Test Results: Voice-Coil Nanopositioning Stages for Two-Photon Polymerization

Test Results: Voice-Coil Nanopositioning Stages for Two-Photon Polymerization

High precision motion control / software support for 3D maskless direct laser writing technology

Laser light has been vilified by Hollywood in many a classical movie, usually as a powerful source of destruction. In 1964, just four years after Theodore Maiman operated the first laser at the Hughes Research Laboratory in California, the James Bond movie Goldfinger already anticipated laser cutting, however not for the betterment of mankind. Later, high-powered light sources made another appearance on the silver screen, as the Dark Side tried to pulverize the Force for good with light sabers and laser cannons. 

Having been predicted by physicists in the early 20th century, it wasn’t until the turn of the 21st century that the laser became a standard tool for material scientists and engineers – from cell phone screen cutting and semiconductor wafer dicing, to improving the surface properties of metals. Still, most of these laser applications revolve around separating, melting or removing materials. What if you could use a laser to literally construct objects?

A miniature Statue of Liberty produced with laser lithography (Image: NanoScribe)

Enter Two-Photon Polymerization

Putting laser beams to work for the creation of 3D objects is no longer science fiction – it is made possible through a process called laser lithography or two-photon polymerization, and it is gaining traction quickly, especially when extremely fine structures and low quantities are required.

Two-photon polymerization (TPP) is based on two-photon absorption (TPA), a third-order nonlinear optical phenomena whereby simultaneous absorption of two photons excites a molecule to a higher energy state – a process that can be exploited to selectively change the material properties of a monomer gel. While TPA was first described in the early 1930’s, it took three more decades to prove the phenomenon with the invention of the laser. 

Photonic crystal produced with laser lithography (Image: Nanoscribe)

Today, TPA is often employed for micro-fabrication purposes, using highly focused coherent light to fabricate sub-diffraction limited structures via laser direct write methods. In practical applications, the advantage of two-photon absorption over one photon absorption is the third-order process, reducing the absorption cross section significantly and allowing for the production of much smaller feature sizes.  Due to the high resolution of the TPP process, features as minute as 100 nanometers, and in some cases even below, can be created. This capability allows the design of extremely lightweight, yet strong structures and objects with high aspect ratios.

Micro manufacturing systems using laser lithography have become quite popular in recent years, due to the flexibility of the maskless 3D lithography method to create true 3D structures and the rise of demand in photonics, bio-technology, micro-optics, and micro-mechanics.

In 3D-TPP micro-fabrication, a photocurable resin is exposed to a highly-focused, 3D-scanned laser beam creating polymerized voxels. At the focal point, the two-photon polymerization process occurs with high spatial confinement while not affecting adjacent volumetric space, leaving structures in the cured volume while adjacent spaces are easily removed, leaving a solid 3D object.  

The laser beam can either be steered by mirrors, or the resin can be mounted on a multi-axis precision positioning stage and moved relative to the stationary beam. A combination of both methods is also possible.  

In either technique, the uniformity of the resulting micro-structure is dependent on spatial positioning tolerances. Because of the nonlinear nature of the photopolymerization process and the very slow scan speeds required for TPP, the precision at which the velocity can be controlled to deliver uniform dose is especially critical. Since we are looking at a 3D process, this also means coordinating motion between multiple nanopositioning stages (or scanner axes) very precisely to deliver the required multi-axis trajectories.

Controlling Motion and Velocity in the Nanometer Range

In this article, we are looking at the motion performance of a compact, voice-coil-driven nanopositioning stage and a software package that allows users to quickly transfer 3D data into a multi-axis motion path for two-photon polymerization laser lithograpy.

The V-308 nanopositioning stage uses a voice-coil linear motor and sub-nm resolution linear encoder for precise, closed-loop motion control.
An XY-combination of the V-308 stage was mounted on a honeycomb breadboard for this test.

Given the high requirements on positional resolution, we first determined the steady stage position noise of the XY stage. The stage was controlled with A-814 Series 4-Axis Motion Controller, using ACS UDMnt Drives, and mounted using three point supports on a honeycomb TMC breadboard placed on an a desk in an office/lab environment on first floor with an adjacent roadway and a standard A/C running.

Smart Processing Commander (SPC) is a single point laser system control software that works from industry standard 2D and 3D CAD file formats. Shown here is a batch recipe setup using a DXF file of the PI Logo and a series of cube structures with alternating layer infills to fabricate woodpile type structures.
3D object in SPC; this advanced software package allows fast and efficient data import and process control.

The baseline in-position stability data were sampled at 100 Hz over 5 seconds, using the integrated high precision PIone linear encoders on both axes (~30 picometer resolution limit).

The graph shows the excellent XY-plane servo stability characteristics of the V-308 XY-stage, observed for 5 seconds. The radially symmetric position noise is on the order of only ±3nm jitter. High servo stability with minimum positional noise is a prerequisite for highly constant velocity especially for low speed motion. Note: the axis units are millimeters, so the last digit indicates nanometers.

