Abstract:
A method is provided for fabricating a thin film of ferrite material upon a variety of substrates wherein pulsed high power laser light impinges upon a target of ferrite material causing localized vaporization of the ferrite material and subsequent deposition upon a proximate substrate. The rapidity with which the target material is vaporized promotes chemical stability during ferrite film growth. Both the target and the substrate are maintained inside a vacuum chamber, and are maintained substantially within a temperature range of 550°-900° C. The equipment needed to practice the method is relatively inexpensive. Furthermore, the method allows for the large-scale production of high quality ferrite films with preferential magnetic texture, wherein the magnetization of the ferrite material aligns in a preferred direction in the plane of the film.

Description:
This is a continuation-in-part application of U.S. Ser. No. 07/750,626, filed Aug. 27, 1991, now U.S. Pat. No. 5,227,204, issued Jul. 13, 1993. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method for the fabrication of ferrite films. 
     BACKGROUND OF THE INVENTION 
     It is known to fabricate ferrite films using a technique called liquid phase epitaxy. In this technique, a substrate is dipped into a molten liquid flux of ferrite material. However, the substrate must be of high purity and regularity, requiring the use of single crystal material. Also, due to the high cost of the equipment used, this approach is expensive. 
     RF sputtering is an alternative method for fabricating ferrite films wherein a beam of ions, e.g., argon ions, impinge upon a target of ferrite material. Evaporation of ferrite material is thereby induced, being subsequently deposited upon a nearby substrate. However, the yield rate of good quality ferrite films is quite low. 
     Another technique involves spinning a substrate while spraying ferrite particles upon the substrate. Relatively poor quality films result. 
     SUMMARY OF THE INVENTION 
     A method for fabricating a thin film of ferrite material upon a variety of substrates is provided wherein pulsed high power laser light impinges upon a target of ferrite material causing localized vaporization and subsequent deposition upon a proximate substrate. The rapidity with which the target material is vaporized promotes chemical stability during film growth. Useful substrate materials include semiconductors, glasses, ceramics, and metals. Particularly useful substrate materials include magnesium oxide (MgO) and strontium gallium oxide (SrGa 12  O 19 ). Other substrate materials include amorphous quartz (SiO 2 ), glass, and (Gd a  Y b  Ca c  R d ) 3  (Ga e  Al f ) 5  O 12 , where a+b+c+d=1, 0≦a≦1, 0≦ab≦1, 0≦c≦1, 0≦d≦1 and R represents any mixture of any number of rare earth elements, including a mixture with only one rare earth element, where e+f=1 and 0≦e≦1, 0≦f≦1, such as Gd 3  Ga 5  O 12  (GGG), for example. Both the target and the substrate are maintained inside a vacuum chamber, where the pressure is preferably maintained at approximately 100-300 millitorr. Also, the target and the substrate are maintained within a temperature range of 550°-900° C. A heated substrate promotes the growth of hexagonal ferrites, for example, wherein the C-axis of a hexagonal ferrite is oriented perpendicularly to the surface of the film. 
     The method allows the use of practically any type of substrate material, so long as the surface of the substrate is smooth, i.e., with a surface roughness measure of no more than about 100Å. Also, the equipment needed to practice the method is relatively inexpensive. Furthermore, the method allows for the large-scale production of high quality ferrite films with preferential magnetic texture. Such high quality ferrite films have commercial application in microwave devices involving non-reciprocal applications, including isolators, circulators, switches, filters, modulators, and limiters. Additionally, the invention provides a technique for growing hexagonal, spinel and garnet ferrites. 
    
    
     DESCRIPTION OF THE DRAWING 
     The invention will be more fully understood from the following detailed description, in conjunction with the accompanying figures, in which: 
     FIG. 1 is a schematic diagram of a deposition system for practicing the method of the invention; and 
     FIG. 2 is a diagrammatic, exaggerated cut-away side view of a ferrite thin film fabricated on a substrate by the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, a high power pulsed laser 10, such as a LAMBDA PHYSIK Model EMG 103 excimer laser, is directed through a quartz window 11 and towards a flat, cylindrical target pellet 12 of ferrite material to be vaporized. The laser 10 produces a pulse train at a rate of 4 Hz, wherein each pulse imparts 250 mJ of laser energy at a wavelength of 248 nm, and at an energy density of approximately 2 J/cm. In a preferred embodiment, a lens 14 is interposed between the laser 10 and the target pellet 12 to further focus and collimate a beam of light 16 produced by the laser 10. The lens has, for example, a 50 cm focal length. 
