Alloy Junction 1952 Protect the Base Attachment During Etching

# Protect the Base Attachment During Etching: A Second Metal Changes the Chemistry

Step 30's etch rate law, $\delta(t) = kt$, assumed a single material sitting in the bath: germanium, dissolving at a steady, purely chemical rate. That assumption holds everywhere on the device except one place — wherever the nickel base tab Step 18 positioned and Step 24 solid-state-bonded to the base region is still exposed to the same bath. A bare nickel surface electrically connected to germanium, both sitting in the same conductive etchant, is not just two materials being etched side by side. It is a galvanic couple: two dissimilar conductors joined by an electrolyte, with their own electrochemical potentials, and that couple can drive a second, non-chemical etching mechanism right at the one location this entire series has most carefully protected — the base contact Step 24's bonding established.

## 1. Two Metals in One Bath Is Not Two Separate Etches

Nickel and germanium sit at different electrochemical potentials in a given etchant. Where they are electrically connected — through the bond Step 24 formed — and both exposed to the same conductive solution, a galvanic current flows between them, driven by that potential difference and limited only by how much the two surfaces resist polarizing against it:

$$ i_{\text{couple}} \;\approx\; \frac{E_{\text{tab}} - E_{\text{Ge}}}{R_{p,\text{tab}} + R_{p,\text{Ge}} + R_{\text{electrolyte}}} $$

This current adds a second attack mechanism exactly where the tab meets the germanium, on top of the ordinary chemical dissolution Step 30 already described — and unlike Step 30's uniform reaction-rate-limited etch, this galvanic contribution is concentrated right at the junction between the two metals, precisely because that is where the driving potential difference and the available current path both exist. A region of the device that was never meant to be etched at all can, if left unprotected, end up attacked faster and more unevenly than the areas this step's chemistry was actually designed for.

## 2. Real Diagram: A Galvanic Couple Forms Wherever the Tab Is Bare

Masked Tab Versus Bare Tab in the Same Bath The galvanic path only exists where the circuit is complete Masked: no galvanic path Ni tab protective mask etchant never reaches the tab Bare: couple is live Ni tab, exposed localized galvanic attack at the bond

## 3. Faraday's Law Turns the Current Into an Extra Etch Depth

A galvanic current is not just an electrical curiosity — by Faraday's law of electrolysis, it corresponds directly to a mass of material dissolved per unit time, which adds an entirely separate term to Step 30's purely chemical removal rate. The total depth lost at an unprotected tab region over an etch exposure of duration $t$ becomes:

$$ \delta_{\text{total}}(t) \;=\; k\,t \;+\; \frac{i_{\text{couple}}\,M}{n\,F\,\rho}\,t $$

where $M$ is the molar mass of the material being galvanically attacked, $n$ is the number of electrons exchanged per ion, $F$ is Faraday's constant, and $\rho$ is its density. The first term is the same uniform chemical etch Step 30 relies on everywhere else on the device; the second term exists only where the galvanic circuit in Section 1 is actually complete, and it can be substantial even over the short exposure Step 30's targeted periphery etch otherwise uses safely. Breaking that circuit — covering the tab and its bonded contact with a mask impervious to the bath before etching begins — reduces $i_{\text{couple}}$ to zero and collapses this equation back down to exactly the chemical-only rate law Step 30 already characterized, which is the entire point of this step.

## Real Diagram: What Masking Actually Saves, Over Time

Depth Lost at the Tab Bond, Masked vs. Unmasked The galvanic term only exists while the circuit stays closed etch exposure time depth lost at the bond masked: chemical term only unmasked: chemical plus galvanic same elapsed time

## Protect the Base Attachment During Etching's Place in the Process Lineage

Protect the Base Attachment During Etching follows directly from Step 30, Perform Post-Alloy Etching, whose chemistry is what actually creates the galvanic risk this step exists to prevent; it precedes Step 32, Rinse and Dry Thoroughly, which will need a mask material that comes off cleanly without leaving its own residue behind. It is the fourth step of Phase 4 and the first point in the entire series where a second, dissimilar metal already bonded to the device — the base tab Step 18 placed and Step 24 secured — actively changes the chemistry of a step that would otherwise treat the whole surface uniformly, turning what Step 30 treated as a single reaction-rate-limited process into two separate mechanisms that only stay separate because this step keeps the galvanic circuit from ever closing.

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