Two-color photo-initiation/inhibition lithographyecee.colorado.edu/~mcleod/pdfs/Two-color...

Two-color photo-initiation/inhibition lithography

Robert R. McLeoda, Benjamin A. Kowalskia & Michael C. Coleb

aUniversity of Colorado, UCB 425, Boulder, CO, USA 80309-0425 bInPhase Technologies, 2000 Pike Road, Longmont, CO 80501

ABSTRACT

Traditional photolithography begins with single-photon absorption of patterned light by a photo-initiator to locally expose a resist. In two-color photo-initiation/inhibition (2PII) lithography, these exposed regions are confined by a surrounding pattern of inhibitors generated by one-photon absorption of a second color in a photo-inhibitor. Like a stencil used to confine spray-paint to a thin, sharp line, the inhibitory pattern acts as a remotely programmable, transient near-field mask to control the size and shape of the modified resist region. The inhibiting species rapidly recombine in the dark, allowing for fast sequential exposures and thus enabling fabrication of complex two- or three-dimensional structures. Keywords: Lithography, Photolithography, Super Resolution, Diffraction Limit, Photopolymer, Optical Data Storage

1.INTRODUCTION Traditional photolithography is founded on one-photon absorption which has the sensitivity required for high-throughput commercial applications but is limited in resolution by diffraction to approximately ¼ the exposure wavelength. This linear diffraction limit can be moderately exceeded via resists initiated with two-photon absorption at the expense of very low sensitivity and throughput1,2. Even finer resolution at even lower throughput is obtained by multi-photon absorption of glass at the focus of a femtosecond pulsed laser 3,4. Inspired by stimulated-emission, depletion (STED) microscopy5, several recent articles6,7,8 reported that the lithography diffraction limit could be similarly bypassed via two one-photon or two-photon processes acting in opposition to obtain resolution far below the linear diffraction limit.

In the transverse plane, the linear diffraction resolution limit is traditionally stated as k λ / NA where λ is the vacuum wavelength, NA is the numerical aperture of the objective lens and k is a process parameter typically in the range of ¼ to 1. In state-of-the-art semiconductor lithography, for example, k ≈ ¼, λ = 193 nm and NA = 1.44 via water immersion, yielding a resolution of ~35 nm. At the other end of the cost spectrum, Blu-rayTM optical data storage operates at k ≈ ½, λ = 405 nm and NA = 0.85, yielding resolution of ~250 nm.

A second diffraction limit is given by the distribution of the focused intensity in the longitudinal direction. This resolution is important in 3D exposures such as micro-stereolithography and multi-layer data storage. The intensity distribution out of the focal plane is completely determined by diffraction and cannot be controlled independently of the transverse field distribution. The traditional approach to reducing this longitudinal feature size is to use a nonlinear material response such as two-photon initiating photopolymers1,2 or nonlinear absorption in glass or other inorganics3,4. However, the longitudinal exposure profile in these cases is still controlled by the transverse profile, limiting the set of 3D patterns that can be produced. Second, the full-width to half-maximum (FWHM) depth resolution is only modestly improved by 35% in the case of two-photon absorption with wavelength held constant or is actually degraded by a factor of 28% in the more common case of two-photon exposure at approximately twice the peak one-photon absorption wavelength9.

This later case is ubiquitous due to the major limit of multi-photon 3D lithography which is the dramatically small absorption cross section, measured in units of Goeppert-Mayer after Nobel laureate Maria Goeppert-Mayer, where 1 GM is 10-50 cm4 s/photon. Nearly all two- or multi-photon 3D fabrication is therefore performed with a single, high

Invited Paper

Advanced Fabrication Technologies for Micro/Nano Optics and Photonics III, edited by Winston V. Schoenfeld,Jian Jim Wang, Marko Loncar, Thomas J. Suleski, Proc. of SPIE Vol. 7591, 759102 · © 2010 SPIE

CCC code: 0277-786X/10/$18 · doi: 10.1117/12.845850

Proc. of SPIE Vol. 7591 759102-1

Downloaded from SPIE Digital Library on 06 Jun 2012 to 198.11.25.124. Terms of Use: http://spiedl.org/terms

NA focused spot from a femtosecond pulsed, amplified Ti:Sapphire laser. High throughput lithography such as mask-projection is therefore impossible, severely limiting the impact of these techniques. Similarly, in optical data storage applications, no multi-photon approach has simultaneously met capacity, transfer rate and power-consumption targets, despite years of academic and industrial research10,11.

