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 Write

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 now 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 [NA: 10x, 25x, 63x]

Two-photon Light Source

• Hight Performance NIR femtosecond laser [FemtoFiber Pro]

Training

Cornell NanoScale's Training Workflow

• Send the tool manager a request for online access to NanoScribe’s NanoGuide Webpage.
• Meet with the tool manager to discuss your project.
• Complete the CULearn training for the tool. 

  • CULearn Training links:

    • RSRCH - CNF - NanoScribe GT2 (NetID Link)
    • RSRCH - CNF - NanoScribe GT2 (GuestID Link)
  • Once you completed the CULearn video and quiz, email the tool manager to schedule a hands-on training session.
• 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. 
• Once the CULearn 

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)

1. Turn the mains power switch to position 1.

2. Wait some seconds until the reset and open buttons blink in alternation and the attention indicator is lit up.

3. Press the reset button, the attention indicator switches off.

4. Switch on the Photonic Professional GT2 Controller by activating the key switch.

5. Switch on the Control PC: Let it boot completely before proceeding, wait until the long on-screen shows up.

6. 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.

7. Switch on the GT power supply.

8. Switch on the z-drive power supply.

9. Switch on the computer monitor.

10. 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)

1. 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.

2. Shut down the computer.

3. Make sure the objective lens is in the lowermost position. Check for the message lower z-limit reached on the touch panel.

4. Switch off the z-drive power supply.

5. Switch off the GT power supply.

6. Check the status of the laser emission; if the backlight of the button start/stop is activated, press it to switch off the emission.

7. Switch off the laser controller by deactivating the key switch.

8. Switch off the Photonic Professional GT2 controller by deactivating the key switch.

9. 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 Sets

The Nanoscribe uses solution sets which are paired to an objective in order to slice a CAD model. In general, each objective has a Solid, Shell and Scaffold, and Swift solution set. 

Comparison of printing solutions by part resolution and print speed. Image from Nanoguide

Alignment & Overlay

GWL Programming

Python Scripting

NanoWrite

Sequence Of Operations

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.
• Transfer to IPA for at least 15 minutes.
• Gently blow-dry the sample (Optional- dependent on size and strength of your structure).
• 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.

Material Specifications for standard printing at CNF

Substrates Specifications:

Indium-Tin Oxide (ITO) Coating Specifications:

Sample Holders available at the CNF

Sample Holder	Substrate Type	Substrate Thickness	Holder Image	CAD Files
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	0.70 mm, 0.70 mm, 0.725 mm		multidill.stl multidill.stp *Note: some dimensions are approximated
2" Wafer	Ø 2 inch Wafer	0.35 – 0.55 mm
10 × Ø 30 mm	Ø 30 mm	0.17 mm

These resins are available at the CNF:

Developer 

• PGMEA (CAS NO. 108-65-6)

Metrology & Post Processing

Aspect Ration & Selectivity

Model UV Curing

• Curing Box [Under Construction]

Silicon Wafer Substrate Production

Standard Type P silicon wafers can be cut to produce 25mm square substrates for use with the 10X and other processes using the CNF DISCO Dicing Saw. Users must provider their own wafer for slicing; for best results, wafers should be 700µm thick +/-25µm. After receiving training on this tool, a program has been saved that is available to dice 6" wafers for use in the Nanoscribe. When using the DISCO Dicing Saw, open the USER directory and select the Nanoscribe6in profile. Please note: this profile is currently set for a 675µm thick wafer; you MUST adjust this parameter for thicker or thinner wafers.

Once you have completed the alignment process (typically aligning only along the flat edge of the wafer is adequate), a total of at least 18 substrates will be produced. Take care to ensure that substrates are free of debris from cutting before using in the NanoScribe. Additional cleaning of substrates via sonication with consecutive baths acetone, IPA, and DI water is recommended.

ITO Borosilicate Substrate Deposition Process 

• Deposition chucks available [Under Construction]

Model Removal & Resin Stripping

Advanced Printing Procedures

Multi-DiLL Printing

Multiple samples can be printed sequentially without the need to open the printer and switch substrates. This is useful for jobs where a large number of samples are needed, particularly if print times are short. The procedure for setting up a Multi-DiLL print will reference this build folder which is available for download: Sample01_02_03.zip The builds are based on these simple STLs: sampleSTLs.zip.

