# Technology & Layer Stack A **LayerStack** describes the vertical cross-section of your fabrication process: which physical materials are deposited on which GDS layers, at what height (`zmin`), and with what thickness. This information is used by simulation exporters (e.g. Tidy3D, MEEP, Lumerical) that consume kfactory layouts. This page covers: | Topic | What you'll learn | | --- | --- | | `LayerLevel` | One physical layer in the process | | `LayerStack` | Collection of `LayerLevel`s | | `Info` | Extra metadata (mesh order, refractive index, etch type, …) | | Attaching a stack to a PDK | Best-practice pattern | | Querying the stack | Per-layer lookups | ## 1 · LayerLevel — one process step `LayerLevel` describes a single physical layer: | Parameter | Type | Meaning | | --- | --- | --- | | `layer` | `(int, int)` or `kdb.LayerInfo` | GDS layer / datatype | | `zmin` | `float` (µm) | Bottom of the slab | | `thickness` | `float` (µm) | Vertical thickness | | `material` | `str` | Material name (passed to simulator) | | `sidewall_angle` | `float` (°) | 0° = vertical, 90° = horizontal | | `info` | `Info` | Optional simulation metadata | ``` import kfactory as kf from kfactory.layer import Info, LayerLevel, LayerStack # Silicon waveguide core: 220 nm thick, starts at z=0 wg_level = LayerLevel( layer=(1, 0), zmin=0.0, thickness=0.22, # µm material="si", sidewall_angle=80.0, # near-vertical etch ) print(wg_level) ``` ``` layer=(1, 0) thickness=0.22 thickness_tolerance=None zmin=0.0 material='si' sidewall_angle=80.0 z_to_bias=None info=Info() ``` ### Info — simulation metadata Pass an `Info` object to attach extra metadata that simulators can consume. Common fields: | Field | Meaning | | --- | --- | | `mesh_order` | Lower = higher priority in mesher (1 overrides 2) | | `refractive_index` | `float` (int/float only; complex must be stored as a string) | | `type` | `"grow"`, `"etch"`, `"implant"`, or `"background"` | ``` wg_level_with_info = LayerLevel( layer=(1, 0), zmin=0.0, thickness=0.22, material="si", sidewall_angle=80.0, info=Info( mesh_order=1, refractive_index=3.47, type="grow", ), ) print(wg_level_with_info.info) ``` ``` Info(mesh_order=1, refractive_index=3.47, type='grow') ``` ## 2 · LayerStack — the full process `LayerStack` is a named collection of `LayerLevel`s. Pass levels as keyword arguments; the keyword name becomes the layer's logical name in the stack. ``` stack = LayerStack( # --- core waveguide layers --- wg=LayerLevel( layer=(1, 0), zmin=0.0, thickness=0.22, material="si", sidewall_angle=80.0, info=Info(mesh_order=1, refractive_index=3.47, type="grow"), ), slab=LayerLevel( layer=(3, 0), zmin=0.0, thickness=0.09, # partial etch leaves 90 nm slab material="si", sidewall_angle=70.0, info=Info(mesh_order=2, refractive_index=3.47, type="grow"), ), # --- oxide cladding --- clad=LayerLevel( layer=(111, 0), zmin=-3.0, thickness=3.22, # 3 µm below + 0.22 µm above wg top material="sio2", info=Info(mesh_order=3, refractive_index=1.44, type="background"), ), # --- metal --- metal=LayerLevel( layer=(41, 0), zmin=0.5, thickness=1.0, material="al", info=Info(mesh_order=1, type="grow"), ), ) print(stack) ``` ``` layers={'wg': LayerLevel(layer=(1, 0), thickness=0.22, thickness_tolerance=None, zmin=0.0, material='si', sidewall_angle=80.0, z_to_bias=None, info=Info(mesh_order=1, refractive_index=3.47, type='grow')), 'slab': LayerLevel(layer=(3, 0), thickness=0.09, thickness_tolerance=None, zmin=0.0, material='si', sidewall_angle=70.0, z_to_bias=None, info=Info(mesh_order=2, refractive_index=3.47, type='grow')), 'clad': LayerLevel(layer=(111, 0), thickness=3.22, thickness_tolerance=None, zmin=-3.0, material='sio2', sidewall_angle=0.0, z_to_bias=None, info=Info(mesh_order=3, refractive_index=1.44, type='background')), 'metal': LayerLevel(layer=(41, 0), thickness=1.0, thickness_tolerance=None, zmin=0.5, material='al', sidewall_angle=0.0, z_to_bias=None, info=Info(mesh_order=1, type='grow'))} ``` ## 3 · Querying the stack `LayerStack` exposes several lookup helpers. Keys are `(layer, datatype)` tuples. ``` print("Thicknesses:") for layer_tuple, t in stack.get_layer_to_thickness().items(): print(f" {layer_tuple}: {t} µm") print("\nMaterials:") for layer_tuple, mat in stack.get_layer_to_material().items(): print(f" {layer_tuple}: {mat}") print("\nZ-min positions:") for layer_tuple, zmin in stack.get_layer_to_zmin().items(): print(f" {layer_tuple}: {zmin} µm") print("\nSidewall angles:") for layer_tuple, angle