Dynamic Performance, Velocity Stability, and Position Error on XY Coordinated Motion for TPP Processes

A) 5µm rectangular polymerization process path

The advanced motion path planning feature allows for excellent velocity control in the areas of interest. The constant combined vector velocity is 100 µm/sec. The center green rectangle has a side length of 5µm. Blue is motion path trajectory. The stages are tuned for TPP smooth scanning performance, the coordinated digital trigger is used to shutter the laser.
Motion Profile and General Performance Characteristics: On each side of the rectangle, the combined vector velocity has been coordinated by the controller to be constant at 100 µm/sec. In advanced path planning, the stages use lead-ins to accelerate and then maintain constant velocity with laser coordinated laser shutter on corners. There are multiple features available for optimized path planning. Observed at left is X-Axis position in time (blue signal) and Y-Axis position in time (green signal).
Zoomed view: Ultra-slow speed rate variation: The view above shows coordinated constant velocity performance with target velocity of 100 µm/sec (yellow) and a rate variation observed at ±5%.
Position error measured during active motion on the X-axis is observed at ~±35 nm.
Position error measured during active motion on the Y-axis is observed at ~±35 nm.

B) 20µm rectangular polymerization process path

Here, a 20 µm square with advanced motion path planning is shown. The constant combined vector velocity is 100 µm/sec. Blue is motion path trajectory. The stages are tuned for TPP smooth scanning performance. A coordinated digital trigger is used to shutter the laser.
In non-active laser processed areas, the velocity moves to shorten process time and in processed areas is tightly regulated. On each side of the rectangle, the combined vector velocity has been coordinated by the controller to be constant at 100 µm/sec. In advanced path planning, the stages use lead-ins to accelerate and then maintain constant velocity with laser coordinated laser shutter on corners. There are multiple features available for optimized path planning. Observed at left is X-Axis position in time (blue signal) and Y-Axis position in time (green signal).

Just as with the 5µm rectangle, the position error was measured to be ~±35nm for both the X and Y axis. The velocity rate variation was observed at ~100 µm/s ±4%.

C) 100µm rectangular polymerization process path and curvilinear coordinated polymerization process path

Lastly, tests with a 100µm rectangle and a curvilinear path were conducted.

In the 100µm rectangle, the observed performances were consistent with previous runs: ~±35nm, positioning error, and 100 µm/sec ±5% rate variation.
The graph shows a curvilinear path inscribed in a 100µm rectangle, again with a constant combined vector velocity of 100 µm/sec. Blue is motion path trajectory.
The combined vector velocity of the curvilinear run has been coordinated by the controller to be constant at 100µm/sec (yellow) with rate variation of ~±3.5% (100 nm/div). In active laser processed areas, the velocity moves in coordinated vector velocity regulated fashion in XY. Observed at left is X-Axis position in time (blue signal) and Y-Axis position in time (green signal).
Error characteristics shown above (100 nm/div). The error is observed at ~±35nm for the X-axis and ~±25nm for the Y-axis.

Summary – Synchronized Low Speed Performance of Voice-Coil Nanopositioning Scanners for 2PP

The goal of this test was to verify if the very low constant path speeds required for high quality results in two-photon polymerization could be achieved by the V-308 voice-coil-driven linear stage. As shown above, the results indicate excellent motion performance with constant vector velocity across all tested paths.

Position errors during active scanning were observed at sub-100 nanometer levels, significantly surpassing the requirements for good TPP results, despite the operating environment and mounting surface for the XY-scanning stage were less than ideal, limiting achievable performance (a lapped granite interface was not available at time of test).

The high motion performance was contributed in part to the employed PIOne encoder technology, providing sub-nm interpolated resolution.  

Additional steps could be taken to further improve the results, if so desired
a) PI’s PILOT algorithm has shown to enhance performance in other critical nanopositioning applications.  

b) Running NanoPWM drivers can reduce noise

c) As mentioned above, mounting the stages on a lapped granite interface will also have a positive effect on precision.

Overall, the tests prove the V-308 voice-coil-driven XY-table combination is a very capable, low cost solution to TPP over short travel ranges.

When Higher Precision and Longer Travel is Required

Planar air bearing XY-tables provide the highest performance when it comes to smootheness of motion, velocity constancy, straightness, flatness, and yaw control. An example, the compact A-311 XY-table is shown below. Detailed performance data is available in this article.

Air bearing XY tables, such as the A-311 compact stage, provide extremely constant motion, straightness and flatness – ideal prerequisites for two photon polymerization.

Integrated Flexure Guide Piezo XY and XYZ Tables

For short travel ranges below 1mm, piezoelectric XY and XYZ flexure tables are available. These types of ultra-precision nanopositioning stages provide the highest dynamics with planarity in the single digit nanometer range.

Basic design of a piezo flexure scanning XY-table. These 2-axis and 3-axis nanopositioning tables  are the gold standard for accuracy, resolution, smoothness, and speed, when travel ranges in the sub 1mm range are sufficient.
+/- 2 nanometer out-of-plane motion of the P-734 flexure-guided, planar nanopositioning XY table

A very compact piezo nanopositioning XYZ stage, model P-611.3SF, featuring 100x100x100µm travel and 0.2nm resolution, has also been employed in Two-Photon Photopolymerization for the Fabrication of Complex-Shaped Particles and Structures in research at the University of Colorado.

More Reading

Subscribe to Tech Blog

Sign up to receive an email of new blog posts.
Please read our Privacy Policy here before sending your request.
Privacy Policy*
spacer
spacer

contact@pi-usa.us

EAST & MIDWEST
(508) 832-3456
WEST
(949) 679-9191

spacer