METALLIZED RARE EARTH GARNET AND METAL SEALS TO SAME

Abstract: A reliable garnet to metal hermetic seal is obtained by metallizing the garnet with successive sputtered layers of tantalum or titanium, molybdenum or tungsten and nickel, followed by soldering or brazing the metallized garnet to a metal member.

Description: The invention relates to a body of garnet material having a surface whereby said body can be affixed by soldering to a metal object. Rare earth garnets such as yttrium aluminium garnet (YAG) and yttrium iron garnet (YIG) exhibit unique optical and magnetic properties which make these materials useful in a variety of applications, such as magnetic bubble domain memory devices; magnetooptic devices, e.g., modulators, optic devices, e.g., lasers; and microwave devices, e.g., circulators. Often, the desired properties reside in an epitaxial mono crystal layer supported on a monocrystalline substrate, and are achieved by complex substitutions of constituents into the garnet crystal structure. The need to mount such devices to metal substrates such as walls or brackets presents a unique bonding problem. In addition to the known incompatibility between oxide and metal surfaces, the garnets themselves are often mechanically or chemically fragile. For example, in one application, the garnet is rectangular in shape, having an aspect ratio of about 6 to 1, thus giving rise to significant stresses due to differential thermal expansion in the body during heating. In addition, heating to temperatures above 400 o C, necessary for most brazing operations, even for a short time, may cause irreversible changes in the magnetic or optical properties, especially of the thin epitaxial layer. In U.S. patent 3,969,086, a garnet is metallized for bonding by sputter depositing a nichrome, molybdenum or chromium layer on the garnet, followed by a copper layer, and then a gold layer. The gold layer is then soldered to the metal to be bonded. In U.S. patent 4,210,389, a YAG laser is first metallized with a sputtered reflective layer of aluminium, gold, silver or rhodium. This layer is then covered with a sputtered layer of either nichrome, titanium or tungsten, followed by a sputtered layer of gold or platinum. This gold or platinum layer is then soldered to a copper mount metallized with gold or platinum. While the above structures apparently provide adequate garnet to metal seals, it is desirable to develop highly adherent, relatively cheap seals which do not rely upon the use of one or more precious metal layers. Accordingly, it is an object of the invention to provide a metallized garnet suitable for soldering or brazing to a metal object, the metallization of which is highly adherent to the garnet substrate and does not contain precious metals. In accordance with the invention, a body of garnet material is characterized by a first layer of metal comprising tantalum, titanium, vanadium, hafnium, zirconium, niobium or alloys thereof, said metal being sputter deposited on at least a portion of said garnet, a second layer of a refractory metal sputter deposited on the first layer, and a third layer of a non-precious solderable metal sputter deposited on the second layer. Also in accordance with the invention the combination of a garnet body sealed to a metal member said metal being bonded by a solder layer to the third solderable layer. Such structures have been found to form highly reliable, adherent, hermetic seals. An embodiment of the invention will be described with reference to the drawings, in which Fig. 1 is a section view of a portion of a garnet material metallized in accordance with the invention; and Fig. 2 is a section view of a portion of a garnet to metal seal formed from the metallized garnet of Fig. 1. Fig. 1 shows a portion of a metallized garnet structure 10 in accordance with the invention, including garnet layer 11, reactive metal layer 12, barrier layer 13 and bonding layer 14. Reactive metal layer 12 is any one of, or an alloy of two or more of the metals tantalum, titanium, vanadium, hafnium, zirconium and niobium. When deposited by the diode sputtering technique described in U.S. patent 3,339,267, such layer forms a chemical bond with the non-metallic constituents in the substrate. Due to the high energy of bond formation of these metals, especially tantalum and titanium, this bond is tenacious and durable. However, because of their reactivity and the stability of their oxides, it is not practicable to solder or braze directly to these metals. Thus, a barrier layer 13, of a refractory metal such as molybdenum or tungsten, is sputter deposited on layer 12. This layer forms a tenacious metal to metal bond with reactive layer 12, but is itself relatively chemically inert. It thus acts as a barrier to protect the subsequent layers from attack by the reactive layer. It does not, however, due to its inertness, provide a strong bond with most solders and brazes. Thus, a solderable layer 14 of nickel is sputter deposited onto the refractory metal layer. Again, the sputter deposition technique referred to results in a strong, tenacious bond between these metal layers. Nickel is chosen as the solderable layer because it is more readily activated by fluxes than are more active solderable metals such as copper, and is notable less expensive than the precious metals, gold, silver, platinum and palladium. The thickness of the layers deposited is not particularly critical, from one-half to one or several microns being adequate to provide a hermetic seal of reliable integrity. By way of example, one such seal has a layer of tantalum one-half micrometer in thickness, followed by layers of molybdenum and nickel, each about one micrometer in thickness. Any solder or braze compatible with the metal member to be bonded and otherwise suitable for the particular application, may be used. Typical solders are tin-silver, tin-lead, tin-bismuth and indium-base alloys. Fig. 2 shows a garnet to metal seal 20, in which garnet layer 21 is bonded to metal layer 26, (e.g. titanium) by means of sputtered tantalum layer 22, sputtered molybdenum layer 23, sputtered nickel layer 24 and tin-silver solder layer 25.

Claims: A body of garnet material whereby said body can be affixed by soldering to a metal object, characterized by a first layer of a metal comprising tantalum, titanium, vanadium, hafnium, zirconium, niobium and alloys thereof, said metal being sputter deposited on at least a portion of said garnet, a second layer of a refractory metal sputter deposited on the first layer, and a third layer of a non-precious, solderable metal sputter deposited on the second layer. The body of garnet material of claim 1, in which the refractory metal is selected from the group consisting of molybdenum and tungsten. The body of garnet material of claim 1, in which the solderable metal is nickel. The body of garnet material of claim 1, in which the reactive metal is selected from the group consisting of tantalum and titanium, the refractory metal is molybdenum and the solderable metal is nickel. The body of garnet material of claim 4, in which the first layer is about one-half micrometer in thickness and the second and third layers are each about one micrometer in thickness. The combination of a garnet body according to one of the claims 1-5 sealed to a metal member said metal member being bonded by a solder layer to the third solderable layer.

Inventors: BRONNES, ROBERT LEWIS, MCKINLAY, JAMES KEVIN, SWEET, RICHARD CLARK, ZWICKER, WALTER CARL
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1985
Main classification: C04B41
IPC codes: G11B11, C04B41, C23C14, B23K1, C04B37