The present invention relates to producing large grain or single crystal films such as Magnesium Oxide (MgO) films.
Magnesium oxide (MgO) thin films have assumed significant importance in recent times due to two major applications, namely, as a protective layer on glass in plasma display panels and as an intermediate buffer layer between a semiconductor substrate (e.g. Si, GaAs) and a ferroelectric film (e.g. PbTiO3) in oxide-based devices on semiconductors. Even more recently it has been discovered that MgO films can be of great value in the fabrication of superconductors and photovoltaic devices. For example, crystalline MgO films can be used as a buffer layer for depositing silicon on inexpensive substrates such as soda-lime glass. While methods such as Sol-Gel and Aluminum Acetylacetonate have been employed to fabricate crystalline films, the grain size of such films has been small. It has therefore been desirable to find a method for depositing large grain to single crystal MgO films for the fabrication of various devices for various electronics, solar, and microelectronic applications. Techniques such as Ion Beam Assisted Deposition have led to textured MgO films of high quality, yet IBAD is an expensive procedure and the grains are neither large nor single crystalline. Inclined Substrate Deposition (IDS) is also commonly used to deposit MgO on glass and while it is textured it is neither large grain or single crystalline. As of the date of this disclosure, no method exists for depositing large grain or single crystalline MgO films on inexpensive substrates; and no method exists that can do so in a way that is cost effective.
Accordingly what is desired is to produce large grain or single crystal films. What is also desired is producing large grain or single crystal films at low temperature, i.e. below the softening temperature of soda-lime glass. It is also desired to obtain large grain or single crystal films on sodalime glass. It is yet another object of this invention to provide large grain or single crystal films in a cost-effective manner.
The present invention provide a method for manufacturing film coating technology using electron beam evaporation technique for the fabrication of high quality templates on glass. This method enables the manufacturing of solar cell or similar electronic and photonic applications. This fabrication technique and optimized process conditions enable rapid development of solar cell substrate for addressing better performance of solar cells.
A method for growing crystalline and preferred oriented films such as but not limited to MgO is provided. Here films of MgO are deposited on substrate such as glass via an electron beam evaporation system. The glass is held in place and at a specific growth temperature to control growth kinetics. Suitable temperatures may range from 300° C. to 600° C. A distance of six to nine inches is held between the substrate and an evaporation source. The evaporation source in one embodiment may be highly dense Mg0 crystal. The deposition rate of the evaporated film on the substrate may be monitored for example by a quartz crystal thickness monitor.
The system that may be used in growing the films may be a vacuum system that includes an electron bean evaporation system. In one embodiment, the vacuum system or chamber is evacuated using a cryopump (1000 l/s) backed by a dry scroll pump. Conditions for evaporating the evaporation source may include having a base pressure in the vacuum chamber of the system be better than 1×10−7 Torr. The oxygen pressure in the chamber may be better than 10−4 Torr.
The vacuum system will have the capability of holding the substrate at the desired specific growth temperature and the desired distance from the evaporation source. In one embodiment, the chamber of the vacuum system can hold 2″ and 4″ wafers on versatile heating stage and could be scaled up for 6″ to 12″ dia. solar cells application. The heating stage in the system can be programmed for substrate temperatures of up to 800° C. covering the temperature range for thin film deposition as well as in-situ annealing, if necessary. The stage can also take smaller samples using mechanical clamping. A thin-film thickness monitor and controller, such as those made by Inficon, Inc., may be used to monitor and control the deposition rate. The thickness of deposited films can be controlled by interfacing thin-film thickness monitor and controller with an e-gun power supply. By using a remote control on the thickness controller, the emission current of the electron gun can be controlled as per the desired coating thickness. After deposition samples are cooled at 30° C./min rate. Films resulting from the process are highly oriented along (111) normal to the substrate surface.
The vacuum system BWI-1000 has a main deposition chamber made from stainless steel equipped with a high power electron gun_of 10 Kwatt. The e-gun is used to deposit different materials including refractory metals and oxides at ease. In one embodiment, the e-gun has four pockets each handling 7 cc volume. See
A permanent magnet is employed to bend and guide the e-beam in a circular path from the filament to the source. Use of longitudinal and transverse sweeping coils makes it possible to raster the beam on the target. The equipment is capable of different predefined sweeping patterns. In one embodiment, the capabilities and other details of the e-gun and the power supply may follow the specifications of an e-gun produced by Telemark, Inc. For instance, the specifications may include an E-Beam Deflection of 270°, a maximum high voltage of −10 kV, lateral coil resistance of 8.5 ohms, longitudinal coil resistance of 7.5 ohms, filament power of 600 watts maximum and water cooling requirements of 2 gpm (min.) at 60° F. An exemplary gun power supply may be a power supply model TT-10. Specifications for power supply may include a maximum output power of 10 kW (1.0 A@10kV), HV (adjustable) of 0 to −10 kV, process control voltage of 0 to +/−10 VDC, current cutback (adj.) of 0.2 to 1 amp, response time of <100 microseconds, gun filament of 5.5 V @ 36 A, a Beam Sweep Lateral Longitudinal of +/−1.5 amp@3W, 3-90 Hz +/−2.0 amps, and cooling by Air.
The material being evaporated is placed in a crucible which occupies about 80% of the crucible volume. The crucibles are water cooled to reduce the heating effects. Additionally, crucible inserts are used to reduce heat flow during deposition thereby allowing higher evaporation rates. The use of appropriate hearth liner also enables a quick change of evaporants with the least contamination levels. Material selection of the hearth is made based on the material being evaporated. The liners commonly available are made up of carbon, tungsten, molybdenum or tantalum.
A substrate is held in a e-beam evaporation system with a source of MgO being coated at 7 μm on a 3.2 mm thick soda-lime glass. Two samples were prepared one at 450° C. and the other at 550° C. both at temperatures below the softening point of ordinary soda-lime glass. on the substrate using an e-gun using the method described above. The result is MgO coatings characterized by x-ray diffraction and scanning electron microscopy and electrical resistivity.
X-ray diffraction results indicated that the coatings fabricated are highly oriented along (111) and thus are highly crystalline. See
While the present invention has been described in conjunction with specific embodiments, those of normal skill in the art will appreciate the modifications and variations can be made without departing from the scope and the spirit of the present invention. Such modifications and variations are envisioned to be within the scope of the appended claims.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/879,275, filed Sep. 18, 2013, entitled “Methods of Depositing Magnesium Oxide (MgO) Films,” which is hereby incorporated by reference in its entirety.
Number | Date | Country | |
---|---|---|---|
61879275 | Sep 2013 | US |