Point Contact Transistor Chemically Etch Germanium
# Chemically Etch the Germanium: Removing the Damage the Grind Could Not Help but Leave
Step 3 ground both faces to their own standards, but grinding cannot finish the job it starts — the abrasive that removes stock also drives a layer of disturbed lattice beneath whatever surface it leaves behind, and no amount of additional grinding removes that layer without simply creating a new one of its own. Step 4 is the operation that actually clears it, and it does so by a mechanism with no counterpart in the grinding step at all: dissolution that runs faster at a protruding peak than in a recessed valley, because a peak sits closer to fresh, unconsumed acid than a valley does. The surface does not just get thinner. It gets smoother, and it gets smoother through a process the previous step could not have substituted for no matter how fine its final abrasive pass.
The etch has to clear a different depth on each face, because each face arrived here carrying a different budget. Step 3 deliberately let the two faces receive different final grinding passes — a coarser one on the base face, where the broad metal contact tolerates it, and a finer one on the contact face, where the point spacing will not. That choice set a damage depth on each face, and this step's etch time has to be sized to clear the deeper of the two if both faces are etched together, or set independently if they are etched in separate operations. An etch duration chosen only against the contact face's shallower damage would leave the base face's deeper damage only partially removed — survivable for a contact that averages over imperfection, but a quiet failure to actually finish what Step 3 started on that face.
What the etch exposes is not just a smoother surface, it is a chemically different one. A ground surface carries mechanically introduced lattice disorder and whatever contamination the grinding operation itself deposited — abrasive fragments, handling residue, oxide formed simply from exposure to air during and after grinding. The etch does not discriminate between removing disordered lattice and removing that surface contamination; both dissolve into the same bath. The surface Step 5 will meet is therefore not the surface Step 3 left behind with the damage subtracted — it is bare germanium with no history on it at all, which is exactly the condition metal deposition needs to form a dependable, low-variability contact rather than one bonding partly to a contaminated interface the process never accounted for.
Over-etching is a real failure mode, not merely a wasted margin. Because the surface actively dissolves rather than merely abrading, every minute the block spends in the bath after the roughness target is reached removes material the device still needs — the body's finished thickness, set back in Step 2, shrinks further with every additional minute in solution. A process that etches for a fixed, generous duration regardless of how close the surface already is to its target trades a predictable amount of germanium thickness for a margin of smoothness the device does not require, repeating in a wet chemical step the same resource logic that governed the machining allowance on the saw.
| Step | Process operation | Input | Output | Specification | Constraint |
|---|---|---|---|---|---|
| 4.1 | Determine required etch depth on each face from Step 3's grinding record | Ground faces from Step 3, grit and damage-depth estimates from 3.3 | Per-face etch depth target | Etch depth at least the estimated subsurface damage depth for that face's final grinding pass | Underestimating either face's damage depth leaves disordered lattice the etch was supposed to remove |
| 4.2 | Prepare the etchant and establish a controlled etch rate | Etch depth targets from 4.1 | Etch process parameters (concentration, temperature, agitation) | Etch rate characterized well enough that time can be used as a reliable control variable | An uncharacterized etch rate makes the stopping decision in 4.4 a guess rather than a measurement |
| 4.3 | Immerse the body and etch for the time required to clear the deeper of the two damage depths | Ground body, etch process parameters from 4.2 | Partially etched body | Etch time sufficient to reach the larger of the two per-face targets from 4.1, if etching both faces together | Stopping at a time sized only to the shallower target leaves the other face's damage incompletely removed |
| 4.4 | Monitor and stop the etch once roughness and depth targets are met on both faces | Partially etched body from 4.3 | Etched body, at or past target on both faces | Measured roughness on the contact face below the point-spacing-derived target from Step 3; base face damage layer fully consumed | Continuing to etch past this point consumes finished-dimension germanium for no benefit either face collects |
| 4.5 | Rinse and dry to leave bare, uncontaminated germanium on both faces | Etched body from 4.4 | Clean, bare germanium surface | No residual etchant, reaction byproduct, or handling contamination remaining on either face | A surface Step 5 meets with residual contamination forms an inconsistent metal-to-germanium interface |
| 4.6 | Re-verify finished body thickness against Step 2's target after all stock removal | Clean body from 4.5, target dimensions from Step 2.1 | Thickness-verified body | Finished thickness within the range Step 2 budgeted, after grinding and etching together | A body thinned past Step 2's lower bound by cumulative removal raises base series resistance the circuit was not designed around |
Step 4 does not merely clean a ground surface; it finishes a job the grind was structurally incapable of finishing on its own. Mechanical abrasion cannot remove its own subsurface damage without generating more of the same kind, which is why the sequence hands this operation to a chemical process with a genuinely different removal mechanism — one that happens to level the surface as a side effect of how it works, rather than as something it has to be separately instructed to do. The two faces still answer to the two different standards Step 3 set for them, and the etch's only new freedom is time: long enough to clear whichever face's damage runs deeper, and no longer than that, because every additional minute spends germanium the rest of the device still has a claim on.