【Knowledge Discover】Front-End Process of Chip Manufacturing — Epitaxial Growth

Chip Manufacturing Epitaxy Growth

      Epitaxial growth is one of the key technologies in crystal growth and semiconductor manufacturing. Its theoretical foundation and process development are fundamentally centered on thermodynamic equilibrium and interface kinetic control.

      From a thermodynamic perspective, the growth mode of epitaxy is determined by the free-energy relationship among the epitaxial layer, interface, and substrate. When the sum of the epitaxial surface free energy and interface free energy is lower than the substrate surface free energy, the system tends to form a wetting state, resulting in two-dimensional layer-by-layer growth (Frank–van der Merwe mode). Conversely, when the former is significantly higher than the latter, three-dimensional island nucleation (Volmer–Weber mode) becomes dominant. Between these two extremes lies the Stranski–Krastanov (S-K) growth mode, in which several monolayers first grow in a two-dimensional manner before strain accumulation induces the formation of three-dimensional islands.

      These thermodynamic criteria apply not only to homoepitaxial systems such as Si/Si and GaAs/GaAs, but also play a critical role in heteroepitaxial systems. For example, lattice-matched systems such as AlGaAs/GaAs and InGaP/GaAs can maintain two-dimensional growth through precise control of growth parameters, whereas mismatched systems such as GaN/sapphire and SiC/Si generally require buffer layers or patterned substrates to release strain and suppress defects.

      At the process level, chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) are currently the two major epitaxial growth techniques. CVD relies on chemical reactions of gaseous precursors on the substrate surface to form thin films, and its variant, metal-organic chemical vapor deposition (MOCVD), has become a major industrial route for III–V and nitride semiconductor growth. In contrast, MBE utilizes ultra-high vacuum and low-rate beam deposition to achieve atomic-level control of thickness and composition, making it particularly advantageous for fabricating quantum wells, superlattices, and other low-dimensional structures.

      In recent years, techniques such as migration-enhanced epitaxy (MEE) and atomic layer deposition (ALD) have further improved interface control precision. ALD, based on self-limiting surface reactions, enables atomic-scale deposition and has been widely applied in the fabrication of high-k dielectric films such as HfO₂ and ZrO₂, ensuring excellent thickness uniformity and interface quality.

      To address lattice mismatch and interface compatibility issues in heteroepitaxy, various advanced approaches have been developed. For instance, buffer-layer engineering in GaN-on-sapphire systems can effectively reduce threading dislocation density, while SiGe/Si systems utilize the S-K growth mode to form pseudomorphic strained layers that improve carrier transport properties. In addition, van der Waals epitaxy has opened new pathways for two-dimensional material integration. Materials such as MoS₂ can be grown on SiO₂/Si substrates through weak interfacial coupling, partially bypassing conventional lattice-matching constraints. Meanwhile, the development of ultra-high-vacuum CVD and gas-source MBE systems has further enhanced strain control capability and interface cleanliness.

Molecular Beam Epitaxy

      Molecular beam epitaxy (MBE) is a precision epitaxial technique operated under ultra-high vacuum conditions. Its core principle involves generating atomic or molecular beams from evaporation sources and depositing them onto a heated substrate surface to achieve controllable single-crystal thin-film growth.

      During growth, the mean free path of atoms and molecules is much larger than the chamber dimensions, allowing particles to travel ballistically without significant gas-phase reactions. Film formation is therefore governed mainly by surface kinetic processes. Incident particles undergo a sequence of surface diffusion, re-evaporation, two-dimensional cluster formation, step-edge incorporation, and step-flow migration. The efficiency of these processes strongly depends on factors such as particle species, beam flux, substrate temperature, and substrate surface orientation. For example, low-vapor-pressure elements such as Si and Ge generally exhibit sticking coefficients close to unity and can therefore be deposited efficiently, whereas high-vapor-pressure elements such as Al and Ga usually require excess supply to compensate for their relatively low sticking coefficients.

      MBE systems are built around ultra-high vacuum (UHV) environments, typically achieving base pressures on the order of 10⁻¹¹ Torr. Residual gases are mainly hydrogen, while vacuum conditions are maintained using combinations of diffusion pumps, turbomolecular pumps, cryopumps, and titanium sublimation ion pumps. Liquid-nitrogen cooling panels surrounding the growth chamber and source flanges help suppress re-evaporation and improve thermal isolation.

      The system is generally constructed from stainless steel and integrates load-lock chambers, transfer chambers, analysis/processing chambers, and growth chambers. Substrate rotation mechanisms are employed to improve thickness and composition uniformity. Flux stability requirements are typically maintained within 1% during operation and within 5% day-to-day, corresponding to temperature control precision better than ±1% at around 1000°C.

