Dislocation Formation Mechanisms in Late-Stage Silicon Crystal Growth

In Czochralski (CZ) silicon single crystal growth, the Dash Necking process reduces the crystal diameter to 2–4 mm during seeding. This enables dislocations induced by thermal shock in the seed crystal to glide to the crystal surface and annihilate, yielding dislocation-free single crystals. Nevertheless, the "dislocation-free" state is not permanent. In the mid-to-late stage of constant-diameter growth, shoulder tapering, and final tail separation from the melt, multiple physical mechanisms can trigger dislocation renucleation and multiplication, ultimately rendering the entire silicon ingot defective and scrapped.

This paper systematically analyzes the root causes of dislocation formation in the late growth stage from six dimensions: thermal stress, solid-liquid interface morphology, constitutional supercooling, process perturbations from diameter fluctuation, impurity precipitation, and thermal shock during tail separation.

1. Accumulated Thermal Stress and Dislocation Multiplication

Thermal stress accumulation and subsequent dislocation multiplication constitute the dominant source of dislocations generated in the late growth phase.

As silicon ingot diameters expand to 300 mm and even 450 mm, alongside increasing crystal length, radial and axial temperature gradients within the single crystal continuously intensify. Significant temperature disparities emerge between the crystal core and peripheral regions; poor heat dissipation at the center causes persistent thermal stress buildup. Once thermal stress exceeds silicon’s Critical Resolved Shear Stress (CRSS) at high temperatures, slip systems on {111} planes along the ⟨110⟩ direction activate and generate glide dislocations.

More critically, dislocations multiply exponentially in accordance with the Alexander–Haasen model. Even an extremely low initial dislocation density will undergo exponential proliferation under sustained high-temperature thermal stress. This means a tiny number of dislocation "seeds" formed late in growth are sufficient to contaminate the entire ingot within a short timeframe, pushing dislocation density beyond acceptable thresholds.

This issue is especially severe for large-diameter silicon ingots. Research confirms that thermally induced dislocations present a critical challenge for both Float-Zone (FZ) and CZ growth when crystal diameters reach 300 mm.

2. Morphological Instability of the Solid-Liquid Interface

The shape of the solid-liquid interface exerts a decisive influence on dislocation nucleation.

In the late stage of constant-diameter growth, falling melt levels inside the crucible increase the exposed height of crucible walls, continuously altering melt convection patterns and heat transfer conditions, which directly modulates solid-liquid interface geometry.

Studies verify that interface deformation induces uneven thermal stress distribution, with stress-concentrated zones serving as preferential sites for dislocation nucleation:

A. Interfaces convex toward the melt create substantial edge shear stress, triggering peripheral dislocations;

B. Concave solid-liquid interfaces are far more prone to dislocation formation and crystal gliding;

C. A planar solid-liquid interface effectively suppresses dislocation generation.

For heavily n-type doped single silicon crystals, evolution of the solid-liquid interface and formation of peripheral {111} facets further expand supercooled zones, interrupt shoulder growth and induce dislocations.

3. Constitutional Supercooling

Constitutional supercooling represents another major mechanism driving dislocation formation late in crystal growth.

Localized severe constitutional supercooling can occur at the solid-liquid interface during constant-diameter growth, shoulder expansion and shoulder tapering. When constitutional supercooling arises, the actual melt temperature ahead of the solid-liquid interface drops below the liquidus temperature, destabilizing the interface to form cellular or dendritic growth fronts. Such unstable interfaces trap melt inclusions, trigger micro-local lattice rotation, and ultimately produce dislocations.

The severity of constitutional supercooling correlates directly with crystal growth velocity, temperature gradients and impurity concentration. In the late growth stage, reduced melt volume continuously enriches residual impurities, drastically elevating the risk of constitutional supercooling.

4. Diameter Fluctuations and Process Perturbations

Stable crystal diameter control during constant-diameter growth is critical; drastic diameter variations introduce dislocation defects.

Sources of diameter fluctuation include three primary categories:

A. Pull rate instability: Pull velocity directly modulates solid-liquid interface geometry and internal crystal stress distribution. Abnormal pull rates or crucible rotation speeds trigger uneven ingot diameters and dislocation formation.

B. Thermal field fluctuations: Intense melt convection, particularly within large-diameter crucibles, generates temperature oscillations and localized remelting of the crystal, inducing diameter deviation.

C. Unstable mechanical transmission: Mechanical vibration or irregular motion of furnace pull mechanisms also acts as a major dislocation source.

For large-diameter single crystal growth, the difficulty of diameter control rises exponentially; minor process disturbances are amplified into stress concentrations sufficient to nucleate dislocations.

5. Inductive Effects of Impurities and Precipitates

Impurities exert dual effects on dislocation behavior in late-stage growth:

Beneficial Effects

Boron, germanium, nitrogen, phosphorus, arsenic and oxygen all suppress dislocations to varying degrees via the dislocation pinning effect. Oxygen atoms pin dislocations to strengthen the silicon lattice and delay glide motion.

Detrimental Effects

Excess or unevenly distributed impurities instead act as dislocation nucleation sites:

A. Excess melt impurities are a leading cause of edge chipping and crystal crown shedding;

B. During cooling, supersaturated interstitial atoms precipitate within the silicon bulk, releasing large quantities of self-interstitial silicon atoms that aggregate to form dislocations in localized regions;

C. Oxide precipitates induce secondary defects including dislocations and stacking faults.

In the late growth phase, segregation effects continuously enrich impurities within the shrinking residual melt, raising impurity concentrations and amplifying the risk of impurity-triggered dislocations.

6. Thermal Shock During Tail Growth and Final Separation from the Melt

Upon completion of constant-diameter growth, immediate separation of the crystal from the melt generates extreme thermal stress that produces dense dislocations and slip lines at the crystal tail, propagating upward for a distance equivalent to one full crystal diameter.

For this reason, a dedicated Tail Growth procedure is mandatory: crystal diameter must be gradually reduced to a fine point prior to melt separation. Tail growth minimizes the effective cross-sectional area subjected to thermal stress at the crystal tail, drastically suppressing dislocation formation.

Nevertheless, tail growth itself carries significant process risks. Improper pull speed or temperature control during tapering introduces thermal shock at the junction between the main crystal body and tail section, nucleating dislocations. For large-diameter ingots (e.g., 18-inch crystals), excessively fast pull rates after melt separation drastically increase final dislocation density.

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