New negative-tone photoresists avoid swelling and distortion

01/01/1997

EQUIPMENT FRONTIERS

New negative-tone photoresists avoid swelling and distortion

Gabi Gruetzner, Simone Fehlberg, Anja Voigt, Micro Resist Technology GmbH, Berlin, Germany

Bernd Loechel, Martina Rothe, Fraunhofer-Institut f?r Siliziumtechnologie, Berlin, Germany

A wide range of photosensitive organic materials are used as photoresists for fabricating integrated circuits. Both positive- and negative-tone resists are needed to simplify and lower the costs of device fabrication. A new generation of negative-tone photoresists avoids several disadvantages of currently used materials for the fabrication of today`s submicron devices.

One major disadvantage of available negative-tone photoresists has been their tendency to swell during aqueous treatments. The swelling makes it impossible to achieve 1:1 printing, so accurate pattern dimensioning can be achieved only by allowing for the swelling during mask generation. Another disadvantage of standard negative-tone resists is their high chemical reactivity with oxygen (from ambient air) during UV exposure. Moreover, most negative resists have to be developed in organic solvents. Because the development process required special equipment to ensure safety and environmental protection, the industry had shown decreased interest in using negative-tone photoresists.

Photoresist manufacturers have attempted to address the problems. For example, Hitachi looked at the disadvantages and developed new negative-tone photoresists especially for applications in high-resolution lithography. A major advantage of Hitachi`s new resists (e.g. Raycast series RD 2000N, RU 1000N, and RG 3000N) was that they did not swell during aqueous treatments and were able to maintain a high resolution. However, polyhydroxysterene (used as polymer in those resists) is an expensive material and provides a relatively low contrast. Also, the applied photoactinic component (PAC) has a very low solubility. Therefore, the maximum layer thickness for the Raycast negative-tone photoresists is less than 1.7 ?m. Furthermore, the PAC is not commercially available and has to be produced with special care [1, 2].

A different approach

Researchers conducted new investigations and further development aimed at overcoming the remaining disadvantages of existing negative-tone photoresists [3, 4]. The result was an improved photoresist family (see Table 1). These materials have a Novolak resin as the polymer and use safer solvents. Simple baking processes (like those for positive-tone Novolak resists) are now used for the negative-tone materials. A high sensitivity to UV light guarantees short exposure times.

Development of the new photoresists takes place in aqueous alkaline solutions. The result is an easy-to-handle resist system using materials that meet the tough requirements of continuing device miniaturization [5]. Moreover, the new materials have proven ideally suited to the application of very thick resist layers [6]. In contrast to the use of image-reversal resists or polyimide, the new negative-tone system allows some process steps to be simplified or even totally omitted. As a result, the new resists ensure high throughput and lower process costs.

The new family of negative-tone photoresists includes four resist types. They differ primarily in the amount of polymer contained, resulting in different viscosities. The polymer matrix used in the ma-N 400 family is always the same, however. No additives were used to adjust the viscosity. Various compositions of the solvents used (mainly ethyl lactate) explain the different viscosities. Depending on the mixture of solvents, different amounts of polymer and PAC can be dissolved to produce different viscosities.

Due to the excellent solubility of PAC in the solvent/polymer mixture, the ma-N 400 photoresists work without chemical amplification. Depending on their specific requirements, process engineers can select negative-tone resists with low viscosity or with very high viscosity (see Table 1). Without using special equipment, the possible layer thickness varies from 0.8-10 ?m (see Fig. 1). High-viscosity resists are especially useful for process applications requiring very thick resist layers.

Figure 1. Variation of thickness vs. rotation speed for each of the four new types of negative-tone photoresist materials in the ma-N family. The materials can be processed with standard lithography equipment that is already used for positive-tone resists.

The new photoresists also allow the steepness of pattern edges to be varied. Edges can be produced with a low undercut or high undercut from the vertical. Thus, customized patterns can be generated inexpensively, even with a difficult topography (for example, with vertical or low-undercut wall profiles).

Photoresist technology

Building on earlier work that led to the development of "deep ultraviolet" negative-tone photoresists, researchers developed a new type of PAC with light absorption shifted into the central part of the UV spectrum (Fig. 2). The ma-N 400 negative-tone resists use those PAC materials, which absorb and are converted by light at wavelengths between 320 and 380 nm [7].

The ma-N 400 resists consist of a special phenolic resin (soluble in acetone and alkaline solutions) as the polymeric bonding agent, an aromatic diazidodiphenyl derivative as PAC, and an environmentally safe solvent (containing a surface smoother). The resists do not need chemical amplification and can be processed on standard lithographic equipment. Process flow is completely compatible with conventional positive-tone resist systems, and no additional tools are required [7].

