feat: add HF cryogenic plasma etching model for SiO2

[gugim2o2o-collab]

Summary

• Add HFCryoEtching process model for pure HF gas cryogenic plasma etching of SiO2
• Add HFCryoParameters with Frenkel-Arrhenius desorption and Arrhenius reaction kinetics
• Register the new model in viennaps.hpp

Physics

Steady-state HF surface coverage (single-species, no passivation layer):

phi_HF = Gamma_HF / (Gamma_HF + k_des(T) + k_r(T) + Y_ie * Gamma_ion)

where

k_des(T) = nu0 * exp(-E_des / kB*T) # Frenkel-Arrhenius desorption k_r(T) = A_r * exp(-E_a / kB*T) # Arrhenius reaction rate

Etch rate (SiO2 only, in nm/s):

v = -(1/rho_SiO2) * ( k_r(T)*phi_HF + Y_ie*Gamma_ion*phi_HF + Y_sp*Gamma_ion )

Anisotropy is achieved without a chemical passivation layer: at cryogenic temperatures, k_r(T) is strongly suppressed on sidewalls (low ion flux), while ion bombardment at the trench bottom overcomes the activation barrier.

New files

• include/viennaps/models/psHFCryoParameters.hpp - parameter struct with Desorption (nu0, E_des), Reaction (A_r, E_a), SiO2, and Ion sub-structs; k_des(T) and k_r(T) helper methods
• include/viennaps/models/psHFCryoEtching.hpp - two-particle model (HFCryoIon, HFCryoEtchant) with HFCryoSurfaceModel computing steady-state coverage and etch velocity

Reviewer notes

• Default parameters (E_des = 0.25 eV, E_a = 0.10 eV, T = 200 K) are reasonable starting points; experimental calibration is needed
• Only Material::SiO2 surfaces are etched; other materials are skipped in calculateVelocities
• Model follows the same structure as CF4O2Etching (particles + surface model + ProcessModelCPU)

[FilipovicLado]

Thanks for contributing this HF cryogenic etching model. The topic is scientifically interesting and potentially valuable for ViennaPS. Low-temperature physisorption, surface diffusion, and ion-assisted activation are all relevant mechanisms for cryogenic SiO2 etching and high-aspect-ratio transport.

Before we can treat this as a ViennaPS model, I think the implementation needs more work in a few areas:

1. Please document the physical derivation and units more rigorously. In particular, the coverage balances combine Gamma_HF = etchantFlux * params.etchantFlux, k_des, k_act_ion = ionActivationFlux * params.ionFlux * A_act, k_r_direct, and k_r in the same denominators/rate expressions. etchantFlux and ionFlux are documented as fluxes in 1e15 / cm^2 / s, while k_des is an Arrhenius desorption rate in 1/s, and Reaction.A_r / DirectReaction.A_r appear to be used as reaction-rate prefactors. It is therefore not yet clear whether these quantities are all consistently normalized to surface-site rates before being combined.
2. The model currently implements its own surface diffusion solve inside psHFCryoEtching.hpp. We are actively working on a shared surface-diffusion framework for ViennaPS in a separate branch, but it has not been merged into master yet. For now, I would prefer not to merge a model-specific surface diffusion implementation that may soon conflict with, or duplicate, the framework-level implementation. Please either keep this part clearly disabled for the initial merge, or wait until the shared surface-diffusion infrastructure is available and then adapt the HF cryo model to use that instead. The goal is to avoid each process model carrying its own diffusion solver, and to make sure future improvements to surface diffusion apply consistently across models.
3. The sidewall/chain logic appears mainly 2D-oriented, but the model is exposed through the normal 2D/3D precompile macro. Please restrict the model to 2D for now or provide a robust 3D formulation.
4. There are a few integration gaps. The model is included in the umbrella header but not in the precompiled model source, and it is not exposed in the Python bindings/stubs. Process metadata is also missing compared with the established models.
5. Please add validation or regression coverage. At minimum analytical/limiting-case tests for coverage behavior and diffusion disabled/enabled behavior.