GLASS TO METAL SEALING PROCESS

Abstract: A process for hermetically sealing a glass to a metal having similar coefficients of thermal expansion uses a single atmosphere containing from about 0.5 to 2.5 volume % of water vapour, 0 to about 5 volume % of hydrogen gas and the balance an 'inert' gas such as nitrogen.

Description: The present invention is directed to a process for fusing a glass to a metal and, specifically, to a single step sealing process that produces strong hermetic glass seals with little or no entrained gas bubble content. It is known in the art to fuse glass to metal to form hermetically sealed electronic components. The reliability of such components is directly dependent on the integrity of the seal. There are two types of such seals employed in the industry which are commonly referred to as "matched seals" with the former referring to seals formed from metals and glass having similar coefficients of thermal expansion. It has been estimated that as many as 80% of such seals utilized in the industry are the matched type. Both types of seals are commonly formed by a three step process including: (1) degassing or decarburizing of the metal surface, (2) oxidation and (3) sealing as disclosed for example, in United Kingdom Patent Application No 2135297A. Such conventional processes require that the metal is first decarburized and then oxidized to form a thin oxide layer on the surface of the metal. A seal is then made by fusing the thin metal oxide layer with molten glass. By the way of example, the above-mentioned United Kingdom Patent Application teaches that the decarburization step is carried out in an atmosphere of 100% hydrogen with less than 5.0% by volume of moisture, preferably at a H₂/H₂O volume ratio in excess of about 50. The oxidation step is carried out in an oxidizing atmosphere composed of a gaseous carrier having an oxidizing agent such as water, carbon dioxide, nitrous oxide and the like and free hydrogen to reduce the oxygen content to thereby favour the formation of Fe₃O₄ (page 2, line 52 to page 3, line 3). The sealing step is carried out using a lean exogas or a N₂-based atmosphere. The temperature of each of the reaction steps can vary over a wide range with examples shown beginning on page 3 employing a different temperature for the decarburizing step from either the oxidizing or sealing steps. Each of these process steps is therefore typically performed at different temperatures in the presence of different gaseous environments which adds greatly to the time and cost of obtaining the desired product. It is therefore an object of the invention to provide a single step process for forming matched gass-to-metal seals, that is to say the process employs a single set of process conditions. According to the present invention there is provided a single step process for fusing a glass to a metal wherein the glass and the metal have similar coefficients of thermal expansion, said process comprising: heating the glass and metal in an atmosphere containing or formed from 0.5 to 2.5 volume % of water vapour, from 0 to 5 volume % of hydrogen gas and the balance an 'inert' gas at a temperature of at least the melting point of the glass for a time sufficient to melt the glass and fuse the melted glass to the metal. In accordance with the present invention the atmosphere in which sealing takes place is controlled within defined limits to prevent condensation in the furnace and to provide a sufficient thin layer of oxide on the metal for proper sealing of the glass to metal. The present process is also economical because it significantly reduces the amount of hydrogen needed to effect sealing. In addition, reduction of the metal to glass sealing process to a single step saves considerable time downtime of the furnace to change gaseous environments is eliminated and parts handling is reduced by about two-thirds. The upper limit placed on the volume percentage of hydrogen, while not particularly critical, is desirable for several reasons. Hydrogen gas is both expensive and its use requires special safety precautions, each of which adds to the cost of conducting the process, especially on a commercial scale. In addition, hydrogen has a tendency to retard formation of the oxide layer on the metal which is necessary for proper sealing. Accordingly, excess amounts of hydrogen well beyond the limits mentioned above can adversely affect the integrity of the seal. The upper limit placed on the amount of water vapour is necessary to prevent thick oxide formation on the metal parts and to prevent condensation in the sealing furnace which can result in furnace shut down and inadequate seals. The preferred amount of water vapour utilized in the process of this invnetion is within the range of from about 1.0 to 1.5 volume % and the preferred amount of hydrogen gas is within the range from about 1.0 to 2.0 volume %. Accordingly, the preferred amount of the inert gas (eg nitrogen) is from about 96.5 to 98.0 volume %. A particularly preferred atmosphere in accordance with this invention comprises 1 volume % each of water vapour and hydrogen and 98 volume % of nitrogen. The sealing process of the invention is preferably conducted at a temperature of from about 1800°F to 1900°F (980 to 1035°C), most preferably about 1850°F (about 1010°C), for approximately 10 to 20 minutes, most preferably about 15 minutes. The selection of metals which may be sealed in accordance with the present invention is, in theory, unlimited with the provision that the metal chosen has a coefficient of thermal expansion sufficiently similar to that of the glass such that excess pressure or compression is not required (ie a matched seal process). However, those skilled in the art will appreciate that the metal must be suitable with regard to other properties such as the ability to form an adherent oxide, the ability to withstand the conditions of the seal without becoming excessively brittle, and the like. More specifically, the difference between the coefficient of thermal expansion for the metal and glass should be no more than about 20 x 10⁻⁷ cm/cm/°C, preferably no more than 10 x 10⁻⁷ cm/cm/°C. The metals which are advantageously employed in the present process include, for example, tungsten, molybdenum, titanium, tantalum and the like and alloys such as chrome-iron alloys, nickel-chrome-iron alloys, nickel-cobalt-iron alloys and the like, with the latter being preferred. A particularly preferred nickel-cobalt-iron alloy is "Kovar" having the designation ASTM F-15. The glasses include borosilicates, lead silicates and borophosphosilicates. Preferred glasses are borosilicates available as Corning glasses 7052, 7040, 7056 and 8830 which have thermal coefficients in the range of from about 46-52 x 10⁻⁷ cm/cm/°C. The process of the present invention may be conducted in any reaction vessel in which the desired process conditions can be maintained, such as a belt furnace. The gaseous compounds (hydrogen gas, water vapour and inert gas) comprising the reaction atmosphere may be fed to the reaction vessel through individual feed lines or, preferably as a combined feed. By the way of example, a mixture of inert gas and 1 volume % of hydrogen gas is bubbled through water at ambient temperatures to control the humidity in the gas mixture to about 1 volume %. Example 1 Commercial metal plates and pins of Kovar ASTM F-15 and borosilicate glass beads (Corning 7052) were assembled into a conventional graphite fixture. The metal and the glass had coefficients of thermal expansion of 57.1-62.1 x 10⁻⁷ cm/cm/°C and 46 x 10⁻⁷ cm/cm/°C, respectively. A conventional belt furnace was purged of air and then filled with a gaseous mixture containing 98.0 volume % of nitrogen gas and 1 volume % of each of water vapour and hydrogen gas. The furnace was heated to a temperature of 1850°F. The assembled parts were then introduced into the furnace without distrubing either the set temperature or the atmosphere and held at that temperature for 15 minutes. The resultant seals were found to have a high degree of hermeticity, resistance to thermal shock, seal strength and has few entrained gas bubbles. In addition, the seals showed acceptable intergranular oxide penetration. Example 2 Example 1 was repeated utilizing metal plates and pins which had been deliberately contaminated by briefly immersing them in lubricating oil. In a conventional sealing procedure, such contaminants would be removed by a decarburizing step. The resulting seals were in all respects comparable to those formed in Example 1.

