inhibitors. It is clear from this data that the absorption bands of these molecules are strongly affected by the position of the sulfonate group, with the DNQ-5-sulfonate compound being better suited to G-line exposure than the DNQ-4-sulfonate due to its strong absorption at this wavelength. An important feature of these DNQ systems is that with increasing exposure to radiation, the absorption bands of these molecules disappear as the compounds are converted to indene-carboxylic acid photoproducts. This decrease in the absorbance of photoproducts of the DNQ photolysis is termed "bleaching." This decrease in absorbance during exposure allows light to propagate to the bottom of thick and strongly absorbing resist films as the exposure proceeds, thus allowing for complete reaction of the DNQ throughput the thickness of a resist film. The choice of alcohol ballast group or backbone to which the DNQ is attached can also have a strong effect on the absorption properties of the compound. If the ballast group or backbone strongly absorbs in the UV wavelength range where the resist will be imaged, then the bleaching effect can be diminished due to the residual absorbance of the ballast or backbone groups.

Figure 10- Examples of absorption spectra for two types of DNQ inhibitors.²

In addition to the desired conversion of DNQ to an indene carboxylic acid photoproduct, there are a number of side reactions that are also possible. An example of some of the possible side reactions of DNQs that have been presented in the literature are shown in Figure 11.³ In general, DNQ-novolac resists are designed such that the dominant photolysis product is the indene carboxylic acid and other side reactions are suppressed. At this point, one may question the source of water that is required for the formation of the indene carboxylic acid in the photoresist. DNQ-novolac resists are in fact slightly hydroscopic, and a low concentration of water is naturally present in resist films processed in moderately humid environments. If DNQ-novolac resists are processed in very dry environments, the slower reaction of the ketene to form the phenol ester with the novolac resin can be promoted. This possibility of ester formation is an important consideration in resist processing. There are also a number of non-conventional resist processing schemes in which these side-reactions are purposely exploited to modify the behavior and imaging performance of DNQ-novolac resists.¹⁷

Figure 11- Examples of possible side reaction of DNQs. Notice that side reactions are possible with any of the three species (DNQ, ketene, and carboxylic acid) in the desired photoreaction sequence.

3. Novolak resins

Novolac resins are formed by the acid or metal ion catalyzed co-condensation of phenols with formaldehydes (see Figure 12).² Acid catalysts are the preferred choice for photoresist applications due to the concern of metal contamination in semiconductor manufacturing. The aromatic phenol ring has three positions, two ortho and one para positions, that are activated toward electrophylic substitution by formaldehyde. In general, mixtures of meta- and para-cresol are used instead of phenol for the production of resist grade novolacs.²

Figure 12- General reaction scheme for the production of novolac resins

The molecular weight of novolacs used for resist production are realtively small. The number average molecular weight (M_n) of resist-grade novolacs is generally between 1000 and 3000, which corresponds to between 8 and 20 repeat units, while the weight average molecular weight (M_w) may be as large as 20,000. From this information, it is clear that the polydispersity of novolac resins used for resist applications can be quite large. The resins have such low molecular weights, that they could be referred to as oligomers or oligomeric mixtures.

The actual structure of novolac resins can be quite complex, even though their molecular weight is relatively low. Kamide and Miyakawa have reported that for an eleven unit phenol-formaldehyde resin, there are 35 unbranched isomers and 2842 branched structures possible.²⁴ This large number of possible polymer structures is a result of the number of different bond-types (o,o-, o,o¢ -, p,o-, and p,o¢ -) that can be formed during the novolac synthesis. The type of linkages present in a novolac resin strongly affects the properties of the formulated resist. Hanabata and coworkers identified four factors concerning the structure of the novolac resin as having key influences in the performance of DNQ-novolac resist:^25-30

· Molecular weight of the resin

· Polydispersity of the resin

· Methylene linkage position

· Ratio of m,p-cresol precursors

The influence of novolac resin architecture on the performance of DNQ-novolac resists is described in more detail in the literature.² The influence of these factors on resist performance have been understood only recently, and fine control over the polymer structure is difficult due to the sensitivity of novolac synthesis to small changes in the experimental conditions. Therefore, control of novolac properties from batch to batch with the accuracy required for photoresist applications has not been generally possible. Most commercial resist suppliers instead resort to blending together a number of different resin batches to produce a resin with relatively constant and reproducible physical properties.² In fact, one technique used to improve the performance of modern high-performance DNQ-novolac resists relies on the separation and blending of various novolac resins and is termed the "tandem novolac" approach.³¹ The first step in making a tandem novolac resin is the fractionation of a polydispersenovolac resin into fractions of different molecular weights. The blended resist resin is then made by recombining the low and high molecular weight components, while omitting the intermediate molecular weight fractions (see Figure 13). The result of this process is a novolac resin, and ultimately a resist, that generally has a higher heat resistance and exposure sensitivity than conventional novolac resins.²

Figure 13 - Gel permeation chromotography traces of a polydisperse novolac resin and a tandem-novolac resin made from this resin by fractionation and remixing.

