Polysilazane Silicon Oxynitride Precursor: Comprehensive Analysis Of Molecular Design, Synthesis Routes, And Advanced Deposition Technologies For High-Performance Dielectric Films

Molecular Composition And Structural Characteristics Of Polysilazane Silicon Oxynitride Precursor Compounds

Polysilazane silicon oxynitride precursor molecules are characterized by Si–N backbone structures with tunable organic substituents and reactive functional groups that govern their vapor-phase reactivity and film-forming behavior. The fundamental repeating units in polysilazanes include [SiH₂]ₙNH– and [SiH₂]ₘO– segments, which upon thermal or plasma-assisted decomposition yield silicon oxynitride phases containing both Si–N and Si–O bonds1. The molecular design of these precursors directly influences the nitrogen-to-oxygen ratio (N:O) in the deposited films, a parameter critical for controlling dielectric constant (typically 4.5–6.5 for SiOxNy) and etch selectivity relative to SiO₂ or Si₃N₄11.

Functionalized cyclosilazanes, as disclosed in recent patent literature, incorporate organoamino groups (–NR₁R₂) attached to silicon centers, enabling high growth rates (>1 Å/cycle) in ALD processes while maintaining low impurity levels2. The general structural formula for these precursors can be represented as cyclic or linear silazanes with the motif (R₂Si–NR')ₙ, where R and R' are independently selected from hydrogen, C₁–C₆ alkyl, vinyl, allyl, or phenyl groups6,10. For instance, hexakis(monohydrocarbylamino)disilanes with the formula (RNH–)₃Si–Si(–NHR)₃ (where R = C₁–C₄ alkyl) have been synthesized by reacting hexachlorodisilane (Cl₃Si–SiCl₃) with excess monohydrocarbylamine, yielding precursors with six reactive amino sites per molecule9. This high degree of functionalization enhances precursor reactivity with co-reactants such as NH₃, O₂, or N₂O during CVD, facilitating conformal coating of high-aspect-ratio (>60:1) structures11.

Hydrazinosilanes represent another important subclass of polysilazane silicon oxynitride precursor, with the general formula [R₁₂N–NH]ₙSi(R₂)₄₋ₙ (n = 1–4), where R₁ = C₁–C₆ alkyl and R₂ = H, alkyl, vinyl, allyl, or phenyl6,10. These compounds are prepared by reacting dichlorosilanes (R₂SiCl₂) with hydrazines (R₁₂N–NH₂), offering advantages such as elimination of ammonium chloride by-products (which can clog reactor lines) and compatibility with low-temperature (<400°C) deposition processes3,4. The N–N bond in hydrazinosilanes provides an additional nitrogen source during film growth, enabling fine-tuning of the N:O ratio in silicon oxynitride films without requiring separate nitrogen-containing co-reactants6.

Polysiloxazanes—hybrid structures containing both Si–N and Si–O linkages in the precursor molecule—are synthesized by co-reacting dihalosilanes with ammonia and water vapor or oxygen, yielding polymers with [SiH₂]ₙNH– and [SiH₂]ₘO– repeating units1. These precursors are particularly suited for producing silicon oxynitride fibers and bulk ceramic shapes, as they can be melt-spun or solution-cast prior to pyrolysis at 800–1200°C in inert or oxidizing atmospheres1,5. The resulting silicon oxynitride materials exhibit nitrogen contents ≥5 mol% and oxygen contents ≥5 mol%, with microstructures ranging from amorphous to nanocrystalline depending on pyrolysis conditions1.

The molecular weight of polysilazane silicon oxynitride precursor polymers typically ranges from 200 to 100,000 Da, with optimal processability achieved in the 1,000–20,000 Da range due to favorable viscosity and volatility characteristics8. Lower-molecular-weight oligomers (200–1,000 Da) are preferred for vapor-phase deposition techniques (CVD, ALD), while higher-molecular-weight polymers (10,000–100,000 Da) are used in solution-based coating and fiber-spinning applications8,16. The degree of polymerization is controlled by reaction stoichiometry, catalyst selection (e.g., basic catalysts for dehydrogenation-driven crosslinking), and thermal treatment conditions during synthesis8,12.

