A METHOD OF HERMETICALLY BONDING OXIDE MATERIALS AT RELATIVE LOW TEMPERATURES

Abstract: A method of hermetically bonding oxide materials. The bonding is performed by means of an intermediate layer of a commercially available lead borate glass at a temperature below the melting point of aluminium. The method requires no previous surface treatment of structuring of the oxide materials, and electrical feedthroughs can be provided in the same process step as the bonding.

Description: Title: A method of hermetically bonding oxide materials at relative low temperatures. Technical Field The invention relates to a method of bonding oxide materials, such as piezoelectric materials, by means of an intermediate layer of a low tempe¬ rature glass, whereby a conductor path pattern of for instance aluminium is applied onto one of the oxide materials, and whereby the used glass has a melting point below 660°C and the maximum process temperature is below 660°C, viz. the melting point of aluminium. The method is for instance to be used for the production of SAW (surface acoustic wave) devices, i.e. surface acoustic components optionally based on lithium niobate and to be used for instance within the area of mobile telecommunication. The method is in particular intended to be used in conjunction with single crystalline piezoelectric oxide materials exhibiting a large degree of anisotrophy with regard to the thermal expansion. In the most general case the thermal expansion coefficient of a given material is a second rank tensor. The actual tensor values at the surface of an arbitra¬ rily cut crystal depend therefore on the orientation (cut) of the crystal under consideration. The latter is of considerable importance in connection with the production of SAW devices since many different piezoelectric crystals in a variety of different crystal orientations are utilized in this area. The method is of particular interest to devices where aluminium is one of the constituents e.g. SAW devices where the conductive electrode mate¬ rial is aluminium, an aluminium-based alloy or an aluminium-containing multilayer structure. Background Art For many commercially available SAW devices, the major fraction of the manufacturing costs is determined by the price of the encapsulation. As components based on surface acoustic waves are inherently sensitive to the quality and purity of the chip surface, these components are encapsu- lated in ceramic packages, cf . Figure 1 . The SAW device is typically a chip of a single crystalline piezoelectric crystal, such as lithium niobate with an aluminium based electrode pattern. The SAW device 1 is mounted in a ceramic housing 2 using a suitable adhesive. The SAW device 1 is con¬ nected to the electrical feedthroughs 4 by means of standard wirebondings 5. The SAW device is hermetically sealed in the ceramic housing 2 by means of a suitable lid 6. Such a package is considerably more expensive than the standard plastic dual-in line package (PDIP) techniques known from the semiconductor industry since the device is relatively non-surface sensitive in this latter case. A novel method of encapsulating SAW devices which either enables the use of standard packages from the semiconductor industry (e.g. PDIP) or renders further encapsulation completely superfluo¬ us will therefore have a considerable influence on the total manufacturing costs of this type of devices. EP No. 0,622,897 discloses a method of producing an encapsulated SAW device. This publication deals with two methods of bonding a substrate to a lid. The substrate and the lid belong to the following group of materials: lithium niobate, lithium tantalate, lithium borate, quartz and glass. The first method is known as anodic bonding and employs an electric field. The second method is known as direct fusing bonding and requires a previous surface treatment of both the substrate and the lid inter alia in order to make the surfaces hydrophillic. The second method requires typically tem¬ peratures exceeding the melting point of aluminium, such as 700°C or higher to provide a strong bond. A disadvantage of the bonding methods is that they both are very sensitive towards surface cleanness and finish. An additional disadvantage is the fact that the provision of electrical feed- throughs requires the inclusion of additional process steps. Furthermore EP No. 0,61 6,426 discloses a method of producing a SAW device based on a laminated structure. This structure comprises two plates, viz. a thin plate from the following group of materials: lithium niobate, lithium tantalate, lithium boron, quartz, and a thick plate from the following group of materials: lithium niobate, lithium tantalate, lithium borate, quartz, borate, amorph carbon and graphite. The laminated struc¬ ture is bonded by direct fusion bonding requiring that both plates are subjected to a previous