in stack.get_layer_to_sidewall_angle().items(): print(f" {layer_tuple}: {angle}°") ``` ``` Thicknesses: (1, 0): 0.22 µm (3, 0): 0.09 µm (111, 0): 3.22 µm (41, 0): 1.0 µm Materials: (1, 0): si (3, 0): si (111, 0): sio2 (41, 0): al Z-min positions: (1, 0): 0.0 µm (3, 0): 0.0 µm (111, 0): -3.0 µm (41, 0): 0.5 µm Sidewall angles: (1, 0): 80.0° (3, 0): 70.0° (111, 0): 0.0° (41, 0): 0.0° ``` Access individual levels by name or by index: ``` wg = stack["wg"] print( f"wg zmin={wg.zmin} µm, thickness={wg.thickness} µm, top={wg.zmin + wg.thickness} µm" ) ``` ``` wg zmin=0.0 µm, thickness=0.22 µm, top=0.22 µm ``` ## 4 · Attaching a stack to a PDK A `LayerStack` is not built into `KCLayout` — you attach it to your PDK module as a module-level constant alongside `LAYER` and the layout object. Best practice: define everything in one module and import `pdk`, `LAYER`, and `STACK` from it. ``` # --- pdk_with_stack.py (inline for demo) --- class LAYER(kf.LayerInfos): WG: kf.kdb.LayerInfo = kf.kdb.LayerInfo(1, 0) SLAB: kf.kdb.LayerInfo = kf.kdb.LayerInfo(3, 0) WGCLAD: kf.kdb.LayerInfo = kf.kdb.LayerInfo(111, 0) METAL: kf.kdb.LayerInfo = kf.kdb.LayerInfo(41, 0) FLOORPLAN: kf.kdb.LayerInfo = kf.kdb.LayerInfo(99, 0) pdk = kf.KCLayout("DEMO_TECH_PDK", infos=LAYER) L = pdk.infos # LayerInfos instance; use for layer objects STACK = LayerStack( wg=LayerLevel( layer=(L.WG.layer, L.WG.datatype), zmin=0.0, thickness=0.22, material="si", sidewall_angle=80.0, info=Info(mesh_order=1, refractive_index=3.47, type="grow"), ), clad=LayerLevel( layer=(L.WGCLAD.layer, L.WGCLAD.datatype), zmin=-3.0, thickness=3.22, material="sio2", info=Info(mesh_order=3, refractive_index=1.44, type="background"), ), ) print(f"PDK: {pdk}") print(f"Stack: {list(STACK.layers.keys())} layers defined") ``` ``` PDK: name='DEMO_TECH_PDK' layout= layer_enclosures=LayerEnclosureModel(root={}) cross_sections={} extrude_configs={} enclosure=KCellEnclosure(enclosures=LayerEnclosureCollection(enclosures=[])) library= factories= virtual_factories= generic_factories={} tkcells={} infos=LAYER(WG=WG (1/0), SLAB=SLAB (3/0), WGCLAD=WGCLAD (111/0), METAL=METAL (41/0), FLOORPLAN=FLOORPLAN (99/0)) layer_stack=LayerStack(layers={}) netlist_layer_mapping={} sparameters_path=None interconnect_cml_path=None constants=Constants() rename_function= thread_lock= info=Info() settings=KCellSettings(version='3.5.0', klayout_version='0.30.12', meta_format='v3') decorators= default_cell_output_type= default_vcell_output_type= connectivity=[] routing_strategies={} technology_file=None layers= Stack: ['wg', 'clad'] layers defined ``` ## 5 · Serialising the stack `to_dict()` returns a plain Python dict — useful for saving to YAML or JSON, or for passing to simulators that don't import kfactory directly. ``` import json d = stack.to_dict() print(json.dumps(d, indent=2, default=str)) ``` ``` { "wg": { "layer": [ 1, 0 ], "thickness": 0.22, "thickness_tolerance": null, "zmin": 0.0, "material": "si", "sidewall_angle": 80.0, "z_to_bias": null, "info": { "mesh_order": 1, "refractive_index": 3.47, "type": "grow" } }, "slab": { "layer": [ 3, 0 ], "thickness": 0.09, "thickness_tolerance": null, "zmin": 0.0, "material": "si", "sidewall_angle": 70.0, "z_to_bias": null, "info": { "mesh_order": 2, "refractive_index": 3.47, "type": "grow" } }, "clad": { "layer": [ 111, 0 ], "thickness": 3.22, "thickness_tolerance": null, "zmin": -3.0, "material": "sio2", "sidewall_angle": 0.0, "z_to_bias": null, "info": { "mesh_order": 3, "refractive_index": 1.44, "type": "background" } }, "metal": { "layer": [ 41, 0 ], "thickness": 1.0, "thickness_tolerance": null, "zmin": 0.5, "material": "al", "sidewall_angle": 0.0, "z_to_bias": null, "info": { "mesh_order": 1, "type": "grow" } } } ``` ## Summary | Class | Key role | | --- | --- | | `LayerLevel` | Single process step: layer, z position, thickness, material | | `Info` | Simulation extras: mesh order, refractive index, etch type | | `LayerStack` | Named dict of `LayerLevel`s; exposes per-layer lookup helpers | Keep `STACK` as a module-level constant in your PDK module alongside `pdk` and `LAYER`. Simulators can then `from my_pdk import pdk, LAYER, STACK` and get everything they need from a single import. ## See Also | Topic | Where | | --- | --- | | Creating a full PDK | [PDK: Creating a PDK]() | | Layer definitions | [Core Concepts: Layers]() | | Cross-sections (port geometry) | [Cross-Sections]() | | KCLayout (owns the cell DB) | [Core Concepts: KCLayout]() |