What is Dip-in Two-photon Lithography ?

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The basic two-photon lithography setup
Figure 1. Basic configuration of two-photon lithography system.

Dip-in two-photon lithography (DTPL) system was developed to overcome some limitations of the conventional TPL system (Figure 1). In dip-in two-photon lithography the lens is in direct contact with (dipped into) the sample. In the conventional TPL the laser is focused into the sample through a thin transparent substrate.  Only transparent substrates could be used in this scenario. The difference between the two techniques is illustrated in Figure 2.

Dip in two-photon lithography Vs Conventional two-photon lithography. The difference between dip in two-photon lithography and conventional two-photon lithography
Figure 2. Dip in two-photon lithography Vs Conventional two-photon lithography

Dip-in vs Conventional Systems

Property Conventional Dip-in
SubstrateTransparent Independent
Photoresist effect of RI is negligibleRI matched resist
Height of structurelimited by substrate thickness and proximity of the lens and substrate Determined by the size of photoresists bath

* RI:refractive index

Substrate

The thickness of the substrate on which sample is placed and the proximity of the lens to the substrate limits the maximum height of structures fabricated in conventional TPL. In TPL the laser passes into the sample through immersion oil, Figure 1. Conventional TPL fabrications relies on transparent substrates. Transparent substrates are used in most conventional TPL fabrications. However there are some reports of conventional TPL over opaque substrate. In such cases a transparent substrate is placed on spacers on top the opaque substrate. The fabrication is carried out by focusing the laser through the transparent substrate into the sample on top of the opaque substrate. DTPL however does not have any dependency on the substrate and opens up TPL to all type of new substrates.

Photoresists

Conventional TPL has little or no dependence on the refractive index of the photoresist. High numerical aperture lenses used in TPL/DTPL systems are highly sensitive to refractive index variations. Changes in refractive indices can affect the position and precision of the laser focus. Imprecise laser focus would lead to loss of resolution of the fabricated structure. Various refractive indices at play during both TPL and DTPL can be seen in Figure 3. Precise fabrication is achieved when the indices n1≈n2≈n3. The absence of immersion oil leads to wayward fabrication during TPL. The refractive index of the photoresists however have little or no effect on conventional TPL.

In case of DTPL the lens is in direct contact with the photoresist. The refractive index (RI) of the photoresist is tuned during its formulation to avoid undesirable refractive effects at the interface of the lens of the photoresist. This is one of the main limitations of DTPL, it requires RI tuning of all materials that are used in DTPL. In Figure 3, n1≈n4 for DTPL.

Figure 3. Various refractive indices at play during TPL and DTPL

Height of the Structure

In conventional TPL there is are restrictions on the maximum height achievable during fabrication. In TPL the lens is situated below the substrate as seen in Figure 4. During fabrication the laser has to pass through the substrate and into the sample. The maximum height of the structure fabricated by TPL would depend on the distance through which the laser can be manipulated along the Zaxis. In this configuration the movement of the laser focus along Z axis is restricted by the substrate. The thickness of the substrate reduces the Z range through which the laser could be focused.

The limited range of Z-movement in conventional TPL restricts the maximum height of the structure that could be fabricated by TPL.
Figure 4. The limited range of Z-movement in conventional TPL restricts the maximum height of the structure that could be fabricated by TPL.

In DTPL the lens is dipped into the photoresist do the lense can move through a greater range inside the photoresist leading to really tall structures. A group of researchers at Lawrence livermore national laboratory (LLNL) in USA produced structures taller than 200 microns with feature sizes less than 150 nm.

Microstructures with heights of more than 200μ and feature size 150 nm fabricated by dip-in two-photon lithography
Figure 5. DTPL fabricated sturctures with more than 200 μ and with feature sizes less than 150 nm. Courtsey LLNL News

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