surface treatment so as to make the surfaces hydrophillic. The laminated structure can also include a silicon based thin film layer of for instance SiO 2 or SiN 4 . In the latter case too the surfaces require a previous treatment so as to make them hydrophillic. EP No. 0,531 ,985 discloses a method of producing an electro-acoustic hybrid integrated circuit. The circuit comprises a semiconductor substrate bonded together with a single crystalline based acoustic element by way of direct fusion bonding using a silicon based intermediate layer. The described bonding method requires a depositing of an Si-based thin film on at least one of the materials to be bonded. Furthermore, the surfaces to be bonded must be subjected to a previous surface treatment so as to make them hydrophillic. Only one of the bonded materials is an oxide material. Moreover, EP No. 0,594, 1 17 discloses a method of producing a piezoelec¬ tric filter. The filter comprises two plates from the following group of materials: lithium niobate, lithium tantalate, lithium borate, quartz, glass or silicon. The plates are bonded by way of direct fusion bonding. Some of the material combinations are based on the use of an intermediate layer of Si, SiO 2 , or SiN 4 . In all cases the surfaces to be bonded are subjected to a pretreatment so as inter alia to make them hydrophillic. In general, bonding processes based on direct fusion bonding or anodic bonding are very sensitive towards the cleanness and finish of the bonding surfaces. These bonding methods furthermore requires a pretreatment to make the bonding surfaces hydrophillic. An additional disadvantage when using direct fusion bonding is the high process temperatures needed to achieve a strong bond. The main disadvantage when utilizing anodic bond¬ ing is the requirement that at least one of the bonding materials must be a conductive substrate for the bonding process to succeed. EP No. 0,449,473 discloses a method of producing a SAW device. The device comprises an aluminium based electrode pattern on a piezoelectric substrate (quartz) . The publication describes a method of bonding a quartz substrate to a lid of quartz or glass. The bonding is carried out by utilizing an intermediate layer of a suitable glass deposited by means of a standard stencil printing technique. The glass used for the intermediate layer is a lead borate type of glass, and the used process temperatures are all below 500°C. The described process requires three heat treatments at different temperatures of a total duration of approximately two days. The final heat treatment employs a pressure of approximately two pounds. A disadvan¬ tage of using quartz as piezoelectric substrate is the relatively low velocity of the SAW on this material thereby limiting the frequency at which the device can be utilized effectively. Depending on the specific application of the SAW device the relatively low electromechanical coupling coefficient of quartz can be an additional disadvantage. US-PS No. 5,337,026 discloses a method of producing SAW devices. The devices comprise aluminium based electrode patterns on a piezoelectric substrate (quartz) . The publication describes a method of bonding two quartz substrates. The method utilizes an intermediate layer of a suitable glass deposited by means of a screen printing technique. Two types of glass with a high content of lead oxide are mentioned. The bonding pro- cesses require three heat treatments at different temperatures below 500°C. The last heat treatment is performed in vacuum in combination with an applied pressure and the total process time is more than 6 hours. The disadvantages of this method are identical to the disadvantages described for EP 449,473. Brief Description of the Invention The object of the invention is to provide a method of bonding oxide mate¬ rials and which is not encumbered with the above disadvantages. A method of the above type is according to the invention characterised by the used piezoelectric oxide materials being of the formula ABO 3 , where A is an alkali or an alkaline earth metal, and B is a transition metal, and by the used glass having a relatively low modulus of elasticity. For a given photolithographic resolution available for the production of the electrode patterns in the SAW devices, the ability to utilize ABO 3 type materials enables the fabrication of encapsulated SAW devices with a higher operat¬ ing frequency than devices based on quartz due to the relatively low SAW velocity in the latter case. An additional advantage is the relatively high electromechanical coupling coefficients available in the ABO 3 type mate¬ rials. According to a particularly advantageous embodiment the used oxide materials are of lithium niobate or lithium tantalate, while the used glass is a lead borate glass. The resulting bonding process can be performed at a temperature as