Micrologic 1960 Diffuse Multiple Transistors Resistors Substrate

# Diffuse Multiple Transistors and Resistors on One Substrate at Once: One Furnace Cycle, Every Component's Value

## 1. Why Tuning One Component's Diffusion Now Changes Every Other Component on the Chip

This step runs the base diffusion across the whole wafer exactly as every earlier series in this project has run it, except that this diffusion now simultaneously sets two different things at once — the base region of every transistor in the layout, and the body of every diffused resistor Step 1 specified by its length-to-width ratio — because both are the same doped region, distinguished only by the shape drawn around them. A transistor's base and a resistor's body were never different materials in this process; they are the identical diffusion, read two different ways by the layout. That identity carries a real design consequence: the sheet resistance this diffusion produces is not a parameter this step can tune independently for the resistors without also retuning the base region of every transistor sharing the wafer, because there is only one furnace cycle and one sheet resistance for the whole chip.

where $\Delta R_{\text{sheet}}$ is the furnace-to-furnace or position-to-position variation in sheet resistance this project already measured in 1958's own Step 2, when a single boat produced a measurable gradient in junction depth from position to position along its length. A resistor's tolerance is therefore inherited directly from a variation this project characterized two series ago for an entirely different reason — there, it threatened junction depth; here, it threatens every resistor value on the chip at once, correlated across the whole wafer rather than independent from one resistor to the next.

## 2. Real Diagram: One Diffusion Cycle, Read Two Different Ways

The cross-section after this diffusion shows the same doped layer under a transistor's base contact and under a resistor's full length — physically indistinguishable except for what shape the oxide mask left open above each one, and what the metal interconnect from this series' later steps will do with each region once it is wired into the circuit.

## 3. Why No Earlier Series Ever Had Two Components Sharing One Diffusion's Fate

Every diffusion this project has documented before this series set the properties of exactly one kind of region — a base, an emitter, a collector contact — and that region belonged to a single transistor whose performance depended on it alone. This step's diffusion simultaneously sets the electrical behavior of every transistor's base and every resistor's value across an entire logic gate, which means a furnace variation that would have been a single device's problem in any earlier series is now a correlated problem across several components at once. The 1958 series worried about furnace position variation because it threatened junction depth on a per-device basis; this series inherits the identical physical variation and discovers it threatens something that did not exist before this series — the ratio between several different components' values, all depending on the same diffusion simultaneously.

Step 2 does not diffuse anything this project has not diffused before; it is the first step in this project's history where one furnace cycle's result is shared, for better or worse, by every component on the chip at once.