Method for brazing two glass components

Abstract: Method for brazing two borosilicate glass components comprising the steps of depositing a first metallic layer on a portion of the surface of each glass element, depositing a second metallic layer on said first metallic layer. Once the bi-metallic layer are deposited on the corresponding surface of the two glass elements, a low temperature solder like SnInAg for example is inserted between the two coated glass surfaces. The final assembly is then heated so as to perform the brazing of the two elements. This method may be used for manufacturing leak tight glass containers encapsulating electronic component in view of being implanted in a human or animal body.

Description: This invention relates to a method of brazing two glass components at low temperature (i.e. under glass melting temperature) in view of producing a leak tight container for encapsulating electronic components. More particularly the method of the present invention may be applied to manufacture a glass encapsulated pressure transducer, which can then be implanted in a human body. Another object of the present invention relates to the encapsulating container obtained with the manufacturing method. A further object of the invention relates to an encapsulated implantable pressure transducer according to the teaching of the manufacturing method. The method for brazing to glass components will be described in relation with the manufacturing of an implantable pressure transducer but may of course be applied to any other fields requiring low temperature leak tight brazing of two glass components so as to manufacture a glass encapsulated implantable device. Many medical applications require implantable measurement devices; for example, intra cranial pressure is a key parameter for regulating the flow rate in a shunt of a shunted hydrocephalic patient. Once implanted, the pressure transducer may be inductively coupled to a reading unit which energises the transducer and allows the transmission of data corresponding to the measured pressure between the transducer and the reading unit. Such implantable systems or devices like pacemakers for example already exist on the market and are generally active components having their own source of energy. They are encapsulated in a titanium container which is usually hermetically sealed thanks to a laser bonding. These existing devices are difficult to manufacture and the use of a titanium container is usually not suitable for passive transducers, which have to be energised by inductive coupling thanks to an external reading unit. The choice of encapsulating electronic components in a glass container has many advantages over the prior art. Firstly the type of the glass may be chosen with an expansion coefficient matching the one of the silicon and therefore no differential expansion occurs between the container and the electronic parts when the sealing operation is performed. Glass is also a good barrier to liquid and thus forms a leak tight container once sealed. Furthermore the glass may be considered as chemically inert and bio compatible and therefore suitable for long term implantation in a human body. It is also to be noted that glass is permeable to RF waves which permits the use of passive telemetry once the device is implanted in a human body. Glass is also transparent to x-ray which may be an advantage or a requirement for medical implanted devices. In order not to damage the electronic components encapsulated in the glass container the brazing should be made at low temperature, preferably less than 200 degrees Celsius. The resulting container should also be leak tight and have suitable mechanical resistance to stress. The aim of the present invention is to solve these problems by providing a method of brazing two glass components at low temperature in order to form a suitable container for encapsulating electronic components. This goal is achieved by a method having the characteristic recited in claim 1. The resulting hermetically sealed glass container is the object of claim 11. Further features and other objects and advantages of this invention will become clear from the following detailed description made with reference to the accompanying drawings illustrating in a schematic and non-limiting way one embodiment of an encapsulated pressure transducer. Fig. 1 is an exploded perspective view of an encapsulated pressure transducer. Fig. 2 is a perspective view of the encapsulated pressure transducer illustrated at fig 1, once the container is sealed. Fig. 3 is perspective exploded view of an alternate embodiment of the base plate of the encapsulated pressure transducer. The method for brazing two glass components at low temperature will now be described in details. The objective is to braze two pieces made of a borosilicate glass for example of the Pyrex type with a low temperature solder so that the resulting bond is leak tight (bubble free and uniformly distributed) and strong enough for the intended applications. This is achieved by first depositing a first thin metallic layer on at least one portion of the borosilicate glass surface. The thickness of this layer is comprised between 50 and 150 nanometers, preferably 100 nanometers (nm). The first metallic layer may be chosen between any metallic element provided that it presents a good adherence to glass. Titanium and chrome are example of materials that may be deposited on glass as a first layer. This first metallic layer may be deposited by know techniques like physical vapor deposition (PVD) or any other method that allows the deposition of a metallic layer on a glass substrate. Next, a second metallic layer suitable for performing brazing operation is deposited above the first metallic layer, this layer having a thickness comprised between 2 and 10 microns (µm), preferably 5 microns (µm). The second metallic layer may be chosen between any metal which does not become oxidized. Different metals like gold (Au), silver (Ag), nickel (Ni) or stainless steel may be used for example to form this second metallic layer. The above mentioned operations are performed on the corresponding borosilicate glass part to which the first part has to be brazed. An optional third metallic layer may be deposited on the second layer, this third layer having the same properties as the second layer. For cost reasons the preferred method uses only two metallic layers but the results would be identical if more than two layers were deposited. The next step consist of positioning a low temperature solder element in between the two second