low as approximately 475°C, which is significantly lower than the melting point of aluminium (660°C). Brief Description of the Drawing The invention is described in greater detail below with reference to the accompanying drawings, in which Fig. 1 illustrates a known encapsulated (ceramic SMD package) SAW devi¬ ce, Fig. 2a is a top view of a SAW device encapsulated according to the invention with the top layer omitted, Fig. 2b is an end view of the device of Fig. 2a, and Fig. 2c is a side view of the device of Fig. 2a. Best Mode for Carrying Out the Invention The description describes the fabrication of a SAW device based on the piezoelectric material lithium niobate as starting point. This material can, however, also be used for other purposes, such as optical waveguides, holograms etc. , and accordingly the following example shall in no regard limit the scope of the invention. The SAW device of Fig. 2 comprises two substrates 1 a and 1 b bonded together by means of an intermediate layer of a suitable glass 2. This glass 2 is electrically insulating and allows therefore electrical feedthroughs 3. These feedthroughs 3 enables electrical access to the hermetically encap¬ sulated SAW device located in the void 4. The substrates are made of oxide materials, such as piezoelectric materials. The piezoelectric oxide materials are of the formula ABO 3 , where A is an alkali or an alkaline earth metal, and B is a transition metal. Piezoelectric oxide materials of a perov- skite or a perovskite-like structure or a helical structure can be used. The SAW device is produced in the following manner: 1 An electrode pattern 3 is provided on top of the substrate 1 a by means of photolitographic techniques known from the semiconductor industry. The material of the electrode pattern 3 can for instance be aluminium or an aluminium based material, such as an aluminium based alloy, optionally in the form of a multilayer structure, optionally on a thin metal film of Cr or Ti. As the substrates 1 a and 1 b may exhibit anisotrophy concerning thermal expansion, it is often impossible to find a glass 2 adapted in all directions to the substrates 1 a and 1 b. In order to minimize the thermally induced residual stress in the ready-made device, it is therefore important to choo¬ se a glass with a low modulus of elasticity. Accordingly, the chosen glass must have a melting point significantly below the melting point (660°C) of aluminium both in order to minimize thermally induced stress and in order to allow the use of aluminium based electrode materials. The commercially available lead borate (PbO-B 2 O 3 glass) SHOTT # 8472 has turned out to be suitable. In this example the intermediate glass layer 2 can for instance be provided by pouring 600°C hot melt of lead borate glass (PbO-B 2 O 3 ) onto an appro¬ ximately 600°C hot substrate 1 b. This glass is available in the form of a fine powder well suited for standard screen - and stencil printing techni¬ ques. The lead borate glass can optionally contain small amounts of AI 2 O 3 and BaO. Since no electrode pattern is present on the substrate 1 b, the precise temperature of the glass melt and the substrate 1 b during the pouring thereon of the glass melt is of minor importance as long as it is above the melting point of the glass which is approximately 350°C, and as long as the glass melt wets the surface of substrate 1 b sufficiently well. An approximate minimum temperature for this process step is 450°C. After an approximately 30 minutes long cool down period of the glass/substrate structure, the glass layer 2 can be grinded down to a uniform thickness of e.g. 100 μm using SiC 1 000 powder and water. When the substrates 1 a and 1 b are to be bonded together only along a portion thereof, such as when a hermetically sealed void 4 is being provi¬ ded, a pattern can be etched into the glass intermediate layer 2 by means of a 10% concentrated nitric acid at a temperature of 50°C. This nitric acid has a very high etch rate of approximately 100 μm/min. for the type of glass in question. The desired structure can be provided by cutting an equivalent pattern in an acid resistive film added to the surface of the glass layer 2. The film can for instance be a 70 μm thick PVC acrylate film: Nitto waferfilm SWT-20. The substrate 1 a with the electrode pattern 3 and the substrate 1 b with the optionally patterned glass intermediate layer 2 are placed on top of each other whereafter the entire structure is subjected to a suitable heat treatment in an oven. The heat treatment is performed at a temperature exceeding 250°C and below 500°C and lasts less than one hour. The resulting glass intermediate layer is slightly fluid and wets the surface of both the substrate 1 a and the electrode pattern 3 with the effect that a hermetically sealed bond is provided with electrical feedthroughs in one process step.

