litho-v1
A direct-write photolithography system with 5um line/space under INR 2,00,000
Our lithography system is based HackerFab CMU's Litho v2.1+ (which is their latest as of September 2026): https://docs.hackerfab.org/home/fab-toolkit/patterning/lithography-stepper-v2.1+-build
Intro
Lithography means writing patterns into stone (Greek: lithos: stone, graphy: to write). Lithography is used in microfabrication to create patterns of a circuit on silicon. Photolithography exposes the pattern of light onto a thin film of photoresist coated on the sample. For positive photoresist, the regions of resist exposed to light undergo a chemical change which makes them soluble in a developer.
Our photolithography system uses a DLP projector with a UV LED modification.
Why DLP?
Masked lithography shines blanket light through a hard mask consisting of transparent and opaque regions. This is used for high-throughput, high-resolution patterning for mass-manufacture of Integrated Circuits. The mask is expensive and slow to make, hence unsuitable for rapid prototyping at a student level. The hard mask itself is made using a maskless lithography system, which can be of 2 types:
Writer: a beam of photons (laser writing) or electrons (EBL: electron beam lithography) is scanned across the sample. Extremely high-resolution (<10nm for EBL), but low-throughput (slow). Used in research. Requires extremely precise motion stages to deflect the beam.
DMD: uses a DMD chip (https://en.wikipedia.org/wiki/Digital_micromirror_device) to selectively block or reflect regions of light from a light source. Does not need beam deflection motion stages, and is much faster than writers because it exposes a large (approx 10mm wide before demagnification) area in a single exposure. However, its resolution is limited by the DMD pixel pitch. It is the perfect compromise for DIY litho: low-cost albeit low-throughput and low-resolution
The DMD chip itself can be found in consumer DLP (https://en.wikipedia.org/wiki/Digital_light_processing) projectors. The DLP projector uses 3 LED light sources (Red, Green, Blue) illuminating the mirror array (DMD) to form the image. This enables swapping the blue LED for a 405nm UV led, which is suitable for exposing photoresist. This would not be possible in a regular LCD projector, as each of the millions of pixels has individual R,G,B leds, which cannot be swapped out for UV.
The silicon microfabrication revolution enabled cheap production of MEMS devices, which in turn makes our lithography system inexpensive!
Optical Specifications
LED Wavelength: 405nm Lens NA: 0.25 DMD pixel pitch: TODO
System Performance
line/space for nominal 5um/5um line array: 4um/6um isolated line widths: TODO XYZ stage repeatability, tiling error, overlay error: TODO
Bill of Materials
INR 2 Lakh Budget
TI DLP Evaluation Module (DLPDLCR471TPEVM) from Mouser
1,10,000
Edmund Optics Beamsplitter (25 x 25mm, 25R/75T, AR Coated)
6,357
Edmund Optics 10X DIN Plan Commercial Grade Objective
20,178
XYZ Stage 40mm. (TODO ADD 60mm one here)
~7,000
Software
1
Basler Pylon
Receiving video feed from industrial camera
Download the latest version of Pylon (the software suite, not the redistributable)
2
TI DLP EVM GUI
Controlling DLP LED intensities
DLPDLC-GUI, NOT Lightcrafter
SOP
These recipes are based on the ones developed at IITBNF (https://www.iitbnf.iitb.ac.in/)
Chemicals
LOR 3b: lift-off resist used for removing un-patterned metal after sputtering. Not photosensitive, but used to provide undercut (seeLiftoff and Undercut in Deposition). TODO Datasheet, MSDS
PPR spin+bake recipe
1
PPR coating
Step-1
Time = 10 s RPM = 500 Acceleration = 250
Step-2
Time = 45 s RPM = 4000 Acceleration = 2000
Step-3
Time = 10 s RPM = 0 Acceleration = 500
2
PPR Bake
Heating
Time = 3 min Temperature = 90 °C
LOR spin+bake recipe
1
LOR coating
Step-1
Time = 10 s RPM = 500 Acceleration = 250
Step-2
Time = 45 s RPM = 4000 Acceleration = 2000
Step-3
Time = 10 s RPM = 0 Acceleration = 500
2
LOR Bake
Heating
Time = 5 min Temperature = 150 °C
Exposure dose = 140mJ/cm^2 is the dose optimized by IITBNF. We don't have capability to measure intensity yet. See LED current mapping
The optimal dose and develop varies with resist age, temperature, humidity etc. A reasonable range is 120-160mJ/cm^2 and 15s to 40s respectively. See
DLP usage
Turn on the projector power supply
Turn on the TODO switch
Open the TI DLP EVM GUI software: it should show the projector connected. If not: Debugging
Debugging
Debug > USB connection
Unplug, replug usb cable
remove HDMI cable
check Device Manager
TODO
LED current mapping
As discovered by TODO on the HackerFab Discord, the current shown in the software maps to actual LED milliamps by TODO formula
Dose Matrix: how to develop a litho recipe for your setup
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