We demonstrate here that two single-photon processes can fabricate structures well below the linear diffraction limit in both the transverse and longitudinal directions. This is applied to both microlithography of polymer structures developed via solvent wash and to 3D fabrication of optical index distributions via polymerization-driven diffusion. The approach is particularly suited to high-throughput 3D fabrication.

2.MATERIALS AND METHODS Stimulated-emission depletion microscopy5, breaks the optical diffraction limit on microscopic resolution by stimulating emission at the periphery of a diffraction-limited excited fluorophore, confining the remaining fluorescence to a sub-diffraction region. Similarly, this lithography method uses two independent, high-throughput processes which act in opposition, as shown in Figure 1(left). Initiating species are generated by single-photon absorption at one wavelength while inhibiting species are generated by single-photon absorption at a second color. The absorption spectra of the photo-initiator and photo-inhibitor are designed to provide independent control over the two processes as shown in Figure 1(center). The inhibitory pattern acts as a transient, programmable 3D near-field mask for the initiating pattern, enabling unprecedented control of feature size and shape in both 2 and 3D. Additionally, the generated inhibiting species rapidly recombine when irradiation with the second wavelength ceases, allowing for fast sequential exposures not limited by memory effects in the material and thus enabling fabrication of complex two- or three-dimensional structures via direct-write lithography (right).

Figure 1. 2PII lithography method and materials. Blue light at 473 nm is focused to a diffraction-limited spot (a) and is absorbed by a photo-initiator, camphorquinone (CQ). Superimposed on this spot is a UV Gauss-Laguerre “donut” mode of 364 nm light which excites a photo-inhibitor, tetraethylthiuram disulfide (TED), suppressing the action of the initiator in the periphery of the pattern. The resulting polymerized region (c) can be much smaller than the diffraction limit. A simplified optical layout is shown on the right including the binary amplitude computer-generated hologram which generates the Gauss-Laguerre mode. Diode-pumped solid state and argon-ion lasers are used for initiation and inhibition, respectively.

The distribution of the resulting polymerized region can be predicted in the steady-state approximation12, to be proportional to (Iblue – β × IUV), where Iblue and IUV are the normalized blue and ultraviolet irradiation intensities, respectively and β is a constant encompassing the ratio of the inhibitor to initiator absorption cross sections, quantum yields, powers and reaction rate constants. As shown in the calculations of Figure 2, as this ratio is increased, the edges of the developed region are increasingly inhibited, reducing the final feature size. However, there is no inhibition at the perfectly dark null of the Gauss-Laguerre mode and thus the polymer development on axis remains

minus

a: Initiation

b: Inhibition

c: Polymerequals

Inhibitor Initiator

TED

λ [nm]

NS

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N

O

O

CQ

Abs

orba

nce

364

nm

473

nm

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constant. This enables independent control over feature size and peak conversion, a particularly attractive feature for robust nanofabrication.

Figure 2. Predicted transverse polymer distribution as a function of β, the ratio of inhibition to initiation. The plot regions show a range of ±w0, the diffraction-limited spot diameter. If β could be increased arbitrarily, the developed polymer feature could be reduced to arbitrarily small dimension while maintaining the same on-axis conversion. The limit to exposed feature size is therefore not diffraction but β, the maximum possible contrast between the two processes.

3.RESULTS 3.1 Two and three dimensional negative-tone lithography

The initiator, camphorquinone (CQ), and inhibitor, tetraethylthiuram disulfide (TED), shown in Figure 1 were combined with triethylene glycol dimethacrylate (TEGDMA), a multifunctional monomer which forms a crosslinked, gelled polymer upon polymerization via a free radical mechanism. Irradiation with blue light excites the CQ and initiates the polymerization via carbon centered radicals whereas irradiation with UV photocleaves the TED, producing sulfur centered dithiocarbamyl (DTC) radicals which terminate the polymerization.