Precursors And Synthesis Routes For Polysilazane Silicon Oxynitride Precursor Materials

Synthesis Of Cyclosilazane And Disilazane Precursors

Cyclosilazane precursors are synthesized via ring-closure reactions of chlorosilanes with ammonia or primary amines, followed by dehydrohalogenation. A representative synthesis involves reacting Si,Si'-diorganyl-N-alkyltetrachlorodisilazanes (RSiCl₂–NR'–SiCl₂R) with excess ammonia in aprotic solvents such as toluene or THF at temperatures of 0–50°C12. The reaction proceeds through nucleophilic substitution of chlorine atoms by amino groups, with concomitant elimination of ammonium chloride (NH₄Cl) as a by-product12. Subsequent dehydrogenation in the presence of strong Lewis bases (e.g., n-butyllithium, sodium hydride) at 50–150°C induces crosslinking between adjacent nitrogen atoms, forming cyclic or polymeric silazane structures with enhanced thermal stability8,12.

For example, the synthesis of hexakis(ethylamino)disilane—a key polysilazane silicon oxynitride precursor—is achieved by reacting hexachlorodisilane with at least 6-fold molar excess of ethylamine (C₂H₅NH₂) in anhydrous toluene at 25°C for 12–24 hours9. The product is isolated by vacuum distillation (boiling point ~180°C at 1 Torr) and exhibits high purity (>99.5% by GC-MS) with chloride content <50 ppm9. This precursor demonstrates excellent thermal stability (decomposition onset >250°C under inert atmosphere) and high reactivity with oxygen-containing co-reactants, making it suitable for low-temperature (<350°C) CVD of silicon oxynitride films9.

Hydrazinosilane Precursor Synthesis

Hydrazinosilanes are prepared by reacting dichlorosilanes with substituted hydrazines (R₁₂N–NH₂) in molar ratios of 1:2 to 1:4 (silane:hydrazine) in aprotic solvents at temperatures of −10°C to 25°C3,6,10. The reaction mechanism involves nucleophilic attack of the hydrazine nitrogen on the silicon center, displacing chloride ions and forming Si–N–N linkages3. A specific example is the synthesis of bis(dimethylhydrazino)dimethylsilane [(Me₂N–NH)₂SiMe₂], prepared by adding 1,1-dimethylhydrazine (Me₂N–NH₂) dropwise to a solution of dichlorodimethylsilane (Me₂SiCl₂) in hexane at 0°C, followed by stirring for 4 hours and removal of precipitated dimethylammonium chloride by filtration6. The product is purified by fractional distillation under reduced pressure (boiling point 68–70°C at 10 Torr) and characterized by ¹H NMR, ¹³C NMR, and ²⁹Si NMR spectroscopy6.

Hydrazinosilane precursors offer the advantage of halogen-free by-products (only volatile amines are released during film deposition), eliminating the need for corrosive HCl scrubbing systems in CVD reactors3,4. Additionally, the N–N bond energy (~160 kJ/mol) is lower than the Si–N bond energy (~355 kJ/mol), facilitating selective bond cleavage during thermal decomposition and enabling precise control over nitrogen incorporation in silicon oxynitride films3.

Polysiloxazane Synthesis Via Co-Reaction Of Dihalosilanes With Ammonia And Oxygen Sources

Polysiloxazanes—precursors containing both Si–N and Si–O linkages—are synthesized by simultaneously reacting dihalosilanes (e.g., dichlorosilane, H₂SiCl₂) with ammonia and water vapor or molecular oxygen in a controlled atmosphere1,5. A typical synthesis protocol involves bubbling ammonia gas (flow rate 100–500 sccm) and water vapor (partial pressure 10–50 Torr) through a solution of dichlorosilane in anhydrous toluene at 50–80°C for 6–12 hours1. The molar ratio of NH₃:H₂O:SiCl₂ is maintained at 4–6:1–2:1 to achieve balanced incorporation of nitrogen and oxygen in the polymer backbone1. The resulting polysiloxazane is isolated by solvent evaporation under vacuum and characterized by FTIR (Si–N stretch at 850–950 cm⁻¹, Si–O stretch at 1000–1100 cm⁻¹) and elemental analysis (typical composition: Si 35–45 wt%, N 15–25 wt%, O 10–20 wt%, H 10–15 wt%)1.

An alternative synthesis route involves reacting silazane oligomers (obtained from dihalosilane + ammonia) with controlled amounts of oxygen or ozone at elevated temperatures (150–250°C), inducing partial oxidation of Si–N bonds to Si–O bonds while preserving the polymer structure5. This method allows precise tuning of the N:O ratio in the precursor, which directly translates to the N:O ratio in the final silicon oxynitride film after pyrolysis5. For instance, treating a polysilazane with 10 mol% O₂ in nitrogen at 200°C for 2 hours yields a polysiloxazane with N:O atomic ratio of ~1.5:1, suitable for depositing silicon oxynitride films with dielectric constant ~5.55.