     The pellet 12 is thermally mounted upon a chuck assembly 18 which is electrically grounded. Also, the chuck assembly 18 is driven by a motor 19 into rotation about a central axis 20 at a rate of, for example, 0.05 revolutions per second. In this embodiment, the normal to the surface of the pellet 12 is inclined by 45° with respect to the axis of rotation of the chuck assembly 18. 
     When a pulse of laser light impinges upon the pellet 12, ferrite material vaporizes, consequently moving towards a substrate 24 and being deposited thereon. The substrate 24 is maintained within a temperature range in accordance with the type of substrate material used, and the type of ferrite to be grown upon the substrate. The substrate 24 is maintained at a distance of approximately 3 cm from the pellet 12 by a substrate holder assembly 26. The substrate holder assembly 26 is electrically isolated. The substrate is maintained within a particular temperature range by means of electrical heaters 30 attached to the block holding the substrate. The temperature is controlled using a temperature measuring device, such as a thermocouple 28. Typically, the holder assembly 26 is maintained at a temperature of 600° C. Additionally, the substrate holder assembly 26 includes a radiation shield 32 to protect the substrate from the effects of ambient ionizing radiation. 
     The assembly 18, the pellet 12, the substrate 24, and the substrate holder assembly 26 reside within a vacuum chamber 34 which is maintained at a pressure of approximately 4×10 -8  torr prior to vaporization and deposition of the ferrite material upon the substrate 24. The partial pressure of oxygen is maintained at about 100 mtorr using a controlled flow of oxygen via tube 36 into the vacuum chamber 34. Thin films (approximately 500 nm) of exclusively c-axis oriented BaFe 12  O 19  can be deposited in situ in less than 30 minutes. 
     More particularly, for example, referring to FIG. 2, to form a film of barium ferrite (BaFe 12  O 19 ) 38, a magnesium oxide (MgO) or sapphire or SrGa 12  O 19  substrate 24 is used. The substrate 24 is maintained within a temperature range of 550°-900° C. If the temperature is not kept generally within this range, the film will tend to include multi-phase structures, such as spinels and other materials of ferrite hexagonal composition. In a preferred embodiment of the method of the invention, an intermediate bonding layer 40 can be used to promote adhesion of the ferrite layer 38 to the substrate 24, as well as to promote desirable crystalline structure. In this instance, the material Mn j  Mg k  (Al m  Zn n  Ga o  Cu p  Mg q  Ni r ) s  Fe t ) 2  O 4  where j+k=1 and 0≦j≦1, 0≦k≦1, where m+n+o+p+q+r=1, and 0≦m≦1, 0≦n≦1, 0≦o≦1, 0≦p≦ 1, 0≦q≦1, 0≦r≦1, and where s+t=1 and 0≦s≦1, 0≦t≦1, such as manganese ferrite (MnFe 2  O 4 ) can aid in the growth of the Barium ferrite film layer 38. 
