The present invention relates to methods for making glass to metal seals for electrical feedthroughs and the like and especially for electrical feedthroughs in implantable medical devices.
It is well known in the glass sealing art to make hermetically sealed electronic components such as electrical feedthroughs. A typical feedthrough consists of an external metal part (a frame or ferrule) into which preformed solid or sintered glass part is sealed. Within the glass part, one or more metal leads (pins) are sealed. Since the reliability of critical implantable medical devices depend on hermetic sealing of various components, the integrity of the glass to metal seals used in battery components and the seal between the internal electrical components and the human body is of paramount importance.
In many implantable medical devices, titanium is used due to its superior corrosion resistance and biocompatability. However, for many glass-metal seals where titanium is a metal component, the glass compositions fail to readily wet the titanium surface and therefore the adhesion of the glass to the metal is compromised. In matched seals, the coefficient of thermal expansion of the glass and metal are similar so that good chemical bonding between the glass and titanium is necessary in order to provide an adequate seal. Thus, for matched seal configurations a poor bond between the titanium and the glass can be fatal to the integrity of the seal. In compression seals where the coefficients of thermal expansion of the various parts is mismatched, sealing is accomplished by establishing a compressive force between the components. However, even for compression seals, the wetting of the glass on the titanium and its adhesion to the titanium can improve the strength and reliability of the seal.
In order to improve the wetting and adhesion of glass and metal, oxide layers are typically formed on the metal in order to change the surface chemistry of the metal. For example, in European Patent Application 0 311 308, a method is disclosed for hermetically sealing a glass to a metal in which an oxide layer is formed on the metal in a one-step process by carrying out the sealing process in an atmosphere which includes an atmosphere of an inert gas such as nitrogen or argon together with water vapor and hydrogen. Also, in Brow et al., Reactions and Bonding Between Glasses and Titanium, Sandia National Laboratories SAND--87-0541C, hermetic seals between titanium and various candidate glasses are disclosed along with the suggestion that oxidation or nitridation may have an effect on seal properties and interfacial chemistries.
It is therefore an object of the present invention to provide a glass to titanium seal having superior sealing properties.
It is also an object of the present invention to provide a method for making a superior glass to titanium seal which can be performed with conventional sealing techniques and equipment.