Two-photon Lithography

Introduction

Direct laser lithography or direct laser writing using two-photon deep ultraviolet laser for additive 3d manufacturing using conventional resin sensitive to two-photon absorption resulting in two-photon polymerization to create volumetric prints from CAD models.

Exposure System

The NanoScribe GT2 Laser Lithography System is an ultramodern two-photon polymerization volumetric maskless printer. It is one of our newest acquisitions here at the Cornell NanoScale Facility (CNF). The tool was purchased in late 2019 and delivered and installed during the second week of January 2020. This was accomplished with the help of a committee of faculty and researchers, and a grant from the NSF. The NanoScribe GT2 can create three-dimensional nanostructures using a NIR femtosecond laser via direct-write onto a photosensitive resin, which is subjected to a non-linear two-photon absorption process. This process involves cross-linking the resin via UV absorption. The laser sets a focal light cone where a concentration of the light intensity defines the exposure focal spot volume or a “3D Pixel.” Using this technique, a CAD design can be broken into an X-Y-Z coordinate system to define the structure pixel by pixel and layer by layer. The exposure side of the system sits in a fine-tuned vibration isolation table and a high-speed ultra-precise piezoelectric stage for movement in X-Y-Z and uses a galvanic mirror defection system for focusing and beam rasterization. Models for printing can be designed using the stand-alone software DeScribe—which comes with the tool—or with any CAD software capable of outputting file formats DXF or STL. The DeScribe software can import these formats.

3D Print Technology & Direct wright

Laser-based direct writing has been around for a few years especially in the semiconductor industry known as maskless lithography (MPL). Typically, the two-dimensional CAD design is transferred onto the substrate that is pre-coated with a photosensitive resin via reduction laser projection. The projected pattern was reduced given the tool's numerical aperture and scan across the surface of the substrate using either a raster or vector format. These techniques are the same ones utilized in 3D direct-write. The only difference is that know you are working with a thickness degree of freedom that can be scanned across the Z-axis. The NanoScribe GT2 has a Piezo Nanopositioning stage and a high-speed galvo scanner. In the case of two-photon lithography, the light source is a pulsed femtosecond fiber laser with the center wavelength being around 780 nm.

Hardware Overview

Diagram of a simplified optical path of a Two-Photon Lithography System.

Utilized Objectives [10x, 25x, 63x]

Sample Holders at the CNF

Two-photon Light Source

Hight Performance NIR femtosecond laser [FemtoFiber Pro]

Tool Training

Cornell NanoScale's Training Workflow

Send the tool manager a request for online access to NanoScribe’s NanoGuide Webpage.
Users are required and responsible for reviewing the services and procedures provided by NanoScribe on the following:
- You will need to log in to NanoGuide to view these materials.
  - Model Creation (CAD)and Import
  - Printing
  - Print with Solution Sets
  - Print Applications
  - Print Strategies
  - Programming Structures
  - Post Print Processes
  - Further REading
Check your STL model for common mesh errors. You can use CAD software capable of mesh verification such as Autodesk Meshmixer or MeshLab
Once your file has been checked for mesh errors, run it through DeScribe to do the conversion. Be sure to select the desired solution set (SF, MF, or LF) for your process.

NanoScribe's Subsystems

Electronics Cabinet
Optics Cabinet [contains the laser, optics, and Galvo]
Emergency Off (EMO) switch
Printing Cabinet [Microscope, Piezo, and Stage]
Power Supply 232
System Controller
System Computer
Laser System Controller
Galvo Stage Power Supply
Monitor, Keyboard, and Mouse
Manual Microscope Controller
Joystick

Power UP and Power DOWN Procedures [STAFF ONLY]

Power UP Procedure

Warning!

"Disregard of the guidelines concerning power-up of the Photonic Professional GT2 may cause damage to the system!"

Apply the following sequence in order to power up the Photonic Professional GT2 (See Fig)

Turn the mains power switch to position 1.
Wait some seconds until the reset and open buttons blink in alternation and the attention indicator is lit up.
Press the reset button, the attention indicator switches off.
Switch on the Photonic Professional GT2 Controller by activating the key switch.
Switch on the Control PC: Let it boot completely before proceeding, wait until the long on-screen shows up.
Switch on the laser controller by activating the key switch. The button start/stop does not have to be pushed (the backlight of the button should be off). The laser emission is controlled via the Photonic Professional GT2 control software NanoWrite.
Switch on the GT power supply.
Switch on the z-drive power supply.
Switch on the computer monitor.
Press the open button in order to release the safety lock and to open the printing cabinet. As long as the safety lock is released the open button is blinking white.
Note: The laser gives acoustic feedback, as soon as its start-up is finished. This can take up to 30 s. After that, the control software NanoWrite may be launched. Make sure that the laser is operational when starting NanoWrite. The laser emission is switched on automatically when NanoWrite starts. The status of laser emission is indicated by the backlight of the button start/stop on the front panel of the laser controller. If the light is activated, the emission is on. You can manually switch on/off the emission by pressing this button. A power calibration of the laser is performed automatically before the first print starts or when NanoWrite receives the first script command.

