Surface Barrier 1953 Initiate Simultaneous Etching

# Initiate Simultaneous Top and Bottom Etching: Why the Two Jets Must Start Together, Not Just Aim Together

Step eleven aligned the two jets on a shared point, and step twelve placed the wafer so that point sits inside the usable region — but neither step controls the one variable this step introduces: when each jet's current actually turns on. If the top jet begins etching even a fraction of a second before the bottom jet, the two etch fronts are no longer symmetric in time, even though they remain perfectly aligned in space. This step is where the fixture's spatial precision from steps eleven and twelve meets a temporal precision requirement neither of those steps addressed.

## 1. Each Jet's Removal Rate Follows Faraday's Law Independently

$$\frac{dd}{dt} = \frac{I\,V_m}{nFA}$$

Each jet dissolves germanium at a rate set by its own current $I$, the molar volume $V_m$, the number of electrons transferred per dissolved atom $n$, Faraday's constant $F$, and the etched area $A$ — step eleven's electrical design applied independently at each face. Because the top and bottom circuits are electrically independent, as step eleven established, nothing in the physics of one jet automatically synchronizes with the other; synchronization has to be imposed externally, by this step's own triggering procedure.

## 2. Real Diagram: A Timing Offset Produces an Asymmetric Pair of Depths

A Start-Time Mismatch Becomes a Depth Mismatch perfectly aligned jets, imperfectly synchronized triggers time → top jet ON top jet etching, full duration bottom jet ON, Δt_start late bottom jet etching, shorter duration d_top ≠ d_bottom, even though both currents match the mismatch comes entirely from Δt_start, not from any current or alignment error this breaks the symmetric convergence steps eleven and twelve were built to deliver this step's trigger procedure is the only thing standing between good hardware and an asymmetric base

## 3. The Timing Tolerance Converts Directly Into a Depth-Mismatch Budget

$$\delta_d = \frac{I\,V_m}{nFA}\,\Delta t_{\text{start}}$$

A start-time offset $\Delta t_{\text{start}}$ between the two jets produces a depth mismatch $\delta_d$ exactly proportional to the single-jet etch rate already derived in Section 1 — meaning the acceptable triggering tolerance for this step is not an arbitrary engineering choice but a direct consequence of how fast each jet already removes material. A process running jets fast enough to finish in a short total etch time has correspondingly less tolerance for a start-time mismatch, because the same $\Delta t_{\text{start}}$ consumes a larger fraction of a shorter total process. This step's trigger circuit has to be designed against the specific etch rate step eleven's current settings established, not against a fixed timing specification independent of it.

Faster Etching Shrinks the Allowed Timing Window the same absolute timing error costs more at a faster etch rate etch current, I → depth mismatch per unit Δt_start δ_d rises linearly with I this process's operating current the trigger tolerance this step enforces has to shrink as the operating current rises

## Initiate Simultaneous Top and Bottom Etching's Place in the Process Lineage

Initiating simultaneous top and bottom etching is step thirteen of Philco's surface-barrier process, immediately after the wafer has been positioned within the fixture and before either jet's progress is monitored toward an endpoint. It is the step that introduces time as an alignment variable alongside the spatial alignment steps eleven and twelve already secured, and it converts step eleven's own current settings into a concrete timing tolerance this step's trigger circuit must meet. Step fourteen, monitoring etch current to endpoint, assumes both jets began under this step's synchronization and tracks their progress from that common, simultaneous start.

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