Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H3/00—Apparatus or processes specially adapted for the manufacture of impedance networks, resonating circuits, resonators
  • H03H3/007—Apparatus or processes specially adapted for the manufacture of impedance networks, resonating circuits, resonators for the manufacture of electromechanical resonators or networks
  • H03H3/08—Apparatus or processes specially adapted for the manufacture of impedance networks, resonating circuits, resonators for the manufacture of electromechanical resonators or networks for the manufacture of resonators or networks using surface acoustic waves

Definitions

• 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.
• SAW surface acoustic wave
• 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.
• 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 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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- Fallss 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.
• 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.
• ABO 3 the used piezoelectric oxide materials
• B is a transition metal
• the used glass having a relatively low modulus of elasticity.
• 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).
• 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.
• Fig. 2c is a side view of the device of Fig. 2a.
• 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:
• 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.
• 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.
• the chosen glass 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.
• 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.
• the glass layer 2 can be grinded down to a uniform thickness of e.g. 100 ⁇ m using SiC 1 000 powder and water.
• 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 ⁇ m/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 ⁇ m 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.

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• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Surface Acoustic Wave Elements And Circuit Networks Thereof (AREA)

Applications Claiming Priority (3)

Publications (1)

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• 1995
  • 1995-11-16 DK DK128095A patent/DK128095A/da not_active Application Discontinuation
• 1996
  • 1996-11-15 EP EP96938027A patent/EP0861521A1/en not_active Ceased
  • 1996-11-15 JP JP9518518A patent/JP2000500111A/ja active Pending
  • 1996-11-15 CN CN 96198351 patent/CN1202283A/zh active Pending
  • 1996-11-15 WO PCT/DK1996/000470 patent/WO1997018628A1/en not_active Application Discontinuation

Non-Patent Citations (1)

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