Figure 2. Irradiation spectrum for the new negative-tone photoresists. The materials have their maximum photosensitivity in the midultraviolet region of the spectrum.

Standard negative-tone photoresists use cyclopentanone or cyclohexanone as solvents. Those compounds are hazardous and toxic - especially if they are used in a cleanroom environment with reduced air exchange. By contrast, the new photoresist family uses relatively harmless ethyl lactate as the solvent.

Application of the new resists is simple. Thin resist layers are easily applied using standard spin coaters. For thicker resist layers, a spin coater with a co-rotating cover can form a 10-?m resist layer in a single process step. The subsequent baking can be done either in a convection oven or on a hot plate. No significant differences in resolution, sensibility, adhesion, and pattern quality have been detected for the new resists.

Lithography throughput

The ma-N 400 negative-tone photoresists can be applied by spin coating, roller coating, and dip coating. An adhesion promoter (e.g. HMDS) should be used. A prebaking step, lasting 15-30 min, is done in a convection oven at temperatures of 85-90?C. Note, however, that ma-N 490 photoresist requires 30-60 min of prebake at the same temperature. Alternatively, a hot plate can be used. In that case, the hot-plate temperature should be set to 80?C, and the wafers should be prebaked for 4-5 min.

Figure 3. Lines and spaces for a pattern produced with an ma-N 400 negative-tone photoresist material. Resolutions of 0.5 ?m have been demonstrated for a thickness of 1 ?m.

In our tests, the resist films were exposed by using vacuum contact printing and standard UV aligners. A 350-W high-pressure mercury lamp was used as the light source. Exposure time depends on the light intensity of the exposure source and the resist film thickness (see Table 1). To develop the photoresist, wafers were subsequently immersed in ma-D 532 developer (metal-ion free) or ma-D 332 (NaOH based). Pattern resolution depends on the resist-film thickness, but a resolution of 0.5 ?m has been achieved (Fig. 3).

For resist-film thicknesses up to 5 ?m, an edge steepness of 90? could be achieved. The crosslinking rate of the resist system (along with photo speed, intensity of the exposure source, and the developer used) influences the profile angle of the pattern. Figure 4 shows a pattern transferred using ma-N 400 (thickness 1.5 ?m) in which a profile angle of 89? was achieved.

Like positive-tone photoresists with high viscosity, the new negative-tone photoresists are very useful for surface-micromachining applications. Difficult topographies (with large differences in feature height) were successfully covered with resists by spin coating and exposed by contact printing after baking. Photoresist patterns could be generated with high precision on the top surface as well as in grooves.

Figure 4. Depth profile achieved with the new photoresist materials. A profile angle of 89? was achieved with a thickness of 1.5 ?m. If necessary, development time can be adjusted to produce undercutting for lift-off applications.

Chemical stability

The crosslinking rate and the type of phenolic resin used give the new photoresists high chemical stability. The fundamental photolithographic reaction during UV exposure takes place in the 4,4`-diazodiphenyl component of the PAC. The photolysis reaction of the 4,4`-diazodiphenyl is shown in Fig. 5. The co-generated diaminodiphenyl product remains in the polymer, and it consists of oligomers connected to each other by methylene bridges. It is assumed that the molecules are connected to the polymer chain by hydrogen bridges between the phenolic group of the polymer and the amino group of the diaminodiphenyl product [8].

Figure 5. Chemical photolysis reaction of the ma-N 400 negative-tone photoresist. During ultraviolet exposure, the fundamental photolithographic reaction takes place in the diazodiphenyl photoactinic component.

Photo-absorption of the generated diaminodiphenyl product is higher than the absorption of the initial 4,4`-diazodiphenyl component, and that can produce a sharp gradient for the depth of the photo reaction. Exposure can also be carried out in such a way that the crosslinking rate is lower at the bottom than at the top of the resist layer. Therefore, depending on the exposure conditions, steep edge profiles can be generated - as well as profiles with high undercut toward the bottom of the resist layer. The intensity of the undercut can be controlled by varying the development time (Table 2).

Lift-off results

Resist side walls with a large undercut are required for lift-off applications. As noted earlier, the ma-N 400 resists make possible fabrication of side walls with high undercut. The influence of the over-developing duration on the resulting undercut for ma-N 490 is illustrated in Fig. 6. Edge profiles with steep edges or with high undercut can be generated by varying the development time. Using a minimum time for development, the resist side walls can be made steep, and only a small undercut occurs in the middle region (see Fig. 4).