Claims: A single step process for fusing a glass to a metal wherein the glass and the metal have similar coefficients of thermal expansion, said process comprising: heating the glass and metal in an atmosphere containing or formed from 0.5 to 2.5 volume % of water vapour, from 0 to 5 volume % of hydrogen gas and the balance an 'inert' gas at a temperature of at least the melting point of the glass for a time sufficient to melt the glass and fuse the melted glass to the metal. A process according to claim 1 wherein the inert gas is selected from nitrogen and argon. A process according to claim 1 or claim 2 wherein the temperature is in the range of from about 1800°F to 1900°F (980 to 1035°C). A process according to any one of the preceding claims, wherein the time of the reaction is from 10 to 20 minutes. A process according to any one of the preceding claims, wherein the amount of water vapour is in the range of from 1.0 to 1.5 volume %. A process according to any one of the preceding claims, wherein the amount of hydrogen is in the range of from 1.0 to 2.0 volume %. A process according to any one of the preceding claims, wherein the metal is selected from the group consisting of tungsten, molybdenum, titanium, tantalum, nickel-cobalt-iron alloys, chrome-iron alloys, and nickel-chrome-iron alloys. A process according to any one of the preceding claims, wherein the glass is selected from the group consisting of borosilicates, lead silicates and borophosphosilicates. A process according to any one of the preceding claims, wherein the difference in the coefficient of thermal expansion of the glass and metal does not exceed 20 x 10⁻⁷ cm/cm/°C. A process according to any one of the preceding claims, wherein the reaction atmosphere contains about 1 volume % of water vapour, about 1 volume % of hydrogen gas and about 98 volume % of nitrogen gas.

Inventors: BANDYOPADHYAY, NIKHILES, KIRSCHNER, MARK J., TAMHANKAR, SATISH S.
Applicants: THE BOC GROUP, INC.
Application year: 1988
Main classification: C03C27
IPC codes: C03C27