layers of the respective borosilicate glass parts and to heat the assembly in a furnace so that the brazing occurs. Any low temperature solder may be used provided that this solder element is compatible with the second metallic layer, presents a good absorptivity and is bubble free. InAg, AgPbSn, SnBi, SnZn or SnInZn amongst other are example of solder which may be used as a solder element for brazing the two glass components. The preferred solder material are low temperature solders such as ln97Ag3 or Sn77.2In20Ag2.8 which melting point are respectively 143°C and 187°C. The optimal solder thickness is between 50 and 100 microns (µm). The assembly is then heated in an oven at a temperature of about 200°C for a time comprised between 1 and 15 minutes, preferably 5 minutes. For medical applications, like the manufacturing of glass encapsulated devices, the use of a SnInAg braze is preferred because the vapor sterilization temperature (often 132°C) is very close to the melting point of InAg. As an alternative, the solder element may be deposited above the second layer either on one element or on both elements to be brazed. Tests have shown that the optimal results were obtained with a first titanium layer, a second gold layer and a SnInAg solder (thickness 50-100µm) and heating during 5 minutes at a temperature of 200°C in an oven under helium atmosphere. In the resulting assembly, the braze was uniformly distributed and bubble free and therefore perfectly leak free. The bonding between the two borosilicate glass parts also presented good mechanical characteristics under strains. Now referring to figure 1, there is shown a container for encapsulating a pressure sensor. For clarity purpose in the accompanying drawings, only the pressure transducer is shown and not the other electronic components which may be installed anywhere inside the glass container. The container comprises a circular base plate 1 made of a borosilicate glass such as Pyrex on which is bonded a pressure transducer 2. The bonding of the pressure transducer on the base plate 1 may be achieved by known techniques like anodic bonding. These known techniques permit the bonding of a silicon component like a pressure transducer over a glass surface and will not be described in detail in the present application. The base plate 1 comprises a hole 3 (see fig. 3) and the pressure transducer is bonded on the base plate 1 so that the membrane of the pressure transducer 2 is located in front of the hole 3. The container further comprises a closed cylindrical cover 4 having radial dimension corresponding to the base plate 1 so as to form a container or capsule once sealed over the base plate. The base plate 1 comprises a bi-metallic layer 5 on at least one portion of its surface. This bi-metallic layer 5 forms, in the example shown, a thin ring located on an outer diameter of the base plate 1. The bi-metallic layer 5 is composed of a first layer of titanium on which a second layer of gold has been deposited. The thickness of the two layers are those mentioned in the above description. The bottom edge 6 of the cover 4 also comprises a portion of its surface, forming a ring on which a bi metallic layer of the same composition is deposited. For sealing the container, thus encapsulating the pressure transducer 2, a low temperature solder braze ring 7, for example made of SnInAg, is inserted between the base plate 1 and the cover 4, the cover 4 is then urged against the base plate 1 so as to close the container. This assembly, shown at fig. 2 is then heated for 5 minutes in an oven under helium atmosphere so as to perform the brazing between the two bi-metallic surfaces located on the base plate 1 and at the bottom of the cover 4. The resulting container contains the pressure transducer 2 and optional other electronic components in a leak tight glass container. Fig 3. shows an alternate embodiment of the encapsulated device in which the pressure transducer 2 is not directly bonded to the base plate 1 but to a glass support 8. The support 8 may be made of a borosilicate glass such as Pyrex for example. In that embodiment, the pressure transducer 2 may be bonded to the Pyrex support 8 at the wafer level (before cutting the wafer) by conventional anodic bonding. The support 8 and the base plate 1 are then brazed together according to the above disclosed method. To that extent a first metallic layer like titanium for example is deposited on at least a portion of the upper face of the base plate 1 and to the corresponding bottom surface of the support 8. Then a second metallic layer for example of gold is deposited on the first metallic layers. A low temperature brazing element having shape and dimension corresponding to the above mentioned bi-metallic layers is then sandwiched between the base plate 1 and the support 8. The brazing is done when the assembly is heated and allows the bonding of both the cover 4 on the base plate 1 and the support 8 on the base plate 1. In that embodiment, the glass support 8 comprises a central hole 9 corresponding to the central hole 3 of the base plate 1. The pressure transducer 2 is first bonded on the glass support 8 so that the membrane of the pressure transducer 2 is positioned above the central hole 9. Then the support 8 is positioned so that the hole 9 in the support 8 and the hole 3 in the base plate 1 coincides on at least one part of their opened surface. In the resulting encapsulated pressure transducer the liquid may interact on the membrane of the pressure transducer through the hole of the base plate 1 and the hole of the glass support 8. In the above description, reference was made to the manufacturing of an implantable pressure transducer, but many other implantable devices (micro pump, flow sensor, chemical sensor) may be encapsulated in a leak tight glass container using the same method of manufacturing and the scope of the claimed invention is not restricted to the above disclosed pressure transducer. The shape and dimensions of the base plate 1 and the corresponding cover 4 may obviously also be different than the cylindrical example shown on fig 2. Depending on the intended application, the container may for example be shaped as a cube, a sphere or any other volume made of two corresponding parts that can be linked together to form a closed container. This encapsulation method in a glass container is particularly well suited for manufacturing inductively coupled implantable medical devices. The use of an inexpensive material is an advantage and the containers are easier and cheaper to manufacture than existing titanium containers.