• Generate STL files as with any standard print job
• Import these files into DeScribe individually and generate a build using your selected solution set. It is important that all files use the same objective, however you may vary between solid and the shell and scaffold solutions.
  • Keep the generated files in their own respective folders - organizing these files is very important to the process
  • In this case, all files related to Sample1 are in a Sample1 folder
• Combine all sample folders to one master folder
  • An example of the file structure is included below:

• Generate a *job.gwl file - this will tell NanoWrite the order of operations and which files to reference
  • The example uses 1_2_3_job.gwl and is stored in the master folder. The gwl file can be created in a text editor (by changing .txt to .gwl or similar) and the content should be formatted as follows:

• In this example, we are printing in positions 4, 5, and 6 on the Multi-DiLL holder.
  • NewStructure: Tells NanoWrite to begin a new print
  • SamplePosition: Tells NanoWrite which position in the sample holder to print
    
  • Wait: Waits for an integer number of seconds to allow the resin to settle after the lens has approached
    
  • MessageOut: Displays text to indicate a new job has started
    
  • include: Opens the build files from the included path
• You may test your gwl file by opening it in DeScribe to verify the paths are correct. Generate a 3D preview (F5) and ensure there are no errors.
  • Note: DeScribe will render all models on top of one another. This is normal behavior, as seen below:

• Copy the folder to the Nanoscribe computer
• Place your substrates in the positions indicated in your gwl file and add resin to them, using an appropriate amount for the sample printed
  • Large deposits of resin may drip in the machine, please ensure a minimal amount is applied
• Insert the correct objective and the Multi-DiLL holder as with standard prints
• Select the position of your first print in NanoWrite. By default, the Multi-DiLL is set to position 5, as seen below. To reduce bubble formation in your print, it is best to change this to your first print position. Note that when clicking the new position on the Multi-DiLL graphic in the NanoWrite "Choose sample holder" dialog box, the view is from the bottom of the holder, so positions are flipped, e.g. position 1 is the top right. Click OK and then approach the sample.

• Load the *job.gwl file in your top folder in NanoWrite, in the case of the example load 1_2_3_job.gwl
• Start print job, the total print time scales linearly with the number of samples in the job

Manual Interface Finding

WARNING: MANUAL INTERFACE FINDING MAY RESULT IN A COLLISION BETWEEN THE SUBSTRATE AND LENS IF DONE IMPROPERLY. IF YOU HAVE CONCERNS, PLEASE CONTACT A STAFF MEMBER BEFORE ATTEMPTING.

Procedure for All Objectives:

1. Load your substrate in a sample holder. If the substrate is thicker or thinner than the holder depth (i.e. 700 microns thick for the Multi-DiLL holder), it is important to measure how much taller or shorter your substrate is.
2. Add resin to your substrate and the objective.
3. Load your sample as you would normally, select the correct holder and click OK.
   1. Do not click Approach Sample at any point.
4. In the advanced camera settings, ensure that the light source is correct for your substrate - for opaque substrates use a Reflective Illumination, but for transparent substrates you will be better served with Transmission Illumination with all objectives.
5. Approach the sample with the manual Z control. You may use coarse control initially until you reach a Z-height on the order of 9000um. For thicker substrates, it is recommended to stop earlier, substract your substrate height above the holder from 9000.
   1. During this step you should see the lens contact the resin, it will be very obvious, looking like a bubble moving across the microscope. Keeping Auto Contrast and Auto Exposure.
6. Using the fine control, move the microscope slowly; you should slowly begin to resolve the substrate.
   1. For a 700um thick substrate this is typically a Z-value greater than 9800um.
   2. Turning off auto-contrast and auto exposure can be helpful; ensuring short exposure times but also smaller Gain (<15) will result in a clearer image.
7. Once the substrate appears in view, turn off the microscope in NanoWrite and open AxioVision.
8. While AxioVision loads, in the NanoWrite Advanced Settings console type ManualControl and click Submit.
9. On the left hand side of the Manual Control interface is a section with Shutter Control. Set the laser power to a low value, typically 5%
10. With AxioVision loaded, open the shutter. If you see a small white dot, this is the laser. If you cannot see it, you are either too close to the substrate and its focal length is past the surface of your substrate. Alternately, you may be too far from the substrate, however this is typically not the case if you have your substrate in focus.
11. Using the fine Z control, slowly adjust the Z location towards your substrate until the white dot just disappears. Note this Z location - switching the microscope control unit to XYZ mode is helpful for this process as the Manual Control interface is not ideal for this step.
12. Close the Shutter in Manual Control.
13. Close AxioVision and Manual Control. Enable the NanoWrite microscope.
    1. Note: closing AxioVision and renabling the microscope will automatically turn on Auto-Contrast, you may wish to disable this again.
14. Your substrate should eventually come into sharp focus, move the substrate on X and Y until you have identified where you would like to print.
15. Modify your _data.gwl file:
    1. It will contain the line FindInterfaceAt $interfacePos, comment this out by adding "%" before the line
    2. If this line is not removed or commented out, the microscope will lose the position and attempt to automatically find the interface
16. Load your print job and begin printing
    1. It may be valuable to have a small object (e.g. a cube) to test the print before starting a longer one. This can help validate your starting point and ensure that your print isn't floating or the laser is focused past your substrate.
17. When your print is complete, click Exchange Sample Holder. This will back the objective from your print as normal.

