3D Lithography: A Game Changer for Flexible Circuits, Study notes of Electronics

Download 3D Lithography: A Game Changer for Flexible Circuits and more Study notes Electronics in PDF only on Docsity!

The Possible Role of 3D Lithography in Flexible

Circuits

Linnell Martinez

MSE 542

Tuesday, April 27, 2006

INTRODUCTION

Recent advances in the area of thin films processing, conductive polymers, and flexible power sources have served as stepping-stones for the development of flexible circuits. The possible applications of such flexible circuits are countless; from flexible smart labels to large roll-up displays, flexible circuits have the potential to be implemented in just about every aspect of our daily lives. Researchers have already developed several prototype devices and materials such as flexible substrates, flexible monochrome displays, and flexible photovoltaic cells amongst many others; however, not much work has been done in the area of creating in-situ three-dimensional (3D) flexible microstructures. As we move to more complex multi-functional flexible systems, 3D lithography by two-photon absorption will play a key role in its development. Conventional two-dimensional (2D) photolithography, the process in which a photosensitive polymer is exposed to UV radiation through a photo-mask, has historically been the desired method of making integrated electronic and optical circuits. Due to the parallel nature of the process, an entire circuit can be patterned with a single flash exposure to UV radiation. The previous method of fabricating 3D structures was based on an arduous layer-by-layer exposure of the resin using conventional photolithography [1]. Recently, a new 3D photolithographic process has been developed and experimentally verified. This process, now widely known as “three dimensional lithography by two-photon absorption” involves the photopolymerization of a resin or polymer stimulated by the non-linear two-

Figure 1.an objective lens for Left: Spatial distribution of two-photon absorption rate with a laser beam focused with !=780nm and N.A.=0.85. Right: Total two-photon absorption per transversal plane [6].

Polymerization Threshold Model:

Image 2. Lateral intensity distribution at the focus calculated byvectorial Debye theory for a linearly polarized (along the x -axis) beam for !=780nm and N.A.=1.4 [7].

An important point to note is that for both illustrated models, the voxel dimensions are very close to the diffraction limit. PHOTO-ACTIVE MATERIALS AND POLYMERIZATION PROCESS In the polymerization process, a photochemical reaction is initiated through a radical mechanism following a two-photon excitation of a photoinitiator. The photoreactive resins that are most commonly used are acrylate monomers or acrylic prepolymers that can be made to crosslink with the use of a radical photoinitiator molecule [8]. The two most popular resins for lithography are SU- and SCR-500, due to their current use in standard chip packaging and conventional lithographic processes. Several other photoactive polymers have been reported by [4, 5]. For a more detailed explanation of the polymerization process please refer to [8]. EXPOSURE AND TRANSLATION SYSTEM Many unique TPA exposure systems currently exist [3, 6, 9, 10], but most have a common set of components necessary to enable the 3D translation and TPA process. A typical setup includes a pump laser, femtosecond pulsed laser, focusing lens, three-axis precision translation stage, shutter, and a computer system. Much of the published work has used a femtosecond Ti:Sapphire laser, mainly due to the absorption spectra of the photoactive resins. The focusing lens is responsible for confining the light to a very small volume so that TPA may occur. Although for some setups a high numerical aperture (NA) lens is desired to minimize the focal volume [6], the work done by [3] shows that it is possible to

Figure 3. Representative setups of lithography systems [1, 6, 9]

The process for writing a pattern with the systems shown above is very simple, and mainly consists of moving the sample along a predefined path, until the entire pattern is exposed. Depending on the structures being fabricated, the exposure might be followed by a development step in order to remove the rest of the polymer which was not excited by the TPA process. EXPERIMENTAL RESULTS AND OBSERVATIONS Using the techniques and systems previously mentioned, several research groups have been able to fabricate, high aspect ratio microstructures, microfluidic devices and many other 3D structures. The following images show some highlights of what has been done with 3D TPA lithography.

Image 4.widths are 200–300 nm. Scale bar: 2 μm. Left : High-aspect-ratio gratings in SU-8. The height of the gratings is 2 Right: Post array in SU-8 Scale bar: 2 μm [9].. 0 μm, and the

Image 5. Attempts in realizing an SU-8 photonic crystal structure with a periodicity of 10 Left: A 132x50x50 μm cage structure made from high-AR pillars and planes. μm [1]. Right:

easily lay down a coat of SU-8 for example and soon after, pattern the interconnect channel by merely scanning the sample or laser beam in the desired pattern. The size and resolution of the patterned structure is directly proportional to the writing speed and voxel dimensions, both of which can be precisely controlled. Although there exists a multitude of other uses for this patterning method for flexible systems, it is important to mention some issues which could hinder it’s use as the desired way of patterning 3D microstructures. One of the main issues with this technology is the very limited materials selection. So far there are two main materials which happen to be used often, and for obvious reasons. SU-8 is a readily available photoresist with a broad spectrum of applications, whereas SCR- 500 is a bit less common, but often used for the purpose of TPA lithography. If the process for the device being fabricated is not compatible with these photoactive materials, then the proposed patterning method can not be used. Since the flexible systems area in general is heavily dependant on the materials choices, much research is being done to come up with viable solutions. Another issue inherent to this patterning technique is its sequential patterning method. This method makes use of a translation stage to move the sample which is being exposed to a laser beam. The finite volume of the focused laser beam means that in order to generate a pattern, the stage will have to be sequentially moved until the desired pattern is fully written. This is the domain in which conventional lithography can not be beat. Although this serial process can slow

things down, the areas being patterned will likely be less than a few centimeters, which would take minutes to write. CONCLUSIONS A brief summary of how two-photon absorption lithography could be used to pattern 3D structures in flexible materials was shown. This method of lithography has given rise to the fabrication of a multitude of 3D microstructures which would have otherwise been unfeasible to manufacture using conventional photolithography. There is still much research being done to try to better estimate the voxel dimensions and the role they play in exposure dose. It must be noted that the key technology which has made 3D lithography possible is the non-linear process of two-photon absorption. Without the quadratic relationship of absorption and incident intensity inherent to TPA, the entire path the light travels would be exposed, thus hindering 3D patterning of the polymer. The summary of experimental results from several works in the literature show that this technology has proven to be an effective way to write 3D microstructures, even well below the diffraction limit.

7. Juodkazis, S., V. Mizeikis, K.K. Seet, M. Miwa, and H. Misawa, "Two- photon lithography of nanorods in SU-8 photoresist," Nanotechnology 16, 846-849 (2005).
8. Yokoyama, S., T. Nakahama, H. Miki, and S. Mashiko, "Two-photon- induced polymerization in a laser gain medium for optical microstructure," Applied Physics Letters 82, 3221-3 (2003).
9. Lee, C.H., T.W. Chang, K.L. Lee, J.Y. Lin, and J. Wang, "Fabricating high-aspect-ratio sub-diffraction-limit structures on silicon with two- photon photopolymerization and reactive ion etching," Applied Physics A: Materials Science & Processing 79, 2027-2031 (2004).
10. Luo, L., C. Li, S. Wang, W. Huang, C. Wu, H. Yang, H. Jiang, Q. Gong, Y. Yang, and S. Feng, "Optical microstructures fabricated by femtosecond laser two-photon polymerization," Journal of Optics A: Pure and Applied Optics 3, 489-492 (2001).