Surface Barrier 1953 Electrolyte Jet Etch Fixture

# Assemble the Electrolyte Jet-Etch Fixture: Where Two Jets Have to Agree on the Same Point

Ten steps have prepared a wafer whose usable area, mapped in step ten, is ready for a process no prior series in this project has needed: two independent etching streams, aimed from opposite faces, that must converge on the same location to leave a well-defined base between them. Every device this project has documented through step ten of the alloy-junction route forms its junctions with tooling that approaches from one side at a time, in sequence. This fixture is the first apparatus in this project's process histories that has to hold two active tools in simultaneous, mirrored register against a single wafer, because the base this process builds exists only where both jets' influence meets.

## 1. Concentricity Between the Two Jets Sets Where the Base Actually Forms

$$\Delta_{xy} = \sqrt{(x_{\text{top}}-x_{\text{bottom}})^2+(y_{\text{top}}-y_{\text{bottom}})^2}$$

The fixture holds the wafer on a common axis between two opposing nozzle assemblies, each one independently adjustable but intended to strike the same $(x,y)$ location on opposite faces. Any lateral mismatch $\Delta_{xy}$ between the two jets' actual aim points does not merely blur the base region — it means the two etch fronts are dissolving toward two different points in space rather than one, so the thinnest remaining germanium does not sit cleanly between two aligned pits but along some skewed, harder-to-characterize path between two offset ones. This is exactly the alignment risk step ten's usable-area margin $m_{\text{alignment}}$ was sized to anticipate, now built into hardware for the first time.

## 2. Real Diagram: A Fixture Built Around One Shared Axis, Not Two Independent Tools

Two Nozzles, One Shared Axis, One Wafer Between Them this is the first fixture in this project's process histories built around simultaneous opposing tools top nozzle adjustable, aimed down wafer, clamped on shared axis bottom nozzle adjustable, aimed up both jets aimed here — one shared point registration pins and a common frame, not two separately mounted tools, keep the point shared

## 3. The Fixture Also Carries the Electrical Circuit Etching Will Need

$$Q = n F \, N_{\text{Ge removed}}$$

Beyond mechanical alignment, this fixture establishes the electrical path the next step's etching current will use: each nozzle assembly doubles as an electrode delivering electrolyte and carrying current to the wafer face it strikes, while the wafer itself is contacted at its edge to complete the circuit. The charge $Q$ passed through this path and the number of germanium atoms $N_{\text{Ge removed}}$ dissolved are related through Faraday's law — the same relationship that will let the next step monitor etch progress electrically rather than only by elapsed time, because this fixture is what makes current, not just electrolyte flow, a controllable variable at each face independently.

The Fixture Carries Electrolyte Flow and Etch Current Together each nozzle is both a jet and an electrode in the same assembly top nozzle = electrolyte jet + electrode current path begins here wafer, edge-contacted to complete circuit bottom nozzle = electrolyte jet + electrode current path returns here Q = nF·N_Ge removed — the next step reads etch depth from this current, not a clock alone

## Assemble the Electrolyte Jet-Etch Fixture's Place in the Process Lineage

Assembling the electrolyte jet-etch fixture is step eleven of Philco's surface-barrier process — the first step with no counterpart anywhere in this project's alloy-junction series, because no prior process this project has documented needed two simultaneous, mirrored tools converging on one point. It is also where step ten's alignment margin stops being a measurement requirement and becomes a hardware specification: the registration this fixture holds is exactly what that margin was computed to protect against failing. Step twelve, positioning the wafer within this fixture, is the step that actually places the usable region step ten mapped onto the shared axis this step has now built.

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