Claims: Claims. 1. A method of bonding oxide materials, such as piezoelectric mate¬ rials, by means of an intermediate layer (2) of a low temperature glass, a conductor pattern of for instance aluminium being applied onto one of the oxide materials, where the used glass has a melting point below 660°C, and where the maximum process temperature is below 660°C, viz. the melting point of aluminium, c h a ra ct e r i s ed by the used piezoelec¬ tric oxide materials being of the formula ABO 3 , where A is an alkali or an alkaline earth metal, and B is a transition metal, and by the used glass having a relatively low modulus of elasticity. 2. A method as claimed in claim 1, c h a ra cte ri se d by the used glass being a lead borate glass (PbO-B 2 O 3 ) optionally containing AI 2 O 3 and BaO. 3. A method as claimed in claim 1, c h a ract e ri sed by the glass melt being poured onto one layer of the oxide material and being grinded down to a desired thickness, such as 100 μm. 4. A method as claimed in claim 2, c h a r a c te ri s e d by etching a pattern into the glass layer, said pattern being masked by providing the glass intermediate layer (2) with an acid resistive film in which the desired pattern is cut out. 5. A method as claimed in any of the preceding claims, c hara cte¬ r i s e d in that the substrate (1a) with the electrode pattern (3) and the substrate (1b) with the optionally patterned glass intermediate layer are positioned opposite one another, whereafter the entire structure is subjec- ted to a heat treatment at a temperature above the melting point of the glass and below the melting point of aluminium. 6. A hermetically sealed device, such as an SAW device, comprising two substrates ( 1 a, 1 b) of oxide materials, such as piezoelectric materials, which are bonded together by means of an intermediate layer (2) of glass, whereby a conductor path pattern is provided on one substrate ( 1 a) and a patterned glass layer is provided on the second substrate ( 1 b) , and where¬ by the substrate (1 a) with the conductor pattern and the substrate ( 1 b) with the glass layer are subsequently positioned opposite one another and subjected to a heat treatment. AMENDED CLAIMS [received by the International Bureau on 07 April 1997 (07.04.97); original claims 1-6 replaced by new claims 1-4 (1 page)] 1. A method of bonding oxide materials, such as piezoelectric mate¬ rials, by means of an intermediate layer (2) of a low temperature glass, a conductor pattern of for instance aluminium being applied onto one of the 5 oxide materials, where the used piezoelectric oxide materials is of the formula AB0 3 , where A is an alkali or an alkaline earth metal and B is a transition metal, and the used glass has a melting point below 660°C (the melting point of aluminium) and a relatively low modulus of elasticity. 2. A method as claimed in claim 1, characte rised by the used 10 glass being a lead borate glass (PbO-B 2 O 3 ) optionally containing AI 2 O 3 and BaO. 3. A method as claimed in claim 1, characterised by the glass melt being poured onto one layer of the oxide material and being grinded down to a desired thickness, and by etching a pattern into said glass layer, 15 said pattern being masked by providing the glass intermediate layer (2) with an acid resistive film in which the desired pattern is cut out. 4. A method as claimed in any of the preceding claims, characte¬ ri s e d in that the substrate (1a) with the electrode pattern (3) and the substrate (1b) with the optionally patterned glass intermediate layer are 20 positioned opposite one another, whereafter the entire structure is subjec¬ ted to a heat treatment at a temperature above the melting point of the glass and below the melting point of aluminium.

Inventors: CHRISTENSEN FRANK K, CHRISTENSEN, FRANK, K.
Applicants: AMP DANMARK AF AMP HOLLAND B V, CHRISTENSEN FRANK K, AMP DANMARK, FILIAL AF AMP-HOLLAND B.V., HOLLAND, CHRISTENSEN, FRANK, K.
Application year: 1996
Main classification: H03H3
IPC codes: C04B37, G02B6, H03H3