Locate the Base Electrically

# Locate the Base Electrically: The Same Technique, a Thousand Times Finer

Step twenty-one used a scanned point probe's rectification polarity to find both junctions along the length of a multi-centimeter ingot, with positional uncertainty of a fraction of a millimeter being entirely acceptable for that step's purpose. This step uses exactly the same physical principle — a metal point contact's forward-conduction sense flipping between n-type and p-type material — but now has to locate the base precisely enough for a wire bond to land on it, and the base itself, after step twenty-seven's etch, may be only a few microns wide. The same measurement technique that worked at millimeter precision for locating a junction-bearing section now has to work at micron precision for locating a wire landing site, and that jump in required resolution is what makes this step a fundamentally harder version of something already done once before.

## 1. Required Probe Resolution Scales With the Target Feature, Not With the Technique

$$\delta x_{\text{probe}} \ll W_B^{\text{(remaining)}}$$

The point-probe rectification technique itself hasn't changed since step twenty-one — what's changed is the ratio between the probe's achievable positioning resolution and the size of the feature it's trying to locate. Locating a junction-bearing section within an ingot many centimeters long tolerates positional uncertainty measured in fractions of a millimeter, because the margin budget established back in step twenty-two was sized at that same coarse scale. Locating the base specifically, now that it may be only microns wide after etching, requires a micromanipulator capable of resolving position at a small fraction of that remaining base width — a resolution requirement orders of magnitude tighter than anything this process has asked of positioning equipment so far.

## 2. Real Diagram: Same Physics, Radically Different Scale

Same Rectification Signal, Two Very Different Scales step twenty-one located a section in an ingot; this step locates a wire landing site in a bar step 21, ingot-scale mm-scale uncertainty, entirely acceptable margin from step 22 absorbs this easily step 29, bar-scale base, microns wide micron-scale precision required the next step bonds exactly here identical physics, a thousandfold tighter precision requirement

## 3. This Step Hands Off a Position, Not Just a Confirmation

$$x_{\text{wire target}} = x_{\text{base}}^{\text{(measured)}} \pm \delta x_{\text{probe}}$$

Unlike step twenty-one, which simply confirmed where junctions sat so a later cut could be planned with margin, this step's output is a specific coordinate the next step's bonding wire will be driven to directly — there's no intermediate margin budget absorbing this step's own measurement uncertainty the way step twenty-two's did for step twenty-one's. Whatever positional error this step carries propagates essentially unfiltered into the wire-bonding step immediately following it, which is why the micromanipulator's resolution, not just the probe's rectification signal, is the parameter this step's success actually depends on.

Required Probe Resolution vs. Target Feature Size δx_probe ≪ W_B — the same technique, scaled down a thousandfold target feature size (log scale) → required probe resolution step 21, ingot section, mm-scale step 29, base, micron-scale a smaller target demands proportionally tighter resolution to locate reliably

## Locate the Base Electrically's Place in the Process Lineage

Locating the base electrically is step twenty-nine of the 1951 grown-junction transistor's full manufacturing sequence — immediately after the bar has been rinsed and dried, and before the base wire is positioned for bonding. It reuses step twenty-one's point-probe rectification technique at a resolution requirement orders of magnitude tighter, converting a confirmatory measurement into a precise coordinate the very next step depends on directly.

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