Power Down Procedure

Warning!

"Disregard of the guidelines concerning power-up of the Photonic Professional GT2 may cause damage to the system!"

Apply the following sequence in order to shut down the Photonic Professional GT2 (See Fig)

Exit NanoWrite. Wait until NanoWrite is shut down. Otherwise, error messages may occur. In general, the laser emission is switched off automatically when exiting NanoWrite.
Shut down the computer.
Make sure the objective lens is in the lowermost position. Check for the message lower z-limit reached on the touch panel.
Switch off the z-drive power supply.
Switch off the GT power supply.
Check the status of the laser emission; if the backlight of the button start/stop is activated, press it to switch off the emission.
Switch off the laser controller by deactivating the key switch.
Switch off the Photonic Professional GT2 controller by deactivating the key switch.
Switch off the Photonic Professional GT2 by turning the mains power switch to position 0.

Warning!

Never switch off the Photonic Professional GT2 Controller or the mains power switch prior to any of the other components. This might cause damage to the system. The emergency shutdown of the system should only be used under dangerous situations. Do not use it as a standard shut down procedure.

CAD & Software

CAD Model

Since NanoScribe only handles the fracturing and slicing of the imported CAD design utilizing its own proprietary General Writing Language (GWL) complex models need to be designed and verified outside of the system and when ready exported as a Standard Triangle Language or Standard Tessellation Language (STL). STLs describe the raw unstructured triangular surface geometry of three-dimensional using normalized units and vertices objects without any color or texture attributes. STLs can be written in both ASCII and binary.

3D CAD Software Available at the CNF

Note: Sofware usage is limited and or restricted depending on the type of license the CNF currently is under contract.

Autodesk
- AutoCAD
- Inventor
- Meshmixer
DeScribe
FreeCAD
MeshLab
LinkCAD

DeScribe

Options for running DeScribe Software

Recipes: DeScribe Tabulated Solution Set based Parameters:

Solution Set	Objective	Resin	Substrate	Sample Holder	Slicing	Hatching	Dev/Rinse
SF	63x	IP-L / IP-DIP	Fused Silica / Borosilicate Substrates	DILL / DiLL Oil	0.3 um	0.2 um	12 min
MF	25x	IP-S	ITO-Coated	DiLL	1 um	0.5 um	20 min
LF	10x	IP-Q	Silicon	DiLL	5 um	1 um	20 min

Alignment & Overlay

GWL Programming

Python Scripting

NanoWrite

Main Operation Steps [ Typical Workflow.]

General Printing: Operational Workflow (SOP) as stated in the NanoGuide

The basic standard operation procedure (SOP) is summarized below. The titles link to pages with more detailed information.
The paragraph headers of this page link to important safety measures. Carefully read through each article to avoid harm to the operator and damage to hardware components. Additionally read through printing considerations, which provides general information for a successful print along with best practice advice.

Check the status of the printer

Make sure the printer components are switched on. If the laser was switched off wait for 30 min before starting a print job to ensure a stable laser output.

Import a CAD model file

Drag and drop an STL file (CAD model file) into DeScribe to start the import wizard.
Center the model and adjust the orientation as well as size. Ensure that it fits within the substrate dimensions and consider the working distance of the objective for the desired print set.
Choose a recipe matching the desired print set and follow the steps given in the import wizard.
Save the GWL print job file.
Preview the print job (shortcut: F5 key).
Alternatively, the print job can be programmed manually.
If the import was performed on a personal computer, copy all GWL files and the subfolder "..._files" to the control computer.

Prepare a sample and change the objective

Prepare the required materials for the intended print set and print job.
Fix the substrate to the sample holder and apply resin.
Check that the resin is free from bubbles.
Select an objective and inspect it for cleanliness. Clean it if necessary.
Use a clean and dry felt ring or resin stop with all objectives except for air objectives.
- Wash felt rings with isopropanol before the first use to remove loose fibers.
Mount objective in the correct position. Use the touch screen to check/select the position if necessary.

Start NanoWrite and insert sample holder

Start NanoWrite.
Wait for the stage calibration warning message.
Ensure sure the stage will not be impeded by obstacles.
Ensure the microscope objective is at the lower z-drive limit.
When it is safe to do so, confirm the warning message by clicking Calibrate.
Wait for the NanoWrite initialization to finish.
When the Exchange Holder dialog appears, carefully insert the sample holder.
Select the correct sample holder and sample position from the dialog box.
Confirm the selection by clicking OK.
Switch on an illumination. Choose reflection illumination for non-transparent substrates.
Start AxioVision software and position it next to NanoWrite to view the print process in real-time.