4. Summary

DNQ-novolac resists have been in use for over 25 years in the semiconductor industry.^2,32 The novolac resin in these resists provides the desired physical properties such as etch resistance that are required in the resist, while the DNQ provides a route to image the material. Although there are only two main components to these resists, there a number of factors that can be used to modify the imaging performance of these resists. Choice and location of ballast or backbone group for the DNQ inhibitor strongly affects the absorbance behavior and dissolution inhibition performance of these compounds. The composition and structure of the novolac resin also strongly impacts the performance of a DNQ-novolac resist. The ultimate goal in designing a DNQ-novolac resist is to design a material that displays a strongly non-linear response in dissolution rate versus exposure and provides good film forming characteristics, thermal stability, and high etch resistance.

5. References

1. R.R. Dammel, personal communication, 1998.
2. R.R. Dammel, Diazonaphthoquinone-based Resists, Bellingham, WA: SPIE Optical Engineering Press, Vol. TT-11, (1993).
3. J.J.Yu, C.C. Meister, G. Vizvary, C.B. Xu, P. Fallon, Proc. SPIE, 3333, at press, (1998).
4. D. Meyerhofer, IEEE Trans. Electron Devices, ED-27, pp. 921, (1980).
5. A. Reiser, H.Y. Shih, T.F. Yeh, J.P. Huang, Angew. Chem. Int. Ed. Engl., 35, 2428, (1996).
6. C.G. Willson, W. Yeuh, M.J. leeson, T. Steinhausler, C.L. McAdams, R.R. Dammel, J.R. Sounik, M. Aslam, R. Vicari, M.T. Sheehan, Proc. SPIE, 3049, 226, (1997).
7. C.L. McAdams, Structure Function Correlation Studies of Dissolution Inhibitors for Novolac-Based Photoresists, Masters Thesis, The University of Texas at Austin, (1996).
8. R.A. Arcus, Proc. SPIE, 631, 124, (1986).
9. M. Hanabata, A. Furuta, Y. Uemura, Proc. SPIE, 771, 85, (1987).
10. M. Hanabata, Y.Uetani, A. Furuta, Proc. SPIE, 920, 349, (1988).
11. T.F. Yeh, H.Y. Shih, A. Reiser, Proc. SPIE, 1672, 204, (1992).
12. T.F. Yeh, H.Y. Shih, A. Reiser, Macromolecules, 25, 5345, (1992).
13. P.C. Tsiartas, L.L. Simpson, A. Qin, C.G. Willson, R.R. Allen, V.J. Krukonis, P.M. Gallagher-Wetmore, Proc. SPIE, 2438, 261, (1995).
14. C.L. Henderson, P.C. Tsiartas, L.L. Simpson, K.D. Clayton, S. Pancholi, A.R. Pawloski, C.G. Willson, Proc. SPIE, 2724, 481, (1996).
15. W.D. Hinsberg, M.L. Gutierrez, Proc. SPIE, 469, 57, (1984).
16. J.P. Huang, T.K. Kwei, A. Reiser, Proc. SPIE, 1086, 74, (1989).
17. C.G. Willson, "Organic Resist Materials," Introduction to Microlithography, Second Edition, Ed. L.F. Thompson, C.G. Willson, and M.J. Bowden, Washington, DC: American Chemical Society, 139, (1994).
18. F.H. Dill, W.P. Hornberger, P.S. Hauge, J.M. Shaw, IEEE Trans. Electron Devices, ED-22(7), 445, (1975).
19. C.L. Henderson, P.C. Tsiartas, S.N. Pancholi, S.A. Chowdhury, K.D. Dombrowski, C.G. Willson, R.R. Dammel, Proc. SPIE, 3049, 805, (1997).
20. C.A. Mack, Microelectronics Manufacturing Technology, 14(1), 36, (1991).
21. M.J. Bowden, "The Lithographic Process: The Physics," Introduction to Microlithography, Second Edition, Ed. L.F. Thompson, C.G. Willson, and M.J. Bowden, Washington, DC: American Chemical Society, 19, (1994).
22. C.G. Willson, R. Miller, D. McKean, N. Clecak, T. Tompkins, D. Hofer, Polymer Eng. & Sci., 23(8), 1012, (1983).
23. A. Reiser, Photoreactive Polymers – The Science and Technology of Resists, New York: J. Wiley and Sons, (1989).
24. K. Kamide, Y. Miyakawa, Makromol. Chem., 179, 359, (1978).
25. A. Furuta, M. Hanabata, Y. Uemura, J. Vac. Sci. Technol., B4, 430, (1986).
26. M. Hanabata, A. Furuta, Y. Uemura, Proc. SPIE, 631, 76, (1986).
27. M. Hanabata, A. Furuta, Y. Uemura, Proc. SPIE, 771, 85, (1987).
28. M. Hanabata, Y. Uetani, A. Furuta, Proc. SPIE, 920, 349, (1988).
29. A. Furuta, M. Hanabata, J. Photopolymer Sci. Tech., 2, 383, (1989).
30. M. Hanabata, A. Furuta, Proc. SPIE, 1262, 476, (1990).
31. M. Hanabata, F. Oi, A. Furuta, Proc. SPIE, 1466, 132, (1991).
32. C.G. Willson, R.A. Dammel, A. Reiser, Proc. SPIE, 3049, 28, (1997).

Contact info:

Clifford L. Henderson

cliff.henderson@chbe.gatech.edu

404 385-0525