2D Etching with MultiParticleProcess (Neutrals + Ions)

This tutorial shows how to use MultiParticleProcess in 2D to model etching driven by multiple particle species.

You will run two cases:

  1. Etching with a single neutral particle species
  2. Etching with neutral + ion particle species, including optional mask etching

The key concept is that MultiParticleProcess computes one flux per particle species, and you provide a rate function that converts these fluxes into a local surface velocity (etch or deposition), optionally depending on the material.

Download Jupyter Notebook: 08_multi_particle_etch.ipynb

1. Import ViennaPS 

import viennaps as ps  # ViennaPS 2D

2. Helper: Create a Masked Trench Domain

We define a small helper function so we can reset the geometry between process steps.

def createTrenchMask():
    extent = 30
    gridDelta = 0.3

    domain = ps.Domain(xExtent=extent, gridDelta=gridDelta)

    # trenchDepth=0 creates a flat bottom surface, maskHeight adds a mask on top
    ps.MakeTrench(domain, trenchWidth=10.0, trenchDepth=0.0, maskHeight=2.0).apply()

    return domain

What this creates

  • A 2D domain of width 30 with resolution 0.3
  • A trench opening of width 10
  • No etched depth in the substrate (trenchDepth=0.0), but a mask layer of height 2.0

This is a simple setup to demonstrate selective etching and mask interaction.

Step 1: Etching with a Single Neutral Particle

3. Initialize Domain and Save Initial State 

domain = createTrenchMask()
domain.saveVolumeMesh("multiParticleEtching_1")

This writes the starting geometry to a VTU file for later comparison.

4. Create the Multi-Particle Model and Add a Neutral Species 

model = ps.MultiParticleProcess()

# one neutral particle species
model.addNeutralParticle(stickingProbability=0.2)

Meaning

  • addNeutralParticle(...) adds a neutral particle source
  • stickingProbability=0.2 controls how often neutrals stick/react at impact

  • With only one species, the flux array will contain:

    • fluxes[0] → neutral flux

5. Define the Etch Rate Function

MultiParticleProcess needs a function that maps local fluxes to a local surface rate.

neutralRate = 1.0

def rateFunction(fluxes, material):
    if material == ps.Material.Mask:
        return 0  # no etching of mask

    return -neutralRate * fluxes[0]

model.setRateFunction(rateFunction)

Notes

  • The function is called for each surface point during the process.
  • material lets you implement selectivity (here: mask is protected).
  • The negative sign means etching (surface moves inward).

  • For this single-neutral case:

    • Only fluxes[0] exists (neutral flux).

6. Run the Process and Save Result 

processDuration = 5.0
ps.Process(domain, model, processDuration).apply()
domain.saveVolumeMesh("multiParticleEtching_2")

You now have an etched profile driven only by neutral transport, with the mask unaffected.

Step 2: Etching with Neutral + Ion Particles

7. Reset Domain 

domain = createTrenchMask()

This restores the original geometry so the second case starts from the same initial state.

8. Create a Model with Neutral and Ion Species 

model = ps.MultiParticleProcess()
model.addNeutralParticle(stickingProbability=0.2)

# add an ion species with a directional angular distribution
model.addIonParticle(sourcePower=500.0, thetaRMin=60.0, thetaRMax=90.0)

Meaning

  • This time the flux array contains two entries:

    • fluxes[0] → neutral flux
    • fluxes[1] → ion flux

  • The ion parameters shape the incoming ion distribution:

    • sourcePower affects the ion intensity (model-dependent)
    • thetaRMin, thetaRMax restrict the angular range (more directional)

9. Define a Combined Rate Function (Selectivity + Weights) 

neutralWeight  = 1.0
ionWeight      = 1.0
maskEtchFactor = 0.1  # 0 = mask fully protected

def rateFunction(fluxes, material):
    neutralFlux = fluxes[0]
    ionFlux = fluxes[1]

    if material == ps.Material.Mask:
        return -maskEtchFactor * ionWeight * ionFlux

    return -(neutralWeight * neutralFlux + ionWeight * ionFlux)

model.setRateFunction(rateFunction)

What this does

  • Substrate etch rate depends on both neutrals and ions.

  • Mask etching is allowed, but reduced:

    • only ions etch the mask
    • scaled by maskEtchFactor

This is a typical pattern:

  • neutrals contribute more to isotropic components
  • ions contribute more to directional components
  • mask etch can be tuned separately

10. Run the Process, Save, and Visualize 

processDuration = 5.0
ps.Process(domain, model, processDuration).apply()
domain.saveVolumeMesh("multiParticleEtching_3")

domain.show()

At this point you can compare:

  • multiParticleEtching_2 (neutral-only etch)
  • multiParticleEtching_3 (neutral + ion, with optional mask erosion)

Download Jupyter Notebook: 08_multi_particle_etch.ipynb