     To obtain cubic ferrites, such as the spinel and garnet (B 3  C 5  O 12 ) ferrites, where A represents a transition metal ion or combination of various transition metal ions, B represents a rare earth ion or yttrium or calcium, for example, and C represents a transition metal ion or combination of various transition metal ions, the substrate 24 consists essentially of MgO maintained at a temperature generally with the range of 500°-950° C. The lower end of this temperature range corresponds to the transition temperature between the amorphous and crystalline phases, and the upper end of the range corresponds to the temperature that is associated with significant ionic diffusion between the substrate and the ferrite film. Since films of high purity are desired, ionic diffusion is to be avoided. Ideally, for the growth of spinal ferrites MgO is the preferred substrate, or one could use the material Mn j  Mg k  (Al m  Zn n  Ga o  Cu p  Mg q  Ni r ) s  Fe t ) 2  O 4  where j+k=1 and 0≦j≦1, 0≦k≦ 1, where m+n+o+p+q+r=1, and 0≦m≦1, 0≦n≦1, 0≦o≦1, 0≦p≦1, 0≦q≦1, 0≦r≦1, and where s=1 and t=0, i.e., no iron (Fe) is present. For garnet ferrites, GGG is a preferred substrate material, or any other material with the formula where (Gd a  Y b  Ca c  R d ) 3  (Ga e  Al f ) 5  O 12 , where a+b+c+d=1, 0≦a≦1, 0≦b≦1, 0≦c≦1, 0≦d≦1, and R represents any mixture of any number of rare earth elements, including a mixture with only one rare element, where e+f=1 and 0≦e≦1, 0≦f≦1. It may be possible to change the composition of the substrate slightly in order to improve the lattice constant mismatch between film and substrate. 
     To form hexagonal films, a substrate with a hexagonal lattice structure is used. For example, if hexagonal barium ferrite is desired, SrGa 12  O 19 , having a hexagonal structure, is preferable. Analogously, to form a film with a cubic lattice structure, a cubic substrate, such as MgO is used. 
     A substrate can be chosen that consists essentially of (Gd a  Y b  Ca c  R d ) 3  (Ga e  Af f ) 5  O 12 , where a+b+c+d=1, 0≦a≦1, 0≦b≦1, 0≦c≦1, 0≦d≦1, and R represents any mixture of any number of rare earth elements, including a mixture with only one rare earth element, where e+f=1 and 0≦e≦1, 0≦f≦1. For example, when a=1 and b=c=d=0, and e=1 and f=0, the substrate is Gd 3  Ga 5  O 12 , which is useful for growing garnet films. Also, when there are equal amounts of Gd and Ca, for example, a=0.5, b=0, c=0.5, d=0. Thus, the substrate consists of Gd 1 .5 Ca 1 .5 Ga 5  O 12 . 
     The rare earth elements include Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. R can be any mixture of rare earth elements, such as the mixture Ce 0 .79 Er 0 .12 Lu 0 .09. Note that the fractions of the components of the mixture add up to 1.0. Thus, when d=e=1 and a=b=c=f=0, the substrate is Ce 2 .37 Er 0 .36 Lu 0 .27 Ga 5  O 12 . The mixture consisting solely of Tm is also possible, where no significant amount of any other rare earth element is present. Thus, the substrate would be Tm 3  Ga 5  O 12 , when a=b=c=f=0. 
     In another embodiment, the substrate consists essentially of Sr x  Ba y  Pb z  (Ga u  Al v ) 12  O 19 , where x+y+z=1 and 0≦x≦1, 0≦y≦1, 0≦z≦1, and where u+v=1 and 0≦u≦1, 0≦v≦1. For example, Sr 0 .5 Pb 0 .5 Ga 4  Al 8  O 19  is a useful substrate material. SrGa 12  O 19  can be used to grow hexagonal ferrites. 
     The material Mn j  Mg k  (Al m  Zn n  Ga o  Cu p  Mg q  Ni r ) s  Fe t ) 2  O 4  where j+k=1 and 0≦j≦1, 0≦k≦1, where m+n+o+p+q+r=1, and 0≦m≦1, 0≦n≦1, 0≦o≦1, 0≦p≦1, 0≦q≦1, 0≦r≦1, and where s+t=1 and 0≦s≦1, 0≦t≦1, can be used as a substrate material, or can be used as an intermediate bonding layer to promote enhanced adhesion and crystal structure of a crystalline ferrite film to be formed, such as a ferrite film with hexagonal or cubic symmetry, e.g., garnet. Amorphous quartz (SiO 2 ) and glass can also be used as substrate materials for growing polycrystalline ferrite films. 
     To achieve crystallinity, the substrate is heated to 750° C. 
     It has been observed that the flow of oxygen within the vacuum chamber is best maintained at a partial pressure of oxygen of approximately 100 mtorr in the vicinity of the substrate during formation of the ferrite film. 
     Other modifications and implementations will occur to those skilled in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the above description is not intended to limit the invention except as indicated in the following claims.