Point Contact 1947 Re Etch the Exposed Upper Face

# Re-Etch the Exposed Upper Face: A Step That Only Fires If a Condition Is Actually Met

Step four's etch was mandatory for every piece that reached it — this step is genuinely conditional, triggered only if step five's metallization actually reached the upper face it was never supposed to touch. Evaporating or electroplating metal onto the underside doesn't happen in a perfectly contained way; depending on the fixture's own geometry, a small amount of metal can travel around an edge or through a gap and land as a thin, unwanted trace on the face that's meant to stay bare germanium until the point contacts arrive. This step exists specifically to catch that contingency, and unlike step four, it doesn't run by default.

## 1. The Decision Is a Direct Comparison Against a Contamination Threshold

$$N_{\text{contam}} \gtrless N_{\text{crit}}$$

Whether this step actually runs comes down to comparing the areal density of stray metal $N_{\text{contam}}$ that actually landed on the upper face against a critical threshold $N_{\text{crit}}$ above which the surface's electrical behavior is measurably disturbed — even a sub-monolayer trace of metal can shift local surface states enough to matter for a surface this sensitive, which is why the threshold here is set far lower than anything that would visibly discolor the germanium. Below that threshold, the upper face is still good; above it, step four's work has to be partially redone.

## 2. Real Diagram: Line-of-Sight Deposition Reaches Where It Isn't Wanted

Evaporated Metal Doesn't Always Stay Where It's Aimed a poorly shielded fixture lets stray atoms reach around the edge evaporation source intended deposit, underside stray trace, upper face — the thing this step checks for whether this trace exceeds the threshold depends on the fixture's own shielding

## 3. The Actual Contamination Level Traces Back to Step Five's Own Geometry

$$N_{\text{contam}} \propto \frac{A_{\text{source}}\cos\theta}{\pi r^2}$$

The amount of stray metal that actually reaches the upper face follows a classic line-of-sight deposition relationship, falling off with the square of distance from the evaporation source and with the cosine of the angle between the source and the exposed surface — a well-shielded fixture geometry, with the upper face turned away from or screened from the source, can make $N_{\text{contam}}$ negligible regardless of how much metal the source emits, while a poorly designed fixture can let a measurable trace through even from a source aimed carefully at the underside alone. This step's outcome, in other words, is really a verdict on how well step five's own fixture did its job, not a fixed cost every assembly has to pay.

Contamination Falls Off Sharply With Shielding Angle good fixture design can push the actual level well below the threshold shielding angle, θ → contamination density, N_contam N_crit — re-etch triggers above this line poor shielding, re-etch required good shielding, step skipped the same step seven can be skipped entirely for a well-designed fixture

## Re-Etch the Exposed Upper Face's Place in the Process Lineage

Re-etching the exposed upper face is step seven of the twenty-five-step sequence documenting the original 1947/48 Bell Labs point-contact transistor — immediately after the base lead was attached and the germanium mounted mechanically, and before the active surface receives any further treatment. It is the first genuinely conditional step in this sequence, running only if step five's metallization geometry let stray metal cross a real contamination threshold onto a face that is meant to stay untouched until the point contacts themselves arrive. Step eight, conditioning the active surface, is the first step to work with the upper face in whatever state this step's decision actually leaves it.

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