Silicon Grown Junction 1954 Attach Bar Ends

# Attach Bar Ends: Gold-Silicon Eutectic Ohmic Solder and Thermal Budget

Attaching the two outer ends of the silicon matchstick to the Kovar header prepared in Step 25 constitutes the first post-growth thermal processing operation near the active transistor core. In germanium technology, soft tin-lead solder ($180\text{ to }230^\circ\text{C}$) was routinely used, but soft solder cannot wet silicon due to its spontaneous native silicon dioxide ($\text{SiO}_2$) skin; furthermore, soft solders fail at the $150^\circ\text{C}$ operating temperatures demanded of silicon devices. Texas Instruments overcame this obstacle by introducing gold-silicon eutectic bonding: heating gold-antimony preforms to $\approx 390\text{ to }410^\circ\text{C}$ ($T_{\text{eutectic}} = 363^\circ\text{C}$) under forming gas to dissolve the surface oxide and form rugged, non-rectifying ohmic joints. Crucially, because solid-state dopant diffusion in silicon is virtually frozen at $400^\circ\text{C}$, the sharp $25\ \mu\text{m}$ base stripe remains completely immune to the junction re-grading that plagued germanium soldering—shifting the primary manufacturing hazard from chemical smearing to mechanical thermal-shock management.

## 1. The Gold-Silicon Eutectic Metallurgy and Non-Rectifying Ohmic Contacts

Establishing robust metallurgical bonds to silicon requires addressing surface oxides and contact resistance:
1. The Native Oxide Obstacle:
Freshly diced silicon instantly forms an amorphous, self-limiting native oxide ($\text{SiO}_2$) $1\text{ to }2\ \text{nm}$ thick upon exposure to atmospheric oxygen. Common soft solders cannot penetrate or wet this ceramic skin. Liquid gold, however, forms a deep eutectic with silicon at $363^\circ\text{C}$, dissolving through thin oxides and alloying directly with the underlying single crystal.
2. Gold-Antimony Eutectic Preforms:
Pure gold alloyed to $n$-type silicon can form an erratic, high-resistance Schottky barrier if not heavily doped. To ensure non-rectifying ohmic behavior, Texas Instruments utilized tiny foil preforms of gold-antimony alloy (99.9% Au, 0.1% Sb, thickness $25\ \mu\text{m}$):
* When heated to $\approx 400^\circ\text{C}$, the preform melts into a liquid Au-Si-Sb eutectic pool.
* As the assembly cools below $363^\circ\text{C}$, excess silicon crystallizes epitaxially back onto the bar ends, incorporating antimony at its solid-solubility limit ($N_D > 10^{19}\ \text{cm}^{-3}$).
* This heavily doped $n^{++}$ regrown layer ensures quantum tunneling across the metal-semiconductor interface, yielding contact resistance $R_c < 0.5\ \Omega$ at both collector and emitter terminals.
3. High-Temperature Operating Robustness:
Unlike low-melting tin-lead solders ($T_m \approx 183^\circ\text{C}$) which suffer thermal fatigue and creep failure above 100°C, the gold-silicon eutectic bond remains mechanically solid and chemically stable up to 363°C, easily fulfilling the silicon transistor’s core commercial promise of continuous operation at 150°C.

## 2. Real Diagram: Gold-Silicon Eutectic Phase Diagram and Suspended Reflow Fixture

## 3. Thermal Budget Comparison: Frozen Diffusion in Silicon vs. Germanium

A crucial thermodynamic divergence separates 1954 silicon manufacturing from 1951 germanium practice:
1. The Frozen Solid-State Diffusion Budget:
In germanium, soldering at $250^\circ\text{C}$ represented a homologous temperature of $T/T_m \approx 0.43$, where dopant diffusion coefficients ($D \approx 10^{-16}\ \text{cm}^2/\text{s}$) could measurably blur a hairline base over prolonged heating. In silicon ($T_m = 1414^\circ\text{C} = 1687\ \text{K}$), soldering at $400^\circ\text{C}$ ($673\ \text{K}$) represents a homologous temperature of only $T/T_m \approx 0.39$:
* Because covalent Si-Si bonds require $E_a \approx 3.7\text{ to }4.0\ \text{eV}$ for impurity diffusion, the diffusion coefficient of antimony and gallium at $400^\circ\text{C}$ is smaller than $10^{-24}\ \text{cm}^2/\text{s}$.
* During the 15-second eutectic reflow dwell, $L_{\text{diff}} = \sqrt{D t} \approx 4 \times 10^{-12}\ \text{m} = 0.04\ \text{\AA}$! The metallurgical boundaries of the $25\ \mu\text{m}$ base stripe do not move by even a single atomic fraction.
2. Thermal Stress and Axial Constraint Relief:
While solid-state diffusion is safely frozen, mechanical thermal stresses are intense. Heating the two bar ends to $400^\circ\text{C}$ causes localized thermal expansion ($\Delta L \approx 5\ \mu\text{m}$). If the Kovar fixture clamped the bar rigidly from both ends, compressive buckling would shear the suspended base stripe.
3. Floating Alignment Fixture:
Technicians mount the matchstick loosely onto the Kovar pedestals under light vertical leaf-spring pressure ($F < 0.1\ \text{N}$), allowing unconstrained axial expansion during reflow. A reducing atmosphere of forming gas ($90\%\ \text{N}_2, 10\%\ \text{H}_2$) prevents copper-Kovar oxidation and scavenges residual oxygen, producing bright, meniscus-filleted eutectic joints without chemical flux.

## Attach Bar Ends's Place in the Process Lineage

Attaching the bar ends is step twenty-six of this project's 1954 silicon grown-junction transistor series — following the preparation of the Kovar-glass lead assembly, and preceding the chemical etching of diamond machining damage. It firmly anchors the silicon matchstick into its package foundation and establishes low-resistance gold-silicon eutectic ohmic contacts to the collector and emitter terminals. Step twenty-seven, etch machining damage, immerses the mounted bar into a nitric-hydrofluoric acid bath to strip away the subsurface micro-cracks and saw damage left by dicing.