Robotic Stacker – WSe₂ Trilayer Gated Heterostructure

This recipe describes the MonArk NSF Quantum Foundry procedure for fabricating a WSe₂ trilayer gated heterostructure using the MonArk robotic stacking system. The process involves sequential pickup and deposition of five flakes (bold indicates gated layers)

Figure 1. Optical image of the as-fabricated heterostructure

Layers:

• Graphite (few-layer)
• hBN (15–18 nm)
• WSe₂ trilayer (contains occasional 4L, 5L+ regions)
• hBN (15–18 nm)
• Graphite (few-layer)

Stamp Preparation

• Use a PDMS-molded stamp fabricated with PC as the polymer (see “Figure 2”).
• Brass molds are employed to shape the PDMS tip, optimized for the MonArk stacker.

MonArk Stacker Axes & Control

Figure 2. Brass mold for MonArk stacker’s PDMS stamp

• A: Vertical stage of stacking finger
• B: Goniometric tilt stage
• C: Rotation stage
• X/Y: Linear translation stages
• Z: Vertical stage for focus adjustment (20× or 50× objective)
• Resolution: 0.001 mm (linear), 0.0013° (angular)

Figure 3. a) top graphite layer, first flake picked up. b) WSe2 3L, third flake picked up. Gold markers on substrate are spaced 100 µm apart.

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Part 1 – Pickup of Flakes

Parameters:

Figure 4. Stamp in contact with electrode substrate during final deposition. b) PC melting off the stamp with a circular front of PC collapsing toward center, denoted by red dotted circle. Notable feature is the finger-like channels indicated by the red arrow. Note image in b) is a different heterostructure.

• Polymer: Polycarbonate (PC) in chloroform
• PC concentration: 7% (w/v)    
• Stage temperature: 150 °C
• Contact diameter: 100–200 µm
• Wait time in contact: 1–2 min
• Pickup speed: ~0.001 mm/s
• Total time per flake: 5–10 min
• Note: A PC concentration of 7% in chloroform was found to provide reliable pickup at 150 °C. Lower concentrations (e.g., ~6%) may require reduced pickup temperatures (~90–120 °C) but can lead to less consistent adhesion depending on flake thickness and stamp geometry.

Figure 5. Optical images of heterostructures with PC deposited onto substrate, a) a successful deposition and b) an example of heterostructure that has been ripped apart due to high melting temperature.

Guidelines:

• Maximize overlap for gated flakes while ensuring electrode access.
• Rotate gated flakes to ~120° separation for three distinct electrodes.
• Position hBN layers to isolate gated flakes and avoid full coverage of subsequent layers.

Tips:

• Alternate between lowering and lifting the stacking finger to avoid sudden movements.
• Look for a grayish color change indicating successful adhesion to the stamp.

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Part 2 – Deposition onto Substrate

Temperature range:

• PC flow temperature: 190 °C
• PC melt temperature: 225 °C

Procedure:

• Align stack so each graphite flake and the WSe₂ trilayer overlap independent electrodes.
• Slowly raise stacking finger in 0.001 mm steps while maintaining PC temperature between flow and melt points.
• Watch for “front” formation (PC detaching toward the stamp center) and finger-like melt channels.
• Continue until the PC fully detaches from the stamp without tearing the heterostructure.

Tips:

• Mold tips have higher curvature than double domes; avoid abrupt sliding at high temperatures.
• Adjust temperature dynamically during deposition to control PC flow rate (~2 °C/sec heating).
• When the contact front passes over the heterostructure, pause or slightly lower the stamp to prevent damage