      Modern MBE systems have evolved from research-oriented platforms to multi-wafer production tools. For example, the Riber 7000 system can accommodate 13 evaporation sources and supports batch processing of 70 six-inch wafers or 140 four-inch wafers, with annual production capacities reaching 24,000 six-inch wafers. Such systems are supplied by companies including Riber and Veeco Instruments.

      One of the major advantages of MBE is its capability for in situ characterization. Reflection high-energy electron diffraction (RHEED) employs a 20 keV electron beam incident at a grazing angle of approximately 1°–3°, generating diffraction patterns on a fluorescent screen that enable real-time monitoring of surface reconstruction, oxide desorption, and growth mode transitions. Two-dimensional growth typically produces streak patterns, whereas three-dimensional growth leads to spotty diffraction features. RHEED is also widely used to calibrate growth rates and alloy compositions.

      In addition, residual gas analyzers (RGA) can detect vacuum leaks and contamination by analyzing residual gas spectra, while ionization gauges and beam flux monitors (BFM) are commonly used for flux calibration and monitoring. Although MBE equipment remains expensive and requires complex maintenance, recent improvements in automation, transfer systems, and process optimization have significantly reduced operational costs. MBE continues to dominate niche high-quality GaAs-based electronic material markets such as AlGaAs/GaAs systems, and its relatively low process hazard has made it the preferred epitaxial technology for many GaAs foundries.

Chemical Vapor Deposition

      Chemical vapor deposition (CVD) is one of the most important epitaxial growth technologies in semiconductor manufacturing. Growth is generally carried out under atmospheric or near-atmospheric pressure conditions, where gaseous precursors are transported into the reaction zone by carrier gases such as hydrogen or nitrogen and react at elevated temperatures to form thin films on the substrate surface.

      Compared with ultra-high-vacuum techniques such as MBE, CVD involves more complex reaction mechanisms, including gas transport, precursor decomposition, intermediate species formation, and surface adsorption. For example, in silicon epitaxy using SiHCl₃ (TCS) as a precursor, multiple Si-H-Cl intermediate species can form, although the overall reaction can be simplified as the conversion of SiHCl₃ and H₂ into Si and HCl.

      Metal-organic chemical vapor deposition (MOCVD) is an important branch of CVD and is widely used for III–V and nitride semiconductor epitaxy. This technique typically employs metal-organic precursors such as trimethylgallium (TMGa), trimethylindium (TMIn), and trimethylaluminum (TMAl), together with group-V hydrides such as AsH₃, PH₃, and NH₃. Because precursor decomposition behaviors differ significantly, parameters such as temperature, pressure, and V/III ratio must be carefully optimized.

      For example, GaN growth generally requires very high V/III ratios because NH₃ exhibits low thermal decomposition efficiency. High ammonia flow helps reduce carbon contamination and improve crystal quality. The primary reaction may be approximated as the reaction between TMGa and NH₃ to form GaN and CH₄, although the actual process involves complex multistep gas-phase and surface reactions.

      A typical MOCVD system consists of gas delivery modules, reaction chambers, and exhaust treatment systems. Gas lines are usually constructed from semiconductor-grade stainless steel and integrated with mass flow controllers, pressure regulators, and pneumatic valves to ensure stable precursor delivery. Reaction chambers are commonly fabricated from quartz or stainless steel and equipped with resistive, inductive, or lamp-heating systems. Both vertical and horizontal reactor configurations are used to accommodate single-wafer or multi-wafer growth for different wafer sizes.

      The exhaust system generally includes particle filters and toxic gas treatment units to ensure operational safety. For metal-organic precursors such as TMGa, temperature-controlled bubblers are employed to maintain stable vapor pressure and continuous precursor supply.

      In compound semiconductor devices, performance is often directly determined by epitaxial layer quality. Unlike conventional silicon devices, which rely heavily on post-processing steps, GaN-based LEDs, RF devices, and power devices are often regarded as “epitaxy-driven devices,” because active structures such as pn junctions, quantum wells, and functional layers are formed primarily during epitaxial growth. Consequently, defect density, interface quality, and doping control strongly influence device performance.

      In the GaN material system, large lattice mismatch with common substrates such as sapphire historically caused high defect densities and difficulties in achieving p-type doping. During the 1980s and 1990s, Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura achieved major breakthroughs through low-temperature buffer layers, optimized growth conditions, and p-type doping control, ultimately enabling the industrialization of high-brightness blue LEDs. Their contributions were recognized with the 2014 Nobel Prize in Physics 2014.