Perform a sample approach

Ensure that the selected sample position is correct. If it is not, double click the respective position on the sample holder icon in NanoWrite.
Click Approach sample to move the objective towards the interface. The interface will be automatically found and the working distance set such that the interface is in focus. The result of the interface approach is displayed in the NanoWrite message log window.
Adjust the brightness and contrast in the AxioVision software.
After each subsequent movement of the stage, the interface position must be verified. Either click Find Interface to autofocus the interface again, or issue FindInterfaceAt commands in the GWL code of the print job. Imported models utilizing block splitting and stitching have the commands included automatically.

Load a print job

Click Load Job and choose a print job (name_job.gwl file).
Wait for loading to finish (indicated by name_job.gwl loaded in the message log window).
If compilation takes a long time, use DeScribe to perform this step (debug -> compile, or shortcut: F5)

Print your structure

For high-precision applications allow ~30 min for the resin and holder to reach thermal equilibrium.
Click Start Job.
Wait until the print job is finished or press Abort to immediately halt the job.

Unload the sample

Click Exchange Holder.
Wait for the Exchange Holder dialog to pop up.
Carefully remove the sample holder.
Confirm the dialog by clicking OK.

Develop the sample

Carefully remove the substrate from the sample holder.
Place the substrate in a developer bath to wash away excess liquid 2PP resin.
Wash away the developer.
Gently blow-dry the sample.
Cure remaining liquid 2PP resin if a shell and scaffold structure was printed.

Clean the objective lens

If an immersion objective was used, it is advised to clean the objective lens directly after printing. This is mandatory if a different 2PP resin will be used with the same objective, or if the print set is changed from oil immersion configuration to DiLL or vice versa.
Cleaning the objective lens between jobs is recommended but is not strictly required if the configuration remains the same for printing subsequent structures.
Objectives should be cleaned at least once a week or after printing several structures.

Leaving the printer

Close NanoWrite. This makes sure that used print commands are set to the default values for the next user.
Only switch off the laser and other printer components if the printer is not in use for a longer period of time.

Multi-DiLL Printing

Multiple print Jobs

Material Specifications for standard printing at CNF

Substrates Specifications:

@ 780 nm	Fused Silica Substrates	ITO-Coated Substrates	Silicon Substrates	Borosilicate Substrates	Microscope Slides
Material	Fused Silica	Soda Lime w/ ITO	Si Substrates	Borosilicate Sub	Microscope Slides
Sample Holder	DiLL	DiLL	DiLL	10 × Ø 30 mm	DiLL
Refractive index	1.454	1.624 [ITO] 1.518 [soda lime]	3.710	1.517	1.518
Dimension	[25, 25, 0.7] mm	[25, 25, 0.7] mm	[25, 25, 0.725] mm	30 mm Ø, 170 µm	[22, 75, 1] mm
Thickness variation	± 25 µm	± 60 µm	± 25 µm	± 10 µm	± 100 µm
Surface finish	DSP	DSP	Polished	DSP	DSP
Density	2.2 g/cm3	2.5 g/cm3	2.3290 g/cm3
Mohs hardness	5.3-6.5	5-6	9-10
Melting point	1400°C	1000°C	3265°C
Thermal expansion coefficient	0.54x10-6 K-1	0.937 W/m·K	2.6 µm/(m·K)
Heat conductivity	1.38 W/(m·K)	0.937 W/(m·K)	149 W/(m·K)
Compatible w/ Solution Set	3D SF	3D MF (3D SF, 2D ML)	3D LF (3D MF, 3D SF, 2D ML)	3D SF Oil	3D SF
Compatible w/ 2PP resin	IP-DIP	IP-S	IP-Q (IP-S, IP-Dip, AZ resin)	IP-L 780 or IP-G 780	None, unless coated
Compatible w/ Objective	63X	20x, 25x , 63x	10x, (20x, 25x, 63x)	63x	63x
Combination & Ref index	-0.058 @ 20°C IP-Dip / -0.025 @ 20°C IP-S	-0.145 @ 20°
Contrast
Cleaning & Prep	O2-plasma & silanization	O2-plasma & silanization	RC1

Indium-Tin Oxide (ITO) Coating Specifications:

ITO	Value
Film Thickness	18 nm ± 5 nm
Film Surface Resistance	100 - 300 Ω
Film Transmittance	≥89%

Sample Holders available at the CNF

Sample Holder

Substrate Type

Substrate Thickness

DiLL

Microscope Slide: 24-26 mm x 50-76 mm

2. Fused Silica Substrate, ITO-Coated substrate, Silicon Substrate: 25 x 25 mm²

3. Borosilicate Substrate: Ø 30 mm

4. Tiny Substrates: 12 mm > Ø 30 mm > 25.4 mm

1.00 mm

0.70 mm, 0.70 mm, 0.725 mm

0.17 mm

0.30 mm

Multi-DiLL

Fused Silica Substrate, ITO-Coated substrate, Silicon Substrate: 25 x 25 mm²