With over-development, the undercut increases - and the resulting patterns are best suited for use with lift-off techniques. In our tests, gold, aluminum, and nickel were used for sputtering, and the metals were deposited on the resist patterns up to a height of 1 ?m. No change in resist quality was observed during the deposition process. After metal, the resist patterns could easily be removed with acetone.

Electroplating processes

The negative-tone resists show excellent stability in both acidic and alkaline plating solutions. The resists can be used in solutions up to a pH value of 13. In one test, copper was electroplated into the resist pattern. Metal was deposited from a sulfuric-acid based plating bath with a pH of 0. A height of 10 ?m was achieved for the copper. No side-wall loss and no underplating were observed during electrodeposition.

Figure 6. Size of undercut vs. time of over-development for a 10-?m coating of the negative-tone photoresist. Side walls with a large undercut can simplify lift-off processes.

Figure 7. Electroplated gold pattern produced using ma-N 400 photoresist as a mold. The micro wheel has a height of 7.5 ?m, produced after two hours of deposition. Prolonged exposure to the alkaline plating solution did not damage the photoresist.

Another electroplating test produced a gold micro-wheel pattern (Fig. 7). The electrodeposition process used an alkaline plating solution working at a pH of 9.5. After two hours of deposition, a pattern height of 7.5 ?m was measured for the gold wheel. An ma-N 490 photoresist generated the mold for this micro component.

Conclusion

A new family of negative-tone photoresists offers ease of use and other important advantages over standard photoresists. No swelling occurs during aqueous treatments, so true pattern dimensions can be achieved without having to make corrections during mask generation. Aqueous alkaline developers are used, so the use of special developers based on environmentally damaging organic solvents can be avoided.

Minimum pattern dimensions of 0.5 ?m can be achieved for a 1-?m thick photoresist layer. Also, using the ma-N 490 photoresist, a maximum layer thickness of 15-20 ?m can be achieved in a single coating step. One useful characteristic of the negative-tone resists is that a large undercut can be created via overdevelopment, so these resists are favored for lift-off processes.

Acknowledgments

The authors wish to thank I. Schmidt for preparing PAC and resist samples, and T. Endrulat for support in generating graphics and photos. This work was financially supported by the Bundesministerium f?r Bildung, Wissenschaft, Forschung und Technologie der Bundesrepublik, Deutschland.

References

1. H. Bottcher, J. Bendix, M. A. Fox, G. Hopf, H.-J. Timpe, "Technical Applications of Photochemistry" pp. 186, 192, 193, Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1991.

2. S. Nonogaki, Polym. J. 19, p. 99, 1987.

3. G. Gruetzner, H. Mischke, J. Bendig, S. Helm, EPP 11, p. 73, 1991.

4. J. Bendig, E. Sauer, S. Helm, G. Gruetzner, P. Keppel, J. Inf. Rec. Mater. 19, 1991.

5. J. Bendig, G. Gruetzner, B. Maerten, H. Mischke, E. Sauer, SUSS-Report Q2/Q3, p. 8, 1992.

6. G. Gruetzner, S. Fehlberg, A. Voigt, I. Schmidt, J. Bendig, Productronic 8, p. 133, 1995.

7. A. Voigt, et al., SPIE Vol. 2438, pp. 413-420, 1995.

8. J. Bendig, G. Gruetzner, Z. Chem. 30, pp. 411-412, 1990.

GABI GRUETZNER received her MS degree in chemistry from the University of Jena, Germany. In 1991 she was one of the founders of Micro Allresist GmbH (now Micro Resist Technology GmbH) in Berlin. Currently, she is manager of the company. Micro Resist Technology GmbH, D 12459 Berlin, Slabystrasse 7a, Germany, ph 49/30-635-0798, fax 49/30-635-0991, e-mail [email protected].

SIMONE FEHLBERG received her MS degree in chemistry from the Technische Fachhochschule Berlin. She joined Micro Resist Technology GmbH in 1992, and is currently working on photoresist and process development.

ANJA VOIGT received her MS degree in chemistry from the Humboldt University of Berlin, Germany. She joined Micro Resist Technology GmbH in 1993, and is now working on photoresist development. In cooperation with Humboldt University, she is investigating the chemical reactions of irradiated negative-tone photoresists.

BERND LOECHEL received his PhD degree from the Free University of Berlin. He later joined the Fraunhofer Institute in Berlin, where he was responsible for silicon chemical-etching technology, and was also involved in x-ray and UV lithography, and galvanoplating. He is now working on three-dimensional UV technology for surface micromachining.

MARTINA ROTHE graduated from Lette-Verein Berlin and then joined the Fraunhofer Institute, where she worked on silicon etching, UV lithography, and measuring techniques. She is currently involved in three-dimensional UV microforming for surface micromachining and x-ray depth lithography.