L. Breaux

Homoepitaxial films grown on Si (100) at 150°C by remote plasma-enhanced chemical vapor deposition

Low‐temperature silicon epitaxy is critical for future generation ultralarge scale integrated circuits and silicon‐based heterostructures. Remote plasma‐enhanced chemical vapor deposition has been applied to achieve silicon homoepitaxy at temperatures as low as 150 °C, which is believed to be the lowest temperature reported to date. Critical to the process are an in situ remote plasma hydrogen cleaning of the substrate surface in an ultrahigh vacuum growth chamber prior to epitaxy, and substitution of thermal energy by remote plasma excitation via argon metastables and energetic electrons to dissociate silane and increase adatom mobility on the surface of the silicon substrate. Excellent crystallinity with very few defects such as dislocations and stacking faults is observed.

Very low defect remote hydrogen plasma clean of Si (100) for homoepitaxy

We discuss a remote hydrogen plasma cleaning technique which is effective at removing carbon, nitrogen and oxygen from a silicon surface that has been pretreated with a wet chemical clean and a final dilute HF dip. It has been found that for best results, air exposure of the wafer after the HF dip must be minimized, as the ability of the H plasma clean to remove oxygen seems to be reduced on wafers which have been exposed to air for several hours. The atomic H supplied by the clean also results in different hydrogen terminated surface reconstructions depending on the substrate temperature. We observe a (1 × 1) RHEED pattern at clean temperatures below 190° C which is believed to be due to disordered silicon monohydride and dihydride termination, a (3 × 1) pattern between 200 and 280° C, corresponding to ordered monohydride and dihydride coverage, transitioning to a (2 × 1) pattern around 300° C due to monohydride termination. A dilute HF dip produces a (1 × 1) pattern due to disordered (3 × 1) cells,i.e. disordered monohydride and dihydride coverage. The passivating effects of the dilute HF dip and of the remote hydrogen plasma clean have also been discussed. The two processes have been found to result in similar hydrogen coverage of the wafer surface which in turn results in similar surface passivation. Both the dilute HF dip and thein situ remote H plasma clean have been found to protect the wafer from contamination for up to 15 min in ambient air and for an indefinite period under vacuum.

Remote plasma-enhanced CVD of silicon: reaction kinetics as a function of growth parameters

The reaction kinetics in Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) have been studied for a chamber pressure of 200 mTorr, rf powers between 4 and 8 W, diluted silane flow rates between 5 and 40 sccm, and temperatures between 190 and 480° C. The observed temperature dependence of growth rate reveals a change in activation energy at 300–325° C, suggesting that hydrogen desorption is the rate limiting step in the deposition reaction. A strong dependence of growth rate on rf power has been attributed in part to the extension of the glow discharge region closer to the substrate at higher rf powers. Growth rate has been shown to increase when the sample is positioned closer to the glow, indicating that the reaction precursor is a short-lived species, probably SiH2 or SiH3. Growth rate has been shown to exhibit a sublinear dependence on silane partial pressure. Oxygen incorporation in the deposited films has been studied using Secondary Ion Mass Spectroscopy (SIMS), and it has been found that the main source of oxygen contamination is the process gases. However, it has also been found that “point of use” purification of the process gases reduces water and oxygen contamination significantly, reducing the oxygen incorporation in the films by an order of magnitude.

