Rhombohedron Epitaxial Growth with Molten Target Sputtering (MTS) at 500°C Substrate Temperature
Materials and Coatings
Rhombohedron Epitaxial Growth with Molten Target Sputtering (MTS) at 500°C Substrate Temperature (LAR-TOPS-275)
A new method to grow epitaxial Si1-xGex single crystal on c-plane sapphire substrates by combining the benefits of sputtering and evaporation systems
Overview
This NASA invention applies to the field of sputtering, depositing SiGe thin films on sapphire substrates. It is a method of modifying the film growth front, enabling epitaxially grown heterostructure devices at lower temperatures and with higher purity, avoiding the conventional time required (2 hours) and high energy consumption thermal soaking at a high temperature (800°C.) By enhancing the kinetic energy of ionized molecules and high flux density, the desired thin film growth using Molten Target Sputtering (MTS) is achieved. Less than 500°C is enough to grow the single crystal Si1-xGex (x=0.85) film because much higher kinetic energy molecules reach the substrate.
The Technology
The Molten Target Sputtering (MTS) method can increase the kinetic energy, the energy latency, and the flux density of sputtered atoms by combining the benefits of both magnetron sputtering and evaporation systems. It does this by a clever, but simple mechanical modification to the magnetron sputtering gun. For the MTS method, only a 1~2mm (depth and width) ring-shape groove is cut between the magnets in order to keep a gap between the copper plate and the sputtering target, as below. This effectively increases the efficiency of magnetron based sputtering machines. The ring enhances the magnetic field intensity and increases the temperature of the target material.
A key difference in design between a conventional sputtering gun and the MTS gun seems very minor, but the ring-shape groove between magnets allows for trapping a portion of magnetic field within the groove. A trapped field creates an additive force to expel the ionized particles (atoms and molecules combined within plasma) by increasing their kinetic energy. The ring groove also serves to increase the target material temperature because the conduction passage of thermal energy to the water-cooled copper sink is interrupted by the empty space of a ring groove below the target. Accordingly, the target surface material is heated to a higher temperature, which in turn increases the flux density by more easily liberating the atoms from the target.
Benefits
- No thermal shadow of the film (minimal thermal gradient) leads to higher device yields
- High crystalline Si1-xGex growth on c-plane sapphire substrate
- Possible to apply to epitaxial growth of group IV semiconductor alloys
- Thick Si1-xGex film (>100m) growth for thermoelectric applications
- Product quality increases and energy usage decreases
- Less energy consumption process (500°C substrate temperature v. 800°C)
Applications
- Major application segments for PVD equipment include microelectronics, storage, solar applications, cutting tools and others such as industrial, optics and packaging. Specific examples are: Specialized equipment like roll to roll, in-line glass and semiconductor production
- Solar cell production
- Thermo-electric generators
- Thin-film coatings used for: - MEMS devices - Roll-to-roll gravure system - Transparent conductive oxide film with high transmittance (at IR and visible region), wide band-gap, and low resistivity
- New epitaxial single crystal thin or thick film growth for next generation semiconductor devices
Technology Details
Materials and Coatings
LAR-TOPS-275
LAR-18730-1
Patent Pending
Similar Results
Single Crystal SiGe/Sapphire Epitaxy
This innovation is based on a new fabrication method that alleviates the thermal loading requirement of the substrate, which previously required surface temperatures within the range of 850 to 900C. Our method employs a new thermal loading requirement of sapphire substrate for growing single crystal SiGe on sapphire substrate, in the range of 450 to 500C. SiGe/sapphire wafers produced via this process show a high reflectivity without the discoloration that appears in low quality films.
Epitaxy of SiGe and Other Compound Semiconductors
Several of the patented methods included in this suite of technologies enable super-hetero-epitaxy of rhombohedral/cubic compound semiconductors on specially oriented trigonal (e.g. sapphire) or hexagonal (e.g. quartz) crystal wafer substrates. This includes alignment of the growth crystal lattice with the underlying substrate lattice to minimize misfit strain-induced dislocation defects in the growing crystal. Thus thicker, defect-free crystal layers can be made. Rhombohedral/Cubic crystal twin defects which is 60 degree rotated on [111] orientation in a rhombohedral/cubic SiGe layer structure can be reduced to well less than 1% by volume, essentially providing a defect-free semiconductor material. Alternately, engineered lattice structures with a high degree of twinning can provide SiGe with improved thermoelectric properties due to the phonon scattering that inhibits thermal conduction without compromising electrical conductivity. Additional patented technologies in this suite provide for physical vapor deposition (PVD) growth methods utilizing molten sputtering targets and thermal control of heated substrates, including electron beam heating, in order to give the atoms in the sputtered vapor or on the substrate surface the energy needed for the desired crystal growth.
The remaining patented technologies enable x-ray diffraction methods for detecting and mapping crystal twin defects across the entire as-grown semiconductor layer. These defects are critical to the performance of any semiconductor device manufactured from such compound semiconductor materials.
High Mobility Transport Layer Structures for Rhombohedral Si/Ge/SiGe Devices
Performance of solar cells and other electronic devices such as transistors can be improved greatly if carrier mobility is increased. Si and Ge have Type-II bandgap alignment in cubically strained and relaxed layers. Quantum well and super lattice with Si, Ge, and SiGe have been good noble structures to build high electron mobility layer and high hole mobility layers. However, the atomic lattice constant of Ge is bigger than that of Si and direct epitaxial growth generates large density of misfit dislocations which decrease carrier mobility and shorten device life time. So it required special buffer layers such as super lattice or gradient indexed layers to grow Ge on Si wafers or Si on Ge wafers. The growth of these buffer layers takes extra effort and time such as post-annealing process to remove dislocations by dislocation gliding inside buffer layer.
This invention is a fabrication method for high mobility layer structures of rhombohedrally aligned SiGe on a trigonal substrate. The invention utilizes C-plane (0001) Sapphire which has a triangle plane, and a Si (Ge) (C) (111) crystal or an alloy of group TV semiconductor (111) crystal grown on the Sapphire.
Electron Beam Heating and Atomic Restructuring of Sapphire Surfaces
This process utilizes an electron beam flood gun to irradiate the sapphire surface as a means of raising the surface temperature. As the electrons collide with the top layers of surface atoms, the substrate absorbs much of the energy through thermalization losses from the electrons. Subsequently, the surface temperature rises and this modifies the atomic surface structure into a form conducive to single crystal SiGe epitaxy. Moreover, while tested on sapphire, this method applies to any other wafer material, providing a broad new means of changing surface temperature and atomic structure independently of the substrate heater.
Double Sided Si(Ge)/Sapphire/III-Nitride Hybrid Structures
III-nitride devices are commonly made on sapphire substrates today for various commercial electronic and optoelectronic applications. Thus, this innovation relates directly to the combination of devices on opposite sides of the sapphire substrate. One possible device combination is to have LEDs one side and solar cells on the other, such as for displays.