The optical system of Figure 1,initially operating at 0.45 NA was used to demonstrate the prediction of Figure 2. The blue power was held constant at 10 mW while the UV was progressively increased from 0, 1, 2.5, 10, and 100 mW. The exposure time was 8 s for each voxel. As shown in Figure 3 left, polymer voxels on glass display the expected reduction below the diffraction limit. The smallest feature, recorded at the highest UV intensity, is shown in Figure 3 right. At 200 nm diameter (indicated by the dashed circle), this demonstrates violation of the diffraction limit by a factor of seven in transverse dimension and a factor of 50 in transverse area.

β=1 β=2 β=8



Figure 3. Low NA results via SEM images of voxels on a glass slide after exposure and solvent wash. The left image confirms the prediction of Figure 2 by reducing the exposed spot diameter via increased inhibiting UV intensity. The smallest feature recorded, shown on the right, has a diameter of 200 nm, a factor of seven below the 1.35 micron diffraction limit.

An oil-immersion, 1.3 NA objective was then used to explore the resolution limit of the system. Ten microwatts of blue and 110 microwatts of UV power were used to create voxels on glass similar to those of Figure 3. An SEM cross-section of one these voxels is compared to the expected profile in Figure 4 left. The agreement is quite good with a single parameter fit of β = 0.75. This indicates both that the system is performing as expected even at these small scales and that there is considerable room for improved resolution by increasing β.

The method extends directly to 3D by translating the initiation/inhibition pattern through a liquid resin in order to fabricate (after solvent wash) air/polymer structures. The coincident foci were translated normal to the glass slide at a velocity of 0.125 mm/s to create 100 nm by 5000 nm polymer columns, a portion of which is shown in Figure 4 right. Note that the cross-section of the feature is equal to that of the isolated voxel on the left. This demonstrates that, as intended, the inhibiting pattern is transient and does not restrict arbitrary 3D lithography.

Reduction below diffraction limit via increased local refinement.

10 μm



Figure 4. High NA results via SEM images of voxels and lines on a glass slide after exposure and solvent wash. The left plot demonstrates lithographic resolution at 1.3 NA showing theoretical diffraction-limited initiation (a) and inhibition profiles (b). The polymer distribution (black squares), measured via an SEM cross-section, agrees well with the predicted profile (c) using a single parameter fit of β = 0.75. The right image is a detail of a preliminary 3D stereo-lithography study showing a portion of a 100 nm by 5 micron tall column that fell to the substrate during solvent wash.

3.2 3D lithography of density/index structures in a solid host

The 2PII chemistry can be applied to a monomer freely diffusing through a 3D polymer matrix host in order to fabricate sub-diffraction integrated optics and data storage systems that self-develop with no wet processing13,14,15. The sequence begins with the CQ initiator that absorbs a blue photon to form initiating radical centers. A superimposed distribution of UV light is absorbed in the TED photoinhibitor to generate inhibiting radicals, restricting polymerization to a region smaller than the diffraction limit. Growth of the polymer chain locally depletes the monomer concentration which is then refreshed by diffusion, increasing the density and index in the polymerizing volume. At the end of the process, a uniform incoherent exposure bleaches all remaining initiator and inhibitor, polymerizing the entire sample and “fixing” the desired index change through crosslinking. With traditional one-photon initiation for use in volume holography, this process has been demonstrated with less than 0.1% polymerization shrinkage, minimizing stress and dimensional changes14. Typical index change is ~0.01 and resolution has been shown to be limited only by the polymer grain size of ~ 5 nm16. Note that mass-transport via diffusion is the critical step; if index change was only due to polymerization, the volume would be homogeneous after the final flood cure.

To demonstrate that this self-developing 3D material approach can be combined with 2PII exposure, we incorporated the photo-initiator and photo-inhibitor of Figure 1 and a high refractive-index monomer into a urethane thermoset matrix. The matrix cures at room temperature from a liquid to a rubbery solid, providing a low scatter, transparent “sponge” for the subsequent exposure and diffusion of the monomer which will be photo-patterned by the 2PII chemistry. We first used an initiating (blue) focus to locally polymerize the monomer resulting in diffusion and diffraction-limited index features (Figure 5, left, top row). We then superimposed an inhibiting (UV) Gaussian focus and demonstrated that increasing UV power reduced and eventually suppressed all polymerization and index change (Figure 5, left, lower rows). Via a UV beam shaped to reduce both the transverse and longitude extent of the polymerization (see Figure 6), this process may enable very high density 3D data storage such as microholographic17 multilayer storage (Figure 5 right).

