MASKLESS LITHOGRAPHY BY PATTERNED HEATING OF PHOTORESISTUSING ULTRA-COMPLIANT THERMAL PROBE ARRAYS
Amar S. Basu, Shamus McNamara, and Yogesh B. Gianchandani
EECS Department, University of Michigan
1301 Beal Avenue, Ann Arbor, MI 48109-2122
With increasing costs of photomasks, maskless lithography is of interest as anavenue for re-configurable manufacturing. This paper describes the use of a
micromachined array of ultra-compliant probes with polyimide shanks and embedded
thin-film heaters (Fig.1) to induce spatially localized thermal cross-linking in AZ5214 E, a widely available photoresist. The technique of localized heating, recently
investigated using lasers1, is similar to traditional image reversal2, except that insteadof patterning the UV exposure, the post-exposure bake is spatially localized. This
contrasts with other methods of scanning probe lithography such as nanoindentation3,4, dip pen patterning5, and charge manipulation6, and represents the only
use of a heated tip to catalyze a localized chemical reaction. In this approach, pixel
sizes can be controlled by the probe temperature and velocity. Using 20-30 mW
power and 2-60 sec residence times, probes with 400 nm tip diameter were used topattern pixels with diameters ranging in size from 400-2000 nm. Resolution can be
improved to the sub-100 nm range by using sharper probe tips and thinner resists.
The probe arrays which enable this technology are fabricated by a low-cost
micromachining process in which a thin film metal bolometer is sandwiched between
two insulating layers of polyimide7. The metal film is molded into an anisotropically
wet-etched notch in the Si substrate to form the scanning tip. The probe is thenreleased from the substrate, flipped out over the die edge, and held in place by athermo-compression bond by a thin film of Au. Probes fabricated from rigidmaterials8 such as Si or SixNy require a force feedback on each tip to prevent damage
to soft samples. The use of ultra-compliant polyimide cantilevers permits the probes
to operate in contact mode (for superior thermal contact) without mechanicalfeedback to control the contact force, and makes scaling to large arrays practical.
This is potentially a low-cost approach to maskless lithography for rapid prototyping
and re-configurable manufacturing.
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References
1. M.C. Kuwahara, J.H. Kim, Proc. Intl. Conf. on MNE, Lugano, 2002, Elsevier.
2. M. Spak, D. Mammato, et al., Photopolymers: Principles, Processes and Materials, Ellenville, 1985,
Society of Plastics Engineers, pp. 247-269.
3. J.H. Bae, et al, Applied Physics Letters 82, 5 (2003).
4. V. F. Dryakhlushin, A.Y. Klimov, Nanotechnology 11, 3 (2000).
5. D. Bullen, X. Wang, Proc. IEEE Intl.Conf. on Micro Electro Mechanical Systems, Kyoto, 2003.
6. S. F. Lyuksyutov, R.A. Vaia, Nature Materials 2, 7 (2003).
7. S. McNamara, A. Basu, J.H. Lee, Y.B. Gianchandani, in Proc. IEEE Intl.Conf. on Micro Electro
Mechanical Systems, Maastricht, 2004 .
8. P..Vettiger,et al., IBM Journal of Research Development 44, (2000).

Fig. 1: SEM image of
micromachined 8-probe array.
Thin film resistors integrated
on each cantilever allow each
tip to be heated independently.
Ultracompliant cantilevers
fabricated from polyimide offer
low contact force even without
mechanical feedback.

(1)

(2)

(3)

Fig. 2: Process flow for maskless thermal lithography. (1) Flood exposure of a wafer spin coated with athermally cross-linkable photoresist. (2) Patterned baking with microprobes. Spatially localized thermal
cross-linking occurs under the heated probe tip (inset) as a result of the flood exposed photoactive
compounds (PACs) and elevated temperatures. (3) The insoluble cross-linked region remains after
development in a standard TMAH-based developer.

Pixel Size (nm)

(b)
1200
1000
80060040020000204060

Heating Time (seconds)

(a)

80(c)
Fig. 3 (a, b) Parallel lithography using microprobe arrays.
Two probe tips spaced 85 µm apart simultaneously
generated the same pattern. The probes tip can be heatedindependently of one another, producing different images.
Thinner photoresists and a sharper tip prevent lateral spreadof heat and result in smaller pixels. (c) Smaller heatingtimes also reduces pixel size, as shown by data obtainedusing 25 mW probe power on a 400 nm resist.