Purification And Stabilization Of Polysilazane Silicon Oxynitride Precursor Compounds

High-purity polysilazane silicon oxynitride precursor materials are essential for semiconductor applications, where metal impurities (Fe, Cu, Na, K) must be maintained below 1 ppm and chloride content below 50 ppm to prevent device degradation4,9. Purification is typically achieved by fractional distillation under high vacuum (0.1–10 Torr) using packed columns with 20–40 theoretical plates, enabling separation of precursor compounds from unreacted starting materials and low-boiling by-products6,9. For thermally sensitive precursors, sublimation at reduced pressure (0.01–0.1 Torr) and temperatures 50–100°C below the decomposition onset is employed6.

Stabilization of polysilazane silicon oxynitride precursor solutions against premature polymerization or hydrolysis is achieved by adding trace amounts (0.01–0.5 wt%) of inhibitors such as phenothiazine, hydroquinone, or tertiary amines (e.g., triethylamine)15. These additives scavenge trace moisture and oxygen, extending the shelf life of precursor formulations from weeks to months when stored under inert atmosphere at 5–25°C15. For liquid precursors used in ALD or pulsed-CVD, the vapor pressure at delivery temperature (typically 40–80°C) should be in the range of 0.1–10 Torr to ensure stable mass flow rates; this is achieved by selecting precursors with boiling points 20–50°C above the delivery temperature2,15.

Chemical Vapor Deposition (CVD) And Atomic Layer Deposition (ALD) Processes Using Polysilazane Silicon Oxynitride Precursor

Thermal CVD Of Silicon Oxynitride Films

Thermal CVD using polysilazane silicon oxynitride precursor compounds is conducted in hot-wall or cold-wall reactors at substrate temperatures of 300–600°C and chamber pressures of 0.1–10 Torr4,9. The precursor is delivered to the reactor either as a neat liquid via direct liquid injection (DLI) or as a vapor by bubbling carrier gas (N₂, Ar, or He at 50–500 sccm) through a heated precursor reservoir maintained at 40–100°C4,9. Co-reactants such as ammonia (NH₃, 10–200 sccm), nitrous oxide (N₂O, 50–500 sccm), or molecular oxygen (O₂, 10–100 sccm) are introduced separately to control the nitrogen and oxygen content in the deposited film4,9.

For example, silicon oxynitride films with composition SiO₁.₂N₀.₆ (N:O atomic ratio ~1:2) and thickness uniformity <3% across 300 mm wafers are deposited using hexakis(ethylamino)disilane precursor at 400°C substrate temperature, 1 Torr chamber pressure, precursor flow rate of 50 mg/min, and N₂O flow rate of 200 sccm9. The deposition rate under these conditions is 15–25 nm/min, with film density of 2.6–2.8 g/cm³ (measured by X-ray reflectometry) and refractive index of 1.65–1.75 at 633 nm (measured by ellipsometry)9. FTIR analysis reveals strong Si–O–N absorption at 900–950 cm⁻¹ and Si–O absorption at 1050–1100 cm⁻¹, confirming the oxynitride composition9.

The use of hydrazinosilane precursors in thermal CVD eliminates the formation of corrosive HCl by-products, as the only volatile species released during precursor decomposition are low-molecular-weight amines (e.g., dimethylamine, ethylamine) which are easily removed by vacuum pumping3,4. This "clean" decomposition pathway reduces particle contamination in the reactor and extends the maintenance interval for chamber components4. Additionally, hydrazinosilane-based processes exhibit excellent step coverage (>95%) on trenches with aspect ratios up to 10:1, attributed to the high surface mobility of precursor fragments at typical deposition temperatures3.

Plasma-Enhanced CVD (PECVD) For Low-Temperature Silicon Oxynitride Deposition

PECVD using polysilazane silicon oxynitride precursor enables film deposition at substrate temperatures as low as 150–350°C, making it compatible with temperature-sensitive substrates such as organic semiconductors, flexible polymers, and BEOL metallization layers2,11. The plasma is generated by applying RF power (13.56 MHz, 50–500 W) or microwave power (2.45 GHz, 500–3000 W) to the process gas mixture, creating reactive radicals and ions that enhance precursor dissociation and film growth kinetics2,[11