100 nm

64 nm FWHM

Theory vs. Experiment

a: Initiation

b: Inhibition

c: Polymerization

Inte

nsity

or p

olym

er d

ensi

ty [A

U]

x [nm]



Figure 5. Demonstration of 2PII lithography of self-developing index structures in a 100 �m thick, flat solid host (left). Blue and UV Gaussian foci were overlapped and the UV power was increased (top to bottom) to suppress the polymerization. Application to 3D micro-holographic data storage (right) via two counter-propagating blue Gaussian foci (UV inhibition pattern not shown).

4.CONCLUSIONS STED microscopy has reached resolution levels previously restricted to scanning electron beams and will undoubtedly enable many discoveries in the biological sciences. From the perspective of optical sciences, however, the impact may be more profound. Multi-photon absorption is a multiplicative process in which the material response is proportional to the optical intensity raised to some integer. Unfortunately, the optical power required grows dramatically with the number of simultaneously absorbed photons, typically restricting application to slow translation of a femtosecond pulsed laser focus. Resonant enhancement can reduce this power requirement at the expense of long relaxation times which degrade resolution when they become comparable to the exposure time. Photochromic absorption with thermal relaxation times measured in minutes is in this latter class.

Conversely, the method reported here and the others recently inspired by STED exploit a new form of nonlinearity in which the material response is proportional to the difference of two intensities. There are three distinct advantages of this approach:

• First, there is no fundamental limit to the smallest feature that can be formed by the difference of two intensities as shown above and verified by the recent STED results. In contrast to multi-photon absorption in which the diffraction limit is reduced a specific amount by the multiplicative process, subtractive processes are not diffraction limited. Many readers may find this statement objectionable. It is obviously true that diffraction influences the size of the subtractive nonlinear responses, but Figure 2 illustrates that it is the ratio of inhibition to initiation that limits the obtainable resolution and thus the limit is not diffraction. Stated differently, given a perfect resist (infinite possible β) and infinite contrast optics (no scatter, etc), the system resolution would be unlimited and thus it is resist and optical contrast that limits resolution, not diffraction.

50 μmPhase microphotograph

~ 1

μm



• Second, subtractive processes offer direct control over the feature shape. Multi-photon absorption reduces feature size but enables no more control over the 3D exposure profile than does linear absorption. As shown in Figure 6, different combinations of the initiating and inhibiting intensity patterns can be combined to dramatically tailor the shape of the altered material region. A particularly important example for 3D exposures such as stereolithography and 3D data storage is the suppression of response in the longitudinal direction. The “bottle beam” inhibition pattern shown in Figure 6 b has been demonstrated in STED microscopy to provide an isotropic 3D point spread function, which is typically ideal. A secondary but useful feature is that the subtractive process results in sharp-edged features that are often more desirable than slowly-decaying Gaussian “tails”.

Figure 6. Predicted 3D confinement of 2PII for β = 1. The top row shows intensity profiles of Gaussian initiation (a), bottle inhibition (b) and donut inhibition (c) beams. The bottom row shows the predicted polymerization profiles for (right to left) donut, bottle, and bottle+donut beams. The scale bars on the lower right show the Rayleigh range and waist radius of the initiating Gaussian beam. Note the significant depth confinement provided by the bottle beam and the sharp edges of the polymerized regions in all cases.

• Thirdly and perhaps most important is that these results can be obtained with no fundamental sensitivity penalty. That is, two highly-sensitive, one-photon processes can be employed to implement a strong nonlinearity. This makes it conceivable that these methods can be applied to high-throughput commercial applications such as semiconductor lithography (using masks) or optical data storage (using a rapidly modulated focus).

Current demonstrations of STED-inspired lithography are in their infancy and there is significant improvement required, particularly in resist materials, to fully realize these potential advances. There have been multiple reported approaches using completely different fundamental chemistry to implement a subtractive nonlinearity, indicating that further exploration of basic chemistry and materials may be fruitful. Finally, the application to large-area mask lithography will require a new generation of multiple-exposure mask design. However, the potential of this fundamentally new class of nonlinearity seems to justify these efforts.

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ACKNOWLEDGEMENTS

The authors are indebted to the NSF for financial support under grants IIP-0750506, IIP-0637355 and ECCS-0636650 as well as funding from Intel Corporation. The 3D solid-host materials were graciously provided InPhase Technologies.

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