Schottky Ttl 1970s Integrate Schottky Diodes into the Standard Ttl Process
# Schottky TTL (early 1970s): Step 5 — Integrate Schottky Diodes into the Standard TTL Bipolar Process
Integrate Schottky Diodes into the Standard TTL Bipolar Process. Adding a Schottky diode to an existing bipolar TTL process was not simply a schematic change — it required a genuine process modification.
## 1. The Process Integration Challenge
Adding a Schottky diode to an existing bipolar TTL process was not simply a schematic change — it required a genuine process modification. The metal-semiconductor contact that forms the Schottky barrier has to be deposited on a clean, lightly doped n-type silicon surface with a controlled, reproducible work-function mismatch; the same metal layer used for an ordinary low-resistance ohmic contact elsewhere on the chip will not behave as a rectifying Schottky contact unless the underlying doping and surface preparation are specifically engineered for it. Early Schottky TTL processes typically used platinum silicide (formed by depositing platinum and reacting it with the silicon surface) as the Schottky metal, chosen for a barrier height that gave a reliably low, stable forward voltage, combined with a lightly doped n-type collector region directly beneath the contact to set the barrier characteristics.
## 2. Real Diagram: Process Cross-Section, Metal Contact to Collector
## 3. Yield and Reproducibility as the Real Engineering Problem
The hard part was not demonstrating a working Schottky diode in the lab — that had been understood for decades — but reproducing a consistent, low-leakage barrier across millions of die on a production line with acceptable yield. Barrier height and leakage current are sensitive to surface contamination, native oxide thickness at the metal-silicon interface, and silicide formation temperature, all of which had to be controlled tightly enough that every clamp diode on every chip turned on within a narrow voltage window relative to the transistor's own base-collector junction; a barrier that drifted too high defeated the clamp's speed benefit, while one that drifted too low or leaked excessively degraded the logic-low noise margin. Texas Instruments and other early Schottky TTL manufacturers treated this silicide process control as proprietary, and it is a large part of why Schottky TTL took until the early 1970s to reach volume production even though the underlying diode physics had been known since the 1930s and the clamping concept itself was straightforward.
## 4. Companion Diagram: Stage Sequence and Trend Summary
## Integrate Schottky Diodes into the Standard TTL Bipolar Process's Place in the Process Lineage
This is step 5 of 20 in the Schottky TTL (early 1970s) series, covering the fabrication challenge of adding a reliable Schottky contact to an existing bipolar TTL process.