Defect microstructure in low temperature epitaxial silicon grown by RPCVD

Defect characterization of epitaxial silicon films grown by low temperature remote plasmaenhanced chemical vapor deposition (RPCVD) under various conditions is discussed. The film morphology and crystallinity have been examined by defect etching/Nomarski optical microscopy and transmission electron microscopy. Prior to epitaxial growth, anex situ wet chemical clean and anin situ remote hydrogen plasma clean were performed to remove the native oxide as well as other surface contaminants such as carbon. A damage-free (100) Si surface with extremely low concentrations of carbon and oxygen as confirmed byin situ Auger electron spectroscopy can be achieved using this cleaning technique at temperatures as low as 250°. Low temperature Si homoepitaxy was achieved by RPCVD on lightly doped (100) Si substrates. Growth parameters such as silane flow rate (partial pressure), chamber pressure, and substrate temperature were varied during epitaxial growth to investigate the dependence of film quality on these parameters. For comparison,in situ remote hydrogen plasma and epitaxial growth were also performed on heavily dopedp-type (100) Si substrates. Finally, the results of epitaxial growth at temperatures as low as 150° are presented.

Defect microstructure in single crystal silicon thin films grown at 150°C–350°C by the remote plasma-enhanced chemical vapor deposition

Defect microstructure in terms of defect density and impurity concentration of epitaxial Si films grown by low temperature Remote Plasma-enchanced Chemical Vapor Deposition (RPCVD) in the temperature range of 150–305° C has been investigated using Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectroscopy (SIMS), Reflection High Energy Electron Diffraction (RHEED) and defect etching/Nomarski microscopy. Defect density in the epitaxial Si films is found to be a strong function of growth temperature in the temperature range under study, indicating that thermal excitation is an important source of energy, in addition to plasma excitation, for driving surface reactions in the RPCVD expitaxial process. Impurity concentrations of H, O and C in the epitaxial films have been determined by SIMS analysis. The trace amounts (∼1 ppm) of oxygen and water vapor in the reactant gas (2%SiH4/He) was identified to be an important source of oxygen in the epitaxial Si films. An oxygen concentration as low as 3 × 1018 cm-3 in the epitaxial Si film grown at 150° C has been achieved through the use of a gas purifier. The higher hydrogen concentration in the films grown at lower temperatures is believed to be due to insufficiently rapid hydrogen desorption from the surface during growth. The results of characterization using TEM and SIMS are discussed to elucidate the atomistic mechanisms of Si epitaxial growth by RPCVD.

The Use of Langmuir Probe Measurements to Study Reaction Kinetics in Remote Plasma-Enhanced Chemical Vapor Deposition of Silicon

The reaction kinetics of Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) have been studied using Langmuir probe measurements of plasma density and plasma potential combined with growth rate data as a function of r-f power, chamber pressure, and substrate bias. An observed increase in growth rate for negative substrate bias suggests that argon ions drive the reaction. The variation of ion density, and plasma potential with r-f power suggests that the flux of argon ions, not their kinetic energy is responsible for increased growth rates at higher r-f powers.

Low-temperature Si in-situ cleaning and homoepitaxy by remote plasma-enhanced chemical vapor deposition

The surface structure of the Si(100) substrate after an in situ remote hydrogen plasma clean, and the defect microstructure of epitaxial Si films grown by Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) in the temperature range of 15O°C-305°C are discussed. Auger Electron Spectroscopy (AES) analysis has been employed to examine the capability of this remote hydrogen plasma clean in terms of removing surface contaminants. Reflection High Energy Electron Diffraction (RHEED) and Transmission Electron Microscopy (TEM) have been utilized to investigate the surface structure in terms of crystallinity and defect generation. A remote hydrogen plasma clean which reduces carbon, oxygen and nitrogen contamination levels to below the detectability of AES analysis, while maintaining a defect-free surface, as indicated by TEM, has been developed. However, excessive plasma power causes defect generation on the Si surface, and the size of the defects is a function of substrate temperature during cleaning in the range of room temperature-305°C. Subsequently, epitaxial Si films grown by RPCVD at various temperatures (150°C-305°C) after optimal remote hydrogen plasma clean have been investigated in terms of defect microstructure and impurity content using RHEED, TEM and Secondary Ion Mass Spectroscopy (SIMS). Epitaxial Si films with very low defect density (4O6 cm2 or less) and low oxygen content (-3x1018 cm3) have been achieved by RPCVD.