Claims: A method of brazing two borosilicate glass pieces in view of obtaining a leak tight bonding between the two glass pieces comprising the steps of: depositing a first metallic layer on at least one portion of the surface of each glass pieces that should be brazed together depositing at least a second metallic layer on the above mentioned first deposited metallic layer inserting a low temperature solder element between the opposite bi-metallic layer surfaces of the glass pieces urging the two glass pieces together and heating the assembly at a temperature allowing the brazing of the solder element, said temperature being less than the glass melting point. Method according to claim 1 characterized in that the solder element is deposited on the second layer of at least one the glass piece to be brazed Method according to one of the preceding claims characterized in that the thickness of the first metallic layer is comprised between 50 and 150 nanometers, preferably 100 nanometers (nm) and in that the thickness of the second metallic layer is comprised between 2 and 10 microns (µm), preferably 5 microns (µm) Method according to one of the preceding claims, characterized in that the fist metallic layer is made of titanium, the second metallic layer of gold and in that the low temperature solder element is made of SnInAg in the following proportions Sn 77.2% In 20& Ag 2.8%. Method according to one of the preceding claims characterized in that the heating is performed in an oven under helium atmosphere at a temperature around 200°C for a period comprised between 1 and 15 minutes, preferably 5 minutes. Method of encapsulating electronic components in a glass container made of at least two corresponding elements that can be linked together to form a closed container characterized in that it comprises the following steps: depositing a first metallic layer on at least a portion of the first element depositing at least a second metallic layer on top of the first mentioned metallic layer depositing the first and the second above mentioned layers on the corresponding surface of the second element forming the container arranging the electronic components inside the container inserting a low temperature solder element in between the two corresponding elements of the container heating the assembly. Method of encapsulation according to claim 6, characterized in that the container comprises a base plate and a corresponding cylindrical cover made of a borosilicate glass and that the first deposited metallic layer is of titanium, the second layer of gold and that the solder is made of SnInAg. Implantable medical device characterized in that the electronic components thereof are encapsulated in a glass container comprising two pieces that are brazed together according to the method of claim 1 so as to form a leak tight container. Implantable pressure sensor characterized in that it comprises a pressure transducer bonded to a borosilicate glass base plate having a hole on a portion of its surface and in that a borosilicate glass cover is brazed at low temperature on the base plate according to the method recited in one of the claims 1 to 5 so as to leak tight seal the cover on the base plate. Implantable pressure sensor according to claim 9, characterized in that the pressure transducer is bonded on a borosilicate glass support and in that the support is brazed according the method of claim 1 on the base plate of a borosilicate glass container comprising a base plate and a corresponding cover. Glass container for encapsulating electronic components in view of being implanted in a human or animal body, characterized in that it comprises a first element and a corresponding cover element brazed together according to the method of claim 1.

Inventors: GINGGEN ALEC, MEUTERLOS STEPHANE, STEINMANN PIERRE-ALBERT, TARDY YANIK, GINGGEN, ALEC, MEUTERLOS, STEPHANE, STEINMANN, PIERRE-ALBERT, TARDY, YANIK, Meuterlos, Stéphane
Applicants: MEDOS SA
Application year: 2000
Main classification: C03C27
IPC codes: A61N1, C03C27