See this short video below which follows the same process:

*Note - the Z values in this video are not necessarily reflective of other prints and should not be used as a reference.

Printing on Porous Substrates

For substrates with holes or other features, it is necessary to manually align the microscope. First, users should follow the Manual Interface Finding procedure. This can be done on any location on your substrate, however an area away from your desired print area is suggested. Once an interface has been determined, manual alignment can begin.

In the following steps, we are assuming that we are looking to find the center of a 1000um hole in the center of a substrate.

1. Move around the microscope until the feature is visible. Ensuring the features are near center of the substrate as possible will prevent bubbles from forming and being moved around the lens.
2. Make sure Center Cursor is on, there should be a dashed blue line on the microscope display. If it is not there, click Center Cursor.
3. Move to one edge of the feature, roughly center, in this case the lowest point of the hole, keeping this in the center of the crosshairs.
4. Move up 1000um on Y by inputting 1000um in the step size box and clicking the up arrow for stage adjustment. Note that in this case picking the Y axis is arbitrary and the X may be used just as easily as a starting point.
5. Realign to the top of the hole, generally 10um increments are sufficient. Record the total distance added or subtracted from the initial 1000um.
6. Move to the new Y center point by inputting half the net distance travelled in Step Size then click the down arrow once e.g. ((1000um-distance correction)/2).
7. Move right on the X axis by current value in Step Size.
8. Align the edge of the hole as with the Y axis. If the center appears to be misaligned with the Y axis, adjustments can be made now to align on this axis.
9. Continue to move side to side and up and down in order to verify that the sample is centered.
10. Record the X and Y coordinates of this center point.
11. For multi-part prints, it is necessary to comment out any instance of FindInterfaceAt  or CenterStage in the _job.gwl and _data.gwl files
    1. In lieu of CenterStage, a new center point can be added manually via the GoToX and GoToY commands, typically in the _job.gwl file.
       1. Formatting is GoToX number where number is the X coordinate of your new center point.

 The CAD models for this substrate are available for download in STL and STP format, as well as a drawing of the substrate.

The short video below demonstrates the process for a 25x25 mm substrate with a 1mm center hole:

These substrates were printed via an SLA process at 50um layer height and subsequently polished to a 3um finish.

Applications

• Microfluidics
• Micromechanics
• Biomedical Engineering
• Micro-electro-mechanical systems
• Mechanical metamaterials
• Micro-optics
• Photonic metamaterials and Plasmonics

Publications & Gallery

• Study highlights promise of 3D printing for electrochemical reactors [04/2021]

• Nano-sized McGraw Tower features 161 steps, chimes [05/2021]

• Biologically inspired micro-robotic swimmers remotely controlled by ultrasound waves [09/2021]

• Building the Nano-3D McGraw Clock Tower [12/2021] 
  

Other Files

Users may wish to store their new and used 25mm square substrates. A set of STL files is provided for users to download and 3D print; each holder can fit 20 substrates.

substrateholder_top.stl    substrateholder_base.stl

It is recommended that these be printed with an SLA or similar process, as tolerances are tighter than allowable by FDM printing.

Other Sites in the NNCI Network with a NanoScribe

• Stanford University: Stanford Nanofabrication Facility

• Harvard University: Center for Nanoscale Systems

• Georgia Tech: Institute for Electronics and Nanotechnology Micro/Nano Fabrication Facility
• University of Kentucky: Center for Nanoscale Science and Engineering
• University of Pennsylvania: SINGH Center for Nanotechnology

Note: A large part of the background information comes from NanoScribe's NanoGuide, Wikipedia, and ZEISS .